Promotion of Cross-Linking and Resulting Suppression of Tar Evolution in Potassium-Catalyzed Pyrolysis of Woody Biomass

Acid-washed woody biomass (cedar) was impregnated with K2CO3 and then pyrolyzed at 550 °C in a fixed bed reactor, in which extra-particle pyrolysis of volatile matter was well suppressed. The tar yield decreased from 0.42 to 0.034 kg/kg-daf with increasing K loading (mK,0) within the range of 0–5.0 mol K/kg-daf. The K-catalyzed condensation by dehydration, dehydrogenation, and dealkylation reactions formed H2O, H2, and gaseous hydrocarbons (GHCs), respectively, producing intermonomer-unit cross-links (MUCs) and thereby decreasing the yield of tar as monomers and oligomers. According to the conversion of K2CO3 (34–57%), three catalytic cycles were estimated with the conversion/regeneration of K2CO3, KOH, and alkoxides/phenoxides, formation of inter-MUCs and that of the above-mentioned gases. The relationship between the amount of inter-MUCs formed during the pyrolysis (assumed to be 30% of that of H2O, H2, and GHCs on a molar basis) and reduction in the tar yield was considered by applying the Bethe lattice model, which is often employed for analyzing degradation and repolymerization of polymer, coal, and biomass. The model described the above relationship semiquantitatively. The efficiency of the K loading for the tar reduction, represented by the derivative of the tar yield (Ytar) with respect to mK,0, i.e., dYtar/dmK,0, was decreased by a factor of about 340 while mK,0 increased from 0 to 5.0 mol K/kg daf. Such a large factor was explained by decreases in the two derivatives, dYtar/dYinter-MUC (Yinter-MUC; the amount of inter-MUCs) and dYinter-MUC/dmK,0, quantitatively. The decrease in the dYinter-MUC/dmK,0 (factor ≈ 30) was mainly due to the depletion of functional groups that underwent condensation reactions while that in the dYtar/dYinter-MUC (factor ≈ 11.6) arose from the nature of network polymer such as approximation by the Bethe lattice.