Linjun Yang

Activated carbon (AC) adsorption is an effective method for abatement of volatile organic compounds (VOCs) in coal-fired power plants. The adsorption behaviors of VOCs (toluene and chlorobenzene) for ACs (coconut shell-based AC (CSAC) and wood-based AC (WAC)) under medium-high temperature (MHT: 90–150 °C) conditions were investigated in a fixed-bed reactor. The results indicated that the adsorption capacity of VOCs was reduced with increasing temperature. CSAC had a higher adsorption capacity and adsorption rate for the two VOCs than WAC at the same adsorption temperature. The adsorption capacity of toluene was within the range of 13.9–49.9 mg/g, while that for chlorobenzene was 26–80.3 mg/g. The content of oxygen-containing groups on the AC surface was increased with acid modification and decreased with alkali modification. The adsorption capacity of AC was improved to varying degrees after chemical modification. Adsorption was mainly controlled by physical adsorption, whereas the effects of chemisorption became evident with increasing temperature. The interactions between oxygen-containing groups and VOCs were more beneficial for improving adsorption capacity than π–π interactions. The pseudofirst-order kinetic model better described the adsorption of VOCs by ACs. The MHT conditions promoted the intraparticle diffusion; lower VOC concentration resulted in the external mass transfer of AC becoming the adsorption rate-limiting step. Increasing the external surface area and micropore volume of AC and enriching the content of surface oxygen-containing groups were useful in enhancing the VOC removal efficiency.

A large amount of water vapor present in low-concentration organic exhaust gases severely inhibited the adsorption of volatile organic compounds (VOCs). In this paper, three coal-based activated carbon samples were prepared by heat treatment, sodium hydroxide impregnation, and ammonia solution impregnation and were analyzed by N2 adsorption isotherm, X-ray photoelectron spectroscopy (XPS), Boehm titration, and water and toluene adsorption isotherms. Besides, the low-concentration toluene adsorption performance of samples was investigated under different humidities, and the desorption activation energies (DAEs) of toluene and water on samples were estimated through temperature programmed desorption (TPD) experiments. Furthermore, the effect of molecule polarity was investigated under 0 and 60 RH%. The results indicated that samples treated by heating (CAC-1) significantly improved the uptake of low-concentration toluene under humid conditions compared to samples modified by alkali impregnation, which is related to surface chemistry. The analysis confirmed that CAC-1 possessed more graphitic (C + N), enhancing the π–π interaction, and possessed lower acidic groups, reducing the bonding sites with water. The Yoon and Nelson (Y–N) model indicated that CAC-1 presented a higher diffusion rate of toluene than CAC under high humidity. TPD experiments further demonstrated that the CAC-1 had smaller DAE of water and larger DAE of toluene, suggesting a stronger hydrophobic and stronger interaction with toluene. In addition, the experiments demonstrated that CAC-1 had superior adsorption performance both for polar and weakly polar VOCs at high humidity, especially for weakly polar VOCs. These conclusions would help to emphasize that activated carbons after heat treatment have better selectivity in removing low-concentration VOCs from high-humid conditions.