EP1680354A1

EP1680354A1 - Hydrogen production from methanol

Info

Publication number: EP1680354A1
Application number: EP04775080A
Authority: EP
Inventors: Erling Rytter
Original assignee: Statoil ASA
Current assignee: Equinor ASA
Priority date: 2003-10-06
Filing date: 2004-10-06
Publication date: 2006-07-19
Also published as: NO20034468D0; WO2005033003A1

Abstract

A method for production of hydrogen from an oxygenated hydrocarbon fuel, comprising the steps of: a) introducing the fuel, steam and an oxygen containing gas into a reformer comprising a catalytic bed, wherein the oxygen containing gas is introduced through one or more tubes made of porous material, inserted into the catalytic bed to form a reformed gas comprising hydrogen, CO, CO2, any inerts and un-reacted reactants, b) removing said reformed gas from the reformer, and c) separating the reformed gas into a hydrogen rich fraction and a hydrogen poor fraction in a separate unit, is described. A plant for performing the method is also described.

Description

HYDROGEN PRODUCTION FROM METHANOL

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a method and a device for the production of hydrogen from an oxygenated hydrocarbon such as methanol, ethanol and the like. The present method and device is inter alia useful for distributed production of hydrogen.

Description of the related art The use of fossil fuels like coal, oil and gas, is widely considered to have contributed to the concentration of CO in the atmosphere. The environmental impact of the raising level of CO₂ is now apparent and has caused a growing concern globally. Accordmg to the Kyoto protocol, a large number of counties have committed to stabilize and reduce the emission of CO₂ to the atmosphere. Accordingly, researchers and scientists around the world are searching for environmental friendly and cost effective energy carriers to substitute fossil fuels.

Hydrogen is expected to become an important energy carrier in the future both for heat and power generation and as a fuel. Hydrogen as such is an environmental friendly energy carrier. The use of hydrogen as a source of energy does not contribute to the emission of environmentally harmful gases.

There are a number of ways to produce hydrogen like electrolysis of water and reforming of various hydrocarbons. Today, steam reforming or autothermal reforming of natural gas is the main source of hydrogen. However, generation of hydrogen from natural gas generates at least as much CO₂ as distributed combustion of the gas. Thus, hydrogen as an environmental friendly energy carrier requires that the CO₂ is captured and safely deposited.

Additionally, today no widespread distribution system exists for use of hydrogen in vehicles or in households. The technical solutions for cost effective and safe distribution of an explosive gas as hydrogen have to be developed. The development of such systems may still take years or even decades. Therefore, there is a need, at least in a transition period, for a simple system for distributed generation of hydrogen, e.g. at fuel stations, onboard vehicles or in buildings.

Reforming of hydrocarbons is normally carried out at a high temperature to give maximum conversion of the feed to the desired products. In reforming of methane the temperature usually is in the range 800 - 1000 °C to avoid excessive so-called methane slip.

For higher hydrocarbons as feed, the reforming temperature is lower, both due to somewhat relaxed equilibrium constraints for the reaction, and also because coking of the catalyst becomes more important. With a naphtha feed, often contemplated as the feed for hydrogen production coupled to fuel cells for CHP (combined heat and power) applications or in vehicles, the temperature in steam reforming usually is in 700 -800 °C range, at least above 650 °C. Methane formation is significant at all these temperatures, and becomes excessive in the low temperature range. As an example consider reforming at 750 °C, 27 bar and steam/carbon = 3 of a feed with the overall composition H/C = 2.2. Then the product on a molar basis at equilibrium will consist of ca. 1.5 parts H₂, 0.4 parts CO₂, 0.25 parts CO and 0.35 parts methane, in other words a significant amount of methane even at 750 °C. If this methane, and a large amount of CO after shift conversion, is carried over to the fuel cell stack, a significant reduction in efficiency will take place due to the large fraction of inerts in the feed gas. Note also that a large amount of steam is needed, a factor that adds complexity and energy consumption to the reforming system.

Another aspect of reforming of methane or naphtha is that the pressure usually is kept at 10-30 bar, in spite of some sacrifice regarding equilibrium conversion, to have an efficient process in terms of volumes of vessels and pipes. For fuel cell applications, the pressure requirement often is relaxed as the systems generally are smaller and it might be optimal to fit the pressure of the reforming unit to the fuel cell stack. A typical operating pressure for PEM fuel cells is 3 bar. Reforming of methanol is much simpler both due to lower temperatures needed for high conversion, and also because it is relatively simple to condense out unconverted methanol that is recycled to the reformer.

Using a hydrocarbon for generating hydrogen by reforming requires the availability of a separate source or storage of water and transfer lines for the water. The source or storage of water and the transfer lines are exposed to freezing at temperatures below 0°C. Additionally a special device is needed for mixing the gaseous fed with water. A simple system for distributed generation of hydrogen based on a hydrocarbon as feed is therefore difficult to achieve.

The use of oxygenated hydrocarbons such as methanol, or ethanol or another oxygenate means two times reforming, first the reforming of natural gas (or other hydrocarbon feed) to syngas (CO + H₂) prior to synthesis of methanol or another oxygenate, and then reforming of methanol to give hydrogen. However, large scale methanol plants are efficient and therefore provides methanol at an adequate price, e.g. below 120 USD/ton. Further, methanol as the feed provides several advantages and is therefore in many aspects an alternative feed for generation of hydrogen by reformation. Firstly, distribution of liquid methanol is easier than distribution of gas. Additionally, a specific advantage of using methanol instead of a hydrocarbon as feed for producing hydrogen by reforming is that the handling of water as co-feed is simplified. The desired amount of water is simply included in the storage tank together with the methanol, either directly mixed in the tank, or premixed with the methanol supply. The freezing point of methanol is as low as -93.9 °C, whereas a 1:1 molar mixture with water freezes at - 84.4 °C and a 2: 1 ratio (wateπmethanol) at - 49,5 °C.

Methanol can be regarded as a substance that has undergone a partial oxidation of methane. Accordingly, less heat is required (compared to methane or naphtha) in the reforming process, and the reformation process may be performed at a lower temperature. This means that there also is less production of by-product like NOx and CO. The last point is particularly important if the hydrogen is going to be used as a fuel for PEM fuel cells as these normally are poisoned by CO. Further, methanol is a liquid and therefore is readily transported to the hydrogen generation site.

Another aspect of PEM fuel cells that often is neglected is that the efficiency drops off as current is drawn from the cells due to reduction in the cell potential. This means that prohibitively large cell stacks might be needed to avoid high load operating conditions. In other words, fuel cells have an efficiency advantage for low loads that to some degree might be offset at high load operation. This adverse effect is enhanced when the fuel is not pure hydrogen, but hydrogen diluted with inerts like CO₂ and/or nitrogen because the cell potential is related to the hydrogen partial pressure through the Nernts equation. Therefore, pure hydrogen, possibly with some steam, will give a higher cell potential and can be operated to high conversion levels. When diluted with inerts, the hydrogen partial pressure rapidly decreases, and a significant portion of the hydrogen therefore leaves the stack in the exhaust gas necessitating an afterburner to be installed. Reforming of methanol to produce hydrogen, in particular for fuel cell applications, is described in the literature. For example, in US 6,180,081, a reactor device is described that consists of an outer shell and a number of burner tubes that is included to provide heat for the reaction. The reactor also contains a catalyst for the reforming reaction, e.g. Cu/ZnO/Al₂O₃, and an arrangement with a porous structure that enables the hydrogen product to be separated from the other product gases and possibly unconverted methanol. Although this reactor produces hydrogen, at least in principle, in pure form, it also has the disadvantages that the heat generated in the burner tubes is not fully utilized for the methanol reforming reaction, and that the combustion gases contain a large amount of nitrogen along with the combustion products (essentially CO₂ and H₂O and any excess air). Besides low energy efficiency, it becomes very inconvenient and costly to separate out the greenhouse gas CO for further use or deposition.

An alternative reactor for reforming of methanol is described in US 5,904,913. In this arrangement, the catalyst is situated in U-tubes, whereas the heat is provided by heat- transfer oil that is flowing along the U-tube bundles. It is further described that the methanol: steam feed gas ratio is between 1:1.13 and 1:2.0, and that a minimum reactor temperature of 280 °C is required for a virtual 100 % conversion of the methanol. If a high- pressure operation, e.g. 20 bar or above, is utilized, then the temperature should be increased further. This patent also describes the well-known fact that the formation of CO will be above the level that is normally accepted for hydrogen as a fuel for a PEM fuel cell (< 50 ppm), and that the level of CO increases with temperature. In an example, the hourly reactor effluent is reported to be 2.2 kg H₂, 13.1 kg CO₂, 3.3 kg H₂O and 0.2 kg CO. Thus, there will in any case be necessary to add one or more small reactors for removal of CO. Catalysts for methanation of CO are described, but not the shift conversion of CO to CO₂. Further, means for generating the necessary heat, or removal of CO₂, have not been included.

The autothermal steam reforming of a hydrocarbon feedstock to produce hydrogen or synthesis gas (CO + H₂), where the oxygen for the reaction is provided by a dense ceramic membrane that conduct oxygen, is described in numerous patents, e.g. EP 1035072A1 and US 5,846,641. This membrane is also referred to as a mixed ionic electronic conductor membrane containing at least one dense ceramic layer. Examples used are with natural gas or methane as feed. However, use of methanol or another alcohol or oxygenate like dimethyl ether as the feed is not considered. The reason for this omission is only speculative, but it is only recently that sufficient oxygen fluxes have been obtained for these materials at temperatures convenient for methanol reforming, let us say below 600 °C.

SUMMARY OF THE INVENTION

As demonstrated above there is still a need for an effective method and a plant for distributed production of hydrogen as an energy carrier.

According to a first aspect there is provided a method for production of hydrogen from an oxygenated hydrocarbon fuel, comprising the steps of: a) introducing the fuel, steam and an oxygen containing gas into a reformer comprising a catalytic bed, wherein the oxygen containing gas is introduced through one or more tubes made of porous material, inserted into the catalytic bed to form a reformed gas comprising hydrogen, CO, CO₂, any inerts and un-reacted reactants, b) removing said reformed gas from the reformer, and c) separating the reformed gas into a hydrogen rich fraction and a hydrogen poor fraction in a separate unit.

The oxygenated hydrocarbon fuel and water is optionally mixed in a storage tank. By mixing the fuel and water a separate water tank is avoided. In cold climate water supply may be a problem due to freezing. This problem is also avoided by mixing water and fuel, as the fuel acts as a anti-freeze solution. The mixture of fuel and water may e.g. be adjusted to give the mixture a freezing point as low as -20 or -30 °C.

According to one embodiment, the reformed gas removed from the reformer in step b) is introduced into a shift reactor to remove CO generated in the reformer according to the water shift reaction: CO + H₂O = CO₂ + H₂, before step c). The water shift reaction will result in a higher yield of hydrogen from the process, and at the same time reduce the amount of CO in the exhaust gas to an acceptable level.

The gas leaving the shift reactor is optionally introduced into a selective oxygenator to remove remaining CO by oxidation to form CO₂.

According to one embodiment, essential pure hydrogen is separated from the reformed gas by means of a selective hydrogen permeable membrane.

According to another embodiment, essential pure hydrogen is separated from the reformed gas by means of pressure swing adsorbtion.

The oxygenated hydrocarbon feed is according to a preferred embodiment, methanol.

The oxygen containing gas is preferably air, oxygen enriched air or oxygen. The use of substantially pure oxygen as the oxygen containing gas will reduce the total gas volume as the inert nitrogen is not introduced into the catalytic bed. The reduced gas volume makes it possible to build a less voluminous plant. The reduced gas volume will also make the separation of gases easier. According to a second aspect, the invention relates to a plant for production of essentially pure hydrogen from an oxygenated hydrocarbon fuel, comprising an autothermal reformer including a catalytic bed, means for feeding the fuel, steam and an oxygen containing gas into the reformer and a separator for separation of substantially pure hydrogen from the remaining gas, wherein the reformer includes tube(s) made of a porous material inserted into the catalytic bed for introduction of the oxygen containing gas.

Optionally, a water gas shift reactor between the reformer and the separator.

A selective oxygenator may optionally be placed between the water gas shift reactor and the separator.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 is a diagram illustrating the ratio of hydrogen to carbon (H₂/C) as a function of the ratio of oxygen to carbon (mol O₂/mol C) in partial oxidation + steam reforming of methanol for different levels of oxygen;

Fig. 2 is a diagram illustrating the heat of reaction, ΔH (kJ/mol), as a function of the ratio of oxygen to carbon (mol O₂/mol C) in partial oxidation + steam reforming of methanol for different levels of oxygen;

Fig. 3 is a flow diagram illustrating a possible process for distributed hydrogen production;

Fig. 4 illustrates a preferred reactor for distributed hydrogen production from methanol; and

Fig. 5 is a diagram illustrating the fuel cell conversion efficiency as a function of hydrogen pressure.

DETAILED DESCRIPTION OF THE INVENTION Reactions for production of hydrogen from methanol comprise the following reactions: Decomposition: I: CH₃OH_(g) = CO + 2 H₂ ( ΔH<>₂₉₈ = 90.6 kJ/mol) Water-gas shift:

H: CO + H₂O_(g) = CO₂ + H₂

Steam reforming:

III: CH₃OH_(g) + H₂O_(g) = CO₂ + 3 H₂ ( ΔH0₂₉₈ = 49.4 kJ/mol)

Partial oxidation:

TV: CH₃OH_(g) + Vi O₂ = CO₂ + 2 H₂ ( ΔH⁰ ₂₉₈ = - 192.2 kJ/mol)

At temperatures in the 200 - 300 °C range, the produced amount of CO is low, and therefore only the two last reactions are considered further. By combining the endothermic steam reforming and the exothermic partial oxidation, an autothermal process can be obtained with a suitable addition of oxygen to the reactor by burning approx. 8 % of the hydrogen produced in reaction III. This effect can be illustrated by the following example:

CH₃OH_(g) + % H₂O_(g) + 1/8 O₂ = CO₂ + 2³/₄ H₂ ( ΔH<>₂₉₈ = -11 kJ/mol)

Here, adding ³/₄ of reaction III and Vi of reaction IV, gives a slightly exothermic total reaction. If we in addition to the heat of reaction also consider that methanol will have to be evaporated into the reactor and that the feed will have to be heated from ambient temperature to the reaction temperature, more oxygen will have to be fed (if other heat sources are not available). The heat of vaporization of methanol is ca. 39 kJ/mol, whereas the heat capacity (Cp) is as an average ca. 60 J/mol*K.

Adding oxygen and performing combined steam reforming and partial oxidation can be illustrated by the diagrams in figures 1 and 2.

From figures 1 and 2 it can be seen that adding 12.5 %, 16.7 %, 20 % or 25 % oxygen compared to the methanol feed, only reduces the production of hydrogen to a moderate degree. The dotted lines in figure 2 shows an autothermal operation, i.e. a thermo-neutral operation with ΔH = 0, corrected for the heat of evaporation (ΔH_vap, upper line) and the heat capacity (C_P*ΔT, lower line) for the methanol to be reformed. It therefore can be seen that it is necessary to increase the oxygen to carbon ratio to ca 0.18 to provide heat for the evaporation and that some further oxygen is needed to encounter for the adiabatic temperature rise of methanol from approximately 25 °C, assuming a reaction temperature close to 250°C. This diagram does not take into account evaporation of the water feed, heating the steam generated and oxygen/air, heat losses and energy recovery from the product gases. Normally, some excess steam will have to be added to prevent catalyst deactivation. All in all, the extra heat requirement for the water/steam system can approach similar values as shown for methanol. With recovery of energy from the product, we see that an O₂/C ratio of 0.20 to 0.35 should provide a system in total heat balance, and that considerably less oxygen is needed if e.g. evaporation is handled separately.

Such an autothermal system , in particular an autothermal reactor, simplifies the whole hydrogen generation considerably as all large heat exchange surfaces can be minimized or eliminated, as the heat is generated internally in the reactor. The oxygen or air can be fed to reactor tlirough a variety of systems, including premixing the gases, through special feeding tubes or nozzles, or by a membrane, inside or prior to the reactor.

One advantage of feeding oxygen instead of air is that the volumes of the process equipment become smaller as the voluminous nitrogen (ca. 80 % in air) is avoided. A further aspect that sometimes have to be considered, is handling of CO . For that purpose it is an advantage that CO can be separated as efficiently as possible from the product. The dominant product gases will be H , CO and excess H O and N₂ if air is used. After condensation of the steam, it therefore is a significant advantage that it is only H₂/CO₂ separation that is left. This separation can be performed by standard techniques using ammine wash, PSA (pressure swing adsorption) or membranes. Figure 3 illustrates a preferred embodiment of the present invention. The preferred feed, methanol, is introduced into the system through a feed line 1 to a storage tank 2. Water is added into the system either as illustrated by water feed line 3 into a line 4 leading from the storage tank to the reforming plant, or by adding water into the storage tank 2 separately or together with the methanol. Methanol and water in line 4 are heated in a heat exchanger 5 before they are introduced into autothermal reformer 6. Air, oxygen enriched air or oxygen is introduced into the autothermal reformer 6 through line 7.

The autothermal reformer 6 is preferably a reformer 6 as illustrated in figure 4. The reformer 6 comprises a catalytic bed 23 through which the reactants flow, and one or more tube(s) 24. These tubes are either porous for distribution of air, oxygen enriched air or oxygen or having a semi permeable membrane allowing transport through the membrane of oxygen but not nitrogen. The existing semi permeable membranes for selective transport of oxygen, are Oxygen ion Transport Membranes (OTM). An OTM membrane is a dense membrane, e.g. in form of a tube, allowing transport of oxygen ions across the membrane.

When air is fed into the tube(s) 24 and methanol and steam is fed into the catalytic bed 23, oxygen will be transported across the membrane into the catalytic bed. In this way air separation occurs inside the reformer to make a compact arrangement. This arrangement also facilitates a design of the reactor that will minimize temperature gradients and thereby reduce the production of by-product and maximize the conversion and thus the production of hydrogen.

The hot gas leaving the reformer 6 in a line 8 is cooled down in a heat exchanger 9 before it is introduced into an optional shift reactor 10, where CO generated in the reformer is removed according to reaction II above.

The gas leaving the shift reactor 10 through line 11 is introduced into an optional selective oxygenator 12 if it is found necessary to remove remaining CO down to the ppm level by oxidation of traces of CO to CO₂. The gas leaving the selective oxygenator in line 13 can be cooled further down in a heat exchanger 14 before it is introduced into a separator 15 which separates hydrogen from CO₂. The separator 15 may be based on any technology suitable for separation of hydrogen and CO₂, such as amine wash, Pressure Swing Adsorption (PSA) or membranes. The configuration of the heat exchangers, e.g. 9 and 14, depends on the actual requirements of the process, like the temperature needed for the catalytic processes in the optional reactors 10 and 12 and the separator 15.

Separated CO₂ leaves the separator 15 through a line 16 for compression or liquefaction and storage or depositing, whereas hydrogen leaves the separator in a line 17. The hydrogen in line 17 may be compressed in a compressor 18. The hydrogen leaving the compressor 18 in line 19 is led to a hydrogen storage 20. Hydrogen in storage 20 is taken out of the storage in a line 21 for further use tlirough a line 21. Alternatively, all or parts of the produced hydrogen may be taken out in a line 22 for direct use in a fuel cell or other uses.

Unconverted methanol and steam may be condensed, e.g. as indicated by the dotted line after the heat exchanger 14 and then recycled through the lines 1 or 3. When the hydrogen in lines 21 or 22 is used, as in fuel cells, any unconverted gas may wholly or partially be recycled whereas rest gas may be burned.

Methanol and steam are effectively reformed to form a reformed gas in temperatures in the reformer in the 250 to 350 °C range. This temperature is sufficiently low to allow simple integration with a PEM fuel cell operated at ca. 80 °C or above, and at the same time gives high methanol conversion. Another option is to operate at a higher temperature, in the 350-600 °C range, if increased fluxes of oxygen transport are facilitated by a higher temperature, as is the case for dense oxygen ion transport membranes.

With the technology of today, the flux through the OTM membrane drops rapidly below approx. 500 °C. The preferred temperatures when using OTM membranes is therefore in the 350 - 600 °C range. This temperature is above the most preferred temperature for reforming of methanol as high conversion temperatures results in the generation of more CO. As it can be seen from the reactions above, generation of CO reduces the yield of hydrogen. Additionally, CO is poisonous for fuel cells. Development of new membranes allowing flux of oxygen at lower temperatures will make it possible to lower the temperature in the reformer 6 to a temperature in the range where the production of CO is lower and thus render the shift converter superfluous.

The inventive method makes it possible to make a compact plant for hydrogen production. As only oxygen (ions) is transported through the tube wall, there will be no nitrogen on the fuel side and in the reformed gas product. T s lowers the total gas flow in the system and makes it easier to separate CO₂ for deposition. This arrangement also facilitates a design of the reactor that will minimize temperature gradients, and thereby reduce by-products and maximize conversion.

The effect of hydrogen dilution through the fuel cell (FC) stack is illustrated in figure 5. The upper curve just illustrates that for a typical total pressure of 3 bar in the FC stack, the pressure is unchanged when the hydrogen is consumed during the series of cells when the feed to the stack is undiluted hydrogen. Such essentially undiluted hydrogen is obtained through the present invention by autothermal reforming of methanol with oxygen followed by condensing of steam to water and separating out CO₂. The two next curves demonstrate the large reduction in the hydrogen partial pressure when hydrogen is obtained by conventional steam reforming of methane or methanol, respectively. Similar partial pressures occur for autothermal reforming with oxygen, whereas the internal nitrogen present when the ATR is performed with air naturally lowers the hydrogen pressure further. It is evident that the conversion in the system must be limited due to drop in efficiency for all cases that contain internal CO₂ or nitrogen. Together with the drop in potential as current is drawn, this gives stacks with many elements in addition to loss of unconverted hydrogen that simply is burned. The consequences for running the reaction at approx. 500 °C and not the preferred 200- 300 °C as mentioned above would be both potentially positive and negative: - Higher reaction rate, i.e. a smaller reactor. - Less equilibrium restrictions, i.e. higher per path conversion. - More CO relative to CO2. A shift converter will probably be needed, but may be necessary in any case. - More energy is needed, i.e. oxygen, to heat the reactants. - More effort is needed to cool down the product gases.

The present method and plant is therefore an attractive solution for distributed production of hydrogen. Even at the reaction temperature mentioned above, the temperature will be 200-300 °C lower than needed for reforming of naphtha.

Claims

P a t e n t c l a i m s 1.

Method for production of hydrogen from an oxygenated hydrocarbon fuel, comprising the steps of: a) introducing the fuel, steam and an oxygen containing gas into a reformer comprising a catalytic bed, wherein the oxygen containing gas is introduced through one or more tubes made of porous material, inserted into the catalytic bed to form a reformed gas comprising hydrogen, CO, CO₂, any inerts and un-reacted reactants, b) removing said reformed gas from the reformer, and c) separating the reformed gas into a hydrogen rich fraction and a hydrogen poor fraction in a separate unit.

2.

The method according to claim 1, wherein the oxygenated hydrocarbon fuel and water is mixed in a storage tank.

3.

The method of claim 1 or 2, wherein the reformed gas removed from the reformer in step b) is introduced into a shift reactor to remove CO generated in the reformer according to the water shift reaction: CO + H₂O = CO + H₂, before step c)

4.

The method of any of the preceding claims, wherein the gas leaving the shift reactor is introduced into a selective oxygenator to remove remaining CO by oxidation to form CO₂.

5.

The method according to any of the claims 1 to 4, wherein essential pure hydrogen is separated from the reformed gas by means of a selective hydrogen permeable membrane.

6.

The method accordmg to any of the claims 1 to 5, wherein essential pure hydrogen is separated from the reformed gas by means of pressure swing adsorbtion.

7.

The method according to any of the preceding claims, wherein the oxygenated hydrocarbon feed is methanol.

8. The method according to any of the preceding claims, wherein the oxygen contaimng gas is air, oxygen enriched air or oxygen.

9.

A plant for production of essentially pure hydrogen from an oxygenated hydrocarbon fuel, comprising an autothermal reformer(6) including a catalytic bed (23), means for feeding the fuel, steam and an oxygen containing gas into the reformer and a separator

(15) for separation of substantially pure hydrogen from the remaining gas, wherein the reformer (6) includes tube(s)(24) made of a porous material inserted into the catalytic bed for introduction of the oxygen containing gas.

10.

The plant according to claim 9, comprising a water gas shift reactor (10) between the reformer (6) and the separator (15).

11.

The plant according to claims 9 or 10, comprising a selective oxygenator (12) between the water gas shift reactor (10) and the separator (15).