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 pathway 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
Arsenic volatilization characteristics of SJS bituminous coal were carried out in a customized isothermal thermogravimetric experimental system at 600−1400 °C under different oxy-fuel atmospheres. The mineralogical and morphological characterization of ash samples were analyzed using XRD and SEM instruments. Different from conventional nonisothermal mass loss curves by TG analyzer, the isothermal mass loss curves of coal did not show a clear process of moisture removal and devolatilization. With the increase of combustion temperature, the isothermal mass loss curve of SJS coal shifted to the left gradually and the burnout time shortened at the same time, whereas the mass loss curves of arsenic showed different tendencies at <900 °C and >900 °C stages. At the <900 °C stage, the effect of O 2 dominated. A higher O 2 ratio led to the larger volatile proportion of arsenic, irrespective of CO 2 content. At the >900 °C stage, the volatilization of arsenic was delayed in oxyfuel condition but with a higher release rate; thus the volatility ratio of arsenic in oxy-fuel combustion was even larger than that of conventional air/flue gas combustion.
The effect of H2O on arsenic release behavior was investigated via experiment and firstprinciples 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 (100) 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.
The effects of the temperature, volatile content, and ash content on the volatilization kinetics of arsenic were discussed. Six Chinese coals were tested in an isothermal thermogravimetric reactor at 600−1300 °C. Sequential chemical extraction analysis was employed to know the speciation transformation of arsenic during coal combustion. Results show that the volatilization ratio of arsenic increases with the temperature, and most arsenic volatilizes between 700 and 1000 °C. The volatile content has a positive effect on the volatilization rate of arsenic in the initial stage of combustion. With the same level of ash content, high-volatile coals (>30%) own a larger volatilization rate of arsenic (0.55−0.6% s −1 ) than the low-volatile coals (<12%) (0.25% s −1 ). In contrast, coals with a higher ash content tend to obtain a lower volatilization rate of arsenic during the process. Despite the effect of volatile and ash contents on the volatilization rate of arsenic, the final volatilization ratio of arsenic largely depends upon its mode of occurrence of arsenic in coal, especially on sulfide-and organic-bound arsenic. A first-order reaction kinetic model can well describe the volatilization characteristics of arsenic. A calculation shows that the activation energy of arsenic volatilization is significantly affected by the volatile content, while the ash content has a tiny effect on it. High-volatile coals have lower activation energy of arsenic volatilization than low-volatile coals.
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