1 THE PRODUCTION OF HYDROGEN AND THE CAPTURE OF CARBON DIOXIDE USING CHEMICAL LOOPING Jason Cleeton 1, Chris Bohn 2, Christoph Müller 1,2, Stuart Scott 1, John Dennis 2 1 Department of Engineering University of Cambridge Trumpington Street Cambridge CB2 1PZ 2 Department of Chemical Engineering University of Cambridge Pembroke Street Cambridge CB2 3RA

2 OVERVIEW H 2 PRODUCTION AND CO 2 CAPTURE VIA REDUCTION AND OXIDATION OF IRON OXIDES BACKGROUND: Phase equilibria and reaction chemistry Thermodynamics and estimation of exergetic efficiency Trade-off between heat output and H 2 output EXPERIMENTAL: CO 2 capture H 2 production Effect of temperature Effect of transition Carbon contamination

3 PHASE EQUILIBRIA Temperature (K) K p = p CO 2 /p CO = p H 2 O /p H 2 ·K W K p = p H 2 O /p H 2 Fe 3 O 4 Fe 2 O 3 FeO Fe Triple Point, 848 K

4 REACTION CHEMISTRY Introduction to the reduction and oxidation reactions Overall, heat is generated Reduction Oxidation Fe 2 O 3 Fe 3 O 4 FeO H2OH2O COCO 2 COCO 2 Fe FeO Fe 3 O 4 airN2N2 Thermally neutral Exothermic H2H2 H2H2 ΔH < 0

5 Coal RED1RED2 OX K GASIFIER K HX Fe 2 O 3 Fe 3 O 4 FeO WaterSat. steam H 2 /H 2 O to cooling/ condensing Syngas Air N 2 to cooling CO 2 /H 2 O to cooling/ condensing Turb 1 Turb 2 Comp 1 Pump 1 Fuel stream H 2 /H 2 O stream OC stream Air stream (3) FLOW SHEET OX1 Biomass

6 DEGREES OF FREEDOM Each reactor is modelled using Gibbs free energy minimisation. Oxidation with air releases most of the heat. This reactor must be hotter than the gasifier, which fixes its temperature. Other reactors aim to run adiabatically (simplify heat integration). Supply a stoichiometric amount of air to complete the re-oxidation to Fe 2 O 3. Control variables are the recycle rate of iron around the loop and the amount of steam added to the reactor producing the H 2. A basis of 1kg/s of coal is assumed. Aiming for a system which Does not require heating utility Produces pure H 2 (avoids carbon deposition during reduction) Maximises the production of H 2 or exergetic efficiency

7 Coal RED1RED2 OX K GASIFIER K HX Fe 2 O 3 Fe 3 O 4 FeO WaterSat. steam H 2 /H 2 O to cooling/ condensing Syngas Air N 2 to cooling CO 2 /H 2 O to cooling/ condensing Turb 1 Turb 2 Comp 1 Pump 1 Fuel stream H 2 /H 2 O stream OC stream Air stream (3) FLOW SHEET OX1 Biomass

8 HEAT INTEGRATION RED = HOT COMPOSITE BLUE = COLD COMPOSITE Gasifier Steam supply Oxidation reactor Product cooling Cooling + condensation Heat Released (MW) Temperature of Heat Curve (K) ΔTΔT

9 OPERATING REGIME Molar Flowrate of Steam into OX1 (mol/s)Oxygen Carrier Recycle Rate (mol Fe/s) + Heat Heat Integrated Incomplete Syngas Conversion

10 EXERGY Molar Flowrate of Steam into OX1 (mol/s) Oxygen Carrier Recycle Rate (mol Fe/s) Definition of Exergy: Condition : Efficiency 1 atm: 48% 1 atm + steam cycle: 54% 10 atm + steam cycle: 58%

11 THERMODYNAMIC SUMMARY The reaction equilibria of iron allow the production of H 2 via a cyclic process. Theoretical exergetic efficiencies are competitive with steam reforming. There is a trade off between producing heat and hydrogen, which can be controlled by how much of the re-oxidation is done by air vs. steam. Two well-mixed reduction reactors were modelled to enable to complete oxidation of CO to CO 2. If this is to be achieved in a single reactor, spatial gradients in concentration are needed.

12 PACKED BED: IDEALIZED CASE Separation of oxides due to concentration gradient Gasifier Fe 2 O 3 (CO 2, Steam, Air) Solid fuel (e.g. wood) (CO, H 2, N 2, tars) Fe 3 O 4 Distance along reactor FeO CO 2 for capture Moving front Schematic, Concentration Fe 3 O 4 + CO ↔ 3 FeO + CO 2 CO CO 2 K p1 K p2

13 EXPERIMENTAL SET-UP Packed bed of iron oxide particles Sand plug Gas outlet to condenser Gas inlet: (CO+CO 2 +N 2 ), N 2 or air H 2 O (l) inlet Thermocouple type K Chamber heated by tape Perforated plate Wire mesh GASES: N 2 CO 2 10 vol. % CO + N 2 Air FLOW: 1 – 2 L/min MEASUREMENT: H 2 (0-30 vol. %) CO 2, CO (0-20 vol. %) CO ( ppm vol.) NDIR Analysers

14 EXPERIMENTAL: LONG BED Complete conversion of entering CO to CO 2 Very high purity H 2 at outlet after sufficiently long purge purgeCO, CO 2 Mole fraction [%] Time [s] Concentration [ppm] H2H2 CO 2 CO CO 2 Production H 2 Production steampurge N 2 Purge

15 EXPERIMENTAL: SHORT BED Slower kinetics for transition from Fe 3 O 4 – FeO Time [s] Mole fraction in effluent [Mole %] CO 2 CO H2H2 K p = p CO 2 /p CO = Total conversion of CO Drop to K P for Fe 3 O 4 – FeO transition Kink in effluent gas curve at K p Reduced rate for Fe 3 O 4 → FeO compared to Fe 2 O 3 → Fe 3 O 4

16 MOVING FRONTS SHARPNESS OF FRONTS: A.Time Constant Fe 3 O 4 to FeO A.Time Constant Fe 2 O 3 to Fe 3 O 4 B.Revised Schematic L, length of bed; v, interstitial velocity at given conditions Fe 2 O 3 Fe 3 O 4 FeO Gradual TransitionSharp Front

17 Fe 2 O 3 -Fe High initial production of H 2, exponential decrease Decrease over entire temperature range Cycle Number H 2 [µmol] 900°C 600°C

18 Fe 2 O 3 -FeO Lower initial production of H 2, but marginal decrease H 2 [µmol] Cycle Number 900°C 750°C 600°C

19 EFFECT OF OXIDATION IN AIR Reoxidation of Fe 3 O 4 to Fe 2 O 3 produces 50°C temperature rise No thermal sintering or adverse effect of oxidation Cycle Number H 2 [µmol] Fe 2 O 3 – FeO Fe 2 O 3 - Fe Fe 3 O 4 – FeO Fe 3 O 4 - Fe

20 CONTAMINATION 600°C, possibility of Boudouard reaction: 2CO ↔ C (s) + CO 2 Deposited C reoxidised in steam, air Little deposition if FeO lowest oxide Time [s] H2H2 [CO] ≈ 4600 ppm vol. [CO 2 ] ≈ 1500 ppm vol. Mole fraction in effluent gas, dry basis [Mole %] Fe - Fe 2 O 3 FeO - Fe 2 O 3 Cycle 2 Time [s] CO 2 mole fraction [Mole %]

21 CONCLUSION PROCESS CHARACTERISTICS Exergetically competitive. Easy trade-off between heat and H 2. Full heat integration possible; no external heating utility requirements. EXPERIMENTAL CO 2 suitable for carbon capture High purity H 2 Several cycles possible with FeO Additional oxidation with air aides process by generating heat Carbon contamination not a problem for FeO

22 ACKNOWLEDGEMENTS EPSRC Gates Cambridge Trust