Diffusional effects in TGA gasification experiments for kinetic determination

Wood matter from pressed oil-stone char gasification rates were measured in a thermogravimetric apparatus to assess the importance of diffusion limitations. Three sets of experiments were conducted at various temperatures, CO2 and CO partial pressures and crucible geometries. The set carried out with a monolayer bed of very fine particles well-exposed to the gas flow was used to evaluate the intrinsic gasification reactivity of the char and to obtain two kinetic equations: an nth-order equation and a Langmuir–Hinshelwood type equation. The other two sets of tests were used to evaluate the external (through a stagnant gas layer) and the internal (through a bed of char particles) diffusion resistances. The results showed that the internal diffusion resistance in a 3 mm deep char bed is quite significant and should not be disregarded in these kinetics determinations. Observed to intrinsic reactivity ratios (i.e. effectiveness factors) as low as 0.2 were calculated based on gasification tests at 950 °C. Furthermore, this study points towards a significant CO inhibition effect due to its accumulation inside the bed.