Development of a fluidized bed cookstove for direct combustion of charcoal dust

Abstract

Conventional direct combustion and gasifier stoves operate a packed bed combustion technology, which is not favorable for combustion of fine solid biomass such as charcoal fines, not least because of the compact nature of these fuels, which block airflow, leading to incomplete combustion. Conversely, fluidized bed combustion technology would effectively combust fine fuels; however, this is largely deployed of the industrial sector for processes such as heating, drying, separation, power generation, among others, and has never been adopted in cookstove designs despite its high-quality combustion and heat transfer potential. This study, therefore, presents the design, modeling, fabrication, and performance evaluation of a fluidized bed cookstove specifically developed for the direct combustion of charcoal dust. The cookstove employed a bubbling fluidized bed combustion mechanism to enhance fuel-air mixing, hence promoting complete combustion and improving heat transfer. The stove design was a result of mathematical modeling using empirical formulae from previous studies, experimental research, and insights from experts, obtained through the use of structured questionnaires. Computational Fluid Dynamics was employed to simulate both the hydrodynamic behaviour and combustion processes within the reactor, thereby predicting the model performance. A prototype was fabricated and tested with five different charcoal species, i.e., Dichrostachys cinerea, Morus Lactea, Piliostigma thonningii, Combretum molle, and Albizia grandibracteata, following ISO 19867-1:2018 testing protocol. Performance indicators assessed included thermal efficiency, firepower, fuel consumption, CO, CO2, and PM2.5 emissions. The cookstove featured a funnel-shaped combustion chamber with a dense phase region (Ø0.106m × 0.119m), a lean phase region (Ø0.212m × 0.064m), and a total reactor height of 0.182m. The combustion chamber was fabricated from 5mm-thick stainless steel, insulated with 30mm of waste glass wool, and enclosed in 1.5mm-thick mild steel cladding. CFD results indicated dense phase particle concentration with no entrainment and a maximum combustion temperature of 726.8℃. The physical prototype achieved an average high power thermal efficiency of 30%, tier 3 for PM2.5 and CO performance. The results demonstrated that the FBC achieved a thermal and emission performance similar to that of ICS (Improved cookstove).