Thermodynamic modelling of the BGLgasification. consideration of alkali metals

Transcription

1 Department of Energy Process Engineering and Chemical Engineering Thermodynamic modelling of the BGLgasification process with particular consideration of alkali metals Stefan Guhl, Bernd Meyer CCT2009 Dresden May 2009 TU Bergakademie Freiberg I Institute of Energy Process Engineering and Chemical Engineering Reiche Zeche I Freiberg I Tel. +49(0)3731/ I Fax +49(0)3731/ evt@iec.tu-freiberg.de I Web

2 Motivation problems regarding volatile ash components, e.g. Na, K, Zn, Pb, Cl, S, in technical processes with reducing atmosphere Entrained-Flow gasification: condensation of volatile ash components on heat exchanger surfaces deposite formation at the raw gas outlet of the BGL-Gasifier internal accumulation of alkalis in blast furnaces affects coke stability, formation of deposit at the refractory lining alkali attack on refractory lining evaluation of the pathway of volatile ash components in the BGL-gasification process by thermochemical calculations 2

3 The BGL - Gasification Process operating pressure: 24 bar raw gas temperature at discharge: 550 C after scrubber: 200 C In: MSW + coal: oxygen: steam: steamoxygen ratio: t/h = MW m³/h STP t/h kg/m³ STP Out: raw gas: Slag: m³/h STP, wf. 3 5 t/h 3

4 Preparation of the Thermochemical Model calculations are based on the Gibbs Energy Minimisation because of several temperature zones and counter-flow of solids and gas within the gasifier interacting equilibrium stages are necessary local equilibrium stages material streams mixer, splitter, iterator interacting of single stages recirculation of volatile ash components between stages is solved by an iterator provides thermodynamic databases data file Thermochemical Model of the BGL-Gasifier 4

5 Preparation of the data file in FactSage TM selection of elements: Ar, Al, C, Ca, Cl, Fe, H, K, Mg, N, Na, O, P, Pb, S, Si, Ti, Zn selection of possible product phases from the databases FACT 5.3, FACToxide, FACTmisc, FACTsalt stoichiometric pure phases ( C_graphite(s), Al 2 O 3 (s) ) gas phase treated as real gas solution phases e.g. ASlag-liq, BAlkCl-ss_rocksalt, BSalt-liquid, Fe-liq data file (database with c p (T), H 298 and S 298 for each stoichiometric substance, parameters for non-ideal solution phases) for equilibrium calculation in SimuSage TM 5

6 Material balance of the BGL-Gasifier 9 samples of feedstock mixture little information about raw gas amounts of main gas components CO, CO 2, H 2, CH 4 known amount and composition of tars, oils, dust particles? 9 samples of slag granule Cu Fe material balance provides amount and elemental composition of tars, oils and dust in the raw gas 6

7 Thermochemical Model 2 isothermal-isobaric equilibrium stages volatilisation of ash components in lower stage and condensation in upper stage, recirculation and accumulation material recirculation solved by iteration tar, oil and dust content of raw gas? assumption of chemical equilibrium composition for all stages? 7

8 Thermochemical Model pyrolysis tars and oils not estimated by chemical equilibrium, considered in bypass ash components of feedstock will not react in upper stage as indicated by chemical equilibrium, assumed as chemical inert in upper stage ash of dust particles in raw gas: condensed volatile ash components partially to raw gas non-volatile ash components considered in bypass split rates depend on condensed phases of the devolatilisation zone and the material balance 8

9 Results composition of slag stream Slag-liquid (91.4 wt.% of slag stream) Iron-liquid (4.8 wt.% of slag stream) Constituents wt.-% Constituents wt.-% SiO Fe CaO C 3.31 Al 2 O Cu 7.07 MgO 2.88 P 4.06 FeO 0.50 FeS 2.06 TiO Na 2 O 2.90 K 2 O 1.03 Carbon (3.8 wt.% of slag stream) CaS 0.61 Constituents wt.-% MgS 0.06 C 100 Na 2 S CaCl NaCl 0.09 MgCl P* *indirectly calculated, not present in solution phase ASlag-liq 9

10 Results volatile species over slag bath partial pressures in bar for species containing: K 0.13 Na Fe 5E-4 Cu 3E-4 Ca 2E-4 Mg 4E-5 partial pressures for alkali species: Species p i in bar Species p i in bar KCl 0.12 NaCl (KCl) (NaCl) K 9.1E-05 Na 5.9E-05 KCN 8.4E-05 NaCN 4.7E-05 KMgCl 3 5.3E-05 NaOH 1.5E-06 KFeCl 3 2.7E-05 NaMgCl 3 1.5E-06 KOH 9.3E-06 NaH 8.2E-07 KH 5.5E-07 Na 2 MgCl 4 3.1E-08 Chlorides are the dominating species for gaseous alkali compounds 10

11 Results - devolatilisation zone condensed phases in devolatilisation zone: BAlkCl-ss_rocksalt (93 wt.-% of condensed phases) Phase constituent wt.-% NaCl 9.1 KCl 90.9 Sum BSalt-liquid (6.3 wt.-% of condensed phases) Phase constituent wt.-% KCl 56.7 NaCl 10.0 MgCl CaCl FeCl Ca 5 HO 13 P 3, Cu 2 S, Spinel, Olivine (0.7 wt.-% of condensed phases) gaseous species in raw gas: Species ppmv KCl 2E-2 NaCl 3E-3 (NaCl) 2 3E-4 (KCl) 2 4E-3 KFeCl 3 2E-2 FeCl 2 6E-3 alkali chlorides dominate the condensed phases minor amounts remain in the gas phase 11

12 Results Distribution of alkalis Feedstock 100 % K 100 % Na Raw gas - gaseous 3E-3 % K 9E-5 % Na Raw gas - dust 53.6 % K 29.2 % Na 255 % K 80 % Na 209% K 9.8% Na Slag 46.4 % K 70.8 % Na 12

13 Conclusion The BGL-gasification process was simulated with thermochemical calculations using the software FactSage TM and SimuSage TM. The model was adjusted to process data and restrictions due to mass transfer and kinetics were considered. The phase separation between slag and iron is well described by the model. The accumulation of alkalis within the gasifier can be estimated, whereas their volatilisation is associated with chlorine. Similar models could be used for the description of the alkali recirculation within blast furnaces. The calculated partial pressures of alkali components within the gasifier could be used for further investigations like interactions of alkali vapours with the refractory lining. 13

14 Thank you for your attention! Stefan Guhl Department of Energy Process Engineering and Chemical Engineering TU Bergakademie Freiberg Reiche Zeche Phone: Freiberg Fax: Germany 14