Experimental comparison of two bench scale units for fast and intermediate pyrolysis

Abstract

Comparison of different literature results for fast pyrolysis of biomass is difficult due to the variety in feedstock, process conditions, analyses, and detail of the provided data. The presented experiments represent a concerted effort to directly compare two bench scale units (10kgh−1) for fast and intermediate pyrolysis on the basis of the same set of biomass feedstock and the same pyrolysis reactor temperature. Hybrid poplar, wheat straw, a blend (hybrid poplar/wheat straw/forest thinnings) and beech wood have been used as feedstocks. Organic liquid yields for intermediate pyrolysis are 17–21 % and that of fast pyrolysis 26–51 %. The advantage of higher organic liquid yields from fast pyrolysis decreases with the ash content of biomass due to catalytically active minerals. It is concluded that the importance of the primary pyrolysis temperature level and the hot vapour residence time is significantly reduced with increasing ash content of the biomass. Carbon balances show that ash is not only increasing the total char yield as inorganic material, but also leads to a higher fraction of carbon material being retained in the char. Analyses of the liquid fractions reveal that pyrolytic degradation appears generally more evolved for the case of intermediate pyrolysis. Moreover, it was observed that there is a correlation between the ratio of CO to CO2 in the permanent gas after pyrolysis and the yield of organic liquids (dry basis). This correlation differs between the two processes and is suitable to predict the organic liquid yield based on permanent gas measurements. Three of the four feedstocks were supplied as part of a Round Robin Test organized by International Agency Bioenergy Task 34 ‘Direct thermal liquefaction’ and the obtained results can be directly compared even beyond the scope of this work.