Methanol fuel stoves as a clean technology for domestic cooking in South Africa

Abstract

Methanol is a potential domestic fuel for South African low-income households that could provide a clean and fire-safe alternative to paraffin. Poorly designed paraffin stoves emit harmful substances black carbon fine particulate matter and are prone to accidental fires. The use, thermal performance and safety of methanol stoves need to be contextualised for South Africa. This study evaluates methanol cookstoves' thermal and emission performance. It highlights the cookstove as a clean technology for domestic cooking to replace paraffin wick stoves. A standard test method for methanol cookstoves has been developed by a South African Bureau of Standards (SABS) Working Group; the proposed test methods were evaluated and documented in the laboratory as part of this thesis research. Three commercially available methanol cookstoves are evaluated using the draft standard test method. A non-pressurised paraffin stove serves as the baseline. The energy loss rate by combined radiative and convective processes is evaluated by measuring the heating and cooling rate using the methanol cookstoves. By combining the heating curve with the derived heat loss rate, we deduce the corrected total cooking power of the fuel/stove combination. Cooking power from methanol combustion is investigated using water heating tests. The equivalent time to boil (time to raise 1 L of water from 20ºC to 100°C) is calculated from the heating rate curve. The hood method is employed to capture the flue gases for the analysis of CO and CO2. The Relative Carbon monoxide Emission (RCE) (new term, defined in the body text) ratio is calculated and compared to the SABS 2% limit value in SANS (1906). Safety tests and evaporation losses during storage are investigated. Different pot finishes (shiny aluminium and black stainless steel) and sizes are used to investigate the influences of pot size and colour on the thermal and emission performance of the stoves. Fuel leakages and safety against conflagration were evaluated by upsetting the stoves through 90º and 180º from the horizontal plane. The results show minor heat losses from radiation and convection during regular operation from both stoves. Hence, the external finish of the pots had a negligible influence on the cooking power. Condensation forming on the bottom of the pot posed a noticeable perturbation leading to inconsistencies in the corrected total cooking power of the stoves. Pot size had a significant influence on the stove performance. In terms of thermal performance, the methanol stoves performed within specifications for the South African market. With the aluminium pot at high power, two methanol fuel stoves (STV01 and STV02) and the paraffin wick stove generated thermal power of more than 1 kW irrespective of pot size. With the small pot, STV03 could not reach the required 1 kW, producing about 0.97 kW. Overall, the paraffin stove produced the highest thermal power with all pot types, followed by STV02, STV01 and STV03. Two methanol fuel stoves (STV01 and STV02) and the paraffin wick stove generated more than 580 W irrespective of pot size. STV03 was unable to reach the 580 W requirement producing ~528 W with the small pot. The paraffin stove generated the highest cooking power of 752 W with the large pot, followed by STV02 with 659 W with the small pot. With the stainless

steel at high power, The paraffin stove produced the highest thermal power irrespective of the pot size and type. STV03 was unable to produce thermal power above 1 kW with any pot size. STV01 generated a thermal power of 1.10 kW with the medium pot, while STV02 generated 1.12 and 1.17 kW with large and small pots. The equivalent time to boil is less than 10 minutes per litre for all three methanol stoves, an acceptable performance range. With the shiny pots, STV01 reported the largest turn-down ratio (best controllability) with large (TDR = 3.7) and medium pots (TDR = 3.1), followed by STV02 and the paraffin stove, both producing TDR values of ~2, irrespective of pot size STV03 produced the poorest TDR, producing TDR of 2 with the medium pot only.STV01 and STV02 reported the largest TDRs with all three black pot sizes. Two of the methanol stoves and the paraffin stove reported Relative Carbon monoxide Emission (RCE) of values exceeding the 2% limit allowed for indoor combustion stoves. At high power, only STV03 has an RCE that complies with the 2% CO/CO2 emission limit set in SABS 1906 (a paraffin stove standard for the safety and emission limits. Overall, the paraffin stove recorded the lowest combustion efficiency, with RCE reaching ~5% with the small shiny pot and 8% with the medium black pot. Further design improvements are needed to bring the carbon monoxide emissions within the health safety limits. Nevertheless, the methanol stoves' overall carbon monoxide emissions are lower than for the reference paraffin stove technology. For all the stoves, the power setting lever requires design improvements to reduce the touching temperature below 42°C. The overall assessment is that methanol-fuelled stoves are a viable alternative to paraffin-fuelled stoves concerning performance and safety.