Mechanism and quantitative description of the reduction behavior of the Fe2O3-SiO2 nanocomposite hot-gas-desulfurization sorbent in coal gas

The $Fe_{2}O_{3}$-$SiO_{2}$ nanocomposite is the active phase of a regenerable honeycomb sorbent developed for removing hydrogen sulfide ($H_{2}S$) and carbonyl sulfide (COS) from hot coal gas produced by air-blown gasifiers in integrated gasification combined cycle (IGCC) plants. Shirai, Kobayashi and Nunokawa (1999) showed that in simulated gasifier gas containing 1-5% steam at 623-723 K the reduction of this sorbent arrests at an oxygen-to-iron ratio $R_{O/Fe}$ of 1.0-1.3, in a mixture of $Fe_{3}O_{4}$, $Fe_{1-x}O$ and iron silicate, with the amorphous $SiO_{2}$ preventing formation of metallic Fe, cementite ($Fe_{3}C$) and carbon, whereas above 773 K fayalite ($Fe_{2}SiO_{4}$) forms and sinters the material. The microscopic mechanism of this suppression - a hypothesized amorphous silica / iron(II) silicate film blocking gas-solid contact on the roughly 10 nm iron oxide primary grains - could not be confirmed; the arrested phase mixture could not be quantitatively apportioned; and cycle-dependent microstructural degradation, flagged by the authors as a risk of further grain growth and sintering when reduction-desulfurization-regeneration cycles are repeated, was not measured. This record asks for a mechanism-level, quantitatively validated description of this reduction behavior. The literature status is partially advanced: the protective-layer mechanism has been directly observed during $H_{2}$ reduction of silica-promoted iron oxides in the Fischer-Tropsch catalyst context, where iron catalysts synthesize hydrocarbons from $CO$/$H_{2}$ (Coombes et al. 2021), and the sorbent's developers measured sulfur-capacity stabilization over about five desulfurization-regeneration cycles (Shirai et al. 2002); the coal-gas-specific mechanism including the $Fe_{3}C$/carbon-suppression leg, the quantitative phase partitioning, and cyclic microstructure evolution remain open.

Background

In integrated gasification combined cycle (IGCC) power generation, coal is gasified (in Japanese designs, by an air-blown entrained-flow gasifier, in which pulverized coal reacts while suspended in the flowing gas) into a fuel gas whose combustibles are CO and $H_{2}$, diluted by $CO_{2}$, $H_{2}O$ and $N_{2}$. Before the gas is used, sulfur compounds (hydrogen sulfide, $H_{2}S$, and carbonyl sulfide, COS) must be removed; wet scrubbing requires cooling the gas and losing its sensible heat, so 'dry' hot-gas cleaning at temperature is attractive for plant efficiency. One regenerable approach, developed by the Central Research Institute of Electric Power Industry (CRIEPI) with Mitsubishi Heavy Industries, is a honeycomb-type (extruded monolith) iron oxide sorbent whose active phase is a $Fe_{2}O_{3}$-$SiO_{2}$ nanocomposite: hematite ($Fe_{2}O_{3}$) primary grains roughly 10 nm across embedded with amorphous (non-crystalline) silica, which acts as binder and structural stabilizer. The sorbent works in a cycle: it is pre-reduced by the coal gas, sulfidized (the reduced oxide reacts with $H_{2}S$/COS to iron sulfide, FeS), then regenerated with an $O_{2}$-bearing gas that converts FeS back to oxide and releases the sulfur as $SO_{2}$. The pre-reduction step is not innocent. Iron oxides reduce stepwise - hematite $Fe_{2}O_{3}$, magnetite $Fe_{3}O_{4}$, wustite $Fe_{1-x}O$ (the non-stoichiometric ferrous oxide), metallic Fe - and the extent of reduction is reported as $R_{O/Fe}$, the atomic oxygen-to-iron ratio obtained by gravimetry (1.5 for hematite, 0 for Fe). Over-reduction is harmful: metallic Fe and cementite ($Fe_{3}C$, an iron carbide) accompany Boudouard carbon deposition ($2CO \rightarrow CO_{2} + C$), which fouls the pores; and reaction of iron oxide with the silica forms fayalite ($Fe_{2}SiO_{4}$, iron(II) orthosilicate), the thermodynamically stable phase in this binary under reducing conditions at elevated temperature, which sinters the sorbent and collapses its surface area. Shirai, Kobayashi and Nunokawa (1999) studied the reduction of this $Fe_{2}O_{3}$-$SiO_{2}$ particle in simulated air-blown gasifier gas (7.5% $H_{2}$, 20% CO, 5% $CO_{2}$, 1-5% $H_{2}O$, $N_{2}$ balance). At 623-723 K the reduction arrested at $R_{O/Fe}$ = 1.0-1.3 in a mixed state of $Fe_{3}O_{4}$, $Fe_{1-x}O$ and a minor iron silicate, and metallic Fe, $Fe_{3}C$ and carbon did not form - unlike $SiO_{2}$-free $Fe_{2}O_{3}$, which reduced to $R_{O/Fe}$ of about 0.23 with clear Fe and $Fe_{3}C$ diffraction peaks. They attributed the arrest to an amorphous silica/iron-silicate film hypothesized to coat the ~10 nm iron oxide primary grains and block gas-solid contact, citing earlier observations that silicate formation retards wustite reduction (Moukassi et al. 1984 on Si-doped wustite single crystals; Shigematsu and Iwai 1993 on dense wustite with $SiO_{2}$/$SiO_{2}$-CaO additions). Above 773 K, fayalite crystallized (completely above 823 K), halving the surface area and enlarging the pores - hence their recommendation to operate below 773 K. Crucially, the protective effect failed in steam-free gas: without $H_{2}O$ the particles gained weight through $Fe_{3}C$ and amorphous carbon deposition, and in dry $H_{2}$-$N_{2}$ they reduced further ($R_{O/Fe}$ 0.6-0.8 at 723 K, with Fe appearing at 873 K as fayalite decomposed). The 1999 work thus established the phenomenon but left its microscopic basis unconfirmed - the authors state that observing the cross-sections of ~10 nm particles without oxidizing them was too difficult - and left the quantitative description incomplete: X-ray diffraction could not separately apportion the arrested mixture into phase fractions of $Fe_{3}O_{4}$, $Fe_{1-x}O$ and $Fe_{2}SiO_{4}$, and the flagged risk of further grain growth and sintering when reduction-desulfurization-regeneration cycles are repeated was not measured.

Problem Statement

For the $Fe_{2}O_{3}$-$SiO_{2}$ nanocomposite hot-gas-desulfurization sorbent (amorphous-silica-bonded hematite with ~10 nm primary grains, in honeycomb form) operating in air-blown coal-gasifier gas (CO-$H_{2}$-$CO_{2}$-$H_{2}O$, $N_{2}$ balance) in and around the 623-823 K range, establish a mechanism-level and quantitatively validated description of its reduction behavior, jointly delivering the following linked results: (i) direct microscopic verification or refutation of the protective-layer mechanism - whether an amorphous silica / iron(II) silicate film forms on the iron oxide primary grains and blocks gas-solid contact, thereby suppressing reduction to metallic Fe and suppressing $Fe_{3}C$ (cementite) and carbon nucleation in CO-bearing gas; (ii) the quantitative phase partitioning among $Fe_{3}O_{4}$ (magnetite), $Fe_{1-x}O$ (the non-stoichiometric ferrous oxide, wustite), and iron(II) silicate in the arrested state, where the gravimetric atomic oxygen-to-iron ratio $R_{O/Fe}$ (1.5 for hematite, 0 for metallic Fe) levels off at about 1.0-1.3, at intermediate temperatures; and (iii) the kinetics and extent of cycle-dependent microstructural change - $Fe_{2}SiO_{4}$ progression, grain growth, sintering, and the consequences for sulfur capacity and mechanical integrity - over repeated reduction-sulfidation-regeneration cycles. The three results are jointly posed. 'Quantitatively validated' requires the (ii) phase fractions to be apportioned by an explicit characterization method and to be consistent with the gravimetric $R_{O/Fe}$. These three results are components of one overarching objective: to make the reduction behavior of this sorbent class mechanism-based and quantitatively characterized, so that its operating window (below the $Fe_{2}SiO_{4}$-forming, sintering threshold) and its cycle lifetime can be specified from established mechanism rather than empirical trial.

References

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