trace element behavior in the fluidized bed gasification of solid recovered fuels – a...

Fuel xxx (2012) xxx–xxx

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Fuel

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Trace element behavior in the fluidized bed gasification of solid recoveredfuels – A thermodynamic study

Jukka Konttinen a,⇑, Rainer Backman b, M. Hupa c, Antero Moilanen d, Esa Kurkela d

a University of Jyväskylä, Department of Chemistry, Renewable Natural Resources and Chemistry of Living Environment, POB 35, FI-40014 University of Jyväskylä, Finlandb Umeå University, Energy Technology and Thermal Process Chemistry, SE-90187 Umeå, Swedenc Åbo Akademi University, Process Chemistry Centre, Combustion and Materials Chemistry Research, Piispankatu 8, FI-20500 Turku, Finlandd VTT Technical Research Centre of Finland, POB 1000, FI-02044 VTT, Finland

h i g h l i g h t s

" A thermodynamic database on trace elements, combining data from several databases." Stepwise introduction of fuel ash components to eliminate unrealistic compounds." Validation of modeling data against experimental data from 1 MWth pilot-gasifier." Critical review and comparison of own results against results from the literature.

a r t i c l e i n f o

Article history:Received 28 May 2012Received in revised form 2 October 2012Accepted 3 October 2012Available online 27 October 2012

Keywords:Trace elementsGasificationThermodynamic equilibriumModelingSolid waste

0016-2361/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.fuel.2012.10.009

⇑ Corresponding author. Tel.: +358 400247445.E-mail address: [email protected] (J. Konttinen).

Please cite this article in press as: Konttinen J estudy. Fuel (2012), http://dx.doi.org/10.1016/j.f

a b s t r a c t

Gasification of biomass and recycled fuels is of particular interest for the efficient production of powerand heat. Trace elements present as impurities in the product gas should be removed very efficiently.The objective of this work has been to develop and test thermodynamic models for the reactions of traceelements with chlorine and sulfur in the gasification processes of recycled fuels. In particular, the chem-ical reactions of trace elements with main thermochemical conversion products, main ash components,and bed and sorbent material are implemented into the model. The possibilities of gas cleaning devices incondensing and removing the trace element compounds are studied by establishing the volatilizationtendency of trace element compounds in reducing gases. The results obtained with the model are com-pared with the measured data of trace elements of gasification experiments using solid recovered fuel asfeedstock. Some corresponding studies in the literature are also critically reviewed and compared. Theobserved discrepancies may be attributed to differences in thermodynamic databases applied and exper-imental arrangements. The method of removing gaseous trace elements by condensation is already in usein the 160 MWth waste gasification plant in Lahti, Finland.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Gasification of biomass and recycled fuels is of particular inter-est for efficient production of power and heat, and probably in thefuture also for the production of liquid biofuels and chemicals. Inthe city of Lahti in Finland, a 60 MWth circulating fluidized bed(CFB) gasification plant, firing waste and biomass fuels, has beenrunning since 1998 [1] in a combined operation with a 360 MWth

pulverized coal boiler. A new waste gasification plant with two80 MWth CFB gasification units has recently been commissionedand has been running for more than 1000 h [2]. Trace elements

ll rights reserved.

t al. Trace element behavior inuel.2012.10.009

as impurities in the product gas exiting the gasifier should be re-moved very efficiently [1,2].

The behavior of trace elements in fluidized bed gasification pro-cesses can be studied with thermodynamic equilibrium calcula-tions. Details about that is given in the next Chapter.

The objective of this work has been to develop and test thermo-dynamic models of the reactions of trace elements with chlorineand sulfur in gasification processes of recycled fuels. In particular,the chemical reactions of trace elements with main thermochemi-cal conversion products, main ash components, and bed and sor-bent material are covered in the model. The main ashcomponents are introduced stepwise to understand their effect inmore detail. A separate database for the thermodynamic equilib-rium calculations of trace elements is presented and used; thisdatabase combines data from several databases in the literature.

the fluidized bed gasification of solid recovered fuels – A thermodynamic

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2 J. Konttinen et al. / Fuel xxx (2012) xxx–xxx

The focus is on possibilities for flue gas cleanup with cyclones, fil-ters or scrubbers used to condense and remove the compoundscontaining trace elements before further use of the product gas.The volatilization tendency of trace element compounds in reduc-ing gases is established and the level of trace elements in the fluegases from different fuels is estimated. Finally, the results obtainedby the model are compared with the measured data of trace ele-ments in gasification experiments [3,4] using solid recovered fuel[5] as feedstock. The results are also compared with other corre-sponding studies reported in the literature. The idea of removingtrace elements by careful condensation is already in practical usein some commercial waste gasification plants [1,2].

2. Combined database vs. previous work

For the work in this paper, five electronically available thermo-dynamic databases have been critically evaluated [3,6]. Data fromall five have been used in the combined project database. The dat-abases are:

(1) The FACT database, Available through FactSage 5.2. CRCT,Centre for Research in Computational Thermochemistry,Ecole Polytechnique, Université de Montréal, Canada [7,8].

(2) HSC Chemistry Version 5.11, Outokumpu Research Oy, Fin-land [9].

(3) The GFE database, Denmark Technical University, Denmark[10].

(4) The SGTE pure substance database. Scientific Group Thermo-data Europe, available through ChemSage, GTT-Technologiesand FactSage [7,8].

(5) Ivtanthermo for Windows, Glushko Thermocenter, Russia[8,11].

The five databases have very different histories which is re-flected in the set of data and the amount of compounds found inthem. All have essentially the same set of data for gaseous C–H–O compounds, but data on gaseous trace element compoundsand condensed species differ considerably. HSC is the largest withalmost 4000 compounds, FACT and SGTE both comprise some 2500compounds, whereas GFE and Ivtanthermo both contain around1500 compounds. All databases are different. Related to this work,Talonen [12] reported a review of the thermodynamic data fortrace elements (As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Tl, V, Mo,Zn) in various data compilations in the above mentioned databases1.�5. It was also shown that the number of included compoundsvaries greatly in the databases and that the thermodynamic dataalso vary considerably for some species. This in turn will greatly af-fect predictions for the speciation of the elements. In their recentreview, Lindberg et al. [13] conclude that one major limitation inusing thermodynamic equilibrium modeling is the lack of compre-hensive databases that contain the thermodynamic data of ashcompounds and phases formed during the thermochemical con-version of fuels. In order to develop a fully consistent thermody-namic database including all phases and compounds for the traceelements, a thermodynamic evaluation of all available experimen-tal thermodynamic data is needed. However, this would require avery extensive study, as all possible compounds of the trace ele-ments with the elements C, H, O, N, Cl, S, Ca, Mg, Na, K, Si, Al,and some others, also should be included.

The idea of selecting and combining data from different dat-abases, including also some data generated in this study, is differ-ent in comparison with the work reported previously in theliterature. When reactions between main ash components andtrace elements are considered, chemical compounds containingmore than one metal play an essential role. Such compounds are,

Please cite this article in press as: Konttinen J et al. Trace element behavior instudy. Fuel (2012), http://dx.doi.org/10.1016/j.fuel.2012.10.009

for example CaBaCl4, PbOSiO2, and Ca3(AsO4). The databases havebeen inspected for such combined compounds [3].

Usually in the literature of thermodynamic equilibrium model-ing of gasification processes, one software (and database) is used,such as different versions of FactSage [14–19] or HSC [20]. The nextparagraph contains a critical review of some selected referenceswith respect to their relevance to the present work.

Kuramochi et al. [14] studied the behavior and emissions of gas-eous H2S and HCl in connection with the gasification of biomassfuels at temperatures of 673–1473 K. Based on a thermodynamicanalysis, they proposed to add potassium-rich biomass to reduceHCl emissions to low levels acceptable for fuel cells. However, thisobservation was not validated against experimental measurementsfrom a biomass gasifier. Increasing potassium in fuel feed can leadto other problems, such as agglomeration, in case of a fluidizedbed. Wei et al. [15] studied the behavior and emissions of gaseouschlorine and alkali metals in connection with biomass gasificationand combustion, under various process temperature and pressureconditions and air-to-fuel ratio. They were able to identify themain gaseous and condensed species between the elements Cl, Kand Na, including chlorides and hydroxides. The fact that theydid not find sulfides or sulfates among these species might be anindication that their calculation system was insufficient. The mod-eling results were not validated against experimental results. Buntand Waanders [16] investigated the behavior of some trace ele-ments (Hg, As, Se, Cd and Pb) in connection with a Lurgi-typefixed-bed gasifier, by using analysis results from residual ash sam-ples, in comparison with FactSage modeled data. In their modelingapproach, they divided the fixed-bed gasifier into drying, devolatil-ization and reduction zones, which were modeled separately usingFactSage and the results were combined in post processing. The so-lid ash-containing samples from inside the gasifier were collectedafter shutting down the gasifier operation and cooling the gasifier.They achieved good agreement between model predictions andmeasurements, with the exception of As. Hg, Cd, Pb, As and Se wereall found to be highly volatile, partitioning into the gas phase. Hgwas found to be the most volatile element in the gas phase inthe form of elemental Hg. They divided the behavior of differenttrace elements into three classes. Class I elements, such as Hg,As, Se, Cd and Pb are volatile (low boiling point). Class II elements,such as Cu, Mo, Ni and Zn are less volatile and partition betweenthe ash and the gaseous phase, with condensation of vaporizedspecies on the surface of ash particles as the gas cools. Class III ele-ments, such as Ba, Co, Cr, Mn and V are the least volatile and re-main in the ash.

Bunt and Waanders [17] also investigated the behavior of someother trace elements (Ba, Co, Cr, Mn and V) in a fixed-bed gasifier,by using data from residual ash samples and FactSage modeling.They found good agreement between most of the metals besidesCr, and good prediction of ash partitioning behavior in connectionwith gasification. The comparison of experimental data [17] of ele-ments remaining condensed in ash against modeling is similar tothe method reported in this paper for fluidized bed gasification.Porbatzki et al [18] studied the release of alkali metals, chlorine,sulfur and heavy metals in connection with gasification of wood,straw and mischanthus. They made modeling predictions usingFactSage (version 5.5) and compared the results against measure-ments of gas-phase species from a laboratory-scale batch fluidizedbed reactor. Besides some gaseous zinc above 900 �C, they did notobserve any gaseous heavy metal species in the product gas, andthe modeling results were in agreement with measurements. Sucha ‘‘clean’’ gas could be possible because of using ‘‘pure’’ biomassfuels, in contrast with waste-type fuels such as those used in thispaper. Yoshiie et al. [19] used a laboratory-scale drop tube to gasifycoal particles and took samples of the ash particles from productgas after cooling and analyzed them for their Pb and Se contents.



J. Konttinen et al. / Fuel xxx (2012) xxx–xxx 3

They investigated the reaction chemistry between the two ele-ments from the experimental results and also using FactSage (ver-sion 6.1) modeling. It is however uncertain how representative theobtained results are, considering the phenomena and flow patternstaking place in a large-scale gasifier.

Diaz et al. [20] reported predictions of the evaporation of traceelements in coal gasification. In their modeling they used the HSCChemistry program [9] (version 4.0). They classified trace elementsin three different groups based on the degree of evaporation of thespecies formed in a gasification reactor and cooling process of theproduct gas. Group A includes those elements that, according tothermodynamic data at equilibrium, could probably be condensedin coal gasification. Mn is classified in this group. Group B containsthose elements that could be totally or partially in the gas phaseunder gas cleaning conditions; they can be divided into two sub-groups depending on whether the cleaning conditions are hot orcold. Co, Be, Sb, As, Cd, Pb, Zn, Ni, V, Cr are elements in this group.Group C contains those elements that could be totally in the gasphase in all possible conditions, including flue gas emissions. Se,Hg and B are the elements that make up this group. Comparisonsof the evaporation degree of different elements between modelingand experimental results were also reported. However, it is knownthat at least the version of HSC used in [20] does not have verystrong numerical subroutines to solve the chemical compositionof the condensed species. Therefore special attention should bepaid when drawing extensive conclusions.

Based on the above review, it can be concluded that when usinglimited or insufficient databases or equilibrium calculation soft-ware, there is a risk of misleading conclusions. As Linderg et al.[13] concluded, in the case of algorithms including trace elements,sufficient databases are not yet available. In such situations, exper-imental validation of the modeling results can be recommended,whenever possible. When it comes to validation, another generalfeature of the work in the literature is that the validation is carriedout against laboratory-scale data. Many times there is a need toscale up the results to process conditions relevant to commercialgasifiers. Since this may not be straightforward, using small-scaledata may once again lead to misleading conclusions. To overcomethe scale-up problem, for example Bunt and Waanders [16,17] di-vided the large-scale fixed-bed gasifier to several individual sub-systems which were modeled separately. The results were thencombined during post-processing.

A recent study by Cui et al. [21] is of particular interest to thiswork. They investigated the conversion of different fuel ash ele-ments from two bagasse-type biomass fuels in bench-scale fluid-ized bed gasification tests. They took samples of filter char, bedmaterial and the product gas stream and analyzed them for a total

Table 1aSummary of process operation conditions in pilot gasification tests at VTT [3].

Balance CFB 1 CFB 2

Gasifier CFB CFBDate September 1997 September 2000Fuel Demolition wood 100% Ewapower REF pellets 70%, wBed material Limestone Sand 75%, limestone 25%

ConditionsPressure, bar 1.1 1.1T bed, C 942 891T freeboard, C 930 888T cyclone average, C 720 816.5T gas cooler average, C 375 611.5T filtration, C 220 394Sorbent No Ca(OH)2

Gas cleaning Cyclone CycloneGas cooler Second cycloneCeramic filter Cooler

Bag filter


of 21 elements including Al, Ca, Fe, K, Mg, Na and Si defined as ma-jor elements, and Ba, Cd, Co, Cr, Cu, Mn, Mo, Ni, P, Sr, Pb, Ti, V, andZn defined as trace elements. They found that most ash particles ormetal elements can be captured by a silicon carbide candle filterthat removes small particles from the gas phase, but some of thevolatile elements pass through the filter and are present in thegas stream. They summarized their analysis presenting the relativeamounts of vapourized and condensed trace elements from exper-imental filter ash samples at filtration temperatures from 640 �C to670 �C. They did not compare their results against thermodynamicmodeling calculations, instead they reported their results accord-ing to the classification proposed by Diaz et al. [20].

3. Calculations

For the development of the thermodynamic database behindthe calculations, thermodynamic data for 29 elements and theircompounds were carefully selected and collected from several dif-ferent databases [3]. This method differs from the work previouslyreported in the literature, where usually one database is used. Adatabase was formed, including data from gas, solid and liquidphases (and liquid solutions), for the elements and their com-pounds that are relevant in reducing gasification conditions.

The combined database, has the following features [3,6]:

Number of elements: 35 (29 included in this paper).Number of solution phases: 4 (gas + three liquids).Number of gaseous species: 200.Number of stoichiometric solid phases: 540.

The 35 elements in the database are divided into four groups:

Main thermochemical conversion elements (9): C, H, O, N(inert), S, Cl, Br�, F�, P.Main ash elements (7): Ca, Mg, K, Na, Al, Si, Fe.Trace elements (EDD) (12): As, Cd, Cr, Cu, Co, Hg, Mn, Ni, Pb, Sb,Tl�, V Other trace elements (7): Ba�, Be�, Mo, Se�, Sn, Ti, Zn.⁄Element not included in this paper.

It was decided to concentrate the reporting of the resultsaround the ‘‘European Dirty Dozen’’ (EDD)-metals and zinc. TheEDD metals are the ones for which emission limits in waste incin-eration are regulated in the EU-directive [22]. The metals of this re-port include As, Cd, Cr, Cu, Co, Hg, Mn, Ni, Pb, Sb and V (Tlexcluded).

The species in the database are selected in the following man-ner: The basic data for the pure gaseous and solid compounds

BFB 1 BFB 2

BFB BFBMarch 2002 March 2002

ood pellets 30% REF Forssa REF ForssaSand 75%, limestone 25% Sand 75%, limestone 25%

1.1 1.1836 849863 871803 812589.5 602410 420No Ca(OH)2

Cyclone (return to reactor) Cyclone (return to reactor)Gas cooler Gas coolerBag filter Bag filter



Table 1bProperties of fuels used in pilot gasification tests at VTT.

Fuel Demolitionwood

REFForssa

EwapowerREF

Woodpellets

Moisture (wt% wet) 12.3 25.89 4.51 7.97Carbon (wt% dry) 45.8 49.46 53.0 50.40Hydrogen (wt% dry) 5.53 6.64 7.5 6.2Nitrogen (wt% dry) 1.62 1.04 0.6 0.1Sulfur (wt% dry) NA

a0.17 0.17 0.03

Chlorine (wt% dry) 0.102 0.748 <0.02 0.59Ash (wt% dry) 8.53 15.34 12.30 0.18

a Not available. Fig. 1. Schematic of the gasifier and gas cooling equilibrium modeling principles[3,4].


are taken from FactSage, altogether 695. These are complementedwith compounds from HSC, SGTE, GFE and Ivtanthermo which arenot in FactSage. These compounds are mainly gaseous trace ele-ment compounds and consist of more than one metallic compo-nent. In the assessment of these compounds the evaluation ofTalonen [12] was used. The liquid phase used is based on the slagphase in FactSage containing SiO2, CaO, FeO, Al2O3 and NiO. It wascomplemented with chlorides and sulfides as ideal components. Asone simplification, argon was used as the inert gas component in-stead of nitrogen. The reason was to exclude species of nitrogen inthe calculation. Nitrogen species should not affect the reactionswith the EDD metals.

Experimental gasification balance data from pilot-scale tests ofsolid recovered fuels (SRFs), wood pellets and demolition wood areutilized as inputs and validation of the calculation results. Thesampling and analysis methods were reported in detail by Niemi-nen and Kurkela [4]. For validation, the contents of trace elementsand main fuel ash elements were available from the four differentpilot test run balances, including the material streams at the

Table 2Equilibrium calculation inputs of the pilot balances. The units are in g/kg ds.

Element BFB 1 BFB 2

A1 A2 A3 A1 A2 A3

C 494.6 494.6 498.35 494.6 494.6 500.4H 120.3 120.3 120.3 124.0 124.0 124.4O 1371.7 1371.7 1381.7 1398.9 1398.9 1420.8S 1.700 2.682 2.682 1.700 2.344 2.344Cl 7.475 7.475 7.511 6.091 6.091 6.146Ca 0.000 0.000 34.26 0.000 0.000 53.32Na 0.000 4.755 6.791 0.000 6.136 9.293K 0.000 3.221 4.992 0.000 3.682 6.428Mg 0.000 3.988 4.292 0.000 2.761 3.232Al 0.000 0.000 17.10 0.000 0.000 18.73Si 0.000 0.000 61.61 0.000 0.000 85.26Fe 0.000 3.528 5.904 0.000 4.755 8.440P 0.000 0.9971 1.057 0.000 1.166 1.258Hg 0.0007 0.0007 0.0007 0.0008 0.0008 0.0008Sn 0.0417 0.0417 0.0419 0.0392 0.0392 0.0395Sb 0.0236 0.0236 0.0237 0.0215 0.0215 0.0217As 0.0020 0.0020 0.0028 0.0020 0.0020 0.0033Cd 0.0026 0.0026 0.0026 0.0019 0.0019 0.0019Pb 0.3431 0.3431 0.3449 0.3208 0.3208 0.3236V 0.0038 0.0038 0.0065 0.0033 0.0033 0.0076Mn 0.2285 0.2285 0.2565 0.2078 0.2078 0.3347Co 0.0024 0.0024 0.0029 0.0024 0.0024 0.0031Ni 0.0180 0.0180 0.0191 0.0210 0.0210 0.0226Cu 0.7430 0.7430 0.7446 0.6395 0.6395 0.6420Zn 0.7095 0.7095 0.7123 0.7251 0.7251 0.7295Mo 0.0290 0.0290 0.0290 0.0290 0.0290 0.0290Cr 0.0978 0.0978 0.0994 0.1133 0.1133 0.1165Ar 1950.2 1950.2 1950.2 1955.8 1955.8 1955.8Ti 1.687 1.687 1.934 1.841 1.841 2.223SiO2 (FB) – – 5000 mol – – 15,000 mCaCO3 (FB) – – 15,000 mol – – 5000 mo


gasification reactor and in the product gas cooling and filtering sys-tem. A summary of the experimental process conditions in the pi-lot gasification tests is shown in Table 1a and Table 1b shows someproperties of the fuels used in these tests. The relative amounts oftrace elements originating from the fuel and other solid feeds inthe gasification tests are shown in Table 2. The measured analysisdata include elements As, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sb, Sn, V, Zn,Hg, and Cl. The corresponding analysis data of the main ash com-ponents Na, K, Ca, Mg, Al, Si, Fe, P and Ti were also available fromtwo test run balances.

The calculations were performed stepwise [3,6]. First the condi-tions inside the gasification reactor (Case A) were studied and thenseparately the conditions of the product gas cooling and filteringsystem (Case B). The principles of modeling, both for the gasifierreactor (Case A) and the gas cooling/cleanup (Case B), are illus-trated in Fig. 1. It was known that the reactions between the mainfuel ash components and trace elements are important; thereforeCase A calculations under the gasifier conditions were divided intothree stages (Case A1, A2 and A3) [3,6]:

CFB 1 CFB 2

A1 A2 A3 A1 A2 A3

458.0 458.0 458.0 522.2 522.2 522.270.99 70.99 70.99 98.56 98.56 98.83974.8 974.8 974.8 972.1 972.1 976.40 0.2824 0.2824 1.280 1.280 1.2801.020 1.020 1.059 5.875 5.875 5.8950.000 0.000 15.73 0.000 0.000 25.520.000 0.5963 0.6409 0.000 2.475 3.6030.000 0.9272 1.002 0.000 1.775 2.7560.000 0.4938 0.7856 0.000 1.248 1.4170.000 0.000 1.604 0.000 0.000 9.3850.000 0.000 27.12 0.000 0.000 13.330.000 1.145 1.202 0.000 0.605 1.9220.000 0.1270 0.1287 0.000 0.5163 0.54931.00001 1.00001 1.00001 0.0001 0.0001 0.00010.0047 0.0047 0.0048 0.0111 0.0110 0.01120.0040 0.0040 0.0040 0.0144 0.0144 0.01440.0034 0.0034 0.0040 0.0033 0.0033 0.00380.0009 0.0009 0.0009 0.0020 0.0020 0.00200.3679 0.3679 0.3679 0.0472 0.0472 0.04820.0024 0.0024 0.0024 0.0050 0.0050 0.00700.1031 0.1031 0.1064 0.1085 0.1085 0.12400.0024 0.0024 0.0021 0.0058 0.0058 0.00600.0217 0.0217 0.0217 0.0090 0.0090 0.00950.0442 0.0442 0.0442 0.3418 0.3418 0.34260.5672 0.5672 0.5672 0.2227 0.2227 0.22430.0007 0.0007 0.0007 0.0042 0.0042 0.00420.0449 0.0449 0.0449 0.0506 0.0506 0.05171353.3 1353.3 1353.3 1300.6 1300.6 1300.61.300 1.300 1.304 0.1104 0.1104 0.2469

ol – – 0.000 mol – – 15000 moll – – 10,000 mol – – 5000 mol



(a)

-12

-10

-8

-6

-4

-2

0

600 700 800 900 1000

log

(pi)

Temperature ( C)

MnCl2

Mn

(b)

-12

-10

-8

-6

-4

-2

0

600 700 800 900 1000

log

(pi)

Temperature ( C)

MnCl2 Mn

(c)

-12

-10

-8

-6

-4

-2

0

600 700 800 900 1000

log

(pi)

Temperature ( C)

MnCl2

Mn

Fig. 2. Partial pressures of the gas compounds containing manganese (Mn) as a function of process temperature for balance BFB 2, (a) Case A1, (b) Case A2 and (c) Case A3 [3].

(a)

0%

20%

40%

60%

80%

100%

Temperature ( C)

MnSalabandite(s)

MnO(s)

Gas (b)

0%

20%

40%

60%

80%

100%

Temperature ( C)

MnO(s)

MnSalabandite(s)

(c)

0%

20%

40%

60%

80%

100%

Temperature ( C)

Mn3Al2Si3O12mnpyrop(s)

Gas

Fig. 3. Percentages of gas and condensed compounds containing manganese (Mn) as a function of process temperature for balance BFB 2, (a) Case A1, (b) Case A2 and (c) CaseA3 [3].


Plestu

Case A1, inputs: The fuel feed main components and the traceelements in fuel (excluding main ash components of fuel) andthe gaseous feeds into the gasifier (air and steam).Case A2, inputs: The same feeds as in Case A1 with the main ashelements of the fuel feed, Na, K, Mg, Fe, P included (Ca, Al and Siare still excluded).Case A3, inputs: The same as in Case A2 with all fuel ash com-ponents included, as well as the bed material feed and thematerial of the fluidized bed gasifier. It is assumed that the

ase cite this article in press as: Konttinen J et al. Trace element behavior indy. Fuel (2012), http://dx.doi.org/10.1016/j.fuel.2012.10.009

fluidized bed material is in huge excess in comparison withother inputs to the system. Since there was no detailed analysisof composition of the bed material available, it was assumedthat the sand material is pure SiO2 and limestone is pure CaCO3,see Table 2.

The idea of stepwise introduction of the main ash componentsinto the calculation system is exceptional when compared to themodeling work reported in the literature [14–20], where usually



(a)

-6

-5

-4

-3

-2

-1

0

600 700 800 900 1000

log

(pi)

(bar

)

Temperature (°C)

H2

CH4

H2OCO CO2

HCl

COS

H2S

(b)

-6

-5

-4

-3

-2

-1

0

600 700 800 900 1000

log

(pi)

(bar

)

Temperature (°C)

H2OH2

CH4

CO2CO

H2S

HCl COS

KCl

NaCl

(c)

-6

-5

-4

-3

-2

-1

0

600 700 800 900 1000

log

(pi)

(bar

)

Temperature (°C)

H2

CH4

CO2H2OCO

HClH2S

COS

NaClHCl

Fig. 4. Partial pressures of the product gas main components as a function of process temperature for balance BFB 2, (a) Case A1, (b) Case A2 and (c) Case A3 [3].

Table 3Summary of the relevant species related to EDD-metals and zinc under different input condition ‘‘Cases A’’ at temperatures of 650–850 �C. (cond. = condensed (solid)).

Metal, phase Case A1 (CHOSCl + Trace elements) Case A2 (CHOSClNaKMgFeP + Trace elements) Case A3 (All)

650 �C 850 �C 650 �C 850 �C 650 �C 850 �C

Arsenic, gas AsS, As, As4O6 AsS, As, As4O6 AsS, As AsS, As AsS, As AsS, AsArsenic, cond. As2S2 - As2S2 - As2S2 -

Cadmium, gas Cd Cd Cd Cd Cd CdCadmium, cond. - - - - - -Chromium, gas - - - - - -

Chromium, cond. (CoO)(Cr2O3), Cr2O3,(NiO)(Cr2O3)

(CoO)(Cr2O3), Cr2O3,(NiO)(Cr2O3)

FeCr2O4 (Na2O)(Cr2O3),FeCr2O4

FeCr2O4 (Na2O)(Cr2O3),FeCr2O4

Copper, gas CuCl, Cu, (CuCl)3 CuCl, Cu, (CuCl)3 CuCl, Cu CuCl, Cu CuCl, Cu CuCl, Cu

Copper, cond. Cu2S Cu2S Cu2S Cu2S Cu2S Cu2SCobalt, gas CoCl2 CoCl2 CoCl2, Co Co CoCl2, Co CoCobalt, cond. (CoO)(Cr2O3) (CoO)(Cr2O3) (CoO)(Fe2O3), CoO CoO (CoO)2(SiO2), CoO (CoO)2(SiO2), CoO

Mercury, gas Hg Hg Hg Hg Hg HgMercury, cond. - - - - - -

Manganese, gas MnCl2 MnCl2 MnCl2 Mn, MnCl2 MnCl2 Mn, MnCl2

Mangan., cond. MnS MnO MnS MnO, MnS Al2MnO4 Mn3Al2Si3O12, MnO

Nickel, gas - - - - - -Nickel, cond. Ni3S2(s2), (NiO)(Cr2O3) Ni3S2(s2), (NiO)(Cr2O3) Ni3S2(s2) Ni3S2(s2) Ni3S2(s2) Ni3S2(s2)

Lead, gas Pb, PbS, PbCl, PbCl2 Pb, PbS, PbCl, PbCl2 Pb, PbS, PbCl Pb, PbS, PbCl Pb, PbS, PbCl, PbCl2 Pb, PbS, PbCl, PbCl2

Lead, cond. - - - - - -

Antimony, gas SbO2H2, SbOH, Sb SbO2H2, SbOH, Sb SbO2H2, SbOH, Sb SbO2H2, SbOH, Sb SbO2H2, SbOH, Sb SbO2H2, SbOH, SbAntimony, cond. - - - - - -

Vanadium, gas - - - - - -Vanadium, cond. V2O3 V2O3 V2O3, (Na2O)2(V2O5) (Na2O)2(V2O5),

V2O3

V2O3, (CaO)3(V2O5) V2O3, (CaO)3(V2O5)

Thallium, gas - - - - - -Thallium, cond. - - - - - -

Zinc, gas Zn, ZnCl2 Zn, ZnCl2 Zn, ZnCl2 Zn, ZnCl2 Zn, ZnCl2 Zn, ZnCl2

Zinc, cond. ZnO - ZnO - Zn2SiO4, ZnAl2O4 -


Please cite this article in press as: Konttinen J et al. Trace element behavior in the fluidized bed gasification of solid recovered fuels – A thermodynamicstudy. Fuel (2012), http://dx.doi.org/10.1016/j.fuel.2012.10.009



the data of all elements are introduced together. A summary of theinput values for each set of pilot-scale conditions in each threecases is shown in Table 2. Significant differences can be seen inthe input amounts of the main ash components.

According to Fig. 1, the product gas is cooled and filtered afterthe gasification reactor. In order to simulate the equilibrium condi-tions of the gasifier product gas, the molar amounts of differentelements in the gas, resulting from Case A calculations for the fourpilot balances, were taken as inputs for Case B calculations. Themolar input amounts are from Case A2 results at 850 �C, sincethese conditions are closest to the real conditions of the chemistryin the gasification reactor. Some of the main ash elements were re-moved from Case B inputs, since these elements too slowly partic-ipate in the chemical reactions at temperatures lower than 850 �C.

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Fig. 5. The contents of elements of interest in gas the phase as a function ofgasification temperature for (a) Case A1, (b) Case A2 and (c) Case A3 for pilotbalance BFB 1 [3].


These elements include Mg, Fe and P (they could also include Ca, Aland Si, but these are not included in Case A2 inputs). These selec-tions can be justified on the basis of the high temperature chemis-try of trace and ash elements of waste-type fuels in severalprevious studies, such as Sandelin and Backman [23] and Kouvoand Backman [24].

4. Results

Figs 2–4 show examples of how the different gaseous and con-densed components for certain EDD-metals are plotted, showingthe manganese species predicted by chemical equilibrium forCases A1–A3. The examples are adapted using data from one ofthe four pilot test run balances, designated as BFB 2 [4]. In the caseof the manganese compounds there are significant differences be-tween Cases A1–A3, as to which species dominate at each temper-ature, especially in the case of solid species. Also the partialpressures of gaseous MnCl2 and Mn are different depending onthe input conditions. The chemistry of sulfur and chlorine is of spe-cial importance concerning the equilibrium species of EDD metals.Fig. 4 shows that the levels of gaseous sulfur (H2S, COS) and chlo-rine (HCl, NaCl, KCl) can vary between different cases. Correspond-ing plots of each EDD metal can be found in [3]. The balance data ofthis test run were generated in a bubbling fluidized bed gasifierusing solid recovered fuel (Tables 1a and 1b) as feedstock at abed temperature of 850 �C.

The complete results using four different pilot test run balanceinput conditions are reported in [3]. Based on those results, Table 3shows a summary of gaseous and condensed trace element speciespredicted by the modeling method at fluidized bed gasificationconditions (temperatures 650–850 �C). Fig. 5 shows clearly howthe different sets of inputs (pilot balance BFB 1, Cases A1–A3 affectthe amounts of vaporized compounds of each element at temper-atures of 600–1000 �C. Significant differences can naturally be seenin all main ash components. What is important is the form of sulfurand chlorine. The resulting differences in the amount of elementsin the gas phase can be seen in practically all trace elements forexample, in Pb, Zn, Mn and Co.

The results indicate that the elements Ca, Al and Si are impor-tant in the equilibrium chemistry [3]. When introducing these ele-ments (present in the fuel ash and in the bed material) into thethermodynamic calculation (Case A3), solid compounds are formedthat are not kinetically realistic [23,24]. Thus assuming a chemicalequilibrium with one set of calculation input data, and including allash components from fuel and other solid feeds, can lead to mis-leading conclusions when trying to predict the behavior of the gas-ification process.

5. Comparison with pilot-scale data

The results of the equilibrium calculation were compared withthe experimental balance data of the EDD trace elements and zinc.The contents of different metal species in the hot product gas andfly ash streams exiting the pilot-gasifier are not available. In thecase of a well-mixed fluidized bed reactor, it can be assumed thatbottom ash samples represent the composition of the solid fluid-ized bed material. Therefore, the metal contents of the bottomash and the equilibrium calculation results are compared as the re-sults of Case A. The results of Case A2 for four different pilot bal-ances are shown in Fig. 6. The results designated as MEAS. arefrom the bottom ash samples and MODEL are the equilibrium cal-culation results, respectively.

In Fig. 6, the contents of different trace elements are normalizedas weight% of the total feed in of each element into gasificationreactor. Some discrepancies can be observed, e.g. with elements




As, Sb and Zn [3]. The calculations predict that these elementsshould not occur in the condensed phase, whereas the measure-ments indicate that they are present in the gasification fluidizedbed. There are many possible reasons for these discrepancies. Theinaccuracy of sampling and analysis of the elements in ashes [4]is one possible reason. In the database, there can be an insufficientnumber of compounds of As, Sb and Zn and the thermodynamicdata of the included compounds can be inaccurate. Also the equi-librium system does not include possible phenomena controlledby chemical kinetics or physical phenomena, such as the possibledeposition of volatile compounds on the surfaces of small solidparticulates exiting the gasification reactor.

The calculation results of the product gas cooling and filteringsystem (Case B) were compared with the experimental trace ele-ment stream data from the product gas filter around temperaturesof 200–450 �C and the results are shown in Fig. 7. The results des-ignated as MEAS. are from the filter ash samples and MODEL arethe equilibrium calculation results, respectively. According to theexperimental results practically all the other trace elements exceptHg have been condensed at the cooled gas conditions at the filter.Significant differences between analyzed and predicted condensedphase are seen in the case of Sb. In the case of cadmium (Cd), thedependence of its condensation on the temperature is very sensi-tive, which might be the reason for the difference in Fig. 7. The sen-sitivity of Cd and Sb compounds between the condensed and gasphase as a function of decreasing gas temperature can be seen inFig. 8, which shows the relative total amount of different trace

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Fig. 6. Weight percentages of condensed metals (EDD-elements and zinc) under fluidizethe contents of trace elements in gasification fluidized bed material (MEAS.) and equilibriin all cases (see Tables 1a and 1b).


elements in the gas phase as a function of temperature, simulatingthe cooling of Case B product gas from 850 �C to 200 �C.

Table 4 shows a comparison of the condensing temperaturedata of different trace elements obtained in this study againstthe results obtained by Cui et al. [21] and Diaz et al. [20,21]. Theresults seem to be in agreement in the case of some of the traceelements, such as Cd, Hg, Mn, Sb. However, there are many discrep-ancies in the case of some other elements. As indicated in thisstudy, the discrepancies can be attributed to the use of insufficientthermodynamic databases and naturally also to different experi-mental arrangements, such as fuels and sampling methods. The re-sults in Table 4 can also be compared with the classificationproposed by Bunt and Waanders [14]. In that case the results aregenerally in agreement, perhaps with the exceptions of As andPb, which according to our study condense already at temperaturessomewhat above 500 �C. However, despite all differences in differ-ent studies, Table 4 indicates that the earlier classifications pro-posed by Bunt and Waanders [16] and Diaz et al. [20,21] on thecondensation temperature behavior of trace elements in the threemain groups may not be applicable to the gasification of biomassand waste-type fuels.

6. Conclusions

In this paper, the behavior of trace elements under fluidized bedgasification conditions is studied with the help of thermodynamic

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Fig. 7. Weight percentages of condensed metals (EDD-elements and zinc) under fluidized bed gasification conditions of the four pilot plant balances. Comparison betweenthe contents of trace elements in gasification fluidized bed material (MEAS.) and equilibrium calculation results (MODEL, Case B).


equilibrium calculations. Experimental gasification balance datafrom pilot-scale tests of solid recovered fuels, wood pellets anddemolition wood are utilized as inputs and for validation of the cal-culation results. For validation, the streams of trace elements andfuel main ash elements were available from the four different pilottest run balances, including the material streams at the gasificationreactor and in the product gas cooling and filtering system. The fo-cus is on the possibilities for flue gas cleanup with cyclones, filtersor scrubbers to allow for the condensation and removal of the com-pounds containing trace elements before further use of the productgas. The method of removing trace elements by careful condensa-tion is already in use in the commercial waste gasification plant atLahti.

For the development of the thermodynamic database behindthe calculations, thermodynamic data for 29 elements and theircompounds were carefully selected and collected from several dat-abases. This method differs from the work reported previously inthe literature, where one database is usually used. ´The formeddatabase includes data from gas, solid and liquid phases (and liquidsolutions) for the elements and their compounds relevant underreducing gasification conditions.

The calculations were performed stepwise. First the conditionsinside the gasification reactor were studied and then separatelyalso the conditions of the product gas cooling and filtering system.The reporting of the results is concentrated around the ‘‘European


Dirty Dozen’’ (EDD)-metals and zinc. The EDD metals of this reportinclude As, Cd, Cr, Cu, Co, Hg, Mn, Ni, Pb, Sb and V (Tl excluded).From the results it can be observed that the elements of the mainash components, namely Ca, Al and Si, have a significant impor-tance to the equilibrium chemistry.

The equilibrium calculation results were compared with theexperimental balance data around the EDD trace elements andzinc. Under fluidized bed gasification conditions at a temperatureof about 850 �C some discrepancies can be observed, e.g. as withAs, Sb and Zn. The calculations predict that these elements shouldnot occur in the condensed phase, whereas the measurementsindicate that they are present in the gasification fluidized bed.There can be many reasons for the discrepancies, such as samplingand analysis, inaccurate and insufficient data on the relevant com-pounds in the database as well as chemical kinetics or physicalphenomena, such as the deposition of volatile compounds. The cal-culation results at lower temperatures (200–450 �C), typical for theproduct gas cooling and filtering system, were compared with theexperimental trace element stream data. According to the experi-mental results practically all the trace elements except Hg con-dense at the cooled gas conditions at the filter.

The results are also compared with corresponding studies in theliterature. The results of condensation temperatures of traceelements from different studies seem to be in agreement inthe case of certain elements, such as Cd, Hg, Mn, Sb. However,



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Fig. 8. Contents of the elements of interest in the gas phase as a function of gasification temperature for (a) BFB 1, (b) BFB 2, (c) CFB 1 and (d) CFB 2.

Table 4Comparison of results of condensing temperatures of trace elements from different studies. Own: Thermodynamic equilibrium modeling combining five databases, experimentaldata from pilot-scale fluidized bed gasification of waste-type fuels. Cui et al. [21]: experimental data from fly ash samples of two bagasse-type biomass fuels, bench-scale fluidizedbed gasification. Diaz et al. [20,21]: Thermodynamic equilibrium modeling for gasification of coal, using HSC chemistry (version 4.0) software and database.

Temperature (�C) Below 200 200–500 500–650 650–800

As Diaz et al. [20,21] OwnCd Own, Diaz et al. [20,21], Cui et al. [21]Cr Diaz et al. [20,21] OwnCo Cui et al. [21] OwnHg Own, Diaz et al. [20,21]Mn Own, Diaz et al. [20,21]Mo Cui et al. [21]Ni Diaz et al. [20,21] OwnPb Diaz et al. [20,21] Own, Cui et al. [21]Sb Own, Diaz et al. [20,21]V Diaz et al. [20,21] OwnZn Diaz et al. [20,21] Own, Cui et al. [21]


the classifications proposed by Bunt and Waanders [16] and Diazet al. [20] on the evaporation behavior of trace elements may notbe directly applicable to the gasification of biomass and waste-typefuels. There may be several reasons for the differences, such as dif-ferent modeling tools and systems and the experimental condi-tions and sampling methods applied.

Acknowledgements

The financial support of TEKES–The Finnish Funding Agency forTechnology and Innovation (research project PERUSKAASU),Carbona Oy, Condens Oy, Fortum Oil and Gas Oy, Foster WheelerEnergia Oy, Pohjolan Voima Oy, Vapo Oy is gratefully


acknowledged. The ongoing project GASIFREAC is financed by theAcademy of Finland, which support is also gratefully acknowl-edged. Financial support from the Swedish National Research Plat-form Bio4Energy is also acknowledged.

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