改性吸附剂喷射脱汞的实验及机理研究

发布时间：2019-05-27 06:06

【摘要】：汞是一种具有较强挥发性、生物累积性及环境持久性的剧毒污染物,而燃煤是最大的人为汞排放源。因此,燃煤汞排放控制己成为当前能源环境领域重要的研究方向。目前,控制燃煤电厂汞排放最成熟可行的技术是活性炭喷射技术(ACI),因为活性炭喷射脱汞过程是包含流动、传热、传质、化学反应在内的复杂两相流反应体系,所以该技术在燃煤电厂的应用还存在较多问题。本文针对汞在吸附剂表面的吸附形态与吸附机理,汞吸附动力学、热力学与吸附平衡特性,影响吸附剂喷射脱汞效率的关键参数及影响机理,吸附剂喷射脱汞机理及协同脱除SO2与NO的潜力,吸附剂喷射脱汞预测模型等方面的科学问题展开研究。制备了原始活性炭(R-AC), NH4Br改性活性炭(NH4Br-AC), NH4Cl改性活性炭(NH4Cl-AC)和飞灰(FA)四种脱汞吸附剂,进行了物理化学表征,在固定床上考察入口Hg0浓度、烟气温度、粒径和烟气组分等因素对汞静态吸附的影响,利用程序升温脱附(TPD)方法研究吸附剂表面的汞吸附形态,分析不同吸附剂的汞吸附机理。结果表明,NH4Br和NH4Cl改性主要以非晶体形态存在于活性炭表面与中孔中；吸附剂的静态汞吸附性能强弱为NH4Br-ACNH4Cl-ACR-ACFA;R-AC对Hg0的吸附以物理吸附为主,存在少量化学吸附,产物为HgO; NH4Br-AC对Hg0的吸附主要是化学吸附,产物为HgBr2; O2促进了R-AC和NH4Br-AC对H酽的吸附；SO2对R-AC和NH4Br-AC的Hg0吸附产生了不同的影响,SO2存在时,NH4Br-AC表面的HgBr2生成量减小,但有少量HgS生成；NO促进了R-AC和NH4Br-AC对Hg0的吸附,因为在吸附剂表面生成了Hg(NO3)2。利用动力学模型研究了气相汞在活性炭和飞灰表面的吸附动力学特性,计算出汞吸附活化能,初始汞吸附速率,并分析汞在活性炭和飞灰表面的吸附热力学及吸附平衡特性。结果表明,汞吸附过程可分为表面吸附和内扩散吸附两个阶段；虽外部传质与内扩散限制汞吸附过程,但汞在活性位点的化学吸附是R-AC和NH4Br-AC汞吸附速率控制步；对于飞灰,外部传质是其汞吸附速率控制步；汞在R-AC和NH4Br-AC表面吸附的活化能分别为-10.06610/mol和-28.068kJ/mol,表明吸附是物理吸附和化学吸附的共同作用；汞在吸附剂表面的初始汞吸附速率与吸附剂的汞吸附容量呈正相关关系；热力学分析表明汞在R-AC和NH4Br-AC表面的吸附均为自发、吸热过程,吸附以物理吸附为主,化学吸附为辅,吸附过程增加了气固界面之间的混乱与复杂程度；汞在R-AC表面的吸附可用Temkin与Langmuir方程较好地描述,汞在NH4Br-AC和FA表面的吸附可用Freundlich方程较好地描述。建立了国内首套模拟烟气夹带流反应器喷射脱汞实验装置,研究了R-AC、NH4Br-AC、NH4Cl-AC及NH4Br改性飞灰(NH4Br-FA)的喷射脱汞特性,考察了入口Hg0浓度,停留时间,烟气温度,吸附剂粒径,喷射量等参数对喷射脱汞的影响,分析了不同吸附剂的喷射脱汞机理。结果表明,提高烟气汞浓度,增加吸附剂停留时间,减小吸附剂粒径,均可提高R-AC和NH4Br-AC的喷射脱汞效率与单位汞吸附量；增加吸附剂的喷射量会提高脱汞率,但降低了单位汞吸附量；烟温升高,R-AC的喷射脱汞率降低,NH4Br-AC的喷射脱汞率提高；NH4Br改性对烟气Hg0的氧化与吸附强于NH4Cl改性；喷射吸附过程中,NH4Br-AC与NH4Cl-AC表面的Br或C1基团均能将烟气中的Hg0分子氧化成HgBr2或HgCl2, HgBr2与HgCl2相比Hg0更容易被吸附；NH4Br-FA的喷射脱汞率较低,其对烟气中Hg0的脱除主要是Hg0的氧化,由于飞灰本身的比表面积与孔隙结构均较差,因而降低了对Hg0的吸附率。在6 kWth燃煤循环流化床-夹带流反应器喷射脱汞实验装置上,研究了贵州无烟煤燃烧的汞排放与分布特性,考察了NH4Br-AC在燃煤烟气中的喷射脱汞特性及协同脱除SO2与NO的潜力。结果表明：贵州无烟煤燃烧后排放的汞主要为颗粒汞,比例为77.34%,气相总汞为22.65%,其中Hg0为10.27%,Hg2+为12.38%；NH4Br-AC停留时间从0.59s增加到1.79s,脱汞率从70.7%增加到90.5%；NH4Br-AC表面的Br官能团显著提高了其对汞的吸附亲和力；喷射NH4Br-AC的SO2协同脱除率达到了30.6%,主要为SO2在NH4Br-AC表面的化学吸附、毛细冷凝及部分SO2被氧化成SO3；喷射NH4Br-AC的NO协同脱除率达到了38%,主要为NO在NH4Br-AC表面的化学吸附及部分NO被氧化成NO2。提出了一个新的活性炭喷射脱汞预测模型,该模型基于外部膜传质与表面吸附过程,考虑了汞质量平衡与吸附等温。结果表明：该模型能合理地预测烟道内的活性炭喷射脱汞效率,可利用该模型对活性炭消耗量进行合理评估；模型参数,包括活性炭浓度,颗粒直径,平衡常数,外部传质系数及活性炭停留时间均对活性炭喷射脱汞效率有重要的影响。
[Abstract]:Mercury is a highly toxic pollutant with strong volatility, bioaccumulation and environmental durability, and coal is the largest source of anthropogenic mercury. Therefore, the emission control of coal-fired mercury has become an important research direction in the current energy environment. At present, the most mature and feasible technology for controlling the mercury discharge of the coal-fired power plant is the activated carbon injection technology (ACI), because the activated carbon injection and demercuration process is a complex two-phase flow reaction system including flow, heat transfer, mass transfer and chemical reaction, The application of the technology in the coal-fired power plant also has more problems. The adsorption and adsorption mechanism of mercury on the surface of the adsorbent, the adsorption kinetics of mercury, the thermodynamic and adsorption equilibrium characteristics, the key parameters and the influencing mechanism of the adsorption and demercuration efficiency of the adsorbent, the mechanism of the demercuration of the adsorbent and the potential of the synergistic removal of SO2 and NO, Research on the scientific problems of the sorbent injection demercuration prediction model and so on. Four demercuration adsorbents of the original activated carbon (R-AC), NH _ 4Br-modified activated carbon (NH4Br-AC), NH4Cl-modified activated carbon (NH4Cl-AC) and fly ash (FA) were prepared. The influence of the factors such as particle size and flue gas composition on the static adsorption of mercury was studied. The adsorption of mercury on the surface of the adsorbent was studied by a program temperature-raising and desorption (TPD) method, and the mercury adsorption mechanism of different adsorbents was analyzed. The results show that the modification of NH _ 4Br and NH _ 4Cl is mainly in the surface of the active carbon and the mesopore in the amorphous form; the static mercury adsorption property of the adsorbent is NH4Br-AC4Cl-ACR-ACFA; the adsorption of the R-AC to the Hg0 is mainly due to the physical adsorption, a small amount of chemical adsorption exists, the product is HgO, and the adsorption of the NH4Br-AC to the Hg0 is mainly chemical adsorption, The product is HgBr2; O2 promotes the adsorption of R-AC and NH 4Br-AC to H-type; SO2 has different effects on the Hg0 adsorption of R-AC and NH _ 4Br-AC; in the presence of SO2, the amount of HgBr2 produced on the surface of the NH4Br-AC is reduced, but a small amount of HgS is generated; and NO promotes the adsorption of the R-AC and the NH4Br-AC to the Hg0, as Hg (NO3)2 is generated on the surface of the adsorbent. The adsorption kinetics of gas-phase mercury on the surface of activated carbon and fly ash were studied by dynamic model. The activation energy, initial mercury adsorption rate and adsorption thermodynamics and adsorption equilibrium of mercury on the surface of activated carbon and fly ash were calculated. The results show that the mercury adsorption process can be divided into two stages: surface adsorption and internal diffusion adsorption; while the external mass transfer and the internal diffusion limit the mercury adsorption process, the chemical adsorption of the mercury in the active site is the control step of the adsorption rate of the R-AC and the NH4Br-AC mercury; for the fly ash, the external mass transfer is the mercury adsorption rate control step; The activation energy of the adsorption of mercury on the surface of R-AC and NH 4Br-AC is-10.06610/ mol and-28.068 kJ/ mol, indicating that the adsorption is a common function of physical adsorption and chemisorption, and the initial mercury adsorption rate of the mercury on the surface of the adsorbent is positively related to the mercury adsorption capacity of the adsorbent; The thermodynamic analysis shows that the adsorption of mercury on the surface of R-AC and NH 4Br-AC is a spontaneous and endothermic process, and the adsorption takes physical adsorption as the main and the chemical adsorption is the auxiliary. The adsorption process increases the confusion and complexity of the gas-solid interface; the adsorption of the mercury on the R-AC surface can be well described by the Temkin and the Langmuir equation, The adsorption of mercury on the surface of the NH4Br-AC and FA can be well described by the Freundlich equation. In this paper, an experimental device for the injection and demercuration of the first simulated flue gas entrained flow reactor in China was established. The mercury-demercuration characteristics of R-AC, NH 4Br-AC, NH4Cl-AC and NH 4Br-modified fly ash (NH4Br-FA) were studied. The concentration of Hg0, the residence time, the temperature of the flue gas and the particle size of the adsorbent were investigated. The effect of injection quantity and other parameters on the injection and demercuration is analyzed, and the mercury removal mechanism of different adsorbents is analyzed. The results show that the mercury concentration of the flue gas is increased, the residence time of the adsorbent is increased, the particle size of the adsorbent is reduced, the mercury absorption efficiency and the unit mercury adsorption capacity of the R-AC and the NH4Br-AC can be improved, the injection amount of the adsorbent can be increased, the mercury removal rate can be improved, the adsorption amount of the unit mercury is reduced, and the smoke temperature is increased, The mercury removal rate of the R-AC is reduced, and the mercury removal rate of the NH4Br-AC is increased; the oxidation and adsorption of the NH 4Br-modified to the flue gas Hg0 are stronger than that of the NH4Cl; during the injection and adsorption process, the Br or C1 groups on the surface of the NH4Br-AC and the NH4Cl-AC can oxidize the Hg0 molecules in the flue gas into HgBr2 or HgCl2, and the Hg0 is more easily absorbed compared with the HgCl2; The mercury removal rate of NH 4Br-FA is low, and the removal of Hg0 in the flue gas is mainly the oxidation of Hg0. Because the specific surface area and the pore structure of the fly ash are both poor, the adsorption rate of Hg0 is reduced. In the 6 kWth coal-fired circulating fluidized-bed-entrained-flow reactor-injection demercuration experiment device, the mercury emission and distribution characteristics of the combustion of anthracite in Guizhou were studied. The mercury-demercuration characteristics of NH _ 4Br-AC in coal-fired flue gas and the potential of co-removal of SO2 and NO were investigated. The results show that the mercury in the burning of the anthracite is mainly granular mercury, the proportion is 77.34%, the total mercury in the gas phase is 22.65%, the Hg0 is 10.27%, the Hg2 + is 12.38%, the residence time of the NH4Br-AC is increased from 0.59s to 1.79s, and the demercuration rate is increased from 70.7% to 90.5%; The Br-functional group of the surface of the NH4Br-AC significantly increased its adsorption affinity to mercury; the synergistic removal rate of the SO2 in the injection of NH4Br-AC reached 30.6%, mainly the chemical adsorption of SO2 on the surface of the NH4Br-AC, the capillary condensation and the partial SO2 being oxidized to SO3; the NO co-removal rate of the sprayed NH4Br-AC reached 38%, The chemical adsorption of NO on the surface of the NH4Br-AC and the partial NO are oxidized to NO2. A new model of activated carbon injection and demercuration is proposed. The model is based on the process of external membrane mass transfer and surface adsorption, and the equilibrium and adsorption of mercury are considered. The results show that the model can reasonably predict the mercury removal efficiency of activated carbon in the flue. The model can be used to evaluate the consumption of activated carbon. The model parameters include the concentration of activated carbon, the particle diameter and the equilibrium constant. The external mass transfer coefficient and the residence time of activated carbon have an important influence on the mercury removal efficiency of activated carbon.
【学位授予单位】：东南大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：X773

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