| With higher proportion in Chinese primary energy structure, the coal is the important source of anthropogenic mercury emissions. At present, the pollution caused by mercury emission from coal combustion is controlled mainly by adsorptive removal including desorption in current pollution control devices and activated carbon injection. However, the lower content in Chinese coal causes lower cooperative efficiency when Hg is removed by the current pollution control devices. Moreover, Hg Cl2 formed by the oxidized Hg0 is scattered in fly ash, desulphurization gypsum and desulfurization solution, which is easy to cause the secondary pollution; cost is higher if Hg0 is removed by activated carbon injection and many oxidized metals. In addition, it is difficult for electrostatic dust collection after the activated carbon is mixed into coal ash. Therefore, the comprehensive utilization of coal ash is affected.Thus, the main solution for removing Hg0 from the flue gas is to study and develop the new sorbent which is provided with wide temperature window, lower price and high desorption efficiency, and stable and is difficult to be polluted again after adsorption removal of Hg0.In this paper, the waste vanadium-titanium steel slag(VTSS) is used as the raw material to innovatively prepare such series of sorbent as KBr(x)/VTSS, KI(y)/VTSS, Fe(x)-Co(y)/VTSS and Fe(x)-Mn(y)/VTSS by impregnation method. Such series of sorbent as Fe-Co-Ti-M and Fe-Co-Ti-M(M=V+Ce+Mg) are prepared by coprecipitation after impregnating VTSS in sulfuric acid. This paper systematically studies Hg0 adsorption performance of sorbent, deeply studies the effect of modifier loading(doping) amount, reaction temperature, SO2 and H2 O and other components of flue gas on Hg0 adsorption performance; focuses on studying the mechanism in which modified VTSS adsorbs Hg0 and the mechanism in which Hg0 adsorption is affected by SO2 and H2 O, by method of XRD, BET, HRTEM, XPS, H2-TPR and other characteristics in combination with experimental data; reveals the critical factor for controlling the adsorption process by reaction kinetics equation simulation for adsorption and desorption, and then studies on the desorption performance of Hgp generated on the sorbent surface after adsorbing Hg0, which provides the theoretical basis for comprehensive utilization after adsorbing Hg0 by sorbent.This paper mainly achieves the following research results:(1) Preparing such sorbents as KBr(x)/VTSS and KI(y)/VTSS by impregnation; Characterizing the sorbents by XRD、SEM、BET and XPS, etc; evaluating on Hg0 adsorption capability of the sorbent; studying on the mechanism in which Hg0 is adsorbed by the sorbents, based on experiment and characteristic results. The result shows:VTSS contains such transition metal oxides as Fe, Ti and V, but their major metal phases are solid solution, thus causing lower Hg0 adsorption performance of VTSS. The Hg0 adsorption performance after loading KBr and KI are impregnated in VTSS has positive correlation with the loading amount of Br and I and the reaction temperature. The adsorption efficiency within 180 min at 120℃ is respectively 53.6%, 39.8% and 17.8% in 5% KBr/VTSS, 3% KBr/VTSS and 1% KBr/VTSS, and 98.2%, 89.7% and 76.3% in 1% KI/VTSS, 0.5% KI/VTSS and 0.3% KI/VTSS. The adsorption efficiency within 180 min in 5% KBr/VTSS is respectively 42.8%, 53.6% and 68.4% at 70℃,120℃ and 180℃. The adsorption efficiency within 180 min in 1% KI/VTSS is respectively 93.6%, 98.2% and 100.0% at 70℃, 120℃ and 180℃.For such series of sorbents as KBr(x)/VTSS and KI(y)/VTSS, the mechanism in which O2 prompts Hg0 removal, is that O2 reacts with KBr and KI to generate Br and I. The activity of KI(y)/VTSS is better than that of KBr(x)/VTSS. The mechanism is that it is easier for O2 to react with KI and then generate I/I2. The efficiency is reduced respectively by 12.1% and 48% after adsorption within 180 min in 5% KBr/ VTSS and 1% KI/VTSS when the amount of O2 is reduced from 6% to 0%. The proportion of Hg2+ at outlet in total HgT out at outlet is increased with increasing reaction temperature. The proportion of Hg2 is respectively 13.9%, 24.7% and 39.6% at 70℃, 120℃ and 180℃ within 180 min in 5% KBr/VTSS; The proportion of Hg2 is respectively 21.4%, 31.5% and 55.8% in 1% KI/VTSS under the same condition above mentioned. The proportion of Hg2+ in total HgT out at outlet decrease with the reaction time for the reason of consuming KBr and KI in reaction, which reduces the active points and further to reduce the speed of reaction among KBr, KI and Hg0.(2) Preparing such series of sorbents as Fe(x)-Co(y)/VTSS ' Fe(x)-Mn(y)/VTSS by impregnation; characterizing the sorbents by XRD、SEM、N2 adsorption, and XPS, etc.; evaluating on Hg0 adsorption capability of the sorbent; studying on the mechanism in which O2 prompts Hg0 adsorption and which SO2 inhibits adsorption, based on experiment and characteristic results. The result shows:Such series of sorbents as Fe(3)-Co(1,2,3)/VTSS and Fe(3)-Mn(1,2,3)/VTSS have good Hg0 adsorption performance in wide-temperature window(150℃-300℃). The mentioned adsorption performance has positive correlation with Co doping amount and reaction temperature. Reaction between Fe and Co or Fe and Mn dramatically enhances Hg0 adsorption efficiency. Co and Mn are main active components. When Hg0 adsorption temperature is raised from 150℃ to 300℃ within 600 min, the efficiency of Fe(3)-Co(3)/VTSS is improved by 19.25% and the adsorption capacity is increased to 135.92ug/g; the efficiency of Fe(3)-Co(2)/VTSS is improved by 19.35% and the adsorption capacity is increased to 134.58ug/g. Increase of the adsorption temperature causes the adsorption efficiency of Fe(x)-Mn(y)/VTSS is firstly improved and then reduced with the doping amount of Mn. The efficiency of Fe(x)-Mn(1,2)/VTSS at such lower temperature as 150℃ and 200℃ is lower than that at 300℃ while the efficiency of Fe(x)-Mn(3)/VTSS is higher than that at 300℃. The reason is that the activity of Mn decreases at higher temperature.O2 in flue gas can prompt Hg0 adsorption of Fe(3)-Co(1,2,3)/VTSS and Fe(3)-Mn(1,2,3)/VTSS, which is consistent with Mars-Maessen mechanism. However, H2 O and SO2 inhibit Hg0 adsorption performance. The inhibition of SO2 has positive correlation with the concentration of SO2 and adsorption reaction temperature. The mechanism in which SO2 inhibits dramatically the sorbents to adsorb Hg0 at higher temperature of 300℃ is that SO2 reacts with the oxides of Fe, Co and Mn to generate sulfate. This reduces concentration of active components. Compared with the case without SO2, Hg adsorption efficiency of Fe(3)-Co(3)/VTSS within 600 min in SO2 at 2000 ppm is reduced respectively by 9.8%, 13.1% and 25.8%. Hg adsorption efficiency of Fe(3)-Mn(3)/VTSS is reduced respectively by 20.0%, 22.4% and 25.5%.(3) Preparing series of Fe-Co-Ti-M and series of Fe-Mn-Ti-M(M=V+Ce+Mg, etc. by co-precipitation; characterizing the sorbents by XRD, SEM, BET and XPS; evaluating Hg0 adsorption capacity of the sorbents; specially focuses on sulfur-resistance and water resistance during Hg0 adsorption of the sorbents and on the optimized sorbents. The study result shows:The presence of Ti an M improves the sulfur-resistance of the sorbents. Moreover, the sulfur-resistance of Fe-Co-Ti-M is better than that of Fe-Mn-Ti-M. For the decreasing amplitude in 2000 ppm SO2 at 300℃with in 600 min,(Fe2Ti0.46Co0.4M0.14)O4 and(Fe2Ti0.23Co0.7M0.07)O4 are respectively 11.0% and 14.0% while(Fe2Ti0.46Mn0.4M0.14)O4 and(Fe2Ti0.23Mn0.7M0.07) O4 are respectively 15.0% and 18.0%.For such series of sorbents as Fe-Co-Ti-M and Fe-Mn-Ti-M, Fe-Co-Ti-M、Fe-Mn-Ti-M narrows the appropriate adsorption temperature window(200-300℃) and reduces the best adsorption temperature(200℃). This is better for Hg0 adsorption at lower temperature. Hg0 adsorption efficiency of(Fe2Ti0.46Co0.4M0.14)O4 at 150℃, 200℃, 300 within 600 min in 6% O2 is respectively 63.39%, 73.97% and 63.02%. Hg0 adsorption capacity of it is respectively 92.00, 111.59 and 93.45 ug/g under the same condition above mentioned, while Hg0 adsorption efficiency of(Fe2Ti0.23Co0.7M0.07)O4 is respectively 68.50%, 79.16% and 70.93% and Hg0 adsorption capacity of it is respectively 100.50, 113.42 and 104.82 ug/g. Under the same condition, Hg0 adsorption efficiency of(Fe2Ti0.46Mn0.4M0.14)O4 is respectively 58.29%, 75.67% and 58.17% and Hg0 adsorption capacity of it is respectively 86.19, 110.56 and 87.84 ug/g while Hg0 adsorption efficiency of(Fe2Ti0.23Mn0.7M0.07) O4 is respectively 60.38%、72.54%、63.10% and Hg0 adsorption capacity of it is respectively 89.22、105.90、95.96 ug/g.(4) Studying the adsorption kinetics of such series of sorbents as Fe(x)Co(y)/VTSS, Fe(x)Mn(y)/VTSS, Fe-Co-Ti-M and Fe-Mn-Ti-M by using pseudo-second order kinetic model; and studying the desorption performance of Hgp on the surface of sorbents by using method of TPD. The study shows:Hg0 can be adsorbed by Fe(x)Co(y)/VTSS, Fe(x)Mn(y)/VTSS, Fe-Co-Ti-M and Fe-Mn-Ti-M. The pseudo-second order kinetic equation simulation result is consistent with the experimental data. The adsorption process is controlled by chemisorption. In Fe(x)-Co(y)/VTSS, the equilibrium adsorption capacity, qe of Fe(3)-Co(3)/VTSS is highest. qe at 150 ℃, 200 ℃ and 300 ℃ is respectively 1109.90, 1397.89 and 2579.89ug/g; In Fe(x)-Mn(y)/VTSS, qe of Fe(3)-Co(3)/VTSS is highest. qe at the temperatures above mentioned is respectively 336.70, 1749.91 and 2172.80 ug/g. qe of(Fe2Ti0.46Co0.4M0.14)O4 is highest in Fe-Co-Ti-M, and(Fe2Ti0.23 Mn 0.7M0.07)O4 in Fe-Mn-Ti-M, at the best reaction temperature of 200℃. They are respectively 2414.64 and 2274.46 ug/g.The desorption temperature and desorption activation energy of Hgp are quite different after generating Hgp on the surface of sorbent by absorbing Hg0. The desorption temperature and desorption activation energy of Hgp on the surface of sorbent are arranged in following sequence: Fe(3)Co(3)/VTSS>(Fe2Ti0.23Co0.7M0.07)O4> Fe(3)Mn(3)/VTSS>(Fe2Ti0.23Mn0.7M0.07)O4. Therefore, such series of sorbents containing Co as e(3)Co(3)/VTSS and(Fe2Ti0.23Co0.7M0.07)O4 are more favorable to actual comprehensive utilization after adsorbing Hg0. The secondary emission of Hg0 can be avoided when Fe(x)Co(y)/VTSS, Fe(x)Mn(y)/VTSS, Fe-Co-Ti-M and Fe-Mn-Ti-M are reused at the temperature lower than 150 ℃ after adsorbing Hg0. |
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