Table 3 reports the results of SSA measurements performed with the fresh samples and with the samples after devolatilisation. The char’s SSA are much larger than that of the correspondent parent materials especially in the case of cocoa shells for which SSA increases of two order of magnitude (Table 3) while for the other single samples and blends the increase is only of one order of magnitude. However, the SSA’s of the blend’s char are, normally, the weighted average of those of the correspondent parent coal and biomass chars (Table 3).
With the uncharred samples the measured SSA’s of the blends are quite lower than that calculated on the basis of a weighted average contribution of the single components in the mixtures. In contrast, the opposite was observed with the chars of the same blends (Table 3) either when subjected to relatively short charring times (Is) or when held in the HTWM reactor for longer times (4s).
The evolution of SSA during sample bum off is shown in Fig. 5 where the measured SSA is reported as a function of carbon conversion (X) in the case of cocoa shells char and of the cocoa shells – coal К blend char. It appears that the specific surface area of cocoa shells char increases of a factor 3 as the carbon conversion changes from 0 to 60% with an almost constant slope. The specific surface area of the blend char, instead, increases much more (about 8 times) as X changes from 0 to 70%: This increase is very strong at the beginning of the combustion process (0<X<10%) then raises less markedly up to X=70%.
Fig. 5. Specific surface area of cocoa shells char and of a char of the blend coal K-cocoa shells as a function of
the sample conversion (d. a.f. b.).