The Electrical Conductivity of Pulverized Fuel Flames

A combustion system has been built in which the combustion products of oxygen and coal gas, having temperatures in the range 2000- 3000°K, can be produced from 2¼ kg/hour of pulverized fuel. The furnace is basically a 25 mm diameter horizontal hole drilled through high density magnesia brick. The hole has been adequately insulated to prevent excessive heat loss due to the high thermal conductivity of the magnesia and to attain flame stabilization. Preheating of the primary or secondary air or oxygen to the burner is successfully done electrically in packed crucilite tubes. This is to obtain sufficient gas preheat so the combustion temperatures required can be obtained with this small scale furnace. The flame temperature is measured by means of a series of W/W-26% Re thermocouples inserted through thefurnace walls. These are shielded by recrystallized alumina sheaths, the tips being of zirconia, and the whole purged with nitrogen. Gas conductivity is measured by means of a water-cooled copper probe inserted along the axis of the flame and towards the burner. Initially a double probe technique was employed but due to current leakage around the walls of the furnace container it was not possible to determine the gas conductivity from these values. The present conductivity probe which has been developed combines tungsten and zirconia electrodes, such that a balance between thermal conductivity and electrical resistivity is maintained in order to prevent burn out and yet retain electrical insulation of the connecting wires. This probe also incorporates a thermocouple for accurate spot temperature measurements. The conductivity of pulverized fuel flames of oxygen and coal-gas, both unseeded and seeded with K2SO4,are presented. Conductivities of the order 1-2 mho/m2- m were obtained without seeding at 2800-2900°K, whilst seeding was necessary to maintain this conductivity at 2500-2600°K. (author)