Fuels from fermentation
In India only animal waste is fed in (there are taboos against using human waste), this produces high quality gas but little amounts of fertiliser.
Biogas can be produced on a large scale, or on a small scale.
For example, a small biogas generator can be used to supply the energy needs of a family, or the waste of a sugar factory or sewage works can be used to power a village or town.
Sugar cane crops can grow fast – 4-5m per year. Their juice is rich in carbohydrates like sucrose. The starches of Maize can be broken down into glucose by using carbohydrase enzymes. If the sugar rich products of sugar cane or maize are fermented with yeast anaerobically, the sugars break down into ethanol and water.
Ethanol is extracted by distillation and used as a car fuel. Some cars (e.g. in Brazil) run on a mixture of ethanol and petrol (called gasohol – 90% petrol, 10% ethanol). Ethanol does not produce toxic gases when burned and so does not pollute as much as other fuels (which produce CO and SO2).
Countries like America do not have enough maize crops to produce large amounts of ethanol, and poorer countries that do have the crops don’t have the money to buy the equipment.
Using a fuel like ethanol is called carbon neutral because you are not contributing to carbon dioxide levels in the air by using it.
There are two main types:
Factors to consider when designing a generator:
Cost – Continuous generators are more expensive as waste has to be mechanically pumped in, and digested material has to be mechanically removed constantly
Convenience – Batch generators are less convenient, as they have to be continually loaded, emptied and cleaned
Efficiency – Gas is produced most quickly at 35°C. If the temp. falls below this, gas production will be slower. Generators in some areas will need to be insulated or kept warm. The generator shouldn’t have any leaks, or gas will be lost.
Position – The waste will smell, so generators should be sited away from homes. The generator is also best located close to the waste source.