Optimization of the structural and surface characteristics of activated carbon for SO2 removal

Coal-based activated carbons (ACs) were prepared from Shenhua lignite by chemical activation of ZnCl2 . The surface functional groups of ACs were modified by NaOH to study the adsorption of SO2 in flue gas. Scanning elec- tron microscope (SEM) was used to observe the surface morphologies,N2 adsorption was used to characterize the pore structures,Fourier transform infrared spectroscopy (FTIR) and Boehm titration were used to investigate the functional groups. The preparation parameters of ACs were discussed based on the response surface methodology (RSM) method, and a prediction model for the surface area was established based on carbonization temperature,carbonization time and heating rate. The optimum conditions for preparation of ACs by RSM were as follows: carbonization temperature of 700 ℃ ,heating rate of 15 ℃ / min,and carbonization time of 3 h. The BET surface area and microporous surface area of the obtained AC were 599 m2 / g and 269 m2 / g,respectively. The effects of NaOH concentration on the pore struc- tures,surface functional groups and SO2 adsorption capacities of ACs were investigated. Results showed that the pore structures were severely damaged after NaOH modification,and the pores were ma2inly distributed in the range of 0. 5-0. 6 nm,0. 7 - 0. 9 nm and 1. 0 - 4. 0 nm. Highest BET surface area (681 m / g) and microporous surface area (292 m2 / g) was reached with treatment by 20% NaOH. This illustrated that low concentration of alkali treatment could expand pores in ACs,while high concentration of alkali might destroy the pore structures,leading to the decrease in surface area and pore volume. The amount of acidic groups decreased with the increasing of alkali concentration, while the basic functional groups increased. AC treated with 30% NaOH exhibited the highest content of basic groups (0. 557 mmol / g). The correlations between the microporous surface area and the content of basic groups with the SO2 adsorption amount were further analyzed. The adsorption capacities of SO2 were positively linearly correlated with the amount of basic groups and microporous surface areas,and the correlation coefficient of basic groups was higher than the microporous surface area. Therefore,the surface basic functional groups and microporous structure were beneficial to the adsorption of SO2 on activated carbon.