COAL CARBONISATION.

New Zealand Herald, Volume LXVIII, Issue 20893, 8 June 1931, Page 5

COAL CARBONISATION.

THE ROTOWARO PLANT.

FEATURES OF THE PROCESS.

FINE FUEL PRODUCED,

Now that the Rotowaro carbonisation plant is in operation it is possible to obtain a really good idea of a treatment process which gives every promise of "■ working a revolution in the New Zealand coal industry.

The plant, which is to be .officially opened on Wednesday by the Minister of Railways, the Hon. W. A. Veitch, and the Minister of Mines, the Hon. A. J. Murdoch, is the first of its kind in the Southern Hemisphere. It was erected primarily to utilise the large surplus of slack produced on the Waikato coalfield, but the possibilities of low-temperature carbonisation in New Zealand go further than that and may extend to the lignite coals of the South Island.

The main product is a high-grade steaming fuel in the form of briquettes which burn without sparks and practically without smoke. Tested in locomotives last week between Auckland and Frankton the briquettes fully came up to expectations. The Railway Department has taken a keen interest in the project., and if the briquettes can be sold at a price which it considers suitable there is little doubt that the department will become a large consumer, as will city industries desiring to avoid causing a smoke nuisance. A German Invention.

■ Faced with only a small market for their slack, owing to tiiie advent of hydroelectric power from Arapuni, four of the Waikato collieries formed, nearly two years ago, a subsidiary company, Waikato Carbonisation, Limited, with a nominal capital of £IOO,OOO, to undertake the treatment of coal by the German Lurgi process. The latter was selected after exhaustive trials of Waikato coal at plants in South Germany. Britain was not a competitor, for the reason that lignite coals, for which the process was designed, are not found in the British Isles.

A site for the plant was selected closft"' to the pit-head of the Rotowaro Mine, and the installation was carried out by German engineers in co-operation with tho company's general manager, Mr. W. G. Heptinstall, who has had wide experience of the process in Canada. After 16 months' work, the plant is nearly completed and the production of briquettes was commenced toward the end of May. Without explanation, the plant itself does not convey much to the visitor. It consists of a great array of cauldrons, pipes, tanks and combustion chambers, some in the open and the others in buildings of steel and brick. Of more obvious interest are numerous belt conveyors by which coal, carbonized residuo and briquettes are carried about and other clever mechanical devices used in handling the various materials. Moderate Heat Used. Carbonisation is continuous, and when in full operation the plant is guaranteed to deal with 250 tons of slack daily. Rather more than half this weight of residual fuel will be produced. No coal is consumed merely for heating purposes when the carbonisers are running; all the heat required to treat the slack is provided by burning the gas given off in the process of carbonisation. This gas is akin to producer-gas, in fact, a small producer plant is used to make -initial supplies for starting up. An important feature of the system is that the temperatures used are comparatively low, the-greatest being about llOOdeg. Fahrenheit, which is very much less than that used in ordinary gasmaking. In consequence, the residue is not at all like gas coke, but is a black material hardly distinguishable from rawslack unless by its rather less shiny appearance. The immediate by-products are the gas already mentioned and tar. The latter is distilled to divide it into oiland pitch, which is used as a binder for the briquettes. The tar distillation plant is not yet in working order, and all the briquettes so far made are bound with imported asphalt pitch. It is not expected that the cool will produce all the pitch needed, and a certain quantity will still have to be bought. Moisture Driven 00. The oil extracted from the tar is estimated to bulk about eighjg.gallons a ton of slack. It is capable of being refined to extract motor ispirit and other _ components, but there is no immediate intention of doing this at Rotowaro, and the oil will be disposed of as it is, probably for heavy fuel purposes. The slack coal is fed into large hoppers at the top of the twin carbonisers, which are over 80ft. high and hold 100 tons each. No lid or trap is provided, the mass of coal forming an effective seal., In the first stage of the process hot burned gas is forced through, driving off 18 per cent, of moisture contained in the coal. The loss of this moisture accounts for the rather strange fact that the residue has a greater heat value than the coal from which it is obtained. From the driers the coal falls into the carbonisers, where gas at a higher temperature is passed through, separating the gas and tar. Lower still, cool gas is circulated to reduce tho temperature of the residue before it reaches the open air. It is then carried to storage hoppers, and i so to the briquetting plant, where it is ' crushed, mixed with pitch, healed and ■ pressed between honeycombed rollers into 3oz. briquettes, which are cooled on wiro- | mesh belts while being conveyed to the i storage bins. Pieces oi Iron Trapped. The cooling and washing of the gas to | remove dust and tar is an elaborate pro- ! cess, but like the drying and carbonising, j is under complete control from a central ' station furnished with a battery of indii cators showing pressures, volumes and | temperatures at all important points. • I Minor details of tlie plant are of interest. A machine used for extracting pieces of iron and steel from the coal is a most ingenious affair. The slack is passed over a roller containing an electromagnet so powerful that a watch brought within a good many feet of it is guaranteed to stop instantly. By some means tho magnetic field is made to cease on the far side of the roller, and njetal fragments, as they are carried round, drop off. The machine is very necessary to prevent such things as drill-ends and pieces of broken chain from getting into the carbonisers. At the outlet of the raw coal bins a cleverly-devised plough is automatically dragged to and fro by a chain, causing a constant mixture of coarse and fine pieces to fall on to a belt. High above, an electrically-operated moving hopper distributes the coal with remarkable evenness into the wide mouths of the driers. Room has been left in the erecting more carbonisers. If the market for briquettes increases as is hoped, more carbonisers will be installed and. an aerial ropeway will be provided to bring slack from the Pukemiro and Renown mines, which are some distance ifway.