Effects of internal air entrainment on combustion characteristics of wood fires

This work was motivated by the need to reduce indoor air pollution resulting from small, unvented, wood fires used for cooking and space heating in developing countries. It was hypothesized that buoyancy-driven air introduced into the middle of such fires can improve combustion efficiency and thus reduce pollutant emissions. A reference fire with a circular, solid insulating bed was compared with an annular fire built on a bed with a hole at the center. Fuel wood pellets were kept within a defined space above the bed by wire mesh fences. An experimental set-up was built that allowed observation of physical fire characteristics, whole-stream filtration of the fire plume to measure particulate matter, and gas sampling for measurement of CO and CO₂. Equal weights of fuel were burnt on both fires from ignition, through smoldering to the end of flaming combustion without recharging. Results indicate that relative to the reference fire and for unit mass of wood burned the annular fire a) was less smoky, b) had a maximum flame half as high, c) produced one third of the particulate emissions, and d) produced only half as much CO.