Char reaction zone

To approach the analysis of, and to be able to comprehend, the complex phenomena of thermochemical conversion of solid fuels some idealization has to be made. For a simplified one-dimensional analysis, there is an analogy between gas-phase combustion and thermochemichal conversion of solid fuels, which is illustrated in Figure 41. Both the gas-phase combustion and the thermochemical conversion is governed by a exothermic reaction which causes a propagating reaction front to move towards the gas fuel and solid fuel, respectively. However, there are also some major differences between the conversion zone and the combustion zone. The conversion front is defined by the thermochemical process closest to the preheat zone, which is not necessarily the char combustion zone, whereas for the flame front is defined by the ignition front. In practice, many times the conversion zone is so thin that the ignition front and the conversion front can not be separated. 

The heat accumulation in the bed surface layer causes the ignition of the char combustion process. The heat is supplied from the over-fire process (see Figure 58C). When the char combustion process commenced, the macroscopic ignition front sustains itself with heat from the exothemic oxidation reactions. Large amounts of the heat released by the char combustion zone are also conducted and radiated away both upwards and downwards in the bed. The downward propagation rate of the macroscopic ignition front is controlled by several factors, such as biofuel moisture content, primary air rate and air temperature [33]. The temperature of the macroscopic propagating char combustion zone is around 1000-1200°C in batch-bed combustion of solid biofuels [38,41]. 

It is feasible to operate with coking coals when a relative high burden of char is maintained in the reaction zone. 

In the char reduction zone, the pyrolysed pellets are partially transformed to gases through the gasification reactions. The remaining pellets (inert char) and the ashes leave the bed through the grate. This can be expressed as ... 

The temperature profiles also show that the temperature below the pyrolysis front is always above 800 °C, i.e. there is no inert char zone. This can mean that the length of the char reduction zone is too short and therefore the gasification reaction is uncompleted. 

Liquid yields of 35-50% on dry feed are typically obtained with higher char yields than fast pyrolysis systems. Conversely, the liquid yields are higher than in slow pyrolysis technologies because of fast removal of vapours from the reaction zone,... 

Figure 4. Scanning electron micrograph of nylon 6/5wt% layered silicate nanocomposite after the ablation test CL char layer RZ porous reaction zone VM virgin material...

The importance of these concepts can be illustrated by the extent to which the pyrolysis reactions contribute to gas produdion. In a moving-bed gasifier (e.g., producer-gas gasifier), the particle is heated through several distinct thermal zones. At the initial heat-up zone, coal carbonization or devolatilization dominates. In the successively hotter zones, char devolatihzation, char gasification, and fixed carbon... 

Kojima, T., Yoshitake, H., Kimura, T., Matsukata, M., and Uemiya, S., Contribution of Local Reactions in the Grid Zone to the Performance of a Jetting Fluidized Bed Gasifier of Coal Char, Energy Fuels, 9 379 (1995)... 

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