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EXP 129

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EXP 129

RICE HUSK ASH MARKET STUDY

ETSU U/00/00061/REP

DTI/Pub URN 03/668

ContractorBronzeoak Ltd

First published 2003© Crown copyright 2003

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This publication contains information which iscommercially valuable to actual or potentialrenewable energy technology exporters. Permissionto lend, give or copy this publication to any personor organisation, in part or in its entirety, is strictlylimited to bona-fide UK exporters.

The work described in this report was carried outunder contract as part of the DTI New andRenewable Energy Programme. The views andjudgements expressed in this report are those of thecontractor and do not necessarily reflect those of theDTI.

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RICE HUSK ASH MARKET STUDY

EXECUTIVE SUMMARY

Objectives

• To determine the current markets for rice husk ash (RHA) through a publication review.

• To evaluate the current and potential value of each market.

• To determine the type and quality of RHA produced from different boilers and relate this tomarket specification.

• To analyse the economics of producing and selling the different types of RHA inconjunction with bioenergy projects.

Introduction

Globally, approximately 600 million tonnes of rice paddy is produced each year. On average 20%of the rice paddy is husk, giving an annual total production of 120 million tonnes. In the majority ofrice producing countries much of the husk produced from the processing of rice is either burnt ordumped as a waste.

The treatment of rice husk as a ‘resource’ for energy production is a departure from the perceptionthat husks present disposal problems. The concept of generating energy from rice husk has greatpotential, particularly in those countries that are primarily dependant on imported oil for theirenergy needs. Rice husks are one of the largest readily available but most under-utilised biomassresources, being an ideal fuel for electricity generation.

Rice husk is unusually high in ash compared to other biomass fuels – close to 20%. The ash is 92 to95% silica, highly porous and lightweight, with a very high external surface area. Its absorbent andinsulating properties are useful to many industrial applications.

RHA is a general term describing all types of ash produced from burning rice husks. In practice, thetype of ash varies considerably according to the burning technique. The silica in the ash undergoesstructural transformations depending on the conditions (time, temperature etc) of combustion. At550°C – 800°C amorphous ash is formed and at temperatures greater than this, crystalline ash isformed. These types of silica have different properties and it is important to produce ash of thecorrect specification for the particular end use.

If a long term sustainable market and price for rice husk ash (RHA) can be established, then theviability of rice husk power or co-generation plants are substantially improved. Many more plantsin the 2 - 5 MW range can become commercially viable around the world and this biomass resourcecan be utilised to a much greater extent than at present.

Potential and current uses of RHA

An extensive literature search has highlighted many uses of RHA. Two main uses have beenidentified, as an insulator in the steel industry and as a pozzolan in the cement industry.


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Steel industry:• RHA is used by the steel industry in the production of high quality flat steel. Flat steel is a

plate product or a hot rolled strip product, typically used for automotive body panels anddomestic 'white goods' products.

• RHA is an excellent insulator, having low thermal conductivity, high melting point, lowbulk density and high porosity. It is this insulating property that makes it an excellent‘tundish powder’. These are powders that are used to insulate the tundish, prevent rapidcooling of the steel and ensure uniform solidification in the continuous casting process.

Cement industry:• Substantial research has been carried out on the use of amorphous silica in the manufacture

of concrete. There are two areas for which RHA is used, in the manufacture of low costbuilding blocks and in the production of high quality cement.

• Ordinary Portland Cement (OPC) is expensive and unaffordable to a large portion of theworld's population. Since OPC is typically the most expensive constituent of concrete, thereplacement of a proportion of it with RHA offers improved concrete affordability,particularly for low-cost housing in developing countries.

The addition of RHA to cement has been found to enhance cement properties:• The addition of RHA speeds up setting time, although the water requirement is greater than

for OPC.• At 35% replacement, RHA cement has improved compressive strength due to its higher

percentage of silica.• RHA cement has improved resistance to acid attack compared to OPC, thought to be due to

the silica present in the RHA which combines with the calcium hydroxide and reduces theamount susceptible to acid attack.

• More recent studies have shown RHA has uses in the manufacture of concrete for themarine environment. Replacing 10% Portland cement with RHA can improve resistance tochloride penetration.

• Several studies have combined fly ash and RHA in various proportions. In general, concretemade with Portland cement containing both RHA and fly ash has a higher compressivestrength than concrete made with Portland cement containing either RHA or fly ash on theirown.

RHA can also replace silica fume in high strength concrete. Silica fume or micro silica is the mostcommonly used mineral admixture in high strength concrete. The major characterisitics of RHA areits high water demand and coarseness compared with condensed silica fume. To solve theseproblems RHA needs to be ground finely into particles of 8-10 µm and a superplasticizer added toreduce water requirement. There are two patents for a ground RHA cement additive that closelymatches the performance of silica fume.

Other less wide spread uses have also been identified:• Due to its insulating properties, RHA has been used in the manufacture of refractory bricks.• There are reports of RHA being used in the manufacture of lightweight insulating boards in

developing countries.• Several studies have been carried out to purify RHA for use in silicon chip manufacture. The

techniques are still being developed, but appear promising.


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• It is known that farmers in Asia will use RHA to prevent insect attack in stored food stuffs,and several scientific studies have been carried out to test the efficacy of this.

• Greenwich University are researching small-scale paddy milling in Bangladesh andVietnam, and the possibility of using RHA for water purification.

• A company in the USA have produced a proto type plant for manufacturing activated carbonfrom RHA, and the major market for this is in water purification.

• There are several reports detailing the use of RHA in vulcanising rubber, although in smallscale experiments.

• There are reports of RHA being used as a soil ameliorant to help break up clay soils andimprove soil structure.

• Husks burnt slowly over a period of six months have been found to be effective as an oilabsorbent and are marketed in California.

Potential and current markets for RHA

Low value or small markets have been identified:• The use of RHA as an oil absorbent is very small and localised and much work would need

to be done to expand the market from currently just one US State, California.• The use of RHA in the manufacture of refractory bricks is too small a market to be

considered as an outlet for large quantities of ash, although it may be suitable for a limitednumber of energy plants.

• Other uses with limited commercial potential, due to localised low value use, are the controlof insect pest in stored food stuffs and as a soil ameliorant.

Markets with potential in the future:• There are potential markets for RHA in the silicon chip industry, which is expanding.

However, the technique of refining RHA to the desired quality has not yet been establishedon a large scale, and it could be many years before such an application is market ready.

• The production of lightweight construction materials and insulation from RHA haspotential, but current use is not widespread and there is limited knowledge of the methodsused.

• The production of activated carbon using RHA has great potential. The share RHA couldplay in this market is not yet clear, and more research on the methodology and cost ofproducing activated carbon from RHA is needed.

• The use in industrial chemical processes such as in the rubber vulcanising process are a longway from being market ready.

Commercially viable markets:Currently the largest and most commercially viable markets appear to be in the concrete and steelindustry.

Steel industry:• The market within the steel industry is well established.• However there are constraints to the expansion of this market due to health issues associated

with using RHA. Crystalline ash, the form preferred by the steel industry, is carcinogenicand the use of RHA is banned in some European countries. This trend is likely to increase.

• There is more scope for development in Asian and Eastern markets, but the future size of themarket is not certain.


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• Some companies are pelletising RHA, and state that this overcomes the health issues. Thereis controversy over whether this is the case, but some steel manufacturers have converted topellets.

Cement industry:• The cement markets are not as well developed as steel, but there is great potential for the use

of RHA in this area.• It is currently not being used to any extent, except in the USA. Two main issues appear to be

limiting its use: lack of awareness of the potential for RHA and the quality of the productitself.

• The cement industry requires amorphous ash, so there are none of the health issuesassociated with crystalline ash.

• The cement industry has to produce a consistent, high quality and standard product. This inturn requires RHA from a controlled combustion environment, to ensure a consistentstandard ash. Producing RHA of the correct quality may cost more than producing normalash due to boiler modifications etc.

• In addition to the use of the ash, it may be possible to generate Certified EmissionReductions (carbon credits) when substituting for Portland cement. Portland cement requiresenormous heating in its manufacture and avoiding the energy (derived from fossil fuels) andthus carbon emitted from its manufacture could generate an additional income stream for theproducers of RHA.

Technical overview

• There is a wide range in the physical and chemical properties of RHA. The chemical andphysical properties of the ash may be influenced by the soil chemistry, paddy variety andfertiliser use.

• The change from amorphous to crystalline ash occurs at approximately 800ºC, although theprocess is often ‘incomplete’ until 900ºC is achieved. All the combustion processes devisedto burn rice husks remain below 1440ºC, which is the RHA melting temperature.

• The most commonly used boilers are based on fixed grate technologies, which tend toproduce ash with high carbon content, high LOI content and high proportions of crystallineto amorphous ash. This type of ash is more suited to the steel industry.

• Suspension fired boilers generally produce more amorphous ash than stoker fired boilersdespite the fact that they may operate at higher temperatures. This is because the operatingtime at high temperatures for suspension fired boilers is comparatively short.

• Commonly, in the production of highly amorphous ash, low temperatures and fairly long“burn-times” are used.

• Fly ash is a fine material and is of higher marketable value since it requires less grindingthan the generally coarser bottom ash.

Conclusions

• Small markets exist for RHA in the manufacture of refractory bricks and as an oil absorbent.• Potential markets in the future include silicon chip manufacture, in the manufacture of

activated carbon, and in the production of lightweight construction materials and insulation.• Currently the largest and most commercially viable markets appear to be in the concrete and

steel industry.


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• The market within the steel industry is well established, but there are constraints to theexpansion of this market due to health issues associated with using crystalline ash.

• The cement markets are not as well developed as steel, but there is great potential for the useof amorphous RHA. Two main issues appear to be limiting its use: lack of awareness of thepotential for RHA and the quality of the product itself. Boiler modifications may be requiredto produce ash of the quality required.

Recommendations for the future

• The best choice would seem to be to produce RHA for the steel industry as this requires noboiler modifications and attracts a high price.

• However, our market study suggests that, whereas growth in the market for RHA to the steelmarket is limited, growth in the market for RHA in the cement industry is growing and ispotentially very large.

• A new entrant to the market place may prefer to target the somewhat less high returns butbetter longer term prospects of the amorphous silica market.


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1. INTRODUCTION TO THE STUDY 1

1.1 Objectives of study: 2

2. GLOBAL RICE PRODUCTION AND USE 3

2.1 Global rice Production 32.2 Factors influencing the use of rice husk 53.1 Steel industry 93.2 Cement and concrete 10

3.2.1 Introduction 103.2.2 Low cost building blocks 113.2.3 Enhanced properties of RHA cement 113.2.3 RHA as a replacement for silica fume in high strength concrete 13

3.3 Refractory bricks 143.4 Lightweight construction materials 143.5 Silicon chips 143.6 Control of insect pests in stored food stuffs 153.7 Water purification 153.8 Vulcanising rubber 153.9 Adsorbent for a gold-thiourea complex 153.10 Ceramics 153.11 Soil ameliorant 163.12 Oil absorbent 163.13 Other uses 16

4. MARKET REVIEW 20

4.1 Introduction 204.2 Low value or small markets 204.3 Potential markets in the future 204.4 Current markets 204.4 Steel 22

4.4.1 Global overview of steel production 224.4.2 Factors affecting the demand for RHA in the steel industry 23• Europe 24• Asia 25• USA 25• Australia 264.4.3 Prices and future trends 26

4.5 Cement and concrete industry 264.5.1 Introduction 264.5.2 RHA as substitute for silica fume 26• USA 27• UK 27• Australia 284.5.2 Future trends and prices 284.5.3 RHA in building block manufacture 28

5. TECHNICAL REVIEW 29

5.1 Introduction 29


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5.2 Overview of husk to ash process 295.2.1 Rice husk as a fuel 295.2.2 Incineration 305.2.3 Boilers with integral combustion 30• Stoker fired 31• Suspension fired 31• Fluidised bed combustors 315.2.4 Gasification 32

5.3 Overview of ash production 325.4 Methods of ash analysis 33

5.5.1 Temperature 335.5.2 Geographical region 34

5.6 Review of influence of combustion method on properties of RHA 345.6.1 Fixed grate boilers 355.6.2 Fluidized bed 365.6.3 Circulating Fluidised Bed (CFB) 365.6.4 Grate versus ‘conventional’ 365.6.5 Gasification 365.6.6 Additional Technology 375.6.7 Special market requirements 37

5.7 Summary of technical analysis 37

6. HEALTH ISSUES 41

6.1 Diseases 416.1.1 Silicosis 416.1.2 Cancer 416.1.3 Other diseases 41

6.2 Exposure limits 426.3 Measures to control exposure 426.4 Health issues in relation to use of RHA 42

7. COST BENEFIT ANALYSIS 43

7.1 Introduction 437.2 RHA Disposal – Negative Benefit 437.3 RHA with Significant Quantity of Crystalline Silica 447.4 RHA with High Amorphous Ash Content 457.5 Concluding Remarks 45

8. POTENTIAL TO EARN CARBON CREDITS 46

8.1 Introduction 468.2 Role of RHA in reducing GHG emissions 468.3 Calculating the value of CERs from Portland cement substitution 46

9. CONCLUSIONS 48

REFERENCES 49

ACKNOWLEDGEMENTS 54

APPENDIX A


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1. INTRODUCTION TO THE STUDY

Rice covers 1% of the earth’s surface and is a primary source of food for billions of people.Globally, approximately 600 million tonnes of rice paddy is produced each year. On average 20%of the rice paddy is husk, giving an annual total production of 120 million tonnes. In the majority ofrice producing countries much of the husk produced from the processing of rice is either burnt ordumped as waste.

Rice husks are one of the largest readily available but most under-utilised biomass resources, beingan ideal fuel for electricity generation. The calorific value varies with rice variety, moisture andbran content but a typical value for husks with 8-10% moisture content and essentially zero bran is15 MJ/kg.

The treatment of rice husk as a ‘resource’ for energy production is a departure from the perceptionthat husks present disposal problems. The concept of generating energy from rice husk has greatpotential, particularly in those countries that are primarily dependent on imported oil for theirenergy needs. For these countries, the use of locally available biomass, including rice husks is ofcrucial importance.

Rice husk is unusually high in ash compared to other biomass fuels – close to 20%. The ash is 92 to95% silica (SiO2), highly porous and lightweight, with a very high external surface area. Itsabsorbent and insulating properties are useful to many industrial applications, and the ash has beenthe subject of many research studies.

If a long term sustainable market and price for rice husk ash (RHA) can be established, then theviability of rice husk power or co-generation plants are substantially improved. A 3 MW powerplant would require 31,000 tonnes of rice husk per year, if operating at a 90% capacity factor. Thiswould result in 5580 tonnes of ash per year. Revenue from selling the ash for beneficial use woulddecrease the pay-back period for the capital needed to build the project. Many more plants in the 2 -5 MW range can become commercially viable around the world and this biomass resource can beutilized to a much greater extent than at present.

Rice husk ash has many applications due to it’s various properties. It is an excellent insulator, sohas applications in industrial processes such as steel foundries, and in the manufacture of insulationfor houses and refractory bricks. It is an active pozzolan and has several applications in the cementand concrete industry. It is also highly absorbent, and is used to absorb oil on hard surfaces andpotentially to filter arsenic from water.

More recently, studies have been carried out to purify it and use it in place of silica in a range ofindustrial uses, including silicon chip manufacture.

RHA is a general term describing all types of ash produced from burning rice husks. In practice, thetype of ash varies considerably according to the burning technique. Two forms predominate incombustion and gasification. The silica in the ash undergoes structural transformations dependingon the temperature regime it undergoes during combustion. At 550°C – 800°C amorphous silica isformed and at greater temperatures, crystalline silica is formed.

These types of silica have different properties and it is important to produce ash of the correctspecification for the particular end use. There are health issues associated with the use of crystalline



ash, inhalation of which can lead to a number of diseases, the most common being silicosis. Thisaffects the potential markets for this type of ash.

1.1 Objectives of study:

An understanding of the market for RHA and the parameters under which it operates will maximiserevenue of husk to energy plants by exploiting the market opportunities available for the RHA.World wide, rice husk fuelled power plants have a high replicability potential if the projecteconomics satisfy the required criteria. This market assessment contributes to the project viabilityand will work as a catalyst in promoting this type of power plant.

The objectives of the study are:

• To determine the current markets for rice ash through a publication review.• To evaluate the current and potential value of each market.• To determine the type and quality of rice husk ash produced from different boilers and relate

this to end-user specification.• To analyse the economics of producing and selling the different types of ash in conjunction

with bioenergy projects.



2. GLOBAL RICE PRODUCTION AND USE

2.1 Global rice Production

Rice is grown on every continent except Antarctica and covers 1% of the earth’s surface. It is aprimary source of food for billions of people, and ranks second to wheat in terms of area andproduction [1]. During growth, rice plants absorb silica from the soil and accumulate it into theirstructures. It is this silica, concentrated by burning at high temperatures removing other elements,which make the ash so valuable.

The annual production of paddy rice (Oryza sativa) globally was 579,500,000 tonnes in 2002 [1].Of this, 95% was produced by 20 countries, as shown in Table 1.

Rice, PaddyProduction in 2002

(t)

Percentage ofTotal PaddyProduction

Husk Produced(20% of total) (t)

Potential AshProduction (18% of

husk) (t)China 177,589,000 30.7% 35,517,800 6,393,204India 123,000,000 21.2% 24,600,000 4,428,000

Indonesia 48,654,048 8.4% 9,730,810 1,751,546Bangladesh 39,000,000 6.7% 7,800,000 1,404,000Viet Nam 31,319,000 5.4% 6,263,800 1,127,484Thailand 27,000,000 4.7% 5,400,000 972,000Myanmar 21,200,000 3.7% 4,240,000 763,200Philippines 12,684,800 2.2% 2,536,960 456,653

Japan 11,264,000 1.9% 2,252,800 405,504Brazil 10,489,400 1.8% 2,097,880 377,618USA 9,616,750 1.7% 1,923,350 346,203

Korea 7,429,000 1.3% 1,485,800 267,444Pakistan 5,776,000 1.0% 1,155,200 207,936

Egypt 5,700,000 1% 1,140,000 205,200Nepal 4,750,000 0.8% 950,000 171,000

Cambodia 4,099,016 0.7% 819,803 147,565Nigeria 3,367,000 0.6% 673,400 121,212

Sri Lanka 2,794,000 0.5% 558,800 100,584Colombia 2,353,440 0.4% 470,688 84,724

Laos 2,300,000 0.4% 460,000 82,800Rest of the World 29,091,358 5.0% 5,818,272 1,047,289

Total (World) 579,476,722 100% 115,895,344 20,861,162

Table 1 Rice paddy, and potential husk and ash production in the 20 highest producingcountries,2002 [1].

Production of rice is dominated by Asia, where rice is the only food crop that can be grown duringthe rainy season in the waterlogged tropical areas.

Most paddy is produced by China (31%) followed by India (21%). Assuming a husk to paddy ratioof 20% [2], and a ash to husk ratio of 18% [3], the total global ash production could be as high as21,000,000 tonnes per year.



Globally, rice production is increasing, with an increase of 10% from 1992 – 2002 (Figure 1). Ofthe top 20 countries only China and Japan produced less rice in 2002 than in 1992. In China, whichproduces the most rice in the world, production fell by 10 000 000 tonnes, or approximately 6% [1].

0

20,000,000

40,000,000

60,000,000

80,000,000

100,000,000

120,000,000

140,000,000

160,000,000

180,000,000

200,000,000

China

USA

To

nn

es

of

Ric

e

Figure 1 Chart showing the 20 highest rice paddy producing countries in 2002.

0

20,000,000

40,000,000

60,000,000

80,000,000

100,000,000

120,000,000

140,000,000

160,000,000

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200,000,000

China

To

nn

es

of

Ric

e

1992

2002

Figure 2 A comparison of rice paddy production in the 20 highest producing countries, 1992and 2002.Yields are affected by several factors, including the agronomy of the crop. This is influenced by thephysical and cultural environments and scale under which the rice is grown (Plate 1). Internationalco-ordination and co-operation in technological advances of rice production is providing



alternatives to the limitations of cultural practices, by the use of chemical fertilisers, insecticides,pesticides and introduction of new varieties. Rice production is often set back by the weather,monsoons and droughts, but the effects of this are increasingly being limited by irrigation and watercontrol systems [3].

The chemical properties of ash arising from rice husks are thought to vary from region. Thedifferences have been attributed to the conditions under which the paddy is grown, such as climate,soil, and use of fertilisers (discussed in Section 5.5.2).

2.2 Factors influencing the use of rice husk

Although the potential global estimate of RHA production is 21,000,000 tonnes (Table 1), theactual scope for utilisation is considerably less. The majority of mills from which the husks aresourced are small and dispersed within developing countries. For example in Indonesia, 93 % allmills produce less than 10,000 t/year [4, 5]. This makes collection of the resource logisticallyproblematical, and currently husks are dumped and burnt in open piles (Plates 3 and 4). The ashproduced is of poor quality and is often used domestically in small quantities for cleaning glasswareand cooking utensils (Plate 10 and Figure 3).

Thus in rural catchment areas the collection of rice husks and security of fuel supply tends to limitthe practical size of biomass power plants. It is estimated that the optimum size of power plants insuch areas is between 2-5 MW, producing up to 10,000 tonnes of ash per year [5].

Larger rice mills such as the Patum rice mill in Thailand produce 320,000 t/year, and already utilisehusk for cogeneration (Plate 2) [6]. In developed countries, where the mills are typically larger,disposal of the husks is a big problem. Burning in open piles is not acceptable on environmentalgrounds, and so the majority of husk is currently going into landfill (Figure 3). The cost of this(discussed in Section 7) erodes the profit of the milling company. This has led to many researchprogrammes into potential end uses of both husk and ash, primarily in the USA.


6

Figure 3 Flow chart showing movements of rice husk and ash under two scenarios – mills with cogeneration or similar facilities, and mills without.

Ash collected forlocal use eg

• cleaningglassware

• control of pestsin stored food

Paddy fields

Rice mills withcogeneration orsimilar facilities

Rice mills withoutcogeneration orsimilar facilities

DomesticMarket

InternationalMarket

ASHFROM

BOILER

HUSKS

LandfillBurnt inopen piles

Localuses

eg• fuel• building

eg• steel industry

eg• steel industry• building block

manufacture• research uses• landfill

Developedcountries

Developingcountries

7

Plate 1 Planting rice in Indonesia

Plate 2 Rice arriving at Patum Rice Mill, Thailand

8

Plate 3 Rice husks piles being burnt in Nicaragua

Plate 4 Close-up of burning rice husk

9

3. PUBLICATION REVIEW OF USES FOR RICE HUSK ASH

3.1 Steel industry

RHA is used by the steel industry in the production of high quality flat steel. Flat steel is a plateproduct or a hot rolled strip product, typically used for automotive body panels and domestic 'whitegoods' products [7].

This type of steel is generally produced by continuous casting, which has replaced the older ingotmethod. In the ingot method molten steel was poured into a large mould where it would be allowedto cool and solidify to form an ingot. The ingot would then be rolled in primary mills, in the firststage of its transformation into a usable steel product. In developed countries this process haslargely been superseded by the continuous casting process (concaster), although the ingot method isretained for certain applications where it is the most suitable way of producing the steel required.Elsewhere this is not always the case, with many of the steel industries of Eastern Europe and Asiastill relying heavily on the old ingot method [7,8].

In continuous casting, a ‘ladle’ of steel, containing more than 200 tonnes of molten metal at1650°C, empties into a tundish, a receptacle that holds the steel and controls its flow in thecontinuous process. From the tundish the steel passes in a controlled manner to a water cooledmould where the outer shell of the steel becomes solidified. The steel is drawn down into a series ofrolls and water sprays, which ensure that it is both rolled into shape and fully solidified at the sametime. At the end of the machine, it is straightened and cut to the required length. Fully formed slabsemerge from the end of this continuous process [7, 8, 9].

It is in continuous casting that RHA plays a role. RHA is an excellent insulator, having low thermalconductivity, high melting point, low bulk density and high porosity. It is this insulating propertythat makes it an excellent ‘tundish powder’. These are powders that are used to insulate the tundish,prevent rapid cooling of the steel and ensure uniform solidification [10]. Traditionally ash is sold inbags which are thrown on to the top of the surface of the tundish of molten steel (Plate 6).Approximately 0.5 to 0.7 kg of RHA is used per tonne of steel produced [11].

There are health issues associated with the use of RHA in the steel industry. Traditionallycrystalline ash is preferred to amorphous. This poses problems as the ash has a tendency to explodeover the operator when it is being thrown on top of the tundish, exposing them to crystalline silicaand possible silicosis (refer to Section 7). A new innovation is the production of pellets from RHAwhich can be much better controlled, and are better from an operational and safety point of view[12, 13].

RCL Ricegrowers Ltd with Biocon in Australia have devised a method for making pellets, althoughno details are publicly available [12]. The National Research Development Centre (NRDC) in Indiahave also devised a method for making pellets, which they claim will spread over the top of themolten steel more easily. Details of the technique are sparse, the husk is first pulverised in a millprior to combustion and then certain chemicals added, the pellets formed and then dried at 350°C[13]. The resulting pellets have the following characteristics:

10

SiO2 90%Bulk density 0.5– 0.6g/ccSize 1–10mm diameterStrength 20 – 50kg/cm2

Porosity 60 – 70%

Table 2 Characteristics of silica nodules produced by the NRDC [13].

However, research by CORUS (formerly British Steel) at the Teeside Technology Centre, has castdoubts on the safety even of pellets [10]. They tested amorphous ash and found that there were fewhealth problems, as there was no crystalline silica, and the ash proved to be equally as good aninsulator as the traditionally used crystalline ash. The problems occurred when emptying thetundish at the end of the process. It was found that the heating of the steel for 4 hours at 1500°C hadtransformed the silica from its amorphous form into cristobalite and tridymite, crystalline forms ofsilica with serious health risks associated (discussed in Section 7). It is likely that the same chemicaltransformation will occur with pellets, and so CORUS do not see them as the ideal solution [10].

There are also issues of steel quality relating to the use of RHA. Although RHA is an excellentinsulator, it will oxidise with elements in steel such as aluminium to form alumina (Al2O3). This is anon-metallic compound that remains in the steel and is a nuisance in future use. Despite this it isstill used in the production of certain steel where its insulating properties are necessary.Approximately ten years ago some tundish powders were produced incorporating a proportion ofRHA, but these are not currently being used [10].

3.2 Cement and concrete

Substantial research has been carried out on the use of amorphous silica in the manufacture ofconcrete. There are two areas for which RHA is used, in the manufacture of low cost buildingblocks and in the production of high quality cement.

3.2.1 Introduction

Concrete is produced by mixing Portland cement with fine aggregate (sand), coarse aggregate(gravel or crushed stone) and water [14] (Figure 4).

Figure 4 The composition of concrete [15]

Approximately 11% of ready mix concrete is Portland cement (Figure 4). It is the binding agentthat holds sand and other aggregates together in a hard, stone-like mass. Cement is made by heatinglimestone and other ingredients to 1450°C in a kiln to produce clinker, this involves the dissociation

11

of calcium carbonate under heat, resulting in lime (calcium hydroxide) and CO2. The lime thencombines with other materials to form clinker, while the CO2 is released to the environment. Thepulverised/ground clinker mixed with gypsum is called Portland cement [14].

Small amounts of admixtures are often added. Admixtures are either naturally occurring compoundsor chemicals produced in an industrial process, which improve the properties of the cement [14].

Most admixtures are pozzolans. A pozzolan is a powdered material, which when added to thecement in a concrete mix reacts with the lime, released by the hydration of the cement, to createcompounds which improve the strength or other properties of the concrete [16, 17, 18]. Pozzolansimprove strength because they are smaller than the cement particles, and can pack in between thecement particles and provide a finer pore structure. RHA is an active pozzolan.

RHA has two roles in concrete manufacture, as a substitute for Portland cement, reducing the costof concrete in the production of low cost building blocks, and as an admixture in the production ofhigh strength concrete. The type of RHA suitable for pozzolanic activity is amorphous rather thancrystalline.

3.2.2 Low cost building blocks

Ordinary Portland cement (OPC) is expensive and unaffordable to a large portion of the world'spopulation. Since OPC is typically the most expensive constituent of concrete, the replacement of aproportion of it with RHA offers improved concrete affordability, particularly for low-cost housingin developing countries [19].

The potential for good but inexpensive housing in developing countries is especially great. Studieshave been carried out all over the world, such as in Guyana, Kenya and Indonesia on the use of lowcost building blocks [20, 21, 22]. Portland cement is not affordable in Kenya and a study showedthat replacing 50% of Portland cement with RHA was effective, and the resultant concrete cost25% less [21].

Using a concrete mix containing 10% cement, 50% aggregate and 40% RHA plus water, anIndonesian company reported that it produced test blocks with an average compressive strength of12N/mm2 [22]. This compares to normal concrete blocks, without RHA, which have an averagecompressive strength of 4.5 to 7N/mm2 or high strength concrete blocks which have a compressivestrength of 10N/mm2. Higher strength concrete with RHA allows lighter weight products to beproduced, such as hollow blocks with enhanced thermal insulation properties, which provide lighterwalls for steel framed buildings (Plate 7). It also leads to reduced quantities of cement andaggregate.

3.2.3 Enhanced properties of RHA cement

Portland cement produces an excess of lime. Adding a pozzolan, such as RHA, which combineswith lime in the presence of water, results in a stable and more amorphous hydrate (calciumsilicate). This is stronger, less permeable and more resistant to chemical attack [23].

A wide variety of environmental circumstances such as reactive aggregate, high sulphate soils,freeze-thaw conditions, and exposure to salt water, de-icing chemicals, and acids are deleterious toconcrete. Laboratory research and field experience has shown that careful use of pozzolans is useful

12

in countering all of these problems. The pozzolan is not just a "filler”, but a strength andperformance enhancing additive. Pulverised fly ash and ground granulated blast furnace slag are themost common pozzolan materials for concrete.

Many studies have been carried out to determine the efficacy of RHA as a pozzolan. They haveconcentrated on the quantity of ash in the mix and the improved characteristics resulting from itsuse. A comprehensive study undertaken by the Asian Institute of Technology in Bangkok, issummarised below, together with other results.

• Comparison of Setting Times

Table 3 compares the setting characteristics of OPC and RHA cement paste [24]. It can be seen thatthe addition of RHA speeds up setting time, although the water requirement is greater than for OPC.

Type ofMortar

WaterRequirement

Initial SettingTime

Final SettingTime

OPC 29 litres 105 minutes 225 minutesRHA 36 1itres 113 minutes 180 minutes

Table 3 Influence of RHA on setting times of cement at a replacement rate of 35% [24].

• Compressive Strengths

Figure 5 compares the compressive strengths of OPC and RHA mortars. At 35 % replacement, theRHA cement had improved early strength and, due to its higher percentage of silica, the RHAcement also had a higher compressive strength at later dates. Other studies have also shown that at28 days RHA cement had significantly greater rates of compressive strength compared with OPC[25]. Highest compressive strength has been obtained when 35% of Portland cement is replacedwith RHA. If 50% is replaced then strength can be considerably reduced [24].

0

10

20

30

40

50

60

3 7 28

Maturity (days)

Co

mp

ress

ive

str

en

gth

(M

Pa

)

OPC

RHA

Figure 5 Compressive strength of RHA cement and OPC [24].

13

• Resistance against Acid Attack

RHA cement was exposed to a mixture of 10% Hydrochloric Acid and 10% Sulphuric Acid andwas found to have more resistance than OPC. It is the silica present in the RHA which combineswith the calcium hydroxide and reduces the amount susceptible to acid attack as well as reducingpermeability .

• Resistance against chlorine

More recent studies have shown RHA has uses in the manufacture of concrete for the marineenvironment [26]. Replacing 10% Portland cement with RHA can improve resistance to chloridepenetration. Capillary suction and accelerated chloride diffusity are also improved by addition ofRHA, (Table 4).

Accelerated chloridediffusity (m2/s10-12)

Electrical resistivity(ohm m)

Effective chloridediffusity (m2/s10-12)

Water –binderratio Control RHA Control RHA Control RHA

0.6 2.4 1.0 31 63 63 2.40.5 1.7 0.3 44 107 2.5 1.50.4 1.3 0.1 53 172 3.6 1.4

Table 4 Effect of RHA on the resistance of concrete against chloride penetration after oneyear of exposure to seawater.

• Mixing RHA and fly ash from coal fired power plants

Several studies have tried combinations of fly ash and RHA. In general, concrete made withPortland cement containing both RHA and fly ash has a higher compressive strength than concretemade with Portland cement containing either RHA or fly ash on their own [27, 28].

3.2.3 RHA as a replacement for silica fume in high strength concrete

Silica fume or micro silica is the most commonly used mineral admixture in high strength concrete[19]. Though difficult (and expensive) to handle, transport, and mix, it has become the chosenfavourite for very high-strength concretes (such as for high rise buildings). The American Societyfor Testing and Materials (ASTM) place RHA in the same class as silica fume – that of a highlyreactive pozzolan. The previously cited results demonstrate that RHA strengthens concrete andRHA could potentially replace silica fume.

Silica fume is a waste product of the silicon metal and ferrosilicon industry. The electro-metallurgical processes involve the reduction of high purity quartz in electric arc furnaces attemperatures of over 2000°C. The silica fume is formed when SiO gas, given off as quartz reduces,mixes with oxygen in the upper parts of the furnace. Here the SiO is oxidised to SiO2, condensinginto the pure spherical particles of silica fume, forming the major part of the smoke or fume fromthe furnace. The silica fume is a super-fine powder of almost pure amorphous silica [29]. Theaverage particle size is 0.15µm in diameter, so every microsphere is 100 times finer than a cementgrain [30].

14

The major characterisitics of RHA are its high water demand and coarseness compared withcondensed silica fume. To solve these problems RHA needs to be ground finely (for at least 1hour15 minutes, depending on the grinding process) into particles of 8-10µm and a superplasticizeradded to reduce water requirement [29]. There are two patents for a ground RHA cement additivethat closely matche the performance of silica fume [31, 32], filed by P.K.Mehta at UC Berkley. In1995 the Pacific Gas and Electric Company restored the Bowman South Arch Dam using two testblocks of RHA concrete using this additive. Three years later there was no evidence of scaling orcracking, yet the surrounding plain Shotcrete was severely damaged [33].

The patents are currently owned by RHA Technology (“RHA Tech”), recently acquired by AlchemixCorporation of Carefree, Arizona. So far only one power plant in the USA has been found that iscapable of producing the right quality of fly ash as a silica fume substitute – Agrilectric Power’ssuspension-fired 13 MW Louisiana plant [34]. The main quality issue is the carbon content, whichfor the RHA produced by Agrilectric, is just 4 – 6%.

3.3 Refractory bricks

Due to its insulating properties, RHA has been used in the manufacture of refractory bricks [11].Refractory bricks are used in furnaces which are exposed to extreme temperatures, such as in blastfurnaces used for producing molten iron and in the production of cement clinker. The market issmall, and other synthetic alternatives are preferred. However a UK company, GORICON,isinterested in using it [35]. Commercial details of prices and quantities are not available.

3.4 Lightweight construction materials

There is anecdotal evidence of RHA being used in the manufacture of lightweight insulating boardsin developing countries [11]. Research at the University of Arkansas has also focused themanufacture of insulation from RHA (Plate 8). The material produced is very low density and solightweight it floats [36].

3.5 Silicon chips

The first step in semi-conductor manufacture is the production of a wafer, a thin round slice ofsemi-conductor material, which is usually silicon. Purified polycrystalline silicon (traditionallycreated from sand) is heated to a molten liquid and a small piece of silicon (seed) placed in themolten liquid. As the seed is pulled from the melt the liquid cools to form a single crystal ingot.This is then ground and sliced to form wafers which are the starting material for manufacturingintegrated circuits [37, 38].

Biocon in Australia have carried out work on purifying amorphous RHA but can only get to about99.9% purity at a great cost, and so Biocon consider that there are no real market opportunities withsilicon chips [12]. However The Indian Space Research Organisation has successfully developedtechnology for producing high purity precipitated silica from RHA and this has a potential use inthe computer industry [38]. A consortium of American and Brazilian scientists have also developedways to extract and purify silicon with the aim of using it in semiconductor manufacture [39]. Acompany in Michigan is purifying RHA into silica suitable for several industries, including siliconchip manufacture [40].

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3.6 Control of insect pests in stored food stuffs

It is known that farmers in Asia will use RHA to prevent insect attack in stored food stuffs. Severalscientific studies have been carried out to test the efficacy of this. The ash used is that from openfires, and so is predominantly crystalline.

Indonesian soy beans are sometimes infested by Graham bean beetles (Callosobruchus analis).RHA has been shown to prevent this by mixing 0.5% ash to soy bean. RHA was shown to be betterthan wood ash and lime, and the report concludes that RHA is ‘highly effective in controlling C.analis beetles’ [41].

It is also thought that RHA can control beetles such as adzuki bean weevil (C. chinensis) whichattacks stored mung beans. RHA was also shown to keep stored potatoes free of the Potato tubermoth (Phthorimaea operculelle) for up to 5 months of storage [42].

It is thought that the insects are irritated by the high levels of silicon and the needle like particles[41].

3.7 Water purification

The use of RHA as a water purifier is generally known, although only one documented study couldbe found [43]. Greenwich University are researching small scale paddy milling in Bangladesh andVietnam, an objective is to find end-uses for the ash, and the possibility of using it for waterpurification. Tests so far have indicated that RHA is inefficient in removing arsenic from water[44]. AgriTech in USA have produced a proto type plant for manufacturing activated carbon fromRHA, and the major market for this is in water purification.

3.8 Vulcanising rubber

There are several reports detailing the use of RHA in vulcanising rubber. In the laboratory RHA hasbeen shown to offer advantages over silica as a vulcanising agent for ethylene-propylene-dieneterpolymer (EPDM), and is recommended as diluent filler for EPDM rubber [45]. No analysis of theash is given so it is not known if it is amorphous or crystalline.

3.9 Adsorbent for a gold-thiourea complex

Gold is often found in nature as a compound with other elements. One way it is extracted is to leachit by pumping suitable fluids through the gold bearing strata. RHA produced by heating rice husksat 300°C has been shown to adsorb more gold-thiourea than the conventionally used activatedcarbon [46]. Ash produced by heating husks to 400° and 500°C was found not to absorb goldthiourea complex.

3.10 Ceramics

There is very little information on the use of RHA in ceramic glazes, other than that it must be pureand high quality [47].

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3.11 Soil ameliorant

There are reports of RHA being used as a soil ameliorant to help break up clay soils and improvesoil structure [11]. Its porous nature also assists with water distribution in the soil. It is not soldwidely on the commercial market for this use, and is a low value market. RHA has no fertilisingpotential as it does not provide the essential nutrients necessary for plant growth. Research in USAhas also been carried out on using it as a potting substrate for bedding plants. RHA was found toincrease the pH of the soil, and so was recommended for use with plants which require alkaline soil,or in situations where acid irrigation water is present [48]. Wadham Biomass Facility, Californiasells its ash to environmental remediation companies as an ingredient in a patented environmentalprocess for treating metals-tainted soil and similar waste streams [49].

3.12 Oil absorbent

Husks burnt slowly over a period of six months have been found to be effective as an oil absorbentand are marketed in California under the trade name ‘Greasweep’ (Plate 9). This is a relativelysmall operation, but there is potential to increase this market. It is thought it is amorphous ash thatis being used [50]. Other research studies have examined the absorption of vacuum pump oil [51]and the reduction of fatty acids in frying oils [52].

3.13 Other uses

There are other uses for RHA which are still in the research stages [11]:• in the manufacture of roof tiles• as a free running agent for fire extinguishing powder• an abrasive filler for tooth paste• a component of fire proof material and insulation• as a beer clarifier• extender filler for paint• production of sodium silicate films [53, 54]

Throughout Asia, RHA is used domestically to clean glassware (Plate 10).

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Plate 5 Rice husk ash at Riceland mill, Thailand.

Plate 6 Bags of ash at Riceland mill, Thailand, for shipping to Germany for use in the steelindustry.

18

Plate 7 ‘Hakkablocks’ building blocks in Indonesia.

Plate 8 A scientist at the University of Arkansas, holds a block of insulating material madefrom RHA.

19

Plate 9 ‘Greasweep’, an oil absorbent made from RHA

Plate 10 Women collecting RHA in Indonesia for domestic use.

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4. MARKET REVIEW

4.1 Introduction

An opportunity matrix of uses and potential for markets is shown in Table 5. It shows that many ofthe potential uses for RHA are not yet market ready or are too small to be significant. Ideally, anexisting high value market is required. No single market fits this criterion, and some compromisehas to be made.

4.2 Small or low value markets

The use of RHA as an oil absorbent is very small and localised and much work needs to be done toexpand the market from currently just one US state, California. The current market prices are $8.56for a 44kg bag, equivalent to US$940 per tonne.

The use of RHA in the manufacture of refractory bricks is too small a market to be considered as anoutlet for large quantities of ash, although it may be suitable for the output of a limited number ofenergy plants.

Other uses with limited commercial potential, due to localised low value use, are the control ofinsect pest in stored food stuffs and as a soil ameliorant. The use of RHA in the manufacture ofhousehold ceramics is also considered to be localised and low value.

4.3 Potential markets in the future

There are potential markets for RHA in the silicon chip industry, which is expanding. However, thetechnique of refining RHA to the desired quality has not yet been established on a large scale, and itcould be many years before such an application is market ready. Potentially the required volumesare large, but although silicon chips command high prices, this is due to the high manufacturingcosts of the chips rather than the low-value cost of the raw materials.

The production of lightweight construction materials and insulation from RHA has potential, butcurrent use is not widespread and there is limited knowledge of the methods used. Further research,such as that being carried out at the University of Arkansas, may develop this commercially, but notin the short term [36].

The production of activated carbon using RHA has great potential, with the global demand foractivated carbon increasing due to more stringent legislation in water purity, and an increasingfocus on water recycling. Current global demand is 800,000 tonnes per year, and prices range fromUS$564 – US$602 per tonne. The share RHA could play in this market is not yet clear, and moreresearch on the methodology and cost of producing activated carbon from RHA is needed [55].The use in industrial chemical processes such as an adsorbent in gold extraction and in the rubbervulcanising process are a long way from being market ready and will be disregarded from thisstudy. It may be that with further research they are found to be ineffective, as is the case of usingRHA as a filter for arsenic removal in water.

4.4 Current markets

The steel and cement industries are identified as having the most potential for a high value, largemarket and are discussed in detail in Sections 4.4 and 4.5.

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Application Current state ofdevelopment

Currentdemand

Potentialdemand

Geographicaluse

Purchaseprice pertonne

Suitability as aMarket

Flat Steel Production Market already inexistence

Medium Decreasing World wide Medium Not expanding.

Concrete manufacture Market in existence,and ongoing research

Low tomedium

High Worldwide Low Expanding and CERpotential

Silica fume replacement Market in existence,and ongoing research

Low High Worldwide High Expanding and CERpotential

Lightweightconstruction materials

Market in existence,and ongoing research

Low Low tomedium

Worldwide Low Currently localised,potential in future

Refractory Bricks Market already inexistence

Low Decreasing Worldwide Medium Small, not expanding.

Manufacture of siliconchips

Research Low High Worldwide Low Not yet market ready,limited potential.

Insect control Research andanecdotal

Low Low Asia Low Low demand, local use

Activated carbon inwater purification

Research Low High Worldwide High Potentially largemarket

Vulcanising process Research - High Worldwide - Not yet market ready

Extraction of gold, andother chemical uses

Research - - - - Not yet market ready

Household ceramicproducts (tiles, glazes)

Anecdotal Low Low Asia Low Little evidence

Soil ameliorant Anecdotal use Low Low Asia Low Low value local use

Oil absorbent Market in existence,and ongoing research

Low Medium Currently USA Medium Potential for marketingas a new product

Table 5 Opportunity matrix of uses and potential markets for RHA

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4.4 Steel

4.4.1 Global overview of steel production

In 2001, approximately 850 million metric tonnes of steel were produced worldwide. Of this, 730million metric tonnes were continuously cast steel, the process for which RHA could be used.Globally, steel production has increased from 1970, but there are fluctuations, with the period 2000-2005 showing a drop of 0.1% on the previous five year period (1995-2000). It is hard to predictfuture growth of the steel industry, but it can be assummed that over the long term production willremain fairly constant. The production of continuous cast steel is likely to increase as developingcountries improve their industrial processes [9].

-1

-0.5

0

0.5

1

1.5

2

2.5

3

1970

- 19

75

1975

- 19

80

1980

- 19

85

1985

- 19

90

1990

- 19

95

1995

- 20

00

2000

-200

5

Figure 6 Average growth rates (% per annum) in world steel production.

Asia and the EU dominate steel production, producing 64% of the worlds continuous cast steel(Figure 6).

Asia43%

European Union21%

USA12%

Rest of world24%

Figure 7 Global production of contiuous cast steel, by area, 2001 [9].

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China is the world's biggest producer of continous cast steel, as shown in Table 6.

Rank CountryContinuously cast steel production

(million metric tonnes)1 China 1302 Japan 100.13 USA 87.34 South Korea 43.25 Germany 436 Russia 307 Italy 25.68 Brazil 24.59 France 18.310 India 17.311 Taiwan 17.112 Spain 15.913 Turkey 1514 Canada 14.915 UK 13.216 Mexico 12.517 Belgium 10.718 South Africa 8.719 Australia 720 Iran 6.9

Table 6 Continuously cast steel production, ranked by country [9].

4.4.2 Factors affecting the demand for RHA in the steel industry

In 1998 a confidential report estimated that global demand for RHA in the steel industry was151,000 tonnes a year. Use in 1998 was dominated by Europe, which accounted for 33% of themarket, with Korea and the USA also having significant shares (Figure 7). The report concludedthat globally RHA demand in the steel industry would increase at an average rate of 6 to 7% peryear, expanding from to 268,000 metric tons per year in 2007 [11].

This was attributed to the continually increasing worldwide production both of steel in general andof higher quality flat steel products in particular, as well as growing industry awareness of thebenefits of RHA.

However a book written at the same time cautioned against the use of ash and stated that “acontroversy concerning possible harmful health effects of crystalline silica in the ash has somewhateroded prices for the ash and has raised doubts concerning future ash markets and prices” [3].

It appears that this controversy has still not been resolved, with reluctance amongst steelmanufacturers to discuss the use of RHA.

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Western Europe33%

South Korea17%

Japan22%

North America23%

Others5%

Figure 8 Global share of RHA in the steel industry, 1998 [11].

• Europe

The 1998 confidential report stated that 62.5% of steel produced in Europe used RHA, and thatglobally the steel industry in Europe accounted for 33% of RHA use, with Germany being thebiggest importer in Western Europe (Figure 8 and Table 7)[11].

Country Tonnes % Global totalGermany 14,100 9%Italy 8000 5%France 6200 4%United Kingdom 5800 4%Spain 4300 3%Belgium 3400 2%Netherlands 2100 1%Sweden 1600 1%Austria 1600 1%Luxembourg 1500 1%Finland 1200 1%

Table 7 RHA use in the steel industry in Western Europe, 1998 [11]

Since 1998 the health issues surrounding ash have become more significant, with Sweden banningthe use of RHA. A survey of the major steel producing companies in Europe met with a limitedresponse. The European steel industry is dominated by just a few main companies, due to a recentspate of mergers. For example, Arcelor, a merger of Aceralia, Arbed and Usinor, the worlds largeststeel manufacturing group, produces 50 million metric tonnes of steel a year, nearly 6% of globalproduction. No comment was available from them regarding the use of RHA.

25

CORUS (formerly British Steel and Hoogovans of the Netherlands) were available for comment.Although there is no specific ban on the use of RHA from the Health and Safety Executive in theUK, there are strict rules for use of crystalline silica-based products [56]. CORUS does not useRHA on a large scale. With manufacturing operations in the UK, Netherlands, Germany, France,Norway and the USA they are responsible for the production of 2% of global steel production.

Eastern Europe may be different with several of the larger steel manufactures setting up plants inthe former East Germany, Poland and the Czech republic. However the health issues may still be aproblem. It is hard to predict the future of the market for RHA in Eastern Europe, but is appears thatthe market in Western Europe is shrinking.

• Asia

Opportunities may be greater in countries that are producing their own RHA, as there will be lowertransport costs. Little is known of the significance of the health issues relating to RHA use in Asiancountries. It is known that there are several proposals for husk to energy plants, for which sales ofash to the steel industry are expected to bring in significant revenue, although it is unlikely that thiswill be sold to the European steel manufacturers.

It is likely that Korea is an end market for some of this ash, as 75% of all steel in Korea uses RHAin processing, although domestic RHA dominates the market. This ready market for domestic ashcould be favourable to husk to energy projects in Korea, which produced 1.3% of the worlds totalpaddy in 2002, and potentially could produce 250,000 tonnes of ash per year (Table 1).

The Thai steel industry does not use RHA as it does not use the continuous casting process requiredfor producing high quality flat steel. It uses rice husks, which provide enough insulation for thelower quality steels requiring shorter casting times. There is anecdotal evidence that in the rarecases when RHA is used, there are health and safety problems with ash becoming airbourne afterbeing tipped on the tundish. There is potential in the future for a market for the ash as the steelindustry develops. Thailand is a major rice producer (number six in the world in 2002, Table 1) andhas a great potential for husk to energy projects and resulting RHA production, but it is likely thatthe ash will be exported rather than used domestically in the steel industry.

There are several proposed husk to energy projects in Thailand, for example, Jpower’s 9.95 MWRoi-et RHA fired plant, due to begin operation in 2003 and ATBiopower is developing several huskto energy projects in Thailand, totalling 90MW, with the RHA being sold to the steel and concretemarkets. Agrilectric has husk to energy projects under development in Brazil and Thailand, and it isproposed that the RHA will be marketed again within the steel industry [34]. Other plants withinAsia include Ban Heng Bee rice mill in Alor Setar in Malaysia, which are planning to pursue ashsales from its energy plant. It estimates that it can make US$179,000 per year from ash sales, mostprobably within the steel industry [57].

• USA

The USA is one of the largest growers of rice in the developed world, and the 1998 confidentialreport highlighted the USA steel industry as being a considerable user of RHA, responsible for 23%of the global market share, some 35,000 tonnes a year (Figure 8) [11]. This ash was produceddomestically, predominantly by Uncle Bens. Uncle Bens also export to Europe, and in 1998 wereresponsible for 20% of RHA imports to Europe [11]. The market for RHA in the steel industry in

26

the USA appears to be saturated. Little is known of the significance of health issues. There are USgovernment safety guidelines concerning working with silica that would apply to RHA [58, 59].

• Australia

Australia is one of the smaller of the world’s rice producers, producing 957,000 tonnes of paddy in2002 [1]. It ranks 19 in the production of continuous cast steel (Table 6), so is not a significantproducer or user of RHA. Despite this, Rice Growers Ltd and Biocon in Australia are planning acogeneration plant at Deniliquin and Biocon plan to manufacture pellets from the crystalline silica[12]. They expect to export 1000 tonnes in pellet form packed into 7.5kg bags, selling directly tosteel manufacturers, either BHP Steel at Port Kembala, NSW, or to Canada, and have plans toexpand the operation.

4.4.3 Prices and future trends

Prices for RHA being sold to the steel industry are commercially sensitive and thus hard todetermine. N.P.Singhania in India is selling RHA at US$150/tonne, delivery at Calcutta port, India[60]. Other estimates of RHA on the world market are approximately $200 per ton of ash, [61]although it has been said that Thai RHA is worth US$300-400/tonne [11].

The impact of the emergence of pellets onto the market is hard to predict. Alsical, a small RHAimporting company based in East Germany, see that the future for RHA lies in overcoming thehealth issues. They are developing a pelleting technique which they hope will be accepted in theEuropean steel industry. The largest RHA dealer world wide is Refratechnik, based in Germany. Amanufacturer of refractory bricks, Refratechnik uses small quantities of RHA itself, but also sellsRHA to the steel industry. Refratechniks import RHA from Thailand, and in 1998 was responsiblefor 33% of global sales, some 50,000 tonnes. Of this, 60% was sold into the European steel industry[11]. Currently Refratechnik controls the market and it is not thought that it has any plans for theproduction of pellets.

If pellets can be shown to be a safe method of both application and disposal of RHA after heating inthe tundish, then the markets in Europe may expand. However, discussions with CORUS imply thatas the use of RHA in the steel industry involves heating to extreme temperatures, crystalline silicawill always form and be a health hazard, regardless of the form in which the RHA was initially.

4.5 Cement and concrete industry

4.5.1 Introduction

In light of economic conditions and the plentiful evidence, both from research and in the field, itseems inevitable that regular and high-volume usage of admixtures will become standard practice inthe concrete industry. Although the concrete industry market for RHA is still being developed, it isconsidered as being potentially a much larger end user than the steel industry. Two markets can bedistinguished; as a substitute for silica fume and in the production of low cost building blocks.

4.5.2 RHA as substitute for silica fume

As discussed in Section 3.3.2, RHA can substitute for silica fume in the manufacture of highstrength concrete. In 2000, 17.75 billion tonnes of cement were produced worldwide [62]. In the

27

UK silica fume is used at a rate of 0.34% of the volume of cement [63]. Extrapolating this figureworldwide results in a potential global market of up to 590,500 tonnes. The potential for this iscurrently being restricted by several factors.

The cement industry has to produce a consistent, high quality and standard product. This in turnrequires RHA from a controlled combustion environment, to ensure a consistent standard ash. Ashof a consistent quality is not readily available and is therefore not used by the cement industry.

There are many other cheaper and more abundant pozzolans available. A waste product from coalfired power stations is pulverised fly ash (PFA). It is abundant and cheap and is therefore often usedas an admixture in high strength concrete. Ground granulated blast furnace slag produced from ironsmelters is also highly pozzolanic and available. However, for high strength and quality, silica fumeis preferred, for which RHA is a potential substitute.

There is little awareness in the cement industry of the enhanced properties of RHA cement,although this being remedied to some extent in the USA.

• USA

Pittsburg Mineral & Environmental Tech. Inc. (PMET), part of Alchemix Corporation, Carefree,Arizona, buys RHA that can be used as a substitute for silica fume in the production of specialistconcrete [64]. It promots the use of RHA in concrete so well that there is currently a shortage.PMET specify the following for use a substitute for silica fume.

Crystalline silica not to exceed 1%Carbon content not to exceed 6%Mean particle size 7-9µm, 95% passing a 45-micron sieve

Table 8 Specification of RHA as a substitute for silica fume.

Ash is not usually this fine, irrespective of the combustion technique. PMET grinds the ash toachieve the fineness required and does not anticipate receiving ash that would be too coarse for itsgrinding process. The only plant supplier to guarantee ash of the correct quality is Agrilectric [34].

Purchase prices of RHA, which meet the technical specifications in Table 8, are from $100 to$120/tonne delivered to a US gulf coast port. This price is before grinding to meet final sizerequirement.

• UK

The cement industry in the UK produced 12 million tonnes in 2001 [64], making it a significantproducer in global terms. The silica fume market was analysed to estimate the potential market forRHA in the UK. Cement manufacturers (Heidelberg Cement, Lafarge), concrete/aggregatesuppliers (Hanson building materials, Tarmac, RMC) and silica fume suppliers (Elkem, INVENSILand Fesil Microsilica) were contacted to determine the size of the silica fume market and potentialfor substitution with RHA.

Although specific quantities and costs for silica fume were not available (due to their commerciallysensitive nature), the market is estimated at between 3000 - 5000 tonnes per year. Currently noRHA is being used in the UK concrete industry. Several companies had been offered the ash but

28

were not using it due to the availability of PFA, granulated blast furnace slag and silica fume.According to one source, the demand for silica fume is higher than supply and there could be anopening for RHA. One of the main concerns with RHA is supply and consistency of quality withinthe product.

• Australia

According to CSR Ready Mix, a major construction materials supplier in Australia and NorthAmerica, there is very limited use of RHA in Australia commercially and it is not used as substitutefor silica fume. Many of the rice growing areas in the eastern part of Australia are distant from thebig markets of Sydney and Melbourne. Up to now it has been difficult for RHA to compete withother supplementary cementitious materials such as fly ash which is commonly used in concrete inSydney and Melbourne (approximately 95% of the pre-mixed concrete volume).

CSR Ready Mix is the biggest user of silica fume in Australia using about 9000 tonnes per annum.There are two suppliers in Australia: Simcoa in Western Australia and Tasmanian Silicon inTasmania. The price depends on how much is being used. Typically cement costs in excess ofAUS$150 per tonne for large users and silica fume is approximately AUS$450.

4.5.2 Future trends and prices

There is great potential for RHA use in the cement industry, but it is currently not being used to anyextent, except in the USA. Two main issues appear to be limiting its use:

• Lack of awareness of the potential for RHA.• Quality.

PMET in USA appear to have overcome both these problems. The patented process [31,32], filedby Dr Mehta, and owned by Alchemix should overcome the quality issue. PMET have also shownthat, if awareness is raised, then there will be a demand for RHA, and as demand increases, soshould prices

However producing RHA of the correct quality may cost more than producing normal ash due toboiler modifications etc, this is discussed in detail in the Cost Benefit Analysis discussed in Section7.

4.5.3 RHA in building block manufacture

In developing countries RHA is used extensively in the production of low cost building blocks, inits raw form as an additive to cement. In India alone approximately 30,000 tonnes of RHA cementis manufactured annually [18]. SDH in Java manufacture lightweight blocks made from RHA,‘Hakkablock’ (Plate 7), and market them for use in high-rise construction where weight ofconstruction materials is important. Prices were not available.

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5. TECHNICAL REVIEW

5.1 Introduction

Commercially, it is important to determine and control the type and quality of rice husk ashproduced. These can vary depending upon the different combustion techniques used. For example,stoker fired boilers tend to produce higher quantities of crystalline ash, whereas similar boilers withsuspension firing produce more amorphous ash. The additional revenue stream provided by the saleof RHA may be the key to an energy projects’ viability. If this is the case the appropriatetechnology should be chosen to produce ash of the required type and quality for the target RHAmarket. For example, the colour of the ash is important for some cement markets where the ashinfluences the colour of the final cementitious product, as well as being a major indicator of thesamples’ residual carbon. For example, from Thailand, ‘blackish’ and ‘whitish’ ash can command$150 and $400 a tonne respectively [11]. RHA can be produced from rice husks by a number ofthermal processes which are described below.

5.2 Overview of husk to ash process

5.2.1 Rice husk as a fuel

The husk surrounding the kernel of rice accounts for approximately 20% by weight of the harvestedgrain (paddy) [65]. The exterior of rice husks are composed of dentate rectangular elements, whichthemselves are composed mostly of silica coated with a thick cuticle and surface hairs. The midregion and inner epidermis contains little silica.

In small single stage mills in developing countries, where bran (the layer within the husk) is notfully separated from the husk, the husk plus bran stream can rise to 25% of the paddy. For largermills, where the husk and bran are fully separated (the type more likely to be providing the husk forelectrical generation), a husk to paddy ratio of 20% is appropriate [65].

Most heating values for rice husk fall in the range 12.5 to 14MJ/kg, lower heating value (LHV). Ifsome bran remains with the husk, a somewhat higher calorific value results. Rice husks have lowmoisture content, generally in the range of 8% to 10% [3, 65]. The following are typical chemicalanalyses of rice husks:

Property Source SourceBulk density (kg/m3) 96-160 67 128 34Length of husks (mm) 2.5-5 67Hardness (Moh’s Scale) 5.5 – 6.5 67Ash 22.24 3 13.2 – 29.0 68Carbon (%) 35.77 3 36.66 34Hydrogen (%) 5.06 3 4.37 34Oxygen (%) 36.59 3 31.68 34Nitrogen (%) 0.32 3 0.23 34Sulphur (%) 0.082 3 0.04 34

Moisture (%) 8.05 3 8.76 34

Table 9 Typical husk analysis from various literature sources

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The high ash content of rice husks and the characteristics of the ash impose restrictions on thedesign of the combustion systems. For example, the ash removal system must be able to remove theash without affecting the combustion characteristics of the furnace (especially if the ash produced ismostly bottom ash). The temperatures must be controlled such that the ash melting temperature ofapproximately 1440ºC is not exceeded and care must be taken that entrained ash does not erodecomponents of the boiler tubes and heat exchangers [3, 65]. This influences the design of thecombustion system, a review of which is presented below.

5.2.2 Incineration

Incineration is the term usually used for deliberate combustion of husk without the extraction ofenergy and encompasses:

• open burning (such as deliberately setting fire to piles of dumped husk),• enclosed burning (typically a chamber made from fire resistant bricks with openings to

allow air to enter and flue gases to leave).

5.2.3 Boilers with integral combustion

For energy recovery from the combustion of fuels, the most common type of combustion systemincorporates heat extraction from the combustion chamber using steel tubes through which watercirculates. In so doing the water removes heat from the combustion chamber while at the same timeincreasing in temperature. This type of boiler is called a “water wall boiler”.

An alternative type uses an uncooled combustion chamber (sometimes called a firebox) connectedto a large drum of water through which tubes are placed to carry the hot exhaust gases from thecombustion chamber to the boiler chimney. This type is called a “fire tube boiler”. Such boilers tendto be less expensive for applications where a boiler size of less than 20tonne/hr and a pressurebelow 20 bar is appropriate.

A variant of the fire tube boiler configuration is one in which the combustion chamber remainsuncooled but the hot gases go to a separate water tube heat exchanger. Sometimes the heatexchanger is called a heat recovery steam generator (HRSG). This configuration avoids a potentialproblem that can occur with high ash fuels which can cause ash build-up in the tubes of fired tubeunits.

For power production using rice husks, water tube boilers are the most common choice. Thecombustion chamber is normally of rectangular cross section. The walls of the chamber are formedeither by tubes welded to each other or with the interstitial space filled with refractory. The tubesmay extend to the base of the chamber or finish at a higher level with uncooled fire-brick wallsfilling the lower area.

The chamber is closed at the base. The type of closure depends on the type of boiler but there isalways a means of extracting ash from the base. This ash is called “bottom ash” to distinguish itfrom “fly ash” which leaves with the hot flue gases and is removed later in the process.

Generally, the chamber tapers at the top before connection to a gas passage where the exiting hotgases pass over additional water or steam filled tubes before release to atmosphere. Sometimessteam or water filled tubes are suspended from the chamber roof into the central combustion zone ofthe chamber.

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Combustion boilers with water cooled tubes for rice husk application may be further sub-dividedinto three main categories: stoker fired, suspension fired and fluidised-bed.

• Stoker fired

Stoker fired boilers employ a grate at the bottom of the combustion chamber. Rice husks are fedabove the grate on which they form a pile where combustion mainly occurs. Secondary combustionof released volatile gases occurs above the pile.

Typically temperatures vary over a wide range but are highest in the pile. As a result the fusiontemperature for ash can be reached. Most ash drops through the grate. The smaller volume residualfly ash is carried away by the flue gases.

• Suspension fired

Suspension firing is an adaptation of the nozzle burners used to burn liquid fuels such as oil. Thisarrangement avoids the need for a grate at the base of the combustion chamber. This has severalpotential advantages including:

− the elimination of an expensive and high maintenance piece of equipment,− improved combustion using finer particles,− easier control of excess air to the combustion chamber,− improved combustion efficiency.

The solid fuel has to be prepared so that it is sufficiently fine to be blown into the combustionchamber such that combustion occurs within the short period of time available whilst the fuel is insuspension. Otherwise, the fuel will fall to the base of the chamber which would then need to have agrate similar to a stoker-fired unit. For rice husks, this means that the husks have to be ground to afine powder before combustion.

• Fluidised bed combustors

The term “fluidised bed combustor” (FBC) encompasses a range of combustion/boiler combinationswhere combustion of the fuel takes place within a bed of inert material that is kept “fluid” by anupward draught of air. The combustion chamber is similar to conventional boilers, such as stokerfired designs, except that the floor of the boiler is covered with numerous air nozzles and some ashremoval outlets. Primary combustion air enters the boiler through the nozzles and in so doing causesthe mix of fuel and inert material to mix continuously in a manner similar to a fluid. The fuel isoften fed from apertures located some distance above the bed. Depending on the ash content of thefuel, additional inert material may also be introduced to ensure that sufficient bed inventory existsfor stable fluidisation. The mixing caused by fluidisation produces a relatively uniform combustiontemperature and avoids the extremes in temperature that occur in other types of combustion. FBCsare conveniently subdivided into “bubbling” and “circulating” types.

Bubbling FBCs have a relatively low fluidising air velocity. This creates a bed which remainswithin the lower part of the combustion chamber (ie there is no deliberate entrainment of fuel andinert bed material in the flue gas). Circulating FBCs employ a higher air velocity which causes aportion of the fluidised bed material, the “lighter” particles, to be transported upward with the fluegas. These particles are “caught” in a cyclone, or similar mechanical separation device, and returned

32

to the main bed, hence the term “circulating”. Circulating FBCs tend to be more efficient thatbubbling beds but the added complexity has resulted in their application only for larger boiler sizes- typically for outputs greater than 150MWth. Relatively few FBCs have been used for rice huskapplications. Where used, bubbling bed types seem to have been employed.

5.2.4 Gasification

Gasification is a type of combustion in which the fuel is heated to release volatiles and convertcarbon to carbon monoxide. The gaseous products in the volatile form a producer gas which canthen be used in a manner similar to gaseous fuels. The heat to produce gasification is normallyderived from the fuel itself. The producer gas contains varying amounts of hydrogen, carbonmonoxide and methane depending on the fuel and the gasifier design.

An important potential advantage of gasification is that the producer gas (after cleaning) can beused as fuel for reciprocating internal combustion engines (ICE) or for gas turbines. This avoids theRankine steam cycle as the means to convert thermal energy to electrical energy and avoids needfor cooling water. Theoretically, a gasifier coupled with an ICE or gas turbine can lead to asignificantly higher energy conversion efficiency. This might be approximately 33% at relativelysmall unit sizes (down to about 1500kWth). At the same size, a steam boiler system might achieveonly 15% conversion efficiency or less.

5.3 Overview of ash production

The different types of combustion have one common characteristic. They all result in the oxidationof most of the “combustible” portion of the husk while leaving the inert portion. The inert portion isgenerally called ash or, after gasification, char. The distinction is somewhat blurred. Originally theterm “char” referred to the uncombusted residue that had not been taken to a sufficiently highenough temperature to change its state, whereas the term “ash” implied that a higher temperatureand change of state had occurred. However, when applied to RHA, the term ash appears to bereserved for all processes apart from gasification irrespective of whether a change of state hasoccurred.

In chemical analyses of husks the term “ash” refers to the chemical constituents of the residual fromcomplete combustion without consideration of the morphology of the components. The term “ash”,in this study refers to the residual of the particular combustion or gasification process whichproduced the ash.

The fine particulate matter which is carried away from the combustion zone by the flue gasproduces fly ash. With stoker and suspension fired boilers this ash is close to 100% amorphoussince the crystalline portion of the ash does not seem to carry in the flue gas. Bottom ash is denserthan fly ash, and for rice husks tends to be more crystalline than the fly ash.

Where fluidised beds and gasifiers are concerned the distinction is not so readily made, since thecombustion occurs at lower temperatures and thus a higher proportion of amorphous ash would beexpected in the bottom ash compared with bottom ash from stoker and suspension boilers.

The proportion of bottom ash to fly ash depends upon the boiler type and operating conditions. Forexample, McBurney Corporation offer a suspension fired boiler with pregrinding of the husks. Thisproduces approximately 10% bottom ash and 90% fly ash [34]. Suspension fired boilers by othermanufacturers, such as Fortum, are expected to produce similar proportions of ash. Combustion

33

units with uncooled chambers, such as challenger units by Advanced Recycling Equipment Incappear to produce nearer to 50% bottom ash and 50% fly ash. At least one manufacturer, TorftechUK Ltd has a compact bed reactor (Torbed® reactor) which produces almost 100% fly ash [68].

For stoker fired boilers 20-30% of the ash is expected to be bottom ash, the remainder fly. Anexample of typical fly ash and bottom ash is shown in Plates 12 (a) and (b).

5.4 Methods of ash analysis

Typically, the ash will contain some unburnt components as well as inert components of the husks.The unburnt component is predominantly carbon. It is typically measured by reheating a sample ofthe ash in an oven. The difference in mass of the sample before and after heating is referred to asthe ‘Loss on Ignition’ (LOI). The LOI value is normally the same as the carbon content of the ash.The carbon content of RHA varies according to the combustion process. RHA analyses from aliterature search and from analyses performed on RHA material for this study indicate carbon (orLOI) values ranging from 1% to 35%. Typically, commercial RHA combustion appears to result inRHA with 5-7% maximum carbon.

The high silica content in the husk may be responsible, in part, for the residual carbon in RHA by‘cocooning’ the carbon such as to prevent air circulating around it or by bonding to the carbon at themolecular level to form silicon carbide. The silica in the rice husks is at the molecular level, and isassociated with water. It occurs in several forms (polymorphs) within the husks. In nature, thepolymorphs of silica (SiO2) are: quartz, cristobalite, tridymite, coesite, stishovite, lechatelerite(silica glass), and opal; the latter two being amorphous [3]. For RHA as a potentially marketableproduct we need only distinguish between amorphous silica and crystalline silica. From thetechnical literature, two forms appear to predominate in combustion and gasification. These arelechatelerite (silica glass), an amorphous form, and cristobalite, a crystalline form. SiO2 can alsooccur in a very fine, submicron form. This form is of the highest commercial value although it isthe most difficult to extract.

The major and trace elements are conventionally expressed as their respective percentage oxidesand may not actually be present in this oxide form. SiO2 is generally determined as ‘total’ SiO2,since the proportion of crystalline to amorphous silica requires further costly analysis, usually by X-Ray Diffraction (XRD). Determining the quantity of these polymorphs is fundamental toinvestigating a market for the ash.

The colour of the ash generally reflects the completeness of the combustion process as well as thestructural composition of the ash. Generally, darker ashes exhibit higher carbon content (with theexception of those that may be darker due to soil chemistry/region (see below). Lighter ashes haveachieved higher carbon burnout, whilst those showing a pinkish tinge have higher crystalline(tridymite or cristobalite) content.

5.5 Factors influencing ash properties

5.5.1 Temperature

XRD patterns of ash combusted at a range of temperatures from 500-1000ºC have shown a changefrom amorphous to crystalline silica at 800ºC, and the peak increased abruptly at 900ºC [13]. Thechange from amorphous to crystalline silica at 800ºC was also found in other studies [69]. InVietnam, a series of experiments using a laboratory oven under conditions designed to simulate the

34

conditions of combustion from a rural facility were carried out [70]. SEM analysis of the ash foundthat the ‘globular’ amorphous silica increased in size from 5-10µm to 10-50µm with risingcombustion temperatures from 500-600ºC. The transition to completely crystalline silica wascomplete by 900ºC.

This can be described by Equation 1 below [20]:

( )

( ) Crist

Crist

CC

Tri

Tri

C

Qtz

Qtz

CC

amorphous

−↑↓

−−

⇒

−+↑↓

−−+

⇒

−↑↓

−⇒−

α

β

αα

ββ

α

βdeg800228

deg1470

21

deg163117

21

deg153deg600400

Equation 1 Transition from amorphous to crystalline silica

Note: �quartz converts to �quartz at 573ºC�quartz converts to tridymite at 870ºCtridymite converts to crystobalite at 1470ºC

These changes affect the structure of the ash. As such, the ‘grindability’ and therefore reactivity ofthe ash is affected since, after grinding, a greater surface area is available for chemical reactions ifthe ash is to be used as a pozzolan. For the steel industry, more crystalline ash is preferred as thisincreases its refractory properties.

5.5.2 Geographical region

It has been reported that chemical variations in husk composition (and consequently ashcomposition) are influenced by such things as the soil chemistry, paddy variety and climate.However, only one report of a change in the physical and chemical properties of ash influenced byregion was found [66]. A variation in colour and trace metal was found in ash from husks burnt indifferent regions, with ash produced from husks from Northern India resulting in a much darker ashthan husks from the US [66]. The colour variation was not related to differences in the carbonremaining in the ash, although it is not known the precise regional features that affected the ash. Itcould be due to the agronomy of the paddy as studies have shown that differences in mineralcomposition of ash can be attributed to fertilizers applied during rice cultivation, with phosphatehaving a negative affect on the quality of the ash in terms of its ability to act as a pozzolan [20]. Ithas also been said that the high K2O found in some ashes could be a consequence of K-richfertilizers used during the paddy cultivation [71].

5.6 Review of influence of combustion method on properties of RHA

The main factors in the various combustion and gasification processes that determine the type ofash produced are time, temperature and turbulence. These effect all chemical changes that occur inthe combustion process including the way the ash morphology is altered.

A broad explanation of combustion techniques was given in Section 5.2. Specific chemical andphysical properties of ash, taken from literature accounts, are described below. Appendix Acompiles the chemical analyses of rice husk ash from the literature review, going back severaldecades. It also includes the analyses of two samples of RHA (one bottom ash and one fly ashsample) obtained specifically for this study. In most of the analyses there were no details ofcombustion or analytical techniques, making it impossible to associate the chemistry of ash directlywith a specific combustion technique. This lack of information may be due in part to many of the

35

analyses having been conducted under laboratory conditions, and also due to the commercialsensitivity of giving exacting technological specifications alongside chemical analyses of ash. Asummary of all the ash analyses is at the end of this chapter.

A study in 1972 compared a range of data for ash composition (Records 5-13 in Appendix 1) [67].The wide range of values was as a result of the variation of purity of the samples and the accuracyof the analytical procedures used. However, since there is no information on different combustiontechniques employed in the husk combustion, or analytical techniques used, it is difficult to tellwhether any of the reported ranges in chemistries seen could be attributed to particular combustiontechniques.

The patent filed by P.K.Mehta, for producing RHA of a quality ideally suited to the cement market,[31,32], describes burning the husk continuously at a low temperature to preserve the amorphousnature of the silica [71]. The method utilizes the fly ash after its separation from the flue gases by amulti cyclone separator. Ash analyses 27-29 in Appendix A are taken from two of P.K. Mehta’sPatents [31, 32].

Commonly, in the production of highly amorphous ash, low temperatures and fairly long “burn-times” are used, as for Mehta’s patent. Other work in India has also concentrated on this technique,and has shown how a two-stage process of combustion could control the chemical and physicalproperties of the resultant ash, increasing its pozzolanic activity by taking the husk through acarbonising process without “flaming”. This type of burning was shown to produce a fine white ashwhich did not ‘carbonize’ [72]. By comparison, a “normal” combustion process (taking the furnacefrom room temperature up to the fixed burning temperature, where it was held until combustion wascompleted) produced a black coloured ash [72]. This same study compared the RHA in terms ofelectrical conductivity and compressive strength tests with concrete. The electrical conductivity isan effective measure of the amorphousness of the ash and showed that the “slow-burn” processproduced significantly more amorphous ash.

Similar results were found in a study in Guyana to ascertain the relationship between operationconditions and ash chemistry produced in terms of ash colour, carbon content and ‘silica activityindex’ (a measure of its pozzalanicity) [20]. Comparing ‘5-hour’ with ‘7-hour’ burn times showedhigher LOI in the shorter burn-time experiments (~6%) compared with ~3% LOI for longer burntimes. In addition, higher percentages of silica were produced over longer combustion periodsalthough no details were given concerning the percentage of amorphous and crystalline ash.

5.6.1 Fixed grate boilers

None of the reports in the literature made specific reference to conventional grate (fixed- ormoving-grate) technology, and although reference to “normal” or “conventional” boilers may wellbe a reference to a grated boiler we cannot assume this in terms of the reported ash properties.However, a sample of ash (“Patum”) from a fixed grate boiler in Thailand was analysed, the resultsof which are given in analysis 31 in Appendix 1. A significant difference between this and otherash samples is the large grain size, with 50% of the sample larger than 0.425mmsq/hole sieve.Compared with the circulating fluidised bed RHA (see “Fortum” ash analysis below) the PatumRHA showed a higher LOI (4.1% versus 2.2%), a higher total carbon content (3% versus 0.5%) andhigher crystalline silica content as one would expect comparing the two technologies. Thecoarseness of the ash samples has market significance, because for the majority of marketablepurposes (steel, cement, absorbent etc) a fine material is preferred, and the grinding of husks beforecombustion or RHA after combustion adds a significant cost to the process.

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5.6.2 Fluidized bed

Since FBCs have a more uniform and lower combustion temperature than stoker boilers, it ispossible that such boilers produce less crystalline ash.

Burning husk in a fluidised bed burner has been found to give mainly amorphous ash with a highspecific surface area [73]. Best results have been obtained by controlling the temperature of theburner via fuel feed rate, with the air supply set at an optimum velocity of 15m/s and thetemperature set at an optimum 750ºC. Comparing the properties of this ash for pozzolanicreactivity with Portland cement, with ash obtained from conventional combustion techniques,(although no description of “conventional” combustion techniques was given) gave excellent resultsin terms of its compressive strength [73].

An analysis from a study involving a series of experiments using a fluidised bed, at differingvelocities, bed temperatures, husk feedrate and excess air levels is presented in analysis 4 inAppendix A [74]. There was no information giving of the proportion of amorphous to crystallineash although the silica ‘recovery’ was high (97.6%) and the carbon content ranged from 1-4%.

5.6.3 Circulating Fluidised Bed (CFB)

The only RHA sample available that can be unequivocally assigned to combustion by a circulatingfluidised bed is “Fortum”, a sample obtained specifically for this study. The sample is a very finematerial, with approximately 50% by volume passing through a 0.150mm sq/hole sieve. It is a palegrey ash (see Plate 12b) compared with the coarser bottom ash from Patum (Plate 12a). It has a lowcarbon content of 0.5% as is often, although not always, the case with pale coloured ash. Generallyone would expect more amorphous ash from CFB combustion since the time spent at highertemperatures tends to be short, and due to the suspended nature of the fuel the temperature is evenlydistributed and does not result in extremely high temperature “hot-spots”. However, the analysis ofthe Fortum ash reveals a fairly high crystalline silica content of 33% crystobalite and 20%(transitional amorphous to crystalline) quartz. In terms of trace elements the Fortum and Patumsamples exhibit similar concentrations.

5.6.4 Grate versus ‘conventional’

The National Research Institute in Chatham, UK is conducting a two year long investigation intoimproving the boiler efficiency of rice furnaces in Bangladesh, with a view to producing RHA of aconsistent quality to sell to the cement industry. The NRI have conducted a series of experimentsboth on the RHA itself and also on blocks made with varying proportions of RHA, substituting forcement, to examine changes in its strength properties. The NRI obtained several samples, mainlyfrom two types of boiler (grate and conventional), however no additional information about theexact type and operating conditions were taken. The results so far show a clear correlation betweenthe types of ash produced, in terms of crystalline vs. amorphous silica content, and the boiler type.The average percentage of crystalline silica in the ash was 75.1% and 17.45% for grated andconventional furnaces respectively.

5.6.5 Gasification

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Literature sources reviewed to date focus more on the relatively high unburnt carbon content ofchar/ash from gasifiers without providing data on the relative amounts of amorphous to crystallineash. The unburnt carbon can exceed 40%. This would preclude use of the ash for other than lowcost uses and may explain why no extensive beneficial use of gasifier ash has been found.

Joseph [25], investigated the combustion processes necessary to burn husks under controlledconditions such that the ash remains mainly amorphous and that the C content is reduced to below15%, in order that it can be used as an additive in lime, bricks or cement. The findings from thisfairly early research concluded that combustion through gasification, rather than through a vortexfurnace produced the better quality ash, and the quality of the ash was further “improved” byvarying the gasification conditions. Significantly, and so far unreported from other publications,they found that variations in collection methods and ash cooling significantly affected the propertiesand characteristics of the ash. Once collected from the gasification system carbon burnout occurredover the proceeding four days in the concrete ash pit, the carbon content of the ash after four dayshad dropped to 7-10% from 26% immediately after collection.

5.6.6 Additional Technology

Torftech, a Canadian based company, supplies Torbed® reactor based rice hull combustion systems.The technology provides ash with a high percentage of amorphous silica for use in the concrete andcement industries. They are able to produce low carbon, high surface area low crystalline ash bymaintaining the temperature of their expanded bed reactors below 850ºC (at 830ºC with anestimated residence time of approximately 5 minutes, no crystallization occurs) [68]. Thetechnology has been a joint venture between Torftech and the University of Western Ontario. Ashanalysis 14 (Appendix A) shows the chemical analysis of the ash produced using their reactortechnology.

5.6.7 Special market requirements

Little data is available regarding ash quality for the currently small and potential markets (sections4.2 and 4.3). For the established market of the steel industry and the emerging market in the cementindustry more data is available, and this is discussed in the cost benefit analysis (section 7).

5.7 Summary of technical analysis

• Indistinct and vague details are often given for boiler descriptions and RHA analyses. Insuch cases the ‘value’ of RHA analyses in a commercial sense is minimal.

• There is a wide range in concentrations of physical and chemical properties of RHA asshown in Table 10 above.

• The change from amorphous to crystalline ash occurs at approximately 800ºC, although theprocess is often ‘incomplete’ until 900ºC is achieved.

• Generally, lighter coloured ash has achieved a more complete carbon burnout.• Al1 combustion processes devised to burn rice husks remain below 1440ºC, which is the

RHA melting temperature.• The chemical and physical properties of the ash may be influenced by the soil chemistry,

paddy variety and fertiliser use.• Suspension fired boilers generally produce more amorphous ash than stoker fired boilers

despite the fact that they may operate at higher temperatures. This is because the operatingtime at high temperatures for suspension fired boilers is comparatively short.

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• Commonly, in the production of highly amorphous ash, low temperatures and fairly long‘burn-times’ are used.

• Fly ash is a fine material and is of higher marketable value since it requires less grindingthan the generally coarser bottom ash.

• Fixed grate technologies tend to produce ash with higher carbon content, higher LOI andhigher proportions of crystalline to amorphous ash.

A summary of the above studies (resulted in full in appendix 1) is shown below.

Determinant % (unless otherwise stated)PH 8.1 - 11SiO3 0.1 – 1.23SiO2 (Total) 62.5 – 97.6SiO2 Amorphous 0.16 – 97.6SiO2 Crystalline <1.0 – 88.4Al2O3 0.01 – 1.01Fe2O3 <0.01 – 2.78CaO 0.1 – 1.31MgO <0.01 – 1.96P2O5 <0.01 – 2.69Na2O <0.01- 1.58K2O 0.1 – 2.54TiO2 <0.01 – 0.03Cl 0.01 – 0.04L.O.I. 1.56 – 5.5Carbon 2.71 – 6.42

Table 10 Summarising the major and trace elements from analyses given in Appendix A

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Plate 10 De-ashing at Patum Rice Mill Thailand

Plate 11 Rice husk boiler at Riceland Mill, Thailand.

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(a)

(b)

Plate 12 Photograph of typical samples of (a) bottom ash from a fixed grate boiler and (b) flyash from a circulating fluidised bed boiler. Both samples are from rice mills in Thailand.

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6. HEALTH ISSUES

All forms of crystalline silica represent a very serious health hazard [58]. The forms that develop athigher temperatures ie cristobalite and tridymite are particularly harmful. Exposure to crystallinesilica via inhalation can lead to a number of diseases, the most common being silicosis.

Amorphous ash does not contain the more harmful forms of silica, but can be a respiratory hazard,particularly if finely ground.

No information specific to RHA is available, and the following applies to crystalline silica from anysource.

6.1 Diseases

6.1.1 Silicosis

Silicosis is the result of the body’s response to the presence of silica dust in the lungs. Therespirable fraction of the dust can penetrate to the alveoli (airsacs) where the exchange of oxygenand carbon dioxide occur. When workers inhale crystalline silica, they land on the alveoli, andwhite blood cells (macrophages) try and remove them. The particles of silica cause the white bloodcells to break open, resulting in fibrotic nodules and scarring on the lungs. Exposure may result inshortness of breath on exercising, possible fever and occasionally bluish skin at the ear lobes or lips.Progression of silicosis leads to fatigue, shortness of breath, loss of appetite and respiratory failurewhich may cause death [59]. There are three classes of silicosis:

• Chronic silicosis

Usually occurs after ten or more years of exposure to silica at relatively low concentrations.

• Accelerated silicosis

Results from exposure to high concentrations of crystalline silica and develops five to ten yearsafter initial exposure.

• Acute silicosis

Occurs where exposure concentrations are the highest and can cause symptoms to develop withinweeks to four or five years after initial exposure.

6.1.2 Cancer

Crystalline silica is classified as carcinogenic to humans, and the International Agency for Researchon Cancer (IARC) concluded that there was “sufficient evidence in humans for the carcinogenity ofcrystalline silica” [75,76].

6.1.3 Other diseases

Silicosis has been shown to increase susceptibility to scleroderma, lupus, arthritis, tuberculosis andkidney disorders [77].

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6.2 Exposure limits

The UK Health and Safety Executive (HSE) has assigned a maximum exposure limit of 0.3mg/m3

for crystalline silica, expressed as an eight hour time weighted average [14]. The USA PermissibleExposure Limit (PEL) is 10mg/m3 divided by the % SiO2 + 2 [15]. This level is considered as beingtoo high and there are additional recommended, but not statutory levels, of 0.1mg/m3 for crystallinesilica, 0.05mg/m3 for cristobalite and 0.05mg/m3 for tridymite [58].

6.3 Measures to control exposure

The UK HSE do not have regulations relating specifically to RHA, and state that the rules for silicashould be followed.

The UK HSE advise that samples should be taken if dust levels are expected, and that they shouldbe regarded as significant of they are above 0.1mg/m3 [56]. In this case dust control levels shouldbe implemented. Capturing and controlling dust at source is always easier than attempting to controlexposure by ventilating the whole area.

Suggestions for controlling dust levels include [59, 77]:

• Dust control programme.• Medical surveillance/disease reporting.• Training and information to workers.• Equipment maintenance programme.• Isolated personal hygiene facilities, eating facilities and a clothing change area.• Record keeping.• Regulated areas/warning signs.

6.4 Health issues in relation to use of RHA

Within the power plant, exposure could occur when de-ashing the boiler, collecting, packaging andtransporting the ash. End uses involving crystalline ash such as in the steel industry and in siliconchip production should be considered carefully. Methods to limit exposure within the steel industryare already being developed. The other major market for RHA, concrete, only uses amorphous ashso carcinogenic heath issues should be minimal.

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7. COST BENEFIT ANALYSIS

7.1 Introduction

In earlier sections of this report, information has been provided on the characteristics of, andpotential uses for RHA. In summary, depending on the combustion method chosen, RHA can varyfrom having a negative value, where costs have to be incurred to dispose of it, to having variouspositive values depending on its quality and the market for it beneficial use.

The main characteristics that determine the potential value of RHA are:

• residual carbon content,• significant quantity of crystalline phases of silica dioxide present• mainly amorphous silica with little or no crystalline phases

Given the premise that a viable rice husk fuelled power plant is to be built, the cost benefit resultingfrom increased expenditure to produce a RHA with higher value than would otherwise result isassessed in this section.

In order to examine the cost benefit analysis for RHA, we have chosen a generic rice husk fuelledpower plant sized to produce 3MWe (net) has been chosen. Unless stated otherwise, the analysesuse the following assumptions:

• ash content of fuel is 18%• annual operating time of plant 7,500 hours• all prices are in US$

In order to put into perspective the additional revenues attributable to RHA use, it is useful to keepin mind that, if electricity has a value of 6 US cent/kWh and all the plant output is sold at this price,the annual revenue from electrical sales is $1,350,000.

7.2 RHA Disposal – Negative Benefit

The predominant reason why RHA would only be suitable for disposal is when its residual carboncontent exceeds 7%. Based on the power generation technologies examined within the scope of thiswork, those that seem to result most often in a higher residual carbon content are gasifiers.

A plant producing 3MWe using gasifier technology will have multiple units because the applicationof gasification to rice husks has not been proven in units larger than about 750kWe. The rice huskconsumption will be approximately 3.75 tonne/hr and the RHA leaving the gasifiers will be 0.675tonne/hr. Over the course of a year of operation, the total RHA produced will be 5063 tonnes.

Typical disposal cost for RHA for transport and disposal at disposal site range from approximately$5/tonne for local disposal in developing countries where land costs are low, to $50/tonne andhigher for disposal to engineered landfills in developed countries. When disposal costs are as lowas $5/tonne, the negative cost of producing RHA which has no beneficial use would be$25,315/year or less than 2% of the revenue from electricity. However, when they are $50/tonne,the negative cost attributable to disposal could be $253,150/year, almost 20% of the revenue fromelectricity.

44

One solution that has been considered for removing carbon from RHA is to use a fluidised bubblingbed incinerator that would burn off the residual carbon in the RHA. A bubbling bed incinerator isestimated to have a capital cost of $315,000. Assuming that operating costs are no more than thecosts to operate the basic power plant and allowing for maintenance costs of about 3% of capitalcost, the annual maintenance cost would be $9450.

The application of this type of unit could reduce the RHA carbon content to below 7%. This wouldeliminate a disposal cost of $25,315 (developing country) to $253,150 (developed country).Allowing for the maintenance costs, the potential benefit from installing the fluidised bed boilercould be $15,865/year (developing country) to $243,700/year (developed country). Based on theadditional capital cost of $315,000, the number of years to recover this amount could be 20 years(developing country) to 1.3 years (developed country).

The above suggests that in developing countries, where sites for RHA disposal are low cost, treatingRHA from gasifiers to reduce residual carbon content is unlikely to be attractive. In developedcountries, where disposal costs are high, such treatment of ash could be attractive provided there isa beneficial use for RHA with below 7% residual carbon exists.

7.3 RHA with Significant Quantity of Crystalline Silica

Combustion of rice husks, typically in stoker fired boilers, where the ash experiences sustainedtemperature above 750°C leads to a significant quantity of crystalline silica in the resultant ash. Thesteel market is preferred for this type of ash.

A stoker boiler power plant designed to produce 3MWe (net) will typically consume 4.11tonne/hrrice husk. This will result in 0.74 tonne/hr RHA. Over a full year of operation, 5550 tonnes of RHAwill be available.

The net value to the producer for RHA sold to the steel industry has been reported to fall in therange $100 to $150/tonne. Based on the mid-range value of $125/tonne, the potential revenue fromproducing RHA acceptable to the steel industry is $694,000/year. This could be more than 55% ofthe revenue stream from sales of electricity and is, therefore, an important commercial objective.

There are no significant additional costs needed to achieve RHA with qualities suitable foracceptance by the steel industry. On the contrary, stoker boiler technology with over grate feedersfor the fuel, and no need to pre-grind the husks, is at the low cost end of the range of equipmentchoices.

It is wise to choose need to choose boiler suppliers who can demonstrate their track record inproducing consistent quality RHA for the steel industry. This could tend to limit competition andhence introduce a hidden cost in the final price. With a typical boiler cost of about $1,200,000, it isclear that a premium of 10% to ensure RHA quality would be recovered in a few months of sales ofRHA.

The only other additional costs for rice husk power plant generation will be related to tests on ashsamples, a higher level of quality control and probably an additional employee to handle the RHAside of the business. Clearly, with revenue from sales of potentially $694,000/year, these costs willoften be no more then 10% of the revenue.

45

7.4 RHA with High Amorphous Ash Content

The only technology to have achieved a good long term record in producing a high amorphousRHA uses suspension firing of pre-ground rice husk ash. Although the ground husk material can beexpected to experience temperatures above 750°C, the time at temperature seems to be insufficientto change the amorphous ash to crystalline form.

The cost of the boiler will be similar to a stoker fired unit of the same size. The main difference willbe that there is no need for a moving grate, but the fuel will be fed pneumatically using burnerssuited to fine particulate material. The extra cost for this approach is mainly associated with huskpreparation which will require rice husk grinders.

Based on a 4.11tonne/hr husk feed, as required for the generic 3MWe (net) plant, installed cost ofhammer mill grinders is estimated to be $143,000. In addition to the capital cost, the grindersrequire considerable maintenance, including replacement screens, hammers and refurbishment ofcutting plates. Based on an annual throughput of 32,500 tonne/year, the maintenance is estimated at$27,000/year. In addition, the hammer mills will consume electricity at an annual rate of772,000kWh. Based on a value of electricity of 6US cent/kWh, the value of the electricity forgrinding the husk is $46,300/year.

The RHA from a plant producing predominantly amorphous ash has a net value of approximately$70/tonne (allowing $30/tonne for freight to market). On this basis the total yearly revenue fromsale of the RHA from the generic plant would be $388,500. After deducting the annual cost formaintaining the hammermills and the value of electricity to run them, the remaining revenue is$315,200. Even allowing for additional work related to managing the sale of the RHA, the paybacktime for the additional equipment needed to produce amorphous RHA will be less than six months.

7.5 Concluding Remarks

Based on the above, the best choice would seem to be to produce RHA for the steel industry as thishas the best returns. However, growth in the market for RHA to the steel market is limited. Growthin the market for RHA in the cement industry is potentially very large. For this reason, a newentrant to the marketplace may prefer to target the somewhat less high returns but better longer termprospects of the cement market.

46

8. POTENTIAL TO EARN CARBON CREDITS

8.1 Introduction

The Kyoto Protocol is part of the UN’s Framework Convention on Climate Change and has set anagenda for reducing global greenhouse gas emissions. If CO2 emissions can be shown and verifiedto be reduced due to different practices, then Certified Emission Reductions (CERs) can begenerated.

If RHA is used in concrete manufacture as a cement substitute then there is the potential to earnCERs [78]. Cement manufacturing is a major source of greenhouse gas emissions, accounting forapproximately 7% to 8% of CO2 globally.There is an emerging market globally for CER’s, with current prices around US$5/tonne of CO2. Itis hard to predict the size and future prices within the market, but using RHA as a cement substitutecan generate CERs, and one company (Alchemix) has already investigated selling these on theinternational market [64].

8.2 Role of RHA in reducing GHG emissions

The cement industry is reducing its CO2 emissions by improving manufacturing processes,concentrating more production in the most efficient plants and using wastes productively asalternative fuels in the cement kiln. Despite this, for every tonne of cement produced, roughly 0.75tonnes of CO2 (greenhouse gas) is released by the burning fuel, and an additional 0.5 tonnes of CO2

is released in the chemical reaction that changes raw material to clinker (calcination).

The potential to earn CERs comes primarily from substituting Portland cement with RHA. Thereare other environmental benefits of substituting Portland cement with RHA. The need for quarryingof primary raw materials is reduced, and overall reductions in emissions of dust, CO2 and acid gasesare attained.

8.3 Calculating the value of CERs from Portland cement substitution

The World Bank Prototype Carbon Fund provides examples of acceptable CERs from substitutingPortland Cement [79]. Their guidelines have been adapted to show the potential income from CERsfor the generic 3MW rice husk to energy power plant used for the Cost Benefit Analysis (Section7).

A 3MW suspension fired boiler plant would typically produce 5550 tonnes annually of RHA.Assuming 50% of RHA produced is sold for cement substitution:

x x 50% =

Emission reductions from substitution of Portland cement are calculated as totalling 1.25 tonnes ofCO2 per tonne of cement substituted, derived as follows [79]:

0.75 tonnes of CO2 per tonne of cement from energy use0.50 tonnes of CO2 per tonne of cement from calcinating limestone

Thus the total annual emission reduction for cement with RHA substitution in cement is:

Rice Husk Ash produced(5555 tonne/yr)

RHA sold for cement substitution(2775 tonne/yr)

47

x x =

Using current estimates of approximately US$5/tonne of CO2, this could provide an additionalannual income stream of US$17,345. This is not significant compared to the potential income fromsales to the steel market of $694,000 and to the cement market of $315,200 (Section 7.3 and 7.4),but could make a difference in a marginal project.

RHA sold for cementsubstitution

(2775 tonne/yr)

1.25 tonnes of CO2

per tonne cementAnnual Emissions

(3469 tonnes CO2/yr)

48

9. CONCLUSIONS

• Small markets exist for RHA in the manufacture of refractory bricks and as an oil absorbent.

• Potential future markets include silicon chip manufacture, the manufacture of activatedcarbon, and in production of lightweight construction materials and insulation.

• Currently the largest and most commercially viable markets appear to be in the concrete andsteel industries.

• The market within the steel industry is well established, but there are constraints to theexpansion of this market due to health issues associated with using crystalline ash.

• The cement markets are not as well developed as steel, but there is great potential for the useof amorphous RHA in this area. Two main issues appear to be limiting its use: lack ofawareness of the potential for RHA and the quality of the product itself. Boilermodifications may be required to produce ash of the quality required.

• The best choice seems to be to produce RHA for the steel industry as this requires no boilermodifications and attracts a high price.

• However growth in the market for RHA to the steel market is limited. Growth in the marketfor RHA in the cement industry appears to be growing and is potentially very large.

• A new entrant to the market place may prefer to target the somewhat less high returns butbetter longer term prospects of the cement market.

49

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54

ACKNOWLEDGEMENTS

Bronzeoak would like to acknowledge the assistance of Charoen Lertrushtakorn, Patum Rice Mill,Bangkok, Thailand and Prasong Limsirichai, Fortum Engineering, Bangkok, Thailand for providingash samples for analysis.

The Geology Department, University of Leicester carried out the X Ray Diffraction analyses andMinton Treherne and Davies Ltd the other analyses.

Dr Nandini Dasgupta and Stephen Graham (Greenwich University) provided data and usefulbackground information.

A55

APPENDIX A

A56

Determinant %unless otherwisestated

N.P.Singhaniahttp://ricehuskash.ebifchina.com

Analysed by Siam City Cement Plc,Saraburi Province, Thailand. Source,[11].

Ron Bailey Jr.Producers Rice MillEnergy Systems Inc. +1(501)767-2100

F. Preto Energy Research Laboratories, energy Mines andResources, Canada

Reference[67]. Year ofanalysis 1870

Boiler details

- -

gasifier Sand fluidised bed, depth range 0.3-0.6m. Temp 650-900oC,fluidising velocity 0.4-2.2m/s, excess air levels 30-95%.Combustion efficiency>97%

Bottom/fly ash?

- - - bottom ash (except where indicated to be fly ash, see below)

pH 8.1 - - - -

SiO3 - 0.18 - 0.25 0.42SiO2 (Total) 88.65 89.29 - 97.55 97.6 (in the fly ash) 93.21

SiO2 Amorphous 0.16 - 90-98 - -

SiO2 Crystalline 88.4 - - - -

Al2O3 4.90 0.54 - 0.00 -

Fe2O3 (FeO3) 0.30 2.45 - 0.19 0.45

FeO2 - - - - -

CaO 0.89 1.09 - 0.59 0.51

MgO 0.96 0.18 - 0.00 0.07

P2O5 - - - 0.44 2.69

Na2O 0.10 0.03 - 0.11 0.3

K2O 0.10 2.43 - - 1.53

TiO2 0 - - - -

CI - 0.01 - - 0.15

-

L.O.I 1.56 3.67 - - -

Fixed Carbon - - - -

Total Carbon - - - -

Carbon 1-4 (fly ash) Size 500 mesh 99.65% passed through #325 sieve

- - -

Specific gravity 5 2.082 - - -

Colour grey grey - - -

Bulk density - 0.39gm/1 - - -

Additional analysis - MnO 0.13 - Ash melting temperature 1400oC

-Notes Sold to steel industry Blaine Fineness 7684cm2/gm Boiler can be operated to

achieve SiO2 amorphouscontent

Range of operating conditions in experimental circumstances.Only one result of chemical composition recorded as below.

A57


Reference[67]. Yearof analysis1871

Reference[67]. Year ofanalysis 1916

Refernce[67]. Yearof analysis1925

Refernce [67]. Year of analysis1928


Refernce [67].Year of analysis1952


Refernce [67].Year of analysis1970

B Foutnier International Centrefor Sustainable Development ofCement & Concrete(CANMET), Canada

Boiler details

- -

-

- - - - -

Torbed Reactor. Gas andcombustion kept at a steady830°C and ash kept below thistemp.

Bottom/fly ash?

-

Assumed to bebottom ash

- - - - - -

Assumed fly ash since TORBEDreactor will produce all fly ash.

pH - - - - - - - - -

SiO3 - - - 1.13 - 0.40 - 0.1 0.1SiO2 (Total) 87.71 96.97 97.3 94.50 95.49 96.62 96.20 91.16 (XRD) 90.7

SiO2 Amorphous - - - - - - - - -

SiO2 Crystalline - - - - - - - - At most, minor cristobalite

Al2O3 - - - - - - - - 0.4

Fe2O3 0.54 - 0.38 Trace 0.94 - - 0.21 0.4

FeO2 - - - - - - - - -

CaO 1.01 0.57 0.43 0.25 0.86 0.32 0.24 0.65 0.4

MgO 1.96 0.12 - 0.23 0.28 0.76 0.24 0.99 0.5

P2O5 0.57 0.57 1.44 0.53 0.36 - 0.46 - 0.4

Na2O 1.58 - - 0.78 - 0.0 - - 0.1

K2O - 0.58 - 1.10 1.88 1.59 0.79 4.75 2.2

TiO2 - - - - - - - - 0.03

CI - - - Trace - 0.42 - - -

L.O.I - - - - - - - - 4.7

Fixed Carbon - - - - - - - - -

Total Carbon - - - - - - - - -

Carbon Size - - - - - - - - 8.3 µm (mean)

Specific gravity - - - - - - - - 2.05

Colour - - - - - - - - -

Bulk density - - - - - - - - -

Additional analysis - - - - - - - - Notes

A58


Reference [80] Charles Weiss Agrielectric Denthep Theppratuangthip Asian Institue ofTechnology,Bangkok,Thailand

Boiler details-

Cyclonic suspension combustion technology. Steam flow 21,000lbs/hr at60psig and 250°F.

Travelling grate stoker with pneumatic spreaders madein Japan.

Simple incinerator

Bottom/fly ash? Assumed to be bottom ash bottom and fly ash used - -

pH - - - -

SiO3 - 0.14 - 0.03SiO2 (Total) 96.01 >90 90% minimum 90.40

SiO2 Amorphous - - - -

SiO2 Crystalline - <1 - -

Al2O3 0.96 <0.01 - 1.01

Fe2O3 0.08 0.032 - 2.78

FeO2 - - - -

CaO 0.3 0.60 - 1.31

MgO 0.28 0.37 - 0.28

P2O5 0.88 0.92 - -

Na2O 0.06 0.14 - 0.78

K2O 0.96 2.3 - 2.40

TiO2 - <0.01 - -

CI - - - -

L.O.I - 4 to 6 - 3.54

Fixed Carbon - - - -

Total Carbon - - - -

Carbon Size - - - -

Specific gravity - - 2.17

Colour - black/grey 3.16

Bulk density - 288kg/m3 -

Additional analysis - NnO 0.12 Unburned carbon 2-7Blaine Fineness

16,976 cm2/g

moisture 3% maximum Notes Husks are pre-ground at the adjacent mill. Boiler code JTS B-8201. Designed for 50 bar,

superheater outlet steam at 405oC, gas -air heater outletat 210oC and 27oC at steam-air heater outlet.

A59


Asian Instituteof Technology,Thailand

Reference [25] Department of MineralScience, Turkeyen,Guyana [20]

Department of MineralScience, Turkeyen,Guyana [20]

Department ofMineral Science,Turkeyen,Guyana [20]

Department of MineralScience, Turkeyen,Guyana [20]

Boiler details - gasifier 5 hour burn 7 hour burn 7 hour burn 5 hour burnBottom/fly ash? bottom ash bottom ash - - - -

pH - - - - - -

SiO3 0.18 - - - - -SiO2 (Total) - - 88.04 94.55 94.26 94.12

SiO2 Amorphous - 'practically' amorphous

- - - -

SiO2 Crystalline - - - - - -

Al2O3 - - - - -

Fe2O3 - - - - -

FeO2

92.28

- - - - -

CaO - - 0.1 1.22 0.91 0.69

MgO 0.18 - 0.26 0.17 0.17 0.17

P2O5 - - 0.69 0.52 0.52 0.7

Na2O 0.03 - 0.3 0.05 0.04 0.0

K2O - - 2.5 0.62 0.72 0.84

TiO2 - - - - - -

CI - - - - - -

L.O.I 3.67 - - - - -

Fixed Carbon - - - - - -

Total Carbon - - - - - -

Carbon 6.42 2.71 3.27 3.1Size - - - - - -

Specific gravity 2.082 - - - - -

Colour - light grey off-white white light grey

Bulk density - - - - - -

Additional analysis Moisture 0.67 - other elements 8.11 other elements 2.87other elements

3.38 other elements 3.44

Blaine Fitneness

7684 (cm2/g) Notes Burnt at approx 900oC followed by retention for

approx 1-11/2 hours at 500-600oC.

A60


Department ofMineral Science,Turkeyen, Guyana

Government Industrial ResearchInstitute, Kyushu, Japan

P.K. Mehta [31] P.K. Mehta [31] P.K. Mehta [20] Bui and Stroeven

Boiler details

5 hour burn

Fluidised bed. Temp of combustioncontrolled by feed rate not air velocity(which is fixed at 15cm/s)

-

- - Eperimental laboratory ovenBottom/fly ash? - fly ash - - - -

pH - - - - - -

SiO3 - - - - - -SiO2 (Total) 91.04 91.69 91.3 93.00 62.5 96.7

SiO2 Amorphous - 100 (XRD) 100 (XRD) 99 (XRD) 90 -

SiO2 Crystalline - - - (XRD) 1 (XRD) 10 -

Al2O3 - 0.14 <0.1 <0.1 <0.1 0.08

Fe2O3 - 0.06 <0.1 <0.1 <0.1 0.03

FeO2 - - - - - -

CaO 0.77 0.58 0.5 0.3 0.2 0.30

MgO 0.17 0.26 - - - 0.16

P2O5 1.00 0.52 - - - -

Na2O 0.04 0.09 0.5 0.4 0.3 -

K2O 0.84 2.54 2.1 0.50 0.1 0.73

TiO2 - 0.01 - - - -

CI - - - - - -

-

L.O.I - 4.18 4.9 5.5 35 -

Fixed Carbon - - - - - -

Total Carbon - - - - - -

Carbon 5.8 - - - - -Size - - - - - -

Specific gravity - - - - - -

Colour off white - - - - -

Bulk density - - - - - -

Additional analysis other elements 6.14 - - - - -Notes These results taken from ash left

smouldering on a heated plate for a fewminutes then heated to 600°C for 15mins.

Analysis from USPatent 4,105,459.1978



Data is average from preheatingat 150°C then 600°C for 10hours and 900°C for 15 hours.

A61

Contact Name: Charoen LertrushtakornCompany Details: Patum Rice Mill and Granary

Public 88 MOO 2 TivanontPathumthaniBangkokThailand

Ash type (bottom/fly?): Bottom AshDetails of Boiler type: Fixed Grate Yoshimine Boiler, moisture between 14-20%, outlet steam 18bars, 360°C. They have2 boiler sizes, 35 and 20 tonnes.Additional Details: Analysed on behalf of Bronzeoak Ltd. Ash Analyses normalised to 100%

Determinant %(unless otherwise stated)

Analytical technique(if known)

pH 10.5 1:2 water extractSiO3 - ICPMSSiO2 (Total) 95.36 ICPMSSiO2 Amorphous 32 XRD

SiO2 Crystalline 61 cristobalite4 quartz

XRD

Al2O3 0.18 ICPMSFe2O3 0.16 ICPMSFeO2 -CaO 0.70 ICPMSMgO 0.36 ICPMSP2O5 0.46 ICPMSNa2O 0.41 ICPMSK2O 1.95 ICPMSTiO2 0.01 ICPMSCl 0.03 ICPMS

L.O.I. 4.1 ICPMSFixed Carbon -Total Carbon 3.0 ICPMS

SIZE MMSQ/HOLE+2.36 0.1+2.00 0.1+1.18 2.0+0.60 24.6+0.425 24.5+0.30 15.1+0.212 11.3+0.150 7.7+0.063 9.5Passing 0.063 5.1

specific gravity -colour -

BULKDENSITYLoose poured 156Vibrated 211compacted 239

Additional AnalysesMoisture 0.5SO3 0.33MnO2 0.08

- = no analysisICPMS – Inductively Coupled Plasma Mass Spectrometer

A62

Contact Name: Prasong LimsirichaiCompany Details: S.T. Fortum Engineering Co., Ltd.

5th Floor SP Building388 Phaholyothin RdBangkok 10400Thailand+662 273 0037 ext. [email protected]

Ash type (bottom/fly?): Fly AshDetails of Boiler type: Circulating Fluidized Bed Boiler, 35t/hr of Chinese make. 399bar.g at 450°C. FD fan4.1375m3/s, ID 14.77m3/s. Cyclonic ash removal system.Additional Details: Analysed on behalf of Bronzeoak Ltd. Ash analysis normalised to 100%

Determinant %(unless otherwise stated)

Analytical technique(if known)

pH 11 1:2 water extractSiO3 - ICPMSSiO2 (Total) 94.29 ICPMSSiO2 Amorphous 46 XRD

SiO2 Crystalline 33 cristobalite20 quartz

XRD

Al2O3 0.61 ICPMSFe2O3 0.24 ICPMSFeO2 - ICPMSCaO 0.67 ICPMSMgO 0.35 ICPMSP2O5 0.79 ICPMSNa2O 0.27 ICPMSK2O 2.25 ICPMSTiO2 0.03 ICPMSCl 0.04

L.O.I. 2.2Fixed Carbon -Total Carbon 0.5

Size (mm sq/hole)+2.36 Nil+2.00 Traces+1.18 0.1+0.60 0.2+0.425 1.9+0.30 6.6+0.212 14.2+0.150 28.7+0.063 43.8Passing 0.063 4.5

specific gravity -colour -

BULKDENSITY(KG/M3)loose poured 404vibrated 473compacted 483

Additional Analysesmoisture 0.2% Loss at 105ºCSO3 0.37MnO2 0.13

- = no analysis

limited distribution – uk companies only on rha/48. rha market...limited distribution – uk...

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