Char particle gasification

The gas, along with entrained ash and char particles, which are subjected to further gasification in the large space above the fluid bed, exit the gasifier at 954—1010°C. The hot gas is passed through a waste-heat boiler to recover the sensible heat, and then through a dry cyclone. SoHd particles are removed in both units. The gas is further cooled and cleaned by wet scmbbing, and if required, an electrostatic precipitator is included in the gas-treatment stream. 

Groeneveld M.J. and Swaaij W.P.M., Gasification of Char Particles with CO2... 

By making use of the physical structure and chemical rate expressions described above it is possible to formulate mass and energy balances for the gasification of a single porous char particle. The mass balances for the diffusing species take the form ... 

Since a char particle typically contains < 2% Ca (w/w), while the char surface is > 200 m /g, the Thiele moduli for the calcium reactions are likely to be much smaller than those associated with char gasification even when the turnover numbers for the reactions are of the same order of magnitude. Thus, we will assume that the sulfur reactions are kinetically controlled while the gasification is diffusion limited. In that case HjS and 00S concentrations... 

Because coal chars are highly microporous, most of the gasification reactions take place inside the char particles. Therefore, diffusion of gas into, and products out of, porous particles is required. The overall diffusion process can be described by the following steps (1) diffusion of the reactant from the bulk gas to the solid surface (film diffusion) (2) diffusion of the reactant from the particle s surface to its interior (internal diffusion) (3) diffusion of the product from the interior to the particle s surface (internal diffusion) and (4) diffusion of the product from the surface to the bulk gas (film diffusion). 

Fig 10. Arrhenius diagram for combustion and C02-gasification of straw char in comparison to other chars (thermobalance data for a char particle size of 50 ... 

Most of the particular difficulties of straw gasification are caused by the high K and Cl-content. The behaviour of these impurities in the successive process steps is therefore of special importance. The selection of a method for their removal is a major process decision. Chlorine volatilisation starts at relatively low pyrolysis temperatures of about 200 C. About half of the chlorine can be removed into the pyrolysis gas up to about 500°C. The rest of the HCl is volatilised together with the potassium at higher temperatures. At lower pyrolysis temperature, K can be kept completely within the char particles together with the residual ash, but the chlorine distributes between char and gas. 

M.J. Groeneveld, W.P.M. van Swaaij, Gasification of char particles with CO2 and H2O, Sixth International Symposium on chemical reaction engineering, Nice, 1980. 

In order to produce activated carbons, it is necessary that the steam gasification progresses under chemical control to allow internal porosity to develop inside the char particles. 

Attempts have been made to predict gasification rates using mathematical models. This area has been briefly reviewed by Rafsanjani et al. (2002) who discuss the use of (what are termed) the grain model, the random pore model, the simple particle model and the volume reaction model. They report that differential mass conservation equations are required for the oxidant gas and char particle. These authors use a simplified mathematical model (the quantise method (QM)) for activation of coal chars when both diffusion and kinetic effects have to be considered. Results are compared with other methods when it is found that QM predictions of rate are more accurate than predictions by the random pore model and the simple particle model. 

Most of the fluidized bed coal combustion and gasification models ignore freeboard reactions of volatiles and char For shallow beds, volatiles bum predominantly in the freeboard In addition the char particles splashing from the bed surface can also react with the oxygen in the freeboard Yates and Rowe (ll8) have proposed a model for the reactions occurring in the freeboard Such an approach can be adopted for modelling the freeboard reactions in the FBC ... 

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... 

Dasappa, S., Paul, P. J., Mukunda, H. S., and Shrinivasa, U., Wood-Char Gasification Experiments and Analysis on Single Particles and Packed Beds, 27th International Symposium on Combustion, The Combustion Institute, 1998, pp 1335-1342. 

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