Release of HCN, NH₃, and HNCO from the Thermal Gas-Phase Cracking of Coal Pyrolysis Tars

The release of HCN, NH 3, and HNCO from the thermal cracking of coal tars produced by rapid pyrolysis has been investigated using a quartz fluidized-bed reactor coupled to a quartz tubular-flow reactor. Primary pyrolysis at 600 °C in the fluidized-bed reactor generated the tars which were subsequently thermally decomposed in the tubular reactor in the temperature range of 600−1000 °C. HNCO was the initial gaseous N-containing species to be evolved, its formation commencing from 600 °C. HNCO was found to be a significant N-containing product of tar cracking and some previous measurements of NH 3 yields during coal pyrolysis are probably the sum of the yields of NH 3 and HNCO. Both HCN and NH 3 start to appear from above 700 °C. While NH 3 reaches a maximum at 850 °C, HCN continues to increase at higher temperatures. It is suggested that NH 3 may be formed from the interactions of N-containing species with donatable H on the soot surface. FTIR analyses of the tars demonstrate that increases in the temperature of pyrolysis result in a decrease in aromatic substitution. Kinetic parameters for the release of tar-N species as HCN were determined by measurement of HCN yields and by assuming that the reaction was first order in tar-N. An overall global rate expression of 10 6 exp[−(140 ± 15)/ RT ] s - 1 was derived from the data. The rate expression suggests that nitrogen release during tar cracking is a complex process.