Wood Char Gasification

Wood-Char Gasification: Experiments And Analysis On Single Particles And Packed Beds

From the article:

Gasification has been studied with the aim of designing reactors, gasifiers, and other combustion systems. In a cocurrent gasifier 1, air and solid fuel move in the same direction, and the flame front moves in the opposite direction. Air first reacts with the solid fuel either in the heterogeneous mode (e.g., in the case of a charcoal gasifier) or with the volatiles generated from the solid fuel in the gas phase, releasing heat and helping in the propagation of a flame front into the unreacted solid aided by axial heat transfer by conduction and radiation. The hot combustion products (CO2 and H2O) are further reduced by the char. These endothermic reactions generate carbon monoxide and hydrogen, and the exit gas can be utilized as a gaseous fuel. Similar processes also occur during fire spread in permeable materials. A number of workers [2–4] have examined the propagation rate of a flame front against airstream through a packed bed of solids such as wood, foam, or biomass. The primary emphasis in these studies has been in predicting the flame spread through the media. Only the first process described earlier, namely, the oxidation, is of importance in predicting the flame spread rate. However, for design and operation of a gasifier, both oxidation and reduction processes are of equal importance. Hence, the present paper is aimed at studying these processes in an isolated single particle and extending the results to a bed of particles to predict the various features of an operating gasifier, namely, the flame front movement, the profiles of different species concentrations and temperature, and the exit gas composition from the gasifier. The present work is limited to charcoal gasification only. The model developed would be of use for understanding and designing biomass gasifiers.

Several designs of wood gasifiers exist [1,5– 7], with modeling aspects addressed by a few [5,8,9] using overall kinetics in a packed bed. Predictions are compared with experiment results by tuning several kinetic and bed-related parameters.

The wood char reactions in CO2–N2 mixtures and O2–N2 mixtures have been studied in detail in our earlier studies [10,11]. The steam–carbon reaction has been studied by several researchers in the late 1940’s and early 1950’s [12–14] for extracting suitable rate expression. Kinetic expressions of varying complexity have been derived by these researchers [14–17] for steam–carbon reaction at temperatures of 1200 to 1500 K and in an environment of mixtures including CO, CO2, and O2. Satyanarayana and Keairns [15] have conducted experiments on char gasification using CO2 andH2O. They show from the results that the rate constants of the C–H2O reaction are about 2.5–5 times faster than of the C– CO2 reaction.