Oxy-fuel combustion effects on trace metals behaviour by equilibrium calculations

摘要

Oxy-fuel combustion is a promising technology for the cogeneration of electricity and a rich stream of CO_2 subjected to direct sequestration. The combination with renewable fuels as biomass could contribute to a large extent in CO_2 reduction in comparison with fossil fuels and therefore, to reduce the greenhouse effect. In this study, the analysis of thermodynamical equilibrium calculations (HSC 6.1) based on Gibbs energy minimization, was employed to compare the environmental impact of trace elements (TEs) emissions in oxy-fuel combustion with that in conventional combustion. Different pure fuels and coal/bio-mass blends were studied under air and oxy-fuel conditions (70%CO_2-30%O_2 and 60%CO_2-40%O_2). The fate of highly volatile trace metals such as Hg, As, Cd and Se, for which Directive 2000/76/EC has established emission limits, was evaluated. The results of this work indicate that, total mercury and cadmium vaporization have been seen to increase during oxyfiring, although results are strongly influenced by the fuel composition. The main differences encountered in TEs speciation during oxyfiring, in contrast to conventional combustion, are related to variations produced in chlorine speciation in the combustion atmosphere (specially with biomass fuels) and also, to the large excess of CO_2 in the oxy-fuel atmosphere which could interact with ash forming compounds, promoting variations in trace metals volatilization.