CHAIRMAN'S ADDRESS - Coke Oven Managers Association

182. THE COKE OVEN MANAGERS’ YEAR-BOOKA REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945ByJ A ClarkeThese articles were serialised in the C.O.M.A. Bulletin between1992 and 1994. We are indebted to John Clarke for his industryand research during his retirement into the work and history ofCOMA. Resulting from this, Council bestowed on him severalyears ago the honorary title of Historian to C.O.M.A.The dissemination of information for the benefit of the coking industryis one of the objectives for which the Association was founded in 1915.This aim has been achieved mainly by providing a platform for thepresentation and discussion of technical papers at regional meetings of theAssociation, but also by publication of the Year-Book, which has becomerecognised worldwide as an authoritative record of advances in the coalcarbonisation process. Progress in the technology of the manufacture ofblast furnace coke and recovery of by-products has resulted from theapplication of scientific study to the various unit processes, which havenow reached high levels of efficiency. Arising from the efforts of Counciland the members over a long period of time, the Association can justifiablyclaim to have made major contributions in support of the progress whichhas been attained.It is proposed to devote this essay, which will be issued in several parts,to the work of the Association during the war years, and summarise someof the contributions which C.O.M.A. made in support of the industryduring that period.One of the problems facing the U.K. coking industry in 1939 was thedesign of suitable anti-glare structures for coke ovens, and the provision ofscreened lighting, to enable plants to operate under war-time conditionswithout attracting attention from enemy aircraft. Urgent attention wasdevoted to this matter prior to the outbreak of war in September 1939, andMr Clarke is an Honorary Fellow and Honorary Life Fellow of C.O.M.AHe was Honorary Secretary, Southern Section 1960/1970 and thenHonorary General Secretary C.O.M.A. from 1970 to 1985. John wasChairman, Midland Section in 1977 and was progressing towards thePresidency when he retired from Orgreave, British Steel in 1985.

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 183.an Anti-Glare Advisory Committee had been set up by the NationalAssociation of Coke and By-Product Plant Owners and the British Iron andSteel Federation, in collaboration with the Home Office, to deal with antiglaremeasures. Several members of C.O.M.A. served on this Committee.A special meeting of the Midland Section of C.O.M.A. was held on9 March 1939. in Sheffield, under the Chairmanship of Mr. T.P. Carr, forthe purpose of considering proposals for screening glare and lighting,introduced by Mr. E.W. Huddy and Mr. F J. Tickner of the Air RaidPrecautions Department of the Home Office. There was an exceptionallylarge attendance at the meeting (including six Past-Presidents, headed bythe Senior Past-President, Mr. A.H. Middleton), and members of theSections. Representatives of the major construction Companies (theWoodall-Duckham Company, Limited, Simon-Carves, Limited and theKoppers Coke Oven Company, Limited) were also present. Variousschemes had been suggested to enable plants to comply with therestrictions which would be imposed in war-time, and it was hoped thatdiscussion of the practical problems envisaged would lead to theinstallation of effective and economic anti-glare systems. A proposal forcomplete screening, involving covering-in of the ovens, was initiallyconsidered to be too costly. An alternative scheme proposed partialscreening of the movable parts separately (ram and larry car), which wouldbe suitably equipped to carry out their tasks without the emission of glarefrom ascension pipes and charging holes, and also enable the pusher sidedoor to be removed with effective screening. Concerning the avoidance ofglare caused by the discharge of coke from the ovens, Mr. Huddy said theonly reliable system would require the installation of a roof over the wholelength of the coke side of the battery. The contractors had been asked toprovide drawings for the partial screening system, and these werecirculated for comment at the meeting. Mr. Huddy said estimates for thecost of the partial screening system had been requested, and a battery hadbeen selected on which to demonstrate the effectiveness of the scheme. Thecost of this experimental installation would be borne by the Government,but before work started it was hoped to have critical comments andobservations from coke oven managers, who would all have to work withan anti-glare system in the event of war.In the comprehensive discussion which followed, members directedqueries and observations to Mr. Huddy and Mr. Tickner. The frank

184. THE COKE OVEN MANAGERS’ YEAR-BOOKexchange of views provided the Home Office representatives and thecontractors with much useful and practical information for incorporationinto the designs.Further consideration of this subject took place at a meeting of theNorthern Section of the Association in Durham on 19 April 1939, whenMr. E.W. Huddy of the Home Office again led a discussion session on thesubject of air raid precautions as applied to coke ovens. Mr. Huddyoutlined the partial screening system which had been discussed by theMidland Section and the problems arising from operating coke ovens underconditions which precluded the emission of glare. He drew attention to theGovernment’s Civil Defence Bill, which would require of premises such ascoke ovens, in the event of war, to take adequate measures during theperiod of darkness to screen any glare or flame, and the Minister couldserve the appropriate notice on the occupier of such premises. It was alsonoted that financial grants, not exceeding one-half of the capital costincurred in taking the above measures, could be awarded by the Treasury tothose factories on which a notice had been served. Mr. Huddy said thatwhen designs for the partial screening scheme were finalised and theequipment was installed on a selected coking plant, they intended toexperiment with it for a month at least, in order to reveal any shortcomings.Mr. Huddy’s introductory remarks led to a wide-ranging discussion ofthe problems involved, to which members and the contractors made usefulcontributions.The Midland Section returned to the subject of anti-glare measures in adiscussion at the Annual General Meeting in Sheffield on18 November 1939. Mr. T.P. Carr (retiring Chairman, and a member of theNational Anti-Glare Committee) introduced the subject, and produceddrawings of various schemes which had been submitted to this Committee.Particular attention in the ensuing discussion was paid to design andinstallation of the anti-glare structure over the coke car track, includingsuitable arrangements for adequate ventilation. It was noted that the use oftubular steel scaffolding and sheeting had enabled coke side anti-glarestructures to be erected rapidly. The coke side at Corby, for instance, with113 ovens, had been completed in a month. Reference was made to theproblems being encountered with glare from the larry car, caused by flamesemitted on charging, which were not adequately screened by the sheetingenclosing the charger car. This covering, it was suggested, exacerbated the

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 185.problem, but it was reported that the Anti-Glare Committee regarded thesheeting as an essential part of the screening system. Procedures forlimiting the production of flame on charging were suggested, such as theuse of steam jets in the ascension pipes and also at the leveller door, to stopgas igniting there. It was noted that at plants where rapid charging of theovens was possible (e.g. with free-running coal in steep-sided hoppers)there appeared to be less likelihood of fire from the charge-holes. In orderto keep air out of the system, and prevent ignition, the charge-hole lidswere replaced as quickly as possible on completion of charging, before thelevelling operation.Reference was made in the discussion to the responsibilities of the cokeovens manager in respect of legal requirements concerning theeffectiveness of the blackout at his plant. It was appreciated that, accordingto the law, the emission of any glare or light was forbidden during thehours of darkness, but the possibility of human error or accident remained,and this could lead to infringement of the statutory requirement. Thismatter was not resolved at the meeting.In practice, the erection of anti-glare screening equipment led to manydifficulties, because of inexperience in these matters. The structures oftenprevented proper observation and operation of the oven machinery,particularly on the coke side, where vision was obscured at intervals bysmoke and steam. Whereas any flame on pusher side doors had to beextinguished immediately during the dusk to dawn period, there was notthe same urgency for remedial action on the coke side, and this situationoften resulted in damage to door frames, buckstays and oven brickwork. By1940/41, when coking plants were subjected to complete shut-down forlong periods during frequent air-raids and warnings, the shortcomings ofthe partial screening system had been demonstrated. On the coke side, theoriginal covers were often too low, lighting was inadequate and ventilationinsufficient, making conditions very unpleasant. The presence of fine dustin the air after pushing also added to the discomfort of the operators. Manyplants were considering the desirability of complete covering-in of theovens, in spite of the cost, so that the plant could continue to operate exceptduring an air-raid. Control of battery temperatures during prolonged “red”warnings proved a difficult task for the foremen and heaters, particularly ifthe enforced shut-down came as dusk fell. Plants varied in their attitude torecovery of production when the all-clear sounded. Some operatorsattempted to make up for losses by pushing and charging as quickly as

186. THE COKE OVEN MANAGERS’ YEAR-BOOKpossible. Others considered it inadvisable to subject the plant to additionalstress, allowing time for some rhythm to be re-established on the battery,and also for the doors to be properly cleaned. Departure from a steadyoperating schedule occasionally led to the incidence of stickers. Damage tooven brickwork and sole blocks also occurred for the same reason. Theseadverse operating conditions gave rise to unwelcome changes in the qualityof by-products, for example increased free carbon in tar, higher levels ofnaphthalene in the gas and changes in the composition of the crudebenzole. Considerable importance was naturally attached to themaintenance of production levels to aid the war effort, and it has to beremembered that coke was in great demand for blast furnaces, gas contractshad to be honoured and toluene was a vital raw material for explosivesmanufacture.The problems of war-time operation featured prominently in theAssociation’s proceedings at that time, and members met to exchangeexperiences and resolve some of the difficulties. At the Annual GeneralMeeting of the Midland Section in Sheffield on 19 November 1941. theincoming Chairman, Mr. W.O. Humphreys (Brookhouse) inaugurated adiscussion on works problems arising from blackout conditions, in place ofthe normal Chairman’s Address. This departure from usual practice gavemembers an opportunity of learning from the discussion of the bestmethods of dealing with prevalent difficulties. There were many usefulcontributions to the proceedings, and subjects considered were glareprevention, suggested procedures for operation of the plant during and afterand air raid, together with a discussion on methods of repairing damagedovens.In a practical paper to the Northern Section of the Association in April1942, Mr. R.J. Barritt and Mr. C.H. Newby considered the problems arisingfrom the operation of coke ovens under war-time blackout conditions. Theyobserved that the unusual troubles being encountered were largely due tosuspension of the regular oven charging and discharging schedule, arisingfrom air raid alerts. Problems had been created by the presence of theblackout shed on the coke side, and included damage to oven soles (fusion,spalling or excessive wear), spalling of oven walls, breakage of doorframes and damage to self-sealing doors, particularly on the coke side.These matters received detailed attention, and methods for effecting repairswere carefully considered. The authors gave their views on the heating of abattery during the period of an “alert”, and procedures for dealing with

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 187.stickers, which were often caused by the coke becoming brittle and fissureddue to prolonged heating. This paper gave rise to an interesting and usefuldiscussion, with members taking advantage of the experiences of expertsfrom one of the leading firms of contractors to the industry.The subject of hot-patching repair of coke oven walls was discussed byMr. S. Tweedy in a paper to the Northern Section in May 1942. War-timeconditions had led to premature deterioration of refractory brickwork, andthe author gave a useful account of the cement-spraying and spray-weldingmethods, together with the trowelling procedure (used mainly for patchingoven door jambs). A description of the three processes was given in somedetail, together with the precautions necessary to achieve optimum results,and the limitations of the various processes were noted.It is, of course, well-known that in recent times tremendous progresshas been made in improving and simplifying many of the hot, dirty andlabour-intensive jobs on coke ovens. War-time operators would have beendelighted to clean their doors with the high-pressure water-jet equipmentnow available, charge the coal using mass-flow larry cars and employ thenew Fosbel ceramic welding system to repair the ovens!During the war, strict controls on the recovery and rectification ofbenzole were enforced by the application of Government Orders. Therewas an urgent need to produce sufficient nitration-grade toluene to meet thedemand for explosives manufacture in the U.K., and coking plants which inpeace-time had produced only motor benzole, were directed to installdistillation plant and fractionating columns to produce pure toluene. Onerecalls that a record had to be made of any occasion when rich gas was notpassed through the benzole scrubbers, and spirit sold as motor benzole wasnot allowed to contain more than 2% by volume of toluene. During theperiod of the war, a number of papers came before the Association dealingwith most aspects of the recovery and rectification of benzole, andprovided members with much useful information.It has long been recognised that maintaining the circulating medium(creosote or petroleum oil) in good condition is one factor of greatimportance if a high recovery of benzole from the gas is to be achieved.The Clayton Cascade oil distillation plant (British Patent 419,545 (1934))was devised for that purpose, taking oil from circulation and separating the

188. THE COKE OVEN MANAGERS’ YEAR-BOOKheavy end as a pitch-like residue, allowing the clean oil to return to thesystem. It is well-known that in repeated circulation through the benzoleplant, creosote behaves in a manner different from that of mineral oil inrespect of its viscosity. Creosote continuously increases in viscosity as itpolymerises and also takes into solution materials such as traces of tar,gum, etc., derived from the gas. Conversely, mineral oil reaches a certainlevel of viscosity and then remains relatively constant. Petroleum oil forbenzole absorption is essentially paraffinic in nature and will only dissolvevery limited amounts of the impurities mentioned above, before depositingmaterial as a sludge on scrubber grids and other parts of the plant.Approximately thirty Clayton plants were installed at U.K. coking plants,and proved very satisfactory. One plant even added dehydrated light tarfrom the electrostatic detarrer to the still feed to augment the oil yield.Occasionally the rated capacity of the plant was to be too low to keep pacewith the thickening of the oil which occurred, due to the factors alreadymentioned. Some plants found it advantageous to distill the oil (e.g. in thetar plant) at a much higher rate than that employed in the Clayton plant,and then when the oil in circulation had reached a satisfactory condition, tobring in the Clayton plant to maintain viscosity at the desired level.In a lecture to the Northern Section in February 1940, Mr. R.H. Larkegave an account of his practical experiences with the Clayton unit installedat the Cleveland Works of Dorman Long and Company. The authorprovided in graphical form the results obtained on the benzole plant duringthe first fourteen weeks of working of the Clayton plant. The steady fall inviscosity of the oil in circulation had given rise to beneficial changes inplant operating parameters and a welcome increase in benzole yield.In April 1940 a paper was read before the Midland Section byMr. G. Claxton and Mr. W.H. Hoffert of the National Benzole Associationinto the causes of formation of sulphur dioxide during the refining ofbenzoles with sulphuric acid. The evolution of sulphur dioxide during therectification process gives rise to the production of an acid benzole, but amore serious effect, particularly in war-time, was the corrosive effect oncondensers and heat exchangers. In order to avoid the trouble, some workshad added caustic soda to the still, and although this had prevented theevolution of sulphur dioxide, occasionally mercaptans had appeared in thedistillate, which exacerbated the problem. It was found that the principalcause of -the trouble was due to decomposition of the compounds formed

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 189.by acid-washing of cyclopentadiene (C 5 H 6 ) and di-cyclopentadiene(C 10 H 12 ), present in crude benzole. Since cyclopentadiene boils at 42 0 C., itcan be eliminated in the forerunnings (together with carbon disulphide,boiling at 46 0 C.), using a Barbet fractionating column or other efficientdevice. However, cyclopentadiene rapidly polymerises to the dimer form(boiling point 170 0 C.) on storage, and accordingly it was recommendedthat cyclopentadiene should be separated from the crude benzoleimmediately on production. When benzole has been stored anddicyclopentadiene has been produced it was found that this compounddepolymerised during distillation, and the monomer then appeared innearly all the spirit fractions.The authors also showed that another means of reducing the acidity wasto use the lowest strength acid necessary to effect the required reduction insulphur content. For an acid of given strength, there was an optimumamount at which sulphur dioxide production was greatest. Beyond themaximum, the quantity of sulphur dioxide produced was shown to beinversely proportional to the acid used.With the realisation of the importance of making the best use of wartimesteel supplies, and of labour necessary to fabricate and erect steelplant, this paper made a valuable contribution to the conservation ofdistillation plant through the avoidance of corrosion.In a paper to the Midland Section in January 1942 (repeated before theSouthern Section in February 1942, Mr. G.J. Greenfield considered theprinciples and practice of benzole recovery from the gas. In his position asAdvisor on Benzole for the Department of Mines, Mr. Greenfield was inthe unique position of having visited every benzole plant at U.K. cokeovens, and his paper provided much useful information covering everyaspect of recovery. He stressed the importance of maintaining a lowviscosity of the absorbing oil, circulation at a rate sufficient to ensurethorough wetting of scrubber grids and cooling of the medium to theappropriate temperature in relation to that of the gas. The desirability ofturbulent flow conditions in oil coolers to achieve maximum efficiency wasdiscussed, and importance was attached to the supply of sufficient coolingwater of suitable quality for optimum results. Efficient stripping of the richoil requires the medium to be at a sufficiently high temperature (usuallyabout 120 0 C.) by heat exchange and closed steam before entering the still,

190. THE COKE OVEN MANAGERS’ YEAR-BOOKwhere open steam completes the process of debenzolising. These matterswere dealt with in some detail, attention also being devoted to steamsupplies and economies made possible by the use of exhaust steam, whereavailable. Other topics considered were conditions for the optimumrecovery of toluene, the testing of debenzolised oil, and the examination ofmethods for assessment of benzole slip.The paper led to a most useful discussion at both Sheffield and Cardiff,as the pages of the 1943 Year-Book show.PART IIAmong the war-time C.O.M.A. papers on benzole was a lengthycontribution by Mr. O.B. Wilson (Manager at Clay Cross coking plant,near Chesterfield, Derbyshire) entitled “Increasing the Make of Benzole”,presented to the Midland Section at a meeting in Sheffield on 21 February1940. Although this paper was preprinted (the present writer recalls havinga copy in his possession) and also appeared, in part, in The Gas WorldCoking Section for March and April 1940, it was not subsequentlypublished in the Year-Book. However, a modified version of the paper,under the title “Searching for more Benzole” was published by the authorin Coke and Smokeless-Fuel Age for May and June 1943, and the lattertwo sources have been used to prepare the following notes. The kindassistance of the Institution of Gas Engineers in providing relevant copiesof The Gas World Coking Section is gratefully acknowledged.The author first considered the quality of coal charged to the ovens,noting that water and ash present in the slack could not make anycontribution to benzole production. He then outlined the process operatedat his works for reducing both ash and moisture in the washed coal chargedto the ovens. The process concerned removal and recovery of the finesolids (less than about ½ mm.) from the coal washery water, which enabledcleaner water to be fed to the washery system and resulted in a betterseparation of the coal from the dirt present in the raw material. Theprocedure adopted by the author was to treat the effluent water from thewashery with starch and lime solutions to give a pH of 9.5. Thiscombination of reagents and alkalinity ensured rapid flocculation andprecipitation of the fine particles in settling tanks, from which a slurry waswithdrawn and dewatered on a vacuum filter. The clean water overflowing

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 191.from the settling tanks was returned by gravity to the washery. The absenceof fine particles from the clean coal permitted water to drain more freely inthe storage bunkers, and the coal carbonised contained 5% less moisturethan previously. Consequently, increases in both throughput and productionof by-products (including benzole) were obtained, together with a reductionin fuel consumption. The author did not reveal how he disposed of therecovered fine solids from the vacuum filter, but noted that having acleaner and drier coal for coking made the process advantageous.Attention was then devoted to the effects of conditions in the coke ovenon benzole production, and the author discussed various methods whichhad been devised to limit the time of contact of gas in the hot free spaceabove the charge and prevent excessive thermal degradation of thehydrocarbons evolved on carbonisation. The gases produced from a chargeof coal being carbonised in a coke oven take the path of least resistance,and the major portion accordingly passes through the coke formed at thewalls of the oven and then travels upwards to the ascension pipe. In doingso, the gases pass from a low- to a high-temperature zone, and becomepartly decomposed, depositing carbon on the coke. Attempts have beenmade in the past to avoid this decomposition of volatiles by drawing off aportion of the products of distillation from the interior of the charge, usinga system of central offtakes devised by Carl Still. Although increasedyields of tar and benzole were obtained by the use of this system, theproducts were not fully aromatic, and their value was thereby reduced. Inthe process patented by Goldschmidt, the extent of thermal decompositionof the gas is controlled by the use of a horizontal channel built into therelatively cool brickwork of the oven top, and connected to the free spaceabove the charge by a series of apertures, through which the gas reaches theascension pipe. The apertures decrease in diameter along the length of thechannel, with the smallest at the ascension pipe end of the oven, permittingfree access of the gas to the protective action of the channel, where thetemperature is some 200 0 C. lower than that of the free space. In thissystem, the residence time of the gas in the hot free space is reduced,leading to increased hydrocarbon yields. According to the author, ovenshaving the Goldschmidt flue were then in use at a limited number of U.K.coking plants, and an increase of about 10% in benzole yield was claimed,compared with conventional practice. In the Beimann main system (whichwas originally evolved as a simple device for equalising the gas pressureover a group of oven chambers), the battery is fitted with two collectingmains, one on either side of the ovens, instead of the usual arrangement of

192. THE COKE OVEN MANAGERS’ YEAR-BOOKa single main on one side only. The mains are of standard construction, andeach oven is equipped with an ascension pipe and liquor-sprayed valve atboth pusher and coke sides, but only one of the mains is fitted with theusual foul gas offtake to the recovery system. The other or “compensating"main (“Ausgleichvorlage” of the original Germany patent) has no outletand serves only to connect each oven chamber with the remainder of thebattery. This system allows gas from ovens in the early stages ofcarbonisation to pass into and along the tops of the ovens which areapproaching the end of coking, and results in the gas pressure beingequalised throughout the whole battery at all times. The amount of gaspassing into or out of the ovens can be controlled by the relative positionsof butterfly valves on the ascension pipes of the two mains. These valvescan be set at a number of positions between fully open and fully closed,and a simple system of operation and control, related to the oven scheduleand other factors, must be devised and implemented to obtain fulladvantage of the Beimann principle. The gas from the Beimann main iscooled by the liquor spray on two occasions -once on entering the main andagain on exit, and the cooling effect of this gas on the oven tops must becarefully adjusted to avoid any danger of spalling or other damage to theupper parts of the chamber. The Beimann main allows the operator tocontrol the temperature of the free space above the charge and enablesincreased yields of tar and benzole to be attained. The author reported thatcareful tests on several batteries had shown increased yields of benzole andtar at 9% and 12% respectively when the Beimann main was in operation.Information was also provided on the effect of gas space temperature onthe composition of the benzole produced, showing that benzene increasedsteadily from 60% to 80% as the free space temperature increased from700 0 C. to l000 o C., but toluene in the spirit recovered fell from 20% to 13%over the same temperature range.The author then devoted attention to the benzol recovery process andstressed the importance attached to cooling and cleaning of the gas prior tothe benzole scrubbers, commenting on the desirable features of watercooledcondensers, and the importance of factors such as turbulent flow inachieving good levels of cooling efficiency. Removal of ammonia from thegas to a high degree of efficiency is a prime necessity, for if this isneglected, then corrosion will take place in the benzole stripping plant,with consequent loss of benzole. Concerning removal of tar fog particlesfrom the gas, the author devoted considerable space to the principles andpractice of the electrostatic system, in which the minute particles present

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 193.are given an electrical charge and then subjected to a strong electrical field,leading to almost complete precipitation of the tar. Examination ofelectrically-precipitated tars showed that they are of lower specific gravitythan the ordinary tar recovered from the plant, and contain liquor of highammonia content together with hydrocarbon oils, large amounts of whichboil in the benzole range and can be recovered with suitable equipment(this system was mentioned in the previous article in this series - C.O.M.A.Bulletin, April 1992, p.l1.). The tars also contain resin-forming materialssuch as coumarone and indene, together with other unsaturated oils andpitch. If such materials were allowed to go forward in the gas stream,thickening of the creosote or petroleum oil used for benzole recoverywould rapidly take place.The author then considered the practical aspects of ensuring efficientrecovery of benzole from the gas, but noted that complete removal couldnever be achieved on the plant, owing to limitations in respect of contacttime between benzole vapour and scrubbing oil, mechanical efficiency ofthe scrubbers and the presence of traces of benzole in the debenzolised oil.In discussing topics such as scrubber design and the counter-currentprinciple of gas washing, oil circulation requirements and the maintenanceof low oil viscosity in relation to benzole recovery, the author stressed theimportance of adequate cooling of both gas and oil, but drew attention tothe need for the oil to be maintained at atemperature 1 to 2 0 C. above that of the gas, to avoid water deposition.Efficient recovery of benzole from the gas demands the circulation ofappropriate volumes of oil, and to illustrate the importance of temperaturein the process, the author presented a graph relating to practice at hisworks, which showed how the oil requirement increased from 55 gallonsper 10,000 cu. ft. at a washing temperature of 10 0 C. to 110 gallons at 25 0 C.in order to maintain efficiency, and posed the question “How manyoperators double the oil in circulation during the hot summer months?”. Agood wash oil should have a high capacity for benzole absorption, a lowviscosity to facilitate oil circulation and covering of the scrubber grids, lowspecific heat to assist in heating and cooling operations, low volatility toreduce distillation loss and a specific gravity which allows of readyseparation from any water present, thus preventing the formation ofemulsions. The oil should be resistant to sludge formation and also tochanges in its characteristics when subjected to repeated heating andcooling.

194. THE COKE OVEN MANAGERS’ YEAR-BOOKThe author then proceeded to describe the laboratory experiments hehad conducted into the causes of thickening of creosote oil, which occursafter repeated circuits round the benzole plant. The oil to be examined washeld at about 190 0 C. and various gases passed through the system. Withoxygen, the viscosity of the oil (Redwood No. 1, temperature unspecified)increased the 39 seconds to 60 seconds after 35 days, but with hydrogensulphide and viscosity had more than doubled (to 80 seconds) after only 14days. The effect of crude coke oven gas (containing 0.7 to 0.9% oxygenand 600 to 900 grains of hydrogen sulphide per 100 cu. ft.) was alsoexamined, and although, as expected, the results were less dramatic thanwith pure gases, the viscosity had nevertheless almost doubled in a month.Washing the oil with alkali and acid to remove basic and unsaturatedconstituents respectively, gave a product which was very resistant tothickening by oxygen and hydrogen sulphide, but the cost of manufactureof such an oil would prohibit its use in practice.In order to maintain the oil in good condition a Wilton pipe still with asingle fractionating column was installed at Clay Cross. In addition to thecreosote oil being regenerated, the feed to the pipe still included coolercondensates and black naphthalene (from steaming out coolers, etc.) as asource of make-up. The author described his experiences with this plant,which produced a light oil and naphthalene in addition to creosote oil, andoutlined the beneficial effects on benzole recovery which resulted from itsuse. To support the aim of increasing the make of benzole, there was a needfor regular laboratory tests on benzole in the gas before and after thebenzole scrubbers, together with information concerning temperatures andvolumes of oil and gas, provided by reliable recording instruments,properly maintained, so that any corrective action could be taken promptlyif necessary.Attention then turned to the debenzolising process, in which the rich oilis raised in temperature by heat exchange and closed steam prior to contactwith live steam in the stripping still, whereby the benzole is carried off asvapour. The author referred to the importance, for efficient stripping, ofadequate preheater surface area in relation to the oil flow, to ensure that asufficiently high temperature (usually about 120 0 C.) is achieved prior to thestill. Reference was made to the economies in steam consumption fordebenzolising which could be achieved where water temperatures alloweda smaller volume of colder scrubbing oil to be circulated, consistent with

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 195.efficient benzole recovery. Also, where it was possible to increase thepreheat temperature to 150 0 C. further benefits accrued, such as increasedbenzole make, better debenzolising and hence improved absorptivecapacity, leading to reduced oil requirements for scrubbing and less steamconsumption in the still.Finally, the author examined the benzole refining processes andtechniques available at that time and commented on procedures whichcould avoid waste of reagents and increase the yield of finished product.The two main considerations involved in benzole refining relate to thenature and amount of sulphur compounds (mainly carbon disulphide andthiophene) existing in the crude, together with the presence of unsaturatedhydrocarbons which, although otherwise acceptable as constituents ofmotor spirit, can give rise to gummy materials under certain conditions.Removal of carbon disulphide (b.p. 46 0 C) can be carried out automaticallyto a high level of efficiency using a Barbet fractionating column. Thebenzole is distilled and a small fraction containing all the carbon disulphideand other low-boiling constituents is removed from the head of the column.Steam requirements for stripping are small, and the only other operatingcost is for water necessary for condensing the “heads” fraction and coolingthe carbon disulphide-free benzole. The author listed the benefits resultingfrom operation of the Barbet column, which included a substantial increasein yield of motor benzole, since only limited acid-washing was then usuallyrequired for the benzole to meet the sulphur specification, and there werealso reductions in the use of acid and soda for rectification. The authoralso discussed the removal of carbon disulphide from benzole by theammonium polysulphide process, patented by Yorkshire Tar Distillers,Limited. The reagent, consisting of approximately equi-molecularproportions of ammonium disulphide, ammonium sulphide and freeammonia, is agitated with the benzole, and after drawing off the reagent awater wash is given, leaving the spirit ready for any further refiningnecessary. The process had become widely adopted by the Company,which supplied the fresh reagent and also accepted the spent material forregeneration at its chemical works. Application of the polysulphideprocess had also enabled increased yields of motor spirit to be obtained,dependent to some extent upon the amount of acid washing which wasnecessary for thiophene removal. As thiophene (C 4 H 4 S) boils at 84 0 C. it isnot possible to separate this compound from benzene (b.p. 80 0 C.) byfractional distillation, and the usual procedure for removal is bysulphonation with strong sulphuric acid. Unfortunately, this treatment also

196. THE COKE OVEN MANAGERS’ YEAR-BOOKremoves unsaturated hydrocarbons present and reduces the yield of motorbenzole.The author referred to the Manvers Main plant designed for thetreatment of acid tar (W.F. Carr, T.G. Woodhouse and W. Green,B.P. 501,077 (1937)). In this process, the tar is agitated with steam torecover the entrained benzole, and then a dilute acid suitable for use in thesulphate plant is run off. Finally, the residue of resinous material is drainedwhilst hot into a shallow tray, where it sets into a brittle solid and can bebroken up for disposal.Tests on plants using this boiling-up technique had shown that asurprisingly large amount of spirit was recovered, and this could be furtheraugmented if soda and water run off from the washer were also treated inthe system, which proved an economical method of disposing of a difficultwaste product.To conclude his review of the refining processes and the theme ofincreasing the make of benzole, the author referred to the inhibitor process,which aims to conserve the unsaturated hydrocarbons present in thebenzole. These compounds are valuable constituents of a motor fuel,provided that their gum-forming tendencies can be controlled.W.H. Hoffert and his colleagues of the National Benzole Associationobserved in laboratory studies some 65 years ago that the formation of gumfrom unsaturateds present in benzole occurs through an initial oxidationstep, and that the addition of quite small amounts (of the order of 0.01 to0.04%) of anti-oxidants would forestall the formation of gum. Theinhibitor can prevent oxidation almost completely for a limited time,known as the induction period, but as the anti-oxidant disappears gumformation then proceeds normally. However, induction periods of a year ormore proved possible by optimising the concentration and nature of theinhibitor, usually phenolic or cresolic in character. In practice, the inhibitoris added continuously during distillation to the benzole on its way tostorage. It is not necessary, therefore, to remove a major portion of theunsaturateds in order to produce a stable spirit, and by restricting acidwashingto the level necessary for thiophene reduction, improved yieldscould be obtained.

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

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 199.also, in another important aspect, differed from full-scale operation, wheremovement of the walls of one oven is affected by any forces beinggenerated in an adjacent chamber. Incorporating a small additional safetymeasure, Dr. Koppers therefore concluded that coals generating pressuresof more than 1.1 lb./sq.in. (at the bulk density obtaining) should not becarbonised in 4.5 metre coke ovens.Reference was then made by the author to the studies of Koppers andJenkner of the pressures developed by coals during carbonisation. For thispurpose these workers designed a small coking chamber heated from bothsides and having walls which could move independently of each other,being carried separately on frames supported by rollers (Publication of theKoppers Company, Essen, 1937). On the outside of one wall a hydraulicmeasuring device was fitted, and a record could be made either of thepressure generated or wall movement during carbonisation. With this unit itwas also possible to demonstrate the increase in coking pressure with anincrease in both bulk density and rate of heating, and also illustrate thepressure developed as the plastic layers coalesce in the final stages ofcarbonisation. Coals tested in the oven gave pressures generally between0.1 kg./sq.cm. (1.4 lb./sq.in.) and 1.0 kg./sq.cm. (14.0 lb./sq.in.), but somepoorly-coking coals exhibited much lower pressures.The movable-wall oven was constructed for research purposes, andowing to the cost of operation was not considered suitable for routine use,for which purpose simpler equipment was required. Dr. Foxwell thendevoted attention to some of the laboratory-scale tests which had beendevised to distinguish between coals safe to carbonise and those having apotential for causing damage to oven walls. He noted that no small-scaleequipment had then been devised to register the maximum pressureoccurring when the plastic layers meet, and that the best guide which couldbe expected from laboratory testing was to carbonise the coal under strictlydefined conditions, giving an empirical indication which would then enablethe safety or otherwise of a coal to be deduced.In the Koppers laboratory-scale apparatus (Koppers Mitteilung, 1930,Volume 12, p.1) an 80 gram sample of the coal to be tested (air-dried, andcrushed to below 1 mm) is contained in an asbestos-lined heat-resistingsteel crucible, 60 mm internal diameter, heated from below with astandardised gas flame. A standard bulk density of 740/760 kg./cu.m. (dry)

200. THE COKE OVEN MANAGERS’ YEAR-BOOKis employed for each test. A steel piston is placed on top of the coal, whichis subjected to a pressure of 1 kg./sq.cm. by a 10 kg. weight and leversystem. Any movement of the piston by the expansive force within the coalduring carbonisation is measured on a recording chart, which also denotesthe contraction occurring in the course of the 4-hour test period. From theoutline of the chart, Koppers gave a guide, based on experience, concerningthe swelling characteristics of the coal and its potential for causing damageto oven walls.The Nedelmann apparatus for the determination of coking pressure(Brennstoff-Chemie, 1931, Volume 12, p.42), although derived from theKoppers laboratory-scale equipment, differs from it in using 120 grams ofcoal contained in a steel crucible of 80 mm internal diameter, andmeasuring on a gauge the pressure set up by the coal on carbonisation. Thecoal sample at standard bulk density is loaded into the crucible, which has alining of asbestos paper, and heat is applied unidirectionally by a Mekerburner under closely-defined conditions over a period of three hours, to afinishing temperature of 900 0 C at the bottom and 700 0 C at the top of thecharge. The pressure developed within the coal is transmitted through apiston covering the whole area of the test crucible, to a lever arm pivotedbetween the plunger and a pressure recorder, reading directly in kg./sq.cm.Based on the maximum pressure recorded in the Nedelmann apparatus,tentative safety limits were proposed; beyond these levels the coal wasconsidered dangerous. Nedelmann also proposed that coals showing lessthan 3% of shrinkage should be regarded as dangerous, or likely to giveproblems in pushing the coke.The importance of bulk density of the charge in relation to developmentof coking pressure was recognised at an early stage, and Koppers designeda large-scale swelling apparatus (H. Koppers, publication of KoppersGmbH, Essen, 1937) in which the behaviour of coals at various densitiescould be assessed. This equipment was designed to record only pressuresgenerated in the coal charge which are in excess of the safety limit of0.08 kg./sq.cm. for coke oven walls, set previously by Koppers andJenkner. The apparatus differed from the earlier laboratory-scale equipmentmainly in respect of size of the components. A crucible of 200 mmdiameter with perforated base was used, and coal of the required bulkdensity charged to a height of 200 mm (i.e. half-oven width,approximately). On top of the coal charge was placed a plunger connected

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 201.to a lever, with a movable weight, and a pen attached to the lever recordedits movement. To avoid compressing the coal after charging the crucible,the lever was supported by an adjustable screw arrangement. The weightcounteracting the force of the expansion was so arranged that the lever armcould not be raised until a pressure of 0.08 kg./sq.cm. was reached. Heatwas applied only to the base of the crucible, and carbonisation wascompleted in about 16 hours. If the pressure exerted by the coal duringcarbonisation was greater than 0.08 kg./sq.cm., the plunger and lever werelifted above the zero line on the recording chart. Any coals showing apressure in excess of the above limit were considered unsafe forcarbonisation. Dr. Koppers recommended that coals should be tested in themanner described at several bulk densities covering the range which mightbe anticipated in practice. It is, of course, well-known that there is a widevariation in bulk density (particularly of wet-charged coal) in differentparts of a coke-oven chamber, and Dr. Foxwell mentioned the adverseeffect on this situation which could be caused by a deflection fromhorizontal of the leveller bar. His paper included an illustration of theextensive oven damage at one plant, where the coal located in the coke sideof the ovens was compressed due to a sagging leveller beam, and theresultant high bulk density increased the coking pressure to a dangerousstate.Dr. Foxwell then referred to three papers dealing with coking pressurewhich had been presented at a meeting of the American Gas Association inMay 1938. Mr. C.C. Russell of the American Koppers Company had drawnattention to the difficulties inherent in relating the results of small-scalecoking pressure tests to commercial coke oven plant operation. Referencewas made to the shortcomings of existing expansion tests, which failed toreproduce the temporary maximum pressure set up during coalescence ofthe plastic layers towards the end of carbonisation. The American KoppersCompany had designed a movable-wall oven, heated from both sides, inwhich full-scale carbonisation could be simulated. One wall of the ovenwas mounted on roller-bearing wheels, allowing movement to take placeand the volume change of the charge under various loads to be measured.Dr. Foxwell indicated that very little work had been carried out with theoven at that stage; however, the work would be followed with interest.Mr. W.T. Brown of the Bethlehem Steel Company had devised testingequipment taking a charge of up to 52 lb. of coal, which was carbonisedunder a pressure of 1.5 lb./sq.in., following the work of Dr. Koppers on thesafety limit of pressure allowable for coke oven walls. Brown confirmed

202. THE COKE OVEN MANAGERS’ YEAR-BOOKearlier findings that some coals which were safe when subjected to longslow coking could be dangerous when carbonised more rapidly. Owing tovariations in characteristics of coals and the coking conditions from oneplant to another, it was necessary for each installation to set its ownpermissible limits for the expansion properties of the coals used. In thethird paper, Mr. V.J. Altieri described an apparatus which measuredsimultaneously the change in depth and width of a coal charge duringcarbonisation. The charge width could be varied up to a maximum of 8 in.by adjustment of a cold rectangular side-wall mounted on ball bearings.The charge of 4 lb. of coal was heated from one side only, and a gaugerecorded any horizontal expansion or contraction. The equipment includedprovision for any downward pressure to be applied, and a chart recordedthe changes in depth of the charge during the coking period. Altierireported the following lateral expansion pressures on certain coals he hadtested:lb.sq.in. kg./sq.cm.A safe coal blend 0.8 0.056A medium-volatile coal 2.3 0.16A low-volatile expanding coal 5.8 0.41Dr. Foxwell also mentioned the “vertical slot” steel test oven which hadbeen designed by the United States Bureau of Mines (R.I. 3403). Thisoven, taking a charge of 200 lb. of coal, was heated electrically on one sideonly; the other wall was movable and connected to a compression springwhich enabled a lateral pressure to be applied to counteract the forcegenerated by the charge during coking. However, the one-sided heatingarrangement with this oven did not permit simulation of the reallydangerous condition which can occur in full-scale practice when the twoplastic layers unite.In concluding his paper, Dr. Foxwell listed the coalfields of Kent,Durham, Somerset and South Wales as areas in the U.K. where coalsdangerous to coke ovens might be found. Experience has shown that themajority of British coking coals are quite safe for carbonisation undernormal conditions of oven operation, but the author noted that the existenceof some dangerous coals should be recognised. Procedures for theirdetection had been described. The importance of bulk density of coal in theoven and its relevance to coking pressure was again stressed, and measures

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 203.for control (such as finer grinding of the charge and adjustment of moisturecontent) were duly noted. Reference was also made to the reduction inpressure which could be achieved by the addition to the blend of dull coalof high durain content (a particularly effective material), or inert mattersuch as finely crushed coke, provided that these components wereintimately mixed with the charge.There was no report in the 1940 C.O.M.A. Year-Book of the extensivediscussion which followed presentation of Dr. Foxwell’s paper. For thisinformation it is necessary to consult the relevant pages of The Gas World(Coking Section), and I am indebted to Mr. Frank Lodge (a formerHonorary General Secretary of the Association) for kindly loaning me hispersonal copies of that publication, which have been used to prepare asummary of some of the points raised.The Chairman at the meeting (Mr. W.N. Warwick) referred to the greatdifficulties of simulating full-scale coking practice in laboratory studies,and hoped that practical investigations would follow from the workreported. He noted that some coals which were normally considered toshrink could occasionally cause difficulties in pushing, in spite ofsatisfactory wall-to-wall and cross-wall heating, and advocated the use of arecording ammeter to show the power required to initiate coke movementand to indicate the scale of any pushing difficulties. He also mentioned thepossible effect of the chosen oven schedule on any problems arising inpushing. Mr. Warwick considered it was desirable to test the individualseams of coal at the colliery supplying slack for the ovens, to assess anyvariations in swelling properties which might cause problems at the plant.Mr. A.H. Middleton, Mr. E.M. Myers, Dr. F.S. Sinnatt and Dr. J.H.Jones all considered Dr. Foxwell’s inclusion of Durham coals in thedangerous category was incorrect. Durham coals had been used forcokemaking for many years without any damage to the ovens, and largetonnages of this coal had been exported and used for coking abroad withoutany reported problems. Some clients particularly specified coal of thevolatile content stated by Dr. Foxwell to be in the danger zone.Considerable doubt was expressed by Mr. Middleton on the value of theKoppers laboratory-scale equipment in providing useful practicalinformation to guide the coke-maker. In contrast, Mr. B.E. Blackledgereported that in South Wales, at Margam steelworks, the apparatus was in

204. THE COKE OVEN MANAGERS’ YEAR-BOOKroutine use for coal testing, and also provided a useful guide to them indeciding the amount of dangerous coal which could be used in their blend.Particular attention was paid at Margam to the charge bulk density, evenwith a blend regarded as safe to carbonise, and the coal was crushed to giveover 90% through 1 / 8 ” mesh.Mr. H. Dean recalled his experiences with one South Wales coal whichbehaved normally on carbonisation until about the 14th hour after charging.As the plastic layers coalesced, the ascension pipe became partiallyblocked with material which was produced by swelling of coal on top ofthe charge. No damage was done to the brickwork and the charges pushednormally. The cure was found in reducing the height of the charge to allowthe coal to swell into the space above. Mr. Dean also mentioned hisexperiences with the Still internal suction process (see Bulletin, September1992, p.7) and noticed that on piercing the charge prior to insertion of thesuction tubes, a large volume of gas was released from the centre of thecharge. It appeared that plastic layers were formed almost immediatelyafter charging, and consequently a large internal pressure could beexpected at the start of carbonisation, but Dr. Foxwell’s graph had notgiven that indication.Mr. H.M. Spiers commented that some of the laboratory tests classifiedcertain coals as dangerous when they were being successfully carbonised inmodern high temperature ovens, implying that the criteria used forevaluation of the results or the methods adopted for the tests were at fault.Mr. Spiers considered the Nedelmann test was more likely to correlate withpractice, because in that apparatus the coal sample is allowed to develop itsown pressure, whereas in the Koppers laboratory test the coal is underapplied pressure throughout. Mr. Spiers observed that there is no directrelationship between the swelling characteristics of coals and their powersto generate pressure on carbonisation. Highly swelling coals of over 30%volatile matter seldom gave any indication of coking pressure at normalcharge densities, but the coals of 25-30% volatile matter were moredifficult to classify. Mr. Spiers said it was desirable that samples of coalsshould be assembled and tested, covering the range from undoubtedlydangerous through to those which were safe, together with informationrelating to carbonising conditions leading to development of dangerousproperties. He thought it might be possible to correlate the behaviour of thecoals with their ultimate composition.

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 205.Dr. R.A. Mott reported that the work of the Midland Coke ResearchCommittee had shown that some, and possibly all of the tests whichDr. Foxwell had described could not be applied directly to British coals,and that modification of the standard procedures was required. Dr. Mottreiterated the views expressed by Mr. Spiers that the inherent swellingproperties of coals are not the sole factor, and not even the most importantfactor concerning potential damage to oven walls. He considered thedominant factor to be the shrinkage of coked coal. At one end of the cokingscale there were coals of high volatile matter (about 35%) and highswelling power, which could be safely carbonised because the shrinkage ofthe coked mass more than compensated for the thrust on the oven wallscreated by the plastic layers. At the other end of the scale, coals of about17% volatile matter would only be weakly-swelling, but of very limitedshrinkage, and the coking of such coals could be expected to lead todifficulties in discharge. Dr. Molt considered that as shrinkage is theprimary factor, dangerous coals were likely to be found among the coals of17-25% volatile matter. Some of the tests described by Dr. Foxwell drewcomments from Dr. Mott, who listed certain of the modifications which theMidland Coke Research Committee was advocating if the results obtainedwere to be of service to the industry.Dr. J. Taylor considered that it was almost impossible to devise alaboratory apparatus which would duplicate working practice sufficientlywell to distinguish between dangerous and safe coals. He found certainpoints to be almost insuperable, and some of the difficulties involved wereoutlined. The coal at the centre of a freshly-charged oven is some 9 in.from the heating walls, and before the coal in that area begins to swell, thematerial on either side has been converted to semi-coke and secondarycontraction has commenced. Examination of a half-oven width piece ofDurham coke would show that the texture at the inner end was much moreopen that at the cauliflower end, indicating that the coal had room toexpand at the oven centre. As swelling pressure decreases very rapidly ifthe coal is allowed to expand, Dr. Taylor thought it important that any testequipment should measure the swelling pressure under the conditionsexisting in a coke oven. This was extremely difficult to achieve,particularly in view of the effect of secondary contraction on the level ofpressure developed. Dr. Taylor considered that the large secondarycontraction occurring with Durham coals enabled these highly-swellingcharges to be coked safely. In his experience, the plastic layer extendedover a range of 390 0 -530 0 C, but the actual swelling which occurred was

206. THE COKE OVEN MANAGERS’ YEAR-BOOKconfined to the last 20-30 o . Before swelling commences a considerableinitial contraction of the plastic coal occurs, allowing the neighbouringswelling layer to expand, with a corresponding influence on the pressureexerted. It would be difficult in test equipment to reproduce the samerelation between these parts of the plastic layer as exists in a full-scaleoven. The rate of heating is very important in relation to swelling pressure.As the plastic layer moves away from the heating wall a variation in therate of heating occurs, thus causing a change in the swelling pressuredeveloped. Dr. Taylor considered it would be very difficult for this to bereproduced under laboratory conditions. In his view it was desirable tobuild a special test oven of the type devised by Dr. Koppers in order totackle the problem and produce reliable information.Replying to the Chairman, Dr. Foxwell said there was undoubtedlysome small movement of the oven walls backwards and forwards in normaloperation, but he considered the oven pushing schedule chosen would havelittle effect where dangerously swelling coals were concerned. If a coal issuspected of being dangerous, it is necessary to remove the danger by usingmethods outlined in the paper and not rely on setting up equal and oppositepressures on either side of a wall. The charging schedule should be selectedto avoid having an empty oven next to one where the coal is exerting itsmaximum transitory pressure.Dr. Foxwell said there appeared to have been some misapprehension inthe minds of Mr. Middleton and those members who had spoken in defenceof the safety of Durham coals in carbonisation practice. The phrase he hadused was not intended to convey the meaning that most of the coals inKent, Durham, Somerset and South Wales were dangerous, but rather thatif dangerously swelling coals occur, it would be in the areas named that onewould first look for them. Regarding the Koppers laboratory-scaleapparatus, Dr. Foxwell said it was evident that the results obtained by thosewho had criticised the method had not inspired confidence in its reliability.Mr. Blackledge had supported use of the apparatus, but Mr. Middleton andothers had condemned it. His own experience was that the method requiredcare and some experience before it could be applied successfully. Heagreed, however, that there were instances in which guidance given by theKoppers and other small-scale apparatus had not been confirmed inpractice.

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 207.Dr. Foxwell said Mr. Dean’s observation that there is a considerablepressure developed by gas trapped in the interior of the charge had beenborne out by some American workers at the Bureau of Mines. Using twocylindrical steel retorts, one holding 90 lb. of coal and the other 180 lb., gaspressures developed after the plastic layer had moved away from the retortwall, and increased irregularly as carbonisation proceeded. Maximumpressures between 60 and 90 lb./sq.in. developed with low-volatile coals,and neither carbonisation temperature nor retort size modified these valuesappreciably. With medium-volatile coals pressures of from 6 to 54 lb./sq.in.developed. High-volatile coals gave much lower pressures, generally notexceeding 1 lb./sq.in. The pressures quoted indicated that the forcegenerated within the plastic layer could be supplemented to a serious extentby the pressure occurring within the uncarbonised coal. Such centralpressures would usually lead to the production of frothy coke, as thepressure was released by the coal expanding upwards into the free space.Replying to Mr. Spiers, Dr. Foxwell said it was evident that the cokeoven contractors were fully aware of the dangerous properties of certaincoals, and that a number of coke oven plants also realise that some of theircoals could be unsafe if carbonised individually. Dr. Foxwell did not agreethat the Nedelmann test is more likely to be capable of correlation withpractice than the Koppers test, because both are empirical and neither thepressures set up nor the extent of expansion or contraction is likely to bearany relation to the actual values attained in a coke oven.Dr. Foxwell said it had to be recognised that small-scale empirical testscould not be regarded as completely reliable. The efforts of those likeDr. Mott who were endeavouring to improve that reliability were to becommended. Dr. Mott had suggested certain modifications to the Kopperslaboratory-scale apparatus and in the method of conducting the test, and theresults of this work would be awaited with interest. One of Dr. Mott’sproposals concerned the size of coal used, and involved discarding the finersizes before making the test, and with this suggestion Dr. Foxwell said hecould not agree, owing to the possibility of a change occurring in thepetrological composition. Dr. Foxwell said his personal view of the testingof the expansive pressure of coals was that British investigators would bewell advised to make a fresh start. The laboratory-scale apparatus allneglected the fundamental factor of the transient swelling pressure of fullscalepractice, although that pressure may not be invariably set up.

208. THE COKE OVEN MANAGERS’ YEAR-BOOKDr. Foxwell believed that much depended upon the shrinkage of the coal,and if, as Dr. Taylor had pointed out, the coal shrinks sufficiently to permitthe plastic layer to expand, the dangerous transient swelling pressure maybe inappreciable. Under other circumstances the effect could be disastrous.Dr. Foxwell said in his paper he had shown that with a given coal at lowbulk density the transient swelling pressure was scarcely noticeable, andwith that low bulk density there would be greater shrinkage of the coke.For reliable and comprehensive information concerning the study ofcoking pressures, Dr. Foxwell said it was necessary to have equipmentusing two-sided heating, and he recommended use of the movable-wall testoven of Koppers design for that purpose. Given a reliable large-scalemethod, the next step would be to adapt one or more of the empirical testsin such a way that they would show when there was any likelihood ofdanger from a coal. Such coals could then be sent for testing in the largerapparatus.PART IVIn 1930 H. Koppers A.-G., Essen, published details of the laboratoryscaleapparatus which had been devised to distinguish between coalshaving the potential to generate dangerous coking pressures on the walls ofoven chambers, and those which could be carbonised safely (KoppersMitteilung, Volume 12, No.1, 1-23 (1930)). The apparatus was employedextensively in Germany, but Dr. G.E. Foxwell was the first coke oventechnologist in the U.K. to use the equipment for the assessment of Britishcoals. His work on this subject was presented in a paper to a meeting of theAssociation in March 1939 (see C.O.M.A. Bulletin, May 1993) andaroused considerable interest. However, in the discussion of the paper,Dr. Foxwell was criticised for including Durham in his list of areas wherecoals possibly dangerous to coke ovens could occur, because carbonisationof Durham coking coals in modern ovens had been practised for manyyears, and no problems had been experienced from coking pressure. Doubtswere, therefore, expressed concerning the reliability of indications given bythe Koppers small-scale apparatus. Dr. Foxwell had used the Koppersapparatus in accordance with the procedures laid down by the Essenlaboratory, but he agreed that indications provided by small-scale swellingtests had not always been confirmed in practice. In discussion,Dr. R.A. Mott expressed the view that modification of the standard method

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 209.was necessary to enable British coals to be assessed with some measure ofconfidence in respect of their coking pressure characteristics.In May 1940 Dr. R.A. Mott's paper “Coals Dangerous to Coke Ovens”,based on the work of the Midland Coke Research Committee waspresented for discussion before a joint meeting of the Northern Section ofthe Association and the Institute of Fuel at Newcastle-upon-Tyne. Acomprehensive report of the Committee’s researches on this subject, underthe title “The Assessment of Coals Liable to Damage Oven Walls” hadbeen published a few months earlier by Dr. R.A. Mott andMr. C.E. Spooner in Fuel, Volume 18, 329-344 & 371-379, 1939. TheCommittee’s investigations had started at the request of a coke ovenconstruction company, which was experiencing some difficulty inevaluating the results of tests on a Durham coal, using the large-scaleversion of Koppers swelling apparatus. Through the co-operation of theKoppers Coke Oven Company, Sheffield, the Committee was providedwith a laboratory-scale Koppers apparatus and also the loan of a large-scaleswelling test apparatus, located at the Orgreave coking plant, with facilitiesprovided by the United Steel Companies. (It may be of interest to recallthat some fifty years ago, in the days of shoe-string budgets for U.K. coaland coke research, donations “in kind” were appreciated as much asfinancial contributions by local coke-makers to the work of the Committee.See R.A. Mott, 1963 C.O.M.A. Year-Book, p.200.)The difficulties of interpretation of the results obtained with the largescaleswelling test apparatus on the Durham coal were traced to blockageof the gas outlets with plastic coal, which caused abnormal internalpressure and resultant swelling. When the coal charge in the crucible wassurrounded by thick asbestos paper the blockages did not occur, and theresults then obtained showed that although the coal developed excessiveplasticity and swelling on carbonisation, a dangerous pressure was notproduced, and it was unnecessary, therefore, to reduce the bulk density andrate of carbonisation in the full-scale plant. It appeared from theCommittee’s enquiries that several Durham coals were being labelled asunsuitable for carbonisation, and foreign buyers of Durham coking and gascoals regarded some of these as dangerous, based on results obtained insmall-scale swelling tests. Similar indications had been shown when coalsfrom Somersetshire, Kent and Monmouthshire were examined. In view ofthe experiences with the Koppers large-scale swelling test. Dr. Mottconsidered that many of the “dangerous” labels acquired from small-scale

210. THE COKE OVEN MANAGERS’ YEAR-BOOKtest results were probably erroneous, and accordingly a critical examinationof the apparatus and technique was undertaken. For the preliminaryinvestigations into the small-scale Koppers test, the Durham coal whichhad given spurious results in the large-scale Koppers test were used, inconjunction with a South Wales coal which was known to be dangerous.Both coals, when tested in the small-scale Koppers apparatus, using thestandard method, were classified in the “dangerous” category, as theyswelled above the datum line and gave only a small final contraction. Aninvestigation into the heat distribution in the coal charge, when using thestandard method for the small-scale Koppers test, showed that a maximumtemperature of only 600 0 C was reached, and as the temperature at the topof the crucible did not exceed 400 0 C after four hours heating, the coal wasnot passing completely through the plastic range. It was considereddesirable for the charge to reach a temperature of 900 o C in order that thefull effect of the contraction occurring in the later stages of carbonisationcould be revealed. Changes were, therefore, made to the lower portion ofthe furnace by replacing the firebrick construction with high temperatureinsulating brick (Gibbons H.T.I.). The combustion space and number ofwaste gas offtakes were increased, allowing higher temperatures to bereached with the 1 / 2 ” Teclu gas burner. In a further modification, the upperportion of the firebrick furnace surrounding the test crucible was alsoreplaced by insulating brick, enabling a maximum temperature ofapproximately 940 0 C to be reached and the top of the charge to rise to750 0 C. These changes reduced the time taken for a test from four to twohours. The next stage was to devise procedures which would result in theDurham coal producing a “safe” curve, but permit the South Wales coal toretain its “dangerous” label. This objective was achieved by the simpleprocedure of removing the dust less than 20 mesh I.M.M. (1/40”) from thesample prepared for testing, thus ensuring that abnormal pressures were notgenerated owing to fine coal blocking the interstices and restricting gasescape.Enquiries made by the Committee to coke oven construction firmsresulted in the supply of further U.K. coals which were being classed asdangerous, together with American and German coals, some of which wereknown to have damaged oven walls. Forty coals were assembled, and inaddition to the usual proximate analysis and calorific value were alsoexamined by the Sheffield laboratory coking test (slow rate of heating) andthe B.S. crucible swelling test (fast rate of heating). The majority of thesamples were subjected to examination by the small-scale Koppers test in

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 211.its original form, and compared with the results obtained by using twomodified versions of the apparatus. The conditions of tests were as follows:Old StandardTest 1(firebrick)ModifiedTest 2(firebrick)ModificationTest 3(insulatingfirebrick)Number of waste gas offtakes 4 8 8Size of coal (I.M.M. mesh) -10 10 – 20 10 – 20Pressure (kg/sq/cm.) 1.16 1.16 0.5Time of test (hours) 4 2 2Bulk density, dry basis,(kg.cu.m.) 750 750 750(lb.cu.ft.) 46.8 46.8 46.8Examination of the results obtained on the forty coals by the threemethods of testing, carried out in conjunction with the known industrialbehaviour of the coals, confirmed the reliability of indications provided bythe modified test 3 procedures, and enabled a dangerous coal to becategorised as one which swelled above the zero line, and gave a finalcontraction of less than 8 mm. (equivalent to 6%) under the conditionsspecified.The importance of bulk density of the charge in relation to thedevelopment of coking pressure on carbonisation has long been recognised,and Dr. Mott briefly reviewed the factors involved. The bulk density of amass of coal varies according to its moisture content and size grading, withdry coal showing the greatest weight per cubic foot. Bulk density increaseswith coarseness of crushing. When water is added to dry coal, the bulkdensity decreases to a minimum value at approximately 5 - 7% moisture(depending upon coal rank), and then rises again as the moisture content isincreased, but does not attain the value obtained for dry coal. The bulkdensity of the charge in the oven is also influenced by the height throughwhich it is dropped, and the consolidation which occurs due to the staticpressure exerted by a superimposed column of coal. Other factors, such asrigidity of the leveller beam, can affect the bulk density. Dr. Mottconsidered that dry slack would give a bulk density in the oven of

212. THE COKE OVEN MANAGERS’ YEAR-BOOKapproximately 50 lb./cu.ft., and a coarsely-crushed wet slack a value ofabout 46 lb./cu.ft., with lower values if finely-crushed. In practice, a bulkdensity of between 42 and 50 lb./cu.ft. would be expected.As the tests on the forty coal samples had been carried out at a standardbulk density of 46.8 lb./cu.ft. (750 kg./cu.m.), it was considered desirableto evaluate the effect of change of bulk density on the swelling propertiesof coal in the small-scale Koppers apparatus. Tests were, therefore, carriedout using some of the coals previously examined, at bulk densities rangingfrom 40.6 to 56.2 lb./cu.ft. (650 to 900 kg./cu.m.). The percentagecontraction after two hours was plotted graphically against the bulkdensity, when it was found that for most of the coals a series of parallelstraight lines was obtained, but for very strongly-swelling coals the slope ofthe line was abnormal. As a coal giving a final contraction of less than8 mm. (6%) was classed as dangerous when tested under modified test 3conditions, a “danger line” was drawn on the graph at that level, andenabled the maximum safe bulk density to be interpolated. The graphshowed that the coals known to have damaged oven walls would bedangerous even if coked at very low bulk densities - e.g. 39 lb./cu.ft. for theAmerican coal, 40.5 lb./cu.ft. for the German coal and 44 lb./cu.ft. for theSouth Wales coal, with volatile matter (Parr basis) of 17.2%, 20.0% and25.5% respectively. Blending would, therefore, be essential if these coalswere intended for coke-making. One of the Durham coals tested (26.8%volatile matter, Parr basis) was classified as potentially dangerous at53 lb./cu.ft., but for the other Durham coals examined (of volatile matter26.0%, 26.5% and 29.2%, Parr basis) a bulk density of 55 lb./cu.ft. wasconsidered permissible, which would allow dry slack to be carbonisedsafely. A Kent coal of 25.5% volatile matter (Parr basis) was shown to bedangerous if coked in a dry state, but could be carbonised alone safely as afinely-crushed wet slack.Dr. Mott’s paper included a graph on which the percentage contractionafter two hours in the Koppers small-scale apparatus was plotted againstvolatile matter (Parr basis) for coals selected from the range of fortypreviously examined. A constant bulk density of 46.8 lb./cu.ft.(750 kg./cu.m.) was used, and it was found that the points fell within aband, the lower limit of which crossed the danger line at 25.5% volatilematter (Parr basis), i.e., coals having over 25.5% volatile matter (Parrbasis) would be safe to carbonise at the bulk density stated. Based on the

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 213.values for contraction of coals which had been tested at various bulkdensities, the lower limit of volatile matter (Parr basis) for safety could beset at 27.0% and 28.5% for bulk densities of 49.9 lb./cu.ft. (800 kg./cu.m.)and 53.1 lb./cu.ft. (850 kg./cu.m.) respectively. The graph indicated that thecontraction occurring in an oven (as with the modified Koppers test) isroughly proportional to the volatile matter of the coal. The danger to whichoven walls may be subjected can be regarded as a balance between thecontraction taking place in the coke and the swelling power of the coal asmeasured at a slow rate of heating, such as the Sheffield laboratory cokingcoal (l 0 C/min.), which is approximately equivalent to full-scale practice.Dr. Mott drew attention to the difficulty of evaluating coals whichexhibit very strong swelling properties (.e.g. one of the Durham coals, anda powerfully-swelling coal from Somerset), particularly as the rate ofheating for the Koppers small-scale test is much faster than commercialpractice. The results obtained with the modified test 3 procedure on thesehighly-swelling coals were compared with the indications provided by thelarge-scale Koppers apparatus, in which the swelling properties areassessed using a bed of coal 8 inches thick and a rate of heating comparablewith full-scale plant (see Bulletin, May 1993, p.22). Examination of thedata obtained by the two methods showed that the conclusions drawn fromthe small-scale test results were correct, and that coals of over 25.5%volatile matter (Parr basis) may be considered safe at a bulk density of46.8 lb./cu.ft. The Somerset coal, however, was the only one for which thesmall-scale test proved unsatisfactory, and this was due to the extremelyhigh swelling power (228% in the Sheffield laboratory coking test) whichthis remarkable coal possessed. When a charge of this coal, crushed to100% through l / 2 inch was carbonised in a 5 cwt. experimental coke oven ata bulk density of 49.9 lb./cu.ft. (800 kg./cu.m.) it swelled into the ovenarch, but produced a strong coke which was easily discharged.Dr. Mott referred to the limitations of the tests which had been devisedto assess the liability of coals to damage oven walls. No universallysatisfactorytest could be designed without an understanding of theunderlying cause of oven damage. All the data which had been obtainedshowed that swelling is not the principal indication of a dangerous coal, butthat the contraction occurring in the Koppers test, both small and largescale,is a satisfactory guide. The shrinkage of coke in an oven is roughlyproportional to the volatile matter and shrinkage cracks begin to appear as

214. THE COKE OVEN MANAGERS’ YEAR-BOOKsoon as the coal has passed through the plastic range and been converted tosemi-coke. The swelling pressure generated in the plastic layer is opposedby the shrinkage in the semi-coke, and a dangerous pressure on the ovenwall is only set up if the shrinkage does not balance the swelling. The causeof damage is not high swelling power alone, but high swelling poweraccompanied by low shrinkage. Low shrinkage associated with low volatilematter is a more dominant factor than high swelling power, which may befound in high-volatile, but safe, coals.The author summarised the characteristics of the most dangerous of thecoals which had been tested in the investigation, noting that all had acalorific value exceeding 15,750 B.Th.U./lb. (Parr basis) and usually over15,850 B.Th.U./lb. (Parr basis), which is an exceptionally high figure, andprovides an additional characteristic for their recognition. The coals had avolatile matter in the range from 17% to 25.5% (Parr basis), with high B.S.swelling numbers. However, two of the most dangerous coals were nonswellingat the flow rate of heating used in the Sheffield laboratory cokingtest, and a further three were only moderately swelling, indicating thatswelling power, per se is not the cause of the danger associated with thesecoals. One result of the Committee’s work had been to emphasise the fairlynarrow range within which dangerous coals lie, and also that undueimportance had previously been attached to the swelling behaviour of acoal and insufficient to the contraction in a pressure swelling test. Since thecontraction under constant load can only be measured satisfactorily in theKoppers swelling test, this apparatus was recommended for adoption inpreference to other equipment (such as the Nedelmann or Baum-Heuserapparatus) which had some disadvantages. As the dangerous properties of acoal are related to bulk density, Dr. Mott suggested that tests should bemade at bulk densities of 43.7, 46.8 and 49.9 lb./cu.ft., when the percentagecontraction in the three tests should lie on a straight line. The bulk densityat which the coal would be dangerous could then be deduced, using thevalue at 46.8 lb./cu.ft. where wet slack was carbonised and 49.9 lb./cu.ft.for a dry charge.PART VThe paper “Coals Dangerous to Coke Ovens” by Dr. R.A. Mott (seeC.O.M.A. Bulletin, September 1993, p.4) which had been presented before

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 215.the Northern Section of the Association and the Institute of Fuel inNewcastle in May 1940, was followed by an interesting discussion. Someof the points raised are summarised in the paragraphs which follow.Mr. S. Tweedy observed that Durham coals had been carbonised by theConsett Iron Company for many years in both old and new ovens (wideand narrow chambers) under a variety of conditions, but no problems dueto swelling pressure had ever been experienced, although Dr. Mott hadpointed out that some Durham coals had been classed as “dangerous” or“likely to be dangerous” by the standard Koppers swelling test. Althoughpractical carbonising experience indicated that Durham coals are harmless,the results of the small-scale Koppers test had occasionally had an adverseeffect upon the sale of these coals, because some purchasers relied on theindications given by the Koppers apparatus. Dr. Mott had now providedevidence from his researches that Durham coals are not dangerous whencarbonised. Mr. Tweedy asked Dr. Mott if he had any informationconcerning certain coals used in Germany which exhibited a harmfultemporary expansion. Reports indicated that carbonisation of these coalshad resulted in damage to the oven walls or buckstays, but as the coke hadcontracted sufficiently, no difficulty had been experienced in pushing theovens. Mr. Tweedy said he considered that the maximum pressuredeveloped under constant volume might possibly give an indication of suchcoals, and asked Dr. Mott to comment on this suggestion. Mr. Tweedyasked if the samples of coal used in the investigations had been oven-driedprior to making the tests, and if so, for what length of time, as it was wellknownthat this process can give rise to oxidation, which would invalidatethe results.Mr. P.C. Pope said the conclusion he drew from Dr. Mott’s paper wasthat whilst the work was of great service, it was more useful in a negativerather than a positive direction. The various swelling tests of Koppers,Nedelmann and Baum-Heuser gave certain indications, but it wasquestionable whether they gave sufficiently reliable information uponwhich a Company could base a contract for the building of coke ovens. Thetests did not appear to be sufficiently accurate for that purpose, and withborderline coals the position was particularly difficult. There wasinsufficient indication concerning the safety of using various coals in ovensof different widths, rates of heating and carbonisation temperatures. It was,therefore, hoped that Dr. Mott would continue his work and devise a

216. THE COKE OVEN MANAGERS’ YEAR-BOOKmethod which would provide more practical and reliable results undervarious operating conditions. Mr. Pope asked Dr. Mott if the coals receivedfor testing from Germany and the U.S.A. had been received in sealed tinsin an inert atmosphere to avoid oxidation which might otherwise occur.Dr. F. Booley asked Dr. Mott how the bulk density of the coal at thesole level of an oven had been determined.Mr. G.H. Harrison enquired if the Koppers apparatus had been designedwith any relation to oven practice, particularly in respect of the freesurface, which amounts to 4 sq.ft. per ton of coal in a modem oven, but foran old-style oven would be 6 sq.ft. per ton. He considered that unless theapparatus was related to free surface, then useful results could not beexpected. Mr. Harrison also asked Dr. Mott if he had any experience withthe Koppers apparatus using sponge-producing coals, as he (Mr. Harrison)considered that these would probably give rise to a high expansion result inthat equipment.Dr. G.E. Foxwell submitted a lengthy written contribution to the paper.He noted that some coals had, in the past, been classified as dangerousalthough they had not actually damaged coke oven walls, and he stressedthe need for a reliable method for assessment of the conditions under whichany given coal would become dangerous. Dr. Foxwell reported that whenhe used the standard Koppers apparatus on a regular basis he was usuallyable to decide with some certainty whether a coal was, or was not, likely tobe dangerous, but there were many borderline coals for which the test didnot give a clear indication of their “possibly dangerous” behaviour oncarbonisation. From the viewpoint of the contractor building coke ovens,such coals would have to be regarded as dangerous. Dr. Foxwell wrote thatthe impression arising from reading the paper was that in order to avoid theerroneous labelling of certain coals, Dr. Mott had set out to whitewash asmany as possible of coals classified by the Koppers test as “possiblydangerous”. He had succeeded by modifying the test, but the questionswhich arose were whether the modifications which had been introducednow gave an accurate assessment of the coals, or whether the original testwas more accurate, or whether either test could be accepted. Commentingon the modifications which Dr. Mott had made to the standard procedurefor conducting a Koppers swelling test, Dr. Foxwell noted that the materialbelow 20-mesh I.M.M. sieve was being removed from the crushed sample

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 Coke, 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

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 219.plant. It was an achievement to have so modified the apparatus andtechnique that the coals concerned did not then behave anomalously.Similarly, it was useful to have dealt satisfactorily with some dangerouscoals which were labelled as safe by the original procedure. Themodifications introduced by Dr. Mott had enabled the test to discriminatemore precisely between safe and dangerous coals, but certain limitationsremained, particularly with highly-swelling coals. Mr. Spiers consideredthat coals having a volatile matter of less than 25% would always beviewed with suspicion until reliable evidence of their safety incarbonisation practice could be provided. Concerning coals in the range of25 to 30% volatile matter, Mr. Spiers concluded that complete reliability inthe indications by the modified apparatus when testing these coals had notbeen achieved, as among such coals it was suspected that there existed anumber having dangerous properties.Mr. Spiers observed that the indications concerning maximum safe bulkdensity found by Dr. Mott could not be accepted, owing to the large densitydifferences which exist between the coal in the upper and lower portions ofthe charge. If a coal could be carbonised with safety at a bulk density notexceeding 47.5 lb./cu.ft., and the average bulk density was represented bythat value, the coal at the oven sole would probably have a density of49.5 lb./cu.ft., based on Dr. Mott’s data. Accordingly, the safety limit forthe whole charge would have to be set at 45 lb./cu.ft. or even less, and afurther margin of safety would be required to accommodate any accidentalincreases in bulk density due to coarser crushing arising from hammerwear, or other causes. Mr. Spiers considered that evidence of maximumsafe bulk density which was derived from tests on a graded laboratorysample of coal, as specified for the modified Koppers test, might not bevalid in full-scale practice. Accordingly, coke oven contractors werejustified in adopting a cautious approach when coals of an unusual naturewere involved, and this attitude applied particularly to the Kent andSomerset coals examined by Dr. Mott, in spite of his views that such coalswould only be regarded as dangerous at a bulk density in excess of50 lb./cu.ft.Mr. G.J. Greenfield said it appeared from Dr. Mott’s paper that none ofthe tests for the assessment of the dangerous characteristics of coals couldbe regarded as universally applicable. The modified form of Koppersapparatus which Dr. Mott had devised gave satisfactory results with certain

220. THE COKE OVEN MANAGERS’ YEAR-BOOKcoals, but he asked if complete reliance could now be placed on this test. InMr. Greenfield’s view, Dr. Koppers had caused considerable unnecessaryconcern to a number of coke oven managers by maintaining that his testcould distinguish between safe and dangerous coals, when in factDr. Mott’s investigations had disclosed a number of inherent defects in thestandard apparatus and procedure. Mr. Greenfield said another ofDr. Koppers’ fallacies concerned his view that because some coals werefound to show a minimum bulk density at 5 to 7% moisture, therefore allcoals follow that pattern. Dr. Mott had also accepted that view, overlookingthe findings of Greenfield and Dummett (C.O.M.A. Year-Book, 1935,p.129) that certain coals showed a minimum bulk density which occurred atvery different moisture levels, depending upon whether the coal wasuncompressed, was compressed by dropping a standard distance, or hadrecently been so dropped and then shaken out again. Their investigations atThorncliffe Coal Distillation, Limited, using a South Yorkshire cokingslack, showed a minimum bulk density (lbs. of wet coal per cu.ft.) at 10%moisture if the coal was uncompressed, but at 4% moisture when dropped adistance of 11 feet, as into an oven. However, as conditions varied fromone works to another, Mr. Greenfield observed that one could notgeneralise on the matter, and each plant would be advised to conduct itsown tests on the coals used.Replying to the points made in the discussion, Dr. Mott agreed withMr. Tweedy that cokes produced from the carbonisation of dangerous coalsare not necessarily difficult to push out of the oven. Any damage to thewalls occurs in the early stages of carbonisation, but particularly at the timewhen the two plastic layers meet. The marked increase in pressuredeveloped at that stage may be termed temporary expansion, forsubsequently the coke may shrink from the walls. It appeared that the mostdangerous coals behaved in that way in the oven, but Dr. Mott said that asthey had been unable to trace any firm relationship between maximumswelling pressure at constant volume and the degree of dangerousness of aseries of coals, the value for maximum swelling pressure should be usedwith caution, although it is often significant. The coals used had been airdriedprior to testing, but as they were of high rank, oxidation was unlikelyto have affected the results.In reply to Mr. Pope, Dr. Mott said the fact remained that the Koppers,Nedelmann and Baum-Heuser tests had been used by coke oven contractors

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 221.for the basing of guarantees. A modern fast-coking oven is less substantialin construction than the old-style smaller and slower-coking type, and theproblem of coking pressure had only been experienced with ovens ofmodern design. Dr. Mott considered it was asking too much to infer fromsmall-scale tests at what rate of heating in a full-scale oven a coal would bedangerous. It was important to recognise which coals were dangerous atspecific bulk densities, and then to ensure that they were not coked aloneabove a critical bulk density. Normally the precaution would be taken toblend a dangerous coal with other coals to ensure safety, and fine crushingwould be used to reduce the bulk density. The foreign coal samples werewell-sealed in cans and drums, completely filled. In one case oxidation wassuspected, as the coal had been in storage for some time before despatch,but a second sample of freshly-mined coal gave substantially the sameresults. Dr. Mott thought it unlikely that oxidation would interfere with thetests on these high-rank coals.In reply to Dr. Booley, Dr. Mott said the value for bulk density of thecoal at the sole of an oven was based on a series of small-scale tests inwhich coal was dropped through various heights, and its bulk densitydetermined. In other tests, static loads were applied to the coal after it hadbeen dropped, a load of 50 lbs./sq.ft. representing 1 ft. column of coal. Thevalues given in the paper were calculated from a series of curvesrepresenting bulk densities after dropping from different heights and undervarious static loads.Referring to the query from Mr. Harrison, no comparison could bemade between the conditions of carbonisation in an oven and the smallscaleKoppers tests, in which undirectional heating is employed. The test isempirical, and the results are only significant if both apparatus andprocedure are carefully standardised. He did not think the test would behelpful in distinguishing sponge-forming coals, for which the SheffieldLaboratory Coking Test would be a more useful guide.Concerning Dr. Foxwell’s observation that they had set out towhitewash certain coals, classified by the standard Koppers apparatus as“possibly dangerous”, Dr. Mott said that had not been their aim, but mainlyto standardise a test of importance in the coking industry. Dr. Foxwell hadargued that screening-out the fine material from the crushed sample wouldchange the petrographic nature of the coal, and Dr. Mott agreed that this

222. THE COKE OVEN MANAGERS’ YEAR-BOOKprocedure would certainly remove all the fusain, but this would give asmall factor of safety in the results. To test the assertion that the amount ofdurain would be altered by using a sized fraction, mixtures containing 10,20 and 30% of durain of 1 / 2 ” to 1 / 4 ” size were prepared, crushed to pass 10-mesh I.M.M. sieve, and the durain determined in the through-10 mesh,10 to 20 mesh, and through-20 mesh sizes. The results, which are givenbelow, indicate that there are negligible changes in the durain content byselecting coal of 10 to 20 mesh for the test.Original duraincontentDurain in crushed coal of sizemesh (I.M.M.)-10 10-20 -20% % % %10 12 10 1220 23 23 2330 28 34 24The rate of heating in the test is at least ten times as fast as in thenormal coking practice, and the effect on the dust is consequently muchgreater than that occurring at industrial rates of carbonisation. Since theremoval of dust from the crushed samples of Durham coals made thembehave in the Koppers test in a manner consistent with the carbonisation of“safe” coals in practice, Dr. Mott considered that the procedure wasjustified, and he submitted that those concerned would welcome thesafeguard given by making frequent use of the sample and rapid test whichhad been described.Dr. Mott reiterated his view that the contraction occurring in theKoppers test provided the best indication of a dangerous coal. He saidDr. Foxwell considered that the illustrations in the paper by Davies andWheeler showed that when the temperature in the oven centre reached500 0 C. and the plastic layer had disappeared, there was only slightevidence of cracks. This interpretation was incorrect, as the minor crackscould be seen clearly, and merely opened out as the temperature increased.The major cracks had been completely formed. Dr. Mott agreed that thecoke comes away from the walls only towards the end of the coking period,but this was no evidence that no contraction has occurred until the plasticlayer has disappeared. The U.S. Bureau of Mines (Report of Investigations3451, by Auvil, Davis and McCartney, (1939)) had measured the

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 223.contraction of a charge from the sides, and also the cracks in coke atdifferent positions, using their sole-heated oven for assessing swellingcoals, and found that the shrinkage shown by cracks in the coke is alwaysgreater than the shrinkage from the walls. Dr. Mott disagreed withDr. Foxwell’s statement that dangerous pressure on the walls isconcentrated in the period between the meeting of the plastic layers andtheir disappearance. The pressure is at its maximum at that time, but it mayhave occurred over a long period, and the transient higher pressure whenthe layers coalesce, though important, is not the only factor involved.Dr. Mott said he sympathised with the desire to devise a test in whichheating of a coal charge from both sides could be achieved. However, hesuggested that the simple test which had been put forward and standardisedfor dangerous coals from Germany, America and South Wales, and appliedto a greater variety of coals than had been previously used, would satisfymost of those concerned with this matter. Dr. Mott said they did not claimabsolute perfection for the swelling test in its modified form, butconsidered that Dr. Foxwell’s criticisms were ill-founded.Replying to Mr. Spiers, Dr. Mott observed that, for obvious reasons, theviews expressed in the paper concerning the assessment of dangerous coalscould not be confirmed by full-scale carbonisation. It had been possible todraw some logical conclusions from their experiments on various scales(short of full-scale) and it was hoped that a logical approach to the problemof coking possibly dangerous coals would be adopted by plant operators sothat, with the accumulation of information and evidence, final practicalproof of the value of the work could be provided. Where a works was facedwith the situation of carbonising a “possibly dangerous” coal alone, or notundertaking coke-making, the decision to proceed would depend largely onmaintaining the maximum bulk density of the charge below that indicatedby the modified test as dangerous for the coal. It should not be difficult toarrange for the moisture content and the degree of crushing to be controlledto give a reasonable factor of safety. Alternatively, blending would benecessary. Even if coals of 25% to 30% volatile matter are not dangerous,the Kent and Somerset coals have such strongly-swelling characteristicsthat trouble with sponge formation could occur on carbonisation, leading tovariation in both free space above the charge and degree of “cracking” ofthe gas. In these cases, it would be useful to blend with poorly-coking ornon-coking coals, or with coke dust.

224. THE COKE OVEN MANAGERS’ YEAR-BOOKIn reply to Mr. Greenfield, Dr. Mott said they had examined coals fromall areas of Great Britain where problems arising from swelling pressurecould possibly be anticipated, and it was reasonable to conclude that themodified test would give a reliable guide to the possible danger involvedwhen carbonising such coals. Since dangerous coals from areas other thanGreat Britain had also been included in the recommendations made, theconclusions were expected to apply to all types of normal coking coals.However, as freak coals such as those in Somerset are found in differentparts of the world, they need special consideration. In such cases, theprinciples outlined for the cause of danger and the sources of error intesting dangerous coals should be helpful.Dr. Mott said they had not overlooked the work of Greenfield andDummett on bulk density, but the acceptance of the minimum bulk densityfor coal of 7% moisture was based on work of the Midland Coke ResearchCommittee which, for the conditions adopted, agreed with the value givenby. Dr. Koppers. Dr. Mott supported Mr. Greenfield’s view that testsshould be carried out at each plant to confirm the figure.* * * * *The dispute between Dr. Mott and Dr. Foxwell concerning the relativeimportance of the factors of swelling and shrinkage of the charge inrelation to oven damage, which had been raised at the 1940 meeting, led tofurther work by the Midland Coke Research Committee. In May 1941,Dr. Molt and Mr. Spooner led a discussion on the shrinking power of cokeat a meeting of the Midland Section of the Association. The Americanworkers Davis, Auvil and McCartney of the Bureau of Mines had beeninterested for some time in studies of the shrinkage of coke, and theirpublished work was also considered at the meeting. The 1942 C.O.M.A.Year-Book does not include a report of the discussions which took place,but the information can be found in the pages of The Gas World CokingSection for 1941 (kindly made available by Mr. F.E. Lodge) which showthat the two Doctors continued to disagree, and there was also somedifficulty in reconciling the results and conclusions drawn by the Americanworkers with the studies made by Dr. Mott. It is hoped to summarise thiswork in due course.

A REVIEW OF C.O.M.A. ACTIVITIES, 1939-1945 - CLARKE 225.In his various contributions to the discussions, Dr. Foxwell vigorouslypursued the principle that for complete reliability of prediction of thepossibility of development of dangerous swelling properties of a coal oncarbonisation, it is necessary to simulate full-scale conditions. Some yearswere to pass before it became possible to embark on a comprehensiveprogramme concerning the coking pressure characteristics of British coals,using an oven based on the Russell design, to which Dr. Foxwell hadreferred earlier. The research on this important subject was one of the firstmajor investigations to be undertaken by the British Coke ResearchAssociation, which came into existence in 1944. A Committee under theChairmanship of Dr. W. Idris Jones was formed to deal with this work, anda movable-wall oven was erected at the Maritime Coke Works atPontypridd in South Wales, where a wide range of coals were tested. TheCommittee issued its first report in 1948, and at the third conference of theBritish Coke Research Association in London on 15 December 1948, thework was presented to the coking industry for consideration anddiscussion. Some changes were subsequently made to the oven flue designto enable more even heating to be attained, and the oven width wasincreased from 12” to 16 3 / 4 ”. A further report on the work was issued in1952. With the establishment of the new Coke Research Centre atWingerworth, near Chesterfield in 1958, the test oven found a new home,and continued to provide the industry with useful practical information.In the last fifty years a considerable volume of literature on the subjectof coking pressure has appeared, and its importance in carbonisationpractice continues to be recognised. As recently as 1992, on the occasion ofthe Second International Cokemaking Congress in London (Chaired by ourImmediate Past-President, Mr. D.C. Leonard), several important papersdirectly concerned with this subject were presented.The present series of articles in the Bulletin have been making anostalgic survey of C.O.M.A. papers being discussed by the Sections somefifty years ago. However, a change in the pattern is proposed for the nextissue of these notes to permit consideration of the progress in knowledgeconcerning coking pressure which has been achieved in the period sinceDr. Foxwell and Dr. Mott presented their C.O.M.A. papers for discussion.