Development of wood-burning rocket cookstove with forced air-injection

Abstract

Nearly 40% of the world's population still relies on biomass stoves as their primary cooking source. Indoor cooking with biomass stoves produces harmful emissions such as fine particulate matter (PM2.5) and can result in significant cardiovascular and respiratory disease. Air injection technology, in the forms of over-fire, under-fire, and staged (a combination of under- and over-fire), has been used for decades to reduce emissions in small-scale industrial biomass burners. Recently, researchers have begun integrating air-injection into biomass cookstoves, but the number of studies is limited, and improper air-injection can result in worsening stove performance. In this paper, we develop and analyze three wood-burning biomass rocket stove air-injection strategies using computational fluid dynamics and experiments. We consider over-fire and under-fire air-injection independently, and then develop a staged air-injection system that combines the optimal jet characteristics from the purely over-fire and under-fire systems. Results show that significant performance gains, both in terms of reduced emissions and increased burn-rate are achievable with forced air-injection. For a radial over-fire air injection system, jet momentum flux is found to control fuel and air mixing and is the driving metric for reducing particulate emissions. For the under-fire system, excess air ratio was found to control both the emissions rate and firepower. In a comparison of all three air-injection strategies, over-fire air injection was found to have the largest reduction in emissions, under-fire air injection had the largest increase in firepower and reduction in boiling times, and the staged air injection method achieved both improved emissions and burn-rate. Additionally, the combustion start-up phase was observed to account for the majority of emissions in all forced-draft stoves, and is most sensitive to under-fire injection, remaining an area ripe for improvement.