The attention of gas engineers has been forcibly directed to the use of tar as a fuel for the firing of retorts, now that this once high-priced material is suffering, like everything else (but, perhaps, to a more marked extent), by what is called "depression in trade." In fact, it has in many places reached so low a commercial value that it is profitable to burn it as a fuel. Happily, this is not the case at Nottingham; and our interest in tar as a fuel is more experimental, in view of what may happen if a further fall in tar products sets in. I have abandoned the use of steam injection for our experimental tar fires in favor of another system. The steam injectors produce excellent heats, but are rather intermittent in their action, and the steam they require is a serious item, and not always available.
Tar being a pseudo liquid fuel, in arranging for its combustion one has to provide for the 20 to 25 per cent. of solid carbon which it contains, and which is deposited in the furnace as a kind of coke or breeze on the distillation of the volatile portions, which are much more easily consumed than the tar coke.
I have adopted is one that can be readily adapted to an ordinary coke furnace, and be as readily removed, leaving the furnace as before. The diagram conveys some idea of the method adopted. An iron frame, d, standing on legs on the floor just in front of the furnace door, carries three fire tiles on iron bearers. The top one, a, is not moved, and serves to shield the upper face of the tile, b, from the fierce heat radiated from the furnace, and also causes the air that rushes into the furnace between the tiles, a and b, to travel over the upper face of the tile, b, on which the tar flows, thereby keeping it cool, and preventing the tar from bursting into flame until it reaches the edge of the tile, b, over the whole edge of which it is made to run fairly well by a distributing arrangement. A rapid combustion takes place here, but some unconsumed tar falls on to the bed below. About one-third of the grate area is filled up by a fire tile, and on this the tar coke falls. The tile, c, is moved away from time to time, and the tar coke that accumulates in front of it is pushed back on to the fire bars, e, at the back of the furnace, to be there consumed.
Air is thus admitted, by three narrow slot-like openings, to the front of the furnace between the tiles, a, b, and c, and under c and through the fire bars, e. The air openings below are about three times the area of the openings in the front of the furnace; but as the openings between the fire bars and the tiles are always more or less covered by tar coke, it is impossible to say what the effective openings are. This disposition answers admirably, and requires little attention. Three minutes per hour per fire seems to be the average, and the labor is of a very light kind, consisting of clearing the passages between the tiles, and occasionally pushing back the coke on to the fire bars. These latter are not interfered with, and will not require cleaning unless any bricks in the furnace have been melted, when a bed of slag will be found on them.
required for these fires is very small, and less than with coke firing. I find that 0.08 in. vacuum is sufficient with tar fires, and 0.25 in. for coke fires. The fires would require less attention with more draught and larger tar supply, as the apertures do not so easily close with a sharp draught, and the tar is better carried forward into the furnace. A regular feed of tar is required, and considerable difficulty seems to have been experienced in obtaining this. So long as we employed ordinary forms of taps or valves, so long (even with filtration) did we experience difficulties with the flow of viscous tar. But on the construction of valves specially designed for the regulation of its flow, the difficulty immediately disappeared, and there is no longer the slightest trouble on this account. The labor connected with the feeding of furnaces with coke and cleaning fires from clinker is of a very arduous and heavy nature. Eight coke fires are normally considered to be work for one man. A lad could work sixteen of these tar fires.
Considerable attention has been paid to the composition of the furnace gases from the tar fires. The slightest deficiency in the air supply, of course, results in the immediate production of smoke, so that the damper must be set to provide always a sufficient air supply. Under these circumstances of damper, the following analyses of combustion gases from tar fires have been obtained:
No Smoke. CO₂. O. CO. 11.7 5.0 Not determined. 13.3 3.7 " 10.8 5.4 " 14.8 2.5 " 13.5 3.0 " 12.4 5.6 " 12.4 4.6 " 13.1 5.9 " 15.3 1.0 " 10.8 4.0 " 14.0 2.8 " ______ ______ Average 12.9 3.9 (11 analyses) ______ ______ 11.5 Not determined. 14.3 " 14.6 "
Damper adjusted so that a slight smoke was observable in the combustion gases.
CO₂. O. CO. 17.30 None. Not determined. 16.60 " " 16.50 0.1 " 15.80 0.1 " 16.20 1.8 0.7 _______ _____ _____ Average 16.48 0.4 0.7
- Gas Engineer.