Technical Bulletin No. 0096: Information on the Sulfur Content of Bark and Its Contribution to SO2 Emissions When Burned as a Fuel (AQTB)

  There has been conjecture about the sulfur dioxide emissions resulting from the burning of wood residues since current New Source Performance Standards for large power boilers permit the use of the Btu in wood residues in calculating allowable sulfur dioxide emission rates. The concern that has arisen is based on available knowledge of the sulfur content of bark and wood residues which, if stoichiometrically converted to sulfur dioxide would account for sulfur dioxide levels of about 130 ppm or greater in the flue gas, depending on the initial sulfur content of the wood residue. Unfortunately this line of reasoning is apparently the background for selection of an emission factor of 0.375 lbs sulfur dioxide per million Btu of wood residue fired that is included in EPA resource documents which are used extensively for estimating and decision-making in the regulatory process. The attached technical bulletin describes a field study carried out to determine the amount of sulfur dioxide generated when burning representative bark.  The study involved a sulfur fuel balance around the combustion device by determining sulfur content of the bark fed, and sulfur leaving the boiler in solid and gaseous forms, namely ash, fly ash, and sulfur dioxide. The study showed that just over 5% of the sulfur contained in the bark was emitted as sulfur dioxide which amounted to 0.0001 to 0.002 lbs sulfur dioxide per million Btu in the bark fed. The remainder of the sulfur was accounted for in the ash combustion products. The findings support the conclusion that the sulfur content of wood is not stoichiometrically converted to sulfur dioxide, probably because it is present mainly in sulfate form.