During gasification reaction, the ash will be elutriated out the upper part of the bed, however most of the char will be located in the bed surface of the upper part of the bed due to its reduced density and size from the initial condition. The gasification reactivity of the char is less than that of the coal, while the combustion reactivity of the char and the coal have similar values. For the higher carbon conversion, the char recirculation will increased the gasification performance.
In this study, the gasification characteristics is executed when the be surface materials is recirculated into the bottom of the fluidized bed gasifier, which has two flow regimes. The char will be oxidized instead of coal, and most of coal will be pyrolyzed and gasified instead of combustion. Therefore, the gas yield and the carbon conversion will be increased.
Experimental works of coal gasification were carried out in a fluidized bed with 0.05 m ID and 0.4 m height in lower reaction part, and 0.12 m ID and 0.7 m height in upper reaction part.
Coal was fed into the bottom of the gasifier by the screw feeder up to 12 kg/h. The range of operating temperature and pressure are 800 ~ 850 °C and 1 ~ 6 bar, respectively. The gas mixture of O2 and N2 for adjusting the O2/C ratio with maintaining specific superficial gas velocity was introduced into bottom of the gasifier. Steam was also fed into the reactor considering H2O/C ratio as well as controlling the reactor temperature. The proximate analysis of the Kideco coal is volatile matter 38.14%, fixed carbon 39.99%, moisture 10.00%, and ash 11.87%. The ultimate analysis of C, H, N, S and O, is 64.59%, 4.83%, 1.51%, 0.48% and 15.40%, respectively.