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Equilibrium modelling of trace element behaviour in fluidized bed combustion and gasification of coal
J. Aerosol Sei., Vol. 26. Suppl I, pp. $687-$688, 1995 Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00
~- -D e r ~ f l m o n
EQUILIBRIUM MODELLING OF TRACE ELEMENT BEHAVIOUR IN FLU1DIZED BED COMBUSTION AND GASIFICATION OF COAL Jussi Lyyranen and Jorma Jokiniemi VTI" Energy, Aerosol Technology Group, FIN-02044 VTI', Finland Wahab Mojtahedi Enviropower Inc., FIN-02150 ESPOO, Finland Jari Koskinen A. Ahlstrom Corp., FIN-48601 KARHULA, Finland KEYWORDS Modelling, trace elements, fluidized bed, combustion, gasification ABSTRACT The behaviour of the total 17 elements (As, Be, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, Sn, Ti, V and Zn) and their compounds during pressurized fluidized bed combustion and gasification (p=12 or p=20 bar, T=380-1380 °C) of coal were modelled using the ChemSage 3.0 equilibrium (eq) model. At this stage no intermetallic compounds were taken into account. The effect of silicates/minerals on the behaviour of trace elements can also be remarkable. It is possible that the reactions with silicates will bind them to the silicates so strongly that they are unable to vaporise/react further. Therefore, the reactions of the trace elements with mineral species has been taken into account via the input amount of the trace elements given for the eq calculations. Thus only a small fraction of the coal containing trace elements are allowed to volatilize. Based on the eq calculations the considered elements can be divided into four different groups: 1. Highly volatile: do not condense in the temperature range considered 2. Volatile: condense at about 500-600 °C 3. Slightly volatile: condense at about 800-900 °C 4. Not volatile: do not volatilize at all in the temperature range considered The classification of the 17 trace elements into four different groups based on their volatility is presented in table 1. Some general conclusions can be drawn from the table: As, Hg, Sb and Se seem to be very volatile in pressurized gasification. Only Fig seem to be very volatile in pressurized combustion. Co, Mn, Ti and V do not seem to be volatile at all. Cd, Pb, Sn and Zn seem to be more volatile in pressurized gasification than in combustion. The volatilization of Cr seems to increase when the ~, coefficient (relative air/fuel ratio) increases. Table 1. The classification of the 17 trace elements into four groups based on their volatility. Volatility Pressurized combustion, Pressurized gasification, Pressurized gasification, 12 bar, ~,=1.15 12 bar, ~,=0.9 20 bar~ ~,=0.53 Highly volatile H8 As, Fig, Sb As, Fig, Sb, Se Volatile As, Cr Cd, Pb, Sn Cd, Pb, Zn Slightly volatile Cd, Cu, Pb, Ni, Sb, Sn Cr, Cu, Ni Be, Cu, Mo Not volatile Co, Mn, Tit V Co, Mn, Ti, V Cr, Co, Mn, Ni, V When interpreting the eq calculation results it should be remembered that the results are valid only if the equilibrium is attainable. Chemical kinetics, mass transfer etc. can have a major effect on attaining equilibrium in real systems. The possible chemical surface reactions can also change $687
$688
J. LYYR,~NENet
al.
the formation of condensable species. In this work the condensation of gaseous species is interpreted so that whenever the gaseous species condenses as a different species as it initially was present in the gas phase the condensation must include a surface reaction step. These surface reactions usually have several individual steps each of which can be rate determining. The following steps can be separated: mass transfer (diffusion) of gaseous reactant to particulate surface, adsorption of reactant to the particulate surface and reaction to the final product species. The reaction mechanisms of surface reactions can not be described by eq model and need a more specific model. As an example of the eq calculation results the behaviour of antimony in the pressurized fluidized bed combustion (p=12 bar) of coal is presented in fig. 1. Sb appears mainly as SbCl(g) and SbO(g) in the gas phase. The condensation of antimony species starts at about 1120 °C mainly as antimony tetraoxide (SbzO4(cd)).This oxide forms probably through the surface reaction of adsorbed SbCl(g) and SbO(g) on particulate matter with the oxygen present in the gas atmosphere. According to eq model SbzO4(cd) would further oxidize to antimony pentaoxide (Sb2Os(cd)). This reaction can also be kinetically controlled and involve a surface reaction step. The extent of this reaction may be slow because of the low temperature (about 580 °C). 1.00E-03 L...,
SbO(g)
1.00E-04
SbCl(u) "" _
~ 1.00E-05 ~ 1.00E-06
___-------
~ 1.00E-07
SbO2(ed) /
8b204(ed)
-
8b205(ed)
d'
~ 1.00E-08 1.00E-09 1.00E-10 380
1280 1180 1080
980 880 780 Temperature [°C]
680
580
480
380
Fig. 1. The equilibrium amounts of antimony in pressurized fluidized bed combustion of coal, p=12 bar, ~=1.15 and antimony volatility 10 %. The amount of antimony in input in the gas phase as Sb(g) was 8.2.10 * tool. ACKNOWLEDGEMENTS This work has been a part of the Finnish combustion and gasification research program LIEKKI 2. The financial support ofEnviropower, Ahlstr6m and IVO during this project is acknowledged. Also Dr. Joseph Helble is acknowledged for the experimental information on the volatility of trace elements. REFERENCES 1. Clarke, L.B. (1993) The fate of trace elements during coal combustion and gasification: an overview. Fuel 72, 731.
~- -D e r ~ f l m o n
EQUILIBRIUM MODELLING OF TRACE ELEMENT BEHAVIOUR IN FLU1DIZED BED COMBUSTION AND GASIFICATION OF COAL Jussi Lyyranen and Jorma Jokiniemi VTI" Energy, Aerosol Technology Group, FIN-02044 VTI', Finland Wahab Mojtahedi Enviropower Inc., FIN-02150 ESPOO, Finland Jari Koskinen A. Ahlstrom Corp., FIN-48601 KARHULA, Finland KEYWORDS Modelling, trace elements, fluidized bed, combustion, gasification ABSTRACT The behaviour of the total 17 elements (As, Be, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, Sn, Ti, V and Zn) and their compounds during pressurized fluidized bed combustion and gasification (p=12 or p=20 bar, T=380-1380 °C) of coal were modelled using the ChemSage 3.0 equilibrium (eq) model. At this stage no intermetallic compounds were taken into account. The effect of silicates/minerals on the behaviour of trace elements can also be remarkable. It is possible that the reactions with silicates will bind them to the silicates so strongly that they are unable to vaporise/react further. Therefore, the reactions of the trace elements with mineral species has been taken into account via the input amount of the trace elements given for the eq calculations. Thus only a small fraction of the coal containing trace elements are allowed to volatilize. Based on the eq calculations the considered elements can be divided into four different groups: 1. Highly volatile: do not condense in the temperature range considered 2. Volatile: condense at about 500-600 °C 3. Slightly volatile: condense at about 800-900 °C 4. Not volatile: do not volatilize at all in the temperature range considered The classification of the 17 trace elements into four different groups based on their volatility is presented in table 1. Some general conclusions can be drawn from the table: As, Hg, Sb and Se seem to be very volatile in pressurized gasification. Only Fig seem to be very volatile in pressurized combustion. Co, Mn, Ti and V do not seem to be volatile at all. Cd, Pb, Sn and Zn seem to be more volatile in pressurized gasification than in combustion. The volatilization of Cr seems to increase when the ~, coefficient (relative air/fuel ratio) increases. Table 1. The classification of the 17 trace elements into four groups based on their volatility. Volatility Pressurized combustion, Pressurized gasification, Pressurized gasification, 12 bar, ~,=1.15 12 bar, ~,=0.9 20 bar~ ~,=0.53 Highly volatile H8 As, Fig, Sb As, Fig, Sb, Se Volatile As, Cr Cd, Pb, Sn Cd, Pb, Zn Slightly volatile Cd, Cu, Pb, Ni, Sb, Sn Cr, Cu, Ni Be, Cu, Mo Not volatile Co, Mn, Tit V Co, Mn, Ti, V Cr, Co, Mn, Ni, V When interpreting the eq calculation results it should be remembered that the results are valid only if the equilibrium is attainable. Chemical kinetics, mass transfer etc. can have a major effect on attaining equilibrium in real systems. The possible chemical surface reactions can also change $687
$688
J. LYYR,~NENet
al.
the formation of condensable species. In this work the condensation of gaseous species is interpreted so that whenever the gaseous species condenses as a different species as it initially was present in the gas phase the condensation must include a surface reaction step. These surface reactions usually have several individual steps each of which can be rate determining. The following steps can be separated: mass transfer (diffusion) of gaseous reactant to particulate surface, adsorption of reactant to the particulate surface and reaction to the final product species. The reaction mechanisms of surface reactions can not be described by eq model and need a more specific model. As an example of the eq calculation results the behaviour of antimony in the pressurized fluidized bed combustion (p=12 bar) of coal is presented in fig. 1. Sb appears mainly as SbCl(g) and SbO(g) in the gas phase. The condensation of antimony species starts at about 1120 °C mainly as antimony tetraoxide (SbzO4(cd)).This oxide forms probably through the surface reaction of adsorbed SbCl(g) and SbO(g) on particulate matter with the oxygen present in the gas atmosphere. According to eq model SbzO4(cd) would further oxidize to antimony pentaoxide (Sb2Os(cd)). This reaction can also be kinetically controlled and involve a surface reaction step. The extent of this reaction may be slow because of the low temperature (about 580 °C). 1.00E-03 L...,
SbO(g)
1.00E-04
SbCl(u) "" _
~ 1.00E-05 ~ 1.00E-06
___-------
~ 1.00E-07
SbO2(ed) /
8b204(ed)
-
8b205(ed)
d'
~ 1.00E-08 1.00E-09 1.00E-10 380
1280 1180 1080
980 880 780 Temperature [°C]
680
580
480
380
Fig. 1. The equilibrium amounts of antimony in pressurized fluidized bed combustion of coal, p=12 bar, ~=1.15 and antimony volatility 10 %. The amount of antimony in input in the gas phase as Sb(g) was 8.2.10 * tool. ACKNOWLEDGEMENTS This work has been a part of the Finnish combustion and gasification research program LIEKKI 2. The financial support ofEnviropower, Ahlstr6m and IVO during this project is acknowledged. Also Dr. Joseph Helble is acknowledged for the experimental information on the volatility of trace elements. REFERENCES 1. Clarke, L.B. (1993) The fate of trace elements during coal combustion and gasification: an overview. Fuel 72, 731.