Biochar Production With Cook Stoves And Use As A Soil Conditioner In Western Kenya

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Biomass is one of the most important resources in smallholder farms in Africa. It is used for fuel, soil organic matter, construction materials and animal feed. While the main source of fuel in the household is wood, other types of on-farm resources are used. In fact, most fuel sources in Kenya are supplied by non-forest trees and traditional fuels (i.e. dung, crop residues). Removal of biomass from the farm as a source of fuel, could lead to decreasing levels of soil organic matter and nutrient availability. An alternative is the pyrolysis of biomass residues while cooking, which can provide fuel energy and biochar as a soil amendment. The first step to establishing a smallholder farm biochar- bioenergy system is the assessment of biomass resources and their capacity to sustain household energy consumption. We carried out a biomass and energy consumption assessment in 50 household in the Highland of western Kenya, investigating the variation of biomass availability with age, farm size and feedstock. Current household energy consumption patterns were determined as well as whether on-farm biomass energy can sustain the current energy use. This thesis presents data on household energy consumption patterns when a pyrolysis stove is introduced in the household and the percent return of biochar after cooking. Total biomass productivity varied with farm age, with an average of 5.76t farm-1. Current energy consumption per capita averages 11.27GJ yr-1. Wood biomass productivity alone does not currently support energy consumption. However, based on our data total biomass productivity including crop residues, shrub and tree litter can provide 18.27GJ capita-1farm-1yr-1of energy for cooking needs. The introduction of a pyrolysis stove can improve the system even further by providing the necessary energy to cook but also producing an average of 0.5t of biochar per household yr-1. A system that combines the production of biochar and bioenergy may be able to address several constraints facing resource-poor farmers in Africa. Pyrolysis cook stoves in combination with a biochar return to soil may have the potential to provide farmers with an increase in crop productivity and energy efficiency. However, biomass serves more than one purpose in the farm and its removal from the fields as a source of fuel can also have negative effects in the nutrient balance of the soil. Currently farmers use a portion of on-farm biomass as fuel, while the rest is kept in the fields as a source of organic matter. Both the fresh biomass and ash serve as sources of soil amendments. However, both amendments have limitations. The quality of the fresh residue can determine the mineralization and availability of specific nutrients (i.e N); while ash is void of important plant nutrients like N limiting its usefulness as a soil amendment. Therefore, the second objective of this thesis was to investigate the short term effects of applying charred and ashed plant residues in comparison to fresh residue return on maize biomass productivity and nutrition. Fresh biomass was applied at an initial rate of 18.05 g pot-1, while 46-65% less biochar and and 90-96% less ash were applied according to pyrolysis and combustion conversion efficiency for each feedstock. Dry biomass production increased by 400% and 350% with application of biochar and ash, respectively, when compared to fresh biomass from nutrient-poor residues. Important differences existed between different plant residues. Pyrolysis or ashing did not improve crop growth for nutrient-rich amendments such as collard stalks which are best applied as fresh biomass. On the other hand, nutrient-poor feedstocks such as cobs and maize stover are most effective as organic amendments when pyrolyzed or fully combusted to ash.

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Lehmann, Christopher Johannes

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Hobbs, Peter

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Soil and Crop Sciences

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M.S., Soil and Crop Sciences

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Master of Science

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