Low-smoke fuel production via low temperature pyrolysis of lump coal
Kühn, Mathys Johannes
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Coal is used on a large scale in the Kwadela Township, Mpumalanga, South Africa with the means to fulfil the basic energy requirements of low-income households. Energy within the coal is utilised via combustion thereof in household appliances, resulting in significant quantities of air pollution being produced. Consequently, a need exists for a fuel that would still provide sufficient energy to the low-income households, whilst fewer pollutants are released into the atmosphere during combustion. Hence, the suggestion to produce low-smoke fuels from raw coal. The aim of this investigation was therefore to produce a technically feasible low-smoke fuel that would serve as a viable alternative to replace coal. Anthracite, which is known to have an inherently low volatile content that may lead to less air pollutants released during combustion, was also investigated as a possible alternative to low-smoke fuel production from coal. The production of low-smoke fuels can be achieved through thermal decomposition of raw coal. A fundamental understanding of the devolatilisation process requires extensive knowledge regarding the intrinsic properties of the raw coal and its subsequent products formed that include char (low-smoke fuel), water, tar and gas. In an attempt to produce such a low-smoke fuel, a coal sample was acquired from within the Kwadela Township, from which four respective low-smoke fuels were produced: each thermally decomposed at a different temperature. The Kwadela coal sample was found to be a medium rank C bituminous coal rich in inertinite (82vol.% mineral matter free basis), with a high ash content (30.6wt.% air dry basis), typical of South African Highveld seam 4 coals. Kwadela coal devolatilisation behaviour of three particle size fractions (20mm, 30mm and 40mm) was studied under inert, atmospheric pressure conditions in the North-West University (NWU) Fischer Assay setup at temperatures of 450°C, 550°C, 650°C and 750°C. The effects of temperature and coal particle size on the derived pyrolysis products were evaluated, and it could be concluded that final pyrolysis temperature was the dominating factor controlling both product yield and quality. It was found that char yield decreased, while volatile- and gas yields increased significantly with an increase in temperature. In addition, the effects of particle size was deemed negligible throughout the in depth devolatilisation study. Tar evolution increased until a maximum yield (4-5wt.%) was obtained at temperatures ranging between 550°C and 650°C, after which it decreased slightly, due to possible manifestation of secondary cracking reactions at higher temperatures. The gas species evolved were found to consist primarily of H2, CO, CO2 and CH4, of which CO2 was the most predominant. Advanced analytical techniques (Simdis, GC-MS/FID, SEC-UV) were employed to analyse the condensable volatile fractions and revealed that the tar consisted mainly of higher-molecular weight olefins and paraffins. Combustion performance tests of the low-smoke fuels produced, concluded that there are significant differences between the behaviour of coal, anthracite and the four low-smoke fuels produced in this study, from both an emissions and practical performance perspective. The low-smoke fuels investigated consisted of the Kwadela coal having a particle size distribution ranging between 9mm and 60mm, which was devolatilised at the respective temperatures of 450°C, 550°C, 650 °C and 750°C. Emissions investigated included gas emissions (NOx, CO, CO2 and SO2), total suspended particulate matter (TSP) and volatile organic compounds (VOC’s), while the time it took to boil 1L of water, fuel ignition time and total space heating provided constituted the tangible practical performance parameters. Combustion efficiencies of the low-smoke fuels decreased with increasing pyrolysis temperature, with anthracite having the highest efficiency. Substantially lower TSP and VOC emissions were released into the atmosphere during combustion of the anthracite and low-smoke fuels in comparison to coal. NOx and SO2 gas emissions decreased with an increase in pyrolysis temperature, whereas CO and CO2 emissions followed similar trends. Hence, the emissions increased up to a maximum at a devolatilisation temperature of 650°C, followed by a decrease, however quantities were still higher than that measured during raw coal combustion. From an emissions perspective the low-smoke fuel produced at 750°C performed the best, however this fuel is not practically viable as water boiled only after one hour in comparison to the 17 minutes observed for the coal and anthracite. The boiling time for low-smoke fuels produced at 450°C and 550°C were relatively acceptable at 30 minutes. All the low-smoke fuels and anthracite, provided space heat for a longer period than that produced by raw coal. Accordingly the anthracite and low-smoke fuel produced at 550°C is the best practically viable fuel, while the benefits thereof include reductions of approximately 80% and 90% less particulate and volatile organic compound emissions respectively. Reductions of 10% and 35% in SO2 emissions were found for the low-smoke fuel produced at 550°C and the anthracite in comparison to the Kwadela coal. A techno-economic feasibility study regarding a low-smoke fuel production facility in the Secunda area indicated that such a venture would be economically sound by a slight margin only as a result of the low-smoke fuel produced being sold at a very low price. Due to the market for low-smoke fuels being low-income households, the price thereof should be as low as possible. It would cost approximately R1.50/kg anthracite to acquire and transport the fuel from Komatipoort to Secunda. Low-smoke fuels produced (locally in Secunda) from coal, on the other hand, have the possibility to be sold at approximately R1.00/kg, which increases the viability of low-smoke fuels in comparison to anthracite.
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