氟利昂-12预混合燃烧水解研究

【作者】 高红；

【导师】 宁平；

【作者基本信息】 昆明理工大学 ， 环境工程， 2011， 博士

【摘要】 氟利昂(CFCs)是一类重要的臭氧消耗物质(ODS)和温室气体,在臭氧层破坏、气候变化异常和酸雨三大全球性环境问题中,臭氧层破坏及气候变化异常均与氟利昂排放相关。为了解决这一全球性的问题,旨在限制和禁止使用CFCs等ODS的《关于消耗臭氧层物质的蒙特利尔议定书》(About the ozone depleting substance of the Montreal protocol)(以下简称《议定书》)得到世界上160多个国家的支持和批准。尽管CFCs的生产与使用已受到极大限制,但如何处理和利用库存和在用设备中的CFCs成为一个广受关注的焦点问题,开发实用的CFCs降解工艺和设备具有重要的现实意义。以热力学软件Factsage为工具,对二氟二氯甲烷(CFC-12)在不同温度、压力以及甲烷、石油液化气(LPG)、一氧化碳、氢气四种燃料氛围中的平衡组成和产物分布作了深入研究,发现CFC-12在4种燃料体系中都能很完全地转化为HF、HCl和C02,反应的化学亲和势和平衡常数都比没有燃料添加时大得多,这一效果以LPG体系更加显著,结合平衡组成分布考虑,认为LPG是降解CFC-12的最佳燃料。热力学研究显示CFC-12的热力学稳定性并不高,在一定条件下很容易发生裂解。在各种影响因素中,温度对裂解后平衡产物的分布影响很大,其中氯元素的分布特征是1400K以下以C12为主,C1量很小,1950K以上以Cl为主。F元素的分布以CF4为主,1900K后,CF2的量也占有比较明显的优势,2500K时,CF4、CF2同时成为F的主要存在形式。当体系中有水存在时,主要产物则是HF、HCl和CO2,但其形成并非是温度越高越好,相对的低温更有利于HF、HCl和CO2的选择性形成。压力对CFC-12降解反应影响很小,因此,在后续实验中没有考察压力对反应的影响。用量子化学中的密度泛函理论(DFT)进行了反应机理分析。对于CFC-12在LPG燃烧场中的降解反应提出了一个涉及46个物种,包含388个反应的自由基反应机理。对所有基元反应进行量子化学计算予以确认。计算过程使用Materials Studio软件在密度泛函LDA/PWC(DNP)水平下完成,经过筛选,最终得到一个包括113个能垒低于10kcal/mol的CFC-12降解机理。从理论上证实了CFC-12在LPG燃烧场中的降解存在多个低能垒的优势反应通道,它们互相交叉,形成了一个网络状反应通道体系。这一低能垒反应通道网络为CFC-12的快速、彻底降解提供了保障,这一保障来源于燃烧场提供的自由基。LPG燃烧过程中形成的CH3、CH2等烃类自由基和卡宾与CFC-12分子及其碎片之间有很强的反应活性,有效地增加了CFC-12的低能垒分解通道,这说明了选择LPG作为燃料的优越性。该机理揭示了CFC-12降解的前半部分反应主要以形成HCl为主,其通道均为自由基反应；而HF主要在后半部分反应中形成,其来源有很大部分依赖于水解通道。该机理还揭示了CFC-12对LPG的燃烧有抑制,但水解通道的存在降低了对燃烧的抑制,水的存在有利于CFC-12的降解。实验研究从CFC-12、H2O对LPG燃烧特性的影响入手,研究了燃烧处理CFC-12的基本规律,实验结果与理论研究完全一致。CFC-12及H20对LPG-空气混合气的燃烧速率有显著影响,CFC-12严重抑制燃烧,最大使LPG燃烧速率下降77.4%。水的影响与CFC-12类似,但程度较轻。实验证实在LPG燃烧场中,CFC-12分解反应能在瞬间完成,是一个动力学快速反应,通过GC-MS分析,没有发现中间产物富集。由于CFC-12对LPG的燃烧有很强的抑制,要提高CFC-12处理效率(即CFC/LPG),控制目标指向燃烧场的稳定。实验结果表明,CHRFPPF (Combustion and Hydrolysis of Rotational Flow with Partial-Premixed Feeding)工艺较好的解决了这一问题,这一工艺过程在设备上则通过将单环缝隙旋流燃烧器改为双环缝隙旋流燃烧器来实现。LPG燃烧场中加入少量水蒸气有助于提高燃烧场处理CFC-12的能力,一次空气中水含量为14g/Nm3-20g/Nm3时对CFC-12降解有利,超过20g/Nm3后则对CFC-12的降解有负作用,用水在室温下以鼓泡的方式饱和预混空气即可达到理想的效果。氟是重要的战略资源,通过向CFC-12燃烧尾气的吸收液中添加CaCl2,将吸收液中的氟以CaF2的形式沉淀出来,通过分离即可实现CFC-12的资源化利用。实验证实,沉淀物的主要成分是碳酸钙和氟化钙,沉淀物要达到萤石精矿对CaF2品位最低要求的工艺条件是：吸收液pH值3.0以下,CaCl2的投加量取理论需要量的2.5倍。通过实验优化了CHRFPPF工艺参数：预混气体供给燃烧器内环,二次空气供给外环；a=1.2；一次空气A1：二次空气A2=0.4：0.6；一次空气用鼓泡法增加水含量。该条件下CFC/LPG值达到2.02而CFC-12分解率在99.9%以上。该结果较文献报道的最好处理能力(CFC/LPG值)高18.8%,还取消了燃烧场中的电热丝系统,简化了设备,降低了能耗。使用Fluent软件,涡耗散(Eddy-Dissipation)化学反应模型完成了双环预混进口形式的CFC-12分解燃烧器射流燃烧与旋流燃烧的数值模拟。计算结果表明旋流燃烧方式可得到比射流湍流扩散燃烧更短的火焰,有利于减小燃烧器尺寸,降低设备造价。强烈的涡旋气流使整个燃烧器的温度、浓度分布更加均匀,更有利于CFC-12的分解。旋流流场中存在的径向和切向速度分布,使全预混旋流燃烧方式下的CFC-12分解率稍低于部分预混燃烧方式的CFC-12分解率。这与实验吻合的很好。根据CHRFPPF工艺要求,设计制作了一套CFC-12处理能力为2千克/小时的设备,其燃烧过程稳定,CFC-12分解率大于99.9%,处理能力达到设计要求,实现了研究目标,为工业化应用打下了良好基础。以二氟二氯甲烷(CFC-12)为研究对象,以热力学和反应机理等理论研究为基础,自行设计了中试实验装置,结合实验研究,确定了预混合旋流燃烧水解(Combustion and Hydrolysis of Rotational Flow with Partial-Premixed Feeding (CHRFPPF))基本工艺参数。更多还原

【Abstract】 Ozone depletion, abnormal climate change and acid rain are the main global environmental problems. The former two issues are related with the emission of chlorofluorocarbons(CFCs), which is an important ozone depleting substances (ODS) and green house gas. In order to address this global issue of CFCs,’About the ozone depleting substance of the Montreal protocol’(Montreal protocol) has been approved and supported by more than 160 countries to limit and/or prohibit the usage of CFCs and other ODS. It has been an issue on how to deal and utilize the CFCs in storage and current instruments, although production and usage of CFCs have been limited by the Montreal protocol. It is an importance of reality to develop practical technology and instrument to decompose CFCs. In this research, dichlorodifluoromethane (CFC-12) was investigated based on theories of thermodynamics and reaction mechanism. Combined with experimental studies, a technology named’ Combustion and Hydrolysis of Rotational Flow with Partial-Premixed Feeding (CHRFPPF)’, and corresponding pilot-scale instrument were developed to decompose CFCs and recover the byproducts as resource.Factsage was used for thermodynamic analysises. Reactions of CFC-12 with bunkers such as methane, liquefied petrol gas (LPG), carbon monoxide and hydrogen were simulated under different temperature and pressure to figure out the equilibrium compositions and product distribution. It was found that using four fuels mentioned above CFC-12 could be completely transferred to HF, HCl and CO2, and that chemical potential and equilibrium constant were higher than those without any fuel. Among the four fuels, LPG exerted the best performance and desired equilibrium composition, and therefore was selected as the optimum fuel to decompose CFC-12. Thermodynamic analysises indicated that CFC-12 has a low stability and could be easily cracked under certain conditions. Among many influencing factors, temperature is the most important one that would influence the equilibrium distribution of products after crack. Element Cl mainly existed in form of Cl2 with a few Cl when temperature was less than 1400K, and mainly in form of Cl when temperature was higher than 1950K. Most of element F existed in form of CF4. Exceeding 1900K, CF2 also played an important role, and at 2500K F was mainly distributed in CF4 and CF2. When water existed in the reaction system, relative lower temperature was favorable to selectively form HF, HCl and CO2. Pressure exhibited only smaller influence on the decomposition of CFC-12, and therefore would not be considered for further studies.Density functional theory (DFT) was used for reaction mechanism research. A residue reaction mechanism involving 46 species and 388 reactions was proposed to model the reaction of CFC-12 and LPG based on references. All the elemental reactions were confirmed through calculation of quantum chemistry. Calculation at DFT LDA/PWC(DNP) level was conducted using commercial software Materials Studio. Finally,113 possible decomposition pathways of CFC-12 were selected based low energy barrier (<10kcal/mol). This confirmed from theory that many reaction pathways of low energy barrier existed in the LPG combustion field of CFC-12, and they crossed and interacted to provide a netted system of reaction pathways. This netted system of low energy barrier provided the guarantee to quickly and completely decompose CFC-12 as result of the residues produced in LPG combustion field. Hydrocarbon like CH3, CH2 and carbene exerted strong reactivity with the CFC-12 and its fraction, and effectively increased the decomposition pathways of low energy barrier. This confirmed the priority to select LPG as combustion fuel of CFC-12. The proposed mechanism indicated that HCl was mainly formed in the first half of the reaction system based on pathways of residue reaction, while most of HF was formed in the second half of the reaction system based on pathways of hydrolysis. It was also found that CFC-12 inhibited the combustion of LPG, while pathways of hydrolysis promoted the combustion. Presence of water was favorable to decompose CFC-12.Experimental researches were carried out to investigate the combustion characteristics of LPG and the basic laws governing the decomposition of CFC-12. Experimental results were completely in agreements with the theoretical studies. Combustion rate of mixture of LPG and air were dramatically influenced by CFC-12 and H2O. CFC-12 seriously inhibited the combustion with maximum rate decrease of 77.4%. Influence of water on combustion was similar as that of CFC-12 but with a relative lower inhibition. Experiments confirmed that decomposition of CFC-12 in LPG combustion field was a fast dynamic reaction without accumulation of transition products.Because of the strong inhibition of CFC-12 on LPG combustion, in order to increase the proficiency of CFC-12 decomposition, LPG combustion field should be controlled to make it stable. Experimental results indicated that CHRFPPF technology could solve this problem better. CHRFPPF technology was realized by replacing single-annulus-rotational-fluent combustor with double-annulus-rotational-fluent combustor. Smaller amount of water steam was favorable to decompose CFC-12. Water concentration of 14g/Nm3-20g/Nm3 was favorable, and unfavorable when exceeding 20g/Nm3. Water steam bubbled into the pre-mixed air under room temperature could reach the desired efficiency.Element F could be recovered in form of CaF2 from CFC-12 by adding CaCl2 into the absorbent to deposit F from HF. By the separation of CaF2, CFC-12 could be resourced. It was found from experiments that most of the deposits were CaCO3 and CaF2-The conditions of making deposits to meet the standard of high quality fluorite mines were to make pH of absorbent less than 3.0, and add CaCl2 as 2.5 times as the theoretical stoichiometry.Parameters of CHRFPPF technology were optimized using experiments. Pre-mixture were fed into the inside annulus while second air flow fed into outside annulus, the ratio of first to second air flow was 0.4:0.6, and first air flow was wetted by bubbling water before mixing. Under optimum conditions, ratio of CFC/LPG reached 2.02 and the decomposition rate of CFC could be more than 99.9%. This CFC/LPG value was 18.8% higher than the best reported value. Furthermore, the electric heat fuses were removed from the combustor to simplify the equipment and reduce the energy consumption.Fluent was used to simulate the eddy-dissipation reaction model occurred in either ejected or rotational fluent in double-annulus-fluent combustor. Results indicated that combustion occurred in rotational fluent generated fire flame smaller than that occurred in ejected fluent. Rotational fluent were more favorable to reduce the combustor size and cost. Strong eddies caused by rotational fluent distributed the temperature and product concentrations more evenly, and therefore promote the decomposition of CFC-12. Radial and axial distribution of velocity in rotational fluent made the decomposition rate in fully mixed fluent a little smaller than that in partially mixed fluent. The simulation results were consistent with our experiment studies.According to the requirement of CHRFPPF technology, the 2kg/h pilot-scale equipment was designed to decompose CFC-12. This equipment could be operated stably, and reached the design capacity and the decomposition efficiency higher than 99.9%. Based on the experimental researches and theoretical analysises, our research objectives have been realized; and sound bases were provided for further industry application.更多还原