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Reactions and Transformations of Mineral and Nonmineral Inorganic Species during the Entrained Flow Pyrolysis and CO2 Gasification of Low Rank Coals

Tanner, Joanne, Bläsing, Marc, Müller, Michael, Bhattacharya, Sankar
Energy & Fuels 2016 v.30 no.5 pp. 3798-3808
aluminum, calcium, calcium carbonate, calcium oxide, calcium sulfate, carbon, carbon dioxide, coal, ferric oxide, gasification, magnesium, magnesium oxide, nitrogen, phase transition, pyrolysis, silica, silicon, temperature, thermodynamics
The reactions and transformations of mineral and nonmineral inorganic species in Victorian (MOR) and Rhenish (HKT) coals were investigated in a two-stage process under high temperature, entrained flow pyrolysis, and gasification conditions. The parent coals were pyrolyzed at a temperature between 1100 and 1400 °C in 100 vol % nitrogen. The resulting char samples were collected and gasified at their corresponding pyrolysis temperatures in 10–80 vol % CO₂ in N₂. Low temperature (500 °C) ash subsamples from the parent coals, chars, and gasification residues were analyzed for elemental and mineral phase composition. The phase composition analysis was in agreement with the proportions of various inorganic constituents in the elemental analysis. In general, the extent of reaction and phase transformation increased with increasing temperature and carbon conversion, which is related to increasing temperature and CO₂ concentration. The char elemental and phase compositions were similar to those of the corresponding parent coal and consisted predominantly of SiO₂, CaSO₄, and CaCO₃ with minor amounts of MgO and Fe₂O₃ in the MOR samples. Char gasification resulted in consistently increasing reaction and transformation trends, which indicates that thermodynamic equilibrium was not reached. Low temperature gasification of MOR and HKT char samples resulted predominantly in thermal decomposition of CaSO₄, retention of CaCO₃ due to recarbonation, and formation of MgO. The ash composition at high temperature differed based on the amounts of and reactions between various parent coal inorganic constituents. In particular, the fate of Ca and Mg differed markedly between the two coals. For MOR, decomposition of MgO resulted in depletion of Mg at high temperatures, whereas Mg was retained in HKT gasification residues as MgAl₂O₄ and Ca₂MgSi₂O₇ due to higher Si and Al content. CaO from CaSO₄ and CaCO₃ decomposition was retained in MOR samples as Ca₂Fe₂O₅ and Ca₂SiO₄, and in HKT as Ca₂MgSi₂O₇.