Burning wood properly gives off very low amounts of smoke particulates, and many wood-fired appliances are certified for smokeless zones. The Defra Smoke Control Zones website includes a list of exempt appliances, including wood-burning ones.
Other emissions from wood fuel contain virtually no sulphur dioxide and very low levels of nitrous oxides, so won’t cause acid rain.
It’s so important to burn efficiently; use properly seasoned wood (with low moisture content) and make sure that equipment is used properly. Manually fed stoves can produce lots of pollutants if operated badly.
Wood pellets are delivered ready-to-use, with a very low moisture content. Logs need to be either bought seasoned or stacked and stored for long enough to reach a suitable moisture content. New green/wet logs must be stored for 2 years to dry enough (so need space for this).
If logs are burned when too damp, this will affect the efficiency of the stove or boiler, lead to production of tar/acid/etc that damages the equipment, and also leads to poor quality of emissions to the atmosphere.
In traditional log stoves, burning green/wet wood leads to corrosion of the liner & stove by acid, and tar coating the interior. Cast iron stoves are popular as they are less susceptible to corrosion from acid from burning damp wood. But if burning properly seasoned wood (as should always be done anyway), a steel stove will be fine. However, adding a water jacket back boiler causes further problems, even with seasoned wood.
Logs should be burned fiercely with lots of air input until they are almost charcoal, after which the stove can be ‘damped down’. Reducing the air supply too early creates lots of smoke & tar. The key is good ‘secondary combustion’ of the high-energy volatile gases given off by burning wood. Some stoves are fitted with a ‘Lamda’ sensor, to regulate the amount of oxygen added and so optimise efficiency. Avoid burning treated, painted or glued wood, or non-wood waste, as these will give off toxic and polluting gases.
Building regulations require all fuel burners to have a dedicated vent to avoid production of carbon monoxide. The chimney needs an insulated flue to prevent fumes condensing as tar. With complete combustion, wood burns to a small amount of ash, which (unlike coal ash) is an excellent fertiliser.