High Efficient Hydrogen Production through Novel Gas Switching Water Splitting (GSWS) and Reforming (GSR) Using High Iron Content Oxygen Carrier

This study is primarily motivated by the scale-up challenges of pressurized chemical looping applications. One promising alternative reactor concept, among several proposed in the last decade focusing on configurations with no external solid circulation, is the “Gas Switching Technology (GST)”, which has been proposed for power [1] and hydrogen production with integrated CO₂ capture (Gas Switching Combustion “GSC” and Gas Switching Reforming “GSR” respectively [2, 3]). Unlike the conventional chemical looping, this novel technology utilizes a single fluidized bed reactor and avoids the circulation of solid oxygen carrier by alternating the feeds of the oxidizing and reducing gases to complete the different redox reactions involved in the process. With this arrangement, scale-up challenges can be greatly reduced. Experimental demonstration campaigns have proven ease of operation of this concept under atmospheric and pressurized conditions [2-4]. To capitalize on this success, this study extends the GST to the water splitting, based on the steam-iron process, for pure hydrogen production. The suitability of iron-based oxygen carrier to steam methane reforming in the GSR is also further explored in this study.

Gas Switching Water Splitting (GSWS) is a three-step (steam, air and fuel steps) process that utilizes the different oxidation states of iron-based oxygen carrier (Fe₂O₃, Fe₃O₄ and FeO/Fe) to efficiently produce hydrogen from natural gas with integrated CO₂ capture in a single process [5]. The redox cycle starts with the steam step where hydrogen is produced by the steam oxidation of FeO (or Fe) to Fe₃O₄. Subsequently, the air step follows where Fe₃O₄ is fully oxidized by air to Fe₂O₃ in an exothermic reaction that generates the heat needed for the next reduction step using CH₄. At this step, the oxygen carrier is regenerated bringing Fe₂O₃ to FeO (or Fe) with inherent CO₂ capture.

As for the Gas Switching Reforming (GSR) with iron based oxygen carrier, it has also been designed as a three-step process (reduction, reforming and oxidation step) operating in a cyclic mode by alternating fuel, a fuel-steam mixture and air feeds to the reactor. The redox cycle starts with the fuel step where dry CH₄ is fed to reduce the iron based oxygen carrier with significant yield of pure CO₂ ready for storage. The reforming step follows the reduction step, where steam is co-fed with CH₄ to produce syngas. The air step follows to re-oxidize the oxygen carrier and provide the heat required for the consecutive reduction and reforming reactions.

However, gas mixing between the stages takes place when switching between them, thereby reducing the CO₂ capture efficiency, CO₂ purity and H₂ purity. It is therefore important that the stages are run longer to minimize the extent of the mixing. To tackle this issue, oxygen carrier of high active Fe₂O₃ content (70wt%) has been used in this study. The oxygen carrier was produced through spray-drying and calcined at 1300°C. For Gas Switching Water Splitting (GSWS), . GSR substantially increases the CO₂ capture efficiency relative to GSWS, making gas mixing between stages less important. In addition, thermogravimetric analysis (TGA) has shown that the oxygen carrier demonstrates high cyclic stability under relevant reaction conditions.

The experiments were conducted under the GSWS and GSR conditions at atmospheric pressure and elevated temperature from 700°C to 900°C using a 1 kW fluidized bed reactor. Process performance at different conditions was quantified through online gas composition and temperature measurements. The results from the GSWS experiments have shown a low degree of fuel conversion in the reduction step. In addition, the oxygen carrier started to agglomerate after about 34% of reduction (to FeO), thus not allowing the process to utilize even half of the oxygen carrier capacity thereby hampering the gas separation performance of the GSWS process. However, better performance was achieved with GSR, with no particle agglomeration, as the small reduction degree of the oxygen carrier was enough to initiate syngas production when steam was co-fed with methane. As a result, the degree of carbon deposition was also less for GSR than GSWS. In summary, results show that the proposed oxygen carrier is more suitable for reforming (GSR) than water splitting (GSWS) using the novel gas switching reactor concept.

References

[1] A. Zaabout, S. Cloete, S. T. Johansen, M. v. S. Annaland, F. Gallucci, and S. Amini, "Experimental Demonstration of a Novel Gas Switching Combustion Reactor for Power Production with Integrated CO2 Capture," Industrial & Engineering Chemistry Research, vol. 52, no. 39, pp. 14241-14250, Oct 2 2013.

[2] S. A. Wassie, F. Gallucci, A. Zaabout, S. Cloete, S. Amini, and M. van Sint Annaland, "Hydrogen production with integrated CO2 capture in a novel gas switching reforming reactor: Proof-of-concept," International Journal of Hydrogen Energy, vol. 42, no. 21, pp. 14367-14379, 2017/05/25/ 2017.

[3] S. A. Wassie et al., "Hydrogen production with integrated CO2 capture in a membrane assisted gas switching reforming reactor: Proof-of-Concept," International Journal of Hydrogen Energy, vol. 43, no. 12, pp. 6177-6190, 2018/03/22/ 2018.

[4] A. Zaabout, S. Cloete, and S. Amini, "Autothermal operation of a pressurized Gas Switching Combustion with ilmenite ore," International Journal of Greenhouse Gas Control, vol. 63, pp. 175-183, 2017/08/01/ 2017.

[5] D. J. C. T. Sanfilippo, "One-step hydrogen through water splitting with intrinsic CO2 capture in chemical looping," vol. 272, pp. 58-68, 2016.