Fundamentals of Mercury Oxidation in Flue Gas

Abstract

The objective of this project was to understand the importance of and the contribution of gasphase and solid-phase coal constituents in the mercury oxidation reactions. The project involved both experimental and modeling efforts. The team was comprised of the University of Utah, Reaction Engineering International, and the University of Connecticut. The objective was to determine the experimental parameters of importance in the homogeneous and heterogeneous oxidation reactions; validate models; and, improve existing models. Parameters studied include HCl, NOx, and SO 2 concentrations, ash constituents, and temperature.The results suggested that homogeneous mercury oxidation is below 10% which is not consistent with previous data of others and work which was completed early in this research program. Previous data showed oxidation above 10% and up to 100%. However, the previous data are suspect due to apparent oxidation occurring within the sampling system where hypochlorite ion forms in the KCl impinger, which in turn oxidized mercury. Initial tests with entrained iron oxide particles injected into a flame reactor suggest that iron present on fly ash particle surfaces can promote heterogeneous oxidation of mercury in the presence of HCl under entrained flow conditions.Using the data generated above, with homogeneous reactions accounting for less than 10% of the oxidation, comparisons were made to pilot-and full-scale data. The results suggest that heterogeneous reactions, as with the case of iron oxide, and adsorption on solid carbon must be taking place in the full-scale system.Modeling of mercury oxidation using parameters from the literature was conducted to further study the contribution of homogeneous pathways to Hg oxidation in coal combustion systems. Calculations from the literature used rate parameters developed in different studies, in some cases using transition state theory with a range of approaches and basis sets, and in other cases using empirical approaches. To address this, rate constants for the entire 8-step homogeneous Hg oxidation sequence were developed using an internally consistent transition state approach. These rate constants when combined with the appropriate sub-mechanisms produced lower estimates of the overall extent of homogeneous oxidation, further suggesting that heterogeneous pathways play an important role in Hg oxidation in coal-fired systems.3