Effect of Torrefaction on Biomass Structure and Product Distribution from Fast Pyrolysis

Abstract

Torrefaction, a thermal pretreatment process, has been documented to improve the chemical composition of bio-oil produced from fast pyrolysis process. During torrefaction pretreatment, the major constituents of biomass (cellulose; hemicellulose and lignin) undergo various structural and chemical changes that can affect the reaction pathways during fast pyrolysis process and favor the production of certain compounds. The main focus of this study was to understand biomass torrefaction chemistry and determine how it subsequently affects the product distribution from non-catalytic and H+ZSM-5 catalyzed fast pyrolysis. Samples were torrefied at three temperatures (225, 250 and 275 oC) and for three residence times (15, 30 and 45 min), for a total of nine treatments. Loblolly pinewood was used for the study due to its abundant availability in the southeast United States. The structural transformations in the biomass constitutive polymers were evaluated using component analysis, solid state CP/MAS 13C NMR and XRD techniques. Component analysis was carried out to quantify the weight percentage of cellulose, hemicellulose and lignin degradation at different torrefaction severity while, XRD and 13C NMR were used to quantify biomass structural changes like cellulose crystallinities and fractions of carbonyl, aromatic, alkyl, ether and methoxyl carbons in torrefied samples. Torrefaction caused degradation (starting at 225oC-30min) and deacetylation (starting at 225oC-15min) of hemicellulose components. Initial wt. loss of 23% in lignin was observed at 225oC-30min due to de-methoxylation and de-etherification of lignin. Cellulose degradation occurred at higher torrefaction severity (225oC - 45min, 250oC - 30 and 45 min, and 275oC -15, 30 and 45 min) and was accompanied by overall increase in aromaticity of biomass. Py-GC/MS study was carried out to study the chemical composition of pyrolyic vapor from raw and torrefied samples. For non-catalytic pyrolysis, selectivity of aromatic hydrocarbon (HC) and phenolic compounds increased with increase in torrefaction severity while that of furan compounds decreased. These were attributed to increase in aromaticity of biomass, changes in structure of lignin and degradation of hemicellulose, respectively. In case of catalytic pyrolysis, the samples torrefied at 225oC-30min and 250oC-15min resulted in significant increase in aromatic HC and also total carbon yield (approx. 1.6 times higher) as compared to catalytic pyrolysis of raw pine. De-etherification and de-methoxylation of lignin occurred at these torrefaction conditions causing increased yield of phenolic compounds, which in presence of catalyst were dehydrated to form aromatic HC. Aromatic HC yield from catatytic pyrolysis was also found to be directly proportional to wt.% of cellulose and inversely to aromaticity of torrefied biomass.