Abstract:
An improved thermal oxidizer ( 10 ) comprising a combustion chamber ( 62 ), a refractory coated mixing device ( 14 ) within a plenum ( 12 ), a burner ( 26 ) mounted outside the oxidizer ( 10 ) for ready access, and temperature sensing and control equipment ( 86 ). The oxidizer ( 10 ) uses the mixing device ( 14 ) to induce a static pressure drop between the burner ( 26 ) and the oxidizer inlet ( 16 ), and a flow passage conveys preheated gas from the plenum ( 12 ) to the burner ( 26 ). A bend in the combustion chamber ( 92 ) provides for recirculation of combustion gases for more efficient burning. The burner ( 26 ) can be a commercially available unit that can accommodate inlet temperatures of up to 1200° F., allowing efficient operation of the thermal oxidizer ( 10 ).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to the removal of pollution from gas streams, and more particularly, to a direct flame incinerator for thermally oxidizing pollutants within an oxygen-bearing, process gas stream, before the process gas stream is vented to the atmosphere.  
           [0002]    The removal of pollutants and undesirable substances from the gas streams of various manufacturing processes is well known in the prior art. Such undesirable substances include impurities and undesirable byproducts. Release or emission of these substances often must be controlled to conform to requirements of the Clean Air Act.  
           [0003]    One conventional approach for removing or controlling these substances is by thermal oxidization. Thermal oxidation occurs when the process gas, containing sufficient oxygen, is heated to sufficient temperatures for a sufficient length of time. Thermal oxidization converts these substances to the harmless gases carbon dioxide and water vapor.  
           [0004]    It is well known that increasing the level of turbulence in the combustion chamber enhances the thermal oxidation process. Turbulence can be increased by using sharp bends in the combustion chamber, or by using aerodynamic jets induced by mixing devices.  
           [0005]    Typical thermal oxidization temperatures may range to 1800° F., and significant amounts of heat are often required. Most of the heat from the thermal oxidization process is usually recovered and used to preheat the process gas entering the thermal oxidizer. This heat is used to preheat the process gas to temperatures typically in the range of 1000 to 1200° F. Some of the heat for the thermal oxidation process comes from oxidization of the substances controlled. The remainder of the heat required for the thermal oxidization process comes from a burner fueled by either gaseous or liquid fuels.  
           [0006]    In one type of thermal oxidizer, a fan supplies ambient air to the burner for combustion. Commercially available burners are often used with a fan supplying a fixed flow of ambient temperature air for combustion in the burner, and the thermal oxidizer temperature is controlled by varying the fuel flow. This design results in changes in burner stoichiometry as the fuel flow changes. Fortunately, these burners have a stability range wide enough to accommodate changes in burner stoichiometry and changes in thermal oxidizer process variables. Prior art places these burners outside the thermal oxidizer, allowing ready access for maintenance and service. However, these thermal oxidizers are inefficient, requiring one to supply fuel to heat the ambient temperature burner air to the thermal oxidizer operating temperature.  
           [0007]    More efficient thermal oxidizers use oxygen in the preheated process gas stream to supply oxygen for burner combustion. Such oxidizers can save approximately 35% in fuel costs compared to oxidizers using ambient air for burner combustion. These prior art thermal oxidizers, however, require that a specialized burner be located inside the preheated process gas duct. These thermal oxidizers expose the burner, and other equipment such as dampers, to the preheated process gas stream where the high temperatures often cause mechanical failure. Commercially available and inexpensive, refractory-lined, carbon steel burners cannot be used in the preheated gas stream. Design, development and manufacture of specialized burners for these thermal oxidizers is expensive and time consuming. Also, locating the burner inside the process air duct hinders maintenance and servicing of the burners and other equipment.  
           [0008]    An example of the prior art using process gas oxygen for burner combustion is U.S. Pat. No. 4,444,735 issued to Birmingham, et al. on Apr. 24, 1984. However, the thermal oxidizer in Birmingham, et al. has a complex control system with dampers operating in the preheated process gas stream. Metal dampers are prone to failure due to excessive inlet temperatures that may reach 1200° F. The Birmingham, et al. thermal oxidizer also uses a perforated metal mixing device as part of the burner. The mixing device improves combustion by enhancing turbulence in the combustion chamber, and mixing of the preheated process gas with the burner flame. Such mixing devices are commonly used in thermal oxidizers. However, the mixing device used by Birmingham, et al. is prone to overheating and mechanical failure due to high inlet temperatures and exposure to thermal radiation from the hot combustion gases. For this reason, the mixing device material temperature is used as a burner control system variable to limit the temperatures in that oxidizer. If the mixing device were protected with a suitable refractory coating, this limitation could be avoided.  
           [0009]    A similar thermal oxidizer to that in Birmingham, et al. is disclosed in U.S. Pat. No. 4,444,724 issued to Goetschius on Apr. 24, 1984. The Goetschius oxidizer also has a metal mixing device incorporated with the burner, and the mixing device is cooled with process air. This complicated burner is costly to produce. The burner is also difficult to replace or service, because it is located inside the preheated process air duct.  
           [0010]    The thermal oxidizer disclosed in U.S. Pat. No. 5,762,880 issued to Rühl, et al. on Jun. 9, 1998, also uses process gas oxygen for burner combustion. This thermal oxidizer uses instrumentation to measure combustion gas and fuel flow, valves or dampers to control gas flows and fuel flow, and an electronic control system to precisely control burner stoichiometry. The burner also may move relative to the combustion chamber to control airflow to the burner. Such a complicated measurement and control system is expensive and unreliable. For many thermal oxidizers, precise burner stoichiometry control is unnecessary.  
         SUMMARY OF THE INVENTION  
         [0011]    The primary objects and advantages of the present invention are:  
           [0012]    to use preheated, process gas to supply oxygen for combustion in a burner, resulting in substantial fuel savings;  
           [0013]    to use a mixing device to increase turbulence in the combustion chamber for improved thermal oxidizer performance;  
           [0014]    to protect the mixing device with a refractory coating to improve the reliability of the mixing device;  
           [0015]    to use the mixing device as a baffle in the thermal oxidizer to provide sufficient pressure to force the process gas through a burner, to eliminate a separate combustion air source;  
           [0016]    to use a conventional, commercially available burner, located outside the thermal oxidizer, allowing ready access for service and maintenance; and  
           [0017]    to locate the mixing device inside the thermal oxidizer for efficient flow of process gas through the mixing device and through the burner, and to incorporate a bend in the combustion chamber to promote turbulence and efficiently direct combustion gases to a heat exchange means.  
           [0018]    The present invention fulfills the above and other objects by providing a thermal oxidizer for removing combustible substances from an oxygen bearing gas stream, having a combustion chamber containing at least one bend whereby the turbulence within the combustion chamber is improved and the flow direction of the to combustion gases is reversed so that the gases are efficiently removed from the combustion chamber and directed to a heat exchanger. A mixing device located at the entrance to the combustion chamber and disposed axially to the combustion chamber contains a plurality of holes allowing the flow of gas into the combustion chamber. A burner assembly located external to the thermal oxidizer discharges into the mixing device. A flow passage from a plenum to the inlet of the burner allows oxygen bearing gas to be supplied from the plenum. The combustion chamber has gas temperature sensing means and a control unit varies the fuel flow to the burner to obtain and maintain desired operating temperatures within the combustion chamber. The mixing device within the thermal oxidizer may be a metal duct, having a plurality holes in the sides to allow the flow of gas through the wall of the duct, with a refractory coating.  
           [0019]    The above and other objects, features and advantages of the present invention should become even more readily apparent to those skilled in the art upon a reading of the following detailed description in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    In the following detailed description, reference will be made to the attached drawings in which:  
         [0021]    [0021]FIG. 1 shows a sectional view of the thermal oxidizer and heat exchange means; and  
         [0022]    [0022]FIG. 2 shows a sectional view of the thermal oxidizer taken through lines  2 - 2  of FIG. 1. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    For purposes of describing the preferred embodiment, the terminology used in reference to the numbered components in the drawings is as follows:  
                                               8.   Process Gas   44.   Burner Flame       10.   Thermal Oxidizer   46.   Jets of Polluted Process Air       12.   Plenum   50.   Plenum Refractory Coating       14.   Mixing Device   52.   Combustion Chamber       16.   Thermal Oxidizer Inlet       Refractory       20.   Heat Exchange Means   54.   Mixing Device Refractory       22.   Mixing Device Hole   58.   Mixing Device Metal Shell       24.   Gas Flow Passage from   59.   Mixing Device End Plate           Plenum to Burner Inlet   60.   Metal Lining for Hole in       26.   Burner       Refractory       30.   Burner Air Inlet   62.   Combustion Chamber       32.   Burner Fuel Inlet   64.   Combustion Gas       34.   Heat Exchange Means Cold   76.   Control Unit           Side Inlet   86.   Combustion Chamber       36.   Heat Exchange Means Cold       Temperature Sensing Means           Side Exhaust   92.   Bend in Combustion       38.   Heat Exchange Means Hot       Chamber           Side Exhaust   94.   Heat Exchange Means Hot       42.   Burner Fuel Control Valve       Side Inlet                  
 
         [0024]    The thermal oxidizer system of the present invention is illustrated in FIGS. 1 and 2. Referring to FIG. 1, process gas  8  enters heat exchange means  20 . The heat exchange means  20  has a cold, or process gas side, and a hot, or combustion gas side. The cold gas side has an inlet  34  and an exhaust  36 , and the hot gas side has an inlet  94  and an exhaust  38 . As the process gas traverses the heat exchange means  20 , it is passed in indirect heat exchange relationship with combustion gas  64  leaving the combustion chamber  62 . The process gas  8  is thereby preheated, whereby combustion efficiency is increased and the heat required to operate the thermal oxidizer is decreased. The preheated process gas leaves the heat exchange means  20  at the cold side exhaust  36  and enters the thermal oxidizer  10  at the oxidizer inlet  16 .  
         [0025]    The thermal oxidizer has a vertical mixing device  14  mounted in a plenum  12 . The preheated process gas enters the plenum  12  and flows around the mixing device  14 . Some of the heated process gas leaves the plenum  12  through holes  22  in the mixing device  14 . The remainder of the process gas leaves the plenum  12  through a gas flow passage  24 . The gas flow passage  24  may be part of the plenum  12 , or it may be a separate pipe or duct. The passage  24  directs preheated process gas to a burner  26 .  
         [0026]    Ready access to the burner  26  and related equipment is provided for maintenance and servicing. The burner  26  has an air inlet  30  and fuel inlet  32 . Fuel is supplied to the burner  26  through fuel control valve  42 , which is connected to a fuel supply that is not shown. Fuel burned in the burner incinerates the process gas in combustion chamber  62 , producing combustion gas  64 . Combustion chamber  62  has a bend  92  to direct combustion gases from the mixing device  14  to hot-side inlet  94  of heat exchange means  20 . The combustion gas then leaves the combustion chamber  62  and enters the heat exchange means  20 . After passing through the heat exchange means  20 , the combustion gas leaves the heat exchange means through exhaust  38  and is vented to the atmosphere through a stack that is not shown.  
         [0027]    The plenum  12  has a refractory coating  50 . Combustion chamber  62  has a refractory coating  52 .  
         [0028]    The mixing device  14 , shown in FIG. 2, comprises a metal shell  58 , metal end plate  59 , and a refractory coating  54 . The mixing device  14  incorporates holes  22  that form jets  46  of process gas. The jets promote turbulence and mixing of the process gas and burner flame, and thereby enhance combustion.  
         [0029]    Combustion chamber temperature is measured by temperature sensing means  86 . Control unit  76  uses this temperature measurement to adjust the fuel flow through burner fuel control valve  42 .  
         [0030]    Thus, the thermal oxidizer  10  has a burner  26 , a mixing device  14 , and a combustion chamber  62 . The inlet  16  of the thermal oxidizer  10  is connected to the cold-side exhaust  36  of a heat exchanger  20 . The discharge of the combustion chamber  62  is connected to hot-side inlet  94  of the heat exchanger  20 . The cold-side inlet  34  of the heat exchanger  20  is connected to a process gas supply, which is not shown.  
         [0031]    The mixing device  14  is mounted in a plenum  12 . The heat exchanger  20  is set at an angle of approximately 90° to the axis or centerline of the mixing device. Aligning the heat exchanger  20  at a right angle to the mixing device  14  allows the burner  26  to be located outside the oxidizer  10 , plenum  12  and inlet duct. The burner  26  is axially disposed to the mixing device  14 , and the mixing device  14  is axially disposed to combustion chamber  62 .  
         [0032]    The mixing device  14  comprises a metal shell  58 , a metal end plate  59 , and a refractory coating  54 .  
         [0033]    Plenum  12  is a chamber or cavity of sufficient volume to house the mixing device  14 , and provides uniform static pressure, and adequate gas flow, around the mixing device  14 . A flow passage  24  directs preheated process gas from the plenum to air inlet  30  of burner  26 . The flow passage may be a pipe or a duct connected to the plenum, or it may be part of the plenum as shown in FIG. 1. The passage may be located anywhere on the plenum. However, a desirable location for the flow passage is the side of the plenum  12  opposite the plenum inlet. This location allows the process gas to flow completely around the mixing device  14  and cool its outer metal shell  58 .  
         [0034]    During operation the mixing device  14  induces a decrease in static pressure between the plenum  12  and the combustion chamber  62 , and forms jets  46  of process gas that mix with the flame  44  from the burner  26 . The jet-mixing process increases turbulence in both the mixing device  14  and the combustion chamber  62 , enhancing incineration of the pollutants and undesirable substances in the process gas. The decrease in static pressure also induces process gas to flow through flow passage  24  and through burner  26 , and into the mixing device  14  and combustion chamber  62 . The mixing device  14  thus provides the means to induce polluted, process air through the burner  26 , eliminating the separate combustion air fan and motor used in the prior art.  
         [0035]    Holes  22  in metal shell  58  and refractory  54  may be formed using any of a number of various techniques which use the metal shell to accurately and reliably size the holes. The holes must be sized accurately and reliably, because the hole sizes influence the static pressure differences across the holes and across burner  26 . Consistent static pressure differences are required for consistent burner performance and consistent mixing of gas jets  46  and burner flame  44 . The holes  22  shown in FIGS. 1 and 2 are made using sections of metal pipe or tube  60  to line the hole  22  in the refractory  54 . The metal lining in the hole aids in forming the refractory during manufacture, and enhances durability of the refractory  54 . Without the metal lining, relatively cool air jets flowing through the holes  22  may cause erosion of the refractory  54 , or destructive thermal stresses and cracks in the hot refractory. Both erosion and thermal stresses eventually cause loss of refractory which may result in damage to metal shell  58  or plate  59 . Loss of refractory near holes  22  may change the geometry of the holes  22  and thereby change the coefficient of discharge of the holes  22 . A change in the coefficient of discharge of the holes  22  may result in a change of gas flow, or a change in static pressure difference across the holes  22 . Thus, accurate and reliable sizing of holes  22  results in consistent and reliable operation of the thermal oxidizer  10 .  
         [0036]    Jets  46  are located to allow for complete combustion of the fuel supplied to the burner  26 . The jets  46  of process gas are located around the periphery of the mixing device  14 , so jets  46  impinge on each other near the center. Making the jets  46  impinge near the center of the mixing device  14 , or combustion chamber  62 , eliminates excessive penetration of the jets  46 . Excessive penetration may lead to jet impingement on a wall. The impingement of a relatively cool gas jet on a hot refractory wall may damage the refractory by erosion and by inducing thermal stresses, with undesirable results as discussed above.  
         [0037]    It is well known in the prior art that impinging jets  46  result in higher levels of turbulence than a single jet. High levels of turbulence enhance combustion efficiency and thermal destruction of undesirable substances in the process gas. Generally, the more jets  46  and the higher the jet velocity, the higher the level of turbulence. However, the more jets  46 , the smaller the diameter of each jet or the lower the jet velocity, for a given airflow. The smaller the jet or the lower the jet velocity, the shorter the distance the jet will penetrate, other factors being equal.  
         [0038]    The number and size of jets  46  are chosen to ensure sufficient penetration of the process gas jets into the mixing device  14  and combustion chamber  62 , for mixing with the burner flame. Jet penetration varies with the difference in static pressure across the holes  22 , and an unnecessarily high difference in static pressure leads to unnecessarily high power consumption. Jet penetration distance also varies with the size of the holes  22 , and velocities and densities of the process gas and the combustion gas. Jet penetration may be calculated using any one of a number of correlations used in the prior art. Some of the jet holes may be sized differently to penetrate different distances than others, for an even distribution of process gas in the combustion chamber  62 .  
         [0039]    The numbers and sizes of holes  22  in mixing device  14  are chosen to also provide adequate static pressure difference for burner  26 . Calculations indicate that static pressure differences that yield adequate jet penetration distances are also adequate for proper burner operation.  
         [0040]    Refractory  54  may be a castable material similar to cement, or a fibrous refractory, or any other type of refractory. The refractory thickness is sized to provide sufficient insulation to protect metal parts from burner flame  44 .  
         [0041]    The burner  26  is typically a commercially available burner that will accommodate inlet temperatures of up to 1200° F. Most of these burners are made of inexpensive carbon steel and lined with refractory for reliable, high-temperature operation. The burner may be fueled with a gaseous fuel or a liquid fuel, and some burners can operate on either fuel. The burner  26  is located outside the plenum  12  for ready access to the burner and other equipment requiring periodic service and maintenance. Locating the burner outside the process gas duct also allows the burner to be made of inexpensive, low-temperature carbon steel.  
         [0042]    The burner fuel control valve  42  regulates the fuel supply to the burner  26 . Control unit  76  determines the position of the fuel supply valve  42 , and thereby the fuel flow to the burner  26 . Combustion chamber  62  temperature is measured with temperature sensing means  86 . The measured combustion chamber  62  temperature is compared to a desired temperature set in the control unit, and the control unit sets the fuel control valve position to maintain the desired temperature.  
         [0043]    The preferred embodiment uses a bend or turn  92  in combustion chamber  62 , to route combustion gases to the heat exchange means hot-side inlet  94 . The turn increases turbulence and mixing in the combustion chamber  62  near the turn and downstream of the turn, which enhances combustion in that section of the combustion chamber  62 . This bend  92  also results in a combustion chamber  62  folded to a compact size, and easy entry to the heat exchange means  20  inlet. An alternative embodiment, familiar to those skilled in the art, is to eliminate bends in the combustion chamber  62  and redirect the combustion chamber  62  gases toward the heat exchange means with a duct containing a sufficient number of bends.  
         [0044]    In operation, the process gas stream to be incinerated is conveyed through the heat exchange means  20  and thermal oxidizer  10  under the influence of a forced draft fan, which is not shown, disposed upstream of the cold side inlet of the heat exchange means  20 . Alternatively, the process gas stream may be conveyed through the heat exchange means  20  and thermal oxidizer  10  under the influence of an induced draft fan, which is not shown, located at the exhaust  38  of the hot-side of the heat exchange means  20 .  
         [0045]    From the description above, a number of advantages of the present invention become evident:  
         [0046]    (a) process gas oxygen is used for combustion in the burner;  
         [0047]    (b) a burner is located outside the thermal oxidizer for ready maintenance and service;  
         [0048]    (c) the mixing device uses a refractory lining, resulting in extended oxidizer life when compared to components without refractory coatings;  
         [0049]    (d) there are no moving parts in the heated process gas stream, resulting in a reliable control system;  
         [0050]    (e) turbulence and mixing are promoted throughout the combustion chamber, resulting in enhanced combustion; and  
         [0051]    (f) the combustion products are efficiently routed through the combustion chamber and the heat exchange means, with no external ducting required.  
         [0052]    Accordingly, the reader will see that the present thermal oxidizer uses process gas to supply oxygen for combustion in a burner, eliminating a separate combustion air source and resulting in substantial fuel savings. In addition, an effective and reliable mixing device provides turbulence in the combustion chamber  62  for excellent oxidizer performance. Also, a conventional, commercially available burner is used and located outside the thermal oxidizer for ready access and maintenance.  
         [0053]    Although only a few embodiments of the present invention have been described in detail hereinabove, all improvements and modifications to this invention within the scope or equivalents of the claims are included as part of this invention.