Fuels, Furnaces, Refractories and Pyrometry

Unit I

Fuels

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1 Fuels

Fuel is a material which gives out energy on combustion. Many fuels are available of which the main classification can be based upon their state :

1. Solid fuels,

2. Liquid fuels,

3. Gaseous fuels.

Besides the above, the atomic power is also becoming a vital source of energy supply. So, it can be added to the above list to make it four.

Solid fuels

The most important solid fuel is coal. Fallen trees and the other plant remains are attacked by the oxygen of the air. The chief constituent of trees, cellulose, is thus converted into water and various gases, so that it soon rots away and no traces of the remains would be present. But, if the trees and other vegetable matter are submerged under water (due to natural calamities), the plant remains are decomposed, only by the action of bacteria. The decomposition products accumulate until the bacteria itself can exist no longer. Due to certain geological influences causing the land level, relative to the sea, rise or fall, the accumulations of decayed plant material get covered layer by layer, by sediment and rocks. Under the influence of heat and pressure, other changes which are more profound are brought about. Pressures, not only due to the weight of the overlying rocks but also from the lateral earth movements, cause the folding and compression of the seams. Chemical action also results, which includes the elimination of water and the oxides of carbon. All these actions are varied, and also their influences, resulting in the ultimate coal substance consisting of varying degrees of volatile matter. Thus, a large varieties of coals are formed.

The variations in the coals have resulted due to the following reasons :

(i) variations in the original vegetable matter,

(ii) extent of bacterial decay,

(iii) extent of removal of the volatile matter by the pressure and heat.

Continuous coal fields are known in which the character of the coal changes steadily as one proceeds across the coal field. Thus, even in one seam, a range of coals may be present. It is established, that it is the segregation of different parts of similar vegetation that has led to the marked differences in their compositions.

The coals resulting from geological action on vegetable material of differing degrees of bacterial decay will have widely different properties and characteristics.

The heat and pressure experienced by the decaying vegetable matter during coalification, control the properties of the coal to a remarkable degree. It is proved that the deepest coal seams possess relatively less moisture and volatile matter. This is because of the fact that they experience higher pressures from the overlying strata of rocks.

Coals are generally classified into four groups;

(i) Peat

(ii) Lignite

(iii) Bitumenite

(iv) Anthracite

These are in this order of ‘age' of coalification; Anthracite being the oldest.

1. Wood

This is an important domestic fuel. It is normally used in the air-dried condition and contains about 15% moisture. In general, wood consists of majorly cellulose (43%). The advantages of wood as an industrial fuel are its low ash content (0.3-0.6%) and its capacity to yield a clean and long flame.

Much of the furniture industry's wastage is used as a fuel in the industrial boilers and households. Wood can also be successfully gasified.

2. Peat

Peat is a ‘recent' growth consisting of partly decomposed vegetable matter, under temperate marshy conditions. The deposits are of a large variety. The moisture content may be as high as 90%. This poses a great disadvantage because of the problems involved in drying. Thus, in spite of being a low ash fuel, peat remains only as a local and domestic fuel.

3. Lignite and Brown Coal

Lignite can conveniently be regarded as intermediate in composition and character between peat and other coals. Lignites have 30-50% moisture. The lower moisture content lignites are known as brown coals.

4. Bituminous Coal

The term is applied to high and medium volatile matter coals (20-45%). Their coking characteristics are given in Table 1.1

Table 1.1

Most of the industrial coals fall into the bituminous variety. Nearly all the coals exhibit a banded structure. These are classified into dull and bright bands. Stopes classified the bands into four types-vitrain, clarain, durain and fussain. Vitrain and clarain together constitute the bright bands of the coal and durain, the dull and harder variety.

The caking characteristics are imparted to the coal due to the presence of the bright constituents. Durain and fussain are of non-coking nature. Fussain is the dull, friable, silky-looking material found in thin layers in the coal.

Low volatile carbonaceous coals are older bituminous coals. They are intermediate between the bituminous and anthracite varieties. They consist of 10.00 - 20.00% volatile matter and carbon upto 93%.

Anthracite

Anthracite can be defined as the coal containing more than 92.00% carbon and volatile matter not more than 9.1%. Many anthracites occur in folded and highly compressed strata and hence it becomes difficult to detect the banded structure.

2 Coal

Classification of coals

Many schemes for classifying the coals based on proximate or ultimate analysis are formulated. The idea behind any classification is to enable the fuel technologist and the fuel user to assess or predict the properties of a coal, the uses to which it could be put and its behavior during the use.

Gruner and Brame had classified the coals as per their proximate analysis and the types. This classification is no more used.

Rank of coal

The biological and geological processes by which wood and other vegetable matter have been converted into lignite and coals are charted. The chart starts from wood and ends up with anthracite. This is a simple straight line classification. Such a straight line classification is very difficult for, many coals cannot fit into it. The wood lignite coal series has a practical value and the position of the coal within it, is referred to as the ‘rank of the coal'. This is defined as the degree of metamorphosis or coalification that the original woody or other vegetable matter has undergone. Coals that have been produced as a result of having undergone the least change are known as the low rank coals - the extreme examples being the lignites or brown coals.

While proceeding from low rank to the high rank coals, the volatile matter and hydrogen contents decrease whereas the carbon content and the calorific values rise. The caking power rises to a maximum after which it falls off again until anthracite is reached.

Seyler's classification

Seyler's classification is aimed at predicting the nature and properties of coal solely from the analytical data, on the, dry-ash-free, basis. The positions of all the coals are plotted in a diagram using the percentages of

(a) total carbon, and

(b) hydrogen,

as the main parameters.

In this, all the coals are found to occupy positions within a fairly well defined curved band. It is also found that all coals with a specified volatile matter content occupied positions along a straight line. Thus, a series of nearly parallel ‘ISOVOLS' can be constructed across the diagram (Fig. 2.1).

Similarly, coals having any specified calorific value occupied positions along a straight line and thus a series of roughly parallel ‘ISOCALS' can also be constructed across the diagram. The ‘ISOVOLS' and ‘ISOCALS' are roughly at right angles to each other and inclined to the carbon and hydrogen zero lines or axes.

Fig. 2.1 Seyler's classfication of coal

The E.C.E International Coal Classification

The Economic Commission for Europe had worked out a system for classifying coals. It is a two dimensional scheme. The horizontal parameter is based upon volatile matter upto 33% (on a dry-ashfree basis). This parameter divides the coals into classes from 0-9. Each class is divided into groups and sub-groups. The groups are determined according to the caking properties of the coals (by either Free-Swelling index or Raga index), the sub-groups being determined by caking properties (either dilatometer test or Gray-King Assay).

Analysis of coal

Proximate analysis

The procedure for the determination of the percentages of moisture, volatile matter, fixed carbon and ash in a coal sample is called the proximate analysis. The actual practice is to determine the moisture, volatile matter and ash and subtract their sum total from 100, to obtain the fixed carbon. Thus, ‘fixed carbon' is actually a hypothetical concept, derived purely arithmetically. This does not imply the existence of uncombined carbon in coal nor does it bear any direct relationship to the total carbon in the coal, carbon is not a precise constituent of coal.

Moisture

All coals contain free or surface moisture, i.e. water. This might have gone to the coal due to the percolation through the strata and workings of the mine from the surface. This may also be present due to adopting a wet cleaning process or exposure to rain.

The loosely adhering surface moisture can be dried off on exposure to the air. The coal is then known as ‘air-dried'.

In the laboratory, the determination is carried out on a coal sample ground to Is 212 μm or equivalent. The drying is performed at a temperature 105 + 2 oC to a constant weight. The sample used is a 1gm sample (air-dried).

Volatile matter

One gram sample, dried before hand, is heated in a silica crucible at 925 oC for 7 minutes. Then it is cooled in a desiccator and weighed. The loss of weight is expressed as a percentage.

Ash content

Dried coal (1 gram) is completely burned in controlled conditions. The ash is defined as the fully oxidised residue of the mineral matter. This may comprise the loss of water, of hydration of clays, or of carbon dioxide or oxidation of iron sulphide.

The incombustible matter of the coal is largely made up of the mineral matter. The residue of this, on complete combustion is the ash.

Ultimate analysis

Ultimate analysis of the coal is defined as the exact chemical determination of the chemical elements present in it. The elements generally determined are :

(i) Carbon, (ii) Hydrogen, (iii) Nitrogen and (iv) Sulphur.

Carbon and hydrogen

A weighed quantity, generally 1 gram of the coal, is burned in Oxygen at 800 oC. The products of combustion are passed over heated copper oxide so that they are fully oxidised to carbondioxide and water. The oxides of sulphur are removed by passing over heated lead chromate and the chlorine present is absorbed by a silver-guage spiral. The gases are then passed through two weighed vessels containing suitable absorbents, KOH and concentrated H2 SO4 for CO2 and H2O respectively. The increase in the weights of the vessels will give the quantities of ingredients absorbed.

An alternative method involves the burning a coal sample at 1350 oC. The loss of weight gives the total of carbon and hydrogen.

Nitrogen

Nitrogen in the coal is determined by the Kjeldahl method. A 0.1 gram coal sample is decomposed by boiling sulphuric acid containing a catalyst. The nitrogen is converted into ammonia. The ammonia forms ammonium sulphate with the acid. Subsequently ammonia is liberated by the addition of an alkali and steam-distilled into a known amount of standard acid. The excess acid is titrated and the equivalent amount of ammonia is calculated and therefrom, nitrogen.

Sulphur

The sulphur in the coal is determined by heating a 1 gram sample with Eschka mixture (one part anhydrous sodium carbonate and two parts of magnesium oxide by weight). The heating is continued until all the carbonaceous matter is burnt out. Sulphur is retained by the Eschka mixture, from which it is later removed as a soluble sulphate. From this solution the sulphate is obtained as a precipitate of barium sulphate; dried and weighed.

Alternately, a 0.5 gram of coal is burned at 1350 oC and the products of combustion are passed through hydrogen peroxide which is converted into sulphuric acid due to the action of SO2. The amount of sulphuric acid is estimated by titration and therefrom, the amount of sulphur.

Calorific Value

This is a very important property of a fuel for, it expresses the amount of heat generated by the complete combustion of a unit weight of the fuel.

A standard special apparatus is used and the procedure adopted is also standardised. An accurate weight of the fuel (1 gram of the fuel) is burned in an atmosphere of compressed oxygen in a calorimetric bomb, immersed in water. The temperature rise of the water is very accurately measured. The heat generated, is thus calculated, knowing the weight of the water. The result is defined as the ‘gross calorific value at constant volume'.

When the coal is burnt in a furnace for industrial purposes, no element of ‘pressure' is in the case of the above experimental determination is involved. The operative calorific value is the calorific value at constant pressure. The difference between the two is of the order of 1 in 1000 for bitumenous coals and 0.5 part in 1000 for anthracite. Since the accuracy of a laboratory determination itself is of the order of 2 parts in 1000, the difference between the calorific values at constant volume and at constant pressure is ignored.

The calorific value of coal can also be empirically calculated, knowing the ultimate analysis of the coal, as per the formula,

Caking properties of coal

The behavior of a coal when it is heated has a marked effect on its performance and use for different purposes. Only those coals which yield a hard dense coke when heated out of contact with air are useful in producing metallurgical coke. Coals of poorer caking property are needed if a free-burning reactive coke is needed. For many domestic and industrial applications however, feebly caking or non-caking coals are often desirable. Therefore, it is important when selecting coals for certain purposes or judging the relative merits of different coals for use in the same appliance, that we have a measure of its caking properties. There are a number of laboratory tests and each give some measure of the caking properties.

British standard crucible swelling test

to 9 in increments of 0.5. The number thus obtained is known as the ‘Swelling number'.

Gray-King Coke type assay

Coke is produced with a 20 gram coal sample, ground to pass 72 mesh B.S. sieve by carbonising under standard conditions to a final temperature of 600 oC. The coke obtained is defined by its size and appearance if the coal is non-swelling. If the coal is swelling, the coke is defined by the number of parts of electrode carbon powder in 20 parts of mixture that produces a standard coke which is hard and of the same volume as the original 20 gram sample of the coal in the assay tube. There are 17 coke types, designated by letters. Coals of type ‘A' are non-caking and ‘G' give a hard strong coke but do not swell. For coals more swelling than those of type G, subscripts are added i.e., G1 to G10 (Fig. 2.2).

Sufficiently large reserves of coal are estimated in our country. Large (83,000 million tonnes) deposits occur predominantly in Bihar, Bengal, Madhya Pradesh and Assam. All these are of the hard bituminous type. In addition, about 100 million tons of lignite is estimated to occur in Tamil Nadu and Orissa. The Table 2.1 gives the approximate resources of coal,

Table 2.1 Indian Coal Reserves (1972) (in million tones).

Courtesy C.F.T.R.I. Dhanbad

The coal deposits are geologically classified as belonging to the Damuda series of the lower Gondwana family. These are divided into (a) the Barakar measures and (b) the Ranigunj measures.

Fig. 2.2 Types of solid residue from Gray-King low-temperature assay

The coal-seams in Ranigunj are separated by heamatite shales.

Generally, Indian coals are weakly coking to coking in nature. The ash (mineral matter) is found to be distributed as fine particles, making it difficult for treatment by washing. The coals of excellent coking quality, suffer from the drawback of high ash content. Fortunately, all Indian coals are low in sulphur, except those of Assam. Coal analyses are given in Table 2.2.

Table 2.2

Further, large deposits of coking coals are also found near Bokaro (Bihar). Non-coking deposits of voluminous nature are also found near Kothagudem in Andhra Pradesh. Both the Andhra Pradesh coals and Tamil Nadu lignites are being largely utilised for power generation.

With the steep rise in the international crude prices, the Govt. of India is making strenous efforts to exploit the domestic coal reserves to the maximum extent and discouraging the use of oil-firing in industry and power generation. Consequently coal output in our country is increasing every year (See Fig. 2.3).

Fig. 2.3 Coal output in India

The efforts of increasing thermal power and nuclear power production are manifest in Fig. 2.4 which shows that the basic policy of our Government is to save foreign exchange.

Fig. 2.4 Power generation in India

3 Combustion of Solid Fuels

Combustion of solid fuels

Perfect combustion is attained when a fuel is burned with the exact quantity of air required - no more or no less. This is a theoretical aspect and in practice, complete combustion is not ensured unless a substantial excess of air is provided.

The fuel and air should be brought into contact with each other under the right conditions, namely,

(a) an adequate portion of the air at the right places throughout the fue,

(b) adequate time and turbulance must be arranged at a temperature sufficiently high to ensure that the chemical reactions involved are completed within the space available.

Air supply

To bring the air into contact with the fuel and then to remove the products of combustion, there must be a flow of gases throughout the system. Part of the air entering the furnace is needed and consumed in the combustion of the fuel and the unwanted excess is simply heated up. The products of combustion as well as the heated air pass through the remainder of the system. Heat exchange occurs between the products of combustion in the furnace bed, walls and roof and the charge. The gases give up their heat; the furnace charge gets heated up.

Fig. 3.1 Natural draught

If the air entering the furnace is insufficient, all the fuel is not burnt. Thus, a serious wastage of the fuel results. Incomplete combustion may be due to :

(a) insufficient air,

(b) failure to make complete use of the air admitted and

(c) irregularities in the fuel bed.

The remedy is usually, a combination of measures - proper arrangements for draught.

Admission of air into the furnace and its subsequent movement throughout the system are brought about in the following ways :

(i) air may be drawn by the natural draught created by a chimney (see Fig. 3.1).

(ii) air may be drawn by a fan at the chimney base induced draught,

(iii) air may be forced in under pressure, by a fan, the pressure serving to propel the gases onwards through the fuel bed and the chimney draught being used to remove the gases-forced draught,

(iv) air may be forced in through the action of a steam jet in a venturi, the chimney draught being used to remove the gases,

(v) Air may be forced in by a fan, the products of combustion being removed by a second fan or by chimney action in such a way as to leave zero pressure in the combustion space above the fuel bed. This is, by far, the most flexible arrangement. The system is known as balanced draught.

Though zero pressure is referred to, in practice, a slight suction (partial vacum) of the order of 5 cm. W.G. is arranged in front of the grate. This helps reduce or eliminate the emission of smoke or flame from the fire door and overheating of the furnace front that might be caused by a very slight excess of pressure.

The amount of draught depends upon:

(a) the nature of the fuel,

(b) the depth of the fire to be maintained,

(c) the rate of combustion desired,

(d) the design of the boiler and that of the fuels and

(e) the combined resistance of the ancillary plant such as the economisers, air heaters, regenerators etc.

Some fuels require more draught than the others. Low volatile matter fuels like coke, anthracite coal etc., require that most of the air for combustion must be drawn through the fuel bed as primary air; this is subject to the resistance of the fuel bed. With high volatile matter fuels, a considerable proportion of the combustible matter of the fuel is distilled off,