THE CHARACTERISTICS OF COAL CHAR GASIFICATION AT AROUND ASH MELTING TEMPERATURE

The high-temperature CO2 gasification (1173-1773 K) of coal char was investigated by using TGA and SEM and by measuring inner surface area concerning four different coals having much different ash melting temperatures. In regard to the unreacted char, the surface areas of micropores (diameter < 0.01 mum) and mesopore (diameter 0.01-0.1 mum) decreased with increasing temperature but the macropore(diameter > 0.1 mum) surface are a did not. The true gasification rate constant was evaluated based on effectiveness factor prediction. The activation energy thus determined was 399 kJ.mol-1 which is 1.88 times as large as the apparent activation energy value obtained without the correction of effectiveness factor. When carbon conversion was greater than 50%, an unusual reduction in the reaction rate from values expected from the low-temperature gasification rate took place over certain temperature range, in the region of +/-200 K of the maximium reduction temperature T(X) for each char. Since T(X) was about 150 K below the ash softening temperature, ST, of each coal, the rate reduction can be explained by ash melting. The macropore plugging by molten ashes of high viscosity was found at around the temperature T(X). Ash-carbon reactions of char which would help pores to grow were investigated by the product gas analysis and found to become significant above the temperature where complete ash melting takes place.