Wood now ranks as the third most common heating fuel, after gas and electricity, for primary and secondary heating fuel use nationally. According to the United States (U.S.) Census, the number of households using wood heat grew by 34 percent between 2000 and 2010, faster than any other fuel used for residential heating. The northeastern states have seen significantly higher growth in wood used for household heating than the nation at large. The increasing use of wood fuel and specifically wood pellets has raised concerns about potential environmental and public health impacts because little is known about the constituents in these fuels.
Biomass combustion has variable emissions, depending on the types and quality of fuel used, combustion technologies and operating conditions. The quality of the fuel depends mainly on its chemical composition, including water and ash contents, plant species, where it grows (origin), fertilizers and pesticides used, harvesting practices, transport, handling and processing, and blending of plant species type.
The combustion and pyrolysis of wood pellets in stoves produce atmospheric emissions of particulate matter (PM), carbon monoxide (CO), nitrogen oxides (NOx), volatile organic compounds, (VOCs), mineral residues, and to a lesser extent sulfur oxides (SOx).
Biomass with high chlorine content can lead to hydrogen chloride formation during combustion, which can have negative effects on the human respiratory systems and can cause acid rain formation.