Biomass
Practical Action
Biomass use
Solid biomass is widely used in developing countries, mainly for cooking, heating water and
domestic space heating. Biomass is available in varying quantities throughout the developing
world - from densely forested areas in the temperate and tropical regions of the world, to
sparsely vegetated arid regions where collecting wood fuel for household needs is a time
consuming and arduous task.
In past decades the threat of global deforestation, provided a focus for the efficient use of
biomass (as well as introducing alternative fuels) in areas where woodfuel was in particular
shortage. Although domestic fuelwood users can suffer greatly from the effects of deforestation,
it often arises because of land clearing for agricultural use or for commercial timber.
There have been many programmes aimed at developing and disseminating improved stove
technologies to reduce the burden, primarily borne by women, of fuelwood collection as well as
reducing health risks associated with smoke from burning fuelwood. Technologies have also
been introduced to help with the processing of biomass to improve efficiency, allow for easy
transportation or to make it more useable.
Crop and industrial biomass residues are now widely used in many countries to provide
centralised, medium and large-scale production of process heat for electricity production or
other commercial end uses. There are several examples in Indonesia of timber processing plants
using wood waste-fired boilers to provide heat and electricity for their own needs, and
occasionally for sale to other consumers. There are also small scale options to utilising crop
residues.
Combustion theory
For solid biomass to be converted into useful heat energy it has to undergo combustion.
Although there are many different combustion technologies available, the principle of biomass
combustion is essentially the same for each. There are three main stages to the combustion
process:
Drying - all biomass contains moisture, and this moisture has to be driven off before combustion
proper can take place. The heat for drying is supplied by radiation from flames and from the
stored heat in the body of the stove or furnace.
Pyrolysis - the dry biomass is heated and when the temperature reaches between 200ºC and
350ºC the volatile gases are released. These gases mix with oxygen and burn producing a yellow
flame. This process is self-sustaining as the heat from the burning gases is used to dry the fresh
fuel and release further volatile gases. Oxygen has to be provided to sustain this part of the
combustion process. When all the volatiles have been burnt off, charcoal remains.
Oxidation - at about 800ºC the charcoal oxidises or burns. Again oxygen is required, both at the
fire bed for the oxidation of the carbon and, secondly, above the fire bed where it mixes with
carbon monoxide to form carbon dioxide which is given off to the atmosphere.
It is worth bearing in mind that all the above stages can occur within a fire at the same time,
although at low temperatures the first stage only will be underway and later, when all the
volatiles have been burned off and no fresh fuel added, only the final stage will be taking place.
Combustion efficiency varies depending on many factors; fuel, moisture content and calorific
value of fuel, etc. The design of the stove or combustion system also affects overall thermal
efficiency and table 1 below gives an indication of the efficiencies of some typical systems
(including non-biomass systems for comparison).
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