Effect of Recycle of Non-Condensable Pyrolysis Gases on Fractional Catalytic Pyrolysis of Biomass

摘要

Catalytic fast pyrolysis technology is one of the thermochemical platforms that can be used to produce high quality bio-oil and chemicals from biomass feedstocks (Adjaye and Bakhshi, 1995; Agblevor et al., 2010a; Williams and Horne, 1995). The process involves the rapid heating of biomass under inert atmosphere in the presence of a suitable catalyst. The technology has been extensively reviewed in the literature by Huber and Corma (2007), Park et al. (2011) and Taarning et al. (2011). In catalytic pyrolysis, a large fraction of the biomass is converted into non-condensable gases (NCG) at the expense of the liquid product. The gas product yield obtained on a dry biomass feed is about 20–50% depending on the type of catalyst used (Agblevor et al., 2010a; Lappas et al., 2009; Mante et al., 2011; Williams and Horne, 1995; Zhang et al., 2009). It mainly consists of carbon oxides, methane, hydrogen and minor hydrocarbons (Huber and Corma, 2007; Stefanidis et al., 2011; Williams and Horne, 1994). These gases can be recycled into the reactor to assist in fluidization or could be combusted to provide process heat for large scale operations. The concept of recycling NCG can help reduce operational cost and eliminate further downstream processing of the gas stream. Studies have shown that with efficient condensation systems and compressors, the NCG can be effectively recycled to replace the initial inert fluidizing gas (Agblevor et al., 2010b; Bridgwater, 1999). Theoretically, recycling the NCG for fluidization could possibly influence the catalytic pyrolysis of biomass as these gases are capable of acting as co-reactants in the process. Studies have shown that the reducing gas stream (CO, H2, and hydrocarbons) and mild oxidative gas (CO2) affect the thermal decomposition of biomass (Butterman and Castaldi, 2009a; Butterman and Castaldi, 2007; Butterman and Castaldi, 2009b; Carlson et al., 2009; Jindarom et al., 2007; Meesuk et al., 2011; Scahill and Diebold, 1988; Thangalazhy-Gopakumar et al., 2011; Zhang et al., 2011). Zhang et al. (2011) investigated the effect of N2, CO2, CO, CH4 and H2 as carrier gases on the fast pyrolysis of corncob in a fluidized bed reactor. They reported that the liquid yield was dependent on the type of gas used and more oxygen was converted to CO2 and H2O in the presence of CO and H2. Meesuk et al. (2011) also reported that lower liquid yields were achieved in H2 atmosphere from the fast pyrolysis of rice husk in a fluidized bed reactor with or without CoMo/Al2O3 and Ni/LY catalyst. A study by Thangalazhy-Gopakumar et al. (2011) also reported a reduction in carbon yield when the thermogravimetric studies of pine wood chips under helium was changed to H2. However, they did not observe any significant change in the yield when the experiment was conducted with a ZSM-5 zeolite catalyst. Studies by Carlson et al. (2011) on the production of green aromatics and olefins from wood sawdust via catalytic fast pyrolysis using ZSM-5 zeolite showed that co-feeding light hydrocarbons (C2–C3) with wood increased gasoline yields. An earlier work by Scahill and Diebold (1988) also explored the effect of recycling olefins over HZSM-5 zeolite catalyst in the upgrading of pyrolysis vapors from wood. Most of the literature reports are on only the effect of individual gases on the pyrolysis of biomass. Currently, there are no reported findings on the influence of recycling the pyrolysis gas on catalytic pyrolysis of biomass. In this study, the effect of recycling the NCG from the fractional catalytic pyrolysis of hybrid poplar wood using FCC catalyst on the product yields and properties of the bio-oils was investigated.