Desulfurization Performance Of Iron-manganese Based Sorbents At Mid-temperature And Effect Of Ambient Gases On Them

Coke oven gas rich in H₂ and CH4 combining with gasification gas rich in CO and CO₂ are reformed in order to acquire syngas with optimum composition in dual gas coal-based poly-generation technology. Although the step utilization of raw material and rational proportion of energy is reasonably achieved by the production of coke, alcohol ether fuel and power in this technology. The key problem to be solved is pollutant control matching this process. The sulfur removing from hot coal gas is still an important part in it. Iron based desulfurizer was preferable for bulk removal of H₂S in two step desulfurization process due to its low price, quick reaction rate and favorable regenerability. However, the consumption of active component due to formation of iron carbide, unsatisfied stability in cycles and lower desulfurization efficiency made it is unadvisable to utilize single iron oxide as sorbent. So it is necessary to modify the iron based sorbent in order to be used for hot coal gas desulfurization.From economic point of view, the iron-manganese mixed oxides with different molar ratios were firstly prepared by co-precipitation method using ferric nitrate and manganese nitrate as precursors. Then the iron-manganese based sorbents were prepared by mechanical mixing method using iron-manganese mixed-oxide as active component, clay as structural promoter and soluble starch as pore former. It was shown that Fe2O3 and Mn2O3 were presented in mixed-oxide and iron-manganese based sorbent from XRD characterization results. In addition, SiO₂ was also existed in iron-manganese based sorbent.H₂-TPR and CO-TPR of mixed-oxide and sorbents were conducted. It was shown that: the reduction of iron-manganese based sorbent was more difficult than that of mixed-oxide. The XRD characterization of reduced sample showed that Mn2O3 was reduced to MnO, while Fe2O3 was reduced to metallic Fe finally. Fe3O4, Fe and MnO were presented in partially reduced sample. It was found that the sulfidation activity of Fe3O4 and Mn3O4 were bigger than MnO and Fe. The effects of iron-manganese molar ratio, calcination temperature and sulfidation temperature on sulfidation performance of sorbents were studied in a fixed-bed reactor. It was found that su lfidation temperature, crystalline and content of active component were main factors influencing the sulfidation performance of sorbents. 7F3M750 with iron-manganese molar ratio of 7/3 has the best sulfidation behavior at 500oC. The addition of manganese oxide in iron-based sorbent can suppress its sulfidation performance at lower temperature, while improve that at higher temperature. The removal ability of sorbent for COS was also improved. The sintering occurred with calcination temperature increasing from 750 to 920oC which made the specific surface area and pore volume of sorbent reduced. The sulfur capacity of sorbent was reduced due to mass transfer resistance. The sorbent 7F3M750 optimally selected in the test has better mechanical stability and stable sulfidation-regeneration performance in cycles. Trace MnSO4 existed in regenerated 7F3M750 was the residue of incomplete decomposition and hasn't obvious effect on sulfidation performance of sorbent.The effect of ambient gas on sulfidation performance and structural stability of sorbent was investigated. The adsorption of H₂S on active component was repressed by CO₂ due to steric hindrance of its bigger molecular volume which made CO₂ affect sulfidation performance of sorbent negatively. The competitive adsorption between CO and CO₂ improved the sulfidation performance of sorbent to some extent. The carbon deposition from CO disproportionation and the formation of iron carbide had adverse effect on sorbent structural stability. CO₂ improved the structural stability by repressing CO reduction and disproportionation. The effect of H₂O on sulfidation was negative and it was weakened by water shifting reaction, the addition of H₂ in inlet also slightly weakened the side effect of H₂O.According to investigating influencing factors of COS formation, it was found that COS derived from side reaction between H₂S and CO or CO₂. The COS from the reaction between CO and H₂S was dominant. The space velocity had little effect on COS formation and the concentration of COS decreased with the increase of sulfidation temperature.The sulfidation behavior of various active species of iron and manganese at 500oC was studied in N2/COS and N2/H₂/COS gas streams. The results showed that COS could be removed simultaneously by iron-manganese based sorbent in mid-temperature desulfurization process and manganese oxides had bigger affinity to COS compared with iron oxides. The order of reaction ability was MnO > Mn2O3 > Fe3O4 > Fe2O3 > Fe. The sulfide of every active phase had catalytic effect on hydrogenolysis of COS.