DOES SOLID CARBON BURN IN OXYGEN TO GIVE THE GASEOUS INTERMEDIATE CO OR PRODUCE CO2 DIRECTLY - SOME EXPERIMENTS IN A HOT BED OF SAND FLUIDIZED BY AIR

Relatively large spheres of non-porous graphite (diam. 2-12 mm) have b een burned in electrically heated beds of sand, fluidized by air. The rate of reaction of the carbon was derived from measurements of the co ncentrations of CO and CO2 in the gases leaving the bed. The carbon pa rticle's temperature was monitored continuously during its combustion using a very fine thermocouple inserted in the graphite sphere. The te mperature of the burning particle rose whilst its diameter shrank. The observations can be interpreted in terms of CO being the only product of oxidation for graphite particles burning between 1000 and 1400 K i n such a fluidized bed. The oxidation of CO is inhibited by the proxim ity of sand, which provides a large surface area for radicals (e.g. OH and HO2) to recombine. Nevertheless, CO does burn to CO2 close to the carbon at the higher temperatures studied (1400 K), but at similar to 1100 K it mainly diffuses away before burning in say a rising air bub ble. In this case less heat is fed back to the burning carbon. The use of tiny sand particles can give small air velocities over the carbon particle, so that convection of CO away from the carbon is reduced and the full enthalpy of combustion of carbon to CO2 is donated to the ca rbon. It seems likely that, just as radicals are important in oxidisin g CO to CO2, they are also involved in burning solid carbon to CO. (C) 1997 Elsevier Science Ltd.