Low power compact plasma fuel converter

Details Low power compact plasma fuel converter Low power compact plasma fuel converter

1999-11-03

2001-11-27

Mayekar, Kishor (Department: 1741)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

With means applying electromagnetic wave energy or...

C422S186220, C422S186280, C123S003000

Reexamination Certificate

active

06322757

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a plasma fuel converter and more particularly to a low power compact plasma fuel converter employing high voltage and low current.
Plasma fuel converters such as plasmatrons reform hydrocarbons to produce a hydrogen rich gas. DC arc plasmatrons have received particular attention in the prior art. See, for example, U.S. Pat. Nos. 5,425,332 and 5,437,250. DC arc plasmatrons typically operate at low voltage and high current. By operating at high currents and lower voltages, the arc current is high enough that precautions are required to minimize electrode erosion and even melting. High flow rates of cooling water are required to keep the erosion in check. Air flow is required to simultaneously center the discharge on the cathode tip (made of hafnium or other specialized material, embedded in a copper plug) and to move the root of the arc at the anode to minimize erosion at the anode. A constriction is also required to increase the impedance of the discharge (i.e., to operate at high voltages and lower currents than free-flowing arcs). The air flows and the constriction are likely to require operation at elevated pressure (as much as 0.5 bar above ambient pressure), and thus a compressor is likely to be required. Even with these precautions, it is often difficult to extend the lifetime of the electrodes beyond approximately 1,000 hours of operation.
DC plasmatrons also require relatively sophisticated power supplies for stabilization of the arc discharge. Further, DC plasmatrons have a limited capability for low power operation. In some reforming applications, the minimum operating power can be significantly greater than needed resulting in unnecessary power loss. DC arc plasmatrons are typically operated at power levels of one kilowatt or more.
It is therefore desirable to have a plasma fuel converter that does not require a compressor or a sophisticated power supply for stabilization of the arc discharge. It is also desirable to have a plasma fuel converter having longer electrode life and with a capability of lower power operation when lower flow rates of hydrogen rich gas are required.
SUMMARY OF THE INVENTION
In one aspect, the plasma fuel converter of the invention includes an electrically conductive structure forming a first electrode. A second electrode is disposed to create a gap with respect to the first electrode in a reaction chamber. A fuel air mixture is introduced into the gap and a power supply connected to the first and second electrodes provides voltage in the range of approximately 100 volts to 40 kilovolts and current in the range of approximately 10 milliamperes to one ampere to generate a discharge to reform the fuel. The discharge can be a “glow type” discharge, a silent discharge and/or a breakdown discharge. A preferred range for voltage is 200 volts to 20 kilovolts. In a preferred embodiment, the plasma fuel converter includes a reaction extension region to increase residence time in a high temperature zone. An insert in the reaction extension region and in the reaction chamber is provided to increase the temperature of operation. The insert may be metallic or ceramic. A heat exchanger may also be provided to decrease power needed from the power supply.
In this embodiment, it is preferred that the power supply be a current controlled, high voltage power supply such as a power supply including a saturable inductor to limit current. The saturable inductor power supply may be a neon transformer power supply.
The fuel-air mixture is selected for operation between stoichiometric partial oxidation and full combustion depending on conditions and applications. An additional power supply may be provided for simultaneous operation in a low voltage, high current DC arc mode and a high voltage, low current glow discharge mode. The plasma fuel converter may include a plurality of plasma regions to increase hydrogen generation rate. The hydrogen rich gas output of the plasma fuel converter may be brought into contact with a catalyst such as for nitrogen oxide catalyst regeneration.
The plasma fuel converter of the invention reduces or removes the disadvantages associated with DC arc plasmatrons known in the prior art. The disadvantages are overcome by the specially controlled high voltage, low current plasma fuel converter operation. The voltage and current vary over time in such a manner as to limit the current flowing in the plasma. The electrical characteristics of the plasma operation are a voltage range from a few hundred volts and up to 40 kilovolts, and a current range from 10 milliamperes to hundreds of milliamperes. In contrast, the corresponding ranges for DC arc plasmatron fuel reformers are a voltage of around 100 volts and currents starting at 3-5 amperes. A representative high voltage, low current discharge of the plasma fuel converters of the invention has “glow discharge” type features. Typically, this type of atmospheric pressure, high voltage, low current discharge can be made to operate at tens to hundreds of watts of average power. In contrast, DC arc plasmatrons known in the prior art are typically operated at power levels of one kilowatt or more.
The high voltage, low current operation of the high-voltage low-current discharge is maintained by the use of an appropriate power supply such as a conventional AC neon transformer. Neon transformer power supplies use a saturable inductor to limit the current to a relatively low value, on the order of tens to hundreds of milliamps. Such power supplies are also capable of producing open circuit voltages of tens of kilovolts. These power supplies are inexpensive and can be made for the delivery of tens to hundreds of watts.
In contrast, in the case of conventional spark discharges, the capacitive-based power supply delivers a high voltage, short pulse that breaks down the electrode-to-electrode gap and results in a discharge. This breakdown phase is followed by a lower voltage, lower power discharge. Most of the energy is delivered during the relatively long low voltage, low power part of the discharge. The energy delivered per pulsed discharge is small, on the order of tens of millijoules. Average power levels are typically around a few watts which is generally too low for hydrogen production applications.
Thus, in a plasma fuel converter using high-voltage, low-current operation according to the invention, the power that is provided by the discharge can be on the order one-tenth of the minimum power of a compact DC arc plasmatron known in the prior art. The reformer of the invention is therefore appropriate for low hydrogen production rates where it provides enough power to increase substantially the enthalpy of the reactants. Such low rates may be appropriate for some applications, such as catalyst regeneration. High hydrogen production rates are possible by using multiple units. A further increase in hydrogen production rate is possible by increasing the air/fuel ratio and the fuel throughput. Alternatively, the low power, high-voltage low-current plasma is used as an expanded volume igniter and a source of radicals to enhance the partial oxidation reaction with the necessary enthalpy increase provided by other means. These other means include air-fuel chemical reactions and/or heat provided by a heat exchanger. In this mode of operation, substantially all of the enthalpy (at least 80% and preferably 90% or more) is provided by these other means. This mode of operation makes possible higher hydrogen production rates than would otherwise be allowed by constrains on plasma power or generator/battery power supply capability.
In some cases, it may be useful to operate the partial oxidation reaction with additional oxygen (i.e., partial oxidation with an oxygen-to-fuel ratio greater than that for stoichiometric partial oxidation). The oxygen is generally provided by additional air. In this case the hydrogen yield (defined as the fraction of the hydrogen in the fuel that is released in the process) is reduced, but the electrical power requirement is decre

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