Patent Application
20070180769
The present invention relates to reformers for catalytically converting hydrocarbons into hydrogen-containing reformate for use in a fuel cell; more particularly, to methods and apparatus for controlling the ratio of air to fuel during various phases of reformer operation; and most particularly, to a method and apparatus for controlling the air/fuel ratio by measuring the oxygen level in the reformer exhaust stream and feeding back such measurement to a fuel and air supply controller in a closed-loop mode.
Catalytic reformers for converting hydrocarbons (referred to herein as “fuel”) and air to reformate are well known, air being a ready source of oxygen for the reforming process in exothermic mode. Such reformate typically comprises hydrogen, carbon monoxide, nitrogen, and residual hydrocarbons. The flow rates of fuel and air typically are monitored and controlled by electronic control means, such as a programmable controller or a computer.
In the prior art, the desired fuel flow rate is calculated in open-loop control based upon the measured mass air flow rate at the inlet to the system and a resultant base pulse width of a fuel injector. There is no feedback control derived from the degree of accuracy of the resultant air-to-fuel (ANF) ratio. The actual A/F ratio delivered to the reformer catalyst is not known but rather is inferred from the measured inlet air mass flow rate and the expected fuel mass flow rate from the fuel injector. Because of variations in production hardware, the air and fuel control setpoints have associated errors that can result in poor combustion and excess fuel deposition on the interior walls of the reformer during a start-up combustion phase.
Further, prior art reformer controls also monitor the inlet and outlet temperatures of the reformer catalyst during both the combustion warm-up phase and steady-state operation. If either the inlet or outlet temperature exceeds a calibratable threshold, the reformer is shut down and the start-up sequence must be re-initiated. As a result, excess fuel may be deposited on the interior surfaces of the reformer, leading to carbon formation and errant fuel control as the fuel puddle evaporates of pyrolizes over time.
What is needed in the art is an improved means for maintaining at a desired value the ratio of air to fuel being supplied to a hydrocarbon reformer.
What is further needed is such a means wherein a non-intended air/fuel mixture is detected and corrected before an unintended and undesirable thermal excursion occurs.
It is a principal object of the present invention to control the ratio of air to fuel being supplied to a hydrocarbon reformer at a predetermined ratio.
Briefly described, a reformer system in accordance with the invention comprises a conventional hydrocarbon reformer; a controllable fuel supply system; a controllable air supply system; an oxygen sensor disposed downstream of the reformer; and a control means for receiving input from the oxygen sensor and setting the flow values for fuel and air.
During start-up of the reformer, air and fuel are mixed in about a stoichiometric ratio, typically 14.5/1 A/F for a typical alkane fuel, and the AF mixture is combusted ahead of the reformer for typically about 20 seconds, the hot exhaust being passed through the reformer to heat the walls and catalyst. The heat of combustion is maximum at the stoichiometric ratio. After the combustion event, combustion is terminated and the A/F ratio is lowered to, typically, about 5/1 to allow reforming to occur.
Once the desired fuel flow rate for combustion is established it can be stored in computer memory as a starting value for subsequent starting events.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.
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Oxygen sensing means 124 may readily employ a prior art automotive exhaust oxygen sensor such as is widely used in all vehicles manufactured today as part of emissions control systems. Such sensors are well suited to measuring oxygen levels in an exhaust stream from a catalytic hydrocarbon reformer.
It is preferable to locate the exhaust oxygen sensor downstream of the reforming catalyst to permit better mixing and equilibration of the oxidation reaction, resulting in a more accurate measure of free oxygen in the reformer exhaust. A heated-type sensor should be located at a point in the reformer exhaust that will not exceed the maximum allowable temperature for the sensor, typically about 900° C. A non-heated type sensor should be located such that the minimum temperature exceeds about 260° C., with periodic excursions above 450° C. to oxidize any soot deposits that may occur.
A heated-type oxygen sensor typically requires approximately 10 seconds of heating to become active for measuring oxygen. This pre-heating period can be built into the reformer start-up algorithm such that the sensor is heated by an electrical resistance heater prior to beginning the combustion event. An advantage of activating the oxygen sensor prior to the combustion event is that less or no time is then spent in a functional open-loop control at the start of combustion wherein the actual A/F ratio is not measured. It is also possible to use the output from the oxygen sensor before it is completely active to determine the fuel volatility and to correct the fuel flow rate to improve the combustion process, as described in U.S. Pat. Nos. 6,925,861 and 6,938,466, the relevant disclosure of which is incorporated herein by reference.
Oxygen sensors in common use in the prior automotive art fall generally into two categories: switching type and wide range.
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While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.
