GASEOUS FUELS

Natural gas. The main constituents of natural gas are methane (CH4) and ethane (C2H6).

It has calorific value nearly 21000 kJ/m3. Natural gas is used alternately or simultaneously with oil for internal combustion engines.

Coal gas. Mainly consists of hydrogen, carbon monoxide and hydrocarbons. It is prepared by carbonisation of coal. It finds its use in boilers and sometimes used for commercial purposes.

Coke-oven gas. It is obtained during the production of coke by heating the bituminous coal. The volatile content of coal is driven off by heating and major portion of this gas is utilised in heating the ovens. This gas must be thoroughly filtered before using in gas engines.

Blast furnance gas. It is obtained from smelting operation in which air is forced through layers of coke and iron ore, the example being that of pig iron manufacture where this gas is produced as by product and contains about 20% carbon monoxide (CO). After filtering it may be blended with richer gas or used in gas engines directly. The heating value of this gas is very low.

Producer gas. It results from the partial oxidation of coal, coke or peat when they are burnt with an insufficient quantity of air. It is produced in specially designed retorts. It has low heating value and in general is suitable for large installations. It is also used in steel industry for firing open hearth furnaces.

Water or illuminating gas. It is produced by blowing steam into white hot coke or coal.

The decomposition of steam takes place liberating free hydrogen, and oxygen in the steam combines with carbon to form carbon monoxide according to the reaction.

The gas composition varies as the hydrogen content if the coal is used.

Sewer gas. It is obtained from sewage disposal vats in which fermentation and decay occur. It consists of mainly marsh gas (CH4) and is collected at large disposal plants. It works as a fuel for gas engines which in turn drive the plant pumps and agitators. Gaseous fuels are becoming popular because of following advantages they possess.

Advantages :

1. Better control of combustion.

2. Much less excess air is needed for complete combustion.

3. Economy in fuel and more efficiency of furnace operation.

4. Easy maintenance of oxidizing or reducing atmosphere.

5. Cleanliness.

6. No problem of storage if the supply is available from public supply line.

7. The distribution of gaseous fuels even over a wide area is easy through the pipe lines and

as such handling of the fuel is altogether eliminated.

8. Gaseous fuels give economy of heat and produce higher temperatures (as they can be preheated in regenerative furnances and thus heat from hot flue gases can be recovered).

LIQUID FUELS

The chief source of liquid fuels is petroleum which is obtained from wells under the earth’s crust. These fuels have proved more advantageous in comparison to sold fuels in the following respects.

Advantages :

1. Require less space for storage.

2. Higher calorific value.

3. Easy control of consumption.

4. Staff economy.

5. Absence of danger from spontaneous combustion.

6. Easy handling and transportation.

7. Cleanliness.

8. No ash problem.

9. Non-deterioration of the oil in storage.

Petroleum. There are different opinions regarding the origin of petroleum. However, now it is accepted that petroleum has originated probably from organic matter like fish and plant life etc., by bacterial action or by their distillation under pressure and heat. It consists of a mixture of gases, liquids and solid hydrocarbons with small amounts of nitrogen and sulphur compounds. In India, the main sources of Petroleum are Assam and Gujarat. Heavy fuel oil or crude oil is imported and then refined at different refineries. The refining of crude oil supplies the most important product called petrol. Petrol can also be made by polymerization of refinery gases.

Other liquid fuels are kerosene, fuels oils, colloidal fuels and alcohol.

SOLID FUELS

Coal. Its main constituents are carbon, hydrogen, oxygen, nitrogen, sulphur, moisture and ash. Coal passes through different stages during its formation from vegetation. These stages are enumerated and discussed below :

Plant debris—Peat—Lignite—Brown coal—sub-bituminous coal—Bituminous coal—Semibituminous coal—Semi-anthracite coal—Anthracite coal—Graphite.

Peat. It is the first stage in the formation of coal from wood. It contains huge amount of moisture and therefore it is dried for about 1 to 2 months before it is put to use. It is used as a domestic fuel in Europe and for power generation in Russia. In India it does not come in the categories of good fuels.

Lignites and brown coals. These are intermediate stages between peat and coal. They have a woody or often a clay like appearance associated with high moisture, high ash and low heat contents. Lignites are usually amorphous in character and impose transport difficulties as they break easily. They burn with a smoky flame. Some of this type are suitable for local use only.

Bituminous coal. It burns with long yellow and smoky flames and has high percentages of volatile matter. The average calorific value of bituminous coal is about 31350 kJ/kg. It may be of two types, namely caking or noncaking.

Semi-bituminous coal. It is softer than the anthracite. It burns with a very small amount of smoke. It contains 15 to 20 per cent volatile matter and has a tendency to break into small sizes during storage or transportation.

Semi-anthracite. It has less fixed carbon and less lustre as compared to true anthracite and gives out longer and more luminous flames when burnt.

Wood charcoal. It is obtained by destructive distillation of wood. During the process the volatile matter and water are expelled. The physical properties of the residue (charcoal), however depends upon the rate of heating and temperature.

Coke. It consists of carbon, mineral matter with about 2% sulphur and small quantities of hydrogen, nitrogen and phosphorus. It is solid residue left after the destructive distillation of certain kinds of coals. It is smokeless and clear fuel and can be produced by several processes. It is mainly used in blast furnace to produce heat and at the same time to reduce the iron ore.

Briquettes. These are prepared from fine coal or coke by compressing the material under high pressure.

Anthracite. It is very hard coal and has a shining black lustre. It ignites slowly unless the furnace temperature is high. It is non-caking and has high percentage of fixed carbon. It burns either with very short blue flames or without flames. The calorific value of this fuel is high to the tune of 35500 kJ/kg and as such is very suitable for steam generation.

Fuels and Combustion

  Fuel may be chemical or nuclear. Here we shall consider briefly chemical fuels only.

A chemical fuel is a substance which releases heat energy on combustion. The principal combustible elements of each fuel are carbon and hydrogen. Though sulphur is a combustible element too but its presence in the fuel is considered to be undesirable.

·         In chemical thermodynamics the study of systems involving chemical reactions is an important topic. A chemical reaction may be defined as the rearrangement of atoms due to redistribution of electrons. In a chemical reaction the terms, reactants and the products are frequently used. ‘Reactants’ comprise of initial constituents which start the reaction while ‘products’ comprise of final constituents which are formed by the chemical reaction. Although the basic principles which will be discussed in this chapter apply to any chemical reaction, here main attention will be focused on an important type of chemical reaction—combustion.

CLASSIFICATION OF FUELS

Fuels can be classified according to whether:

1. They occur in nature called primary fuels or are prepared called secondary fuels 

2. They are in solid, liquid or gaseous state. The detailed classification of fuels can be given in a summary form as follows 

Type of fuel                        Natural (Primary)        Prepared (Secondary)

Solid                                      Wood                       Coke

             Peat                          Charcoal

             Lignite coal                Briquettes

Liquid                                    Petroleum                 Gasoline
                                                                               Kerosene
                                                                               Fuel oil
                                                                               Alcohol
                                                                               Benzol
                                                                               Shale oil

Gaseous                              Natural gas                Petroleum gas
                                                                               Producer gas
                                                                               Coal gas
                                                                               Coke-oven gas
                                                                               Blast furnace gas
                                                                               Carburetted gas
                                                                               Sewer gas