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
Solid fuels
Solid fuels include wood, charcoal, peat, lignite, coal, coke, etc. The immediate use of all these fuels is for heating and lighting. However, these fuels have a long history of industrial use. 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 is used for generation of electricity. Also Tanga Cement Company uses coal as a source of power to run machines for 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 in proportion to its quantity than is obtained from a corresponding quantity of wood, and has a further advantage of being smokeless. Wood charcoal is often used for cooking and heating, in blacksmithing, etc.
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 use. 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 burning, 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. Anthracite is a very hard black coal and it is the oldest of all types of coal.
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.
Liquid fuels
Liquid fuels include petrol (gasoline) diesel, alcohol (spirit), kerosene (paraffin), liquid hydrogen, etc. Liquid fuels have 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 in wide use 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. Immense heat and pressure resulted in the formation of coal gas and oil.
Energy produced when petroleum products (diesel, petrol, kerosene, natural gas etc) are burned, originated from the sun. This energy was transferred to animals through their consumption of plants or plant products. When the animals died, got buried, and compressed by heat and pressure, they produced oil which gives off that energy when burnt.
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 came from crude oil. Some oil fuel is also used for electricity generation.
Ethanol burns with a clean, non-smoky flame, giving out quite a lot of heat. On a small scale, ethanol can be used as methylated spirit (ethanol mixed with methanol or other compounds) in spirit lamps and stoves. However, ethanol is such a useful fuel that some countries have developed it as a fuel for cars. In countries where ethanol can be produced cheaply, cars have been adapted to use a mixture of petrol and ethanol as fuel.
Brazil has a climate suitable for growing sugarcane. Ethanol produced by fermentation of sugarcane has been used as an alternative fuel to gasoline (petrol), or mixed with gasoline to produce “gasohol”. Currently, about half of Brazil’s cars run on ethanol or “gasohol”. “Gasohol” now accounts for 10% of the gasoline sales in the U.S.A.
The idea about the use of biofuel for fuelling automobiles and other machines has been borrowed by other countries including Tanzania. However, the programme has raised a bitter concern among different activists. Their doubt is that emphasis on growing crops for biofuel production may take up land that could otherwise be used for growing food crops. This, therefore, would mean that there would not be enough land to grow enough food to feed the ever-increasing human population. Hence, hunger will prevail. Notwithstanding all these shouting, biofuel crop production is there to stay!
Gaseous fuels
The use of gaseous fuels for domestic heating is common in urban areas. Compressed gas that is delivered to our homes in steel cylinders is liquefied propane, butane, or mixture of the two. When the valve is opened, the liquid gas vapourizes 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 naturally found in nature: -natural gas -methane from coal mine
- Fuel gas from solid fuels or materials: -gas derived from coal (water gas and producer gas) -gas 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 a solid carbonaceous 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. Carbonaceous 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:
As the carbon dioxide formed rises up through the red-hot coke, it is reduced to carbon monoxide:
Since more heat (406 kJ) is produced in the lower part than is absorbed in the upper part of the furnace (163 kJ), some excess heat is obtained in the long run. This heat keeps the coke hot. 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 good deal of producer gas contains nitrogen, a gas that does not support combustion, it has a lower calorific 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
A 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 posses 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, sulphur dioxides, etc, which pollutes the air.
It must 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 high pyrometric burning effect (highest temperature that can be reached by a 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 or food, 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, MJ/m3, etc. Heating value is commonly determined by use of an instrument called bomb calorimeter.
By custom, 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.
Calorific values of solid, liquid and gaseous fuels
| 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 15ºC, 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 one mole of fuel burns is called heat of combustion. This is often written as
This value can be measured in the laboratory indirectly by burning the fuel to heat water. Simple apparatus is shown in figure bellow. The basic idea is: Heat gained by the
Heat gained by the water = heat given out by the fuel.
Method
These are the steps:
- Pour a measured volume of water into the tin. Since you know its volume you also know its mass (1 cm3 of water has a mass of 1g).
- 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.
Measuring the energy value of a fuel
Calculations
It takes 4.2J of energy to raise the temperature of 1g of water by 1ºC. This constant value is called specific heat capacity of water, usually represented as 4.2Jg-1C-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 given out = 4.2g-1C-1 mass of water (g) its rise in temperature (ºC).
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
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) |
| Results | Results |
| 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 |
| Calculations | Calculations |
| 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 |
Example 2
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.
Experiment
Aim: To find out the energy value of ethanol.
Materials: water, beaker, thermometer, weighing balance, spirit lamp and ethanol.
Procedure:
- 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:
Specimen calculation:
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
Mass of water = 100g
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 | |
| Ethanol | -859 | -1367 |
| Butane | -1827 | -2877 |
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.
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 a 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, oil, etc as a source of mechanical power. In some countries, vehicles have been modified to use natural gas as a source of power. In Tanzania for example, plans are underway to modify car fuel systems so that a natural gas obtained from Songosongo in Kilwa could power cars. This will help a great deal to reduce the cost of running cars on liquid fuels whose price in the world market is continuously escalating. Hydrogen may become an important fuel for cars and homes in the future, as we run out of oil and gas. It has two big advantages:
- Its reaction with oxygen produces just water. No pollution to the environment!
- It is a ‘renewable’ resource. It can be made by electrolysis of acidified water. As cheaper sources of electricity for electrolysis are developed, this may become an attractive option.
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, fuel oil, etc. Most generators use liquid fuels such as petrol and diesel to generate electricity. So, fuels play an important role in electricity production. In Tanzania, coal from Kiwira mines is used for generation of electricity used in Tanga Cement Factory and some industries in Dar es Salaam. This is why escalation of crude oil in the world market results to increased cost of electricity supplied to homes and industries. In developed countries, uranium is used as a fuel to generate electricity which is used at homes and in industries.
Lighting: Kerosene is used in paraffin lamps, tin lamps and hurricane lamps by the rural communities to light homes. The use of paraffin is important in rural areas of Tanzania where 90% of the total population stay and earn their living. It is estimated that only 10% of the population have access to electricity. So, you can see how crucial this fuel is to the majority of the people.
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: manufacture of different kinds of products such as petroleum jelly, nylon and plastic.
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 source 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 destruction, a result that causes many problems to the human society and other organisms as well.
Trees have several advantages apart from providing us with fuels. Trees help in the attraction of rainfall and conservation of water sources in various areas. Trees also help in removing bad gases from air such as carbon dioxide that is emitted to the atmosphere due to various human activities. In so doing, trees help to maintain the balance of gases in the atmosphere.
Trees and other vegetation provide habitats and shelters for wild animals and birds of the air. Presence of trees also help to maintain the survival of microorganisms found in the soil, which are important for the balance of nature. Trees can make our country look beautiful and hence attract local and foreign eco-tourists, a fact which can contribute to our country’s revenue, and economic growth.
Deforestation results to scarcity of rainfall as we are experiencing these years. This is because trees attract rainfall. Scarce rainfall leads to drought. Prolonged drought causes famine. Therefore, people will suffer from famine if they continue to use firewood or charcoal as their sources of fuels.
The other effect is soil erosion, which leads to loss of soil fertility. Trees act as a soil cover, which makes the soil resist the impact of raindrops. Deforestation means removal of the soil cover and hence making the soil bare. It is obvious that tree cutting for firewood or charcoal will expose the soil to agents of soil erosion such as wind, water and animals. This will make the soil more prone to erosion. So long as plants depend on the top soil (which contains more plant nutrients) for survival and existence, an eroded soil will consequently support very few or no vegetation at all. The aftermath of this is soil aridity.
As noted early, trees help absorb excess carbon dioxide produced by respiring living organisms. Cutting down trees will lead to excessive accumulation of carbon dioxide in air. Carbon dioxide, among other gases, is responsible for excessive heating of the earth, a phenomenon called global warming. This is because the gas forms a layer in the atmosphere that acts as a blanket. The layer of carbon dioxide gas so formed prevents heat emitted by the heated earth from escaping to the upper atmosphere. This causes extreme heating of the earth’s surface. Consequences of global warming are many, the worst being drought that could ultimately lead to extinction of plant and animal species.
Vegetation that has dried up due to prolonged drought
In brief, cutting down trees for charcoal and firewood can lead to the following environmental problems:
- prolonged drought spells and hence famine;
- drastic change 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 value;
- broken food chain 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 campaign 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 the bylaws against those people cutting down trees carelessly for charcoal burning. At the same time, the central Government must 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.
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. It plays our favourite songs on the radio and lights our homes. Energy makes our bodies grow and allow our minds to think. 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 source of all energy on earth is the sun.
Forms of energy
Energy exists in many different forms such as heat, light, sound, electrical, etc. The amount of energy can be measured in joules, kilojoules, megajoules, calories, etc. There are many forms of energy, but they can all be put in two categories: Kinetic and Potential.
Forms of energy
| KINETIC ENERGY | 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. |
| Electrical energy is the movement of electrical charges. Everything is made of tiny particles called atoms. Atoms are made of even smaller particles called electrons, protons and neutrons. Applying a force can make some of the electrons move. 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. Light is one type of radiant energy. 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). Nuclear power plants split the nuclei of uranium atoms in a process called fission. The sun combines the nuclei of hydrogen atoms in a process called fusion. Scientists are working creating fusion energy on earth, so that someday there might be fusion power plants. |
| Motion energy is the energy which enables movement of objects and substances from one place to another. Objects and substances move when a force is applied according to Newton’s laws of motion. Wind is an example of motion energy. | Gravitational energy is the energy of position or place. A rock resting at the top of a hill contains gravitational potential energy. Hydropower, such as water in reservoir behind a dam, is an example of gravitational potential energy. |
| Sound energy is the movement of energy through substances in longitudinal (compression/rarefaction) waves. Sound is produced when a force causes an object or substance to vibrate – the energy is transferred through the substance in a wave |
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, e.g. 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.
The Law of Conservation of Energy
Explain the law of conservation of energy
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 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.
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. When the potential energy is all used up, the batteries are dead. In the case of rechargeable batteries, their potential energy is restored through recharging.
The chemical energy in your mobile phone battery can be converted into sound, light, text, etc. The main energy changes that occur in a variety of simple situations are:
- Battery chemical to electrical, sound or light;
- Car engine chemical to mechanical and then kinetic;
- Light bulb electrical to light and heat;
- Parachutist potential to kinetic;
- Solar heat to electrical and kinetic;
- Wind mill kinetic to electrical;
- Running water kinetic to electrical;
- Muscles chemical to kinetic, etc.
What other situations of energy changes do you know? Mention them
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 process, 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.
Experiments on the Conservation of Energy from One Form to Another
Carry out experiments on the conservation of energy from one form to another
Activity 1
Carry out experiments on the conservation of energy from one form to another
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.
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.
