High-purity hydrogen via the sorption-enhanced steam methane reforming reaction over a synthetic CaO-based sorbent and a Ni catalyst

Transcription

1 High-purity hydrogen via the sorption-enhanced steam methane reforming reaction over a synthetic CaO-based sorbent and a Ni catalyst M. Broda a, V. Manovic b, Q. Imtiaz a, A. M. Kierzkowska a, E. J. Anthony c, C. R. Müller a a Laboratory of Energy Science and Engineering, ETH Zurich, Leonhardstrasse 27, 8092 Zurich, Switzerland b CanmetENERGY, Natural Resources Canada, 1 Haanel Drive, Ottawa K1A 1M1, Canada c School of Applied Science, Cranfield University, Bedfordshire MK43 0AL, England 2nd September 2013 Laboratory of Energy Science and Engineering

2 Introduction Global H 2 production (~50 Mt/yr, 2008) Global H 2 use P. Zakkour and G. Cook, CCS industry roadmap-high purity CO 2 sources: final draft sectoral assessment, Carbon Counts, UK,

3 Steam methane reforming accounts for ~50 % of the global hydrogen production Reforming reaction: CH 4 + H 2 O CO + 3H 2 Water-gas-shift reaction: CO + H 2 O CO 2 + H 2 H o 25 ºC= kj/mol H o 25 ºC= - 41 kj/mol Disadvantages: Highly endothermic High operating temperatures catalyst sintering and coke formation Overall process is complex and comprises several unit operations Further purification steps are required to produce high-purity hydrogen, e.g. preferential oxidation (PROX) M. Balat, Possible method for hydrogen production, energy spurces, part A: recovery, utilization, and environmental effect, 2008, 31, nd September 2013 Laboratory of Energy Science and Engineering 3

4 Sorption-enhanced steam methane reforming (SE-SMR) Reforming and shift reaction: CH 4 + H 2 O CO + 3H 2 CO + H 2 O CO 2 + H 2 H o 25 ºC = kj/mol H o 25 ºC = - 41 kj/mol CO 2 absorption reaction, e.g. carbonation of CaO: CO 2 + CaO CaCO 3 H o 25 ºC = -178 kj/mol Overall: CH H 2 O + CaO CaCO H 2 H o 25 ºC = -13 kj/mol Advantages: Reduced operating temperature ~ o C Elimination of the shift reactor and catalyst Reduction, or possibly even elimination, of subsequent purification steps 2nd September 2013 Laboratory of Energy Science and Engineering 4

5 Schematic diagram of the SE-SMR process H 2 Sweep gas CO 2 or H 2 O CaCO 3 (s) Heat Reforming reactor Regenerator CaO (s) CH 4, H 2 O CO 2 5

6 Material options for the SE-SMR reaction: 1. Limestone + reforming catalyst 2. Synthetic CaO-based sorbent + reforming catalyst 3. Bifunctional catalyst - sorbent Materials studied here: Ni-based catalyst (47 wt.% Ni) CaO-based sorbents Limestone Pellets (90 wt.% CaO,10 wt.% cement) 6

7 Synthesis of Ni-based catalyst (hydrotalcite-based) 5. Drying, 70 o C, 72 h 3. ph adjustment to 8.6 using HNO 3 4. Reflux at 80 o C for 16 h 6. Calcination (600 o C, 6 h) 2. Precipitation using NaOH and Na 2 CO 3 1. Aqueous solution of Mg 2+, Ni 2+ and Al 3+ Broda et al., ACS Catal., 2, ,

8 SE-SMR reaction + regeneration Mole fraction H 2, CO 2, CO, CH H 2 1. Pre-breakthrough period CO 2 Complete CH.. 4 conversion and high CH 4 _._. CO H 2 selectivity 3 Fast absorption of CO 2 via the 4 formation of CaCO 3 2. Breakthrough period Fairly sharp decrease in the mole fraction of H 2 and breakthrough of CO 2 3. Post-breakthrough period Time [s] 4. Calcination 8

9 Fixed-bed reactor Particle size: mm Reforming temperature: 550 o C Calcination temperature: 750 o C H 2 O/CH 4 ratio: 4 Broda et al., ACS Catal., 2, ,

10 (N 2, H 2 O - free basis) SE-SMR reaction (Ni-catalyst + pellets) Mole fraction H 2, CO st cycle _.._ 5 th cycle 10 th cycle Time [s] The time at which breakthrough occurs decreased from ~380 to ~280 s, but stabilized after the 5 th cycle. 10

11 (N 2, H 2 O - free basis) SE-SMR reaction (Ni-catalyst + limestone) Mole fraction H 2, CO st cycle _.._ 5 th cycle 10 th cycle Time [s] For limestone, the time at which breakthrough occurs decreased continuously from ~370 s in the 1 st cycle to 95 s in the 10 th cycle. 11

12 H 2 produced (mol) H 2 production pellets limestone Number of cycles After the 8 th cycle, Ni-catalyst + pellets show a stable pre-breakthrough production of H 2 Continuous deactivation of Ni-catalyst + limestone. 12

13 CaO conversion [mol CO 2 / mol CaO] CaO conversion pellets limestone Number of cycles After 10 cycles, the pellets showed a ~110 % higher CaO conversion than the reference limestone. 13

14 Characterisation of the materials Unreacted materials CaO-based pellets Limestone Ni-based catalyst BET surface area [m 2 /g] BJH pore volume [cm 3 /g] After 10 SE-SMR cycles BET surface area [m 2 /g] BJH pore volume [cm 3 /g] The BET surface area and BJH pore volume of limestone drastically decreased over 10 SE-SMR cycles. The Ni-based catalyst and the pellets possessed good thermal stability. 14

15 Structural changes with SE-SMR cycles CaO-based pellets (c) (e) Limestone Ni-based catalyst unreacted Ni particles (d) (f) cycled The initial morphology of the synthetic CO 2 sorbent and Ni-based catalyst did not change appreciably over 10 cycles. The cycled limestone lost its nano-structured morphology completely due to its intrinsic lack of a support. 15

16 Conclusions At a reaction temperature of 550 o C and using a steam-to-methane ratio of 4, equilibrium conversion of methane was achieved for both systems, resulting in the production of high-purity hydrogen (~99%). The pellets showed better performance in the SE-SMR reaction than limestone, demonstrated by a high cyclic CaO conversion, and in a higher quantity of H 2 produced during the pre-breakthrough period. The favourable performance of the pellets was attributed to their stable nano-structured morphology stabilized by homogeneously dispersed mayenite. The cycled limestone lost its nano-structured morphology completely over 10 SE-SMR cycles due to its intrinsic lack of a support component. 16

17 Acknowledgment Swiss National Science Foundation for financial support (Project _135457/1) Electron Microscopy Center of ETH Zurich (EMEZ) for providing access and training to electron microscopes. 17