Backwashing behavior and hydrodynamic performances of granular activated carbon blends.

Ozone-biological activated carbon (O3-BAC) process has been proved to be an efficient and cost-effective technology in advanced treatment of drinking water. However, O3-BAC raises strict requirements in adsorption, hydrodynamic and regeneration performances, which one single activated carbon could hardly all-sided meet. Blending activated carbons seems to be an appropriate and economically feasible method to deal with the issue. Thus, the uniformity and stability of activated carbon blends during water treatment, especially in backwashing process are of great importance. In this paper, cyclic experiments of downward adsorption and upward backwash on 11 typical commercial granular coal-based activated carbons and their blends were carried out in column test. Hydrodynamic performances such as bed expansion rate and bed pressure drop were measured. The uniformity and stability of activated carbon blends were investigated by determining iodine number of samples collected from different heights of activated carbon bed. Then, both traditional regression methods and back-propagation neural network model were utilized to predict superficial velocity at 30% bed expansion rate and maximum bed pressure drop of activated carbon blends. The results indicate that water backwashing process has no effect on the composition proportion of activated carbon blends, and slightly changes the particle distribution of activated carbon bed regarding pore structure and adsorption capacity. A three-layer back-propagation neural network model for superficial velocity at 30% bed expansion rate yields mean relative errors of 2.17%, which is much lower than that given by traditional regression methods such as 5.53% (weighted average), 4.08% (linear) and 4.06% (polynomial). Moreover, the back-propagation neural network model for maximum bed pressure drop yields mean relative errors of 1.37%, which is much lower than that given by traditional regression methods such as 4.31% (weighted average), 4.28% (linear) and 4.22% (polynomial). The non-linear relationships can be accurately identified by the back-propagation neural network model.

Transformation of alkali and alkaline earth metals during the preparation of activated carbon from Zhundong high-alkali coal.

The preparation of activated carbon (AC) is a promising approach for the efficient utilization of Zhundong high-alkali coal. The volatilization and release of alkali and alkaline earth metal (AAEM) species can be effectively inhibited by using a lower operating temperature and a carbon matrix. However, the long time of the pyrolysis and activation process may promote the release of the AAEM from the coal during the process. Therefore, it is necessary to explore the transformation of AAEM during the preparation of AC from Zhundong high-alkali coal, and the cleanness of this process is evaluated accurately. In this study, the evolution of AAEM, distribution, and chemical speciation is characterized before and after the preparation of AC from the coal, and then thermodynamic calculations were performed using FactSage to simulate the transformation of AAEM in the coupled process of pyrolysis and activation. The results showed that in the process of AC preparation, the AAEM species inside the carbon matrix moved towards the surface of the AC with the aid of released volatiles and the activation reaction. Some Na and K species were released due to their weak binding with the carbon matrix and this resulted in the loss of Na and K content, whereas Mg and Ca were closely combined with the carbon matrix and were enriched in the AC. Furthermore, the defects and amorphous structure of the AC prepared with H2O activation were more than that of the AC prepared with CO2 activation, which meant that more of the AAEM species were exposed to the high temperature environment. As a result, the loss of AAEM content in the AC with H2O activation was higher than that in the AC with CO2 activation. In this process, a small amount of highly volatile and corrosive AAEM was produced, and the release of volatile matter and the consumption of the carbon matrix were the main factors for the AAEM loss. Therefore, the preparation of AC from Zhundong high-alkali coal is a viable method for its clean use.

Mechanism of the evolution of pore structure during the preparation of activated carbon from Zhundong high-alkali coal based on gas-solid diffusion and activation reactions.

Zhundong coal can significantly reduce the preparation temperature of activated carbon (AC) due to the high contents of alkali and alkaline earth metals (AAEMs) present in it. Moreover, because of its lower operating temperature and the presence of carbon matrix, Zhundong coal can effectively inhibit the release of AAEM during the preparation of AC. For these reasons, the preparation of AC from Zhundong coal is a promising approach for the clean utilization of Zhundong coal. Accordingly, this study was aimed to investigate optimum conditions for the preparation of AC from Zhundong coal. For this purpose, at first, Raman spectroscopy was used to determine the conditions for an optimal carbonization process using a coal sample; then, the evolution of the pore structure of AC under different conditions was examined by small-angle X-ray scattering (SAXS) and the N2 adsorption analyser. Furthermore, environmental scanning electron microscopy (ESEM) was performed to analyze the surface morphology of AC. Finally, by dividing the activation process into gas-solid diffusion and activation reactions, a mechanism for the evolution of pore structure during the preparation of AC was proposed. The results showed that the char with an amorphous structure and less graphite-like carbon, which was obtained by heating Zhundong coal from room temperature to 600 °C at 5 °C min-1 under the protection of N2 and then maintaining it at this temperature for 60 min, is suitable for the subsequent activation process. At low temperatures, the diffusion of H2O was dominant in the activation process, and the weak gas-solid reaction resulted in poor development of the pore structure; on the other hand, the CO2 activation reaction mainly occurred on the surface of the char due to the poor diffusion of CO2, and then, the produced pores could improve the diffusion of CO2; this led to significant development of the pore structure. With an increase in temperature, the H2O diffusion reaction was enhanced, and the pore structure of AC was completely developed; however, the diffusion of CO2 reduced with an enhancement in the CO2 activation reaction, leading to the consumption of carbon matrix by CO2 gasification instead of pore formation by the CO2 activation reaction. Therefore, proper utilization of the unique characteristics of H2O and CO2 during pore formation is important to control the activation process.