Study On Mercury Removal Characteristics Of SO₂ Activated High Sulfur Petroleum Coke

As the fourth largest pollutant in coal combustion flue gas,mercury has been widely concerned worldwide.Up to now,activated carbon injection(ACI)has been recognized as the most promising control technology for mercury removal from coal combustion flue gas,but the cost is too high.High sulfur petroleum coke,which is a by-product of the petrochemical industry,is appropriate raw material for producing activated carbon because of high carbon content,abundant resource and low price.Based on the carbothermal reduction of SO2,the high sulfur petroleum coke was activited and modified in SO2 containing gas at high temperature to develop a cheap and efficient sulfur-rich mercury removal sorbent.Firstly,SO2 was used as activating agent to prepare mercury removal sorbent from high sulfur petroleum coke in a tube furnace apparatus.The characterization technologies of N2 adsorption/desorption,elemental analysis and X-ray photoelectron spectroscopy(XPS)were applied to analyze the influences of reaction temperature,time and SO2 concentration on physical and chemical properties of SO2 activited petroleum coke.Meanwhile,the mechanism of activation modification and C-SO2 reaction was further investigated.The results show that the gas-phase products were evolved from high sulfur petroleum coke in pyrolysis process,while SO2 reacts with C atoms in the interior of particle accompanied by the release of CO2 and sulfur vapor,which contribute to the increase of specific surface area and porosity.Futhermore,the pore structure is determined by pyrolysis and C-SO2 reaction characteristics.The sulfur and oxygen contents in SO2 activited petroleum coke increase significantly.C-S/C=S bonds are more prone to breakage at elevated temperature,which leads to the escape of sulfur.However,intermediates containing of C-O/S-O/S=O groups increase gradually during the C-SO2 reaction process.Therefore,the content of sulfur becomes lower,while the content of oxygen becomes higher with increasing reaction temperature.The increase of sulffur with reaction time followed a saturation curve that reached a plateau at the time that the system reached steady state conditions.The main sulfur form of SO2 activited petroleum coke is non-oxidized sulfur.S2 is the main species of sulfur in the gas phase at reaction temperature,which applied to prove the absence of the elemental sulfur in the carbons' structure.Compared with raw coke,the content of oxygen functional groups in SO2 activited petroleum coke was increased obviously.Secondly,experimental study of mercury adsorption was carried out in a fixed bed device,and the mercury removal capacity of SO2 activited petroleum coke prepared under different conditions was obtained.Furthermore,the effects of adsorption temperature,inlet Hg0 concentration,flue gas composition and thermal regeneration on mercury removal characteristic and mechanism were investigated.The results show that the pore structure and chemical active site of raw petroleum coke are poor,leading to the terrible mercury removal capacity.SO2 activated petroleum coke shows superior mercury removal capacity,which is determinted by the pore structure,oxygen functional groups and sulfur forms.The mercury removal capacity decreases with the increasing adsorption temperature,because the physisorption by micropore and chemisorption by oxygen/sulfur functional groups are inhibited at higher temperature.The mercury adsorption rate becomes faster,while the initial mercury removal efficiency becomes lower at elevated initial Hg0 concentration.Dynamic fitting in different stages showed that chemical adsorption was the controlling step of the whole process.SO2 combines with unsaturated carbon to form C-S easily,but it may compete with Hg0 on carbonyl functional group,which has unconspicuous effect on mercury removal of SO2 activated petroleum coke.Hg0 on or close to surfaces are oxidized by the adsorbed oxygen.Once Hg0 is oxidized,mercuric ions are more readily bound to sulfur via the Hg-S bond,while some will bind to oxygen.The absorption capacity of regenerated petroleum coke was weakened because of the loss of active sites in the regeneration process.Finally,XPS was applied to compare sulfur forms of raw/mercury-adsorption SO2 activited petroleum coke.SO2 activated commercial activated carbon was treated by high temperature thermal reduction and H2O2 solution impregnation respectively.Beside,the effects of sulfur forms on the mercury removal capacity were investigated.The results show that the content of non-oxidized sulfur in SO2 activated petroleum coke decreased after mercury adsorption,which indicated non-oxidized sulfur may play an important role in mercury removal.The form of sulfur in SO2 activated commercial activated carbon is complex.The oxidized sulfur form was eliminated when the activated carbon was further heated at 800? in N2 atmosphere.After oxidized treatment in H2O2 solution,apart from the disappearance of soluble sulfate,part of non-oxidized sulffur was oxidized as well.Mercury adsorption result show that non-oxidized sulfuur is one of the main active site of Hg0 capture.