Chun Zhu

Adsorption equilibrium, kinetics and mechanism studies of mercury on coal-fired fly ash

Fly ash samples were collected from the electrostatic precipitator (ESP) of a 600MW pulverized coal boiler firing Zhungeer bituminous coal in China to evaluate and explore its mercury adsorption capacity and mechanism. Samples characterization was conducted to feature their morphologies correlated to mercury content, and experimental studies on mercury adsorption in a fixed-bed apparatus were carried out to further verify its mercury adsorption availability. Based on the experimental data, adsorption isotherm was modeled with Langmuir, Freundlich, and Temkin equations. Adsorption kinetic analysis was also performed. The results show that mercury content of fly ash samples is associated with particle size, unburned carbon content and functional groups of Al-O/Si-O or Si-O-Si/Si-O-Al tetrahedron on fly ash. Increase of initial mercury concentration is beneficial to promote mercury adsorption due to the enhancement of mercury diffusion force onto the fly ash surface, mercury intraparticle diffusion rate and initial mercury adsorption rate. Fly ash with medium size displays better mercury adsorption capacity. Smaller particle size results in higher specific surface area, but brings about low specific surface area utilization rate for mercury adsorption. Freundlich isotherm equation presents better fitting result, indicating that fly ash surface is non-uniform. Mercury adsorption on fly ash at 120°C is mainly physisorption enhanced by chemisorption with ΔG at −36.73 kJ/mol. The pseudo-first-order kinetic model can describe the adsorption process more accurately and predict mercury adsorption capacity of fly ash preferably, showing that mercury adsorption on fly ash surface in fixed-bed is controlled dominantly by external mass transfer.

Investigation of mercury adsorption and cyclic mercury retention over MnOx/γ-Al2O3 sorbent

In this study mercury sorbent based on manganese oxides impregnated γ-alumina was synthesized. Mercury retention characteristics were investigated by mercury speciation and thermal desorption experiments. No gaseous mercuric oxide was observed in mercury speciation experiments with a mercury mass balance ratio of 89.11%. Maximum mercury desorption peak at 480 °C indicated that mercury was adsorbed in the form of mercuric oxide. Three cycles of mercury retention were tested with different thermal treatment in-between to evaluate the cyclic performance. Changes in surface phase and manganese chemistry before and after thermal treatment were characterized by XRD and XPS. Deterioration in mercury retention capacity was observed after thermal desorption at 500 °C, which was interpreted with reduced initial adsorption rate calculated by Pseudo-second order kinetic model. XPS studies suggested that atomic ratios of Mn4+/(Mn4++Mn3+) decreased from 73.2% to 32.3% and 33.9% after thermal desorption in N2 and air, respectively. The reduction of MnO2 to Mn2O3 was irreversible thus the mercury retention capacity could not be restored by thermal desorption at high temperatures. Spent sorbents that were reactivated at 200 °C in air without thermal desorption at 500 °C possessed considerable cycling performance for mercury retention due to the preserved Mn4+.