Mercury emissions from coal combustion in fluidized beds under oxy-fuel and air conditions: influence of coal characteristics and O2 concentration

Abstract

In this work, the fate of mercury in a bubbling fluidized bed combustor working under oxy-combustion conditions has been studied and compared with air combustion. The influence of burning three different rank coals, with sulphur content ranging from 0.65 % to 5.17 %, on Hg partitioning has been studied. The presence of limestone as sorbent for SO2 capture as well as the concentration of O2 at the entrance of the combustor were also evaluated. Coal rank does not have a direct influence on coal partitioning. However, sulphur content of the coal is an important parameter to describe not only the high percentage of particle-bound mercury, up to 87 %, in presence of limestone but also the prevalence of Hg(0) as the main species in the gas phase when burning the lignite. The O2/CO2 ratio has little effect on Hg partitioning in experiments carried out in presence of limestone at 925 ºC (the optimum temperature for sorbent sulphating under oxy-combustion conditions) and little differences are found with air combustion at 850 ºC (the optimum temperature for sorbent sulphating under air combustion conditions). Percentage of particle-bound mercury shows a maximum at 925 ºC independently of the coal studied, which it is related with the maximum sulphur retention for each coal at this temperature.