This disclosure relates generally to waste heat recovery systems, and more particularly to a waste heat recovery system for use at a landfill or other industrial site where hot gas generated in combustion processes is exhausted to the atmosphere.
The decomposition of organic matter in landfills produces significant amounts of gas, primarily methane and carbon dioxide, along with trace amounts of other organic gases and certain contaminants. When landfill gas migrates through soil or is released into the atmosphere it presents safety hazards related to the potential to form explosive mixtures of methane and air, and environmental hazards related to the release of methane and other pollutants. Landfill gas can also create nuisance odors within and beyond the landfill boundaries. For these reasons, federal and state regulations require that landfill owners provide positive means to control migration and release of landfill gas. Accordingly, gas collection wells are usually placed vertically in a landfill to collect the gases produced during the decomposition process, and these wells are connected together by a gas pipeline system that transports the collected gas including the entrained contaminants to a convenient location for beneficial use or disposal.
Disposal of the landfill gas is normally accomplished by burning the gas within an enclosed or open flare. Beneficial use of landfill gas can take a variety of forms with the most common being fuel for engines that generate electricity, fuel for landfill leachate evaporation systems, or direct sale of the gas for off-site applications such as fuel for industrial boilers or electrical generators. Government regulations dictate at what temperatures the gas must be burned and for how long the gas must be exposed to the prescribed temperatures based on air quality standards. The regulations are designed to assure that the gas and the contaminants therein are destroyed prior to being released to the atmosphere. Where regulations require the use of an enclosed flare, the landfill gas is typically burned at the bottom of the flare stack, which is designed to maintain the gas undergoing treatment in the combustion process at a relatively high temperature (e.g., usually around 1500° F., typically between 1400° F. to 1800° F. and in some cases between 1200° F. and 2200° F.). The volume of the flare stack is selected to provide enough residence time, such as between 0.3 and 1.5 seconds, to ensure adequate treatment of the components within the gas. The difference in temperature from the bottom of the flare stack to the top of the flare stack is normally quite small, meaning that the exhaust gas ejected out of the top of the flare stack is still very hot and thus contains significant heat energy. Likewise, due to inherently poor thermodynamic efficiency, both internal combustion engines and turbines fueled by landfill gas eject significant heat energy to the atmosphere in the form of exhaust gas at temperatures that are typically in the range of 750° F. to 1150° F. and almost always in the range of 600° F. to 1200° F. Because this energy is simply released to the atmosphere, it is referred to as waste energy or waste heat. Where exhaust gas is at a relatively high temperature such as 600° F. to 2200° F. and the quantity of the hot gas is such that the total energy content amounts to all or a significant portion of that required to operate a desirable downstream process, opportunities exist to beneficially use the waste heat. Regardless of whether a gas is simply flared or employed within a process for beneficial use, very few systems are designed to recover and beneficially utilize any of the waste heat exiting a flare stack or combustion engine at, for example, a landfill.
A waste heat recovery system is coupled to a flare stack or an exhaust stack of a primary process, for example, a landfill gas treatment system, to recover at least a portion of the energy within the exhaust produced by the gas treatment system and provides the recovered energy to a secondary process to thereby reduce the amount of energy needed to be otherwise input into the secondary process. In one embodiment, a waste heat recovery system includes a transfer pipe, an induction fan, a heat exchange unit and a secondary exhaust stack. Generally speaking, the transfer pipe is connected between an exhaust or flare stack of a primary process, such as a landfill gas treatment system, and a secondary process which may be a wastewater treatment unit, a chemical treatment unit or any other process that can utilize the waste heat. The induction fan is positioned within or connected to the transfer pipe and operates to create a draft within the transfer pipe to facilitate movement of some of the exhaust gas from the flare or exhaust stack of the primary process to the heat exchange unit or directly to a secondary process. When used, the heat exchange unit transfers energy in the diverted exhaust gas to the secondary process using for example a heat transfer fluid, and the secondary exhaust stack vents the exhaust gas passed through the heat exchange unit to the atmosphere.
Preferably, the transfer pipe is connected to the flare or exhaust stack of the primary process through a bustle which is designed to operate in conjunction with the induction fan and possibly a control damper to divert exhaust gases to the transfer pipe in a manner that does not significantly affect the back pressure or exhaust gas flow pattern within the flare or exhaust stack. This operation helps to assure that the transfer of exhaust gas from the primary stack to the heat transfer unit does not negatively affect operation of the primary process.
Additionally, a method for recovering waste heat from a primary process includes transferring an amount of exhaust gas from a primary process to a secondary process, utilizing at least some of the energy in the transferred exhaust gas within the secondary process and venting the exhaust gas to, for example, the atmosphere through a secondary exhaust stack. If desired, transferring exhaust gas from the primary process may include using an induction fan and a bustle to create a draft at the exhaust end of the stack of the primary process to facilitate the transfer of the exhaust gas from the stack of the primary process without significantly affecting the back pressure or gas flow within the exhaust stack of the primary process.
During operation, the disclosed system or method recovers energy from one or more primary processes and applies the recovered energy either directly or indirectly to one or more secondary processes without adversely affecting the operation of the primary process or processes. If desired, the disclosed system and method may use the recovered heat energy to treat a variety of wastewater streams, to recover products from wastewater, to chemically treat wastewater, to provide space heating for buildings, etc. The energy recovered from the primary process may be originally generated as a result of the combustion of low grade fuels, such as biogas generated in landfills, and the results of the combustion may be obtained by diverting stack gas from flares or exhaust stacks used in landfill or petroleum operations to a heat transfer system. If desired, however, the diverted stack gases may be used directly in a secondary process to facilitate physical changes and/or chemical reactions within the secondary process.
While the methods and devices described herein are susceptible to various modifications and alternative constructions, certain illustrative embodiments thereof are depicted in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed in the drawings. To the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined by the appended claims.
Referring to
As is known, waste energy (typically in the form of heat energy) is generated by a primary process 12, which may be a landfill gas treatment system, and is typically exhausted out of the flare or exhaust stack 14 to the atmosphere 16. However, the energy recovery system of
The waste energy recovery system 10 may include one or more sensors 26a and 26b located at different positions along the stack 14 that operate to detect one or more conditions of the gas within the exhaust stack 14, such as, for example exhaust gas pressure, combustion temperature, fuel consumption, flow rate, or any other condition of the gas within the exhaust stack 14. The sensors 26a and 26b may be connected to a differential sensor 27 that detects or measures the difference between the measurements made by the sensors 26a and 26b to determine for example the difference in the pressure of the gas in the stack 14 to thereby determine gas flow rate in the stack 14 between the locations measured by the sensors 26a and 26b. The sensor 27 may be connected to a controller 28 which, in turn, controls a variable motor-driven damper 30. The controller 28, which may be communicatively connected to other sensors besides the sensor 27, such as one or more sensors 29 in the primary process 12, implements any desired control routine to operate the damper 30 which, in turn, controls the rate (or volume) of exhaust gas transferred to the secondary system 20 through the transfer pipe 18. If desired, the controller 28 may also or instead be connected to and control the rate of the induced draft fan 22 either in conjunction with or apart from the damper 30 to thereby have further control over the rate at which exhaust gas is transferred from the exhaust stack 14 to the transfer pipe 18.
Generally speaking, the controller 28 operates to ensure that the quantity of the exhaust gas, and the manner by which the exhaust gas is transferred to the secondary process 20 does not adversely affect the performance of the primary process 12 and, in particular, may operate to keep the back pressure in the stack 14 at a desired or acceptable value based on, for example, operational parameters of the primary process 12, such as combustion gas flow, exhaust gas flow, engine speed, etc. The sensors 26a and 26b provide indications of properties of the exhaust gas while the sensors 29 provide performance parameters which indicate the performance of the primary process 10. Examples of primary process performance parameters include, but are not limited to, combustion rate, exhaust gas flow pattern, static pressure at the pickup point, temperature, etc.
The waste energy recovery system 100 depicted in
As depicted in
As depicted in
Additionally, a controller or a control panel 200 may be connected to various components of the system of
The controller 200 may also be communicatively connected to one or more sensors 210, 212 which measure temperature, pressure or other characteristics of the gas within the flare stack 114 and may apply any desired control scheme to control the operation of the modulating damper 158 and the speed of the induced draft fan 122 to control the flow of gas from the flare stack 114, through the transfer pipe 118 and into the heat exchanger 120. In the primary process, where a particular exhaust pattern and/or pressure or pressure differential in the flare stack 114 may be required or at least desired, the controller 200 operates the induction fan 122 and the valve 158 to provide an adequate draft in the transfer pipe 118 to maintain the static pressure at all locations within the bustle 150 at substantially the same values that occur when the heat exchanger 120 is not operating. In this manner, the overall effect on the exhaust flow pattern and flare stack pressure caused by the operation of the heat transfer system will be minimized or eliminated. Accordingly, the primary process will operate normally and is not significantly affected by the bustle 150 and/or the induction fan 122. In this manner, the controller 200 operates to control the amount of heat energy transferred from the exhaust of the primary process to the secondary process by controlling the flow of and exit temperature of the transfer fluid in the heat exchange unit 115, the flow of exhaust gas from the exhaust stack 114 of the primary process to the heat exchanger 120 or both.
While the waste energy recovery system has been described in general terms with respect to a primary process, a heat exchange unit and secondary processes, the waste energy recovery system may specifically be employed in primary processes including, but not limited to, incinerators or flares which emit hot stack gases, engines such as internal combustion engines, turbines and reciprocating engines used in, for example, waste gas disposal systems at landfills and/or petroleum production facilities, etc. Preferably, the fuel used in the primary process is renewable or easily recovered, such as landfill gas. However, the waste heat recovery system may also be used with primary processes which use conventional fuels such as coal, wood, oil and natural gas.
Furthermore, the heat or waste energy recovered from the primary process may be used directly, or indirectly employing a heat exchange unit and recirculated heat transfer fluid in secondary processes which may be, for example, chemical processes or wastewater treatment units, or any combination of these and other desired types of processes. A manner of using the recovered energy indirectly in a secondary process is illustrated in
Alternately, the exhaust gas from the primary process may be used directly in a secondary process. Examples of this use of the waste heat include submerged gas evaporators, spray dryers, sludge dryers or processes that utilize components of the stack gas directly to promote or take part in a chemical reaction. Further, the exhaust gas from the primary process may be provided to venturi devices used for contacting gas and liquid or reactors used to treat wastewater. In some cases, the residual for final disposal or combinations of residual and salable products may be recovered by such systems. Wastewater treatment in submerged hot gas evaporators and venturi devices used for contacting gases and liquid may rely on evaporation and/or any combination of evaporation and chemical reactions between constituents in the primary process exhaust gas and constituents of the wastewater. Alternately, one or more additional reactants may be added to the exhaust gas or directly into the submerged gas or venturi reactor to achieve desired characteristics in a final product and/or residual.
One skilled in the art will recognize that the waste energy recovery system of
The system described with respect to
Referring now to
When desired or needed, a feed pump 322 delivers the wastewater stored in the tanks 318 to a distillation column 324 located on Skid 3, which is used to purify the solvent. In particular, the pump 322 delivers the wastewater to an inlet 325 of the distillation column 324. The location of the inlet 325 in reference to the height of the distillation column 324 is dependent on the design of the distillation column 324, the characteristics of the wastewater and the desired quality of the recovered solvent. The wastewater is at varying temperatures depending on the location along the length of the distillation column 324 the temperature being highest at the bottom and lowest at the top. Mixtures of vapor and liquid in equilibrium at varying temperatures and pressures within the distillation column 324 are increasingly enriched in the solvent at locations above the inlet 325 and are increasingly depleted in the solvent at locations below the inlet 325. Thus, the purity of the solvent is continuously increased in a known manner as the flow approaches the top of the distillation column 324. Concurrently, substances that are less volatile than the solvent, which may or may not include recoverable substances, settle to the bottom of distillation column 324 during the refining process. A pump 326 located at the bottom of the distillation column 324 transfers the less volatile fraction to a bottoms receiver tank 328 where the material may be stored and/or processed in any desired manner. As part of this processing, a pump 330 may be used to transfer all or a portion of the material in the receiver tank 328 to, for example, an evaporator 331 which may further evaporate or condense the material. The output of the evaporator 331, which is illustrated in
Heat is provided to the distillation column 324 via a heat exchanger 332, which supplies heat to the bottom portion of the distillation column 324 to thereby cause the separation of solvent and sludge within the distillation column 324. An air-cooled condenser 336 located at the top of the distillation column 324 cools and condenses the pure or nearly pure solvent and provides this condensed solvent to a receiver tank 338 located on Skid 3. A pump 340 pumps the recovered condensed solvent from the tank 338 to one or more product storage tanks 350 within the tank farm 316, where the purified solvent may be transferred through pump 352 to trucks, railroad cars, pipelines, etc. and delivered to a final destination.
As depicted in
The heat exchange system 300, located on Skid 1, includes a transfer pipe 370 connected to a bustle 371 disposed on or at the top of the flare stack 305 or other exhaust stack associated with a primary process. The flare stack 305 may be, for example, a flare stack of a traditional landfill gas treatment system, may be an exhaust stack of an engine that operates using low grade or contaminated fuels, such as landfill gas, or may be an exhaust stack associated with any other source of heat energy. The bustle 371 captures some of the exhaust gas within the flare stack 305 and delivers this captured exhaust gas via the transfer pipe 370 to a heat exchanger 372. The capture of the exhaust gas is aided by an induction fan 374, or other type of fan, which exhausts gas out of a secondary exhaust stack 380. Because the gas captured by the bustle 371 and ported through the heat exchanger 372 has been fully and completely processed in the flare stack 305 according to applicable regulations, the exhaust from the heat exchanger unit 372 may be released directly to the atmosphere, or may be used in other processes.
Generally speaking, the induction fan 374 operates to draw waste heat gas from the flare stack 305, which in a landfill treatment situation is typically at 1400° F. to 1600° F., and delivers this gas to the heat exchanger 372 at approximately the same temperature. The heat exchanger 372 operates to transfer a portion of the heat energy of the exhaust gas diverted from the flare stack 305 to a process fluid or to a heat transfer fluid within a fluid transfer pipe 382. A combination storage-expansion tank 386 with appropriate control systems is connected to the pipe 382 to assure an adequate supply and not an overfill condition of process fluid or transfer fluid in the pipe 382 and any systems connected to the pipe 382, which in this depiction includes the three heat exchangers 332, 364 and 372. In one case, the operation of the heat exchanger 372 and the transfer fluid may reduce the temperature of the gas in the secondary exhaust stack 380 to approximately 700-730° F., thereby recapturing a great deal of the heat energy within the exhaust gas drawn through the heat transfer unit 372.
A pump 384 pumps the heat transfer fluid in the pipe 382 through various valves to both the heat exchanger 364 and the heat exchanger 332, where the energy in the transfer fluid in the form of heat is used or transferred to other stages of the solvent distillation process or the integrated building heating system as described previously. In particular, after exiting the heat exchanger 372, the transfer fluid within the pipe 382 may be provided at approximately 600° F. to the heat exchanger 364. Some of the heat energy within the heat transfer fluid is transferred to the air provided by the fan 362. In one embodiment, the heat transfer unit 364 may heat the air to approximately 90° F., and this heated air is used for space heating within a building or buildings located close to Skid 2.
Still further, the transfer fluid output from the heat transfer unit 364 which may be at, for example, approximately 500° F. to 575° F. is provided to an input of the heat exchanger 332 where some of the remaining energy in this fluid is transferred to the distillation column 324 and used in the distillation process to recover solvent. The transfer fluid output from the heat exchanger 332, which may be at, for example, approximately 150° F. to 300° F. is then recirculated by the pump 384 back through the heat transfer unit 372 to be reheated to approximately 600° F. While the heat transfer fluid of
As will be noted, in the heat exchange unit 300 of
Of course, the use of waste energy from the flare stack 305 is not limited to a single stage heat transfer process, but can include the use of multiple stages of heat transfer systems connected in series to the output of the flare stack 305 to provide or obtain energy from the flare stack for multiple different processes, or for multiple different uses within the same process, etc.
Still further, as indicated in the process of
Thus, the systems described herein recover waste heat or waste energy from the burning of low-grade or low-cost fuels, such as landfill gases which, heretofore have been simply released to the atmosphere, and do so by placing a heat exchange unit between a flare or exhaust stack of a primary process and one or more components of a secondary process which, preferably, is located close to the flare or exhaust stack. These systems reduce or eliminate entirely the amount of energy that must be independently provided to the secondary process via more costly energy sources. Still further, as noted above, it is preferable to place the secondary process using the recovered waste energy in close proximity to the flare or exhaust stack, such as that of a landfill, to efficiently use the recovered energy. However, placing the secondary process close to the primary process is also desirable because it locates chemical and other wastewater processing systems close to or even on the same land as the primary process, which consolidates these different processes in the same geographical area. In the case of landfills, this consolidation enables commercial processing operations to be collocated on real estate, such as on landfill property, which typically has very little other uses, and thus consolidates the commercial activities associated with processing what are typically considered to be noxious or undesirable fluids (liquids and gases) while simultaneously saving energy in the processing of those fluids. Still further, the use of skids for locating the secondary process close to the primary process enables the secondary processes to be easily moved, changed, etc. during the life of the primary process. In fact, in some cases, it may be desirable to temporarily and/or removably locate one or more secondary processes in close proximity to the primary process to enable easy assembly and installation, and easy disassembly and removal of the secondary processes.
Generally speaking, the height of the slot 408, i.e., the distance between upper and lower edges of the slot 408 on the outer wall of the stack 402, may vary linearly, circularly, arcuately, exponentially or in any other desired manner around the circumference of the stack 402 to achieve the desired effect of transferring exhaust gas from the stack 402 to the transfer pipe 404 uniformly around the circumference of the stack 402. Of course, the bustle 400 and the stack 402 may be designed to provide even or nearly even suction around the outer edge of the stack 402 in other manners. For example, as illustrated in
While two stack and bustle designs are depicted in
Also, while the diameter of the stack 402 is illustrated as being constant along the length of the stack 402 at which the bustle 400 is attached to the stack 402, the diameter of the stack 402 could vary, such as by tapering inwardly, along the length (height) of the stack 402 either before, at or after the location at which the bustle 400 attaches to the stack 402. This tapering feature may be used in conjunction with the slot and wall spacing features described above to force more of the exhaust gas traveling within the stack 402 into the bustle 400 and, thereby, into the transfer pipe 404. Additionally, while the stack 402 and the transfer pipe 404 are illustrated in
While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, it will be apparent to those of ordinary skill in the art that changes, additions or deletions may be made to the disclosed embodiments without departing from the spirit and scope of the invention.
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