430 



NATURE 



[JA- 



and vapours, leaving behind the solid coke, and heat ia j w; 



used 



I lb 



up in hrinfr'^R nbrnit the chnnf 

 (,( iiKil is (|. ( (imposid in the n ■ 

 up in tli(' <|i'i ()inpo>iition and disii!' 

 46a B.Th.U. over and above tlw 

 reactions. The heat withdrawn \ 

 hot fjns and vapours ainnuiits to .;. 

 in the red-hot coke wh. n it is dr.. 



44? H.'l'h.l'., <(n tlinl \\\r h'-at llia( u., 



I her 



111.' 1. 



Ill 



,M'S' in tlif 

 iliii;\i\' lii'ii/ 

 with th.' Ilii- ;; 

 by radiation and (> 

 making Jn all 1884 I 



The thorm.nl valtp li 

 fPin.Tin till' -.^aTni' wintli 

 in a horizontal, verticil, 

 or a rhamber, and ii i 



lion IS ijli.' 4- 324 • 

 setting, however, 

 ntal bench would be 

 -r.sos, 398 B.Th.U. 1 



and 23 B.Th.U. in 



■ rf>nrtions in 

 till' liistillation 

 inclinrd rotort. 

 chirfiv in t! 



th 



in,i<Ic wliich ha\' 

 flu;!!!"!-- altaiiKxl in nioil> 

 iito;t sf'ttin^- cpiotod al- 



>- 4- 1884 = 31 12 ; now, I 10. 01 ^as 

 14,200 B.Th.U. in its combustion, 

 ,,.. heat to carbonise 45 lb. of coal, 

 the coal would require 2 19 per cent. 

 of its weight of coke to carbonise it, whilst if the whole 

 of till" lii.it of (oniluistion could I)c used in the retort, 

 8-6 piT (int. would li<' sufficient. 



A f.iir idi\i of the iinnoniies thai .-iri' postiiliL" can be 

 obtaini-d h\ stat iiiL^ lln- li'Jit \\^<A in thi' sdiiiij^ ruid retort 

 in percentages : — 



economies have brc 



bonising fuel to \\\i- 

 In the liori/ont.il 

 heat used won' ' ' 

 coke gives an 

 so would give ..u.. 

 or, in other words. 



Used in retort 



Used in setting \ 



\l 



Decomposition and 



distillation 

 Escaping in gas and 



vapours .. 

 In hot coke 



Flue gases 

 Radiation and con- 

 vection ... 



1 3. Ash 



B.Th.U. 

 per lb 



462 



324 



442 



1463 



398 

 23 



Per cent, of 

 heat used 



«57 



r39-3 



io'4 



132 



47'2"j 



I2 8r6°"7 

 07.1 



so that 393 per cent, of the heat is used in the retort, and 

 607 in the setting, the item which overshadows all others 

 being the 47-2 per cent, which escapes up the chimney 

 in the hot flue gases. 



It is evident that the first step towards economy is to 

 be found in a better utilisation of the heat in the setting, 

 so as to abstract so far as possible the heat from the 

 products of combustion, and this is done by regeneration, 

 which reduces the flue gases by more than 300° C. in 

 temperature, and brings down the loss due to this item from 

 47-2 to 25-2 per cent., whilst feeding the producers with 

 red-hot coke from the retort effects a further economy, with 

 the result that the fuel used falls from 21-9 to 14-8 per 

 cent, and even lower. 



I'ndi r these conditions the percentage of heat used in 

 doing the work of carbonisation would be largely increased, 

 and the chart would be as follows : — 



Used in retort 



I: 



I. 



Used in setting-! 2. 



Decomposition and distillation 



Escaping in gas and vapours... 



In hot coke 



Flue gases 



Radiation anpl convection 



Ash 



21-41 

 I3S[ 

 i8-9l 



54-1 



25-2] 



199 -45-9 

 oS| 



so that more than one-half the heat is utilised in work. 



In the most modern practice results as low as 10-24 

 per cent, of the weight of the coal carbonised have been 

 quoted, whilst in vertical retorts and chamber carbonisation 

 12 to 15 per cent, is the usual figure, these advances being 

 made by utilising hot coke in the producers, more perfect 

 regeneration, and reduction of the radiation. 



The factor which endows all carbonisation problems with 

 especial difficulty is that we are dealing with a bodv of 

 such varying composition that no two samples are alike, 

 whilst the conditions under which we are decomposing 

 them vary from minute to minute. 



The conduction of heat through a substance like the 

 NO. 2204, VOL. 88] 



rcclay retort is a determ 



s, as the r'">'ii'!""'= "v;..t .. . ,.„, .....;. 



;ire total!;. >vc that can L 



making minations in 



In any caiortin«lric deieimination the on' 



'ce is continuously cooled by the calorii: 

 v.iuM tin heat poured into the other side is very differei 

 in effect to the mass of heated material existing in th' 

 flue.s surrounding the working retort. 



The rate at which heat is transmitted under working, 

 conditions depends upon the degree of heat in the flu 

 and outer walls of the retort, the higher the temper 

 the more rapid being the transmission, whilst the <; 

 cnce between the temperature of the outer and inner ski: 

 of the retort is a factor of the greatest importance ; tl. 

 greater the diflfcrence, i.e., the cooler the inner skin -- 

 the mass in contact with it, and the hotter the out' r 

 in the flue the more rapidlv will the heat pass. -\„ - 

 if transmissioi ith the character o 



with its porosi ^ ith the temperatur 



and ltni,;li of time for which it has been baked, so tha 

 it is impossihle to give any definite figure as to the rat 

 of conductivity or transmission which shall hold good i 

 all cases. Determinations based upon the rate of tran- 

 mission at comparatively low temperatures m.-iv Kf di^ 

 carded at once as valueless, but Mr. G. B- -infi 



the conducting power of firebrick, and can con- 



clusion that one square foot of firebrick, one inch thick 

 passed 659 centigrade pound units, or ii-86 B.Th.U. p* 

 hour for each degree centigrade of difference between th 

 sides of the brick, when these differences were of th' 

 magnitude of 200-300° C. 



My own opinion is that at the ordinary working ten- 

 perature of a retort under gasworks conditions the amoun' 

 of heat transmitted approximates to 25 B.Th.U. per square 

 foot of surface for each 1° C. difference in the temperature 

 of the outer and inner surface of the retort, and that this 

 is not seriously affected by the thickness of the fireclay, 

 as conduction is so slow with a retort 3 inches thick that 

 it is probably only the internal portion that is cooled to 

 any great degree when a fresh charge is fed into a properly 

 heated retort, and the mass of fireclay acts as a store of 

 heat, so that the heat has only a short travel. 



In a horizontal retort ready for charging, the temperature 

 of the inner walls will approximate to 1000° C. (1832° F.V 

 and the flue temperature to 1100° C. (2012° F.), and th' 

 fireclay walls of the retort will conduct the heat at .' 

 rate which approaches to 25 B.Th.U. per square foot pe: 

 hour for each degree centigrade difference in the two 

 surfaces, so that during the first two hours, when the 

 averarjp temperature of the inner side of the retort walls, 

 cooled by the charge and by the retort having been opened, 

 will not be more than 800° C. (1472° F.), the amount of 

 heat pa.ssing through the walls into the charge will b 

 25 X (1100 — 800) = 7500 B.Th.U. per square foot of surface, 

 whilst by the fifth hour, when the inner side of the wall 

 of the retort has risen to 950° C. (1742' F.^, the amount 

 passing will be — 



25X(iioo-95o) = 3750 B.Th.U., 



or only half the amount passed in the '-arli-^r period, tlv 

 average being approximately 5625 B.Th.U. per hour, which. 

 taking the heat units needed for the actions taking plac- 

 in the retort as 1228 B.Th.U. per lb., gives a carl^nisini; 

 value for a six-hour charge of 12 tons per 1000 square fee: 

 of retort surface. 



The diminution in the quantity of heat passing through 

 the walls of the retort during the last stages of carbonisa- 

 tion docs not affect the rate at which the still uncarbonised 

 core of coal is being heated, as the envelope of coke sur- 

 rounding it has reached nearly the same temperature 

 as the walls of the retort, and forms a store of heat, 

 whilst in the carbonising mass during the first part of 

 the distillation the volume of gas evolved is so large that 

 it carries off from the contents of the retort a large propor- 

 tion of the heat, and so keeps down the temperature of the 

 mass until the later stages of the carbonisation. 



It has become the custom to speak of the temperature 

 of carbonisation being high merely because the tempera- 

 tures in the flues and in contact with the walls of the 

 retort are high, and to speak of the products of hi^h 



