Technical Market Support » Metallurgical Coal
The stamp charging of coal has emerged as a widely-used, effective treatment to improve the quality of coke produced in coke-making operations. Traditionally, expensive prime hard coking coal has been used to make coke with the required properties to operate blast furnaces effectively. Through the use of stamped charging technology cheaper, lower quality coals can be used in larger amounts increasing the flexibility and cost efficiency of coke-making operations along with maximising oven throughput. An important consideration for the use of a coal in a stamped charged operation is weighing up the cost of the energy used for stamping with the improvement in coke quality and reduction in raw material costs.
The defining factor in determining a coal's suitability for stamp-charging treatment is the stampability, which is the amount of energy required to pack the coal to a target bulk density. Coal properties and factors which can influence stampability include coal type and rank, moisture content and size distribution. One purpose of this project is to develop a small-scale stampability apparatus for determining the stampability of single coals and coal blends. As well as devising standard procedure for the operation of the apparatus. The project also aims to improve the understanding of the relationship between bulk density and coke quality at small scale for Australian Coals.
The objectives of this project were to:
· Develop a standard procedure to determine the stampability of a coal or coal blend;
· Demonstrate the use of stamp charging at small scale to investigate coals being considered for stamp charging operations;
· Relate the stampability of a coal to coal quality; and to
· Improve understanding of the relationship between coal bulk density and coke quality.
The stampability of single coals has been found to be related to the moisture at which the coal is tested, the size distribution and coal type. In general coals with a low Hardgrove Index (HGI) were found to require lower energy to stamp. Stamping is also less energy intensive when the size distribution (measured by Rosin Rammler n) is broader. The stampability of a coal blend is related to the moisture and size distribution of the resulting blend. However the limited data in this project does not allow a conclusion to be drawn on which parameter is of greater significance in a blending scenario. The maximum compressive strength of the coal cake occurs at an optimum moisture and this moisture differs coal by coal. The compressive strength is also influenced by the size distribution of the feed. However it is important to ensure that increased moisture to create a stable coal cake does not require excess energy for carbonisation. As with the stampability the compressive strength of coal blends is related to the moisture and size distribution of the coals.
All coals tested in this project showed improvement in coke quality with stamped charging. Though the improvement in coke quality did differ depending on the individual coal and the improvement in quality for some coals did plateau as the charge density increased. Coal G was shown to produce coke that has adhesion controlled strength. For these coals the mechanism for improvement in coke quality is via increased contact between coal particles with increasing bulk density. This type of coal is more likely to be of higher rank (lower volatile matter) and are more easily stamped, this is very favourable as significant improvement in coke quality can be achieved whilst lower energy is required for stamping. The improvement in coke quality for Coals F, H, I & J with stamp charging is due to reduced porosity. Despite the significant improvement in the quality for coal H, the initial quality was very low. Also this coal is low rank (high volatile matter), which means that more energy would be required for stamping. From the testing of coal blends it is evident that the addition of small proportions of a hard coking coal are able to improve the structure of a coke which can be further improved by stamped charging.