Biomass
Biomass is a bioenergy fuel source which when burnt provides heat energy for distribution or conversion into electrical energy. Biomass or biofuels take many forms to include: wood from forestry; energy crops such as rapeseed oil; food waste and industrial waste. Biomass fuels do release CO2 when they are burnt, but have a “closed carbon cycle with no net increase in atmospheric CO2 levels” (Biomass Energy Centre), provided they are managed sustainably, as the same amount of CO2 is taken up when the replacement biomass is grown.
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Overview
Biomass is organic matter that was recently living, such as wood, straw, crops, algae, sewage sludge, animal litter or other biological waste.
In general biomass in the domestic sector almost always refers to wood fuel, which is only sustainable if it comes from renewable forest sources. Biomass systems can have high levels of efficiency, typically 60-80% in ranges, pellet stoves, log stoves and log boilers.
Biomass fuel usually takes the form of wood pellets or wood chips. The burning of wood is considered to be a carbon neutral process since the CO2 released when energy is generated, is balanced by that absorbed during the fuel's production (i.e. replacement tree growth). It is most cost effective when a local fuel source is used, which also helps to reduce transport pollution which might otherwise be associated with the solid fuel.
The major drawback is the physical size and installation cost of the fuel storage and delivery system. A 20kW thermal boiler typically consumes 0.6m3 of wood chip daily in winter, and the volume of one tonne of dried wood is about 6m3.
Definitions
The table belows list some of the terms used with biomass and biofuels.
| Term | Definition |
|---|---|
| Accumulator Tank | A buffer vessel or thermal store |
| Auxiliary Boiler | A boiler, usually fossil fuel, that assists the biomass system to meet the peak load. As peak load is infrequent, a biomass boiler sized to meet the base load can be installed to provide a large capital saving. The auxiliary boiler is sized at the difference between the base and peak load. |
| Back-Up Boiler | A second boiler, usually fossil fuel, used to provide 100% back-up to the biomass system. |
| Base Load | The minimum heat demand from a system which is maintained throughout a defined period. |
| Buffer Vessel | A vessel that captures residual heat on boiler shut-down preventing frequent boiler start-ups to improve system efficiency. It will also reduce the energy required from an auxiliary boiler. Usually has a simple on/off control strategy. |
| Capacity Factor | The ratio of the actual heating plant output over a period of time and its output if it had operated at full capacity for the same period of time. |
| Thermal Store | A vessel that is charged by the biomass boiler when the boiler output exceeds the load demand enabling a small biomass boiler to meet demand. A progressive control strategy using several temperature sensors will control the boiler output. |
| Turndown Ratio | The ratio of the maximum output of a boiler to its minimum output. |
Suggested Reading
CIBSE: AM15 Biomass Heating (here )
Carbon Trust: CTG012 Biomass heating, A practical guide for potential users
BESA: TR/38 Installation of Biofuel Heating (here )
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