Moderate-temperature (~300 C) fixed-bed purification of high-calorific (CO-rich) coal-derived fuel gas: kinetic enhancement of regenerable zinc-based desulfurization and sodium-aluminate halide capture

The 2008 study of Kobayashi, Shirai and Nunokawa (Powder Technology 180, 178-183) proposed a moderate-temperature (~300 C) fixed-bed dry purification system for high-calorific, oxygen-blown (CO-rich) coal-derived fuel gas - desulfurization with a regenerable zinc-ferrite sorbent and halide capture with a sodium-aluminate sorbent - and identified five linked requirements standing between its experimental findings and a practical system: sulfidation kinetics at ~300 C sufficient for sustained sub-ppm sulfur; higher halide-capture kinetics and single-pass sodium utilization (beyond ~22% at the 1-ppm breakthrough threshold and ~75% at complete breakthrough); demonstrated multi-cycle regeneration durability under pressurized CO-rich coal gas; bed designs mitigating incomplete single-pass conversion at acceptable pressure drop; and quantitative screening of alternative regenerable chemistries for the equilibrium-versus-kinetics and carbon-deposition trade-off. Later literature - chiefly the authors' own CRIEPI program - resolved the underlying purification objective at 450 C by exhaust-gas blending, a carbon-deposition-resistant sulfate-derived honeycomb zinc ferrite, near-completely utilized sodium-aluminate halide beds, and a bench-scale multi-cycle demonstration on real O2/CO2-blown gasifier syngas; moderate-temperature (~300 C) results elsewhere (ZnO-K2CO3/Al2O3, ZnMoO4) remain atmospheric, lower-space-velocity, single-cycle, or in non-CO-rich gas. The problem - achieving the five linked requirements at the ~300 C operating point in pressurized CO-rich coal gas - remains open.

Background

Coal can be converted to power or fuels by gasification: the coal reacts with oxygen and steam to produce a synthesis gas, which in an integrated gasification combined cycle (IGCC) plant is burned in a gas turbine, and in an integrated gasification fuel cell (IGFC) plant is fed to fuel cells such as the molten carbonate fuel cell (MCFC), whose nickel electrodes and molten-salt electrolyte are poisoned by trace sulfur and halide gases. Oxygen-blown gasification (pure oxygen instead of air) yields a nitrogen-free, high-calorific fuel gas dominated by carbon monoxide (CO of order half the volume), which is also the natural feed for synthetic fuel production; it typically carries a thousand or more ppm of H2S, some COS, and tens to hundreds of ppm of HCl. Removing these to sub-ppm levels while the gas is still hot - dry (hot/warm) gas cleanup with solid sorbents in fixed beds instead of the conventional cold wet scrubber - avoids the efficiency penalty of cooling and reheating the gas and the associated wastewater.

The sorbent chemistry: zinc ferrite, ZnFe2O4, often as a composite with silica, captures H2S by sulfidation, forming ZnS and iron sulfides, and can be regenerated by oxidation back to oxide while releasing SO2, so one bed can in principle run many sulfidation-regeneration cycles. Sodium aluminate, NaAlO2, captures HCl as NaCl with alumina as byproduct and is the standard alkali-based halide sorbent. Performance is measured by fixed-bed breakthrough testing: gas passes through a packed bed at a stated gas hourly space velocity (GHSV, volumetric flow divided by bed volume), and the useful life is the time until the outlet concentration crosses a threshold, commonly 1 ppm; the fraction of stoichiometric sorbent capacity used at that point is the usable conversion. In CO-rich gas a competing chemistry appears at higher temperature: the Boudouard disproportionation $2\mathrm{CO}=\mathrm{CO}_{2}+\mathrm{C}(\mathrm{s})$ deposits solid carbon in the bed and fouls the sorbent.

Kobayashi, Shirai and Nunokawa (2008) tested both steps of a moderate-temperature (~300 C) fixed-bed purification train for oxygen-blown coal gas (simulated gas: CO ~55 vol%, H2 ~22 vol%, CO2 ~3 vol%, H2O ~3 vol%, N2 balance, H2S 0.1 vol%, at 0.98 MPa). They found the central dilemma: at 300 C the zinc-ferrite-silica bed held outlet total sulfur below 1 ppm only briefly before kinetic breakthrough (at 350 C it held ~2 ppm to about one fifth of capacity; performance rose with temperature), whereas at 450 C, where kinetics suffice, effluent CO fell by 26 vol% with CO2 rising 13 vol% - the 2:1 signature of Boudouard carbon deposition, with degraded desulfurization. The NaAlO2 halide bed at ~300 C, 0.10 MPa and 18,000 h-1 kept 200 ppm inlet HCl below 1 ppm at the outlet for 570 min, but only ~22% of the sodium was converted at that threshold and conversion saturated at ~75% at complete breakthrough - incomplete single-pass utilization. They concluded that a practical moderate-temperature system required kinetic enhancement of the zinc sorbent, better halide-sorbent utilization, demonstrated multi-cycle regenerability under pressurized CO-rich gas, bed designs mitigating incomplete conversion at acceptable pressure drop, and screening of alternative regenerable chemistries (zinc titanate, copper-based) against the equilibrium, kinetics and carbon-deposition trade-off.

The subsequent literature is dominated by the authors' own program at CRIEPI (the Central Research Institute of Electric Power Industry, the Japanese electricity industry's research laboratory), which re-routed the operating point to 450 C for the oxy-fuel IGCC application (an IGCC variant with an O2/CO2-blown gasifier, so the gasifier gas carries very little nitrogen and is amenable to CO2 capture): exhaust-gas (steam/CO2) blending kinetically retards carbon deposition; a sulfate-precursor honeycomb zinc-ferrite sorbent (a monolithic block perforated by parallel square channels instead of packed pellets) survives severe start-up conditions with <1 wt% carbon where the former formulation collapsed with ~25 wt%; sodium-aluminate halide sorbents of their SAS series (grades E, G and K) reach 93-98% sodium-to-chloride conversion at 450 C and ~4000 h-1; and an integrated bench facility ran 46 h with five automatic sulfidation-regeneration-reduction cycles on real O2/CO2-blown gasifier syngas (44% CO, 248 ppm H2S) at 0.98 MPa with sub-ppm H2S/COS and ~0.09 ppm HCl (Kobayashi and Akiho 2016, 2017; Kobayashi and Akiho 2017a; Kobayashi et al. 2019, 2019a), summarized in Kobayashi's 2021 monograph on dry syngas purification. Adjacent work at genuinely moderate temperature exists but under easier conditions: ZnO promoted with K2CO3/Al2O3 holds H2S and COS below 0.1 mgS/m3 at 300 C, but at atmospheric pressure, 2000 h-1 and without regeneration evidence (Shangguan et al. 2013); a ZnMoO4 mixed oxide sulfides at 350 C and regenerates completely at 500 C with 31 wt% sulfur capacity, but in H2S/H2 rather than CO-rich gas (Girard et al. 2015); zinc ferrite made by sol-gel auto-combustion (a wet-chemistry route in which a combustion-synthesized gel yields a nanoscale oxide) for moderate-temperature service was already under study contemporaneously (Zhang et al. 2007). Reviews of hot-gas desulphurization (Meng et al. 2010) and of IGCC status (Wolfersdorf and Meyer 2017) record that regenerable-sorbent dry cleanup at moderate temperature remains a development target rather than an established capability. What no published work demonstrates is the source question's core: sustained sub-ppm sulfur at ~300 C in pressurized CO-rich coal gas with a regenerable zinc-based sorbent, together with halide-sorbent utilization at ~300 C beyond the ~75% single-pass limit at high space velocity.

Problem Statement

Establish the sorbent-and-bed conditions under which a fixed-bed, regenerable dry purification system can operate at moderate temperature at the ~300 C target operating point (the ~300 C value is binding; the 300-350 C band spanned by the source's own moderate-temperature tests marks the moderate-temperature regime rather than widening the target) on pressurized, oxygen-blown (CO-rich) coal-derived fuel gas - representative conditions being those of the source tests (the sulfur-bed tests were run at ~0.98 MPa; the halide test at ~0.10 MPa and 18,000 h-1): CO of order half the gas volume, with H2 ~20-25 vol% and several vol% each of CO2 and H2O; H2S of order 0.1 vol% with COS present; HCl of order 200 ppm; total pressure of order 1 MPa; and space velocities from a few thousand h-1 (sulfur beds) to 18,000 h-1 (halide beds). Five results are required, and they jointly serve one objective: a practical ~300 C purification train delivering sustained sub-ppm sulfur and halide with practical bed volumes, cycle lengths and pressure drop.

(1) A zinc-containing regenerable sorbent whose sulfidation kinetics at ~300 C are fast enough that, at the representative sulfur-bed space velocity (a few thousand h-1) and total pressure of order 1 MPa, outlet total sulfur (H2S + COS) stays below 1 ppm up to at least one fifth of the sorbent's stoichiometric sulfur capacity - the fraction the source reached only at 350 C - rather than only briefly as observed at 300 C in the source tests.

(2) A halide sorbent whose capture kinetics and single-pass usable conversion at ~300 C, high space velocity and the representative ~1 MPa pressure both exceed the observed values (the source's halide test was run at ~0.10 MPa) - conversion at the 1-ppm breakthrough threshold above ~22% and maximum single-pass conversion above ~75% - or an alternative regenerable halide sorbent that likewise meets the same thresholds (~22% at the 1-ppm breakthrough threshold and ~75% maximum single-pass conversion) at ~300 C, high space velocity and the representative ~1 MPa pressure.

(3) Multi-cycle sulfidation-regeneration durability, under representative pressurized CO-rich coal gas at ~300 C, of both the zinc-based sorbent and the halide sorbent selected under (2), including post-regeneration breakthrough profiles that return the outlet below the 1-ppm threshold after each regeneration over at least five sulfidation-regeneration cycles.

(4) Reactor- and bed-design measures (staging, pellet or honeycomb geometry, contact time) that raise single-pass usable conversion to the levels required by results (1) and (2) at the stated space velocities, while keeping pressure drop per unit bed depth and sorbent mass per unit gas treated no greater than the source's own packed beds - the zinc-ferrite-silica sulfur bed and the sodium-aluminate halide bed - at the same gas velocities.

(5) Quantitative comparison of alternative regenerable chemistries (zinc titanate, copper-based and others) for the trade-off between thermodynamic equilibrium, reaction kinetics, and carbon deposition at ~300 C in CO-rich feed, concluding in a shortlist that states, for each candidate chemistry, whether it meets result (1)'s breakthrough criterion at ~300 C without Boudouard carbon deposition.

The target operating point is ~300 C rather than anywhere in the 300-350 C moderate-temperature band spanned by the source's own tests, in which the source located the kinetic limitation its system concept must overcome. Results obtained above that band - such as the 450 C exhaust-blended process of the authors' later program (see Background), the higher-temperature point at which the source itself observed the Boudouard carbon-deposition signature - address a different operating point.

References

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