Patent Application
20130255549
Solid fuel burners are notorious for non-ideal flow behavior such as clumping. Fuel clumping has been associated with variable resistance to undergrate air flow. Fuel clumping may be visualized as a formation of “hills” 114 and “valleys” 116 in fuel 106 on the grate 104. The hills 114 typically have high resistance to airflow, and the valleys 116 typically have low resistance to airflow. Additionally, airflow may be affected by proximity to the walls 102. A result of this variable resistance to airflow is that there may be less airflow than desirable in regions 118 above the hills 114, and more airflow than desirable in regions 120 above the valleys 116. Moreover, the solid fuel 106 typically volatilizes responsive to high temperatures from combustion, and it is the volatilized, gas phase components that actually burn. There may be more volatilization above the hills 114 than the valleys 116, which may further add to the disparity in composition between the regions 118 above the hills 114 and the regions 120 above the valleys.
The non-homogeneity of the regions 118, 120 leads to two undesirable conditions. Regions 118 with low airflow tend not to have enough oxygen for complete combustion. This results in cooler temperatures and high output of carbon monoxide (CO) and other products of incomplete combustion. Conversely, excess airflow in the regions 120 causes high temperatures and relatively high concentrations of oxygen and nitrogen, both of which tend to cause formation of oxides of nitrogen (NOx).
Manufacturers and operators of solid fuel burners 101 have attempted to ameliorate the problems associated with non-homogeneity by introducing overfire or secondary air above the grate 104 and the fuel 106 with one or more overfire air sources 122. The overfire air is typically introduced at high velocity to help mixing of the regions 118, 120. Unfortunately, while overfire air may provide more oxygen to complete combustion of CO to carbon dioxide (CO2), it may not affect or can even make more severe the formation of NOx. Moreover, it is typical that overfire air is added in excess. Excess overfire air reduces the temperature of flue gases 112 and can reduce thermodynamic efficiency of processes driven by the heat produced by combustion. Reduced thermodynamic efficiency may generally require burning more fuel to create a desired output, or may reduce the amount of the output for a given amount of fuel. Finally, the ability to deliver overfire air across a wide grate 104 is limited by the amount of inertia that can be imparted on the overfire air and the distance it can travel through buoyant forces associated with the combustion.
What is needed is a technology that can improve uniformity or homogeneity of reactive gases associated with a solid fuel burner. It is also desirable to improve gas homogeneity with minimum cooling of exit gas temperature. Finally, some applications by benefiting from improved homogeneity across a grate having dimensions larger than what may be addressed by overfire air.
According to an embodiment, a solid fuel burner may be provided with a system for providing electrodynamic homogenization. The solid fuel burner may include a grate configured to support a burning solid fuel and an underfire air source configured to deliver underfire air to the burning solid fuel from below the grate. The system for providing electrodynamic homogenization may include an electrode (one or more electrodes) configured to apply an electric field to the burning solid fuel or a region proximate the burning solid fuel. The electric field, which may include a time-varying electric field, may be selected to cause mixing and homogenization of volatilized fractions of the solid fuel, combustion gases, and air. The improved mixing and homogenization may result in reduced emission of carbon monoxide (CO), reduced emission of oxides of nitrogen (NOx), reduced oxygen in flue gas, increased temperature of flue gas, and/or allow for a larger grate surface.
According to an embodiment, a solid fuel burner may include a system for providing electrodynamic homogenization. The system may include a grate configured to support a burning solid fuel and an underfire air source configured to deliver underfire air to the burning solid fuel from below the grate. An electrode (one or more electrodes) may be configured to apply an electric field to the burning solid fuel or a region proximate the burning solid fuel. The electric field, which may include a time-varying electric field, may be selected to cause mixing and homogenization of volatilized fractions of the solid fuel, combustion gases, and air. The improved mixing and homogenization may result in reduced emission of carbon monoxide (CO), reduced emission of oxides of nitrogen (NOx), reduced oxygen in flue gas, increased temperature of flue gas, and/or allow for a larger grate surface.
According to another embodiment, a method for operating a solid fuel burner may include delivering underfire combustion air below a grate, burning solid fuel on the grate with the combustion air in a combustion reaction, and homogenizing a mixture of volatilized solid fuel and underfire combustion air in the combustion reaction by applying an electric field with at least one electrode disposed above the grate or comprising the grate. The electric field may include a time-varying electric field.
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.
Various electrode embodiments are contemplated. As illustrated in the embodiment 201, the electrode 202 may be disposed above the grate 104 and the solid fuel 106.
The electrode 202, 302, 402 may include a plurality of electrodes. Such a plurality may include plural electrodes 202 located in the combustion volume, plural grate electrodes 302 located in the combustion volume and/or plural electrodes 402 located outside the combustion volume. Plural electrodes may also include combinations of two or more of the electrodes 202, 302, 402 indicated diagrammatically in
Referring to
The solid fuel burner 201, 301, 401 may include an overfire air source 122 configured to deliver overfire air above the grate 104. Application of the electric field by the electrode 202, 302, 402 may result in a reduction in the amount of overfire air required to meet emission requirements compared to a system not including the electrode 202, 302, 402 and/or not providing electrodynamic homogenization. Application of the electric field by the electrode 202, 302, 402 may result in a reduction in an amount of underfire or undergrate air required to meet emission requirements compared to a system not including the electrode 202, 302, 402 and/or not providing electrodynamic homogenization. Similarly, application of the electric field by the electrode 202, 302, 402 may result in a reduction in the amount of total air required to meet emission requirements compared to a system not including the electrode 202, 302, 402 and/or not providing electrodynamic homogenization. Finally, the application of the electric field by the electrode 202, 302, 402 may result in a reduction in an emission of one or more of oxides of nitrogen (NOx) and carbon monoxide (CO) from the solid fuel 106 burning compared to a system not including the electrode 202, 302, 402 and/or not providing electrodynamic homogenization.
According to embodiments, the application of the electric field by the electrode 202, 302, 402 may result in heat release nearer the solid fuel 106 compared to a system not including the electrode 202, 302, 402 and/or not providing electrodynamic homogenization. The release of heat nearer the solid fuel 106 may provide enhanced drying of the solid fuel 106. This may allow the use of lower grade fuels, reduced pre-processing of fuel, and/or may allow the use of fuels that cannot normally be fired without application of heat from a second combustion reaction (e.g., co-firing with natural gas).
The solid fuel burner 201, 301, 401 may include an electrode controller 204 operatively coupled to the electrode(s) 202, 302, 402 and configured to determine an electrode 202, 302, 402 voltage or charge concentration corresponding to the electric field. The electrode controller 204 may include one or more of a state machine, a field-programmable gate array, a microcontroller, or discrete components configured to determine the electric field.
The solid fuel burner 201, 301, 401 may include an amplifier or voltage multiplier 206 operatively coupled to the electrode controller 204 and the electrode(s) 202, 302, 402, or included in the electrode controller 204 and operatively coupled to the electrode(s) 202, 302, 402. The amplifier or voltage multiplier 206 may be configured to output an operating voltage waveform to the electrode(s) 202, 302, 402 responsive to a logic level digital or low voltage analog signal received from the electrode controller 204.
According to embodiments, the electric field may include a time-varying electric field and the voltage may similarly correspond to a time-varying voltage applied to the electrode(s). For example, the time-varying electric field may include an electric field that varies according to an alternating current (AC) voltage waveform applied to the electrode(s). The time-varying voltage may include a sinusoidal, square wave, sawtooth wave, triangular wave, truncated triangular wave, logarithmic, or exponential waveform. Various voltages may be used. For example, the time-varying voltage applied to the electrode(s) may include a periodic voltage having an amplitude of 4000 to 115,000 volts (or ±4000 to 115,000 volts). The time-varying voltage may include a periodic voltage having a frequency of 50 to 800 Hertz, for example. According to some embodiments, the time-varying voltage can have a periodic frequency of 200 Hertz to 300 Hertz.
According to embodiments, the solid fuel burner 201, 301, 401 may include one or more sensors (not shown) operatively coupled to the electrode controller 204 and configured to measure one or more characteristics of the burning of the solid fuel 106, the flame, or combustion gas produced by the burning solid fuel 106. For example, the one or more sensors (not shown) may be configured to measure a variable characteristic of a completeness of combustion or a fuel 106 characteristic. The electrode controller 204 may be configured to select an electric field characteristic to increase gas mixing when the completeness of combustion is lower than a target value or when the fuel 106 characteristic corresponds to a need to increase mixing.
The solid fuel burner 201 (and variants 301, 401) may include a mechanical or pneumatic stoker (not shown) configured to deliver the solid fuel 106 to the grate 104 assembly.
The electrode controller 204 may be configured to control one or more of an overfire air 122 flow, the underfire air 108 flow, or a rate of fuel delivered by a stoker. Alternatively or additionally, the solid fuel burner 201, 301, 401 may include one or more of an overfire air controller (not shown), an underfire air controller (not shown), or a stoker controller (not shown) operatively coupled to the electrode controller 204.
The solid fuel burner 201, 301, 401 may include a physical gap (not shown) between a stoker (not shown) and the solid fuel 106 on the grate 104, the gap being configured to reduce or eliminate current leakage from the electric field through fuel carried by the stoker (not shown). Optionally, the solid fuel burner 201, 301, 401 may include a fuel cache (not shown) operatively coupled to a fuel stoker (not shown) and electrical insulation (not shown) between the fuel cache (not shown) and a support structure (not shown) The fuel cache (not shown) and the electrical insulation (not shown) may be configured to reduce or eliminate current leakage from the electric field through the stoker (not shown) and fuel positioned near a stoker intake (not shown).
Burning of various types of solid fuels are contemplated to benefit from electrodynamic homogenization. For example, the solid fuel 106 may include at least one of a biomass fuel, coal, a tire-derived fuel (TDF), municipal solid waste (MSW), refuse derived fuel (RDF), or a hazardous solid waste.
Proceeding to step 504, underfire combustion air may be fed from below the grate. In step 506, solid fuel on the grate may be burned with at least the underfire combustion air in a combustion reaction. Burning the solid fuel may include burning the solid fuel in a combustion volume defined by a wall.
Proceeding to step 508, a mixture of volatilized solid fuel and underfire combustion air above the fuel is homogenized by applying an electric field with at least one electrode. Various electrode arrangements are contemplated. For example, applying the electric field with at least one electrode may include applying an electric field with at least one electrode disposed inside the combustion volume. For example, the at least one electrode may be disposed above the grate. Alternatively or additionally, the at least one electrode may include the grate. According to embodiments, applying the electric field with at least one electrode may include applying the electric field with at least one electrode disposed outside the combustion volume. The at least one electrode may include a single electrode, or may include a plurality of electrodes. The plurality of electrodes may include a plurality of electrodes disposed similarly, for example, all electrodes being above the grate, all electrodes including portions of the grate, or all electrodes being disposed outside the combustion volume. Alternatively, a plurality of electrodes may include one or more electrodes above the grate, one or more electrodes comprising the grate, and/or one or more electrodes disposed outside the combustion volume.
Step 508 may include operating an electrode controller to determine the electric field. In some embodiments, the electric field may be a DC electric field or an intermittently applied DC electric field. Alternatively, the electric field may include a time-varying electric field. Operating the electrode controller may include amplifying a logic level digital or low voltage analog signal received from the electrode controller to an operating voltage placed on the at least one electrode. Additionally or alternatively, operating the electrode controller may include one or more of operating a state machine, operating a field-programmable gate array, operating a microcontroller, or operating discrete components configured to determine (optionally time-varying) electric field.
A time-varying electric field may include an electric field that varies according to an alternating current (AC) voltage waveform applied to the electrode(s). The time-varying electric field may include a sinusoidal, square wave, sawtooth wave, triangular wave, truncated triangular wave, logarithmic, or exponential waveform.
Applying a time-varying voltage to the electrode to produce a time-varying electric field in step 508 may include applying a periodic voltage having an amplitude of 4000 to 115,000 volts. Applying a time-varying voltage to the electrode to produce a time-varying electric field may include applying a time-varying periodic voltage having a frequency of 50 to 800 Hertz. According to some embodiments, the time-varying voltage can have a periodic frequency of 200 Hertz to 300 Hertz.
Optionally, the method 501 may include operating one or more sensors operatively coupled to the electrode controller to measure one or more characteristics of the combustion reaction. For example, operating one or more sensors may include measuring a variable characteristic of a completeness of combustion. Operating the electrode controller in step 508 may include selecting the electric field to increase the homogenization when the completeness of combustion is lower than a target value.
The electrodynamic homogenization may increase uniformity in oxygen concentration above the grate. The differences may be caused by solid fuel pile depth variations across the grate, the differences include a more oxidizing atmosphere above regions of the grate carrying a small solid fuel pile depth and a more reducing atmosphere above regions of the grate carrying a large solid fuel pile depth.
The application of the electric field by the electrode in step 508 may further increase the release of heat near the fuel. This may be used to dry wet fuel, pre-heat difficult-to-burn fuel, or otherwise improve fuel flexibility.
Proceeding to step 510, overfire or secondary air may be applied over the burning fuel on the grate. For example, this may include operating an overfire air source. Operating the overfire air source may include delivering sufficient overfire air to substantially complete combustion of the solid fuel. The application of the electric field by the electrode may results in a reduction in the amount of overfire air required to meet emission requirements compared to a system not including the electrode. Additionally or alternatively, the electrodynamic homogenization provided by the application of the electric field by the electrode may result in a reduction in an amount of underfire air required to meet emission requirements compared to a system not including the electrode. Moreover, the application of the electric field by the electrode may results in a reduction in an amount of total air required to meet emission requirements compared to a system not including the electrode.
The application of the electric field by the electrode may result in a reduction in an emission of one or more of oxides of nitrogen (NOx) and carbon monoxide (CO) from the solid fuel burning compared to a system not including the electrode to apply the electric field.
The method 501 may optionally include controlling one or more of an overfire air flow, the underfire air flow, or a rate of fuel delivered by the stoker. Additionally or alternatively, the method 501 may include communicating, from an electrode controller, with one or more of an overfire air controller, an underfire air controller or a stoker controller.
The solid fuel may include a biomass fuel, coal, tire-derived fuel (TDF), or other solid fuel. As mentioned above, fuel flexibility may be improved by the electrodynamic homogenization.
In cases where the solid fuel is delivered to the grate with a stoker, current leakage from the electric field through the solid fuel may be reduced or eliminated by maintaining an air gap between the stoker and the solid fuel on the grate. Additionally or alternatively, current leakage from the electric field through the fuel may be reduced or eliminated by delivering electrically isolated fuel to a fuel cache, maintaining electrical insulation between the fuel cache and a support structure and between the stoker and the support structure. The stoker may deliver the solid fuel from the electrically isolated fuel cache.
Proceeding to step 512, heat from the combustion may be supplied. For example, the heat may be supplied to an electrical generation system, a chemical process, or to provide domestic heating.
The method for operating a solid fuel burner 301 may include operating one or more sensors operatively coupled to an electrode controller to measure one or more characteristics of the combustion reaction. Operating one or more sensors may include measuring a variable characteristic of a completeness of combustion. Operating the electrode controller may include selecting the time-varying electric field to increase the homogenization when the completeness of combustion is lower than a target value. At least one sensor (not shown) may be disposed to sense a condition proximate the burning fuel or a combustion gas above the burning fuel. The first sensor may be operatively coupled to the electrode controller via a sensor signal transmission path (not shown). The at least one sensor (not shown) may be configured to sense a combustion parameter of the burning fuel or the combustion gas above the burning fuel. For example the at least one sensor may include one or more of a flame luminance sensor, a photo-sensor, an infrared sensor, a fuel flow sensor, a temperature sensor, a flue gas temperature sensor, a radio frequency sensor, and/or an airflow sensor.
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. 61/640,695, entitled “SOLID FUEL BURNER WITH ELECTRODYNAMIC HOMOGENIZATION”, filed Apr. 30, 2012; and from U.S. Provisional Patent Application No. 61/616,223, entitled “MULTIPLE FUEL COMBUSTION SYSTEM AND METHOD”, filed Mar. 27, 2012; which, to the extent not inconsistent with the disclosure herein, are incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61640695 | Apr 2012 | US | |
| 61616223 | Mar 2012 | US |
