Patent application title: Methods of Improving thermal transfer within a hydrocarbon reformig system
Jeffrey L. Barber (Golden, CO, US)
Jeremiah J. Cronin (Longmont, CO, US)
CBH2 TECHNOLOGIES, INC.
IPC8 Class: AC10G3504FI
Class name: First stage is reforming reforming in all stages catalyst in at least one stage
Publication date: 2009-02-26
Patent application number: 20090050522
A process for supplementing steam reformation with hydrogen, oxygen and
heat from dissociation of hydrogen peroxide (H2O2). Steam
reforming suffers from starting the overall system in an endothermic
manner which leads to long start times and limited turndown capability in
delivering high quality hydrogen. Using a material such as hydrogen
peroxide in a controlled manner allows for instant generation of
oxygen-enriched steam, an essential state in reforming hydrocarbons. It
additionally, offers the opportunity to relieve parasitic power
requirements associated with compressing air to reach the O2 density
required for the reforming process and eliminates, or minimizes (as a
function of overall power system design requirements) post process
cleanup of nitrogen-oxides.
1. A method comprising:a. contacting a hydrogen peroxide solution with
water to produce heat, steam and oxygen; and,b. supplying at least one of
the heat, steam and oxygen to a hydrocarbon reforming system.
2. The method of claim 1, wherein a system of contacting the hydrogen peroxide solution and water is external to the hydrocarbon reforming system.
3. The method of claim 1, wherein a system of contacting the hydrogen peroxide solution and water is integrated internally with the hydrocarbon reforming system.
4. The method of claim 1, wherein a chemical reaction process path of the reforming system is subdivided into at least two parallel process paths, wherein each parallel process path is isolated from any other process path, and wherein the parallel process paths have a uniform geometry and an interior accessible by at least one end, and wherein each interior contains at least two catalytic beds.
9. The method of claim 4, wherein the reforming system uses only hydrogen peroxide as an oxygen source.
10. The method of claim 4, wherein the reforming system uses only air as an oxygen source.
11. The method of claim 4, wherein at least one heat exchange device is placed in a discharge gas stream of the reforming system and is interconnected by at least one rod to at least one of the process paths.
12. The method of claim 1, wherein the diluted hydrogen peroxide solution has a concentration of between about 60% by weight and about 70% by weight.
15. The method of claim 1, wherein the diluted hydrogen peroxide solution has a concentration between about 35% by weight and about 55% by weight.
16. The method of claim 1, further comprising:contacting the steam and oxygen with a hydrocarbon fuel to form a hydrogen reformate stream and,supplying the hot hydrogen reformate stream to a second hydrocarbon reforming device.
18. The method of claim 16, wherein the hydrocarbon is liquid ethanol.
19. The method of claim 16, wherein a chemical reaction process path of the reforming system is subdivided into at least two parallel process paths.
20. The method of claim 19, wherein each parallel process path is isolated from any other process path, and wherein the parallel process paths have a uniform geometry and an interior accessible by at least one end, and wherein each interior contains at least two catalytic beds, and wherein at least one element interconnects the at least two catalytic beds.
22. The method of claim 20, wherein the element is composed of at least two materials.
23. The method of claim 22, wherein the element is an assembly of at least one rod and at least one plate in contact with the at least one rod.
24. The method of claim 23, wherein the reforming system uses only hydrogen peroxide as an oxygen source.
25. The method of claim 19, wherein at least one heat exchange device is placed in a discharge gas stream of the reforming system and is interconnected by at least one rod to at least one of the process paths.
26. The method of claim 25, wherein the reforming device uses only hydrogen peroxide as an oxygen source.
27. The method of claim 16, wherein the diluted hydrogen peroxide solution has a concentration of between about 60% by weight and about 70% by weight.
29. The method of claim 16, wherein the diluted hydrogen peroxide solution has a concentration between about 35% by weight and about 55% by weight.
FIELD OF THE INVENTION
The invention resides in the field of hydrogen generation through the reformation of hydrocarbon fuels.
BACKGROUND OF THE INVENTION
Fuel cells are being considered for many applications beyond vehicle transportation, including stationary and transportable electrical power plants. These applications, unlike industrial or automotive vehicles, function predominately as steady state operations over significant time periods, although there are transient loads applied to these electrical power generation systems. Given relatively stable operating loads and conditions, the principle technical approach to providing a hydrogen feed stream to the fuel cell stack has been an effort to implement an endothermic process generically referred to as steam reforming. This process has been used by industry, in one embodiment or another, for nearly 100 years to generate hydrogen from hydrocarbons, primarily from methane (CH4) found in natural gas.
For transportable power generation systems, liquid hydrocarbon fuels are the preferred fuels due to the higher hydrogen densities offered by these liquids over any compressed gas, such as methane. A problem with using an endothermic reforming process is that this type of system does not have the ability to follow large increases in load demand. Primarily because of this problem, steam reformation has been largely abandoned for transportation applications dominated by highly fluctuating transient loads. Steam reforming, in different system configurations, is still being pursued as a possible method to generate hydrogen for those applications in which transient load changes are significantly less frequent, such as stationary/transportable electrical power generation. However, even for these applications, meeting transient load demands is still problematic due to the inherent endothermic investment that needs to be made at the very beginning of the reforming process.
One solution that has been proposed to compensate for the inherently slow response characteristics of steam reforming is to capture, divert and store under pressure a portion of the final hydrogen feed stream product during moments of excess production for latter use during sudden electrical load increases. While it is certainly possible to store a portion of the gaseous hydrogen feed stream in this manner, this intermittent hydrogen source is only useful in meeting dynamic load demands placed upon a Fuel Cell Power System (FCPS). This approach does not provide any assistance in overcoming the fundamental problem of steam reforming, which is the up-front endothermic investment required to create a subsequent increase in overall hydrogen production.
Traditional as well as newer steam reforming processes also suffer in their ability to respond to rapid increases in hydrogen demand due to the significant cleanup difficulties of the reactant gases. All of these reforming processes rely upon air as the oxygen source. While this source is readily available, the component makeup of air causes significant problems in these systems. While oxygen makes up approximately 21% of air by volume, nitrogen comprises approximately 78% of air by volume. This low oxygen ratio causes several problems including the formation of undesirable chemical compound formations. One particularly significant problem is that steam reforming requires that the air be compressed to differing pressures depending upon the particular steam reforming process being utilized. Small-scale gas compression, required for FCPS output, ranging from 5 kW to 500 kW is typically inefficient, resulting in large parasitic electrical loads to the FCPS. The large presence of nitrogen in the air also increases parasitic energy losses from the compressor, because the nitrogen must be compressed as well.
The presence of nitrogen leads to other undesirable consequences in a typical steam reforming processes. One of these undesirable consequences is the chemical formation of nitrogen-oxides (NOx), primarily NO and NO2. NOx formations start during the initial hydrocarbon oxidation phase. The formation of these compounds in turn causes two other problems with respect to overall system efficiency. First, NOx is a heavily regulated pollutant and must be catalytically cleaned in an elevated thermal environment. This process further consumes hydrogen as the oxygen bonds are stripped from the NOx as it progresses to a final mixture of N2+CO2+H2O, Second, NOx formation causes the loss of available oxygen to fully oxidize carbon, which releases the sought after hydrogen. Consequently, the steam reforming processes which use air, experience further increases in parasitic compressor losses when additional air is compressed and introduced into the overall process to compensate for the loss of oxygen due to NOx formation.
Carbon Monoxide (CO) concentrations above a few parts per million are unacceptable to the long-term reliability and durability of the PEM Membrane Electrode Assembly (MEA). As such, a CO contaminated hydrogen gas stream must be exposed to a water-gas shift reaction to convert CO to carbon dioxide (CO2), which is acceptable to the MEA. When air is used as the oxygen source, the presence of nitrogen not only creates the previously described NOx pollutants, but also further encourages the formation of CO due to the reduced availability of oxygen to fully react with the carbon. As the percentage of CO formation increases for any given gas flow rate, the water-gas shift reaction bed must also grow in size. Elevated temperatures are needed for this catalytic bed to function correctly. Thus, as the bed mass increases, it will take longer to elevate the bed, or a new section of the bed, to the necessary temperature. Until this event occurs, the product gas stream can not be supplied to a PEM fuel cell due to the contamination caused by the excessive CO levels. This high CO gas stream must be diverted away from the fuel cell, once again inhibiting the reformer's ability to meet sudden increases in hydrogen demand as well as further deteriorating the reformer's overall conversion efficiency.
In order to accomplish an increase in the hydrogen reforming rate, steam reformers require that more steam be made available, which is the crux of the slow response characteristics of this reforming process. Typically, this endothermic investment needs to be made by heating liquid water to steam at the very beginning of the reforming process. A downstream hydrogen reformate holding tank is only able to provide a limited amount of time for the primary reforming system to boil additional water. The actual amount of time available to boil this water is dependent upon the size and storage pressure of the holding tank.
Another considerable obstacle that must be addressed in configuring a hydrogen or reformate holding tank is that while it is possible to supplement the hydrogen source for the typical duty cycle, it becomes much more difficult to do so when non-typical duty cycles are considered. If electrical power generation cannot fail, such as hospital emergency or telecommunications power backup, then the secondary hydrogen storage system must be forced to grow in size and pressure in order to meet all high demand scenarios. While this is possible, the size increase adds to the cost, complexity and maintenance of the entire fuel delivery system.
Another difficulty that steam reforming faces is the limited degree of adjustability in the output rate of the hydrogen reformate. The ratio of maximum output to minimum output is typically referred to as the turndown ratio. Current steam and/or autothermal type reforming systems have a turndown capability between 2 or 3 to 1 compared to typical power generation which has a turndown between 5 and 10 to 1. While it may be possible to improve the reformer's turndown, there are fundamental obstacles that resist this change. Steam based reforming of hydrogen from hydrocarbons that uses air as the oxidation source, must provide a means for maximized hydrogen conversion as well as the NOx, CO and sulfur-based compound cleanup described above. This clean up is currently performed by specialized, sequential catalytic beds that need to be at specific minimum/maximum temperature range to function correctly. One means of providing increased turndown capability is to subdivide the fixed catalytic processing volume(s) into several smaller mass, parallel processing paths or processing sections. Even with a significantly small catalytic mass, which a subdivided Flow Section would allow, starting a new reformer or catalytic bed would take several minutes to reach the minimum operational temperatures required to produce hydrogen compatible with a PEM fuel cell. For applications requiring a fast startup and electricity delivery capability, this delay in electrical power generation is unacceptable.
Thus, a reformation process that could quickly and efficiently follow increases in transient load demand is desired. Preferably, such a system would not rely on air as a source of oxygen and would be highly adjustable between the minimum and maximum system output.
SUMMARY OF THE INVENTION
The present invention provides methods of using hydrogen peroxide to aid and improve the ability of a hydrocarbon reforming process to meet fast startup and dynamic changes in hydrogen production.
In one embodiment, the present invention provides a method of contacting a hydrogen peroxide solution with water to produce heat, steam and oxygen. These products are supplied to a hydrocarbon reforming system. The system for contacting the hydrogen peroxide solution and water may be either external to the hydrocarbon reforming system or integrated internally into the hydrocarbon reforming system.
In a preferred embodiment, the chemical reaction process pathways of the reforming system are subdivided into at least two parallel process paths. Each parallel process pathway may be isolated from the other process pathways. The parallel process paths preferably have a uniform geometry and an interior accessible by at least one end. The interior of the process pathways optionally contains at least two catalytic beds. In a specific embodiment, an element interconnects the catalytic beds. The element may be composed of at least two materials, such as an assembly of at least one rod and at least one plate in contact with the at least one rod.
The reforming system supplemented with the products of the dissociation of hydrogen peroxide may use only hydrogen peroxide as an oxygen source or may use only air as an oxygen source.
The method may also be implemented in a reformer system that includes at least one heat exchange device placed in a discharge gas stream of the reforming system and interconnected by at least one rod to at least one of the process paths.
The hydrogen peroxide solution used in the methods of the present invention is preferably a diluted hydrogen peroxide solution having a concentration of between about 60% by weight and about 50% by weight. The hydrogen peroxide used may also be concentrated from a more dilute solution. In this manner, a hydrogen peroxide having a concentration below about 50% by weight is concentrated to form a diluted hydrogen peroxide solution having a concentration of between about 50% by weight and about 70% by weight prior to contacting the hydrogen peroxide and water solution with a disassociating catalyst to form oxygen, hydrogen and heat. The processes may also be carried out using a diluted hydrogen peroxide solution having a concentration between about 35% by weight and about 55% by weight.
In a specific embodiment, these processes include contacting the steam and oxygen generated by the reaction of the water and hydrogen peroxide with a hydrocarbon fuel to form a hydrogen reformate stream which is then supplied to the a second hydrocarbon reforming device. The hydrocarbons used in this system are preferably at least one of methane, methanol, ethanol, gasoline, diesel, DME, JP5, and JP8. Most preferably, the hydrocarbon is liquid ethanol. Similarly, the chemical reaction process pathways of this reforming system may be subdivided into at least two parallel process paths which may be isolated from the other process pathways. The parallel process pathways preferably have a uniform geometry and an accessible interior containing at least two catalytic beds. Preferably, at least one element composed of at least two materials interconnects the catalytic beds. The elements are typically an assembly of at least one rod and at least one plate in contact with the rod. This reforming system may also use either hydrogen peroxide as the sole oxygen source or air as the sole oxygen source.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a traditional autothermal type steam reformation of hydrogen from hydrocarbon fuels.
FIG. 2 is a block diagram of a process of the present invention in which hydrogen peroxide is used as a source of oxygen, heat and steam to support a traditional reforming process.
FIG. 3 is a block diagram of a process of the present invention in which hydrogen peroxide reacts with a hydrocarbon fuel to create a hot gaseous stream of hydrogen and by-products, which are introduced into the inlet of the primary, air consuming, reformer.
FIG. 4 is a cross section of a design layout for a system of the present invention that integrates hydrogen peroxide delivery with an air dependent reformer.
FIG. 5 is a cross section of a design layout for a system of the present invention that integrates hydrogen peroxide delivery with a hydrocarbon fuel into a stand alone reforming system.
FIG. 6 is a cross-section of a design layout of different passive thermal transfer devices useful in the methods and systems of the present invention.
FIG. 7 is a cross-section of a design layout of a specific thermal transfer embodiment of the present invention.
FIG. 8 is a cross-section of a design layout of a device of the present invention used for passive thermal transfer from one reforming section to another.
FIG. 9 is a process block diagram for the creation and use of concentrated hydrogen peroxide according to methods of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The ability of an air based reforming system to respond to any increase in hydrogen production demand is highly dependant upon that system's ability to provide appropriate thermal increases to the water feed stock, catalytic beds and other thermal masses of the containment components. The rate at which the thermal management system must respond is identical to the rate at which demand for hydrogen production changes. One system level approach to supplying more hydrogen for transient load increases, while the reformer's thermal management system struggles with maintaining the necessary process conditions, is to produce and store excess pressurized hydrogen in holding tanks.
An alternative approach to obtaining supplemental hydrogen from a holding tank would be to introduce a supplemental, exothermic oxygen source at the beginning of the primary reforming process. To be effective, the oxygen source would ideally need to be free or substantially free of contaminates such as nitrogen and other compounds that either create a cleanup requirement or potentially damage the various catalytic beds or the fuel cell MEA. Furthermore, for any such supplemental oxygen generation/delivery subsystem to be effective, it must not only generate oxygen but also generate sufficient heat to ideally flash an excess quantity of liquid water into steam. These compounds can then be subsequently used to accelerate the reforming of the hydrocarbon fuel, which is itself a highly exothermic reaction. Such an approach meets both the immediate load following demand for increased hydrogen, and accelerates the addition of new thermal energy to move the primary reforming system to a higher rate of hydrogen production. When the primary reforming system has been brought up to temperature and can sustain the higher flow rate demand on its own, the supplemental oxygen, heat and steam system may be turned off.
Solid and liquid oxidizers are a traditional source for oxygen. When activated, most solid or liquid oxidizers release considerable quantities of thermal energy as they disassociate and release the chemically stored oxygen. The supplemental oxygen generation/delivery system needs to capture this energy for use in vaporizing water into steam and to deliver either the thermal energy or the desired steam directly to the steam reforming section.
While there are several solid oxidizers available, many of which have been used to provide emergency oxygen in airplanes, submarines or emergency rescue gear, a preferred approach utilizes a liquid oxidizer. Most preferably, the liquid oxidizer is diluted hydrogen peroxide (H2O2). Diluted hydrogen peroxide is a mixture of hydrogen peroxide and water. The mixture may also include stabilizers. The relative concentration by weight of peroxide varies from standard plant production of approximately 70% down to about 3% found in many over the counter preparations for household use. A H2O2 concentration of between about 65% by weight to about 45% by weight is the preferred concentration to best meet the requirements of the chemical reactions, as well as the general safety and handling requirements of any large market implementation. The dissociation of this compound (H2O2→Heat+H2O+O.sup.-) is highly exothermic and directly generates the three desired components of water, oxygen and thermal energy.
The use of hydrogen peroxide as the oxidizer is advantageous over the use of other oxidizers as there is no need to capture thermal energy and transfer it to liquid water. Instead, water content is directly vaporized without the need for heat exchangers and pumps, thereby avoiding further system cost and complexity. An additional benefit is that there are no contaminates that must be cleaned up in the primary reforming system. In particular, there is no nitrogen released. The lack of additional nitrogen in the supplemental oxygen stream prevents increasing the NOx formation rate within the primary reforming system, though total process mass does increase. Instead, the NOx cleanup catalysts of the primary reforming system can be brought up to the necessary operating temperatures as the NOx free reactants generate and distribute exothermic energy throughout the entire reformer system, prior to shutting down a supplemental oxygen system.
In selecting a solid or liquid oxidizer for this use, the impact of any associated chemical contaminates that would be released and how those contaminates would affect the catalytic beds or the fuel cell membrane must be considered.
FIG. 1 depicts one of the most recent process evolutions of steam reforming, specifically autothermal reforming, that the present invention can be applied to. Referring to FIG. 2, a holding tank for the hydrogen peroxide/water mixture (1) feeds a pressurizing pump (2) that provides the necessary energy to allow for the controlled micro-injection (60) of this liquid. The quantity of the peroxide and water mixture that is injected (60) into the Reaction Pressure Vessel (5a) is determined and controlled by the System Controller (4). The products of the dissociation of peroxide can be immediately introduced into the Primary Steam Reforming Section (10) or the flow rate of these products can be modified by an intermediate control valve (20). The additional hydrocarbon fuel required may be provided by the primary reforming system's fuel delivery mechanism.
One specific embodiment is shown as a process block diagram in FIG. 3. In this embodiment, a similar hydrogen peroxide and water mixture tank (1) holds the mixture and feeds it to the Pressurizing Pump (2) for subsequent injection (60) into the Reaction Pressure Vessel (5b). However, in this approach the Reaction Pressure Vessel (5b) is reconfigured to accept pressurized (9), micro-injection (80) of the hydrocarbon fuel in use by the Primary Steam Reformer (10), which is stored in a separate tank (8). The catalysts of the Reaction Pressure Vessel (5b) need to be reconfigured to provide not only for hydrogen peroxide dissociation as depicted in FIG. 2, but also for the subsequent catalytic reduction and water gas shift reactions required to reform the hydrocarbon fuel. The injection rates are controlled by a similar System Controller (4). The thermally energetic gas stream from the Reaction Pressure Vessel (5b) is either directly introduced at the appropriate process point of the Primary Steam Reformer (10) or an intermediate Flow Control Valve (20) may be used to fine tune this flow rate if needed. In either configuration, FIG. 2 or FIG. 3, there is a hot, thermally energetic gas flow that is introduced into the Primary Reforming System (10). This gas stream is immediately available to increase the rate of hydrogen production through the Primary Reforming System either directly, as in the system depicted in FIG. 3, or indirectly, as depicted in FIG. 2. For either configuration, the exothermic nature of the hydrogen peroxide reactions provide a means to accelerate the rate in which the catalytic beds in the Primary Reforming System are brought up to their functional temperature range or facilitates maintaining of the temperature range in those beds as the total mass flow is increased.
When an oxidizer is used that does not contain nitrogen or other contaminates, it is not necessary for the catalytic beds, which are required to cleanup these formations, to be immediately and fully functional. These beds can initially be well below their minimum operational temperature without causing an adverse effect upon the quality of the hydrogen reformate feed stream provided to the fuel cell. As long as the contamination-free oxygen source is used, these beds can be brought up to the particular minimum temperature ranges over time, in preparation to perform their particular cleanup function. The catalytic beds need to be at their operational functional level whenever the transition is made from the oxidizer back to air as the oxygen source.
As such, the current Flow Section through the reformer system can either be further heated to provide for increased reforming rates, or an entirely new Flow Section can be the recipient of all or part of the contamination-free reformate stream. In this latter process, the new "cold" Flow Section could be quickly brought thermally online in preparation for eventually performing all of the oxidation, water gas shift reactions and cleanup required when air is used as the oxidation source. During this time period, the supplemental hydrogen stream is immediately usable by the fuel cell stack, allowing for the entire Fuel Cell Power System (FCPS) to perform load following power production.
When a PEM type fuel cell is used, CO will be one of the incompatible compounds generated by these processes. The formation of CO can be controlled in the new Flow Section through both the manipulation of the oxidizer/water to fuel ratio as well as reliance upon the final CO clean up stage of the primary reforming system. The final CO to CO2 conversion process, typically referred to as the low temperature water-gas shift and the Selective Oxidation or Preferential Oxidation (PROX) process (FIG. 1), occur at reduced temperatures, compared to those found in the upstream hydrocarbon reduction/oxidation beds. These components may need to be adapted, relocated, and/or duplicated to be able to fully perform their function of CO cleanup when peroxide is being used to bring up a new Flow Section through the steam reforming system.
In a preferred embodiment, depicted in FIG. 4, the primary steam reforming system is configured such that there were multiple, insulated, independently selectable Flow Sections (10a, 10b, 10c) through all or the majority of the temperature critical oxidation and gas cleanup sections. The number and relative size differences of these separate sections, if any, are a function of the overall Fuel Cell Power System (FCPS) duty cycle. The ratio between the FCPS turndown capability (y:1) and the primary reformer turndown (x:1) is a design/analysis starting point to determine the actual number and configuration of required Flow Sections, namely (y/x). For example, if the FCPS needs to work over a 6 to 1 turndown range in power output (say 60 kWe maximum to 10 kWe minimum) but a reforming process can only manage a 2 to 1 turndown (60 kWe to 30 kWe), then the total volume of the 60 kWe capable reformer would need to be divided into a minimum total of 6/2=3 Flow Sections, (i.e., three, 20 kWe Flow Sections). As the FCPS increased its demand for hydrogen from 10 kWe to 30 kWe, the reformer will increase output from 50% in just one section to meet the kWe flow demand to 100% in the first section and 50% output in a second section. At 100% of the FCPS power production of 60 kWe, all three Flow Sections will be at their 100% hydrogen production capacity. There are many significant system level difficulties faced by current reforming systems when they are commanded to transition from lower hydrogen production levels to higher levels. The use of hydrogen peroxide in the processes of the present invention is specifically intended to facilitate that transition.
Cost is always a design issue for consumer-oriented products. Two broad approaches of addressing cost through design are to minimize the total number of components and to identify simpler manufacturing and assembly approaches to those remaining components. The need to meet specific performance requirements is typically a countering design factor that has the tendency to increase cost. With that perspective, the oxygen assist approach of the present invention, which is generally characterized by the process block diagram in FIG. 2, represents a simpler system. Since the primary Reformer System (10) already has a fuel delivery system (80), as well as volumes within its structure for catalysts (40, 90, 100, 110), duplicating those components in a Hydrocarbon Oxidation/Reduction Vessel (5b), as illustrated in FIG. 3, adds to overall system cost and complexity. Indeed, to minimize the addition of more components, it may be preferable to remove the Reaction Pressure Vessel (5a) of FIG. 2 and integrate the remaining components of FIG. 2 into the Primary Reforming System (10). The peroxide/water injector (60) and the necessary catalysis and/or electrical heated ignition grid (70) to initiate the dissociation of the peroxide could be integrated directly into each Flow Section (10a, 10b, 10c) of the Primary Reforming System (10). The fuel injection/delivery mechanism (80) would need to be located subsequently along each Flow Section. Since the oxidation process of the hydrocarbon fuel is exothermic, a means of transferring that thermal energy gain to the endothermic generation of steam is provided and/or relocated to the Flow Section structure. By using an approach that encourages internal thermal conduction, thermal loss to the environment is minimized. To realize this goal, a series of mechanical rods (50), shaped to maximize thermal transfer, may be embedded with the various catalysts thereby mechanically interconnecting the exothermic oxidation and/or reduction catalyst beds (90) to the endothermic steam generation bed (40). The thermal transfer rods (50) may connect to other catalytic beds as required to facilitate creating and/or maintaining elevated temperatures of the gas clean up catalysts (100), (110). When the Flow Section and the associated beds are up to temperature to allow for transfer from peroxide as the sole oxygen source to eventually the ambient air, water can be pressure mist injected (30) into the vaporization bed (40). A throttle valve (20), used for control of external air flow and/or air/steam flow through the Flow Section, may then be opened to gradually transition the Flow Section from peroxide to 100% air as the oxygen source.
The following description illustrates how the above process of using hydrogen peroxide as a thermal/process assist functions in a hydrocarbon reforming system, starting from a "cold" non-operating state. Referring to FIG. 4, to allow for near immediate delivery of fuel cell safe, hydrogen reformate, all throttle control valves (20a, 20b, 20c) remain closed (as illustrated by throttle control valve, 20a) isolating all of the reforming Flow Sections from ambient air and its associated nitrogen contaminates. The peroxide/water injector (60) of at least one Flow Section begins to pressure inject this solution in a manner that encourages fine misting of the liquid. Thermal rise of the Flow Section begins as the hydrogen peroxide mist makes contact with the Dissociation Catalyst (70), which may also contain an electrical heating grid/ignition element to further ensure dissociation. The hydrocarbon fuel is then pressure injected (80) down stream of the Dissociation Catalyst/Ignition Element in a manner that encourages fine misting of the fuel. Pre-heating of the minute quantities of fuel, immediately prior to injection, would further accelerate the initial startup sequence by enhancing the thermal content of the fuel and improving the misting properties during injection by reducing the droplet size. A similar pre-heating of the small quantities of the hydrogen peroxide/water solution, just prior to its pressurized injection into the Flow Section is another means to facilitate its dissociation and release of thermal energy to the Flow Section.
The dissociated peroxide, (steam and oxygen mix) with the injected hydrocarbon fuel enter the reduction and/or oxidation catalyst bed (90) to begin stripping hydrogen atoms from the hydrocarbon. Since this reaction is highly exothermic, efforts are typically made to transfer this energy to the task of pre-heating the input fluids, primarily the water and the hydrocarbon fuel. To this end, thermal conduction rods (50) begin the transfer of thermal energy to the steam generating beds (40) in preparation for the transition back to air as the oxygen source. While the Primary Reforming System (10) may continue to use other external, thermal transfer means to heat process water, the depicted approach (30, 40) assists the reforming system (10) to quickly adjust to smaller increases in hydrogen production demands without the need to inject any hydrogen peroxide/water solution. The thermal conduction rods (50) may also be extended from the highly exothermic oxidation/reduction beds (90) to down stream catalytic beds (100, 110) used for gas clean up such as NOx to N2 and CO to CO2 conversion. The geometry, material construction, location and member of these rods (50, 50a, 50b, 50c of FIG. 6) can be adjusted, based upon process analysis to best meet the thermal transfer needs while minimizing cost. Thermal insulating sleeves (120), can be added to sections of the thermal conducting rods (50a, 50b, 50c of FIG. 6) to either avoid excess thermal transfer to a particular catalyst bed (100, FIG. 4) or to ensure sufficient thermal transfer to catalytic beds further downstream (110).
As the catalytic beds in those Flow Sections are actively brought up to operating thermal conditions, using hydrogen peroxide as the sole oxygen source, the flow control valve begins to open in a manner similar to that depicted by valve 20c, to allow ambient air or air/steam to begin to mix in with the reforming process. In turn, the rate at which the hydrogen peroxide/water solution is injected (60) would be proportionally reduced. Eventually, the active Flow Section would be operating entirely on air as the oxygen source.
At this point, the Primary Reforming System (10) may continue to operate on the active Flow Sections or bring a non-active Flow Section online using the process described above. The new Flow Section (for example 10b) is isolated from the external air or air/steam gas flow by fully closing the flow control valve (20b). As thermal conditions in Flow Section (10b) reach those required to allow for air operation, the flow control valve (20b) begins to open, initiating the transition to 100% air operation in that Flow Section.
At any point in time during the Primary Reforming System's (10) operation on 100% air, one or more of the Flow Sections may have additional, intermittent amounts of hydrogen/peroxide solution injected (60) into the Flow Sections to meet sudden increased demands for hydrogen production (transient load demands). Likewise, additional quantities of water may be injected (30) into the vaporization beds (40) to ensure sufficient process steam was also present for these transient load conditions.
There are those applications in which the cost of fully duplicated hydrogen generating systems, as depicted in the process diagram of FIG. 3, is necessary and justified. One example is those instances in which the hydrogen peroxide/water supported reforming is the only reforming system, as described in copending U.S. patent application Ser. No. 10/884,771 which is incorporated herein by reference. Other application requirements may require a much greater thermal transfer rate to heat the catalytic beds of the Primary Reforming System. This higher thermal transfer rate would be possible by introducing a much hotter gas stream into the Primary Reforming System than that provided by the previously described dissociation of hydrogen peroxide. The source of this much hotter gas stream (700 degree C. versus 100-200 degree C.) could be the fully oxidized/reduced hydrocarbon gaseous fuel stream exiting from the Pressure Reaction Vessel (5b) as illustrated in the process diagram of FIG. 3. FIG. 5 illustrates a design layout of how the components listed in the FIG. 3 process diagram could be configured. Since this approach does create a stand alone reforming system, it would need to have nearly identical functionality as depicted in the FIG. 4 reforming system.
The primary differences would be that the Reaction Vessel (5b) needs to be included and since all of the input compounds are liquids and the output product is a gas stream, the vessel (5b) is not a flow through design as shown in the Flow Sections (10a, 10b, 10c) of FIG. 4. The other components depicted in reforming Flow Sections of FIG. 4 need to be duplicated in the self contained reformer (5b) in order to preserve the necessary functionality. Hydrogen peroxide/water solution is injected (60) into the Vessel (5b) to be dissociated by catalysts and/or heated electrical grid (70). The hydrocarbon fuel is injected (80) and mixed with the dissociated hydrogen peroxide prior to entering oxidation/reduction catalytic bed (90). Thermal conduction rods (50) passively transfer the thermal energy from this exothermic bed, upstream to a steam generation bed. Additional water may be injected (30) into this thermal mass bed (40) to provide additional process steam when needed.
FIG. 5 does indicate additional catalytic beds (100, 110) for gas cleanup that may or may not be included in a particular system configuration. If the Reaction Vessel (5b) is the only reforming system, then these catalysts need to perform the preferential oxidation (PROX) and water gas shift of CO to CO2 in order to provide a fuel cell safe hydrogen fuel stream. There is however, no need for any NOx cleanup due to the absence of nitrogen in this process. In the embodiment employing a second system reformer (i.e. the Primary Reforming System (10)) it is advantageous to minimize the thermal loss to additional catalyst mass (100, 110) in the Reaction Vessel (5b) by leaving it out and relying upon similar catalysts in the Primary Reforming System to complete the gas clean up. This approach provides the Primary Reforming System with the hottest, most energetic gas possible for heat and brings the thermal mass of the primary system to operational temperature.
The effective thermal transfer of the heats of dissociation and combustion from the point of generation to the regions in need of thermal energy is an important determinant to the success of the assisted reforming approaches of the present invention. One approach to this energy transfer described above utilizes internal, thermal conductive structures (50, and 50a, 50b, 50c of FIG. 6) to transfer heat from high temperature exothermic catalyst beds (90) to endothermic beds (40, 100, 110) to minimize thermal loss to the environment and to accelerate thermal conduction to regions where the energy is needed.
Steam reforming of hydrocarbons utilizes several different catalytic beds to accomplish different tasks including steam generation, hydrocarbon fuel oxidation, NOx cleanup, and water gas shift for CO to CO2 conversion. All of these beds need to be at elevated temperatures in order for them to function as designed. Of the beds listed, only the catalyst beds and/or regions in which carbon oxidation/reduction is occurring will have an exothermic release of energy. This exothermic release has been traditionally used to heat the follow on catalytic beds to their required temperature range. A traditional mechanism to accomplish this thermal transfer has been the hot gas molecules themselves as they travel through the various subsystems of the steam reformer.
Because steam based reforming is being applied to generate hydrogen for fuel cells, and for PEM type fuel cells in particular, the product gas stream from the reformer can not be delivered to the fuel cell until the gas has been cleaned of CO, and NOx, assuming that there is no sulfur present. The clean up of these pollutants cannot occur to the degree necessary until the various catalytic beds are up to their operational temperatures. Steam reforming has another consideration that internal, thermal conduction structures would be of value. In particular, the heating and eventual formation of steam is a process that is upstream of the exothermic oxidation of the hydrocarbon fuel. Consequently, the use of the gas flow to transfer thermal energy is not possible. However, thermal conduction structures would be able to efficiently and cheaply transport the thermal energy against the flow stream. Effective thermal transfer to the upstream steam generation bed allows for reduced parasitic loads used to pump and heat water external to the reforming process. This will also allow for an increase in the reforming system's ability to generate steam, thereby improving the overall load response capability.
Referring to FIG. 6, the ideal shape and surface area ratios of the heat transfer structures (160) for heat absorption from the oxidation bed (90) and the preferred shape for the thermal release into the steam generation bed (40) or the gas cleanup beds (100, 110) are not identical in moving along the length of the thermal transfer rod (270). The actual number, size and spacing of heat transfer plate structures (160) is determined by the thermodynamic needs of the reforming system and the material limitations that are imposed. In one preferred embodiment, a core of thermally conducting aluminum (140, FIG. 6), or similar material, is wrapped with a thin exterior layer of temperature-resistant stainless steel or similar type material (150). The stainless steel sheath prevents the aluminum from reacting with the hot gases, and contains any aluminum that may melt or flow. Solid rods of temperature resistant stainless steel may be used (130, FIG. 6), though such types of steel offer poor thermal conductivity and use a considerable amount of relatively expensive metal. Due to the very large temperature extremes that occur within the catalytic beds, a bimetallic composite structure with significantly different thermal growth coefficients will need to be carefully designed to handle the differential thermal growth. Thermal insulators (120) may be used to preferentially direct thermal conduction past one region (100, FIG. 4) or regions to another region requiring an input of thermal energy (110). This approach aids in overall thermal management of the collective beds and preferentially accelerates the heating of selected beds (110 for example) during initial system startup or activation of a new Flow Section.
A preferred approach to implementing passive, thermal conductive heat transfer structures located within various catalytic and thermal mass beds is illustrated in FIG. 7. These transfer structures (50) need to be placed inside a thermally insulated (210) vessel (10) in such a fashion that the thermal and/or catalytic beds are well packed around them. In consideration of the need for a low cost assembly process, i.e., one that could be fully automated, a thermal transfer structure similar to (50c), best meets those requirements. This structure consists of a series of separate pieces each of which can be axially assembled within the Flow Section (10). Specifically, a thermal conductive mechanism of one or more thermal transfer rods (270) using a bi-material construction with an inner core (140) selected to maximize thermal transfer, and protected from the high temperature environment by a thin, thermally-resistant sheath (150) is preferred. A stop or machined feature (171, 172 respectively) is placed in the Flow Section vessel (10) to locate the thermal structure end plate (181 or 182 respectively). The end plate (181) provides location for one or more thermal transfer rods (270), thermal transfer plates (160) and bed separators (200). The thermal transfer rod(s) (270) can have a constant diameter along its length, or a continuing, sequential reduction in the cross-sectional area along its length to facilitate the sequential press fit placement of components (160), (200), (120) during assembly. One means to fully automate the assembly of the entire structure is to press one or more thermal transfer rods (270) into the first end plate (182) and insert the structure into the reaction vessel (10) until it locates to the stop (172). With the assembly (10, 182, 270) in a vertical position and connected to a vibrator mechanism, and with the free end of the rods (270) rigidly located to the walls of the Flow Section vessel (10), (by an external assembly device), the first section of the first bed (110) can be added while vibrating the structure to ensure the necessary degree of compaction. The first thermal transfer plate (160) is pressed into location after the desired level of catalyst bed material has been added, further compacting the bed if necessary. This is repeated sequentially for the remaining sections of catalyst beds (110, 100, 90), dividing and separating each by a correctly spaced thermal transfer plate (160). Thermal insulators (120) can be used to preferentially allow heating of one bed faster or to a greater degree than another catalytic bed. An example would be the desire to heat the low temperature water gas shift bed (110) for CO to CO2 conversion as quickly as possible to ensure minimal CO being entrained with the gas outflow stream. A bed separator (200) may be added to provide a mixing volume (192) for the injected hydrocarbon fuel (80) and the steam/oxygen mixture. A similar volume (191) can be created for injecting and dispersing the hydrogen peroxide/water solution (60) upstream of the dissociation elements (70). The final end plate (181) and retaining ring (171) may be installed to prevent movement of the assembled catalytic beds, independent of the vessels orientation or vibration during shipment or use. The Flow Section (10) is then enclosed with a cap (280).
Another passive, low cost, thermal transfer mechanism that greatly decreases the time required to bring an adjacent, non-active Flow Section fully up to operating temperature is depicted in FIG. 8. FIG. 8 shows the most downstream components of any of the above described Flow Sections, whether it represents a configuration as depicted in FIG. 4 or FIG. 5.
As previously described, the ability of a steam reforming system, whether utilizing air or another oxidizer source, to generate a fuel cell safe hydrogen feed stream is dependent upon being able to bring the temperatures of the various catalytic beds up to operational temperatures. A subdivision of the steam reforming flow path into separate, independent Flow Sections has been presented in this disclosure as a means to reduce the mass, and therefore the time required, to heat just a portion of these critical catalytic beds to the necessary temperatures. Another addition to that concept is the utilization of the thermally energetic gaseous flow that has left the active Flow Sections to pre-warm the beds of the inactive sections. Once the reformate gas stream (250) has left a Flow Section, its thermal energy must be reduced from many hundreds of degrees Centigrade to less than approximately 80 degree C. FIG. 8 illustrates a method to capture some of this unwanted thermal energy using a heat exchange device (240) in the gas stream and to passively transfer (230) this energy back "upstream" into the inactive Flow Sections. FIG. 8 shows two such Flow Sections, the primary Section and other Sections. The Primary Flow Section is considered to be the first Flow Section that is ignited during every startup of the hydrogen gas generation system.
Once the product gases have left the catalytic beds of the Primary Flow Section, the thermal energy of that gas stream is lost to the Primary Flow Section. In order to recover additional benefit from the thermal energy of that gas stream, additional components are required. This additional recovery may be accomplished by using a gaseous compatible heat exchanger (240) device to transfer thermal energy from the exhaust (250) of the Primary Flow Section to secondary, tertiary flow sections, and so on, into adjacent Flow Sections using conduction through transfer rods (230) into heat transfer plates (220). Thermal energy applied to the heat transfer plates (220) will be conducted into the structure end plates (180), beginning the thermal heating of the adjacent Flow Section and associated catalytic beds through the previously described thermal transfer structures (50).
The reformate discharge region (250) of each Flow Section needs to be configured to facilitate the attachment of a discharge manifold (260) that interconnects all of the various Flow Sections into a common discharge flue/manifold. The gaseous heat exchanger is preferably located in the common flue area of the discharge manifold, thereby offering the opportunity to transfer thermal energy to all possible Flow Sections. This presents the ability to preferentially conduct thermal energy from the primary Flow Section to the secondary Flow Section by selecting unique cross-sectional areas of the thermal transfer rods (230). Preferential energy transfer to sequential, non-primary Flow Sections can be accomplished by designing the thermal transfer rods to each subsequent Flow Section to have different thermal conductance or capacitance. This approach is especially effective if each Flow Section has an independent external airflow control valve (20). A closed airflow control valve allows for no cooling gases (air) to flow past the catalytic beds of an adjacent Flow Section, the ability to transfer thermal energy to that Flow Section is significantly improved. As the secondary Flow Section is brought online, thermal energy will quickly stop flowing into that Section as the exothermic output exceeds the conducted thermal energy potential of (240), (230) and (220). At this point, the tertiary Flow Section receives the majority of the recovered thermal energy of the combined gaseous discharge streams of the primary and secondary Flow Sections. FIG. 8 does not show a thermal transfer rod (230) or a thermal plate (220) inserted into the Primary Flow Section. Though these devices may be placed in this Section, this is only beneficial to starting the system if there is an external or alternative heating means applied to the gaseous heat exchanger (240). Otherwise, a physical connection to the thermal transfer structure (50) of the Primary Flow Section increases the time required to bring the various beds of that Flow Section up to operational temperatures.
A final mechanism which dramatically increases the rate of thermal transfer to the Flow Section of either process configuration represented by FIG. 2 or FIG. 3 is the concentration level of the hydrogen peroxide itself. The greater the water content in the diluted hydrogen peroxide solution, the greater will be the portion of the dissociation energy absorbed by water. At some point, the quantity of water may become so great that there is insufficient energy to vaporize all of the water without an additional heating source. One source of this additional heat may be one of the active Flow Sections previously described utilizing the various passive thermal transfer mechanisms previously described. Hydrogen peroxide commonly ranges in concentrations from about 70% by weight to about 3% by weight. A 70% by weight peroxide is the typical output from production plant and is aggressively and highly exothermic in its dissociation. The concentration levels compatible with the hydrocarbon reforming processes of this invention are preferably between about 55% by weight and about 40% by weight. Public safety issues would tend to strongly encourage the use of the lower concentration levels of hydrogen peroxide if not further reducing those levels below about 40% hydrogen peroxide by weight. As previously described, once the reforming process is occurring, there is an over abundance of thermal energy available for use. While catalysts are more commonly used to dissociate hydrogen peroxide, thermal energy, in and of itself is sufficient to dissociate hydrogen peroxide.
The only difficulty with low concentration levels of hydrogen peroxide would be during the initial startup of a cold reforming system. At "cold" start up conditions, the diluted hydrogen peroxide solution would still dissociate but the overall temperature will be reduced due to the presence of greater water concentrations and the lack of any additional heat from the reforming process. One solution to this problem, for either type of reforming system generally described by FIG. 2 or FIG. 3, is the use of high concentrations of hydrogen peroxide to generate an initial large thermal surge within the Flow Section (10). Then, as other thermal increases occur from the early chemical oxidation/reduction reactions of the hydrocarbon fuel, a transition can be made back to the more dilute hydrogen peroxide to continue the start of the Flow Section. While it is possible to manually fill two separate hydrogen peroxide tanks, one with a low concentration and the other with the higher concentration, this solution is only marginally better from a public safety perspective because there is less total volume of the higher concentration hydrogen peroxide being handled and shipped around the country.
A preferred approach to address the thermal starting issues with highly dilute hydrogen peroxide includes the use of a concentrated quantity of the peroxide specifically for starting. To avoid the economic impracticality of two different concentrations of hydrogen peroxide available at all fueling stations, a portion of the diluted peroxide could be diverted from the system holding tank (1, FIG. 9) and then thermally heated (13) to evaporate off excess water, thereby increasing the concentration of the peroxide. This system is illustrated in FIG. 9. Since hydrogen peroxide has a boiling point about 50° C. greater than water, it is relatively easy to preferentially evaporate water from the dilute mixture, thereby increasing the concentration of peroxide over water.
The concentrated peroxide is then exclusively used to start the entire reforming process. By design, the concentration of the hydrogen peroxide is sufficiently high to completely vaporize all water present upon its dissociation as well as contain sufficient thermal energy to begin the heating of the Flow Section components.
The concentration of peroxide needed may be determined through various approaches including measurement of specific conductance, pH, vapor pressure or density. All of these values vary in a quantitatively measurable and predictable fashion as the relative percentage of hydrogen peroxide varies. A direct or indirect measurement approach will need to be taken to determine the peroxide concentration. Concentrating the hydrogen peroxide beyond about 65% to about 70% peroxide by weight is not beneficial. In fact, beyond this concentration, there is insufficient water (steam) to oxidize a fuel such as ethanol. The need for subsequent humidification of the final hydrogen feed stream, further increases the minimum amount of water required. Additionally, peroxide in concentrations greater than 70% becomes an increasingly aggressive oxidizer and a potential health hazard if it comes into contact with skin. Measurement and control efforts (15 and 4) would need to be taken to assure that a maximum percent concentration as determined by the system requirements is not exceed.
There are several potential thermal sources (14) that can be used to evaporate excess water from the diluted hydrogen peroxide/water solution. The best approach varies depending upon the system requirements of the application in which the Fuel Cell Power System is installed. For an automotive application in which there is a significant electron storage capacity, (regenerative braking, for example) an electrical heating element would likely be the simplest and most cost efficient means of providing the thermal energy (14) necessary to evaporate the excess water. For other applications, the hot reformate gas stream can be the thermal energy (14) and allowed to pass through a heat exchanger, thereby transferring the necessary heat to vaporize the water directly. A more indirect approach is to pass the coolant from existing FCPS heat exchangers (14) through a coiled tube inside the hydrogen peroxide Concentrating Vessel (13). Two or more of the above methods can be integrated to meet multiple application requirements. For example, passive methods can be the primary mechanism used to concentrate the peroxide in order to minimize parasitic energy consumption. A back up system of indirect electrical element heating may be used as an emergency starting capability.
The Hydrogen Peroxide Concentrating Vessel (13) needs to have several features. First among these is the need for the material to be compatible with concentrated H2O2. Secondly, the vessel would need to be sealed, or sealable, from the environment as well as pressure capable since the fluids are being heated, which will increase the vapor pressure. Since vapor pressure, as a function of temperature and percent concentration of peroxide is one means to determine concentration, this is one preferable approach to concentration determination (15, 4). Upon heating the peroxide in the concentration vessel to a measured, predetermined temperature value, there will be some unique vapor pressure specific to the percent concentration of peroxide to water. At a predetermined, and/or controller-selectable vapor pressure point, selected to maximize the rate of water evaporation or oppositely minimize the quantity of hydrogen peroxide contained in the trap vapor, a vent (16) could be opened to vent the water vapor. The continued heating and subsequent venting of vapor would continue until a determination was made that the hydrogen peroxide solution was at the necessary percent concentration. The vessel also needs to include a safety vent mechanism in the case of run away pressure buildup. A second peroxide concentrating vessel may be added to ensure an adequate supply for all start/stop scenarios. While one vessel, which has previously concentrated its quantity of peroxide to the necessary level, is being used to start the reformer system, a second vessel is active in concentrating its portion of diluted peroxide.
In order to minimize the loss of peroxide during this process, temperatures well away from the boiling point of peroxide may be selected. The process controller is able to select the most appropriate temperature value and adjust that value as the concentrating process proceeds or the starting demands change. Because there is substantial run time after each start of the FCPS, the concentration of peroxide need not be an aggressive accelerated process.
The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain the best mode known for practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with various modifications required by the particular applications or uses of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.
Patent applications by Jeffrey L. Barber, Golden, CO US
Patent applications by Jeremiah J. Cronin, Longmont, CO US
Patent applications by CBH2 TECHNOLOGIES, INC.
Patent applications in class Catalyst in at least one stage
Patent applications in all subclasses Catalyst in at least one stage