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Energy Fuels 2010, 24, 4193–4205 : DOI:10.1021/ef100482nPublished on Web 07/22/2010

Fate of Alkali Metals and Phosphorus of Rapeseed Cake in Circulating Fluidized Bed

Boiler Part 2: Cocombustion with Coal

Patrycja Piotrowska,*,† Maria Zevenhoven,† Kent Davidsson,‡ Mikko Hupa,† Lars-Erik Amand,‡

Vesna Bari�si�c,§ and Edgardo Coda Zabetta§

†Process Chemistry Centre, Abo Akademi University, Turku, Finland, ‡Department of Energy and Environment,Division of Energy Technology, Chalmers University of Technology, Gothenburg, Sweden, and §R&D Department,

Foster Wheeler Energia Oy, Varkaus, Finland

Received April 16, 2010. Revised Manuscript Received July 2, 2010

This paper is part 2 in a series of two papers describing the fate of alkali metals and phosphorus duringcocombustion of rapeseed cake pellets with different fuels in a 12 MWth CFB boiler. In the first part(Piotrowska, P.; Zevenhoven, M.; Davidsson, K.; Hupa, M.; Amand, L.-E.; Bari�si�c, V.; Coda Zabetta, E.Energy Fuels 2010, 24, 333-345), woodwas applied as a base fuel for the cocombustion tests. In this secondpaper, coal was used. Cocombustion with coal has been proven to be a strategy to improve the combustionof rapeseed cake. This paper presents the fate of alkali metals and phosphorus during successfulcocombustion of up to 25% of rapeseed cake pellets on an energy basis with coal. Tests with and withoutaddition of limestone were performed. The fuels were analyzed according to standard fuel analyses andchemical fractionation. Elemental analyses of outgoing streams were performed bymeans of wet chemicalanalysis. In addition, SEM/EDX analyses of outgoing ashes and deposit samples collected with a depositprobe were performed. The SO2 and HCl emissions were analyzed. Mass balances were calculated for allcocombustion tests. Gaseous alkali chlorides were measured before the convective pass at a flue gastemperature of 800 �C using an in situ alkali chloride monitor (IACM). At the same place HCl and SO2

were measured, and deposit samples were collected with a deposit probe. Rapeseed cake cocombustioncaused an increase in alkali metals and phosphorus. However, no heavy bed agglomeration or depositswere observed. This is due to interactions between alkali metals and aluminum silicates from coal. Noformation of gaseous alkali metal chlorides was detected in the beginning of the convection pass by meansof IACM. Phosphorus was present in the deposit samples up to about 9 wt%P2O5 in the leeward side of thedeposit probe when no lime was supplied to the combustion chamber. Addition of limestone resulted in ahigher deposition rate and lowered emissions of HCl and SO2.

Introduction

One of the key targets set by the European Council is toincrease the renewable energy share to 20% in EU energyconsumption by 2020.1 The consequent tendency has been toreplace partially fossil fuels with biomass in the energy sector.Combustion of biomass, and related operating problems, hasbeen extensively reported in the literature.2-8 The propertiesof biomass fuel, e.g., high volatile, high alkali metals, and low

carbon content, require specially designed boilers. Opera-tional problems during biomass combustion, such as agglom-eration, slagging, fouling, and corrosion, are related to ashchemistry. Several authors (e.g., refs 9-11) have reported thatthese problems could be counteracted when cocombustingbiomasswith coal. The cocombustion of biomass in coal-firedutility boilers was identified by Hughes et al.12 as a mostpromising technology, and its technical, economical, andenvironmental advantageshavebeendiscussedbyDemirbas.13

However, before implementation, the combustion propertiesof each biomass-coal mixture needs to be determined experi-mentally.13-15

It is well-known that the main ash constituents responsiblefor the combustion problems are alkali metals and chlorine.Recently there has also been interest in phosphorus. It is oneof the macronutrients and is present in substantial amounts

*To whom correspondence should be addressed. Telephone: þ358 2215 3507. Fax: þ358 2 215 4962. E-mail: [email protected].(1) Commission of the European Communities.Communication from

the Commission to the European Parliament, the Council, the EuropeanEconomic and Social Committee and the Committee of the Regions, 20 20by 2020 Europe’s climate change opportunity, COM (2008) 30 final,Brussels, Belgium, January 23, 2008.(2) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel

Process. Technol. 1998, 54, 17–46.(3) Baxter, L. L.;Miles, T. R.;Miles, T. R., Jr.; Jenkins, B.M.;Milne,

T.; Dayton, D.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol. 1998,54, 47–78.(4) Werther, J.; Saenger,M.; Hartge, E.-U.; Ogada, T.; Siagi, Z.Prog.

Energy Combust. Sci. 2000, 26, 1–27.(5) Zevenhoven-Onderwater, M.; €Ohman, M.; Skrifvars, B.-J.;

Backman, R.; Nordin, A.; Hupa, M. Energy Fuels 2006, 20, 818–824.(6) €Ohman,M.; Nordin, A.; Skrifvars, B.-J.; Backman, R.; Hupa,M.

Energy Fuels 2000, 14, 169–178.(7) Davidsson, K. O.; Amand, L.-E.; Steenari, B.-M.; Elled, A.-L.;

Eskilsson, D.; Leckner, B. Chem. Eng. Sci. 2008, 63 (21), 5314–5329.(8) Frandsen, F. J. Fuel 2005, 84, 1277–1294.

(9) Davidsson,K. O.; Amand, L.-E.; Elled, A.-L.; Leckner, B.EnergyFuels 2007, 21, 3180–3188.

(10) Ferrer, E.; Aho, M.; Silvennoinen, J.; Nurminen, R.-V. FuelProcess. Technol. 2005, 87, 33–44.

(11) Aho,M.; Gil, A.; Taipale, R.; Vainikka, P.; Vesala, H. Fuel 2008,87, 58–69.

(12) Hughes, E. E.; Tillman, D. A. Fuel Process. Technol. 1998, 54,127–142.

(13) Demirbas, A. Energy Convers. Manage. 2003, 44, 1465–1479.(14) Hupa, M. Fuel 2005, 84, 1312–1319.(15) Skrifvars, B.-J.; Backman,R.;Hupa,M.; Sfiris,G.; Abyhammar,

T.; Lyngfelt, A. Fuel 1998, 77, 65–70.

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Energy Fuels 2010, 24, 4193–4205 : DOI:10.1021/ef100482n Piotrowska et al.

mainly in seed-originating biomass fuels. Its release duringcombustion was investigated by Novakovi�c et al.,16 whereasthe influence on agglomeration and slagging was investigatedby Bari�si�c et al.,17 Lindstr€om et al.,18 and Bostr€om et al.19 Itspresence in thedepositswas reportedbyWigley et al.20 and thecontribution to fine particle formation by Tissari et al.21

In the present paper, the results of cocombustion tests ofrapeseed cake with coal are presented. The rapeseed cake wasa residue from rapeseed methyl ester (RME) production.South African bituminous coal was applied as a base fuel.The combustion properties of rapeseed cake/meal have al-ready been investigated to some extent before.17,19,21-25Mostof the references deal with rapeseed cake cocombustion withbiomass. However, only two articles refer to the cocombus-tion with coal. Kandefer et al.25 reported experiments per-formed in a grate furnace, whereas, Nevalainen et al.22

reported tests in a bench scale (50 kW) fluidized bed boiler.Both authors agreed that combustion of rapeseed cake withcoal is possible with no big operational problems.

This paper focuses on alkali metals and phosphorus, sincethose are the main ash forming elements present in the rape-seed cake. The fate of alkali metals and phosphorus isdetermined by showing the elemental mass balances and thedistribution of elements in the individual ash outflows. Stan-dard fuel characterization is supplemented with chemicalfractionation in order to show chemical forms of ash formingmatter when entering the boiler. This may help to understandthe combustion behavior as well as the formation of agglom-eration and deposits. The results are supplementedwith SEM/EDX analyses of ash samples and measurements of gaseousalkali metal chlorides, HCl, and SO2 in the beginning of theconvection pass. At the same position, deposit samples werecollected on steel rings using an air-cooled probe and furtheranalyzed bymeans of SEM/EDX. The description of agglom-eration and deposit formation phenomenon is done.

Experimental Setup

Cocombustion Tests. Rapeseed cake cocombustion tests withcoal were performed in a 12 MWth circulating fluidized bed(CFB) boiler at Chalmers University of Technology. The sche-matic diagram of the boiler is shown in Figure 1. The combus-tion chamber (1) has a cross section of 2.25 m2 and a height of13.6 m. The fuels were mixed before being fed through a fuelchute (2). Primary air was introduced to the bottom of the bed

and secondary air 2.2 m above the bottom plate. The bedtemperature was held at 849 �C with an experimental standarddeviation of 1 �Cat the bottomand at 852( 8 �Cat the top of thebed. The total pressure drop in the riser was 7.7 ( 0.3 kPa andthe ratio of primary air flow to total air flow was 50.0 ( 0.3%.The excess air ratio was held at 1.21. During the tests, the boilerwas operated at 6.0 ( 0.1 MWth, which resulted in a superficialflue gas velocity at the top of the riser of 4.6 ( 0.1 m/s and acirculation of material via the primary cyclone (3) and particlereturn leg (4) back to the boiler. There is approximately 3000 kgof bed material in the system (50% in the riser section and50% in the cyclone leg and particle seal). The flue gas tempera-ture was 832 ( 4 �C when entering the convection pass (9), andthe fly ashes were collected in the secondary cyclone (10)and the bag house filter (11) where the temperature droppedto 159 ( 2 �C.

During the tests, SouthAfrican bituminous coal was used as abase fuel. The rapeseed cake (RC) pellets, a residue from biodieselproduction, was provided by Emmelev S/A (Denmark). Theexperimental plan consisted of three tests during which the RCshare and limestone addition were used as varying parameters aslisted in Table 1. The test codes, used in the paper, consist ofletters, indicating base fuel and limestone addition, and of num-bers which denote the energy fraction of rapeseed cake (e.g.,CNL25 indicates that coal was used as the base fuel and nolimestone was added while 25 en% of rapeseed cake wascocombusted).

Two tests of different RC ratio, 18% and 28% (energy basis),were performed with limestone continuously being fed to thechamber (CL18 andCL28), the duration of which were 12.5 and22 h, respectively. Between the tests, coal with minimal lime-stone flow (16 kg/h) was combusted for 5 days. Complete bedremoval was carried out 2 days prior to test CL18 (the first coaland RC cocombustion test). Coal with limestone addition wascombusted for 15 h prior to the test CL18.

During the last test (CNL25), the limestone supply was shutoff and 25% of the rapeseed cake on an energy basis wascocombusted during approximately 23 h. It followed testCL28 with a 1.5 h break of coal monocombustion. At the endof the experiment, a reference test with only coal was carried out(CNL) for approximately 2 h.During all tests, sand consisting of98.9% silica (SiO2) was used as bed material, with a particlediameter of 106-125 μm. The inorganic contaminations weremainly Al, Ca, Mg, Fe, and K.26 The limestone was taken from

Figure 1. The schematic diagram of the 12 MWth CFB boiler atChalmers University of Technology: (1) combustion chamber;(2) fuel feed chute; (3) primary cyclone; (4) particle return leg;(5) bottom ash sampling spot; (6) loop seal; (7) heat exchanger;(8) deposits sampling spot and position of IACM upstream of theconvection pass; (9) convection pass; (10) secondary cyclone;(11) bag house filter; (12) flue gases measuring spot; (13) flue gasfan; (14) bag house filter ash sampling; (15) secondary cyclone ashsampling; (16) recirculatingmaterial sampling spot; (17) bottom ashremoval; (18) flue gas recirculation fan; (19) air fan; (20) fuelbunkers; (21) sand bin; and (22) limestone bin.

(16) Novakovi�c, A.; van Lith, S. C.; Frandsen, F. J.; Jensen, P. A.;Holgersen, L. B. Energy Fuels 2009, 23, 3423–3428.(17) Bari�si�c, V.; Amand, L.-E.; Coda Zabetta, E. The role of lime-

stone in preventing agglomeration and slagging duringCFBcombustionof high phosphorus fuels. Presented at World Bioenergy, J€onk€oping,Sweden, May 2008.(18) Lindstr€om, E.; Sandstr€om,M.; Bostr€om,D.; €Ohman,M.Energy

Fuels 2007, 21, 710–717.(19) Bostr€om, D.; Eriksson, G.; Boman, C.; €Ohman,M.Energy Fuels

2009, 23, 2700–2706.(20) Wigley, F.;Williamson, J.;Malmgren, A.; Riley,G.Fuel Process.

Technol. 2007, 88, 1148–1154.(21) Tissari, J.; Sippula, O.; Kouki, J.; Vuori, K.; Jokiniemi, J.Energy

Fuels 2008, 22, 2033–2042.(22) Nevalainen, H.; Leino, T.; Tourunen, A.; Hiltunen, M.; Coda

Zabetta, E. Proceedings of the 9th International Conference on Circulat-ing Fluidized Beds, Hamburg, Germany, May 2008.(23) Eriksson, G.; Hedman, H.; Bostr€om, D.; Petersson, E.; Backman,

R.; €Ohman, M. Energy Fuels 2009, 23, 3930–3939.(24) Piotrowska, P.; Zevenhoven, M.; Davidsson, K.; Hupa, M.;

Amand, L.-E.; Bari�si�c, V.; Coda Zabetta, E. Energy Fuels 2010, 24,333–345.(25) Kandefer, S.; Baron, J.; Olek, M. Co-burning of rapeseed cake

and hard coal in a grate furnace. Ochrona Powietrza i ProblemyOdpadow 2006, 3, 69–76.

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Ignaberga in the south of Sweden and contains more than 93%of calcium carbonate.27 Bottom ash, circulating material, andfly ash samples were collected after approximately 12 h and atthe end of each cocombustion test.

Sampling andAnalysis of Fuels andAshSamples.The fuel flowwas continuously recorded, and fuel samples were taken oneach day of performed experiments. The collected samples ofeach fuel were mixed afterward and used for standard fuelanalyses and chemical fractionation. Standard fuel analyseswere performed by an external accredited laboratory accordingto Swedish standards. In addition, chemical fractionation wasapplied to the fuels in order to determine the chemical forms ofthe ash forming matter entering the boiler. The principle ofchemical fractionation was given by Benson et al. for coal.28 Themethod was later modified by Baxter29 and finally adapted bythe Laboratory of Combustion Chemistry at Abo AkademiUniversity.30-33 The procedure is also called stepwise leachingsince it is based on consecutive leaching by water (H2O), 1 Mammonium acetate (NH4Ac), and 1M hydrochloric acid (HCl).The increasingly aggressive solvents leach untreated fuel sam-ples into a series of four fractions. The untreated samples, thethree liquid fractions, and the remaining solids were analyzed byan external laboratory for ash forming elements.

Samples from all outgoing solid material streams, bottomash, secondary cyclone, and bag filter ash, were collected at theend of each test. Bottom ash samples were collected 0.7 m abovethe nozzles (5 in Figure 1), and fly ash samples were collected atboth the secondary cyclone (15) and bag filter (14). In addition,cyclone leg samples were taken (16). Collected ash samples wereanalyzed quantitatively according to ASTMD3683 and ASTMD3682 by an external laboratory. Additionally, semiquantita-tive analyses of the solid samples were performed by means ofSEM/EDX. Hereto, ash samples were mounted on carbon tapeand covered with a thin carbon layer. Also cross sections of

bottom ash particles and circulating material particles werestudied by means of SEM/EDX. For this purpose the particleswere embedded in epoxy, ground andpolished on silicon carbidegrinding paper with ethanol to obtain cross sections and asmooth surface for SEM/EDX analyses. Elemental X-ray mapswere taken at a magnification of 100� showing the elementaldistribution in the cross-sectioned particles. Spot analyses ofparticles were done with EDX at a higher magnification 300�and 500�. The SEM pictures were taken in the backscattermode in which the quantification can be performed.

Deposit samples were collected on steel rings (outer diameter38 mm) which were fitted on an air-cooled probe situated in themiddle of the flue-gas stream before the convection pass (8). Thesurface temperature of the steel rings was set to 480 �Cduring alltests to simulate a superheater tube. The rings were weighedbefore and after exposure to the flue gas. The difference inweight was used to calculate the rate of deposit build-up (RBU).The exposure time was 12 h for test CL18 and 22 h in the case oftwo other tests (CL28 and CNL25). Semiquantitative analysesof the deposit samples from the windward (0�) and leeward(180�) side of the deposit ringwere performed bymeans of SEM/EDX. Also the side of the deposit ring where the deposit was thethinnest (about 50� from the windward side) was analyzed.A schematic view of three sides of interest on the deposit ringis shown in Figure 2. No deposit sample was taken during testCNL when only coal was combusted.

Analyses of Gases. At the beginning of the convection pass(8 in Figure 1), gaseous HCl and SO2 concentrations weremeasured with Fourier transform-infrared (FT-IR) spectros-copy. In addition, the concentration of gaseous alkali chlorideswas continuously measured at this position using the in situalkali chlorides monitor (IACM) developed by Vattenfall. Thisinstrument uses a sampling time of 5-10 s. The detection limit ata 5 m measuring length (width of the flue gas channel) is 1 ppmfor KCl and NaCl.34,35 After the bag filter (12), the emission ofHCl was measured by means of FT-IR and SO2 with a non-dispersive ultraviolet (NDUV) analyzer; the results were usedfor determination of the elemental balance over the boiler.

Table 1. Rapeseed Cake and Coal Cocombustion Tests

fuel loadc [kg ds/h] ash with fuelc [kg ash/h]

test name test duration [h] RCa ratio [%en]b RCa coal RCa coal

limestone[kg/h]

bed regeneration[kg sand /h] ash balanced

CL18 12 18 197 687 14.0 91.6 96.1 2.1 69.6CL28 22 28 325 650 22.9 88.7 92.8 1.6 86.3CNL25 23 25 301 667 24.3 93.8 0.0 102.7 92.9CNL 2 0 0 838 0.0 118.0 0.0 190.7

aRapeseed cake. b [%en], percentage on energy basis. cFuel load and ash content in the fuel measured at ChUT. d (ashout/ashin) � 100%.

Figure 2.Three areas of interest for analysis on a deposit probe ring.

(26) Eklund, A.; Brus, E.; €Ohman, M.; Hedman, H.; Bostr€om, D.;Nordin, A. Utv€ardering av Hyttsand som b€addsand i FB-anl€aggningarF€orstudie och laboratorief€ors€ok. Report of V€armeforsk SerVice AB,2003, 832, ISSN 0282-3772.(27) Mattisson, T.; Lyngfelt, A. Thermochim. Acta 1999, 325, 59–67.(28) Benson, S. A.; Holm, P. L. Ind. Eng. Chem. Prod. Res. Dev. 1985,

24, 145–149.(29) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.;

Jenkins, B. M.; Oden, L. L. Alkali deposits found in biomass powerplants - a preliminary investigation of their extent and nature. SummaryReport for National Renewable Energy Laboratory, 1995.(30) Skrifvars, B.-J.; Blomquist, J.-P.; Hupa, M.; Backman, R. Pre-

dicting the ash behavior during biomass combustion in FBC conditionsby combining advanced fuel analysis with thermodynamic multicompo-nent equilibrium calculations. Proceedings of the 15th Annual Interna-tional Pittsburgh Coal Conference, Pittsburgh, PA, 1998.(31) Zevenhoven-Onderwater, M. Ash-Forming Matter in Biomass

Fuels. Academic Dissertation, Department of Chemical Engineering,Abo Akademi University, Turku, Finland, 2001, ISBN 952-12-0813-9.(32) Zevenhoven,M.; Yrjas, P.; Backman, R.; Skrifvars, B.-J.; Hupa,

M. The AboAkademi database-fuel characterization.Proceedings of the18th International Conference on Fluidized Bed Combustion, Toronto,Ontario, Canada, May 2005.(33) Zevenhoven,M.; Skrifvars, B.-J.; Yrjas, P.; Hupa,M.;Nuutinen,

L.; Laitinen, R. Searching for improved characterization of ash formingmatter in biomass. Proceedings of the 16th International Conference onFluidized Bed Combustion, Reno, NV, May 2001.

(34) Kassman, H.; Andersson, C.; H€ogberg, J.; Amand, L.-E.;Davidsson, K. Gas Phase Alkali Chlorides and Deposits during Co-Combustion of Coal and Biomass. Proceedings of 19th InternationalConference on Fluidized Bed Combustion, Vienna, Austria, May 2006.

(35) Andersson, C. A method and a device for measuring, by photo-spectrometry, the concentration of harmful gases in the fumes through aheat-producing plant. European Patent EP 1221036, 2006.

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Results

Fuel Analyses. Standard Fuel Analyses. The standard fuelanalyses are shown in Table 2. Compared to coal, the carboncontent of rapeseed cake (RC) is lower. However, it is com-parable with woody fuels. The sulfur and nitrogen content inRC are much higher compared to the coal. On the otherhand, the ash content in RC is half of the ash content in coalwith big differences in elemental analysis. RC pellets contain9 times more potassium, 7 times more sodium, and 12 timesmore phosphorus than coal. In rapeseed cake, the chlorinecontent is some 0.26 wt % db, whereas in coal chlorine isbelow the detection limit (500 ppm). Calcium is on a similarlevel in both fuels During the experiments, the moisturecontent of the fuels was between 7 and 12%when introducedto the boiler.

Chemical Fractionation. In Figure 3 the chemical fractio-nation results are presented. The figure shows the elementalcomposition of the ash forming matter in fuels divided in thefour obtained fractions: water-soluble, ammonium acetatesoluble, hydrochloric acid soluble, and nonsoluble. Thequantitative analyses of the elements leached in the indivi-dual fractions are presented as gram/kilogram of dry fuel.The mass balance closure for chemical fractionation forindividual elements is satisfactory (RC, 1.07 ( 0.18; coal,0.88 ( 0.21) (The elemental mass balance was calculated asthe sum of the element concentrations in four analyzedfractions to the concentration of the element in the untreatedfuel sample. Experimental standard deviations are shown;chlorinewas excluded from the calculations for coal, since nochlorine was found in the untreated fuel sample).

As mentioned in part 1,24 nearly 50% of RC ash formingmatter is leached by water and consists of alkali metals,phosphorus, sulfur, and chlorine. The rest is mainly leachedin ammonium acetate (∼30%), consisting almost entirely ofphosphorus, potassium, calcium, and magnesium. Phos-phorus is present in all three liquid fractions, with 70% ofthe total analyzed phosphorus present in the ammonium

acetate and acid soluble fraction. In contrast, nearly 90% ofash forming matter in the coal is not soluble at all andconsists mainly of silicon and aluminum. Potassium andsodium aremainly present in the solid residue (90%and 80%of the total analyzed content, respectively). There is also asmall amount of phosphorus in coal, 60% of which can befound in the acid soluble fraction, followed by 20% presentin the solid residue. In both fuels more than 80% of the totalanalyzed calcium can be found in both ammonium acetateand acid soluble fractions, with an equal distribution be-tween them.

Ash Balance over the Boiler. Total Ash Recovery. The ashforming matter entering the boiler originates mainly fromcoal as shown in Table 1. The high ash content of coal resultsin up to 4 times higher coal ash inflow compared to RC ashinflow during test CNL25. The total ash balance was calcu-lated for each test and is also shown in Table 1 as the ratio ofinert ash outflow to inert ash inflow presented as a percen-tage (out/in %). The ash forming matter entering the boilerwith fuels but also silica entering as regenerated bedmaterialand limestone were taken into account when calculatingthe ash balance. Complete calcination of CaCO3 with fur-ther formation of CaSO4was assumed. The amount of sulfurreacting with calcium oxide was determined based on thesulfur retention in the boiler. The closure of the ash balanceforRC cocombustion tests was nearly 70% for test CL18 andwas increasing substantially for the following tests. The shorttest CNL does not allow reliable ash balance calculations,but some indication of the cleaning effect can be observed.The total ash outflow after the RC supply to the boiler wasshut off was much higher than the ash inflow.

Elemental Mass Balance. Elemental flows were calculatedusing the quantitative elemental analyses of the incomingand outgoing streams, in the samemanner as in part 1.24 Theresults are shown in Figure 4. The figure is divided intobottom ash, secondary cyclone ash, bag filter ash, andemission fractions of elemental inflow. The sum of thoseflows gives the mass balance (outflow/inflow [%]). The diff-erence between the sum and 100% is marked on the graph asan accumulated ash fraction in the system; this naturally alsoincludes any possible measurement errors. The concentra-tion of chlorine in the bottom ash samples was not analyzedbecause of the low detection limit (500 ppm) of the method.The balance exceeding 100% indicates cleaning of the boilerwalls and/or heat exchanger pipes in the convection pass.The values presented in Figure 4 give an indication of thepathways that ash forming matter was taking through theboiler. During all tests no heavy accumulation of ash leadingto defluidization was observed.

Out of the two fly ash streams (bag filter and secondarycyclone), the secondary cyclone stream is the main ashstream, where about 40% of entering alkali metals andphosphorus was found. During test CL18 also more than40% of potassium inflow can be found in the bag filter ashwhere also nearly 60% of total chlorine inflow can be found.During tests with the addition of limestone (CL18 andCNL28), ∼40% accumulation of calcium and sulfur can beobserved but also accumulation of phosphorus. About 30%of total magnesium and sodium inflow seem to be accumu-lating. On the contrary, when limestone feeding was shut off(test CNL25), the accumulation of phosphorus decreasedand the alkali metals balance is near 100%.Moreover duringthis test, the cleaning of the boiler can be observed in the caseof calcium, sulfur, and chlorine forwhich the balance exceeds

Table 2. Properties of Fuels

fuel RCa coal

ash [% db] 7.5b 14.0c

HHV [MJ/kg db] 22.16 28.02LHV [MJ/kg db] 20.67 27.09

Ultimate AnalysisCd [% db] 49.9 69.3Hd [% db] 6.9 4.4Nd [% db] 5.1 1.8O(calculated) [% db] 29.6 10.0Se [% db] 0.72 0.47Cl f [% db] 0.26 <DL

Elemental AnalysisSi [mg/kg db] 261 32 900Al [mg/kg db] 43 20 200Fe [mg/kg db] 152 2690Ti [mg/kg db] 4 1180Mn [mg/kg db] 60 58Ca [mg/kg db] 7040 7470Mg [mg/kg db] 4500 2030P [mg/kg db] 11 500 915Na [mg/kg db] 4660 637K [mg/kg db] 12 300 1310

aRapeseed cake. bAshed at 550 �C according to CEN TS 14775.cAshed at 1000 �C according to SS 187157, utg 1. dAccording tostandard ASTM D3178-79. eAccording to standard SS-187177, utg 1.fAccording to standard SS-187185; ar, as received; db, on dry basis; DL,below detection limit of 500 ppm.

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100%. During test CNL25, the secondary cyclone ash flowdropped down (from 65 for test CL28 to 58 kg/h) but thebag filter ash flow slightly increased (from31 for test CL28 to37 kg/h).

Bottom Ash and CirculatingMaterial. Samples taken fromthe bottom of the bed are representative for bottom ashsamples whereas samples from the cyclone leg are repre-sentative for circulating material. Both types of sampleswere investigated by means of wet chemical analyses andSEM/EDX. The wet chemical analyses results are shown inFigure 5 where themain ash elements are shown as oxides onwt%db. Silicon has been omitted tomake the diagram easierto read; the concentration of SiO2 in the bottom ash samplescloses the bars on average in 102 wt % ((3%) and mainlyoriginates from the silica sand used in the fluidized bed. Theresults show the two studied phenomena: (1) cofiring ofrapeseed cake, the influence of RC addition and (2) influenceof limestone.

Cofiring of Rapeseed Cake. Composition. Figure 5 showssome difference between cyclone leg and bottombed samplesfor test CNL25. In the cyclone leg sample, the elementalconcentration of potassiumwas 24%higher (0.93 wt%K2O),magnesium was about 34% higher (1.03 wt %MgO) than inthe bottom bed sample, whereas the concentration of phos-phorus was 49% higher (1.55 wt %P2O5). Rapeseed cake

combustion had some influence on the bottom ash composi-tion. Two times higher concentrations of P2O5 and Na2O(1.04 and 0.44wt%, respectively) andmore than 50%higherconcentrations of SO3, K2O, and MgO were observed in thebottom ash sample taken at the end of test CNL25 (5.37,0.75, and 0.77 wt %, respectively) in comparison to the coalreference case.

Particle Studies. In Figures 6 and 7, the overall elementalX-ray maps of the cross-section of cyclone leg particles areshown. Figure 6 shows results for the coal combustion case(coal_ref.) and in Figure 7 for test CNL25, left picture. InFigure 6, potassium can be observed only together withsilicon and aluminum. Sodium was present in amountsbelow the detection limit of the X-ray detector. Phosphorusis present in the sample; however, its signal is much lessintensive than in the X-ray maps for test CNL25 (Figure 7,left picture). In the coal reference case, phosphorus occursmainly togetherwith calciumand somemagnesium.Calciumsulfate particles can be observed. In X-ray maps for testCNL25, also single particles of calcium sulfate are present.Some potassium is present together with aluminum silicatesaswell as within ash layers deposited on bedmaterial but alsoseparate ash particles. Phosphorus is found together withalkali metals with small concentrations of earth alkalinemetals. Sodium is probably associated with ash particles rich

Figure 3. Chemical fractionation results for rapeseed cake and coal.

Figure 4. Elemental mass balances shown as outflow/inflow of the element in [%]. Accumulation (þ error) bar calculated as a difference of themass balance closure and 100% (Δ = 100 [%] - out/in [%]).

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in phosphorus. During coal combustion (Figure 6), the ashlayer formed on the bed material particles consist mainly ofaluminum and calcium with some phosphorus. The additionof RC (Figure 7, left) increased the concentration of phos-phorus and alkali metals that are present in separate ashparticles.

Influence of Limestone. Composition. Figure 5 shows somedifference between cyclone leg andbottombed samples for testCL28. In the cyclone leg sample, the elemental concentrationof potassium was 12% higher (0.80 wt %K2O), sulfur wasabout 24% higher than in the bottom bed sample (6.49wt %SO3) whereas the concentration of phosphorus was22% higher (1.01 wt %P2O5). Obviously, the composition ofbottom ash sample changes for the test when limestone wassupplied to the bed (CL28) due to higher concentrations ofcalcium entering the combustion chamber with CaCO3 (46%higher concentration in the bottom ash sample for test CL28was found, 24.6 wt %CaO). The concentration of sodium andphosphorus is lower by about 20% (0.83 wt %P2O5 and0.34 wt %Na2O). The rest of the main elements stay withinthe 10% change in concentration.

Particle Studies. In Figure 7 (picture on the right), theoverall elemental X-ray maps of cyclone leg particles crosssections are shown for sample collected at the end of testCL28. It can be seen that the sample is dominated by calciumsulfates, but also individual potassium aluminum silicatescan be observed. Particle coatings are dominated mainly byearth alkaline metals (Ca, Mg).

In Figure 8, the magnified cross-section of particles foundin the bottom of the bed during RC cocombustion testswithout limestone addition (Particle 1 and Particle 2) andwith limestone addition (Particle 3 and Particle 4) is shown.In the bottom ash samples for both tests, two types ofparticles could be observed: with significant coating andscarce coating. Both of those two typical types of particlesare shown in the paper for each test. A thick coating (Particle1 and Particle 3) could suggest old particles that underwentdifferent transformations, whereas scarce deposit (Particle 2and Particle 4) could indicate the initial coating formation.

Both Particles 1 and 2 indicate heterogeneous coating builtup during RC addition compared to the test when limestone

is supplied to the combustion chamber. Coatings consist ofcalcium, aluminum silicates (Particle 1). Only small concen-trations of alkali metals are present together with phosphorusin the coatings (Particle 2). During limestone addition, coat-ings consist entirely of calcium silicates (Particle 3). In theoutercoating, the bigger share of aluminum can be observed. Onlya small amount of phosphorus in the outer layer is present(Particle 3 and 4).

Measurements in the Beginning of Convection Pass. InFigure 9, concentrations of gaseous components, alkalinechlorine, i.e., KCl þ NaCl, HCl, and SO2 in dry flue gas at6% of O2, are shown for all tests. During all tests, the alklinechlorine concentration was below 1 ppmv. The rate of depositbuild-up has not been measured for the coal reference test.During test CL28, the deposit from the windward side hasmost probably fallen off, resulting in an unexpected smallerRBU when compared to test CL18. Figure 10 shows theelemental, semiquantitative compositions analysis results ofthe windward side, leeward side, and side deposits.

Cofiring of Rapeseed Cake. Gas Measurements. The maindifference between pure coal combustion and RC cofiringtest (CNL25) can be noticed in HCl concentration reaching150 ppmv at 6% O2 (see Figure 9). SO2 concentrations seemnot to be affected by RC addition. Also gaseous alkalinechlorides are unchanged and found below 1 ppmv in bothcases. During test CNL25, 0.0398 g of deposit was collectedin 22 h resulting in an RBU of 1.01 g/(m2 h).

Deposits. Cofiring 25 en% of RC with coal (CNL25)resulted in deposits mainly consisting of aluminum silicates(from nearly 50 wt % in the windward deposit to morethan 70 wt % in the side deposit) and a mixture of calciumand potassium salts of sulfates and phosphates (see Figure 10).In all samples, chlorine was below 0.15 wt % of the mainanalyzed oxides. The highest concentration of potassium wasfound on the windward and leeward side of the deposit ring(7 wt % in both), whereas, the highest concentration ofphosphorus (9 wt %) was found in the leeward side deposit.Sodium stayed at the level of 3 wt %Na2O in both the wind-ward and leeward deposits. The highest concentration ofcalcium was found in the windward side deposit reaching9 wt %CaO.

Figure 5. Elemental composition of bottom ash and circulating material determined quantitatively. Silicon is not included in the graph due tosilicon fed to the boiler. LOI corresponds to loss on ignition correspondingmainly to carbonates present in the sample. Coal_ref stands for coalreference; here the bottom ash sample was taken prior to the test CL28 after 5 days of coal firing.

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Influence of Limestone. Gas Measurements. When com-paring tests CL28 (limestone was supplied to the combustionchamber) with CNL25 (no limestone was supplied to thecombustion chamber) (see Figure 9), the obvious loweredSO2 concentration can be observed and also lower HClemission. A higher RBU can be observed, but no change inthe (KCl þ NaCl) concentrations is visible.

Deposits. The windward side deposit composition for testCL28 was not analyzed since the deposit most probably felloff before the sample was taken out. For the reason only thecomposition of leeward side and side deposit for both tests,

CL28 and CNL25, could be compared in order to study thelimestone influence.

The composition of the deposits has only slightly changedwhen limestone was added. Obviously, calcium concentra-tions are doubled for deposit collected from the leeward sideand for the side deposit it was tripled. In the leeward sidedeposit, sulfur and phosphorus concentrations have beenlowered to 4.6 and 3.2 wt % of oxides. Limestone additionresulted also in lower potassium and sodium in the leewardside deposit. The side deposits do not show large differencesin composition for the test with limestone addition. Theobvious increase of calcium and the decrease of silicon andaluminum are observed; only the concentration of phos-phorus doubled (6.0 wt %P2O5) compared to CNL25.

Fly Ash. Influence of Limestone. Composition. Fly ashfrom the boiler can be divided into two fractions, secondarycyclone and bag filter ash. The quantitatively determined ele-mental composition of fly ash samples is shown in Figure 11.The flow into the secondary cyclone has decreased by7.5 kg/h for CNL25 compared to CL28 (during CNL25,58 kg/h). The main difference in the secondary cyclonecomposition between test CNL25 and test CL28 was foundin the calcium concentration, which is nearly 2 times higherduring test CL28. Sulfur has increased by 65% to 4.74wt %SO3. Although the chlorine concentration slightlyincreased, it stayed below 0.2 wt % in both tests. Theconcentrations of all the other main ash oxides decreasedon average by 20% during test CL28 compared to concen-trations during test CNL25, being most probably the effectof dilution with lime. The exceptions were potassium andiron. The concentration of potassium can be assumed toremain on the same level in both tests (a decrease of 6% inconcentration was observed during test CL28 compared totests CNL25). The iron concentration has decreased by 45%during test CNL25when compared toCL28. In bag filter ashcomposition during limestone addition (CL28), the calciumgoes up to 13.20 wt %CaO, sulfur increased to 3.75 wt %SO3,and chlorine slightly increases but still stays below 1 wt% inall tests in bag filter ash, beingmost probablywithin the errorlimits. All other main ash elements stay at a similar level.Only a sodium decrease could be observed (by almost 30%).The ash flow into the bag filter ash has increased by 5 kg/hreaching 37 kg/h during test CNL25 compared to CL28.

Particle Studies. Spot analysis of secondary cyclone andbag filter ash are shown in Figure 12. The top pictures showsecondary cyclone samples, while the bottom pictures showbag filter ash samples. During both tests, secondary cycloneash consists mainly of aluminum silicates. However, the spotanalysis also show that calcium sulfates and uniform mag-nesium-potassium-phosphates particles can be observedduring the test with lime addition. Bag filter ash spot analysesdo not show large differences. In both cases the ash compo-sition is dominated by aluminum silicates; during limestoneaddition the obvious formation of calcium sulfates wasconfirmed by presence of CaSO4 (Figure 12).

Discussion

Ash Forming Matter. The main ash forming elementsentering the boiler during rapeseed cake cocombustion testswith coal in the ratios studied here are silicon and aluminumoriginating from coal. An increase in the share of rapeseedcake significantly increases the input of alkali metals. Thisresults in the increase of (K2OþNa2O) weight per unit heat

Figure 6. Cross-section of particles from the cyclone leg. The coalreference sample was collected after 5 days of coal firing prior to testCL28. The images are squares with the side equal to 900 μm.

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of dry fuel [kg(K2OþNa2O)/GJHHV] from 0.09 in the case ofpure coal to 0.25 for the test CNL25. According to Mileset al.,28 the value of such an order indicates risk of slaggingand fouling in biomass combustion. Another aspect thatneeds to be taken into consideration during rapeseed cakecocombustion tests is the high concentration of phosphorus.As studied by Novakovi�c et al.16 and Bostr€om et al.19 thiselement, next to chlorine (Knudsen et al.36 and Westberget al.37) may have a crucial influence on the release of potas-sium during combustion. According to Bostr€om et al.,19 thehigh affinity of phosphorus toward potassium may lead toformation of low melting potassium-phosphate salts. Theformation of potassium-phosphates in combustion was alsomentioned by Tissari et al.21 who included phosphorus in thefate of potassium during combustion of agricultural fuels.

Bari�si�c et al.17 reported that the formation of which could becounteracted by limestone addition. Sodium in combustion issuspected to generally follow the fate of potassium and alsosodium phosphates may form when available from the fuel.

Chemical fractionation results give a good indication onthe possible associations of inorganic constituents present inthe fuels and thusmay be useful when predicting their releaseupon combustion. Leaching with water dissolves simplesoluble salts, usually alkali salts that are often volatile duringcombustion. With ammonium acetate the ion exchangeablepart of the fuel ash forming matter is dissolved, usuallyrepresenting organically associated metal ions. In the finalstep, the HCl acid soluble inorganic salts are leached ororganically associated nonmetals like phosphorus or sulfurseem also to be soluble in the HCl treatment; in the rest(insoluble) of the fraction mainly silicates are present butalso sulfur and chlorine when covalently bonded to theorganic structure of the fuel.

Hereto, analyzed coal consists mainly of insoluble siliconand aluminum indicating the presence of clays. The major

Figure 7.Cross-section of particles from the cyclone leg. Samples collected at the end of the test CNL25 (left picture) and CL28 (right picture).The images are squares with the side equal to 900 μm.

(36) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Energy Fuels2004, 18, 1385–1399.(37) Westberg, H. M.; Bystr€om, M.; Leckner, B. Energy Fuels 2003,

17, 18–28.

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part of the alkalimetals was found dissolved at lowpHor notdissolved at all, indicating an association with the silicates.The small amount of phosphorus leached at lowpH indicatespossible association with includedminerals, e.g., calcium phos-phates. Calcium leached by ammonium acetate may be mainlybound to the carboxyl groups or possibly on ion-exchange siteswith the clays.28 Acid soluble calcium is associated withexcluded minerals, most likely as a carbonate or oxide.28

In contrast, biomass contains much less minerals andconsists more of the nutrients required for proper develop-ment of living cells. As mentioned in part 1,24 the rapeseedcake’s ash forming matter was mainly leached by waterindicating simple salts like alkali metal chlorides, sulfates,and phosphates. Phosphorus found in the other fractions isthought to be organically associated. As shown in previousstudies,38 around 70% of total phosphorus present in RC isbelieved to be associated with phytic acid (myo-inositolhexaphosphate) and its degradation products (mainly inosi-tol pentaphosphate). The solubility of phytic acid dependsstrongly on the type of complexes that phytic acid is formingwith cations and proteins.39,40 The leaching mechanism of

phytic acid salts present in RC was not investigated in detailin the present work. Stoichiometric calculations suggest thatalso calcium associated with carboxylic groups is present inthe fuel. A substantial amount of sulfur was found in the restof the fraction indicating that RC contained covalentlybonded sulfur thatmaybe releasedduring combustionas SO2.

Figure 8. SEM/EDX spot analysis of typical bed particle cross sections of bottom ash samples. Titanium and manganese and sulfur (below1wt%) are excluded from the graphs. Particle 1 andParticle 2 coated bedmaterial was collected at the end of test CNL25; Particle 3 andParticle4 coated bed material was collected at the end of test CL28.

Figure 9. SO2, HCl, and KCl þ NaCl entering the convection passand the rate of deposit built-up (RBU). During combustion of thecoal, no deposit sample was taken.

(38) Pontoppidan, K.; Pettersson, D.; Sandberg, A.-S. Anim. FeedSci. Technol. 2007, 132, 137–147.(39) Cheryan,M.CRCCrit. Rev. FoodSciNutr. 1980, 13 (4), 297–335.(40) Shahidi, F., Ed. Canola and Rapeseed: Production, Chemistry,

Nutrition and Processing Technology; Van Nostrand Reinhold CompanyInc. (AVI Book): New York, 1990, ISBN 0-442-00295-5.

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Ash Balance. The closure of the ash balance (Table 1) forRC cocombustion tests is within 70-90% ashout/ashin indi-cating no significant ash accumulation in the system. How-ever, the mass balance closure for CNL of 190% suggeststhat, anyhow, some ash accumulation took place in theboiler and was removed from the plant during the coalmonocombustion case.

No significant differences in the distribution over differentash fractions of alkali metals and phosphorus were observedbetween the tests. The closure of the alkali metals massbalance in the order of 75%/65% (for potassium and so-dium, respectively) during CL28 may indicate the influenceof limestone addition on alkali metals accumulation. In part1 of the series,24 during rapeseed cake cocombustion withwood, limestone was found to indirectly facilitate the for-mation of (K þ Na)Cl. However, in the cocombustion testswith coal, no such a conclusion can be made since theformation of alkali metal chlorides is not confirmed byIACM (Figure 9). Alkali metals were mainly found in thesecondary cyclone ash indicating their associationwith parti-cles larger than 10 μm. It was not influenced by an increase of

the rapeseed cake share by 10% during limestone addition(CL18 andCL28) neither by limestone addition (CNL25 andCL28). However, when no lime was added (CNL25), theinflow to bag filter increased indicating formation of finerparticles (smaller than 10 μm).41 The phosphorus massbalance closure for test CNL25 is very good; the main partwas found in the secondary cyclone (particles g10 μm).41

The SEM/EDX analyses suggest the formation of Mg-K-phosphates but also of mixed particles of aluminumsilicates and phosphate salts (Figure 12). Both ash outflows(secondary cyclone and bag filter ash) were dominated bycoal originating aluminum silicates. SEM/EDX and wetchemical analysis gave the elemental composition of theinvestigated samples; however, in order to have a closer lookat the formation of phosphates, the chemical forms of theelements should be analyzed.

Figure 10.Overall area SEM/EDX analyses of deposit samples. Carbon is excluded from the graph due to the carbon coating. No sample fortest CL28 from the windward side deposit was analyzed; the deposit most probably fell off before the sample was taken out. Cr2O3 is excludedfrom the graph (below 1 wt %).

Figure 11.Elemental composition of secondary cyclone and bag filter ash determined quantitatively. LOI stands for lost on ignitionmainly dueto carbonates present in the sample. Manganese was excluded from the graph (below 1 wt % MnO).

(41) Elled, A.-L. Co-combustion of Biomass and Waste Fuels in aFluidisedBedBoiler-Fuel Synergism.AcademicDissertation,Depart-ment of Energy and Environment, Chalmers University of Technology,Gothenburg, Sweden, 2008, ISBN 978-91-7385-139-8.

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Themain difference in the distribution between fly ash andemissions is observed for sulfur and chlorine. During CNL25,SO2 and HCl emissions have significantly increased whencompared to the tests with addition of limestone CL28. Theincrease of sulfur dioxide emission can be easily explainedwiththe limestone inflow shut off and, as a consequence, lack of cal-cium for capture of sulfur. The high part of calcium and sulfurfound in the bottom ash fraction originates from calciumsulfate, and the high outflow could be explained with thehigher bed regeneration as shown in Table 1. The lower con-centrations of HCl during tests with limestone addition (CL18and CL28) compared to test CNL28 could be assigned to thereaction of calcium-based sorbent with hydrochloric acid, aswas extensively investigated by Partanen et al.42-44 The influ-ence of limestone on the decrease of HCl concentrations wasalso observed in the beginning of the convection pass, whichsupports the above hypothesis of chlorine capture by lime.However, it remains unexplained why in the present work the

reduction of HCl in the beginning of the convective pass canbe observed during limestone addition, whereas it was not

Figure 12. SEM/EDX spot surface analysis of secondary cyclone and bag filter ash. For test with no lime (CNL25) and test with limeaddition (CL28).

Figure 13. Picture of windward side deposit ring on the depositprobe taken during tests CNL25.

(42) Partanen, J.; Backman, P; Backman, R.; Hupa,M. Fuel 2005, 84,1664–1673.(43) Partanen, J.; Backman, P.; Backman,R.;Hupa,M.Fuel 2005, 84,

1674–1684.(44) Partanen, J.; Backman, P.; Backman,R.;Hupa,M.Fuel 2005, 84,

1685–1694.

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observed in part 1 during the rapeseed cake cocombustionwith wood.24

Chlorine captured by lime during tests CL18 and CL28was most probably released when the lime inflow was shutoff (CNL28) and resulted in the mass balance closure higherthan 100%. The accumulation of chlorine in the form ofalkali metals most probably did not take place since IACMresults do not confirm formation of alkali metal chlorides.

Agglomeration Tendencies. In part 1 of this series,24 ag-glomeration could be found when cocombusting wood withrapeseed cake. The gaseous alkali compounds released fromboth wood and rapeseed cake possibly reacted with the sandbed, forming a sticky coating which combined with mostlikelymoltenRCash particles and lead to agglomeration andconsequently to defluidzation. This phenomenon has beendescribed in part 1.24 In the present study, rapeseed cake wascocombusted with coal. During all tests no heavy agglom-eration leading to defluidization was observed. Also inSEM pictures (Figure 8), no agglomerates could be seen. Inaddition, the clear difference between the silica core and thecoating can be observed suggesting that probably no chemi-cal reaction took place between bed material and alkalivapors released from fuel. Apparently different processestook place when compared to the cocombustion of RC withwood. In coal, alkali metals were found mainly connected tothe silica minerals (as illustrated in Figure 3), a stable formthat will not contribute to the formation of volatile alkalicompounds in fluidized bed conditions. This implies that thepotential amount of gaseous alkali compounds available forreactions was substantially lower during the cocombustionwith coal when compared to the cocombustion with wood.

In addition, the alkali released by RC, which could con-tribute to sticky layer formation with the sand bed, may bescavenged in the bed by ash compounds originating fromcoal, for example, by alkali silicates present in the coal formingrather harmless K-Al silicates (e.g., reactions 3 and 4). Thehigher melting temperatures of alkali metals and aluminumsilicate mixtures than of alkali silicates are reported in theliterature45 and thus being desirable. X-raymaps of circulatingmaterial (Figure 7) show the presence of aluminum togetherwith main RC ash elements supporting the theory of coalash and RC ash interactions. Another factor that could haveinfluenced cocombustion experiments is theRCash to coal ashratio. The 4-fold higher coal ash flow compared to RC ashinflow can be observed (Table 1) and might have acted as adiluting agent for low-melting RC ash particles.

The coating found on bed particles consisted of ashelements typically present in coal ash (Figures 6-8). Ag-glomeration phenomenon during coal combustion has beendescribed in detail byManzoori et al.46,47 who has suggestedthe transport of ash from the char surface to the siliconparticle via collision. Also in the present studies, the layersaround the silica core are most probably the result ofphysical processes rather than the chemical reaction since aclear difference between the particle core and deposited ashcan be observed. The deposition of ash on the bed particles isdependent on the fuel particle temperature during burning,whereas the agglomeration itself depends more on the aver-age bed temperature, as stated by Manzoori47 and Bartels

et al.48 This could give an explanation to thick layers ofdeposited ash and no agglomerates formed, meaning thedeposited ash particles cooled down to the bed materialparticles temperature and most probably underwent homo-genization (€Ohman et al.6 and Brus et al.49).

During limestone addition, the presence of calcium sul-fates and potassium aluminum silicates is suggested by theX-ray maps. The coatings on bed material are dominated byearth alkaline metals. The detailed SEM/EDX analysis ofbed material coatings (Figure 8) show that during limestoneaddition, the layer close to the silica core consists almostentirely of calcium silicates (Particle 3). The formation of thecalcium rich layer may be caused by the interaction of limewith bed material as described more in part 1 of the series24

and Partanen et al.50 Second, fuel ash may deposit on bedmaterial particles as in the case of tests with no lime.Phosphorus can be seen in the outer layer of coatings. Themass ratio of phosphorus entering with rapeseed cake tophosphorus entering with coal ash for test CL28 is largerthan 6, implicating that phosphorus seen in graphs based onEDX originates mainly from rapeseed cake. During cofiringtests with the addition of limestone, the formation of calciumphosphate particles is expected to take place.

Fouling Tendencies. The formation of alkali metal chlor-ides and its influence on fouling and corrosion duringcombustion is extensively reported.36,37,51,52 As shown inFigure 9 during RC cofiring with coal, no formation ofgaseous (KCl þ NaCl) took place. On the contrary, duringcocombustion of RC, from the same supplier, with wood,24

the significant increase of alkali metal chlorides in theconvective pass was recorded with the use of IACM. Thisimplies the beneficial influence of coal ash on alkali metalchlorides formation, as already has been pointed out inliterature by Ferrer et al.10 Alkali metal chlorides could bedirectly released from the fuel or they could form accordingto reaction 137

KOHðgÞþHClðgÞ f KClðgÞþH2OðgÞ ð1ÞThere are three plausible mechanisms that could result in thereduced concentrations of alkali metal chlorides. They in-volve either reaction with the produced alkali metal com-pound or the reaction with available MOH

4MClðgÞþ 2SO2ðgÞþ 2H2OðgÞþO2ðgÞ f 2M2SO4ðsÞþ 4HClðgÞ ð2Þ

Al2O3�2SiO2ðsÞþ 2MClðgÞþH2OðgÞ f 2MAlSiO4ðsÞ

þ 2HClðgÞ ð3Þ

Al2O3�2SiO2ðsÞþ 2MOHðgÞ f 2MAlSiO4ðsÞþH2OðgÞ

ð4Þwhere M stands for K or Na.

(45) Zheng, Y.; Jensen, P. A.; Jensen, A. D.; Sander, B.; Junker, H.Fuel 2007, 86, 1008–1020.(46) Manzoori, A. R.; Agarwal, P. K. Fuel 1993, 72, 1069–1075.(47) Manzoori, A. R.; Agarwal, P. K. Fuel 1992, 71, 513–522.

(48) Bartels, M.; Lin, W.; Nijenhuis, J.; Kapteijn, F.; van Ommen,J. R. Prog. Energy Combust. 2008, 34, 633–666.

(49) Brus, E.; €Ohman,M.;Nordin,A.Energy Fuels 2005, 19, 825–832.(50) Partanen, J.; Backman, P.; Hupa, M. Brief Commun. Combust.

Flame 2002, 130, 376–380.(51) Davidsson, K. O.; Amand, L.-E.; Leckner, B.; Kovacevik, B.;

Svane, M.; Hagstr€om, M.; Pettersson, J. B. C.; Pettersson, J.; Asteman,H.; Svensson, J.-E.; Johansson, L.-G. Energy Fuels 2007, 21, 71–81.

(52) Skrifvars, B.-J.; Laur�en, T.; Backman,R.; Hupa,M. In Impact ofMineral Impurities in Solid Fuel Combustion; Kluwer Academic/PlenumPublishers: New York, 1999.

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The sulfation of alkali metal chlorides has been patented53

and discussed in detail in the literature by, e.g., Brostr€omet al.,54 Kassman et al.,34 and Aho et al.,55 whereas thecapture of alkali metals by one of the aluminum silicatescommonly present in South African coals, kaolin (Ahoet al.11), was extensively investigated by Tran et al.56 Thereaction between aluminum silicates and potassium chlorideand hydroxide are plausible mechanisms. Both result in thecapture of potassium and directly (reaction 3) or indirectly(reaction 4) in the formation of HCl. The reaction betweenalkali metal sulfates and aluminum silicates could take placeas well; however, as reported by Tran et al.,57 it is a very slowreaction and may have a minor influence on the alkaliscapture in the current studies. It is uncertain whether theformation of alkali metal chlorides did take place initially.The presence of aluminum silicates and sulfur may havedestroyed the gaseous compounds before detection with theIACM was possible, e.g., according to reactions 2-4. How-ever, during tests with limestone addition (CL18, CL28),reaction 2 most probably did not take place due to theformation of calcium sulfate (CaSO4) and thus almost noSO2 was available in the flue gas for the reaction with (K þNa)Cl . However, no significant increase of alkali metalchlorides concentrationwas observed in the convection pass.In part 1,24 it was observed that the formation of (KþNa)Clwas indirectly favored during the tests with limestone addi-tion, since there was no sulfur available for the reaction withalkali metal chlorides (reaction 2); this was not observed inthe present case, indicating a significant influence of alumi-num silicates from coal ash on alkali metal compounds (e.g.,reactions 3 and/or 4). Even the interaction of alkali metalswith phosphates could be considered as stated by Bostr€omet al.19 This may result in the formation of alkali metal phos-phate particles instead of releasing alkali to the gas phase.

In the present studies, almost no deposits were collected inthe case of the test CNL25. The ring looked clean, and thewindward side of the deposit is shown Figure 13. Thisobservation suggests that the fly ash produced during co-combustion of coal with rapeseed cake pellets do not buildup on the deposit rings; the ash is simply removed by the fluegas stream and is collected in the secondary cyclone and baghouse filter. It seems that no deposition by inertial impactiontakes place57 (mainly particles of 10 μm or larger) or that thecoal ash has erosive properties that lead to tube cleaning.

The rates of deposit build-up (Figure 8) were low. Duringcoal co-combustion, the RBU values are of a smaller orderthan the values reported for the first part of the experimentsfor which wood was used as a base fuel24 (2.6 g/m2 h wasreported when 12%on energy basis of RCwas cocombustedwithout limestone addition for 12 h). Limestone additionhas increased the RBU and as could be seen from depositanalyses resulted only in the increase of calcium in thedeposits. As was mentioned above, during test CL28 thedeposit from thewindward side hasmost probably fallen off,

resulting in a smaller RBU than during test CL18, duringwhich a smaller energy % of RC was cocombusted.

Conclusions

The fate of alkali metals and phosphorus was investigatedduring the cocombustion tests of rapeseed cake pellets withcoal. Tests were successfully carried out in a 12 MWthcirculating fluidized bed boiler. In contrast to the cocombus-tion tests with wood, described in part 1 of this series,24 a highconcentration of alkali metals and phosphorus in rapeseedcake did not cause operational problems. This is most prob-ably the positive effect of South African bituminous coal ash.Chemical fractionation showed that alkali metals present inthe rapeseed cake aremainlywater-soluble. This indicates thatduring combustion they could be easily volatilized and formalkalimetal chlorides, since a high concentration of chlorine ispresent in the rapeseed cake as well. However, the formationof alkali metal chlorides could not be observed neither in thetests with limestone addition nor without. This is expected tobe the result of alkali metals interactions with aluminumsilicates in coal ash.No bed agglomeration could be observed,and coatings on bed material were dominated by coal ash. Inaddition, the 4-fold higher coal ash flow compared to RC ashinflow might have acted as a diluting agent for low-meltingRC ash particles.

Only small concentrations of alkali metals were presenttogether with phosphorus in the coatings on bed materialparticles. Separate rapeseed cake ash particles rich in alkalimetals and phosphorus could be observed in the X-ray mapsof circulatingmaterial. During limestone addition, the forma-tion of calcium silicates around the silica based bed materialparticles took place. SEM/EDXanalysis of secondary cycloneash suggested the formation of Mg-K-phosphates. Both ashoutflows (secondary cyclone and bag filter ash) were domi-nated by coal originating aluminum silicates. The fly ashproduced during cocombustion of coal with rapeseed cakepellets did not build up on the deposit rings. Limestone addi-tion lowered the HCl, and SO2 emissions, however, increasedthe rate of deposit build up.

The alkali metals and phosphorus mass balance closure forthe test without limestone addition (CNL25) was very good.During test CNL25, the increased inflow to the bag filter wasrecorded which indicates the formation of fine particles(smaller than 10 μm). The elemental mass balance closure inthe tests with limestone addition (CL18 andCL28) indicates asmall accumulation of phosphorus, most probably bound tocalcium.

These tests proved successful cocombustion of up to 25en% rapeseed cake with coal in a semi-industrial CFB boilerwith and without limestone addition. The cocombustion withcoal seems to be an advantageous way of rapeseed cakecombustion. However, long-term effects on the operation ofthe boiler need to be studied.

Acknowledgment. This study was carried out as part of theSAFEC project. The financial support of the Academy ofFinland, the Graduate School of Chemical Engineering is grate-fully acknowledged.Results of IACMcould be usedwith permis-sion of Vattenfall AB, which is highly appreciated. Specialthankfulness goes to Matti Hiltunen for his key contribution inthe early phases of this work. Linus Silvander, Tor Laur�en, andPiia Lepp€asalo are kindly acknowledged for help with theexperimental work. AkademiskaHus and themeasurement teamat theChUTarekindly acknowledged for their continuous effortsto operate the boiler.

(53) Andersson, C. A method for operating a heat-producing plantfor burning chlorine-containing fuels, European Patent EP 1354167,2006.(54) Brostr€om,M.; Kassman, H.; Helgesson, A.; Berg, M.; Andersson,

C.; Backman, R.; Nordin, A. Fuel Process. Technol. 2007, 88, 1171–1177.(55) Aho, M.; Vainikka, P.; Taipale, R.; Yrjas, P. Fuel 2008, 87, 647–

654.(56) Tran,K.-Q.; Isa,K.; Steenari, B.-M.; Lindqvist, O.Fuel 2005, 84,

169–175.(57) Baxter, L. L.; DeSollar, R. W. Fuel 1993, 72, 1411–1418.

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Fate of Alkali Metals and Phosphorus of Rapeseed Cake in ...users.abo.fi/mzevenho/portfolj/publikationer/refereed papers/ef2010… · reported tests in a bench scale (50 kW) fluidized