Fuels - part 2

Anthracite

Anthracite coals are nearly pure carbon, are very hard and dense, have a very high lustre, and contain but little hydrogen or volatile matter. They burn with a slight flame, form no smoke, have no caking properties, and are difficult to ignite. Their specific gravity is high, being nearly 1.75 in good Lehigh coal. They have a calorific value of from 9000 to 9500 C.
Between bituminous and anthracite coals are a number of semi-anthracites, which cannot be classed in either variety.

Coal deteriorates considerably when stored, owing to the escape of some of its volatile constituents. There is a popular idea that wetting coal before burning increases its heating capacity but this is a fallacy, for a loss of heat results. The average composition of various coals is here tabulated for comparison: -

Charcoal is made by the dry distillation of wood, at a temperatureof from 4000 to 4500 C. This is done in heaps, or in closed retorts. All the volatile matter is driven off, and the residue consists of carbon and the inorganic constituents of the wood. Good charcoal is porous, brittle, with conchoidal fracture, and retains the form of the wood, but has only about three-fourths of the volume and usually about 20 per cent of the weight of wood. It burns with but slight flame, without smoke, and is easily ignited. Containing but little sulphur or phosphorus, it is especially useful in making some high grades of iron and steel. Its calorific intensity is about 7000 C.
In this country the most of the charcoal is made by burning wood in "charcoal pits." The wood is heaped in a hemispherical pile around a central opening, and covered with earth and sod, leaving only a few small draught holes near

the bottom. Then it is ignited at the centre and allowed to burn until the whole pile is on fire. A smoldering combustion takes place, largely at the expense of the oxygen and hydrogen of the wood fibre, forming water, carbon dioxide and volatile hydrocarbons, which escape. The draught holes are then all closed and the pit is kept carefully covered
until the fire smothers and the charcoal is cold. By carbonizing in pits nearly all the volatile matter is lost, or at best, only a part of the tar is saved and the yield of charcoal is only 20 per cent by weight of thewood. But if the process is carried on in retorts, a large amount of gas, pyroligneous acid, and tar is collected (see p. 257), and about 30 percent of charcoal is obtained, together with nearly 40 per cent of pyroligneous acid and 4 per cent of tar.

Coke is made by the destructive distillation of coal. It has a silvery white lustre, an open, porous structure, and a metallic ring when struck. It contains all the ash-forming materials of the coal, but nearly all volatile matter and sulphur have been eliminated.
For metallurgical purposes it must be sufficiently strong to sustain the weight of the charge in the furnace without crushing. The calorific value is from 8000 to 8500 C. It burns without smoke and with but little flame, and does not cake. It is made in kilns of two general types: - The "bee-hive" coke oven (Fig. 13) is made of brick, with a circular opening (A) at the top and a door (B) at the side, through which the coke is drawn. A part of the coal is burned, ill order to carbonize the remainder. As a rule, no attempt is made to save the volatile products or the tar. The yield of coke amounts to only 60 or G6 per cent of the weight of the coal. Coking ovens in which the by-products are saved, are. much used in Germany, and to a slight extent in this country. There are several
kinds, but the Otto-Hoffmann, the Simon-Carves, and the Semet-Solvay ovens are most used. In these, the ammonia and coal tar are recovered, and a coke suitable for metallurgical purposes is obtained. The waste gas is employed to heat the retorts.

The Otto-Hoffmann oven is shown in Fig. 14. The retorts are narrow chambers (0) about 30 feet long, ;) feet high, and 22 inches wide, having doors at each end, and heated by vertical flues (T, T) in the walls. Coal is charged through (F, F), while the gases and
tar pass off through (A, A) to the hydraulic main (V, V). The gas for heating enters from pipe (G), mixes with hot air from the regenerator (R), and burns in the flue (S) under the retorts, the flame passing up through the flues (T, T), and down through (T', T') to
(S'), from which the products of combustion pass through the regenerator (R') and heat it. After a time, the flow of gases is reversed, the producer gas enters through (G '), and air through (R'), burning together in (S'), while the products of combustion escape through (R). The volatile matter given off from the coal, passes through (V) to washers and scrubbers (see Illuminating Gas), which remove tar and ammonia, while the gas is stored in a holder, to be led, later, through (G, G'), and burned under the retorts.

The Simon-Carves oven (Fig. 15) is also a long, narrow retort (A) with doors at each end, but the heating flues (F; F) are set horizontally in the retort walls. The volatile matter escapes from the retort through (B), passes to the washer and scrubber, whence the
purified gas goes to the holder, from which it is drawn as needed, through (G), and burned with hot air.

lining and backed by a heavy brick retaining wall; this supports the weight of the roof arch, and also holds the heat during the drawing and charging of the retort. Thus the flue walls can be made much thinner than in the ovens previously mentioned, and the Oven works more rapidly, giving a larger yield of coke, and will coke coals which are low in volatile matter. The lining can easily be replaced without rebuilding the entire oven. The retorts are usually about 30 feet long by 16 inches wide, and 51/2 feet deep, and hold about 41/2 tons of coal at each charge. No regenerative heating is used, the
heat being retained in the walls between the retorts. A number of these ovens have been recently introduced into this country mill give excellent results.