View Artice Online Chem Soc Rev Review Article Table 2 Comparison of three pyrolysis techniques Process conditions Products Pyrolysis technology Residence time Heating rate Temperature (C) Char (% Bio-oil (% Gases (% Conventional 5-30 min <50C min-1 400-600 <35 <30 <40 Fast pyrolysis <5s ~1000℃s-1 400-600 <25 <75 <20 Flash pyrolysis <0.1s ~1000℃s-1 650-900 <20 <20 <70 Those secondary reactions could take place either inside or 2.1.2.Fast pyrolysis of cellulose.Cellulose is the most outside the biomass particles.The intra-particle vapor-solid extensively studied component in lignocellulose due to its 8S:2t:509126/603 interactions are particularly important for large size particles abundance and the simplicity of its structure.The degree of (>0.5 cm).Thus biomass particle size <2 mm has been crystallinity and the dimensions of the crystallites are the most recommended for maximum bio-oil yield.34.64 important properties related to the stability and reactivity of Earlier efforts to understand the fundamentals of thermal cellulose.Each repeating unit of cellulose has three hydroxyl pyrolysis of lignocellulose were mainly focused on the global groups;those hydroxyl groups form either intramolecular or kinetic modeling development using thermogravimetric and intermolecular hydrogen bonds,which are highly relevant to differential scanning calorimetry techniques with products and the single-chain conformation and stiffness.The intermolecular intermediates lumped according to the phase and molecular hydrogen bonding in cellulose is responsible for the sheet-like weight.65-68 The conversion was defined by the weight loss nature of the native polymer.ss while the products were lumped into char,tar,and gases.The Cellulose,together with cellobiose,a-cyclodextrin,glucose, thermal decomposition behavior of the three main components and levoglucosan,are widely employed in mechanism studies of of lignocellulose,namely,cellulose,43.69-77 hemicellulose,77-1 cellulose fast pyrolysis.4Free radical mechanisms,5900 and lignin,were investigated to decouple the complexity in concerted mechanisms,and ionic mechanisms both chemistry and kinetic models.Generally the three main have been proposed for cellulose pyrolysis.Cellulose transforms components were assumed to decompose independently,and to a liquid before its degradation and then decomposes in two volatiles are evolved from cellulose and hemicellulose while char pathways.One directly leads to certain small molecular pro- is mainly from lignin.23 In most of the reports,process para- ducts such as furan,levoglucosan,glycolaldehyde,and hydroxyl meters such as particle size,heating rate,and pyrolysis temperature acetone,.74while the other pathway forms low-degree oligo- 410 were discussed and optimized to achieve high liquid yields.Here,mers.The low-degree oligomers can further break down to we mainly focus on the development of understanding the chem- form furan,light oxygenates,char,permanent gases,and istry and molecular products of pyrolysis.In thermogravimetric levoglucosan (Fig.3).727 compounds including char have analysis(TGA)studies,it was found that pyrolysis of hemicellulose been identified using GC-MS analysis of pyrolyzed cellulose and and cellulose occurred quickly.462,65.7 Hemicellulose mainly its surrogates.74 The major products are levoglucosan,hydroxy- decomposed at 220-315 C,and cellulose decomposed mainly at acetaldehyde,furfural,formic acid,acetic acid,and aldehyde 315-400C(Fig.2).However,lignin is more difficult to decompose compounds.54,74,108,109 and the weight loss occurred in a wide temperature range Initially,levoglucosan is generated in its liquid form in (160-900C)with generation of high solid residue(Fig.2).57 Next, cellulose pyrolysis and then some of it volatilizes to be a primary we will further summarize the chemistry of the reaction occurring volatile product.It can also undergo condensed-phase secondary during pyrolysis of the individual component in lignocellulose. pyrolysis to fomm pyrans and light oxygenates (Fig.) Various small linear oxygenates have been formed from gradual decomposition of levoglucosan.73,94.112 It is interesting that 3.0 100 hemicellulose levoglucosan itself is relatively stable and does not break down cellulose when pyrolyzed alone.37.13 The secondary decomposition of lignin 2.5 80 levoglucosan was found induced by the pyrolysis vapors from 2.0 cellulose and lignin and inhibited by the xylan-derived vapor 60 1.5 Dehydration and isomerization of levoglucosan lead to the 40 formation of other anhydro-monosaccharides.These anhydro- 1.0 monosaccharides may either re-polymerize to form anhydro- 20 0.5 oligomers or further transform to smaller oxygenates by fragmentation/retro-aldol condensation,dehydration,decarbonyl- 0.0 ation,or decarboxylation. 200 400 600 800 Char is obviously an undesired product in CFP.The secondary Temperature (C) reaction of primary pyrolysis products was found to increase Fig.2 Pyrolysis curves of hemicellulose.cellulose,and lignin from TGA the char yield.5.14 Re-polymerization and secondary pyrolysis (Adapted with permission from Yang et al.Fuel.2007.86.1781-1788.77 of levoglucosan was found to be an important pathway for char Copyright 2007 Elsevier.) formation.11113 Increasing the residence time of volatiles 7598|Chem.Soc.Rev.2014.43.7594-7623 This joumal is The Royal Society of Chemistry 20147598 | Chem. Soc. Rev., 2014, 43, 7594--7623 This journal is © The Royal Society of Chemistry 2014 Those secondary reactions could take place either inside or outside the biomass particles. The intra-particle vapor–solid interactions are particularly important for large size particles (40.5 cm). Thus biomass particle size o2 mm has been recommended for maximum bio-oil yield.54,64 Earlier efforts to understand the fundamentals of thermal pyrolysis of lignocellulose were mainly focused on the global kinetic modeling development using thermogravimetric and differential scanning calorimetry techniques with products and intermediates lumped according to the phase and molecular weight.65–68 The conversion was defined by the weight loss while the products were lumped into char, tar, and gases. The thermal decomposition behavior of the three main components of lignocellulose, namely, cellulose,43,69–77 hemicellulose,77–81 and lignin,43,77,82–91 were investigated to decouple the complexity in both chemistry and kinetic models. Generally the three main components were assumed to decompose independently, and volatiles are evolved from cellulose and hemicellulose while char is mainly from lignin.92,93 In most of the reports, process para￾meters such as particle size, heating rate, and pyrolysis temperature were discussed and optimized to achieve high liquid yields. Here, we mainly focus on the development of understanding the chem￾istry and molecular products of pyrolysis. In thermogravimetric analysis (TGA) studies, it was found that pyrolysis of hemicellulose and cellulose occurred quickly.14,62,65,77 Hemicellulose mainly decomposed at 220–315 1C, and cellulose decomposed mainly at 315–400 1C (Fig. 2). However, lignin is more difficult to decompose and the weight loss occurred in a wide temperature range (160–900 1C) with generation of high solid residue (Fig. 2).65,77 Next, we will further summarize the chemistry of the reaction occurring during pyrolysis of the individual component in lignocellulose. 2.1.2. Fast pyrolysis of cellulose. Cellulose is the most extensively studied component in lignocellulose due to its abundance and the simplicity of its structure. The degree of crystallinity and the dimensions of the crystallites are the most important properties related to the stability and reactivity of cellulose. Each repeating unit of cellulose has three hydroxyl groups; those hydroxyl groups form either intramolecular or intermolecular hydrogen bonds, which are highly relevant to the single-chain conformation and stiffness. The intermolecular hydrogen bonding in cellulose is responsible for the sheet-like nature of the native polymer.58 Cellulose, together with cellobiose, a-cyclodextrin, glucose, and levoglucosan, are widely employed in mechanism studies of cellulose fast pyrolysis.73,74,94–98 Free radical mechanisms,75,99,100 concerted mechanisms,94,101–104 and ionic mechanisms105,106 have been proposed for cellulose pyrolysis. Cellulose transforms to a liquid before its degradation and then decomposes in two pathways. One directly leads to certain small molecular pro￾ducts such as furan, levoglucosan, glycolaldehyde, and hydroxyl acetone,70,74 while the other pathway forms low-degree oligo￾mers. The low-degree oligomers can further break down to form furan, light oxygenates, char, permanent gases, and levoglucosan (Fig. 3).67,107 27 compounds including char have been identified using GC-MS analysis of pyrolyzed cellulose and its surrogates.74 The major products are levoglucosan, hydroxy￾acetaldehyde, furfural, formic acid, acetic acid, and aldehyde compounds.54,74,108,109 Initially, levoglucosan is generated in its liquid form in cellulose pyrolysis and then some of it volatilizes to be a primary volatile product. It can also undergo condensed-phase secondary pyrolysis to form pyrans and light oxygenates (Fig. 4).69,74,107,110,111 Various small linear oxygenates have been formed from gradual decomposition of levoglucosan.73,94,112 It is interesting that levoglucosan itself is relatively stable and does not break down when pyrolyzed alone.67,113 The secondary decomposition of levoglucosan was found induced by the pyrolysis vapors from cellulose and lignin and inhibited by the xylan-derived vapor.98 Dehydration and isomerization of levoglucosan lead to the formation of other anhydro-monosaccharides. These anhydro￾monosaccharides may either re-polymerize to form anhydro￾oligomers or further transform to smaller oxygenates by fragmentation/retro-aldol condensation, dehydration, decarbonyl￾ation, or decarboxylation.69 Char is obviously an undesired product in CFP. The secondary reaction of primary pyrolysis products was found to increase the char yield.95,114 Re-polymerization and secondary pyrolysis of levoglucosan was found to be an important pathway for char formation.111,113 Increasing the residence time of volatiles Table 2 Comparison of three pyrolysis techniques Pyrolysis technology Process conditions Products Residence time Heating rate Temperature (1C) Char (%) Bio-oil (%) Gases (%) Conventional 5–30 min o50 1C min1 400–600 o35 o30 o40 Fast pyrolysis o5 s B1000 1C s1 400–600 o25 o75 o20 Flash pyrolysis o0.1 s B1000 1C s1 650–900 o20 o20 o70 Fig. 2 Pyrolysis curves of hemicellulose, cellulose, and lignin from TGA. (Adapted with permission from Yang et al., Fuel, 2007, 86, 1781–1788.77 Copyright 2007 Elsevier.) Chem Soc Rev Review Article Published on 07 May 2014. Downloaded by Shanghai Jiaotong University on 18/02/2016 07:32:58. View Article Online