Secondary char formation in the catalytic pyrolysis of biomass

Samples of four types of wood and pure cellulose, both untreated and impregnated with salt (Na₂CO₃, K₂CO₃, NaCl, KC1), were pyrolysed. Two experimental systems with different geometries and secondary reaction patterns were used, viz. a McBain thermogravimetric balance and a Gray-King retort. The substrates were pyrolysed under a stream of nitrogen in the thermobalance and in some of the Gray-King runs, using a modified retort. Salt impregnation was found to modify weight loss rates and to increase the charcoal yield in the presence of an inert carrier gas in both experimental systems. Longer residence times of volatiles in the hot zone gave rise to larger charcoal yields from untreated substrates. However, long residence times of volatile matter over Na₂CO₃-impregnated cellulose were found to be detrimental to char formation. These results indicate that primary volatiles may undergo secondary reactions through competitive pathways, either polymerizing to form char or cracking to form lighter volatiles. Long residence times of light volatiles appear to enhance the latter pathway in the presence of Na₂CO₃.