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市政污泥是城市生活污水处理过程中的副产物,近年来,污泥产量急剧增加。2020年我国市政污泥年产量已经超过6×107 t (以80%含水率计) [1],预计2025年将超过9×107 t (以80%含水率计) [2]。污泥含有丰富的碳元素,有机碳的质量分数为222.74~406.03 g·kg−1[3-5],资源化利用潜力巨大。解决迫在眉睫的污泥问题,实现污泥碳素资源化,对于推动碳达峰和碳中和目标的实现具有现实意义。
市政污泥热解作为一种十分高效的污泥处理和资源回收方式受到了广泛研究[6]。污泥在热解过程中不仅能实现减量化,并且会产生含碳增值产品 (如生物炭) 。研究发现,市政污泥热解过程中碳素产物的形成与污泥中含碳官能团演变有关[7]。DE等[8]发现相比于温度低于400 ℃时,污泥热解生物炭在400~700 ℃的温度区间中脂肪族基团更少而石墨碳结构更多,一定程度上证明了生物炭的结构随温度的上升更加稳定。WANG等[9]通过红外光谱 (FTIR) 发现在700 ℃时污泥热解产物中的羰基已经消失,热稳定性得到了提高。此外,在高温下,生物炭中C-O键及-OH键等含碳氧官能团数量减少,吸附吸能减弱[10]。目前对于污泥热解过程中含碳官能团的半定量分析研究较少,对污泥热解过程中从低温到高温阶段含碳官能团的演化规律分析研究比较匮乏。同时,污泥热解不同阶段速率存在差异,通过动力学方法可以表征污泥热解不同阶段趋势。REN等[11]研究污泥热解活化能发现热解温度越高,反应越慢。徐新宇等[12]分析污泥热解活化能发现随着温度的升高,热解反应受阻,但加入芬顿试剂混合热解后,热解反应加快。因此通过动力学方法可以有效分析污泥热解过程,探究热解规律,研究污泥热解碳素转化机制。
本研究拟通过热重分析和热解动力学分析市政污泥热解特性,并基于傅里叶红外光谱分析仪 (FTIR) 和X射线光电子能谱 (XPS) 对市政污泥热解过程中含碳官能团进行定性半定量分析,旨在探明市政污泥热解过程热解特性及主要含碳官能团演化机制,为市政污泥热解机理及碳形态转化研究提供理论基础。
市政污泥热解特性及含碳官能团演化过程分析
Pyrolysis characteristics of municipal sludge and transformation of carbon-containing functional groups during pyrolysis process
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摘要: 我国市政污泥年产量大,研究其热解机理和碳形态转化可为实现污泥无害化和资源化提供理论基础。采用热重、动力学拟合、红外光谱 (FTIR) 、X射线光电子能谱 (XPS) 等检测和分析方法,对市政污泥热解特性、活化能变化和含碳官能团进行分析。结果表明:市政污泥热解过程中含碳官能团的生成在不同的阶段存在差异,进一步采用Kissinger-Akahira-Sunose (KAS) 法、Flynn-Wall-Ozawa (FWO) 法和Friedman法进行动力学分析,其活化能结果显示在转化率为0.40~0.70时,市政污泥热解速率加快,污泥中含碳官能团加速发生热解转化。FTIR和XPS结果表明,在400 ℃以下时,醇羟基的分解是导致污泥结构中羟基减少的主要原因。在400 ℃时,酚羟基和C=O键相对质量分数最多,而在500 ℃时,羧基和C-O键相对质量分数最高。C=C键和C-N键随温度升高持续减少,C-C结构则先减少后增加,在500 ℃时相对质量分数最低,而芳香结构随温度的上升持续增加。在400~600 ℃内,污泥残炭的含碳氧官能团较多,吸附性和稳定性较高。本研究结果可为市政污泥热解机理及碳形态转化提供参考。Abstract: The annual production of municipal sludge in China is massive. The study of its pyrolysis mechanism and the transformation of carbon-containing functional groups can provide a theoretical basis for the reduction of the sludge harmlessness and the resource utilization. Thermal gravity, kinetic fitting, fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and other detection and analysis methods were used to analyze the pyrolysis characteristics, activation energy changes and carbon-containing functional groups of municipal sludge. The results showed that there were differences in the generation of carbon-containing functional groups in different stages during the pyrolysis of municipal sludge. Kissinger-Akahira-Sunose (KAS), Flynn-Wall-Ozawa (FWO) and Friedman methods were used for further kinetic analysis. The results of activation energy showed that when the conversion rate was between 0.40 and 0.70, the pyrolysis rate of municipal sludge was accelerated, and the carbon-containing functional groups in the sludge were accelerated to undergo pyrolysis conversion. The results of FTIR and XPS showed that in the range of 30-400 ℃, the decomposition of alcoholic hydroxyl groups was the main reason for the reduction of hydroxyl groups in the sludge structure. At 400 ℃, the relative mass fraction of phenolic hydroxyl groups and C=O bonds was the highest, while at 500°C, the relative mass fraction of carboxyl groups and C-O bonds was the highest. The C=C bond and C-N bond decreased continuously with the increase of temperature, while the C-C structure first decreased and then increased, and the relative mass fraction was the lowest at 500 ℃. Meanwhile, the aromatic structure increased with the increase of temperature. In the range of 400-600 ℃, the carbon and oxygen functional groups of the sludge residue carbon were more, and the adsorption and stability were higher. This study analyzed the pyrolysis characteristics of municipal sludge and the evolution process of carbon-containing functional groups, providing a theoretical basis for the study of municipal sludge pyrolysis mechanism and carbon form transformation.
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Key words:
- municipal sludge /
- pyrolysis /
- dynamics /
- carbon-containing functional group /
- carbon conversion
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表 1 市政污泥工业分析和元素分析
Table 1. Industrial analysis and elemental analysis of municipal sludge
% 工业分析 元素分析 水分 挥发分 灰分 固定碳 C H O* N S 4.48 47.56 45.15 2.81 20.14 3.51 27.33 3.32 0.55 注:*表示氧元素质量分数由差减法计算得来。 表 2 表观活化能结果
Table 2. Results of apparent activation energy
转化率 KAS FWO Friedman 活化能/(kJ·mol−1) R2 活化能/(kJ·mol−1) R2 活化能/(kJ·mol−1) R2 0.20 163.84 0.938 3 190.18 0.958 7 164.22 0.945 8 0.30 321.85 0.992 9 347.32 0.994 3 319.66 0.994 7 0.40 405.98 0.990 7 432.04 0.992 4 405.14 0.992 5 0.45 394.11 0.977 4 423.26 0.981 9 396.96 0.980 9 0.50 357.80 0.965 3 391.49 0.973 1 365.41 0.970 6 0.55 320.19 0.953 2 358.84 0.965 2 332.37 0.960 7 0.60 277.49 0.937 0 321.99 0.955 8 294.44 0.948 2 0.65 253.37 0.913 8 303.51 0.942 6 274.49 0.930 6 0.70 238.02 0.891 5 293.78 0.930 9 262.88 0.914 3 表 3 不同温度下市政污泥热解后产物的相对质量分数
Table 3. Relative mass fraction of municipal sludge pyrolysis products at different temperatures
热解温度/ ℃ C-C/C=C C-O -COO C=O 相对质量分数/% 结合能/eV 相对质量分数/% 结合能/eV 相对质量分数/% 结合能/eV 相对质量分数/% 结合能/eV 30 58.09 284.80 28.49 286.38 13.42 288.28 — — 200 57.55 284.80 32.93 286.28 9.52 288.28 — — 300 56.27 284.80 36.40 286.08 7.33 288.48 — — 400 36.02 284.80 28.18 285.58 15.47 288.80 20.33 287.34 500 30.82 284.80 40.90 285.45 16.57 289.50 11.70 287.00 600 41.97 284.80 34.65 285.45 13.25 288.42 10.14 286.72 700 62.30 284.80 33.17 285.96 4.53 289.46 — — 注:表中“—”表示未检出。 -
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