Coupled Reduction–Sulfidation Mechanism and Kinetics of Zinc Ferrite Hot-Gas Desulfurization Sorbents in Reducing Coal Gas

Zinc ferrite ($\mathrm{ZnFe_2O_4}$) is a regenerable sorbent for removing ppm-level hydrogen sulfide from hot, reducing coal-derived gas. Kobayashi, Shirai, and Nunokawa (2000) established its phase-level chemistry by in situ X-ray diffraction and $^{57}\mathrm{Fe}$ Mössbauer spectroscopy: reduction proceeds by zinc cation migration, producing $\mathrm{ZnO}$ plus a zinc-dislocated spinel $\mathrm{Zn}_x\mathrm{Fe}_{3-x}\mathrm{O}_4$; at about 80 ppm $\mathrm{H_2S}$ only zinc sulfidizes, while at 500 ppm iron sulfides also appear. Their phase-level study left the quantitative layer undetermined: the rate laws and activation parameters of the coupled reduction and sulfidation processes, the dependence of the cation-redistribution extent $x$ on reduction conditions and its effect on Zn/Fe selectivity, the identity of the iron sulfides (including a Mössbauer doublet with pyrite-like parameters), the contribution of a direct spinel-to-$\mathrm{ZnS}$+$\mathrm{Fe_3O_4}$ sulfidation route, and a complete kinetic model under realistic coal-gas mixtures. The authors' 2002 sequel resolved the phase behavior in CO-containing simulated coal gas and identified pyrrhotite-type (iron-deficient $\mathrm{Fe_{1-x}S}$) and sphalerite-type (zinc-blende-structure) sulfides, and independent later work confirmed that both $\mathrm{ZnO}$ and ferrite-hosted zinc act as sulfidation sources; the phase-resolved coupled kinetics, the quantitative cation-redistribution correlation, and a complete coupled kinetic model nevertheless remain unresolved in the literature, and the zinc-only to zinc-plus-iron sulfidation boundary function remains an open question attributed to the 2002 sequel literature, addressed as part of component (2) below. The open problem, addressed in components (1)–(5), is the quantitative, predictive kinetic description of zinc ferrite transformation in hot coal gas.

Background

Integrated gasification combined cycle (IGCC) power plants and other coal-gas utilization systems generate a fuel gas containing hydrogen sulfide ($\mathrm{H_2S}$) at levels from tens to hundreds of ppmv, which must be removed before the gas reaches turbines or synthesis catalysts. In dry (hot-gas) desulfurization, the gas is contacted at roughly 350–600 °C with a metal-oxide sorbent that binds sulfur as a solid sulfide and can be regenerated by oxidation. Zinc oxide ($\mathrm{ZnO}$) offers favorable sulfidation thermodynamics—equilibrium $\mathrm{H_2S}$ concentrations in the ppmv range—but suffers from slow kinetics, reduction to volatile elemental zinc in strongly reducing gas, and sintering; iron oxides sulfidize and regenerate quickly but leave a less favorable residual $\mathrm{H_2S}$ equilibrium. Zinc ferrite ($\mathrm{ZnFe_2O_4}$, the mineral franklinite; a normal cubic spinel in which $\mathrm{Zn^{2+}}$ occupies tetrahedral A sites and $\mathrm{Fe^{3+}}$ occupies octahedral B sites) combines the deep-removal thermodynamics of zinc with the regenerability of iron and was intensively developed as a regenerable hot-gas sorbent.

In a reducing environment the two metals do not behave independently, and this complicates the reaction pathway. Kobayashi, Shirai, and Nunokawa (Ind. Eng. Chem. Res. 2000, 39, 1934–1943) used in situ X-ray diffraction (XRD — diffraction patterns recorded continuously at the reaction temperature, so that crystalline phases are identified while they coexist at operating conditions rather than after cooling) and room-temperature $^{57}\mathrm{Fe}$ Mössbauer spectroscopy (a nuclear spectroscopy whose sextet and doublet sub-spectra encode the valence and magnetic environment of iron atoms, allowing iron-containing phases to be quantified even when their XRD reflections overlap) to follow $\mathrm{ZnFe_2O_4}$ at 550 °C in $\mathrm{H_2}$/$\mathrm{H_2O}$/$\mathrm{N_2}$ mixtures and in simulated coal gas containing CO and $\mathrm{CO_2}$. They showed that reduction proceeds not by clean decomposition into separate oxides but by zinc cation migration out of the spinel: crystalline zincite ($\mathrm{ZnO}$) forms together with an iron-rich, zinc-dislocated spinel $\mathrm{Zn}_x\mathrm{Fe}_{3-x}\mathrm{O}_4$ (a zinc-substituted magnetite whose Mössbauer parameters were tabulated across the solid-solution series by Kanzaki, Kitayama, and Shimokoshi, J. Am. Ceram. Soc. 1993, 76, 1491–1494), with wüstite ($\mathrm{FeO}$) appearing only under stronger reduction. On subsequent exposure to $\mathrm{H_2S}$ at low concentration (about 80 ppm), only zinc sulfidizes—both the wurtzite and zinc-blende (sphalerite) polymorphs of $\mathrm{ZnS}$ grow while all iron-bearing oxides remain unchanged—whereas at 500 ppm $\mathrm{H_2S}$ iron sulfides also form, including a cubic sulfide with incorporated zinc and a Mössbauer doublet with parameters resembling pyrite ($\mathrm{FeS_2}$) that could not be uniquely assigned.

Particle-level kinetics of zinc ferrite sorbents had been measured before this work at higher $\mathrm{H_2S}$ concentrations and at the level of overall pellet conversion (Focht, Ranade, and Harrison, Chem. Eng. Sci. 1988, 43, 3005–3013; Woods, Gangwal, Harrison, and Jothimurugesan, Ind. Eng. Chem. Res. 1991, 30, 100–107), and the same CRIEPI group had earlier studied the desulfurization performance and pore structure of zinc-ferrite-containing sorbents (Kobayashi, Shirai, and Nunokawa, Energy & Fuels 1997, 11, 887–896). What the 2000 study did not determine—and what later work still has not delivered—is the phase-resolved, quantitative kinetic description of the coupled processes at desulfurization-relevant sulfur loadings: the rates at which reduction (with its cation redistribution) and sulfidation of the resulting oxide reservoirs proceed, how the zinc-dislocation extent depends on reduction conditions, the exact sulfur speciation at elevated $\mathrm{H_2S}$, and how all of this combines into a predictive model under realistic transient coal-gas compositions.

Problem Statement

The problem, arising from the phase-level findings of Kobayashi, Shirai, and Nunokawa (Ind. Eng. Chem. Res. 2000, 39, 1934–1943), which left the quantitative kinetic layer undetermined and which the subsequent literature has not yet discharged, is to complete the quantitative, mechanistically grounded description of zinc ferrite transformation during hot desulfurization of reducing coal-derived gas, so that sorbent behavior can be predicted rather than screened empirically. Concretely, determine the following in the hot-gas desulfurization regime (reducing coal gas, temperatures of the order of 450–600 °C, $\mathrm{H_2S}$ partial pressures from the tens-of-ppm regime upward, as exemplified by the 550 °C conditions of the source study):

(1) The kinetics and relative rates of the two coupled processes—reduction of $\mathrm{ZnFe_2O_4}$, including zinc cation migration forming $\mathrm{ZnO}$ and the zinc-dislocated spinel $\mathrm{Zn}_x\mathrm{Fe}_{3-x}\mathrm{O}_4$, and sulfidation of the resulting oxide reservoirs by $\mathrm{H_2S}$—as rate laws with activation parameters sufficient to predict time-dependent phase evolution under transient gas compositions.

(2) The quantitative dependence of the zinc-dislocation extent $x$ in $\mathrm{Zn}_x\mathrm{Fe}_{3-x}\mathrm{O}_4$ on reduction conditions ($\mathrm{H_2}$ partial pressure, CO content, temperature, time) and its effect on the zinc-versus-iron sulfidation selectivity, including the boundary function separating zinc-only from zinc-plus-iron sulfidation (the $\mathrm{H_2S}$ partial pressure at which iron sulfides begin to form, as a function of temperature and gas composition).

(3) The speciation and stoichiometry of the iron sulfides formed at elevated $\mathrm{H_2S}$ partial pressure—including an unambiguous assignment of the Mössbauer doublet with pyrite-like parameters and the extent of zinc incorporation into the cubic sulfide—established under conditions free of cooling-atmosphere artifacts.

(4) The quantitative contribution of the direct sulfidation channel of the ferrite/spinel phase (zinc extraction from $\mathrm{ZnFe_2O_4}$ or $\mathrm{Zn}_x\mathrm{Fe}_{3-x}\mathrm{O}_4$ to $\mathrm{ZnS}$, leaving magnetite) relative to the decomposition-then-sulfidation channel.

(5) A complete kinetic model coupling reduction, cation redistribution, and sulfidation under realistic coal-gas mixtures ($\mathrm{CO}$/$\mathrm{CO_2}$/$\mathrm{H_2}$/$\mathrm{H_2O}$ with variable $\mathrm{H_2S}$ partial pressure), suitable for predictive sorbent design.

These five components share one scientific objective—a quantitative, predictive account of zinc ferrite sorbent chemistry in hot reducing gas.

References

1. Makoto Kobayashi, Hiromi Shirai, and Makoto Nunokawa, Elucidation of Sulfidation Mechanisms of Zinc Ferrite in a Reductive Gas Environment by in Situ X-ray Diffraction Analysis and Mössbauer Spectroscopy, Industrial & Engineering Chemistry Research 39 (2000) 1934–1943, DOI 10.1021/ie990456t, https://doi.org/10.1021/ie990456t
2. Makoto Kobayashi, Hiromi Shirai, and Makoto Nunokawa, High-Temperature Sulfidation Behavior of Reduced Zinc Ferrite in Simulated Coal Gas Revealed by in Situ X-ray Diffraction Analysis and Mössbauer Spectroscopy, Energy & Fuels 16 (2002) 601–607, DOI 10.1021/ef010178m, https://doi.org/10.1021/ef010178m
3. Makoto Kobayashi, Hiromi Shirai, and Makoto Nunokawa, Estimation of Multiple-Cycle Desulfurization Performance for Extremely Low-Concentration Sulfur Removal with Sorbent Containing Zinc Ferrite–Silicon Dioxide Composite Powder, Energy & Fuels 16 (2002) 1378–1386, DOI 10.1021/ef020024m, https://doi.org/10.1021/ef020024m
4. Makoto Kobayashi, Hiromi Shirai, and Makoto Nunokawa, Investigation on Desulfurization Performance and Pore Structure of Sorbents Containing Zinc Ferrite, Energy & Fuels 11 (1997) 887–896, DOI 10.1021/ef960231k, https://doi.org/10.1021/ef960231k
5. G. D. Focht, P. V. Ranade, and D. P. Harrison, High-temperature desulfurization using zinc ferrite: Reduction and sulfidation kinetics, Chemical Engineering Science 43 (1988) 3005–3013, DOI 10.1016/0009-2509(88)80053-8, https://doi.org/10.1016/0009-2509(88)80053-8
6. Mark C. Woods, Santosh K. Gangwal, Douglas P. Harrison, and Kandaswamy Jothimurugesan, Kinetics of the reactions of a zinc ferrite sorbent in high-temperature coal gas desulfurization, Industrial & Engineering Chemistry Research 30 (1991) 100–107, DOI 10.1021/ie00049a015, https://doi.org/10.1021/ie00049a015
7. Tadao Kanzaki, Kenzo Kitayama, and Kazuo Shimokoshi, Mössbauer Spectroscopy Studies on Zn-Bearing Ferrite, Journal of the American Ceramic Society 76 (1993) 1491–1494, DOI 10.1111/j.1151-2916.1993.tb03930.x, https://doi.org/10.1111/j.1151-2916.1993.tb03930.x
8. Xiaobo Min, Bosheng Zhou, Yong Ke, Liyuan Chai, Ke Xue, Chun Zhang, Zongwen Zhao, and Chen Shen, Sulfidation behavior of ZnFe2O4 roasted with pyrite: Sulfur inducing and sulfur–oxygen interface exchange mechanism, Applied Surface Science 371 (2016) 67–73, DOI 10.1016/j.apsusc.2016.02.229, https://doi.org/10.1016/j.apsusc.2016.02.229
9. Christian Frilund, Minna Kotilainen, José Barros Lorenzo, Pertti Lintunen, and Kimmo Kaunisto, Steel Manufacturing EAF Dust as a Potential Adsorbent for Hydrogen Sulfide Removal, Energy & Fuels 36 (2022) 3695–3703, DOI 10.1021/acs.energyfuels.1c04235, https://doi.org/10.1021/acs.energyfuels.1c04235
10. Lu Li, Hongbo Zhang, Pin Zhou, Xianglong Meng, Lizhong Liu, Jinping Jia, and Tonghua Sun, Three dimensional ordered macroporous zinc ferrite composited silica sorbents with promotional desulfurization and regeneration activity at mid-high temperature, Applied Surface Science 470 (2019) 177–186, DOI 10.1016/j.apsusc.2018.11.033, https://doi.org/10.1016/j.apsusc.2018.11.033
11. Junwei Han, Wei Liu, Wenqing Qin, Tianfu Zhang, Ziyong Chang, and Kai Xue, Effects of sodium salts on the sulfidation of lead smelting slag, Minerals Engineering 108 (2017) 1–11, DOI 10.1016/j.mineng.2017.01.007, https://doi.org/10.1016/j.mineng.2017.01.007
12. Longgong Xia, Shuheng Cao, Qihou Li, Xingwu Lu, and Zhihong Liu, Co-treatment of copper smelting slag and gypsum residue for valuable metals and sulfur recovery, Resources, Conservation and Recycling 183 (2022) 106360, DOI 10.1016/j.resconrec.2022.106360, https://doi.org/10.1016/j.resconrec.2022.106360
13. Makoto Kobayashi and Yoshinobu Nakao, Inhibition and elimination of carbon deposition in dry gas desulfurization process under oxy-fuel IGCC derived coal gas environment, Fuel 152 (2015) 19–28, DOI 10.1016/j.fuel.2015.02.092, https://doi.org/10.1016/j.fuel.2015.02.092
14. Makoto Kobayashi and Hiroyuki Akiho, Performance evaluation of honeycomb shaped sorbent for sulfur removal in advanced oxy-fuel IGCC power generation plant, Fuel 203 (2017) 1015–1025, DOI 10.1016/j.fuel.2017.02.087, https://doi.org/10.1016/j.fuel.2017.02.087
15. Makoto Kobayashi, Hiroyuki Akiho, Yasushi Ozawa, and Akira Nakajima, Evaluation of dry acid gas removal process on bench scale test facility coupled with syngas produced by O2/CO2-blown gasifier, Fuel 250 (2019) 144–153, DOI 10.1016/j.fuel.2019.03.136, https://doi.org/10.1016/j.fuel.2019.03.136
16. Makoto Kobayashi, Development of sorbents used in dry syngas purification, in Dry Syngas Purification Processes for Coal Gasification Systems, Elsevier, 2021, ISBN 978-0-12-818866-8, DOI 10.1016/B978-0-12-818866-8.00004-5, https://doi.org/10.1016/B978-0-12-818866-8.00004-5
17. Bing Zeng, He Li, Tao Huang, Changjun Liu, Hairong Yue, and Bin Liang, Kinetic Study on the Sulfidation and Regeneration of Manganese-Based Regenerable Sorbent for High Temperature H2S Removal, Industrial & Engineering Chemistry Research 54 (2015) 1179–1188, DOI 10.1021/ie503233a, https://doi.org/10.1021/ie503233a
18. Dicka Ar Rahim, Wei Fang, Haryo Wibowo, Dwi Hantoko, Herri Susanto, Kunio Yoshikawa, Yingjie Zhong, and Mi Yan, Review of high temperature H2S removal from syngas: Perspectives on downstream process integration, Chemical Engineering and Processing – Process Intensification 183 (2023) 109258, DOI 10.1016/j.cep.2022.109258, https://doi.org/10.1016/j.cep.2022.109258