Homogeneous oxidation of mercury during combustion: A comparison between combustion in air and oxy-combustion

Since oxy-combustion is a relatively new technology, many problems associated with the behavior of coal in this novel combustion process still need to be solved. In particular, there is a lack of knowledge regarding the behavior of toxic trace elements, and there is a need to determine whether trace element distribution among the by-products and emissions to the environment, is comparable with their conduct during combustion in air. Recent work (1) suggests that some differences can be expected regarding environmental and technological problems. Mercury is the trace element whose behavior in oxy-combustion is attracting most interest because it is well known that coal-fired power plants are the greatest source of mercury emissions to the environment (2-3). If the composition of the mercury species in gas phase during oxy-fuel combustion differed from combustion in air, the efficiency of mercury retention by fly ashes or the capture in Flue Gas Desulfuration Systems (FGD) (if any) might differ when compared with combustion in air. Furthermore, the use of control systems of NOx (if any) and the presence of a flue gas enriched in oxygen could modify the common partitioning of this pollutant. Apart from the distribution of mercury among ashes and by-products from desulfuration processes, and the potential emissions of mercury to the air, mercury might also cause operational problems in oxy-combustion plants as it could accumulate in the CO2 in the processing unit (4). Differences in the behavior of mercury in oxy-combustion processes can be expected with respect to the air coal combustion processes. Hence there is a need to understand the speciation of mercury that occurs in the flue gas and that will condition mercury retention in gas cleaning systems. The present work focuses on the impact of different proportions of O2 and CO2 in combination with NOx, SO2, H2O and HCl present in flue gases, upon mercury speciation.