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富氧富水蒸汽条件下流化床燃烧高氮燃料的NOx排放特性
Alternative TitleCharacteristics of NOx emission from fluidized bed combustion of high-nitrogen fuel in atmospheres rich in O2 and steam
朱传强
Subtype硕士
Thesis Advisor许光文  ;  刘晓星
2014-05
Degree Grantor中国科学院研究生院
Degree Discipline化学工艺
Keyword富氧燃烧   富蒸汽气氛   低nox燃烧   高水分燃料   富氮燃料
Abstract高水分燃料,例如褐煤,污泥,轻工业生物质残渣等直接燃烧时,燃烧过程生成大量水蒸汽并存在于燃烧空间,尤其在富氧燃烧时反应器内水蒸汽含量会更高,大量水蒸汽必定与燃烧过程相互作用,影响NOx生成及燃料燃烧效率。尽管在富含水蒸汽条件下燃料燃烧的NOx排放和生成机理已引起关注,而对富氧燃烧条件下水蒸汽对高N燃料的NOx生成的研究较少。本文以相对富N的白酒糟为典型燃料,改变燃烧反应气中的O2浓度,研究水蒸汽对流化床燃烧的NOx生成特性的影响并解析其机理。主要研究内容和结果如下: (1) 白酒糟富氧燃烧。大量水蒸汽在燃烧气氛中的存在原则上可降低NOx的排放,但该影响与燃烧反应气中O2浓度(21~50 vol.%)对NOx生成的影响相互竞争。当氧气浓度低于约35 vol.%时,加入水蒸汽能降低NOx生成;而当氧气浓度高于约35 vol.%时,加入水蒸汽促进了NOx生成。提高燃烧温度可最终使NOx排放量随温度升高而降低,燃烧气氛中水蒸汽的存在延迟了这种降低作用的发生,使在高氧气浓度下NOx生成随温度升高而降低的现象发生在更低的温度下。增大水蒸汽添加量在燃烧反应气中具有较高氧气浓度时反而致使NOx生成量增大。当燃烧反应气中氧气浓度为25 vol.%,40 vol.%和50 vol.%时,增加水蒸汽/燃料质量比使NOx生成从增大变为减小的转折分别发生在蒸汽/燃料质量比为0.0、0.50和0.80。加入水蒸汽后烟气中可燃气体的燃尽率略有降低,由于水蒸汽的增加缩短了反应器中气体停留时间。 (2) 挥发分与半焦分别燃烧。燃料热解时水蒸汽的存在抑制了燃料挥发分的析出和燃料N向挥发分的迁移,使得半焦含氮量稍有增加。燃料氮向NOx的转化主要来自于挥发分N,约占70%,半焦对NOx具有明显的还原作用。比较白酒糟燃烧与半焦和挥发分的单独燃烧表明,强还原(低O2浓度外加水蒸汽)或氧化气氛(高O2浓度)促进了挥发分和半焦的相互作用。 (3) 富氧富水蒸汽条件下燃料燃烧的NOx生成机理。基于实验结果与文献报道的富水蒸汽存在条件下NOx的生成路径,水蒸汽在富氧条件下对燃料燃烧的NOx生成的作用受燃烧反应气中氧气浓度和燃烧气氛中水蒸汽含量对燃烧影响的协调竞争而变化,实质上改变了系统中的H+和OH-的平衡,在较低的温度和氧气浓度下,能产生大量的H+自由基,增强燃烧气氛的还原能力,降低NOx生成,而在高温高氧气浓度下下生成更多的OH-,从而促进NOx生成。
Other AbstractFor combustion of fuels with high moisture contents such as lignite, sewage sludge and many biomass wastes (process residues) from light industry processes, water vapor has to be formed to make the combustion atmosphere rich in vapor. The content of vapor or steam in the combustion atmosphere must be rather higher for the combustion in O2-enriched agents. Although some efforts have been made to clarify how the steam affects the combustion and the emission of nitrogen oxides (NOx) for air combustion, almost no study has been conducted on how the NOx emission varies with the conditions of oxy-fuel combustion in steam-rich atmospheres. This work is devoted to such an understanding by taking distilled spirit lees as the typical N-rich fuel, and the following summarizes the major research contents and obtained results. (1) Combustion of distilled spirits lees in oxygen-rich atmospheres. When keeping the excessive air ratio (ER) at about 1.2, the additionally added steam into the combustion atmosphere caused respectively the lower and higher NOx concentration and fuel-N conversion into NOx than the combustion without additional steam addition at the oxygen concentration below and above about 35 vol.% in the combustion gas or agent. With raising the combustion temperature the NOx emission (shown by both NOx concentration and its corresponding fuel-N conversion) would finally tend to decrease, and the presence of steam in the combustion atmosphere delayed the occurrence of such a tuning of NOx emission from increase to decrease so that the turning appeared at the higher temperature. Elevating the O2 concentration in the combustion agent lowered the occurrence temperature for such a decrease in NOx emission with raising temperature. The critical or turning mass ratio of the added steam against fuel (S/F) which turns the variation of NOx emission with raising S/F from increasing to decreasing was bigger for the higher O2 concentration in the combustion agent. Such a turning S/F ratio was 0.0, 0.5 and 0.8 for the tested O2 concentrations of 25 vol.%, 40 vol.% and 50 vol.% in the combustion agent, respectively. The burnout rate of combustible gases at the reactor exit appeared slightly lower when steam was additionally added into the combustion atmosphere, as a result of shortened gas residence time inside the reactor by the added steam. (2) Separate combustion of volatile and char. The presence of additional steam in fuel pyrolysis suppressed the release of fuel volatile and the transfer of N from fuel (char) into volatile so that the N content in char was larger. The NOx emission for combustion of distilled spirit lees came mainly from the volatile combustion, which was about 70% of the totally formed NOx. Char showed obvious catalytic reduction of NOx formed in volatile burning. This interaction between char and volatile-combustion NOx appeared stronger in the higher reduction (low O2 content and with steam adddition) or oxidation (higher O2 content) combustion atmospheres. (3) Mechanism of NOx formation in atmospheres rich in O2 and steam. Analyzing the pathways of NOx formation based on experimental results and literature reports found that the influence of steam on NOx formation in O2-rich atmospheres is subject to the competition of the influences from varying the contents of steam and oxygen in the atmosphere. This is essentially subject to the variation in the concentration balance between H+ and OH- radicals. At lower temperature and O2 content in the combustion agent, more H+ was formed to increase the reduction capability of the atmosphere and thus to lower the NOx emission, whereas more OH- was formed to increase the oxidation capability of the atmosphere to raise the NOx emission.
Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/15554
Collection研究所(批量导入)
Recommended Citation
GB/T 7714
朱传强. 富氧富水蒸汽条件下流化床燃烧高氮燃料的NOx排放特性[D]. 中国科学院研究生院,2014.
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