1.1 Background
Biomass is the largest energy source in Kenya and provides 69% of the Country‘s overall energy requirements, petroleum and electricity accounting for about 22% and 9% respectively. About 55% of this is from farmlands as woody biomass, crop residue, and animal waste, and the remaining 45% is from forests [1]. A national household budget survey [2] carried out from September 2015 to August 2016 showed that most households use this biomass as fuel. In the 11.4 million households surveyed, the primary source of cooking fuel was firewood (54.6%), charcoal (14.6%), kerosene (14%), and liquefied petroleum gas (LPG) (13.4%). In the urban areas, households mainly used kerosene (29%), LPG (27.6), charcoal (21.9%), and firewood (16.1%). A study done in 2018 showed that Kenyan households using LPG as their primary cooking option also use one or more traditional fuels like charcoal (47%), wood (17%), and kerosene (18%) [3].
Arid and semi-arid lands (ASALs) cover more than 80% of the land and are a major source of woodfuel. However, the woodfuel is extracted in an uncontrolled and unmanaged manner, with natural regeneration as the way of recovery [4, 5]. To mitigate climate change, and provide sustainable energy for all, Kenya has developed policies to promote a transition to clean cooking fuels. These include promoting LPG and ethanol in urban areas, clean biomass cookstoves in rural areas, and alternative fuels such as compressed biomass and biogas [6, 1]. There is also recent legislation [7] that seeks to promote efficient and sustainable production (e.g. growing fast-maturing trees for energy production and biofuels), distribution, and marketing of biomass energy resources.
There is a growing demand for charcoal in Kenya caused mainly by population growth and urbanization, and about 2.5 million tonnes are produced annually [8]. Several advantages make charcoal as a fuel attractive for cooking compared to un-carbonized biomass. Its calorific value is roughly double that of un-carbonized material (its higher heating value per unit mass is approximately 30 MJ/kg with 5% moisture content compared to approximately 15 MJ/kg of firewood with 15% moisture content). Charcoal can be stored for long periods because it cannot get damaged by rain or moisture, it is available in the local market in small quantities, and it can be burned in inexpensive stoves [9]. The cost per kg (and per megajoule) of LPG, charcoal, and wood pellets is about US$ 1.50 (US$ 0.03), US$ 0.47 (US$ 0.02), and US$ 0.38 (US$ 0.02) respectively [3, 10-12].
The artisanal Kenya ceramic jiko (KCJ) cookstove, developed in 1984, has been widely accepted. 34% of all households in Kenya own it (40% urban and 31% rural), and about 343,000 units are sold each year with an annual market value of US$ 1.3 million. Households that use charcoal primarily use approximately 395 kg of charcoal per year [3]. The thermal efficiency of KCJ and similar stoves is below 27% [13, 14]. Factory-assembled modern charcoal cookstoves e.g. Jikokoa, JikoFresh, and SuperSaver, are increasingly being introduced [3].
The most widely used method for converting wood to charcoal in Kenya is the earth-mound and pit kilns with 10-14% wood-to-charcoal conversion efficiency. Although kilns with efficiencies of up to 30% have been promoted, e.g. improved earth Kiln, Casamance, and Brazillian masonry (or “beehive”) kiln, their actual use in charcoal production is very minimal [8]. The conversion of 1 kg of wood, therefore, yields only 0.1 – 0.3 kg of charcoal. In other words, to produce 1 kg of charcoal with a specific energy content of 30 MJ/kg, 3.3 - 10 kg of firewood is required, which would otherwise have a total energy content of 53 - 160 MJ [9, 15].
To save the energy currently being lost when converting wood to charcoal, technologies e.g top-lit-up-draft (TLUD) gasifier cookstoves, that provide clean cooking energy from biomass pellets at thermal efficiencies greater than 50% [16, 17] are available. Compared with the direct combustion of solid biomass, gasifier cookstoves have higher thermal efficiency and fewer pollutant emissions since the syngas created from solid biomass is easily mixed with air to generate heat [18].
To densify lignocellulosic biomass, the first machine in the process line is a chipper. The wood chips are dried to about 10% (wet basis) in a rotary drum dryer that can use woodfuel. After drying, hammer mills are used to grind the chips to a screen size of 3.2 to 6.4 mm, a particle size suitable for pelleting. The ground biomass is then compacted using a ring or flat die pelletizer. Pellet density ranges from 1000 to 1200 kg/m3. Pellets are then sifted over a screen to remove fines and weighed before storage [19, 20]. Various factors that affect the economic viability of pellet production have been studied [21-23].
Pellets production in Kenya is still in its nascent stages. To promote pellets as a cooking fuel, SNV and EcoZoom undertook pilot studies in 2014 - 2015, aiming at increasing access and use of pellet stoves in urban and peri-urban markets. Pellets were tested on both locally manufactured gasifier stoves (WISDOM and SCODE) and imported ones (Philips and TERI). A key challenge observed in the adoption of pellet-cookstoves was the high upfront cost of the stoves (e.g. a WISDOM gasifier stove costed US$ 34) [3]. Other factors also affect the adoption of pellet gasifier cookstoves, and households will generally adopt a fuel stacking model [24-30, 12]. Ongoing pellet production and gasifier cookstoves initiatives in Kenya include: Lean Energy Solutions Ltd., Iko Briq Ltd., Power Spot Ltd., Wisdom Innovations Ltd., and KIRDI [3, 10, 31, 32].
This study compared the energy efficiency and emissions of a natural-draft gasifier cookstove using wood pellets to contemporary charcoal cookstoves in Kenya.
1.2 Limitations of the study
Field tests to determine performance of cookstoves in kitchens were not conducted.
It was beyond the scope of the study to determine to what extent the targeted communities would adopt the pellet cookstoves if promoted. Adoption of cookstoves is dependent on many factors, including availability and affordability of high-quality cookstoves, reliability and accessibility of fuel supply, safety and usefulness of the cookstove in application, technical/industrial support for stove manufacturers, innovative funding mechanisms for consumers, awareness, and post-sales interventions, and multi-stakeholder collaboration [25-30].
A few firms are already manufacturing biomass briquettes and pellets in Kenya from industrial wastes like sugarcane bagasse and sawdust, although at a small scale [3, 10]. However, we did not investigate their experiences, especially regarding biomass availability, cost of investment, operation and maintenance, location and plant capacity, logistics, and energy costs [20, 21].