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

Use of Hydrogen as fuel in fuel cell and its challenges:

Use of Hydrogen as fuel in fuel cell and its challenges:

Hydrogen is the simplest and lightest element. Storage is one of the greatest problems for hydrogen. It leaks very easily from container meant for storage, no mater how strong and no matter how well insulated. Therefore, hydrogen in storage tanks always evaporates, at a rate of at least 1.7 percent per day.

Another important property of hydrogen is it is very reactive in nature. When hydrogen gas comes into contact with metal surfaces it decomposes into hydrogen atoms, which are so very small that they can penetrate metal. This causes structural changes that make the metal brittle.

One of the largest problems perhaps hydrogen fuel cell transportation has is its fuel tank size. In gaseous form of hydrogen, a volume of 238,000 litres gas is necessary to replace the same energy capacity of 20 gallons of petrol (gasoline). One option is to compress the gas. Because of gas’s low density property, compressed gas does not give a car as useful a as of gasoline as far as storage volume is concerned. Moreover, a compressed hydrogen fuel tank would be at risk of developing pressure leaks either through accidents or through normal wear and such leaks could result in dangerous explosions.

In case, the hydrogen is liquefied, the liquid hydrogen would give a density of 0.07 grams per cubic centimeter. In that case, it may require almost the four times volume of gasoline for a given amount of energy release. Besides, there are many difficulties in storing liquid hydrogen. Liquid hydrogen is cold enough to freeze air. Accidents may occur from pressure build-ups resulting from plugged valves. Besides, energy costs of liquefying the gas and refrigerating it also to be considered while calculating economy.

Other option may be considered is the use of powdered metals to store the hydrogen in the form of metal hydrides. The volume of stored metal hydrides would be little more than that of the metals themselves; but storing in this form, hydrogen would be far less reactive. However, the weight of the metals will make the storage tank very heavy.

As far as production of hydrogen is concerned, hydrogen does not freely occur in nature in useful quantities. Therefore hydrogen must be split from molecules, either molecules of methane derived from fossil fuels or from water. Currently, most hydrogen is produced by the treatment of methane with steam (the equation is CH4 (g) + H2O + e > 3H2(g) + CO(g)). The CO(g) in this equation is carbon monoxide gas, which is a byproduct of the reaction. Again the production of CO, which converts into CO2 is a greenhouse gas – not environment friendly option. Again, at present we do not have viable technology to obtain hydrogen from water, other than electrolysis – which is not energy saving option.

Therefore, as of now, it is a challenge before us to use hydrogen economically, efficiently and environment-friendly way. As lot research activities are going on in this field, very soon positive favorable result could be seen.

Principle of Hydrogen fuel cell:

The hydrogen fuel cell is an electrochemical energy conversion device. Hydrogen and oxygen are fed into opposite sides of a cell, which are separated by a membrane permeable to hydrogen ions but not electrons. Hydrogen gas molecules entering the anode side of the cell are ionized in the presence of a catalyst to form protons and electrons. The protons pass through the membrane to combine with the oxygen and electrons to produce water at the cathode. The electrons flow through an external circuit from the anode to the cathode, creating an electrical current, which powers an electric load such as a motor.

Alternative Fuels:

Alternative Fuels:

Alternative fuels are derived from resources other than petroleum. Some are produced domestically, reducing our dependence on imported oil, and some are derived from renewable sources. Often, they produce less pollution than gasoline or diesel. Some of the important alternative fuels are:

(1) Ethanol is produced domestically from corn and other crops and produces less greenhouse gas emissions than conventional fuels.

(2) Bio-diesel is derived from vegetable oils and animal fats. It usually produces less air pollutants than petroleum-based diesel.

(3) Natural gas is a fossil fuel that generates less air pollutants and greenhouse gases.

(4) Propane, also called liquefied petroleum gas (LPG), is a domestically abundant fossil fuel that generates less harmful air pollutants and greenhouse gases.

(5) Hydrogen can be produced domestically from fossil fuels (such as coal), nuclear power, or renewable resources, such as hydropower. Fuel cell vehicles powered by pure hydrogen emit no harmful air pollutants.