Markedly Different Decomposition Temperature and Products of Biomass Pyrolysis at Low Temperature—Differentiation of Acids in Their Effects on Pretreatment

Pine as a softwood and poplar as a hardwood pretreated with hydrochloric acid (HCl), phosphoric acid (H3PO4), and hypophosphorous acid (H3PO2) are studied for the pyrolytic properties and products in thermogravimetry (TG) and fixed bed reactor. The pyrolysis performances are pronouncedly distinguished due to the compositional and structural changes induced by the acid pretreatments. Reduction in the mineral content in the biomass feedstocks by pretreatment with the acids results in significant changes in the pyrolytic products. The residual P in the H3PO2-pretreated biomass apparently catalyzed the biomass deeper dehydration in pyrolysis compared to the other two mineral acids. TG analysis shows a shift of the temperature of maximum mass loss (Tmax) by more than 40 °C to lower temperature in the decomposition of the H3PO2-pretreated biomass from that of the untreated and the HCl- and H3PO4-pretreated biomass. Inspired by the striking differences in TG profiles of biomass pretreated by the three acids, thermal pyrolysis of pretreated biomass was carried out in a fixed bed reactor aimed at producing biochemicals at low temperatures (330 °C and 400 °C). The liquid products obtained from the fixed bed reactor show remarkably different major anhydrosugars as a result of pretreatment by the three acids. While phenolics dominate in the collected pyrolysis liquid from untreated biomass samples, biomass pretreated with all three acids results in substantially reduced phenolics in the bio-oils. The reduction in phenolic compounds in the bio-oil may be attributed to the reduction in mineral content in the feedstock. Consequently, the yields of anhydrosugars, mainly levoglucosan (LG) and levoglucosenone (LGO) are increased. LG yields of 20.9–28.5% from the cellulose content are obtained from HCl- and H3PO4-pretreated pine/poplar, with very low LGO yield (less than 1.7%). However, H3PO2-pretreated biomass is selective to produce LGO, especially at 330 °C. LGO yields of 7.4% and 6.7% are obtained from H3PO2-pretreated pine and poplar, respectively.