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CHEMISTRY: FORM TWO: Topic 4 - FUELS AND ENERGY
TOPIC 4: FUELS AND ENERGY
A fuel is a substance that can be combusted or burnt to release energy as a byproduct. The energy can be in the form of heat, light, electricity, sound etc. This energy can be harnessed to power machines or used for other purposes such as heating or lighting. Combustion is the burning of fuel with energy released as a byproduct. Fuel is a very important substance for the existence of a modern man. Examples of fuels include petroleum products (petrol, diesel, fuel oil, kerosene, spirits, etc), natural gas, coal, wood, charcoal, producer gas, water gas, etc.
Different Sources of Fuels
Identify different sources of fuels
There are many types of substances that are used as fuels. The fuels exist as solids, liquids or gases. The most common substances that are used as fuels in Tanzania include wood, wood charcoal, coal, petroleum products and natural gas. These fuels are obtained from different sources as analysed below:
- Wood: wood is obtained from logs or poles of trees. The wood used as fuel in Tanzania is obtained from natural and artificial forests. Wood fuel is mainly used in rural areas where there are no alternative fuels. Wood is also a major source of fuel used by government institutions such as schools, colleges, hospitals, and military institutions.
- Charcoal: This fuel is made by heating certain substances such as wood and bones in a limited supply of air. Wood charcoal is the main source of fuel in urban areas and in some townships.
- Coal: coal used in Tanzania is mined at Kiwira coal mines. It is used indirectly for generating electricity or directly for powering machines in processing and manufacturing industries and factories. The electricity generated from coal is used in such industries as Tanga cement and several other industries in Dar es Salaam.
- Natural gas: This gaseous fuel is mined at Songosongo in Kilwa (Lindi region), located in southern Tanzania. The gas is used as a fuel at homes and in small industries. It is also used to generate electricity that is used in various manufacturing and processing industries. The electricity generated from this gas is also sold to Tanzania Electricity Supply Company (TANESCO) who distributes the energy to its various clients.
- Petroleum products (kerosene, diesel, petrol, fuel oil, fuel gas, etc.) These petroleum fractions are obtained from crude oil by the process of fractional distillation of crude oil (petroleum). Diesel, petrol and oil are used in vehicles and other machines. Kerosene is used in kerosene lamps and stoves for heating at homes and for other general purposes.
Methods of Obtaining Fuels from Locally Available Materials
Describe methods of obtaining fuels from locally available materials
Fuels can be classified into three groups according to the physical state of the fuel. A fuel can be in any of the three states of matter namely, solid, liquid or gaseous state.
Fuels According to their States
Classify fuels according to their states
The experiment gave these results for ethanol and butane. Make sure you understand the calculations:
Experimental results for heat determination
|Ethanol (burned in a spirit lamp)||Butane (burned in a butane cigarette lighter)|
|Mass of ethanol used: 0.9g||Mass of butane: 0.32g|
|Mass of water used: 200g||Mass of water used: 200g|
|Temperature rise: 20ºC||Temperature rise: 12ºC|
|Heat given out = or 16.8KJ||Heat given out = = 10080J or10.08KJ|
|The formula mass of ethanol is 46. 0.9g gives out 16.8KJ of energy. So, 46g gives out of energy||The formula mass of butane is 58. 0.32 gives out 10.08KJ of energy. So, 58g gives out KJ of energy|
|So, H combustion for ethanol is -859KJ/mol||So, H combustion for butane is -1827 KJ/mol|
Determination of energy (calorific) value of ethanol
The energy/heating/calorific value of a fuel refers to the amount of heat given out when a specific amount of fuel is burned.
Aim: To find out the energy value of ethanol.
Materials: water, beaker, thermometer, weighing balance, spirit lamp and ethanol.
- Pour a known volume of water into a beaker.
- Measure the temperature of the water.
- Fill the spirit lamp with enough ethanol.
- Weight the mass of both the ethanol and the lamp.
- Light the lamp and let it continue burning for a few minutes before putting it off.
- Measure the water temperature again, to find the increase.
- Reweigh the ethanol and its container to find how much ethanol was burned.
Record the following:
- Mass of spirit lamp + ethanol (initially)
- Mass of spirit lamp + ethanol (finally)
- Mass of ethanol burned
- Final temperature of water
- Initial temperature of water
- Rise in temperature of water
- Mass of water
The amount of heat (q) released by ethanol is given by:
Mass of lamp and ethanol initially = 50g
Mass of lamp and ethanol finally = 49.5g
Mass of ethanol burned = 50.0 — 49.5 = 0.5g
Final temperature of water = 42ºC
Initial temperature of water = 20ºC
Rise in temperature = 42ºC – 20ºC = 22ºC
Specific heat capacity of water = 4.2 Jg-1C-1
Heat given out = Mass of water Xspecific heat capacity Xtemperature rise
Repeat similar procedures with kerosene, charcoal, coal, firewood etc. and compare your results. Which fuel has more energy per gram? That is the most efficient fuel.
How reliable is the experiment?
The following table compares the experimental results with values from data book.
|Fuel||Heat of combustion in KJ/mol|
|From the experiment||From a data book|
Note the big difference! The experimental results are almost 40% lower for both fuels. Why do you think there is such a big difference? There are two reasons for this:
- Heat loss: Not all the heat from the burning fuel is transferred to the water. Some is lost to the air, and some to the container that holds the fuel.
- Incomplete combustion: In case of a complete combustion, all the carbon in a fuel is converted to carbon dioxide. But here combustion is incomplete. Some carbon is deposited as soot on the bottom of the lamp and some converted to carbon monoxide. For example, when butane burns, a mixture of all these reactions may take place:
The less oxygen there is, the more carbon monoxide and carbon will form.
Renewable energy sources include biomass, geothermal energy, hydroelectric power, solar energy, wind energy, and chemical energy from wood and charcoal. These are called renewable energy sources because they are replenished within a short time. Day after day, the sun shines, wind blows, river flows and trees are planted. We use renewable energy sources mainly to generate electricity.
In Tanzania most of the energy comes from non-renewable sources. Coal, petroleum, natural gas, propane and uranium are examples of non-renewable energy sources. These fuels are used to generate electricity, heat our homes, move our cars and manufacture many kinds of products. These resources are called non-renewable because they cannot be replenished within a short time. They run out eventually. Once, for example, coal or petroleum is depleted, it may take millions of years to be replaced. So, these are non-renewable energy sources.
Biogas is a gaseous fuel produced by the decomposition of organic matter (biomass). Under anaerobic conditions, bacteria feed on waste organic products, such as animal manure and straw, and make them decay. The product formed from this decay is called biogas, which consists mainly of methane, though other gases such as carbon dioxide, ammonia, etc, may also be produced in very small quantities. The biogas produced can be used as a fuel for cooking, heating, etc.
Raw materials for biogas production may be obtained from a variety of sources, which include livestock and poultry wastes, crop residues, food processing and paper wastes, and materials such as aquatic weeds, water hyacinth, filamentous algae, and seaweeds.
The Working Mechanism of Biogas Plant
Explain the working mechanism of biogas plant
The organic waste products are fed in a biogas plant. Prior to feeding the material into the plant, the raw material (domestic poultry wastes and manure) to water ratio should be adjusted to 1:1 i.e. 100 kg of excreta to 100 kg of water. Then adequate population of both the acid-forming and methanogenic bacteria are added.
The bacteria anaerobically feed on the liquid slurry in the digester. The major product of this microbial decomposition is biogas, which largely contain methane gas. The gas so produced is collected in the gas holder and then taped off. The gas is used as a fuel for cooking, heating and other general purposes.
The biological and chemical conditions necessary for biogas production
Domestic sewage and animal and poultry wastes are examples of the nitrogen-rich materials that provide nutrients for the growth and multiplication of the anaerobic organisms. On the other hand, nitrogen-poor materials like green grass, maize stovers, etc are rich in carbohydrates that are essential for gas production. However, excess availability of nitrogen leads to the formation of ammonia gas, the concentration of which inhibits further microbial growth. This can be corrected by dilution or adding just enough of the nitrogen-rich materials at the beginning.
In practice it is important to maintain, by weight, a C:N close to 30:1 for achieving an optimum rate of digestion. The C:N can be manipulated by combining materials low in carbon with those that are high in nitrogen, and vice versa.
A pH range for substantial anaerobic digestion is 6.0 – 8.0. Efficient digestion occurs at a pH near to neutral (pH 7.0). Low pH may be corrected by dilution or by addition of lime.
To ensure maximum digestion, stirring of the fermentation material is necessary. Agitation (stirring) can be done either mechanically with a plunger or by means of rotational spraying of fresh organic wastes. Agitation ensures exposure of new surfaces to bacterial action. It also promotes uniform dispersion of the organic materials throughout the fermentation liquor, thereby accelerating digestion.
A Model of Biogas Plant
Construct a model of biogas plant
The biogas plant consists of two components: the digester (or fermentation tank) and a gas holder. The digester is a cube-shaped or cylindrical waterproof container with an inlet into which the fermentable mixture is introduced in the form of liquid slurry. The gas holder is normally an airproof steel container that floats on the fermentation mix. By floating like a ball on the fermentation mix, the gas holder cuts off air to the digester (anaerobiosis) and collects the gas generated. As a safety measure, it is common to bury the digester in the ground or to use a green house covering.
Structure of the biogas plant
The Use of Biogas in Environmental Conservation
Explain the use of biogas in environmental conservation
Environmental conservation is a major concern in life. We need to live in a clean and health environment so as to enjoy our lives better. The use of biogas as an alternative source of energy is essential in environmental conservation due to a number of reasons. These are some of the reasons:
- Biogas does not produce much smoke or ash, which could otherwise pollute the atmosphere or land. When the gas is burned it produces very little smoke and no ash as compared to other sources of fuel such as wood.
- The use of biogas for cooking and heating prevents the cutting down of trees to harvest firewood, or burn charcoal for fuel, a practice that could result to soil erosion, drought, etc. Hence, using the biogas as fuel helps to conserve the environment as no more cutting of trees may be done.
- Using cow dung, poultry manure and other excreta for biogas production helps keep the environment clean because these materials are put into alternative use instead of just being dumped on land, a fact that could lead to pollution of the environment.
- Some biomass employed in biogas production is toxic and harmful. By letting these materials be digested by bacteria, they may be turned into non-toxic materials that are harmless to humans, plants, animals and soil.
- The excreta used for production of biogas produce foul smell if not properly disposed of. Using this excrete to generate biogas means no more bad smell in air.
- Health hazards are associated with the use of sludge from untreated human excreta as fertilizer. In general, a digestion time of 14 days at 35ºC is effective in killing the enteric bacterial pathogens and the enteric group of viruses. In this context, therefore, biogas production would provide a public health benefit beyond that of any other treatment in managing the rural health and environment of developing countries.