The optimization, evaluation, and design of a side-feed wood-burning cookstove with fan-driven secondary air injection.

Thesis (Master's)--University of Washington, 20193 billion people still rely on open-fires and/or traditional cookstoves that burn unprocessed biomass fuels to cook and provide heat for their homes. These traditional cooking practices require large amounts of fuel and emit high-levels of harmful pollutants that have long lasting health, social, economic, and environmental impacts. The emissions from traditional cooking practices is the leading cause of household air pollution (HAP), which is the world’s single greatest environmental health risk to the human population, causing 3.8 million premature deaths each year and sickening many more. Improved cookstoves that burn locally available unprocessed biomass fuels (e.g., wood, charcoal, dung, and agricultural-waste), referred to as intermediate cookstoves, can provide significant reductions in household PM2.5 and CO concentrations, alleviating health risks. These stoves can also reduce the amount of fuel a household needs to procure on a daily-basis, saving households time and money. The design of intermediate cookstoves that effectively reduce in-home emissions and household fuel-consumption requires sophisticated knowledge of solid-fuel combustion and cookstove thermodynamics. Secondary air injection is often used in cookstoves to improve fuel-to-air mixing to achieve comprehensive reductions in emissions. The design of secondary air injection systems requires complex and time intensive experimental investigation to optimize numerous jet design parameters. In this thesis, the development of a secondary air injection system design tool for fan-driven systems in side-feed wood-burning cookstoves (i.e., rocket-stoves) is detailed. We find that jet configurations where jet radial penetration lengths approach the mid-line of a cylindrical cross-flow result in a maximum reduction in PM2.5 emissions and provide a good compromise between jet injection energy (representative of operational cost), quality of cross-flow mixing characteristics, and thermal-efficiency; supporting the results of previous investigations into secondary air injection optimization in furnace and gas turbine combustion chambers. The design tool is applied to the development of a solar-powered fan-driven secondary air injection system for the KuniokoaTM, a natural-draft, side-feed, wood-burning cookstove, manufactured and sold in East-Africa by BURN Manufacturing. The resulting system was integrated into a prototype stove, referred to as the Kunikoa-TurboTM. Laboratory performance testing of this prototype was performed at operating conditions typical of in-home use in rural-Kenya. Testing results suggest the Kuniokoa-TurboTM reduces PM2.5 emissions by 97% compared to measured household-kitchen PM2.5 concentrations in households in rural-Kenya using traditional three-stone-fires. Compared to the natural-draft KuniokoaTM, we find that the Kuniokoa-TurboTM significantly reduces PM2.5 emissions at a stove firepower greater than 3kW and is effective in reducing PM2.5 emissions throughout a wide range of fuel-types and burn-rates, corresponding to Tier 3 emissions performance for a stove firepower between 0.75kW-5.5kW