(784b) Membrane Reactors for Carbon Capture and Hydrogen Production in IGCC Plants: Simulation, Cost Analysis and Design Optimization

In recent years, incentives for the sustainable use of fossil fuels (e.g., coal) have risen in the United States and worldwide. These incentives are motivated by the changes in global climate that have been associated with the emissions of greenhouse gases, such as carbon dioxide (CO₂), to the atmosphere during fossil fuel combustion. Particularly, for coal-based power generation, the challenge is to mitigate such emissions efficiently and economically. Integrated Gasification Combined Cycle (IGCC) plants are an emerging technology for coal-based electricity production that allow CO₂ capture with higher efficiency and lower penalty on cost of electricity when compared to conventional pulverized-coal plants. This presentation focuses on the alternative of using water gas shift (WGS) membrane reactors for pre-combustion carbon capture and hydrogen (H₂) production in IGCC plants. Specifically, a stand-alone membrane reactor model, which will be ultimately integrated into the IGCC flowsheet, is introduced for simulation, cost analysis and optimization studies.

Regarding the modeling task, a one-dimensional and nonisothermal WGS membrane reactor model is developed. This membrane reactor is equipped with a H₂-selective zeolite membrane that holds potential to be used in the WGS environment of coal-based gasification plants. Specifically, a shell and tube reactor design is considered, in which the catalyst is packed in the tube side, a thin membrane layer is placed on the surface of the tube wall and the sweep gas flows in the shell side to lower the H₂ partial pressure of this side. Also, an inner tube, where a coolant flows, is used to provide the necessary cooling that ensures the reactor temperature remains within certain bounds set by the high-temperature WGS catalyst (Fe-Cr-based) operation. The model assumes the reactor operates at steady state and considers constant pressure, adiabatic shell with respect to the environment, plug-flow operation and neglects axial dispersion. Also, the flux through the zeolite membrane is described by a combination of activated and Knudsen diffusion components.

The developed model is simulated using operating conditions consistent with the IGCC plant, including the syngas feed from the gasifier after steam injection, WGS reactor temperature and pressure. The simulation results showed that configurations with co/counter-current flow for the shell (sweep) and co-current flow for the jacket (coolant), relative to the tube (feed), produced a CO₂ rich stream (retentate) and a H₂ rich stream (permeate) that can be further processed for storage and gas turbine power generation, respectively. These product streams satisfied the constraints reported by the U.S. DOE, such as the R&D goal of 90% CO₂ capture and the desired H₂ recovery value of 95% [1], for the retentate and permeate, respectively. These configurations also maintained the reactor temperature under the operating limit associated with the selected WGS catalyst. Using the successful case studies as benchmark, novel R&D targets for the zeolite membrane characteristics (permeance, selectivity) and cost to meet these constraints are established and compared to the requirements for other materials reported by the DOE [1]. Finally, the optimization of the membrane reactor design, considering the obtained membrane characteristics from the cost analysis, is performed. This optimization has the objectives of maximizing the reactor performance and minimizing the membrane use (and thus its cost) by placing it in the optimal location. The previously introduced optimization problem for the isothermal case [2,3] is extended to incorporate the nonisothermal nature of the problem, including the addition of the constraint on the upper limit of the reactor temperature.

References

[1] John J. Marano. Integration of H₂ separation membranes with CO₂ capture & compression. Report to DOE, Contract No. DE-AC26-05NT41816, 2010.

[2] Fernando V. Lima, Prodromos Daoutidis, Michael Tsapatsis, and John J. Marano. Modeling and optimization of membrane reactors for carbon capture in Integrated Gasification Combined Cycle units. Ind. Eng. Chem. Res., 51(15):5480-5489, 2012.

[3] Fernando V. Lima, Prodromos Daoutidis, Michael Tsapatsis, and John J. Marano. Modeling and optimization of membrane reactors for carbon capture in IGCC units. In AIChE Annual Meeting, Minneapolis, MN, October 2011.