Combustion efficiency and emissions analysis for a school wood energy system in interior Alaska

Abstract

A recent expansion in wood energy use at schools in Alaska has resulted in more than a dozen wood energy systems in operation. However, few have been evaluated for fuel efficiency and pollution impacts, both of which can be examined via combustion gas analysis. In this research, we monitored the wood energy system at a public school during winter heating conditions. Wood energy parameters were sampled on three occasions during early, mid, and late winter in northern Alaska. Combustion gas was sampled for a range of parameters that indicated boiler performance, including gas emmissions of oxygen (O₂), carbon dioxide (CO₂), carbon monoxide (CO), excess air, combustion efficiency, and stack temperature, which were monitored over 6 days. We observed differences in combustion gas composition between seasons as well as the response of combustion efficiency to gas concentrations. Combustion efficiency most strongly correlated with excess air (R² = 0.693), but poorly correlated with stack temperature (R² = 0.005). The primary combustion gases (O₂, CO₂, and CO) were moderately correlated with combustion efficiency (with R² values of 0.40, 0.56, and 0.55, respectively). Seasonal differences were found between early, mid, and late winter, with generally less variation in combustion gas contents occurring during late winter. Mean combustion gas concentrations also varied with heating season. In all cases, mid-winter means were significantly different than early and late winter values. This research found that more efficient combustion of wood fuels should lead to cost savings, especially during early and late heating seasons. The findings should also be relevant to those of other wood-energy-using schools (in Alaska and elsewhere) that experience severe mid-winter conditions coupled with milder shoulder seasons.

Citation

Nicholls, David L.; Huang, Daisy. 2020. Combustion efficiency and emissions analysis for a school wood energy system in interior Alaska. Res. Pap. PNW-RP-616. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 18 p.