Patent Application
20070187372
This invention relates to plasma fuel reformers (plasmatrons) and more particularly to such plasmatrons that operate with high enthalpy and low power.
Plasma reformers, often referred to as plasmatrons, are well-known devices for reforming hydrocarbons to generate a hydrogen rich gas that includes hydrogen, carbon monoxide and light hydrocarbons through the use of a plasma discharge. Plasmatrons are known that use low current and high voltage discharges to provide significant advantages as described in U.S. Pat. No. 6,881,386 and published U.S. patent application 2005/0210877, of which some of the inventors of the present application are co-inventors. The contents of these patent documents are incorporated by reference herein.
It is often desired to operate plasmatrons in an “incomplete pyrolysis” mode, that is, at an oxygen/carbon ratio less than one. Known plasmatrons do not operate effectively in this mode because of the low temperature of the plasma stream. The known low power plasmatrons can operate at an O/C ratio of approximately one to produce a hydrogen rich gas including H2, CO and N2 with temperatures of approximately 900 degrees C. Higher temperatures can be achieved by increasing the O/C ratio to 3 or higher (complete combustion), but the product gas will contain mainly CO2, H2O and N2 which is not desirable.
However, for some applications it is advantageous to operate a plasmatron at an O/C less than one (or an O/C much less than one, approaching zero) thereby producing H2 as well as significant amounts of hydrocarbons according to the chemical reaction:
CmHn→xH2+CmHn-2x
A promising application for this mode of operation is the “selective catalytic reduction” of diesel exhaust emissions by hydrocarbons (SCR-HC).
Compared to other NOx elimination technologies, SCR-HC has the advantage of continuous operation and the use of hydrogen and hydrocarbons as the reducing agents.
The low power plasmatrons referred to above are usually employed as ignitors and therefore have low average enthalpy. For example, at a diesel flow rate of 0.1 g/s the amount of air required for partial oxidation at an O/C of approximately 1 would be approximately 25 liters per minute. At an average power of 200 W such operation corresponds to an enthalpy of approximately 0.47 MJ/m3. The average air temperature at this enthalpy would be approximately 300° C. If higher total flow rates were required, the enthalpy and corresponding air temperature would be even lower. To operate in the incomplete pyrolysis mode the temperature required for the destruction of hydrocarbons would be in the range of 1500-2000° C.
It is therefore an object of the present invention to provide a plasmatron capable of high enthalpy, low power operation in an incomplete pyrolysis mode.
In one aspect, a high enthalpy, low power plasma reformer according to the invention includes an annular ground electrode including an air intake manifold and helical structure within the annular electrode for directing air helically upward along the ground electrode in a heat transfer relation to cool the electrode and to preheat the air. A high voltage electrode is spaced from the ground electrode to create a gap through which the preheated air flows, and the high voltage electrode includes a passage for delivering hydrocarbon fuel through an atomizer into an arc discharge region. The high voltage discharge is initiated in the gap between high voltage and grounded electrodes. The preheated air is injected through tangential channels to create swirl flow and to rotate and stretch the arc thus producing the volume discharge region. The fuel injected through the high voltage electrode with a flow rate up to 2 g/s is partially pyrolyzed to produce hydrogen rich gas in the electric arc discharge region within the annular ground electrode. In one embodiment, air is introduced into the air intake manifold at a rate in the range of 8-15 liters per minute. It is preferred for “partial pyrolysis” mode that the average temperature in the arc discharge region would be in a range of approximately 1500-2000° C. The hydrogen rich gas may include H2, CH4, CO, N2 and hydrocarbons of C2-C4 groups (such as C2H2, C2H4, C3H6 etc.) In a preferred embodiment, the power is approximately 500 watts, the air flow rate is approximately 8 liters per minute and the enthalpy is approximately 3.7MJ/m3 that corresponds to a temperature of 2300° C.
In yet another preferred embodiment of this aspect of the invention, there is a downstream portion of the plasmatron including an air inlet for the introduction of additional air to achieve the desired overall oxygen/carbon ratio and to prevent soot formation.
In yet another aspect, a high enthalpy, low power plasma reformer includes a short annular ground electrode including an air intake manifold and the annular electrode includes structure for guiding air. A high voltage electrode is spaced from the ground electrode to create a gap through which the air flows and the high voltage electrode also includes a passage for delivering hydrocarbon fuel to an atomizer. The ground electrode forms an arc discharge region where the fuel is reformed. A further section downstream from the electric arc discharge region includes additional air and fuel (up to 2 g/s) introduction structure allowing generation of hydrogen rich gas with a desired total O/C ratio. The additional air and fuel also allows for production of hydrogen rich gas with selected composition (from deep pyrolysis at O/C<<1, to combustion at O/C>3) at a wide dynamic range of total flowrates. In a preferred embodiment of this aspect of the invention the short annular ground electrode has a length in the range of 5-10 mm. A preferred embodiment operates at an O/C ratio in the range of 1-1.6 at a fuel flow rate of approximately 0.1 g/s and an air flow rate of approximately 25 liters/min. An appropriate power level of operation is 500-700 watts. In this mode of operation the thermal effect of the partial oxidation reaction is added to the high enthalpy of the plasma stream.
In still another embodiment of this aspect of the invention, the annular ground electrode includes an insert made of a high temperature alloy. Water cooling may also be provided. In yet another embodiment the plasmatron further includes a thermally insulating section for reaction initiation and stabilization located between the electric arc discharge region and the further downstream section where additional air and fuel are introduced.
The high enthalpy, low power plasma reformers according to the invention provide better mixing of the air and fuel and decrease the likelihood of soot formation because of an increased rate of fuel vaporization, the volume of the air-fuel mixture and flow velocity in the reaction channel. Moreover, high flow velocities and temperatures improve fuel atomization. The higher enthalpy of the present designs allows ignition of an air-fuel mixture in a wide range of O/C ratios.
The present designs also create conditions conducive to a fast start of partial oxidation reactions (at O/C equals one) and the immediate production of hydrogen that is beneficial for other chemical processes, for example the HC-SCR process. An important advantage of the invention is that the designs disclosed herein allow for operation in an endothermic mode of incomplete pyrolysis at O/C ratios less than one. The plasmatron reformer designs disclosed herein provide for flexibility because the plasmatrons can operate in different modes: as a fuel vaporizer (O/C much less than one), “incomplete pyrolysis” (O/C less than one), or an oxidation reaction from partial oxidation up to complete combustion (O/C greater than or equal to one).
With reference first to
In operation, plasma air, at a flow rate in the range of approximately 8-15 liters per minute, is injected into the ground electrode 20 through the air manifold 22 and is caused to revolve upwardly inside the ground electrode 20 by the helical structure 24 to provide efficient air cooling of ground electrode 20's inner surface (the air is, of course, preheated in the process). This preheated air is then injected into the gap 26 between the ground electrode 20 and the high voltage electrode 12 through tangential channels and creates a low power volume discharge in the region 28. The internal diameter of each of the channels is approximately 1-1.5 mm.
Liquid fuel with flow rate up to 2 g/s injected into the high speed plasma air stream (having an average temperature in the range of approximately 1500-2000° C.) is efficiently atomized, vaporized and partially pyrolyzed to produce hydrogen rich gas containing H2, CH4, CO, hydrocarbons of C2-C4 groups and N2 Air cooling of the ground electrode in combination with the endothermic nature of the chemical reaction prevents excessive erosion of the ground electrode 20 surface.
Additional air may be injected through the manifold 32 into a downstream air section 30 if desired to correct total O/C ratio and to prevent soot formation. The plasmatron shown in
Another embodiment of the invention is shown in
The embodiment of
The plasmatrons of the invention allow a high level of temperature to be achieved by significantly increasing electrical power with the simultaneous decrease of air flow rate. For example, at a power of 500 W and an airflow rate of 8 liters/min, the air enthalpy would be 3.7MJ/m3 and a temperature of 2300° C. In contrast, at a power level of 500 watts but with an air flow rate of 15 liters/min the enthalpy would drop to 2 MJ/m3 and the temperature would be reduced to 1400° C. Hot air velocity at these temperature levels would be in the range of 150-200 m/sec which is sufficient for adequate fine fuel atomization. In experiments, the inventors have been able to atomize up to 2 g/s of fuel with an excellent quality of atomization.
It is recognized that modifications and variations will occur to those of skill in the art and it is intended that all such modifications and variations be included within the scope of the appended claims.
