CONSERVATION OF FUEL.

Otago Daily Times, Issue 20703, 29 April 1929, Page 17

CONSERVATION OF FUEL.

LOW TEMPERATURE CARBONISATION. THE MACLAURIN SYSTEM. (From Our Own Correspondent.; LONDON,'March 19. By a previous mail I was able to send out some particulars of the Maclaurin low carbonisation plant which has been installed by the Glasgow Corporation with the primary object of encouraging the public to use smokeless fuel. It is one of the few systems which is entirely satisfactory for the purpose for which it has been established. I am now in possession of an article by Dr Robert" Maclaurin, which appeared in the Journal of the Royal Technical College and which was read before the Scientific Society. In this Dr Maclaurin goes fully into the theory of the low carbonisation of coal and the methods employed in his system. He points out in regard to the conservation of our fuel resources that for domestic use in this country about 35.000. tons of coal are consumed per annum. If this were carbonised in Maclaurin plant, it would yield 19,250,000 tons of smokeless fuel, 500.000. gallons of oil, 350,000 tons of sulphate of ammonia, and gas containing 2,450,000,000 therms. The heat obtained from the smokeless fuel and the gas evolved would be sufficient to supply the same heat to domestic consumers as they presently obtain from the raw coal they burn. Recent advances in carbonisation are in the direction of steaming retorts (says Dr Maclaurin in the course of his article). This gives rise to the production of a considerable quantity of water gas while at the same time carrying away the oil with less decomposition. It also increases the yield of ammonia. It, however, tends to burn out some of the coke and to impair its quality. It is very difficult in high temperature retorts to obtain a good domestic fuel as some portions of the coke obtained are almost certain to have been heated to temperatures so high that the resultant product is extremely difficult to ignite. In low temperature carbonisation without prolonging the heating unduly it is rather difficult to avoid having a friable coke. There is also a liability to obtain coke with a tarry centre. The low temperature coke presently being offered as a domestic fuel retains about 10 per cent, of the volatile matter of the coal. Ordinary high temperature coke contains between 3 per cent, and 4 per cent. EASE OF IGNITION. ~ Tl *? c oke made by gradual heating in the Maclaurin producer also contains between 3 per cent, and a per cent., but yet it is quite easily ignited. Evidently the percentage of volatile matter left docs not determine the ease of ignition, iiic very high temperature to which the outer layers of coke are heated in gasworks retorts .seems to inhibit ease of ignition. The reason for this change in the combustibility of the carbon residue is at present obscure. What is now known as the Maclaurin Carbonising Plant embodies the first attempt to carbonise coal for coke production by a purely internal heating process. Ihe heating in this ease is so gradual that a coke of entirely different type from ordinaiy gasworks coke has been obtained. It has also been possible to obtain a coke hard enough for blast furnace use by carbonising non or lowcooking bituminous coals. The plant in which this has been attempted resembles m many ways a miniature blast furnace being fully 40 feet high, and about 8 feet diameter internally. The plant holds between. 24 and 30 tons of coal, and has a normal through-put'of 20 tons per day. Blast enters the producer about 20 feet below the charging boll. Protn this point upwards the temperature of the, fuel diminishes till the bell is reached, when the entering coal is at the normal atmospheric temperature. FOUR ZONES. From the air ports upwards the plant con bt considered to consist of four zones - (a) combustion zone; (b) ammoniamaking zone; (c) tM distilling zone; (d) the condensing zone. Where blast enters, the temperature is kept between TOOdeg and OOOdeg C., according to the type of coke desired. The producer gas made passing upwards losses heat to the down-coming fuel. Each hour the coke made from one ton of coal is withdrawn at the bottom, and each layer of fuel in the plant drops approximately one foot. The hourly change of the temperature of the gases surrounding -the fuel is not more than 40deg C., and the coal therefore, is in the plant fully 12 hours before it reaches oOOdeg C. This gradual heating allows the heat to penetrate to the centre of the coal. The oils have practically all been driven off by this time, ami as they pass up through the decreasing temperature little decomposition is possible. The fuel deprived of its oil continues to drop a foot per hour until it has reached the combustion zone. [n this passage it is all the time i 4 n contact with gas containing hydrogen and water vapour, so that a considerable rpiantity of ammonia is produced. In this zone the volatile matter is reduced u \ t only from 3 per cent, to 5 per cent, remains. When the air blast is kept low, relatively to the fuel going through, a low combustion zone temperature is maintained, and a black, easily ignited fuel is obtained. If the blast is kept relatively high a grey coke, not easily' ignited, is obtained. Below the combustion zone the fuel is steadily cooled by a current of steam blown in at the discharging doors. The steam, as it passes up, cools the downcoming coke, and, as it- nears the high temperature of the combustion zone, part of it is decomposed by the carbon of the coke, producing water gas. There are, therefore, throe qualities of gas made in the different zones. Water gas is made in the lower part of the plant: producer gas where the air blast enters; and distillation gas in the upper regions. The califoric value of the mixed gases is about 240 B.Th.U. per cubic feet. SOME OF. THE DIFFICULTIES. Before the author began experimenting at Port Dundas, with the assistance of the Glasgow Corporation, he was warned by those best acquainted with the difficulties of coal carbonisation that it was unlikely that the plant would run for more than a week, for the brick work would have to he pulled down in order to remove the mass of semi-coked and tarry fuel which wa r likely to be formed in the upper part of the plant. The author recognised this difficulty, and based his hope of succeeding on the statement of Percy—that coals when slowly heated lose to a considerable extent their power of coking. Provision was also made in the upper part of the plant for collecting the oils which were certain to condense on the upper part of the fuel, in a well or trough, at the top of the brickwork, so that these oils could not trickle down to the hotter fuel below. The author felt convinced that the lack of proper provision to trap the condensing oils was the cause of all previous failures to coke coal successfully in plant of this type. If the oils were not so trapped they would he certain to find their,way down into the heated region, and from there would be driven up again after partial decomposition, and at each return they would be of a more viscous nature. The second objection raised, and one i which had reason in it, was this. If I by slow heating coking was avoided, ; then friable coke would almost certainly result. The author believed it wbnld be possible to work in such a way as jto avoid the extreme of rapid in- ! tumescence while not interfering with the strength of the resultant coke.

NATURE OF THE FUEL. The starting of a large untried plant, with enormous risks of failure and explosion, is not a very pleasant experience. From the first, however, the plant worked successfully, clearly indicating that an excellent smokeless fuel could be obtained in this way, and that sticking-up in the plant could be avoided. The only trouble that appeared was a minor one, and was caused by the coke dropping into the water seal while very hot. This resulted in a sparky fuel being obtained. This difficulty has been obviated. The coke obtained varies according to the nature of the coal used. With coals of a coking index below 10 no difficulties at all occur, and the coke still retains something of the strrXare of the coal put in. That the coke made <Ss not in any way friable is evident from the fact that the average resistance to crushing of 34 samples tested was So4lb to the square inch. Much of the coke made in the Grangemouth plant has been tried in blast furnaces and has been found strong enough to bear the burden. In view of recent American blast furnace experience it would seem probable that this black, easily ignited coke should prove an ideal fuel for blast furnace work. It is possible to make a grey coke by • regulation of the air blast, but although this coke is somewhat harder, I am doubtful if it is as good for blast furnace purposes as the black coke. Black coke or grey coke can be obtained at will simply by regulation of the air valve, or, if desired, the plant can be run as a gas producer, in which case the fuel is burned out to ash.