In this article we will discuss about the classification or type of fuels: 1. Solid Fuels 2. Liquid Fuels and 3. Gaseous Fuels.
The fuel is a material which when once raised to its ignition temperature continues to burn if sufficient oxygen or air is available. The principle constituents of any fuel are carbon and hydrogen. Fuels may be solid, liquid or gaseous. They may be natural or artificially prepared.
Classification # 1. Solid Fuels:
i. Wood:
Except in industries in which a large amount of waste wood is available, this material is not generally considered as a commercial fuel. The heat value of wood varies both with kind of wood and with the water content.
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ii. Charcoal:
It is prepared by the dry distillation of wood. The process is generally carried on in retorts so that the by-products resulting from distillation are recovered. The by-products are methyl alcohol, acetic acid, acetone, gaseous compounds and tar.
Charcoal is an excellent metallurgical fuel on account of the total absence of sulphur. But it is costly. It has the property of absorbing 12% to 15% moisture from atmosphere which lowers its heating value.
iii. Peat:
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It is a mixture of water and decayed vegetable matter. When air is dry then it varies greatly in its physical aspect. It burns fully without smoke or soot formation. It is used as fuel in gas producer plant. Peat was used on Swedish state railways for generating steam.
iv. Coal:
This includes all types of natural solid fuels from lignite to anthracite.
v. Coke:
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It is the result of the destructive distillation of certain soft coals in closed retorts. Two methods are in use today in making metallurgical coke, the old type beehive oven and the other is the byproduct of oven. In these methods coke is the main product and in the by-product coke oven gas is the by-product.
In making of metallurgical coke the raw coals are carefully selected in order that the proper kind of coke is manufactured. In the making of illuminating gas, the opposite view rules, the coal being selected for its gas content, the coke quality being secondary.
The coking property of fuel depends upon the presence of certain compounds which upon vaporization form a residue that acts as a cementing material and converts the loose particles of coal into solid, mechanically strong, masses of various sizes.
Coking is done at two distinct temperature levels called high temperature carbonization and low temperature carbonization. The former is the standard method for metallurgical coke temperatures from 1000°C to 1200°C are reached. Low temperature carbonization ranges from 550° to 750°C and its primary function is to produce a smokeless fuel from soft coals and the coke made is relatively weak.
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A secondary result is that more than double the yield of light oil and tar is obtained. The result of coking is to drive out the coal, the volatile matter and the moisture. The theoretical yield is the sum of the fixed carbon and ash in the original fuel. But owing to incomplete coking commercial coke contains generally some gas and the quenching of coke at the end of the process confers upon it some hygroscopic water.
vi. Briquetted Coal:
Small coal and slack of all types produced in mining are made available as fuel by briquetting of the dried coal by pressure alone. This type of coal gives satisfactory strong briquettes without the addition of a binder.
vii. Pulverised Fuel:
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Low grade fuel is efficiently burnt by pulverising it. This method has many advantages such as flexibility of control, complete combustion with less excess air and high temperature of flame and the nature of the flame (oxidising or reducing) is easily controlled.
The average result of high temperature and low temperature carbonization is as shown in the following table 8-1:
The usual ways of determining the composition of coal are:
(i) By proximate analysis
(ii) By ultimate analysis.
The proximate analysis consists of determining:
(i) Moisture content
(ii) Volatile matter
(iii) Fixed carbon
(iv) Ash.
The ultimate analysis consists of determining the percentages of the ultimate constituents:
(i) Carbon
(ii) Hydrogen
(iii) Oxygen
(iv) Sulphur
(v) Nitrogen
(vi) Ash.
The proximate analysis is used for commercial purposes as it furnishes data regarding commercial important properties of coal, while ultimate analysis is used for calculative and scientific work.
The calorific value of elementary fuels at 0°C and 760 mm of mercury are given below:
Classification # 2. Liquid Fuels:
Liquid fuels are much commonly employed in internal combustion engines. They are also used for steam releasing purposes.
Some of the advantages of liquid fuels are:
(i) Higher calorific value
(ii) Economy in space
(iii) Easy control of consumption
(iv) Cleanliness
(v) Economy in staff
(vi) Elimination of wear and tear of grate bars
(vii) Ease of lighting up or shutting of operation.
Characteristic Necessary in a Liquid Fuel:
The oil which is used as a fuel must have high calorific value, must not give inflammable vapour until a temperature well above any likely to be attained in use, is reached, must not be too viscous at ordinary temperatures and it should be free from water or solid materials in suspension.
Petroleum:
From crude petroleum bulk of oil fuels ranging from petrols to heavy oil is obtained. It varies in colour from a light yellow to almost black; some oils are highly mobile while others are thick and viscous. Its specific gravity varies considerably; any value from 0.77 to 1.0.
The flash point may be from below the freezing point of water upto 160°C. (The viscosity varies greatly from district to district and it also varies for the samples from the same district. It increases with the rise of specific gravity but no connection can be traced between viscosity and specific gravity. Increase of temperature causes a rapid decrease in the viscosity, while sluggish oil flows freely if its temperature is raised slightly.)
Specific heat decreases with a rise in specific gravity.
By a combination of chemical treatment and refined methods of fractional distillation a large number of hydrocarbons have been obtained from petroleums and it has been established that all natural oils consist of mixtures of numerous hydrocarbons belonging to various well recognised series and members of atleast eight series have been identified.
The general chemical formulas for the eight series are:
The lower members of the series are frequently gases and further up they soon pass into easily condensable liquids and finally through more and more stable liquids until solid substances are reached. Increase in molecular complexity is then accompanied by rise in boiling point in the case of liquids (or of melting point in case of solids) rise in specific gravity and viscosity.
Let us consider paraffin hydrocarbons.
First four are gases (i.e., n = 1 to 4). From 5 to 18 are liquid and then gradually transformation takes place from 18 to 21 after which they are solids known as paraffin waxes.
The volatile fuels petrol, benzol and kerosene are mixtures of hydrocarbons belonging to paraffin, aromatic and naphthalene series. In petrol we find benzene, toluene and xylene of aromatic series, pentane, hexane, heptane, octane, nonane, decane and undecane of paraffin series and cyclo hexane, hexahydrotoluene and hexahydroxylen of naphthalene series.
Two outstanding types of petroleum are met with paraffin base and asphalt base. From paraffin base oils, by means of distillation we get high grade lubricating oils, paraffin wax, motor fuel, burning oils (suitable for paraffin engines) and diesel fuel, while asphalt oil is worked solely for the production of fuel and its residue is the various grades of asphalt.
Classification # 3. Gaseous Fuels:
The following are important advantages of gaseous fuels over solid fuels:
(i) The gas can be produced at one point and clean gas is distributed over a wide area to various implements.
(ii) Combustion of gaseous fuel permits the exact control in quantity, length of flame, nature of flame (reducing or oxidizing) and temperature.
(iii) Smoke and ash are eliminated.
(iv) Greater economy in high temperature work.
(v) Cleanliness.
(vi) Low grade fuel can be successfully used by gasification and we recover important by-products.
Gaseous fuels are of two types:
(i) Gases of fixed composition such as acetylene C2H2, methane CH4, ethylene C2H2, ethane C2H2, etc.
(ii) Composite industrial gases such as blast furnace gas, coke oven gas, producer gas, illuminating gas, blue water gas, carbureted water gas, Dounson gas, etc.
The following table 8-5 gives the properties of fixed composition of gaseous fuels.
The following table 8-6 gives the average composition, calorific value and combustion data for industrial fuels.
A brief discussion of the characteristics of some of the composite industrial gases is given below:
i. Blast Furnace Gas:
This is a by-product in the production of pig iron in the blast furnace. This gas serves as a fuel in steel works for power generation in gas engines or for steam raising in boilers and for preheating the blast for furnace. It is used as a fuel for metallurgical furnaces.
The gas leaving the blast furnace has a high dust content, the proportion of which varies with the operation of the furnace. Most of the dust is deposited in flues and dust catchers; before it can be used as a fuel, it must be washed to free it from the remaining dust.
ii. Coke Oven Gas and Illuminating Gas (Coal Gas):
They result from the same operation of the destructive distillation of coal in closed retorts. Coal gas is made principally in two types of retorts, horizontal intermittent or vertical continuous; while coke oven gas is obtained as a by-product in the manufacture of coke. Both these gases are used for industrial heating and power purposes.
iii. Water Gas:
When steam is blown through incandescent bed of fuel containing carbon, blue water gas results. This gas is a composite mixture of CO, CO2 and H2. It contains no unsaturated hydrocarbon and burns with a blue flame. It is used for heating. In order to make blue water gas fit for use as a domestic lighting gas, unsaturated hydrocarbons are added and the gas is known as carbureted water gas.
iv. Producer Gas:
It is a gas obtained from the incomplete combustion of coke or charcoal in a current of air. It consists of CO and N2 and its analysis should be 38.4% CO and 65.2% N2; but actually the gas obtained from the producer plant different from the theoretical one because the production of gas depends on-
(a) The temperature,
(b) Pressure,
(c) Size of coal and
(d) Velocity of air passing through the producer.
It has got a very low calorific value because the principal constituent is nitrogen which is an inert gas.
v. Dounson Gas or Mixed Gas:
It is a mixture of producer gas and water gas. It consists of hydrogen, carbon monoxide, carbon dioxide and nitrogen. Its calorific value depends upon the method of its production.
vi. Natural Gas:
It consists of methane together with hydrogen, other hydrocarbons and some impurities. It is used for domestic and industrial heating.