biomass firing in cfb

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Combustion of Different Types of Biomass inCFB Boilers

Matti Hiltunen

RD Partners

Kotka, Finland

Vesna Barišić, Edgardo Coda Zabetta

Foster Wheeler – R&D Department

Varkaus, Finland

Presented at

16th European Biomass Conference

Valencia

Spain

June 2−6, 2008



COMBUSTION OF DIFFERENT TYPES OF BIOMASS IN CFB BOILERS

Hiltunen, M., Senior adviser, RD Partners, Virsumaentie 91, FI-48600 KOTKA, Finland,[email protected]

Barišić, V., Senior Researcher, Foster Wheeler, Relanderinkatu 2, FI-78201 VARKAUS,

Finland, [email protected] Zabetta, E., Research Manager, Foster Wheeler, Relanderinkatu 2, FI-78201

VARKAUS, Finland, [email protected]

ABSTRACT: The interest to use biomasses as fuels has increased strongly during the last years as a mean to reducethe CO2 emissions of energy production. Compared to conventional fuels like coal and peat, biomass fuels are moredifficult. Fuel quality varies, moisture can be high, fuel handling and feeding are more demanding. With such problems, the most common technologies for industrial combustion of solid biomass fuels are: bubbling fluidized bed boilers (BFB), circulating fluidized bed boilers (CFB) and grate fired boilers. Of these, the technologies based onfluidized bed (BFB and CFB) are becoming increasingly popular. As a drawback, biomass-fired B/CFBs may sufferfrom bed agglomeration. Ash composition together with sulfur and chlorine contents in biomass fuels are the mainfactors having an impact on the risk of bed agglomeration in fluidized bed boilers, and on the rate of boiler fouling,deposit formation, slagging, and superheater corrosion. On the basis of ash composition, the biomass fuels can be

divided into three groups having significant differences in combustion. This paper reviews in light of experiencewhat are the main differences in fuel and ash properties and how the fuels from each group can be fired in CFB boilers.Keywords: biomass composition, ash, circulating fluidized bed (CFB)

1 INTRODUCTION

The demand for biomass is growing and prices areincreasing drastically. Thus, all types of biomass will beconsidered as fuels in the near future. Analyzing nationaland European policies as well as the targets forrenewable energy, UNECE/FAO in cooperation with theUniversity of Hamburg recently showed [1] that huge

extra amounts of wood will be required in Europe in thefuture if the targets set by European Commission were to be met.

Compared to conventional fuels like coal and peat, biomass fuels are more difficult. Fuel quality variesseasonally and regionally, moisture can be high, fuelhandling and feeding are more demanding, and in biomass-fired boilers fouling, formation of deposits,slagging, and superheater corrosion are common problems.

With respect to problems addressed above, the mostcommon technologies for industrial combustion of solid biomass fuels are: bubbling fluidized bed boilers (BFB),circulating fluidized bed boilers (CFB) and grate fired

boilers. Of these, the technologies based on fluidized bed(BFB and CFB) are becoming increasingly popular.However, biomass-fired B/CFBs may suffer from ash-related problems. Ash composition together with sulfurand chlorine contents in biomass fuels are the mainfactors having an impact on the risk of bedagglomeration in fluidized bed boilers, and on the rate of boiler fouling, deposit formation, slagging, andsuperheater corrosion. On the basis of ash composition,the biomass fuels can be divided into groups havingsignificant differences in combustion properties.

This paper reviews, in light of experience, what arethe main differences among biomass fuel groups in fueland ash properties, and how the fuels from each group

can be fired in CFB boilers.

2 BIOMASS FUELS

The standard CEN/TS 14961 [2] classifies biomassresources in the following main categories:1. Woody biomass,2. Herbaceous biomass,3. Fruit biomass,All categories also include sub-categories, intentional

blends and unintentional mixtures.The standard reports technical specifications of

properties for commercial fuels like wood pellets, briquettes, chips, sawdust, log wood, olive cake andstraw bales. The specified properties includecharacteristics such as dimensions of the fuel, moisture,ash contents, bulk density, net calorific value, nitrogenand chlorine contents.

The standard also lists typical values and typicalvariations of chemical properties for the solid biomassfuels. The properties include ash content, gross and netcalorific values, main combustible elements (C, H, O, N,S), halogens (Cl and F), main ash forming elements andminor ash forming elements in dry fuel. The chemical

properties are given for quite many solid biomasses likeconiferous and deciduous woods and barks, loggingresidues, short rotation coppices, straws and grains ofselected cereals, grasses, selected husks, stalks, trash andfruit biomasses.

Even though an important information and data forvarious solid biomass fuels are given in standardizedformat, from the combustion point of view thisclassification is not able to quantify the fuel properties ofvarious biomasses in sufficient detail.

3 ASHES OF BIOMASS FUELS

From the combustion point of view, biomass fuelscan be divided into three groups on the basis of their ashcomposition:



1. Biomasses with Ca, K rich and Si lean ash2. Biomasses with Si rich and Ca, K lean ash3. Biomasses with Ca, K and P rich ash

Most woody fuels belong to group 1. Rice husk,

bagasse or spring harvested reed canary grass areexamples of biomass fuels in group 2, sunflower seedand rapeseed cakes are fuels in group 3.

4 BIOMASSES WITH ASHES RICH IN CALCIUMAND POTASSIUM, LEAN IN SILICA

Woody biomass fuels have low content of nitrogen(0,3–0,7 w-% in dry solids), sulphur (0,03–0,05 w-% indry solids) and ash (0,1–6 w-% in dry solids) comparedto many fossil fuels. Moisture contents of these fuels areoften high, up to 50–80 %, reducing net calorific value offuel.

The ash of woody biomass is typically rich incalcium (Ca) and potassium (K), see Table 1. Thecontent of CaO in wood and bark ash is 30–50 w-%,although large variations exist. K 2O concentrations areup to 15 w-% in ash, but maximum values can be evenhigher. Also MgO is one of the main ash components inwoody ash, 4–10 w-% in ash. The contents of sodium(Na) are usually low, Na2O is usually below 3 w-% inwood biomass ash. If Na2O contents are high, the reasonis usually contamination, or the plants have been growingon salty soil.

SiO2 exists naturally in some wood species likespruce and aspen. High contents of Al2O3 and SiO2 indicate contamination by aluminium silicates from soil

[3].The composition of biomass ash is strongly

dependent on the species and part of the biomass plant,e.g. trunk, bark, braches, tops and needles have differentash compositions. The nutrients available, soil quality,fertilizers and weather conditions have also significantimpact on the ash composition. Deviations of several tensof percent from the mean value of each element arecommon in biomass ashes.

Table I: Examples of ash compositions of coal, peat,conifer bark and forest residue [4 ]. Coal and peat wereashed at 815 ºC while bark and forest residue at 575 ºC

Polishcoal

Peat BarkConifers

Forestresidue

w-% w-% w-% w-%

SiO2 47,7 32,1 4,8 11,6

Al2O3 23,8 17,3 2,8 2,0

Fe2O3 9,5 18,8 1,5 1,8

CaO 3,8 15,1 45 40

MgO 2,9 2,5 5,2 4,8

K 2O 2,4 1,4 8,0 9,2

Na2O 1,1 0,5 0,9 0,6

P2O5 0,4 3,7 4,2 4,4

Others 8,4 8,6 27,6 25,6

Olive cake, for example, belongs to this group of biomasses by its composition and combustion properties,although it is classified as fruit biomass in the CEN/TS14961 standard.

Biomass ashes are very fine, a few µm in particle

size. Ca- and K-containing ashes deposit easily onsurfaces, causing fouling of e.g. superheaters, formingCaO, CaSO4 and K 2SO4 rich deposits that harden if notremoved frequently by soot blowing. The deposits canharden in the superheater area. In the economizer sectionflue gas temperatures are low, below 500ºC, and thedeposits remain usually loose and easily removable bysoot blowing.

MgSO4 is not stable at the boiler conditions and itdoes not actively participate to the deposit formation.

Chlorine in the fuel makes the fouling even worse,and induces the risk of high temperature corrosion in thesuperheaters. The chlorine content in trunk wood is low,0,01–0,02 w-% in dry wood. In bark the chlorine content

is slightly higher, 0,02–0,03 w-%. In forest residues(branches, tops of trees) and short rotation coppice likewillow the chlorine contents are 0,02–0,03 w-%, i.e. atthe same level with bark.

The clear exception is eucalypt bark that can containvery high contents of chlorine. Concentrations up to 0,98w-% have been analyzed, although normally the valuesare 0,2–0,3 w-%.

During fluidized bed combustion potassium andcalcium from biomass ash can react with quartz (SiO2)from the bed sand already at the normal operationtemperatures of 700−900 °C, forming a layer of Ca,K-silicate onto the bed particle. The layer becomes thickerin time, and the particle size increases. The layer is

sticky, and the bed particles can agglomerate togetherincreasing the bed particle size further. During unsteadyoperation of the boiler, or on an occasional excursion tohigh temperature, the whole bed can sinter. The bedagglomeration can be controlled by keeping the bedalkali contents low enough by regularly discharging the bed ash and feeding fresh sand into the bed. Thechemistry of fuel ash - bed sand interactions is howevercomplicated. It is usually useful to minimize the quartzcontents in the make-up sand.

The ash melting point temperatures of woody fuelashes vary in a wide range. The wide range correlateswith variations in ash composition. In general, the higherthe fuel alkali and chlorine contents, the lower are the

sintering and initial deformation temperatures.Wood ash starts to form agglomerates and to sinter

between 900 ºC and 1000 ºC in combustion conditions.Coal and peat ashes are usually trouble free at thesetemperatures, even if the melting point temperatures arein the same range with biomass fuels. Coal or peat is co-fired with biomass in many multi-fuel fired boilers.

The woody biomass ashes are in general much morereactive than the ashes of fossil fuels. Lower reactivity ofcoal and peat ashes is connected to a composition withmainly quartz and various silicate-based minerals, likealuminium silicates, calcium silicates and alkali silicates,and iron oxides. Calcium and alkali in these minerals arenot in free, reactive form like they are in biomass ashes.

Accordingly they are quite inert at the conditions offluidized bed combustion.Even though combustion of woody biomass fuels is

more challenging compared to coal and peat wood



combustion technologies are nowadays well establishedowing to continuous boiler and material development.Thanks to a good operational practice developed byexperiences, nowadays a number of woody biomass fired boilers are successfully operated by skilled operators

with high availability, over 8000 h/a, for example in the pulp and paper industries. This know-how comes from alot of learning.

When woody biomass fuels are fired in highefficiency boilers with high steam temperatures and pressures, the negative properties of biomass fuels andtheir ashes are amplified. The boiler fouling, depositformation and corrosion are elevated compared to coal or peat fired boilers.

When firing woody biomass besides the compositionof ash it is crucial to know reactivities of the ashcomponents. Biomass fuels are often co-fired with otherfuels. It is also important to understand the interactions ofashes and reactions of flue gas components with the ash

components [5].Virgin wood biomass fuels are commonly used in

boilers of conventional design, i.e. principally similarthan the boilers designed for fossil fuels. Radiativeunprotected superheaters are used above the combustionzone. The final superheater stage is nowadays usually

INTREX®

superheater in Foster Wheeler CFB boilers.The furnace and boiler are dimensioned to take intoaccount the lower heat value of biomass fuel and higherflue gas flow rates. Steam temperatures up to 540 ºC and pressures over 100 bar are common.

During the past 30 years Foster Wheeler has bookedover 300 CFB boilers ranging from 7 to nearly 1000MWth. Of these, nearly 50 are designed for biomass

(co-)firing with a total cumulative capacity of 6247MWth. Note that in this paper, peat is not included among biomass fuels. The boilers include industrial CFB boilers,utility boilers and hot water boilers. Figure 1 shows amodern biomass fired 240 MWth CFB boiler SöderenergiAB, Igelsta (92 kg/s, 540ºC, 90 bar) in Södertälje,Sweden. The boiler will start operation in 2009, and itwill be the largest biomass fired boiler in Sweden. Thedesign fuels are biomass, recycled fuel pellets anddemolition wood.

Figure 1: Cross-section of a modern biomass fired 240MWth CFB boiler Söderenergi AB, Igelsta (92 kg/s,

540ºC, 90 bar) in Södertälje, Sweden.

5 BIOMASSES WITH ASHES RICH IN SILICA,LEAN IN CALCIUM AND POTASSIUM

The fuels in group 2 are very diverse by chemicalcomposition and combustion properties. Most fuels in

this group belong to herbaceous, or agricultural biofuels.Some of the fuels, like straws of cereals have alsorelatively high potassium (K) and chlorine (Cl) contents.Rice husk and bagasse have very high SiO2 contents inash, see Table 2.

Table II: Examples of ash compositions of wheat straw,rice straw, rice husk and bagasse [5, 6] ; ashed at 575 ºC

Wheatstraw

Ricestraw

Ricehusk

Bagasse

w-% w-% w-% w-%SiO2 59,9 69,9 95,4 73,0Al2O3 0,8 0,3 0,1 5,0

Fe2O3 0,5 0,2 0,1 2,5CaO 7,3 3,4 0,4 6,2MgO 1,8 1,6 0,3 2,1K 2O 16,9 15,3 1,8 3,9

Na2O 0,4 0,4 0,0 0,3P2O5 2,3 1,5 0,5 1,0

Others 10,1 7,4 1,4 6,0

The composition of biomass ash is stronglydependent on the species and part of the biomass plant.The available nutrients, soil quality, fertilizers andweather conditions have significant impact on thecontents of potassium, sodium, chlorine and phosphorusespecially in agro-biomass ashes.

In straw the content of potassium as well as theleachability of potassium (and sodium) correlate withchlorine content and leachability of chlorine. This property can be used to reduce alkali and chlorinecontents of agro-biomasses during harvesting andstorage. On rainy years the chlorine content in agro- biomass like straws is lower than in dry years. Water-soluble chlorides are partly washed away by rain.

Time of harvesting is an important for multi-annualcrops; for example, harvesting dry reed canary grass inspring in Northern countries produces crops with lowcontent of chlorine and potassium compared to theautumn harvest. From the combustion point of view, theash properties are significantly improved in springharvested reed canary grass.

Straws of cereals have about 5–10 w-% ash. SiO2 isthe main ash component, but variations are large. Theother main ash components are K 2O 5–30 w-%, CaO 4– 14 w-%. Occasionally, also Na2O and P2O5 contents arequite high, up to 10 w-% and 8 w-%, respectively.

The chlorine contents in straws are high compared towoody fuels: up to 2 w-% in dry wheat straw has beenanalyzed. Chlorine content in rice husk is relatively low,commonly below 0,1 w-%, but in rice straws content ofchlorine can be as high as 0,7 w-%.

The ash melting properties of straws of cereals arechallenging. The sintering temperatures are in the range700–900 ºC, and ash softening points below 1000 ºC.Complete melting happens often below 1200 ºC. Fromexperience, straw is known as a reactive but difficult fuelwith high fouling, slagging and corrosion properties.



The bed agglomeration mechanism during straw(type 2 ash) CFB combustion is different compared towoody biomass (type 1 ash) fired boilers. During strawcombustion the bed agglomeration is caused by separatesticky and partly molten ash particles, and not by a sticky

alkali and calcium silicate layer that is gradually formedon the bed particles like during wood combustion. Themolten straw ash particles consist on potassium chlorideand low melting potassium silicates formed in reactions between potassium and silica present inherently in thefuel ash. Therefore, the quality of bed material does nothave any significant impact on the bed agglomerationrate with fuels like straw [8].

Foster Wheeler’s 77.5 MWth CFB boiler in Grenå,Denmark (29 kg/s, 92 bar, 505 ºC) has been co-firingstraw with coal since 1992. Shredded straw is blown pneumatically into return legs below the two hotcyclones. The straw is mixed with the hot circulationmaterial flowing into furnace from the loop seals.

At the beginning of the boiler operation, Grenå CFBhas experienced severe fouling and chlorine induced hightemperature corrosion problems in the convectivesuperheaters due to relatively high superheat temperature(505 ºC) of the steam. At late 1990´s the final superheater

in the convection cage was replaced with an INTREX®

superheater. Thereafter, superheater problems weresignificantly reduced.

Straw firing in large-scale industrial CFB boilers isdifficult, however it is feasible if co-fired in small shareswith fuels like coal.

Another fuel that belongs to the same type 2 ash isrice husk with the ash contents from 15–25 w-% in drysolids, which is higher compared to many other biomass

fuels. Rice husk ash contains over 90% SiO2 in mostcases, making it very different from the straws of othercereals and even the rice straw ash. Potassium andcalcium contents in rice husk ashes are low compared torice straw, containing up to 15% K 2O and 3,5% CaO.Thanks to its exceptional ash composition, the ashmelting temperatures of rice husk are very high, about1500 ºC.

The armour-like SiO2 -based structure of rice huskremains during the ashing process, as Skrifvars et al. [7]have shown. They found that when rice husk was burnedalone, ash particles did not stick onto heat-exchangersurfaces and caused no significant fouling or slagging.When co-fired with other fuels, the rice husk ash

particles in the fly ash seems to be able to keep the boilersurfaces without fouling, even if a fouling fuel such aseucalypt bark is co-fired with rice husk. This cleaningeffect by rice husk ash is probably more a physical effectthan anything else [7].

Rice husk is commonly used as a fuel in countriesfarming rice. Grate firing and fluidized bed firing are inuse. Rice husk is a special biomass fuel that can be co-fired quite easily also in CFB power plants. It does notcause fouling or slagging, but has slightly erosive effectdue to a large particle size and sharp edged SiO2 particlesin ash. The erosivity of rice husk is high enough to “sand blast” to some extent the boiler when co-fired with forexample wood bark that have ash with fouling

propensity.Rice husk storing is difficult and it requires huge

storage volumes due to a low bulk density. Therefore,rice husk is a seasonal fuel. In large boilers rice husk is

co-fired with other fuels like coal and bark. FosterWheeler has supplied several such boilers in Asia, e.g.two 370 MWth CFB boilers in Tha Toom, Thailand(134/122 kg/s, 161/35 bar, 542/542 ºC).

6 BIOMASSES WITH CA, K AND P RICH ASH

Sunflower stalk ash and rapeseed expeller ash fromfood production are examples of the third type of agro- biomass ash, having K 2O, CaO and P2O5 as the major ashcomponents, see Table 3.

Table III: Examples of ash compositions of sunflowerstalk and rapeseed expeller [6, 9]; ashed at 575 ºC

Sunflowerstalk

Rapeseedexpeller

w-% w-%SiO2 3,1 0,0Al2O3 0,1 0,0Fe2O3 0,2 0,3CaO 6,6 15,0MgO 4,3 9,0K 2O 27,5 22,8

Na2O 0,0 0,0P2O5 18,5 41,1

Others 39,7 11,8

The available nutrients, soil quality and fertilizershave significant impact on the contents of potassium,sodium and phosphorus in the ashes of this biomassgroup as well.

Also these agro-biomasses contain some chlorineincreasing the risk of chlorine induced high temperaturecorrosion of superheaters.

The ash melting temperatures are in the same rangeas straw. Sintering may start at about 700 ºC, and the ashis completely molten below 1200 ºC.

Sunflower residues and rapeseed cake are mainlyused as cattle meal, and they have been not utilized muchas a fuel. However, if the production of liquid biofuelsfrom these plants will increase, also the use of residuesfrom the fuel production may increase.

According to quite limited combustion experience sofar, these fuels are very fouling [9], but they can be co-fired with coal in moderate shares in normal highefficiency CFB boilers [10].

There are also indications that combustion of highalkali, high phosphorous fuels is possible in CFB boilerswith Ca-additive such as limestone. The role of limestonecan be summarized as: 1) to provide calcium for thereaction with phosphorous forming high-temperature-melting calcium phosphates instead of low-temperature-melting potassium phosphates, and 2) to coat silica particles preventing the reaction of potassium (calcium) phosphates and silica originating from the fluidizingsand, which can form low-temperature-melting potassium (calcium) silicates, especially relevant forfluidized bed combustion conditions [11].



7 CONCLUSIONS

Solid biomass fuels can be ranked in three classes based on ash composition. The fuels in each group showsimilarities in fluidized bed combustion, e.g. in the

mechanisms of bed agglomeration. Between the threegroups fuels have significant differences in combustion properties. The ranking of the fuels based on ashcomposition is useful when the combustion properties ofa biomass fuel are evaluated and predicted. Theclassification is helpful also when ash reactions inmultifuel systems are to be predicted.

8 REFERENCES

[1] Mantau, U. et al; Wood resources availability anddemands. Implications of renewable energy policies. UNECE/FAO policy forum on

“Opportunities and impacts of bioenergy policiesand targets on the forest and other sectors: what isthe future contribution of wood to meeting UNECEregion’s energy needs?” , Geneva, 10 Oct. 2007.

[2] CEN/TS 14961, Solid biofuels – Fuel specificationsand classes.

[3] Werkelin, J., Distribution of ash-forming elementsin four trees of different species. Master´s thesis.report 02-5. Åbo Akademi, Faculty of ChemicalEngineering, Process Chemistry Group, 2002.

[4] Alakangas, E; Suomessa käytettävien polttoaineiden ominaisuuksia (Properties of fuelsused in Finland), VTT Research Notes 2045. Espoo2000. (in Finnish).

[5] Hupa, M.; Ash behaviour in fluidized bedcombustion – recent research highlights.Circulating Fluidized Bed Technology IX, May 13-16, 2008. Hamburg, Germany. Proceedings. p.845−856.

[6] http://www.ecn.nl/phyllis/ Database forcomposition of biomass and waste.

[7] Skrifvars, B-J et al; The fouling behavior of ricehusk ash in fluidized-bed combustion. 2. Pilot-scaleand full scale measurements. Energy & Fuels 2005,Vol 19 No 4, p. 1512−1519.

[8] Erhardsson, T. et al; Bed agglomeration risk relatedto combustion of cultivated fuels (wheat straw, reedcanary grass, industrial hemp) in commercial bed

materials. Värmeforsk Report 998. Stockholm, November 2006. (in Swedish).

[9] Nevalainen H. et al; Deposits and emissions duringthe co-combustion of biodiesel residue with coaland biomass in a CFB pilot. Circulating FluidizedBed Technology IX, May 13-16, 2008. Hamburg,Germany. Proceedings.p.863 – 868.

[10] Derda, P. et al; Fate of alkali metals during co-combustion of biodiesel residues with coal in asemi-industrial CFB boiler. Circulating FluidizedBed Technology IX, May 13-16, 2008. Hamburg,Germany. Proceedings. p 857–862.

[11] Barišić V. et al; The role of limestone in preventingagglomeration and slagging during CFB

combustion of high-phosphorous fuels. To be presented at World Bioenergy 2008, 27−29 May2008, Jönköping, Sweden.

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