Changes in pyrolysis characteristics of agricultural residues before and after water washing
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摘要: 首先对花生秸秆、高粱秸秆和芦苇三种农业废弃物进行了水洗预处理，再利用固定床反应器进行了热解实验，借此考察水洗前后碱金属和碱土金属（AAEMs）和纤维组成的变化及其对农业废弃物热解特性的影响。研究发现，水洗可脱除52.7%−92.6%的钾和一半左右的中性溶解物（NDS）。AAEMs和NDS的脱除对热解农业废弃物热解产生综合影响，含AAEMs和NDS较多的花生秸秆影响尤为明显。AAEMs的脱除遏制脱羧基、脱羰基、脱氢及挥发分二次缩聚反应，而NDS的脱除直接影响气体和液体的产率和组成。对于所有三种农业废弃物，水洗后均有利于提高生物油和生物炭产率，但气体产率降低；水洗后生物油中含氧化合物增多，其中，糖类和呋喃类增幅最大，而烃类、含氮化合物减少。水洗后，花生秸秆的CO2、CO和CH4产率均明显降低，而生物油中长链脂肪酸的占比增大。Abstract: Three agricultural residues (peanut straw, sorghum stalk and reed) were first pretreated by water washing, with which the pyrolysis experiments were carried out in a fixed bed reactor to investigate how the pyrolysis characteristics are changed by removing the alkali earth metals (AAEMs) and a part of fibrous components by water washing. The result shows that the water washing removes 52.7%–92.6% potassium and approximately half of neutral detergent solute (NDS) from three agricultural residues. The removal of AAEMs and NDS has a complex influence on the pyrolysis of agricultural residues, especially for peanut straw due to its higher contents of both AAEMs and NDS. However, the removal of AAEMs has an inhibitory effect on the reactions such as decarboxylation, decarbonylation, dehydrogenation and polycondensation, and the elution of NDS directly affects the yields and composition of gas and liquid products. For all three agricultural residues, overall, the water washing promotes the production of bio-oil and bio-char, but it lowers the gas yield. The bio-oils generated from the water-washed agricultural residues have higher proportions of oxygenates, especially sugars and furans, with lower proportions of hydrocarbons and nitrogenous compounds. Besides, after washing, the yields of CO2, CO and CH4 with peanut straw decrease significantly, while the proportion of long-chain fatty acids in bio oil increases.
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Key words:
- agricultural residues /
- pyrolysis /
- water washing /
- inorganic matter /
- fibrous components
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图 1 水洗前后农业废弃物的热重分析
Figure 1 TGA/DTG analyses of the agricultural residues before and after water washing
图 2 水洗前后农业废弃物700 ℃下主要热解气体的释放速率
Figure 2 Release rates of main pyrolysis gases from the agricultural residues before and after washing at 700 ℃
图 3 水洗前后农业废弃物700 ℃下热解生物油中各类化合物GC-MS分析相对峰面积
Figure 3 GC-MS relative peak areas (%) of different classes of compounds in the bio-oils produced from pyrolysis of the three agricultural residues before and after washing
表 1 水洗前后农业废弃物的工业分析和元素分析
Table 1 Proximate and ultimate analyses of the agricultural residues before and after water washing
Residues Proximate analysis (%, dry basis) Ultimate analysis (%, dry basis) volatile ash fixed carbona C H N S Oa PS 66.39 17.04 16.57 46.51 6.79 2.77 0.06 43.87 W-PS 72.87 15.50 11.63 49.46 6.92 2.34 0.00 41.28 SS 72.20 9.12 18.68 48.10 6.79 0.59 0.00 44.52 W-SS 76.77 7.06 16.17 48.10 6.38 0.54 0.00 44.98 RD 73.03 7.97 19.00 47.64 6.63 0.83 0.00 44.90 W-RD 77.51 6.54 15.95 48.94 6.44 0.66 0.00 43.96 a: by difference 
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表 2 水洗前后农业废弃物中的无机矿物质含量
Table 2 Contents of AAEMs in agricultural residues before and after water washing
Residues AAEMs (%, dry basis) Na K Ca Mg PS 0.08 1.79 1.51 1.05 W-PS 0.03 0.70 1.60 0.81 SS 0.03 1.10 0.34 0.19 W-SS 0.02 0.63 0.32 0.16 RD 0.02 0.27 0.23 0.06 W-RD 0.01 0.02 0.16 0.02
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表 3 水洗前后农业废弃物的纤维组成
Table 3 Contents of fibrous components in agricultural residues before and after water washing
Residues Fibrous components (%, dry basisa) NDS hemicellulose cellulose lignin silicate PS 47.4 14.3 26.8 7.6 3.9 W-PS 29.4
(21.0)21.3
(15.2)34.7
(24.8)11.1
(7.9)3.5
(2.4)SS 16.8 29.9 23.6 24.8 4.9 W-SS 9.5
(7.9)30.7
(25.5)28.1
(23.4)26.7
(22.2)5.0
(4.2)RD 15.6 24.3 42.1 12.5 5.5 W-RD 8.9
(8.0)27.6
(24.7)44.5
(39.9)13.9
(12.4)5.1
(4.6)a: data in parenthesis are based on the mass of dried raw biomass
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表 4 水洗前后农业废弃物热解总产物分布及各气体产率
Table 4 Distributions of overall products and yields of individual gases produced by pyrolysis of the three agricultural residues before and after water washing (%, dry biomass basis)
Products PS W-PS SS W-SS RD W-RD Overall products gas 22.9 17.1 17.9 14.8 15.9 12.3 water 16.2 17.5 24.2 26.1 21.6 23.9 bio-oil 24.3 29.5 27.6 32.0 31.5 37.2 biochar 35.5 34.7 29.3 26.3 29.3 25.3 total 98.9 98.8 99.0 99.2 98.3 98.7 Gases CO2 14.50 10.92 11.15 8.79 9.19 6.99 CO 6.22 4.43 5.15 4.58 4.98 3.94 H2 0.46 0.41 0.33 0.32 0.31 0.24 CH4 1.12 0.75 0.92 0.80 1.08 0.93 C2−C3 0.64 0.58 0.41 0.31 0.38 0.26
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表 5 水洗前后农业废弃物热解生物油中部分主要化合物的GC-MS分析相对峰面积（%）
Table 5 GC-MS relative peak areas (RPAs) of some main compounds in pyrolysis bio-oils of the three agricultural residues before and after washing (%)
Group/Compound Molecular formula PS W-PS SS W-SS RD W-RD Fatty acids Hexadecenoic acid C16H30O2 − 1.8 − − − − Hexadecanoic acid C16H32O2 − 6.7 1.0 − − 0.5 Methyl hexadecanoate C17H34O2 1.1 0.4 0.1 − − − Oleic acid C18H34O2 − 0.9 1.2 − − − Octadecanoic acid C18H36O2 − 0.4 0.4 − − − Sugars 1,4,3,6-dianhydro-α-d-glucopyranose C6H8O4 0.7 2.8 1.3 1.3 0.8 1.1 1,6-anhydro-β-d-glucopyranos C6H10O5 − − 0.4 4.7 1.2 5.1 D-allose C6H12O6 − 3.3 0.1 − 1.2 6.1 Cyclic hydrocarbons 1,3,5-cycloheptatriene C7H8 3.1 − − − 0.5 − 1,3,5,7-cyclooctatetraene C8H8 5.0 − − − 0.9 − Trimethylbicyclo heptane C10H18 1.4 2.0 − − − − Ethylcyclododecane C14H28 0.9 − 0.1 − − − Nitrogenous compounds Dihydro-trimethyl- oxazole C6H11NO 13.9 − − − − − 1-pyrrolidinylacetic acid C6H11NO2 11.8 − − − − − (Z)-9-octadecenoamide C18H35NO − 0.9 0.4 0.7 − 0.8 (Z)-13-docosenamide C22H43NO − 0.9 0.4 0.7 − 0.7
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表 6 水洗前后农业废弃物热解生物油中部分化合物的产率
Table 6 Yields of some main compounds in the bio-oils produced from the three agricultural residues before and after washing (%, dry biomass basis)
Compound PS W-PS SS W-SS RD W-RD Acetic acid 1.10 1.77 4.07 3.86 4.19 3.28 Hexadecanoic acid 0.00 0.35 0.02 0.00 0.00 0.01 Octadecanoic acid 0.00 0.07 0.04 0.00 0.00 0.00 Oleic acid 0.00 0.15 0.47 0.00 0.00 0.00 Hydroxyacetone 0.13 0.38 0.60 0.55 0.62 0.45 Furfural 0.11 0.29 0.52 1.36 0.83 0.79 2-furanmethanol 0.08 0.34 0.33 0.93 0.41 0.78 Guaiacol 0.00 0.10 0.22 0.26 0.16 0.31 Phenol 0.10 0.20 0.29 0.34 0.13 0.14 Cresols 0.10 0.35 0.21 0.31 0.13 0.30 Xylenol 0.07 0.14 0.16 0.29 0.14 0.14 Benzene 0.09 0.06 0.00 0.00 0.00 0.00 Toluene 0.13 0.13 0.10 0.04 0.07 0.24 Xylene 0.00 0.00 0.02 0.00 0.00 0.00
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