Performance evaluation of a low-emissions stove using a biomass and coal blend

Abstract

Anthropogenic activities, particularly coal combustion for industrial and domestic purposes, contribute significantly to air pollution, posing severe health risks. In developing countries like South Africa, indoor air pollution from coal combustion is a pressing concern. Identified pollutants from coal combustion include particulate matter (PM), carbon oxides (COx), nitrogen oxides (NOₓ), and sulphur dioxide (SO2). A significant global population relies on solid fuels like coal for energy, especially in rural South Africa. Transitioning to cleaner energy sources and advanced stove technologies is crucial to mitigate health risks associated with domestic coal combustion, as studies reveal challenges in reducing emissions even with improved designs of open fires and braziers. The NWU semi-continuous stove underwent combustion experiments using coal discards and torrefied wood blended extrudates under high-power and low-power conditions. Higher biomass content in fuels leads to decreased ignition times in both high and low-power experiments. Highpower settings accelerate combustion, resulting in shorter ignition times. Burn rates increased with increased biomass content in pellets. 0% biomass pellets displayed an average burn rate of 23 g/min, and 100% biomass pellets displayed an average burn rate of 77 g/min. The stove shows higher biomass consumption rates but maintains similar coal consumption rates compared to other stoves. Power output increases with biomass content, mainly corresponding to higher burn rates. 0% biomass pellets displayed a peak power output of 10 kW, and 100% biomass pellets displayed a peak power output of 30 kW. Differences from similar research on coal stoves are attributed to stove design and fuel characteristics. The mass of fuel needed to raise water temperature increases with biomass content in high-power experiments but decreases beyond 50% in low-power experiments. Cooking efficiencies, the ratio between the energy output of the stove and energy absorbed by the cooking pot, ranged from 1.5% to 5.5%. An inverse relationship between the energy output during the cooking phase and cooking efficiency was observed, indicating diminishing returns at higher energy outputs. System efficiency decreases with increasing biomass content, and it is sensitive to combustion dynamics and excess oxygen in high-power experiments. The system efficiency ranged from 77% to 96%. Low-power experiments generally displayed higher system efficiency. In high-power experiments, CO and CO2 emission factors were higher compared to low-power experiments, decreasing with pellets with low biomass content and increasing with higher biomass content. CO emission factors ranged from 0.9 g/MJ to 4.4 g/MJ, and CO2 emission factors ranged from 87 g/MJ to 97 g/MJ. NOₓ emission factors decreased with decreasing nitrogen content in the fuel. NOx emissions also decreased at high temperatures, attributed to flue gas iii recycling reducing NOₓ through reactions with hydrocarbons. NOx emission factors ranged from 0.03 g/MJ to 0.11 g/MJ. SO2 emission factors initially decreased with biomass content up to 50%, then increased due to limited reactions with calcium species in the parent coal, decreasing sulphur retention. Low-power experiments generally exhibited lower SO2 emission factors. SO2 emission factors ranged from 0.11 g/MJ to 0.22 g/MJ. The particulate matter emissions initially decreased with biomass addition and increased with higher biomass content, with high-power experiments showing higher PM emissions influenced by flue gas velocity and weak pellet integrity. PM emission factors ranged from 0.04 g/MJ to 5.2 g/MJ. The NWU semi-continuous stove effectively burned blended fuel with up to 25% biomass.