煤燃烧中NOx的来源和抑制其生成的有效措施

煤燃烧中产生的ＮＯ＿ｘ是不可忽视的大气污染源。根据其生成机理，它主要来自煤中氮的氧化以及高温下空气中Ｎ＿２和Ｏ＿２的反应。烟气中ＮＯ＿ｘ的浓度范围很宽，从２００至２５００ｍｇ／ｍ￣３，影响因素有燃烧温度、空气过剩系数、煤种和燃烧器类型等。在煤燃烧中采取适当措施，可以有效地降低ＮＯ＿ｘ浓度，如降低燃烧温度、分段燃烧、优化空气过剩系数、烟气部分循环和选用含氨量低的煤等。     

鼓泡流化床煤燃烧中燃料氮转化的研究 （Ⅱ）机理解释

本文讨论了流化床煤燃烧中ＮＯ＿ｘ和Ｎ＿２Ｏ生成与分解机理，提出了鼓泡流化床中ＮＯ＿ｘ及Ｎ＿２Ｏ的主要生成机理和生成过程。     

煤粉颗粒粒度对低温NOx和SO2生成特性的影响

在实验室水平管式电炉上, 在较低温升速率和低温燃烧 （３００℃～５５０℃） 的条件下分别对合山高硫烟煤和晋城烟煤的四种不同粒度煤样的燃料ＮＯｘ 和有机硫分ＳＯ２ 的生成特性进行了研究,并且得到了在当前试验条件下颗粒粒度与燃料ＮＯｘ 和有机硫分ＳＯ２ 生成特性的定性关系及结论． 还研究了以ＣＯ为主的还原气氛在煤燃烧过程中对固氮和固硫所产生的影响以及煤粉超细化燃烧方式下ＮＯｘ 和ＳＯ２ 的生成特性．     

混煤燃烧与动力配煤

论述了混煤燃烧特性、结渣特性、污染生成特性等;研究表明,煤炭混配后对煤粉着火、稳燃,降低锅炉的结渣、沾污和积灰,煤热值以及ＮＯｘ、ＳＯｘ的生成及排放等均有影响;提出动力配煤将是我国燃煤电厂燃料使用的发展方向。     

鼓泡流化床煤燃烧中燃料氮转化的研究（Ⅰ）影响因素

本文研究了各种操作因素如床温、过量氧率、煤种、脱硫、燃料粒径等对流化床煤燃烧中燃料氮转化为ＮＯ＿ｘ和Ｎ＿２Ｏ的影响，为找到减少流化床煤燃烧中氮氧化物的排放措施提供依据。     

氢—二十一世纪的清洁能源

1　前言目前 ,我们使用的燃料主要是石化燃料 ,其中包括有石油、煤、天然气。石化燃料的广泛使用给我们的环境带来了许多不利的方面 ,如大气中ＣＯ2浓度的升高所造成的温室效应 ,气候反常以及工厂、汽车排放的尾气中的ＮＯｘ/ＳＯｘ 所带来的酸雨现象 ,严重影响着人类的生存、生产及生态环境。因此 ,为了减少排放ＣＯ2 、ＮＯｘ、ＳＯｘ 等废气 ,寻求新的可以替代石化燃料的清洁能源成为当今世界各国的热门研究课题。随着世界人口的增长 ,“三Ｅ问题”(经济—能源—环境 )日益成为全球性的问题 ,能源问题不但影响着经济发展 ,而且还直接影响…     

矿物质对煤焦燃烧过程中NO释放规律的影响

在石英固定床反应器上研究了煤焦燃烧过程中矿物质在不同燃烧条件下对NO释放规律的影响.结果表明：煤中的矿物质对燃料氮转化为NO有显著的影响,其影响与矿物质的组成和燃烧条件有关,碱金属Na、K催化半焦氮的氧化在较低的温度下进行并降低半焦氮对于NO的转化率,而Ca、Fe在低温燃烧条件下增加NO的排放,高温时使NO的排放降低;矿物质惰质组分的存在使NO的排放增加;随着煤阶的升高,半焦的反应性降低,燃料氮对于NO的转化率增大;燃料氮的转化率随燃烧温度的升高而增加,但达到极大值后又趋于降低;矿物质对于NO排放量的影响决定于矿物质对于半焦氮的氧化以及半焦还原NO反应催化作用的相对大小.     

燃煤电厂脱硫脱硝研究（I）燃煤过程SO2和NOx的形成机制及排放

本文阐述了煤燃烧过程ＳＯ２和ＮＯｘ的来源及形成机制,分析了影响燃烧过程ＳＯ２和ＮＯｘ排放量的有关因素,提出了正确估算ＳＯ２和ＮＯｘ排放量的近似方法,概述了减少ＳＯ和ＮＯｘ排放量的主要途径。     

循环流化床N_2O排放与控制实验研究

在小型石英玻璃循环流化床内研究了Ｎ２Ｏ和ＮＯ排放控制技术，通过调节床内二次风率的大小及注入二次风的高度、给煤点的位置以及床内注入不含氮的燃料气体，研究了Ｎ２Ｏ和ＮＯ排放特性，分级燃烧能同时降低Ｎ２Ｏ和ＮＯ排放量，二次风率越大，Ｎ２Ｏ和ＮＯ排放量降低幅度越明显，提高二次风注入位置有利于控制Ｎ２Ｏ和ＮＯ排放。从料腿上部给煤能显著降低Ｎ２Ｏ排放，但会适量引起ＮＯ排放的增加。可采用多点给煤，调节给煤分配，来保证Ｎ２Ｏ和ＮＯ排放量均能同时满足环境要求。向床内注入不含氮的燃料气体能同时降低Ｎ２Ｏ和ＮＯ排放量，降低幅度和注入燃料气的流量、位置相关     

汽车尾气转化催化剂发展新趋势

随着汽车发动机新技术的应用及环保法规的日益严格 ,汽车尾气转化催化剂将呈以下发展趋势。首先 ,为提高燃料燃烧效率和减少ＣＯ排放 ,汽车发动机将逐渐采用贫燃技术。据称 ,该发动机比常规发动机的燃料经济性高出 2 0 %～ 2 5%。由于氧气过剩 ,因而将ＮＯｘ 还原脱除就成为一技术难题。目前正在研究的解决方案包括ＮＯｘ 捕集、选择性还原和电热催化剂等。该技术可望于 2 0 0 1年前在欧洲工业化。其次是设计发动机冷启动时能快速预热的催化剂。在欧洲和北美 ,汽车排放污染物主要是在催化转化器预热之前的早期排放引起的。在今后 6年中 ,美…     

煤燃烧过程中NOx的生成和还原

燃煤过程中NO     

Effects of gas temperature fluctuation on the NO release from pulverized coal particle during char combustion

The effects of gas temperature fluctuation on the NO release from pulverized coal particle during char combustion are investigated. The computed results show that the NO formation through the heterogeneous oxidation and reduction reactions is influenced by the gas temperature fluctuation for the particles with initial diameters of 10-50 μm. The gas temperature fluctuation leads to faster NO release from the particle. The heterogeneous NO formation during the char combustion is further enhanced by the increase in the fluctuation amplitude of gas temperature.     

Influence of mineral matter in coal on decomposition of NO over coal chars and emission of NO during char combustion

NO-char reaction and char combustion in the presence and absence of mineral matter were studied in a quartz fixed bed reactor. Eight chars were prepared in a fluidized bed at 950 °C from four Chinese coals that were directly carbonized without pretreatment or were first deashed before carbonization. The decomposition of NO over these coal-derived chars was studied in Ar, CO/Ar and O₂/Ar atmospheres, respectively. The results show that NO is more easily reduced on chars from the raw coals than on their corresponding deashed coal chars. Mineral matter affects the enhancement both of CO and O₂ on the reduction of NO over coal chars. Alkali metal Na in mineral matter remarkably catalyzes NO-char reaction, while Fe promotes NO reduction with CO significantly. The effect of mineral matter on the emission of NO during char combustion was also investigated. The results show that the mineral constituents with catalytic activities for NO-char reaction result in the decrease of NO emission, whereas mineral constituents without catalytic activities lead to the increase of NO emission. Correlation between the effects of mineral matter on NO-char reaction and NO emission during char combustion was also discussed.     

Modeling of in-line low-NOx calciners—a parametric study

Simulations with a heterogeneous model of an in-line low-NO_x calciner, based on non-isothermal diffusion-reaction models for char combustion and limestone calcination combined with a kinetic model for NO formation and reduction, are reported. The analysis shows that the most important hydrodynamic parameter is the mixing rate of preheated combustion air into the sub-stoichiometric suspension leaving the reducing zone and the most important combustion parameter is the char reactivity. Also, the calcination rate modifies very much the temperature in the calciner, char and limestone conversion and NO emission. Carbon monoxide is a key component for the reduction of NO and reliable data for the kinetics of NO reduction by CO over CaO are very important for the prediction of the NO emission. The internal surface area of char and limestone particles influences the combustion and calcination rates and thereby the char and limestone conversion and the NO emission.     

Experimental study of the reduction mechanisms of NO emission in decoupling combustion of coal

Decoupling combustion realized by dividing the coal combustion process into coal pyrolysis and the combustion of volatiles and char can reduce NO emission from coal fired stoves. In this study, decoupling combustion process in coal fired stoves was simulated experimentally with a specially designed dual-bed model reactor. The reactor can simulate various combustion sub-steps occurring in a real stove. The experimental results suggest that NO reduction by burning char may be the major contribution for NO reduction in decoupling combustion process. Compared with normal combustion, up to 40% NO emission can be reduced in decoupling combustion process.     

分解炉空气分级燃烧及NOx排放特性研究

随着我国经济的飞速发展,作为重要基础材料的水泥产品需求量极大且趋于稳定。水泥生产过程中的NOx排放与燃煤火电厂和汽车尾气产生的NOx排放已成为空气污染的主要来源,而分解炉是降低水泥生产工艺中NOx排放的有效设备。笔者在引入高温烟气的模拟分解炉内进行空气分级燃烧试验,研究配风位置、配风比例以及石灰石/煤比例对分解炉内燃烧和NOx排放特性的影响规律。试验稳定过程中,高温烟气发生装置的给煤量和配风量保持不变。此时,高温烟气发生装置的时间平均温度为911℃,其产生的高温烟气温度稳定在750℃左右,高温烟气中NOx主要以NO和N2O的形式存在,其浓度分别为261.49×10^-6和12.96×10^-6。该股高温烟气将模拟实际回转窑产生的烟气进入分解炉内。在分解炉的上部区域(距离顶部0~2 000 mm区域)的温度为800~1 000℃,与实际分解炉运行温度一致,排放烟气中NOx主要以NO和N2O形式存在。随着中间配风位置的下移,煤粉燃烧放热区域下移,而顶部区域的石灰石吸热量变化较小,则原有热量平衡被打破且原有吸热量高于现有放热量,导致顶部区域内燃烧温度降低。此时,还原气氛中煤粉燃烧和石灰石分解反应时间均变长,导致NOx的还原反应更加充分。但石灰石分解产生的氧化钙(CaO)作为中间产物会促进NO的生成反应,其反应时间增加也促进了NO的生成;另一方面,石灰石作为催化剂参与焦炭和挥发分还原NO的反应过程,分解炉顶部区域的温度下降使得该还原反应变弱。综上,NO的最终排放浓度是以上反应的综合结果。随着配风位置的下移,该变化对NO的生成作用更加明显,故NO的排放浓度逐渐升高。当一级风量与二级风量的配风比例降低时,分解炉上部区域的煤粉燃烧份额减少和石灰石分解量降低,而分解炉下部区域的煤粉燃烧份额增加和未分解的石灰石份额增加,但石灰石的吸热增加量高于燃烧增加份额的放热量,因此分解炉内整体温度均降低。分解炉内NO浓度是由石灰石催化的氧化过程和还原过程综合决定的。一级风量变小时,尾部CO浓度随之增加,烟气中NO浓度呈现降低的趋势。当石灰石/煤比例增加时,分解炉内沿程温度逐渐下降。随着石灰石给粉量增加,分解炉内石灰石受热分解产生的CaO浓度增加,CaO催化NO还原反应更剧烈,从而NO浓度逐渐降低。而石灰石给粉量增加和分解炉温度降低的过程导致尾部的CO浓度升高。     

Conversion of volatile-nitrogen and char-nitrogen to NO during combustion

The design of low emission combustion chambers using low NO_x strategies involving staged burning or stratified combustion requires a detailed understanding of the combustion processes of the fuel volatiles and char burning. In this paper some aspects of the combustion of coal-volatiles and char are considered. The extreme cases of volatile combustion, namely premixed and diffusive burning are examined in order to consider the range of NO_x reduction options available to the combustion chamber designer. A similar set of situations is examined for char burning and the release of the fuel-nitrogen to form NO.

The implications of the processes are considered in two practical applications, those of the high temperature combustion found in pulverised coal burning and in a lower temperature regime of the conditions under fluidised bed combustion. In the case of pulverised coal flame the degree of mixedness of the volatiles played a dominant part in determining the extent of NO formation whilst the role of char-nitrogen is only to form NO and NO reduction is limited because of the short residence time and low char concentrations at the end of the reaction zone. In a circulating fluidised bed combustor it was concluded that a different situation can arise. If the bed is sufficiently large enough to give a residence time of several seconds, then the NO initially formed in the fluidised bed is reduced by the carbon in the top of the bed and the riser under steady state conditions and its concentration at the exit can be estimated by equilibrium calculations.     

Experimental study of NO reduction over biomass char

Some biomass fuels produce more NO_x than coal on the basis of heating value, giving rise to the necessity and importance of controlling NO_x emission in biomass combustion. The present study investigated the NO reduction over biomass char in a fixed bed quartz reactor in the temperature range of 973–1173 K. The reaction rates of three biomass chars (sawdust, rice husk and corn straw) with NO were compared with Datong bituminous coal char. The results show that the reaction orders of biomass chars for NO are of fractional order and independent of temperature. Biomass chars are more active in reducing NO than coal char. The characteristics of biomass char affect NO conversion. Biomass char formed at high pyrolysis temperature, especially large in particle size, is less active in reducing NO. To some extent, increase of reaction temperature and char loading enhance NO conversion. There exists an optimum bed height for the highest NO conversion. Moreover, NO reduction over biomass char is also enhanced in the presence of CO, O₂ and SO₂.     

Catalytic effect of Na–Fe on NO–char reaction and NO emission during coal char combustion

The catalytic reduction of NO over coal-derived chars and the catalytic effect of Na–Fe on NO emission during char combustion were investigated in a quartz fixed bed reactor. The catalytic characteristics of Na and Fe in the NO–char reaction were studied and compared in detail. The results show that the catalytic activity of Na depends greatly on its loading amount, while the activity of Fe is more sensitive to temperature. Na–Fe composite catalysts were also prepared with chars as support. Synergistic effect was found both in the reduction of NO and the char combustion. The Na–Fe composite catalysts exhibit significantly higher catalytic activity than the mono-metallic catalysts with the same metal loading amount. It is intriguing to note that the effectiveness of the catalysts on reducing NO emission during char combustion is in the same order as that in the NO–char reaction, i.e. the chars with catalysts not only have high activity in NO–char reaction but also emit less NO during their combustion.     

Numerical analysis of pulverized coal combustion characteristics using advanced low-NOx burner

A three-dimensional numerical simulation is applied to a pulverized coal combustion field in a test furnace equipped with an advanced low-NO_x burner called CI-α burner, and the detailed combustion characteristics are investigated. In addition, the validities of the existing NO_x formation and reduction models are examined. The results show that a recirculation flow is formed in the high-gas-temperature region near the CI-α burner outlet, and this lengthens the residence time of coal particles in this high-temperature region, promotes the evolution of volatile matter and the progress of char reaction, and produces an extremely low-O₂ region for effective NO reduction. It is also found that, by lessening the effect of NO reduction in Levy et al.'s model and taking the NO formation from char N into account, the accuracy of the NO prediction is improved. The efficiency factor of the conversion of char N to NO affects the total NO concentration downstream after the injection of staged combustion air.