【作者】 刘勇；

【导师】 高翔； 郑成航；

【作者基本信息】 浙江大学 ， 工程热物理， 2018， 硕士

【摘要】 在绿色发展的方针指导下,燃煤电厂大气污染物排放受到严格控制,燃煤电厂相继加装选择性催化还原脱硝系统(SCR),在实现NOx高效脱除的同时,烟气中的SO3浓度升高。SO3不仅污染大气环境,而且给电厂运行造成严重危害,如形成硫酸氢铵使SCR催化剂中毒和空预器堵塞等。对于高硫煤电厂,SO3引起的问题更加严重。燃煤烟气SO3的控制逐渐引起重视并成为研究的热点。目前燃煤电厂主要采用低低温电除尘器、湿式电除尘器和湿法脱硫塔等污染物脱除设备协同控制SO3。这些方法虽然能在一定程度上减少SO3的排放,但是无法减少硫酸氢铵的生成,为此本文提出在SCR系统前喷射碱基吸收剂脱除S03的方法,并开展了小试和中试实验研究。首先进行了碱基吸收剂的SO3吸收性能研究。比较了钙基吸收剂中的Ca(OH)2,镁基吸收剂中的Mg(OH)2,钠基吸收剂中的Na2CO3,这三种有代表性的碱基吸收剂的SO3吸收性能。发现300-400℃C范围内,对SO3的吸收性能:Na2CO3>Ca(OH)2≈Mg(OH)2。Na2CO3 的吸收性能优于 Ca(OH)2 和 Mg(OH)2 主要有两方面的原因。一方面是Na2CO3与SO3发生反应所需的活化能较小,相同温度下,气固界面反应速率更快。另一方面Na2CO3与SO3反应过程中不断释放出CO2,产物层为多孔结构,产物层的生长对气体扩散造成的阻力较小,有助于SO3扩散到未反应的吸收剂表面。而Ca(OH)2和Mg(OH)2与SO3反应后生成的产物较为致密,阻碍了 SO3向颗粒内表面扩散。同时研究了碱基吸收剂的SO2吸收性能。与Ca(OH)2和Mg(OH)2相比,Na2CO3对SO3的吸收效果明显大于SO2,可以减少SO2对吸收剂的消耗。其次针对优选的碱基吸收剂碳酸钠,研究了温度、SO3浓度、粒径、CO2浓度、水蒸气等关键参数对SO3吸收性能的影响规律。150-300℃范围内,温度升高,碳酸钠的SO3吸收量增大,300℃以上,吸收量对温度的变化不敏感,此时吸收量受到SO3扩散速率的限制。提高SO3浓度,SO3吸收量明显增大,SO3浓度提高至1200 ppm以上,吸收量仍可继续增大。减小碳酸钠粒径,SO3吸收量增大,平均粒径小于50 d时,继续减小粒径,吸收量不再明显增大。烟气中CO2的存在会降低碳酸钠的SO3吸收量,但是影响很小,这是由于烟气中的CO2会对产物中的CO2向主气流中扩散造成一定的阻力。300℃以下,烟气中加入水蒸气,SO3的吸收量明显增加,可能是水蒸气的存在促进了 SO3向H2SO4转变,使反应机制发生了改变;300℃以上,水蒸气对SO3的吸收性能影响较小。发现300℃以上,外扩散是SO3吸收速率的限制因素,这是由Na2CO3与SO3的高反应活性和烟气中SO3浓度较低所共同决定的。建立了 SO3吸收速率的外扩散控制模型,得到了 SO3传质系数。提出增大吸收剂在烟气中的数目浓度、提高吸收剂分散均匀性等强化SO3脱除的关键方法。最后建立了 Na2CPO3溶液喷射脱除SO3的中试实验平台,研究了碳酸钠粒径分布、Na2CO3:SO3摩尔比、停留时间、烟气温度和SO3浓度等操作参数对SO3脱除效率的影响规律。中试试验结果表明,烟温300℃以上,碳酸钠溶液喷射后迅速蒸干,粒径分布集中在2μm左右,10dm以下的颗粒在99%以上,减小了SO3扩散到颗粒表面的阻力,提高了吸收剂利用率和SO3脱除效率。SO3脱除效率随Na2CO3:SO3摩尔比、停留时间和SO3浓度的增大而增大,对温度的变化不敏感。Na2CO3:SO3摩尔比=4:1,停留时间3.66 s时,SO3脱除效率可达96.4%。建立了以主要操作参数为变量的碱基吸收剂喷射脱除SO3效率模型,与实验结果拟合程度较高,可用于指导工程应用。更多还原

【Abstract】 Under the guidance of the principle of "green development",the emission of atmospheric pollutants from coal-fired power plants is strictly controlled,and coal-fired power plants successively upgrade the selective catalytic reduction denitration system(SCR),resulting in the efficient removal of NOx.However,the SO3 concentration is increased.SO3 not only pollutes the atmosphere,but also poses serious hazards to the operation of the power plant,such as the formation of ammonium bisulfite to poison the SCR catalyst and block the air preheater.For high-sulfur coal power plants,the problems caused by SO3 are more serious.The control of coal-fired flue gas SO3 has.gradually attracted attention and become a research hotspot.At present,coal-fired power plants mainly use low-low-temperature electrostatic precipitators,wet electrostatic precipitators,and wet desulfurization towers and other pollutant removal equipment to cooperatively control SO3.Although these methods can reduce SO3 emissions to some extent,they cannot reduce the production of ammonium bisulfite.Therefore,this paper proposes a method to remove SO3 before the SCR system by alkaline sorbent injection,and conducts laboratory experiments and pilot experiments.First,the absorption performance of alkaline sorbents with SO3 was studied.The SO3 absorption performance of three representative alkaline sorbent:Ca(OH)2 in calcium-based sorbents,Mg(OH)2 in magnesium-based sorbents,Na2CO3 in sodium-based sorbents were compared.The absorption performance of SO3 in the range of 300-400℃ was found to be:Na2CO3>Ca(OH)2≈Mg(OH)2.Absorption performance of Na2CO3 was superior to Ca(OH)2 and Mg(OH)2 for two main reasons.On the one hand,the activation energy required for Na2CO3 to react with SO3 was smaller,so at the same temperature,the gas-solid interface reaction rate was faster.On the other hand,CO2 was continuously released during the reaction of Na2CO3 with SO3,and the product layer was a porous structure.The growth of the product layer caused less resistance to gas diffusion and help the diffusion of SO3 to the unreacted absorbent surface.However,the product of Ca(OH)2 and Mg(OH)2 reacting with SO3 was denser and hinder the further diffusion of SO3.SO2 absorption performance of alkaline sorbents was also studied.Compared with Ca(OH)2 and Mg(OH)2,the absorption performance of Na2CO3 on SO3 was significantly greater than that of SO2,which can reduce the consumption of absorbent by SO2.Second,the effect of temperature,SO3 concentration,particle size,CO2 concentration,and water vapor on the absorption performance of SO3 was studied for the preferred alkaline sorbent sodium carbonate.It was found that within the temperature range of 150-300℃,the SO3 absorption of sodium carbonate increased with the increase of temperature.Above 300℃,the absorption was insensitive to the temperature change.At this time,the absorption was limited by the diffusion rate of SO3.Increasing the SO3 concentration significantly increased the SO3 absorption and increaseds the SO3 concentration to more than 1200 ppm,and the absorption can continue to increase.When the particle size of sodium carbonate was reduced,the absorption of SO3 increased.When the average particle size was less than 50 μm,the absorption no longer increased significantly while the particle size continued to decrease.The presence of CO2 in the flue gas reduced the SO3 absorption,but the effect was small.It was due to the release of CO2 after Na2CO3 absorbed SO3,and the CO2 in the flue gas would cause some resistance to the diffusion of CO2 in the product into the main gas flow.The addition of water vapor to the flue gas increased the absorption of SO3 when the temperature was lower than 300℃.It may be that the presence of water vapor promoted the conversion of SO3 to H2SO4 and the reaction mechanism was changed.Above 300℃,the effect of water vapor absorption on SO3 was not significant.Through the above studies,it was found that,above 300℃,the absorption efficiency of SO3 by Na2CO3 was mainly limited by external diffusion,which was determined by the high reactivity of Na2CO3 with SO3 and the lowconcentration of SO3 in the flue gas.An externaldiffusion control model for SO3 absorption rate was established and the mass transfer coefficient of SO3 was obtained.The key methods for enhancing the removal of SO3,such as increasing the number concentration of the absorbent in the flue gas and increasing the dispersion uniformity of the absorbent,were proposed.Finally,a pilot experimental platform for SO3 removal by injection of Na2CO3 solution was established.The operation parameters that influence the removal efficiency of SO3 such as sodium carbonate particle size distribution,Na2CO3:SO3 molar ratio,residence time,flue gas temperature,and SO3 concentration were studied.The pilot test results showed that the smoke temperature above 300℃,the sodium carbonate solution was rapidly evaporated after spraying,the particle size distribution was concentrated at about 2 μm,the particles below 10 μm were above 99%,greatly reduced the diffusion resistance of SO3 to the surface of the particle,and increased the removal efficiency of SO3 and the utilization of absorbent.SO3 removal efficiency increased with increasing Na2CO3:SO3 molar ratio,residence time and SO3 concentration and was insensitive to temperature changes.When the molar ratio of Na2CO3 to SO3 was 4:1 and the residence time was 3.66 s,the removal efficiency of SO3 was 96.4%.The SO3 efficiency model was established by using the main operating parameters of alkaline sorbent injection as variables,which has a good degree of fitting to the experimental results and can be used to guide engineering applications.更多还原