Patent Application
20110129393
The present invention relates to hydrocarbon reformers; more particularly, to apparatus for vaporizing fuel entering a hydrocarbon reformer; and most particularly, to an electrically heated contact fuel vaporizer (EHCV) for a reformer wherein a flow director extends into the vaporizing space to direct the flow of vapor and wherein, in one aspect of the invention, air may be introduced at start-up to cause spontaneous fuel combustion for warm-up of the reformer.
In the art of catalytic reforming of hydrocarbons to generate hydrogen, it is known to provide an EHCV apparatus ahead of the reforming catalyst to vaporize fuel for reforming. Subsequent to vaporization, the fuel typically is mixed with a controlled amount of air to provide an optimum fuel/air mixture for reforming, which mixture ratio is below the Lower Explosive Limit (LEL) for the particular fuel being used.
Especially upon startup of a reformer, a high thermal load is required to vaporize liquid fuel entering the reformer at ambient temperature. It is known to provide a vaporizer having a designated surface for vaporization of the fuel upon which the liquid fuel is impinged during operation of the reformer. Upon start-up, the surface must be heated by separate means, typically by an electrically-powered heater such as a glow plug or cartridge rod which is de-energized when a sufficient temperature increase is achieved. At equilibrium reformer operating temperatures, the surface typically receives sufficient waste heat to achieve reliable vaporization without further supplemental heating.
In the prior art, ECHVs for vaporizing diesel fuel have used different methods to add heat to the fuel, typically a metering device that flows fuel to an open ended heating chamber consisting of a thin wall tube with the cartridge rod heater centered inside. The tube and heater forms an annulus about 1.0 mm in radial size extending the length of the working portion of the rod heater. The open end (outlet end) delivers the heated fuel in vapor form to a mixing chamber of the reforming device where air is mixed with the fuel vapor for reforming.
Some disadvantages of such a prior art system are:
a) the EHCV is mounted by means of a dual ferrule fitting, welded to the high thermal mass end plate of the mixing chamber of the reformer. This mounting system creates a seal between the EHCV and the chamber but does not thermally isolate the two and creates an undesirably large contact area between the EHCV and the reformer. The thermal mass of the end plate creates a heat sink which cools the vapor, reducing the efficiency of the device;
b) the 1.0 mm annulus is too large even for the maximum required fuel flow of the application, which has the effect of minimizing surface contact between the liquid fuel and the rod heater and creates a thicker fluid film layer requiring longer residence time; the overly-large annulus also causes hot spots on the heater which are not being cooled by fuel or vapor, which impacts durability of the heater;
c) fuel vapor enters the mixing chamber at low velocity, adversely impacting air/fuel mixing;
d) the outer walls of the EHCV are exposed to cold reformer inlet air in the mixing chamber;
e) outer fuel heating chamber walls are cooled by convection, thereby wasting energy; cooled chamber walls condense vapor by conduction, and condensed vapor droplets cause the prior art EHCV to sputter liquid fuel;
f) because of the large annulus and multiple sources of heat loss, power requirement is high, typically about 400 W, and vaporized fuel output is relatively inefficient.
At startup of a reformer, the reforming catalyst must be heated to achieve catalyzing temperature, typically to about 500° C. It is known in the prior art to provide a spark or other igniter mechanism extending through a wall of the reformer into a combustion chamber between the prior art EHCV and the reforming catalyst. For a short period, a combustible fuel/air mixture is formed in the combustion chamber and ignited by the igniter, the hot combustion gases then passing through the reforming catalyst. When catalyzing temperature is reached, ignition is suspended and the fuel/air ratio is adjusted for reforming.
Providing an igniter in the hot zone of a reformer presents significant engineering and materials challenges. A standard automotive spark plug is not suitable as the continuously hot environment causes corrosion and failure, thus expensive or exotic materials of construction are required; a spark-ignition device is easily fouled by carbon deposits, leading to ignition failure; the igniter mounting requires additional bosses on the reformer housing, which are additionally expensive and undesirable; the igniter requires power to operate in addition to the power required for the EHCV; and the igniter itself adds to the cost and complexity of the reformer.
What is needed in the art is an EHCV that provides high-efficiency high-volume vaporizing of diesel fuel and also eliminates the need for a separate igniter in a hydrocarbon reformer.
It is a principal object of the present invention to vaporize fuel more efficiently.
It is a further object of the present invention to eliminate the need for a separate igniter in a hydrocarbon reformer being supplied by an improved EHCV in accordance with the present invention.
Briefly described, an improved EHCV for a catalytic hydrocarbon reformer includes an electrically-powered rod heater surrounded by a first tube defining a first annulus therebetween. In one aspect of the invention, the first annulus is about 0.2 mm in radial dimension and having an open end defining an exit from the EHCV. A spiral flow director is disposed in the first annulus to direct flow of fuel and vapor in a helical path around the rod heater through the EHCV. A tubular insulative housing surrounds the first tube, preferably comprising an outer housing tube and a thermal barrier tube disposed between the outer housing tube and the first tube.
A first inlet port for introduction of liquid fuel at the inlet end of the EHCV extends through the insulative housing and the first tube into the first annulus to provide fuel to the heating rod for vaporization.
In one aspect of the invention, the helical path for directing the flow of fuel through the EHCV may be formed as a channel in either the internal surface of the first tube, the outer surface of the rod heater, or partially in both. Further, multiple rod heaters may be chained in parallel with one or more cross-passages connecting the heaters, fluidically, to increase the heating capacity of the EHCV.
In a second embodiment useful in reformers having no combustion igniter, a second inlet port and internal passages including a second annulus between the thermal barrier tube and the first tube permit controlled entry of oxygen, preferably in the form of air, into the first annulus near the exit end thereof to mix with hot, vaporized fuel exiting the vaporizer, thereby creating a fuel/air mixture above the LEL which spontaneously combusts to form hot gases for heating the reforming catalyst as in the prior art. When a sufficient temperature is achieved in the reformer, air flow into the EHCV is suspended, extinguishing combustion, and fuel flow rate is adjusted for reforming.
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 exemplifications set out herein illustrate currently preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.
Referring to
Fuel flows along heater surface 24, confined by a concentric first tube 25 in a first annular flow space 27 and is discharged at exit 26 into a mixing chamber ahead of a catalytic hydrocarbon reformer. In radial dimension, an annulus of flow space 27 is less than 1.0 mm, and preferably is about 0.2 mm, in contrast to a typical prior art EHCV wherein this dimension is about 1.0 mm. The reduced annulus height of flow space 27 results in faster heating of a thinner layer of fuel.
A flow director 28, which may take the form of a helical wire or a raised helical rib on heating element 12 or on an ID of first tube 25, is disposed in flow space 27 and causes fuel and fuel vapor to follow a helical path through flow space 27, which path serves to prevent channeling as is known to occur in prior art EHCV devices, thus defining a longer contact path for fuel against surface 24 and distributing fuel and fuel vapor more evenly over surface 24, thereby preventing formation of undesirable hot spots. Further, fuel vapor exits EHCV 10 with a swirling motion, thus improving mixing with introduced air in the mixing chamber. The spiral pattern of flow director 28 may be formed having a constant pitch as shown in
In one aspect of the invention, tube 25 is formed, at least in part, of an insulative material to minimize heat losses from the heated fuel.
Preferably, an outer tubular wall 31 defining a housing extension is swaged or otherwise attached to housing 14 and closes against first tubular wall 25 near the exit end thereof. Outer tubular wall 31 may be a low mass structural member for sealing the vaporizer while minimizing parasitic heat transfer it because of its low mass structure.
A second tubular wall 38 may extend longitudinally from housing 14, as for example, from a step 39 in housing 14 as shown in
Preferably, in mounting the EHCV to a reformer, the contact area between the EHCV and the reformer is reduced in comparison to prior art mountings to minimize heat loss from the EHCV.
Referring to
Referring now to
Referring now to
In operation of either EHCV 10 or EHCV 10′, whenever vaporized fuel is required and the temperature of surface 24 is below a predetermined lower limit, heating element 12 is energized to raise the temperature of surface 24. When the ambient temperature in EHCV 10 or 10′ is sufficient to maintain vaporization of fuel, heating element is de-energized.
In operation of EHCV 10′, when the temperature within reformer 30 is insufficient to cause reforming catalysis of vaporized fuel, combustible gas 36 is injected through openings 42, forming a combustible fuel/air mixture that combusts spontaneously in chamber 62 to form hot gases that are passed through reformer 30. When the reformer attains catalysis temperature, flow of combustible gas 36 is terminated, and flow of reforming air 52 is adjusted to provide an optimal fuel/air mixture for reforming.
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.
