Patent Application
20130118077
The temperature of water-saturated steam reformer reformate, which is the dew point of the reformate, is used to calculate the steam/carbon ratio, to provide an indication for routine maintenance or repair, and optionally as feedback to control a controller's positioning of a metering valve that provides natural gas to a steam/natural gas ejector that feeds the steam reformer.
One example of a system in which steam reformers are preferred (rather than autothermal reformers or catalytic partial oxidizers, etc.) is in providing reformate gas to feed a fuel cell power plant. The steam reformer is particularly suited to high temperature fuel cells, such as phosphoric acid fuel cells, where the steam reformer can be supplied with steam stripped from coolant water circulating through the fuel cell. If a separate boiler is used, the waste heat from the fuel cell can supply the energy necessary to produce the required steam for the reformer. The steam/carbon ratio of a steam reformer is critical since too low a ratio subjects the reformer to potential carbon buildup that can result in blockage of the reformer, and shut down of the fuel cell power plant. Too high a steam/carbon ratio reduces the overall electrical efficiency with respect to the input fuel. A high steam/carbon ratio also increases the pressure drop across the fuel processor and reduces the availability of high grade heat for the customer.
Since the measurement of steam is both expensive and unreliable, prior art systems have utilized ejectors to maintain a suitably controlled steam/carbon ratio, as the demand for flow of reformate varies, without measuring either. Examples are U.S. Pat. Nos. 3,585,078 and 4,002,805, both incorporated herein by reference. Both systems use an ejector having a controllable, metering orifice. The position of the needle (sometimes referred to as a pintle or a plug) is controlled in various ways by fuel cell load current and reformer temperature. Systems of this sort have sometimes been referred to as passive, steam/carbon ratio control systems.
The passive, ejector steam/carbon control methodology has heretofore proven itself to be satisfactory. Such systems do tend to veer away from the designed, optimal range of steam/carbon ratios over time. However, there is no means by which to monitor changes in the steam/carbon ratio, throughout the life of the power plant, except by on-site diagnosis using actual measurements of steam and carbon in the reformate feed.
The ejector is usually designed to achieve a relatively high but safe ratio, such as around 3.5, to ensure that carbon buildup is avoided. But that results in lower efficiency and power output.
Therefore, the measurement of a steam/carbon ratio other than by measuring the steam and carbon components of effluent feed, displaying that ratio, and/or providing a closed loop, feedback control system to maintain the steam/carbon ratio within desired limits, are both desirable for fuel processing systems utilizing steam reformers.
It has been determined that the steam/carbon ratio of steam reformer feed can be closely approximated by calculation, or reference to a table, from the dew point of the reformate flowing toward the fuel cell. The superheated reformate must be cooled in apparatus to reach its dew point. In turn, it has been found that the dew point of the steam reformer effluent can be determined from reformate gas that has passed through a water counterflow separator scrubber, in an ammonia removal system, which cools the superheated reformate and ensures saturation of the reformate with water, thereby identifying the temperature of the reformate as the dew point of the reformate.
The steam/carbon ratio calculated from the scrubbed reformate dew point is made available for display, thus allowing regular maintenance intervals to include determination of whether the steam/carbon ratio is within desired limits, track trends of the ratio over periods of time, and detect failures, such as steam leaks. Additionally, once the ratio is calculated, the controller can readily adjust, or bias, the admission of fuel to the ejector to return the steam/carbon ratio to within its desired limits. This allows setting the ejector design ratio to a low ratio, close to 2.5, resulting in improved electrical efficiency and generation of electrical power.
In any system which does not utilize an ammonia removal system, the reformate can be cooled by a heat exchanger which is variably controlled in response to temperature and pressure so as to limit the cooling to reaching the dew point.
Other variations will become more apparent in the light of the following detailed description of exemplary embodiments, as illustrated in the accompanying drawings.
An exemplary embodiment is shown in
The controller determines the signal in the line 24 partially in response to a signal in a line 37 from a current sensor 38 indicative of current flow through the load 39 of a fuel cell 40. The controller 25 provides the signal on line 24 also in response to a signal on a line 41 from a temperature sensor 42 which indicates the temperature of the reformer. Thus, the controller 25 controls the area of the ejector metering orifice 19 in response to the fuel cell power plant load current and the temperature of the steam reformer, and also in response to the dew point of the reformate, which is indicative of steam/carbon ratio, according to the disclosure hereinafter.
The reformate gas generated by the steam reformer 10 passes through a conduit 43 to a water gas shift reactor 44, in which water and CO react to form more hydrogen as well as CO2.
The reformate is cooled in an apparatus, such as a separator scrubber 46, which receives reformate at an inlet 47 from a conduit 48. The separator scrubber 46 is part of an ammonia removal subsystem 49. The separator scrubber 46 includes an ammonia dissolving bed 50 which comprises a packed bed of high surface area medium such as open cell foam, unitary integral ceramic foam, honey-comb materials, corrugated materials, metal or ceramic packing, or wire mesh structures. In an alternative design, the bed may be a fluidized bed using a fluidizable material.
The reformate is passed through a manifold 52 so that the reformate passes upwardly through the packed bed 50 as water flows downwardly through the packed bed 50 from a water distribution mechanism 54, such as orifices, disposed above the packed bed. The water that flows downwardly through the packed bed is collected in an accumulator 55 and passed through a conduit 56 to a pump 57. The pump 57 forces the water upwardly, through a heat exchanger 58 and a deionizer 59, and then through a conduit 60 to the water distribution mechanism 54. The pump 57 may be regulated by a signal on a line 63 from the controller 25. The accumulator 55 may have an overflow line 65, and a makeup water feed, not shown.
The deionizer 59 (sometimes referred to as a demineralizer) includes an ion exchange bed, such as a cation exchange resin, and preferably a hydrogen form cation exchange resin to remove ammonia, which is harmful in phosphoric acid fuel cells and proton exchange membrane fuel cells. The outlet 68 of scrubber 46 may include a mist eliminator which prevents water droplets from moving along with the reformate stream in a conduit 69.
The temperature of the reformate in the conduit 69 is determined by a temperature sensor 71, which passes a signal over a line 72 to the controller 25. The temperature indication on the line 72 may be utilized by the controller to control the temperature of the water in the conduit 60 being fed to the water distribution mechanism 54 in the scrubber 46, by means of controlling the pump 57 by signals on the line 63 or by controlling a heat exchanger bypass valve 74 by means of a signal on a line 75. Control of the water temperature flowing downwardly through the scrubber 46 will adjust, inversely, the amount of ammonia dissolved into the water. A separator scrubber system of this type is disclosed in Patent Publication WO 2008/036090 A1, incorporated herein by reference.
Because the water distribution mechanism 54 supplies excessive water, the reformate gas passing through the mist eliminator 68 is saturated with water. If the temperature of the water supplied to the bed 50 cools the superheated reformate entering the inlet 47 to its dew point, there will be no water evaporated into or condensed from the reformate passing from the manifold 52 upwardly through the bed 50. In such a case, the temperature of the reformate in the conduit 69 will be the dew point of the reformate.
A level sensor 87 provides a signal on a line 88 to the controller 25. If the controller senses an increasing water level, the temperature at sensor 71 will be lower than the true dew point, so the calculated steam/carbon ratio needs to be adjusted higher; if the signal on the line 88 indicates a decreasing water level, the calculated steam/carbon ratio needs to be adjusted lower. In addition, whenever there is an increase or a decrease in the water level, the temperature of the water in the conduit 60 must be increased or reduced by adjusting the valve 74 by means of the signal on the line 75. The temperature of the gas represents its dew point, so long as the water level in the accumulator 55 remains constant.
From the dew point indicated by the signal on the line 72, the controller can calculate a close approximation of the steam/carbon ratio of the reformate in the conduit 69. This can be presented in a display, such as the display 84 associated with the controller 25. From this, service personnel can track the drifting of the steam/carbon ratio over time, or detect if the steam/carbon ratio goes out of preferred limits.
In addition, the controller 25 can utilize the dew point indicated by the signal on the line 72 to bias or adjust the steam/carbon ratio. The relationship between stem/carbon ratio and dewpoint is illustrated in
With either embodiment of the steam/carbon ratio measurement and control system, it is possible to adjust the steam ejector 13 so as to run the steam/carbon ratio a small fraction above 2.50 in steady state, which increases the efficiency. During transients, (such as changes in the load current), the steam/carbon ration may be raised by some amount to ensure that it won't drop below about 2.50. Thus, in a system having an ammonia scrubber, the steam/carbon ratio can be measured and displayed and can be controlled to a very close approximation, utilizing only apparatus which is already in place. However, in any system in which an ammonia scrubber is not in place, the invention may be utilized simply by cooling the reformate to its dew point, and utilizing the temperature of the dew point to closely approximate the steam/carbon ratio, display it, and use it to control the amount of steam and fuel being presented to the ejector.
In
If the system is running at atmospheric pressure, the pressure at the temperature sensor 71 will vary only by a few inches of water, which is insignificant in this system; thus, the pressure sensor 94 would be unnecessary in such a case. The pressure of the reformate in the conduit 69 could also be determined from system operational parameters.
Since changes and variations of the disclosed embodiments may be made without departing from the concept's intent it is not intended to limit the disclosure other than as required by the appended claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2010/001890 | 7/2/2010 | WO | 00 | 12/28/2012 |
