Mercury removal mechanism of AC prepared by one-step activation with ZnCl2

Abstract

In this work, activated carbon with ZnO laden on the surface was prepared by ZnCl2-one-step activation of different types of biomass (corn straw, bamboo powder and sawdust) and employed for mercury removal in the coal gas. During the activation, a part of ZnCl2 is converted into ZnO and laden on activated carbon, which greatly promotes the mercury removal performance with the presence of H2S. The ZnO content varies greatly with the content of SiO2 in the biomass due to its inactivation of ZnO and formation of Zn2SiO4. Carbonyl, which is important to mercury removal is largely retained after the activation at 500 °C and the amount of carbonyl seems to be positively correlated with the specific surface area (related to the lignin content). The temperature programmed desorption method was applied to further analyse the mercury adsorption in the coal gas. The results showed that ZnO and carbonyl play an important role in mercury removal, while the physisorption is not significant and only accounts for approximately 7% at 130 °C. The mechanism study showed that ZnO serves as a catalytic active site and catalyses the Hg0 into HgS with the presence of H2S. While the CO group of carbonyl acts as adsorption sites and turns into CO after the adsorption. This study provided a promising preparation method of mercury sorbents in the coal gas, and the influence of surface functional groups was also investigated in detail.