Special Report No. 09-02: Sulfur Capture in Combination Bark Boilers – An Update

Factors affecting the in situ sulfur capture within combustion units that cofire biomass and sulfur-containing fuels or waste gases are examined. A review of published literature and analysis of test results for several combination boilers and pilot/laboratory scale furnaces suggests significant capture of the fuel sulfur occurs in bark/biomass-fired boilers of the grate design while little capture occurs in fluidized bed combustors and utility boilers that burn mainly coal with small amounts of wood. The most likely explanation for these observations is a mechanism involving adsorption of sulfur dioxide and oxygen on the activated-carbon-like surfaces of combustion ashes generated in grate-fired bark/biomass boilers, followed by oxidation of sulfur dioxide to sulfur trioxide on this surface and subsequent reaction with adsorbed moisture (water) to form sulfuric acid.