H.—34b

The total yield of spirit from the tar and gas varies from 2 to 4 gallons per ton of coal carbonised, and it has proved itself under test to be an excellent motor-fuel. Much greater yields of tar spirit can be obtained, however, by hydrogenation of the tar and the 20 gallons of tar referred to above would, in this manner, produce 14 gallons of motor-spirit and 6 gallons of Diesel oil. It is cleai that if motor-spirit is to be obtained in large quantities by the low-temperature process, it is in the hydrogenation of tar that it must be sought. Nevertheless, the scale of production which would be possible in New Zealand owing to the limited market for the carbonized residue, either as coke or as briquettes, is such that the necessary capital costs of hydrogenation would not be justified. It must be noted that in low-temperature carbonization the main product is coke, not tar. It will be seen from the above that there is little hope of effectively producing large quantities of petrol in New Zealand as by-products of distillation of coal, since, at the most, not more than 5 per cent, by weight of the coal is converted into petrol. Hydrogenation. This, as the name indicates, is a method by which hydrogen gas is made to unite chemically with the solid coal, giving a liquid oil. The process will be understood by reference to the following table, which gives a comparison of the principal types of coal used in New Zealand and imported oil fuels : —

It will be seen that the oils have a greater calorific value than even the best of the coals. Another difference is that the ratio of hydrogen to carbon, from the figures in the ultimate analyses, is 1 to 14 or 15 in the case of the coals, and Ito6or 7 in the case of the oils. The process of hydrogenation of coal consists in bringing about a combination of hydrogen gas and coal substance, so as to increase the ratio of hydrogen to carbon. The coal substance is thus changed from solid to liquid form, and in practice a point can be reached which represents the conversion of 70 per cent, by weight into petrol. High pressures are required, up to 3,000 lb. per square inch, and temperatures of from 400° to 450° 0. To increase the speed of the reaction between the hydrogen and the coal substance numerous catalysts have been tried, among the most successful being iron sesquioxide and compounds of molybdenum. Briefly, the process of hydrogenation consists in grinding the coal finely, mixing it with selected catalysts (substances that permit or expedite a chemical reaction while themselves unchanged at the end), and a small quantity of heavy oil and pumping it to a reaction chamber kept at the required temperature to which compressed hydrogen gas is also admitted. The coal absorbs hydrogen and is changed into a liquid which flows from the reaction chamber, and is distilled to remove the petrol. The heavier-oil fraction remaining is treated a second time with hydrogen, until some 70 per cent, of weight of the total coal substance has been converted into petrol. Approximately 20,000 cubic feet of hydrogen are required for each ton of coal hydrogenated. The hydrogen may be prepared from water-gas, a mixture principally of hydrogen, 50 per cent., and carbon monoxide, 40 per cent., obtained by blowing steam over incandescent coke. The watergas, together with steam, is passed over suitable catalytic materials, when the steam and the carbon monoxide react to form hydrogen and carbon dioxide. The latter is removed by washing, and further treatment gives hydrogen of satisfactory purity. Some hydrogen may also be prepared by suitable treatment of the fixed gases resulting from the hydrogenation process. The preparation of hydrogen and the power requirements of the plant bring the total consumption of coal to 3| tons for every ton of petrol produced. The complexity and cost of the plant and operations may be judged from the fact that of the total estimated production cost of petrol by this method in New Zealand only about one-seventh represents the cost of the raw coal. Hydrogenation Overseas. Though the theory of hydrogenation is simple, the practice is difficult and a large capital cost is involved. As is well known, the Imperial Chemical Industries have made researches on the subject during the last ten years which are credibly reported to have involved an expenditure of one million pounds sterling. Work has also been carried out in Germany by the Interessen-Gemeinschaft, or 1.G., but they operate chiefly on lignite tars and raw lignites.

3

Water lost Gross Ultimate Analysis on Dry Ash-free Material. 105° C., | Calorific Value i.e., Free B.Th.U. per Moisture. Pound. Carbon. Hydrogen. Oxygen. Nitrogen. Sulphur. L I Per Cent. Bituminous coal (Millerton) 1*3 15,000 84-4 5-6 6-8 1*1 2-1 Sub-bituminous coal (Wai- 15-0 10,500 73-2 5-4 20-0 1-1 0-3 kato Field or Southern) Lignite (Mataura) .. 30-40 7,500 67-2 5-5 25-6 1 1 0-6 v J Crude petroleum .. .. 19,000 84-85 11*5- 2-0 12-5 Fuel oil from crude .. .. 19,000 86 12 2-0 First-grade petrol .. .. 20,000 85-3 14-7