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垃圾热处置技术是近来广泛应用的先进垃圾处理技术,与传统的垃圾填埋技术相比,热处置技术因具有占地面积小、能量回收率高,重复周期短,资源占用率低等优点,近年来在我国得到巨大发展[1-3],在2017年,垃圾热处置率已经达到39.3%[4]。但是垃圾热处置过程中除了废气废水废渣,还有重金属、氯苯类、氯酚类、二噁英等有毒污染物的排放[5-6],垃圾热处置过程产生的环境污染问题亟待解决。
二噁英因其极高的毒性一直备受人们关注,热处置过程中二噁英的来源主要包括三部分:(1)垃圾中原有的;(2)高温气相合成;(3)低温异相合成,其中低温异相合成包括从头合成和前驱物合成两种。其中垃圾中原有的二噁英含量很低,高温气相合成和低温异相合成是二噁英形成的主要途径。
氯苯类化合物(CBz)本身也是一种有机污染物,同时作为二噁英合成的重要前驱物,参与二噁英的高温气相合成以及低温前驱物合成途径,是二噁英合成的重要来源[7-8]。在污染物排放特性相关研究中发现,CBz的排放量通常是二噁英的1000倍以上[9];在前驱物合成二噁英的相关研究中发现,1,2,4-三氯苯(1,2,4-TrCBz)排放量能达到其他氯苯类化合物10倍以上[10],同时生成的二噁英浓度及毒性当量高于其他氯苯类[11]。基于以上研究结论,1,2,4-TrCBz在氯苯类、二噁英的排放特性研究中是比较合适的替代物。
在对氯苯类、二噁英的排放控制的研究中发现气氛条件对二噁英的生成和降解有重要影响。Addink 等发现在无氧条件下碳和飞灰的混合物在加热条件下没有进行从头合成途径生成二噁英[12];Milligan 和Altwicker 研究发现飞灰在氮气气氛下提供前驱物低温异相合成所需条件同样几乎没有二噁英生成[13];Hagenmaier 等研究发现,在高温氮气气氛下二噁英可被降解[14];Addink 和Olie 研究发现,在热处置过程中,二噁英的排放量随着氧气含量的增加而增加[12];Floyd研究发现,在热处置过程中减少二噁英的排放量的最佳燃烧条件和CO量密切相关[15];Wen-Tsung Hung等研究发现,在焚烧过程炉中添加氧化铁除去CO时,二噁英的排放量明显减少[16];汤元君,董隽等研究发现,热处置过程二噁英的排放量受氧化还原性气氛的影响,热解气化产生的还原性气氛有效降低了二噁英的排放量,生成总量主要呈现为气化<热解<焚饶的趋势[17-18]。气氛条件可能是热解、气化技术在热处置过程中可以有效降低含氯有机污染物的排放量的重要原因。
虽然国内外在对热处置过程中气氛条件与氯苯类、二噁英的排放关联已经做了很多研究,但是大多研究集中于热处置过程中的二噁英的低温异相合成途径,高温段的相关研究较少,而且主要为针对实验现象的分析,还原性气氛对抑制二噁英和前驱物生成的相关反应机理和反应路径尚未探明。在高温段实现含氯污染物的有效降解,有利于降低烟气中HCl含量,减少低温异相合成二噁英的氯源,达到污染物减排、发电效率提升的目的[19-21]。
本文以1,2,4-TrCBz为实验对象,探究其在中高温段(550—850℃)与生活垃圾热解气化产生的两种主要还原性气体H2和CO的反应机理。首先开展了H2和CO与1,2,4-TrCBz在中高温段的相关实验,通过实验现象分析了还原性气体与1,2,4-TrCBz可能存在的反应路径,再采用量子化学原理和Gaussian 09W软件模拟了各反应路径的反应体系结构变化和能量变化,分析比较了各路径的竞争关系,通过实验与模拟的结合以探寻相关反应机理。
氯苯类还原性气氛下高温降解反应机理
Degradation mechanism of chlorobenzenes in reducing atmosphere at high temperatures
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摘要: 氯苯类化合物(CBz)是生活垃圾热处置过程中存在的重要污染物,热解、气化技术能够有效降低热处置过程的CBz排放。为了探究其在热解、气化环境下的降解机理,以1,2,4-三氯苯(1,2,4-TrCBz)为研究对象,进行了1,2,4-TrCBz在中高温段(550—850 ℃)还原性气氛(H2,CO等)下的降解特性实验,同时通过量子化学计算,利用Gaussian 09W软件模拟了降解过程中可能存在的多条反应路径,并比较了各路径的竞争关系。结果表明温度越高,1,2,4-TrCBz的降解效率越高。降解效率在750 ℃和850 ℃时,分别为14.07%和60.27%,1,2,4-TrCBz分子的C—Cl键解离能在370 kJ·mol−1左右,温度在650 ℃以下时,环境提供的热量不足以使C—Cl键断裂,H2降低了40 kJ·mol−1左右的C—Cl键断裂所需的能量,提高了降解反应速率。单独的CO不参与1,2,4-TrCBz的降解反应,实验降解特性与N2气氛相似。CO与H2共存时,在相同温度下,降解表现呈现H2>H2+CO>CO的规律,CO的存在提高了H2近20 kJ·mol−1的降解反应活化能,从而降低了反应速率。1,2,4-TrCBz的降解过程存在3条有效降解路径,反应更趋向于通过生成1,3-二氯苯(1,3-DCBz)的路径进行。Abstract: Chlorobenzenes (CBz) are important pollutants of municipal solid waste (MSW) during incineration. Pyrolysis and gasification technology can effectively reduce the emission of CBz. In order to explore the degradation mechanism of CBz in the pyrolysis and gasification environment, experimented the degradation characteristics of 1,2,4-TrCBz in reducing atmosphere (H2, CO, etc.) at moderate and high temperatures (550—850 ℃). At the same time, several possible reaction paths were simulated by Gaussian 09W software through quantum chemistry calculation, and compared the competitive relationship of each path. The results show that the higher the temperature, the higher the degradation efficiency of 1,2,4-TrCBz. The degradation efficiency is 14.07% and 60.27% at 750 ℃ and 850 ℃, respectively. The C—Cl bond’s dissociation energy of 1,2,4-TrCBz molecule is about 370 kJ·mol−1. when the temperature is below 650 ℃, the heat provided by the environment is not enough to break the C—Cl bond. H2 reduces the energy required for C—Cl bond breaking at about 40 kJ·mol−1 and improves the degradation reaction rate. CO did not participate in the degradation of 1,2,4-TrCBz alone, and the experimental degradation characteristics were similar to those in N2 atmosphere. When CO and H2 coexist, the degradation performances show the rule of H2 > H2+CO > CO, at the same temperature. The presence of CO increases the activation energy of H2 degradation reaction by nearly 20 kJ·mol−1, which reduces the reaction rate. There are three effective degradation paths in the degradation process of 1,2,4-TrCBz, and the reaction is more likely to take place through the pathway of 1,3-Dichlorobenzene (1,3-DCBz).
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表 1 各实验工况气体组分摩尔分数
Table 1. Initial mole fraction of reactants in experiments
工况
Experimental condition气氛 Atmosphere N2/% H2/% CO/% 工况1 100 0 0 工况2 97 3 0 工况3 94 6 0 工况4 94 0 6 工况5 94 3 3 表 2 1,2,4-TrCBz分子键解离能
Table 2. Molecular bond dissociation energy of 1,2,4-TrCBz
键位
Chemical bonds键解离能/(kJ · mol−1)
Bond dissociation energyC1—Cl10 367.43 C2—Cl11 364.67 C4—Cl12 371.89 C3—H7 481.87 C5—H8 480.67 C6—H9 476.34 C1—C6 660.96 表 3 1,2,4-TrCBz分子C—Cl键原子FED
Table 3. FED of C—Cl bond atom of 1,2,4-TrCBz
原子
Atoms$ {\mathrm{F}\mathrm{E}\mathrm{D}}_{\mathrm{H}\mathrm{O}\mathrm{M}\mathrm{O}}^{2} $ $ {\mathrm{F}\mathrm{E}\mathrm{D}}_{\mathrm{L}\mathrm{U}\mathrm{M}\mathrm{O}}^{2} $ $ {\mathrm{F}\mathrm{E}\mathrm{D}}_{\mathrm{H}\mathrm{O}\mathrm{M}\mathrm{O}}^{2}+{\mathrm{F}\mathrm{E}\mathrm{D}}_{\mathrm{L}\mathrm{U}\mathrm{M}\mathrm{O}}^{2} $ C1 1.96×10−1 2.99×10−2 2.26×10−1 C2 9.38×10−2 1.24×10−1 2.18×10−1 C4 1.59×10−1 3.82×10−2 1.97×10−1 Cl10 1.75×10−1 3.95×10−3 1.78×10−1 Cl11 7.19×10−2 1.53×10−2 8.72×10−2 Cl12 1.53×10−1 4.93×10−3 1.58×10−1 表 4 路径能垒
Table 4. Pathway energy barriers
路径
Reaction pathways活化能/(kJ ·mol−1)
$ \Delta H $ 吉布斯自由能/(kJ· mol−1)
$ \Delta G $ $\mathrm{1,2},4-\mathrm{T}\mathrm{r}\mathrm{C}\mathrm{B}\mathrm{z}+{\mathrm{H} }_{2}\longrightarrow{{\rm{TS1}}}\longrightarrow{{\rm{P1}}}$ $ 316.02 $ $ -107.53 $ $\mathrm{1,2},4-\mathrm{T}\mathrm{r}\mathrm{C}\mathrm{B}\mathrm{z}+{\mathrm{H} }_{2}\longrightarrow{{\rm{TS2}}}\longrightarrow{{\rm{P2}}}$ $ 314.42 $ $ -107.52 $ $\mathrm{1,2},4-\mathrm{T}\mathrm{r}\mathrm{C}\mathrm{B}\mathrm{z}+{\mathrm{H} }_{2}\longrightarrow{{\rm{TS3}}}\longrightarrow{{\rm{P3}}}$ $ 316.43 $ $ -95.53 $ $\mathrm{1,2},4-\mathrm{T}\mathrm{r}\mathrm{C}\mathrm{B}\mathrm{z}+{\rm{CO}}\longrightarrow{{\rm{TS4}}}\longrightarrow{{\rm{P4}}}$ $ 235.66 $ $ 113.74 $ $\mathrm{1,2},4-\mathrm{T}\mathrm{r}\mathrm{C}\mathrm{B}\mathrm{z}+{\rm{CO}}\longrightarrow{{\rm{TS5}}}\longrightarrow{{\rm{P5}}}$ $ 234.85 $ $ 114.02 $ $\mathrm{1,2},4-\mathrm{T}\mathrm{r}\mathrm{C}\mathrm{B}\mathrm{z}+{\rm{CO}}\to{{\rm{TS6}}}\to{{\rm{P6}}}$ $ 243.93 $ $ 102.67 $ 表 5 路径能垒
Table 5. Pathway energy barriers
路径
Reaction pathways活化能/(kJ · mol−1) $ \Delta H $ 吉布斯自由能/(kJ · mol−1)
$ \Delta G $ $ 1,2,4-\mathrm{T}\mathrm{r}\mathrm{C}\mathrm{B}\mathrm{z}+{\mathrm{H}}_{2}+{\rm{CO}} \longrightarrow {\rm{TS4}} \longrightarrow {\rm{P4}} \longrightarrow {\rm{TS4a}} \longrightarrow {\rm{P4a}} $ $ 336.92 $ $ -107.53 $ $ 1,2,4-\mathrm{T}\mathrm{r}\mathrm{C}\mathrm{B}\mathrm{z}+{\mathrm{H}}_{2}+{\rm{CO}} \longrightarrow {\rm{TS4}} \longrightarrow {\rm{P4}} \longrightarrow {\rm{TS4b}} \longrightarrow {\rm{P4b}} $ $ 284.59 $ $ -10.74 $ $ 1,2,4-\mathrm{T}\mathrm{r}\mathrm{C}\mathrm{B}\mathrm{z}+{\mathrm{H}}_{2}+{\rm{CO}} \longrightarrow {\rm{TS4}} \longrightarrow {\rm{P4}} \longrightarrow {\rm{TS4c}} \longrightarrow {\rm{P4c}} $ $ 294.37 $ $ -7.52 $ -
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