Carbon Molecular Sieve Membrane as a True One Box Unit for Large Scale Hydrogen Production

Abstract

IGCC coal-fired power plants show promise for environmentally-benign power generation. In these plants coal is gasified to syngas then processed in a water gas-shift (WGS) reactor to maximize the hydrogen/CO{sub 2} content. The gas stream can then be separated into a hydrogen rich stream for power generation and/or further purified for sale as a chemical and a CO{sub 2} rich stream for the purpose of carbon capture and storage (CCS). Today, the separation is accomplished using conventional absorption/desorption processes with post CO{sub 2} compression. However, significant process complexity and energy penalties accrue with this approach, accounting for ~20% of the capital cost and ~27% parasitic energy consumption. Ideally, a “one-box” process is preferred in which the syngas is fed directly to the WGS reactor without gas pre-treatment, converting the CO to hydrogen in the presence of H{sub 2}S and other impurities and delivering a clean hydrogen product for power generation or other uses. The development of such a process is the primary goal of this project. Our proposed process includes a catalytic membrane reactor (MR) that makes use of a hydrogen-selective, carbon molecular sieve (CMS) membrane, and a sulfur-tolerant Co/Mo/Al{sub 2}O{sub 3} catalyst. The membrane reactor’s behavior has beenmore » investigated with a bench top unit for different experimental conditions and compared with the modeling results. The model is used to further investigate the design features of the proposed process. CO conversion >99% and hydrogen recovery >90% are feasible under the operating pressures available from IGCC. More importantly, the CMS membrane has demonstrated excellent selectivity for hydrogen over H{sub 2}S (>100), and shown no flux loss in the presence of a synthetic material, i.e., naphthalene. In summary, the proposed process has been successfully demonstrated with the bench-top reactor. In parallel we have successfully designed and fabricated a full-scale CMS membrane and module for the proposed application. This full-scale membrane element is a 3 diameter with 30L, composed of ~85 single CMS membrane tubes. The membrane tubes and bundles have demonstrated satisfactory thermal, hydrothermal, thermal cycling and chemical stabilities under an environment simulating the temperature, pressure and contaminant levels encountered in our proposed process. More importantly, the membrane module packed with the CMS bundle was tested for over 30 pressure cycles between ambient pressure and >300 -600 psi at 200 to 300°C without mechanical degradation. Finally, internal baffles have been designed and installed to improve flow distribution within the module, which delivered ≥90% separation efficiency in comparison with the efficiency achieved with single membrane tubes. In summary, the full-scale CMS membrane element and module have been successfully developed and tested satisfactorily for our proposed one-box application; a test quantity of elements/modules have been fabricated for field testing. Multiple field tests have been performed under this project at National Carbon Capture Center (NCCC). The separation efficiency and performance stability of our full-scale membrane elements have been verified in testing conducted for times ranging from 100 to >250 hours of continuous exposure to coal/biomass gasifier off-gas for hydrogen enrichment with no gas pre-treatment for contaminants removal. In particular, tar-like contaminants were effectively rejected by the membrane with no evidence of fouling. In addition, testing was conducted using a hybrid membrane system, i.e., the CMS membrane in conjunction with the palladium membrane, to demonstrate that 99+% H{sub 2} purity and a high degree of CO{sub 2} capture could be achieved. In summary, the stability and performance of the full-scale hydrogen selective CMS membrane/module has been verified in multiple field tests in the presence of coal/biomass gasifier off-gas under this project. A promising process scheme has been developed for power generation and/or hydrogen coproduction with CCS based upon our proposed process. Our preliminary economic analysis indicates about 10% reduction in the required electricity selling price and ~40% cost reduction in CCS on per ton CO{sub 2} can be achieved in comparison with the base case involving conventional WGS with a two-stage Selexsol® for CCS. Long term field tests (e.g., >1,000 hrs) with the incorporation of the catalyst for the WGS membrane reactor and more in-depth analysis of the process scheme are recommended for the future study.« less