Environmental aspects of wood residue combustion in forest products industry boilers

We conducted a comprehensive review of air emissions resulting from burning wood residues in industrial boilers and potential methods to control these emissions. This report compares average emissions with similar data published by the U.S. Environmental Protection Agency for the burning of fossil fuels coal, oil, and natural gas in industrial boilers. As compared with coal or oil combustion, wood combustion in boilers generally leads to lower emissions of trace metals, hydrochloric acid, sulfur dioxide (SO2), and nitrogen oxides (NOx); higher emissions of carbon monoxide, polyaromatic hydrocarbons, and total volatile organic compounds; and comparable emissions of particulate matter and polychlorinated dibenzo-dioxins and -furans (PCDDs/Fs) (both of which are highly dependent on the efficiency of the ultimate particulate matter control device). Most importantly, wood combustion is carbon dioxide-neutral, a distinct advantage over fossil fuel combustion. Firing wood in stoker units with sulfur-containing fuels, such as coal and oil, leads to a reduction in expected SO2 emissions because of the high carbon and alkali content of most wood ash, and cofiring wood with coal also has some benefits for NOx reduction. This report also discusses the generation and types of combustion ashes resulting from wood burning in mostly combination boilers in the United States and Canada, and provides an overview of ash management practices and the salient characteristics of such ashes relative to their trace metal, organic, and PCDD/F contents.