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A fuel is a substance that burns at a fast but controllable rate. The fuels we use today are usually mixtures of many different compounds. These compounds are generally composed of molecules that contain carbon and hydrogen atoms only, or carbon, hydrogen and oxygen atoms. (Some contain other elements, such as sulfur.)
When these fuels burn in air, the main reaction occurring is:
fuel + oxygen → carbon dioxide + water + energy
Fuels like these do not burn unless oxygen or air is present. (Air is about 20 % oxygen.) They also need a spark to start the reaction off – that is, to ignite the fuel. This is shown in Figure 1.
Figure 1 - What is needed for a fuel to burn?
The heat energy produced when a fuel burns can be used for a range of important purposes. These include:
No wonder we need fuels!
Figure 2 - Manufacturing glass. Sand and limestone are two of the main raw materials used to make glass. These and the other raw materials required for the particular type of glass being manufactured are heated together in a furnace to over 1000⁰C. The fuel burned to produce this heat energy is usually natural gas.
Accessed: 15 October 2010
Figure 3 - Opening an inspection door in a furnace (called a boiler) at Loy Yang Power Station in Victoria. The fuel being burned is coal. The furnaces in which the coal is burned are almost as high as a 12-storey building
CREDIT: Loy Yang Power Station
The fuels we use today ultimately come from plants and other living organisms. Some were produced by and from organisms that lived a very long time ago. Others are being produced right now. We will discuss ancient and newly formed fuels next.
The coal, oil and natural gas we use today were produced from organisms that lived millions of years ago. These organisms included tiny aquatic organisms and plants that lived in swampy regions and even in ancient forests, which over time were covered with layer after layer of wet sediments (mud).
These layers of sediments pressed down on them. Over time the sediments turned into rock. With the pressure and heat at those depths, and the action of anaerobic bacteria (bacteria that live without oxygen), the remains of the organisms were slowly changed into oily liquids and gas, or else into coal. What was formed depended on the actual conditions and type of organisms.
You no doubt have seen images of what happens when crude oil spills into a marine environment. But do you know how crude oil was produced in the first place?
When aquatic organisms died near the bottom of stagnant bodies of water or the bottom of a deep lake or marine basin and were rapidly covered with mud, they were slowly changed into natural gas, oils and tars. The formation of these substances started up to 200 million years ago. See Figures 4 and 5.
Figure 4 - How the formation of oil and natural gas under the sea began, millions of years ago.
Accessed: 18 November 2010
Figure 5 - The organisms were slowly covered with sediments and subjected to heat and pressure and the action of anaerobic bacteria. Over time, this changed them into oil and gas. The oil and gas seeped through porous rock (rock that has small spaces through which substances can migrate), but could not seep through non-porous rock. As a result they collected in various ‘pockets’ under the non-porous rock.
Accessed: 18 November 2010
Coal is a complex mixture of carbon, water and a number of other substances. Figure 6 shows the different stages of coal formation.
Younger coal is quite soft and wet and is called peat (Figure 7). Because its water content is relatively high, peat does not burn well. Older coal contains less water than peat, and is harder. The coal formed over a longer time is called lignite, or brown coal (Figure 8). The oldest coal is commonly known as black coal (Figure 9). It is harder again. Since it contains even less water, it burns best.
Figure 6 - A diagram illustrating the process of coal formation. The label ‘coal’ refers to black coal.
Accessed: 27 October 2010
Figure 7 - Notice how wet it is, and how close it is to the surface.
Accessed: 18 November 2010
Figure 8 - These mining engineers are standing on a bed of brown coal located in the Latrobe Valley, Victoria. In Australia brown coal is quite close to the surface, which makes it much easier to mine. All the miners need to do is to dig a giant hole and scoop it out. (This is known as open-cut mining.)
CREDIT: Loy Yang Power Station
Figure 9 - A sample of black coal. There are deposits of black coal all over Australia, but about 97 % of the black coal we use or export comes from New South Wales and Queensland.
Source: http://www.faqs.org/photodict/photofiles/list/8930/12180black_coal.jpg Accessed: 28 October 2010
Australia has vast deposits of brown coal and black coal. The brown coal found in Australia is up to about 50 millions of years old while our black coal is on average about 250 million years old. A high proportion of this coal is used as a fuel to generate electricity (see Figure 3).
Because of their ancient origins, coal, crude oil and natural gas are termed fossil fuels. Interestingly, small fossils are often found in coal. Because they took so long to form, and are in limited supply, they are a non-renewable energy resource.
The fuels we use today that come from organisms that are living today or have recently been harvested, are called biofuels. Because we can keep producing them, biofuels are classified as renewable energy resources.
Table 1 lists some common biofuels and how they are produced.
Table 1 Examples of biofuels
|Biofuel||How is it produced?|
|Biogas||Biogas, which mostly is methane gas, is produced by the action of bacteria on rotting animal manure, human sewage and rotting garbage. The source is sealed so the biogas cannot escape. It is collected through a system of pipes. Before it is used it is treated to remove foul-smelling gases that are present in small amounts.|
|Bioethanol||One way in which bioethanol is by the fermentation of sugars present in sugar cane or in the waste products from a sugar refinery. It also can be obtained by treating grains such as corn and wheat, wheat stubble, etc.|
|Biodiesel*||Biodiesel is produced by the chemical treatment of oils obtained from plants, microalgae and other organisms. The treatment includes a reaction with an alcohol, usually methanol.|
Note: *The diesel that comes from crude oil is increasingly being called petrodiesel to distinguish it from biodiesel.
As you have just learned, fossil fuels and biofuels ultimately come from living organisms. Most of these organisms depended on photosynthesis to produce the glucose they needed to survive. The main difference between fossil fuels and biofuels is the time it took to produce the fuels. In the case of fossil fuels, the process of creating the fuel from the organisms that first used the energy from the Sun, took millions of years.
The overall energy transformations involved in using either kind of fuel to drive a vehicle shown in Figure 9.
Figure 10 - The main energy transformations involved in producing and using a fuel that is used for driving a vehicle
This flow chart shows that all the energy we obtain from fuels, whether ancient or new, originally comes from the Sun. So when we burn natural gas, for example, we are really using light energy from the Sun that shone on the Earth millions of years ago!
Some of these are listed in Table 2.
Table 2 Some advantages and disadvantages of fossil fuels
1 They are ‘energy-rich’ fuels. That is, you obtain a large amount of energy per kilogram of the fuel.
Moreover, not much energy is needed to process fossil fuels before they are used.
1 When they burn, they produce a large amount of greenhouse gases (mainly carbon dioxide and steam). So they contribute to the problem of global warming.
2 Some also produce other harmful substances when they are purified or when they are burned. These include acidic gases that dissolve in the moisture in the air and cause rain and snow to be acidic. When acid rain and acid snow fall down onto waterways and soils, they kill many plants, fish and other organisms. They can destroy whole ecosystems.
3 Coal is wet. A lot of the energy that might be obtained from burning coal is wasted in evaporating the water.
4 Because they are all energy-rich, there is always a high danger of fire and explosions when they are extracted, processed, transported and stored. If a coal bed catches alight, it can take months to put the fire out.
2 They are abundant. In Australia, for example, we have vast quantities of coal that should last for hundreds more years. This means the coal can be used to generate electricity on a large scale at relatively low cost.
5 They are still a non-renewable energy resource, so they will eventually run out.
3 Crude oil and natural gas flow easily so can be piped to the nearest refinery to be processed. In the case of natural gas, it can be piped directly from where it is purified to a gas-fired power station.
6 There is a great danger of oil spills, which can damage the environment and kill or harm many living things. Crude oil is toxic and contains carcinogenic (cancer-causing) compounds. There also is a risk of toxic gas leakages.
4 Because crude oil and natural gas are mixtures, they can be separated into different products that suit particular purposes. For example, kerosene is a blend that suits aircraft.
7 There are many other uses for the chemicals present in crude oil and natural gas. They are the raw materials used to produce plastics, dyes, pharmaceuticals, and so on. So burning then as a fuel means less is available to produce thousands of very useful substances.
Biogas is probably one of the most useful and sustainable biofuels there is. Its main advantages are listed in Table 3.
Table 3 The main advantages of using biogas
1 It is a renewable fuel that can be collected on a small scale, such as in small rural communities and on individual farms, and on a larger scale, such as from landfill sites and sewage treatment plants in large cities. This means it does not have to be transported large distances and it can be collected and used by even the poorest communities.
Figure 11 - A domestic biogas collection system in Africa
2 It makes use of waste. It does not require land that might be used to grow crops or cutting down precious rainforests. Therefore poor communities are not deprived of essential food, and the rich ecosystems within rainforests are not endangered.
Figure 12 - Destroying a rainforest to grow food or products such as palm oil, which is used in the manufacture of cosmetics, processed foods and biodiesel.
3 Biogas is emitted by rotting animal wastes and rubbish, whether we use it or not. One problem with this is that it smells foul. More importantly, it means methane is released into the air. The methane molecule is about 20 times more potent at ‘trapping’ heat than the carbon dioxide molecule. So it is much better to burn it and produce carbon dioxide than to allow it to enter the air as it is. Therefore this helps solve the problem of global warming.
A worded equation for the combustion of methane is:
methane + oxygen → carbon dioxide + water + energy
Collecting and burning biogas also reduces the risk of build-up of methane gas under buildings and roadways. Methane seeping from landfill sites, and so on, has been known to cause explosions and loss of life.
Figure 13 - Models of the methane molecule and the carbon dioxide molecule.
Carbon dioxide CO2
For each molecule of methane burned, one molecule of carbon dioxide and two molecules of water are produced.
|4 Biogas is a gas so can easily be piped to where it is burned.|
5 The energy produced by burning biogas can be used for heating or for generating electricity, or both. It even can be used as a transport fuel to drive buses and cars.
Figure 14 - A biogas cogeneration plant – that is, a plant where the biogas is used to generate electricity and heat at the same time.
Disadvantages of biogas include:
Bioethanol, commonly known as alcohol, has the chemical formula C2H5OH.
From Figure 15 and its chemical formula, it can be seen that the molecule contains two carbon atoms, a total of six hydrogen atoms and one oxygen atom.
Figure 15 - A model of the ethanol molecule
Table 4 lists some of the main advantages and disadvantages of bioethanol.
Table 4 Some advantages and disadvantages of bioethanol
|1 When pure bioethanol is burned, it burns ‘cleanly’. That is, no smoke is produced. The main products are carbon dioxide and water. (Small amounts of other substances also are produced.)||
1 Bioethanol absorbs moisture out of the air if it is not properly sealed from it, which means it is difficult to keep it pure. The presence of water also affects the amount of energy obtained per kilogram of this fuel.
2 It still produces greenhouse gases when it burns. Moreover, it is not as ‘energy-rich’ as fossil fuels. Therefore, when compared with them, it produces a greater amount of greenhouse gases than fossil fuels for a given amount of energy produced when they are burned.
|2 Bioethanol can be blended with petrol in different proportions to suit different vehicles. This reduces the consumption of fossil fuels.||
3 While oils such as those present in petrol help protect engine parts of motor vehicles from corrosion, the presence of bioethanol, especially if it also contains water, can be a problem.
In addition, bioethanol vaporises more easily than petrol and its vapours can cause problems. For this reason, for most vehicles in Australia, it is generally advisable to use blends that contain less than 10 per cent bioethanol.
|3 Bioethanol can be produced from wastes, such as those produced in sugar refineries.||
4 Transporting wastes to where they can be treated and the impure bioethanol to where it is purified, consumes more fuel and produces more greenhouse gases than may be justified in many cases.
In general it has been found that it takes more energy to produce, purify and transport each kilogram of bioethanol than is obtained from it when it is burned as a fuel.*
5 The production of bioethanol consumes a lot of water.
|4 Bioethanol can be produced from a wide range of plant material.||
6 There are other important uses of bioethanol besides as a transport fuel. It is used to manufacture a wide range of useful chemicals, including solvent cleaners. Burning it as a fuel means less is available for producing a wide range of other useful substances.
7 No country in the world, including Australia, has enough arable land to grow the amount of crops that would be needed if we were to replace fossil fuels with bioethanol, even if none of the land was used for food production. Therefore only relatively small amounts can ever be produced.
Note: * Earlier ethanol refineries, places where bioethanol was purified by distillation, consumed more energy than could be obtained back from the final product. However, special distillation techniques have been developed to overcome this problem.
Biodiesel is a mixture of molecules, most of which belong to a family of organic compounds called fatty acid methyl esters. In the industry this is abbreviated to FAME.
Figure 16 - This is a model of a typical fatty acid methyl ester (FAME) molecule. Not all molecules present in biodiesel have a ‘bend’ in them like this one.
The main difference between biodiesel molecules and petrodiesel molecules is that biodiesel molecules contain two oxygen atoms close to one end (shown here as red spheres). Petrodiesel molecules are chains of carbon and hydrogen atoms only.
Accessed: 23 November 2010
Biodiesel is produced by reacting vegetable oils or fats with an alcohol. Glycerol also is produced in this reaction. When the alcohol used is methanol (commonly called methyl alcohol), the biodiesel produced is in the form of fatty acid methyl esters. The reaction is called a transesterification reaction. A simplified worded equation for the reaction is:
vegetable oil + alcohol → biodiesel + glycerol
Table 5 - Some advantages and disadvantages of biodiesel
1 As a fuel for vehicles, biodiesel performs just as well as petrodiesel when used at warm temperatures. It performs much better than a vegetable oil.
2 Engines last longer when biodiesel is used rather than petrodiesel.
1 Biodiesel does not perform as well as petrodiesel when it is cold. It becomes too viscous to flow properly to the combustion chamber in the engine.
2 Biodiesel can adversely affect rubber parts.
3 Biodiesel is more likely to attract moisture than petrodiesel. This can cause problems in the performance of the fuel and its ability to flow in cold weather, and cause corrosion.
|3 Burning biodiesel produces less greenhouse gases and less other pollution per kilogram than petrodiesel.||4 Where biodiesel is produced from waste fats and oils, greenhouse gases are produced when collecting and transporting them. Where biodiesel is produced from oil crops, greenhouse gases are produced when planting and harvesting the crops and transporting them to the processing plants.|
|4 Biodiesel is less flammable than petrodiesel, so there is less risk of fires when storing, transporting and using it.||5 Biodiesel is more difficult to ignite in the combustion chamber in an engine in cold weather than petrodiesel.|
|5 Biodiesel is more easily broken down and is less toxic than petrodiesel, if it is spilt and seeps into waterways, soils, and so on.||6 Biodiesel cannot be transported via pipelines, because it can form a gel in them and block them when it is cold. So it must be transported by truck.|
|6 Biodiesel can be produced from waste fats and oils.||
7 Processing the wastes require more production steps and more energy.
8 The by-product of making biodiesel is glycerol, which has many uses, including in cosmetics.
9 If too much biodiesel is produced, more glycerol would be produced than can be used. (It also is produced when soap is made.)