Patent Application
20070154359
The invention generally relates to a de-sulfurization system and more particularly relates to an in situ method of conditioning a fixed bed de-sulfurizer.
A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. There are many different types of fuel cells, such as a solid oxide fuel cell (SOFC), a molten carbonate fuel cell, a phosphoric acid fuel cell, a methanol fuel cell and a proton exchange member (PEM) fuel cell.
As a more specific example, a PEM fuel cell includes a PEM membrane, which permits only protons to pass between an anode and a cathode of the fuel cell. A typical PEM fuel cell may employ polysulfonic-acid-based ionomers and operate in the 50° Celsius (C) to 75° temperature range. Another type of PEM fuel cell may employ a phosphoric-acid-based polybenziamidazole (PBI) membrane that operates in the 150° to 200° temperature range.
At the anode of the PEM fuel cell, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:
H2→2H++2e− at the anode of the cell, and Equation 1
O2+4H++4e−→2H2O at the cathode of the cell. Equation 2
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM.
The fuel cell stack is one out of many components of a typical fuel cell system, such as a cooling subsystem, a cell voltage monitoring subsystem, a control subsystem, a power conditioning subsystem, etc. The particular design of each of these subsystems is a function of the application that the fuel cell system serves.
In an embodiment of the invention, a technique includes providing an agent to remove sulfur compounds from a gaseous hydrocarbon and conditioning the agent. The conditioning includes communicating a first hydrocarbon flow to the agent and monitoring a second hydrocarbon flow that is produced by the communication. Based on a characteristic of the second hydrocarbon flow, a determination is made whether the conditioning is complete.
In another embodiment of the invention, a fuel cell system includes a de-sulfurization tank, a reformer, a pressure monitoring device and a fuel cell stack. The de-sulfurization tank includes an agent that is adapted to remove sulfur compounds and is capable of undergoing a conditioning cycle. The tank includes an inlet to receive a first hydrocarbon flow and an outlet to provide a second hydrocarbon flow. The reformer is adapted to convert the second hydrocarbon flow into a reformate flow, which is received by the fuel cell stack. The pressure monitoring device monitors a pressure of the second hydrocarbon flow, and a circuit of the fuel cell system is coupled to the pressure monitoring device to determine whether the conditioning of the tank is complete based on the pressure.
Advantages and other features of the invention will become apparent from the following drawing, description and claims.
Referring to
The incoming hydrocarbon flow may contain various sulfur compounds, such as intentionally added odorants for purposes of facilitating leak detection, as well as residual sulfur compounds that are left over from the well and processing plant. As a more specific example, the incoming hydrocarbon flow may contain mercaptans, thiophenes, H2S and COS. These sulfur compounds have the potential of harming components of the fuel cell system 10 if not significantly removed. For example, the sulfur compounds may poison the reformer as well as poison the catalysts in the membrane electrode assemblies (MEAs) of the fuel cell stack. Therefore, the fuel cell system 10 includes a de-sulfurization tank 70 to, as its name implies, remove sulfur compounds from the incoming hydrocarbon flow. The de-sulfurization tank 70 may include multiple fixed bed agents, one of which may contain an adsorbent agent bed, such as a zeolite-based agent, which must undergo a “conditioning cycle” (described further below) before the hydrocarbon flow that exits an outlet 72 of the tank 70 matches the incoming hydrocarbon flow to the tank 70. Thus, while being conditioned, the de-sulfurization tank 70 may not be able to supply an adequate hydrocarbon flow to the rest of the fuel cell system, thereby potentially causing errant operation of the fuel cell system. However, as described further below, the fuel cell system 10 includes certain features to detect the state of the de-sulfurization tank 70 for purpose of determining when conditioning of the tank 70 is complete.
The conditioning cycle is attributable to the high affinity that the zeolite-based agent has for the hydrocarbon molecules of the incoming hydrocarbon flow. Therefore, although the zeolite-based agent attracts (via absorption, chemisorption, physisorption or a combination of these mechanisms) sulfur-containing molecules from the flow, the pores of the zeolite-based agent initially attract a considerable amount of hydrocarbon molecules. Therefore, in order for the de-sulfurization tank 70 to finction as intended, the zeolite-based agent must become saturated with the hydrocarbons (i.e., conditioned) before the outgoing flow rate from the tank 70 matches its incoming flow rate.
In other embodiments of the invention, as will be appreciated by one skilled in the art, other de-sulfurization agents, other than zeolite-based agents, which have a high affinity for hydrocarbon molecules and need to undergo a conditioning cycle may be used in place of the zeolite-based agent as described in this application.
Thus, when the de-sulfurization tank 70 is new or has not been used for a significant period of time, a relatively large pressure drop occurs across the tank 70 between its inlet and outlet. In this state of the de-sulfurization tank 70, the flow from the tank 70 is unpredictable and is greatly reduced from the flow that enters the tank 70. This may cause errant operation of the downstream components that receive the outgoing flow from the de-sulfurization tank 70 if the existence of the conditioning cycle is unrecognized. At the conclusion of the conditioning cycle, the inlet and outlet pressures of the tank 70 are virtually the same, as well as its inlet and outlet flow rates.
Therefore, in accordance with embodiments of the invention described herein, the fuel cell system 10 has features (further described below) to detect the end of a conditioning cycle so that normal operations of the fuel cell system 10 may commence. It is noted that in the context of this application, the “conditioning of the tank 70” and the “conditioning of the zeolite-based agent” are used interchangeably
During its normal power producing operation, the fuel cell stack 20 receives its incoming fuel flow (provided by a fuel source 60) and oxidant flow (provided by an oxidant source 50, such as an air blower) at an anode inlet 22 and a cathode inlet 24, respectively. Inside the fuel cell stack 20, the fuel flow is routed from the anode inlet 22, through the anode flow channels of the fuel cell stack 20 and appears as anode exhaust at an anode outlet 28. It is noted that the anode exhaust may be routed back through the fuel cell stack 20 in accordance with some embodiments of the invention. In other embodiments of the invention, however, the anode exhaust may not be rerouted through the fuel cell stack 20. Furthermore, in accordance with some embodiments of the invention, the fuel cell stack 20 may be “dead-headed,” which means that the anode chamber of the fuel cell stack 20 is closed off so that no anode exhaust leaves the fuel cell stack 20. Thus, many variations are possible and are within the scope of the appended claims.
The oxidant flow is communicated from the oxidant inlet 24, through the cathode flow channels of the fuel cell stack 20 and appears as cathode exhaust at an oxidant outlet 26 of the fuel cell stack 20. It is noted that, depending on the particular embodiment of the invention, the cathode exhaust may be routed to a flare or oxidizer; or, alternatively, the cathode exhaust may be rerouted back through the fuel cell stack 20. In other embodiments of the invention, the cathode exhaust may be routed to a fuel processor 80 of the fuel cell system 10 to at least provide some of the air for the fuel processor 80.
Stack output terminals 30 of the fuel cell stack 20 provide a DC output voltage, a voltage that may be regulated to a particular DC level or to a particular AC voltage, depending on the type of load to the system 10.
The de-sulfurization tank 70 is part of the fuel source 60 that supplies the fuel to the anode inlet 22 of the fuel cell stack 20. The fuel source 60 also includes a fuel processor 80 that receives the outgoing flow from the de-sulfurization tank 70 and provides a reformate flow (i.e., the fuel flow to the stack 20) at an outlet 82 of the fuel processor 80. As an example, the fuel processor 80 may mix the incoming flow with steam for purposes of aiding an autothermal reformer or a steam reformer of the fuel processor 80. Besides the autothermal reformer or the steam reformer, the fuel processor 80 may include, as examples, low temperature shift (LTS) and high temperature shift (HTS) reactors as well as a preferential oxidation (PROX) reactor, in accordance with some embodiments of the invention.
For purposes of inducing a continuous flow through the de-sulfurization tank 70 during the conditioning cycle, the fuel source 60 includes a blower 75 that has its suction inlet connected to an outlet 72 of the de-sulfurization tank 70. The outlet of the blower 75 is connected to the inlet of the fuel processor 80. Thus, the blower 75 is controlled to establish a suction on the outlet 72 of the de-sulfurization tank 70 during the conditioning cycle. In some embodiments of the invention, the blower 75 may be a variable speed blower whose speed is varied during the normal course of operation of the fuel cell system 10; and the speed of the blower 75 may be set at its maximum for the duration of the conditioning cycle.
Among its other features, the fuel cell system 10 may include a controller 54 that regulates various operations of the fuel cell system 10. In this regard, the controller 54 may include a processor 56 (one or more microprocessors or microcontrollers, for example) that is coupled to a memory 58. The memory may store, for example, instructions that when executed by the processor 56 cause the processor 56 to perform various techniques, including the techniques that are disclosed herein. The controller 54 includes input terminals 55 that receive various status signals, indications of commands, etc. from the components of the fuel cell system 10. In response to the inputs received at the input terminals 55, the controller 54 produces various control signals on its output terminals 53 for purposes of controlling motors, controlling valves, communicating with other entities, etc.
For purposes of starting up the fuel cell system 10, the controller 54 determines when the conditioning of the de-sulfurization tank 70 is complete. One way to accomplish this is to measure a certain length of time, and when the time expires, it is assumed that the de-sulfurization tank 70 is conditioned. However, this technique may not be reliable.
As described herein, more reliable techniques to determine whether the conditioning cycle is complete involve monitoring a characteristic of the outgoing flow from the de-sulfurization tank 70.
For example, the outgoing flow rate from the de-sulfurization tank 70 may be monitored to detect the end of the conditioning cycle in accordance with some embodiments of the invention. Referring to
Alternatively, a pressure associated with the flow from de-sulfurization tank 70 may be monitored for purposes of detecting the completion of the conditioning cycle. For example,
Referring back to
In accordance with some embodiments of the invention, the pressure monitoring device 73 may be a pressure switch that provides a given signal at the output terminal 74 when the crossing of a predetermined pressure threshold is detected . Alternatively, the pressure monitoring device 73 may be a pressure sensor that provides an analog or digital signal at the output terminal 74, which is indicative of the measured pressure. As yet another example, the pressure monitoring device 73 may be a flow meter in other embodiments of the invention. It is noted that due to its relative cost and ease of use (little or no calibration, for example), the pressure switch may be the most desirable choice, although many variations of the pressure monitoring device are possible and within the scope of the appended claims.
Referring to
Many different embodiments of the invention, other than embodiments specifically described herein, are contemplated and are within the scope of the appended claims. For example, the fuel cell system 10 may use one of a variety of different fuel cell technologies. As non-limiting examples, the fuel cell stack 20 may include PEM-based fuel cells, alkaline-based fuel cells, phosphoric acid-based fuel cells, molten carbonate fuel cells or solid fuel oxide fuel cells (SOFCs). Furthermore, although a fuel cell system is described herein, the de-sulfurization bed conditioning and conditioning detection that are described herein may be used in connection with systems other than fuel cell systems, which use a hydrocarbon that passes though a sulfur-removing agent. Thus, many variations are possible and are within the scope of the appended claims.
While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.
