Single Particle and Packed Bed Combustion in Modern Gasifier Stoves—Density Effects

Abstract

This article is concerned with a study of an unusual effect due to density of biomass pellets in modern stoves based on close-coupled gasification-combustion process. The two processes, namely, flaming with volatiles and glowing of the char show different effects. The mass flux of the fuel bears a constant ratio with the air flow rate of gasification during the flaming process and is independent of particle density; char glowing process shows a distinct effect of density. The bed temperatures also have similar features: during flaming, they are identical, but distinct in the char burn (gasification) regime. For the cases, wood char and pellet char, the densities are 350, 990 kg/m³, and the burn rates are 2.5 and 3.5 g/min with the bed temperatures being 1380 and 1502 K, respectively. A number of experiments on practical stoves showed wood char combustion rates of 2.5 ± 0.5 g/min and pellet char burn rates of 3.5 ± 0.5 g/min. In pursuit of the resolution of the differences, experimental data on single particle combustion for forced convection and ambient temperatures effects have been obtained. Single particle char combustion rate with air show a near-d ² law and surface and core temperatures are identical for both wood and pellet char. A model based on diffusion controlled heat release—radiation—convection balance is set up. Explanation of the observed results needs to include the ash build-up over the char. This model is then used to explain observed behavior in the packed bed; the different packing densities of the biomass chars leading to different heat release rates per unit bed volume are deduced as the cause of the differences in burn rate and bed temperatures.

Keywords:

• Ash effect
• Biomass gasifier stove
• Char mode
• Char particle combustion
• Flaming mode

Notes

*Moisture and ash-free basis.

V _p –primary air flow rate, f–flaming, c–char.

**Beyond the range of K-Type thermocouple used.

*Averaged over more than three experiments.

m = measured; other temperatures estimated.