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气体添加剂对选择性非催化还原脱硝反应过程影响的研究
Effects of Gaseous Additives for Selective Non-Catalytic Reduction of NOx
【作者】 曹庆喜；
【导师】 吴少华；
【作者基本信息】 哈尔滨工业大学 ， 热能工程， 2009， 博士
【摘要】 选择性非催化还原（SNCR）脱硝技术具有成本适中、脱硝效率中等、在老机组采用相对容易等特点,在我国的研究和应用处于刚刚起步阶段,具有一定的应用前景。对SNCR脱硝性能的一个严重制约是其脱硝温度窗口相对狭窄,本文的研究工作围绕开发能够拓宽SNCR脱硝温度窗的廉价易得的气体添加剂展开。首先在电加热的管式流动反应器试验研究了气体添加剂CO、CH4和H2对SNCR反应中NO还原的影响以及NH3和添加剂的转化规律。CO、CH4和H2都能使SNCR脱硝温度窗向低温移动。综合考虑最佳温度降低幅度、温度窗宽度和脱硝效率等因素,添加剂CH4的性能最好,其主要缺点是会引起较高的CO排放,在最佳脱硝温度附近CO排放达到最高,此时CH4向CO的转化率在50%以上。在对单一成分添加剂研究的基础上,为了给采用煤气化气等工业混合气做添加剂提供理论基础和指导,本文对由CO、CH4和H2构成的复合添加剂进行了研究。发现当复合添加剂中各组分浓度相差不十分悬殊时,CO和CH4构成的复合添加剂使SNCR温度窗改变的效果与单独添加其组分CH4的效果比较接近,CO组分所起到的作用相对较小。H2和CH4构成的复合添加剂的影响与其各组分单独作用有比较明显的差别,说明H2和CH4在复合添加剂中都起到了重要的作用。当CO和H2构成的复合添加剂中CO的浓度不多于H2时,H2对复合添加剂的性能起决定性作用。当CO的浓度大于H2的浓度时,CO组分在复合添加剂中所起的作用趋于明显和重要。对CO、CH4和H2构成的复合添加剂,CH4和H2组分起到主导作用,而CO的影响相对较小。为了分析添加剂影响SNCR的反应机理,并对此做出预测,综合前人的建模成果及最近的研究进展,构建了添加剂CO、CH4和H2参与的SNCR基元反应机理,通过与本文试验结果的对照和对反应机理的分析,参考相关文献,针对机理中某些基元反应的动力学参数进行了修正,使之能对试验结果做出更准确的预测。机理分析表明添加剂CO、CH4和H2主要是通过自身氧化过程中形成链分支反应,促进OH等活性基的生成来影响SNCR反应的。由于添加剂氧化形成链反应的反应途径和反应速率不同,造成了它们对SNCR反应的影响有所不同。对加入复合添加剂的SNCR脱硝反应的作用机理分析表明添加剂CO、CH4和H2共存时,添加剂自身的消耗及其影响SNCR脱硝的关键反应与各添加剂单独作用时是相同的。组分CO、CH4和H2对复合添加剂性能影响有区别的主要原因是这三者氧化竞争OH,并引发链反应促进OH生成的能力不同。采用本文的基元反应机理和前人提出的混合模型对添加剂参与的SNCR脱硝反应进行了计算,发现当采用H2做添加剂时,如果还原气体（NH3和添加剂H2）与模拟烟气不能快速混合,最大脱硝效率会大幅度的下降,这对试验结果做出了合理的解释。本文采用前人提出的两步总包反应和修正反应温度来体现添加剂影响的方法,通过对基元反应模型的计算结果进行数据回归,得到了添加剂参与的SNCR脱硝总包反应模型的动力学参数。并通过与基元反应机理的计算结果和试验结果的比较验证了模型的可靠性。在以上研究的基础上,采用本文发展的总包反应模型,借助CFD软件Fluent对一台600MW的电站锅炉上化学动力学和流动混合等物理过程共同控制的SNCR脱硝过程进行数值模拟,考察了大型锅炉上采用气体添加剂改善SNCR脱硝性能的效果。与工业试验测量结果及其设计值的比较表明计算模型能够对大型电站锅炉上SNCR脱硝过程进行比较准确的预测。小型反应器上的研究表明添加剂能够大大加快低温下SNCR脱硝反应速率,进而显著提高脱硝效率并降低NH3漏失。大型锅炉上的计算结果显示添加剂能显著减轻NH3漏失。不采用添加剂时NH3漏失最高达59μL/L,采用CO添加剂能够使NH3漏失降低到14μL/L以下。不同负荷下SNCR脱硝效率在27 35%之间,采用CO添加剂也没能使脱硝效率有大幅度的提高,采用CO前后脱硝效率的变化幅度小于两个百分点。大型锅炉上添加剂的作用效果需要今后进一步研究。
【Abstract】 Among the developed technologies for reducing emissions of NOx, selective non-catalytic reduction （SNCR） is characterized by considerably lower capital cost, moderate NOx removal efficiency and easy installation. A serious limitation of this process is that the temperature range over which NOx can be reduced is relatively narrow. This research focuses on developing cheaper and easily available gas additives which could widen the SNCR temperature window.Firstly, the influences of CO, CH4 and H2 on NOx reduction and the transform of NH3 and the additives in SNCR process were investigated experimentally in an electricity-heated tube reactor. All the three additives can shift the temperature window to lower temperature. Comprehensive considering the decreasing extent of the optimal temperatue for NOx removal, the width of the temperature window and NOx removal efficiency, CH4 is the most effective additive of the three, and its disadvantage is to causes CO emission due to its incomplete oxidation, the maximum conversion rate of CH4 to CO could be more than 50% near the optimum temperature for NOx reduction.Based on the study of single component additive, the composite additives composed of CO, CH4 and H2 were investigated experimentally in order to providing a solid background for the application of coal gas and other industrial gas mixtures as additives in SNCR process. The results show that while the mole ratio of the components is comparative, the effects of composite additive composed of CO and CH4 on SNCR temperature window is closed to that of its component CH4. The contribution of CO component is relatively minor. The temperature window with composite additive composed of H2 and CH4 is distinct from that with its each component, so both H2 and CH4 component make important contributions. While the fraction of CO is no more than that of H2 in composite additives composed of them, the performance of composite additives is dominated by H2 component; while the fraction of CO becomes larger, the influence of CO component becomes notable. The performance of composite additive composed of CO, CH4 and H2 depends mainly on CH4 and H2 component. The function of CO component is relatively minor.To analysis the reaction mechanism and predict the influence of the additives, a primary elementary reaction mechanism for promoted SNCR process by CO, CH4 and H2 was developed. By reaction mechanism analysis, the kinetic data of several elementary reactions was revised referring the related literatures, so as to make more exact predictions of the experimental results. By mechanism analysis, the effects of these additives on NOx reduction are achieved principally by promoting the production of OH and other radicals through chain reaction in their own oxidation process. The difference of the additives in changing the temperature window may be attributed to the distinction of their reaction path and reaction rates. While these additives are coexisted, their consumptions and their influences on SNCR are achieved by the same way as they are added individually. The distinct contribution of CO, CH4 or H2 component on the property of composite additive are mainly caused by their different competence with respect to reacting with OH and producing OH radical by chain reaction.A simple approach for mixing previously proposed together with the elementary reaction mechanism were used to calculation the SNCR process promoted by the additives. The results show while H2 additive is used, if the mix of reducing gas（NH3 together with H2） and the simulated flue gas is not fast enough, the maximum NOx removal efficiency will decrease remarkably, which give a reasonable interpretation on the experimental results.Two steps overall reactions for SNCR process previously proposed and the method for considering the effects of additives by adjusting the reaction temperature were adopted. The kinetic data of the overall reactions model for SNCR process promoted by additives was obtained by data regression based on the calculation results of the elementary reaction model. The overall reactions model is validated by experimental results and the predicted results of the elementary reaction model.Numerical simulation of SNCR process controlled by fluid-dynamic and chemical kinetics of reactants in a 600MW utility boiler is performed using CFD code Fluent together with the overall reactions model developed in this work. The numerical model was validated by industry experimental results and the design value. The researches in laboratory scale reactor indicate that the rate of SNCR reaction can be enhanced by the investigated additives greatly, so higher NOx removal efficiency and lower NH3-slip can be obtained under low temperature. The calculation results in 600MW utility boiler show that NH3-slip can be abated significantly by injecting some additives to furnace. The maximum predicted NH3-slip is 59μL/L while no additive is used, and it decreases to less than 14μL/L while CO additive is injected. The predicted NOx removal efficiency is between 27 and 35% under different boiler load. The NOx removal efficiency is not raised largly by injecting CO and its variation caused by CO addition is less than 2%. However the effects of additives on SNCR process in large scale boilers need further researches in future.