Abstract:
An energy recovery system permits recovering energy from fumes. The system employs a heat engine such as a Stirling engine, and a supplemental combustible fuel. A combustor receives the paint fumes as well as the supplemental fuel from a fuel supply. The fuel supply includes a fuel throttle regulating the fuel mass flow rate. An air blower provides air to the combustor. The heat engine includes a heater receiving heat from the combustor. A temperature sensor detects the temperature of the heater, while a controller operatively controls the fuel throttle to vary the fuel mass flow rate based on the temperature of the heater.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present patent application claims priority to U.S. patent application Ser. No. 11/217,899 filed on Sep. 1, 2005. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to energy recovery systems, and more particularly relates to the use of heat engines such as Stirling engines for energy recovery. 
       BACKGROUND OF THE INVENTION 
       [0003]    Paint is generally a solid pigment dissolved in a volatile liquid solvent. When the paint is sprayed on a surface, the volatile solvent evaporates while the solid pigment settles on the surface. These volatile solvent vapors, commonly referred to as paint fumes, are hazardous and may not be discharged to the atmosphere. Accordingly, the paint fumes are generally scrubbed and incinerated. While it may appear that, with newly developed means to concentrate the solvent vapors in the scrubbing gas, such waste products could be combusted to provide energy, the concentration of solvents in the paint fumes can range from a few parts per million (ppm) to thousands of ppm, resulting in a heat value that greatly varies. Therefore, it is difficult to recover energy from these paint fumes due to the varying levels of combustible solvents. 
         [0004]    Accordingly, there exists a need to provide an energy recovery system that is capable of recovering energy from concentrated paint fumes despite variations in solvent concentration. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The present invention provides an energy recovery system capable of recovering energy from paint fumes and other combustible volatile agents. The system employs a heat engine such as a Stirling engine, and a supplemental combustible fuel used in conjunction with the paint fumes. Generally, a combustor receives the paint fumes as well as the supplemental fuel from a fuel supply. The fuel supply includes a fuel throttle regulating the fuel mass flow rate. An air blower provides air to the combustor. The heat engine includes a heater receiving heat from the combustor. A temperature sensor detects the temperature of the heater, while a controller operatively controls the fuel throttle to vary the fuel mass flow rate based on the temperature of the heater. 
         [0006]    Accordingly to more detailed aspects, the controller varies the fuel mass flow rate to maintain a generally constant temperature of the heater. The paint fumes would typically be provided at a constant mass flow rate, although the concentration of solvent vapor in the paint fumes varies from a minimum level to a maximum level. The system is thus designed such that the maximum level of solvent vapor does not over heat the heat engine. For example, the heat engine may be sized to utilize the maximum level of solvent vapor, or the mass flow rate of the paint fumes may be fixed at a level to prevent over heating. Similarly, the system is designed such that the highest equivalence ratio (air fuel ratio to stoichiometric ratio, described later herein) does not exceed the lean blow-out limit, and such that the lowest equivalence ratio does not exceed the rich over-heat limit. 
         [0007]    Additionally, the energy recovery system may include an air throttle regulating the air mass flow rate. The controller may operatively control the air throttle to regulate the air mass flow rate based on the position of the fuel throttle. Additionally, the energy recovery system preferably includes an oxygen sensor detecting the level of oxygen in the exhaust. Thus, the controller may also operatively control the air throttle based on the level of oxygen in the exhaust. In this manner, the equivalence ratio can be kept at a generally constant level, thereby preventing the combustor from reaching the lean blow-out limit or the rich over-heat limit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings: 
           [0009]    The FIGURE is a schematic depiction of an energy recovery system constructed in accordance with the teachings of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    An energy recovery system  20  has been schematically depicted in accordance with the teachings of the present invention. The energy recovery system  20  generally is employed for recovering energy from paint fumes  22  which are collected from an area in which painting occurs. It will be recognized that the energy recovering system  20  may be employed with numerous other combustible agents, solvents or fumes, and the system  20  will be described in connection with paint fumes  22  as one example. Recently, a process has been developed whereby the paint fumes are scrubbed with nitrogen and the mixture of solvent vapors and nitrogen is fed to a concentrator (not shown), in which the concentration of the solvent vapors is increased until the mixture (referred to herein as paint fumes  22 ) has a sufficiently high heat value to serve as fuel. The concentrator supplies a constant mass flow rate of paint fumes  22 , but as noted above, the concentration of solvents in the fumes  22  varies widely between a minimum level to a maximum level. Typically, the composition of the solvents in the paint fumes  22  does not change appreciably, although the system can be adjusted to accommodate some variation in solvent composition. As such, the solvent heat value and the stoichiometric air-fuel ratio are roughly constant. 
         [0011]    The fumes  22  are provided to a heat engine  24  for recovery of energy. The heat engine  24  used in conjunction with the energy recovery system  20  can comprise a Stirling cycle heat engine similar to those previously developed by the Assignee of the present invention, STM Power, Inc., including those described in U.S. Pat. Nos. 4,996,841; 5,074,114; 5,611,201; 5,706,659; 5,722,239; 5,771,694; 5,813,229; 5,836,846; 5,864,770; the disclosures of which are hereby incorporated by reference in their entirety. 
         [0012]    Generally, the heat engine  24  includes a combustor  26 , a heater  28  and a recuperator  30 , as is well know in the art. These devices are disclosed in detail in the aforementioned patents, and a preferred combustor has been developed by the Assignee STM Power, Inc., as disclosed in U.S. Pat. No. 5,921,764, the disclosure of which is incorporated herein by reference in its entirety. These elements, including the combustor  26 , may be separately formed from the engine  24 , or may be integrated therein such as is disclosed in U.S. Pat. Nos. 5,074,114 and 5,388,409, the disclosures of which are hereby incorporated by reference in their entirety. Similarly, a preferred construction of the heater  28  is shown in U.S. Pat. No. 6,282,895, the disclosure of which is hereby incorporated by reference in its entirety. 
         [0013]    In order to overcome the limitations imposed by the varying concentration of solvent vapor in the paint fumes  22 , the heat engine  24  and its combustor  26  are also supplied with supplemental fuel  32 . The fuel  32  is a combustible fuel, preferably a gas such as natural gas, propane, or some other high-quality fuel. A fuel supply  34  includes a pressure regulator  36  and a fuel throttle  38  for regulating the mass flow rate of the fuel  32  delivered to the combustor  26 . The combustor  26  burns the paint fumes  22  which are mixed with the supplemental fuel  32 . A blower  40  provides air  42  to the combustor  26  for mixing with the fumes  22  and fuel  32 . Generally, a constant mass flow rate of combustion air  42  may be supplied by the blower  40  to the combustor  26 , although the air mass flow rate can also be controlled as will be discussed in more detail below. The products of combustion from the combustor  26  flow through the heater  28  and recuperator  30 , which extract heat energy therefrom. The products of combustion are then passed out of the heat engine  24  as exhaust  54 . 
         [0014]    In order to accommodate the variances in the concentration of solvent vapor in the paint fumes  22 , and hence variations in the heat value of the fumes  22 , a sensor  46  is provided in communication with the heater  28  to sense the temperature thereof. The sensor  46  may be attached to tubes contained within the heater  28  having the working fluid that is heated by the combustor  26 . A temperature signal  48  is sent to a controller  50 , which in turn is operatively connected to the fuel throttle  38 . The temperature sensor  46  is preferably a proportional, integral, derivative (PID) type sensor suitable for close loop control as is well known in the art. In the preferred construction, the controller  50  operates the fuel throttle  52  in order to maintain a generally constant temperature in the heater  28 . The term generally constant, as used herein, means a variation of less than plus or minus 5%, or ±50 degrees Celsius. Accordingly, based on the temperature of the heater  28 , an appropriate amount of supplemental fuel  32  may be provided to the heat engine  24  in order to extract energy from paint fumes  22 . 
         [0015]    It will be recognized, that even when no supplemental fuel  32  is provided, the maximum concentration of the solvent vapor in the fumes  22  must not result in over heating of the engine  24 . Accordingly, the maximum level of solvent concentration is identified beforehand and the system is designed to prevent overheating. For example, the size of the heat engine  24  may be selected based on this maximum level. Further, multiple heat engines  24  may be employed, and the stream of paint fumes  22  can be split to supply each heat engine of the system  20 . Likewise, the mass flow rate of the paint fumes  22  emanating from the concentrator may be selected based on the capacity of the heat engine  24 . 
         [0016]    As is known in the art, the ratio of combustion air  42  to the mixed fuel  22 ,  32  often differs from the stoichiometric ratio, and the ratio of the air-to-fuel ratio to the stoichiometric ratio is referred to as the equivalence ratio. As such, when the equivalence ratio is above one, the engine is running “lean”, and when it is less than one the engine is running “rich”. When the fuel throttle  38  is run to maintain a constant temperature of the heater  28 , and when the air mass flow rate is constant, the equivalence ratio (λ) may be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   λ 
                   = 
                   
                     
                       
                         
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         [0000]    where {dot over (m)} is mass flow rate, C is mass fraction of solvents in the fumes  22 , ρ is stoichiometric mass air/fuel ratio, h is heat value, {dot over (Q)} is total fuel heat input to the combustor  26 , and λ is the equivalence ratio. Subscript s refers to solvents, f refers to fumes  22 , g refers to gas  32 , and refers to air  42 . 
         [0017]    As such, the behavior of the air to fuel ratio (and hence λ) depends on the sign of the expression (ρ g h s −ρ s h g ). If this expression is positive, then increasing solvent concentration will lean out the combustion. If this expression is negative, then increasing solvent concentration will enrich the combustion. Therefore, the highest and lowest equivalence ratios are calculated for each of the two above-noted situations, (i.e. where increasing solvent concentration either leans out the combustion or enriches the combustion). 
         [0018]    Accordingly, the highest equivalence ratio  2  should not exceed the lean blow-out limit (i.e. the amount of combustible fuel is insufficient to support combustion), and the lowest equivalence ratio should not exceed the rich overheat limit (i.e. the amount of combustible fuel is too high to support combustion). Thus, the system  20  may be designed to accommodate these limitations. For example, the controller  50  may operate the fuel throttle  38  to regulate the air to fuel ratio to avoid exceed either of these limits. Likewise, the heat engine  24  may be cycled on and off. Most preferably, these two requirements may be met by modulating the mass flow rate of the air  42 . 
         [0019]    As shown in the FIGURE, the controller  50  uses a control signal  60  to operate the air throttle  44  and regulate the air to fuel ratio. The air throttle  44  may be controlled based on the predetermined behavior of the equivalence ratio λ as effected by the operation/position of the fuel throttle  38 . However, the energy system  20  preferably includes an oxygen sensor  56  for use in controlling the air throttle  44 . The oxygen sensor  56  is positioned downstream of the heat engine  24  and the recuperator  30  to sense the level of oxygen in the exhaust  54 . The oxygen sensor  56  is preferably a PID-type sensor. A signal  58  indicative of the level of oxygen in the exhaust  54  is sent to the controller  50 , which in turn may use this data to operate the air throttle  44 . In particular, the air throttle  44  may be operated to maintain a constant oxygen level in the exhaust  54 . Similarly, the controller  50  may operate the air throttle  44  in order to maintain a constant equivalent ratio, or at least to ensure that the equivalence ratio λ does not exceed either the lean blow-out limit or the rich over heat limit as previously discussed. 
         [0020]    Accordingly, it will be recognized by those skilled in the art that the energy recovery system  20  of the present invention allows for recovery of energy from paint fumes  22  having varying levels of solvent concentration, and thus varying levels of heat energy. A heat engine such as a Stirling cycle heat engine, provides a reliable and efficient method for extracting heat from the paint fumes by combining the fumes with a supplemental fuel. This, in combination with a feedback control loop tied to the heater of the heat engine, allows a constant tube temperature to be maintained within the heater to ensure reliable recovery of energy from the paint fumes. The system may readily be tailored to prevent overheating of the engine, and with the addition of an air throttle, and preferably an oxygen sensor in the exhaust pathway, increased control over the operating parameters of the energy recovery system  20  may be readily achieved. 
         [0021]    The foregoing description of various embodiments of the invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Numerous modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.