This invention relates to fuel humidifiers and pre-heaters, and in more particular applications, to fuel humidifiers and preheaters for use in fuel cell systems, particularly molten-carbonate fuel cell systems.
A molten-carbonate fuel cell (MCFC) produces electricity by reacting hydrogen with oxygen and carbon dioxide. The electrolyte of an MCFC is molten mixture of alkali carbonates, which form a highly conductive salt at high temperatures (typically 600 to 700° C.). The carbonate CO32− is formed at the cathode by reacting carbon dioxide with oxygen, and passes through the electrolyte to the anode, where it reacts with hydrogen to form water. The electrons formed at the cathode do not pass through the electrolyte, and thus pass through an external circuit. The cathode, anode, and net reactions are summarized as follows:
O2+2CO2+4e− 2CO32 (Cathode)
H2+2CO32− 2H2O+2CO2+4e− (Anode)
H2+½O2+CO2 H2O+CO2 (Net)
The hydrogen is usually generated from a hydrocarbon (i.e., natural gas, propane, coal, etc.) internal to or upstream of the MCFC via the highly endothermic steam reforming reaction. In either case, a fuel needs to be pre-heated and humidified to a specified temperature using a hot gas (for example cathode exhaust gas (CEG)). In addition, an MCFC typically requires long start-up times (i.e., transient conditions on the order of days), and can be operated under a variety of load conditions. These conditions can consist of at least variable steam to fuel ratios, steam and fuel flow rates, and variable hot gas inlet temperatures and flow rates. An example of this is shown in Table 1. Cases 4 and 5 represent two stages of start-up (end and beginning, respectively) with the remaining cases comprising various electrical power outputs.
There is a continuing need for new and improved systems and methods for humidifying and heating the fuel for fuel cell systems, and in particular for MCFC systems.
In accordance with one feature of the invention, a fuel humidifier/pre-heater unit is provided for pre-heating and humidifying a fuel flow provided by a fuel supply.
According to one feature of the invention, a fuel cell system is provided and includes a molten-carbonate fuel cell and a fuel humidifier/pre-heater unit for pre-heating and humidifying a fuel flow to the molten-carbonate fuel cell.
In one feature, the unit includes a steam generator, a water bypass, a liquid/steam mixer, and a mixture heater. The steam generator includes a water flow path in heat transfer relation with a hot fluid flow path to generate a vaporized water flow. The liquid/steam mixer is connected downstream from the water flow path to receive the vaporized water flow therefrom and downstream from the water bypass to receive a liquid water flow therefrom. The mixture heater includes a mixture flow path in heat transfer relation with a hot fluid flow path, the mixture flow path being connected downstream from the liquid/vapor mixer and the fuel supply.
According to one feature, the hot fluid flow path of the steam generator is located downstream from the hot fluid flow path of the mixture heater with respect to a hot fluid flow.
According to another feature, the hot fluid flow path of the mixture heater is located downstream from the hot fluid flow path of the steam generator with respect to a hot fluid flow.
In one feature, the unit further includes a water bypass control valve connected upstream of the steam generator and the water bypass to selectively direct liquid water flows thereto.
In accordance with one feature, the steam generator and the liquid/steam mixer are an integrated unit.
In accordance with another feature, the liquid/steam mixer is located external from the steam generator.
According to one feature, the unit further includes a steam/fuel mixer connected downstream from the liquid/steam mixer to receive a superheated steam flow therefrom, downstream from the fuel supply to receive a fuel flow therefrom, and upstream from the mixture flow path to supply a steam/fuel mixture thereto.
In one feature, the unit further includes a fuel bypass, and a fuel/humidified fuel mixer connected downstream from the mixture heater to receive a humidified fuel flow therefrom and connected downstream from the fuel bypass to receive a fuel flow therefrom. In a further feature, the unit further includes a fuel bypass control valve connected upstream from the fuel bypass and the mixture heater with respect to the fuel flow.
According to one feature, the steam generator includes a helical-wound tube with a water inlet and a steam outlet. The water inlet is located vertically lower than the steam outlet.
In accordance with one form of the invention, a method is provided for humidifying and pre-heating a fuel flow. The method includes the steps of:
In one feature, the superheating of step c) and the heating of step e) are accomplished by transferring heat from a hot fluid flow to the water flow in step c) and the mixture flow in step e).
According to one feature, the hot fluid flow transfers heat in step c) before it transfers heat in step e).
According to another feature, the hot fluid flow transfers heat in step e) before it transfers heat in step c).
In one feature, step d) includes mixing the water flow resulting from step c) with all of the fuel flow.
In accordance with one feature, step c) includes selectively adjusting the first and second portions of the water flow to achieve a desired temperature for the mixture flow resulting from step e).
According to one feature, the method further includes the step of f) modulating the temp of the humidified and pre-heated fuel flow by mixing the mixture flow resulting from step d) with a second portion of the fuel flow that has not been heated in step e).
In one feature, step f) includes selectively adjusting the first and second portions of the fuel flow to achieve a desired temperature for the mixture flow resulting from step f). In a further feature, step c) includes selectively adjusting the first and second portions of the water flow to achieve the desired temperature for the mixture flow resulting from step f).
In one feature, step c) further comprises superheating the first portion prior to mixing the first and second portions.
Other objects, features, and advantages of the invention will become apparent from a review of the entire specification, including the appended claims and drawings.
Two embodiments of a fuel humidifier/pre-heater unit or system 10 are shown in
Moreover, steam may be generated and mixed with the fuel to obtain the desired temperature. However, the steam may not be able to reach the required temperature, as in the continued example (see Table 2). For cases 1, 2, 3 and 6, a fuel pre-heater is required.
A major difficulty of this invention is the control mechanism. The hot and cold (i.e., fuel and steam) fluids do not necessarily follow the same turn down ratio. For example, Table 1 shows the fraction of CEG and saturated mixture (after the steam and fuel have been fully mixed) relative to max operation. To accommodate the variable nature of turndown and start-up, the humidified fuel temperature delivered to the stack or reformer can be controlled by bypassing all or a portion of the fuel. However, during start-up, the hot gas temperature can increase to temperatures too high for even 100% fuel bypass. Table 1 shows that the beginning (case 5) and end (case 4) of start-up have the same flow rates, but a difference in Entering Temperature Differential (ETD) between the fuel/steam and the CEG of 184° C. With an adequate steam generator 12 for the beginning of start-up, and an adequate mixture heater 14 (to heat the fuel and steam) for full load, the mixture temperature in case 4 cannot be lowered by fuel bypass alone (see Table 3 for mixture heater performance).
Ideally, only low temperature valves are used to control the system. The first suggested control embodiment is to use a fuel bypass 16 in conjunction with a liquid water bypass 18 around or to the end of the steam generator (see
The second suggested control embodiment is to use only the water bypass 18 to control the humidified fuel temperature (see
Relative to the hot gas, the design shows the mixture heater 14 placed upstream of the steam generator 12. This is not essential to the design. Any combination of the mixture heater 14 placed upstream or downstream of the steam generator 12 and a co-current or counter-current generator 12 may be used.
Features of the mixture heater 14 included or potentially included in the design are:
Features of the steam generator 12 included or potentially includes in the design are:
Features of the fuel and water bypass controls 22,24,26 include or potentially include:
The attemperator 20 is used to control the steam outlet temperature. Its unique features included or potentially included in the design are:
Features of the fuel/steam mixers 28 include or potentially include:
The type of line and branch, and flanged entrance and exit connections are not integral to the design. Other connections could be used (i.e., compression fittings/branches, threaded fittings/branches, etc.).
The housing 30 shown in
Minimizing pressure drop could lead to poor distribution of either the fuel or hot gas streams. Flow straighteners (i.e., perforated sheets, conical diffusers, slotted discs, etc.) could be used to correct this problem.
When a helical tube steam generator 12 is used, it may be desirable to provide a domed shaped baffle 58 to direct the hot gas into the annulus 46.
The system 10 is particularly useful for supplying the fuel to a MCFC fuel cell system, shown schematically at 60.
Number | Date | Country | |
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60558285 | Mar 2004 | US |