In tropical countries, plants grow fast. Sugar cane grows about 4-5 metres in a year. It has juice high in carbs, particularly sucrose. Maize is another fast grower. We can break the starch in the maize kernels down into glucose using the enzyme carbohydrase
If sugar rich products from cane and maize are fermented anaerobically with yeast, the sugars are broken down incompletely to give ethanol and water. You can extract the ethanol from the products of fermentation by distillation, and you can use it in cars as a fuel
Car engines need special modification to be able to use pure ethanol as a fuel, but it is not a major job. Many cars can run on a mixture of petrol and ethanol without any problems at all.
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In some ways ethanol is an ideal fuel: It is efficient and does not produce toxic gases when you burn it. It is much less polluting than conventional fuels which produce Carbon Monoxide, sulfur dioxide and nitrogen oxides. In addition, you can mix ethanol with conventional petrol to make a fuel known as gasohol. This is being done increasingly in the US and it reduces pollution levels considerably. However there is still some pollution from the petrol part of the mix
Using ethanol as a fuel is known as Carbon Neutral. This means no overall increase in CO2 in the atmospere when you burn Ethanol. The orginal plants removed CO2 from the air during photosynthesis. When you burn the ethanol, you simply return it.
The biggest difficulty with using plant based fuels for our cars is that it takes a lot of plant material to produce the ethanol. As a result, the use of ethanol as a fuel has been largely limited to countries with enough space and a suitable climate to grow lots of plant material as fast as possible
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Brazil was the trailblazer for ethanol as a fuel when oil prices shot up in the 70's. The brazilians grew their own 'green petrol' and slashed the money paid out on oil imports which were crippling the economy of the country. They were very successful, and in the 80's, 90% of the cars produced in Brazil had ethanol-powered engines.
However, when oil prices dropped again the Brazilian government couldn't afford to subsidise ethanol as a fuel. As a result they began to move back to petrol driven cars.
Now people al over the world are worried about the environmental problems caused by the burning of fossil fuels. Interest in clean alternatives such as ethanol is soaring. Brazil is again taking the lead, supporting countries such as India with advice on technology for producing ethanol from plants
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In America, the use of gasohol is increasing all the time. A lot of ethanol is fermented and distilled from maize grown within the US itself. However as the use of ethanol grows the Americans are also importing ethanol from places like Brazil and the Caribbean. In 2004, the US imported 160 million gallons of ethanol on top of its own production
The main problem is finding enough ethanol. If we Europeans added 5% ethanol to our fuel it was reduce CO2 emissions, but we would need 7.5 billion litres of it a year. Thats more than half the total production level in Brazil.
The methods of ethanol production we use at the moment leave large quantities of unused cellulose from the plant material. To make ethanol production work finically in the long term, we need to find a way to use this cellulose. We might develop biogas generators which can break down the excess cellulose into methane, another useful fuel.
Genetically engineered bacteria or enzymes may be able to break down the cellulose into straw and hay and make it available for yeast to make more ethanol.