Experimental Study On Mercury Adsorption By Activated Carbon Under Oxy-combustion Flue Gas

The greenhouse effect is a global environmental disaster.CO2 is the main gas that causes this effect,and coal-fired power station is one of the largest CO2 emission sources.Oxygen-enriched combustion technology is considered to be one of the carbon capture technologies,with the most economic advantage and application prospect,but the mercury in its combustion flue gas is not only a serious threat to human health and ecological environment,but also a security problem because of corrosion between mercury and aluminum metal of compressed equipments.Activated carbon injection(ACI)has been recognized as the most promising technique for mercury removal,but the complex flue gas compositions will have important effects on the activity of mercury removal.Therefore,in this paper,a commercial activated carbon was selected to study the influence rules and mechanism of the single and compound existence of various flue gases on mercury removal in a fixed-bed device.Firstly,the effects of CO2,NO,SO2 and HCl on the mercury adsorption by activated carbon were investigated,respectively.The results show that high concentration of CO2 had little effect on the adsorption of mercury,while 6%O2 could significantly enhance the mercury removal rate of activated carbon,so that the Hg0 breakthrough rate was reduced by about 30%.SO2 produced strong inhibitory effect on mercury adsorption of activated carbon,which became stronger with the increase of SO2 concentration.On the one hand,SO2 would compete with Hg0 on the active sites and made the fine holes blocked to hinder the diffusion and adsorption of Hg0.On the other hand,SO2 can release the mercury that has been adsorbed on the surface of activated carbon in the form of Hg0.Lower concentration of NO(25ppm)could greatly enhance the effect of Hg0 removal,because NO prompts the generation of NO2,effectively removing Hg0,to increase the contents of Hg2(NO3)2 or HgO on the surface of activated carbon.HCl has a significant effect on the mercury adsorption of activated carbon,in which the homogeneous oxidation was only 3%to 4%,while the heterogeneous oxidation adsorption occupied an absolute high proportion.HCl promotes the formation of c-cl to oxidize Hg0 to Hg2+,and increases the HgCl2 content on the surface of activated carbon.Secondly,the influences of three different water vaor concentrations(8%,15%,23%)with different types of acidic gases(SO2?NO?HCl)on mercury removal were investigated.The results show that H2O could significantly inhibit mercury removal because of pore blockage and competition adsorption.In addition,the free electron(e-)in activated carbon can combine with Hg2+ through water vapor adsorbed on the surface of AC so that mercury released back into flue gas in the form of Hg0.In the presence of SO2,low H2O concentration showed promoting effect,however,this promoting effect disappeared while H2O concentration became higher.It could be due to high H2O concentration made competition adsorption strengthen and pore blockage more serious,apart from this,H2SO4 was generated due to the coexistence of SO2,O2 and H2O,which also led to the pore blockage.In the presence of NO,the low concentration of H2O attenuated the promoting effect of NO on the mercury adsorption while high concentration had no effect at all.This may be bacsuse of waxing and waning between facilitating roles of acidic oxidation and negative effects of H2O.In the presence of HCl,the mercury removal effect was initially inhibited and then promoted when H2O concentration rose from 8%to 23%.The inhibition trend may resulted from reinforced competition adsorption and reverse occurrence of Deacon Reaction.The changes of H2O concentration almost had no effect the mercury breakthrough rate under the simulated oxy-fuel combustion flue gas.Finally,the effects of initial mercury concentration,height of bed layer and adsorption temperature on mercury removal by activated carbon was explored under simulated oxygen enriched combustion flue gas,and the experimental results were analyzed with four simplified kinetic models.The selected three different initial mercury concentration and bed height had no effect on mercury removal,while increasing temperature significantly inhibited the adsorption of mercury,and also affected mercury morphology on the surface of activated carbon.The kinetic results show that the adsorption process of mercury was controlled by chemical adsorption and external mass transfer.