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

Understanding fuel combustion process:

Understanding fuel combustion process:

(a) Fuels are chemical substances which may be burned in presence of oxygen to generate energy in the form of mostly heat. They mainly consist of carbon and hydrogen. Small quantity of sulfur is also present in fuel as contamination. Solid, liquid and gaseous fuels are used by various systems. Coke and coal are solid fuels, petrol, diesel, kerosene is liquid and LPG, CNG are the example of gaseous fuel.

C + O2 = CO2, (here C and O2 are reactants and CO2 is the product of burning fuel)

(b) Each fuel burn at a particular temperature, called ignition temperature. For starting the combustion process each fuel should be brought above its ignition temperature. An appropriate air-fuel ratio is also necessary to maintain in order to get desired result. The minimum ignition temperature at atmospheric pressure for some fuel is: (i) Carbon is 400 degree Celsius; (ii) Hydrogen is 580 degree Celsius; (iii) Carbon monoxide (CO) is 610 degree Celsius; (iv) Methane (CH4) is 630 degree Celsius; (v) Gasoline is 260 degree Celsius.

(c) As air is the major ingredient in any burning process, the air-fuel ratio is the term frequently used in the analysis of combustion process for any fuel. It is usually expressed on a mass basis, i.e., Mass of air required / mass of fuel burnt.

(d) Fuel combustion process is the process when a particular fuel is burnt completely, i.e., all carbon present in the fuel converts into carbon dioxide (CO2), all hydrogen converts into water (H2O) and all sulfur converts to sulfur dioxide (SO2). Theoretically, in completely burnt process, no un-burned residue of carbon, hydrogen should be present and process should not produce any carbon monoxide (CO). For any internal combustion engine, complete combustion is desirable, as energy conversion is maximized and exhaust gas characteristics is improved, thereby engine efficiency and less pollution.

(e) It is desirable to use more air than the actual requirement for the complete combustion of any fuel; to prevent chance of any incomplete combustion. Excess air is also needed to control rise in temperature of the combustion chamber.

(f) Energy is an inherent property of a system by which work is done. Any system at a given set of conditions has certain energy content. The concept of energy is derived to describe a number of processes such as conversion of work to heat. Joule (J) is the unit in SI system. Other units of energy / heat are: (i) 1 cal (calorie)= 4.1868 J; (ii) 1 kcal= 4186.8 J; (iii) 1 Btu (British thermal unit)= 1055.05 J; (iv) 1 ft.lbf= 1.35582 J; (v) 1 kJ= 1000 J; (vi) 1 hp.h (horsepower.hour)= 2,684,520 J; (vii) 1 kWh= 3,600,000 J.

(g) For any combustion energy is transferred to heat and heating value for any system is the amount of energy released when a fuel is burned completely.

Conservation of fossil fuels Fossil fuels take millions of years to make. We are using up the fuels that were made more than 300 million years ago. Once they are gone they are gone. It is best not to waste fossil fuels. They are not renewable; they can not be made again. We can save fossil fuels by conserving energy.

Heat value of coal:

Heat value of coal:

The heat value of or the energy value is the amount of energy that can be converted into the actual heating ability of the coal sample. The heat value is very important factor for any fuel, goes for conversion of energy or actual usefulness. By heat value the quality of coal or any fuel measured.

The basic parameters of coal sample analysis are moisture, volatile matter, ash, and fixed carbon. Moisture, ash, or volatile matter do not participate and creation of energy when coal is burnt; so the heat value is more depending on the fixed carbon a sample has. To be precise, heat value or heat of combustion is the energy released as heat when a compound undergoes complete combustion with oxygen. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide (CO2), water (H2O) and since the reaction is exothermic, heat is generated. The quantity of heat generated is expressed as calorific value (Q), which is the heat liberated by its complete combustion with oxygen. It is expressed in terms of kcal/kg or Btu/lb (in British unit).

Generally, formula applies for calculation of calorific value Q is

Q = 337C + 1442(H - O/8) + 93S

Where, Q is the calorific value.

C is the fixed carbon content in the sample.

H is the hydrogen content.

O is the oxygen content.

S is the sulfur content in the sample.

Therefore, the quality of coal, in other words, the heat value of coal can be improved by reduction of ash content in coal sample. Coal beneficiation or washing of coal can reduce ash in coal.