Xenobiotic particle emission formation in fixed-bed domestic coal combustion

Abstract

Abstract : Combustion of solid fuels have been reportedly associated with several cases of cardiovascular, respiratory and neurological diseases. Furthermore, it is suggested that premature deaths and morbidity cases may be prevented by the use of clean energy technologies than coal and wood burning. In South Africa, despite 87% of households being connected to the grid, over 50% and 75% of households in urban and rural areas continue to depend on wood and coal for major household activities respectively. In 2004 the Department of Minerals and Energy advocated for the use of toplit updraft method (TLUD) as an interim clean coal burning technology. The initiative has been associated with the reduction of visible smoke/particulate matter (~80%) and coal consumption. However, studies suggest that there might be other emissions other than PM at household or regional level from such technologies which may be detrimental to health and environmental performances, which requires further investigations. This present study has investigated the emissions of xenobiotic pollutants emitted from small-scale combustion technology currently in use and advocated to be used in South African low-income settlements. The study focused on emission factors from small-scale coal combustion technologies, influence of coal properties on emissions, particle size distribution at different combustion phases, the physicochemical properties of emitted particles, risk assessment of BTEX emissions, toxicity and cellular uptake of particulate matter from coal combustion. All experiments were carried out in a laboratory environment in order to minimize contribution of other environmental contaminants, which were not intended for the scope of the project. The emission factors of CO2, CO, and NOx ranged from 98–102 g MJ-1, 4.1–6.4 g MJ-1, and 75–195 mg MJ-1, respectively. Particulate matter (PM2.5 and PM10) emissions were in the range 1.3–3.3 g MJ- 1 for the BLUD method, and 0.2–0.7 g MJ-1 for the TLUD method, for both field and lab-designed stoves. When employing the TLUD method, emission factors of PM2.5/PM10 reduced by up to 80% compared to the business as usual BLUD method. The fuel moisture content (from 2.4 wt.% to 8.6 wt.%) led to an 18 and 30% decrease in fire-power when using the TLUD and BLUD methods, respectively. Measured carbon monoxide (CO) emission factors (EF) increased with an increase in moisture content, while carbon dioxide (CO2) EF remained unchanged. The use of A-grade coal resulted in 49% increase in PM emissions compared with D-grade coal at high ventilation rates. Particles from the top-lit updraft (TLUD) showed an ultrafine geometric mean diameter centred at approximately 109 ± 18.4 nm for the ignition phase, 54.9 ± 5.9 nm for the pyrolysis/flaming phase, and 31.1 ± 5.1 nm for the coking phase. In studying the physical properties of smoke particles from coal burning three types of shapes were classified, viz., spherical organic particles with similar characteristics to tarballs (ignition), soot particles (flaming), and mineral particles (char-burning). Elemental composition was determined using ICP-MS and it was found that 55%, 28% and 17% of the selected elements (Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu 63, Zn) were emitted during the char burning, flaming and ignition phase, respectively. However, during ignition most emitted element were K and Si, which may be used as markers for the simultaneous combustion of wood and coal. When using near-field breathing zone concentration of BTEX in a simulated exposure scenario applicable for a typical coal burning South African household, the cancer risk for adult males iii iii and females, was found to be 1.1 -1.2 and 110-120 folds higher than the US EPA designated risk severity indicator [1E-6], respectively. All four TEX compounds recorded the Hazard Quotient [HQ] of less than 1, indicating a low risk of developing related non-carcinogenic health effects. Lastly, smoke particles emitted from the TLUD ignition from a high-ventilated stove were used to determine the cytotoxicity and cellular uptake. Particles from three combustion phases showed mild toxicity on the bronchial epithelial cells at highest concentration of 150 μg/ml while lower concentrations (20 μg/ml, 60 μg/ml 100 μg/ml) did not indicate significant toxicity on the cells. The results of this study have shown that the emissions of health and environment damaging may be significantly reduced by the correct selection of stove to fuel combinations. The PM emissions are directly proportional to the stove type, fuel properties and ignition method. Stoves with higher ventilation rate ignited using fuel with low moisture content and lit with the TLUD ignition method, produces optimum performance on the reduction of PM2.5 and PM10. However, the use of fire ignition method and coal with different moisture content did not show a convincing reduction on the gaseous emissions. Therefore, it remains important when promoting clean coal burning technologies to take cognisance on non-conclusive information of emission performance, particularly on gaseous emissions. It was further, established that particles emitted from domestic coal burning are of small diameter with an average mean of less than 100 nm. This is concerning due to the longer residence time of small particles on air spaces and higher deposition rate in the lower respiratory track of the exposed individual. The emitted particles low to medium temperature produced organic enriched spherical particle with similar characteristics to tarballs. The emissions of BTEX remains a worrying subject given the high probability of cancer under a defined exposure scenario defined under this project. Therefore, it remains important not to rule out the potential harm associated with gaseous emissions when promoting the technology (use of TLUD ignition method).Ph.D. (Energy Studies