Patent Application
20080222913
1. Field of the Invention
This invention relates to a method and equipment for reducing contaminants such as volatile organic compounds (VOC's) and carbon monoxide (CO) normally present in dryer offgas that is discharged into the atmosphere from a moist organic product drying process. The equipment includes a product dryer, recuperative thermal oxidizing apparatus, a furnace having a burner, which serves to deliver hot products of combustion to the thermal oxidizing apparatus, and a gas-to-gas heat exchanger of the indirect type having a hot gas side and a cool gas side, hereinafter referred to as the primary heat exchanger, for bringing the hot gaseous output from the thermal oxidizing apparatus that is ultimately discharged into the atmosphere into indirect heat exchange relationship with recycle dryer offgas to increase the temperature of the recycle dryer offgas prior to its reentry into the dryer.
Efficient thermal oxidation of VOC's and CO requires correlation of four factors occurring simultaneously:
1) Adequate temperature;
2) Adequate oxygen concentration;
3) Adequate residence time; and
4) Adequate turbulence.
In the present process, a non-preheated portion of the dryer offgas is directed to the burner, while another preheated portion of the dryer offgas that is removed from the stream thereof after passage through the primary heat exchanger is directed to the thermal oxidizing apparatus. The flow rates of the non-preheated portion of the dryer offgas and the preheated portion of the dryer offgas, and the input of fuel to the furnace are controlled and adjusted to provide a hot gaseous output from the thermal oxidizing apparatus that is at a temperature of at least about 1600° F. with an optimum 5% oxygen content by volume, which are sufficiently high to substantially oxidize VOC's and CO in dryer offgas that is discharged into the atmosphere. Introduction of the non-preheated portion of the dryer offgas into the burner also lowers the flame temperature thereby reducing the nitrogen plus oxygen based compounds (NOx's) of the hot products of combustion emanating from the furnace.
2. Description of the Prior Art
Dryers have been used for many years to lower the moisture content of a variety of organic products, such as grain, including distiller's grain and the like, which nominally may have a water content as high as 60-75%. The recent emergence of ethanol plants producing substantial quantities of moist distiller's grain as output residue requiring drying for further commercial use, has rekindled interest in more efficient drying processes while, at the same time, necessitating that dryer offgas discharged into the atmosphere contain reduced amounts of VOC's, CO and NOx's.
Commercial drying equipment has been previously designed and constructed to dry organic products to a predetermined acceptable level, which is normally about 10% moisture by weight, wet basis. It has been known for some time to incorporate thermal oxidizing apparatus in processes and equipment for drying moist organic products in order to lower the VOC and CO content of the product output from the dryer. For systems that utilize recuperative thermal oxidation processes (as opposed to end-of-pipe regenerative thermal oxidation processes) that are similar to one another, these processes have primarily involved the rudimentary steps of bypassing a non-preheated first portion of the dryer offgas to a mixing chamber interposed between a furnace and thermal oxidizing apparatus. A second portion of the offgas has been heat exchanged against the gaseous output from the thermal oxidizer, before being recycled back to the dryer.
In order to reduce the VOC and CO content of dryer offgas introduced into the atmosphere employing a thermal oxidizer, the hot gaseous Output from the oxidizer should be at least about 1600° F. and the oxygen concentration should be at least about 5% by volume. Heretofore, the temperature of the output from the thermal oxidizer has been limited to temperatures in the order of 1400° F. when the oxygen concentration is increased to 5% by volume; hence, VOC and CO reduction has not been optimum.
Even though residence time of the offgas being oxidized was not restricted and gas turbulence not a significant factor, it was not heretofore feasible to adequately control both temperature of the thermal oxidizer, and its oxygen concentration, in order to significantly lower the VOC and CO content of the offgas introduced into the atmosphere. The temperature and the oxygen concentration could be controlled individually, but not simultaneously for most efficient operation of the thermal oxidizing apparatus.
In the prior art one-stage dryer offgas recuperative thermal oxidation processes of
The cool offgas portion was directed to a mixing chamber forming a part of conventional recuperative thermal oxidizing equipment that normally included a burner connected to a furnace that, in turn, was connected to a mixing chamber joined to a thermal oxidizer. Alternatively, a portion of the cool offgas directed to the mixing chamber could be redirected to the burner in the function of flue gas recycle for NOx reduction. Sources of natural gas fuel and combustion air were supplied to the burner. The hot gaseous output of the thermal oxidizer, after being directed through a tempering chamber, which reduced the temperature of the gaseous output, was introduced into the hot gas side of the primary heat exchanger. A portion of the hot gaseous output exiting from the hot side of the primary heat exchanger was returned to the tempering chamber, while the remainder of the hot gaseous output exiting from the hot side of the primary heat exchanger was discharged into the atmosphere.
In the system shown in
The present invention provides a method of significantly reducing the VOC and CO content of dryer offgas that is discharged into the atmosphere from a moist organic product drying process using a recuperative thermal oxidizing apparatus having an input of hot products of combustion from a furnace, along with controlled proportions of non-preheated and preheated dryer offgas, thereby producing a hot gaseous output ultimately destined for atmospheric discharge. Flue gas recirculation (FGR) is used to reduce the NOx content of the burner gases directed into the thermal oxidizer by directing a sufficient quantity of dryer offgas, without preheating thereof into the burner to lower the burner flame temperature thereby limiting NOx production in the burner.
Even where residence time and degree of turbulence are the same in the prior art single-stage recuperative thermal oxidation processes as compared with the two-stage recuperative thermal oxidation of the present invention, simultaneous attainment of adequate temperature and oxygen concentration is not possible in the prior art processes, which is necessary for efficient thermal oxidation. The present two-stage design solves this dilemma.
In the preferred method of this invention, the dryer offgas is separated into first and second portions with the first portion thereof being directed to the burner of the furnace connected to the thermal oxidizer. The second portion of the dryer offgas is preheated by bringing that portion into indirect heat exchange with the hot gaseous output from the thermal oxidizing apparatus. The preheated dryer offgas is separated into two portions with the larger portion recycled back to the dryer and the smaller portion directed to the mixing chamber of the thermal oxidizer.
The non-preheated portion of dryer offgas is directed to the burner along with fuel and combustion air, whereby the hot products of combustion from the furnace have a limited NOx content. An amount of the preheated dryer offgas is removed from the primary heat exchanger output and mixed with the hot products of combustion emanating from the furnace. The quantities of fuel and combustion air supplied to the burner, the relative quantities of non-preheated offgas used for NOx control and preheated offgas directed to the mixing chamber, and the temperature of the preheated offgas directed to the mixing chamber are all closely controlled and correlated to assure that the temperature of the hot gaseous output from the thermal oxidizing apparatus is at least about 1600° F. with an oxygen concentration of at least about 5.0%. In particular, it is preferred that the weighted average temperature of the non-preheated offgas used for NOx control and preheated offgas directed to the mixing chamber be of the order of 600-650° F. Alternatively, a portion of the controlled quantity of combustion air may be injected into the furnace or mixing chamber.
It is also preferred that the hot gaseous output from the thermal oxidizing apparatus be directed through a tempering chamber to somewhat reduce the temperature of the hot gaseous output before introduction into the hot side of the primary heat exchanger that preheats the second portion of the dryer offgas.
Introduction of a controlled proportion of non-preheated dryer offgas into the burner maintains the temperature of the furnace gases at a low enough level that thermal NOx production by the burner is limited. In order to maximize the weighted average temperature of the dryer offgas entering the thermal oxidation process, thus resulting in the highest possible outlet temperature from the thermal oxidizer, the quantity of the non-preheated portion of the dryer offgas directed to the burner, as compared to the preheated portion of the dryer offgas directed to the mixing chamber, is preferably the minimum needed to meet NOx reduction requirements by the method of flue gas recirculation.
In the preferred process, the temperature of the dryer offgas emerging from the dryer is nominally in the range of 200-250° F. The temperature of non-preheated dryer offgas directed to the burner for NOx control is essentially the same. The temperature of the portion of dryer offgas recycled to the dryer is increased in the cold side of the primary heat exchanger by an amount of about 450-500° F., and preferably to a level of about 650-750° F. Similarly, the temperature of the preheated offgas returned to the thermal oxidizing apparatus is of the order of 650-750° F. The temperature of the hot gaseous output from the thermal oxidizing apparatus is reduced in the tempering chamber to a temperature that is slightly lower than the maximum allowable temperature of the primary heat exchanger. The purpose of this temperature reduction is to protect the primary heat exchanger from damage while allowing the primary heat exchanger to be made from commercially available, reasonably affordable materials.
Apparatus for reducing the VOC and CO content of dryer offgas that is discharged into the atmosphere from a moist organic product drying process is shown schematically in
Heated gas enters the dryer product and gas inlet 20 through duct 42 joined to the outlet of the cool gas side of an elongated, transversely rectangular, indirect, gas-to-gas heat exchanger herein referred to as the primary heat exchanger 44. The connection of duct 42 to the primary heat exchanger is via the primary heat exchanger cool gas side outlet plenum 106. Dryer offgas exits rotary dryer 12 through dryer product and gas outlet 22 and passes through the dropout chamber 26 to the centrifugal separator 24. The offgas goes overhead from the centrifugal separator 24 via ducts 46 and 48 to suitable cyclone separators. Duct 46 leads to a pair of cyclones 50 and 52, while duct 48 leads to a pair of cyclones 54 and 56. Relatively particle-free offgas exits from cyclones 50-56 through respective ducts 58 and 60. Duct 58 leads to the inlet of recycle fan 59 and duct 60 leads to the inlet of recycle fan 61. The outlets of recycle fans 59 and 61 are transitioned together to a common offgas return duct 62 that leads to the primary heat exchanger cool gas side inlet plenum 63, which communicates with the cool gas side of primary heat exchanger 44 and duct 108 leading to the FGR fan 110.
Except for a relatively small flow rate of dryer offgas that is conveyed through duct 108 to the FOR fan 110 then to the burner 80 via the FGR fan outlet damper 112 and duct 109, the balance of dryer offgas flows from the primary heat exchanger cool gas side inlet plenum 63 into the cool gas side of the primary heat exchanger where this portion of the offgas is substantially heated before exiting the primary heat exchanger into the primary heat exchanger cool gas side outlet plenum 106. Heated dryer offgas emanating from the cool side of the primary heat exchanger and entering the primary heat exchanger cool gas side outlet plenum 106 is divided into two streams. The larger of the two streams emanating from plenum 106 enters the dryer product and gas inlet 20 through duct 42 and repeats the path previously described. The smaller of the two streams emanating from plenum 106 enters the inlet of the vapor injection fan 105 via duct 104, and is then conveyed to the mixing chamber of the combination furnace and mixing chamber 82 via duct 107.
Recycle fans 59 and 61, interposed between ducts 58, 60 and 62, provide the motive forces for circulating dryer heat transfer media and dryer offgas through the cyclones 50-56, the ducts 46, 48, 58, 60, 62 and 42, the cool gas side of primary heat exchanger 44, dryer 12, dropout chamber 26 and centrifugal separator 24. Particulate materials collected by cyclones 50-56 are introduced into the cooling drum product inlet 32 via conveyor 30. Product separated from the gas streams by the dropout chamber 26 and centrifugal separator 24 is conveyed via conveyors 28 and 30 into the cooling drum product inlet 32.
Vapor injection fan 105 provides the motive force for conveying heated dryer offgas from the primary heat exchanger cool gas side outlet plenum 106 to the combination furnace and mixing chamber 82 via ducts 104 and 107.
FGR fan 110 provides the motive force for conveying non-preheated dryer offgas from the primary heat exchanger cool gas side inlet plenum 63 to the burner 80 via ducts 108 and 109 and through FGR fan outlet damper 112.
Air from the atmosphere enters the cooling drum air inlet and product outlet 36, flows through the length of the cooling drum 34, exits from the cooling drum air outlet and product inlet 32 via duct 64 passing over the top of cooling drum 34, and into the dirty gas inlet 65 of bag house 66. Small quantities of particulate matter and moisture become entrained in the air as it passes through the cooling drum 34. Heat is transferred from the hot product to the air as it passes through the cooling drum 34. This heat transfer results in the product being cooled and the air being somewhat warmed. The warmed air is cleaned of particulate matter in bag house 66. The clean, warm air exits bag house 66 and is conveyed by duct 68 to the cooling air fan inlet damper 71, then to the cooling air fan 72, which provides the motive force for moving air through the cooling drum 34, bag house 66, ducts 64 and 68, and cooling air fan inlet damper 71. The clean, warm air exits the cooling air fan 72 and is conveyed via duct 70 to the cool gas side air inlet of an indirect gas-to-gas heat exchanger herein referred to as the combustion air heater 74. The clean, warm air is indirectly heated as it passes through the cool gas side of the combustion air heater 74, which receives heat from hot gases passing through the hot gas side of the combustion air heater, which process is subsequently described. The clean, heated air exits the cool gas side air outlet of the combustion air heater 74 and is conveyed via duct 76 to the combustion air fan 78, then through the combustion air fan outlet damper 79, then to the burner 80 via duct 77. The combustion air fan 78 provides the motive force for moving the air though the combustion air heater 74, the combustion air fan outlet damper 79, the burner 80, and ducts 70, 76 and 77. Fuel, which is typically natural gas, enters the burner 80 via the fuel inlet pipe 84 and the fuel flow rate control valve 85. The fuel and clean, heated air exit the burner 80, and are combusted in the furnace chamber of the combination furnace and mixing chamber 82. Alternatively, fuels other than natural gas can be used including propane, light and heavy fuel oils, and solid fuels.
The hot products of combustion from the furnace chamber and heated dryer offgas from duct 107 enter the mixing chamber of the combination furnace and mixing chamber 82 and are well-mixed therein prior to moving to and through the thermal oxidizer 86. If desired, a second thermal oxidizer 88 may be provided in series flow relationship with the thermal oxidizer 86 to provide additional residence time of the thermal oxidizer process. The hot gaseous output from the thermal oxidizer 88 is conveyed into the tempering chamber 90. A portion of the gas that passes through the hot gas side of the primary heat exchanger 44, is conveyed via ducts 100 and 101 into the tempering chamber 90.
The gaseous products from the thermal oxidizer 88 and duct 11 that separately enter the tempering chamber 90 are well-mixed therein prior to moving into and through the primary heat exchanger hot gas side inlet transition 43 and then into and through the hot gas side of the primary heat exchanger 44 where heat is indirectly transferred from this gaseous mixture to the dryer offgas moving in counterflow direction on the cool gas side of the primary heat exchanger. During this process the gaseous mixture on the hot gas side of the primary heat exchanger 44 is substantially cooled before exiting the hot gas side of the primary heat exchanger into the primary heat exchanger hot gas side outlet plenum 45. The cooled gaseous mixture entering the primary heat exchanger hot gas side outlet plenum 45 is divided into two streams. One stream of the cooled gaseous mixture enters the hot gas side gas inlet of the combustion air heater 74 and is further cooled as heat is indirectly transferred from this gaseous mixture to the combustion air passing through the cool gas side of the combustion air heater. This stream of cooled gaseous mixture exits the hot gas side of the combustion air heater 74, passes through the combustion air heater hot gas side outlet transition 92, duct 94, exhaust fan 96, and exits to atmosphere through the stack 98. The exhaust fan 96 provides the motive force for moving the gaseous products and mixtures through the combination furnace and blending chamber 82, thermal oxidizers 86 and 88, tempering chamber 90, the hot gas side of the primary heat exchanger 44, the hot gas side of the combustion air heater 74, the stack 98, and the associated ducts, plenums and transitions, 43, 45, 92 and 94.
The other stream of the cooled gaseous mixture that emanates from the primary heat exchanger hot gas side outlet plenum 45 enters the tempering fan 102 via duct 100, then passes through the tempering fan outlet damper 103 and into the tempering chamber 90 via duct 101. The flow rate of the cooled gaseous mixture entering the tempering chamber 90 via duct 101 is controlled by the tempering fan outlet damper 103 to regulate the temperature of the gaseous mixture emanating from the tempering chamber to a target temperature that is somewhat lower than the maximum allowable temperature of gases entering the primary heat exchanger 44.
The flow rate of dryer offgas that is conveyed through duct 10 to the burner 80 is controlled by the FGR fan outlet damper 112 as required to regulate and minimize the production of thermal NOx from the burner utilizing the principle of flue gas recirculation. Alternatively, other known technologies can be used to regulate and minimize the production of thermal NOx from the burner.
A supplementary combustion air duct 114, connected between burner 80 and a shroud surrounding furnace and mixing chamber 82 provides a supplemental quantity of combustion air when the quantity provided through duct 76 is insufficient. A portion of the heat emanating from the hot furnace shell is transferred to the supplemental combustion air supply in direct contact with the furnace shell and this quantity of heat is therefore conserved in the process rather than being lost to the environment.
In operation, a moist organic product to be dried, such as grain, including distiller's grain resulting from ethanol production, animal or fish byproducts, municipal sludge, forage materials, or wood byproducts, is directed from source 16 to the conveyor 18 for delivery into the inlet 20 of rotary dryer 12. Dryer offgas that has been substantially heated in the cool gas side of the primary heat exchanger 44 is commingled with the moist product in the rotary dryer 12 in a direct-contact heat exchange process, during which process the moist product receives heat and rejects moisture in the form of steam and the heated dryer offgas rejects heat and is cooled, and integrates the moisture rejected by the product into its composition. The dried product and offgas emitted from the outlet 22 of dryer 12 is delivered into the dropout chamber 26, which separates a large fraction of the dried product from the gaseous content of the dryer output. The offgas emitted from the dropout chamber 26 along with the remaining fraction of entrained dried product moves into the centrifugal separator 24, which separates another fraction of the dried product from the gaseous content. The dried products captured in the dropout chamber 26 and the centrifugal separator 24 enter the dropout chamber conveyor 28 and are conveyed to the cooling drum product inlet 32 by conveyor 30. The dried product output from cooling drum 34 is directed to the bucket elevator 40 by conveyor 38 for removal from apparatus 10.
In an exemplification of a preferred process, as depicted in the flow diagram of
Atmospheric-derived combustion air, nominally at a temperature of about 100° F. after passing through cooling drum 34, and that is introduced into burner 80 through duct 77 and damper 79, preferably is at a temperature of about 250° F. as a result of having been brought into heat exchange relationship in combustion air heater 74 with the hot gaseous output from the primary heat exchanger 44. Use of non-preheated offgas supplied to burner 80 limits the flame temperature of the burner, thereby deterring formation of undesirable NOx compounds.
The heated offgas exiting from the primary heat exchanger 44 through plenum 106 and returned to dryer 12 via duct 42, in the exemplary process of
In the single-stage prior art process of
In the single-stage prior art process of
In the single-stage prior art process of
In the two-stage process of this invention, as shown schematically in
The two-stage process of this invention allows selective variation of the relative proportions of non-preheated dryer offgas and preheated dryer offgas introduced into the burner/thermal oxidation apparatus as required to optimize the control of VOC's, CO, and NOx's.
The single-stage thermal oxidation processes of
In the table below, the common features of the three prior art single-stage thermal oxidation processes of
1. The dryer processes are identical. All the mass and energy inputs and outputs to and from the dryers are identical.
2. The dryer offgas enters the dryer's furnace/mixing chamber for thermal oxidation of pollutants. This is the furnace that provides the heat that drives the thermal oxidation process and dryer's evaporation process.
3. The offgas mass flow rates of the dryers are identical.
4. The dryers are indirectly fired with a primary heat exchanger between the thermal oxidizer and the dryer. Hot gases from the thermal oxidizer transfer thermal energy to the primary heat exchanger, which transfers this thermal energy to the dryer heat transfer media.
5. The dryer heat transfer media consists primarily of steam, which is the same moisture that has been evaporated from the product being dried.
6. The combustion air source temperatures are identical.
7. The atmospheric exhaust temperatures are identical.
8. The radiation and convection losses from the dryer, thermal oxidizer and primary heat exchanger are identical.
9. The mass flow rates of air leaks into the dryer system are identical.
10. For each of the four systems, the mass flow rates and energy flow rates are thermodynamically balanced within each individual process, which includes the processes of mixing, combustion, separating, heat transfer, and drying.
11. For each of the four systems, the mass flow rates for each individual constituent (N2, O2, CO, H2O) are mathematically balanced within each individual process.
The present non-provisional application claims the benefit of U.S. Provisional Patent Application No. 60/906,651, entitled TWO-STAGE THERMAL OXIDATION OF DRYER OFFGAS, filed Mar. 13, 2007, which is specifically incorporated herein by reference thereto.
| Number | Date | Country | |
|---|---|---|---|
| 60906651 | Mar 2007 | US |
