The effect of particle size on the performance of low-smoke fuels in coal stoves

Abstract

The link between exposure to household air pollution (HAP) and ill-health has been consistently drawn in the literature. The World Health Organisation (WHO) places the burden of disease from HAP at an estimated 4.3 million premature fatalities a year; almost 600 000 of these occur in Africa. The 2011 South African census reported that approximately 3.9 million households in the country rely on a form of solid fuel as their primary energy source, despite interventions by government, including the policy of universal electrification. For low-income households in the colder climates of the Highveld, coal is the most common solid fuel which is burnt in a variety of devices, including cast iron stoves. This persistent use of coal is not only associated with poor health, but has also been identified as a major source of local ambient air pollution. A low-smoke fuel (LSF) has been previously considered, at government level, as a way to quell the effects of domestic coal use. This work aimed at determining the effect of the fuel particle size on the performance of such a low-smoke fuel when used in a common cast iron stove. A bulk coal sample was acquired from one such community and was segregated into its constituent sizes then subjected to partial devolatilisation at 550°C to produce an LSF. Four fuel sizes - 15, 20, 30, and 40 mm, as well as a composite of the sizes - were tested, against their untreated coal analogues, in pre-set combustion tests to evaluate the thermal- and emissions performance of each fuel. Thermal performance assessment metrics included ignition time, water boiling time, and heat transfer- and combustion efficiencies, while the emissions considered were CO, SO2, NOx, particulate matter (PM) and volatile organic compounds (VOCs). Ignition times were found to decrease from coals to LSFs, and to decrease with increasing particle size; the 40mmLSF (a 40 mm devolatilised coal or LSF) ignited twice as fast as the 15mmLSF. The effects of fuel type on the water boiling time were only observed in the later stages of the burn cycle, with the LSF boiling a 2 L batch of water in an average 20 minutes, while the coals reported an average boiling time of 24 minutes. In addition to boiling water faster than the coals, the LSF boiling times were found to decrease with increasing particle size. Heat transfer efficiencies showed no significant variation with fuel type or particle size, with the average efficiency for the coals being 66.4 %, while that of the LSFs was 65.8 %. The fuels’ performance was better gauged by the combustion efficiency, which was found to improve marginally from the coal fuels to the LSFs, and to increase with increasing particle size. Emission factors (EFs) for NOx and SO2 were found to depend on the fuel nitrogen and sulphur content as well as the combustion conditions. Increased combustion efficiency, which increased with particle size, was found to lead to higher SO2EF and NOxEF. The PM and VOC emissions showed a strong dependence on the ash content and volatile matter yield, which both increased with increasing particle size. The emission factors of both VOCs and PM were found to be inversely correlated with particle size, and decreased from coals to LSF as a result of the pretreatment. This work contributes to the emissions and performance inventories from South African domestic coal combustion. The insight gained lends itself to the production of coal alternatives, the pairing of these fuels with suitable combustion devices, and may be consulted as part of the growing body of literature which impacts on low-income household energisation policies and interventions.