Maintaining oxygen/carbon ratio with temperature controlled valve

Abstract

Fuel from a source (6) passes through a hydrogen desulfurizer (8) and a proportioning mixing valve (10) to a CPO (31), the temperature of the output of the CPO being monitored (50) to provide a signal (51) which a controller (52) utilizes to adjust the valve (10). The output of the CPO may be passed through a water gas shift reactor (35) and a preferential CO oxidizer (40) to provide fuel to a fuel cell system (26). The air provided to the valve (10) may be humidified, such as by an enthalpy recovery device (21) receiving the oxidant outflow (24) from the fuel cell system.

Description

TECHNICAL FIELD

This invention relates to using a fuel/air ratio valve at the inlet of a catalytic partial oxidizer (CPO) to control CPO exit temperature in the presence of disturbances, such as changes in air or fuel composition or temperature.

BACKGROUND ART

The conversion of hydrocarbon fuel such as natural gas or propane to a hydrogen-rich gas is particularly useful to provide fuel for a fuel cell power plant. Catalytic partial oxidation (CPO) reactors require that the oxygen to carbon (fuel) molar ratios be maintained within a narrow range in order to maintain proper reactor operating temperatures. If the temperature of the catalyst is not maintained between about 750° C. and 850° C., the production of hydrogen falls off. Furthermore, if the temperature of the catalyst rises above about 900° C., the catalyst is damaged and production is permanently impaired.

To maintain nominal catalyst temperature, it is necessary to carefully control the air and fuel flow rates to maintain proper molar ratios. However, the composition of the fuel stream at the CPO inlet can change due to changes in the composition of an external fuel supply, or due to internal changes resulting from a hydrodesulfurization process. If humidified air is used at the inlet of the CPO, the manner of its humidification, such as from an enthalpy recovery device, can cause variations in the humidity of the air from time to time, causing lowering of oxygen partial pressure.

None of the prior art systems are sensitive to the varying compositions of the fuel and air. Mass flow controllers are very expensive and tend to have a high pressure drop. In having both a fuel blower and an air blower with variable speed drives, variation in the speed of either blower may disturb the rate of flow from the other blower, which can result in interaction of controls or instability.

DISCLOSURE OF INVENTION

Objects of the invention include: precise control of the oxygen/carbon ratio at the inlet of a CPO; controlling the fuel/air mixture at the inlet to a CPO in a manner which is insensitive to changes in the compositions of the inlet gases; improved generation of hydrogen-rich gas in a CPO; improved generation of fuel for a fuel cell power plant; and an inexpensive and accurate methodology for controlling the oxygen/carbon ratio at the inlet of a CPO.

According to the present invention, the temperature of the reformate generated in a CPO is utilized to control a mixing valve that determines the proportion of fuel and air at the inlet of the CPO. In further accord with the invention, a proportional/integral controller is used to convert temperature at the outlet of the CPO into a control signal for the valve. If desired, signal modification may be used to accommodate the time differential of temperature sensing vs. the proportioning of the molar ratio of carbon to oxygen at the input to the CPO.

Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole figure herein is a simplified schematic diagram of a system generating hydrogen-rich reformate for a fuel cell system employing the present invention.

MODE(S) FOR CARRYING OUT THE INVENTION

Referring to the figure, a fuel supply includes a source 6 of natural gas or liquified petroleum gas which provides fuel over a conduit 7 to a hydrodesulfurizer 8, the output of which on a conduit 9 is provided to a proportioning mixing valve 10 which has two inlets 11, 12 and one outlet 13. The inlet 11 receives fuel from the hydrodesulfurizer 8. The inlet 12 is connected by a conduit 20 through a pump 22 to an energy recovery device 21 that receives unhumidified air drawn by the pump 22 and derives heat and moisture from air in a conduit 24 at the oxidant outlet 25 of a fuel cell system 26. Some of the heat and humidity in the air exhausting from the fuel cell within the conduit 24 is transferred to the air passing through the enthalpy recovery device 21, before the exhaust air passes to ambient 28. Humidified air from the pump 22 is passed in a conduit 29 to the air inlet 45 of the fuel cell system as well as in the conduit 20 to the valve inlet 12.

The valve 10 provides an air/fuel mixture in a conduit 30 to a catalytic partial oxidizer 31 (CPO). Therein, the fuel air mixture is converted to a mixture which is, on a dry basis, essentially 37% hydrogen, 15% CO, 4% CO₂ and traces of other gases including unconverted hydrocarbons. This reformate is passed over a conduit 34 through a water gas shift reactor 35 which also receives water over a conduit 36. In the water gas shift reactor 35, the water and carbon monoxide are converted to CO₂ and hydrogen, thereby enriching the flow within a conduit 39. Then, a preferential CO oxidizer 40 converts significant amounts of the remaining CO into CO₂, which is less innocuous to the catalyst in a fuel cell. The hydrogen-rich gas in a conduit 41, sometimes referred to as “syngas”, is passed through the anode flow fields of the fuel cell system 26, the exhaust 27 of which may be returned in a fuel recycle loop 24, and occasionally purged, all as is conventional, and the details of which are not critical to the present invention.

According to the invention, a temperature sensor 50 provides a temperature signal on a line 51 to a controller 52, which in turn determines, via a signal on a line 53, the positioning of the proportioning mixing valve 10 so as to provide a substantially perfect ratio of air to fuel, as is determined by the temperature in the conduit 34. In the controller 52, the signal on the line 51 may be processed through a proportional and integral gain.

In certain implementations of the present invention, the air in the conduit 20 may be unhumidified air, within the purview of the invention. Typically, the blower 22 will operate at substantially constant speed.

The description with respect to the figure is exemplary merely, and indicative of a known system in which the present invention can be utilized to great advantage. The invention may be utilized in completely different systems, it sufficing that the temperature of the outflow of a CPO is utilized to control the proportions of air and fuel entering the CPO as reactants.

Thus, although the invention has been shown and described with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the invention.

Claims

1. A catalytic partial oxidizer system comprising:

a catalytic partial oxidizer having an inlet and an outlet;

a fuel supply;

a source of air;

a proportioning mixing valve having two inlets and an outlet and controllable so as to determine the proportion of gases at the respective inlets which will be provided to the outlet, one inlet being connected to said fuel supply and the other inlet being connected to said source of air, the outlet of said valve being connected to the inlet of said CPO;

a temperature sensor for providing a signal indicative of the temperature of flow at the outlet of said CPO; and

a controller responsive to said signal to control said valve and thereby control the proportions of gas at the respective inlets which are provided through said valve to said CPO.

2. A system according to claim 1 wherein:

said controller processes said signal with proportional and integral gain.

3. A system according to claim 1 wherein:

the outflow of said CPO is provided to a fuel cell system having a cathode exhaust passing through an enthalpy recovery device which comprises said source of air.

4. A system according to claim 1 wherein:

said source of air is a source of humidified air.

5. A system according to claim 1 wherein:

said fuel supply provides fuel selected from natural gas and liquified petroleum gas.

6. A system according to claim 1 wherein:

said fuel supply includes a hydrodesulfurizer.