Rotary kilns, or calciners, are used to cause thermal decomposition, phase transition, or removal of volatile fractions from ores and other solid materials in the presence of air. The most familiar calcine (a word that refers to any product produced by the process) is Portland cement, which is produced from limestone (calcium carbonate) as the decomposition product calcium oxide.
Typically, temperature vs. time should be carefully controlled in rotary kilns to provide sufficient energy without exceeding a melting point of the product. Lime kilns (as Portland cement-producing rotary kilns may be commonly referred to) tend to be somewhat tricky to operate for producing optimum product. Temperatures that are too high or too low, or treatment times that are too long or too short can result in sub-par product that does not yield favorable market demand or price. Operation of rotary kilns may be complicated by variations in co-fired fuels (such as tires) that are introduced into the kilns along with raw materials for producing the product.
What is needed is a technology that can maintain consistent desirable kiln conditions and/or adapt to changing operating conditions, while minimizing undesirable flue gases such as oxides of nitrogen (NOx) and carbon monoxide (CO).
According to an embodiment, a rotary kiln may include a stationary burner configured to output one or more fuels and combustion air to support a flame in an inclined rotary shell having an upper end and a lower end and configured to rotate around the stationary burner and the flame to convey a feedstock, reaction intermediates, and a calcined product along the inside of the inclined rotary shell from the upper end to the lower end, and to output a calcined product at the lower end. At least one electrode may be mechanically coupled to the stationary burner and operatively coupled to the flame, the electrode(s) being configured to apply a high voltage or an electric field corresponding to the high voltage to the flame. The voltage or a time variation of the voltage may be controlled to maintain a desired flame characteristic and/or a desired time vs. temperature profile in the reactants and products conveyed through the rotary kiln.
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. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here.
According to an embodiment at least one electrode 116 is operatively coupled to the stationary burner 102 and/or the flame 104. The at least one electrode 116 is configured to apply a high voltage or an electric field corresponding to the high voltage to or proximate to the flame 104. The electrode or electrodes 116 are arranged along a portion of the inclined rotary shell 106 such that the electrode(s) are substantially not subject to contact with the transported process material 112. For example, the electrode(s) 116 can be arranged substantially along an axis of rotation of the inclined rotary shell 106, and to occupy a fraction of the radius of the inclined rotary shell 106 around the axis of rotation of the inclined rotary shell 106. Additionally or alternatively, the at least one electrode 116 can be arranged along a region above the axis of rotation of the inclined rotary shell 106 and having a lateral extent above the axis of rotation and not intersecting the inclined rotary shell 106 or clinker (process material 112) carried by the inclined rotary shell 106.
According to an embodiment, at least one electrical lead 118 is configured to supply the voltage to the at least one electrode 116, the at least one electrical lead 118 being operatively coupled to the stationary burner 102. The at least one electrical lead 118 is mechanically coupled to the outside of stationary burner 102. For example, the at least one electrical lead 118 can be mechanically coupled to a refractory material 120 disposed along the outside of the stationary burner 102. The at least one electrical lead 118 can alternatively be carried inside the refractory material 120 disposed along the outside of the stationary burner 102. Alternatively, the at least one electrical lead 118 can be carried inside the stationary burner 102, such as along the inside of a conduit, inside a non-conductive fuel delivery channel, and/or inside a combustion air delivery channel. The at least one electrical lead 118 can be cooled by a fluid flow (not shown) inside or peripheral to the at least one electrical lead 118.
According to an embodiment, a voltage source 122 is configured to provide the voltage to the at least one electrode 116. The voltage source 122 can be configured to provide a time-varying voltage to the at least one electrode 116 such as a chopped DC voltage, an alternating current voltage, or an alternating current voltage superimposed over a DC-bias voltage. The voltage source 122 can be configured to apply a periodic voltage waveform to the at least one electrode 116. The periodic voltage waveform can be characterized by a frequency between 200 and 800 Hertz, for example. The periodic voltage waveform can include a high voltage waveform between 0-40,000 volts for, example. For the example of an AC waveform, the periodic voltage waveform can be ±2000 to ±100,000 volts, for example. Other frequencies and/or other voltages can be substituted without departing from the spirit or scope of the claims.
According to an embodiment, a control interface 124 is configured to control the voltage source 122. For example, the control interface 124 can control the voltage source 122 to maintain the quality of the calcined product 114, to provide immunity from changes in fuel, to compensate for variations in fuel flow rate, to compensate for changes in environment, and/or or to minimize of one or more components of a flue gas 126.
According to an embodiment, the inclined rotary shell 106 is electrically grounded. A feedstock introduction apparatus (not shown) can be included at an upper end 108 of the inclined rotary shell 106. A calcined product 114 receiving apparatus (not shown) can be included at the lower end 110 of the inclined rotary shell 106. The at least one electrode 116 can be mechanically coupled to the feedstock introduction apparatus (not shown) and operatively coupled to the stationary burner 102 and the flame 104 along an axis of rotation of the inclined rotary shell 106 by electromagnetic interaction (arrangement not shown).
The at least one electrode 116 can be configured to minimize or make substantially constant a time to which the process material 112 is subject to radiation heat transfer by causing the flame 104 to occupy a small volume proximate to the process flow.
In continuous flow processes where it is desired to subject all of the process material 112 to uniform and consistent convective heating from the flame 104, variations in heating modes, such as found in region 204 or radiation heating, such as that corresponding to region 206, can be undesirable.
According to embodiments, a solution to the shortcomings of the prior art may be to confine the flame 104 into a relatively small or at least a constant region to minimize variable radiation/convective heating of the process material 112. By confining the flame 104, the convection heating zone of the first region 202 (which may provide substantially constant heat flux) can be maximized. According to other embodiments, the variable heating of the second region 204 as described in conjunction with
According to an embodiment, as shown in
According to an embodiment, a second region 206 farther down the inclined rotary shell 106 corresponds to a region of flame confinement, or “squish,” wherein substantially the entire (emissive portion of the) combustion process occurs in a limited volume. As will be appreciated, application of voltage, charge, and or electric fields to the flame 104 provides the illustrated confinement. In the second region 206, the process material receives significant radiation heating. In some embodiments, radiation heating can form the dominant heat transfer mode (or H-mode) in the region 206. In some embodiments, substantially all the heat transfer in the region 206 is associated with radiation heat transfer.
After passing the region 206, the process material enters a third region 208, according to an embodiment. For example, the region 208 corresponds to the region shown in
In contrast to the situation depicted in
According to an embodiment, the charge electrode 402 is configured to apply a charge or voltage to the flame 104. One or more field electrodes 404 are configured to cooperate with the applied charge or voltage to “confine” or “squish” the flame 104. In essence, squishing the flame 104 causes the flame 104 to occupy a relatively small volume. The small volume of the flame 104 corresponds, for example, to the second region 206 shown in the embodiment 301 of
A voltage source 122 (shown in
Additionally or alternatively, voltage(s) applied to the charge electrode 402 and the one or more field electrodes 404 can be configured to cooperate to cause the shape of the flame 104 to be affected by electrical charge induced mixing. Such enhanced mixing causes more rapid combustion and/or a change in flame emissivity compared to a flame without electrical charge induced mixing. For example, electrical charge induced mixed flames have been found to exhibit turbulence artifacts and corresponding increased fuel-air surface area. Electrically mixed flames can occupy a reduced volume (e.g., corresponding to the second region 206 shown in
Beginning with step 702, the rotary kiln receives process material, or feedstock, at an upper end of and inclined rotary kiln shell. For example, the feedstock can include an ore or other solid material such as limestone (calcium carbonate), hydrated or wet titanium oxide, or a wet or damp solid fuel. Proceeding to step 704, the rotary kiln shell is rotated to convey the process material along the inside and bottom of the inclined rotary kiln shell. Gravity provides a driving force for causing the process material to make its way along the inclination of the rotary kiln shell such that the rotation of the kiln shell conveys the process material past the stationary burner. Proceeding to step 706, a stationary burner supports a flame inside the rotating rotary kiln shell. Step 706 can include outputting one or more fuels and combustion air from the stationary burner to support the flame. Heated gas including flue gas from the flame moves substantially parallel to the axis of the rotating kiln shell to convectively heat the process material. The process material thus receives heat from the supported flame.
Proceeding to step 708, at least one electrode is supported inside the rotary kiln shell and operatively coupled to the flame. The electrode(s) is supported in a substantially constant relationship to a stationary burner. As may be appreciated from the descriptions above, various electrode arrangements are contemplated. Step 708 can include one or the other, or both of steps 710 and 712. Step 710 includes supporting at least one charge electrode proximate the flame. Step 712 includes supporting at least one field electrode in substantially constant relationship to the stationary burner.
Proceeding to step 714, one or more voltages are applied to at least one electrode. Step 714 includes applying high voltage to at the least one electrode operatively coupled to the stationary burner and to the flame. Applying the high voltage can include applying a least one electrical field to the flame to cause the flame to adopt a selected geometry, selected characteristic, or selected geometry and characteristic. Causing the flame to adopt a selected geometry, selected characteristic, or selected geometry and characteristic can include selecting a flame to cause a substantially constant temperature exposure to the conveyed feedstock and reaction intermediates during a calcining process.
Applying a voltage in step 714 can include applying a high voltage. Applying the high voltage can include applying a waveform having a periodic frequency between 200 and 800 Hertz, at between ±2000 and ±40,000 volts, for example. Applying the voltage can include conveying less than about 1 milliampere of current. Applying the high voltage can include applying a sinusoidal, square, triangular, truncated triangular, sawtooth, or logarithmic voltage waveform. Applying the high voltage can further include operating at least one of a waveform generator, a voltage inverter, or a voltage multiplier to produce a high voltage alternating current (not shown).
Step 714 can include one or the other, or both of steps 716 and 718. In step 716, one or more voltages are applied to a charge electrode. In step 718, one or more voltages are applied to a field electrode. Applying one or more voltages to a field electrode in step 718 can include applying the voltage(s) synchronously with the one or more voltages applied to the charge electrode in step 716.
The method 701 can further include operating at least one of a waveform generator, a voltage inverter, and/or or a voltage multiplier to produce a high voltage alternating current applied in step 714.
Proceeding to step 720, the flame is squished responsive to the applied voltage(s), according to an embodiment. For example, synchronously applying voltages in steps 716 and 718 causes the flame to be repelled from the field electrode and to be squished into a smaller volume than the flame would occupy without the applied voltages. Additionally or alternatively, applying the voltage(s) in step 714 causes the flame to be turbulently mixed such that step 720 includes causing the flame to occupy a reduced volume compared to a flame with reduced turbulent mixing. Moreover, applying the one or more voltages in step 714 causes electrically enhanced mixing of fuel and oxidizer, and can cause fuel particles or molecules to travel a shorter distance before being consumed by the flame compared to not applying the one or more voltages. Such reduced travel distance corresponds to squishing the flame in step 720.
According to an embodiment, applying a high voltage in step 714 includes applying a least one electrical field to the flame to cause the flame to adopt a selected geometry, selected characteristic, or selected geometry and characteristic. According to an embodiment, the selected geometry, selected characteristic, or selected geometry and characteristic can include a flame selected to cause a substantially constant temperature exposure to the conveyed feedstock and reaction intermediates during the calcining process. Additionally or alternatively, the selected geometry, selected characteristic, or selected geometry and characteristic can include a compact flame configured to substantially complete combustion between the stationary burner and the at least one electrode. Additionally or alternatively, the selected geometry, selected characteristic, or selected geometry and characteristic can include a compact flame selected to minimize a time during which a reaction intermediate or calcined product is exposed to a temperature above a desired calcining temperature. Additionally or alternatively, the selected geometry, selected characteristic, or selected geometry and characteristic can include a compact flame selected to maximize a time during which a reaction intermediate or calcined product is exposed to a desired calcining temperature.
Proceeding to step 722, according to an embodiment, the proportion of heat delivered by convective heating is increased in proportion to radiation heat transfer. That is, the smaller flame may not expose the process material to thermal radiation for as long as a flame in a rotary kiln not including an applied voltage, charge, or electric field.
Proceeding to step 724, the product is output. For example, the product may include Portland cement, dehydrated titanium oxide, or a dried solid fuel.
According to an embodiment, during the method 701, the stationary burner is maintained in substantial electrical isolation from ground and from voltages other than the voltage applied to the flame. According to another embodiment, the inclined rotary body is held at electrical ground. According to another embodiment, the stationary burner can be isolated or insulated from high voltage.
According to an embodiment a high-consistency calcine material is made by a process including inputting a stream of raw material to a rotating process vessel, supporting a flame with a stationary burner disposed within the rotating process vessel, and operating an electric field application system to impress one or more electric fields upon the flame and a region near the flame. The flame is disposed to heat the raw material, an intermediate product reacted from the raw material, and/or the high-consistency calcine material made from the intermediate product. The electric field application system is configured to control a spatial distribution of a radiant energy source comprising the flame by controlling the one or more electric fields. The spatial distribution corresponds substantially to radiation heating received by the process material. The high-consistency calcined product can include Portland cement, dehydrated titanium oxide, or a dried solid fuel.
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. 61/661,744, entitled “FLAME ENHANCEMENT FOR A ROTARY KILN”, filed Jun. 19, 2012; which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.
Number | Date | Country | |
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61661744 | Jun 2012 | US |