Fate of Mercury in Volatiles and Char during in Situ Gasification Chemical-Looping Combustion of Coal

Abstract

Mercury emission is an important issue during in-situ gasification chemical-looping combustion (iG-CLC) of coal. This work focused on experimentally “isolating” two elementary subprocesses (coal pyrolysis and char gasification) during iG-CLC of coal, identifying mercury distribution within the two subprocesses, and examining the effects of a hematite oxygen carrier (OC) on the mercury fate. The mercury measurement accuracy was carefully ensured by comparing online measurements (by a VM 3000 instrument) and benchmark measurements (by the standard Ontario Hydro Method, ASTM D6784) as well as repeated tests (10 times for each case). The mercury mass balance was 115% for the entire iG-CLC. A total of 44.7% of the mercury was released as the gas phase form within the coal pyrolysis process at a typical CLC operation temperature (950 °C), whereas 13.4% was released during the char gasification process. The release rate and amount of mercury were minimally affected by the presence of OC; however, the OC promoted the conversion of Hg0(g) to Hg2+(g). Only a small amount of mercury was absorbed by the OC and transported into the air reactor along with carbon residue, released as Hg0(g) and Hg2+(g) or remained in the OC and coal ash as particulate mercury.