Thermodynamic prediction of the effect of repeated recirculation of cooled flue gases on the content of major, minor, and trace compounds in oxy-coal combustion products

Abstract

This study investigates changes in the composition of oxy-coal combustion products resulting from the recirculation of cooled flue gas (FGR) at 20, 60, 100, 200, and 300 oC^{\ o}C oC (containing 70–95 mol% CO2).  It presents the results of thermodynamic calculations describing changes in the content of the major, minor, and trace components of flue gases, ash and condensate. The results reflect a scenario of starting the oxy-coal combustion system in a fluidized bed boiler using low-purity oxygen from an air separation unit.  This work demonstrates that in FGR loop the major species, i.e., Ar and N2, as well as the minor, e.g., Cl2, PbCl4, HgCl2, and CrOCl3, are accumulated. After nine FGR loops cooled to 300 oC^{\ o}C oC , marked increases in concentrations were observed: ZnCl2 and HCl (3-fold), as well as CrO2(OH)2 (2.5-fold). The ash that was formed contained, among others, CaSO4, SiO2, CaMgSi2O6, MgSiO3, ZnFe2O4, and MgCr2O4, whose mass changed in successive reactors as a result of the repeated FGR. Depending on the temperature of the cooling reactor, flue gases were subjected to recirculation and the main component of the condensate was H2O or H2SO4· 6H2O. The condensate contained chloride salts, e.g., PbCl2, KCl, and ZnCl2, as well as sulfate salts, i.e., K2SO4 and Na2SO4, in smaller amounts. A consequence of the nine-fold FGR cooled to T≤200 oCT\le200\ ^oCT≤200 oC was, among others, a percentage mass increase in ZnCl2 in the condensate. The less cooling applied to flue gases, the more likely the occurrence of sulfates was in the condensate.