Chemistry
Fuels And Energy
Chemistry is all around us. Did you know that everything is made out of chemicals? Any reaction taking place in body cells of all living and non living organisms correlates with chemistry. In chemistry we study materials that make up the earth and universe. Chemistry is sometimes called the central science because it bridges other natural sciences, including physics, geology and biology. Therefore, when talking about chemistry, we are referring to life on Earth.
Fuels And Energy
Introduction
A fuel is any substance that can be combusted or burnt to release energy as a byproduct. The energy can be in the form of heat, light, electricity, and sound. This energy can be harnessed to power machines or be used for other purposes such as heating or lighting. Examples of fuels include petroleum products (petrol, diesel, fuel oil, kerosene, and spirits), natural gas, coal, wood, charcoal, producer gas, and water gas. In this chapter, you will learn about fuel sources, categories of fuels, uses of fuels, conservation of energy, and renewable energy. By the end of the chapter, you should be able to describe methods of obtaining fuels from locally available materials, and classify fuels according to their states and efficiency. You should also be able to assess the environmental effects of using charcoal and firewood as a source of fuels, and carry out experiments on the conversion of energy from one form to another. Lastly but not least, you should be able to construct a model biogas plant, and explain the use of biogas in environmental conservation
Fuel Sources
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:
Different Sources of Fuels
Identify different sources of fuels
1. 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.
2. Charcoal, This fuel is made by burning 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.
3. 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
4.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
5. Petroleum products (kerosene, diesel, petrol, fuel oil, and fuel gas), 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
Methods of making charcoal
When certain organic matters are heated in a limited supply of air, a black solid residue called charcoal is obtained. The organic matter can be from plant or animal sources, for example, wood or animal bones. Heating a substance in a limited supply of air is called destructive distillation.
Wood or bone charcoal is made by the process of destructive distillation of wood or bones, respectively. Charcoal is largely pure carbon. The entry of air during carbonization (destructive distillation) process is controlled so that the organic materials do not burn down to ashes as in a conventional fire, but instead decompose to form charcoal
Procedure for making wood charcoal
- Cut wood into small pieces.
- Arrange the wood pieces into a pile of wood on the ground
- Cover the pieces of wood with soil, leaving one open space for setting a fire
- Set fire to the wood and then cover the open space with soil. Make sure that the wood is burning
- After the wood is burned, uncover the soil and pull out the black solid substance underneath. This is charcoal
Coal formation
Coal is formed from the remains of lush vegetation that once grew in warm shallow coastal swamps. The following are the stages in the process of coal formation:
- The dead vegetation collects in the bottom of the swamp. It may start to decay. But decay soon stops, because the microbes that cause it need oxygen, and the oxygen dissolved in the stagnant, warm water is quickly depleted
- The vegetation is buried under debris
- Over hundreds of thousands of years, the environment changes. Seas flood the swamps. Heavy layers of the sediment pile up on the dead vegetation, squeezing out gas and water and turning it into peat
- As the peat is buried deeper, the increasing heat and pressure compress it progressively to form different types of coal
- As the process continues, the coal gets harder and more compact. Its carbon content also increases, giving different types of coal. Table 4.1 shows a summary of the stages in the process:
As carbon content increases so does energy given out per unit weight. But hard coal tends to have higher sulphur content, hence likely to cause environmental pollution. When burnt, the sulphur in the coal produces sulphur dioxide gas that is released into the atmosphere, causing air pollution.Sulphur + Oxygen=Sulphur dioxide
Categories of Fuels
Fuels can be classified into three groups according to the physical states of the fuels. 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
1. Solid fuels, Solid fuels include wood, charcoal, peat, lignite, coal, and coke. The immediate use of all these fuels is for heating and lighting. However, these fuels have a long history of industrial uses.
Coal was the fuel for the industrial revolution, from firing furnaces to running steam locomotives and trains. Wood was extensively used to run locomotives. Coal is still used for generation of power until now. For example, in Tanzania, the coal mined at Kiwira-Mbeya is used for generation of electricity. Also, Tanga Cement Company uses coal as a source of power to run machines for the production of cement
Wood is used as a solid fuel for cooking, heating or, occasionally, as a source of power in steam engines. The use of wood as a fuel source for home heating is as old as civilization itself. Wood fuel is still common throughout much of the world. It is the main source of energy in rural areas.
Wood charcoal yields a large amount of heat and has a further advantage of being smokeless. Wood charcoal is often used for cooking and heating, and in blacksmithing.
Animal charcoal is used for sugar refining, water purification, purification of factory air and for removing colouring matter from solutions and from brown sugar. Animal charcoal is made by destructive distillation of animal bones.
Coke is a fuel of great industrial uses. Coke is obtained by destructive distillation of coal. Most of the coke produced in industry is used as a reducing agent in the production of metals such as pig iron. A substantial amount of coke is also used for making industrial gases such as water gas and producer gas. Coke is a better fuel than coal because when it is burnt, it produces a clean and smokeless flame. When coal is used as a fuel, it produces many toxic gases during burning. Coke has high heat content and leaves very little ash
Coal is a complex mixture of substances, and its composition varies from one place to another. It depends on coal's age and condition under which it was formed
When coal is heated in a limited supply of air, it decomposes. This thermal decomposition is called destructive distillation of coal. The products are coke, coal tar, ammoniacal liquor and coal gas
2. Liquid fuels, Liquid fuels include petrol (gasoline) diesel, spirit, kerosene (paraffin), and liquid hydrogen. Liquid fuels have an advantage over solid fuels because they produce no solid ashes, and can be regulated by automatic devices. They are relatively more convenient to handle, store and transport than solid fuels.
Most liquid fuels are derived from fossils. Fossil fuels include coal, natural gas and petroleum. These fuels are formed from remains of sea plants and animals which lived millions of years ago. The remains became buried under layers of sediment. The immense heat and pressure resulted in the formation of coal gas and oil.
Petroleum fuels are used in cars and in various other machines. Fuels used in cars and lories (petrol and diesel), kerosene (for jet aircraft) and fuel oil (for ships), all come from crude oil. Some oil fuel is also used for electricity generation.
Spirit burns with a clean, non-smoky flame, giving out quite a lot of heat. On a small scale, spirit can be obtained as methylated spirit (ethanol mixed with methanol or other compounds) in spirit lamps and stoves.
3. Gaseous fuels, The use of gaseous fuels for domestic heating is common in urban areas. The compressed gas that is delivered to our homes in steel cylinders is liquefied propane, butane, or a mixture of the two. When the valve is opened, the liquid gas vaporizes quickly into gas and passes through a pipe to the stove. Gaseous fuels are the most convenient fuels to handle, transport and store.
The following is a list of types of gaseous fuels:
- Fuel obtained naturally such as natural gas and methane from coal mine
- Fuel gas from solid fuels or materials which are derived from coal (water gas and producer gas) together with those derived from wastes and biomass (biogas)
- Fuel gas made from petroleum
Gaseous fuels used in industry
Producer gas and water gas are important industrial fuels
Producer gas, Producer gas is produced by burning the solid carbonated fuel, such as coke, in a limited supply of air in a producer furnace. The reaction is exothermic and this makes coke to get hotter. Carbonated fuels are fuels that contain a high proportion of carbon. The producer gas is a mixture of carbon monoxide and nitrogen.
When air, mixed with a little steam, is passed through the inlet in the lower part of the furnace, the coke (carbon) combines with oxygen (from air) to form carbon dioxide:
The symbol ΔH is called enthalpy change (or energy change) and the negative sign shows that the energy is liberated (produced)
As the carbon dioxide formed rises up through the red-hot coke, it is reduced to carbon monoxide:
The positive sign shows that the energy is consumed (absorbed)
The nitrogen gas in the air is not affected at all during the process. Hence, the overall reaction equation may be represented as follows:
As a fuel, producer gas burns to give out carbon dioxide.
Because a producer gas contains nitrogen, a gas that does not support combustion, it has a lower energy value compared to water gas. See Table 4.2 for comparison.
Water gas, Water gas is produced by passing steam over white-hot coke at 1000 °C. The gas is a mixture of hydrogen and carbon monoxide. The reaction is endothermic, causing the coke to cool.
Water gas burns as a fuel to give carbon dioxide and steam
However, carbon monoxide is a very poisonous gas. The gas made from petroleum or coal contains some carbon monoxide, which makes it poisonous. Natural gas is safer and efficient, as it contains no carbon monoxide
Characteristics of a good fuel
Good fuel burns easily to produce a large amount of energy. Fuels differ greatly in quality. There are certain characteristics, which make a good fuel. After all, there is no fuel among the different fuels known that possesses all the virtues that a good fuel should have. Generally, a good fuel has the following characteristics:
- It should be environmentally friendly (not harm the environment) in the course of its production and use, that is, it should not produce harmful or toxic products such as much smoke, carbon dioxide, carbon monoxide, and sulphur dioxides, which pollute the air
- It should be affordable to most people, i.e. it must be cheap
- It should not emit or produce dangerous by-products such as poisonous fumes, vapour or gases
- It should have high calorific value, i.e. it must burn easily and produce a tremendous quantity of heat energy per unit mass of the fuel
- It should be easy and safe to transport, store, handle and use
- It should be readily available in large quantities and easily accessible
- It should have a high pyrometric burning effect (the highest temperature that can be reached by the burning fuel). Normally gaseous fuels have the highest pyrometric effect as compared to liquid and solid fuels
- It should have a moderate velocity of combustion (the rate at which it burns) to ensure a steady and continuous supply of heat
- A good fuel should have an average ignition point (temperature to which the fuel must be heated before it starts burning). A low ignition point is not good because it makes the fuel catch fire easily, which is hazardous, while high ignition point makes it difficult to start a fire with the fuel
- A good fuel should have a low content of non-combustible material, which is left as ash or soot when the fuel burns. A high content of no-combustible material tends to lower the heat value of the fuel
Calorific values of fuels
The heating value or calorific value of a substance, usually a fuel, is the amount of heat released during the combustion of a specific amount of it. The calorific value is a characteristic of each substance. It is measured in units of energy per unit of substance, usually mass, such as Kcal/Kg, J/g, KJ/Kg, KJ/Mol, and MJ/m3. Heating value is commonly determined by use of an instrument called bomb calorimeter
The basic calorific value for solid and liquid fuels is the gross calorific value at constant volume, and for gaseous fuels, it is the gross calorific value at constant pressure
| Solid and liquid fuels | Calorific value (MJ/kg) |
| Alcohols | |
| Ethanol | 30 |
| Methanol | 23 |
| Coal and coal products | |
| Anthracite (4% water) | 36 |
| Coal tar fuels | 36–41 |
| General purpose coal (5–10% water) | 32–42 |
| High volatile coking coals (4% water) | 35 |
| Low temperature coke (15% water) | 26 |
| Medium-volatile coking coal (1% water) | 37 |
| Steam coal (1% water) | 36 |
| Peat | |
| Peat (20% water) | 16 |
| Petroleum and petroleum products | |
| Diesel fuel | 46 |
| Gas oil | 46 |
| Heavy fuel oil | 43 |
| Kerosene | 47 |
| Light distillate | 48 |
| Light fuel oil | 44 |
| Medium fuel oil | 43 |
| Petrol | 44.80–46.9 |
| Wood | |
| Wood (15% water) | 16 |
| Gaseous fuels at 15 ºC, 101.325 kPa, dry | Calorific value (MJ/m3) |
| Coal gas coke oven (debenzolized) | 20 |
| Coal gas low temperature | 34 |
| Commercial butane | 118 |
| Commercial propane | 94 |
| North sea gas, natural | 39 |
| Producer gas coal | 6 |
| Producer gas coke | 5 |
| Water gas carburetted | 19 |
| Water gas blue | 11 |
Measuring the heat given out by fuels
We burn fuels to provide us with heat energy. The more heat a fuel gives out the better. The amount of heat given out when a specified amount of fuel burns is called heat of combustion. This is often written as ΔHcombustion. This value can be measured in the laboratory indirectly by burning the fuel to heat water. Simple apparatus is shown in Figure 4.1. The basic idea is: Heat gained by the water = heat given out by the fuel
Experiment 4.1
Aim: To measure the amount of heat given out by a fuel
Materials: thermometer, beaker, spirit burner, common balance, iron stand, and water
Procedure
- Pour a measured volume of water into a beaker. Since you know its volume you also know its mass (1 cm3 of water has a mass of 1 g)
- Weigh the fuel and its container
- Measure the temperature of the water.
- Light the fuel and let it burn for a few minutes
- Measure the water temperature again, to find the increase.
- Reweigh the fuel and container to find how much fuel was burned.
Calculations
It takes 4.2 J of energy to raise the temperature of 1g of water by 1 ºC. This constant value is called specific heat capacity of water, usually represented as 4.2 J g-1 °C-1 (4.2 joules per gram per centigrade).
So, you can calculate the energy given out when the fuel burns by using this equation: Energy = specific heat capacity × mass × change in temperature That means
Then since you know what mass of fuel you burned you can work out the energy that would be given out by burning one mole of it
Example 1 , Table 4.3 shows the results obtained by burning ethanol and butane. Study them carefully and learn how to calculate the calorific values of ethanol and butane
| Ethanol (burned in a spirit lamp) | Butane (burned in a butane cigarette lighter) |
| Results | Results |
| Mass of ethanol used: 0.9 g | Mass of butane: 0.32 g |
| Mass of water used: 200 g | Mass of water used: 200 g |
| Temperature rise: 20 ºC | Temperature rise: 12 ºC |
| Calculations | Calculations |
| Heat given out = 4.2 × 200 × 20 =16800 J or 16.8 kJ | Heat given out = 4.2 × 200 × 12 = 10080 J or 10.08 kJ |
| The formula mass of ethanol (C2H5OH) is 46. 0.9 g gives out 16.8 KJ of energy 46 g gives out, X kJ of energy So, –859 kJ of energy | The formula mass of butane (C4H10) is 58. 0.32 gives out 10.08 kJ of energy. 58 g will give out X kJ of energy So, –1827 kJ of energy |
| So, ΔHcombustion for ethanol is -859 kJ/mol | So, ΔHcombustion 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.
Example 2, A 0.5 g sample of ethanol was burnt in a spirit lamp and raised the temperature of 100 ml of water from 20 oC to 42 oC. If the molecular weight of ethanol is 46 g mol–1, calculate the amount of energy liberated when 0.5 g of ethanol were burnt.
The amount of heat (q) released by ethanol is given by:
The following table compares the experimental results with values from data book.
| Fuel | Heat of combustion in kJ mol–1 | |
| From the experiment | From a data book | |
| Ethanol | -859 | -1367 |
| Butane | -1827 | -2877 |
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.
Uses of Fuels
Uses of Fuel
List uses of fuels
You have already learned different types of fuels and their energy values. Fuels can be put into several uses. The use of a given kind of fuel for a particular function depends on the economic value of that use. Generally, the uses of fuels include the following:
- Source of mechanical power: Vehicles, machines and several other devices are powered by fuels such as diesel, petrol, and oil, as a source of mechanical power. In some countries, vehicles have been modified to use natural gas as a source of power
- Cooking and heating: Fuels like wood, liquefied gas (propane or butane or a mixture of the two), charcoal and kerosene are burned to provide energy for cooking and heating. When burned, these substances provide enough heat to cook food and even heat different substance at home. Inhabitants of cold countries in temperate regions of the world burn different kinds of fuels to produce heat for heating homes and water
- Generation of electricity: The machines and devices responsible for electricity production and supply are fuelled by heavy liquid fuels such as diesel. Most generators use liquid fuels such as petrol and diesel to generate electricity. In Tanzania, coal from Kiwira mines is used for generation of electricity used in Tanga Cement Factory and some industries in Dar es Salaam
- Lighting: Kerosene is used in paraffin lamps, tin lamps and hurricane lamps to light homes. The use of paraffin is important in some rural areas of Tanzania.
- Industrial uses: Industrial operations such as welding and metal fabrication make use of oxyacetylene flame which produces extremely high heat to melt and cut metals.
- Other alternative uses: Manufacturing of different kinds of products such as petroleum jelly, nylon and plastics
The Environmental Effects on Using Charcoal and Firewood as Source of Fuels
Assess the environmental effects on using charcoal and firewood as source of fuels
Trees are the most common sources of fuels in developing countries like Tanzania. Fuels from trees are mainly used for domestic purposes. People cut down trees for firewood and for burning charcoal that is mainly supplied to urban areas to be used as fuel
Because of the rapidly growing human population, the demand for trees as a source of fuel has ever increased to the extent that this resource is no longer sustainable. The act of cutting down trees for firewood, charcoal, timber, and for obtaining logs that are shipped to overseas has made this resource to be depleted. This leads to environmental degradation, a result that causes many problems to human society and other organisms as well. The process of cutting trees without replanting other trees is known as deforestation. Deforestation results in the scarcity of rainfall as we are experiencing these years. This is because trees attract rainfall. Scarce rainfall leads to drought. Prolonged drought causes famine because the soil cannot support crops anymore as shown in Figure 4.2. Therefore, people will suffer from famine if they continue to use firewood or charcoal as their sources of fuels
In brief, cutting down trees for charcoal and firewood can lead to the following environmental problems:
- Prolonged drought spells and hence famine
- Drastic changes in rainfall patterns
- Global warming and climate change
- Increased soil erosion and rapid depletion of soil nutrients
- Increased aridity and desertification
- Loss of valuable species of economic or medicinal values
- Broken food chains and reduced ecosystem stability
- Destruction of animal habitats and shelters
- Extinction of animal, microbial and plant species; and
- Loss of biodiversity
Therefore, it is important to plant more trees and to reduce our dependence on trees for fuels in order to improve our environment. Tree planting campaigns should be a regular practice and the trees that have already been planted should be cared for. Natural forests should be conserved. Local Governments should be encouraged to make and enforce bylaws against those people cutting down trees carelessly for charcoal burning. At the same time, the Central Government should look for the alternative energy sources for her citizens urgently
Continued use of trees for fuels will end up our life on earth. Let us take actions to conserve our environment so that we continue living a healthy life.
Conservation of Energy
Energy is defined as the ability to do work or bring about change. Energy makes changes; it does things for us. It moves cars along the road, and boats over the water. It bakes cakes in the oven and keeps ice frozen in the freezer. Energy is used for playing our favourite songs in the radio and lighting our homes. Energy makes our bodies grow. People have learned how to change energy from one form to another so that we can do work more easily and live more comfortably. The main source of all energy on earth is the sun
Forms of energy
Energy exists in many different forms such as heat, light, sound, and electrical energy (Table 4.5). The amount of energy can be measured in joules, kilojoules, megajoules, and calories. There are many forms of energy, but they can all be put in two categories: Kinetic and Potential Energy. Kinetic energy is energy in motion of waves, electrons, atoms, molecules, substances, and objects. Potential energy is stored energy and the energy of position-gravitational energy
| Kinetic energy | Potential energy | |
| Electrical energy is the movement of electrical charges. Electrical charges moving through a wire is called electricity. Lightning is another example of electrical energy. | Chemical energy is energy stored in the bonds of atoms and molecules. This energy holds these particles together. Biomass, petroleum, natural gas, and propane are examples of stored chemical energy. | |
| Radiant energy is electromagnetic energy that travels in transverse waves. Radiant energy includes visible light, x-rays, gamma rays and radio waves. Solar energy is an example of radiant energy. | Stored mechanical energy is energy stored in objects by the application of a force. Compressed springs and stretched rubber bands are examples of mechanical energy. | |
| Thermal energy, or heat energy, is the internal energy in substances caused by the vibration and movement of the atoms and molecules within substances. Geothermal energy is an example of thermal energy. | Nuclear energy is energy stored in the nucleus of an atom - the energy that holds the nucleus together. The energy can be released when the nuclei are combined or when a nucleus splits apart (disintegrates). |
Kinetic energy is energy in motion. Its existence can be shown by winds, ocean currents, running water, moving machines or a falling body. Potential energy is energy at rest. It is found stored in different forms, for example, in coal, petroleum and natural gas, batteries and muscles. Such energy does not work so long as it is stored. It is capable of doing work when it is converted to other forms of energy such as heat, light or radiation
Energy conversion (Energy changes)
Can energy be created or destroyed? When wood or charcoal is burned, it appears as if energy is destroyed and wasted. In fact, the energy in these kinds of fuels is not destroyed when the fuels are burned. It is simply converted to other forms of energy such as heat and light. When you are seated on a desk in class, you are possessing potential energy. When you stand up and walk away from the classroom, you are transforming the potential (chemical) energy in your muscles to kinetic energy
The Law of Conservation of Energy
Explain the law of conservation of energy
states that energy can neither be created nor destroyed but it can only be changed from one form to another. When we use energy, it does not disappear. We simply convert it from one form to another
Experiments on the Conservation of Energy from One Form to Another
Carry out experiments on the conservation of energy from one form to another
A car engine burns petrol, converting the chemical energy in the petrol to mechanical energy and again from mechanical energy to kinetic energy. Solar cells change radiant energy into electrical energy. Energy changes its form, but the total amount of energy stays the same
Potential (chemical) energy in a dry cell is converted to electrical energy which is finally converted to sound energy in radio speakers. In a tape record player, the same chemical energy is ultimately converted to kinetic energy to drive the cassettes
All energy changes that occur during chemical and physical changes must conform to the Law of Conservation of Energy, that is, energy can only be changed from one form into its equivalent of another form with no total loss or gain.
The most common form of energy in chemistry is the heat change. A chemical reaction must involve some change in energy. As the reaction occurs, chemical bonds of reactant molecules are broken while those of the product molecules are formed. Energy is given out when a chemical bond forms and it is consumed when a bond is broken
Take an example of combustion (respiration) of glucose in living cells:
During respiration, the bonds of glucose and oxygen are broken down while those of carbon dioxide and water are formed. Heat is absorbed when chemical bonds are broken and it is released when the bonds are formed. The total amount of heat absorbed by the reactants is equal that released by the products. Heat absorbed is given a positive sign (+ve) while heat given out is assigned a negative sign (-ve). So the total energy change is equal to zero. This means that no energy has been created or destroyed
Renewable Energy Biogas
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, the 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, and ammonia, may also be produced in very small quantities. The biogas produced can be used as a fuel for cooking, and heating
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, 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. Then the 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 contains methane gas. The gas so produced is collected in the gas holder and then tapped 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, and maize stovers, 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.
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 (Figure 4.3). The digester is a cube-shaped or cylindrical waterproof container with an inlet into which the fermentable mixture is introduced in the form of a 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 greenhouse covering
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 healthy 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 in soil erosion, and drought. 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 help 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 used in biogas production is toxic and harmful. By letting these materials to 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 the production of biogas produce foul smell if not properly disposed of. Using these excreta 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 35 ºC 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
Chapter summary
A fuel is any substance that can be combusted or burnt to release energy as a by-product
Fuels can be classified according to their states as solid fuels, liquid fuels or gaseous fuels. Producer gas and water gas are important industrial fuels.
The heating value or calorific value of a fuel is the amount of heat released during the combustion of a specific amount of it. The amount of heat given out when one mole of fuel burns is called heat of combustion.
Some uses of fuels include generation of mechanical power, cooking and heating, generation of electricity, and lighting.
Dependency on firewood as a source of energy can lead to environmental problems such as prolonged drought spells, global warming, soil erosion, desertification, and loss of biodiversity.
Energy is the ability to do work or bring about change. There are many forms of energy, but they can all be put in two categories: Kinetic energy and Potential energy.
Kinetic energy is energy in motion of waves, electrons, atoms, molecules, substances, and objects
Potential energy is stored energy and the energy of position - gravitational energy. Renewable energy sources include biomass, geothermal energy, hydroelectric power, solar energy, wind energy, and chemical energy from wood and charcoal
Non-renewable energy sources include coal, petroleum, natural gas, propane and uranium.
Biogas is a gaseous fuel produced by the decomposition of organic matter (biomass). Use of biogas as a source of energy conserves the environment
Review questions
Question Time 4
A.1. ______ is any substance that can be combusted or burned to release energy
- Food
- Fuel
- Coal
- Wood
2. Wood charcoal is made by the process called________.
- fractional distillation of wood
- destructive distillation of wood
- burning
- decarbonation
3. Three of these substances have something in common. Which is the odd one out?
- Wood
- Coal
- Natural gas
- Water
4. Which of the following liquid fuels is not commonly used as a source of energy in Tanzania?
- Petrol
- Diesel
- Paraffin
- Biodiesel
5. Which is not a necessary condition for biogas production?
- A carbon-nitrogen ratio (C:N) close to 30:1
- A pH ranging between 6.0 and 8.0
- Agitation (stirring)
- Spraying warm water on fermentation material
B. Write True for correct statements and False for wrong statements
- Coal is formed from the remains of dead animals.
- Hard coal tends to have higher sulphur content than soft coal.
- Cutting down trees carelessly may lead to famine.
- Water gas and producer gas are gaseous fuels used in industry.
- Calorific value of a fuel is the amount of energy produced when 1 kg of that fuel is completely burned in air.
- Energy is the ability to do work or bring about change.
- The source of all energy on earth is the sun.
- When mobile battery energy is converted to light some energy is lost.
- During chemical reactions, the chemical bonds of the reactants are broken while those of the products are formed
- Coal is renewable energy.
3. Define the term fuel
4.Give two examples in each of the following types of fuels:
- Liquid fuels
- Solid fuels
- Gaseous fuels
5. briefly
- Name any four (4) sources of energy in Tanzania.
- Briefly explain how coal is formed.
- Explain the procedure for making wood charcoal.
- What do you understand by the term heating value or calorific value of a fuel?
- How can you use the calorific value to explain the efficiency of a fuel?
- Explain why liquid fuel is more efficient than a solid fuel.
- Explain any four uses of fuels.
- Why are the results obtained from the experimental determination of energy values in the laboratory very different from the actual values?
- Define the term energy.
- State any five (5) forms of energy.
- Give any four (4) examples of energy changes.
- State the Law of Conservation of Energy
- Using relevant examples, explain the fact that "energy cannot be created nor destroyed but it can only be transformed from one form to another"
- Define the term heat of combustion
- Describe the structure of a biogas plant.
- Explain why the use of biogas as a fuel is said to help in environmental conservation. Give concrete examples where necessary
6. Name two (2) gaseous fuels used in industries. State the composition of each of the two gases and explain how each is formed
7. List down three (3) advantages of liquid fuels over solid fuels
8. State five (5) characteristics of a good fuel
9. Assess the environmental effects on using firewood and charcoal as sources of fuel, basing your argument on:
- soil erosion
- global warming
- drought and famine
- air pollution
- ecosystem balance
10. Calor is a hydrocarbon called propane, C3H8. In an experiment, 1 gram of propane was used to heat 1000 cm3 of water. The temperature of the water rose from 20 ºC to 32 ºC.
- By how much degrees did the water temperature rise?
- What amount of energy is given out when one mole of propane burns? (C =12, H =1)
11. Describe how you would attempt to find, by experiment, the quantity of heat liberated when one gram of liquid methanol, CH3OH, is burnt in air
12. The table below shows the heat liberated when 1.0 g of each of the three alcohols is burnt in air. For each of these alcohols, calculate the heat of combustion in KJ per mole
| Alcohol | Heat evolved |
| Methanol (CH3OH) | 22.6 kJ |
| Ethanol (C2H5OH) | 29.7 kJ |
| Propanol (C3H7OH) | 33.4 kJ |
13.Define the following terms
- Renewable energy
- Non-renewable energy
REFERENCES
- Gallagher, R. & Ingram, P. (2000). Complete Chemistry. Oxford University Press. London.
- Harwood, R. (2003). Chemistry (2nd ed.). Cambridge University Press. London.
- Tanzania Institute of Education (1995). Secondary School Chemistry, Book One. NPC (KIUTA). Dar es Salaam.
- SCSU, MoEVT-Zanzibar. (2008). Chemistry for Zanzibar Secondary Schools, Forms 3 & 4. Oxford University Press. Dar es Salaam
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