Reaction Characteristic Of Sulfur And Mercury Based On Synergetic Control During Coal Pyrolysis Process

Mercury has been categorized as a priority global environmental pollutant because of its strong mobility,neurotoxicity,bioaccumulation,and persistence in the environment for long duration.Coal-fired power plants are the major anthropogenic source of mercury emissions worldwide.China,as the world's biggest coal producer and consumer,has been committed to control coal-fired mercury emission.Many techniques have been developed for Hg0 removal from coal-fired flue gas,and the most of them are based on adsorption or oxidation absorption.The adsorption method use sorbent to capture mercury and the removal efficiency has strong association with sorbent properties.In order to obtain efficient,durable and stable removal efficiency,chemical modification of sorbent raw materials is necessary.Sorbent injection device upstream of a particulate control device must be installed to spray the sorbents into flue gas.However,it will undoubtedly increase the burden of subsequent particulate control device.Besides,the sorbents aren't recyclable and decrease the quality of fly ash.Undoubtedly,the end treatment units of coal-fired flue gas become more complex and the high operation cost limits the industrial application of those approaches,particularly in developing countries.As a supplement and development of flue gas pollutants control techniques,it is essential to develop the managements of its source and the process control in the entire gas production.The content and occurrence form of sulfur performs a positively correlated with that of mercury.During coal pyrolysis,the release regularity of sulfur and mercury are highly correlated.Hance,the source and process cooperative control mechanism of sulfur and mercury is investigated.And the study not only enriches the multi-pollutant control strategy but also is conducive to effectively reduce the cost of environmental governance.Three coals(XLT,EEDS and WH)with different coal rank and sulfur content were selected as the experimental samples.The mercury species and content,pyrolysis atmosphere and temperature were studied on a fixed-bed reactor combining with modern analytical technology.The results illustrated that the acid treatment method separated Hg into three modes:organic-bound Hg,pyrite-bound Hg and mineral-bound Hg.According to the results of experiments,the release temperature range for thermal unstable and stable organic-bound Hg was 250-400? and 500-6500?,and the content of organic-bound Hg was in the range of 35%-55%.The release temperature range for pyrite-bound Hg was 500-700?,with the content in the range of 15%-45%.The release temperature for mineral-bound Hg was above 700? with content about 10%-35%.In comparison to the inert atmosphere,weak oxidizing atmosphere increased the sulfur and mercury removal from coals.The oxygen environment appeared to facilitate decomposition of sulfur and mercury containing compounds at a low temperature.When the coals were thermally upgraded to 500? under a weak oxidizing environment by fast pyrolysis,40%-60%of sulfur and more than 75%of mercury in those coals could be removed with slightly char yield loss.Meanwhile,the pore structure of char was improved during fast pyrolysis process.Based on the recovered sulfur resource and Hg0 synergistic removal,the Fe2O3/Semi-Coke sorbent was developed for H2S catalytic oxidation.The sorbent had mainly micropore with surface area of 321.281 m2/g and pore volume of 0.163 cm3/g.The ?-Fe2O3 was major activity component and played a dominant role during adsorption process.The sorbent exhibited the best catalytic performance and high stability at 180? which the breakthrough time and release sulfur capacity reached 827 min and 21.08%,respectively.Higher temperature and larger O2/H2S mole ratio in feed gas easily caused the deep oxidation of H2S or/and elemental sulfur.The reduction atmosphere(CO and H2)could react with O2 easily,which was unconductive to H2S catalystic oxidation.In the presence of CO,a small amount of by-product COS were generated.The mechanism and deactivation of H2S catalytic oxidation reaction were also studied in detail.Firstly,O2(g)and H2S(g)were adsorbed on the surface of sorbent to form O2(ads)and H2S(ads);subsequently,the H2S could react with Fe2O3 to form S0 and FeS2;finally,the O2(ads)was consumed to oxidize the FeS into S0 and Fe3+.Moreover,the H2S(ads)could be oxidation directly by O2(ads)to form S0.The Fe3+and O2(ads)played an important role in the selective oxidation reaction.And sulfur condensation on the sorbent surface,which greatly decreased the sulfur area and pore volume of sorbent,was the main reason for sorbent deactivation.The sorbent was used to remove Hg0 from the pyrolysis gas and the effects of temperature and gas components on the removal performance were also investigated.The sorbent exhibited excellent Hg0 removal performance from 60 to 150?.However,the removal activity was limited and removal efficience decreased significantly with the temperature increase over 150?.When a certain concentration of oxygen was introduced to the reactor,the Hg0 removal efficiency was over 95%at 150?.It was indicated that the temperature window of Hg0 removal in presence of H2S was effectively increased.The introduced oxygen could promoted to produce more activity sulfur(S*)and then the capture efficiency of Hg0 remarkable increased.H and CO had a negative effect on Hg removal in presence of H2S and O2,because of competitions for O2 and limiting H2S catalytic oxidation.H2O inhibited the removal of Hg0,mainly due to competitive adsorption with H2S.HgS(black)was the main stable spciation of Hg0 on the sorbent surface.Experimental and density functional theory(DFT)studies proved that Hg0 removal over the sorbent followed the Eley-Rideal mechanism.The adsorbed H2S was dissociated into HS and S on the?-Fe2O3(001)surface.And then the activity sulfur reacted with adsorbed Hg0 to form HgS(black).The introduced oxygen increased the active sites on the surface of adsorbed substrate,which could enhance the H2S oxidation performance.The number of activity sulfur(S*)had jumped,as a result the removal efficiency of Hg0 significantly increased.