CHAIRMAN'S ADDRESS - Coke Oven Managers Association

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A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 217.prior to carrying out the test, and he (Dr. Foxwell) objected to this actionon the grounds that the proportions of bright and dull coal in the sampletested would differ from the original composition. The dust below 20-meshwould contain all the fusain in the sample, and also as the bright coal ismore friable than the durain, the portion tested would be deficient in theconstituents responsible for the coking power of the coal. Regarding theheating of the apparatus, Dr. Foxwell indicated that he had not experiencedany difficulty in burning the gas completely with the four openings of thestandard apparatus. Concerning the interpretation of the test results,Dr. Foxwell disputed the view expressed by Dr. Mott, who considered thatthe contraction of the coke is a more important indication of dangerousswelling pressure than any expansion which occurs on carbonisation.Dr. Mott had referred in his paper to the work of Davies and Wheeler (TheFormation of Fractures in <strong>Coke</strong>, C.O.M.A. Year-Book, 1932, p.81) and hadconcluded from the illustrations provided in that paper, that shrinkagecracks begin as soon as the coke has passed through the plastic layer stage.In Dr. Foxwell’s interpretation, the illustrations showed that even when thecentre of the charge had reached 500 0 C. and the plastic layer haddisappeared (and with it all internal pressure) only slight evidence of crackswas observed, and none of the major contraction and cracks leading tocauliflower ends. The illustrations in the paper by Davies and Wheelershowed evidence of only small cracks in the lateral direction and none ofcontraction in the longitudinal direction from wall to oven centre, prior todisappearance of the plastic layer. Dr. Foxwell commented that, as everycoke oven operator is aware, the coke comes away from the walls onlytowards the end of the carbonising period, and very strong evidence couldbe provided in favour of the view that until the plastic layer hasdisappeared no considerable contraction occurs, except possibly with ahigh-volatile coal. Consequently, if the coke does not contract appreciablyin the wall to oven centre direction until the plastic layer has disappeared,then contraction cannot be related to dangerous swelling pressures.Dr. Foxwell observed that, in general, generation of any dangerousswelling pressure usually occurs in the final stages of carbonisation, and isconfined to the period between the coalescence of the two plastic layersand their disappearance. Towards the end of the coking process the widthof the plastic layer is doubled, and the rate of heating of the coal in thecentral zone rapidly increases as heat is applied from both sidessimultaneously. A sharp increase in the rate of gas evolution occurs, with a
218. THE COKE OVEN MANAGERS’ YEAR-BOOKgreater resistance to its escape, and a temporary abnormal pressure may,therefore, be generated at this stage.Dr. Foxwell agreed with Dr. Mott’s view concerning the limitations ofthe various tests which had been devised to measure the liability of a coalto damage oven walls. All the tests were open to error, and Dr. Foxwellconsidered that for a reliable assessment of the behaviour of a coal oncarbonisation it was necessary to devise an apparatus in which arectangular section of coal having the width of a full-scale coke ovenchamber, and of the required bulk density, could be heated from both sidesat the conventional rate of carbonisation. Dr. Foxwell drew attention to apaper which had recently been published by Mr. C.C. Russell of theAmerican Koppers Company (American Institute of Mining andMetallurgical Engineers. Technical Publication 1118, 1939), reporting theresults of tests using a movable-wall oven similar to the design of Koppersand Jenkner. Some of the conclusions drawn from the work were (1) easeof pushing the coke cannot be construed as indicating that no dangerouspressures have occurred during the coking period; (2) the pressuredeveloped during carbonisation reaches a maximum when the two plasticlayers meet. In some cases this can amount to five times that registered inthe earlier part of the coking period; (3) this behaviour cannot bereproduced in tests where the coal is heated from one side only; (4) sincebone-dry coal appears to have a fairly uniform bulk density of about54 lb./cu.ft., this value should be the minimum for use in coking pressuretest procedure. Dr. Foxwell concluded that theoretical argument andpractical experience, coupled with the test results produced by theAmerican Koppers Company, all combined to indicate that Dr. Mott’sprocedures were unreliable and could not be accepted as a proper basis forassessment.Mr. H.M. Spiers commented that Dr. Mott had shown greatdetermination in his efforts to devise a test which could be relied upon todistinguish between safe and dangerous coals. He had largely succeeded inremoving part of the uncertainty which had previously existed indiscriminating between the two types, and had taken matters to a moreadvanced stage than that reached by earlier workers. However, muchremained to be accomplished. It was fortunate that Dr. Mott had had accessto a number of coals which were classified as unsafe by the standardKoppers small-scale tests, but had been carbonised without trouble on the
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CHAIRMAN'S ADDRESS - Coke Oven Managers Association

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