Source of information: ISIJ International, Vol. 31 (1991), No. 5, pp. 449-457
Recently, reserves of good coking coals have become less available and comparatively more expensive. Resources are being extended by the use of coal blends with different coking properties and/or selective additivies. Coal preheating technology emerged a technique to overcome some of these problems.
It
has several advantages including: increases in coke oven productivity, improvements in quality of metallurgical coke, greater uniformity of charge, less air pollution by using a closed charging system, levelling of the charge, a saving in energy because dry coal is
more efficient in the preheater than in
a coke oven, and the possibility of using poorer and cheaper coking coals. The disadvantages of
this technique are the handling of fine and hot coal, the carry-over and the preheater fines.
Currently, the preheating process is being re-considered in combination with dry-cooling of coke in
a European
Research Project called "Jumbo Coking Reactor", which is based on past and current experience of development of
modern coke making technology.
This study reviews preheating technology as a means to widen the range of coking coals including not only the high-
volatile coals which are more abundant, but also semi-anthracite and petroleum coke.
Given all the above benefits, it may well be asked why, after at least two decades of development, preheating is not used throughout the world more widely than at present (the appearance of publications on the subject has declined recently). Despite much apparent enthusiasm by research workers, contractors and at least some commercial users of pre-heating, the technique is by no means generally applied at coking plants. This is due mainly to economic factors such as the decline in coke demand. But, there are also reasons of a more technical kind. Hesitation to introduce a new technique that certainly adds to the complexity of coke oven operation must play a large part in preventing a more widespread adoption of preheating. In a survey of BCRA research on preheating) 26) the main disadvantages are stated to be the problem of handling hot coal (the pollution and oxidation risks), carry-over, and the fines from the preheating and handling plant. The use of a charging car carries risks of pollution during the filling of the hoppers and of erratic coal flow; in pipeline charging with steam (nitrogen is Now preferred, however), the plugging of nozzles and condensation present problems. Difficulties in the use of chain conveyors for transporting the hot coal arise from increased resistance to motion falls of coal at steep angles, rapid wear, inadequate sealing, and troublesome maintenance. Further problems include: The high rate of gas evolution during charging; the danger of over-filling the oven; the possibility of generating an explosive mixture in the charging car hoppers; the pressure rise in the oven and the greater difficulty in controlling the level of the charge in pipeline charging; the smaller shrinkage of the charge(necessitating an adjustment to the vertical heat distribution in the flues) ; and the solids carry-over, which is adversely affected by air ingress during gravity charging and depends on operating conditions and pipe entry configuration in pipeline charging. German workers consider the drawbacks as being the need for additional equipment (with its cost and the greater complexity of operation) and the carry-over problem (leading to a fall in the tar quality and the danger of blockage of gas mains). The latter can be minimized By adding a binder to the coal or dealt with by the use of a separate " charging main "; but these mean respectively a smaller increase in the bulk density of the charge and an increase in cost. According to these workers, carry-over is better avoided by combining preheating with stamp-charging. Of these drawbacks, the carry-over is perhaps the most important. Several studies have been devoted to the control of carry-over and techniques t reduce it. A few coking plants have abandoned preheating after initial enthusiasm and in 2 instances reasons have been published. At the Redcar works of the British Steel Corporation, where a whole series of disasters due to poor engineering led to irretrievable damage to newly-built coke ovens using the Coaltek preheating and pipeline charging system, the reasons given for returning to wet charges for rebuilt plant were: (a) There was a need to guarantee the performance and lifetime of the ovens. (b) The economic advantage in using local high-volatile coals (which anyway had deteriorated in quality) had disappeared because of the availability of cheap imported coals. (c) The type of coal blend being used to produce high-quality, un-reactive coke for the giant Redcar blast furnace was possibly not ideally suited to preheating. (d) Preheating meant relatively high manning levels and maintenance costs (the effect of which was to increase the cost of the coke if high output was not consistently maintained). At Iscor's Pretoria works in South Africa, an early version of the Otto-Simon Carves Thermo charge system was installed, but pre-heating has since been abandoned as uneconomic (running costs were high, the coke reactivity tended to increase and tar quality and pollution posed fairly severe problems). Partial briquetting of the charge has been preferred as a means of improving the coke quality. It should also be noted in this context that Nippon Steel has selected Precarbon at its Muroran plant. Elsewhere, at Oita, Kawasaki plant at Chiba and NKK plant at Fukuyama partial drying to a controlled moisture content of the charge (4 to 5 wt % moisture) is used with heat from CDQ. This is a simpler technology with benefits qualitatively similar to several of those of preheating.