Modern Gasworks Practice

THE DRY PURIFICATION OF COAL GAS 585 General Working Points It is seldom advisable to put natural bog-ore into operation in the state in which it arrives, for some difficulty may be experienced in getting the material to commence working. When trouble is experienced in this way, the admixture of a small proportion of material which has previously been in use will generally overcome the difficulty. It is as well to emphasize that the activity of the material is not necessarily in direct proportion to its ir on content. V ery rauch depends upon its physical condition, and it sliould be in as fine a state of division as possible. When oxide is discharged from a purifier, after the preliminary foulings, it should not be returned to the box before all lumps are broken and the whole reduced to a powdery nature. Thorough pulverization. will increase the efficiency of an oxide very considerably. Experiments have shown that whilst one sample of unpowdéted oxide after tour absorptions contained 37-3 per cent, of sulphur, the same material in the pulverized state contained 47-1 per cent., or an increase in sulphur of 27 per cent. The chief difficulty is that of ensuring that the smaller lumps, about half an inch. in diameter, shall be effcctively powdered. With. hånd labour alone the task is most laborious, and the only satisfactory result is obtained with one of the many special disintegrators now to be had. Oxide may be prevented from caking heavily in the purifiers by adding a small proportion of coarse sawdust on each occasion that the material is changed ; whilst it should. also be maintained in an alkaline condition. Even though ammonia be allowed to travel forward from the scrubber, the oxide may befound to give an acid reaction ; and, whether acid or neutral, enough finely powdered slaked lime should be added to the material to make it alkaline before it is returned to the purifier. The importance of water content has already been referred to and, as there is, perhaps, a general tendency to employ oxide containing too much moisture rather than. too little, special attention should be given to this point. Unfortunately, such questions as space -velocity and rate of travel of gas are too frequently govemed by the apparatus at the engineer’s disposal; and, more often than not, the capacity of his available plant is very much below the recognized working basis. In general, however, calculations as to the quantity of material which will be required for the purpose of purification during a year’s working may be based on an allowance of 25 to 30 cubic feet of oxide per million cubic feet of gas made per annum. Whatever care may be bestowed upon the purification plant, it is occasionally the experience of every engineer to find that a perceptible trace of sulphuretted hydrogen is getting through into the finished gas. When such is the case, quick action is usually necessary, and the folio wing three remedies are suggested :— (1) If a small proportion of air is injected into the gas at the outlet of the purifiers, the trace will very frequently be temporarily cleared. (2) If the material in the purifiers is obviously at fault, the top tier of the final box may be rendered extremely active pro tem. by being covered with a 2-inch layer ■composed of fresh oxide and chemically prepared material. The last named is