The effect of heating rates between 1 and 1000 K/s on coal pyrolysis in helium at 1.2 bar has been examined in a wire-mesh pyrolysis apparatus. The apparatus used a microcomputer for feedback temperature control and had a sweep gas flow through the sample holder to give rapid product removal. A significant increase in total volatile yields between 1 and 1000 K/s was found for three of the four coals tested, provided that sufficient time was allowed at the peak temperature for reactions to run to completion. For Pittsburgh No. 8, the only coal for which tar measurements were made, the increase in total volatiles could mostly be attributed to an increase in tar yield. Experiments under vacuum confirmed the effect of heating rate observed in helium, suggesting that the lower volatile yield observed at 1 K/s is not an artifact of external mass-transfer resistance.
IntroductionWhen early wire-mesh reactors were found to give greater volatile yields than traditional pyrolysis retorts (e.g. Fischer Assay, Gray-King) it was not immediately apparent whether this was entirely as a result of a reduction in secondary, char-forming reactions or, a t least in part, an effect of the faster heating rates used. In the wire-mesh experiments (probably the best-known being by Howard and associates' at MIT), products had only a thin layer of pyrolyzing coal to pass through and were quenched almost immediately on exit. This gave relatively little opportunity for secondary reactions compared to a retort, in which volatiles have to traverse a relatively large bed of coal and then the heated zone beyond before being cooled.A direct test of the effect of heating rate in the wiremesh reactor was not possible at the time. The MIT apparatus used a pulse of direct current from a bank of batteries to heat the sample holder, and the lowest heating rate that could be obtained was about 100 K/s. Slow heating experiments in this apparatus might also have been affected by secondary reactions, because volatile products were allowed to circulate freely in the same vessel as the sample holder and significant cracking and char deposition could have taken place if the wire-mesh was hot for long periods. Both of these problems associated with extending wire-mesh operation to low heating rates have been ad-
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