The main emissions from a wood fired plant burning clean wood from forests or coppice will consist largely of water vapour and carbon dioxide (plus nitrogen and oxygen from the combustion air). The emissions will also contain traces of carbon monoxide, particulates and volatile organic compounds. However, these emissions are not exclusively produced by burning wood. They are also produced when fossil fuels like gas and oil are burned to produce energy. However, this is not a reason to be complacent. Instead it highlights the needs to consider seriously the appliance that the wood is burned in and the quality of the wood fuel that is used.
On a more positive note, wood smoke contains almost no sulphur dioxide and very little in the way of nitrous oxides. This means acid rain is not produced as a result of burning wood. In addition, it should be noted that the carbon dioxide released when wood is burned is the same as that absorbed by the tree when it was growing.
The Effect of Poor Burning Techniques
Wood can release pollutants into the atmosphere when poor burning techniques are used. The hazards include smoke and carbon monoxide.
Smoke is particles of unburned fuel which form as a result of incomplete combustion. It contains harmful pollutants that can trigger coughs, runny noses, headaches and eye and throat irritation. Unburned fuel can also be deposited as creosote in the chimney, providing the potential for a chimney fire.
Smoke can also contribute to smog which is carcinogenic, harmful to lung tissue and can worsen respiratory and cardiovascular problems especially if their size is smaller than 10 microns (PM10s).
Dark or smelly smoke drifting from a chimney means the wood is not burning completely. The best fire is a hot one with very little visible smoke and no smell of smoke indoors. If you smell smoke indoors, it means the wood stove or fireplace isn't venting properly - it may be a block in the chimney, a faulty damper or competition with a range hood. These are fire hazards and could lead to carbon monoxide poisoning.
Wood-smoke emissions can be reduced, indoors and out, by using efficient (and well maintained) appliances and well seasoned (i.e. dry) wood. However, even with perfect wood, a chimney may not work under certain conditions or not at all and spillage may occur. 'Spillage' is where the smoke comes back into the room instead of going up the flue and is dangerous to occupants, but there may also be problems with poor dispersal of the smoke at the top of the chimney and hence around the building.
The most common causes of poor chimney draught are: relining (which reduces the internal size) following a chimney inspection where the flue has been condemned; reduction in the stack height for safety reasons (crumbling brickwork); modification to the fireplace for aesthetic reasons (maybe new owner); reduction of the inlet (ventilation) air due to home improvements reducing leakage; growth of surrounding trees creating a barrier; prevailing winds causing down draught. All these problems can be solved by the installation of a good quality chimney fan onto the chimney top. An electrically driven chimney fan, will provide controllable updraught where the natural buoyancy is insufficient or erratic. Installation of a chimney fan does not negate the need for adequate combustion air to the appliance nor should it be an excuse not to fit a carbon monoxide detector. However, a chimney fan fitted with a temperature sensor and appropriate controller can provide additional safety by emitting an audible alarm when the temperature drops (fire needs refuelling or the temperature rises excessively (chimney fire in progress).
Chimney fans with adjustable speed control have the added advantage of improving the efficiency of the appliance by creating exactly the right amount of draught at all times. Excessive draught causes the wood to burn faster and can cool the flame which leads to more smoke.
Carbon monoxide (CO) is generated wherever combustion of a carbon based fuel occurs so that all gas, oil, coal and wood boilers as well as wood burning stoves have the potential to produce dangerous levels of CO gas if installed or operated incorrectly.
CO reduces the blood's ability to supply necessary oxygen to the body's tissues, which can cause stress to the heart. When inhaled at higher levels, CO may cause fatigue, headaches, dizziness, nausea, confusion and disorientation and, at very high levels unconsciousness and death. As it breaks down rapidly its affects are most apparent in confined spaces.
To avoid the risk of carbon monoxide poisoning stoves should be fitted professionally and be inspected and cleaned every year. In addition, chimneys should be swept at least once a year. It is also a good idea to fit a carbon monoxide detector and a smoke alarm in any room where wood or any other fuel is burned.
The following are recommended to improve the efficiency of appliances and reduce any potential pollution:-
- Burn small, hot fires - they produce much less smoke than ones that are left to smoulder.
- Burn seasoned wood. This is wood which has been allowed to try over at least one summer season. Green wood is a major culprit in the creation of creosote.
- Burn logs that are 10 -15 cm (4-6 inches) in diameter. Fires burn better with more surface area exposed to the flame.
- Never burn rubbish, plastics, cardboard, glossy paper or polystyrene.
- Never burn wood that has been taken from salt water. Chlorine combines with the smoke to produce dioxins and furans, which are dangerous carcinogens.
- Never burn treated or painted wood, particulate-board or plywood.
- Store wood outside, off the ground and covered. Bringing green wood indoors to dry can promote the growth of allergy-causing mould spores circulating indoors.
- Use a high-efficiency wood stove, fireplace or insert that is certified to produce low emissions.
- Have the systems installed by a professional and have it inspected and cleaned every year.
- Make sure the fire is getting enough air - check that the air inlet is open wide enough to keep the fire burning briskly, resulting in complete combustion and less smoke.
Wood contains little ash, with clean wood typically producing less than 1%. Being high in potash it is a good garden fertiliser.
Clean burning appliances with secondary and tertiary burn systems produce virtually no smoke which also means no creosote production, making them much safer than an open fire and some of the older wood burning stoves. Of all wood burning appliances, pellet stoves and boilers produce some of the lowest emissions and the highest efficiencies.
Smoke Control Areas
Within a smoke control zone, wood can only be burnt on Exempted Appliances pursuant to Smoke Control (Exempted Fireplaces) Orders. It is illegal to burn wood in any other device and illegal to deliver wood (for burning) to properties within the area. The companies producing Exempted Appliances (wood burning stoves) include:
Clearview, Vermont Castings, Dovre Castings Ltd, Dunsley Yorkshire Stoves, Morso, Osier Jotul and Euroheat Distributors.
Further information on smoke control zones can be obtained from Environmental Heath Officers.
As a source of renewable energy wood fuel has huge environmental benefits. However, care must be taken to ensure that no unnecessary emissions are released into the atmosphere and that carbon monoxide does not build up in the home.
The National Energy Foundation is grateful for the help of the National Fireplace Association (www.nfa.org.uk) and Exhausto Ltd (www.exhausto.co.uk), suppliers of chimney fans for their assistance in putting this page together.