Home CHEMISTRY TOPIC 4: FUEL AND ENERGY | CHEMISTRY FORM 2

TOPIC 4: FUEL AND ENERGY | CHEMISTRY FORM 2

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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.

Fuel Sources

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:
  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 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.
  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. The electricity
    generated from this gas is also sold to Tanzania Electricity Supply
    Company (TANESCO) who distributes the energy to its various clients.
  5. 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
Methods of making charcoal
When
we heat certain organic matter in a limited supply of air, we obtain a
black, solid residue called charcoal. The organic matter can be from
plant or animal sources for example, wood or animal bones. Heating a
substance in 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 material does not burn down to ash as in
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 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 the 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 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
    bellow shows a summary of the stages in the process:
Stages of formation of different types of coal
Name of coal Carbon content
Peat 60%
Pressure and Heat Lignite 70% Hardness
Bituminous coal 80%
Anthracite 95%
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.S(s)+O2(g)->S02(g)
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
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.

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