An investigation of the recovery of volatiles from Waterberg coal.

Abstract

South Africa has large coal reserves in world terms and the coal mining industry serves an important role in supporting the South African economy. Currently, most of the coal is mined in Witbank and Highveld Coalfields, where it occurs in relatively thick seams, near the surface. However, these coalfields are fast becoming depleted and new techniques are required to utilise the large coal resources still available in South Africa, especially in the Waterberg region, where a significant number of low-grade seams may be mined but discarded. The Waterberg region contains approximately 40% of the remaining coal in South Africa. However, the lithology of most of the coal seams is such that separation of the gangue minerals by density is difficult. There are many problems associated with mining in the Waterberg region, which include lack of infrastructure, lack of water sources, environmental issues (groundwater pollution) and socio-economic issues (Jeffrey, 2005). The experimental part of the project was aimed at investigating the recovery of chemicals from Waterberg coal which has a high mineral content. The design and simulation part of the project utilised the data obtained from laboratory experiments, to determine if a reactor with counter-current flow of gas and solids could be used to recover the volatile material from the coal. It was envisaged that the volatile material would be removed in the upper zone of the reactor, by hot gas generated by combustion of char in the lower zone. It was also envisaged that the air for combustion would be pre-heated, using heat exchange with the exit gas, as practiced in power stations. A mass and energy balance calculation was done over the entire system, to determine the limits of mineral content (expressed as ash content). Since, a large quantity of energy is required to raise the temperature to the point where volatiles can be separated, particularly if there is a high mineral content, the proportion of char is important, to provide sufficient energy to drive the process. It was envisaged that heat exchange would conserve energy and that it may be possible to use coal containing a relatively high mineral content, which would normally be discarded. Laboratory experiments were conducted on single coal pieces, which were first subjected to pyrolysis in a horizontal tube, which was inserted into a muffle furnace. A stream of nitrogen was used to transport the volatile material to an external collection vial, submerged in an ice bath. Some of the volatile material condensed in the cooler portions of the reactor, but it was recovered by subsequent dissolution in a solvent. The remaining char was then heated in a furnace with air, to burn the carbon and to determine the mass of ash remaining. The temperature and time for pyrolysis and combustion were varied, to determine suitable ! ii! operating conditions. The liquid products were analysed using the Karl Fischer method and GCMS analysis Four series of experiments were conducted, having a different variables changed. The first variable that was investigated, was the size of the coal sample. Single coal particles, with varying mineral content, were used. The mass of the coal piece was about 40g and the dimensions were about 50mm x 25mm x 30mm) The combustion experiments showed that these particles were not fully combusted, as they had an unreacted core. The subsequent series of experiments were conducted on single particles with a mass in the range 7g to 23g. The pyrolysis experiments show that significant amounts of the liquid product condensed inside the reactor tube. The combustion experiments showed that the structure of the high ash samples was maintained, whilst the low ash samples disintegrated. This is an important factor as the coal dust would cause channelling of the gas flowing up the reactor. The second variable that was investigated was the effect of time. The results showed that 30 minutes was sufficient to extract the hydrocarbons. It was noted that the coal with a high mineral content (> 60% ash) contained a relatively large amount of volatiles (17 %), and very little water (< 2 %). The GCMS analysis showed the presence of large proportion of phenols. Phenols are an important commodity and have a wide range of industrial applications. Karl Fischer analysis revealed that the overall water content was low and this made the liquid product easier to upgrade. The experimental results were then utilised in a mass and energy balance calculation, to simulate the proposed counter-current reactor and external heat exchangers. Two scenarios were investigated; The first was with the pyrolysis temperature set at 600°C and the combustion zone set at 800°C. The data from samples with varying mineral content was analysed. (The ash content of the feed ranged from 25.7% to 65.8%). The amount of excess energy available reduced as the mineral content increased, but the process still worked when the ash content of the combusted material reached 65.8%. The second scenario was with worst coal (65.8% ash), the pyrolysis temperature set to 700°C, and the combustion temperature set at 950°C. The results showed that there was still sufficient energy released by the combustion of the char to operate the process, but there was a smaller excess of energy. Additional water was needed to reduce the temperature of the product stream (hydrocarbons). It should be noted that loss of energy from the high temperature zones by conduction was not considered and that some of the ‘excess energy’ will be needed.