No abstract

This study focuses on the extrusion of discarded coal fines from the Highveld coalfield together with recycled lowdensity polyethylene (LDPE) and polypropylene (PP) which are used as binders to produce agglomerates with better handling properties than the coal fines for industrial use. The binder content varied between 5 and 100 wt %. The barrels of the twin screw extruder were kept at a temperature of 220 °C to melt the binders while forcing the mixture through a 10 mm die. The extrudates containing 10% or more binder were strong and homogeneous, while a 5% binder addition proved to be too low to produce homogeneous extrudates. The extrudates containing 10% LDPE and 10% PP showed compressive strengths of 17.5 and 7.9 MPa before breaking, respectively. The rest of the extrudates (>10% plastic addition) did not break but merely flattened as the plastic load increased. The compressive strength of all the extrudates showed no significant difference after being exposed to water. Furthermore, the extrudates absorbed less than 5% water after being submerged for 24 h. Thermogravimetric analysis of the extrudates was conducted under a nitrogen atmosphere up to 900 °C. Three iso-conversional methods, Kissinger−Akahira−Sunose, Starink, and Flynn−Wall−Ozawa, were used to determine the activation energy of the extrudates and raw materials. The lower activation energy and conversion temperatures found for the extrudates indicate a synergy between plastic and coal fines when the extrudates are pyrolyzed. Results from this study suggest that the coextrusion of recycled plastic with coal fines will produce solid carbonaceous fuels with high hydrophobicity, heating value, and high mechanical strength compared to coal fines.

A novel 'green coal' product formulation has recently been developed and the utilization concept tested at the North-West University coal research laboratories. Hydrothermal liquefaction was used to produce bio-oil and biomass char from sweet sorghum bagasse at operating temperatures ranging between 280 and 300°C, and the resultant char was mixed in various ratios (0, 0.25, 0.50, 0.75, and 1) with fine medium-rank C bituminous discard coal (<212 m) and CaCO 3 (1-5 wt%). The mixtures were pressed into 12 × 12 mm pellets using an LRX press at a pressure of 4 bar and gasified using CO 2 at atmospheric pressure and temperatures ranging between 800 and 1000°C. Kinetic parameters obtained from the experimental data showed that the reaction rate of the biochar was an order of magnitude higher than that of raw coal, with the blend containing 3 wt% CaCO 3 having the fastest reaction rate. In order to study the effect of temperature and catalyst on the retention of elemental sulphur during combustion of the various pellets, a combustion set-up consisting of a furnace, glass bayonet-type reactor, Liebig cooler, liquid traps, and an SO 2 gas analyser was used, with experiments conducted at temperatures between 500 and 800°C. As expected, sulphur retention was low for the raw coal and biochar blends, but increased significantly to between 56 and 86%, decreasing with increasing temperature, in the runs with added metal catalyst/sorbent. A simulation using FactSage TM predicted that >50% of the pyritic sulphur entering the fixed-bed gasifier would be removed from the gaseous phase as insoluble CaSO 4 when operated in a catalytic gasification mode at a temperature of 800°C, which is in good agreement with the experimental findings.Coal briquette, catalytic gasification, reactivity, sulphur retention.

South African collieries generate approximately 31 Mt of fine and ultrafine coal annually, with the majority of the ultrafine fraction discarded in slurry ponds and underground workings. Use can be made of this energy source through briquetting, thereby alleviating the handling problems associated with fine coal. Briquettes of inertinite-rich high-ash coal when combined lignosulphonate and resin have shown promising mechanical strength, therefore requiring reactivity analysis. In this study, chars derived from lump coal, binderless briquettes, and lignosulphonate-and resin-bound briquettes were subjected to CO 2 gasification at 875, 900, 925, 950, and 1000°C. Binder addition brought about no distinct difference in char reactivity. The briquetted chars showed approximately double the reactivity of lump coal chars. The increase in micropore surface area derived during the devolatilization process is postulated to be the major contributor to the increased reactivity of the briquettes. No significant differences were observed between the activation energy of the lump coal and manufactured briquettes, with values ranging between 222-229 kJ/mole. Industrial implementation of fine coal briquetting in South Africa will result not only in an increase in coal resources, but also reduce environmental concerns linked to fine coal discards.