Experimental and kinetic studies of gas-phase mercury adsorption by raw and bromine modified activated carbon

Abstract

Gas-phase mercury capture performance of raw activated carbon (R-AC) and bromine modified activated carbon (AC–Br) was evaluated in a fixed-bed reactor. Effect of flue gas temperature on mercury adsorption efficiency was explored. Sorbent characterization was conducted to feature their morphologies correlated to mercury adsorption efficiency. Adsorption kinetic and thermodynamic analysis of fixed-bed experimental results was also carried out to explore the different mercury adsorption mechanisms of R-AC and AC–Br. The results show that vapor mercury adsorption capacity of AC–Br increases with temperature, but R-AC demonstrates opposite results. The bromine ion (Br−) remains in the mesopore and the surface of R-AC in the form of amorphous during modification process, leading to the reduction of mesopore volume and the increase of micropore volume. The increase of active site (Br−) on activated carbon improves chemisorption of vapor mercury on AC–Br at 150°C and 200°C. The kinetic analysis shows that mercury adsorption on active sites is the adsorption rate controlling step, in which it can be divided into two steps: surface adsorption and intraparticle diffusion adsorption. Vapor mercury adsorption on AC–Br needs more activated energy due to the enhancement of chemisorption. The initial mercury adsorption rate of AC–Br increases with temperature, which is different from that of R-AC. The thermodynamic analysis shows that the adsorption of gas-phase mercury on both R-AC and AC–Br is spontaneous, endothermic and mainly physical in nature enhanced by chemisorption. The bromine modification enhances chemisorption of vapor mercury on AC–Br leading to the increase of complexity degree of mercury adsorption process.