Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/329

IKON 313 to be satisfactorily extracted and a saleable pig produced ; in this case the actual heat consumption remains about the same, but a less amount of carbon dioxide and a larger quantity of carbon oxide escape in the waste gases ; this is particularly noticeable when anthracite is used as fuel instead of coke, as in many American fur naces. Thus, in a series of observations parallel with those detailed above, but made with a smaller furnace (using coke and calcined Cleveland ironstone), the size of which was insufficient to cause the most economical action possible (48 feet in height instead of 80), the average composition of the gases by weight was i.e., they contained much less carbon dioxide relatively to the car bon oxide than those escaping from the larger furnace, whilst the amount of carbon burnt in the form of coke was increased from 1-020 parts to 1 318 parts per unit of weight of pig (or from about 22 "3 to 28 8 cwts. of coke per ton of pig, the coke containing about 91 5 per cent, of carbon and 8 5 per cent, of moisture and ash, &c.). Of this extra fuel consumption part only was expended to generate the heat requisite for the furnace requirements, owing to the for mation of less carbon dioxide and more carbon oxide ; the re mainder produced the effect of sensibly raising the temperature of the exit gases, which escaped at the average temperature of 452 C. instead of 332 C. , thus carrying out of the furnace much more heat than was brought in by the blast. , If by the use of a hotter blast more heat is brought into the furnace by this means than will supply the loss of heat in the waste gases experienced with the less highly heated blast, one of three things must result : either less coke will be requisite to produce the heat necessary for the various furnace requirements ; or the surplus heat will bo carried out in the waste gases, they being at a higher temperature; or the fuel will be burnt to less advantage, a smaller proportion of carbon dioxide being formed and a larger one of car bon oxide. As to what does actually take place in such a case, opinions are somewhat divided ; for, whilst most iron masters main tain that a sensible diminution in fuel requisite per ton of iron is always occasioned by employing a hotter blast, others (especially Bell) consider that their experience proves that in certain cases, e.g. , with Cleveland ore, no actual saving in fuel accompanies the use of blast at temperatures above about 500 (see 21). The results obtained by Bell with blast-furnaces using Cleveland ironstone and coke are applicable, with appropriate modifications, to furnaces employing different ores and fuel. For instance, Crossley found (Journal Ironand Steel Inst., 1871, ii. 157) that a furnace 67 feet in height, 19 wide (boshes), and 13,124 cubic feet capacity, smelting Askam ha?matite and Red Bay ore at Askam-in-Furness, produced grey Bessemer pig with 22J cwt. of coke to the ton of iron (1-1375 per unit of weight of pig), the heat requirements during the process being as follows (the mode of representation adopted above being employed, and the heat viewed by Bell as consumed during the reaction of carbon on the carbon dioxide of the flux being left out of account) : Comparatively constant heat requirements. Reduction of ferric oxide 1C55 ,, carbon from carbon oxide 90 silicon (phosphorus and sulphur practically absent) 120 Ile nt curried away by tuyere water ." 91 ,, ,, molten pig 330 ,, ,, radiation, <tc 2G8 25CO More variable heat requirements. Heat absorbed during causticizing of lime 171 ,, ,, decomposition of moisture of blast 23S ,, evaporation of water in coke l(i ,, ,, ,, ,, ore 27 Heat carried out by molten slag 413 865 3425 This amount of heat was supplied us follows : Total carbon of coke burnt to carbon oxide l-0fi25x2400=2. 1 >.>0 Portion of ditto further burnt to carbon dioxide 1(!2 x5600= 907 Heat brought in by blast 577 Less heat earned out in waste gase? In this case the carbon carried out as dioxide in the escaping gases was only 20 2 per cent, of the total carbon therein contained; 60 that here the fuel was burnt to less advantage than in the most economically worked Cleveland iron furnaces. This, however, was partly due to the fact that in order to make very grey Bessemer pig a larger quantity of coke was employed than would suffice to make forge iron, viz. , about 2 cwts. per ton extra ; so that forge iron could be run in the same, furnace with an expenditure of about 21 f cwts., or 1-0825 parts of coke per unit weight of pig. Other things remaining the same, this would represent an amount of total carbon burnt equal to about 97 per unit weight of pig, which must, therefore, be burnt in the following way to give the same heat as before, viz., 2550 + 907 = 3457 due to the fuel : Since the carbon in the carbon dioxide of the limestone was 0555, the total carbon as dioxide in the gases would thus be -2016 + 0555 = "257, whilst the total carbon as oxide and dioxide would be 0-97 + 0-0555 -0-040 = 0-9855 (0 040 being the carbon in the pig iron, which contained 4 per cent, of carbon) ; so that the escaping gases would contain, when forge iron was being made, about 26 per cent, of the total carbon present therein as carbon dioxide and 74 per cent, as carbon oxide, still indicating a fuel expendi ture less economical than in the most favourably arranged Cleve land furnaces in which 30 to 33 per cent, of the total carbon in the escaping gases is contained as dioxide ; presumably this is due either to the smaller height of the furnace (67 feet only) or to the more difficult reducibility of the ore used. On the other hand, the Wrbna (Eisenerz) charcoal furnaces smelting spathic ore (which is somewhat more readily reducible than calcined Cleveland ironstone) were fotind to give the following values, reduced as before to a unit of weight of pig (white iron) (Tunner, Journ. I. and S. Inst., 1873, p. 433): Comparatively constant requirements. Reduction of ore and of carbon in pig 1C20 ,, silicon 50 Heat carried away by tuyere water and radiation, A:c 192 pig ironi 340 More variable requirements. Evaporation of water in charcoal Decomposition of moisture of blast Heat carried out by slag, 1 part carrying out 440 units of heat (Rinman) -2202 This heat was furnished as follows : carried out in escaping gases 2940 2610 The fuel burnt was charcoal containing about 63 parts of carbon per unit weight of pig, which in order to produce 2727 heat units must have been burnt as follows : Heat due to burning of O GS carbon to carbon oxide 0-63x2400 = 1512 further combustion of 0-217 carbon to carbon dioxido 0-217x5000 = 1215 2727 Of this 63 parts of carbon burnt 045 was taken up again by the iron, leaving 5S5 as the total quantity of carbon escaping in the waste gases. Hence ~~ or 371 per cent, of the carbon was burnt to dioxide, and 62 9 per cent, to carbon oxide, which represents a more economical use of the fuel than in the Cleveland furnaces, not withstanding the smaller size of the Wrbna furnace. Bell has also calculated (Chemical Phmomena, p. 420) for the same furnace very much the same numbers, the furnace requirements being estimated by him as slightly in excess of Tiinner s valuation. Calculations as to the development and appropriation of heat in the Cedar Point anthracite furnace, U.S., founded on the same principles as the above cited examples have been made by "Wither- bee (Transactions Am. Inst. Mining Engineers}; the consumption of anthracite as compared with charcoal in American furnaces is always greater, indicating a smaller proportion of carbon dioxide ultimately formed. As far as the data at present extant go, it does not appear that in any furnace yet constructed burning coke, charcoal, anthracite, or coal, upwards of 40 per cent, of the carbon in the issuing gases is, on an average, contained as dioxide, the remainder being oxide, although somewhat larger amounts are occasionally found as exceptional occur rences. Admitting that |- of the carbon of the fuel is burnt to dioxide and f to oxide, the heat evolution per unit of carbon burnt would be | x 8000 +-f x 2400 = 4640 instead of 8000, which would be developed were all burnt to dioxide ; that is, the " duty " actually per formed by the fuel would be 58 per cent, of the possible maximum amount ; so that even in such a furnace the con sumption of fuel would be at least f= I 72 times the 1 This item is somewhat larger than Bell s figure, as the temperature of the charcoal furnace hearth is somewhat higher (Rinman). XIII. 40