Pyrolysis and ignition of single wooden spheres heated in high-temperature streams of air

The ignition of single wooden spheres heated in a hot air stream has been investigated experimentally. Three species of wood were used. The single wooden spheres were oven-dried and exposed to various temperatures and streams of air with different flow rates. The orientation of the grains in the wood was kept either perpendicular or parallel to the air stream. The temperature inside a wooden sphere was monitored by embedded thermocouples. The orientation of grains in the hot air stream had a notable influence on the ignition time, particularly with air at 673 and 773 K. At 873 K neither the flow rate of air nor the orientation of the grain had any effect on the ignition time. Glowing reaction was observed prior to ignition for wooden spheres heated to 773 K, but flaming ignition was found for air at 873 K. A correlation for estimating the ignition times was proposed. A simple one-dimensional model for the pyrolysis of wood, taking account of the heat of pyrolysis and the evaporation of moisture, is capable of predicting the mass loss quite well, but is less satisfactory for predicting the temperature in the period prior to flaming ignition.