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Palladium membranes in solar steam reforming
Palladium membranes in solar steam reforming
10
A. Giaconia ENEA Research Centre, Rome, Italy
10.1
Introduction: what is steam reforming?
Palladium membranes can be exploited in several innovative hydrogen production routes. Besides solar-aided water splitting processes (electrolysis and thermochemical cycles), membranes can also be applied in solar reforming. Today more than 75% of industrial hydrogen production is obtained by reforming of hydrocarbons, typically natural gas and oil derivatives. The process most employed is steam reforming according to the following general reaction scheme: CnH2n+2 + 2nH2O → nCO2 + (3n + 1)H2 where CnH2n+2 is the hydrocarbon feedstock which can even be replaced by an oxygenated chemical (e.g. ethanol or glycerol) when the fossil fuel is replaced by a biomass-derived fuel. In the case of methane (obtained, for example, from natural gas or biogas) the following reactions take place: CH4 + H2O → CO + 3H2 (steam reforming reaction, ΔH = +206 kJ/mol) CO + H2O → CO2 + H2 (water-gas-shift (WGS) reaction, ΔH = −41 kJ/mol) Clearly, the process is highly heat demanding for steam generation (an excess of steam is required to prevent reactor fouling and guarantee stable operation, and also to drive the high-temperature steam reforming reactor that is usually operated at temperatures higher than 800°C). Hence, the process heat is supplied by the additional aid of fuel, but it can alternatively be supplied by an external carbon-free source, such as a concentrating solar power (CSP) plant.
10.2
The use of solar energy in steam reforming
The application of solar-thermal power from CSP plants to directly drive heat demanding thermochemical conversion is considered one of the most sensible ways to exploit solar energy. This approach not only reduces the carbon footprint of chemical conversion, but also allows the chemical storage of solar energy in the form of chemical energy of products. Moreover, in fuel refineries it is possible to improve the Palladium Membrane Technology for Hydrogen Production, Carbon Capture and Other Applications. http://dx.doi.org/10.1533/9781782422419.2.215 Copyright © 2015 A. Giaconia, Published by Elsevier Ltd. All rights reserved.
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Palladium Membrane Technology
heat value and the environmental impact (i.e. the overall “quality”) of the primary feedstock by the aid of “solar energy”. Different projects have been carried out so far dealing with hydrocarbon conversion to hydrogen powered by concentrated solar energy, including methane steam reforming, dry reforming and cracking. Particularly, the solar methane steam reforming process was successfully demonstrated in a solar receiver reactor (called a volumetric reformer) operating at high temperature (> 850°C).1,2
10.3
The use of palladium membranes in solar steam reforming
Palladium is the preferred metal for use in metallic membranes, because it is soluble to hydrogen over a wide range of temperatures and, due to its selectivity, produces high purity hydrogen. Palladium membranes can be classified into: • unsupported (dense) • metallic supported • ceramic (porous) supported.
Dense unsupported metallic membranes consist of rolled Pd (or Pd-Ag) foils, where the Pd (Pd-Ag) membrane is the only material that separates the two sides (permeate and retentate). In order to be mechanically and thermally stable, the thickness of the membrane is typically in the order of 50–200 μm. This fact has two severe drawbacks; on the one hand, the thicker the membrane is, the lower the flux rates that are achieved. On the other hand, thicker membranes lead to higher material costs, since palladium is an expensive noble metal. On the other hand, in supported membranes a thin Pd layer is used and the palladium cost becomes very small compared to the support/module cost. Therefore, unsupported membranes are currently not predominantly adopted in large scale applications, although self-supported thick tubes are actually a large commercial market for smaller scale H2 purification and production. In the metallic supported membranes, the thin hydrogen permeable palladium layer (usually < 5 μm) is coated on a metal support, either a dense support made of materials with high hydrogen permeability, or a porous metal support with low resistance for hydrogen transport. The function of the support is only to support the Pd layer which is deposited on the feed (retentate) side. One of the most commonly adopted techniques consists of porous stainless steel (PSS) supported palladium membrane. The main advantages that PSS offers are mechanical stability, and a similar support thermal expansion coefficient to Pd, which ensures good mechanical properties during temperature variations and easiness of assembling in a module. A non-metallic barrier layer is required between the Pd and the metal support in order to avoid Pd diffusion in the support material. The other kind of supported membranes, porous ceramic, are commercially available in the form of tubes or hollow fibres. Even though ceramic supported membranes
Palladium membranes in solar steam reforming
217
are more fragile than metallic ones, they offer benefits concerning the reduction of required palladium, since the selective layers are thinner thanks to the higher surface quality they provide. Moreover, ceramic supported membranes do not require an intermetallic layer. In general, selective metal-based membranes also allow recovery of high-grade hydrogen and increased conversion through steam reforming when the reactor is operated at relatively low operating temperatures (<650°C). The integration with membranes avoids dedicated hydrogen separation and purification units, to further improve the compactness and enhance the conversion, despite the thermodynamic limitations of low-temperature steam reforming. Hence, suitable membranes are being identified and developed for the on-site separation of hydrogen from a reaction mixture at 400–550°C, and the typical steam reforming environment comprising H2/CH4/CO2/CO and steam. In particular, the following hydrogen selective membrane types with a Pd-based layer deposited on a porous support can be considered: • Pd (alloy) supported on asymmetric PSS, with an intermetallic ceramic layer and a limited Pd layer thickness (< 5 μm) • Pd (and Pd-Ag) on porous ceramic supports prepared by electroless plating (EP) • Pd (and Pd-Ag) on porous ceramic supports by “two-layers” deposition, obtained by combination of EP with physical vapour deposition.
For comparison, the performance of developed membranes is benchmarked with selfsupported Pd-Ag membranes, consisting of a rolled Pd-Ag foil (thickness > 50 μm). All developed membranes are tested under the typical conditions of the membrane reactor (400–550°C, 1–10 bar). At the end of this activity, the most suitable and reliable membranes are selected to be tested in the membrane reactors.
10.4
Examples of solar steam reforming technology using palladium membranes
The application of high-temperature thermal storage in CSP plants leads to significant benefits, especially in the case of solar-powered chemical plants. Particularly, molten nitrate mixtures such as NaNO3/KNO3 (60/40 w/w) have been proposed as CSP heat storage media at temperatures up to 550–600°C.3,4 Indeed, this fluid reaches storage efficiencies higher than 99% and the possibility to provide 24 h/24 h solar heat at constant rate, mainly thanks to its low thermal conductivity and high heat capacity per unit volume. For this reason, this molten salt mixture is often called “solar salt,” and it is being positively used as a solar heat carrier and heat storage medium in the Solar-Two pilot tower plant in California, and other more recent commercial installations in Spain and Italy.4–6 Clearly, the utilization of such molten salts can also ensure constant-rate solar heat supply for an energy demanding industrial chemical process such as steam reforming,4 with the continuous solar energy supply to enhance the process management avoiding daily start-up and shut-down operations despite the intermittent primary source.
218
Palladium Membrane Technology
An innovative solar steam reformer powered with molten nitrates is being developed in the framework of the European project CoMETHy (Compact Multifuel-Energy to Hydrogen converter) co-funded by the European Union’s Seventh Framework Programme (FP7/2007–2013) under the Fuel Cells and Hydrogen Joint Technology Initiative (EC Grant Agreement No. 279075). In this case, the molten salts are used to transfer the heat collected from the CSP plant and other possible back up heat sources to the steam reforming plant. This enables operating the reformer at temperatures lower than 550°C, and selective Pd-based membranes can be applied to recover high-grade hydrogen and increase conversion despite the thermodynamic limitations. Compared to a typical steam reforming process, this steam reforming technology operates at lower temperatures, from typically 850–950°C down to 400–550°C, with a consequent significant gain in material costs since no special steel alloy for hightemperature operation is required. The high-temperature furnace is then replaced by a flameless heat exchanger, heated by a liquid molten salts stream, making the whole reactor envelope more compact. Additionally, by operating at lower temperatures, it is possible to combine steam reforming and water-gas-shift (WGS) reactions in a single stage at 400–550°C, resulting in a low outlet CO content (<<10 %vol) and reduction of the reformer heat duty. The integration with membranes avoids dedicated hydrogen separation and purification units, to further improve the compactness and to enhance the conversion despite the thermodynamic limitations of low-temperature reforming. When the thermochemical plant is powered by solar heat, in principle there will be no combustion in the whole process, and no CO2-containing flue gases emitted to the atmosphere: this will lead to a reduction of fuel consumption and greenhouse gas (GHG) emissions, being in the order of 40% to more than 50% less than the conventional route.4 The reduction of fossil fuel consumption will make the hydrogen production cost less subject to the fossil (e.g. NG) price in solar steam reforming; hence, a breakeven point can be predicted for the hydrogen production cost, resulting in a solar route economically more convenient than the fossil-based one.7 The same solar steam reforming process applied to biomass-derived fuels (i.e. biofuels like biogas, bioethanol, etc.) allows totally renewable hydrogen production. An additional advantage of the use of molten salts as heat transfer fluid for steam reforming is when some stand-by periods of the plant are foreseen, as is the case of small-medium scale reformers for hydrogen refuelling stations. In this case, the continuous recirculation of molten salts eases the overall process management by maintaining all plant components (e.g. catalyst, membrane) at working temperature (400–550°C) during stand-by periods: this minimizes start-up periods and materials ageing resulting from thermal cycling. Within the operating range (400–550°C, 1–10 bar), in general, composite membranes (with a few microns thick Pd-based layer deposited on a porous metal or ceramic support) or self-supported membranes (consisting of a rolled Pd-Ag foil with thickness > 50 μm) can be considered for hydrogen removal from the molten salts heated reactor.
Palladium membranes in solar steam reforming
219
As for the integration of the catalyst with the membrane, two options can be considered: • a multi-stage membrane reformer (MSMR), where the membrane is external to the reactor, and • an integrated membrane reactor (IMR), where the membrane is integrated with the catalyst and the heat exchanger.
The MSMR scheme consists of a number of reformers, similar to that shown in Figure 13.2, where thermodynamic equilibrium is approached, each one followed by a membrane separation module.4 Thus, a step-by-step increase of methane conversion is obtained in the reactors thanks to the hydrogen removal in the intermediate membrane units. A three-stage process scheme is reported in Figure 13.3. It is possible to demonstrate that the larger the number of reactor/membrane stages, the higher is the overall thermal efficiency of the process, as shown in Figure 13.4: indeed, in a single-stage process (i.e. a single-pass reactor like that represented in Figure 13.2) a large amount of steam should be generated to produce the hydrogen corresponding to thermodynamic equilibrium; on the other hand, in membrane reactors the same amount of steam allows larger hydrogen production rates resulting from the higher methane conversion.4 Therefore, the application of membrane reactors in solar steam reforming increases the complexity of the chemical plant, but significantly reduces the size and costs of the power plant (heat supplier) by improving overall thermal efficiency. This is an important point, considering that in solar-powered thermochemical processes the CSP plant is always a major cost item3,4 so the application of membranes will make the overall plant more competitive. The above considerations are effective in the case of an IMR too. Clearly, the IMR (with respect to the MSMR) is more compact and leads to higher efficiency (and feedstock conversion) on a single pass. However, the development of this reactor involves a considerable engineering challenge, with several design and mechanical issues, dealing with the catalyst/membrane coupling and the harmonization of the three key transport mechanisms: heat transfer from the molten salts to the catalyst bed, reaction kinetics and hydrogen permeance through the membrane.
References 1. S. Moeller, D. Kaucic and C. Sattler (2006), ASME Journal of Solar Energy Engineering, 128 16–23. 2. A. Berman, R.K. Karn and M. Epstein (2006), Energy & Fuels, 20 455–462. 3. A. Giaconia, R. Grena, M. Lanchi, R. Liberatore and P. Tarquini (2007), International Journal of Hydrogen Energy, 32 469–481. 4. A. Giaconia, M. De Falco, G. Caputo, R. Grena, P. Tarquini and L. Marrelli (2008), AIChE Journal, 54 1932–1944. 5. U. Herrmann, B. Kelly and H. Price (2004), Energy, 29 883–893.
220
Palladium Membrane Technology
6. U. Herrmann and D.W. Kearney (2002), ASME Journal of Solar Energy Engineering, 124 145–151. 7. S. Moeller, D. Kaucic and C. Sattler (2006), ASME Journal of Solar Energy Engineering, 128 16–23.
10
A. Giaconia ENEA Research Centre, Rome, Italy
10.1
Introduction: what is steam reforming?
Palladium membranes can be exploited in several innovative hydrogen production routes. Besides solar-aided water splitting processes (electrolysis and thermochemical cycles), membranes can also be applied in solar reforming. Today more than 75% of industrial hydrogen production is obtained by reforming of hydrocarbons, typically natural gas and oil derivatives. The process most employed is steam reforming according to the following general reaction scheme: CnH2n+2 + 2nH2O → nCO2 + (3n + 1)H2 where CnH2n+2 is the hydrocarbon feedstock which can even be replaced by an oxygenated chemical (e.g. ethanol or glycerol) when the fossil fuel is replaced by a biomass-derived fuel. In the case of methane (obtained, for example, from natural gas or biogas) the following reactions take place: CH4 + H2O → CO + 3H2 (steam reforming reaction, ΔH = +206 kJ/mol) CO + H2O → CO2 + H2 (water-gas-shift (WGS) reaction, ΔH = −41 kJ/mol) Clearly, the process is highly heat demanding for steam generation (an excess of steam is required to prevent reactor fouling and guarantee stable operation, and also to drive the high-temperature steam reforming reactor that is usually operated at temperatures higher than 800°C). Hence, the process heat is supplied by the additional aid of fuel, but it can alternatively be supplied by an external carbon-free source, such as a concentrating solar power (CSP) plant.
10.2
The use of solar energy in steam reforming
The application of solar-thermal power from CSP plants to directly drive heat demanding thermochemical conversion is considered one of the most sensible ways to exploit solar energy. This approach not only reduces the carbon footprint of chemical conversion, but also allows the chemical storage of solar energy in the form of chemical energy of products. Moreover, in fuel refineries it is possible to improve the Palladium Membrane Technology for Hydrogen Production, Carbon Capture and Other Applications. http://dx.doi.org/10.1533/9781782422419.2.215 Copyright © 2015 A. Giaconia, Published by Elsevier Ltd. All rights reserved.
216
Palladium Membrane Technology
heat value and the environmental impact (i.e. the overall “quality”) of the primary feedstock by the aid of “solar energy”. Different projects have been carried out so far dealing with hydrocarbon conversion to hydrogen powered by concentrated solar energy, including methane steam reforming, dry reforming and cracking. Particularly, the solar methane steam reforming process was successfully demonstrated in a solar receiver reactor (called a volumetric reformer) operating at high temperature (> 850°C).1,2
10.3
The use of palladium membranes in solar steam reforming
Palladium is the preferred metal for use in metallic membranes, because it is soluble to hydrogen over a wide range of temperatures and, due to its selectivity, produces high purity hydrogen. Palladium membranes can be classified into: • unsupported (dense) • metallic supported • ceramic (porous) supported.
Dense unsupported metallic membranes consist of rolled Pd (or Pd-Ag) foils, where the Pd (Pd-Ag) membrane is the only material that separates the two sides (permeate and retentate). In order to be mechanically and thermally stable, the thickness of the membrane is typically in the order of 50–200 μm. This fact has two severe drawbacks; on the one hand, the thicker the membrane is, the lower the flux rates that are achieved. On the other hand, thicker membranes lead to higher material costs, since palladium is an expensive noble metal. On the other hand, in supported membranes a thin Pd layer is used and the palladium cost becomes very small compared to the support/module cost. Therefore, unsupported membranes are currently not predominantly adopted in large scale applications, although self-supported thick tubes are actually a large commercial market for smaller scale H2 purification and production. In the metallic supported membranes, the thin hydrogen permeable palladium layer (usually < 5 μm) is coated on a metal support, either a dense support made of materials with high hydrogen permeability, or a porous metal support with low resistance for hydrogen transport. The function of the support is only to support the Pd layer which is deposited on the feed (retentate) side. One of the most commonly adopted techniques consists of porous stainless steel (PSS) supported palladium membrane. The main advantages that PSS offers are mechanical stability, and a similar support thermal expansion coefficient to Pd, which ensures good mechanical properties during temperature variations and easiness of assembling in a module. A non-metallic barrier layer is required between the Pd and the metal support in order to avoid Pd diffusion in the support material. The other kind of supported membranes, porous ceramic, are commercially available in the form of tubes or hollow fibres. Even though ceramic supported membranes
Palladium membranes in solar steam reforming
217
are more fragile than metallic ones, they offer benefits concerning the reduction of required palladium, since the selective layers are thinner thanks to the higher surface quality they provide. Moreover, ceramic supported membranes do not require an intermetallic layer. In general, selective metal-based membranes also allow recovery of high-grade hydrogen and increased conversion through steam reforming when the reactor is operated at relatively low operating temperatures (<650°C). The integration with membranes avoids dedicated hydrogen separation and purification units, to further improve the compactness and enhance the conversion, despite the thermodynamic limitations of low-temperature steam reforming. Hence, suitable membranes are being identified and developed for the on-site separation of hydrogen from a reaction mixture at 400–550°C, and the typical steam reforming environment comprising H2/CH4/CO2/CO and steam. In particular, the following hydrogen selective membrane types with a Pd-based layer deposited on a porous support can be considered: • Pd (alloy) supported on asymmetric PSS, with an intermetallic ceramic layer and a limited Pd layer thickness (< 5 μm) • Pd (and Pd-Ag) on porous ceramic supports prepared by electroless plating (EP) • Pd (and Pd-Ag) on porous ceramic supports by “two-layers” deposition, obtained by combination of EP with physical vapour deposition.
For comparison, the performance of developed membranes is benchmarked with selfsupported Pd-Ag membranes, consisting of a rolled Pd-Ag foil (thickness > 50 μm). All developed membranes are tested under the typical conditions of the membrane reactor (400–550°C, 1–10 bar). At the end of this activity, the most suitable and reliable membranes are selected to be tested in the membrane reactors.
10.4
Examples of solar steam reforming technology using palladium membranes
The application of high-temperature thermal storage in CSP plants leads to significant benefits, especially in the case of solar-powered chemical plants. Particularly, molten nitrate mixtures such as NaNO3/KNO3 (60/40 w/w) have been proposed as CSP heat storage media at temperatures up to 550–600°C.3,4 Indeed, this fluid reaches storage efficiencies higher than 99% and the possibility to provide 24 h/24 h solar heat at constant rate, mainly thanks to its low thermal conductivity and high heat capacity per unit volume. For this reason, this molten salt mixture is often called “solar salt,” and it is being positively used as a solar heat carrier and heat storage medium in the Solar-Two pilot tower plant in California, and other more recent commercial installations in Spain and Italy.4–6 Clearly, the utilization of such molten salts can also ensure constant-rate solar heat supply for an energy demanding industrial chemical process such as steam reforming,4 with the continuous solar energy supply to enhance the process management avoiding daily start-up and shut-down operations despite the intermittent primary source.
218
Palladium Membrane Technology
An innovative solar steam reformer powered with molten nitrates is being developed in the framework of the European project CoMETHy (Compact Multifuel-Energy to Hydrogen converter) co-funded by the European Union’s Seventh Framework Programme (FP7/2007–2013) under the Fuel Cells and Hydrogen Joint Technology Initiative (EC Grant Agreement No. 279075). In this case, the molten salts are used to transfer the heat collected from the CSP plant and other possible back up heat sources to the steam reforming plant. This enables operating the reformer at temperatures lower than 550°C, and selective Pd-based membranes can be applied to recover high-grade hydrogen and increase conversion despite the thermodynamic limitations. Compared to a typical steam reforming process, this steam reforming technology operates at lower temperatures, from typically 850–950°C down to 400–550°C, with a consequent significant gain in material costs since no special steel alloy for hightemperature operation is required. The high-temperature furnace is then replaced by a flameless heat exchanger, heated by a liquid molten salts stream, making the whole reactor envelope more compact. Additionally, by operating at lower temperatures, it is possible to combine steam reforming and water-gas-shift (WGS) reactions in a single stage at 400–550°C, resulting in a low outlet CO content (<<10 %vol) and reduction of the reformer heat duty. The integration with membranes avoids dedicated hydrogen separation and purification units, to further improve the compactness and to enhance the conversion despite the thermodynamic limitations of low-temperature reforming. When the thermochemical plant is powered by solar heat, in principle there will be no combustion in the whole process, and no CO2-containing flue gases emitted to the atmosphere: this will lead to a reduction of fuel consumption and greenhouse gas (GHG) emissions, being in the order of 40% to more than 50% less than the conventional route.4 The reduction of fossil fuel consumption will make the hydrogen production cost less subject to the fossil (e.g. NG) price in solar steam reforming; hence, a breakeven point can be predicted for the hydrogen production cost, resulting in a solar route economically more convenient than the fossil-based one.7 The same solar steam reforming process applied to biomass-derived fuels (i.e. biofuels like biogas, bioethanol, etc.) allows totally renewable hydrogen production. An additional advantage of the use of molten salts as heat transfer fluid for steam reforming is when some stand-by periods of the plant are foreseen, as is the case of small-medium scale reformers for hydrogen refuelling stations. In this case, the continuous recirculation of molten salts eases the overall process management by maintaining all plant components (e.g. catalyst, membrane) at working temperature (400–550°C) during stand-by periods: this minimizes start-up periods and materials ageing resulting from thermal cycling. Within the operating range (400–550°C, 1–10 bar), in general, composite membranes (with a few microns thick Pd-based layer deposited on a porous metal or ceramic support) or self-supported membranes (consisting of a rolled Pd-Ag foil with thickness > 50 μm) can be considered for hydrogen removal from the molten salts heated reactor.
Palladium membranes in solar steam reforming
219
As for the integration of the catalyst with the membrane, two options can be considered: • a multi-stage membrane reformer (MSMR), where the membrane is external to the reactor, and • an integrated membrane reactor (IMR), where the membrane is integrated with the catalyst and the heat exchanger.
The MSMR scheme consists of a number of reformers, similar to that shown in Figure 13.2, where thermodynamic equilibrium is approached, each one followed by a membrane separation module.4 Thus, a step-by-step increase of methane conversion is obtained in the reactors thanks to the hydrogen removal in the intermediate membrane units. A three-stage process scheme is reported in Figure 13.3. It is possible to demonstrate that the larger the number of reactor/membrane stages, the higher is the overall thermal efficiency of the process, as shown in Figure 13.4: indeed, in a single-stage process (i.e. a single-pass reactor like that represented in Figure 13.2) a large amount of steam should be generated to produce the hydrogen corresponding to thermodynamic equilibrium; on the other hand, in membrane reactors the same amount of steam allows larger hydrogen production rates resulting from the higher methane conversion.4 Therefore, the application of membrane reactors in solar steam reforming increases the complexity of the chemical plant, but significantly reduces the size and costs of the power plant (heat supplier) by improving overall thermal efficiency. This is an important point, considering that in solar-powered thermochemical processes the CSP plant is always a major cost item3,4 so the application of membranes will make the overall plant more competitive. The above considerations are effective in the case of an IMR too. Clearly, the IMR (with respect to the MSMR) is more compact and leads to higher efficiency (and feedstock conversion) on a single pass. However, the development of this reactor involves a considerable engineering challenge, with several design and mechanical issues, dealing with the catalyst/membrane coupling and the harmonization of the three key transport mechanisms: heat transfer from the molten salts to the catalyst bed, reaction kinetics and hydrogen permeance through the membrane.
References 1. S. Moeller, D. Kaucic and C. Sattler (2006), ASME Journal of Solar Energy Engineering, 128 16–23. 2. A. Berman, R.K. Karn and M. Epstein (2006), Energy & Fuels, 20 455–462. 3. A. Giaconia, R. Grena, M. Lanchi, R. Liberatore and P. Tarquini (2007), International Journal of Hydrogen Energy, 32 469–481. 4. A. Giaconia, M. De Falco, G. Caputo, R. Grena, P. Tarquini and L. Marrelli (2008), AIChE Journal, 54 1932–1944. 5. U. Herrmann, B. Kelly and H. Price (2004), Energy, 29 883–893.
220
Palladium Membrane Technology
6. U. Herrmann and D.W. Kearney (2002), ASME Journal of Solar Energy Engineering, 124 145–151. 7. S. Moeller, D. Kaucic and C. Sattler (2006), ASME Journal of Solar Energy Engineering, 128 16–23.