Isothermal partial oxidative pyrolysis mechanisms of solid particles from biomass gasification
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摘要: 在固定床反应器中考察了不同气氛下PBG恒温热解特性的差异,结合XPS与13C NMR等技术手段分析了400 ℃恒温热解条件下PBG固相产物的化学结构变化。结果表明,PBG在400 ℃恒温热解时,生物质气化燃气(BAG)与N2气氛下更易生成焦油,其析出量分别为50.71与37.45 mg/g,而临氧燃气气氛(BAG+2% O2)下焦油析出量仅为11.96 mg/g,说明适量O2的存在可有效抑制焦油的生成。进一步进行化学结构分析表明,在燃气(BAG)恒温热解条件下,PBG主要发生以脱氢脱氧为主的芳香化缩聚反应,易形成焦油类的大分子多环芳烃;而在临氧燃气(BAG+2% O2)恒温热解条件下,PBG表面的有机基团易与O2发生表面氧化反应,生成表面含氧官能团,在一定程度上抑制了芳香环缩聚反应,进而有利于降低焦油类物质的产率。因此,在生物质气化燃气实际高温过滤过程中适当添加氧(如:2% O2),可有效降低PBG焦油收率,且不会形成大分子多环芳烃,有助于解决粗燃气过滤的过滤介质堵塞问题。Abstract: Isothermal pyrolytic characteristics of PBG at 400℃ under different reaction atmospheres in a horizontal tubular quartz reactor were investigated and compared. Meanwhile, chemical structures of PBG and its pyrolysis solid products were also analyzed with the help of XPS and 13C NMR methods. The results indicate that tar yields derived from PBG pyrolysis are 50.71 and 37.45 mg/g under biomass air gasification (BAG) and N2 atmospheres, respectively, while 11.96 mg/g under BAG+2%O2 atmosphere, which indicates that the presence of oxygen can inhibit the production of tar. Furthermore, the dominant reaction is characterized as the polycondensed aromatization involving dehydrogenation and deoxygenation in PBG under BAG atmosphere, tending to the formation of heavy organic compounds such as tar. While, surface oxygen-containing organic functional groups can be generated via the oxidation reaction between some surface functional groups of PBG and O2 in PBG under BAG+2%O2 atmosphere, inhibiting the aromatization of aromatic clusters and the formation of heavy organic compounds such as tar to some extent. Thus, the introduction of a limited amount of oxygen may be helpful for solving the pipe blockage during hot gas filtration of the raw BAG gas.
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表 1 PBG的工业分析及元素分析
Table 1 Proximate and ultimate analyses of PBG
Sample Proximate analysis wdb/% Ultimate analysis wdb/% M A V FC C H S Oa N PBG 2.51 19.58 16.84 61.07 56.88 3.64 0.93 18.28 0.69 a:calculation by difference 表 2 PBG原样及不同气氛下热解固体产物表面碳官能团的相对含量
Table 2 The relative amounts of carbon functionalities in PBG and its pyrolysis solid products derived from different reaction atmospheres
Sample Relative amount w/% Rb C-C & C=C C-H C-O C=O COOH total OFGsa PBG 8 72 13 1 6 20 0.11 BAG-solid product 39 46 10 1 4 15 0.85 BAG+2%O2-solid product 10 66 12 5 7 24 0.15 N2-solid product 10 74 8 4 4 16 0.14 a:total OFGs=the sum of oxygen-containing functional groups; b:R= the ratio of “C-C & C=C” to “C-H” 表 3 PBG及其在不同气氛下热解固相产物13C NMR碳官能团分布
Table 3 Distribution of carbon functionalities determined by 13C NMR in PBG and its pyrolysis solid products derived from different reaction atmospheres
Skeletons
Moieties
δCarbonyls Aromatics Alkyls C=O
(202)COOH
(187, 178)Ar-O
(167, 153)Ar-C
(140, 126)Ar-H
(113, 101)R-O
(93, 76, 56)CH2
(40, 31)CH3
(20, 13)PBG 0.82 2.75 7.26 45.99 22.11 5.45 7.04 8.58 BAG-solid product 0.00 0.00 6.28 73.50 15.06 0.00 0.00 5.16 BAG+2%O2-solid product 0.00 1.58 8.49 59.27 21.83 6.57 0.00 2.26 N2-solid product 0.00 0.98 6.63 52.87 26.50 9.44 1.11 2.47 Ar-O: O attached to aromatics; Ar-C: C attached to aromatics; Ar-H: H attached to aromatics; R-O: O attached to aliphatics 表 4 PBG及其在不同气氛下热解固相产物芳香度、芳香环边缘位点的碳原子占比、每个芳香簇所含最小碳原子数
Table 4 Aromaticity, fractions of aromatic edge carbons and minimum number of carbons per aromatic cluster in PBG and its pyrolysis solid products derived from different reaction atmospheres
Sample Aromaticity XAr-H Xedge, min Xedge, maX nC, min PBG 75.36 0.29 0.39 0.72 >12 C BAG-solid product 94.85 0.16 0.23 0.28 >77 C BAG+2%O2-solid product 89.59 0.24 0.34 0.45 >29 C N2-solid product 86.00 0.31 0.39 0.55 >20 C -
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