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

Abstract

The release of HCN, NH3, 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 NH3 yields during coal pyrolysis are probably the sum of the yields of NH3 and HNCO. Both HCN and NH3 start to appear from above 700 °C. While NH3 reaches a maximum at 850 °C, HCN continues to increase at higher temperatures. It is suggested that NH3 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 106 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.