In the dry gas desulfurization step of the oxy-fuel integrated gasification combined cycle (IGCC) concept, regenerable zinc ferrite sorbents remove H2S and COS from hot coal syngas in cyclic fixed-bed operation, and the main threat during plant start-up, when recirculated exhaust gas is unavailable, is carbon deposition by the Boudouard reaction, whose thermodynamic driving force is summarized by RB=PCO2/(PCO)2R_B = P_{\mathrm{CO_2}}/(P_{\mathrm{CO}})^2. The originating study (Kobayashi and Nakao, Fuel, 2015) showed in pressure-compensated laboratory tests at 9.8 bar and 450 °C that start-up-condition reduction deposits at most about 0.8 wt% carbon, that oxidative regeneration removes it to ≤0.03 wt% within the ordinary regeneration period, and that desulfurization breakthrough performance is unimpaired; it left open the behavior at the actual plant pressure of 27.5 bar, in the honeycomb-monolith geometry of the real process, and over repeated start-up cycles. Later work by the same group closed the geometry and actual-syngas components at 0.98 MPa, introducing an improved sulfate-derived honeycomb sorbent with intrinsic carbon resistance and demonstrating five sulfidation-regeneration-reduction cycles on real O2/CO2-blown gasifier syngas without detrimental carbon deposition. Every published experiment, however, remains at 0.98 MPa with the pressure gap bridged only by matching RBR_B, and cyclic testing stopped at five cycles. The problem is the experimental verification at full plant pressure (2.75 MPa absolute) of actual carbon deposition behavior, the kinetics of carbon removal during regeneration, and total-pressure effects on sorbent reduction and oxidation beyond the RBR_B equivalence, together with the long-term effect of repeated start-up cycles on carbon accumulation, regeneration efficiency, and desulfurization performance in the process-representative honeycomb sorbent.

Background

Integrated gasification combined cycle (IGCC) power plants gasify coal and burn the resulting syngas in a gas turbine. In the oxy-fuel IGCC variant, the gasifier is blown with an O2/CO2 mixture and the gas turbine runs as a semi-closed cycle with oxy-fuel combustion, so that the exhaust is mainly CO2 and H2O and CO2 can be captured by condensation alone. A key enabling technology for this concept is dry gas desulfurization: instead of wet scrubbing at low temperature, hot syngas (about 450 °C) is passed through fixed beds of a regenerable zinc ferrite (ZnFe2O4) sorbent that chemically absorbs H2S and COS during a sulfidation step, releases the absorbed sulfur as SO2 during an oxidative regeneration step with dilute O2 in N2, and is then returned to the working state by a reduction step with syngas; three reactors are switched cyclically so that desulfurization is continuous. In the plant-scale process the sorbent is shaped as a honeycomb monolith, a ceramic block with many parallel square flow channels, rather than packed granules.

Carbon deposition threatens this process through the Boudouard reaction, 2CO→CO2+C2\mathrm{CO}\rightarrow\mathrm{CO_2}+\mathrm{C}, which deposits solid carbon on the sorbent from CO-rich gas. Its thermodynamic driving force is summarized by the parameter RB=PCO2/(PCO)2R_B = P_{\mathrm{CO_2}}/(P_{\mathrm{CO}})^2 built from the CO and CO2 partial pressures, with lower RBR_B meaning a stronger tendency to deposit carbon (with pressures in bar, RBR_B carries units of bar⁻¹). During normal operation a small stream of recirculated exhaust gas, rich in CO2 and H2O, is mixed into the syngas, raising RBR_B out of the carbon-deposition region; during plant start-up, however, the gas turbine is not yet running on coal-derived fuel, no exhaust is available, and the raw O2/CO2-blown gasifier syngas has very high CO and very low CO2 and H2O, giving RBR_B as low as roughly 0.01.

The source study (Kobayashi and Nakao, 2015) quantified this start-up risk experimentally in a laboratory fixed bed at 450 °C. Because the plant operates at 27.5 bar while the apparatus was limited to 9.8 bar, the authors used a pressure-compensation methodology: the test gas composition at 9.8 bar was adjusted so that RBR_B matched the plant value, which assumes that carbon deposition depends only on RBR_B. With granular zinc ferrite at RBR_B between 0.013 and 0.033, at most about 0.8 wt% carbon deposited during a 3-hour start-up reduction; oxidative regeneration with 1.5 vol% O2 in N2 removed it to ≤0.03 wt% within the ordinary regeneration period; and the H2S/COS breakthrough performance afterwards was not degraded (breakthrough is the point in a fixed-bed run where H2S or COS first appear in the treated outlet gas, so unchanged breakthrough behavior means the desulfurization capacity was not impaired). The study's closing qualifications list what that methodology could not establish: the actual deposition behavior, carbon-removal kinetics, and total-pressure effects on sorbent reduction and oxidation under the full plant pressure; the honeycomb-monolith geometry used in the actual process; and the long-term effect of repeated start-up cycles.

Subsequent work by the same group substantially advanced two of these components. A sulfate-derived zinc ferrite formulation, which retains a small residual sulfur content from sulfate raw materials acting as an intrinsic inhibitor of carbon formation, was shown in 2016 to deposit about five times less carbon than the former nitrate-derived formulation and to regenerate to below 0.01 wt% carbon over two cycles. In 2017 the honeycomb geometry was tested directly: the former nitrate-derived honeycomb structurally collapsed under the severe start-up condition (RBR_B = 0.0134 at 0.98 MPa absolute), with carbon that could not be eliminated even by 10 hours of oxidation, whereas the improved sulfate-derived honeycomb stayed structurally intact through two full reduction-sulfidation-regeneration cycles, kept deposited carbon ≤1.0 wt% and fully burnable within the ordinary regeneration period, and showed unchanged breakthrough behavior. In 2019 the process was demonstrated at bench scale on actual O2/CO2-blown gasifier syngas at 450 °C and 0.92–0.98 MPa absolute: after an initial roughly 4-hour run, a 46-hour campaign completed five sulfidation-regeneration-reduction cycles with the honeycomb sorbents, with no detrimental carbon deposition and stable sulfur release and regeneration. The same papers state what remains: every test was still performed at about 0.98 MPa rather than the plant pressure of 2.75 MPa, and five cycles do not establish the tens-to-hundreds-of-cycle behavior needed to fix sorbent lifetime.

Problem Statement

Experimentally verify, under the actual full plant pressure of 2.75 MPa absolute (27.5 bar) rather than the pressure-compensated 9.8 bar (0.98 MPa absolute) laboratory conditions, the carbon-deposition and regeneration behavior of the regenerable zinc ferrite desulfurization sorbent in the dry gas desulfurization step of the oxy-fuel integrated gasification combined cycle (IGCC), in which coal is gasified with an O2/CO2 blast and the hot syngas (about 450 °C) is passed through fixed beds of a regenerable zinc ferrite (ZnFe2O4) sorbent that absorbs H2S and COS during a sulfidation step, releases the absorbed sulfur as SO2 during an oxidative regeneration step with dilute O2, and is returned to the working state by a reduction step with syngas, with reactors switched cyclically so that desulfurization is continuous. The sorbent must be in the process-representative honeycomb-monolith configuration (a ceramic block with many parallel square flow channels, as used in the actual plant, rather than packed granules), with representative start-up syngas (no exhaust recirculation, RB≤0.013R_B \le 0.013) and exhaust-mixed steady syngas; here RB=PCO2/(PCO)2R_B = P_{\mathrm{CO_2}}/(P_{\mathrm{CO}})^2 is the thermodynamic parameter of the Boudouard reaction 2CO→CO2+C2\mathrm{CO}\rightarrow\mathrm{CO_2}+\mathrm{C} (carbon deposition from CO-rich gas), computed from the CO2 and CO partial pressures (with pressures in bar, RBR_B carries units of bar⁻¹; lower RBR_B means a stronger tendency to deposit carbon). The required results are: (i) the actual carbon deposition behavior during start-up-condition reduction at full pressure, quantified by direct carbon analysis of the sorbent; (ii) the kinetics of carbon removal during oxidative regeneration at full pressure; (iii) the effects of total pressure on sorbent reduction and oxidation beyond the thermodynamic parameter RBR_B — that is, whether the pressure-compensation equivalence, which matches RBR_B alone across the 9.8 bar to 27.5 bar gap, is valid or must be corrected for pressure-dependent kinetics and transport; and (iv) the long-term effect of repeated start-up/desulfurization cycles on carbon accumulation, sorbent regeneration efficiency, and desulfurization performance, measured as H2S and COS breakthrough (the point in a fixed-bed run where H2S or COS first appear in the treated outlet gas), over a cycle count and duration sufficient to establish the trend of these quantities — that is, enough consecutive start-up/desulfurization cycles to determine whether carbon accumulation, regeneration efficiency, and breakthrough performance remain stable, degrade monotonically, or converge to a steady state, the published record reaching only five cycles. These four required results jointly settle one question: whether the laboratory-validated carbon-deposition control of the process transfers to the operating plant, which is the condition for finalizing the regeneration-step schedule and sorbent lifetime design of the dry desulfurization unit; all four are mandatory and a partial subset does not resolve it.

References

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