Technical Market Support » Metallurgical Coal
This commissioned review examines the current status and future directions for pulverised coal injection (PCI) into blast furnaces used in the production of pig iron. It has been undertaken to assist the Australian coal industry to identify possible future industry funded research. This research should aid the coal industry in determining the advantages and disadvantages of various types of coal for used as PCI coal.
Prior to starting the review of the literature a wide spectrum of Australian coal producers were asked for their views on research needs based on their efforts to market coal in this developing market. All those actively selling coal in this market expressed concerns on the level of understanding of the value of their coal in the market and the impact of coal quality on coal PCI performance. The information obtained was used to identify the areas to be covered in this review.
An extensive review of the literature has been conducted to determine the current level of understanding of the impact of pulverised coal injection or granulated coal injection (GCI) on the blast furnace process.
The subsequent impact on the whole steelworks operation is examined, but not in detail as the overall impact is in the area of reduced operating costs. These operating costs are very dependent on a particular steelworks operation and this information is not freely available in the literature.
Blast furnace technology is of critical importance to the crude steel industry and is continually undergoing refinements to improve productivity and reduce operating costs. The past improvements in productivity, coke consumption and fuel use within the steelworks have been hastened due to the competition in the world steel markets.
Pulverised coal injection, while not new technology, is one such process refinement that is being implemented in steelworks around the world. Increased injection of coal is also being driven by the need to extend the life of ageing coke ovens and therefore reduce the need for newer higher cost coke ovens. The injection of coal into the blast furnace has been shown to:
- increase the productivity of the blast furnace, ie: the amount of hot metal produced per day by the blast furnace;
- reduce the consumption of the more expensive coking coals by replacement with cheaper soft coking or thermal coals;
- assist in maintaining furnace stability; and
- improve the consistency of the quality of hot metal and reduce the silicon content of the pig iron.
The current rates of coal injection vary with European steelworks achieving 180 to 215kg of coal per tonne of hot metal (kg/tHM), while Japan's five major steel producers are averaging around 83.5kg/tHM in 1993, down from 84.7kg/tHM in 1992, with the maximum of 183kg/tHM achieved by Kobe Steel at the Kobe works.
The decline in PCI use in Japan was due to reduced production demand leading to operators lowering the productivity of blast furnaces. Currently, 25 out of 31 blast furnaces operating in Japan are equipped for PCI.
By the year 2000 all Japanese blast furnaces will be equipped for PCI, at which time the coal requirement for PCI in Japan will approach 16 million tonne per year compared to current requirements of 6 million tonne per year.
Pulverised coal injection, while not new technology, is a process improvement that is being implemented in steelworks around the world to improve the productivity of blast furnaces and reduce operating costs. It has been demonstrated both theoretically and in operating blast furnaces that injection rates of over 200kg/tHM can be achieved.
At injection rates greater than about 160kg/tHM special attention to burden charging is required to ensure furnace stability and there is a requirement for greater oxygen enrichment of the hot blast to maintain raceway temperatures and reduce gas flow within the furnace.
The two main areas of concern to the Australian coal industry relate toe the handleability of the coal, mainly pulverised coal, and the combustibility of the coal.
Reports of handling problems, mostly blockages in transfer lines, in dense phase transport systems are common. Explanation of the mechanisms of dense phase transport blockages are difficult as it is not always clear whether they occur spontaneously or gradually.
Work done by ACIRL and Wollongong University did indicate softer coals, due to the greater level fines in the pulverised coal when milled under the same conditions of the transfer system. Attention to the mill settings to match the characteristics of the coal will greatly reduce these problems.
Other mechanisms, such as those due to the plastic nature of some coals which may lead to blockages, have not been examined in the literature.
The perceived need for rapid combustion of the injected coal has prompted some blast furnace operators in the past to pulverise the coal very fine and to choose highly reactive coals. It is now recognised that complete combustion of the coal is not possible at high injection rates and in some cases low burnout can be beneficial.
An example of this is lower blast momentum due to a lower amount of combustion in the tuyere, and this lower momentum reduces the degradation of coke in the deadman region.
The entry of unburnt coal char into the burden is inevitable at high injection rates. It is desirable that this clear participate in the ore reduction reactions in preference to the coke, in order that bed permeability be maintained.
Limited research indicated that this may be the case. A better understanding into the interactions of char with the gases, liquid metal and fine coke around deadman region will be obtained by examining the relative reactivates of coal char and coke fines.
If coke reactivity is more than an order of magnitude lower than char reacitvity then the differences in coal char reactivity due to coal types may not be significant to the phenomena occurring in this region.
The chemical and physical phenomena that take place within the blast furnace are complex. To understand the impact of increasing PCI rates and coal quality on these phenomena requires large scale tuyere combustion rigs, investigations on operating blast furnaces and the use of powerful mathematical modelling techniques.
These investigations are being conducted by most steelworks around the world including Australia where BHP, at their Newcastle laboratories, are in the early stages of their research program.
The needs of the Australian coal industry to identify the relative merits of their coals in this developing market and the potential injection rates at various steelworks can be met by:
- Investigating other mechanisms that may lead to blockages of transfer lines, such as those due to the plastic nature of coal. This could be examined by impacting a stream of pulverised coal onto a plate place at different angles to the stream. Several coals with different plastic properties should be examined.
- Conducting research into the relative reactivates of char from various coals and fine cokes. At high injection rates the relative reactivates of the coal char and fine coke will be important in determining the likely impact of char and/or fine coke on burden permeability. If the fine coke reactivity is found to be an order of magnitude lower than that of coal char then changes in char reacitvity due to coal type may not be significant.
- Developing a generic steelworks mass and energy model to allow producers to better understand the future requirements of their customers for coking and PCI coals.
- Developing a 'simplified' model of the blast furnace process that evaluates the impact of coal quality on the mass and energy balances within the furnace without the complexities of the phenomena relating to gas flows.
Adding to the fundamental understanding of char formation and reactivity will not only aid Australian producers it will also be of benefit to those undertaking the more complex modelling of the impact of PCI on blast furnace operation.
Ideally, the generic model of steelworks operations could incorporate the 'simplified' blast furnace model by the use of a process simulation package, such as Aspen Plus. Since these packages are not easy to use this would increase the modelling effort and skills required for successful completion.
The end product of such a project would be a package that could be used in a similar way to the Coal Quality Impact Model used for the evaluation of coals for use in power stations.
An alternative simple approach to both models would be to use spreadsheets or similar software for the calculation of the various relationships.
To ensure that there is acceptance of any models developed requires the relationships used and assumptions made to be reported in the literature so that open comment can be made on limitations.
The 'simplified' blast furnace model would allow producers and blast furnace operators to compare the likely impact of coal properties on various aspects of blast furnace operation.This could be done through the use of indices that can be related to, but are not modelled, the blast furnace operation.