Patent Application
20160123577
According to an embodiment, a combustion system includes a solid fuel burner disposed within an enclosure. The burner is configured to sustain a combustion reaction of the solid fuel and an oxidant. Two or more electrodes are also positioned within the enclosure. A first electrode is positioned to impart an electrical potential or charge to the combustion reaction. A second electrode is positioned within the enclosure above and/or lateral from the solid fuel. By applying a selected voltage between the electrodes, the shape of the combustion reaction can be adjusted in a selected manner.
According to an embodiment, the second electrode is fixed to an inner wall of the enclosure by a support. The combustion system can include a cooling apparatus that cools the support. Cooling the support increases the electrical resistance between the inner wall and the second electrode, thereby inhibiting an unwanted flow of current between the second electrode and the inner wall of the enclosure.
According to an embodiment, a method includes supporting a solid fuel in a combustion volume defined by an enclosure, supplying an oxidant to the combustion volume, and sustaining a combustion reaction of the solid fuel and the oxidant within the combustion volume. The method further includes adjusting a height of the combustion reaction by applying an electrical potential between the combustion reaction and a first electrode positioned above the solid fuel and fixed to the enclosure. The method can further include cooling a support that fixes the first electrode to the enclosure.
According to an embodiment, a system includes an enclosure defining a combustion volume,a grate configured to support a solid fuel, and an oxidant source configured to supply oxidant to the combustion volume to support a combustion reaction of the solid fuel and the oxidant. An electrode is positioned above the grate and is fixed to an inner wall of the enclosure by a support. A cooling apparatus is configured to cool the support. A voltage supply is coupled to the grate and the electrode and is configured to adjust a shape of the combustion reaction by applying an electrical potential between the electrode and the grate.
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Other embodiments may be used and/or other changes may be made without departing from the spirit or scope of the disclosure.
The enclosure 102 is, for example, a furnace including one or more walls that at least partially enclose the combustion volume 104. The enclosure 102 can include multiple layers of various materials selected to withstand very high temperatures. For example, an innermost layer of the enclosure 102 can include a ceramic material such as an aluminosilicate material.
The solid fuel burner 106 is configured to hold a solid fuel within the combustion volume 104. The solid fuel burner 106 also includes an oxidant source that supplies and oxidant to the combustion volume 104. The solid fuel burner 106 supports the combustion reaction 108 of the solid fuel and the oxidant.
According to an embodiment, the solid fuel burner 106 includes an electrode that is positioned to impart a voltage or charge to the combustion reaction 108. In one example, the electrode is a grate on which the solid fuel is disposed. Alternatively, the electrode can be separate from the grate and can be positioned so that it is in contact with the combustion reaction 108.
The electrodes 112, 114 are fixed to an inner wall of the enclosure 102. The electrodes 112, 114 can be disposed above and or laterally from the solid fuel and opposite from each other within the enclosure 102. The electrodes 112, 114 can include a ceramic material that becomes increasingly conductive as the temperature increases and that is capable of withstanding very high temperatures. The ceramic material can include silicon carbide, an aluminosilicate material, or other suitable ceramic material. Alternatively, the electrodes 112, 114 can include a conductive material such as one or more refractory metals.
The voltage supply 116 is configured to apply an electrical potential between the electrode of the solid fuel burner 106 and one or both of the electrodes 112, 114. The electrical potential can be applied via the connection lines 118, 120, 122. When the electrical potential is applied between the solid fuel burner 106 and the electrodes 112, 114, the shape of the combustion reaction 108 can be manipulated. Is often desirable to decrease a length of the combustion reaction 108 and to broaden the combustion reaction 108. By applying the electrical potential between the solid fuel burner 106 and the electrodes 112, 114, the length of the combustion reaction 108 can be decreased and the width of the combustion reaction 108 can be increased by drawing the combustion reaction 108 toward the electrodes 112, 114. This can decrease the amount of undesirable byproducts such as oxides of nitrogen (NOx) and carbon monoxide (CO).
According to an embodiment, the electrical potential can have a magnitude greater than 10,000 V. According to an embodiment, the electrical potential can have a magnitude greater than 45,000 V. The electrical potential can have a time varying waveform. The timeframe waveform can include one or more of sawtooth waveforms, square waves, sine waves, and combinations thereof.
According to an embodiment, the grate 206 acts as both a support that holds the solid fuel 208 and an electrode that can be used to electrically adjust the shape of the combustion reaction 108 in conjunction with the electrodes 112, 114. In particular, when a voltage is applied to the grate 206, a voltage or charge is imparted to the combustion reaction 108. According to an embodiment, when the electrodes 112, 114 and the grate 206 are charged to opposite potentials, an electric field is established between the grate 206 and the electrodes 112, 114. The shape of the combustion reaction 108 can follow the electric field lines. The grate 206 can act as a primary electrode, establishing an electric field with the wall electrodes 112, 114.
According to an embodiment, the electrodes 112, 114 are positioned above and lateral to the grate 206. This can provide multiple benefits including both the reduction in the generation of undesirable byproducts, and the attraction of particulates in directions selected to prevent output up the flue. Thus, not only are undesirable byproducts reduced, but the output of particulates up the flue is also reduced. While a particular placement of the electrodes 112, 114 is disclosed in
According to an embodiment, the grate 206 can include a mild steel plate, having rounded corners to minimize electrical arcing. Combustion air flows from under the grate 206. The grate 206 can include ¼ inch holes on 1 inch centers, square pitch. Alternatively, the grate 206 can be a ceramic material that becomes conductive at high temperatures.
According to an embodiment, the enclosure 102 can include six inches of aluminosilicate ceramic fiberboard that thermally insulates the combustion volume 104 from an exterior of the enclosure 102. The innermost 1 inch of the enclosure 102 can withstand 3000° F., and the remaining 5 inches can withstand 2300° F.
According to an embodiment, the electrodes 112, 114 can be plate electrodes made from a ceramic material. The ceramic material can include silicon carbide, an aluminosillicate material, or other suitable ceramic material. The electrodes 112, 114 can includes shapes other than plates. Likewise, the electrodes 112, 114 can be positioned within the combustion volume 104 in other locations and orientations relative to the grate 206 than is depicted in
According to an embodiment, the solid fuel combustion system 200 operates at firing rates between 0.04-1.75 MMBTU/h. Two variable speed screw feeders can deliver fuel to the grate 206. The oxidant source 210 can include a blower with a variable speed control. The oxidant source 210 can supply the combustion volume 104 with air from beneath a grate 206. The oxidant source 210 can include a damper that controls backpressure on the oxidant source 210.
According to an embodiment, the enclosure 102 can include a door and a sight port, which can be used to monitor the combustion volume 104. The sight port can be covered with welding glass to allow safe, high-contrast viewing. The combustion reaction 108 can be inspected through the sight port to determine if the applied electric fields are impacting the flame shape and capture video for further analysis.
According to an embodiment, the voltage supply 116 can deliver up to 50 kV at up to 12 mA. The voltage supply 116 can include multiple individual voltage supplies. According to an embodiment, the voltage supply 116 can include a waveform generator that can generate square, sinusoidal, sawtooth, and other types of waveforms. The voltage supply 116 can drive the electrodes 112, 114 and the grate 206 such that the grate is 180° out of phase with the electrodes 112, 114, effectively doubling the maximum potential supplied to the system.
A control circuit 212 can control the voltage supply 116 via a connection line 214. Alternatively, the voltage supply 116 can be manually controlled by a technician.
According to an embodiment, the solid fuel 208 can include an organic biomass fuel. The solid fuel 208 can include wood, such as fir pellets. Alternatively, the solid fuel 208 can include other types of solid fuel such as hog fuel, refuse derived fuel (RDF), and lignite coal
In
Because at very high temperatures, ceramic materials can become conductive, it is possible for an inner wall of the enclosure 102, the support 202, and the electrode 112 to become sufficiently conductive that a significant leakage current flows from the electrode 112 through the support 202 to the enclosure 102. This is problematic for several reasons. First of all, the leakage current significantly increases the amount of electrical power consumed when applying the electrical potential between the electrodes 112, 114 and the grate 206. Second of all, the leakage current causes a large drop in the electrical potential between the electrodes 112, 114 and the grate 206. The decrease in electrical potential can become so significant that the shape of the combustion reaction 108 can no longer be manipulated in the desired manner, leading to an increase in undesirable byproducts and the decrease in the effectiveness of the combustion system 300.
Accordingly, the cooling apparatus 302 is positioned proximate to the support 202 so that the cooling apparatus 302 can cool the support 202. By cooling the support 202, the electrical conductivity of the support 202 remains comparatively low. With the conductivity of the support 202 being comparatively low, the leakage current between the electrode 112 and the enclosure 102 is relatively small. With a small leakage current, there is not a significant power consumption due to leakage. Furthermore, the electrical potential between the electrodes 112, 114 and the grate 206 can remain sufficiently high so that the shape of the combustion reaction 108 can be manipulated in the desired manner.
While
The bracket 406 is fixed to an inner wall of the enclosure 102. The electrode 112 is also fixed to the bracket 406. In this way, the bracket 406 fixes the electrode 112 to the enclosure 102 within the combustion volume 104. According to an embodiment, the bracket 406 includes a ceramic material that can withstand extremely high temperatures. However as discussed previously in relation to
Accordingly the cooling tube 402 is placed adjacent to the bracket 406. The cooling tube 402 includes an inner channel 404 through which a cooling fluid 408 passes. As a cooling fluid 408 passes through the inner channel 404 of the cooling tube 402, heat is transferred from the bracket 406 to the cooling tube 402. In this way, the bracket 406 is cooled by the transfer of heat from the bracket 406 to the cooling tube 402. Because heat is transferred from the bracket 406 to the cooling tube 402, the temperature of the bracket 406 remains at a comparatively low temperature with respect to the electrode 112. Because the temperature of the bracket 406 is comparatively low, the electrical conductivity of the bracket 406 is also comparatively low. This leads to a comparatively low leakage current from the electrode 112 to the enclosure 102.
According to an embodiment, it is desirable for the temperature of the electrode 112 to remain comparatively high so that the electrical conductivity of the electrode 112 remains comparatively high. With the electrical conductivity of the electrode 112 comparatively high, the electrode 112 can be held at a high voltage thereby creating a high electric field between the electrode 112 and the grate 206. The combustion reaction 108 follows the electric field lines toward the electrode 112.
According to an embodiment, the cooling tube 402 extends along a surface of the bracket 406 in a serpentine fashion such that a large surface area of the bracket 406 is in contact with or proximal to the cooling tube 402. As the cooling fluid 408 passes through the serpentine cooling tube 402, much heat is transferred from the bracket 406 to the cooling fluid 408 in the inner channel 404 of the cooling tube 402. In this way, the cooling tube 402 helps to maintain the bracket 406 at a comparatively low temperature.
According to an embodiment, the cooling tube 402 includes quartz. Alternatively, the cooling tube 402 can include any other suitable material that can withstand high temperatures within the combustion volume 104. According to an embodiment, the cooling fluid 408 is air. After the cooling fluid 408 is passed through the cooling tube 402, the cooling fluid 408 flows into the combustion volume 104. The spent cooling fluid 408 can include an oxidant. In an alternative embodiment, the cooling fluid 408 is a liquid such as water. Alternatively, the cooling fluid 408 can be a gas other than air. The cooling fluid 408 can also be working fluid.
While
Specific embodiments may be made by reference to the following examples:
Open air testing revealed strong electric field effects on the shape of combustion reactions showing reductions in combustion reaction height by 28% and dilation in combustion reaction width by 33%; this squashing and stretching of the combustion reaction was a desired goal of the research.
Testing with the furnace door closed showed significant reductions in opacity. In order to see the effect, oxygen was reduced until the opacity was increased to 82%. When an electric field was applied, the opacity was cut nearly in half to 48%. Such results bode well for increasing capacity, and reducing oxygen concentrations without attendant CO production. A reduction in oxygen (O2) also is likely to reduce NOx substantially.
Ceramic electrodes were also developed which are capable of tolerating the furnace environment. Ceramic surfaces are not normally conducting; however, as some ceramics reach furnace temperature they become sufficiently conductive and make excellent in-furnace electrodes, robust to the physical, chemical, and temperature environment of an operating furnace. However, the high temperature conductivity of various ceramics is a two-edged sword. When the thermal insulation in the furnace reaches operating temperatures, they also become electrically conductive for very high voltages, and the resulting current draw makes establishing an electric field inside the furnace difficult.
Therefore, isolation of the ceramic electrodes from the furnace walls using cooled surfaces is being worked on. Such cooled surfaces represent a very minor cooling load (currently estimated at 0.1% of the total thermal load or less) as the cooling is needed only at limited isolation points. Moreover, any convenient fluid may be used for cooling such as water, steam, or air. In preliminary tests of this method with a scaled-down electrode, the current drawn by the ceramic electrode was reduced from 870 μA down to 200 μA with a stack temperature ranging from 1100° F. to 1300° F. This 77% reduction in current is a 95% reduction in power consumption from 7.5 W to 0.4 W. Also, as a result of the sufficiently low current, a less severe voltage drop was observed, which means than an electric field can be successfully established inside the hot furnace.
Wood pellets, hog fuel, refuse derived fuel (RDF), and lignite coal generated baseline emissions. Constant firing rates allowed the system to reach steady state and generate baseline data. Data was also collected under transient conditions. Table 2 shows a snapshot of baseline data for wood pellets, hog fuel, lignite, and RDF around a firing rate of 0.5 MMBTU/h.
The visual effects of electrodynamic combustion control on a flame were apparent during open-door testing when the furnace was near room temperature. The flame shape was manipulated and an initial reduction in opacity from 82% to 48% was observed. However, the opacity reduction diminished as internal surfaces warmed, and at high temperatures, visual inspection of the flame with activated electric fields showed a diminished ability to affect flame shape. The cause was traced to current leakage. When the furnace was run and a stack temperature of 1300° F. was reached, all three electrodes were drawing nearly 850 μA. Using these numbers and Ohm's law, V=IR, one can calculate a voltage drop of approximately 9.5 kV, indicating that the electrodes in the furnace were charged to a potential of, at most, 500 V—an insufficient voltage for flame manipulation.
Evidently, even highly resistive materials are insufficiently resistive at elevated insulation temperatures. Resistivity vs. temperature had previously correlated for a variety of insulating materials with a megaohm meter and found to be adequate. However, it is observed that as the voltage increases to the 50 kV range and beyond, even small current leakage across a large surface area is sufficient to reduce the electric field strength and hamper the desired flame response.
Subsequent testing with a high-voltage probe showed that current was leaking to ground through the thermal insulation. To investigate the current draw at room temperature, a 10 MΩ resistor was placed on each electrode inlet and measured the voltage drop across the resistors using a high-voltage probe and a multimeter. The grate electrode was drawing 44 μA, and each wall electrode was drawing 12 μA. After installing a scaled-down electrode with cooled supports, no electric potential was measured on the insulating walls at room temperature, and the ceramic electrode measured 9.5 kV. The cooled electrodes also measured 0 kV. At a firing rate of 0.15 MMBTU/h and a stack temperature of 1097.1° F., the voltage drop was 2.94 kV, the current draw was 194 μA, and the power consumption was 0.38 W.
While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.
The present application claims priority benefit from U.S. Provisional Patent Application No. 62/074,453, entitled “SOLID FUEL SYSTEM WITH ELECTRODYNAMIC COMBUSTION CONTROL”, filed Nov. 3, 2014 (docket number 2651-259-02); co-pending at the time of filing; which, to the extent not inconsistent with the description herein, is incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62074453 | Nov 2014 | US |
