Mechanism of low nox emission in circulating fluidized bed decoupling combustion

博士学位论文 Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion 作者姓名: 指导教师: DO HAI SAM 许光文 (研究员，中国科学院过程工程研究所) 高士秋 (研究员，中国科学院过程工程研究所) 学位类别: 工学博士 学科专业: 化学工程 培养单位: 中国科学院过程工程研究所 2018 年 月 Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion A dissertation submitted to University of Chinese Academy of Sciences in partial fulfillment of the requirement for the degree of Doctor of Philosophy in Chemical Engineering By Hai-Sam Do Supervisors: Professor Guangwen Xu Professor Shiqiu Gao Institute of Process Engineering Chinese Academy of Sciences June 2018 摘 要 摘 要 循环流化床解耦燃烧（CFBDC）实现低氮氧化物（NOx）排放的技术可行性 已在处理白酒糟（DSL）的工业示范装置上得到了很好验证。低 NOx 排放被认 为是由包括焦炭，焦油和热解气（py-gas）的 DSL 热解产物在燃烧器中再燃时共 同还原 NOx 的结果。为揭示 CFBDC 系统中还原 NOx 的机理，本研究采用实验 室规模的沉降炉（DTR）反应器，模拟 CFBDC 再燃条件，研究了生物质半焦、 焦油和热解气对 NO 的还原能力，并因此概算了 CFBDC 中各热解产物组分的潜 在作用。 本论文在 500℃下热解 DSL 制备测试用半焦和焦油，py-gas 根据实验所得热 解气的组成采用钢瓶气配制。为确保 NO 被充分还原，大多数实验在反应物总质 量给料速度 0.15g / 条件下进行。第 章首先研究了 NO 还原效率（ηe）随反 应物进料速率、再燃化学计量比（SR）、反应温度、停留时间和初始烟气组成的 变化。结果表明，在相同质量流速条件下，焦油较半焦和热解气更能抑制 NO 的 产生。热解气中 CO 的存在抑制了均相 NO 还原反应，导致热解气的 ηe 较低。对 于由 DSL 制备的半焦和焦油，高温和高的初始 NO、CO 浓度促进其对 NO 的还 原。 本章的主要结论是： 通过热解产物再燃烧获得最高 ηe 的适当 SR 值为 0.6-0.8。 基于上述结果，第 章研究了焦炭，焦油和热解气对 NO 还原的协同效应。 结果表明，NO 还原物的总质量流速一定时，焦炭/热解气混合物（二元反应物） 比其它混合物能够实现最佳的协同 NO 还原效果，且还原效率随着热解气比例的 增加而升高。而只有当焦油比例降低至 26％时，焦油/热解气或焦油/半焦混合物 才产生积极作用。此外，热解产物在还原 NO 过程中，焦炭与某些物质（如 H2， CxHy）之间存在有效的相互作用，其协同效应与反应物中 C，H 元素与进料 NO 的摩尔比密切相关（CH/NO 比）。 第 章进一步研究了来源于其他燃料如木屑（SD）和先锋（XF）褐煤的焦 炭和焦油的还原能力。在一定的还原物质量比率下（0.15g/min），SD 焦炭或 XF 褐煤焦由于灰分含量较低（具有催化物质），还原 NO 效率低于 DSL 焦炭，但 SD 焦油在三种焦油中实现的 ηe 最高。值得注意的是，焦油总是表现出比焦炭更 好的 NO 还原能力。以苯酚、苯、乙酸、乙酸甲酯和庚烷等作为模型焦油化合物 I 循环流化床解耦燃烧过程低 NOx 排放机理研究 还原 NO 的实验表明， 焦油中的苯酚发挥的作用显著。无论是生物质焦油还是 煤焦油，含有至少一个芳香环（如苯酚，苯）的化合物组分都是还原 NO 的主要 贡献者。 本研究的结果将对应用 CFBDC 技术处理富 N 燃料在操作运行方面具有重 要指导意义。考虑到热解产生的焦油是降低 CFBDC 中 NOx 排放的主导因素， 建议今后研究集中于焦油还原 NO 的动力学，以及焦油与其他成分的协同作用。 另外，热解温度对不同反应物还原 NO 活性的影响也值得研究。 关键词：热解，NO 还原，再燃，解耦燃烧，循环流化床，生物质，煤，低 NOx 燃烧，下降管反应器。 II Abstract Abstract The technical feasibility of low-NOx circulating fluidized-bed decoupling combustion (CFBDC) has been well proved in an industrial demonstration plant treating distilled spirit lees (DSL) The lowered NOx emission was believed to result from the combined contributions of DSL pyrolysis products including char, tar, and pyrolysis gas (py-gas) to the reduction of NOx via their reburning in the riser combustor of the CFBDC system In order to further understand the mechanism of NOx reduction in CFBDC, this study is devoted to investigating the capabilities of biomass char, tar and py-gas for NO reduction through experiments in a lab-scale drop-tube reactor (DTR) that simulates the reburning conditions occurring in CFBDC The work performed pyrolysis of DSL at 500 °C to produce the tested char and tar reactants while py-gas was prepared by mixing pure gases from cylinders according to the analyzed py-gas composition In order to ensure the sufficient reduction of NO, a total mass feeding rate of reagents, which is 0.15 g/min, was adopted for most experiments We first investigated in Chapter the variations of acquired NO reduction efficiency (ηe) with major parameters including reagent feeding rate, reburning stoichiometric ratio (SR), reaction temperature, residence time, and initial flue gas composition It was found that tar enabled the best NO reduction in comparison to char and py-gas did at the same mass feeding rate of reagent (0.15 g/min) The presence of CO in py-gas inhibited the homogeneous NO reduction reactions to cause lower ηe For DSL-derived char and tar, their realized ηe were facilitated through by higher temperatures and higher initial NO and CO concentrations The main conclusion of this chapter is that the suitable SR values for obtaining the highest ηe by reburning of pyrolysis products were found to be 0.6 − 0.8 Based on preceding results, the synergetic effects among char, tar and py-gas reagents on NO reduction were evaluated and discussed in Chapter The comparison at given total mass flow rate of NO-reduction reagent indicated that the char/py-gas (binary reagent) enabled the best synergetic NO reduction than the others did Its realized efficiency elevated with increasing of the py-gas proportion The tar/py-gas or tar/char mixture caused a positive effect only when the tar proportion was necessarily lowered to about 26% In addition, there existed obvious interactions between char and some species in py-gas (i.e., H2, CxHy) for NO reduction by pyrolysis products The III Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion synergetic effects were closely related to the molar ratio of C and H elements in reagents over the fed NO (CH/NO ratio) The NO reduction capabilities of char and tar reagents derived from other fuels such as sawdust (SD) and Xianfeng (XF) lignite were further investigated in Chapter At the specified mass flow rate of reductant, say, 0.15 g/min, the SD char or XF lignite char were less efficient than the DSL char did for reducing NO because of the lower contents of ash (containing catalytic matters) in the SD and XF lignite chars However, the SD tar enabled the highest ηe among the three tested tars Above all, tar as an attractive reagent always exhibits the better NO reduction than char does Testing model tar compounds including phenol, benzene, acetic acid, methyl acetate and heptane for NO removal revealed that phenol plays an important role in enabling the good NO reduction by the SD tar Our major understanding from testing the NO reduction by tar is that the compounds containing at least one aromatic ring (e.g phenol, benzene) are the major contributor for reducing NO in either biomass tar or coal tar In conclusion, the results of this study would be significant in the operation of CFBDC technology treating N-rich fuel By considering the pyrolysis-generated tar as a dominant factor in lowering NOx emission in a CFBDC system, further studies are suggested to focus on kinetic analysis of the NO reduction by tars and also on the combined action of tar with other reagents Additionally, the effect of pyrolysis temperature on NO reduction activity by various reagents should be investigated Key words: pyrolysis, NO reduction, reburning, decoupling combustion, circulating fluidized bed, biomass, coal, low-NOx combustion, drop-tube reactor IV Table of Contents Table of Contents Chapter 1  Introduction 1  1.1  Background 1  1.2  Objectives and Significance 3  1.3  Thesis Outline 4  Chapter 2  Literature Review 7  2.1  Nitric Oxides 7  2.1.1  Sources of NOx 7  2.1.2  NOx Emission in China 7  2.2  Low-NOx Emission Strategy 9  2.2.1  NOx Formation During Fuel Combustion 9  2.2.2  NOx Reduction Technologies 11  2.3  Decoupling Combustion (DC) for Lowering NOx Emission 14  2.3.1  Principle of Decoupling and DC Technology 14  2.3.2  Low-NOx Emission in Grate-Based DC 16  2.3.3  Low-NOx Emission in CFBDC 18  Chapter 3  Material and Methodology 25  3.1  Preparation of NO-Reduction Reagents 25  3.1.1  Feedstock Material 25  3.1.2  Pyrolysis Setup and Procedure 25  3.1.3  Characteristics of NO-Reduction Reagents 28  3.2  Experimental Drop-Tube Reactor for NO-Reduction Evaluation 30  3.2.1  Main Chamber 30  3.2.2  Heating Control System 31  3.2.3  Reagent-Feeding System 32  3.2.4  Flue-Gas Supplying System 35  3.2.5  Sampling and Analyzing System 36  3.3  Experimental Procedure 37  3.3.1  Procedure and Analysis 37  3.3.2  Validation of Experimental Setup Conditions 39  Chapter 4  NO Reduction by Biomass Pyrolysis Products 45  4.1  Introduction 45  4.2  Experimental Conditions 45  V Mechanism of Low-NOx Emission in Circulating Fluidized-Bed Decoupling Combustion 4.3  Results and Discussion 46  4.3.1  NO Reduction Varying with Reagent Feeding Rate 46  4.3.2  NO Reduction Varying with SR 49  4.3.3  NO Reduction Varying with Reaction Temperature 53  4.3.4  NO Reduction Varying with Residence Time 54  4.3.5  NO Reduction Varying with Flue Gas Composition 55  4.4  Conclusions 61  Chapter 5  Synergetic Effect Among Pyrolysis Products in Reducing NO .63  5.1  Introduction 63  5.2  Experimental Conditions 63  5.3  Results and Discussion 65  5.3.1  Synergetic Effect of Binary Reagent 65  5.3.2  Synergetic NO Reduction Varying with Reaction Temperature 70  5.3.3  Synergetic NO Reduction Varying with Residence Time 71  5.3.4  Synergetic NO Reduction Varying with Gas Species 72  5.4  Conclusion 76  Chapter 6  NO Reduction by Reagents Derived from Different Fuels 77  6.1  Introduction 77  6.2  Materials and Experimental Conditions 77  6.2.1  Materials 77  6.2.2  Experimental Conditions 79  6.3  Results and Discussion 81  6.3.1  NO Reduction by Char Reagents and Effect of Ash Content 81  6.3.2  NO Reduction by Tar Reagents and Model Tar Compounds 88  6.4  Conclusions 98  Chapter 7  Conclusions and Recommendations 99  7.1  Conclusions 99  7.2  Innovation 101  7.3  Recommendations for Future Work 101  Nomenclatures .103  References 105  Appendix A  Chemical Compositions of Tested Tar Reagents 115  A.1 GC–MS Spectra 115  VI .. .Mechanism of Low- NOx Emission in Circulating Fluidized- Bed Decoupling Combustion A dissertation submitted to University of Chinese Academy of Sciences in partial fulfillment of the requirement... NOx emissions, accounting for 28.4%, 34.0%, and 25.4% of the total NOx emissions in 2010, respectively (Zhao et al., 2013b) In 2014, Mechanism of Low- NOx Emission in Circulating Fluidized- Bed Decoupling. .. to the reburning condition in CFBDC due to the limitations Mechanism of Low- NOx Emission in Circulating Fluidized- Bed Decoupling Combustion of batch or semi-batch reactors adopted In addition,