By G.R. Gavalas
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Extra info for Coal Pyrolysis (Coal Science & Technology)
At the temperature of pyrolysis it is produced as a vapor but at room temperature it becomes a viscous liquid or solid. The tar liquids consist largely of dimers of smaller fragments generated by the primary bond dissociation reactions. In an ideal experimental setup, once released from the coal particles the tar molecules are removed from the high temperature region escaping secondary reactions. The captive sample and entrained flow techniques approximate this desirable operation. 9 shows the cumulative tar yield at two pyrolysis times, 20s and 80s, as a function of temperature.
Unfortunately, the relative addition rates at positions like (a) and (a~~) have not yet been measured. In coal pyrolysis, reactions of the type (a) can be the source of various hydrocarbon gases via the corresponding radicals. For example the CH ' radical 3 produced in (a) will evolve as methane after hydrogen abstraction from some suitable site. Reaction (b) is important as a low activation energy route to the dissociation of methylene bridges. 2 refer to addition to the benzene ring y U y + x· (}x (Ai) where X = H,CH 3 ,C H (i=1,2,3) and Y is H or an alkyl chain or bridge.
In both cases the rate determining step was the dissociation of a benzylic hydrogen. Assuming equal A-factors, the activation energy in the ortho-compound must be lower than that of the meta-compound by about 2 kcal/g-mol. All the above calculations refer to a temperature of 300 oK. To calculate the heats of reaction at higher temperatures, ego 800 oK, requires the pertinent heat capacities. These can be computed by group additivity using the data of reference 30. 2 above. 26 cal/g-mol oK. 03. 08.