CHAIRMAN'S ADDRESS - Coke Oven Managers Association

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A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 197.PART IIIOne aspect of coking practice which has continued to attract attention inthe industry is the subject of coking pressure. The incidence of damage tocoke oven brickwork due to the development of wall pressures during thecarbonisation of some coals or blends first came into prominence followingthe replacement of semi-silica by high-silica bricks in oven construction,which allowed higher flue temperatures of 1350 0 C. or more to be employedand faster coking rates to be achieved. No problems had been encounteredwith coal charges subjected to long slow coking at flue temperatures ofabout 1200 0 C., even though they were carbonised in a compressed state,with a bulk density in the oven of some 60 lb./cu.ft. (dry basis). Thedifficulties due to “swelling pressures”, as they were called, were firstencountered in Germany, and by the middle 1920s had become a chronicproblem there. In the U.S.A. also this difficulty became apparent, andstudies were being undertaken to discover which coals were safe tocarbonise and which could prove dangerous. This topic was first seriouslyconsidered and discussed by C.O.M.A. members in March 1939, whenDr. G.E. Foxwell gave a paper “Dangerous Swelling Pressures inCarbonisation Practice”, which included some striking illustrationsindicating the extensive damage which can occur to oven brickwork whencertain coals are coked. In May 1940 Dr R.A. Mott presented a paper“Coals Dangerous to Coke Ovens”, based on the work of the Midland CokeResearch Committee. Both these contributions were presented at jointmeetings of the Northern Section of the Association and the Institute ofFuel (now Energy) at Newcastle-upon-Tyne, and gave rise to considerableinterest and discussion.In his paper, Dr. Foxwell explained the conditions necessary for thegeneration of coking pressure during carbonisation by referring to hispioneer researches on the plastic state of coals (Fuel, 1924, Volume 3,p.122). Foxwell devised laboratory equipment to measure the change inresistance to the flow of a stream of inert gas caused by the back-pressuredeveloped in a column of coal undergoing carbonisation. The variation ofresistance to gas flow with increasing temperature of carbonisation enableda “plastic curve” to be drawn, and from this work Foxwell showed thatcoking coals become partly fused and are plastic over a temperature range,usually between 350 0 C and 500 0 C, depending upon the coal rank. Whenindividual particles of coking coal are heated through these temperatures,
198. THE COKE OVEN MANAGERS’ YEAR-BOOKcoalescence occurs, and there is increasing resistance to the passage of gas.Poorly-coking coals, when heated as above, form masses of roundedglobules which cohere only loosely together, allowing gas formed bydecomposition to escape easily. In a newly-charged coke oven, as the coalis rapidly heated and reaches a temperature of about 350 C, a layer ofplastic coal is formed at each wall. These layers travel inwards towardseach other as carbonisation proceeds, maintaining a position roughlyparallel to the oven walls. At the top and bottom of the charge additionalplastic layers are formed by radiation of heat from the oven roof and thehot sole of the chamber respectively. During this time the gas evolved bydecomposition of the coal must escape from the plastic layers eitherthrough the hot coke being formed at the walls or the uncarbonised coalsituated between the layers. If the permeability of the plastic layers issufficiently high, the gas generated can find its way relatively easily to theascension pipe. However, if the coal produces a plastic layer which isalmost impermeable to the gas being produced within it, a pressure will becreated until a level is reached which allows the gas to escape. Damage tothe structural brickwork can then occur, as the pressure generated forcesthe coke against the oven walls. There is a possibility also that the coal inthe cool portion of the charge may become surrounded by a plasticenvelope of semi-carbonised coal, and any pressure arising from a build-upof gas between the plastic layers would then augment the force generatedwithin the layers. High coking pressures are known to occur with somecoals as the plastic layers coalesce at the end of the coking period. At thisstage, the material remaining in the centre of the charge, enriched bycondensed tarry compounds, is being heated rapidly on all sidessimultaneously, with consequent increases in both plasticity and rate ofevolution of gas. Under these conditions, there can be considerableresistance to the escape of gas being produced, and a dangerous peakpressure may be set up in these final stages of carbonisation.Dr. Foxwell referred to the large-scale experiments of Koppers andJenkner (Glückauf, 1931, Volume 67, p.353) who had set out to determinethe pressure which a coke oven wall could withstand before deformationoccurred. In a test wall 4.5 metres high and 2 metres long, constructed fromnormal coke oven silica shapes, and loaded from the top to correspond withthe weight of the brick superstructure found in practice, it was found thatcracks first appeared at a pressure of about 0.09 kg./sq.cm. (approximately1.3 lb./sq.in.), but these experiments were conducted on cold walls and
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CHAIRMAN'S ADDRESS - Coke Oven Managers Association

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