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
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
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.
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.
The natural growth rate of the chosen crop.
The quality of the soil, the hours of sunlight, the temperature, rainfall and drainage
Planting density, cutting cycle and whether fertilizer is applied.
The table below shows typical crop yields and energy content of some biofuels.
Crop Yields and Energy Content
Source UK Natural Environment Research Council (NERC),
Towards a Sustainable Energy Economy (TSEC) programme
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)
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.
Because of the poor energy conversion efficiencies of biomass
fuels, practical generating systems often employ co-firing with coal to
achieve reasonable utilisation of the generating plant.
While biomass crops provide an
environmentally friendly fuel source for generating electrical energy,
their cultivation occupies land which may be better employed for food
production. See also Carbon Footprints