In sub-Saharan Africa, over 90% of the population rely on wood and charcoal for cooking, and the spiralling demand for biomass fuel is leading to deforestation and land degradation. Collecting fuel wood is also a timeconsuming and wearisome business for millions of African women, who are forced to walk long distances in search of wood to cook the family’s evening meal.

Part of the solution lies in introducing more efficient cooking methods, and this is why many organizations are now working with communities to develop and adopt improved cooking stoves. But how do these compare with other stoves? Mary Njenga and Miyuki Iiyama of the World Agroforestry Centre, Cecilia Sundberg of the Swedish University of Agricultural Sciences (SLU) and K. Röing de Nowina of the International Institute of Tropical Agriculture (IITA) put three different types of stoves to the test in a research programme supported by the Swedish Research Council.

“There were two things we wanted to find out,” says Mary. “First, we compared the efficiency of a gasifier with the Hifadhi improved stove and the traditional three-stone cooking stove, using Grevillea robusta prunings. Second, we looked at the properties of three feedstocks which are commonly available in rural Kenya.” These were Grevillea prunings, maize cobs and coconut shells. The participatory cooking experiment took place in Embu County, an agricultural district on the flanks of Mt Kenya where farmers grow Grevillea for timber. Five households were invited to cook the traditional meal of maize, ugali, and kale using the three different types of stove and feedstock.

A woman using the Gasifier Cookstove in Embu, Kenya

Domestic gasification is a process where organic material is burnt with controlled quantities of oxygen. This produces gas for cooking and a by-product, charcoal, which can either be used as a cooking fuel or for soil improvement. The other two cooking methods – the Hifadhi stove and the traditional three-stone stove – convert biomass to heat and ash.

The research found that gasifiers use 40% less fuel than traditional three-stone stoves and 27% less than the Hifadhi improved stoves. They cook faster than the other two stoves and produce a useful by-product, yielding 200 grams of charcoal for every kilogram of wood burnt. “They also produce much lower emissions,” says Mary, “and that’s very important from the point of view of the health of the women and children, who often spend time in the kitchen with their mothers.” Gasifiers emit approximately half the carbon monoxide of improved and three-stone stoves and 90% less particulate matter. In the trials to assess the value of different feedstocks, the researchers found that Grevillea prunings burned for longer than maize cobs and coconut husks. When used in the gasifier, the Grevillea prunings provided cooking heat for 50 minutes – 20 minutes longer than was needed to cook the traditional meal of ugali and kale. In contrast, maize cobs lasted just 20 minutes. In terms of the quantities of charcoal that they produced, coconut husks and maize cobs were marginally more efficient than Grevillea.

“The research suggests that gasifiers are the most efficient of the three types of stoves,” says Mary. “They help to save fuel, they cook more rapidly and they produce fewer dangerous emissions.” However, they do have two drawbacks. They are expensive and there is the safety issue to consider. The gasifier used in these experiments is made out of galvanized steel, which gets extremely hot. These drawbacks could be addressed by improving the design and by raising awareness about the long-term benefits outweighing the initial cost.

“Despite these drawbacks, it is clear that gasifiers have many advantages,” says Mary. “They can be easily integrated into agroforestry systems as you only need very small pieces of wood – such as the prunings of Grevillea – and they produce a useful byproduct.”

References

Njenga M, Iiyama M, Neufeldt H, Helander H, Larsson A, Röing de Nowina K, de Leeuw J, Jamndass R, Sundberg C (forthcoming) Cooking with a gasifier saves time and fuel, produces charcoal as a by-product and reduces household indoor air pollution in Kenya. Njenga M, Iiyama M, de Leeuw J, Röing de Nowina K, Kätterer T, Kimutai G, Sundberg C. 2015. Cooking with a gasifier saves fuel and time, reduces smoke and produces charcoal for another cooking. Miti. The Tree Business Magazine for Africa. No. 26 April-June 2015.