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脱除烟气中二恶英类物质的实验研究
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摘要
随着我国经济的发展、城市规模的扩大,城市生活垃圾清运量逐年增加。目前,处理城市垃圾主要有填埋、堆肥和焚烧三大方法。其中,焚烧法以无害化程度高、减容量大、占地面积小、处理及时,并且可回收热能等优点,已逐渐成为处理城市垃圾的主要方法。但由于垃圾成分复杂,焚烧后会产生大量有毒有害物质,其中以二恶英的毒性与危害最为突出。本课题针对当前垃圾焚烧炉脱除二恶英成本高,技术难度大等问题,采用前人研究制备的高活性吸收剂,在固定床试验中研究了影响二恶英类物质去除效率的若干因素,并进行了理论推导与验证。另外,还进行了液相条件下复合吸收剂脱除二恶英类物质的实验研究和产物分析。本课题的研究对发展适合我国的二恶英类物质的控制技术具有一定的理论意义和应用价值。
     实验利用粉煤灰和石灰为主要物质,制备了可用于脱除1,2,4-TCB的改性吸收剂。比表面积和孔结构测试分析结果显示:吸收剂的比表面积和孔容积与制备原料相比增加了10倍左右,孔径大部分在3~40nm之间,属于中等孔隙。扫描电镜和X-射线能谱分析表明,Ca(OH)2在吸收剂表面的相对含量高于吸收剂中Ca的平均含量,添加剂在吸收剂表面得到了很大程度的分散。在固定床实验系统上主要研究了温度、入口浓度、氧气含量、添加剂含量以及停留时间和吸收剂反应时间对脱除效率的影响。平行实验表明,吸收剂具有较好的脱除1,2,4-三氯苯的效果。产物分析结果显示:主要的中间产物为2,6-重(1,1-二甲基)-4-甲基-苯酚,未检测到二恶英类物质,反应的最终产物为C02和H20,氯元素被高活性吸收剂表面的碱性物质吸收,阻断了二恶英类物质生成路径。
     另外,在液相条件下进行了复合吸收剂脱除1,2,4-三氯苯的实验,研究了吸收剂配比、初始pH值、温度、三氯苯进口浓度等因素对复合吸收剂脱除性1,2,4-三氯苯性能的影响,研究结果表明复合吸收剂具有较好的脱除1,2,4-三氯苯的性能。实验得出复合吸收剂脱除三氯苯最佳脱除条件:吸收剂配比为2,反应温度50℃,pH值为10,氧气含量10%,在最佳条件下,前10分钟的脱除效率高于90%。通过产物分析得出主要的中间产物是4-甲基-1-戊醇和丁二酸,反应的最终产物为CO2和H2O。根据产物分析推测,反应机理为三氯苯与复合吸收剂发生脱氯和氧化两步反应,脱氯反应为卤代芳烃的亲核取代反应,氧化反应为酚或氯酚与氧化剂M之间的反应。
With the development of economy and the expansion of China's cities, the amount of MSW clean-up has increased year by year. At present in the world, landfill, composting and incineration are the main MSW processing methods Among these three methods, MSW incineration technique has become more and more widely used, because it has advantages in hazard-free treatment, more volume reduction, less area occupying and heat energy recycle. However, due to the complicated matrix, MSW using the incineration technique produces toxic and harmful substances. Dioxin is the most persistent and toxic substance produced after burning, which is regarded as potential threat to human health. It's very difficult and expansive in dealing with dioxin. Instead, highly active absorbent was prepared in this study. The effects of temperature, oxygen content, pH, etc. were investigated using a fixed-bed experimental system. The structure of the product was analyzed by GC-MS. Additionally, the experiments of removal of dioxins type chemicals were carried out in NaClO2 solution. This study has some theoretical meaning and application value for developing the technique of dioxin control.
     Fly ash and CaO are the main constituents of the modified absorbents, which have the capability of removal 1,2,4-trichlorobenzene. The analysis results of specific surface area and pore structure tests showed that the specific surface area and pore volume of fly ash increased about 10 times after digestion at certain hydrothermal condition by calcium oxide. The sizes of most of the micro pores were in the range of 3-40 nm, which was regarded as the medium pore. The SEM and X-ray energy spectrum analysis showed that:the relative average content of Ca(OH)2 on the absorbents surface was higher than that inside. The additives were dispersed in the absorbent surface evenly.
     The effects of temperature, inlet 1,2,4-TCB concentration, oxygen content, additive content etc. on the removal efficiency were investigated using a fixed-bed experimental system. The results showed that the absorbents had preferable removal capability of 1,2,4-TCB from flue gas. The result of product analysis showed that 2,6-Bis-[1,1-Dimethylethyl]-4-methyl-Phenol was the major intermediate product in the reaction; CO2 and H2O were the eventual products; Dioxins were not detected by GC/MS. This maybe because the formation routs were blocked off after Cl was absorbed by the highly reactive absorbent, which was decomposed from trichlorobenzene.
     The experiments of 1,2,4-TCB removal were carried out in NaClO2 solution. The effects of solution concentration, initial pH value, reaction temperature, inlet 1,2,4-TCB concentration etc. on 1,2,4-TCB removal performance were studied in the paper. The results showed that the absorbents had preferable removal capability of 1,2,4-TCB from flue gas.The optimum experimental conditions of removing TCB are the ratio of NaClO2/NaClO in complex absorbent is 2, the reaction temperature is 50℃, and pH of reaction medium is 10. The TCB removal efficiency above 90% was achieved under optimized conditions. There are two steps in this reaction. In the first, Ispo-substitution reaction between TCB and nucleophile reaction might occur in alkaline medium, the product are chlorophenols and 4-methyl-1-pentanol. Then chlorophenols and 4-methyl-1-pentanol would be oxidized by "M" such as ClO2 and ClO2. The result shows that main intermediate products are 4-methyl-1-pentanol and succinic and the end product are H2O and CO2.
引文
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