Mercury emission and speciation in fly ash from a 35 MWth large pilot boiler of oxyfuel combustion with different flue gas recycle

•The distribution of Hg in a 35 MWth large pilot boiler of oxyfuel combustion was investigated.•Higher concentration of gaseous mercury is contained in oxyfuel combustion flue gas.•The oxyfuel combustion atmosphere enhances the conversion of Hg0 to Hgp.•Trigonal red HgS is the main mercury species in air combustion fly ash.•Mercury bound to organic matter (Hg-OM) and trigonal red HgS are the main species in the oxyfuel combustion fly ash.

The behaviour of mercury is investigated in a 35 MWth large pilot boiler of oxyfuel combustion with dry and wet flue gas recycle (FGR). The mercury species in fly ash particles as well as the involved reaction mechanism are systematically investigated. The results suggest that higher concentration of gas-phase mercury (Hgg) is contained in oxyfuel combustion flue gas. The conversion of Hgg to particulate associated mercury (Hgp) is enhanced in oxyfuel combustion. Different mercury species are presented in fly ash collected from air and oxyfuel combustion: trigonal red HgS is the main mercury species in air combustion; mercury bound to organic matter (Hg-OM) and trigonal red HgS are both dominant in the samples from oxyfuel combustion. The prominent difference is the significantly increase of Hg-OM in oxyfuel combustion atmosphere. The oxyfuel combustion atmosphere would favor the formation of oxygen-rich functional groups, especially the CO group, which is responsible for the adsorption of mercury and formation of Hg-OM in fly ash.