Molten carbonate fuel cells (MCFCs) operating on coal- or waste-derived fuel gas require deep halide cleanup: HCl (and HF) react with the molten alkali-carbonate electrolyte, forming alkali chlorides and fluorides that are retained in and progressively alter the melt, corrode cell and balance-of-plant hardware, and deplete electrolyte over time. The system-level design target in general use — HCl below about 1 ppm — traces to 1980s gasifier/MCFC plant evaluations rather than to a demonstrated cell-level threshold, and the allowable halide level for extended MCFC operation has never been definitively established. This problem asks for that definitive ceiling: the maximum HCl (and HF) concentration in the anode fuel feed that is compatible with a practical stack lifetime on the order of 40,000 hours, including the coupled limits set by cumulative chloride and fluoride retention in the carbonate electrolyte and by corrosion of cell materials. Existing evidence is partial and small-format: single-cell tests found 10 ppm HCl harmless over several thousand hours, small-format 'button'-cell tests (disc-shaped test cells) in carbonate-fuel-cell CO2-capture service found 0.2–0.5 ppm HCl harmless over roughly 900 hours with degradation onset near 40 ppm after about 400 hours, and thermochemical models project negligible electrolyte loss over 10–15 years only at ppb-level HCl. No stack-scale, lifetime-scale study establishes the ceiling; the sub-ppm specification continues to circulate as a cited but unverified design assumption.

Background

A molten carbonate fuel cell (MCFC) is a high-temperature fuel cell operating near 650 °C, in which a molten lithium/potassium (or lithium/sodium) carbonate electrolyte immobilized in a porous ceramic matrix separates a porous nickel anode from a nickel-oxide cathode; the cell is attractive for coal- and waste-gas applications because it internally reforms hydrocarbons, tolerates carbon monoxide, and operates at efficiency gains in integrated gasification fuel cell (IGFC) concepts. Bench-scale MCFCs have run for tens of thousands of hours, and commercial units target lifetimes on the order of 40,000 hours (Morita et al., 2010).

Coal-derived fuel gas is, however, a contaminated feed: besides hydrogen sulfide (H2S) and carbonyl sulfide (COS), it carries halides — principally HCl, at levels from a few to hundreds of ppm depending on coal chloride content and gasifier conditions — together with HF and particulate alkali halides. In the hot-gas cleanup train that IGFC economics favor, sodium-based sorbents (for example sodium aluminate, NaAlO2, on alumina supports) react HCl to NaCl and can be regenerated and reused; Nunokawa et al. (2008) developed such a reusable molded sodium-aluminate sorbent and sized a moving-bed reactor around a continuous outlet HCl target below 1 ppm. Hot-gas HCl cleanup has since been reviewed repeatedly (Ohtsuka et al., 2009; Dou et al., 2012; Dolan et al., 2012; Marcantonio et al., 2021).

The specific design value that such cleanup trains must meet is the fuel cell's halide tolerance. The sub-ppm (<1 ppm) HCl target commonly used for MCFC feedstock descends from 1980s DOE evaluations of gasification and gas cleanup for molten carbonate fuel cell power plants (Jablonski et al., 1982) and contemporaneous chloride-cleanup studies, and it has propagated through the literature as a cited specification. Its physico-chemical basis is sound in direction: HCl and HF convert alkali carbonates to alkali halides, M2CO3 + 2 HX → 2 MX + H2O + CO2, and the halide ions have little thermodynamic driving force to leave the melt, so they accumulate, shifting electrolyte composition (melting behavior, ionic conductivity, wettability), attacking electrodes and hardware, and, in the long run, consuming electrolyte. But the quantitative lifetime-scale ceiling — how much chloride and fluoride an operating cell can actually absorb, and at what inlet concentration the cumulative damage stays acceptable over a 40,000-hour-class life — has never been pinned down. In practice the question resurfaces on both sides of the cell: on the anode side for coal- and biogas-fed MCFCs, where cleanup polishing to a cited 1 ppm criterion is standard (Micoli et al., 2013; Song et al., 2013), and on the cathode side in the modern application of MCFCs as electrochemical membranes for CO2 capture from HCl-bearing flue gas, where FuelCell Energy and partners have run a dedicated contaminant-tolerance program (Ghezel-Ayagh et al., 2014; Jolly et al., 2015). The target of this problem is the anode fuel-side halide ceiling; the cathode-side capture application is context only and no lifetime-scale ceiling has been established. Cell-level data accumulated so far suggest the single-cell HCl tolerance exceeds 1 ppm — 10 ppm was harmless over several thousand hours (Watanabe et al., 2006) — yet the published contaminant limits for capture service were still described as being finalized, and stack-scale, lifetime-scale, multi-contaminant validation remains missing. By contrast with halides, sulfur poisoning of MCFCs (H2S, SO2) has received detailed experimental and mechanistic treatment, dominating post-2008 contaminant research on this cell type.

Problem Statement

Establish the maximum halide concentration — principally HCl, and also HF and other halide species present in coal- or waste-derived fuel gas — that a molten carbonate fuel cell can tolerate in its anode feed over a practical stack lifetime on the order of 40,000 hours, without unacceptable irreversible performance loss. For definiteness, the binding acceptance criterion is the standard end-of-life criterion for this cell type — a 10 % loss of the initial cell voltage over 40,000 hours (equivalently a voltage-decay rate of 0.25 % per 1,000 hours) — so that the ceiling is the maximum inlet halide concentration at which the projected decay over 40,000 hours stays within that limit; electrolyte-loss and corrosion limits are mandatory secondary checks that may lower, but not raise, the ceiling. In other words, replace the current unverified sub-ppm (<1 ppm) HCl design assumption with a definitive, evidence-based allowable long-term halide concentration ceiling, reported for the principal halide HCl as the primary single value and, as secondary bounds at the same criterion, for HF and for the shift caused by a co-present contaminant such as H2S. The resolution must provide, as mandatory parts rather than optional context: (i) cumulative chloride and fluoride retention in, and depletion of, the molten carbonate electrolyte; (ii) the corresponding corrosion limits of electrodes and hardware; and (iii) the ceiling value itself, reported as a single concentration together with its uncertainty, each evaluated against the same 40,000-hour, 10 % end-of-life criterion.

References

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