【作者】 牛胜利；

【导师】 路春美；

【作者基本信息】 山东大学 ， 热能工程， 2011， 博士

【摘要】 燃煤电厂排放SO2和NO引起严重的环境问题,亟待开发能保障电厂经济性的SO2和NO协同脱除技术。初步研究表明,羧酸类有机钙盐可对这两种污染物进行联合脱除,但对有机钙盐热解、脱硫以及脱硝等反应机理认识还存在许多不完善地方,不利于有机钙盐对燃煤电厂SO2和NO协同脱除工业化应用。本文以丙酸钙(Calcium Propionate,CP)、丙酸调质氢氧化钙产物(Modified Calcium hydroxide by Propionic acid, MCP)以及丙酸调质氢氧化钙与氧化镁产物(Modified Calcium hydroxide and Magnesium oxide by Propionic acid, MCMP)这3种有机钙盐为研究对象,从实验、动力学参数计算和机理分析等角度,对它们的热解、固硫和脱硝机理以及协同脱除SO2和NO反应特性和反应机理进行探索。在热分析天平实验系统上研究有机钙盐热解特性,通过非预置模型法和预置模型法计算热解过程动力学参数,揭示有机钙盐热解机理。与碳酸钙(Calcium Carbonate, CC)不同,有机钙盐热解曲线由有机气体和CO2析出构成。在一定范围内提高O2浓度或者降低升温速率,有机钙盐热解曲线向低温段推进。相对于O2/N2气氛,有机钙盐CO2析出在O2/CO2气氛下向高温阶段延迟。析出的有机气体在还原性气氛下可以促成再燃脱硝,固体钙基产物能够进行固硫,在理论上保证有机钙盐对SO2和NO协同脱除。扫描电镜微观结构分析表明,相比较无机钙,有机钙盐固体热解产物颗粒度更小,结构更为疏松。根据非预置模型法的Ozawa-Flynn-Wall法和Vyazovkin法计算所得MCP和MCMP在N2气氛下热解表观活化能数值接近,MCP值分别为146-735kJ/mol和138-761kJ/mol, MCMP值分别为370-474kJ/mol和375-490kJ/mol, Avrami理论计算MCP和MCMP反应级数值分别为0.050-0.386和0.090-0.649。根据Ozawa-Flynn-Wall法计算CP和CC在O2/N2气氛下热解过程表观活化能,CP和CC值分别为83-346kJ/mol和193-202kJ/mol,根据Avrami理论计算反应级数,CP和CC值分别为0.061-0.608和1.647-2.084。模型预置法的Coats-Redfern法计算CP和MCP热解动力学参数表明,4级化学反应模型(C4)可以解释CP和MCP在O2/N2以及O2/CO2气氛下热解时,第二失重阶段热解机理,而1级和4级扩散模型(D1和D4)分别揭示它们在O2/N2和O2/CO2气氛下第三失重阶段热解机理,同时,O2/CO2气氛下计算的表观活化能数值明显高于O2/N2气氛下的相应值。在快速智能定硫仪实验系统上表征CP、MCP和MCMP固硫率,得出有机钙盐对煤燃烧过程固硫特性。1323K时,以钙硫摩尔比(Ca/S)为1和1.5的量添加CP,龙口褐煤(brown coal, BC)固硫率分别为69.80%和57.08%。1223K和1323K时,Ca/S为2的MCP对聊城贫煤(lean coal, LC)固硫率分别为73.46%和65.40%。无机钙效果不好,1323K时,Ca/S为2的CC对BC和LC固硫率分别只有34.08%和40.07%。通过热分析天平实验系统,分析CP和MCP固硫过程中CaO转化率,探讨有机钙盐与烟气中SO2作用的固硫特性,并借助等效粒子模型分析固硫过程的机理特征。1323K时,CP和MCP的CaO转化率分别为44.32%和54.95%,是CC相应值的5.49倍和6.80倍。采用等效粒子模型对表面化学反应控制阶段的Gfp(χ)～t以及产物层扩散控制阶段的Pfp(χ)～t进行拟合,能取得良好线性关系。高温下拟合直线斜率得到提高,固硫反应进程得到强化。在沉降炉实验系统上研究CP、MCP以及MCMP再燃还原NO特性。1323K时,CP、MCP和MCMP效率分别为79.65%、76.36%和72.65%,与生物质的效率值相当,并且远高于煤粉的脱硝效率。为达到较好脱硝效果,有机钙盐再燃比应维持在25%附近,O2浓度不宜超过4%,并且应保证0.65s左右停留时间。氨气和尿素的选择性非催化还原脱硝进程都有非常明显“温度窗口”,分别在1273K和1223K达到脱硝峰值,氨气的氨氮摩尔比为1.75和1.25时,效率值为85.34%和79.32%,尿素的氨氮摩尔比为2和1.5时,效率值为78.89%和70.19%。综合考虑NO还原率和氨剂有效利用率,氨气和尿素的氨氮摩尔比以1.5-2为宜。提高O2浓度,氨气和尿素的脱硝强度都遭到削弱,同时反应区应保持0.60s左右停留时间。CP和MCMP先进再燃脱硝效率明显高于基本再燃和选择性非催化还原相应值,再燃比为19.83%、氨氮摩尔比为0.8时,CP和MCMP在1273K最高效率值分别为93.37%和91.74%。在再燃燃料和氨气共同作用下,先进再燃“温度窗口”明显拓宽,并且O2浓度从2%提高到6%,脱硝效率降低不再明显,同时,以氨氮摩尔比为0.8的量添加氨气,就能保证CP和MCMP先进再燃的脱硝效率接近同等条件下的最高值。在固定床实验系统上研究CP、MCP和MCMP对煤燃烧过程中SO2和NO协同脱除特性和反应规律。在1073-1373K温度区间,这3种有机钙盐均能表现出很好的SO2脱除效果。Ca/S为2时,CP对BC和LC的SO2脱除率最高值分别为66.01%和71.72%,MCP的SO2脱除率最高值分别为67.20%和69.85%,MCMP的SO2脱除率最高值分别为70.72%和67.06%,均高于同等条件下CaO相应值。有机钙盐对NO的脱除表现在1173K以上温度区,Ca/S为2.5时,CP对BC和LC的NO脱除率最高值分别为49.38%和50.15%,MCP的NO脱除率最高值分别为47.57%和56.44%,MCMP的NO脱除率最高值分别为46.19%和56.67%。同时,添加有机钙盐后,煤粉的着火温度、失重峰温度以及转化率曲线向低温区移动,并且失重峰降低,失重半峰宽值增大。预置模型法的动力学参数计算表明,有机钙盐的添加,降低煤燃烧过程的表观活化能,使反应易于进行。在沉降炉实验系统上研究CP对烟气中SO2和NO的协同脱除特性。在1500×10-6的SO2作用下,随着O2浓度的变化,丙酸钙基本再燃的脱硝趋势与不含SO2时一样,但脱硝效率比不含SO2时有所提高。钙基固硫过程是一个需氧过程,在2-6%的O2浓度范围内,提高其值,能够强化丙酸钙对SO2的脱除能力。与基本再燃一样,SO2同样能够强化先进再燃的脱硝能力,但由于它自身的效率已较高,所以在1273K时,1500×10-6的SO2仅将其效率值提高1.96%(O2浓度4%)和2.03%(O2浓度6%)。但另一方面,氨气的加入并未对SO2的脱除产生明显的影响,1273K时,在2%、4%和6%的O2浓度条件下,先进再燃的SO2脱除效率仅比基本再燃时分别提高1.19%、0.67%和0.53%。在耦合戊酮和小分子碳氢化合物燃烧模型以及它们与NO相互反应模型基础上,建立包含453个基元反应和110种反应物质的反应机理,通过动力学模拟软件Chemkin,描述丙酸根类有机钙盐基本再燃脱硝反应本质。基元反应H+O2=O+OH对基本再燃脱硝进程影响最大,它产生的链锁反应能强化HCO、CH3、CH2、CH2CO、CH2O、CH2OH、CH3O、HOCHO等与NO的反应。对选择性非催化还原进程有重要影响的连锁分支反应系数ζ为0.29,这个值能够保证反应自维持进行,而NH2+NO=NNH+OH和NH2+NO=N,+H2O对氨剂脱除NO作用最大。在再燃燃料和氨剂共同作用下,反应H+O2=O+OH对NO还原的敏感性系数,相对于其它反应来说,其值更大,同时反应NH2+NO=NNH+OH和NH2+NO=N2+H2O对于NO浓度改变(向减小方向发展)所作的贡献比例也较选择性非催化还原时大。另一方面,C1型碳氢化合物对NO的还原能力要强于C2型,所以当需要平衡氨气与碳氢化合物的效果以达到优化反应进程的目的时,尽量使用C1型小分子化合物,而对于反应物质为C5等较大的分子来说,则应促进其向尽量小的碳氢化合物转变。在SO2的作用下,不仅H+O2=O+OH、C2H2+O=HCCO+H等原有基元反应活性有所提高,而且催生H+SO2=HOSO等新的基元反应,它们能够产生一系列的链锁反应,使得O以及OH等活性基团的浓度大大提高,从而强化脱硝进程。同时,含硫中间产物直接参与到NO的还原反应中,引发SN+NO=N2+SO、CH2(S)+NO=HCN+OH等新的基元反应,这些都使得当烟气中添加SO2时,丙酸钙基本再燃以及先进再燃的脱硝效率有所提高。更多还原

【Abstract】 Sulfur dioxide (SO2) and nitric oxide (NO), which are relased from coal fired power plants, have brought about serious environmental problems and it is urgent to explore economical technologies to reduce SO2 and NO simultaneously. Previous studies proved that the calcium based carboxylic materials could be used to abate these two pollutant gases together. However, the mechanisms for thermal decomposition, desulfurization and denitrification of calcium based organic compounds have not been well acknowledged.The calcium propionate (CP), modified calcium hydroxide by propionic acid (MCP) and modified calcium hydroxide and magnesium oxide by propionic acid (MCMP) are mainly mentioned in this study and their reaction characteristics and mechanisms on SO2 and NO dual reduction are investigated from experiments, kinetic calculations and mechanism analysis.Thermal decomposition characteristics are investigated through thermogravimetric (TG) and the kinetic parameters are calculated through model-free and model-fitting approach. Different from calcium carbonate (CC), there are two mass loss segments, which are the release of the organic gas and carbon dioxide (CO2), during the thermal decomposition process of CP, MCP and MCMP. Enriching oxygen concentration or reducing temperature heating rate, thermal events of the calcium based organic compounds are prompted towards lower temperature zone. At the same time, O2/CO2 atmosphere, which is compared with O2/N2 atmosphre, leads the CO2 release segment moving higher temperature zone. The released organic gas favors reburning for NO reduction and the solid residual product could capture SO2. Meanwhile, the scanning electron microscrope analysis shows that the grain diameter of CP and MCMP is obviously smaller than the one of CaCO3 after calcinations at 1173K. Also, the whole structure of these two calcium based organic compounds is more dispersed and the pole is more magnified.The values of apparent activation energy, which are calculated through model-free approach of Ozawa-Flynn-Wall method and Vyazovkin method, are close to each other. The values for MCP are 146-735kJ/mol and 138-761 kJ/mol and for MCMP are 370-474kJ/mol and 375-490kJ/mol. The reaction orders, calculated through Avrami theory, are 0.050-0.386 and 0.090-0.649, respectively, for MCP and MCMP. Based on Ozawa-Flynn-Wall method, values of apparent activation energy for CP and CC under O2/N2 atmosphere are 83-346kJ/mol and 193-202kJ/mol and their reaction orders are 0.061-0.608 and 1.647-2.084 from Avrami theory. Kinetic calculations through model-fitting approach of Coats-Redfern method show that reaction model of the fourth order of chemical reaction (C4) properly describes the reaction mechanism of the second mass loss segment for both CP and MCP under either O2/N2 or O2/CO2 atmosphere and for the third mass loss segment of CP and MCP, one-way transport of diffusion mechanism (D1) and Ginstling-Brounshtein equation of diffusion mechanism (D4) are fit for the O2/N2 atmosphere and O2/CO2 atmosphere, respectively. At the same time, the apparent activation energy values achieved under O2/CO2 atmosphere are much higher than the ones achieved under O2/N2 atmosphere.Performances of CP, MCP and MCMP on desulfurization during coal combustion are labeled through the fast intelligent sulfur fixing experimental system. At 1323K, the desulfurization efficiency of 69.80% and 57.08% for brown coal (BC) could be achieved by CP at Ca/S equaling to 1 and 1.5, respectively. At 1223K and 1323K, desulfurization efficiency of 73.46% and 65.40% could be achieved for lean coal (LC) if MCP is added at Ca/S of 2. At the same time, the inorganic compound of CC behaves poorly and at 1323K, the desulfurization efficiency of only 34.08% and 40.07% could be achieved, respectively, for BC and LC, if CC is added at Ca/S equaling to 2.Characteristics of calcium based organic compounds on sulfur fixing are investigated through analyzing the CaO conversion of CP and MCP on thermogravimetric analyzer. At 1323K, the values are 44.32% and 54.95% for CP and MCP, which are 5.49 times and 6.80 times of CC. The modified grain model is applied to deal with the data of both the surface reaction segment of Gfp(χ)～t and the product layer diffusion reaction segment of Pfp(χ)～t and the satisfying linear relationship is achieved through the linear regression process. In higher temperature zone, the slope of the regression line is increased and the sulfation progress gets strength.Investigations on NO reduction through CP, MCP and MCMP reburning are conducted on the drop tube experimental system. At 1323K, NO reduction efficiency of 79.65%,76.36% and 72.65% could be achieved through basic reburning (BR) of CP, MCP and MCMP, respectively. These values are comparable with the ones of biomass and obviously higher than the ones of coal powder. At the same time, for satisfying outcome, the reburnign fuel fraction should be kept about 25%, oxygen concentration is normally lower than 4% and the residence time of 0.65s is acquired. There is a distinct "temperature window" for both ammonia based and urea based selective non-catalytic reduction (SNCR). At 1273K, ammonia performs best and the NO reduction efficiency is 85.34% and 79.32%, respectively, at mole ratio of N-reducing agent to NO of 1.75 and 1.25. At 1223K, urea achieves the best performances and the efficiency is 78.89% and 70.19%, respectively, at mole ratio of N-reducing agent to NO of 2 and 1.5. Taking NO reduction efficiency and ammonia reagent utilization into consideration, mole ratio of N-reducing agent to NO is normally 1.5-2. Increaing O2 concentration, the NO reduction is weakened for both ammonia and urea. Also, residence time is required about 0.60s. NO reduction achieved during CP and MCMP advanced reburning (AR) is obviously high than the one in SNCR and BR. At reburning fuel fraction of 19.83% and mole ratio of N-reducing agent to NO of 0.8, CP and MCMP achieve the best performances at 1273K and their values are 93.37% and 91.74%, respectively. The "temperature window" is obviously broadened and as oxygen concentration is increased from 2% to 6% in AR, NO reduction gets a lower depression compared with BR. At the same time, quantity of ammonia reagent injection, which is required at mole ratio of N-reducing agent to NO of 0.8 in AR, could show satisfying outcome.Characteristics of CP, MCP and MCMP on SO2 and NO dual reduction during BC and LC combustion process are investigated on fixed bed experimental system. From 1073K to 1373K, CP, MCP and MCMP perform well on desulfurization. At mole ratio of calcium compounds to sulfur of 2. CP acquires the highest SO2 reduction efficiency of 66.01% and 71.12%. The values for MCP are 67.20% and 69.85% and for MCMP are 70.72% and 67.06%. And all these values are higher than the ones of CaO. NO reduction of calcium based organic compounds exists in temperature zone higher than 1173K. At mole ratio of calcium compounds to sulfur of 2.5, the highest efficiency of CP for BC and LC is 49.38% and 50.15%. The values of MCP are 47.57% and 56.44% and of MCMP are 46.19% and 56.67%. With addition calcium based organic compound, the ignition temperature, the maximum mass loss rate temperature and the conversion curve are shifted towards lower temperature zone. Also, the maximum mass loss rate is reduced and the half peak width is broadened. Kinetic calculations through model-fitting approach shows that calcium based organic compounds decrease the apparent activation energy to make the coal combustion more easily occur.Performances of CP on SO2 and NO dual reduction in post combustion flue gas are conducted on the drop tube furnace experimental system. Under the effect of 1500×10-6 of SO2, the CP basic reburning performance is improved. Calcium sulfuration needs oxygen participation and as oxygen concentration is increased from 2% to 6%, desulfurization of CP is continuously reinforced. Like basic reburning, SO2 intensifies NO reduction of advanced reburning. As AR itself could gain high efficiency, promotion of 1500×10-6 SO2 on NO reduction at 1273K is only 1.96% and 2.03%, respectively, for oxygen concentration of 4% and 6%. However, addition of ammonia makes little sense on SO2 reduction. At 1273K, improvement of SO2 reduction during advanced reburning over basic reburning is just 1.19%,0.67% and 0.53% for oxygen concentration of 2%, 4% and 6%, respectively.With the combination of 3-pentanone combustion model, small molecule hydrocarbon compounds combustion model and reaction model between hydrocarbon compounds and NO, the kinetic model, which includes 453 elementary reactions and 110 species, is established to describe the NO reduction reaction mechanism by propionic acid based compounds reburning.Elementary reaction H+O2=O+OH makes the greatest contribution to NO reduction and it could supply plenty of active radical OH through chain reaction. As a result, the NO reductions through HCO, CH3, CH2, CH2CO, CH2O, CH2OH, CH3O, HOCHO get strength. The key parameter for NO reduction of the chain branching ratio is 0.29 in this investigation and this value could guarantee the process self-sustaining. At the same time, elementary reactions of NH2+NO=NNH+OH and NH2+NO=N2+H2O play the most important role in NO reduction. Under the combined effect of reburning fuel and ammonia, sensitivity coefficient of elementary reaction H+O2=O+OH gains the largest value. Also, NH2+NO=NNH+OH and NH2+NO=N2+H2O make a greater contribution to NO in comparison with SNCR. Meanwhile, C1 hydrocarbon compounds show a stronger ability in NO reduction than C2 hydrocarbon compounds.Under the effect of SO2, not only the existent elementary reactions of H+O2=O+OH and C2H2+O=HCCO+H get strength, but also new elementary reactions of H+SO2=HOSO are generated. More active radical, like OH and O, are produced and the NO reduction is subsequently intensified. At the same time, the sulfur-containing intermediate products are participated in NO reduction, triggering a series of new reactions, like, SN+NO=N2+SO, and CH2 (S)+NO=HCN+OH. All of these factors make contributions to improvement in NO reduction with SO2 addition during basic reburning and advanced reburning.更多还原