Chlorine in Coal Combustion and Cofiring
AUTHOR: Robert M Davidson
DATE: November 2005
This report considers two main topics concerning the chemistry of chlorine in coal combustion. One is mercury in coal combustion flue gas and the possible role of chlorine in its abatement or capture. The second is cofiring coal with biomass and wastes. This has raised questions concerning potentially greater problems with fouling and corrosion, especially when cofiring straw, and possible higher emissions of chlorinated organic compounds, especially when cofiring municipal solid waste (MSW) and refuse-derived fuels (RDF). It now seems to be a generally accepted belief that the chlorine content of the coal controls the oxidation state of mercury in coal combustion flue gas.
The report examines the role of coal chlorine on the speciation of mercury in coal combustion flue gas. Firstly, the experimental and plant data are examined to determine whether they support a strong effect of coal chlorine on the speciation and capture of mercury in flue gas. Although there has been much work on the modelling of the gas phase mercury chemistry based on the kinetics of homogeneous reactions, there is little, or no, evidence on the extent to which these reactions take place in coal combustion flue gas. The model predictions based on both homogeneous and heterogeneous mechanisms are examined to see how well they replicate the available data. It is concluded that coal chlorine is one of the factors that affect mercury oxidation but it does not seem to be the determining factor. The evidence for chlorine induced corrosion shows that cofiring coal with high chlorine biomass fuels can produce an overall benefit. Not only does coal protect from corrosion, it also appears to reduce potential emissions of PCDD/Fs.