Patent Application
20050053816
This invention relates to a catalyzed burner which can combust the anode exhaust stream from a polymer electrolyte membrane (PEM) fuel cell to produce heat for use in a PEM fuel cell power plant.
Polymer electrolyte membrane (PEM) fuel cells operate at relatively low temperatures, typically in the range of about 100° F. (38° C.) to about 200° F. (93.3° C.), and often at essentially ambient pressure. A PEM cell anode exhaust gas stream primarily contains water, carbon dioxide and small amounts of hydrogen. For efficiency and emission reasons, the fuel remaining in the anode exhaust gas stream after it passes through the fuel cell power plant cells should be used in the operation of the PEM cell power plant. However, this cannot be done with a conventional metal burner. The inability to utilize the anode exhaust gas stream from a PEM fuel cell power plant to provide additional energy for operation results from: a) the high water and CO2 content in the anode exhaust stream; and b) the low hydrogen content of the anode exhaust stream. In addition, the high turn down ratio of flows required exceeds conventional burner capabilities.
It would be desirable to be able to utlize an anode exhaust gas stream in a PEM fuel cell power plant to provide energy for operating the power plant in order to improve system efficiency, and to provide reduced power plant emissions levels.
This invention relates to a burner which is operative to combust the anode exhaust stream of a PEM fuel cell power plant to provide energy for operation of the power plant.
A PEM fuel cell power plant is a low temperature power plant, and operates at a temperature in the range of about 100° F. (38° C.) to about 200° F. (93.3° C.), and preferably at about 180° F. (82.2° C.), and preferably at essentially ambient pressures. For PEM fuel cells using any form of steam reformer, steam production from the cell stack waste heat is not an option, as it is with 400° F. (204° C.) phosphoric add cells, so alternative steam production methods are required. As a result, the anode exhaust energy is the prime source for heat to create steam, but the anode exhaust consists largely of a small amount of H2, with CO2, water vapor and, in the case of autothermal reformer, catalytic partial oxidation reformer, or partial oxidation reformer units, some N2. The hydrogen in the anode exhaust stream is typically below the normal combustibility level, thus we employ a catalyzed porous burner to burn the anode exhaust gas stream.
The burner of this invention enables combustion of the PEM cell anode exhaust gas stream thus producing heat that can be used for producing steam for a reformer in the fuel cell power plant, or for other purposes in operating the power plant, or in its environs. The burner of this invention is impervious to damage from exposure to gasoline or gasoline combustion products which may be utilized during start up of the power plant. The burner of this invention includes a catalyzed porous ceramic open cell foam burner member. The catalyst which is coated on the burner can be platinum, rhodium, or palladium, and combinations thereof. The burner body is preferably an open cell metallic or ceramic foam which provides an open cell porosity that is in the range of about 70% to about 90%. With this degree of porosity, the majority of combustion of the gas stream takes place internally of the burner body, and the pressure drop from the inlet to the outlet of the burner body can be as low as about two to about three inches water at operating conditions. This degree of porosity also allows the burner operating pressures to be at essentially ambient pressure. The operating temperature of the burner can be as high as about 1,700° F. (927° C.), but is preferably less than 1,195° F. (646° C.).
The burner of this invention is particularly useful in mobile environs which utilize a PEM power plant to produce electricity on demand, which demand may vary. One such mobile environ is an automobile, bus, or other vehicles. Operating vehicles with electricity provided by PEM fuel cells wherein the anode exhaust gas stream produced by the cell stack is burned to provide heat for the system, requires that the burner have a relatively high turn down ratio. The phrase “turn down ratio” refers to the ratio of the maximum fuel and air flow rate to the minimum fuel and air flow rate. The burner of this invention has a 10:1 turn down ratio as compared to a conventional burner turn down ratio of 3:1, and the 10:1 turn down ratio cannot be met by conventional burners because of blow-off, flashback or extinction problems that conventional burners encounter. In addition, in automotive applications which operate at or near ambient pressures, overall system efficiency requires that the system inlet to outlet pressure drop including the burner be kept at a minimum.
It is therefore an object of this invention to provide a catalytic burner which is operative to combust the exhaust gas stream from the anode side of a PEM fuel cell power plant.
It is a further object of this invention to provide a burner of the character described that is not adversely affected by gasoline combustion products or PEM fuel cell power plant anode bypass gas during start-up of the power plant.
It is another object of this invention to provide a burner of the character described which has a high open cell porosity thus providing a very large catalyzed surface area per unit volume of the burner.
It is still a further object of this invention to provide a burner of the character described which can be operated at essentially ambient pressures and has a low burner inlet to burner outlet pressure drop.
These and other objects of the invention will become more readily apparent from the following detailed description of embodiments of the invention when considered in conjunction with the accompanying drawings, in which:
Referring now to the drawings, there is shown in
During startup the fuel gas stream bypasses the stack by being bled off from the line 38 through a line 52 which connects to the burner/mixer steam generator station 14 in order to provide additional fuel for heat up and to minimize emissions. A valve 54 serves to control the flow of fuel through the line 52, the valve 54 being actuated by a fuel cell power plant operating processor controller (not shown). Burner exhaust from the station 14 is removed from the station 14 via line 56 that directs the exhaust stream to a condenser 58 where water is condensed out of the exhaust stream. The water condensate is transferred from the condenser 58 to the water tank 48 through a line 60, and the dehydrated exhaust stream is vented from the power plant 2 through a vent 62. Water from the water storage tank 48 is fed to the steam generator station 14 through a line 64.
Once the fuel cell power plant 12 achieves operating temperature, the valve 54 will be closed and the valve 66 in a line 68 will be opened by the power plant controller. The line 68 directs the fuel cell stack anode exhaust stream to the station 14 wherein any residual hydrogen and hydrocarbons in the anode exhaust stream are combusted. The anode exhaust stream contains hydrogen, water and hydrocarbons. During startup of the power plant 12, the station 14 can be provided with air through line 70 and raw fuel for combustion through line 72 as well as anode bypass gas provided through line 52. The fuel can be natural gas, gasoline, ethanol, methanol, hydrogen or some other combustible material. Air is always provided to the station 14 through line 70 irregardless of the source of the combustible fuel.
Referring now to
Referring now to
The gasoline start up burner has two purposes. During start up, prior to operation of the catalytic burner, it is used to produce hot gas for steam generation. It does this by mixing finely atomized gasoline droplets with air, and burning the gasoline. Gasoline is introduced into the burner by means of a pressure atomizing fuel injector and mixed with the air which enters through a swirler and a series of primary and secondary dilution holes. Proper sizing and placement of the air entry holes produces a stable recirculation zone in the vicinity of the fuel injector which ensures stable combustion without the need to actuate an igniter once ignition has taken place. This also produces complete combustion of the fuel and a relatively even exit temperature profile.
The other purpose of the gasoline burner is as an air/anode exhaust mixer which premixes air and anode exhaust gas prior to combustion on the catalytic burner. The start burner functions in this mixer mode during normal power plant operation when the remaining hydrogen in the anode exhaust is burned on the catalytic burner to produce the steam needed for power plant operation.
During startup of the fuel processing system, the hot gas from the gasoline burner 74 is used to transfer heat into water which is pumped by a circulating pump 78 through the first heat exchanger 82 and thence through a second heat exchanger 88 and a third heat exchanger 89. The circulating pump flow rate is sufficiently high to maintain two-phase flow in the heat exchangers 82, 88 and 89 at all times. The two phase (liquid/gas) component flow which is maintained, simplifies control requirements and limits heat exchanger size. This two-phase flow stream is pumped into a steam accumulator 76, where the liquid water is recirculated back through the heat exchangers 82, 88 and 89, while saturated steam is extracted from the accumulator 76 for use in the fuel processing system. Feed water to the circulating pump 78 is provided to maintain the liquid level in the accumulator at appropriate levels. As the fuel processing system begins to generate low-quality reformate, this reformate bypasses the anode of the fuel cell and is fed into the mixing section of the gasoline burner 74 to be combusted.
During normal operation, the fuel cell anode exhaust is supplied to the burner/mixer 74 together with air. The burner/mixer 74 functions as an air/anode exhaust mixer. After mixing of the fuel cell anode exhaust with air, the resultant mixture is fed into the catalytic burner 2, without reducing its ability to operate as a gasoline burner during the start up phase. The anode exhaust mixture is combusted catalytically in the catalytic burner 2. Radiant and convective heat from the catalytic burner 2 is transferred to the heat exchanger coils 88, with the remainder of the convective heat transfer occurring in the heat exchanger 89. As during startup operation, the circulating pump 78 maintains two-phase flow in the heat exchangers and saturated steam is extracted from the accumulator 76.
It will be readily appreciated that the burner of this invention will enable the use of anode exhaust to be used as a source of heat for producing steam for operating a PEM fuel cell power plant due to the inclusion of a catalytic burner in the assembly. The inclusion of an auxiliary gasoline or other conventional hydrocarbon fuel burner allows the catalyzed burner to bring the fuel cell power plant up to operating temperatures prior to the use of the anode exhaust stream as a source of energy to produce steam for the power plant. The inclusion of an air swirler in the auxiliary burner portion of the assembly enables adequate mixture of air with the anode exhaust stream prior to combustion in the catalytic burner part of the assembly.
Since many changes and variations of the disclosed embodiment of the invention may be made without departing from the inventive concept, it is not intended to limit the invention otherwise than as required by the appended claims.
