تمیز کردن دودکش گاز توسط پرتو الکترونی با انرژی بالا - مدلسازی و تحلیل حساسیت

The removal of sulfur and nitrogen oxides from flue gases using high energy electron beams is based on the generation of excited molecules when the flue gas is bombarded by accelerated electrons. The excited molecules undergo ionization, dissociation and electron attachment to yield reactive species (ions, metastables, free radicals and electrons) which interact with the flue gas components. A complex mathematical model was built-up, which includes the main chemical processes in both gas and liquid phases together with the droplets generation and thermodynamic equilibrium between the two phases. The simulation results are in good agreement with the experimental data gathered from literature. Modeling the formation of liquid droplets and the adjacent physico-chemical phenomena provide a better understanding of the process and a more accurate interpretation of the experimental results. The model enables the investigation of the treatment efficiency's sensitivity upon the main operating parameters. A fractional three level factorial white experiment was designed using as parameters the irradiation dose, the water vapor content and the nitrogen oxide initial concentration of the flue gases. The removal yield of SO2 is rather insensitive to the said parameters, while, on the contrary, the removal yield of NO is very sensitive.

Growing population and the rise of industrial activities have taken their toll on the quality of the environment. The pollutants emitted from industrial facilities, power stations, residential heating systems and engine vehicles have adverse effects on human health, cause stratospheric ozone depletion, which in turn leads to climate change, and contaminate soil and water, leading to acidification and eutrophication [12]. Conventional methods for the removal of sulfur and nitrogen oxides such as flue gas desulfurization and selective catalytic reduction [14] have long proved their high removal efficiencies [29]. However, this achievement is accompanied by large energy consumption and space requirements resulting in soaring investment and operating costs [28]. Thus, new methods have been devised for the abatement of sulfur and nitrogen oxides from flue gases. The electron beam flue gas treatment (EBFGT) is a relatively new procedure, developed in the late 1970s by the Ebara Corporation, in which the pollutants are subjected to ionizing radiation leading to the formation of a high-quality fertilizer mixture [8]. While achieving high removal efficiencies for both sulfur and nitrogen oxides, the process can be extended to the treatment of other gaseous pollutants and liquid effluents [16] and [21]. Compared with more traditional methods, EBFGT has the advantage of scalability and simplicity in addition to being an easily controllable process [5]. The technology has gradually achieved some level of market penetration, at first with the construction of a series of pilot plant installations in Japan, USA, Germany and Poland, among others, and, more recently, with the development of two industrial facilities in Poland and China [7]. However, the technology suffers, just like the conventional treatment methods, from large energy requirements [2] and from reliability issues associated with the continuous operation of high energy electron accelerators [20]. Consequently, numerous investigations have been made into the possibility of reducing the energy consumption for the process: employing hybrid irradiation methods such as combined microwave and electron beam treatment [18], turning to alternative non thermal plasma generation methods [10], fitting the plasma reactor with a catalytic layer [15], using a variety of additives such as ammonia, hydrogen peroxide [1], natural gas and hydrated lime [25]. The potential use of medium energy accelerators has also been investigated [5], with the provision that a dispersed liquid phase should be introduced in the reactor before the beginning of the irradiation treatment. Another method to reduce the energy consumption and the operating costs is the investigation of more appropriate reactor configurations [23], either experimentally or through the use of mathematical modeling [8]. The first mathematical models developed started from the simplest reaction systems, formed only of N2, O2 and NO, considering as little as 29 chemical reactions [22] and have been gradually improved to include over 850 chemical reactions in the gas phase [26]. However, the size of the kinetic system greatly impacts the computational capacity so, more recent modeling studies have only taken into consideration a fraction of these chemical reactions [6] and [11] or have resorted to empirical or semi-empirical approaches [9]. Despite early interest in modeling the liquid phase phenomena taking place during irradiation [19], the low liquid to gas ratio experimentally observed [32] has lead researchers to neglect the formation and behavior of this liquid phase in their modeling efforts. However, experimental evidence shows that the introduction of fine water droplets, even in small amounts, in the irradiation chamber can lead to serious energy savings and lowers the operating costs [5]. The aim of the current paper is to advocate a complex mathematical model, considering 90 gas phase and 32 liquid phase chemical reactions that can accurately describe the behavior of the sulfur and nitrogen oxides subjected to electron beam irradiation. The model is, then, used to investigate the treatment efficiency's sensitivity upon the main operating parameters.

The results obtained from the proposed new mathematical model are in good agreement with published experimental data from literature. The model predicts with good accuracy the performances obtained experimentally for a relatively large array of operating conditions: in the majority of cases the departure model – experiment is within the experimental error range. Moreover, the model is more accurate in predicting the sulfur dioxide behavior, as its removal pathway, despite being more complex, involves fewer radio-chemical reactions. In the best case scenario, the predicted nitrogen and sulfur oxides' removal efficiencies are very similar to those obtained experimentally for a residence time of 4.11 s and an irradiation dose of 10.2 kGy being closer than those predicted by the empirical model in the case of nitrogen oxide. Nitrogen oxide's slightly better removal can be explained by the low initial concentration of this pollutant and the high irradiation rate. The factorial white experiment has proved the capacity of this new mathematical model to capture the sensitivity of the process, showing that both these parameters have a marked effect on the removal efficiency of the nitrogen oxides: even small positive variations in initial pollutant concentration (in the range of tens of ppm) lead to significantly poorer performance for the process, in good agreement with the experiments. The increase in irradiation dose and humidity content have a notable beneficial effect on the abatement of nitrogen oxide from flue gases, the former being almost entirely removed from the gas in the most favorable scenario (high humidity and irradiation dose and low initial concentration). However, increasing the irradiation dose has a negative effect on the energy consumption of the process and may lead to maintenance problems for the accelerators in the long-run. In addition to this, the water vapor content reduces the temperature of the flue gases and poses problems for the equipment, both factors leading to greater operating and investment costs. The sulfur dioxide's removal efficiency shows small improvements with the increase of irradiation dose and humidity content of the flue gases. The removal of sulfur dioxide is negatively impacted by the addition of a less than stoichiometrical quantity of ammonia, the small nitrogen oxide initial concentration and the relatively short residence time. The new mathematical model proposed for characterizing the abatement of sulfur and nitrogen oxides can predict in a consistent manner the overall process efficiency of the irradiation beam treatment for flue gases while showing appropriate sensitivity against the main operating parameters.