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Perspectives on the Future of Cokemaking Stuart Nicol and Bob Durie
A few years ago there was talk of the demise of the coke ovens, at least in the developed countries, in favour of new steel-making technologies. This now seems less likely and this article provides an analysis of the future of cokemaking and blast furnace steelmaking.
Background With the advent of a more enviromentally aware society, traditional cokemaking has come under close scrutiny from regulatory authorities with regard to the production of fugitive pollutants and their influence both on plant operators and the community at large. This rightful attention has unfortunately had the effect whereby legislation, or threatened legislation, has impeded the introduction of new facilities. This concern has developed to an extent where the future of ironmaking by the integrated blast furnace route is seriously threatened. The consequences of this have grave implications for not only the coal industry but also for the overall Australian economy. Revenue to Australia from export coking coal amounts to some $3.5 billion in contrast to the lower figure of $2.3 billion for coals consigned to the energy sector (Fig. 1). For this reason, it is important to understand the current status of the coking industry and the reasons leading to its recent problems.
The Production of Coke and its Use Metallurgical coke is traditionally produced by heating carefully formulated blends of coals by indirect heat transfer in a sealed oven. The process is essentially a batch process in which the coked product is 'pushed' from the oven by means of hydraulic rams after a prescribed residence time. This period is
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coal blend and oven type dependent and may lie in the range 12-24 hours. The ovens in which the coal-to-coke transformation takes place consist of a series of refractory lined slots which may be up to 0.45 m in width, 6 m high and 16 m long (Fig. 2). Because of the number of battery openings (at the top for charging and at both ends for pushing), the traditional process is prone to leakage of environmentally unacceptable materials to the atmosphere. These materials may be in the form of dust, condensable tars and liquids and gas, particularly in the case of ageing ovens where door seal closures are a particular problem. The coke so produced is subsequently quenched-screened and fed to the ironmaking blast furnace. Within the blast furnace (Fig. 3), the coke not only acts as a reductant and fuel but also, by virtue of its placement in layers (slits), it provides a means of ensuring adequate permeability. This permeability is necessary to enable reducing gases to have access to the ferrous burden. For this reason coke must have specific physical characteristics that ensure that strength is maintained as the coke reacts and descends in the blast furnace. This is necessar y to enable it to support the imposed loads at high temperatures and physicochemical properties which permit the satisfactory progression of the complex chemical reactions within the furnace. Furthermore, the trend toward large blast furnaces and associated coke replacement by pulverised coal injec-
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tion imposes even more stringent quality requirements on coke than in the case of smaller blast furnaces due, in part, to the increased hypsometric loads and chemical reactions leading to physical degradation (solution loss reactions). Coke quality and coke availability is therefore fundamental to the operation of the existing blast furnace route to steel making.
The Current Situation The age of coke ovens Worldwide, it has been generally recognised that the life of coke ovens is around 20-30 years. On this basis, by the year 2001 the majority of the existing coke oven capacity will be obsolete. Unless this capacity is replaced, it has been estimated that within the next decade there will be a shortfall in world coke supply of between 15-20 mt/a. The range arises because of various estimates of the likelihood of success at increasing coke oven life and the outcome of restructuring measures within the steel industry.
Pulverised Coal Injection The growing popularity of direct coal injection into the blast furnace as a means of reducing the demand for coke has undoubtedly served as an opiate to alter the focus of thinking away from the central problem of long-term coke supplies. PCI, while old technology, has been embraced by the modern steel industry to the extent that significant
coke replacement has been possible. Fig. 4 illustrates this trend. [Specific coke rate = tonnes of coke per tonne of hot metal] However, there is a lower limit for achievable coke replacement by PCI because of the requirement for blast furnace permeability. At the present time, just what this limit is remains a matter of conjecture. There is also some evidence to suggest that this 'limiting coke' will need to have superior physico-chemical characteristics but, as yet, this is based on relatively scant data. The interested reader is referred to the recent article by Thomson et al published in the preceding issue of this journal, for further information on this subject.
Coking coal availability Generally speaking, the low availability of traditional coking coals (Fig. 5) and hence the premium these demand has led the market to a certain degree of apprehension over the potential for price rises as well as exposure to vulnerability of supply. These factors have contributed to the present-day enthusiasm for alternative 'cokeless' steelmaking technologies.
New steel making technologies Nashan points out that in the 1960s it was considered that direct reduction processes would eliminate coke based metallurgy. This task proved more difficult, technically and economically, than expected to the extent that direct
reduction currently represents only about 8% of world steel production. Direct reduction processes generally require ver y high grade iron ore (≼ 67% Fe). Hence the development of alternative steel making processes is likely to be determined in part by the availability of suitable iron ore supplies. The availability of such ores is extremely limited without the incorporation of an expensive beneficiation step which, until recently, has not been adopted in Australia. Furthermore, from an Australian iron ore export point of view, there are only limited iron ore resources that are amenable to this type of beneficiation. It is evident therefore that not only is the Australian coal industry economically sensitive to the fate of cokemaking because of its inexorable relationship with the blast furnace, but that similar implications exist for the iron ore industry.
The Need for Cokemaking R&D Against this salutor y or cautionar y background and the forgiving flexibility and reliability of the blast furnace (as well as the large capital investments involved), there are signs of a growing worldwide awareness of the need to invest in alternative approaches to cokemaking with the aim of reducing the environmental impact of coke production and hopefully the capital costs, as well as extending the range of coals used in coke making. The objective is to safeguard the existing low risk tech-
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nological and financial investment in blast furnace technology as part of a broad macroeconomic portfolio. Indicators of such activity are: ■ the German development [Bertling et al] of coke ovens employing superlarge coking chambers with rigid side wallsfor use as single production units (Jumbo reactors), ■ renewed interest in non by-products recovery ovens [Clarimboli et al], ■ a new European Research Centre for Cairbonisation R&D [Nashan], ■ developing climate for the acceptance of and investment in new technology. Other factors that are stimulating interest in the development of new or improved 'clean' coke making processes include: ■ the anticipated growth in steel demand and a coke shortfall of ~20 mt/a by the year 2001 [Nashan], ■ recognition of potential business opportunities for merchant coke ovens, ■ recognition of a need to support the future of the Australian raw materials (iron ore and metallurgical coal) export industry, and ■ as indicated above, the need to service and protect the existing capital outlay in blast furnace technology. In addition, there is also the need to utilise optimally the coking coal fines
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produced as a result of the coal mining/preparation process. These fines not only give rise to problems in the coal cleaning process but also lead to consignment moisture problems (when wet) and dusting problems (when dry). The possibility of treating this material by pyrolytic processing to produce a handleable added-value product needs also to be considered. In Australia in recent years, coke R&D effort has been reduced because of the fact that the domestic steel industry is largely tied to specific captive coal supplies. Therefore, process development has been neglected owing to the attention afforded to optimisation of the coking behaviour of those particular coal sources in site-specific circumstances.
Alternative Cokemaking Technologies Formed Coke Processes Coke production, traditionally, is a batch process. Over the years, there have been numerous attempts at the development of continuous cokemaking processes. Such developments were widely reported in the 1960s, driven by concerns over dwindling traditional coking coal reser ves and by the large price differentials between
coking and steam coal and less by the environmental drivers of the current era. Similarly, attempts at low temperature carbonisation processes were curtailed by World War 2, after which much of the concern about liquid fuels supplies was diminished. Many, but not all, of these processes were technically viable but were casualties of the politicoeconomic circumstance of the day. One notable group of processes that fell into this category were the numerous socalled formed coke processes. In essence, these processes consisted of two stages. The first was usually a low temperature carbonisation step performed at around 500ºC, followed by a high temperature carbonisation stage at temperatures in excess of 1000ºC. A briquetting stage was often employed prior to high temperature carbonisation. Numerous alternative formed cokemaking processes have been developed (but only a few put to the test at full scale) and details of many of these are provided in a Battelle report published in 1979. Such processes include: ■ Ancit Process (Netherlands) ■ DKS Process (Japan) ■ Auscoke Process (Australia) ■ Consol-BNR Process (US) ■ BFL Process (Germany)
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Clean Coke Process (US) INIEX Process (Belgium) ■ ‘Thin bed' Process (Russia) ■ CTC Process (US) ■ Cauldepon Process (US) There are many additions that could be made to the above list. Of these it is worthwhile to note that Japanese steel mills are currently funding a large R&D effort aimed at the development of super coke ovens for productivity and environmental enhancement (Scope 21 Project). The objectives of the project are energy savings of 20% and a productivity increase of 300%. It is appropriate to ref lect, at this point, on some of the reasons for the former demise of interest in formed coke type processes but it must also be recognised that it is rare to find such information in a well documented form. However, the following general statements can be made: ■ ’irresolvable technical problems' and ’economic downturns' are examples of typically used phraseology in reviews of past endeavours, ■ the economics of the process developments were based on the 1970s price differentials between coking and steam coals, ■ the need to build expensive pilot plants operating at least 10 t/h to provide material of sufficient quantity ■ ■
for blast furnace trials, the unwillingness of blast furnace operators to risk trials which might beperceived to interfere with routine production. On the positive side of the picture, formed coke processes are continuous and enclosed – and are therefore environmentally more acceptable. This is evidenced by the growing number of processes of this type receiving approval from the United States Environmental Protection Agency (EPA). Critics have argued that formed coke processes merely transfer part of the coking operation from the coke oven to the blast furnace. The evidence for this is tenuous but, in either case, it is a better proposition from the point of view of environmental emissions to concentrate efforts on reducing emissions to a single source rather than to tackle the multiplicity of problems associated with coke oven leakage. The product quality, from a carbonisation point of view, has been described as very acceptable. However, concerns have been expressed by blast furnace operators about the effects of the closely sized briquetted product on blast furnace gas distribution. Tateoka of Nippon Steel Corporation, in a summary of events, concluded '...formed coke manufacturing processes might ■
become the only process that allows the blast furnace to continue existence even if it were to cause a decline in productivity and a rise in fuel rate'. The evidence for the qualifying statements, however, was not made totally clear. It is interesting to note that the newly developed CTC process [Wolfe] has received environmental approval in the US by the EPA and that plans are well advanced toward the full scale commercialisation of this technology. The CTC process involves the use of heated screws reactors for the low temperature carbonisation step, followed by the production of large briquettes which can be subsequently crushed to produce coke of an appropriate size distribution for traditional blast furnace use. Interestingly, the project involves Australian interests and a European engineering firm with steelmaking connections (Koninklijke Hoogovens). Capital expenditure in excess of $200 million is involved.
Non-recovery coke ovens There has been a recent resurgence of interest in the production of coke in improved (to minimise environmental problems) non-recovery coke ovens. The possibility of manufacturing coke without the economic offset of by-product processing and use has been revis-
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ited. The motivation for this interest includes: â– a need for new coke making facilities to meet demand (in China and India) and the cost of collecting and processing the volatiles, â– use of low shaft (small) blast furnaces with less demand on coke quality (strength). Non-recover y cokemaking, which must not be confused with the old beehive type ovens, is a process in which the oven chambers are in a horizontal configuration. Volatile matter is recycled and combusted within sole flues located beneath the oven floor, thereby providing additional heating. Because the ovens operate on negative pressure, door leaks are non-existent. New generation non-recovery ovens are claimed to be environmentally friendly and incorporate incineration systems which are claimed to destroy 99.99% of toxic hydrocarbon emissions. The technology is particularly suited to the production of foundr y coke from single coals. However, it remains to be determined if blends of coal, as used in slot ovens, make coke of equal quality in nonrecovery ovens. The principal concerns with non-recovery type ovens rest with the high residence times required (~24 h) and whether the use of a freely expanding bed is compatible with the production of strong dense coke. Nonetheless, in India, it has been estimated that approximately 1.5 mt of coke were produced by beehive type ovens in 1990.
Conclusion In summary, it is reassuring to know that some forward thinking is evident in this area of such importance to the Australian economy and that, hopefully, the predictions made by Kahn in 1990 and illustrated in Fig. 6 are seen to eventuate. In spite of current interest in 'cokeless' ironmaking technologies, it would seem probable that a reliance on blast furnace technology will continue well into the next century. PCI injection is likely to alleviate, but not eliminate, the need for additional supplies of coke. The ageing of existing coke ovens, with the attendant environmental issues, implies that a technological imperative exists to develop alternative environmentally acceptable cokemaking methods. Because of Australia's dominant position
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as a supplier of coking coal to the international marketplace, it would seem appropriate that it be both mindful of and supportive of such endeavours.
References Battelle Report, 1979. Technology of the formed coke processes. Pt VI, p.1-57. Bertling, H, Rohde, W, and Louis, G, 1994. Jumbo coking reactor - first operational experience with a new cokemaking technology. Proc . Ironmaking Conference, vol. 53, p. 165. Ciarimboli, J P, and Sturgulewski, R,1992. Non-recovery coke ovens. Ironmkg. Proc. AIME, 51, pp.601-6. Kahn, P P, 1990. The future of cokemaking. Proc. International Symposium on the Future of Ironmaking Processes. Hamilton, Ontario, pp.8-15.
Nashan. C. 1992. European Development Centre for Cokemaking Technology - tasks and targets. Proccedings Ist International Cokemaking Congress. p.19. Tateoka, M. 1992. Future technical devclopments in cokemaking: a Japanese view. Proc. 2nd International Cokemaking Congress. Essen, p.9 1. Thomson, A, Zulli, P, McCarthy, M, Horrocks, K, 1996. Pulverised coal injection in iron making blast furnaces. The Australian Coal Review. no. 1. pp.42 - 8. Wolfe, R, 1996. CTC Continuous Cokemaking Process. Proc 3rd International Cokemaking Congress, Ghent, Belgium.
Stuart Nicol is the Principal of Novatech Consulting Pty Ltd. Bob Durie is an Honorary Research Fellow with the CSIRO Divsion of Coal and Energy Technology.