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大颗粒煤燃料氮迁移特性的实验研究与层燃NOx数值模拟
中文摘要

我国能源结构以煤为主,煤炭直接燃烧是主要的利用方式。燃煤工业锅炉是仅次于燃煤电厂的第二大耗煤大户,量大面广的燃煤链条炉排锅炉氮氧化物的排放不可忽视。链条炉排锅炉大颗粒煤层燃的过程与煤粉悬浮燃烧有很大差异,大颗粒煤在热解、焦炭氧化、焦炭异相还原NO等反应过程中燃料氮的迁移转化十分复杂,但并未得到足够的重视,亦缺乏深入的研究。燃煤链条炉排锅炉现有的脱硝技术大多直接借鉴电站锅炉,脱硝成本高,经济性不佳,适合燃煤链条炉排锅炉清洁高效燃烧的运行方案十分缺乏。 本文针对大颗粒煤热解过程中氮官能团的迁移转化规律和含氮小分子气体HCN和NH₃的析出规律进行实验研究,采用水平管式炉实验台,结合XPS、FTIR等测试方法,研究了不同煤种、不同热解温度、不同粒径等因素的影响。热解时煤中不同氮官能团会发生相互转化并分解为含氮小分子气体HCN和NH₃。随着热解温度升高,煤中氮官能团总量下降,原煤热解析出的HCN和NH₃增加。煤粉在堆积方式热解时,HCN与焦炭发生二次反应生成更多的NH₃。大颗粒煤与煤粉堆积方式相比,颗粒内部煤焦结构更加致密,热解气在焦炭孔隙内停留时间更长, HCN与焦炭发生二次反应更剧烈,NH₃生成量更大。在研究燃煤链条炉排锅炉层燃NO〓模型时不能直接套用煤粉热解数据,需要采用大颗粒煤热解实验获得含氮小分子气体分布的相关参数。 本文针对大颗粒煤焦氧化过程中焦炭氮的迁移转化规律进行了实验研究,采用水平管式炉实验台,结合XPS、testo烟气分析仪,研究了不同氧化温度、不同煤种、不同氧浓度、不同颗粒粒径等因素的影响。随着温度升高,焦炭氮氧化生成NO的转化率先增大后减小。在低温下,焦炭氧化生成大量含氧官能团促进了焦炭异相还原NO。在高温下,焦炭颗粒表面气化反应增强存在CO氛围,焦炭表面CO与煤焦表面C(N)作用生成C(NCO)促进NO的还原。随着煤阶的升高,焦炭反应性降低,焦炭孔隙内部还原NO的活性位减少,焦炭氮氧化生成NO增多。大颗粒焦炭氧化生成的NO主要被颗粒内部自身焦炭还原。大颗粒焦炭因内部孔隙结构致密,增大了气体扩散阻力,在颗粒内部形成低氧环境,有利于生成含氧官能团促进焦炭异相还原NO;同时,内部焦炭氮氧化生成的NO在向外扩散过程中在焦炭内部停留时间更长,增加了被焦炭异相还原的比例。根据实验数据,分别考虑了焦炭氧化速率、温度和粒径的影响,建立了大颗粒煤焦炭氮氧化生成NO的数学模型。 本文针对大颗粒煤焦异相还原NO规律进行了实验研究,采用固定床实验台,结合XPS、XRD、testo烟气分析仪,研究了不同温度、不同煤种、不同颗粒粒径、炭层厚度等因素的影响。热解温度升高后,焦炭晶体结构更加有序,异相反应的活性位减少,焦炭还原NO速率变慢,同时,焦炭中氧官能团总量下降,进一步减弱了焦炭氧官能团对焦炭异相还原NO的促进作用。与煤粉焦相比,大颗粒煤焦的反应表面积减小,还原NO速率减慢。大颗粒煤焦炭层厚度增大后,参与反应的焦炭活性位增多,同时增加了NO在炭层中停留时间,有利于焦炭还原NO。根据实验数据建立了大颗粒煤焦还原NO的数学模型。 为揭示燃煤链条炉排锅炉层燃过程中NO生成规律,本文采用单元体炉进行了实验研究,研究了不同煤层厚度、不同配风方式、不同颗粒粒径以及粒径分层燃烧方式等因素的影响。床层表面NO生成为典型双峰结构,第一个峰值主要为挥发分中含氮小分子气体HCN和NH₃氧化形成,第二个峰值为床层燃尽阶段焦炭氮氧化形成,双峰之间的谷值是由于焦炭层对NO的还原而形成。煤层厚度减小,焦炭层还原NO的程度减弱,生成更多的NO。采用推迟配风方式,床层的中前部氧量少,氧化生成NO少,同时焦炭层还原NO的程度加强,有利于抑制NO的生成。煤焦粒径增大后,焦炭层还原NO速率减小,床层生成NO增多。粒径分层燃烧时,床层上部焦炭粒径较小,有利于焦炭层还原NO。通过对两台链条炉排锅炉进行了配风调试,采用推迟配风和煤层厚度增大的运行方式,均增强了焦炭层对床层NO〓的还原,降低了锅炉尾部NO〓排放浓度,试验取得了低NO〓调控的预期效果。 本文在团队燃煤链条炉排锅炉燃烧模型的基础上建立了层燃NO〓模型,并应用单元体炉的层燃实验对模型准确性进行了验证。应用本文所提出的层燃NO〓模型,以一台在用的链条炉排锅炉为例,通过对床层过量空气系数、炉拱形状和空气分级燃烧的数值分析发现:降低床层过量空气系数,增加焦炭还原层厚度,有利于减少床层NO的生成;并利用人字形后拱将尾部焦炭氧化生成的NO导流到前拱下方与热解产生的NH₃反应,实现炉内还原NO的效果,最后得到链条炉排锅炉拱风组合低NO〓燃烧运行原理,为实际锅炉低NO〓燃烧改造提供技术支撑。 关键词:大颗粒煤层燃、燃料氮迁移、NO〓模型、拱风组合、低NO〓燃烧

英文摘要

The primary energy structure of China is based on coal, most of which are utilized by direct combustion. Industrial boiler ranks in the second place of coal consumption, only less than the coal-fired power plant. Thus the NO〓 emission problems should not be ignored for the coal-fired grate boiler. The process of macro coal particle grate combustion in the grate boiler is very different from the pulverized coal combustion. The fuel nitrogen migrations and conversions are very complicated in the macro coal particle utilization processes including pyrolysis, char oxidation and heterogeneous reduction over char. However these processes have not gained enough attentions and are lack of in-depth study. The nitrogen removal technologies in the grate boiler nowadays primarily learned from the power plant boiler, of which the nitrogen removal cost is high. The clean and effective ways of combustion in the grate boiler are needed. The trends of nitrogen function groups migration and release of HCN and NH₃ for macro coal particle pyrolysis were experimentally investigated. A horizontal fixed bed was used, combining the XPS and FTIR detection methods, to study effects of coal type, pyrolysis temperature and particle sizes. The nitrogen function groups mutual convert and decompose to light gas-N like HCN and NH₃ during coal pyrolysis. With the rising pyrolysis temperature, the total amount of nitrogen function groups decrease and the released HCN and NH₃ amount increase. When the pulverized coal was piled up during pyrolysis process, the more amount of NH₃ was released due to secondary reaction between char and HCN. The pyrolysis for the macro coal particle produced more NH₃. Because the macro char particle is denser, the residence time for the pyrolysis gases staying in the char pores becomes longer, the secondary reaction is more severe and more HCN converts to NH₃. In the study of grate combustion in coal-fired grate boiler, pulverized coal pyrolysis data cannot be directly applied to NO〓 model. The NO〓 model requires macro coal particle pyrolysis experiment to obtain related parameters of the distribution of light gas-N. The trends of char nitrogen migration for macro char particle oxidation process were experimentally investigated. A horizontal fixed bed was used, combining the XPS and testo flue gas analyzer, to study effects of combustion temperature, coal type, oxygen concentration and particle sizes. With the increasing temperature, The NO from char oxidation first increases then decreases. The low-temperature oxidation of char produces large amount of oxygenated function groups, promoting the NO reduction over char. At high temperatures, the gasification reaction on the char surface becomes more severe. Meanwhile on the char surface the CO reacts with C(N) yielding C(NCO), which promotes the NO reduction. With coal rank increasing, the char reactivity decreases, the active sites for NO reduction in in char pores decreases and the NO produced by char nitrogen oxidation increases. In macro char particle oxidation process the NO reduced over char is due to the in-site reduction by carbon on the inner surface of the char pores. The macro char particle is denser and there are large amount of oxygenated function groups in the char pores with low oxygen environment, promoting the NO reduction over char. Meanwhile the residence time for the NO staying in the char pores becomes longer, and more NO converts to N₂. Based on experimental results, given the effects of char oxidation rate, temperature and particle sizes, a numerical model for the NO formation by the char nitrogen oxidation in the macro coal particle was established. The rates of NO heterogeneous reduction over macro char particle were experimentally investigated. A fixed bed reactor was used, combining the XPS, XRD and testo flue gas analyzer, to study effects of temperature, coal type, particle sizes and the height of char bed. With an increase in pyrolysis temperature, the crystal structure of char becomes ordered, while the active sites for heterogeneous reduces and the rate of NO reduction over char is lowered. Meanwhile the total amount of oxygenated function groups decreases, which lowers the promotion of the NO heterogeneous reduction by the oxygenated function groups. The reactive surface area for the larger particle with identical weight is less than the smaller particle, thus the NO reduction rate is slower for the larger one. With an increase in the char bed height, the involved carbon active sites increase, meanwhile the residence time for the NO in the char bed also increases, these factors are favorable for the NO reduction over char. Based on experimental results, a calculation model for the NO reduction over macro particle was built. In order to reveal the NO formation characteristics in the process of macro coal particle grate combustion in the grate boiler. A unit boiler reactor was used to study effects of coal bed height, air distribution ways, particle sizes and layered bed with multiple particle sizes. The NO formation on the char bed surface shows typical two-peak structure, the first peak is due to the oxidation of HCN and NH₃ in the volatile, the second peak is due to the char nitrogen oxidation during the burn out stage of the grate combustion. The lowest value between the two peaks is due to the NO reduction over char. The decrease in the height of coal bed lowers the char reduction layer, decreasing the NO reduction amount, and the NO from coal bed grows. The delay air distribution decreases the front oxygen, leading to less NO by oxidation. Meanwhile the reaction between char reduction layer and NO becomes more severe which is favorable for the NO reduction. With increasing char particle, the NO reduction rate decreases, the effect of char reduction layer is weakened and more NO formed. On the condition of layered bed with multiple particle sizes, the upper smaller char particles are favorable of NO reduction. By using delay air distribution and thicker coal bed height operation mode, the reaction between char reduction layer and NO becomes more severe and two sets of chain grate boiler’s NO〓 emission are reduced. The expected results of the low NO〓 control test have been achieved. Finally a NO〓 model for grate combustion in the grate boiler is built, based on previous works of numerical modeling. The numerical results were validated by the unit boiler experimental results. An in-use industrial grate boiler was numerically investigated. Some conclusions are obtained through the numerical analysis of the excess coefficient for coal bed, the arch shape and the air staged combustion. First reducing the excess coefficient and increasing the height of char reduction layer can reduce the NO formation from the char bed. Second, utilizing the back arch to get some of the NO formed by char oxidation to the front position beneath the front arch, forcing them to react with NH₃, lowers the NO in the gaseous space. Finally a mechanism of low NO〓 combustion operation based on the combination of arch shape and air distribution in grate boiler is obtained, which offers a solid technical support for the low NO〓 operation improvement of the realistic boilers. Key words: Macro coal particle grate combustion, Fuel nitrogen migration, NO〓 model. Combination of arch shape and air distribution. Low NO〓 combustion

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