Pyrolysis of Collie Coal Briquettes To Produce Char as a Metallurgical Reductant

摘要

Western Australia lacks coking coal but has a substantial reserve of high-moisture sub-bituminous coal of noncoking nature in the Collie basin. To diversify Collie coal uses, one desirable option is to briquette the coal followed by pyrolysis to make char as a metallurgical reductant, thus extending the applications of Collie coal to the metallurgical industry. This study investigates the pyrolysis behavior of Collie coal briquette and determines the optimal pyrolysis conditions to produce char from Collie coal briquette as a potential reductant for metallurgical applications. Collie coal briquettes formed using a patented binderless briquetting process were pyrolyzed in a laboratory furnace at temperatures ranging from 500 to 1400℃ with a heating rate of 10℃ min~(-1) and a holding time of 1 h at the peak pyrolysis temperatures. The corresponding briquette fines were also pyrolyzed to study the importance of transport effect during pyrolysis of the large briquettes. The pyrolysis time needed for complete devolatilization was found to be well less than the 1 h holding time employed. As expected, the briquette char yield and reactivity decrease with an increasing pyrolysis temperature. The mechanical strength of the char initially increases with the pyrolysis temperature, levels off at around 1000℃, and then decreases with a further temperature increase. Dependent upon the pyrolysis temperature, briquette chars generally have low contents of ash (< 13%), sulfur (< 0.6%), and reabsorbed equilibrium moisture (< 8%), as well as a volatile matter content of ～0.4—16.9%. The compressive strength of the chars varies from 6 to 15 MPa, and the reactivity index toward CO_2 is in the range of 14.5-38.8% (ECE-INCAR reactivity test). The experimental results indicate that chars produced from Collie coal briquettes are potentially suitable for metallurgical applications. These chars broadly satisfy the requirements of the moisture content (<12%), ash content (<15%), sulfur content (<1.5%), volatile matter content (<2%), mechanical strength (6.9-30 MPa), and reactivity index (around 2.6—28%). On the basis of the considerations for char productivity and properties, the optimal pyrolysis temperatures for Collie coal briquettes to produce char as metallurgical reductants appear to be 800-900℃.