The redox process for producing hydrogen from woody biomass

Available online at www.sciencedirect.com

International Journal of Hydrogen Energy 28 (2003) 491 – 498 www.elsevier.com/locate/ijhydene

The redox process for producing hydrogen from woody biomass R. Sime∗ , J. Kuehni, L. D’Souza, E. Elizondo, S. Biollaz Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

Abstract Hydrogen may be made from biomass fuel gas using a method of cyclic reduction and oxidation (REDOX) of iron oxides. The production of hydrogen using REDOX technology has been modelled. These studies showed that the hydrogen production e4ciency depends signi5cantly on the gasi5er fuel gas composition and the thermochemical properties of the REDOX material. A lab scale REDOX system was developed to provide experimental data. Good agreement between experimental and theoretical data was obtained. The inherent thermodynamic constraints of the REDOX process limit the maximum e4ciency. The REDOX hydrogen process is signi5cantly less e4cient and more costly than conventional hydrogen technology. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Biomass; PSA; Hydrogen; REDOX; Gasi5cation

1. Introduction Concerns over the sustainability and environmental impact of our current energy infrastructure have focussed attention on obtaining alternative strategies. One alternative being explored is to develop a hydrogen energy infrastructure. Today, most of the hydrogen that is produced is from fossil fuels and is primarily used for chemical applications. It is thought that future applications will also include hydrogen as an energy carrier. As fossil fuels are quickly being depleted, long-term sustainability will depend on producing hydrogen from non-fossil energy sources. In this work, biomass is used as a renewable hydrocarbon source to supply the fuel gas. REDOX technology was initially developed during the late 19th and early 20th century for the production of hydrogen and nitrogen from coal. The technology was based on cyclic reduction and oxidation of iron oxides. In the 5rst stage the metal oxide material was reduced using fuel gas (Fig. 1). The material was then re-oxidized in a second stage by either steam (for hydrogen production) or air (for nitrogen production) [1]. ∗

Corresponding author. Fax: +41-56-3102199. E-mail address: [email protected] (R. Sime).

Fig. 1. Basic REDOX process diagram.

The REDOX technology development was eventually abandoned as other technologies such as pressure swing adsorption (PSA) and cryogenic separation began to dominate. In recent years there has been a renewed interest in developing the REDOX technology [2,3]. Key requirements for commercial success are high e4ciency and low capital costs. Preferably the e4ciency would be greater or equal to the e4ciency obtained from standard hydrogen technologies such as PSA. Other modelling studies [4] have shown that PSA technologies in combination with advanced gasi5er technologies may obtain

0360-3199/03/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 2 ) 0 0 1 4 6 - 5

492

R. Sime et al. / International Journal of Hydrogen Energy 28 (2003) 491 – 498

50 – 60% hydrogen production e4ciency with installations up to 2000 MWth . 2. Modelling The REDOX process was modelled using an Excel spreadsheet. The model uses known gasi5cation data together with the metal oxide equilibrium properties to calculate the performance of the REDOX system. When the fuel gas from the gasi5er is passed through the bed, it is partially oxidised by the metal oxide material to form depleted fuel gas. The composition of the depleted fuel gas depends on the equilibrium properties of the metal oxide material. The depleted fuel gas is then fully combusted with the addition of air. The heat generated is used to produce steam for the second stage. In stage 2, steam is used to re-oxidize the metal oxide material. A gas stream containing a mixture of hydrogen and steam is produced. The composition of the steam-hydrogen gas depends on the equilibrium properties of the metal oxide material. After condensing the steam, pure hydrogen remains. The heat generated by cooling the steam-hydrogen gas stream may be used for fuel drying. Examples of the relevant chemical equilibrium are

Fig. 2. Experimental equipment. The REDOX bed is a 5xed bed electrically heated reactor.

Stage 1

Fe3 O4 + CO + H2 ←→FeO + CO2 + H2 O; Stage 2

FeO + H2 O ←→Fe3 O4 + H2 : For the modelling studies the following was assumed: • Su4cient metal oxide material is present to enable the REDOX reactions to proceed to equilibrium. This is achieved by operating with shallow cycles so that excess material is still available at the end of reduction. • The temperature of the fuel gas, REDOX bed and steam are maintained at 1073 K (800◦ C). • The pressure is approximately atmospheric. • Energy losses from the gasi5er are minimal (=5%). • The combustion boiler e4ciency is less than 80% LHV. • Energy recovered from cooling the steam hydrogen mixture will be used to dry the biomass. Providing su4cient energy for biomass drying becomes increasingly di4cult with increasing hydrogen production e4ciency. Preliminary ASPEN calculations have shown that recovering some of the latent heat may be required for e4cient systems. • The iron oxide material is stable for a large number of cycles. Studies have shown that iron oxide does degrade over days [2]. This degradation is not su4cient to dramatically aNect the e4ciency, provided that the material is replaced regularly. 3. Experimental An objective of this work was to maximize the conversion of fuel gas to hydrogen. Experimental equipment for

studying the REDOX hydrogen process was designed and constructed (Fig. 2). The equipment is suitable for measuring the conversion of fuel gas to hydrogen. Synthetic fuel gas is mixed with water in the evaporator where it is heated to 400◦ C. The humidi5ed fuel gas is then heated in the furnace to 800◦ C before passing through the bed material. After passing through the bed the fuel gas is depleted having been oxidized by the bed material. On cooling to 4◦ C the water condenses. The gas composition was measured using a Prima 600 mass spectrometer. Water Oow was measured using a Sartorius mass balance. Gas Oow was measured using a Bronkhorst gas Oow-meter. Gas composition, water Oow, gas Oow and temperature are monitored in real time using Labview software. The energy Oow within the REDOX component (Fig. 1) may be represented by Fig. 3. Energy is transferred between each stage of the cycle via two routes, oxygen transfer and steam production. The experimental set-up does not have a combustion heated boiler device for steam production. Instead, the steam is produced from electricity. To obtain a correct e4ciency value it is necessary to limit the amount of steam added during the second stage (Fig. 4). The limit is calculated from the theoretical amount (combustion boiler e4ciency ¡ 80% LHV) of steam that can be produced from the energy available in the depleted fuel gas. Electrical heating of the bed is necessary because the small reactor has a very high surface area per volume. On a larger system, only insulation would be required. The electricity consumed in heating the bed is not included in the e4ciency calculation. Furthermore, electrical heating

R. Sime et al. / International Journal of Hydrogen Energy 28 (2003) 491 – 498

493

Fig. 3. Energy Oow in the REDOX component.

Fig. 4. Energy Oow in the experimental set up.

ensures no heat losses. Therefore, the measured e4ciency data may be regarded as an upper limit. A typical experimental procedure is as follows: Iron oxide pellets (1–5 mm diameter, 300 g) are loaded into the

REDOX bed. The pellets were commercially made from Fe3 O4 powder using 10% CaCO3 as a binder. The furnace is electrically heated to 1073 K (800◦ C). In the 5rst stage, synthetic fuel gas mixed with steam is passed through the

494

R. Sime et al. / International Journal of Hydrogen Energy 28 (2003) 491 – 498

Composition of the depleted gas [%]

stage 1

flush

stage 2

flush

stage 1

flush

stage 2

100 90 80 N2 CO2 CO H2 CH4

70 60 50 40 30 20 10 0 37

47

57

67

77

Time [min]

Fig. 5. The exit gas composition during typical REDOX cycles. 850 840

Temperature [˚C]

830

Oxidation

Reduction

820 810 800 790 780 770 760 750 0

20

40

60

80

100

Time [min]

Fig. 6. The change in temperature for a typical experiment.

REDOX material (Fig. 5). In a second stage, steam mixed with nitrogen are passed through the bed material in the opposite direction to the fuel gas (counter current). The water spray nozzle used in this evaporator requires a gas Oow to operate correctly. Nitrogen is added to improve the spray distribution in the evaporator and the gas Oow rate measured. This increases the measurement accuracy. Nitrogen inclusion is particular to accurate data collection of the experimental set up and is not normally required by REDOX systems. Between each stage the system is Oushed with nitrogen to avoid excessive mixing of the stages. In a real system the streams would be separated (Fig. 1). The temperature changes during the cycle (Fig. 6) are due to endothermic reactions in the 5rst stage followed by exothermic reactions in the second stage. These temperature changes have a slight negative eNect on the e4ciency. However, they are unavoidable in real systems, hence the measured e4ciency is closer to reality.

If starting from either pure magnetite (Fe3 O4 ) or wuestite (Fex O) there will be an initial imbalance between the mass of oxygen transferred to and from the material. Only after mass balance has occurred can real e4ciency data be obtained. It normally takes a number of cycles for the process to stabilize. In Fig. 7, cycles 4 and 5 have achieved oxygen balance. The measurement of the oxygen transferred (Fig. 7) to and from the bed requires an accurate knowledge of the gas stream properties in and out of the bed. The gas Oow and composition coming out of the bed are measured (see previous text for the method). The input gas composition is known before the start of the experiment and is kept constant. The input gas Oow rate is calculated based on an assumption of carbon species balance. Number of moles (CO + CO2 + CH4 )in = Number of moles (CO + CO2 + CH4 )out :

R. Sime et al. / International Journal of Hydrogen Energy 28 (2003) 491 – 498

495

Magnetite 328 326

Mass change [g]

324 322 320 318 316 314 312 Wuestite 310 0

1

2

3

4

5

Cycle number

Fig. 7. Change in mass during the REDOX cycle (steam is not in excess). This value is calculated from the experimental oxygen balance.

This assumption only holds true if there is no carbon species deposition in the bed. To check the validity of this assumption a number of premixed fuel gases containing nitrogen were passed through the bed. The ratio of carbon/nitrogen was checked over a range of conditions. No measurable carbon species deposition occurred. The input steam Oow rate is known from the measurement of water consumption using a mass balance. The steam Oow rate out of the bed is calculated using hydrogen species balance. Number of moles (H2 + H2 O + 2 × CH4 )in =Number of moles (H2 + H2 O + 2 × CH4 )out : The primary task of the equipment is to measure the fuel gas to hydrogen conversion e4ciency. The fuel gas to hydrogen conversion e4ciency is calculated as follows: EC =

Hydrogen HHV × moles hydrogen × 100% : Fuel gas HHV × moles fuel gas

Note: the overall e4ciency of biomass to hydrogen is a calculated value based on known gasi5cation e4ciencies and measured REDOX e4ciencies. It has not been measured by direct experiments.

4. Results and discussion The gasi5er fuel gas quality is an important issue for the REDOX technology development. The eNect of the fuel gas composition has been modelled. The modelling showed that the potential hydrogen production e4ciency is strongly dependent on the fully oxidized-components/fuel-components ratio of the fuel gas ((CO2 + H2 O)=(CO + H2 ) = O=F). For conditions where steam availability is not limiting, the chemical conversion relates to the diNerence between the initial and 5nal O/F ratio of the fuel gas.

The initial O/F ratio is determined by the gasi5er and the biomass feedstock. The 5nal O/F ratio is determined by the thermochemical properties of the metal oxide material. The REDOX reactions are reversible. Therefore, when the fuel gas passes through the bed, fuel gas oxidation proceeds to an equilibrium composition (i.e. not all of the fuel gas is oxidized). Ideally, the diNerence between the initial and 5nal O/F ratios should be as large as possible. In reality, the availability of steam for the re-oxidation of the metal oxide is limiting for conditions where the diNerence in the O/F ratios are large. Not enough energy remains in the depleted fuel gas for steam production. The eNect of the fuel gas CO2 =CO ratio on the chemical conversion was measured over a range of CO–CO2 mixtures (Fig. 8). Note, gas mixtures containing just CO2 and CO are technically simpler to study; there is only one equilibrium, no reforming and no shift conversion reaction. Good agreement was obtained between experimental results and modelled predictions. The modelled line assumes an average metal oxide equilibrium CO2 =CO ratio of 3.2 (Table 1). Another important consideration is the moisture content of the biomass. A high moisture content results in a poor fuel gas, high in combustion gases, (CO2 and H2 O) and low in fuel gases (H2 and CO). To experimentally demonstrate this problem, standard fuel gas was mixed with a range of steam concentrations. This had a detrimental eNect (Fig. 9) of fuel gas to hydrogen conversion e4ciency. The option to cool the fuel gas, condense the water and then reheat (to 1073 K) lessens the eNect of fuel gas steam content, but it also involves e4ciency loss. The requirement to dry the biomass prior to gasi5cation puts REDOX technology at a disadvantage relative to PSA technology. The PSA systems require the presence of steam for reforming and shift conversion. Hence, moisture in the biomass has only a minimal impact for PSA technology. During these studies, two synthetic fuel gases were used to represent typical fuel gases. The 5rst was representative

496

R. Sime et al. / International Journal of Hydrogen Energy 28 (2003) 491 – 498

Fuel gas to hydrogen conversion efficiency (%, HHV)

80 Experimental Modelled Linear (Experimental) Linear (Modelled)

70 60 50 40 30 20 10 0

0

0.5

1

1.5

2

2.5

3

CO2/CO ratio

Fig. 8. ENect of the CO2 =CO ratio on the conversion of fuel gas to hydrogen. Table 1 Measured C/F data Initial CO2 =CO

Final CO2 =CO

0.67

3.26 3.17 3.12 3.08 3.31 3.24 3.24 3.19 xP = 3:20

1.24 1.90 2.50

of fuel gas from standard air gasi5cation. The mole fraction dry composition used was N2 0.639, H2 0.07, CO 0.12, CO2 0.151 and CH4 0.02. The second was based on the indirect gasi5cation concept of Vienna University named FICFB (Fast Internally Circulating Fluidised Bed) [5]. Indirect gasi5ers use a separate combustor to provide heat for the endothermic gasi5cation reactions. The mole fraction dry composition used was H2 0.378, CO 0.351, CO2 0.162 and CH4 0.109. Both fuel gases are mixed with a suitable amount of steam before passing through the REDOX bed. The conversion e4ciency for the standard fuel gas (10% H2 Og ) was 30%. The indirect gasi5er fuel gas (16% H2 Og ) was higher with 48% e4ciency. Both e4ciencies could be improved if the methane in the fuel gas were better used. Currently, most (¿ 95%) of the methane entering the bed passes through unreformed. Increasing the reforming capability can be accomplished either by pre-reforming using a commercial catalyst. A way to increase the diNerence between the initial and 5nal O/F ratios is to increase the 5nal O/F ratio by modifying the metal oxide material. This may be achieved by using

mixed metal oxides. A high metal oxide O/F ratio would allow strong depletion of the gasi5er gas by changing the 5nal ratio of gases. However, for the reverse reaction this means that only a small fraction of the steam will be converted to hydrogen. A goal therefore must be to 5nd the optimum O/F ratio. The modelling predicts that under ideal conditions an overall biomass to hydrogen e4ciency of 45% should be considered an upper limit. The option of using FICFB gasi5cation and PSA technology was modelled using ASPEN. The calculated e4ciency was 60% and compared well with other studies [4]. Economic analyses of two technology options were considered, one using REDOX and the other PSA. Both options used FICFB indirect gasi5cation [5]. The economic calculations assume: • • • • • • • • •

Biomass cost 2 US$/GJ. Labour cost 32 US$/h. Interest 10%. Loan period 20 years. Electricity US$0:05=kWh. Operating capacity 90%. Contingency 10% of equipment costs. Buildings and structures 10% of equipment costs. Engineering 15% of equipment costs.

In the World there are no large REDOX vessels to base the capital cost on. Therefore, the costs are based on the relative cost per volume of a Ouidised bed gasi5er (Both systems operate at a similar temperature). Hydrogen production costs from REDOX and PSA are shown in Fig. 10. Data from other studies using BCL indirect gasi5cation [4] have been included for comparison. For REDOX, three scenarios have been considered. In the 5rst scenario (REDOX-100%), the capital cost of the REDOX component was calculated assuming an energy density of 0:35 MWth =m3 (total thermal power in the fuel gas/total

R. Sime et al. / International Journal of Hydrogen Energy 28 (2003) 491 – 498

497

Fuel gas to hydrogen conversion efficiency (%, HHV)

40 35 30 25 20 15 10 5 0 0

5

10

15

20

25

Steam content of fuel gas %

Fig. 9. The eNect of fuel gas steam content on the conversion e4ciency. The mole fraction dry composition of the fuel gas was N2 0.639, H2 0.07, CO 0.12, CO2 0.151 and CH4 0.02.

Hydrogen production cost (US$/GJ)

90 FICFB-PSA BCL-PSA FICFB-REDOX 100% FICFB-REDOX 50% FICFB-REDOX 10%

80 70 60 50 40 30 20 10 0

10

100

1000

10000

Biomass input (MWth HHV)

Fig. 10. Hydrogen production cost from REDOX and PSA.

volume of the REDOX beds) given by Hacker et al. [6]. This results in a very high hydrogen production cost. The second scenario (REDOX 50%) shows the eNect of halving the cost of the REDOX component. In the 5nal scenario (REDOX 10%) the cost of the REDOX component is reduced to 10%. Even using this extreme scenario the hydrogen production costs are still at least 60% higher than the PSA options. Using standard PSA technology, the equipment needed to convert fuel gas to hydrogen makes a relatively modest contribution to the total investment [4]. For REDOX, the capital costs are much higher because of the low energy density in the REDOX bed.

by the fuel gas O/F ratio. REDOX hydrogen technology faces signi5cant technical and economic challenges. The ef5ciency is low and will remain low due to thermodynamic constraints. REDOX reactors will require exceptionally high capital cost.

5. Conclusion

References

Good agreement between modelling and experimental was obtained. The e4ciency was shown to be inOuenced

[1] Messerschmitt A. Hydrogen and nitrogen or a mixture of the two gases. Ger 1916; 291603.

Acknowledgements The authors would like to thank the following people: Samuel Stucki, Thomas Marti, Peter Hottinger and Peter Binkert. Financial support by the Swiss O4ce of Energy is gratefully acknowledged.

498

R. Sime et al. / International Journal of Hydrogen Energy 28 (2003) 491 – 498

[2] Hacker V, Faleschini G, Fuchs H, Fankhauser R, Simader G, Ghaemi M, Spreitz B, Friedrich K. Usage of biomass gas for fuel cells by the SIR process. Power Sources 1998;71:226–30. [3] Kodama T, Watanabe Y, Miuro S, Sato M, Kitayama Y. Reactive and selective redox systems of Ni (II) Ferrite for a two-step CO and H2 cycle from carbon and water. Energy 1996;21(12):1147–56. [4] Faaij A, Hamelinck C, Larson E, Kreutz T. Production of methanol and hydrogen from biomass via advanced conversion

concepts—preliminary results. Biomass for Energy and Industry, First World Conference and Technology Exhibition, 2000. [5] Hofbauer H, Stoiber H, Veronik G. Gasi5cation of organic material in a novel Ouidization bed system. Proceedings of the First SCEJ Symposium on Fluidization, Tokyo, 1995. p. 291–9. [6] Hacker V, Frankhauser R, Faleschini G, Fuchs H, Friedrich K, Muhr M, Kordesch K. Hydrogen production by steam–iron process. Power Sources 2000;86:531–5.