Biomass - How It Works
Biomass, also known as biofuels or bioenergy, is obtained from organic matter, either directly from plants or indirectly from industrial, commercial, domestic or agricultural products.
The use of biomass is generally classed as a ‘carbon-neutral’ process because the carbon dioxide released during the generation of energy is balanced by that absorbed by plants during their growth. However, it is important to account for any other energy inputs that may affect this carbon-neutral balance on a case-by-case basis, for example any use of fertiliser, or energy consumed in vehicles when harvesting or transporting the biomass to its point of use.
How it works
Biomass falls into three main groups:
- Dependent resources: These are the co-products and waste generated from agricultural, industrial and commercial processes. This includes forest products, waste wood, straw, slurry, chicken litter and industrial and municipal wastes (such as food processing wastes). For example, for every tonne of wheat harvested, a certain amount of ‘waste’ straw is created, or for every tree felled to make furniture, a certain percentage cannot be used. These co-products can be utilised as biomass fuels, for example, in combustion.
- Dedicated energy crops: These are short-rotation crops, such as coppice, miscanthus, willow and poplar, which are grown specifically to generate biomass fuel.
- Multi-functional crops: These are crops that can be used to create different types of energy. For example, the ears of wheat can be used to create fuel (including bioethanol and biodiesel), while straw can be used to generate electricity.
Biomass can be converted into heat and electricity in a number of ways. Depending on its source, these processes include burning, pyrolysis (the decomposition or transformation of a compound caused by heat), gasification (the conversion of solid biomass into a gaseous fuel), anaerobic digestion (the decomposition of an organic biodegradable material by bacterial action in the absence of air, and in warm, moist conditions) or fermentation.
Woody biomass
Energy can be derived from woody biomass sources (including forest products, waste wood and straw) using combustion systems, which can be used to heat anything from a domestic stove or hot water system to an entire community. Biomass can also be used on its own or by co-firing it with fossil fuels in power stations, reducing greenhouse gas emissions by replacing a component of the fossil fuel required. In industrial or agricultural use, boilers fuelled by woody biomass such as cardboard, wood and waste pellets or straw can help reduce waste removal costs.
Biogas, landfill gas and fermentation harness the natural process of anaerobic digestion.
Biogas
Biogas is generated from concentrations of sewage or manure. These are usually in the form of slurry comprising mostly water (almost 95 per cent). The slurry is fed into a digester, either continuously or in batches. Digestion takes from about 10 days up to several weeks, at a temperature of 35°C.
Landfill gas
Landfill gas arises from waste deposited underground in landfill sites. Biodegradable organic waste decomposes anaerobically to produce a gas that is roughly an even mixture of carbon dioxide and methane. The methane content gives it the potential as a fuel, which can then be used to generate electricity or to provide process heat. The amount of gas available from a landfill site depends on the type of waste, moisture content, temperature, acidity of the waste and the design of the site. Gas is drawn up from vertical or horizontal wells through a system of pipes. The generating equipment is usually contained within the same area as the extraction plant.
Fermentation
Fermentation occurs when anaerobic digestion converts sugars into ethanol with the use of micro-organisms, usually yeast. Bioethanol can be used as a transport fuel by mixing it with petrol or using it directly in a modified combustion engine. Sugar cane or beet is the most efficient source but potatoes, corn, wheat and barley can also be used. Processes that produce bioethanol from woody material, such as forestry residues, energy crops and waste paper, are also approaching commercial viability and a number of pilot plants are proposed for the UK.
Biodiesel can be made from vegetable oils, animal fats or recycled cooking oils. However, the production of biodiesel requires a high amount of energy, offsetting its ability to reduce carbon emissions. However, it still provides an improvement over fossil fuels, typically reducing lifecycle carbon dioxide emissions by over 60 per cent (source: British Biogen).