Abstract:
An asphalt plant includes a plurality of asphalt processing components including a first set of components producing volatile emissions and a second set of components requiring process heat. A separate central burner assembly is adapted to supply heat energy in the form of heated gas to satisfy the process heat requirements of the second set of components. A first duct system is provided which includes a fan and which is adapted to capture a portion of the volatile emissions produced by the first set of components and to convey the captured emissions into the central burner assembly for mitigation. A second duct system is provided and includes a fan and is adapted to convey heated gas from the central burner assembly to the second set of components.

Description:
This application is a Continuation-in-Part of U.S. application Ser. No. 09/045,212, filed Mar. 20, 1998 now U.S. Pat. No. 6,146,007, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to asphalt plants. More specifically, the present invention relates to an asphalt plant which captures and mitigates fugitive emissions and which uses a centralized media burner to supply process heat energy to the various plant components. 
     BACKGROUND OF THE INVENTION 
     On asphalt plants it is desirable to have a variety of air pollution control measures. The asphalt making process, by its very nature of heating and processing the bituminous asphalt components, produces a considerable quantity of undesirable hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), particulate matter and other emissions which constitute the unfortunate signature plume of an asphalt plant, commonly referred to as “blue smoke.” In addition to being a source of air pollution, asphalt plants are noisy and visually unappealing, owing to their network of open conveyors, hoppers, bins, blowers and other heating and material handling equipment. Accordingly, asphalt plants in general are regarded as quite a nuisance, especially in and around residential areas. 
     The typical asphalt plant has high energy requirements. The drum dryer/mixer typically includes a gas burner to dry the aggregate material and to heat the mixing zone to foster adequate mixing of the aggregate with the liquid asphalt. The asphalt material contained in the asphalt storage tanks must be constantly heated to maintain the asphalt cement in its liquid state, and thus another gas burner or similar heating system is required in order to constantly heat the storage tanks. Thus, burner emission are created at both the asphalt storage tanks and at the drum dryer/mixer. 
     Moreover, the volatile components of the heated asphalt cement as well as the finished asphalt create a certain amount of fugitive emissions as the asphalt components and the finished asphalt are stored, mixed, and transported through the plant. Furthermore, the asphalt cement storage tanks and the asphalt storage silos are usually vented in order to prevent undue pressure build up, especially on hot days, which further complicates the fugitive emission problem. Additional fugitive emissions are created when the finished asphalt material is loaded onto trucks for transport to a job site. 
     One approach to alleviating the fugitive emission problem has been to enclose portions or all of the plant in order to minimize the amount of leakage from the ductwork and conveyors in the plant. Such an approach, an example of which is described more fully in U.S. Pat. No. 5,620,249, does not provide an improved mitigation system and is typically best suited for applications in which the plant can be made very compact, which is not always feasible. 
     Attempts have also been made to apply flameless media burner technology to asphalt plants. Media burner technology uses a bed or matrix of ceramic materials which act as a flame arrestor, thereby controlling the rate and temperature of the combustion process. Externally mixed fuel is added to the media burner, which is pre-heated until a self-sustaining combustion is initiated. Ideally, a very efficient centralized media burner should be able to supply heat to the various process components, so that the maximum amount of energy is extracted from the consumed fuel. Unfortunately, existing media burner technology has proven unsatisfactory for asphalt processing plants. The externally mixed fuel components have proven to be too explosive for safe, everyday applications. 
     Accordingly, there exists a continuing need for an improved asphalt plants. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, an asphalt plant includes a plurality of asphalt processing components, with the plurality of processing components including a first set of components producing volatile emission and a second set of components requiring process heat. A central burner assembly is disposed separately from each of the plurality of processing components, with the central burner assembly being adapted to supply heat energy in the form of heated gas to satisfy the process heat requirements of the second component set. A first duct system is in flow communication with the first component set and the central burner assembly, with the first duct system including a fan and being adapted to capture a portion of the volatile emissions produced by the first component set and to convey the captured emissions into the central burner assembly for mitigation. A second duct system is in flow communication with each of the components in the second component set and the central burner assembly, with the second duct system including a fan and being adapted to convey heated gas from the central burner assembly to the second component set. 
     In further accordance with a preferred embodiment, the first set of components may include an asphalt cement storage tank, an asphalt storage silo, a drum dryer/mixer having a mixing zone, and/or a truck loading area having a substantially sealed enclosure. At least one component of the first set of components may include an enclosure connected to the first duct system. 
     The central burner assembly may include an air inlet plenum, with the first duct system being connected to the air inlet plenum. Further, the second component set may include a rotary drum dryer/mixer, and the second duct system may include an insulated portion for conveying heat to the drum dryer/mixer. The asphalt plant may include a cement storage tank, with each of the storage tank and the central burner assembly including a heat exchange unit, with the heat exchange units being adapted to scavenge heat from the central burner assembly and convey the heat to the storage tank. 
     Preferably, the central burner assembly comprises a media burner having an enclosed combustion chamber defined in part by a top wall, a bottom wall, and an interconnecting sidewall. A portion of the combustion chamber preferably contains a matrix of ceramic members. The media burner preferably includes a fuel delivery system, with the internal delivery system being adjustable to permit the fuel to be injected at different locations within the media burner. 
     Still preferably, the first duct system may be connected to an air plenum for delivering captured fugitive emissions to the media burner, and an air valve may be provided for controlling the flow of air from the air plenum to the combustion chamber. The air valve may include a baffle slidably mounted adjacent an air inlet opening in the bottom wall, with the baffle being moveable between an open position removed from the air inlet opening and a closed position covering the air inlet opening. 
     In accordance with another aspect of the invention, a central media burner is provided for providing process heat in the form of heated gas to a selected set of asphalt processing components, with the central media burner being separate from each of the processing components. The central media burner comprises an enclosed combustion chamber defined in part by a top wall, a bottom wall, and an interconnecting sidewall. A portion of the combustion chamber contains a matrix of ceramic members, with at least one of the walls defining a gas outlet. The combustion chamber is adapted to supply thermal energy in the form of heated gas. An adjustable internal fuel delivery system delivers fuel to a selected location in the combustion chamber portion, and an air inlet plenum delivers combustion air to the combustion chamber. A duct system communicates the heated gas from the gas outlet to the selected set of processing components, whereby the heated gas is supplied through the duct system to each of the processing components to satisfy the process heat requirements thereof. 
     In accordance with yet another aspect of the invention, an asphalt plant comprises a plurality of asphalt processing components, with a first set of components producing volatile emissions, and a second set of components requiring process heat energy. A central media burner produces process heat energy in the form of heated gases, with the media burner having an enclosed combustion chamber defined in part by a top wall, a bottom wall, and an interconnecting sidewall, with a portion of the combustion chamber containing a matrix of flame arresting ceramic members. The central media burner, which is separate from each of the plurality of asphalt processing components, also includes an inlet and an outlet. A fuel delivery system is provided to deliver combustion fuel directly to the combustion chamber. A first duct system including a fan is in flow communication with the first component set and the central media burner inlet, and captures a portion of the volatile emissions produced by the first component set and conveys the captured emissions to the inlet of the central media burner for mitigation. A second duct system includes a fan and is in flow communication with the second component set and the central media burner outlet, and conveys heat energy from the outlet of the central media burner to the second component set. 
     In accordance with a still further aspect of the invention, an asphalt plant comprises a drum dryer/mixer requiring heat energy and producing volatile emissions, a flameless media burner assembly, a first duct system and a second duct system. The media burner assembly is remote from the drum dryer/mixer, with the media burner assembly having an enclosed combustion chamber and a fuel delivery system extending into the combustion chamber. A portion of the combustion chamber houses a matrix of ceramic members, and the fuel delivery system has a fuel port to convey fuel to the combustion chamber portion. The first duct system is in flow communication with the drum dryer/mixer and the media burner assembly, with the first duct system including a fan and being adapted to capture a portion of the volatile emissions produced by the drum dryer/mixer and to convey the captured emissions into the media burner assembly for mitigation. The second duct system is in flow communication with the drum dryer/mixer and the media burner assembly, with the second duct system including a fan and being adapted to convey heat energy in the form of heated gas from the media burner assembly to the drum dryer/mixer. 
     In accordance with yet another aspect of the invention, an asphalt plant comprises an asphalt processing component that produces volatile emissions and that requires process heat, a flameless burner assembly, and a first and second duct system. The flameless burner assembly supplies heat energy to the processing component, with the flameless burner assembly being separate and spaced apart from the processing component. The first duct system is in flow communication with the processing component and the flameless burner assembly and captures at least a portion of the volatile emissions produced by the processing component and conveys the captured emissions into the flameless burner for mitigation. The second duct system is in communication with the processing component and the flameless burner assembly, and supplies heat energy from the flameless burner assembly to the second component set to thereby substantially satisfy the process heat requirements thereof. 
     These and other objects, features and advantages of the present invention will become readily apparent to those skilled in the art upon a reading of the following description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan diagrammatic view of an asphalt plant incorporating the features of the present invention; 
     FIG. 2 is a fragmentary elevational view of the drum dryer/mixer and the baghouse filter illustrating the insulated duct from the media burner routed into the mixing zone of the drum dryer/mixer; 
     FIG. 3 is an enlarged elevational view in cross-section of the media burner having the internal add fuel injection system; 
     FIG. 4 is an enlarged fragmentary elevational view of the media burner internal fuel injection system; 
     FIG. 5 is an enlarged fragmentary elevational view taken along lines  5 — 5  of FIG. 4; and 
     FIG. 6 is an enlarged fragmentary plan view taken along lines  6 — 6  of FIG. 5 illustrating the air valve assembly. 
    
    
     DETAILED DESCRIPTION 
     The following detailed description is not intended to limit the invention to the precise form or forms disclosed. The embodiments described in detail have been chosen in order to best explain the principles of the invention so that others skilled in the art may follow its teachings. 
     Referring now to the drawings, FIGS. 1 and 2 illustrate an asphalt plant incorporating features of the present invention and generally referred to by the reference numeral  10 . The asphalt plant typically includes a variety of plant processing components, such as those components outlined in more detail in U.S. Pat. No. 5,620,249, the disclosure of which is incorporated herein by reference. Asphalt plant  10  typically includes a rotating drum dryer/mixer  12 . The drum dryer/mixer  12  is preferably of the counterflow design, although a parallel flow drunk dryer/mixer could also be used. Asphalt plant  10  also typically includes a plurality of virgin aggregate silos  14 , a recycled asphalt product (RAP) storage bin  16 , and a virgin aggregate hopper  18 . A conveyor  20  is provided to transport the virgin aggregate to the drum dryer/mixer  12 , while a RAP conveyor  22  is provided to transport the RAP to the drum dryer/mixer  12 . The conveyors  20 ,  22  may be slat conveyors or other conventional designs. Each conveyor  20 ,  22  is preferably enclosed by a duct  24 ,  26 , respectively. One or more asphalt cement storage tanks  28  are provided which supply liquid asphalt to the drum dryer/mixer  12  via a feed line  30  as is well known in the art. 
     Finished hot mix asphalt produced in the drum dryer/mixer  12  is conveyed to a batcher silo  32  by a bucket conveyor  34 , from where the asphalt is transferred to one or more loadout silos  36  by a conveyor  38 . The bucket conveyor  34  and the conveyor  38  are each enclosed by a duct  40 ,  42 , respectively. The loadout silos are preferably mounted over an enclosure  44  sized to receive a transport vehicle (not shown). Each of the drum dryer/mixer  12 , the conveyors  20   22  [ 22 ,  24 ],  34 ,  38 , and the silos  32  and  36  are likely to release volatile emissions, which are captured by a portion of the duct systems  24 ,  26 ,  40 ,  42  and the enclosure  44 . The captured emissions are routed to a return duct  46 , and then to a central burner  48  as outlined below. Another return duct  47  is provided which routes captured emissions from the storage tanks  28  to the central burner  48  as will be discussed in greater detail below. 
     The central burner  48  is preferably a media burner employing flameless combustion technology. A more complete explanation of flameless combustion technology can be found in U.S. Pat. No. 5,165,884, the disclosure of which is incorporated herein by reference. The return duct  46  is connected to the burner  48  for routing the captured emissions within the duct  46  to the burner  48  for mitigation as will be explained in greater detail below. Burner  48  includes an insulated duct  50  which routes heat energy to the drum dryer/mixer  12 . Additional heat energy may be routed to other components as needed using additional ducts (not shown). Each of the above mentioned ducts preferably is insulated and includes one or more dampers for closing portions of the ducts during plant start up or as may otherwise be required. 
     As shown in FIG. 2, a filter or baghouse  52  is provided for capturing particulate emission from the drum dryer/mixer  12  in a manner well known in the art. An insulated duct  54  routes the exiting gas stream from the drum dryer/mixer  12  to the baghouse  52 , and duct  54  is also connected to return duct  46  for routing emissions to the burner  48 . The heat energy from the drum dryer/mixer  12 , which has been routed through the insulated duct  50 , enters the interior of the drum dryer/mixer  12  at an exit point  56 . 
     Also as shown in FIG. 2, the drum dryer/mixer  12  preferably includes a collar  58  for introducing RAP into the drum dryer/mixer  12 , a discharge hood  60  for routing finished hot mix asphalt out of the drum dryer/mixer  12 , and an insulated duct  62  having a damper  64  that connects the drum dryer/mixer  12  to the stack  66  of the baghouse  54 . A fan  68  in conjunction with a damper  70  controls the flow of gases from the mixing zone  73  of the drum dryer/mixer  12  to the insulated duct  50  via an insulated duct  72 . Another damper  74  controls the flow of gases into the duct  50 . 
     Referring now to FIG. 3, media burner  48  includes a top wall  76 , a bottom wall  78 , and continuous sidewalls  80  enclosing an internal combustion chamber  82 . A plurality of ceramic members  83 , such as saddles, balls, or other shapes, are disposed within the combustion chamber  82 . The ceramic members  83  function to control the combustion process and will exhibit very high thermal inertia. The ceramic members may be any suitable shape, such as saddle shaped, round or spherically shaped, or “dog bone” shaped. An air inlet plenum  84 , which is connected to outside air as well as to the return ducts  46  and  47 , is provided for routing air and captured emissions to the combustion chamber via an air inlet valve assembly  86 . The plenum  84  includes an auger  85  to permit periodic removal of the ceramic members  83 , which may be released through the valve assembly  86  if needed. 
     A fuel delivery assembly  88  is provided for routing combustion fuel to the combustion chamber  82 , and includes a fuel manifold  89  and a plurality of fuel injection lances or rods  90 . The sidewall  80  of burner  48  includes a heat exchange unit  91  having a plurality of oil lines  92  which scavenge heat from the burner  48 . The oil lines  92  route heated oil to a heat exchanger  94  on each of the asphalt cement storage tanks  28  via a feed line  96 , which helps to maintain the asphalt within the storage tanks  28  in a liquid state. Burner  48  also includes a hot air outlet  97  connected to the insulated duct  50 , a pre-heater  98  for heating the burner in preparation for start up, and a system of thermocouples  100 . 
     As shown in FIGS. 3-5, the fuel rods  90  are arranged in a plurality of rows. Each fuel rod  90  includes an outer tube  102  having a sidewall  104  enclosing a chamber  106 . A plurality of fuel ports, for example,  108   a ,  108   b ,  108   c , . . .  108   n , are provided in the sidewall  104 . An inner conduit  110  is slidably disposed within each of the outer tubes  102 , with each conduit  110  including a fuel flow passage  112  terminating in an orifice  114 . The fuel passage  112  is connected to the fuel manifold  89  by a flexible hose  116  connected to an inlet end  117  of the conduit  110 . Each inner conduit  110  includes an adjustable locking collar  118 , which permits the inner conduit  110  to be adjusted relative to the outer tube  102 . A pair of spaced apart seals  120 ,  122  are connected to an outlet end  124  of the inner conduit  110 , with the orifice  114  being located between the seals  120 ,  122 . Accordingly, fuel from the fuel manifold  88  is routed through the flexible hose  116 , into the fuel passage  112 , and into that the portion of the chamber  106  dictated by the present location of the inner conduit  110  (i.e., the present location of the seals  120 ,  122 ) relative to the outer tube  102 . The fuel exits the chamber  106  via the closest adjacent fuel port  108   a ,  108   b ,  108   c , or  108   n , again depending on the position of the inner conduit  110  relative to the outer tube  102 . 
     Referring now to FIGS.  4 — 6 , the air valve assembly  86  includes a plurality of valves, for example  86   a ,  86   b ,  86   c , and  86   d , each of which is shown in a different position in FIG. 6. A plurality of spaced apart holes  125  are provided in the bottom wall  78  of the burner  48 , which holes  125  communicate air from the air inlet plenum  84  to the combustion chamber  82 . A baffle member  126  is slidably mounted to the bottom wall  78  and also includes a plurality of spaced apart holes  128 , which are spaced to match the spacing of holes  125 . Accordingly, the amount of air flowing through the holes  125  can be controlled by sliding the baffle member  126  back and forth on the bottom wall  78  having the holes  125 . For example, the air flow can be maximized by sliding the baffle member  126  to the position of valve  86   a  at the top of FIG. 6, or minimized by sliding the baffle member  126  to the position of valve  86   d  at the bottom of FIG. 6, with valves  86   b  and  86   c  being shown in intermediate positions. 
     Although certain exemplary embodiments constructed in accordance with the teachings of the invention have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all embodiments of the teachings of the invention fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.