Torrefaction and HZSM-5 catalyst combination improving pyrolytic products of cedarwood

Abstract: Fast pyrolysis of biomass is a promising technology for bio-energy because of its high liquid yield and low cost. The liquid product of fast pyrolysis is called bio-oil, which is complex mixtures of water and various organic compounds. Bio-oil has many advantages such as little sulfur and nitrogen content, highly potential values, readily stored and transported properties. However, the high acidity, low heating values and high oxygen and water contents in bio-oil limit its broaden applications. Catalytic pyrolysis and catalytic upgrading of pyrolysis vapors have been used to improve the quality of bio-oil. However, there is a crucial challenge: Rapid deactivation of catalyst caused by serious coking. The relevant studies have shown that catalyst deactivation has a direct relationship with the high content of oxygenated compounds in pyrolysis volatile caused by high oxygen content in biomass. In addition, thermal pretreatment of biomass, torrefaction, has been shown to improve the quality of bio-oil by lowering the oxygen content and enhancing the aromatic yield. So, in this paper, the couple effects of torrefaction and catalytic pyrolysis on characteristics of pyrolytic products of cedarwood have been investigated. Pretreament of cedarwood via torrefaction was performed in a tube furnace at varying reaction temperature(200, 230, 260, 290℃) with a residence time of 30min. The torrefied cedarwood were characterized by elemental analysis and proximate analysis. The results showed that increasing torrefaction temperature caused the increase of carbon content from 48.16% to 54.3%, the oxygen content decreased from 44.33% to 35.65%. And the torrefied cedarwood product has a brown/black color, reduced volatile content and increased energy density: 21.25 MJ/kg (after 30 min reaction time at 290℃) versus 18.13MJ/kg for untreated cedarwood. Then the torrefied cedarwood were subsequently catalytically fast pyrolyzed over HZSM-5 in a vertical tubular reactor at 550℃ with a residence time of 30 min. The gas products of pyrolysis were analyzed by chromatograph(GC), and liquid products were analyzed by gas chromatography mass spectrometry (GC-MS). Torrefaction caused deacetylation and decomposition of hemicellulose, cleavage of ester linkages and demethoxylation of lignin. And the experimental results show that after coupled torrefaction and catalytic pyrolysis, increasing the torrefaction temperature caused the coke yield decreased; the content of CO in gas product decreased from 53.69% to 40.84%, the content of H2 and CO2 increased by 43.1% and 35.04% respectively, and the content of CH4 had no obvious change. As for liquid products, with the increasing of torrefaction temperature, the content of acid significantly deceased, aromatic yield increased , and enrichment of phenols. The result indicated that the couple effects between torrefaction and catalytic pyrolysis are very important for upgrading of bio-oil. However, severe torrefaction can lead the coke yield increased and aromatic yield reduced. So the optimal torrefaction condition of cedarwood is 230-260℃.