This invention relates to fuel-gas generation systems for fuel cells and hydrogen generation equipment.
Fuel cells are increasingly becoming an alternative way of producing electricity for use in commercial and industrial establishments, electric vehicles, and homes. However, their rapid assimilation into society is being hindered by the high costs and hazards associated with using pure hydrogen as a source of fuel in the fuel cell, and the complexity of small-scale fuel processors that are incorporated into the fuel cell system. Various methods of producing a hydrogen-rich air stream for use as fuel in a fuel cell, by using easily available hydrocarbon fuels such as natural gas or gasoline as a raw-product, are currently under development. The successful development of such systems to avoid current problems will greatly facilitate the wider acceptance of fuel cells as a commercially viable source of energy.
Many existing fuel cell systems are, furthermore, currently not economically feasible due to the large number of components that go into their fabrication, which greatly add to the cost and complexity of maintaining such systems. These systems may also be very complicated to operate and maintain on an on-going basis. Because of their current arrangement, these systems are also relatively inefficient with respect to the quantity of fuel cell fuel-gas actually produced.
There is therefore a need for a fuel cell fuel-gas generation system which has fewer parts, is easier to fabricate and maintain, and which operates at a higher efficiency than currently available fuel cell fuel-gas generation systems.
In this specification, a xe2x80x9cwaste gas oxidizerxe2x80x9d (WGO) means a device wherein unused hydrogen rich fuel-gas or unrecovered hydrogen is oxidized before being vented to the atmosphere or to other post-treatment devices. The unused hydrogen rich fuel-gas may be from the anode of a fuel cell, and the unrecovered hydrogen may be from, for example, a Thermal Swing Absorber (TSA) or Pressure Swing Absorber (PSA). The unused or unrecovered hydrogen is sometimes referred to as tail gas or waste gas. In a preferred embodiment, the WGO is an anode-off gas oxidizer (AGO).
The waste gas may consist mostly of hydrogen, carbon monoxide, carbon dioxide, light hydrocarbons (such as methane), and water vapor. Oxygen is preferably added to enable the hydrogen, carbon monoxide, and hydrocarbons to be oxidized to water and carbon dioxide. The required oxygen may be provided either in the form of air or unreacted cathode off gas, or a mixture thereof, from a fuel cell which contains enough oxygen for the oxidation reaction.
Typically, a WGO may comprise an enclosed volume with a first inlet for the hydrogen-rich anode off gas or tail gas, a second inlet for the oxygen containing cathode off gas, a means for igniting the combustible mixture of hydrogen rich anode off gas and the oxygen containing cathode off gas within the enclosed space, and an outlet for the products of combustion from the enclosed space. The reaction of the hydrogen, carbon monoxide, and hydrocarbons in the anode off gas with the oxygen in the cathode off gas takes place in the enclosed volume of the WGO and the unreacted products leave the enclosed volume through the product outlet.
The enclosed volume can be within a pressure vessel or a pipe or a tube, which may be constructed of steel, stainless steel, steel alloy or another suitable metal. It could also be non-metallic such as glass, composite insulation, ceramics etc. The two inlets and the outlet can be either formed integrally with the vessel or they can be separate components which are attached by welding, soldering, brazing etc. The enclosed volume is preferably large enough to provide the required residence for the oxidation reaction to take place to the required degree of completion. The ignition means can consist of any suitable means for initiating and maintaining an oxidation reaction such as a spark-igniter, a flame rod, a hot electric resistance wire, or a heated metallic or ceramic matrix.
The WGO can be started up and brought up to operating temperature using auxiliary hydrocarbon fuels such as methane, propane etc. After the WGO reaches the operating temperature, the anode off gas or the tail gas can be introduced into the reaction chamber for oxidation of the hydrogen, carbon monoxide and the hydrocarbons to carbon dioxide and water. The auxiliary fuel can then be turned off and the WGO operating temperature can be maintained by the oxidation of the hydrogen, carbon monoxide and hydrocarbons in the anode off gas or tail gas to carbon dioxide and water.
In this specification, an xe2x80x9cautothermal reformerxe2x80x9d (ATR) is a device for the conversion of a mixture of hydrocarbon, steam, and oxygen to a hydrogen-rich gas. The hydrogen rich gas may or may not also contain carbon monoxide as a byproduct. An ATR may or may not utilize catalysts for carrying out the above conversion. However, the use of catalysts in the ATR reduces the average operating temperature of the conversion reaction.
In an ATR, the primary reactions which facilitate the conversion of the hydrocarbon to a hydrogen rich gas are a partial oxidation reaction and steam methane reforming (SMR) reaction. If catalysts are used for the conversion, the partial oxidation reaction is generally referred to as a catalytic partial oxidation (CPO) reaction. The CPO reaction for the conversion of methane is:
CH4+0.5(O2)xe2x86x92CO+2(H2)+heat
The CPO reaction is exothermic and therefore has the advantage of very fast response to a change in the hydrogen demand from the fuel cell. If a catalyst is not used, the operating temperature is higher.
The second reaction that takes place in an ATR is the SMR reaction which is described by the following chemical reaction:
CH4+H2O+heatxe2x86x92CO+3H2
This reaction is highly endothermic and may take place without a catalyst. However, a catalyst is typically used to enable the reaction to take place at a lower. The SMR reaction provides a higher quality of hydrogen in response to fuel cell hydrogen-load demand and improves the process efficiency. Heat energy for the endothermic SMR reaction is provided by direct heat transfer and heat from the partial oxidation of the hydrocarbon in the CPO reaction described above. Therefore, in an ATR, the exothermic CPO reaction is balanced by the endothermic heat of the SMR reaction.
The combination of the CPO and the SMR reactions in the ATR provides a gas stream with a higher concentration of hydrogen than that produced by the CPO reaction alone. However, this combination also provides a faster response to fuel cell hydrogen load demands than is possible with a SMR reaction alone.
While the ATR consists predominantly of the CPO and SMR reactions, some water gas shift (WGS) reactions may also occur within the ATR as described by the following chemical equation:
CO+H2Oxe2x86x92CO2+H2+heat
The WGS reaction reacts some of the CO generated during the CPO reaction with some of the steam to produce additional hydrogen.
Separate catalysts can be used for the CPO reaction and the SMR reactions. Alternatively, a combined catalyst in which both reactions take place can also be used. According to one aspect of the invention, there is provided an integrated reactor for producing fuel gas for a fuel cell, the integrated reactor comprising: an waste gas oxidizer (WGO) assembly having an associated WGO chamber, an inlet, an outlet and a flow path for exothermic gases produced in the WGO chamber; and an autothermal reactor (ATR) assembly located at least partially in the WGO chamber, the ATR assembly having an inlet means and an outlet means for process gases flowing therethrough and a catalyst bed intermediate the inlet and outlet means, at least part of the inlet means of the ATR assembly being located in the flow path of the WGO chamber.
In one aspect, the present invention relates to an integrated reactor configuration for the production of a fuel cell fuel-gas. More particularly, the invention provides for the integration of an autothermal reformer (ATR) assembly into an waste gas oxidizer (WGO) assembly. One of the benefits of integrating an autothermal reformer assembly into the waste gas oxidizer assembly is to enhance thermal integration so that the higher temperature heat generated during the operation of the waste gas oxidizer assembly can be used to advantage. This higher temperature heat may be transferred into the steam reforming section of the ATR assembly, allowing for decreased air consumption within the partial oxidation section of the ATR assembly. The lower air consumption increases the overall process efficiency and enhances the system operating characteristics.
The configuration of the integrated reactor of the invention, comprising the autothermal reformer (ATR) assembly within the waste gas oxidizer (WGO) assembly, has important applications in fuel processing subsystems that operate at under-oxidized stoichiometric ratios (SR) between 0.00 and 0.30. It has been found that the practical thermal neutral point (TNP) with heat loss considerations is at stoichiometric ratios of approximately 0.20 to 0.25 SR. The thermal neutral point is the operation point at which no net heat is generated within the ATR.
The addition of oxidant to the reactant mixture generates the heat necessary to sustain the endothermic reforming reaction and compensate for heat losses. The thermal neutral point refers to the minimum amount of oxidant addition necessary to balance the endothermic reforming loads and the exothermic partial oxidation reaction.
One important application for the invention is its use in fuel cell systems. These applications require fuel-processing subsystems that simultaneously meet high efficiency characteristics, low equipment costs, and flexible operation.
The integrated reactor of the invention has certain distinct advantages when compared with state-of-the-art systems. One such advantage is that the innovative integrated configuration of the reactor allows for operation of the autothermal reformer assembly using lower amounts of oxidant or air. This, in turn, results in the attainment of higher efficiencies because less fuel is directly processed with oxygen (for example, net 3 moles H2 per mole of CH4), and more fuel is directly processed with steam (net 4 moles H2 per mole of CH4). Additionally, the waste heat from the anode off-gas combustion is used by direct heat transfer to supply heat to the endothermic reaction in the steam reformer. Another benefit of the invention is that the integrated reactor configuration facilitates the transfer of heat within the reactor such that high quality (high temperature) heat generated in the combustor of the WGO assembly is used to preheat the process gas entering the ATR to heat the process gases which are flowing through the steam reforming section of the ATR assembly.
It will be noted that, although existing autothermal reformers for fuel processing may use the waste gas oxidizer reactor to generate steam, there is no direct thermal integration between the heat produced by the waste gas oxidizer with the process gas entering and flowing through the autothermal reactor. The process gases in conventional autothermal reactor systems are typically pre-heated only by heat exchange with the exiting product gas from the autothermal reformer itself, but receive no heat directly from the waste gas oxidizer. In conventional systems, moreover, steam generated by the waste gas oxidizer, or fuel/steam mixtures pre-heated by the waste gas oxidizer, occur separately and discretely, and are thereafter sent to the autothermal reformer. It will also be noted that, although existing steam methane reformers for fuel processing may use the waste gas oxidation reaction to directly heat the SMR catalyst, these reactors do not use CPO catalysts to provide the additional benefits of the ATR process.
Since the ATR assembly is integrated within the WGO assembly, more difficult fuels to reform, such as gasoline and diesel fuels, may be easily handled. In addition, the added flexibility of ATR introduces the ability to control the thermal environment of the reforming process in two ways. First , this environment can be controlled directly by increasing or decreasing the amount of air added to the process gases entering the ATR section and, second, the environment can also be controlled by increasing or decreasing the combustion intensity within the combustion section. The integrated ATR/WGO assembly may be connected to a plate type heat exchanger that functions to preheat the reformer process gases by heat exchange with the ATR section product gases as they exit the ATR section and prior to entering downstream reactors. Another unique characteristic of this embodiment is that the ATR process gases and the WGO combustion gases flow essentially in a counterflow configuration.
In one embodiment of the invention, the integrated autothermal reactor includes an external jacket in which process gases in the ATR assembly are pre-heated prior to entering the catalytic beds of the ATR assembly reactor zones. In a preferred embodiment, this ATR assembly is fully integrated with the primary WGO assembly such that heat generated by the WGO assembly combustion process is in contact with the external jacket of the ATR assembly. This may be considered as the pre-heating jacket in which the process gases, such as steam, fuel and oxidant, are heated. This heat can be used to increase temperature and to vaporize liquid fuels and/or water. In addition, embodiments of the invention provide an ATR reactor assembly within the WGO assembly whereby a primary steam generation jacket is also provided such that the thermal output from the WGO assembly is used to heat the primary steam used in the ATR assembly. In such an embodiment, therefore, heat generated by the WGO assembly first preheats both the process gases entering the ATR assembly as well as later vaporizing the water/steam which is a component of the process gases.
In yet a further embodiment, the autothermal reformer assembly may include one or a series of heat transfer elements, which may be appropriately located between the WGO and the SMR reactor zones, for example, between the catalytic chambers in a monolith catalyst container, and these heat transfer elements facilitate heat conduction directly into the process gases flowing within the SMR section of the autothermal reformer assembly. The heat transfer elements may comprise metal or other highly conductive components, such as heat pipes, that are appropriately shaped and located within the integrated reactor, to maximize heat conduction.
The invention will be described with further reference to the attached drawings.