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COKE OVEN DESIGN – THE PAST, THE PRESENT AND THE FUTURE By L A Watson Read before the Northern Section on 15 February 2000 Introduction The by-product coke oven was born in the nineteenth century, developed progressively through the twentieth century and prepares to enter the twenty-first century as a mature technology. Except in the fields of battery control and automation, where designers and operators alike must continue to advance towards full automation of all functions, there is limited scope remaining for further technical advances in the design of the battery itself. The new batteries that will enter service within the next few years will, in all probability, represent the final generation of by-product coke ovens. As the cokemaking technology has matured in recent years there have been profound changes in the coke oven design and construction industry. History has been reversed with the merger of all of the famous old coke oven builders into a single entity. In this paper the development of the by-product coke oven is traced over the years and the product of that development is considered, namely a new facility planned for construction in Germany, the very first twenty-first century coke plant. The Birth of the By-Product Coke Oven The coking or carbonisation of coal has been practised for centuries and for the past 250 years coke ovens in one form or another have been used for the purpose. What we have come to regard as the traditional vertical slot-type oven chamber evolved from the beehive oven with the construction of the Smet ovens in Belgium in the 1850s. By 1861 the Coppée slot-type oven with vertical heating flues was established more or less as the industrial standard of its time. This classical slot-type nonrecovery oven was constructed by several companies, including Dr C Otto Mr Watson is Managing Director of OSC Process Engineering Limited.
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of Germany. Bus-flues were introduced to collect the raw gas from the oven chambers and to distribute it equally to the heating flues. Boilers were installed to absorb the heat from the battery flue gases. Thus the waste heat battery emerged, and examples of waste heat batteries continued in operation until quite recently. Around 1861 Knab took out patents for an oven which was to be heated externally only, with the gas produced being cleaned of tar and ammonia before being returned to the battery for combustion in the heating flues – the conceptual by-product coke oven. This design was adopted by a Monsieur Carvès who constructed a total of 119 ovens at Bessèges in France between 1867 and 1879. This was truly the first by-product coke oven facility. Monsieur Carvès, of course, entered into a partnership with Mr Simon of Manchester and Simon-Carvès was formed with the construction of the Crook battery in 1881. In Germany, Dr C Otto adapted their Otto-Coppée design to a byproduct coke oven and installed their first battery also in 1881. By 1883 both Simon-Carvès and Dr C Otto were using external regenerators to preheat the combustion air; however, there was still one more major development left to emerge. For many years Heinrich Koppers had been a chief engineer within Dr C Otto and the development of the recuperative preheating of combustion air, underjet gas distribution and many other important elements of the emerging technology were attributable to him, as well as other famous names such as Hoffmann, Hilgenstock and Still. Heinrich Koppers left Dr C Otto in 1899 to form his own company and introduced the individual regenerator oven in 1904. This remarkable advance in the technology was adopted universally and provided the blueprint for the regenerative slot-type by-product recovery oven that has dominated the cokemaking scene to this day. In order to understand current day developments, it is important to note that another great pioneer of the coke industry left Dr C Otto to form his own coke plant construction company in 1898. His name of course was Carl Still. Carl Still’s greatest contribution was to the by-product plant technology, and Firma Carl Still was the acknowledged world leader in this field for almost a century. Nevertheless, his contribution to the development of the by-product coke oven should not be overlooked. He constructed the first 6m oven in 1926, an amazing accomplishment when one realises that the 6m high capacity oven did not emerge as an industry standard until the 1960s. Carl Still also developed the multistage heating
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flue (sometimes known as the Christmas tree) which for environmental reaons finds favour in the year 2000. The twin-flue heating system became the hallmark of Dr C Otto and Simon-Carves. The patent for the twin-flue was acquired by Dr C Otto in 1903; however, the technology was not applied until 1912, when it was used in a battery near Saltburn of the Skinningrove Iron Company Limited. Dr C Otto, Firma Carl Still and Heinrich Koppers dominated the coke oven construction industry around the world through most of the twentieth century. There were of course many other important players including Simon-Carves, Semet Solvay, Wilputte, Didier, Becker, and in the latter half of the century Gyprokoks within the old Eastern bloc, Nippon Steel in Japan and the Anshan Institute in China.
Carl Still
02.08.1868 – 02.08.1951 Gründer der Firma Carl Still, Recklinghausen
Dr. Carlos Otto 1838 - 1897
Heinrich Koppers geb.: 23, Nov. 1872 gest.: 5, Sept. 1941
In 1974, Heinrich Koppers was acquired by Krupp and this company Krupp Koppers subsequently acquired Wilputte in the USA. In the same year the coke oven division of Simon-Carves was acquired by Dr C Otto. In 1979, Dr C Otto was acquired by the German steel company Saltzgitter, who in 1985 sold it on to its main rival Firma Carl Still to form a new company Still Otto. The following year, in 1986, Still Otto was acquired by Thyssen, who subsequently acquired the coke oven interests of Didier Engineering, resulting in the formation of Thyssen Still Otto Anlagentechnik in 1993.
Fried, Krupp Krupp Fried, 1811 1811
KruppUhde Uhde Krupp
merger
Krupp Krupp Chemieanlagenbau Chemieanlagenbau 1961 1961
1996
merger
Krupp Uhde coke oven division
merger
KruppKoppers Koppers Krupp 1974 1974
HeinrichKoppers Koppers Heinrich 1901 1901
by Still
take over
spin-off
Thyssen Krupp Krupp AG AG Thyssen 1999 1999
ThyssenKrupp Krupp Thyssen EnCoke EnCoke 1999 1999
spin-off
DrC COtto Otto& &Comp. Comp. Dr 1872 1872
Still Otto Otto Still 1985 1985
FirmaCarl CarlStill Still Firma 1898 1898
ThyssenStill Still Otto Otto Thyssen 1987 1987
merger
merger
Family Tree
by Thyssen
take over
Didier Didier
Aug, Thyssen Thyssen Aug, 1891 1891
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The final chapter of this rationalisation process opened during 1999 with the merger of the Thyssen and Krupp Groups. As part of this merger, the Krupp Koppers Group and the Thyssen Still Otto Group combined to form a new company, ThyssenKupp EnCoke. The company has its headquarters in Bochum, in the offices of the former Dr C Otto & Comp GmbH. This new Group envelops the technologies and designs of Otto, Still, Koppers, Didier, Simon-Carves, Wilputte, and others that have been lost in the mists of time. One way or another the new Group represents the majority of the ovens that are operating around the world. Remarkably there have been no fundamental developments in the design of the by-product coke oven since the great pioneers founded the technology all of those years ago. Vertical slot-type oven chambers with vertical heating flues connected to individual regenerator chambers has continued to provide the basis for the various by-product coke oven designs that have been promoted over the years. The individual design and construction companies each had their own particular distinguishing features. Otto was noted for its twin-flue heating wall and its underjet rich gas heating system. The company favoured the use of fireclay in the regenerator walls and oven roof, with a sliding joint at the interface between the silica and the fireclay. Over the past twenty-five years they have employed an air-stage heating system, wherein gas (lean or rich) together with a portion of the combustion air, is introduced at the base of the flue and the remaining portion of the combustion air is introduced at a higher level within the flue. Koppers traditionally favoured an all-silica oven. They, like Dr C Otto, employed the twin-flue design but with a different regenerator arrangement, connected to the heating walls through the so-called “scissor flues”. The most notable and enduring feature of the Koppers design was of course the recirculation port located at the base of each twin flue. As the gas and combustion air entered the base of the heating flue, it entrained a part of the waste gas flow from the downstream leg of the twin flue. This dilution of the combustion process reduced the intensity of the flame, thereby enhancing the uniformity of vertical heating and coincidentally reducing the NOx content of the flue gases. This latter benefit, of course, was not a consideration in 1927 when the feature was initially introduced, but emerged as an advantage when environmental issues came to the fore. The Koppers twin-flue incorporated a rather novel hairpin arrangement
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which enabled the effective height of the flue to be adjusted, within limits. In recent years, with the introduction of the Combiflame heating system, Koppers moved progressively towards the Otto design, with fireclay regenerator walls, through regenerators (no scissor flue) and air-stage heating. Unlike Otto, who were dedicated to the underjet system, Koppers employed both gas-gun and underjet methods of rich gas heating, depending on the client’s preference. Carl Still remained consistent in their design approach throughout the century. The multistage “Christmas tree” heating system was the hallmark of the design, and the combustion air was delivered to the various stages up the height of the heating flue through hollow binders or headers. A horizontal bus flue connected the vertical flues to provide a half divided heating arrangement. Correspondingly, the regenerators were half divided. Unlike Otto and Koppers, Carl Still only employed the gas-gun method of rich gas heating. The design remained popular until the early 1990s when the last of the Carl Still ovens were constructed. Didier Engineering was not a major force within the coke oven design and construction industry. However, during the 1970s and 1980s, they were to disturb the equilibrium of the major three with the introduction of their so-called “Grouped Flue” design. Within this design, the heating wall was divided into groups of four flues (or two flues in the case of the ends). Each group was equipped with its own bus flue and thus each heating unit could be adjusted externally without recourse to internal orifice plates or sliding bricks. This was and remains an elegant concept and one which found great favour with plant operators. They built batteries for Dofasco in Canada, Republic Steel in USA, AIS (BHP) Port Kembla in Australia and Dunkirk (SOLLAC) in France. In reality these batteries were quite difficult to build and maintain and the complexity of the refractory structure was such that cross leakage could be a problem, particularly in lean gas heating. Nevertheless, they were at least prepared to pursue a significant design development, whilst the major three continued over the years to allow their long established designs to evolve. Coke oven designers are market driven and they have over the years responded to the demands of their customers, or more realistically to the pressures on their customers. There have been four factors that have influenced the modern design of the by-product coke oven; energy efficiency, coke quality, operating costs and environmental control. Of these four factors, environmental control
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has been and continues to be the driving force behind design development, and many of the improvements relating to energy efficiency, coke quality and operating costs have emanated from environmental control issues. ThyssenKrupp EnCoke has been awarded the contract to design a new coke plant facility for its sister organisation ThyssenKrupp in Germany. The plant will produce 2.5 million tonnes per year of blast furnace coke and is intended to represent the very latest state of the art in coke oven design. It will be the first twenty-first century coke plant and will set the industrial standard for the foreseeable future. It will be constructed within the industrial heartlands of Germany and must therefore comply with some of the most demanding and exacting environmental control standards in the world. Moreover, it will be the first design to emerge from the new design organisation and should provide a distillation of the best ideas and technology available from within the old organisations. The designers have responded to the pressures from the customer to reduce the number of ovens to a minimum by increasing the length and height of each chamber to the practical limit. The maximum length of the chamber is determined by the ability of the pusher machine to level and push the charge. The height is limited by wall stability and the depth of roof required to achieve an acceptable wall strength. At the new facility the roof will be more than 1.7m deep. The greater the capacity of the oven chamber, the fewer the number of charges and pushes that are required to achieve the throughput. Moreover, there will be fewer openings (doors, chargehole lids, ascension pipe caps and spigot joints) through which emissions may escape. The oven size is therefore driven to a large extent by environmental considerations; however, fewer ovens require fewer operators and thus operating costs are affected. As part of the integrated steelworks the batteries will be fired exclusively on blast furnace gas, reverting to rich gas firing only in time of emergency. For this reason the ovens will be of the gas-gun design. Had it been ThyssenKrupp Steel’s intention to operate the batteries on rich gas on a regular basis then an underjet design would have been employed in order to achieve the distributive control necessary in a modern heating system.
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The consensus amongst the designers in Bochum has emerged in favour of the conventional twin-flue heating system, with through regenerators. Stability of the refractory structure is an important consideration in this respect, as is the twin-flue heating unit concept for heating control purposes. The regenerator walls will be constructed using fireclay shapes, over two-thirds of the height, above which silica will be used. A sliding joint will be constructed at the interface between the two materials. This is generally in accordance with both the Otto and Koppers standard of recent years and it enables the correct material to be employed; silica in the hot zone where it is strong and stable, and fireclay in the lower zone where the temperature is lower and fluctuates with each reversal. It is imperative in the compound oven that the regenerator walls should remain tight throughout the life of the battery, and in this regard the fireclay/silica construction is an essential feature of the design. Within the carbonisation process the objective is to achieve uniform carbonisation of a homogeneous charge in order to achieve consistent coke quality, optimum energy efficiency and low pushing emissions. Ideally, at the end of the carbonisation cycle, just prior to the push, the temperature of the coke throughout the centre of the charge (equi-distant from the heating walls) should be the same, around 1050oC. In order to accomplish this it is essential to provide a homogeneous charge of consistent bulk density. This depends primarily on coal preparation and charging techniques, and whilst this falls outside the scope of this paper, it is worthwhile noting that major advances have been made in this area during recent years. In order to achieve uniform temperature conditions within the batch it is necessary to adjust and control the horizontal and vertical temperature profiles of each heating wall, along the length of the battery. The designers have accomplished this in recent years through the promotion of the “heating� unit concept in which each individual hairpin flue complete with its respective upstream and downstream regenerator segment can be controlled separately. Thus for each heating wall the number of heating units corresponds to the number of hairpin flues. For the new facility the designers have extended the heating unit concept by introducing a division wall through each individual regenerator. Not only does this provide greater control of the air distribution in the vertical direction, it also more importantly facilitates adjustments to compensate for the long and short ducts between the regenerators and the flues, thereby enabling the excess air to be reduced, with further benefit to energy efficiency and NOx levels.
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B E HEIZUNG S SC HE M A T K S / S C HWAC HG AS I S o h le /
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S ta ffe l
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S ta ffe l
S o h le
S ta ffe l
S o h le
S ta ffe l
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Abga s
Abga s
(A b g a s )
(Lu ft)
(G a s )
Abga s (G a s )
Lu ft (A b g a s )
Lu ft (A b g a s )
Gas (A b g a s )
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S ta ffe l
S o h le /
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AB GAS LU F T S CHW ACHGAS
(A b g a s )
Abga s (Lu ft)
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Gas
Abga s
(A b g a s )
(Lu ft)
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The new facility will operate exclusively on lean gas (enriched blast furnace gas). Rich gas firing will be accommodated as required by the side entry gas-gun facility – essentially an adaptation of the Carl Still design. From a height in excess of 10m, it will be difficult to change siliminite gas nozzles; however, techniques have been developed to enable this to be accomplished. It is interesting to note that the coke side and pusher side end flues will be equipped with their own gas channels, so that the quantity of gas to these flues can be adjusted independently. As a standard for the future, however, the designers will usually adopt the underjet system where batteries are to be fired exclusively on rich gas. During the latter part of the twentieth century there have been major advances in the science and application of the battery bracing system. The system performs two distinct functions; expansion control during the heating-up period, and refractory restraint during the lifetime of the battery thereafter. The forces employed through the bracing system during heat-up are substantially greater than those required during normal operations. Moreover, the longitudinal bracing has an important function during heatup, but is more or less passive during normal operations. Essentially, the buckstays themselves are employed as spring elements, reacting against the top and bottom tie rods. The individual elements of the bracing system act against the buckstays to restrain; the regenerator fireclay, the regenerator and below sole silica (including the gas channels), the heating walls and the oven roof. Each element is controlled through coil springs and adjusted to prescribed loads, to assure the optimum restraining force at all points within the refractory structure. The obsession with novel door designs which gripped the industry in the 1980s and early 1990s has diminished and it is now generally recognised that a well designed heavy duty haematite or cast steel door is best able to provide the long-term consistent performance required by a coke plant. Fully adjustable spring-loaded sealing strip systems are now almost universally standard; however, there remains a choice between the ‘Z’-type sealing strip (FLEXZED) and the membrane-type sealing arrangement (FLEXIT). Both doors have advantages and disadvantages. The new facility in Germany will probably be equipped with the FLEXIT door through customer preference; however, the popularity of the FLEXZED door is unlikely to diminish in the coming years.
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FLEXIT® Coke Oven Doors
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For decades it has been the ambition of designers to provide individual oven chamber pressure control. This has profound implications for the integrity of the battery and for the control of emissions. A novel yet simple system has been developed by DMT in Germany and trialled on old 4m ovens in Duisburg. This PROven system will be employed on the new facility in Germany, and assuming it is successful, will be adopted on new batteries and as a retrofit around the world. In essence the traditional pullman valve is replaced by a so-called FixCup in which the liquor level can be varied automatically to throw a constant back pressure on the oven chamber. Conventionally the gas collecting main pressure is maintained at sufficient pressure to ensure that suction conditions do not develop at the base of the oven chamber at the end of the carbonisation cycle. In reality this means that the majority of the ovens within the battery, particularly those that have been freshly charged, are held at unnecessarily high pressure, thereby placing extreme demands on the emission control abilities of oven doors, charge hole lids, spigot joints and ascension pipe caps. Moreover, the pressure must also be contained by the refractory brickwork. Cross wall leakage and more significantly oven roof leakage can occur as the battery ages or in the case of serious disruption of operating schedules. The PROven system enables pressures to be substantially reduced, thereby facilitating to a marked degree the emission control tasks. With the PROven system the gas collecting main will normally operate under a slight suction, which of course facilitates the charging process. The new battery will incorporate a pusher side offtake and the secondary offtake during charging will be provided through the now wellestablished ‘U’ tube connecting adjacent ovens by the fifth hole. Charging aspiration will be provided by means of high pressure liquor, and whilst the design currently incorporates water-sealed ascension pipe caps, as confidence in the PROven system builds these may be eliminated in favour of mechanically sealed lids. There is increasing evidence that drain blockages and overspill from water-sealed caps is leading to refractory damage within batteries and thus the designers will try to avoid this feature in the future.
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Start of coking
End of coking
Off the main
FixCup
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With the advances in computer techniques and software applications there have been meaningful advances in battery management and control systems in recent years. Within the coke plant, field equipment rather than the processing or software capabilities has been the limiting factor, and as this becomes more robust and reliable we can expect to see major advances within the early years of the new century. Total battery control with “no-man” operation, including battery machines, is the designer’s ambition. Several plants in the Far East are already progressing steadily towards this goal and it is intended that the new facility in Germany should be provided with manual, semi-automatic and fully automatic control modes. There are many imperatives driving the industry towards total automation; health and safety, environmental, economics, quality. From the coke oven designer’s standpoint, however, the greatest incentive is operational consistency, constant carbonising schedules, consistent heating control, regular cleaning and scheduled preventative maintenance. Unlike the battery itself, the control and management systems are not the exclusive domain of the coke oven design and construction companies. With modern day software applications and the ready availability of proprietary field equipment, many coke plants are developing their own systems. The ThyssenKrupp EnCoke “Cokemaster” combines and integrates many of the well-proven modules that have been developed over recent years, including battery heat input control, chamber wall temperature measurement, pushing schedule management, coal management, coke management, on-line gas treatment control system and management reporting systems. To this integrated control system will be added the additional modules, including the oven machine control modules. Conclusion Except for the small quantity of coke that is consumed by the foundry industry and the non-ferrous industry as a reductant carbon, production of coke is geared almost exclusively to steelmaking and in particular to servicing the blast furnace. Thus, whilst the by-product oven may not see the end of the twenty-first century, it is likely to be around for another decade or two. The age profile of the existing coke ovens around the world
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is such that there must be at least one more major resurgence in the coke plant construction industry. ThyssenKrupp Steel’s new facility in Germany will represent in all respects the very latest “state of the art” in coke oven design. Compared to Crook in 1881, the fundamentals of the basic design are more or less the same and it is reasonable to observe that there has been no outstanding advance in the technology since the days of the great pioneers. The major advance has, of course, been in the capacity of individual chambers and latterly in the field of control and automation. Even Heinrich Koppers may have found it hard to imagine a battery of 90 m3 chambers, operated by centrally controlled robot machines. Assuming that by-product ovens continue to be built for another twenty years then further major advances in design may be unlikely. At 20m between door plugs, there is little scope for a further increase in the length of the chamber. At a chamber height of 8.34m the limit has probably been reached, beyond which the structural design of the battery would have to be re-engineered – and this is unlikely to be economically viable. The heating system combined with the most modern battery control systems will ensure that carbonisation conditions are near perfect, with no wasted energy. Waste gas temperatures cannot be reduced further; radiation losses are not a major factor in the energy balance and are being reduced through the use of modern insulation materials. Accordingly there is little scope for improvements in energy consumption. Control of emissions from the new facility will be best world practice, and whilst further design improvements will be pursued in the future, these must by definition be at the margin. There will, however, be further advances in the control and automatic operation of the ovens, although this is unlikely to affect in any significant way the design of the ovens themselves. The non-recovery oven will gain some favour in the years to come. However, the economic case for this alternative technology is not compelling in all cases, and whilst disposal routes for tar and light oils continue to exists, the by-product oven is likely to prevail as the dominant coking technology – particularly within integrated steelwork. If necessary, the by-product plant will be adapted to convert the by-products to energy that is usable within the steelworks, either through thermal or catalytic techniques or through the generation of electrical power.
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Continuous coking processes emerge from time to time and a viable technology may well be found. Given the timeframe normally required for the commercialisation of new technologies, it is highly improbable that any new process, however attractive, will have any significant impact on the dominance of the by-product oven over the next 20 years. Thus as we proceed into the new century, almost 120 years after the construction of the by-product coke oven battery at Crook, we can conclude that the latest “state of the art” as represented by the new ThyssenKrupp Steel batteries in Germany will take the by-product oven close to the practical limits of refinement, capacity and performance. There will always be room for improvement; however, these will in future be at the margin and will be affected as much through the support equipment, control systems, machines and gas treatment plant, as by the design of the ovens themselves. In essence the new design can be expected to represent the industrial standard for the next decades, beyond which a fundamental shift in steelmaking technology may be expected.