Embodiments described herein concern catalytic combustion of hydrogen. One embodiment concerns catalytic combustion of hydrogen in the fuel cell exhaust of a fuel cell powered vehicle.
Polymer Electrolyte Membrane (PEM) fuel cells emit exhaust gas during operation that is primarily air, water and hydrogen. The concentration of hydrogen in the exhaust from a PEM fuel cell during steady state operation is relatively low. A simple calculation using the commonly accepted hydrogen utilization, see for instance U.S. Pat. No. 6,569,549, and air utilization gives a concentration of hydrogen in the combined anode and cathode exhaust stream of about 1%. For short periods during a change in operation of the PEM fuel cell, the amount of hydrogen emitted can be significantly larger than is emitted during steady state operation. Hydrogen in fuel cell exhaust can create a risk of uncontrolled rapid hydrogen combustion.
Embodiments described herein relate to systems and methods for reducing hydrogen concentration in a stream of gas that may be exhaust from a fuel cell. One embodiment provides a catalytic hydrogen combustor comprising a monolith that forms a plurality of passageways that extend along a flow direction from an inlet to an outlet. A first coating containing hydrogen combustion catalyst is disposed on walls of the monolith passages in a first zone of the monolith that extends from the inlet to a transition location between the inlet and the outlet. A second coating containing hydrogen combustion catalyst is disposed on walls of the monolith passages in a second zone of the monolith that extends from the transition location to the outlet. The second coating is formulated to catalyze hydrogen combustion at a greater rate than the first coating.
Another embodiment provides a method of reducing an amount of hydrogen in fuel cell exhaust for steady state operation exhaust flow that contains a low concentration of hydrogen and for exhaust flow containing a short duration pulse of higher concentration hydrogen. The method includes directing the flow of exhaust through a first catalytic combustion zone in which the exhaust is exposed to a first hydrogen combustion catalyst. Subsequently, the flow of exhaust is directed through a second catalytic combustion zone in which the exhaust is exposed to a second hydrogen combustion catalyst. The second catalytic combustion zone is configured and the second hydrogen combustion catalyst is formulated to catalyze combustion of substantially all hydrogen in a flow of fuel cell vehicle exhaust during steady state operation. The first catalytic combustion zone is configured and the first hydrogen combustion catalyst is formulated to catalyze combustion that at least substantially reduces the concentration of hydrogen in exhaust containing a pulse of higher concentration hydrogen.
Embodiments described herein concern catalytic combustion of hydrogen and an apparatus that accomplishes that combustion. More particularly, the embodiments concern catalytic combustion of hydrogen in a gas stream in which the concentration of hydrogen is not constant.
PEM fuel cells emit exhaust gas that contains at least hydrogen, oxygen, water and nitrogen. A catalytic hydrogen combustor in a fuel cell exhaust should initiate combustion of hydrogen in fuel cell exhaust gas at a low temperature (ignition temperature) and should operate at a low temperature during the fuel cell operation. Catalyzed hydrogen combustion releases heat that raises the temperature of the exhaust gas, catalyst and surrounding structures. Overheating of a catalytic combustor can result in two undesirable consequences. High temperatures can sinter the catalytic component on the supporting monolith diminishing the effectiveness of the catalytic component to catalyze hydrogen combustion. Also, should temperatures reach the auto-ignition temperature of hydrogen, rapid and uncontrolled combustion of hydrogen could result.
The honeycomb element 16 of the monolith 12 allows gas to flow through the honeycomb element 16 from the inlet end 22 to the outlet end 24 and presents surfaces that form passages through which gas flows. The honeycomb element 16 may form a series of connected chambers that extend from the inlet end 22 to the outlet end 24 and the chambers should be sized and configured such that at least substantially all of the gas passing through the honeycomb element 16 will pass over surfaces of the honeycomb element 16 sufficiently to allow a catalyst on the surface to catalyze combustion of hydrogen in the gas passing through the honeycomb element 16. The configuration of the honeycomb element 16 is not limited to any configuration or configurations. The honeycomb element 16 shown by
The monolith 12 conducts heat and has sufficient mass and specific heat so that heat released by hydrogen combustion does not raise the temperature of the monolith 12 to an unacceptable level. The honeycomb 16 of the monolith 12 is formed of corrugated stainless steel foil that is approximately .04 mm thick. The monolith 12 may be of the LS type supplied by Emitec USA.
The honeycomb element 16 is coated by a ceramic and hydrogen combustion catalyst coating on the surfaces that define the channels 18. The hydrogen combustion catalyst is platinum and the supporting ceramic is gamma alumina. As discussed below, the amount of heat created by catalyzed hydrogen combustion in a region of the monolith depends on the concentration of hydrogen in the fuel cell exhaust and the concentration of hydrogen combustion catalyst particles on the surface of the monolith. The structure of a monolith with a platinum and gamma aluminum coating is known to those of skill in the art and is described by U.S. Pat. No. 7,610,752, which is owned by the assignee of this application, and is incorporated by reference herein.
Fuel cells that power vehicles operate for extended periods, minutes to hours at a steady state condition emitting exhaust gas of substantially constant composition. This steady state operation creates exhaust gas typically having a hydrogen concentration of less than one percent hydrogen in the combined anode and cathode fuel cell exhausts. During transitions in fuel cell load and operation, the fuel cell can emit a pulse of hydrogen into the exhaust gas that can result in the exhaust gas comprising ten to twenty percent hydrogen during for a period that lasts several seconds.
As shown by
An embodiment of a catalytic hydrogen combustor may comprise a monolith that forms a plurality of passageways that extend along a flow direction from an inlet to an outlet, a first coating containing hydrogen combustion catalyst may be disposed on surfaces of the monolith that are adjacent to passageways in a first zone of the monolith between the inlet and a transition location between the inlet and the outlet, a second coating containing hydrogen combustion catalyst may be disposed on surfaces of the monolith that are adjacent to passageways in a second zone of the monolith between the transition location and the outlet and the second coating formulated to catalyze hydrogen combustion at a greater rate than the first coating.
The first coating of the embodiment of a catalytic hydrogen combustor may comprise ceramic and a hydrogen combustion catalyst and the second coating may comprise ceramic and a hydrogen combustion catalyst. The ceramic of the first coating may be gamma alumina and the hydrogen combustion catalyst of the first coating may be platinum. The ceramic of the second coating may be gamma alumina and the hydrogen combustion catalyst of the second coating may be platinum. The weight percent of platinum in the second coating may be greater than the weight percent of platinum in the first coating. The weight percent of platinum in the second coating may be approximately 2 and the weight percent of platinum in the first coating may be approximately 0.33.
An embodiment of a catalytic hydrogen combustor may comprise a monolith that forms a plurality of passageways that extend along a flow direction from an inlet to an outlet and the monolith may comprise a honeycomb element and an outer shell that surrounds the honeycomb element from the inlet to the outlet. A first coating containing hydrogen combustion catalyst may be disposed on surfaces of the monolith that are adjacent to passageways in a first zone of the monolith between the inlet and a transition location between the inlet and the outlet, a second coating containing hydrogen combustion catalyst may be disposed on surfaces of the monolith that are adjacent to passageways in a second zone of the monolith between the transition location and the outlet and the second coating may be formulated to catalyze hydrogen combustion at a greater rate than the first coating. The first coating of the embodiment of a catalytic hydrogen combustor may comprise ceramic and a hydrogen combustion catalyst and the second coating may comprise ceramic and a hydrogen combustion catalyst. The ceramic of the first coating may be gamma alumina and the hydrogen combustion catalyst of the first coating may be platinum and the wherein the ceramic of the second coating may be gamma alumina and the hydrogen combustion catalyst of the second coating may be platinum. The weight percent of platinum in the second coating may be greater than the weight percent of platinum in the first coating. The weight percent of platinum in the second coating may be greater than the weight percent of platinum in the first coating. The weight percent of platinum in the second coating may be approximately 2 and the weight percent of platinum in the first coating may be approximately 0.33.
A method of reducing the amount of hydrogen in fuel cell exhaust for steady state operation exhaust flow that contains a low concentration of hydrogen and for exhaust flow containing a short duration pulse of higher concentration hydrogen may comprise directing the flow of fuel cell exhaust through a first catalytic combustion zone in which the exhaust is exposed to a first hydrogen combustion catalyst, subsequently directing the flow of fuel cell exhaust through a second catalytic combustion zone in which the exhaust is exposed to a second hydrogen combustion catalyst, the second catalytic combustion zone may be configured and the second hydrogen combustion catalyst may be formulated to catalyze combustion of substantially all hydrogen in a flow of fuel cell vehicle exhaust during steady state operation, and the first catalytic combustion zone may be configured and the first hydrogen combustion catalyst may be formulated to catalyze combustion that at least substantially reduces the concentration of hydrogen in exhaust containing a pulse of higher concentration hydrogen. The concentration of hydrogen in a pulse of higher concentration hydrogen may be reduced by the first catalytic combustion zone to an amount that is substantially the same as the concentration of hydrogen in fuel cell exhaust during steady state operation. The first hydrogen combustion catalyst and the second hydrogen combustion catalyst may each comprise a coating of platinum and gamma alumina and the weight percent of platinum of the first hydrogen combustion catalyst may be less than the weight percent of platinum of the second hydrogen combustion catalyst. The first hydrogen combustion catalyst may comprise a coating of platinum and gamma alumina having a weight percent of platinum of approximately 0.33. The second hydrogen combustion catalyst may comprise a coating of platinum and gamma alumina having a weight percent of platinum of approximately 2.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US12/44507 | 6/28/2012 | WO | 00 | 12/23/2014 |