Biomass can be burned to produce electricity, hot water or hot air. Direct combustion is the simplest and most common method of generating energy from biomass. For small-scale applications, the fuel usually takes the form of wood pellets, chips and logs.
Wood pellets are a compact form of wood, with low moisture content & high energy density. Larger applications use fuels such a miscanthus and willow, which are short rotation crops.
The energy content of the fuel used depends on the moisture content. A higher moisture level will slow down the combustion process, as the water has to boil off before the fuel burns.
Wood is not the only form of biomass that can be burnt to produce energy. Other forms of biomass produced by farmers are by-products of conventional agricultural activity. They include ‘dry’ agricultural wastes such as straw, damaged grain or seeds, even animal bedding has been tried as a fuel. These wastes vary in their efficiency, but for farmers who burn these wastes for heat, they can represent a useful and cheap fuel source.
‘Wet’ agricultural wastes such as manure can also be used to generate heat and electricity by a process called Anaerobic Digestion.
In biomass electricity generation, biomass is combusted and heat is produced, which is then used to generate electricity.
Biomass heating systems produce either hot water or hot air. The biomass is combusted in a furnace, where it heats either air or water. Hot air production is used primarily for space heating, and hot water can be used either as part of a domestic hot water system or for heating systems such as central heating and underfloor heating. In hot water systems, a heat exchange system can be used to heat water tanks etc.
Combined Heat and Power (CHP)
Combined heat and power systems can also be used – the biomass is burned to produce both electricity and heat. CHP is an efficient way to use the biomass, as it uses the waste heat from generation as useful energy.