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