Browsing by Author "Wang, Chunbo"
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Item Open Access The combined effect of H2O and SO2 on the simultaneous calcination/sulfation reaction in CFBs(Wiley, 2019-01-10) Chen, Liang; Wang, Chunbo; Zhao, Fan; Zou, Chan; Anthony, Edward J.The combined effect of H2O and SO2 on the reaction kinetics and pore structure of limestone during simultaneous calcination/sulfation reactions under circulating fluidized bed (CFB) conditions was first studied in a constant‐temperature reactor. H2O can accelerate the sulfation reaction rate in the slow‐sulfation stage significantly but has a smaller effect in the fast‐sulfation stage. H2O can also accelerate the calcination of CaCO3, and should be considered as a catalyst, since the activation energy for the calcination reaction was lower in the presence of H2O. When the limestone particles are calcining, SO2 in the flue gas can react with CaO on the outer particle layer and the resulting CaSO4 blocks the CaO pores, increases the diffusion resistance of CO2 and, in consequence, decreases the calcination rate of CaCO3. Here, gases containing 15% H2O and 0.3% SO2 are shown to increase the calcination rate. This means that the accelerating effect of 15% H2O on CaCO3 decomposition is stronger than the impeding effect caused by 0.3% SO2. The calcination rate of limestone particles was controlled by both the intrinsic reaction and the CO2 diffusion rate in the pores, but the intrinsic reaction rate played a major role as indicated by the effectiveness factors determined in this work. This may explain the synergic effect of H2O and SO2 on CaCO3 decomposition observed here. Finally, the effect of H2O and SO2 on sulfur capture in a 600 MWe CFB boiler burning petroleum coke is also analyzed. The sulfation performance of limestone evaluated by simultaneous calcination/sulfation is shown to be much higher than that by sulfation of CaO. Based on our calculations, a novel use of the wet flue gas recycle method was put forward to improve the sulfur capture performance for high‐sulfur, low‐moisture fuels such as petroleum coke.Item Open Access The effect of H2O on formation mechanism of arsenic oxide during arsenopyrite oxidation: Experimental and theoretical analysis(Elsevier, 2019-12-07) Zou, Chan; Wang, Chunbo; Chen, Liang; Zhang, Yue; Xing, Jiaying; Anthony, Edward J.The effect of H2O on arsenic release behavior was investigated via experiment and first-principles density functional theory (DFT). The experimental results show that sulfide-bound arsenic is the main form present in coal, and that H2O has a positive influence on the release of arsenic during coal combustion. Furthermore, DFT calculations were performed to investigate the mechanism for H2O influence on arsenic oxidation. Thermodynamic and kinetic analyses were also conducted to study the influence of temperature on the reaction process. From thermodynamic analysis, arsenic oxide formation on the FeS2 (1 0 0) surface with and without H2O weakens with increasing temperature. In addition, the equilibrium constant for the reaction with H2O addition is slightly higher than that for the reaction without H2O, which suggests that the degree of the chemical reaction in the presence of H2O should increase. From kinetic analysis, the reaction rate constants increase with temperature, and the activation energy of the arsenic oxide formation reaction with and without H2O is 100.72 kJ/mol and 124.08 kJ/mol, respectively. This indicates that H2O adsorption on the surface can decrease the energy barrier and accelerate the reaction forming arsenic oxide. Based on the thermodynamic and kinetic analyses, it can be concluded that temperature has an inhibitory influence on reaction equilibrium and positive influence on the reaction rate. The experiment and calculation results explain the influence of H2O on the formation mechanism of arsenic oxide and provide a theoretical foundation for the emission and control of arsenic.Item Open Access Effect of H2O on the volatilization characteristics of arsenic during isothermal O2/CO2 combustion(Elsevier, 2019-03-06) Zou, Chan; Wang, Chunbo; Liu, Huimin; Chen, Liang; Anthony, Edward J.The effect of H2O on the volatilization behavior of arsenic in coal was studied under O2/CO2 combustion conditions at 800–1300 ºC, which covers the effective range of coal combustion temperatures appropriate for conventional coal combustion technologies. By controlling the combustion time of the coal, the volatilization percentage and rate of As emissions versus time were obtained. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were used to study the evolution of minerals with and without H2O under O2/CO2 combustion conditions. The effect of CO2 on As volatilization was first investigated and it was found that increasing CO2 concentrations inhibits the volatilization of As, with this effect decreasing with increasing temperature. When a fraction of the CO2 was replaced with H2O, the volatilization of As increased, but the positive effect of H2O also decreased with increasing temperature. The volatilization percentage of As with 30% H2O was 6.1% higher than that without H2O at 800 ºC, while it was only 2.7% higher at 1300 ºC. When the concentration of H2O increased from 0 to 30%, the peak value of the As volatilization rate increased and the time needed to reach the peak value decreased. The volatilization characteristics of As for three coals were very similar, which demonstrates that the effect of H2O was not limited to only one specific coal.Item Open Access The kinetics and pore structure of sorbents during the simultaneous calcination/sulfation of limestone in CFB(Elsevier, 2017-07-14) Chen, Liang; Wang, Chunbo; Wang, Ziming; Anthony, Edward J.The interaction of calcination and sulfation in the simultaneous calcination/sulfation of limestone sorbent under circulating fluidized bed boiler conditions was studied. A specially designed constant-temperature reactor which can stop the reaction at a given time was employed. When limestone entered the furnace of mixed gases of CO2, O2, SO2, etc., its weight went down first, then up, so there was a minimum weight point. The whole reaction period could be divided into two stages by this minimum weight point, named the weight-loss stage and the weight-growth stage, which were dominated by the calcination reaction and by the sulfation reaction, respectively. In the weight-loss stage, the sulfation reaction took place and CaSO4 formed simultaneously together with limestone calcination as long as SO2 was present. In the weight-growth stage, the sulfation ratio at 60 min in simultaneous calcination/sulfation is 30.7% higher than that in the sequential calcination then sulfation process. The weight loss rate of limestone calcined in the presence of SO2 was lower than that without SO2 present but the final weight was higher. The calcination of limestone was slowed by the presence of SO2; a probable mechanism was proposed, namely that the CaSO4 formed may fill or plug the pores in the CaO layer, and impede the transfer of CO2 and, therefore, retard the calcination reaction. This mechanism was supported by the observation that the effective diffusion coefficient of CO2 in CaO produced in the presence of SO2 was reduced. The impeding effect increased with increasing SO2 concentration (0–3000 ppm), while, when the particle size decreased from 0.4–0.45 mm to 0.2–0.25 mm, the calcination rate of limestone was higher, no matter whether there was SO2 present or not. The impeding effect was less pronounced at 880 °C than at 850 °C. The reason for this appears to be the fact that there was less CaSO4 formed at 880 °C and, therefore, fewer pores of the particle were filled or plugged.Item Open Access Modelling the simultaneous calcination/sulfation behavior of limestone under circulating fluidized bed combustion conditions(Elsevier, 2019-08-28) Chen, Liang; Wang, Chunbo; Tong, Shuai; Anthony, Edward J.;The simultaneous calcination/sulfation (SCS) reaction is the realistic reaction process for limestone use in CFB boilers. A SCS reaction model based on the randomly-overlapped pore concept, which takes into consideration the calcination of CaCO3, the sulfation of CaO and the sintering effect simultaneously, was developed. The results of this model fit well with the results from the thermogravimetric analyzer (TGA) tests and, thus this model was used to study the characteristics of the SCS reaction. The SCS reaction consists of a mass-loss stage and a mass-growth stage, and the two stages are seperated by a minimum mass point. The mass-loss stage is dominated by the calcination of CaCO3, while the mass-growth stage is dominated by the sulfation of CaO. The minimum mass point is a balance point of the mass change caused by the two reactions. The calcination reaction occurred in a layer of the particle. As the calcination reaction progresses, the reaction front moves inward and a CaO layer is formed. The SO2 in the calcination atmosphere can react with the CaO layer and produce CaSO4. The CaSO4 can fill the pores of the CaO layer and narrow the pore width, increase the CO2 diffusion resistance and consequently slow the calcination reaction. The sulfation reaction becomes slower as the reaction progresses. There was an upper limit to the sulfation conversion, which is much higher in the outer layer of the particle. For a typical particle with a radius of 200 μm, the sulfation reaction ceases in the inner part (0-150 μm) of the particle due to the exhaustion of SO2, while in the outer part of the particle (150-200 μm), the decrease of the sulfation rate is caused by the simultaneous decline of the reaction surface area, surface Ca2+ ion concentration and SO2 concentration.Item Open Access Review of arsenic behavior during coal combustion: Volatilization, transformation, emission and removal technologies(Elsevier, 2018-04-17) Wang, Chunbo; Liu, Huimin; Zhang, Yue; Zou, Chan; Anthony, Edward J.Growing public awareness of the environmental impact of coal combustion has raised serious concerns about the various hazardous trace elements produced by coal firing. Arsenic deserves special attention due to its toxicity, volatility, bioaccumulation in the environment, and potential carcinogenic properties. As the main anthropogenic source of arsenic is coal combustion, its behavior in power plants is of concern. Unlike mercury, arsenic behavior in coal combustion has not been subjected to systematic, in-depth research. Different researchers have reached opposing conclusions about the behavior of arsenic in combustion systems and, as yet, there is relatively little research on arsenic removal technologies. In this paper, the volatilization, transformation, and emission behavior of arsenic and its removal technologies are discussed in depth. Factors affecting the volatilization characteristics of arsenic are summarized, including temperature, pressure, mode of occurrence of arsenic, coal rank, mineral matter, and the sulfur and chlorine content of the fuel. The behavior of arsenic during oxy-fuel combustion and the effect of combustion atmosphere (O2, CO2, SO2 and H2O(g)) are also reviewed in detail. In order to better understand the pathways of arsenic in a power plant environment, a particular focus in this work is the transformation mechanism of ultra-fine ash particles and the partitioning behavior of arsenic. Finally, the effects of air pollution control devices (APCDs) on arsenic emissions are examined, along with the effectiveness of flue gas arsenic removal technologies with different kinds of adsorbents, including calcium-based adsorbents, metal oxides, activated carbon, and fly ash.Item Open Access The simultaneous calcination/sulfation reaction of limestone under oxy-fuel CFB conditions(Elsevier, 2018-10-16) Chen, Liang; Wang, Chunbo; Yan, Guangjing; Zhao, Fan; Anthony, Edward J.Using a customized thermogravimetric analyzer, the characteristics of the simultaneous calcination/sulfation reaction of limestone (the simultaneous reaction) under oxy-fuel circulating fluidized bed (CFB) boiler conditions were investigated. The results were compared with the calcination-then-sulfation reaction (the sequential reaction) that has been widely adopted by previous investigators. The sample mass in the simultaneous reaction was higher than that in the sequential reaction. With the increase of SO2 concentration (0–0.9%), the mass difference between the two reaction scenarios increased; while with the increase of temperature (890–950 °C), the difference became smaller. Calcination in the presence of SO2 was slower than that without SO2. With the increase of SO2 concentration, the pore volume of the calcined CaO decreased, and the effectiveness factors of the calcination reaction also declined. This indicates when CaSO4 forms, the pores in CaO were filled or blocked, thus increasing the internal resistance to CO2. Because the simultaneous process is the real one in CFB boilers, and it shows different characteristics from the sequential reaction, all investigations of CaO sulfation in CFB should follow this approach. Also in this work, the effects of SO2 concentration, temperature and H2O on the simultaneous reaction were studied. The sulfation ratio in the simultaneous reaction increased with higher SO2 concentration. Compared with that in the absence of H2O, 8% H2O in flue gas significantly improved sulfation. In the tested range (890–950 °C), the optimum temperature for sulfation was around 890 °C. The sulfation rate in the mass-loss stage was higher than that in the fast sulfation stage, which is likely due to the continuous generation of nascent CaO in this stage.Item Open Access Simultaneous removal of SO2 and NOx by a new combined spray-and-scattered-bubble technology based on preozonation: from lab scale to pilot scale(Elsevier, 2019-03-28) Si, Tong; Wang, Chunbo; Yan, Xuenan; Zhang, Yue; Ren, Yujie; Hu, Jian; Anthony, Edward J.A new technology (called here, spray-and-scattered-bubble technology) based on preozonation was designed and tested for simultaneous removal of SO2 and NOx from power plant flue gas. It combines the advantages of the common spray tower and the jet bubble reactor, in which the flue gas experiences an initial SO2/NOx removal in the spray zone and then undergoes further removal in the bubble zone. Factors that affect the simultaneous removal of SO2/NOx were investigated through lab-scale experiments, by varying the O3/NO molar ratio, liquid/gas ratio and the immersion depth. The results showed the removal of SO2 and NOx can be significantly improved as compared to a separate spray column or bubble reactor, by as much as 17%, for the spray column and 18% for the bubble reactor for NOx and 11% for the spray column, and 13% for the bubble reactor for SO2, for liquid/gas ratio of 4 dm3/m3 or immersion depth of 100 mm. The O3/NO molar ratio had little effect on the SO2 removal, but it strongly affected the removal efficiency of NOx especially when it was less than 1.0. Both the liquid/gas ratio and immersion depth demonstrated a positive correlation with the removal efficiency. However, a balance must be maintained between efficiency and economics, since the liquid/gas ratio directly influences the performance and number of the circulating pumps, and the depth is closely related to the flue gas pressure drop, and both factors affect energy requirements. To further confirm its industrial feasibility, a 30 h test using real coal-fired flue gas was conducted in a pilot-scale experimental facility (flue gas volume of 5000 Nm3/h). Increasing SO2 concentration in flue gas can promote the removal efficiency of NOx, but the SO2 removal was almost complete under all conditions tested. Finally, taking a 300 MW unit as an example,- the total energy cost of this new technology is estimated as being 10% lower than that of the common spray tower technology, based on an analysis using Aspen Plus™, with the largest difference reflected in the energy requirements of the circulating pumps and the ozonizer. Over all, the new technology offers the joint advantages of reducing emissions and saving energy.Item Open Access Vaporization model for arsenic during single-particle coal combustion: Model development(Elsevier, 2019-03-15) Liu, Huimin; Wang, Chunbo; Zhang, Yue; Zou, Chan; Anthony, Edward J.The kinetic parameters for chemical reactions associated with the vaporization of arsenic species are rarely reported due to the difficulties in obtaining suitably purified arsenic compounds as well as the issues associated with the extreme toxicity of many arsenic species. Here, we used a single-particle coal combustion model combined with a vaporization yield model of arsenic fitted by experimental data, which was used to determine the activation energy and frequency factor of the oxidation/decomposition reactions of arsenic species in this work, namely: As-org, FeAsS, FeAsO4 and Ca3(AsO4)2. The combustion kinetics of volatile/char and arsenic thermodynamic properties were used to model the vaporization zone and intensity of emissions for arsenic compounds. The results show that the reaction kinetic parameters of these arsenic species could be determined within an order of magnitude despite the variation of compositions in the coal sample and temperature, and this approach provides a new method to determine the reaction kinetics of hazardous elements such as As. Combining the vaporization yield and reaction kinetics of arsenic species with the single-particle coal combustion model, a novel vaporization model of arsenic was developed. With this model, the temporal evolution of combustion parameters (temperature, conversion ratio of coal, particle porosity, flue gas concentration) as well as arsenic vaporization ratio and As2O3(g) concentration can be predicted at the microscopic level.