A kinetic investigation of unimolecular reactions involving trace metals at post-combustion flue gas conditions

Abstract

Environmental contextUnderstanding trace metal speciation in coal combustion flue gases is imperative to the design of effective capture technologies to prevent their release into the atmosphere. Unfortunately much of the kinetics that dictate trace metal speciation are not known and the current study focuses for the first time on the kinetics for three reactions involving mercury and one involving selenium. Rate constant expressions are provided over a broad temperature range (i.e. 298–2000 K), indicative of post-combustion flue gas conditions. AbstractAb-initio methods were carried out to calculate forward and reverse rate constant data for the following reactions: Hg + Cl2 ↔ HgCl2, HgCl + Cl ↔ HgCl2, Hg + O ↔ HgO, and Se + H2 ↔ SeH2. Theoretical predictions of bond distances, vibrational frequencies and enthalpies of reaction are compared to available experimental data to determine the level of theory most appropriate for predicting kinetic parameters. The pseudopotentials ECP60MDF and RECP60VDZ were used for mercury in combination with B3LYP or QCISD(T) methods whereas the complete 6–311++G(3df,3pd) Pople basis set with the CCSD(T) method was used for selenium. Potential energy curves for each reaction were constructed and a variational approach along with RRKM theory was used to predict rate constants from 298 to 2000 K. Reactions HgCl + Cl ↔ HgCl2 and Hg + O ↔ HgO were found to have a strong negative temperature dependence, whereas the insertion reactions Hg + Cl2 ↔ HgCl2 and Se + H2 ↔ SeH2 were found to proceed very slowly with large pre-exponential factors.