Important Fuels and their Energy content

Important Fuels and their Energy content:

1. Coal –

Coal is a rock formed from ancient plants that have been decomposed and heated under pressure underground. This process is called metamorphism. Coal varies widely in its composition, as it is a complicated mixture of hydrocarbons and carbohydrates, with small amounts of nitrogen, sulphur, water and minerals. It must be mined from the ground, either from deep mines (which creates spoil heaps), or by open-cast surface mining (which devastates the landscape). There are enough reserves for 200–250 years at the present rate of consumption.

Coal burns in air with a yellow smoky flame, leaving ash behind. The energy content of coal depends upon its type. The heat of combustion of brown coal or lignite is about 25 kJ/g, but the heat of combustion of bituminous coal (used in industry) and anthracite (used in homes) is about 32 kJ/g. When coal burns, it produces water and carbon dioxide. It also produces harmful sulphur dioxide, carbon monoxide, oxides of nitrogen (known as NOx), hydrocarbons and soot.

Coal cannot be used in cars and aircraft, but it can be converted into gaseous or liquid fuel. This fuel can be used to power diesel engines, and German aircraft flew using it in the Second World War. The“liquefaction plants” needed to make it produce carcinogenic (cancer-causing) hydrocarbons.

2. Natural Gas –

Natural gas is nearly always found associated with oil (see the Oil sheet). It consists almost entirely of methane. For example, the Frigg gas field in the North Sea produces gas which is 95% methane and 4% ethane, with only traces of other hydrocarbons. It has no smell and does not contain carbon monoxide, so it is not poisonous. However, it can asphyxiate (suffocate), and so an artificial smell is added before it is distributed so that leaks can be detected.

Natural gas ignites easily, and will cause explosions if sufficiently large quantities escape. It is a relatively clean fuel, and produces only carbon dioxide and water when it burns completely. Its heat of combustion is –890.3 kJ/mol, equivalent to an energy content of 55.6 kJ/g. There are sufficient reserves of natural gas for only 20 years at the present rate of consumption. However, rotting rubbish in landfill sites generates it, and some companies are now pumping it out for sale. It also can be produced by deliberate production of “biogas”, which is about 50% methane. Biogas is produced from decaying organic matter in biogas digesters, and is particularly popular in India and China.

Natural gas is not very portable, and is usually delivered to the customer through underground pipes. It can be liquefied to form NGL (natural gas liquid). This is different from LPG (liquid petroleum gas), which is a liquefied by-product of oil refineries). NGL (and LPG) can be used to power vehicles, and some experimental aircraft.

3. Ethanol –

Ethanol is a member of a large class of compounds called alcohols. It is a colourless liquid (boiling point 78ºC) which will mix with water. It can be made by reacting ethene (a product of crude oil) and water at 300ºC and high pressure with phosphoric acid as a catalyst. It can also be produced from carbohydrates, such as sugar, by fermentation using yeast. This is the method of choice for producing alcoholic drinks.

Ethanol burns very easily with a pale yellow flame to form carbon dioxide and water. Its heat of combustion is –1367.3 kJ/mol, which is equivalent to an energy content of 29.6 kJ/g. Alcohol has been used as a fuel for a long time, for example in spirit lamps and as methylated spirits. It is relatively safe and easy to transport, as it is a liquid. Petrol can be blended with up to 20% ethanol for use in an ordinary car engine without adjustment. Recently, ethanol has been produced by fermentation in industrial quantities to power vehicles. Brazil, in its Proalcohol programme, produces 3,200 million litres of ethanol (“Gasohol”) per year from crops such as sugar cane, sorghum and cassava. Brazil has also designed and manufactured cars to run on pure ethanol. The fuel is more expensive than petrol, but effectively is a renewable energy source and does not have to be imported. However, it is doubtful that hungry people see this as a benefit.

4. Hydrogen –

Hydrogen is the most abundant element in the Universe, but on Earth free hydrogen is less than one part per million of the atmosphere. However, it is abundant on Earth in the form of water, which is an almost inexhaustible supply of the element.

Several million tonnes of hydrogen are manufactured in the world each year, including 500,000 tonnes per year in the U.K. It can be made by the electrolysis of brine (sodium chloride solution), but this process is expensive and inefficient. Other methods include passing steam over white-hot coke (the Bosch or Water Gas Process), and the oxidation of natural gas using a catalyst. It is also found as a by-product in oil refineries.

When hydrogen burns completely, the only product is water. The heat of combustion for hydrogen is –285.8 kJ/mol, which is equivalent to an energy content of 142.9 kJ/g. Hydrogen is very easy to ignite, and caused dreadful accidents (such as the famous “Hindenburg” disaster) when used to lift airships in the days before helium became plentiful. Hydrogen is stored in gas cylinders or liquid containers, allowing it to be used in some domestic or industrial situations where natural gas might be used. It would be difficult to use in cars or aircraft in this way, but large quantities can be absorbed by expensive metals such as palladium, and released later by warming.

5. Summary - Average energy content of coal, oil and natural gas:

Crude Oil - 47.9 kJ/g

Natural Gas - 55.6 kJ/g

Lignite Coal - 25 kJ/g

Bituminous and Anthracite coal - 32 kJ/g

Carbon emission from bio-fuels:

Carbon emission from bio-fuels:

Bio-fuels are the fuels of solid, liquid or gaseous in nature, which has been derived from bio-mass – recently living organisms or their metabolic byproducts. Thus, it could be oils from plants, manure from cows, wood from trees etc. For example, bio-gas (i.e., gas produced by the biological breakdown of organic matter in the absence of oxygen); bio-ethanol; bio-diesel; straight vegetable oil etc., are the bio-fuels. It is a renewable energy source, mostly have agricultural based, unlike other natural resources such as petroleum, coal and nuclear fuels.

It has been seen that certain social and environmental benefits bio-fuels has as compare to use of fossil fuels, such as reduction of greenhouse gas emission, increased national energy security, increased rural earnings and development and above all, reduction of use of fossil fuel.

Bio-fuels and other forms of renewable energy are thought to be ‘carbon neutral’ or ‘carbon negative’. Carbon neutral or carbon negative is the difference of quantum of carbon produced and emitted to the atmosphere when these are used as fuels and the quantum of carbon absorbed in the process of their growth. If both are same, is called carbon neutral or if quantum of carbon absorbed through photo-synthesis is more than the emission is called carbon negative. Both the cases are advantageous towards environment point of view and reduction of global warming.

Strictly speaking, bio-fuels are neither carbon neutral nor carbon negative. This is because extra energy is required to grow crops and process them into fuel. This extra energy releases extra carbon to atmosphere as emission. For example, plants require fertilizer to grow, requires energy for transportation and processing; this extra energy releases carbon to the atmosphere as emission. Therefore, this emission aspect is to be debated, whether we are really gaining in respect of carbon emission, by using bio-fuel. However, the arable lands can be better utilized if people shift towards bio-fuels; so the rural earnings. The poorly irrigated land mass also can be taken up for cultivation.

Petroleum / crude oil / natural gas (exploration, production, processing etc.):

Petroleum / crude oil / natural gas (exploration, production, processing etc.):

Petroleum or more commonly known as Crude oil, available naturally below surface of the earth; used by human being as fuel and applicable for production of various chemicals. Modern society heavily depends on Petroleum / crude oil inclusive of natural gas. Crude oil has become most significant fuel and raw material for production of chemicals in present day world. Because of higher demand for hydrocarbons, pressure on exploration, production and further processing of crude oil / natural gas is tremendous and due this the prices of crude oil have touched sky level recently.

The main activities of the petroleum industry are: (a) Exploration of crude oil and natural gas; (b) Production of crude and natural gas; (c) Processing consists of refining crude for gasoline, diesel, lubricants and further production other down stream chemicals by cracking; (d) Marketing and distribution of gasoline, diesel, lubricants and other processed chemicals.

A. Exploration: Petroleum / crude oil / natural gas are extracted mainly from oil wells found in various oil fields spread all over the world. Various methods are employed in exploration of crude and natural gas and to optimize the process techno-economically. Most exploration process depends on highly sophisticated technology to detect and determine the extent of these deposits. Exploration is initiated with gravity survey, magnetic survey to detect large scale features of the sub-surface geology of the area of investigation. More detailed seismic surveys are conducted at the areas of interest (known as leads). Seismic surveys are basically based on the principle of velocity / time taken by sound wave on reflection at various formations of ground strata, as velocity differs at various interfaces. Finally, when a prospect has been identified, evaluated and passes the selection criteria, exploration wells are drilled to know the presence of oil or gas.

B. Oil / gas Production: Next stage after exploration is production or recovery from the oil reservoir identified after exploration. (a) Primary recovery of oil and gas is done when underground pressure in the oil reservoir is sufficient. This pressure forces the oil to bring it to surface. (b) In Secondary oil recovery various techniques are employed, as this is done when pressure is low in the underground oil reservoir. Pumps such as, beam pumps, electrical submersible pumps are used to bring the oil to the surface. Water injection, gas lift (when CO2 or air is used) and natural gas re-injection are some of the methods also applied when required. (c) Tertiary oil recovery is mainly done to maintain recovery from very low pressure oil reservoir; by reduction of viscosity of underground oil by injecting heat, vapour, surfactants, solvents or miscible gases (CO2).

C. Processing of crude: Processing of crude is done in oil refineries; where more useful petroleum products such as gasoline, kerosene, diesel, LPG (liquefied petroleum gas) etc., are produced by a process known as fractional distillation. Products having lower boiling point leaves from top and higher boiling point products leave the fractioning column at the bottom. Therefore, light distillates are LPG, gasoline, naptha; middle distillates are kerosene, diesel and heavy distillates including residuum are fuel oil, lubricating oils, wax and tar.

D. Petroleum is the major source of organic chemicals. In petrochemical industry major chemicals are derived from two constituents of crude oil – Xylene and Naptha. These chemical raw materials are broken down into basic chemicals such as Butadiene. Basic chemicals are made into finished chemicals or chemical products either direct conversion or through an intermediate. For example, butadiene can be directly made into synthetic rubber or butadiene may be processed into an intermediate such as acetone and later acetone may be used for manufacturing other finished polymers.

• Raw material – Naphtha, Xylene.

• Basic chemicals – Butadiene, Ethylene, Propylene etc.

• Intermediates – Acetone, Phenol, Acrylonitrile, Vinyl acetate, Ethanol etc.

• Polymers – PVC, Polyester fiber, Polystyrene, Polyethylene etc.

E. Major Crude oil and natural gas producing countries are: (a) Saudi Arabia (an OPEC country); (b) Russia; (c) USA; (d) Iran (an OPEC country); (e) China; (f) Mexico; (g) Canada; (h) UAE (an OPEC country); (i) Venezuela (an OPEC country); (j) Norway; (k) Kuwait (an OPEC country); (l) Nigeria (an OPEC country); (m) Brazil; (n) Algeria (an OPEC country); (o) Iraq.