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Burning any fuel produces a range of emissions in the flue gases. Carbon dioxide and water vapour are inevitable, but using modern, efficient equipment, clean fuel and best practice others can be kept to a minimum

Combustion of biomass fuels

Burning biomass, or any other fuel, in inappropriate, or badly maintained equipment, or under poor operating conditions can give rise to a number of potential emissions. These can include particulate matter (PM), NOx, CO and other carbon containing compounds.

Most biomass, including wood, is composed of roughly 50% carbon by weight, 40% oxygen and 5% hydrogen. Under ideal combustion conditions these are completely converted to carbon dioxide (CO2) and water vapour (H2O). In addition there can be about 0.3% nitrogen, 0.1% sulphur, 0.1% chlorine, and trace quantities of various minerals such as calcium, potassium, silicon, phosphorus and sodium. The levels of these, and other, elements depend on many factors, including the environment the material was grown in, the species, any contaminants in the soil, water or air, etc. The chemical composition of different parts of a plant also varies; for example higher levels of minerals in bark, lead to increased ash production, while there is more nitrogen and sulphur in green waste and brash. This underlines the importance of obtaining high quality woodfuel, with a high proportion of clean, white stem wood.

The principal concerns about emissions and the impact of combustion systems on air quality are in relation to carbon dioxide and carbon monoxide (CO2 and CO), small particulates (PM10 and PM2.5; particles smaller than 10 microns and 2.5 microns respectively), oxides of nitrogen (NOx) and sulphur dioxide (SO2).

CO2 is a greenhouse gas and an inevitable consequence of burning any organic (carbon containing) material. Under conditions of insufficient oxygen supply, or incomplete combustion carbon monoxide (CO) can also be formed as well as fine particles of unburned carbon, or soot. In a well designed combustion system, which allows sufficient time and turbulence within the combustion chamber for complete combustion, levels of CO within the flue gases can be kept to a minimum. The incorporation of a Lambda oxygen sensor in the flue of many modern systems helps ensure optimum operating parameters.

NOx formation arises as a result of several mechanisms. Nitrogen in the fuel is oxidized, leading to fuel NOx. In addition, in any high temperature combustion process in the presence of air, atmospheric nitrogen can also be oxidized, leading to thermal NOx. This process is temperature dependent and rises rapidly above about 1,100ºC.

The level of sulphur in virgin wood is generally very low, typically around 0.1%, or an order of magnitude lower than in fossil fuels like coal and fuel oil. Much of the sulphur condenses onto the fly ash particles as sulphates, although a significant proportion can be emitted as SO2 and SO3.

As any gardener knows, plants need a number of minerals to thrive, including potassium (K) and phosphorus (P). These are present at trace concentrations in wood, but at higher levels in the bark and leaves. Other minerals can be taken up into growing biomass if present in the soil, water or atmosphere. Most of these will contribute to the ash in the form of various salts. Larger particles fall through the grate as bottom ash, while smaller particles can be carried up with the flue gases, together with any particulate unburned carbon, as fly ash. There are a range of filters available to catch such particles, include cyclones, electrostatic precipitators, and high efficiency fabric filters. It must be noted, however, that even a small amount of soil inclusion in the fuel as a result of poor storage or handling practice, will lead to greatly increased levels of ash, once again underlining the importance of obtaining quality fuel.

There can also be chlorine present in the wood, typically at levels of around 0.1%. However in some environments this can be significantly higher. Wood grown or stored near to the sea, for example, can pick up chlorine from salt spray. This chlorine can form salts of some of the minerals, such as sodium chloride (NaCl) or potassium chloride (KCl) or hydrogen chloride (HCl) and can also enhance the production of a number of harmful halogenated compounds.

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