Hydrogen-membrane reactors

What it does

Hydrogen membrane reactors are being studied for power production with carbon capture and storage (CCS). They are used for pre-combustion decarbonisation: the removal of CO₂ from the fossil fuel before combustion takes place. The membrane reactor produces hydrogen used as fuel gas at a low pressure and a steam and CO₂ rich stream at high pressure. The separated hydrogen will be used as fuel in a gas-turbine combined-cycle plant to generate electricity at high efficiency. The implementation of a membrane reactor in a pre-combustion natural gas combined cycle scheme for electricity generation with CO₂ capture is depicted below.

Power generation with CO₂ capture using a membrane reactor, simplified schematic.

Typically natural gas first passes a pre-reformer to convert the heavier parts of the natural gas, and to increase the hydrogen concentration. The reformate then enters the membrane reformer. Hydrogen from the membrane reformer is used in the combustion chamber of the gas turbine. The remaining CO₂ stream is, after clean-up, sent to CO₂ compression and storage. Heat is supplied to the membrane reformer by means of combustion of natural gas or hydrogen (indicated by the Q-arrow). This combustion can occur at atmospheric pressure, or can be integrated with the gas turbine. Flue gas from the gas turbine is used in a steam cycle for generation of steam, which is used for additional power production and for the membrane reactor. Typically the feed pressure of the membrane reactor equals the pressure at which natural gas is available from high pressure pipelines, i.e. 40-45 bars. Permeate pressure is typically in the range of 5 to 10 bar.

Other configurations are also studied, e.g. upstream the membrane reactor a natural-gas fuelled auto thermal reformer or gas heated reformer (GHR), in these cases combined with a membrane water-gas-shift (MWGS) reactor. Moreover, MWGS for use in an integrated gasification combined cycle is part of the research. System assessment studies are used to optimize reactor configurations toward lowest carbon capture penalties and highest system efficiencies.

How it works

Membrane reactors allow for conversion of natural gas into two separate streams of H₂ (for power production) and CO₂ (for underground storage).The conversion is done catalytically through steam reforming and/or the water-gas shift reaction. Due to the in situ removal of hydrogen, the reaction equilibriums of the reforming and water gas shift reactions, shown in equations 1 - 3, are shifted to higher conversions through Le Chatelier’s principle.

The principle of a hydrogen-membrane reactor is shown below.

Principle of a hydrogen-membrane reactor.

 The other concept, the MWGS reactor is different with respect that only the water-gas shift reaction is carried out, next to permeation of hydrogen as in the membrane reformer. Because of the mild exothermic nature, heat supply is not required.

Hydrogen membranes exist that selectively permeate hydrogen between 573 and 873 K with permeances in the order of 10-6 mol/m2/s/Pa, see link.

What it is

A hydrogen-membrane reactor unit comprises large steel tubes or vessels comprising hydrogen-selective membranes and catalyst material. ECN has designed and constructed a unique process development unit (PDU) for testing membrane reactors that can be implemented in power plants with CO₂ capture. It has a capacity of 8 tubular membranes of 50 cm long, and can be operated up to 40 bar and 600°C. The PDU will demonstrate the feasibility of membrane reactors at a bench scale size. The PDU is typically operated with 150/400 ln/min wet feed flowrate (2.5/6.7kWth product H₂) for reforming and shift, respectively. The operating window is 25-400% (maximum capacity is 25 kW H₂). It will be used for testing of reactor concepts, and the study of membranes and catalysts. More information can be downloaded here (PDF, 1.20 MB).

Applications

Hydrogen-membrane reactors can separate hydrogen from CO₂ and CO from syngas produced by (auto-thermal) reformers from natural gas, and from coal gas produced by coal gasifiers. As such, electricity, hydrogen, or both can be produced without CO₂ emissions to the atmosphere. Depending on the requirements, the process can be designed for partial or near-complete capture of CO₂. Hydrogen-membrane reactors are also used for process intensification in general and pure hydrogen production for the chemical industry in particular.

Advantages

Hydrogen selective membranes offer the possibility of combining reaction and separation of the hydrogen in a single stage at high temperature and pressure to overcome the equilibrium limitations experienced in conventional reactor configurations for the production of hydrogen. The reforming reaction is endothermic and can, with this technique, be forced to completion at lower temperature than normal (typically 500-600°C). The shift reaction being exothermic can be forced to completion at higher temperature (300-450°C). Membrane reactors allow one-step reforming, or a single intermediate water gas shift reaction, with hydrogen separation (the permeate) leaving behind a retentate gas which is predominantly CO₂ and steam, with some unconverted methane, CO or H₂. After clean-up, condensation of the steam leaves a concentrated CO₂ stream at high pressure, reducing the compression energy for transport and storage. The need for multiple shift reaction stages is avoided. Moreover, process intensification with membrane reactors allows for more compact units, lower investment cost, higher yields and reduced energy cost.

Technology status (2009)

Membrane-steam reforming and membrane-water gas shift has been demonstrated in a single membrane reactor. Gas separation studies with palladium membranes have been successfully conducted with the PDU and M-WGS experiments are ongoing. System calculations show that the technology is now already competitive with conventional capture technology. Nevertheless, we expect to need several years of development before the technology can be scaled up to a pilot unit. Research topics include efficiency improvement, scale-up and reliability issues.

Projects

Since 2006 ECN has been developing the technology in close co-operation with other industries and research institutes. Research projects from which results of the development have been or will be disseminated include:

The CACHET project

The CATO project

The CATO-2 project,

The websites of these projects are periodically updated with results and research plans.

Selected publications

Jansen, D., Dijkstra, J. W., van den Brink, R. W., Peters, T. A., Stange, M., Bredesen, R. Goldbach, A., Xu, H. Y., Gottschalk, A., Doukelis, A. (2009) Hydrogen membrane reactors for CO₂ capture, Energy Procedia 1/1, pp. 253-260.

Pieterse, J.A.Z., Boon J., Delft van Y.C., Dijkstra, J.W., Brink van den R.W. On the potential of Nickel Catalysts for Steam Reforming in Membrane Reactors, Accepted for publication in Catalysis Today, 2009.

Dijkstra, J. W; Jansen, D; Brink, R. W. van den; Peters, T. A.; Stange, M.; Bredesen, R.; Goldbach, A.; Xu, H.; Gottschalk, A.; Tlatlik, S. & Doukelis, A. (2009) , Development of hydrogen membrane reactors for CO₂ capture, Anon., Carbon Dioxide Capture for Storage in Deep Geologic Formations, Volume 3, series ed. L. I. Eide, CPL Press.

Dijkstra, J. W.; Jansen, D. (2004) Novel concepts for CO₂ capture, Energy 29/9-10, pp. 1249-1257, Science direct

Dijkstra, J.W., Kluiters, S.T.A. Reactor for reforming and steam reforming of natural gas for synthesis gas manufacture. Patent WO/2007/142518. 26pp.

Jansen, D., Dijkstra, J.W., De Groot, A. Shift membrane burner/fuel cell combination. Patent WO/2004/021495, 18 pp.