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Combustion, fuels and hydrocarbons. When a substance burns, it is said to combust . Combustion is a rapid reaction between a substance and oxygen that releases heat and light energy. . A fuel is a substance that reacts with oxygen (combusts) to release useful energy.
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Combustion, fuels and hydrocarbons When a substance burns, it is said to combust. Combustion is a rapid reaction between a substance and oxygen that releases heat and light energy. A fuel is a substance that reacts with oxygen (combusts) to release useful energy. Many fractions obtained from crude oil are used as fuels because they contain hydrocarbons that burn easily and release a large amount of useful energy.
carbon dioxide + + hydrocarbon oxygen water Complete combustion of hydrocarbons Plenty of air is needed to provide enough oxygen for a hydrocarbon fuel to burn completely. The blue flame of a gas hob or a Bunsen burner is an example of complete combustion of a hydrocarbon (in this case, natural gas). What are the products of thecomplete combustion of a hydrocarbon?
carbon dioxide + + propane oxygen water + + C3H8 5O2 3CO2 4H20 Complete combustion of propane Propane is a hydrocarbon used in camping gas. What is the equation for its combustion? How would the equations change if butane was used?
Incomplete combustion of hydrocarbons If there is a shortage of air (oxygen), incomplete combustion of hydrocarbons takes place. Instead of producing just carbon dioxide and water, incomplete combustion also produces carbon monoxide and/or carbon (soot). It also releases less energy than complete combustion. Carbon monoxide is a poisonous gas because it reduces the ability of blood to carry oxygen.
Alternative fuels How soon will we be using alternative fuels?
The need for alternative fuels Most vehicles in the world use petrol or diesel as a fuel. These are produced from crude oil, a fossil fuel. Although fossil fuels are convenient sources of energy, they are very polluting, and will one day run out. As a result, some people have already begun using alternative fuels to power their vehicles, such as biofuels and hydrogen. Why is it important to develop and use these fuels before oil supplies run out?
What are biofuels? Biofuels are renewable fuels produced from plant material, such as agricultural crops. Two types of biofuel used in vehicles are bioethanol and biodiesel. They can be safely combined with normal petrol or diesel and used in conventional engines to reduce levels of harmful emissions without causing engine damage.
What is bioethanol? Bioethanol is an alcohol produced by the natural fermentation of the carbohydrates (such as starch) in sugar beet/cane or wheat crops. ‘Flexi-Fuel’ vehicles, fitted with modified fuel injection systems, can run on E85 fuel (85% bioethanol, 15% petrol), which cuts carbon dioxide emissions by 70% compared to normal petrol-engine cars.
What is biodiesel? Biodiesel is produced by chemically reacting vegetable oils or animal fats with alcohol and a catalyst. The process can be completed in 12 hours. Biodiesel can be mixed with conventional diesel, which significantly reduces emissions, especially toxic hydrocarbons, particulates and carbon monoxide. There are few garages in the UK that sell biodiesel. Home-made fuels, usually from waste vegetable oils, are heavily taxed.
Advantages of biofuels What are some of the advantages of using biofuels? • Biofuels are carbon neutral: the carbon released during combustion comes from the carbon dioxide the plants took in when they were growing. • Storage, transport and distribution costs are low as biofuels can be handled in the same way as conventional fuels. • By-products of production, such as pressed seedcake, can be burnt in power stations instead of fossil fuels or used aanimal feed.
Disadvantages of biofuels What are some of the disadvantages of using biofuels? • Although biofuels themselves produce relatively little when combusted, their production needs energy from fossil fuels. • There are few UK producers of biofuels, and only small quantities of fuel are made. Biofuels therefore need to be imported, mainly from Brazil and South-East Asia. • The high demand for land to plant biofuel crops can lead to deforestation and habitat loss, for example in Malaysia.
What are hydrogen fuel cells? Hydrogen fuel cells are electrochemical devices like batteries, which produce electricity by converting hydrogen and oxygen into water. A big advantage of hydrogen fuel cells is that they do not create any pollution: the only emission they produce is water vapour. The chemicals inside fuel cells are constantly replenished, so unlike batteries, fuel cells never run flat or need recharging.
Disadvantages of fuel cells What are some of the disadvantages of fuel cells? • The production, storage and distribution of hydrogen for use in fuel cells relies heavily on energy from fossil fuels. • Pure hydrogen is expensive and highly flammable. One way around these problems is to replace hydrogen with a more convenient substance. Methanol, natural gas and propane have all shown potential for use in fuel cells.
Glossary (1/2) • biodiesel – A biofuel made by chemically reacting plant oils or animal fats with alcohol and a catalyst. • bioethanol – A biofuel made from the natural fermentation of the carbohydrates in plants such as sugar cane/beet and wheat. • biofuel – A renewable fuel made from biological material, such as plant matter. • combustion – A rapid reaction between a substance and oxygen that produces heat and light energy. • complete – The type of combustion that only produces carbon dioxide and water.
Glossary (2/2) • fuel – A substance that provides useful energy when it burns. • fuel cell – A device that produces electricity from a chemical reaction, e.g. between hydrogen and oxygen. • hydrocarbon – A molecule containing only hydrogen and carbon. • incomplete – The type of combustion that produces carbon monoxide and/or soot in addition to carbon dioxide and water.
