SEWGS

What it does

Sorption-Enhanced Water Gas Shift (SEWGS) is a pre-combustion CO₂ capture process that removes the carbon dioxide (CO₂) and carbon monoxide (CO) from a syngas or coal derived gas at elevated pressure and temperatures of about 400 °C. It produces a hot stream of hydrogen and steam, which can be directly fed to a gas turbine, and a cooled stream of relatively pure CO₂, which can be compressed and transported to a subsurface injection location. The SEWGS process consumes steam, cooling water and electricity, but the amounts are smaller than for conventional pre-combustion technologies.

How it works

SEWGS is a combination of a chemical reaction and a gas separation. The CO is converted to CO₂ according to the so-called water gas shift reaction

CO + H₂O <---> CO₂ + H₂

The CO₂ is removed from the gas by adsorption on a solid sorbent.

The water-gas shift reaction is a chemical equilibrium reaction. Hence, only a part of the CO could be converted to CO₂ at elevated temperature. The trick is to combine the chemical reaction with the CO₂ capture in one reactor. As CO₂ is adsorbed on the sorbent, it is taken out from the gas. According to Le Chatelier’s Principle, more CO will react with steam as long as CO₂ is taken out from the gas phase. This is the principle of the SEWGS. By taking out all CO₂ from the gas, all CO is removed simultaneously, and a relatively pure stream of hydrogen and steam is produced. When the sorbent is saturated with CO₂, it is regenerated so that it can be re-used. Ten thousand to a hundred thousand of cycles of sorption and regeneration will need to be accomplished. More information on the process can be found at: the SEWGS process.

The SEWGS principle; reaction and adsorption at high pressure (upper) and desorption at low pressure (lower)

What it is

A SEWGS unit comprises large steel vessels filled with small tablets of the sorbent and catalyst material. While the syngas or coal gas is passed through some of these vessels, the sorbent is loaded with CO₂, and H₂ is produced. In other vessels the sorbent material is being regenerated, producing CO₂. By switching of valves, the operation mode of the vessels is periodically changed. A condensor is required for knocking out the steam present in the CO₂ product gas. In some configurations, a compressor for CO₂ is required.

Applications

SEWGS can remove 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 full capture of CO₂. Other applications are being investigated.

Advantages

SEWGS does neither require cooling of the syngas nor reheating of the hydrogen produced. It can replace the conventional CO₂ removal by Low Temperature Water Gas Shift, cooling, condensing, washing, and reheating. Calculations by others confirm that SEWGS benefits from substantial higher efficiencies and better process economies (see CACHET and CAESAR projects). Furthermore, the solid sorbent material has important economic, safety and environmental advantages over liquid solvents or sorbents, as it is inexpensive, does not need to be replenished, can be used for extended periods, is non-hazardous and is not emitted directly or indirectly to the atmosphere.

Limitations

It is not feasible to retrofit SEWGS, like any other pre-combustion technology, to existing natural-gas fired power plants and pulverised coal power plants.

Technology status (2009)

The full SEWGS process has been demonstrated in our lab for thousands of cycles using clean gases. Although calculations show that the technology is now already competitive with conventional capture technology, we expect to need another four years of development before the technology can be scaled up to a pilot unit. Research topics include efficiency improvement, reliability issues and effects of impurities. Besides the SEWGS technology, gas turbines need to be modified for operation on hydrogen instead of natural gas or coal gas. Also, autothermal reformers will need to be scaled up from the scale of existing gas-to-liquid plants to the size of power plants.

In December 2007 the multi-column SEWGS test rig was opened by Ruud Lubbers. (photo by courtesy of Eric Sitters)

 Projects

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

The CACHET project

The CAESAR project

The CATO project

The CATO-2 project,

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

Scientific publications

E. van Selow, P. Cobden, R. van den Brink, A. Wright, V. White, P. Hinderink, J. Hufton. Pilot-scale development of the sorption enhanced water gas shift process. In: Carbon Dioxide Capture for Storage in Deep Geologic Formations, Volume 3, L.I. Eide (ed.), CPL Press and BP, to be published.

E.R. van Selow, P.D. Cobden, P.A. Verbraeken, J.R. Hufton, R.W. van den Brink. Carbon capture by sorption enhanced water gas shift reaction process using hydrotalcite-based material. Ind. Eng. Chem. Res. 48, 9, 4184.

S. Walspurger, L. Boels, P.D. Cobden, G.D. Elzinga, W.G. Haije, and R.W. van den Brink (2008). The crucial role of the K+-Aluminium oxide interaction in K+-promoted alumina- and hydrotalcite-based materials for CO2 sorption at high temperatures. ChemSusChem 1, 643 – 650.

P.D. Cobden, P. van Beurden, H.Th.J. Reijers, G.D. Elzinga, S.C.A. Kluiters, J.W. Dijkstra, D. Jansen, R.W. van den Brink (2007). Sorption-enhanced hydrogen production for pre-combustion CO2 capture: Thermodynamic analysis and experimental results. Int. J. Greenhouse Gas Cntrl. 1, 170 – 179.

An ECN leaflet about the SEWGS process can be downloaded here (PDF, 750 kB).