【作者】 田红景；

【导师】 郭庆杰；

【作者基本信息】 青岛科技大学 ， 化学工程， 2010， 博士

【摘要】 包含CO₂在内的温室气体的排放是人类目前所必须面临的严峻问题之一。化学链燃烧技术是一种新型的、具有鲜明节能减排特色的燃烧技术,它具有CO₂内分离性质,无需外加CO₂分离装置即可捕获得到高纯度的CO₂气体。在化学链燃烧中,燃料不直接与空气接触,而是通过载氧体颗粒在空气反应器和燃料反应器之间的循环转化实现氧和热量的传递,完成燃料和空气的无火焰燃烧过程。这种新型的燃烧方式基于两步化学反应,实现了能量的梯级利用,具有更高的能量利用效率,同时,也抑制了反应中NOx气体的生成。载氧体的应用是化学链燃烧系统中的关键环节。目前,用于化学链燃烧系统中的传统载氧体主要是Ni、Fe、Cu和Co等金属的氧化物。这些载氧体颗粒具有良好的反应活性和循环特性,但单位质量载氧能力弱,价格昂贵且破碎后的颗粒物对环境存在二次污染。硫酸钙由于价格便宜,单位质量载氧能力强,对环境友好等特点,被视为很有发展前景的载氧体材料。本文采用Gibbs自由能最小化原理,探讨多种操作参数对CaSO₄向CaS的转化率、含硫气体的释放量和含碳产物沉积量的影响。模拟结果发现,反应温度的增大会抑制积碳的发生,但同时也会导致H2S气体向SO2气体的转化,给烟气脱硫过程增加了困难。操作压力的增加不仅会促进积碳的发生,破坏高温对积碳的抑制作用,也会导致高温区间内含硫气体释放量的增加,所以反应器内高压条件应尽量避免。对于氧原子过量系数,处在0.2⁰.8范围内时,氧原子过量系数减小会使含硫气体释放量得到有效抑制,但对于0.9¹.4的区间,在温度低于700℃时,减小氧原子过量系数对含硫气体释放量影响很小。氧化反应器内,较高的反应温度会降低CaS向CaSO₄的转化率,尤其在氧原子过量系数较小时这种现象更为明显;反应器内的O2应保持略有过量,较大的氧原子过量系数会促进CaS向CaSO₄的转化率。采用浸渍法实现了Ni、Fe离子在CaSO₄载氧体颗粒表面的分散,浸渍后的载氧体同气体燃料和固体燃料的反应性能同浸渍前相比明显改善;CaSO₄载氧体对Ni、Fe离子的浸渍量越大,同气体燃料和固体燃料的反应性能越好。复合型载氧体同污泥半焦颗粒和玉米秸秆半焦颗粒的反应性能远远高于它同煤焦颗粒的反应性能,说明了污泥半焦颗粒尤其是玉米秸秆半焦颗粒更适合应用于固体燃料直接使用的化学链燃烧系统。通过向采用强酸处理过的CaSO₄载氧体中加入CaCO3纳米颗粒作为固硫剂,可大大改善钙基载氧体的循环性能。氮气气氛下,CaSO₄载氧体颗粒在反应温度为1250℃时呈现缓慢分解,在温度达到1300℃时分解速率明显加快。空气反应器中控制反应温度在1300℃以下可使CaSO₄载氧体的热分解反应被避免;在CaSO₄载氧体颗粒的热分解过程中,产物CaO的生长活化能低于成核活化能,产物核一经形成便可迅速成长,反应对应的活化能值随CaSO₄转化率的增大而单调减小。采用双外推法和Popescu法计算得到无任何副反应干扰、CaSO₄颗粒处于原始状态下的活化能值为992.15 kJ/mol,推导得CaSO₄颗粒的热分解机理最可能是成核与生长机理,其最概然机理函数为[ ? ln（1 ? X）]2。还原性气体分压对钙基载氧体还原过程中含硫气体释放量的影响很大,还原性气体分压越大,反应过程中含硫气体释放量越小。即使在1000℃或更高的反应温度,只要保持CO或H2的气体分压在50 kPa以上,可充分抑制反应过程中含硫气体的释放,甚至可使CaSO₄向CaS的转化率达到100 %。CaSO₄载氧体的循环流量对系统热效率、还原反应器出口气体中CO₂与水蒸汽体积浓度和系统自给热影响较大。CaSO₄载氧体的循环流量越大,还原反应器出口气体中CO₂与水蒸汽体积浓度越高,但随着CaSO₄载氧体的循环流量的增加,系统热效率增大的速率已越来越慢,当热效率达到40 %时,系统热效率的提升空间已较小。更多还原

【Abstract】 Nowadays it is generally accepted that the increasing greenhouse gas emissions, the typical one being carbon dioxide, is one of the serious problems we must face. To solve the problem, chemical-looping combustion （CLC） technology has been proposed as a new technology which would satisfy the capture of CO₂ with few energy losses. It has the inherited characteristics of CO₂ separation. High pure CO₂ can be obtained and captured without any additional CO₂ separation units. In the CLC system, the fuel does not react with the air directly. Both the oxygen from the air to the fuel and the heat from the air reactor to the fuel reactor are transferred by the oxygen carrier to realize the flameless combustion between the fuel and air. Therefore, the formation of NOx gases in the system is efficiently inhibited. In addition, the CLC systems coupled with the gas turbine or some other integrated power systems would be more potentially efficient than the systems with conventional combustion technology.The choice of oxygen carrier is a key for the performance of the CLC system. The current oxygen carriers are metal oxides, such as the oxides of nickel, iron, copper and cobalt, are characterized by high reactivity and good regeneration stability. However, the costs of these metal-oxide oxygen carriers are high but their oxygen carrying ability are low per unit mass. In addition, some leakage of the metal oxide particles becomes second pollution sources to the environment. Therefore, it is necessary to find a new kind of oxygen carrier. Recently, calcium sulfate（CaSO₄） is considered as a novel oxygen carrier because it has some obvious advatanges. Firstly, CaSO₄ is cheaper due to vast gypsum resources all over the world. Secondly, compared with the metal oxides, CaSO₄ has a relatively high oxygen carrying capacity. Thirdly, as a nonmetal sulfate, it is much more friendly to the environment. Therefore, it is very suitable for the fluidized bed reactor of chemical-looping combustion system.Based on the minimization of the total Gibbs free energy for all species, the effects of many factors on the conversion of CaSO₄ to CaS, the amount of the released sulfurous gases and the deposited solid carbonaceous products are discussed. It is indicated from the simulated results that higher reacting temperature inhibits the deposition of carbon but promotes the conversion of H2S to SO2. However, the operating pressure contradicts the temperature for the effects on the amounts of the deposited carbon. The occurrence of deposited carbon is promoted at higher operating pressure. In addition, the higher operating pressure results in the more amounts of the released sulfurous gases. Therefore, the high operating pressure in the fuel reactor should be avoided when possible. Moreover, the effects of the oxygen excess ratio on the carbon deposition and sulfur release are not neglected. When the oxygen excess ratio increases from 0.2 to 0.8, the released amount of the sulfurous gases greatly grows. However, when the oxygen excess ratio rises from 0.9 to 1.4, the effects of reducing oxygen excess ratio on the released amount of the sulfurous gases is quite small, especially when the reacting temperature is less than 700℃. In the air reactor, the higher reacting temperature reduces the conversion of CaS to CaSO₄ especially when the oxygen excess ratio is smaller than 0.75. The oxygen in the reactor should remain a little excessive because the bigger oxygen excess ratio is helpful to the conversion of CaS to CaSO₄.Two kinds of compound oxygen carrier samples have been prepared by the incipient wet impregnation method with the saturated solution of nickel nitrate and iron nitrate respectively as the active-phase precursor on the surfaces of calcium sulfate particles. The reactivity of oxygen carrier with both the gaseous and solid fuels is much better than before impregnation. The higher impregnation amount improves obviously the reactivity of CaSO₄ oxygen carrier with gaseous and solid fuels. The reactivity of compound oxygen carrier with sludge char or corn straw char is much better than with coal char. This indicates that sludge char and corn straw char are more suitable in CLC system using solid fuels. In addition, it is found that after the pre-treatment of CaSO₄ oxygen carrier with strong acids, the addition of CaCO3 nanoparticles greatly improved the recycle ability of calcium based oxygen carrier.The thermal decomposition behavior of calcium based oxygen carrier in nitrogen atmosphere is also studied. CaSO₄ particles begin to decompose slowly when the reacting temperature achieves 1250℃. The decomposition rate obviously accelerates once the reacting temperature increases to 1300℃. In the air reactor, the decomposition of CaSO₄ can be avoided at the temperature lower than 1300℃. In the decomposition, the growth activated energy of produced CaO is lower than the nucleation activated energy of CaO. Therefore, once the nucleation of CaO occurs in the surface of the oxygen carrier, it can grow rapidly. The activated energy reduces monotonically with the increasing of conversion of CaSO₄. Through the double-extrapolated method and Popescu method, the activated energy of the decomposition of CaSO₄ without any disturbance of side reactions is calculated to be 992.15 kJ/mol. The most possible decomposition mechanism of CaSO₄ is nucleation and nuclei growth mechanism. The most likely mechanism function is characterized by [ ? ln（1 ? X）]2.It is observed that the partial pressure of the reductive gases has a significant effect on the amount of the released sulfurous gases. The high partial pressure of the reductive gases is beneficial to reduce the released amount of sulfurous gases. When the reacting temperature increase to 1000℃or higher, the release of sulfurous gases can be inhibited fully if the partial pressure of CO or H2 maintains above 50 kPa. In particular, the conversion of CaSO₄ to CaS can reach 100 %.The flow rate of circulating CaSO₄ oxygen carrier is important to the system heat efficiency, the concentration of CO₂ and H2O in the gases emitted from the fuel reactor and the heat integration of the system. The great flow rate of circulating CaSO₄ oxygen carrier increases the concentration of CO₂ and H2O in the gases from the fuel reactor. Similarly, the heat efficiency of the system is improved monotonically with the increasing of the flow rate of circulating CaSO₄. However, the increasing rate of the heat efficiency slows down when the flow rate of circulating CaSO₄ increases. When the heat efficiency reaches 40 %, its increasing range is quite slight.更多还原