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The formation of dioxins in combustion systems

机译：燃烧系统中二恶英的形成

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摘要

Flame chemistry in incineration systems involves the formation of many organic products of incomplete combustion, including chlorinated species such as polychlorinated biphenyls (PCB), polychlorinated dibenzo-p-dioxins (PCDD), and polychlorinated dibenzofurans (PCDF). Because the latter are of environmental concern, a great deal of research has been expended on understanding their formation. There are two temperature windows in which they can form: the "homogeneous" route between 500 and 800℃ and the "heterogeneous" one at 200 to 400℃. Homogeneous reactions, which are the result of the pyrolytic rearrangement of chlorinated precursors, such as chlorophenols and chlorobenzenes in the gas phase, have not been researched as extensively as the heterogeneous mechanism. Heterogeneous formation is a catalysed reaction, which takes place on the ash or soot particles present in combustion systems. There are conflicting views regarding the relative amounts of PCDD/F formed from precursors such as chlorophenols in comparison with the de novo process during commercial operations. The de novo reactions involve the oxidation and chlorination of any unburned carbon in the particulates. The reaction pathways for de novo PCDD/F are based on preexisting 3-ring carbon skeletons; single-ring chlorinated precursors are not intermediates. The formation process is driven by oxidation, and the rate is related to carbon burnoff. Dechlorination and decomposition proceed at elevated temperatures. The reaction appears to take place on the global (external) surface of the particles, but is determined by their carbon and chlorine contents. During de novo formation, chlorine is an active agent, either as chloride in the solid phase or as atomic chlorine in the gas. There is always a contribution from solid-phase chlorine, and gas-phase chlorine is active only at higher concentrations when the solid phase is depleted. An excess of chlorine appears to inhibit de novo formation. Different mechanisms are postulated for PCDD and PCDF. Single- and multiring species chlorinate differently, along different paths. The single-ring compounds formed are comparatively loosely held, while the multiring species tend to be strongly held. Much of the PCDD/F formed is retained on the solid surface and is unable to equilibrate with the gas phase under flue gas conditions. De novo catalysis is due mainly to copper, although iron and other metals are active at lower rates. Copper catalyses the oxidation of carbon, as well as the chlorination and dechlorination of organic products. In its two oxidation states it also acts as a "shuttle" for chlorine between gas and solid. The catalytic effect can be poisoned by sulfur or nitrogen compounds, such as sulfur dioxide and urea. All the formation models proposed to date both for the homo- and heterogeneous routes are inadequate, no doubt as a result of the complexity of the processes. The homogeneous route needs more fundamental research, and as regards the de novo route, more attention needs to be paid to the composition and nature of the ash's surface, including particle size and carbon/catalyst disposition.
机译：焚化系统中的火焰化学涉及许多不完全燃烧的有机产物的形成，包括多氯联苯（PCB），多氯二苯并对二恶英（PCDD）和多氯二苯并呋喃（PCDF）等氯化物。由于后者与环境有关，因此已经在了解其形成方面进行了大量研究。在其中可以形成两个温度窗口：500到800℃之间的“均匀”路径和200到400℃之间的“异质”路径。均相反应是气相中氯化前体（如氯酚和氯苯）发生热解重排的结果，但尚未对其均相机理进行深入研究。非均相形成是催化反应，其发生在燃烧系统中存在的灰或烟尘颗粒上。关于与商业操作中的从头工艺相比，由前体（例如氯酚）形成的PCDD / F的相对量存在争议。从头反应涉及颗粒中任何未燃烧的碳的氧化和氯化。从头PCDD / F的反应途径基于预先存在的3环碳骨架；单环氯化前体不是中间体。形成过程受氧化驱动，其速率与碳燃烧有关。脱氯和分解在高温下进行。该反应似乎发生在颗粒的整体（外部）表面，但由其碳和氯含量决定。在从头形成过程中，氯是一种活性剂，无论是固相中的氯化物还是气体中的原子氯。固相氯总是有贡献，而固相贫化时，气相氯仅在较高浓度下才有活性。过量的氯似乎抑制了从头形成。假定PCDD和PCDF具有不同的机制。单环和多环物种沿不同的路径进行不同的氯化。形成的单环化合物相对松散地固定，而多环物质则倾向于牢固固定。形成的大部分PCDD / F保留在固体表面上，在烟气条件下无法与气相平衡。从头催化主要是由于铜，尽管铁和其他金属的活性较低。铜催化碳的氧化，以及有机产品的氯化和脱氯。在其两种氧化态下，它还充当气体和固体之间氯气的“梭”。催化作用可能会被二氧化硫和尿素等硫或氮化合物所毒害。迄今为止，所有建议的同构和异构路线的形成模型都是不充分的，这无疑是由于工艺的复杂性造成的。均质路线需要更基础的研究，就从头路线而言，需要更多地关注灰分表面的组成和性质，包括粒径和碳/催化剂的处置。

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  • 来源
    《Combustion and Flame》 |2004年第3期|p.398-427|共30页
  • 作者

    B.R. Stanmore;

  • 作者单位

    Department of Chemical Engineering, University of Queensland, Qld 4072, Australia;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 燃料与燃烧;
  • 关键词

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