Evaluating energy efficiency and emissions of charred biomass used as a fuel for household cooking in rural Kenya

Abstract: In sub-Saharan Africa a large share of the energy use utilize biomass as a fuel. In some countries more than 90 percent of the energy use is biomass. This energy is primarily used for cooking, heating and drying. Cooking food on an open fire or using a traditional stove will combust the firewood inefficiently and leads to pollution in the form of particulate matter, carbon monoxide and other hazardous pollutants. Indoor pollution has serious health effects and especially women and children are affected by this since they spend more time in the kitchens compared to men. More efficient combustion would lead to less harmful pollution to women and children in these rural areas. There are different kinds of stoves on the market and one of them is the gasifier stove which allows the biomass to go through pyrolysis in a separate step before complete combustion. If the charred biomass is harvested before complete combustion it can be saved for later use. This stove will result in cleaner and more energy efficient combustion compared to the traditional 3-stone-fire. The aim of this study has been to evaluate the charred biomass harvested from this gasifier stove in terms of energy use efficiency, emissions and cooking time. The charred biomass was compared to conventional charcoal bought at the local market. The charred biomass investigated is charred Grevillea prunings from the Grevillea Robusta tree, charred coconut husks ( Cocos nucifera) and charred maize cobs (Zea mays). They were tested by cooking a meal consisting of two dishes at five different households for different kinds of charred biomass and conventional charcoal as a reference. Using charred Grevillea prunings gives an energy saving up to 31 percent while charred coconut husks gives up to 11 percent energy saved compared to the 3-stone-fire. Charred maize cobs was only up to 2 percent more energy efficient than conventional charcoal due to its low energy density and fast burning rate. In most cases there was no significant difference between the emissions of the different charred fuel types. Only charred maize cobs resulted in significantly higher emissions than the other fuels. Household B deviated from the others households and had higher emissions. In conclusion the different types of charred biomass are good fuels for cooking. Charred maize cobs are less valuable since they require a higher rate of refilling of fuel during cooking and do not result in better energy use efficiency compared to conventional charcoal. There were no significant differences between the different types of charred biomass and conventional charcoal in emissions except for a few cases where charred maize cobs had a slightly higher level of emission compared to the others. CO 2- levels were so low that there was no risk of harmful concentrations in any way. PM 2.5-emissions levels were safe, but the CO-emissions levels for charred maize cobs were close to levels were symptoms might show.