Kinetic Model and Simulation of Promoted Selective Non-catalytic Reduction by Sodium Carbonate

Abstract The detailed kinetic model of selective non-catalytic reduction （SNCR） of nitric oxide, including so-dium species reactions, was deyeloped on the basis of recent studies on thermal DeNOx mechanism, NOxOUTmechanism and promotion mechanism of Na2CO3. The model was validated by comparison with several experi-mental findings, thus providing an effective tool for the primary and promoted SNCR process simulation. Experimental and simulated results show part-per-million level of sodium carbonate enhances NO removal efficiency andextend the effective SNCR temperature range in comparison with use of a nitrogen agent alone. The kinetic modeling, sensitivity and rate-of-production analysis suggest that the performance improvement can be explained as ho-mogeneous sodium species reactions producing more reactive OH radicals. The net result of sodium species reac-tions is conversion of H2O and inactive HO2 radicals into reactive OH radicals, i.e. H2O＋HO2=3OH, which enhances the SNCR performance of nitrogen agents by mainly increasing the production rate of NH2 radicals. More-over, N2O and CO are eliminated diversely via the reactions Na＋N20=NaO＋N2, NaO＋CO=Na＋CO2 andNaO2＋CO =NaO＋CO2, in.the pro.moted SNCR process, especially in the NOxOUT process.

Experimental and Modeling Study on de-NO_x Characteristics of Selective Non-catalytic Reduction in O_2/CO_2 Atmosphere

An experimental study of thermal de-NOx using NH3 as reductant in 02/C02 atmosphere with the effect of S02 and different additives was performed in a drop tube furnace. Results show that the optimum temperature win- dow is 841-1184 ℃, and the optimum reaction temperature is about 900 ℃ with a de-NOx efficiency of 95.4%. A certain amount of S02 has an inhibiting effect on NO reduction. The effect of additives, including Na2C03, C2H5OH and FeCI3, on NO reduction by NH3 is also explored. The addition of Na2CO3 and FeCI3 is useful to widen the tem- perature window and shift the reaction to lower temperature for the efficiency is increased from 30.5% to 74.0% and 67.4% respectively at 800 ℃. Qualitatively, the modeling results using a detailed kinetic modeling mecha- nism represent well most of the process features. The effect of Na2CO3, C2H5OH and FeCI3 addition can be reproduced well by the Na2C03, C2H5OH and Fe（CO）5 sub-mechanism respectively. The reaction mechanism analysis shows that the effects of these additives on NO reduction are achieved mainly by promoting the produc- tion of OH radicals at lower temperature.

Control of nitrogen oxides emission by selective non-catalytic reduction in preheating section during iron ore pellets production

Reducing the NO_(x) emission from pelletizing process is of great importance to the green development of iron and steel industry.The flue gas temperature of preheating(PH)section during grate-kiln iron ore pelletizing process typically ranges within 850–1050℃,which meets the temperature requirements of selective non-catalytic reduction(SNCR)for NO_(x).The in-bed SNCR behavior of NO_(x) in the PH section was investigated,and the influence of relevant parameters was revealed.Results show that with the flue gas temperature rising,the denitration rate reached a peak value and then declined,where the appropriate temperature range was 950–1000℃.Increasing the NH_(3)/NO ratio(NSR)contributed to improving the denitration rate,and the appropriate NSR was 1.0.Oxygen content in the flue gas also showed an important influence on denitration rate,which reached a peak value and then dropped with the oxygen content rising.Under the condition of 18 vol.%oxygen content,the denitration reaction mainly occurred in the form of 4NO+4NH_(3)+O_(2)=4N_(2)+6H_(2)O.For restricting the competitive reaction of NH_(3) oxidation,the oxygen content in flue gas of PH section should be kept at an appropriate range.In general,the denitration rate reached about 25%in the PH section through spraying ammonia.

N₂O Formation in Selective Non-catalytic NO_xReduction Processes

Nitrous oxide is not an environmentally regulated species in the U.S., but it does participate in the stratospheric ozone chemistry and contributes to the greenhouse effect. Nitrous oxide has been found to be a by-product of the selective non-catalytic reduction process. Chemical kinetic calculations demonstrated that the formation of nitrous oxide in the urea-based selective non-catalytic reduction process is linked to the conversion of NO by cyano species released from the process parent compounds. This conversion occurs within in temperature window between 850 and 1050℃. With urea injection, nitrous oxide emissions represent up to 20 percent conversion of the NOx reduced. The amount of nitrous oxide formed depends primarily on the process temperature, the amount of chemical injected, the initial NOx level, and the carbon monoxide level in the gas stream. These observations, which were based on the chemical kinetics of the process, should be considered in designing selective non-catalytic reduction systems to minimize nitrous oxide by- product formation.

Numerical Simulation of Urea Based SNCR Process in a Trinal-Sprayed Precalciner

In order to study the combustion characteristics,NOx emission and NH3 slip in a new trinal-sprayed precalciner,the simulations of combustion and aqueous urea solution based selective non-catalytic reduction(SNCR)process were conducted by computational fluid dynamics in this precalciner,the effects of different injection heights,different injection flow rates and stratified injection under different flow rates on SNCR process were studied.The results showed that the flow field was symmetrically distributed in the precalciner,and the flue gas from the rotary kiln formed the recirculation region on both sides of the cone body,which increased the residence time of the solid particles.The temperature was mainly between 1100 K and 1250 K in the middle and upper column of the precalciner,which met the demand of the pulverized coal combustion and raw material decomposition.The concentration of NO at the outlet of the precalciner was 559 ppm,moreover,different injection heights and different injection flow rates had a strong influence on NOX removal efficiency and NH3 slip.The aqueous urea solution should be injected at SNCR-1 to prolong the residence time of NH3,and injection flow rate had an optimal flow rate but not the higher the better.When the injection flow rate under stratified injection was 0.019 kg/s,which could play a better optimization role on NO removal efficiency on the basic of the injection flow rate.In consideration of cost effective,a stratified injection with an injection flow rate of 0.019 kg/s and an injection height of 20 m,25 m and 30 m was suggested as a compromise of a satisfactory NOx reduction rates and reasonable NH3 slip.Under this condition,numerical simulation result showed that NOx concentration at the outlet of precalciner was 297.27 mg/Nm3 and NH3 slip was 4.67 mg/Nm3,meeting emission standard.

Experiment and mechanism investigation on advanced reburning for NO_x reduction:influence of CO and temperature

Pulverized coal reburning, ammonia injection and advanced reburning in a pilot scale drop tube furnace were inves- tigated. Premix of petroleum gas, air and NH3 were burned in a porous gas burner to generate the needed flue gas. Four kinds of pulverized coal were fed as reburning fuel at constant rate of 1g/min. The coal reburning process parameters including 15%~25% reburn heat input, temperature range from 1100 °C to 1400 °C and also the carbon in fly ash, coal fineness, reburn zone stoichiometric ratio, etc. were investigated. On the condition of 25% reburn heat input, maximum of 47% NO reduction with Yanzhou coal was obtained by pure coal reburning. Optimal temperature for reburning is about 1300 °C and fuel-rich stoichiometric ratio is essential; coal fineness can slightly enhance the reburning ability. The temperature window for ammonia injection is about 700 °C^1100 °C. CO can improve the NH3 ability at lower temperature. During advanced reburning, 72.9% NO reduction was measured. To achieve more than 70% NO reduction, Selective Non-catalytic NOx Reduction (SNCR) should need NH3/NO stoichiometric ratio larger than 5, while advanced reburning only uses common dose of ammonia as in conventional SNCR technology. Mechanism study shows the oxidization of CO can improve the decomposition of H2O, which will rich the radical pools igniting the whole reactions at lower temperatures.

SNCR脉动式喷射脱硝数值模拟研究

选择性非催化还原(selective non-catalytic reduction,SNCR)脱硝技术,在无需催化剂的条件下直接将还原剂氨或尿素喷入烟气往往效率不是很高,还原剂与烟气的混合是制约脱硝效率的关键因素之一。以某90t/h层燃炉二次燃烧室为研究对象,运用Fluent软件对炉膛内的烟气流动、传热传质、脱硝反应进行数值模拟,并在不改变氨氮摩尔比和喷枪布置位点的情况下对还原剂喷射方式进行了优化研究。研究结果表明:当还原剂采用脉动速度喷射方式、喷枪角度水平向下45°时可以得到最好的脱硝效果,氨的均匀性指数为0.549,脱硝效率达到58.13%。研究结果为SNCR脱硝技术的优化提供了理论依据和技术参考。

大型燃煤锅炉SNCR过程数值研究

基于Fluent软件平台,对410t/h燃煤锅炉中选择性非催化还原(SNCR)过程进行建模和模拟计算.计算结果与实验测量数据吻合很好,阐明了利用这种方法预测大空间、复杂温度场、复杂流场的锅炉炉膛中氮氧化物生成及还原过程的可行性.计算结果表明,在还原剂与氮氧化物初期混合条件有限的情况下,脱硝效果决定于物质的湍流扩散和温度场之间的相互作用.研究在不同喷射截面上温度和氨氮摩尔比对SNCR过程的影响,结果表明喷射截面应该取在平均温度位于SNCR"温度窗口"中部的截面所对应的高度处,在该工况下脱硝率在42%以上,且随氨氮摩尔比由1.0提高到2.2,脱硝率增长约58%.综合考虑尾部漏氨,氨氮摩尔比应该控制在1.4以下.

循环流化床烟气SNCR脱硝机理与实验

采用电加热循环流化床锅炉模拟实际烟气,在炉膛出口喷入氨水或同时添加H2、CH4进行脱硝,实验结果表明:最佳停留时间、氨氮物质的量之比分别为0.6,s和1.5,氨水最佳反应温度为920,℃,添加H2最佳温度降低至750,℃,添加CH4最佳温度降低至840,℃.同时为深入认识其化学反应规律,采用基元反应动力学模型对氨水与NOx反应进行了模拟计算,模拟结果表明,该基元反应模型较好地解释了反应机理.

联合空气分级与SNCR在300MW锅炉上的应用

在一台300Mw燃煤锅炉上应用联合空气分级与选择性非催化还原（SNCR）技术，分析该联合脱硝技术的脱硝效果，探讨各种喷枪组合以及喷射点选择对脱硝的影响．实验结果表明，在应用联合脱硝技术后，该锅炉的NO。排放质量浓度为186mg／m^2，联合脱硝效率达到75．2％，其中SNCR系统的单独脱硝效率为46．8％．采用长短枪配合的喷射方案，将还原剂从炉膛温度为950℃～1100℃的位置喷入，可以获得较高的脱硝效率和较低的氨逃逸量．SNCR脱硝系统的投运对飞灰碳质量分数、炉渣碳质量分数没有显著的影响，但会造成排烟温度上升3．5℃，锅炉效率降低约0．15％．

气态氨作还原剂的SNCR脱硝工艺的试验研究与模拟

在自行研制的试验台上对氨气作还原剂的SNCR脱硝工艺进行了实验研究,并利用化学反应动力学软件Chemkin 4.1进行了模拟。通过试验发现,采用自制改进型槽缝式TB系列喷嘴内部喷入的方式明显优于侧喷,提高了NOx的去除率。以NOx去除率高于50%为标准,试验所得温度窗口为863~937℃。随着氨氮比的增大,NOx的去除率和氨的泄漏量增大,当氨氮比大于1.0后,NOx去除率的随氨氮比增大的幅度减小,而氨的泄漏量增大幅度增大。利用Chemkin 4.1的模拟所得的温度窗口和NOx去除率与实验基本相符,通过对详细机理中重要基元的分析,得出了各温度下的主要反应途径。

CFB锅炉SNCR烟气脱硝空预器堵塞原因分析及处理

选择性非催化还原(SNCR)法是目前CFB锅炉常用的烟气脱硝技术。对一台锅炉空气预热器堵塞的原因进行了研究,对堵塞物成分、喷氨、燃煤硫分、空气预热器吹灰等进行了分析。通过煤种调整、喷氨改造、合理投运空预器吹灰等手段,成功解决了SNCR脱硝导致空预器堵塞的问题。

基于支持向量机的电站锅炉SNCR系统建模

将支持向量机方法应用于电站锅炉SNCR系统的参数调节,建立了锅炉热态参数和SNCR系统参数与脱硝率之间的关系模型,并与神经网络方法预测结果进行了对比.结果表明,平均相对误差和均方根误差都较后者降低了60%以上,同时线性相关系数r也提高了11%.通过该模型研究了典型工况下尿素用量和稀释水流量对脱硝率的影响,将其与机理性研究得到的结论进行比对,表明该模型所包含的信息很好地反映了样本数据中的规律.最后,研究了两个重要参数——核参数和边界参数对预测性能的影响,发现核参数取值应在[2,6],此时,误差水平较低且对边界参数不敏感.

低氮燃烧加SNCR脱硝技术在超临界锅炉上的应用

介绍了燃料燃烧过程中NOx生成原理及控制技术,结合某600 MW超临界机组锅炉脱硝项目,分析了低氮燃烧加SNCR来减少NOx排放的工艺流程和技术特点,该系统的投产为其以后的推广做出了范例。

600MW超临界锅炉SNCR烟气脱硝系统的启动调试

介绍了600MW超临界锅炉SNCR烟气脱硝原理及脱硝工艺,分析了锅炉燃烧褐煤时尿素溶液流量、喷射枪的位置和机组负荷等参数对NOx排放浓度的影响,给出一般运行工况下的操作指导,初步得出不同负荷下的优化运行经验,总结了SNCR烟气脱硝工程的调试经验,并指出调试过程需要注意的问题。

MMT添加剂解决CO对SNCR的抑制作用

通过实验研究MMT和CO添加剂对SNCR反应的影响,着重研究MMT和CO在协同作用下对SNCR脱硝效率和CO排放浓度的影响,通过实验数据分析MMT添加剂在SNCR体系中的反应机理.结果表明:喷入模拟烟气中MMT添加剂的最佳质量浓度为40mg/L,将此质量浓度的MMT添加到以尿素为还原剂的SNCR反应体系中,可以使反应温度窗口拓宽约30℃,最佳反应温度为900℃,对应的脱硝效率得到提高;CO的存在会使SNCR反应窗口向低温区移动,反应窗口宽度变窄,最大脱硝效率降低,且CO浓度越高,变化越明显.当模拟烟气中CO的体积分数从0增加到800μL/L时,NO脱除率从83%下降到75%,最佳脱除温度从950℃下降至800℃;在存在CO的SNCR体系中,MMT添加剂不仅可以提高低温区SNCR反应的脱硝效率,而且能够在高温区抵减CO对SNCR脱硝的抑制作用,扩宽脱硝温度窗口宽度至240℃,在反应窗口内提高脱硝效率,使得最大NO脱除率在CO体积分数为0μL/L时从83%增大至89%;在CO体积分数为200μL/L时,最大NO脱除率从80%增大至85%;在CO体积分数为500μL/L时,最大NO脱除率从79%增大至85%;在CO体积分数为800μL/L时,最大NO脱除率从75%增大至83%.

基于燃烧控制和SNCR＋烟气CFB的循环流化床锅炉超低排放示范

循环流化床工业锅炉也要具备超低排放的能力。除了深度挖掘循环流化床燃烧自身低污染排放潜力之外,还可以对烟气进一步控制处理。利用炉内低氮燃烧,配合低成本的选择性非催化还原(SNCR)控制NOx;利用炉内脱硫产生的CaO在烟气循环流化床吸收塔中吸收SO_2和其他污染物,有一定的优势,为此开展了工程示范。示范在燃用Ⅱ类烟煤的75 t/h蒸汽工业锅炉进行。运行表明,炉内不投石灰石时,即使不投SNCR,NOx能控制在50 mg/m3以内,此时烟气循环流化床吸收塔必须喷消石灰浆,吸收塔出口SO_2浓度有一定的波动。炉内投石灰石后,NOx排放因受其催化浓度迅速增加,此时需及时投入SNCR;烟道上增湿活化、控制烟气循环流化床吸收塔中的温度,能够稳定地将SO_2控制在35 mg/m3。可见,基于燃烧控制和SNCR+烟气CFB控制,循环流化床锅炉在高效运行的同时,烟气污染物能可靠地实现超低排放。

干态氨法SNCR烟气脱硝工艺设计及经济性分析

设计了一套适用于300 MW级CFB锅炉干态氨法SNCR烟气脱硝装置,其处理烟气量为100万Nm^3/h,NO_x含量为390 mg/Nm^3,系统脱硝效率不低于50%,烟气中NO_x排放浓度≤195 mg/Nm^3。该系统经济性分析结果表明,经济性受氨水市场价格影响较大,在当前氨水价格及脱硝电价补贴条件下,该系统总投资约400万元,年运行费用约1 117.9万元,年综合收益约667.6万元。

碱回收锅炉脱硝技术综述

主要论述了当前碱回收锅炉脱硝的技术方案,从反应原理、工艺方法、项目组织、生产运行和达到的结果简述了各种方法的适应性,从而推断出低温SCR脱硝工艺在碱回收锅炉脱硝中具有较大的推广前景。

350t/d垃圾焚烧炉污泥掺混燃烧与SNCR脱硝特性的数值模拟

以某台350t/d垃圾焚烧炉为研究对象,利用CFD数值模拟方法,研究掺混不同比例半干化污泥(含水率60%)下的燃烧特性以及污染物排放特性,并对选择性非催化还原(selective non catalytic reduction,SNCR)脱硝喷枪位置进行了优化研究。结果表明:一烟道烟气温度与污泥掺混量成反比,氧含量与污泥掺混量成正相关,在3%~13%的污泥掺混量中,7%是比较合适的污泥掺混量,污泥掺混量低于10%时,第一烟道高温区的燃烧状态能达到二恶英控制的燃烧要求。NO_x生成量与污泥掺混量成正相关,在10%掺混量时仅采用SNCR方法焚烧炉出口烟气NO_x含量高达278.63mg/m^3,未能达到排放标准。但通过对SNCR喷枪位置进行调整,可利用烟气涡旋回流提高脱硝效率,相同氨氮比下出口烟气NO_x含量降低到245.25mg/m^3。模拟计算得出不同污泥掺混量时的温度和NO_x的变化趋势,提出适宜的污泥掺混量及SNCR喷枪布置优化方案,可为垃圾焚烧炉污泥掺混焚烧及SNCR脱硝提供参考。