Combustion of wooden spheres-experiments and model analysis

Abstract

Studies on combustion of wooden spheres have been made towards understanding their role in wood gas generators. Experiments on 0.01, 0.015, 0.02 and 0.025 m dia spheres show two regimes of combustion-flaming and glowing. During flaming combustion, the sphere decreases in diameter by about 10% and loses 75-80% of its weight, this reduction being related to loss of volatiles only. Simulation experiments performed by inert heating of the wooden spheres to temperatures of about 350°C to cause loss of volatiles confirms the above result. The glowing zone combustion involves the remaining weight loss of 20% and diameter variation following the d²-1aw. The mass loss correlations follow mass loss rate (kg/s) = k'diameter (m), k = 7 × 10^-4 for flaming zone and 7 × 10^-5 for glowing zone. A theory for flaming combustion involving the movement of an exothermic pyrolisis front into the sphere and other elements similar to droplet combustion theory has been evolved. The theory of glowing combustion of the porous char has been evolved following the model description of Howard. Both the theories include augmentation of heat and mass transfer due to free convection in an approximate way. Predictions from both the theories show that combustion times and weight loss time plots are represented reasonably well by the theories.