Biomass
Growing crops of wood or other kinds of biomass to burn as fuel for generating electricity has some appeal as a means of utilising the solar energy captured by photosynthesis for electrical power generation. The main advantages of biofuels are that they are renewable energy resources which ultimately do not contribute to global warming. Just as with fossil fuels however, burning biomass fuel to raise steam in conventional electricity generating plants also generates greenhouse gases. In the case of biomass however, the process of growing the new biomass is based on photosynthesis which uses energy captured from sunlight to extract CO2 from the atmosphere and convert it into the combustible organic compounds thus offsetting the greenhouse gas generated by burning.Unfortunately the logistics often defeat the arguments for using biomass. The energy content of biomass is relatively low and vast areas of land are needed for cultivating the fuels and furthermore a lot of energy is required to harvest and move the crops to the power station. For long term sustainability the ash containing mineral nutrients also needs to be returned to the land.
Despite this, by 2030 biomass-fuelled electricity production is projected to triple and provide 2% of world total, 4% in OECD Europe, as a result of government policies to promote renewables.
Biofuels
Fast growing woods and grasses are ideal sources of biomass fuels, but it is also possible to use agricultural and even household and industrial wastes. In Australia and Latin America sugar cane pulp, known as bagasse, is burned as a valuable energy source as a by-product of the sugar production. Unfortunately the energy content of all of these fuels is only about half as much as coal.The energy content is also dependant on the moisture content of the biomass. Before it can be used the fuel must be dried to reduce the moisture content and energy content is usually specified for dry weight of fuel after removal of moisture. Some fuels such as grasses may be left to dry in the field others may be oven dried. Depending on moisture content and end-use, the biomass may also be processed into pellets or briquettes in the field or at a processing facility.
- Calorific Values
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The energy content of biomass fuels, as harvested, is highly variable
due to the variability of the moisture content of the fuel as well as
the nature of the crop itself. Woods and grasses as harvested may have
calorific values of only 10 GJ/tonne due their high moisture content,
typically 50%, but the calorific value may be improved by drying the
fuel. Natural air drying can reduce the moisture content to as low as
20% and oven drying to even lower levels enabling calorific values of 18
GJ/tonne or more to be achieved.
So that "like for like" comparisons can be made and for consistency, calorific values for biomass are usually specified for dried fuel with minimal moisture content thus: GigaJoules per Oven Dried Tonne (GJ/ODT).
Industrial and domestic wastes have much lower energy content. As might be expected the energy content of waste from rich countries is much higher than that from poorer countries whose waste may not be suitable for use in electrical generating plants.
For comparison, the corresponding calorific value of good quality coal is around 30 GJ/tonne.
- Crop Yields The measure for crop yields is usually standardised as Oven Dried Tonnes per unit area per year (ODT/ha/yr). Like the energy content, the crop yield also varies widely depending on the type of crop, where it is grown and how it is managed. The following key factors influence the yield.
- Type of crop The natural growth rate of the chosen crop.
- Location The quality of the soil, the hours of sunlight, the temperature, rainfall and drainage
- Crop management Planting density, cutting cycle and whether fertilizer is applied.
The table below shows typical crop yields and energy content of some biofuels.
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Crop Yields and Energy Content |
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Source UK Natural Environment Research Council (NERC),
Towards a Sustainable Energy Economy (TSEC) programme
Fuel Requirements
As an example, a 10 MegaWatt power generating plant will deliver 87,600 MWh (315,360 GigaJoules) of electrical energy in a year assuming no down time. Because the efficiency of electricity generating plant fuelled by biomass is typically only around 35% (See Generating Efficiency), the energy content of a year's supply of the biomass fuel consumed by the 10MWatt plant must be at least 250 MWh or 900,000 GigaJoules. Since the typical energy content of biomass is about 18 GJ/tonne, it will require 50,000 tons of biomass fuel to provide this energy. With typical crop yields of around 10 ODT/ha/yr, a 10 MegaWatt generating plant would need 5,000 hectares (12,350 acres) of good quality land for biomass planting to keep it supplied with sufficient fuel.This compares with less than 1 hectare (2.5 acres) of land required to support two 5 MegaWatt wind turbines. (See Wind Farms)
Electricity Generating Plant
Generating plant fuelled by biomass uses conventional steam turbine electricity generating plant as used in coal fired power stations with modifications to the combustion chamber and fuel handling systems to handle the bulkier fuel.
