epsrc thermal management sheffield drying tech feb 2010

8/11/2019 EPSRC Thermal Management Sheffield Drying Tech Feb 2010

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EPSRC Thermal Management ofIndustrial Processes

A Review of Drying Technologies

(February 2010)

Report Prepared by :SUWIC, Sheffield University

Researchers : Dr Hanning Li, Dr K Finney

Investigators : Professor Jim SwithenbankProfessor Vida N Sharifi

Sheffield University Waste Incineration Centre (SUWIC)Department of Chemical and Process Engineering

Sheffield University


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Executive Summary

In accordance with the EPSRC grant proposal, Sheffield University has conducted an extensive

literature review on biomass drying and an evaluation of the drying process. This report presentsthe results obtained from our literature review work. Various sources of information were used in

order to compile this report. These included websites, journal publications, reports and

communications with manufacturers and industry.

The main topics covered in this review work include:

Biomass Drying (benefits and drawbacks) Drying equipment and processes

Assessment of drying technologies when using flue gases

Assessment of drying technologies when using superheated steam

Costs and environmental impacts of drying biomass

Recent technological developments when using low temperature heat sources for drying

biomass

This report present the findings from the above review work.

Acknowledgements:The authors would like to thank the Engineering and Physical Science Research Council (EPSRC

Thermal Management of Industrial Processes Consortium) for their financial and technical support

for this research work.


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List of Contents

1. Introduction ..................................................................................................................................4 2. Classification and Properties of Biofuels .....................................................................................4

2.1 Benefits of Drying Biomass for Combustion and Gasification ..........................................7 2.2 Drawback of Dried Fuel ...................................................................................................... 8

2.3 Background of Biomass Drying........................................................................................ 10

3. Drying Process ........................................................................................................................... 11 3.1 Stages of Drying................................................................................................................ 12

4. Drying Equipment and Processes .............................................................................................. 13 4.1 Rotary Dryer ..................................................................................................................... 13 4.2 Flash Dryer........................................................................................................................ 16 4.3 Fluidized Bed Dryer ..........................................................................................................18 4.4 Sprout Bed Dryer ..............................................................................................................19 4.5 Belt Dryer.......................................................................................................................... 20

5. Assessment of Dryer Technologies using Flue Gas ...................................................................21 5.1 Performance ...................................................................................................................... 21

5.2 Heat Recovery ................................................................................................................... 23 5.3 Fire Safety in Dryers .........................................................................................................25 5.4 Environmental Aspects ..................................................................................................... 25 5.5 Cost ...................................................................................................................................27 5.6 Dryer Selection .................................................................................................................29

6. Superheated Steam Systems.......................................................................................................31 6.1 Dryer Type ........................................................................................................................ 31 6.2 Advantages and Disadvantages .........................................................................................33 6.3 Capacities of Dryers..........................................................................................................35 6.4 Performances..................................................................................................................... 35 6.5 Heat Recovery ................................................................................................................... 36

6.6 Cost ...................................................................................................................................37 6.7 Environmental Aspects ..................................................................................................... 38

7. Recent Development in Low Temperature Heat Sources for Biomass Drying.......................... 39 7.1 DRY-REX......................................................................................................................... 40 7.2 SRE- Renergi LTD Dryer..................................................................................................43 7.3 Microwave ........................................................................................................................ 45

8. Conclusions ................................................................................................................................46 9. References ..................................................................................................................................48


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1. Introduction

Dry biomass fuel provides significant benefits to combustion and gasification, but they must be

balanced against increased capital and operation costs. Currently, several methods have beenestablished and some promising technologies are being investigated.

2. Classification and Properties of Biomass

Material of biological origin, excluding material embedded in geological formations and

transformed to fossil fuels, is called biomass (Holmberg, 2007). As a renewable energy source,

biomass is derived from living organisms, such as wood, herbaceous crops and waste. Woody

biomass is composed of bark, forest residues, sawdust and cutter shavings. Herbaceous biomass

is grown from numerous types of plants, including miscanthus, switch grass, hemp, corn, poplar,

willow, sorghum and sugarcane. Biomass waste includes construction wood, crushed or chipped

used wood, used paper, pulp and paper sludge, municipal solid waste (MSW), manufacturing

waste and landfill gas. Figure 2-1 shows some samples of solid biomasses.

Biomass is carbon based matter and is composed of a mixture of organic molecules containing

hydrogen, oxygen, nitrogen and also small quantities of other atoms, including alkali, alkalineearth and heavy metals. Table 2-1 presents the properties of biomass fuels.


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Bark Wood chips Pulp and paper slu

Crushed construction wood Sawdust Sugarcane bagasse

Figure 2-1. Examples of biomass materials.


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Table 2-1. Typical physical and thermochemical properties of selected wet biomass fuels (Bruce and Sinclair, 1996).

Pulp and Wood Residues Milled Lignite Sugarcane

Paper

sludge Bark

Fuel

Chips Peat Bagass

Moisture % wet basis 50 - 65 30 - 60 45 - 55 45 - 55 30 - 50 48 - 52

HHV, dry basis MJ/kg 15 - 19 19 - 25 19 - 21 19 - 21 16.6-24.3 18.6-20.3

HHV, wet basis MJ/kg 6 @35% 11@50% 10@50% 10@50% 12@40% 10@50%

Bulk Density (wet

basis) kg/m3 500-900 290-380 260-320 300-400 650-780 80-130

Volatiles % dry wt - 69 - 76 70 - 85 60 - 70 50 - 60 84

Ultimate analysisCarbon % dry wt 25 - 50 55 50.6 50 - 60 41.5-61.4 43.2-49.0

Hydrogen % dry wt 3 - 6 5.8 6.2 5.0 - 6.5 3.4 - 4.6 5.9 - 6.6

Oxygen % dry wt 19 - 38 39 43 30 - 40 11.3-19.7 43.4-48.0

Nitrogen % dry wt 2 - 5 0.1 0.1 1.0 - 2.5 0.7 - 1.1 0.1 - 0.41

Sulphur % dry wt 2 - 48 0.1 0.02 0.1 - 0.2 1.0 - 2.4 trace

Ash % dry wt 3 - 50 3 0.1 2 - 10 11.9-40.6 1.4 - 2.9


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2.1 Benefits of Drying Biomass for Combustion and Gasification

It is commonly accepted that wet fuel consumes some heat of combustion to evaporate water in the

fuel. As shown in Table 2-1, the average moisture content of biomass is typically between

55-60% (wet basis) depending on the biomass type. In addition to the high average moisture

content, the weather, season and storage time may cause drastic deviations in the bark moisture.

The moisture content of sawdust may vary from air-dry to 70% wet basis (Holmberg, 2007). The

high moisture in the biomass will have a negative effect on combustion and gasification operations.

The water should be removed from the biomass to improve the quality of combustion and

gasification.

The heating value (i.e. HHV, as shown in Table 2-1) is dependent on biomass moisture, where a

lower moisture content in the biomass increases the heating value. As a result of drying, energy

input into the boiler may be increased without increasing the fuel input, or the fuel input into the

boiler may be decreased to get the same energy input, as in the case of moist fuel. The basic

functions of the combustion control and burner management systems are to maintain constant

steam flow or pressure under varying loads, through proper input of fuel and to maintain safe and

efficient operation throughout the boilers load range. Compared with several other fuels (e.g. oil,

natural gas and coal), the heating value of biomass varies as a result of varying moisture content.

Generally, it is possible to control large changes in fuel quality during the combustion, but such

boilers set high requirements for the process control. Drier biomass will significantly reduce the

range of varying water content, facilitating process controls in combustion processes.

With dry fuel, all the heat of combustion goes into heating the air and products, leading to a flame

temperature of 2300-2500F (1260-1370C), while green wood has a combustion temperature ofabout 1800F (982C) (FBT, Inc., 1994). The increased flame temperature is beneficial in many

aspects. First, the higher flame temperature means that there is a larger temperature gradient in

the boiler for radiant heat transfer. More heat transfer takes place for the same boiler tube area,

increasing steam production. In new boilers designed for dried fuel, the boiler can be smaller

because less heat transfer area is needed.

The high flame temperature with dry fuel could achieve more complete combustion of the fuel,


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resulting in lower carbon monoxide (CO) levels and less flyash leaving the boiler. More

complete combustion also means more heat is released from the fuel. In a new boiler, the fire

box and the downstream ash handing system can be smaller.

With better combustion, the extra air can be reduced. This reduction in excess air means less heatof combustion goes into heating air. Using less excess air also reduces sensible heat losses with

the flue gases, hence increasing the boiler efficiency. The forced draft (FD) fan, which provides

air for combustion, will consume less power with less excess air. Likewise, the induced draft (ID)

fan, which draws the flue gas out of the boiler and through the pollution control equipment, will

require less power.

There will be an increase of up to 5-15% in the overall thermal efficiency, and possibly up to a50-60% increase in steam production (Wade, 1998).

2.2 Drawback of Dried Biomass Fuel

Drying biomass is expensive and the additional costs may discourage the use of dried biomass fuel.

Figure 2-2 presents the costs for wood (ELECTROWATT-EKONO, 2003). The total production

cost of pellets ranges from 52.2 to 81.3 /t (without drying) and from 73.5 to 94.6 /t (with drying).

The drying costs are dependent on the technology used and range from 25 to 29 /t pellets. The

production costs are also dependent on the annual operational time. If a plant is run in a 3 shifts,

7 days a week mode, the production costs are approx 84 /t pellets. Other operational modes and the

associated costs include: 3 shifts and 5 days a week = 90 /t pellets, 2 shifts and 5 days a week = 101

/tpellets and 1 shift and 5 days a week = 133 /t pellets.

The drying is an additional operational cost in the process and power industries, even though thismay be offset by using smaller boilers, air emissions equipment and fuel handling equipment. If

the required biomass is over dried, the energy consumption and maintenance costs could be

significantly increased. The additional complexity may also affect the overall system operation.


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Without drying With drying

Figure 2-2 Pellet costs without/with drying

In addition to the cost concerns, there are other operational and environmental issues, which must

be addressed when using wet biomass material.

Ash deposition on heat exchanger tubes is commonly known to reduce the heat transfer, which will

subsequently result in a reduction in plant thermal efficiency. Ash deposition on heat exchanger

tubes is also a precursor for corrosion and fouling processes in heat exchangers. Shao, et al .(2009) found that the dried biomass increased the ash deposition rate during biomass combustion.

Such ash deposits from dried biomass decreases the amount of biomass available for the

combustion and increases the maintenance costs (i.e. plant downtime to remove deposits).

Burning dried biomass fuel results in higher combustion temperatures in the boiler. However, as

the flame temperature increases, it approaches the fusion temperature of the ash. If the ash starts

to flow and form slag, this can seriously affect the boiler operation. Usually the flowingtemperature of the ash is safely above the flame temperature, but when contaminants from

construction debris or salts are mixed with the fuel, the flowing temperature can be lower.

The major problems of slagging are associated with alkali metal content, principally sodium and

potassium, both common in biomass fuels with high ratio of alkaline metal oxides to silica. As

dried fuel is burnt in the furnace, the increased ash leads to high concentrations of sodium and

potassium and thus a tendency to slagging at lower temperatures.


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Slagging can seriously affect the boiler operation. As a consequence, heat transfer tube life-time

is reduced and maintenance costs are increased. New material can be designed for corrosion

resistance, but the cost is a concern.

With drier fuel, the higher furnace temperatures will tend to increase the formation of nitrogen

oxides, though modern over fire air systems could minimise NOx concentration (compared to

older designs). When NOx emissions from the firing of biomass fuel (even after drying), are

compared with the natural gas-fired conventional burners, the levels are quite similar. However

NOx emissions from biomass fired plants are significantly lower when compared with the oil and

coal fired systems. Low NOx-burners are an active area of development and some natural gas and

oil burners are capable of very low emissions, significantly lower than that from a conventionallydesigned biomass boiler.

2.3 Background of Biomass Drying

Even though biomass drying is not common before combustion at the moment, commercial

biomass dryers were used at pulp and paper mills in the 1970s and 1980s. The dominant

combustion technique for biofuels at that time was grate firing. This type of boiler can handle

fuels with varying moisture contents, but ideally a moisture content of 30-40% wet basis should be

used (Wimmerstedt 1995). The main reason for investments in manufacturing and designing

dryers in the 1970s and 80s was probably the high oil prices resulting from two oil crises. In

some cases, the moist fuel also decreased the boiler capacity so much that it became reasonable to

install a dryer in combination with the boiler. In the 1970s and 1980s, industrial dryers were

direct flue gas dryers (Holmberg, 2007). Flue gases were either taken directly from the boiler or

generated in a separate flue gas burner. The most common dryer types were drum dryers andflash dryers.

Since the 1970s, fluidized bed boilers have replaced grate firing as a combustion technique

(Huhtinen, 1999). Compared with grate firing, the fluidized bed boiler is a more suitable

combustion technique for moist biofuels. However, the use of moist fuel decreases the energy

efficiency of the power plant. One solution could integrate the existing flue gas dryers with

fluidized bed boilers. Bad operating experiences, environmental considerations and economic


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factors may block the integration development.

Biomass is a burnable material with a heterogeneous particle size. It is also typical that the

biomass flow contains stones and sand. It is obvious that the properties of biofuel set tough

requirements for the operation of the dryer. For some dryer types (e.g. flash dryers), themaintenance cost may also be high. It is presumable that the operational experiences of the flue

gas dryers have not always been satisfactory and bad experiences have supported the decision to

develop the dryer.

All types of wood material contain volatile organic material that may be emitted together with the

water vapour, as shown in Table 2-1. The emissions of biomass drying are greatly affected by the

drying temperature, when it exceeds 100C (Holmberg, 2007). Below 100C, emissions arereported to be low (Spets, 2004). The drying temperatures of flue gas dryers are clearly higher

than 100C. Exhaust gases or unclean condensates must be treated after the dryer if they contain

high concentration of emissions. Treatment increases drying costs and the treatment of the

exhaust gas may be a cost issue.

3. Drying Process

Biomass drying systems consist of three principal factors: drying medium, heat supply and dryer

type. Drying medium are mostly flue gas, air and steam. Heat supplies into the biomass are

operated through convection (direct dryers), conduction (indirect dryers) or the combination of

direct and indirect dryers. The type of dryers includes rotary dryers, flash dryers, belt dryers and

fluidized bed dryers, among others. Rotary and flash dryers are mostly used in industry today.

In direct-heated dryers, hot air, flue gas or superheated steam is in contact with the biomass

material. The hot air, flue gas or superheated steam loses its sensible heat and provides the latent

heat of evaporation to dry the materials. The air also removes the water vapour. The material

can be agitated by mechanical devices or by fluidized air. The super-heated steam remains above

its saturation without condensation.

Indirect drying separates the biomass material from the heat source (either hot air or superheated

steam) by a heat exchange surface. As a result, the latent heat of evaporation of the water vapour


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is easy to recover since the water vapour is not diluted by air. With super-heated steam drying,

the dryer can be designed to produce steam at a desired pressure.

3.1 Stages of Drying

There are several steps for drying. First, the material must be heated up to the wet bulb

temperature to produce a driving force for water to leave the wet material. Next, any surface

moisture is evaporated very quickly. Once all the surface moisture is removed, the material must

be heated to drive water from the inside of biomass to the surface so it can evaporate. This

happens when the rate of drying drops as the material remains close to the wet bulb temperature.

Once the material is completely dry, it begins to heat up to the surrounding temperature, because

water is no longer present to keep the temperature low.

These steps are important for drying a combustible material. High temperatures are desirable to

increase heat transfer and minimize equipment size; but at same time, the fuel ignition could be a

concern. By understanding these steps involved with biomass drying, fast drying at high

temperatures can be exploited with minimal fire risk.

Significant fire risks mostly occur in two instances. The first is after the surface moisture has

been evaporated, but before an appreciable amount of water has been driven out from inside the

biomass. During this very short period, no water vapour at the surface keeps the fuel particle

cool, leading to its surface quickly heating up while the inside remains cool. If the surface

remains hot for a long enough time, the biomass can ignite even if it is not completely dry.

However, once the inside of the biomass starts to drive water to the surface, this supply of

moisture will keep it cool until it is completely dry (Intercontinental Engineering, Ltd. 1980).

Over-dried biomass is also a fire risk. If the biomass loses all its moisture, it will begin to warm

and can ignite when it reaches its combustion temperature. Generally speaking, dryers are notdesigned to completely dry material.

When a material still has moisture associated with it, its temperature will be very close to the wet

bulb temperature of air as evaporation occurs, regardless of air temperature. Therefore, a very

hot air stream can be used to dry biomass in a co-current flow process because the hot air is

introduced to the dryer along with the wet biomass.


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increasing dryness. Figure 4-1 shows internal structure of flights in a typical rotary dryer made

by Aeroglide Corporation and GEA Barr-Rosin .

Figure 4-1. Internal construction of a rotary dryer made by Aeroglide Co. and GEA Barr-Rosin

http://www.aeroglide.com/rotary-dryers.php ; http://www.barr-rosin.com/products/products.asp

There are several types of rotary dryers, but the most widely-used is the directly-heated single-pass

rotary dryer. In this type of dryer, hot air or gas is in contact with biomass material inside a

rotating drum to induce the evaporation of the moisture. The rotation of the drum, with the aid of

flights, lifts the solids in the dryer so they fall down through the hot gas, promoting better heat and

mass transfer. If contamination is not a concern, hot flue gas can also be fed directly into the

dryer. The heat in the hot air or gas evaporates the water and consequently, the gas temperature is

rapidly reduced as it leaves the dryer. The exhaust gases leaving the dryer may pass through a

cyclone, multi-cyclone, gashouse filter, scrubber or electrostatic precipitator to remove any fine

material entrained in the air. An ID fan may or may not be required depending on the dryer

configuration. If one is needed, it is usually placed after the emission control equipment to

reduce erosion of the fan. It may also be placed before the first cyclone to provide the pressure

drop through downstream equipment. Figure 4-2 is a schematic diagram of rotary dryer and its


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associated processes. The biomass and hot air normally flow co-currently through the dryer, so

the hottest gases come in contact with the wettest material, as shown in Figure 4-2. For materials

where temperature is not a concern, the flue gas and solid flow in opposite directions, called

counter-current flow, so the driest solids are exposed to the hottest gases with lowest humidity.

This counter-current flow configuration produces the lowest moisture in the biomass as it leavesthe dryer, but this exposes essentially dry material to a high flue gas temperature, which could

increase the fire risk.

Figure 4-2. Schematic diagram of co-current operation in a rotary dryer.

The basic, single-pass rotary dryer design can be modified to allow three passes of the air and

biomass through the dryer. The material first enters an inner cylinder with the hot air. Smaller

and drier material is quickly blown through the cylinder into a larger concentric cylinder for the

second pass. Larger material is moved and tumbled with the aid of flights. After the secondpass, the air and material pass back up the outermost cylinder of the dryer and are removed. The

triple-pass design works best with biomass smaller than an inch, because larger material can cause

plugging. Single-pass dryers can take larger material.

Another commonly applied technology is indirectly heated rotary dryers, which use a heat source

steam or hot air passing through the outer wall of the dryer or through an inner central shaft to

heat the dryer by conduction. This is more common with materials that would be contaminated

by direct contact with flue gases or with materials that react with air. A hybrid direct/indirect


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rotary dryer also exists where very hot flue gases enter the dryer through a central shaft and

initially provide heat indirectly by conduction, then the same gases pass through the dryer coming

into direct contact with the wet material. During the second pass, the indirect heating warms the

flue gas and solids. In this way, a high flue gas or burner temperature can be used for heating,

while reducing the fire risk by limiting the temperature of the gas in direct contact with biomass.

The inlet gas temperature to rotary biomass dryers can vary from 230 to 1100C and the outlet

temperatures vary from 70 to 110C. Most dryers have outlet temperatures higher than 100C to

prevent the condensation of acids and resins. Retention times in rotary dryers can be less than a

minute for small particles, and 10-30 minutes for larger materials (Intercontinental Engineering,

Ltd., 1980; Haapanen, et al ., 1983).

The efficiency of the dryer is largely dependent on the differential temperatures between the inlet

and exhaust gas, although the heat transfer rate is also influenced by the relationship between the

design of flights and the speed of rotation. Irrespective of the gas and material temperatures,

however, the drying (or residence) time may be critical, as this is governed by the rate of diffusion

of the water from the core to the surface of the material.

Numerous applications of rotary dryers have been made in the drying of sludges from municipal

waste water treatment plants. Relative to other sludge drying processes, the dryer produces large

quantities of exhaust gases containing odorous compounds. Other applications include pulp and

paper, saw mills and board mills. The manufacturers supplying various rotary dryers include

M-E-C Inc, Rader Inc., Raytheon (Stearns-Roger Division) and ABB Raymond (Bartlett-Snow).

4.2 Flash Dryer

The pneumatic or flash dryer dries biomass rapidly owing to the easy removal of free moisture orwhere any required diffusion to the surface occurs readily; drying takes place in a matter of

seconds. Wet material is mixed with a stream of heated air (or other gas), which conveys it through

a drying duct where high heat and mass transfer rates rapidly dry the product. Flash or pneumatic

dryers are followed by a cyclone. The gas passes through a scrubber to remove entrained

particulate material. A simplified process of a flash dryer (without a scrubber) is shown in Figure

4-3.


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The flash dryer provides short drying times and the equipment is more compact than a rotary dryer.

The flash dryer requires smaller sizes of biomass so as to transport and suspend it by an air stream.

Some dry biomass can be mixed with the wet inlet material to improve material handing. Flash

dryers have been used not only for wood waste, but also for peat, bagasse, lignite and other solids.

Gas temperatures are slightly lower for flash dryers than for rotary dryers, but they still operate attemperatures above the combustion point. The solids retention time in a flash dryer is generally

less than 30 seconds.

Manufacturers include Flakt, of Sweden, Raymond Division of ABB Raymond, Ahlstrom of

Finland, Williams Patent Crusher and Pulverizer Co., Inc. of St. Louis, MI, F.L. Smidth, of

Denmark (Figure 4-4).

Figure 4-3 flash dryer configuration


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Fluidized bed dryers are mostly used in steam drying processes and co-location near process plants

or power plants. The manufacturers include GEA Barr-Rosin Inc, Canada; GEA Process

Engineering Inc; Anhydro.

Figure 4-5. Schematic diagram and manufactured fluidized bed dryers.

http://www.anhydro.com/content/us/products/dryers/fluid_bed_dryers

http://www.barr-rosin.com/products/fluid-bed-dryer.asp

4.4 Sprout Bed Dryer

Sprouted dryers (cascade dryers) are commonly used for dying grain, although they can be used

for other types of biomass. The material is introduced by a flowing stream or screw driver and

the drying media is introduced at the bottom (Figure 4-6). The feedstock then falls down to the

bottom and is lifted again. The material is let out through the opening holes on the side of

chamber. The residence time is generally a couple of minutes.

The original cascade dryer design was by Bahco of Sweden. The original Bahco group is now

part of the ABB group which have retained the rights for the rest of the world. Others include

Hercules, Canada and ESI Inc. of Kenneshaw, GA. The cascade dryer is mostly be used for

wood waste.


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Figure 4-6. Sprouted dryer (Berghel et al., 2008).

4.5 Belt Dryer

In belt dryers (Figure 4-7), the feedstock is spread onto a moving perforated conveyor to dry the

material in a continuous process. Fans blow the drying medium through the conveyor and

feedstock. If multiple conveyors are used they can be in series or stacked (i.e. multi-pass).

Belt dryers are very versatile and can handle a wide range of materials. Recently, belt dryers are

frequently used in low temperature operations to save energy, as well as to reduce air emission and

fire hazards. Belt dryers are provided by Swiss Combi, Bruks Klckner, Mabarex, Andritz Fiber.

Figure 4-7 Drying plant in Nufri, Spain. Plant type: Belt drying plant. Input: Sludge from fruit

juice plant. Heat Generation: waste heat, hot water. Evaporation: 3.2-5.3 t/h. Start up: 2005


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5. Assessment of Dryer Technologies using Flue Gas

Thermal drying systems for moist fuels can be grouped into two categories, flue gas/air type and

steam type. Flue gas/air dryers use the sensible heat in the products of combustion to accomplish

the drying and are a well established technology. The heat supplied in the gases, together withthe moisture driven off, is directed first to the emission control equipment and then to the stack for

release. Steam dryers use the latent heat in steam to accomplish the drying. The steam

originating from the moisture driven off from the fuel is recirculated as the heat transfer medium.

They are a more recent development, and come in a variety of forms. Their assessment will be

discussed in next section.

The following introduces the advantages, limitation, manufactures, capacity, operationalconditions and costs of each dryer under flue gas/air as drying medium.

5.1 Performance

Rotary Dryer

Rotary dryers are less sensitive to particle size and can accept the hot flue gases. They have low

maintenance costs. The material moisture is hard to control in rotary dryers because of the long

lag time (Fredrikson, 1984). Rotary dryers also present a fire hazard and require the most space.

Compared with single-pass dryers, triple-pass dryers have higher capital costs, higher maintenance

costs, higher blower costs and pose more of a fire risk (Intercontinential Engineering, Ltd. 1980).

The retention time for the smallest particles can be as short as 30s, while for most of the material it

is in the order of 20-30 minutes. Common design features affecting retention time include single

and triple pass, and dense versus open internal flighting. The single pass dryer is less prone to

plugging and can handle a wider range of particle sizes; a triple-pass dryer uses concentriccylinders to increase the gas path length and generally cost less. Some designs recirculate

55-65% of the flue gas to lower the gas temperature at the inlet to the dryer and reduce the

generation of blue haze fine particulates composed of the more volatile components of the

material being dried, which is particularly problematic in the case of wood.

Rotary dryers are suitable for materials which are in the form of free flowing solids and fibrous

particles of up to 125mm (5"), granules, pellets, broken filter cakes and powders. Additionally,


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Sprout Dryer

Sprout dryers are similar to flash dryers, except they can handle slightly larger particles.

However, for a good cascading effect in the dryer, the particle size must be fairly uniform. Like

other direct air-heated dryers, heat recovery is difficult and expensive.

Moisture reduction is normally similar to that with rotary dryers, from 50-60% down to 20-40%

range, wet basis. Retention time is in the order of 2 minutes. Based on 4 cascade dryer

applications, Bruce and Sinclair (1996) summarized oven dry throughput rates ranging from

1.4-24.5 t/h. The dryer performance is summarized in Table 5- 1.

Belt DryerBelt dryers are better suited to take advantage of waste heat recovery opportunities because they

operate at lower temperatures than rotary dryers. Rotary dryers, for example, typically require

inlet temperatures of at 260C for drying, but more optimally operate around 400C. In contrast,

the inlet temperature of at least one commercially available vacuum dryer can be as low as 10C

above ambient, although more typically belt dryers operate at higher temperatures between about

90C and 200C. Because of their lower temperatures, belt dryers can even be used in

conjunction with a boiler stack economizer to take maximum advantage of heat recovery from

boiler flue gas. In this scenario, an economizer first recovers heat from the boiler flue gas, then

the exhaust from the economizer is used for fuel drying.

Their lower temperature also means that there is a lower fire risk. Emissions of volatile organic

compounds (VOCs) from the dryer will also be lower. An advantage of belt dryers over many

other dryer types is that the material is not agitated. This means there may be fewer particulates

in its emissions. On the other hand, fines may need to be screened out first and added back into

the dryer at a later point, since they can fall through the belts perforations.

5.2 Heat Recovery

Energy efficiency in air drying can be improved by using heat exchangers, recirculating exhaust

gases, multistage drying and heat pumps.

In heat exchangers, the heat is transferred from the exhausted gas through the wall of a heat


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exchanger to the inlet gas. The exhaust gas is usually not saturated, so part of it can be

recirculated into the inlet of the dryer. Because the exhaust gas is generally warm, the energy is

not needed for heating recycled exhausted gas. Overall, the drying efficiency can be improved.

Besides, the exhaust gas, if high enough in oxygen, can be also used as preheated burner air.

The latent heat of water vapour in exhaust gases can be recycled to heat inlet material.

Wimmerstedt (1999) studied a case that a flue gas dryer was not co-located with other plants.

The dryer was operated with a dew point for exiting gas of about 80C. The gas can be used for

material pre-heating in a direct contact process. If there is a demand for the material to be

heating to a low temperature, a considerable part of the latent heat can be recovered. An

assessment of the heat consumption in one such plant gave a specific heat consumption of 3140

kJ/kg evaporated water. About 80% of the waste energy could be recovered for district heatingbut the outlet water temperature was as low as 50C.

Multistage drying can be used when high inlet temperatures are a concern. Instead of diluting the

entire hot gas stream with cold air to reduce the temperature, some of hot gas can be introduced to

later stages of the dryer to boost the air temperature. As a result, less dilution air is used. A

run-around coil can be used where the physical layout of the dryer does not allow the exhaust gas

to be close to the inlet gas. A heat carrier, such as antifreeze, oil, or any kind of heating fluid is

first pumped through a heat exchanger coil in the exhaust gas duct, then through a heat exchanger

in the inlet air duct to release its heat to the inlet air. The disadvantage is that two heat

exchangers are needed, but this is sometimes cheaper than running extra duct work.

A heat pump is similar to a run-around coil. Because the heat pump uses a refrigerant and

compressor, it can recover part of the latent heat of vaporization by condensing or dehumidifying

the exhaust gas and then provide this heat to the inlet air at a higher temperature. Heat pumps

could improve the efficiency, but capital costs for the compressor can be very high with significant

compressor energy requirements. Heat pumps are also generally limited to providing heat at no

more than 60-66C.


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5.3 Fire Safety in Dryers

Fire safety refers to precautions that are taken to prevent or reduce the likelihood of a fire. Fires

start when a flammable and/or a combustible material with an adequate supply of oxygen is heated

to an ignition temperature.

Combustible organic vapours are generally released from wood at temperatures from 200C,

where auto-ignition temperatures are generally 260-280C. Most dryers however can operate at

much higher temperatures to keep the air temperature greater than the biomass surface temperature.

This increases the drying rate, but also increases the fire risk in the dryer. For this reason, all

dryers are designed to minimize fire risk and are equipped with fire suppression systems.

In general, air drying has a potential high fire risk, because of the high amounts of oxygen in the

air supply. This can be reduced by limiting the amount of excess air or by recirculating exhaust

gases to the dryer inlet. Recirculation of exhaust gas also increases the thermal efficiency of the

dryer.

Flue gas dryers can operate at higher temperatures than air dryers, because flue gases have a lower

oxygen content (Intercontinental Engineering, Ltd, 1980). Compared with air or flue gas dryers,

superheated steam drying processes have a low fire risk. One risk however is the hot dried

biomass coming into contact with air after drying (Haapanen 1983, Wardrop Engineering, Inc.

1990).

As mentioned above, one important issue causing a fire hazard is the high temperature. One

effective method is a reduction in the operational temperature. For example, an operation

temperature of less than 100C could significantly reduce the likelihood of a fire. Various drying

processes at low temperatures will be discussed in Section 9. The maximum temperature that can

be used at the dryer inlet is limited by the burning temperature of dry wood in air around

260-290C. The flue gas can reach high temperatures, since they are depleted in oxygen relative

to air (typically 5-10% O 2 by volume).

5.4 Environmental Aspects

The exhaust gas from a biomass dryer may need to be treated before release to atmosphere. If


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flue gas is used for drying wet fuels, the outlet gas from the dryer may contain SO 2, CO 2, CO,

particulate matter (PM) and hydrocarbons. Table 5-2 shows emissions data after secondary

cyclones for drying softwood.

Table 5-2. Softwood dryer emissions data (Wade, 1998).

A survey by Bruce and Sinclair (1996) indicated that for rotary dryers, total PM leaving the dryer

is lower than entering. In some cases, the reduction is as high as 50-80%. To achieve a

reduction in air emissions, the equipment is required to associate with the process. The first piece

of equipment after the rotary or flash dryers is a primary cyclone to separate the biomass fromexhaust gas stream. A set of multicyclones can follow the primary cyclone to remove some PM.

Cyclones, however, are not very effective for very small particles and thus a baghouse or wet

scrubber may be needed to remove small particles.

Some of particles may be removed in part by cyclones and wet scrubbers. However, condensable

volatiles can not be removed by dust collectors and appear in the stack emissions and result in

undesirable blue haze. At dryer temperatures higher than 180-220C, condensable resins and

organics will be released. After leaving the dryer, the condensed resins and organic acids form

aerosol, called as blue haze (Wade, 1998). These condensable organics are also counted as

particular matter (Bruce and Sinclair, 1996). The most effective method to control fine PM and

aerosols are wet electrostatic precipitators. The others include filter bed and electro-filter bed

scrubbers.

The volatile emissions mostly consist of monoterpenes. Monoterpenes are naturally emitted from

wood and have boiling points of 150-190C. The major components are -pinene and -pinene.


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Photochemical reactions of monoterpenes with nitrogen oxides form low level ozone. Ozone is a

strong oxidant and a component of smog. In high concentrations, ozone is responsible for

impaired lung function in human populations, crop damage and is believed to be responsible for

forest damage in Europe and North America. Volatile emissions control can be achieved by

lowering peak temperatures, recycling exhaust gases leaving the dryer and reducing the amount offine material in the dryer feed material and its residence time.

5.5 Cost

In general, three kinds of cost information are available in the literature.

The first kind of cost information is the dryer equipment alone, i.e. flue gas dryer equipment:

rotary, cascade, and flash, and so on. The second kind of cost information is the complete costs,

including equipment and installation costs. The third one is the operating cost, such as costs by

electricity, water and gas, etc.

The costs may change according to the different dryer types, installation and the retrofit of the

dryer. During the course of this study, the costs for different dryers were identified:

Rotary Dryer

- Single pass rotary dryer = $20/kg/h, three pass rotary dryer = $18/kg/h.

- The dryer cost for 7.6 t/h wet wood chips = $323.000 (included installation cost);

- The dryer cost for 15-130 MW = $1.5-5.3 million (complete unit);

- Steam & Roger rotary single pass dryer = $26-47 /kg/h (complete unit);

- MEC rotary single pass dryer = $24-65 /kg/h (complete unit);

- Aeroglide rotary dryer = $17-31 /kg/h (complete unit);

- Heil rotary dryer = $32-88 /kg/h (complete unit);- Biomass dryer = $38,000 /t/h(42/kg/h) (included installation cost);

- Biomass flue gas dryer for 55t/h = $5.4 million(complete unit);

The heat requirements were 3,000-8,1000 kJ/kg of water removed., with most estimated in

3500-4700 kJ/kg

Flash Dryer

- For a disk dryer, the cost was estimated at $5.4; the capital cost of a flash dryer was $18-35 /kg/h.


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- The total cost of both the equipment and installation of a 15-130MW flash dryer for was

$550-1600 /kg/h.

- The cost of the dryer for bark = $350 /kg/h.

-There are two line Flakt flash dryer installed at the Assi Lvholmen Linerboard mill in Pitea,

Sweden. One unit supplies a lime kiln, the other, a recovery boiler converted to a power boiler.The units cost 17.5 M$US in 1980, or the equivalent of 24.5M$US at the end 1995 for both the 6

and a 13 t/h capacity, or about 9.5 and 15MUS$ respectively. A second article describes a Flakt

flash dryer installation at E. B. Eddy Ltd, in Espanola, Ontario, which cost 14MCdn$ in 1986 and

handles about 18 t/h of wood waste (Wade, 1998).

Cascade Dryer

During the course of this study, technical articles providing capital costs for three cascade dryerinstallations were identified:

Cascades Inc., East Angus, Quebec cost 36M$Cdn in 1992, or the equivalent of 32M$US at the

end 1995. Included were both cascade and flash drying and suspension firing of part of the fuel.

The cascade dryer with a throughput of about 9.0BDt/h accounted for about 5M$US;

Fletcher Challenge, Crofton, BC, cost 8.5M$Cdn in 1986, or the equivalent of 7.2M$US at the

end 1995. The cascade dryer had a throughput of about 36BDt/h.

Alabama River Pulp, Claiborne, AL, cost 6.3M$US in 1992, or the equivalent of 6.6M$US at the

end 1995. The cascade dryer had a throughput of about 32BDt/h.

Bruce and Sinclair(1997) summarized the costs for three kind of dryers and found that the total

installed costs for the complete dryer systems were very similar, if the wood fuel handling and

storage is excluded. This cost information is summarised in table 5-3. It should be emphasised

that the material handling equipment such as conveyors, feeders and bins is not included in any of

the cost information presented. These costs and retrofits where space is limited can add

substantially to the total cost of an installation.


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Table 5-3. Capital Cost of Flue Gas Dryers (a)( Bruce and Sinclair, 1996).

Type

Moisture Content

In,% - Out,%

Equipment Cost

k$/t/h

Total Installed Cost (b)

k$/t/h

Rotary 55 40 80 - 45 370 - 160Sprouted 55 40 70 - 45 360 - 200

Flash 55 15 180 - 70 860 - 330

Notes: a - based on all the boiler flue gas entering at 300C and leaving the dryer at 105C.

b - the first value being for about 4 t/h, the second about 35 t/h.

Besides equipment and installation costs, the operating costs are important concerns. The main

components to the operating costs are those for power and maintenance. Power consumptionbased on oven dry throughput are: 8-14 kWh/t for rotary dryers; 15-20 kWh/t for cascade; and

16-38 kWh/t for flash dryers, the latter depending on the size reduction required, which is a large

consumer of power. Size reduction power is highly variable depending on the equipment used,

size distribution of the feed, product size, type of material being pulverised, species and initial

moisture content. Summary data presented in a reference indicates a range of 10-100 kWh/t or

more.

In the absence of data encountered in the literature on the maintenance costs of equipment, an

allowance of 2% of total installed cost of the drying system equipment is suggested as the basis for

preliminary evaluation purposes.

5.6 Dryer Selection

The selection of dryers depends on a particular application. In general, water evaporation rate,biomass property, biomass size, operation temperature and heating resource availability are

important in the selection. Meanwhile, environmental controls and safety are important

considerations in the dryer design.

For flue gas/air dryers, the selection of dryers is highly depended on the size of the biomass

material. For flash dryers, a small particle size is needed for moving air or steam to suspend the

particles. Triple-pass rotary dryers will accept larger material, but may experience plugging by


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very large material. For large or variable material, a single-pass rotary dryer might be the best

option. In general, reducing the size of the material may be a good option for the drying process,

but it is an energy-intensive operation.

In the selection of dryers, the advantages and disadvantages of various dryers are alwaysconsidered. The significant advantages of rotary dryers include the fact that they are less

sensitive to material size, they operate at high temperatures to reduce drying time, their wide range

of evaporation rates and their easy installation. The major drawback is that these possess the

greatest fire hazard, since the high temperature operation is mostly applied in rotary dryers. Air

emissions need to be controlled. Heat recovery is difficult in rotary dryer. Flash dryers are

more compact and easier to control, but require a small particle size. Air emissions again need to

be highly controlled. Both flash and cascade dryers are used for high capacity water removal.Belt dryers are currently adopted in low temperature operations, which present a lower fire risk,

reduced air emission and low energy consumption but require a large footprint. In general, rotary

dryers are the most commonly selected. Technical knowledge of equipment configurations,

installations and operations could provide useful information for new users. These are

summarised in Table 5-*.

Table 5-4. Summary of considerations in choosing a dryer.

Dryer

Type

Requires

small

particles

Heat

recovery

Fire

Hazard

Air

Emission

Drying

Temperature

(C)

Evaporation

(t/h)

Rotary No Difficult High medium 200-600 3-23

Belt No Easy Low Low 150-700 4.8-17

Flash Yes Difficult Medium High 160-280 2-41

Cascade No Difficult Medium medium 30-150 0.5-40

The capital costs of various dryers are often comparable. However, a belt dryer operated at low

temperatures may require less equipment for treatment of emissions; so for new installations the

overall cost may be less. The operation and maintenance costs of belt dryer are higher than for

other dryers. In general, multi-pass dryers are more complex than single-pass dryers and so have

greater operation and maintenance costs.


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6. Superheated Steam Systems

Using steam to dry moist fuels has attracted recent interest, because of the advantages of the low

risk of fires, high energy efficiency and better environmental control.

The main components of a steam drying system are the fuel feeder, the flash dryer ducts or fluid

bed depending on the concept, cyclones, blower for recirculating the steam and a heat exchanger to

heat the superheated steam. The recirculating steam can be near atmospheric pressure or at a

higher pressure. The steam is in a superheated condition to provide the thermal driving force

necessary to evaporate the moisture in the fuel. The source of heat can be either very hot gases or

high pressure steam. Instead of air, superheated steam is used to suspend the solids and provide

the heat. Generally, the wet material is mixed with enough superheated steam to dry the materialand end with steam.

The superheated steam dryers have some key advantages compared to air dryers. No oxidation or

combustion reactions are possible. Steam dryers have higher drying rates than air and gas dryers.

Steam drying also avoids the danger of fire or explosions and allows toxic or valuable liquids to be

separated in condensers. However, the systems are more complex and even a small steam

leakage is devastating to the energy efficiency of the steam dryer.

6.1 Dryer Type

In the 1980s, an energy efficient steam drying technique using superheated steam or turbine

backpressure steam was developed in Sweden by Modo-Chemetics for drying wood pulp, bark,

agricultural products, peat and other biomass materials under pressure. Since that time, different

forms of steam dryers have been developed, tested and in some cases, commercialised. Thisstudy introduces different approaches to steam drying. Steam drying technologies may be

atmospheric or pressurised, have short or long residence times and be large or small. Simplified

block diagrams of various types are presented in Fig 6-1.


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a. Basic IVO steam dryer b. IVO steam dryer with fluidized bed

c. Niro steam dryer d. Luri steam dryer

Figure 6-1. Simplified block diagrams of various types of superheated steam dryers.

The first is the basic IVO dryer made by Imatran Voima Oy (IVO), where biomass material is

mixed with recycled superheated steam, as shown in Figure 6-1 a. The superheated steam andbiomass pass through a flash tube and the solids are separated from the steam in a cyclone, the

same as in a flash dryer. Most of steam is recycled through a fan to provide the force to suspend

the solid material and then the steam passes through a heat exchanger to increase its temperature.

The extra steam can be condensed to recapture the latent heat, compressed to a higher temperature,

or with high pressure operations, steam can be injected into a gas turbine to increase the power

output (Hulkkon et al. 1991).

The second IVO superheated steam dryer is a bed mixing dryer, as shown in Figure 6-1 b. It can

be used with a fluidized bed gasifier or boiler. Some of the hot bed material from the combustion

chamber is mixed with the wet biomass in steam. The sensible heat from the bed material

evaporates the water from the fuel. The fuel can then be fed, along with the bed material back

into the process, while the steam can be recycled, with the excess steam being used for other

applications (Hulkkonen et al., 1991).

Niro of Denmark has commercialised its superheated steam fluid bed dryers and have numerous


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installations. The fluid bed operates under pressure of 200-300kPag (29-44 psig) by circulating

steam in a closed vessel. Wet material is fed into the first of 16 vertical cells arranged around a

centrally located heat exchanger. In the cell, the material is kept in suspension by superheated

steam entering from the bottom. The material moves successively through each of the 16 cells and

is finally discharged by a screw conveyor. Niro installations are mainly applied to the drying ofsludge and other by product materials in the agricultural industry as a part of reprocessing.

The Lurgi technology from Germany uses a fluidised bed consisting itself of the material being

dried. The material to be dried must be capable of assuming and maintaining a granular form, but

many materials, according to Lurgi, exhibit this property, including: peat, lignite, sewage sludge,

pulp and paper mill sludge, agricultural by products, ore and mineral sediments. Heating of the

wet material and reheating the superheated steam is done using a heat exchanger tube bundlewhich is immersed in the bed material and provides very high rates of heat transfer.

A steam drying system developed by Modo-Chemetics for wood waste fuels was adapted from

earlier work on pulp drying and was promoted in the early 1980s for fuel drying. Only one such

system was installed, and is no longer in service. This technology, similar to Stork's, is no longer

being offered.

6.2 Advantages and Disadvantages

The main advantage of superheated steam dryers is that the latent heat of vaporization from drying

can be recovered and no heat losses occur compared with heating air for drying. There are

normally no air emissions from superheated dryers because all the vapours, including organic

species, are condensed. Nevertheless, wastewater treatment should be considered. Superheated

steam dryers have higher heat transfer rates and faster drying. The presence of an inert steam

atmosphere has no fire hazard. The IVO mixed bed steam dryer does not need heat exchangersfor drying. A high-pressure IVO dryer integrated with a gas turbine eliminates the wastewater

stream by combusting the organics in the turbine.

The heat source and temperature for drying are important for dryer selection. Compared with

flue gas, a process stream for heating exchange is energy efficient, but it requires the capital

investment in heat exchangers and the interactions between the dryer and process. Superheated

steam dryers require a high-temperature heat source. The selection and design should consider


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6.3 Capacities of Dryers

Besides the advantages and disadvantages of each dryer, the capacity and costs in industrial

applications are critical parameters to determine the selection of dryers. Table 6-2 lists the

capacity and operation parameters of industrial dryers (EPRI).

Table 6-2. Capacities and performances of steam dryers (Bruce and Sinclair, 1996).

Company Dryer

Evaporative

Capacity,

t/h

Operating

Pressure in

Dryer, kPag

Steam Pressure

of Heat Source,

kPag

Average

Residence

Time, s

IVO Flush 0.7(1) 300-2200 - 2-3

IVO Flush 5.6(2) ~0 - 2-3Niro Fluidized bed 1-40 200-300 1500-2800 300

Lurgi Fluidized bed >150 15 - 25 400 - 900 -

1 - One 0.7 t/h pilot plant, and range of pressures tested

2 - One demonstration plant of 5.6 t/h

6.4 Performances

The superheated nature of the recirculating drying steam is achieved by a heat exchange with the

heat source. From Lurgi's literature, the high pressure steam required for lignite and peat steam

dryers is 400-900kPag, giving dryer steam temperatures of 120-150C, assuming a 40C

difference. For agricultural and wood residues, the corresponding pressure is 1000-2500kPag,

giving dryer steam temperatures of 144-186C, again assuming a 40C difference.

The minimum temperature of the recirculating drying steam is a function of the operating pressureof the dryer. For efficient use of the dryer capacity, the steam must remain superheated. A 20C

above saturation temperature is commonly accepted, though equipment suppliers will have their

own criteria.

The throughput rate, evaporation capacity and other design and performance data of a Niro system

described in the literature are presented in Table 6-3.


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Table 6.3. Design and performance data of a Niro A/S steam dryer.

Evaporation (t/h) 27.2

Capacity (t/h) 10.8

moisture, Feed % 70.8

moisture, Discharge % 10

Thermal requirements:

without heat recovery (GJ/t_evap) 3.45

with latent heat recovery (GJ/t_evap) 0.4

Blower power (kWh/t_evap) 67

6.5 Heat Recovery

In superheated steam drying, the latent heat of vaporization is easier to recover because the water

vapour that leaves the fuel is not diluted by air, so it can be condensed directly to recover the heat.

Depending on dryer and plant configuration, there are several possibilities for heat recovery. If

power plants could provide hot water for heating, superheated steam processes can be operated at

atmospheric pressure. If process steam is required, the dryer must either be operated at a higher

pressure or the steam from the dryer must be compressed to increase the temperature. The steam

from the dryer can be used directly or it can be condensed on the outside of the boiler tubes to

produce clean steam without any impurities.

In the case of a steam dryer incorporated in a process industry or a district heating plant, the steam

from the dryer can be recovered at high temperatures and can be further used in a process industry

or in a DHS at high temperatures, as shown in Figure 6-2 (Wimmerstedt, 1999). From a

thermodynamic point of view, the heated steam to the dryer should be extracted from the turbine at

as low a pressure as possible, to ensure maximum cogeneration. The waste heat from the dryer is

mainly used for feed water heating, which gives more cogenerated electricity than using it for

district heating (Wimmerstedt, 1996). The IVO Bed Mixing Dryer could be included in this case.

The heat to the dryer is supplied directly from the boiler via the hot bed material. The product

steam is used for heating the district heating water. The entire recovered heat load reduces the

basis for cogeneration. The coupling accordingly gets the same function as increasing the


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Table 6-4. Wastewater characteristics from steam dryers.

The non condensable components in the dryer steam include rich-terpene, CO 2 and lesser amounts

of H2, CO, CH 4, and C 2 - C4 (Fagerns, 1996). Those components can be disposed of by firing in

the boiler or furnace receiving the dried fuel (EPRI).

7. Recent Development in Low Temperature Heat Sources for

Biomass Drying

In thermal drying, moisture in wet biomass is evaporated by introduction of a hot gas (air)/steam

stream. With typical temperatures averaging 350C, the dried product contains 70-90% solids by

weight. Typical thermal drying installations include unit operations like blending of wet and

dried solids, drying, dried solids handling and exhaust gas cleaning. Most designs require wet

scrubbers and dry solids collectors to prevent particulates and odours from polluting the air. As a

result, most thermal drying plants are large, complex systems associated with high capital

investment and operating costs. Furthermore, control and handling of dust emissions and

high-fire risks are a problem with all thermal dryers.

Biomass drying using low temperature heat sources offset the high capital and operating costs

associated with traditional drying methods. Under a low-temperature operation environment,

dust emissions and fires risks could be significantly reduced. As reported so far, industrial dryers


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available for low temperature heat sources are provided by Swiss Combi, Bruks Klckner,

Mabarex (Thermal Energy), Andritz Fiber and Svensk Rkgasenergi (SRE), among others.

Those dryers have capacities of 500-4000 kg/h under operation temperatures of 30-110C. The

following section introduces some typical applications of dryers using low temperature heat

sources. Besides, microwave dryers are also briefly introduced as a potential commercial dyers.

7.1 DRY-REX

Thermal Energy International Ltd has installed a low temperature biomass drying system,

DRY-REX, at Uniforet pulp and paper mill in Quebec to dry sludge into high efficiency biofuel.The system processed 200 wet tons of sludge a day containing 22% solids (thus 78% moisture),

evaporating 5 tons/hr of liquid from the sludge at low temperature with virtually no emissions.

The DRY-REX dryer is an ambient temperature system, which uses a patented, two-step,

integrated granulating-drying process (Barre and Bilodeau, 1999). The feed stream from the

dewatering system is first processed in a granulator on top of the dryer. Granulation reduces wet

residue bulk, optimizes particle size and solids distribution for easier handling and exposes

maximum surface area for fast drying. The dryer consists of a totally enclosed structural frame

with several stacked drying zones, each with its own solids conveying system and air distribution

assembly, as shown in Fig. 7-1. The continuous tunnel-type convection dryer uses a vacuum,

forced-air stream and air temperature above 5C as the main driving force. The 50% of the drying

air is recirculated to reduce the quantity of exhaust air. The residence time is 15-30 minutes for

sludge and wood chips. Thermal energy for water evaporation is 50-150 kWh/t water evaporated,

much less than other dryers, as shown in Table 7-1. The capital costs, which include supply,

installation, operation, maintenances and energy, are 2.4-6.5 U.S.$/wet ton (in 1999) for sludge

and 0.5-1.5 U.S.$/wet ton (in 1999) for bark. The identified problem was that temperaturesabove 5C limited the absorption of water. Barre and Bilodeau (1999) suggested that the

temperature should be raised to 20C.


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solid product, enabling mills to eliminate the dust problem and to bypass the screw presses.

Table 7-2 Typical results in the pulp and paper industry.

For drying bark, tests were done at several mills. The pilot dryer in the tests was fed at an

average rate of 42 wt% solids (as low as 27.6wt% solid) and delivered on average 75 wt% solids.

There were no significant differences between drying raw and shredded bark, but bark particles

should be limited to 75x75x20 mm or less to prevent potential accumulation and obstructions in

the dryer. Drying tests were also done on mixtures of bark and sludge with various mixing ratios.

The results revealed that drying bark alone and drying the mixture of sludge and bark were

essentially identical. Table 7-3 illustrated the effects of feeding dried bark into the existing power

plant.


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Table 7-3 Drying converts wet residuals into a profitable energy that reduces consumption of fossil

fuels and lowers the emission of greenhouse gases.

7.2 SRE- Renergi LTD Dryer

Svensk Rkgasenergi (SRE), a subsidiary of Opcon AB, the energy and environmental technology

Group, signed an agreement concerning delivery of its new energy-efficient low-temperature drier

for biomass System Renergi LTD, to Corbat SA, a Swiss sawmill. SREs low-temperature drier

will be used to produce pellets.

The Renergi LTD low-temperature dryer can use heat at temperatures of 50C to dry sawdust,

chips, bark and similar material. Because of the low temperatures the heat can be recovered from

waste energy sources. This also saves significant the operating costs. The Renergi LTD dryer is

based on the counter-flow principle. Material is fed from the top of the dryer and removed at the

bottom. The heart of the dryer system consists of two drying cells. The task of the drying cell is

to provide optimal exposure to the sawdust so that its water content can be effectively taken up by

the dry air that passes over the it. The wheels on the dryer cell consist of 16 shovels that are

designed to spread the sawdust into the air. This ensures efficient exposure of the solids to the air,

which can absorb the water at a high rate. The long-term retention time in each drying cell

guarantee homogeneous drying.

The fan sucks in the outside air via the air battery LB1, as shown on Figure 7-2, which raises the

air temperature. The dry and warm air offers good conditions for effective drying. The water

content of the sawdust is absorbed in TC1 by the dry air until the air is saturated with water and


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leaves the drying cell with a lower temperature. Air passes through an air filter (LF1), which

removes sawdust before the air is heated once again via LB2. Now the air can perform a

secondary drying process in TC2, leaving the dryer via the LF2 air filter. Once this drying batch

is ready, the process returns to emptying and refilling of dryer cells. Table 7-4 shows the

performance of this dryer. There is no further information regarding its operation andmaintenance.

Figure 7-2. Functional diagram and cell structure of Renergi LTD Dryer.

Table 7-4. Performance of the RENERGI LTD low temperature dryer (Incoming fresh air to dryer:

Temperature: 0C, Water content in per kg of dry gas: 2 g/kg, Enthalpy: 5 kJ per kg of dry gas /

Saw dust: 55 % moisture content in incoming saw dust, 10 % in outgoing saw dust.


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7.3 Microwave

Conventional drying of wood is the most energy-intensive and costly process. Conventional

wood dryers function under the basis of convective heat transfer from hot air to the surface of

wood followed by conductive heat transfer from the surface to the centre of wood. These dryersrequire considerable amounts of energy and long drying times in order to evaporate water from the

wood. Unlike the conventional dryers, where heat is applied externally to the surface of the

material, microwave simultaneously heats the bulk of the material. When properly designed,

microwave drying systems have several advantages over conventional methods including a

reduction in the drying time, high energy efficiency, improvements in product quality for various

industrial applications, a reduction in fire hazards and lower air emission. Microwave drying of

wood products, however, has not been used to a larger extent in wood industries mainly due to theinsufficient knowledge of the complex interaction between wood and process parameters during

drying as well as the higher investment expenses.

The technology of microwave heating and drying in the field of wood products started to be used

in the early 1960s. Microwave drying and heating processes can be operated at temperatures

below 100C, which is beneficial in terms of energy savings, safety operations and emission

control. In addition, drying wood with the use of microwaves is faster than conventional drying.

Antti (1999) has shown that the microwave drying of pine and spruce is 20-30 times faster than

with conventional methods.

The wood drying rate through using microwaves mainly depends on the frequency of microwave

irritation, the wood material, the temperature and the wood sizes (Hansson, 2007). At a

frequency of 2.4 GHz, a drying rate of 0.4 fractional moisture per hour could be reached for boards

of spruce and beech (Hansson, 2007) and a 25-mm-thick Korean red pine could be dried in less

than one and half hours (Lee, 2003). With a frequency of 915 MHz and a manipulated

microwave input power and hot air, a 25-mm-thick pine plank could be dried in less than three

hours (McAlister and Resch, 1971). A prototype continuous microwave dryer for softwood

structural lumber could dry 50-mm-thick hemlock and Douglas fir in 5-10 hours (Hansson, 2007).

It is known that drying by microwave technology is a high energy consumption process even

though it saves thermal energy. To improve energy efficiency, Turner (1994) pointed out the

suitability of combined microwave and convective drying. Lee (2003) investigated the


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performance of combined microwave and convective drying of wood. In his study, Korean red

pine was selected as material. To study efficiency, Lee (2003) compared specific moisture

evaporation rates (SMER). In general, the average value of SMER in microwave drying was

found to be about 1 kg/kWh (Lee, 2003). As shown in Table 7-5, evaporation rates were very

low if combined microwave and convective heat was used. To improve energy efficiency, theexhaust gas should be recycled to inlet.

Table 7-5. Performance characteristics of combined microwave and convective drying Korean red

pine at a constant hot air energy input of 1kWh per one hour without heat recovery.

For wood drying using microwaves, the effect on wood hardness and structure is a primary

concern. Vongpradubchai and Rattanadecho (2009) investigated wood properties after

microwave treatment. SEM results demonstrated that microwave dried specimens has a better

micro structure arrangement because of uniform energy absorption, heat and moisture distribution.

In addition, the microwave heating offers better mechanical properties with high strength and little

deterioration in its long term performance with higher quality than conventional methods.

8. Conclusions

The use of dried biomass could provide significant benefits to operation of boilers. It will increase

boiler efficiency, lower emissions and improve operation. The drying mediums used are air, flue

gases and superheated steam. The dryers are commonly rotary dryers, flash dryers, belt dryers


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and fluidized bed dryers. Drying by air and flue gases mostly takes place in rotary, flash, belt and

cascade dryers. Superheated steam is mostly used for drying biomass in flash and fluidized bed

dryers.

The selection of a dryer depends on its particular application. In general, water evaporation rate,biomass properties (including size), operating temperature and heating resource availability are

important in the selection. Meanwhile, environmental controls and safety are important concerns

in the dryer design.

Rotary dryers are the types used with air or flue gases. The significant advantages include less

sensitivity to material size, a wide range of evaporation rates, easier installation and an abundance

of applications. The high temperature operation is mostly applied in rotary dryers to acceleratedrying. The significant drawback is the fire risk and significant air emissions due to the high

temperature operation. Flash dryers are more compact and easier to control compared to rotary

dryers, but require a small particle size. Both flash and cascade dryers can be used in high

capacity water removal. Heat recovery is difficult in rotary dryers, flash tube dryers and cascade

dryers. Belt dryers are currently adopted in low temperature operations, as they pose less of a fire

risk, emit fewer air pollutants and have a low energy consumption, despite their requirements of a

large footprint.

Compared with air dryers, the main advantage of superheated steam dryers is that the latent heat of

vaporization from drying can be recovered. There are normally no air emissions from

superheated dryers because all the vapour, including organic species, is condensed. The presence

of an inert steam atmosphere has no fire hazard. The disadvantages include the small particle

size that is required for mixing, the high capital costs for a stainless steel pressure vessel and the

wastewater treatment. Superheated steam dryers require a high-temperature heat source. Most

superheated steam dryers were constructed near process and power plants, since hot water and flue

gases provide the resources of high temperature and energy. As a superheated steam dryer, the

IVO mixed bed steam dryer does not need heat exchangers for drying. A high-pressure IVO

dryer integrated with a gas turbine eliminates the wastewater stream by combusting the organics in

the turbine.

Drying biomass at low temperatures (


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drying are belt dryers and two cell dryers. Both are operated under vacuum conditions.

Microwave dryers may need more development and research before more widespread applications

for biomass drying.

9. References

Antti, A. L. (1999). Heating and drying wood using microwave power, Ph.D. Thesis. Vol. 35.

Lule University of Technology, Division of Wood Physics.

Barre, L. and M. Bilodeau(1999), Drying residuals at low temperature with the Dry-Rex dryer,

Pulp and Paper Canada, 1999

Berghel, J., Lars Nilsson, Roger Renstrom (2008), Particle mixing and residence time when

drying sawdust in a continuous spouted bed, Chemical Engineering and Processing 47: 12461251

Bruce, D. M. and M. S. Sinclair (1996). H.A. SIMONS LTD, Thermal Drying of Wet Fuels:

Opportunities and Technology, Final Report( TR-107109 4269-01)

ELECTROWATT-EKONO (UK) LTD 2003, Maximising the Potential of Wood use for Energy

Generation in Ireland.

FBT, Inc., (1994), Fluidized bed Combustion and Gasification: A Guide for Biomass waste

Generators, Southerneastern Regional Biomass Energy System. Work performed by FBT, Inc.,

Chattanooga, TN

Fredrikson, R.W. (August 1984), Utilisation of Wood Waste as Fuel for Rotary and Flash Tube

Wood Dryer Operation. Biomass Fuel Drying conference Proceedings; Aug. 8, 1984, Superior,

Wisconsin. St. Paul, MN: Office of Special Programs, University of Minnesota; pp.1-16

Hansson, Lars (2007): Microwave Treatment of Wood. Doctoral thesis, Lule University of

Technology, Skellefte, Sweden.

Haapanen, A.P.; Heikkila, L.; Ljias, M.; Valkamo, P.; (1983), Enso uses Flash-directed, pulverized


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CONFERENCE HAMBURG

Turner I. W. (1994). A study of the power density distribution generated during the combined

microwave and convective drying of softwood. 9 th International Drying Symposium, Gold Coast,

Australia, August 1-4.

Vongpradubchai S. and P. Rattanadecho (2009). The microwave processing of wood using a

continuous microwave belt drier, Chemical Engineering and Processing 48: 9971003

Wade A. Amos, Report on Biomass Drying Technology, NREL 1998

Wimmerstedt R. (1995) Drying of peat and biofuels. In: Mujumdar AS, editor. Handbook of

industrial drying. New York: Marcel-Decker; p. 809-859

Wimmerstedt R., J. Hager (1996), Steam drying: modeling and applications, Drying Technol. 14 (5)

10991119.

Wimmerstedt R. (1999) Recent advances in biofuel drying, Chem. Eng. Process. 38 (46), p

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epsrc thermal management sheffield drying tech feb 2010

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