Ash partitioning during the oxy–fuel combustion of lignite and its dependence on the recirculation of flue gas impurities (H2O, HCl and SO2)

Oxy–fuel combustion of a brown coal (i.e. lignite) has been carried out at 1000 °C to experimentally examine the vaporisation of organically bound metals and the agglomeration of ash particles as a function of the concentration of gaseous impurities including H2O, HCl and SO2 in ∼27% O2 balanced with CO2. The properties of bulk ash and individual metals were investigated intensively. Particularly, attention was paid to Na which is notorious for fouling and to organically bound Al which has been less studied. The results indicate that, the organically bound metals, although possessing a very low content in the raw coal, are vital for the agglomeration of ash particles, which are also highly sensitive to the loading of gas impurities in flue gas. HCl recirculation is the most crucial factor promoting the vaporisation of metals via chlorination. Apart from alkali metals, the organically bound Al and Ti were also vaporised noticeably. Recirculation of SO2 promoted the sulfation of Na to condense into liquid droplet which increased fine ash yield. Co-existence of bulk HCl and SO2 played a synergetic role in the sufation of Na via an initial chlorination of the char-bound Na. In contrast, co-existence of steam with HCl and SO2 favored the formation of Na alumino-silicates, which are favorable for ash agglomeration.