 to minimize air quality impacts using best available control technology (BACT).
Particulates and Health Impacts From Wood Combustion
Particulate matter is the solid matter or very fine droplets that result from combustion or through secondary physical and chemical conversions in the atmospheric. Visible at larger sizes and invisible in its smallest form, particulates are the air pollutant of greatest concern when considering wood combustion. Particulate matter is a generic term and is defined without regard to chemical form in the Clean Air Act (Section 109). Small particulates range in size from 10 micrometers to 2.5 microns (PM10, PM2.5) and less in diameter. The smallest particles cause greatest human health concern, since small particles can lodge deep within the lungs when inhaled and remain airborne for longer periods and distances.
Many wood-fired boilers employ a “tall” stack of sufficient height to disperse emissions and reduce ground-level concentrations of particulate matter (and other pollutants) to mandated levels. Elaborate cyclonic and electrostatic precipitators (dry and wet) scrub the exhaust and collect particulates in the modern wood-fired operations. Therefore, most references to older wood burning systems may overstate wood combustion emissions relative to corresponding gas and oil systems of the same period. Specific wood-fired operations use the best available control technology (BACT) to ensure that they meet emission limits, prevent pollutant and associated deterioration of air quality.
Wood energy is the only traditional industrial fuel that does not substantially add to atmospheric carbon dioxide in the long term, if wood supply comes from sustainable sources and forest acreage is maintained. In the context of greenhouse gases, wood presents an advantage over all of the other fuel sources discussed in this article. While the combustion of wood and fossil fuels result in carbon dioxide and water vapor, over time the carbon stored in solid wood, roots, limbs and soil can offset the carbon release in energy production. With wood combustion there is a release of carbon to the atmosphere, which is later taken up in subsequent tree growth.
Carbon dioxide release from fossil fuel combustion – oil, natural gas and coal – represent ancient carbon deposits (millions of year old), the carbon from wood combustion differs in that it is derived from recently living plant matter. Therefore, the carbon emitted from burning wood is reabsorbed as new trees grow. As long as trees are replanted at least as fast as wood is burned, wood-to-energy power plants will not significantly increase greenhouse gas levels. Fossil fuels emissions, in contrast, cannot be compensated by the same tree growth since there is not enough land to plant in trees necessary to compensate for the worldwide burning of fossil fuels.
Since fuel and transportation costs comprise the largest operational costs associated with combustion-based steam electrical generation plants, wood’s rather low-energy content (compared to fossil fuels) puts it at a disadvantage in competitive markets. Pelletization helps reduce bulk and increase energy density. Also experimentation with torrification -a thermal process to convert biomass into a coal-like material,- yieldis better fuel characteristics than the original biomass. Admittedly, there is not enough wood supply to supply the total energy needs of society, so wood will likely be an interim solution as other renewable technologies and low-carbon energy sources become economically viable.
Adapted from: Air Emissions from Modern Wood Energy Systems Biomass Energy Research Center factsheet. 2 p.
Publication date: Jan. 1, 2019
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