(155b) Kinetic Modeling of Gas-Phase Mercury Oxidation by Chlorine and Bromine In Combustion Effluents During Oxy-Combustion

Around 29 Gt of CO₂ are emitted annually worldwide. The main sources of these emissions are transportation and electricity-generating plants, which use coal and fuel combustion. Since CO₂ capturing techniques are still expensive, in order to reduce CO₂ emissions from fuel combustion, oxy-combustion has been proposed as a solution, with the aim of obtaining combustion flue gases with higher concentrations of CO₂ that are easier to remove. The implementation of oxy-combustion affects the speciation of the rest of the combustion products, including the speciation of mercury. Different techniques have been proposed for the control of mercury emissions, due to its high toxicity. In this work, the addition of halogens (chlorine and bromine) to the flue gases have been studied in order to oxidize mercury from its elemental form to oxidized form – HgX_2.  As these mercury halides are water soluble, it is feasible to capture the HgX using conventional air pollution control devices. An elementary reaction mechanism has been developed for modeling the oxidation of mercury by the addition of halides, in both regular combustion and oxy-combustion conditions. Thermochemical properties and kinetic parameters have been calculated for Hg and Halogen – NO_x species using B3LYP and CBS-QB3 methods. The elementary reaction mechanism has been constructed with use of chemical activation analysis for association and addition reactions with quantum RRK analysis for k(E) and Master Equation analysis for fall-off. Results indicate that the presence of NO_x inhibits Hg oxidation by chlorine addition, through several catalytic cycles for loss of Cl.  As for bromine addition, similar catalytic cycles are observed, but in this case, the overall thermochemistry favours more mercury oxidation. Therefore, results show that during oxy-combustion conditions, Hg oxidation is enhanced, since flue gases contain lower concentrations of NO_x.