RELEASE OF HCN, NH3, AND HNCO FROM THE THERMAL GAS-PHASE CRACKING OF COAL PYROLYSIS TARS

The release of HCN, NH3, and HNCO from the thermal cracking of coal ta rs produced by rapid pyrolysis has been investigated using a quartz fl uidized-bed reactor coupled to a quartz tubular-flow reactor. Primary pyrolysis at 600 degrees C in the fluidized-bed reactor generated the tars which were subsequently thermally decomposed in the tubular react or in the temperature range of 600-1000 degrees C. HNCO was the initia l gaseous N-containing species to be evolved, its formation commencing from 600 degrees C. HNCO was found to be a significant N-containing p roduct of tar cracking and some previous measurements of NH3 yields du ring coal pyrolysis are probably the sum of the yields of NH3 and HNCO . Both HCN and NH3 start to appear from above 700 degrees C. While NH3 reaches a maximum at 850 degrees C, HCN continues to increase at high er temperatures. It is suggested that NH3 may be formed from the inter actions of N-containing species with. donatable H on the soot surface. FTIR analyses of the tars demonstrate that increases in the temperatu re 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 sugge sts that nitrogen release during tar cracking is a complex process.