Fluctuating fossil fuel costs and concerns about climate change have thrust low-cost, domestic renewables, like wood, into the energy spotlight. The enactment of North Carolina’s Renewable Energy Portfolio has increased the interest and opportunities to burn wood fuel to make electricity, heat, and steam. Favorable policies that incentives heat and energy production have facilitated a right-sized domestic market for wood energy. This factsheet reviews the air quality impacts of supplementing fossil fuels with woody biomass and current regulation on emissions from wood-fired plants.
Complete Combustion and Heat Capture Is Key
Burning wood in a power plant or industrial operation is among the cleanest solid energy sources currently available. Complete and high temperature combustion under controlled conditions ensures low emissions from wood energy. Regulations require the monitoring of stack gases and particulate emissions to limit and reduce the amount of pollution.
Some people may envision smoky wildfires, wood stoves and residential fireplaces when they think of wood energy. But today’s wood burning facilities are light years ahead of the 1970s version wood stoves, standard fireplaces and wood fires which can pose problems for local air quality.
Variations in wood combustion efficiency result from age of equipment, design and variations in moisture, fuel size, combustion temperature and air mixture. When wood burns incompletely or slowly it emits smoke, ash, odors and windborne particulates that can impact regional air quality and human health.
Industrial Wood Energy
Industry is by far the largest user of wood energy, including power plants, factories, industrial facilities (especially paper mills). Fossil fuel conversion to wood energy has been motivated by cost-savings and availability of woody residues. Most wood-processing facilities fully utilize their internal residue stream of wood sources for steam, heat and electric energy production (in some cases capturing efficiencies that approach 80 -90 % versus traditional powerplants that make be closer to 35-40 %). Wood’s cost-saving benefits may get an even greater boost with renewable energy and climate change policy that avoids traditional fossil fuel energy sources. Regardless of the motivation, wood is a valuable fuel and feedstock. Current and future users will be motivated to fully combust and extract the maximum amount of energy (and value), while minimizing pollution.
Clean Air Standards
All industrial fuel users must comply with clean air standards and regulations regardless of the fuel type. Industrial combustion facilities are regulated by the United States Environmental Protection Agency and NC Division of Air Quality. These agencies manage air quality, through regulation, permits, and enforcement to ensure that new or existing facilities minimize air quality degradation. Regulations apply to all major combustion facilities, including specific emissions related to wood fuel combustion. It is important to note that emission standards are strictest for larger-scale facilities.
Figure 1 illustrates that a wood-fired power plant with modern emission control equipment emits 1⁄500 to 1⁄280 of the emissions of a wood stove, for the same amount of wood fuel input.
There are several air pollutants that any industrial facility must control regardless of fuel used. The most common regulated pollutants for oil, gas, coal and wood are nitrous oxides (NOx), carbon monoxide (CO, a product of incomplete combustion), sulfur dioxide (SO2), mercury (Hg), and particulate matter (very small, airborne particles). Greenhouse gases too have become a significant concern in recent years. In this section we will discuss several fuel sources in the context of each of these pollutants. The following table provides a comparison among various fuel emissions.
Natural gas is considered the cleanest industrial fuel because it can be burned to generate low levels of NOx, CO, SO2, and particulate matter, in comparison to other fossil fuels. Of late, natural gas has become very inexpensive an