546756 Multifunctional Catalytic Materials with CO2 Capture and Oxygen Transfer Functionalities for High-Purity H2 Production Via Sorption Enhanced Chemical Looping Steam Methane Reforming

Wednesday, June 5, 2019
Texas Ballroom Prefunction Area (Grand Hyatt San Antonio)
Andy Antzara1, Eleni Heracleous2 and Angeliki Lemonidou1, (1)Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece, (2)School of Science & Technology, International Hellenic University (IHU), Thessaloniki, Greece

Introduction

Decreasing availability of fossil resources, together with the environmental issues related to the fossil-derived greenhouse gas emissions, have dictated the necessity for further improvements in the efficiency of energy production processes as well as the transition towards sustainable resources. In this context, hydrogen is heavily supported as an energy carrier, as it is expected to play a key role in the necessary transition from fossil fuels to a sustainable energy future. Likewise, hydrogen demands as a primary industrial gas are continuously increasing in the petroleum refining, mainly driven by clean fuel regulations. Hydrogen however, is produced almost entirely from fossil fuels. Steam reforming of methane (SMR), the dominant large–scale technology for hydrogen production, accounts for almost 50% of the worldwide production. It is evident that a novel technology for large–scale hydrogen production is required.

At this direction, Sorption enhanced chemical looping steam methane reforming (SE-CL-SMR) is considered as an alternative method for the efficient production of pure hydrogen, combining chemical looping steam methane reforming with in-situ CO2 capture [1,2]. In this process, the reformer contains a CO2 sorbent and an oxygen transfer material (OTM). In the first step of this cyclic process, the oxide is reduced by CH4 and serves as the reforming catalyst. The reaction proceeds under near autothermal conditions due to the heat released by the strongly exothermic carbonation. In a second step, the saturated sorbent is regenerated, with the energy provided by the exothermic OTM re-oxidation [3-5]. Aiming at reducing the complexity and cost of the proposed process, we have developed a hybrid multi-functional catalytic material that combines good oxygen transfer characteristics, significant reforming activity and stable CO2 sorption capacity in a single pellet. Herein, we demonstrate experimentally the feasibility of the SE-CL-SMR process over the NiO/CaO-based hybrid material. The obtained results are compared to the performance of a mechanical mixture of a NiO/ZrO2 OTM with a CaO/ZrO2 CO2 sorbent with the same NiO/CaO composition under identical experimental conditions.

 

Experimental

Material synthesis: The NiO-based OTM was prepared via wet impregnation using a commercial ZrO2 support with 40 wt% NiO loading [6]. The CaO-based CO2­ sorbent was prepared via a sol-gel auto-combustion route, using citric acid as a combustion agent and ZrO2 as structural promoter, with CaO concentration of 66 wt% [7]. The hybrid material was prepared with the same method used for the sorbent synthesis, with ZrO2 as structural promoter and CaO and NiO concentration of 40 wt% and 25 wt% respectively. 

SE-CL-SMR experiments: The SE-CL-SMR experiments were conducted over the hybrid material, as well as over a mechanical mixture of NiO/ZrO2 with CaO/ZrO2 at fixed NiO/CaO ratio. For these tests, the materials were initially exposed to CH4/steam at 650°C, S/C ratio 3 and GHSV 400 h-1 until saturation in CO2. After saturation, the feed was switched to air for reoxidation of Ni and regeneration of CaCO3 at 800°C.

 

Results and discussion

The composition of the reformate gas on dry basis during the reforming stage for the hybrid material is shown in Figure 1a. During the pre-breakthrough period, where NiO reduction, reforming and water gas shift reactions are carried out simultaneously with in-situ CO2 capture, very high H2 concentration was observed (~95%). Similarly high H2 purity and pre-breakthrough duration were recorded for the OTM/sorbent mechanical mixture (not shown for brevity). Pre-breakthrough period was followed by a transitional period, where the sorbent was gradually saturated (breakthrough period) leading to an increase of CO, CO2 and CH4 and decrease of H2 concentration. Although complete CH4 conversion is reached from the beginning of the reforming stage, H2 yield is relatively low (84%) during the first ~10min, indicating that during this short period part of CH4 is consumed for NiO reduction. After complete NiO reduction, the reactor runs under conditions of simple SE-SMR process with higher H2 yield (90%), approaching the thermodynamic equilibrium of SE-SMR (Figure 1b). Even after CaO saturation, the reformate gas obtained in the exit of the reactor has composition that approaches that of conventional reforming (post-breakthrough), indicating that the reforming activity of Ni in the hybrid material remains unchanged even after the decrease of surface area due to CaCO3 formation.

After switching to air flow and for almost 20 minutes no additional external heating was applied to the reactor, since the heat generated by Ni reoxidation was more than enough to sharply increase the temperature of the solid, while meeting more than 33% of the heat requirements for the strongly endothermic sorbent regeneration reaction. The use of the hybrid material allowed achieving higher CaCO3 decomposition without external heating compared to the mechanical mixture for the same NiO/CaO molar ratio (33% vs ~27% – Figure 1c) even though the maximum temperature reached in the reactor during Ni oxidation was lower (~780°C vs 800°C) (Figure 1d). This can be attributed to the direct transfer of heat due to close contact between the two solid phases in the hybrid material, eliminating the conduction losses that exist in the case of the mechanical mixture. It is important to highlight that the use of the hybrid material reduced the volume of the bed by ~25% compared to the OTM/sorbent mechanical mixture. In addition, the hybrid material exhibited excellent stability without deterioration in its performance for 10 consecutive SE–CL–SMR cycles

Figure 1. Product concentration (a), CH4 conversion and H2 yield (b) as a function of time during the reforming stage  and CO2 desorption (c) and temperatures profiles (d1–d3) as a function of time during the regeneration stage  (Reforming stage: T=650°C, S/C=3, NiO/CaO=0.5, GHSV=400 h-1, Regeneration stage: T=800°C, air flow)

 

Post-reaction BET measurements of the material after 10 reforming/regeneration cycles showed that the surface area of the spent hybrid material after the reforming carbonation stage of the 10th cycle (with NiO and CaO in their reduced and carbonated form respectively) decreased significantly compare to those of the fresh materials due to formation of CaCO3 (from 12.8 to 5.7 m2/g). Compared however to the pure CO2 sorbent at similar state (Table 1), the hybrid material maintains higher surface area for similar carbonation conversion of CaO, indicating that Ni in the hybrid materials serves also as CaO binder. This is related to the high capturing performance of the hybrid material during SE–CL–SMR, similar to that of the OTM/sorbent mechanical mixture. After complete regeneration, the surface area of the materials is restored to even higher value (16.9 m2/g) than that of the fresh material. This could be ascribed to gradual restructuring of the materials due to the consecutive capture and evolution of CO2 with cycles.

Table 1: BET surface area and pore volume of CaO/NiO–based hybrid material and CaO–based CO2 sorbent before and after SE–CL–SMR experiments

  

CO2 Sorbent

Surface area  (m2/g)

Pore volume  (m2/g)

66 wt.% CaO/CaZrO3

 

 

fresh–oxidized/calcined

15.3

0.05

used–reduced/carbonated

3.9

0.02

used–oxidized/calcined

14.0

0.04

Hybrid OTM/CO2 Sorbent

 

 

40 wt.% CaO–25 wt.% NiO/CaZrO3

 

 

fresh–oxidized/calcined

12.8

0.06

used–reduced/carbonated

5.7

0.31

used–oxidized/calcined

16.9

0.14

Conclusions

The synthesis and use of a hybrid material with combined functionalities in the SE-CL-SMR reaction demonstrated the feasibility and high potential of the proposed process. The developed hybrid catalyst demonstrated very high catalytic activity and sorption capacity, resulting in the production of high purity H2 (~95%), comparable to that obtained over a mechanical mixture of OTM and CO2 sorbent. During the initial stage of regeneration, the heat generated by Ni reoxidation was adequate to sharply increase the temperature of the solid, causing the decomposition of a substantial part of the saturated CaCO3 without providing external heating. The use of the multi-functional hybrid material significantly reduced the volume of the catalytic bed by at least 25% compared to the mechanical mixture while obtaining similar results. The results clearly demonstrate that the use of a hybrid material in the SE-CL-SMR process decreases the operating costs and renders the process more attractive for commercialization.

 

References

[1]  V. Dupont, Á.B. Ross, I. Hanley, M. V. Twigg, Int. J. Hydrogen Energy 2007, 32, 67–79.

[2]  A. Antzara, E. Heracleous, E., D.B. Bukur, A.A. Lemonidou, Int. J. Greenh. Gas Control. 2015, 32, 115–128.

[3]  A. Antzara, E. Heracleous, A.A. Lemonidou, Appl. Energy 2016, 180, 457–471.

[4]  J. Fermoso, M.V. Gil, F. Rubiera, D. Chen, ChemSusChem. 2014, 7, 3063–3077.

[5]  M. Rydén, P. Ramos, Fuel Process. Technol. 2012, 96, 27–36.

[6]  L. Silvester, A. Antzara, G. Boskovic, E. Heracleous, A.A. Lemonidou, D.B. Bukur, Int. J. Hydrogen Energy. 2015, 40, 7490–7501.

[7]  A. Antzara, E. Heracleous, A.A. Lemonidou, Applied Energy. 2015, 156, 331–343.

 


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