Coal flash pyrolysis: secondary cracking of tar vapours in the range 870–2000 K

Abstract

Tar free from influences of the original coal or char was cracked in two reaction systems, one using tar vapour in tubular quartz reactors at 900–1400 K and reactor residence times of ≈ 0.2 and ≈ 1 s, the other using tar aerosol in a shock tube at 1100–2000 K and residence time ≈ 1 ms. In the latter system the aerosol evaporated rapidly after passage of the shock front and the tar behaved kinetically as a vapour. Yields of light hydrocarbons including C 6H 6 and CO from tar cracking were determined as a function of temperature. Maximum yields of individual species agreed well between the two reactors but the temperatures of maximum yield depended on residence time. Hydrocarbon yields on a daf coal basis were very similar to those obtained previously by direct flash pyrolysis of the same coal at 870–1270 K. Kinetic analyses of the formation of C 2H 4, C 3H 6 and C 2H 2 gave activation energies of 220–260 kJ mol −1, similar to those for formation of the same hydrocarbons from n-hexadecane. The precursors of the alkenes could be polymethylene groups in the tar. CH 4, C 6H 6 and CO all had low activation energies of formation, reflecting the many different functional groups capable of eliminating these molecules.