High-sulfur
petroleum coke is potentially one of the most promising
materials for developing efficient and low-cost sorbents for Hg0 removal from flue gas. In this study, three distinct SO2-activated high-sulfur petroleum cokes were prepared at 650–850
°C via different sulfurization protocols, including SO2-impregnated petroleum coke (PC-S), SO2-impregnated coke
produced with an additional reductive heat-treatment process in N2 (PC-NS), and SO2-impregnated coke prepared with
the presence of SO2 in the gas during the cooling process
(PC-SC). The SO2-activated cokes were characterized with
nitrogen adsorption/desorption measurements, X-ray photoelectron spectroscopy,
thermogravimetric analysis, and their mercury removal performances
were investigated in a fixed-bed reactor at 80–200 °C
with an initial mercury concentration of 35–120 μg/m3. To explore the influence of nonoxidized sulfur species on
Hg0 removal, particular focus was given to the comparison
of the reduced sulfur, sulfide sulfur, and elemental sulfur formed
during different sulfurization processes, as well as the consequent
differences in Hg0 adsorption characteristics. It found
that the sulfur incorporated into the carbon matrix of PC-S by interaction
with SO2 at 650 °C was dominantly organic sulfide.
Thermal treatment in N2 at 800 °C led to an addition
of reduced sulfur in PC-NS. An increment of 0.74 at. % elemental sulfur
was observed in PC-SC after exposure to SO2 during the
cooling step. The Hg0 adsorption capacity of PC-SC at low
temperatures was significantly enhanced, yielding a mercury capacity
of 622 μg/g at 80 °C. However, elemental sulfur suffered
from severe temperature sensitivity for mercury binding. The sample
PC-NS which contained more reduced sulfur showed the highest Hg0 adsorption capacity at elevated temperatures above 160 °C.
Furthermore, the increment in inlet mercury concentration would lead
to higher adsorption capacities and faster adsorption rates. According
to the R
L obtained from Langmuir isotherms,
the Hg0 adsorption reactivity can be arranged as follows:
PC-NS > PC-S > PC-SC. The mercury temperature-programmed desorption
results suggested that sulfur species not only had an effect on Hg0 adsorption capacity but also played an essential role in
the thermal stability of the adsorbed mercury compounds. Mercury was
principally associated with elemental sulfur in the form of metacinnabar
and combined with thermal-stable organic sulfide in cinnabar or organomercuric
compounds Hg–SR.