Modern Gasworks Practice

THE DRY PURIFICATION OF COAL GAS 571 Of the impurities named, the gas in normal instances contains, when distributed, the folio wing quantities :— Sulphuretted hydrogen .... nil. . Ammonia ______________________..... nil. Sulphur compounds (other than H2S) . . 35 to 50 grains per 100 cubio feet. - Hydrocyanic aoid . . . . . nil to 10 „ „ „ „ Carbon dioxide ______________________..... 750 to 1,150 „ „ „ „ Where a proportion of carburetted or blue water gas is added to the coal gas the mixture (owing to the lower sulphur content of water gas) contains a smaller percentage of sulphur impurities than does a neat coal gas. Purified carburetted water gas contains on an average from 10 to 15 grains of sulphur compounds per 100 cubic feet. The abolition of the sulphur clauses in 1905 did much to simplify the process of gas purification, in that lime as a purifying medium is now largely dispensed with. The somewhat erratic operation of this material and the uncertainty of its working were such as to prove a constant anxiety to the works manager. In addition, considerable expense was incurred in order to effect the removal of an impurity present in quantities so small as to render it practically innocuous. To-day oxide purification for the removal of sulphuretted hydrogen is of primary interest to the gas engineer, although the use of lime has been by no means universally discarded, particularly in so far as some of the smaller undertakings are concerned. REMOVAL OF SULPHURETTED HYDROGEN The elimination from the gas of this impurity presents no great difficulties, but the manner in which the necessary plant is operated varies considerably with the taste of the engineer. Of the original quantity of sulphuretted hydrogen the bulk remains in the gas after the latter has passed the washers and serubbers, and is invariably removed by causing the gas to pass in contact with. layers of special material in a moist condition. The chief materials employed for sulphuretted hydrogen removal have, in the past, been hydrated lime, hydrated oxide of iron, and manganese dioxide in the form of Weldon mud. Natural ferric oxide (bog-ore) is still largely made use of, but in recent years there has been a tendency to turn to various artificial preparations, which may be obtained more cheaply and which., in many cases, have inereased activity as compared with bog-ore. Natural Oxide of Iron Natural oxide of iron, or bog-ore, occurs in peaty deposits in large quantities in Ireland, Holland, and Belgium, also to a limited extent in England, chiefly in the ncighbourhood of Westbury (Wiltshire). In its natural form the ore contains from 50 to 60 per cent, of water, and between 30 to 35 per cent, of ferric oxide (Fe2O3). In addition to iron compounds the virgin material contains about 15 to 25 per cent, of inert matter, consisting chiefly of vegetable substances and silica. In the dried state the analysis of an oxide of good qnality will vary between the folio wing limits:—