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Biofuels Potential use with Fuel Cells
and Environmental Aspects Introduction The most common use of biofuels is the combustion of fire wood,
which accounts for approximately 90% of primary energy production in the
developing world. This is a cheap but wasteful use of the resource, and results
in deforestation and detriment to human health. Urban air pollution in developing countries is
worsening rapidly. Other methods of use are prefered which provide
higher value forms of energy such as electricity.
It has been estimated that digestion, gasification, and fermentation of biomass to biogas and liquid biofuels has the potential to meet 14% of the world's energy needs, and 35% of the developing world's energy needs. This constitutes a decentralised energy supply from renewable resources. Furthermore, use of biofuels may provide a route for collection, treatment, and effective utilisation of wastes. The implementation of technology and innovative techniques in areas such as waste water treatment and agriculture may have widespread benefits. |
Index of technical reviews |
However, the practical availability and energy density of biofuels are lower than fossil fuels, and the effective utilistion of all the products of the process is essential for economic viability. This includes:
- Provision of heating, cooling, and lighting energy
- Provision of a stabilised organic material which makes an excellent fertiliser
- Displacement of fossil fuel depletion and global warming due to CO2 emissions
- Reduction in global warming due to capture of methane emissions
- Reduction of pollution of rivers and soils
- Reduction of deforestation and erosion
- Conservation and improvement of soil conditions
- Improvements in sanitation and reduced risk of disease
In industrialised countries the savings in costs of waste disposal, and the value of the fertilisers, exceeds the value of the energy (at current prices) significantly. Maximising the income from potential sources such as these is essential.
- Municipal solid waste (landfill gas)
- Sewage
- Farm slurry
- Process wastes
Anaerobic digestion occurs at 30-40C and the methane yield is sensitive to climate and the size of the digester. Small sewage digesters may be viable in warm climates but not in cold climates. If the digestion takes place at sufficiently high temperature (70C) pathogens are destroyed, again reducing the risk of disease.
The accessible resource from farm wastes is approximately 3x that from sewage. The yield of biogas is more consistent enabling higher plant load factors.
Process wastes include those from the paper, food, and chemical industries; distilleries and breweries, and sugar refineries.
Biogas digesters can range from a 1m3 household unit, through a 10m3 farm unit, to a 2000m3 commercial vessel or large waste water system. Small, low cost plants are used in the developing world, but the cost is still too high for high utilisation. Development requirements include low cost:
- Digestion techniques to increase yield
- Manure handling for farm units
- Systems for process heating
Fuel cells with efficiencies up to 60% offer a potential solution, but the cost of the fuel cell systems, including the fuel cleaning equipment and reformers must be reduced. Standard designs are required to reduce capital costs and financial risk. However, flexibility and modularity are required to enable standard designs to be matched to the energy demand and fuel availability. The high purity of hydrogen gas required before it can be used in low temperature fuel cells adds to the fuel processing equipment complexity and cost. The high temperature fuel cells offer a potential long term solution.
In summary the factors influencing the economic viability include:
- Input material
- Local climate
- Efficiency of the digestion and energy generation process
- Cost of the digesters, fuel processing equipment, prime movers, and heat and power distribution equipment
- Local processes and implementation possibilities
- Energy prices and markets for residues
- Government environmental policies and legislation
The process is CO2 neutral and tree planting may have environmental benefits. However, the fertility of the soil may be degraded.
Starch crops such as grain and root crops are hydrolysed before fermentation.
Cellulose crops, wood and wastes, are milled and hydrolysed before fermentation
Heat is required for the fermentation of all sources, and the process is very oxygen hungry. However, the technology is low cost and the resulting fuel in a convenient form.
The use of these sources displaces the depletion of fossil fuels, and CO2 production, but the resource has competing uses including animal feed, soil amendments, industrial feedstock, and commercial products. Specific energy crops are required for ethanol to become a major component in the energy mix. However, the introduction of energy crops must not be allowed to overwhelm indigenous species, risk the spread of plant disease, or entail excessive use of nitrogen fertilisers or pesticides.
The SOFC type of fuel cell may use ethanol directly.
The main attraction of methanol lies in it's possible use for transport fuel cells, or direct methanol fuel cells if development of the latter is successful.
Integration of energy, agriculture, and agroforestry is required using resource use planning. The provision of energy and waste water treatment for urban communities requires special attention.
The aim is for low cost, small scale, decentralised technology, which can be installed quickly and cheaply. This precludes fuel cells at present but in ten years time they may be widely viable.
- Technolgy of Biogas Production and Application in Rural Areas; World Energy Conference 1989
- Green Energy: Biomass Fuels and the Environment; United Nations 1991
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