Emissions from wood combustion
Wood is made up of cellulose, hemicellulose, lignin, trace quantities of inorganic elements and water. The compounds cellulose and lignin are formed mainly from carbon, oxygen and hydrogen. As with other carbon based fuels, when wood is burned in air, the main emissions are carbon dioxide and water vapour, with relatively much smaller emissions of carbon monoxide gas, organic gaseous carbon compounds, nitrogen oxides and particulate matter (smoke). The amounts of each of these will be influenced by the stove technology and fuel quality. The ash left behind is comprised of unburnt carbon, potash (potassium salts) and other inorganic material and can be used as a garden fertiliser.
clearSkies defines the maximum emissions concentrations for:
CO – carbon monoxide;
PM – particulate matter;
OGC – organic gaseous compounds; and
NOx – nitrogen oxides
Carbon dioxide (CO2)
Carbon dioxide emissions are not prescribed by clearSkies, but a description is included here to explain the difference between greenhouse gases and regulated air pollutants. Carbon dioxide is a greenhouse gas, not a pollutant regulated for the protection of human health out of doors. The carbon released is from the current carbon cycle, i.e. the wood takes it from the air during growth unlike fossil fuels, where the carbon has been out of atmospheric circulation for millions of years. Because of this, the carbon dioxide from burning biomass (e.g. wood) is treated by the United Nations Framework Convention on Climate Change and by UK government legislation and policy as not contributing to man-made climate change, as long as it is replanted.
Carbon Monoxide (CO)
Carbon monoxide is formed when combustion is incomplete. It is a pollutant regulated in UK air quality legislation. It is a colourless, odourless gas which in high enough concentrations can lead to unconsciousness and death and so stove users should have a carbon monoxide alarm in the room to warn of any leaks. It is rapidly dispersed in the air and there are no instances of carbon monoxide from combustion causing fatalities out of doors. There are no concerns over meeting the air quality objective for carbon monoxide in the UK.
Particulate Matter (PM)
Particulate matter emissions (smoke) are also a product of incomplete combustion and are predominantly solid carbon and condensed liquid carbon compounds, along with very small amounts of ash. A portion of it is in the size fractions PM10 (less than 10 millionths of a meter across) and PM2.5 (less than 2.5 millionths of a meter across). The smallest sizes can penetrate deep into the lungs and are linked to adverse health effects and so the concentrations of particulate matter in these size fractions in the air are regulated by law. Emissions of PM are highest for open fires and older technology stoves. Other major sources include cooking, transport, industrial processes, construction, agriculture and bonfires. There are concerns over meeting the government target for PM2.5 in built-up areas.
Organic Gaseous Compounds (OGC)
Organic gaseous compounds is a group of carbon compounds emitted as a result of incomplete combustion. Some of these are harmful and they contribute to poor air quality. When they cool and condense, they contribute to concentrations of particulate matter in the air.
Nitrogen Oxides (NOx)
Nitrogen oxides (NOx) are comprised of two colourless, odourless gases nitric oxide (NO) and nitrogen dioxide (NO2). These are formed from both the nitrogen in the air and from the small quantity of nitrogen in the fuel. NOx is mainly emitted from higher temperature combustion such as natural gas flames and vehicle engines but are also released from wood combustion. Nitrogen dioxide causes adverse health effects and is a regulated pollutant. The UK air quality objective is not being met at some locations close to roads due to vehicle emissions.