Prediction of distribution of trace elements under Oxy-fuel combustion condition using Victorian brown coals

Abstract

In reducing CO2 emissions from the energy sector, power generation using Oxy-fuel combustion has become a promising technology. The issue of trace element emissions during coal combustion, however, has important implication on the operation of Oxy-fuel combustors using different types of coals, as the inclusion of excessive amounts of these elements in the gas is harmful to the health and environment with additional implications for CO2 transport and storage. Though a number of studies have been carried out to investigate the distribution of trace elements in air combustion, limited data are available on Oxy-fuel combustion for different grades of coals. In this study, thermodynamic equilibrium calculations, using FactSage software, were performed to predict the trace elements (Cr, As, Se and Hg) speciation during Oxy-fuel combustion of Victorian brown coal at different temperatures (800–1400°C). These predicted trace element emissions from Oxy-fuel combustion were also compared with that from air combustion. It was found that the amount of toxic hexavalent chromium (Cr6+) species, such as CrO2(OH)2, in the vapour phase was greater for Oxy-fuel combustion than that for air combustion. This is due to the presence of water vapour in the Oxy-fuel atmosphere. At low temperature the distributions of toxic arsenic and selenium species in Oxy-fuel combustion showed almost the same behaviour as in air combustion. The model also indicates that the mercury emissions were marginally greater for Oxy-fuel combustion than for air combustion. These results are important for the selection of operating conditions to the emitted trace elements within permissible limits.