Patent Publication Number: US-6701855-B2

Title: Process for the pyrolysis of medical waste and other waste materials

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/385,772 having a filing date of Jun. 3, 2002, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to a process for the pyrolysis of waste materials particularly medical waste. More particularly, the invention relates to a pyrolysis process, wherein the waste material is placed in a sealed container. The sealed container is inserted in a load chamber and the waste material is subjected to the process of pyrolysis. 
     In recent years, government agencies, industries, and other organizations have had to address various problems relating to the handling and processing of organic waste materials including chemical and biological products. The disposal of medical waste is a particularly difficult problem, because of the presence of infectious bacteria, viruses, and other pathogens in the waste. It has been found that heating such organic waste materials to extremely high temperatures causes the components to thermally decompose. The heat energy converts the chemical components of the waste material (primarily carbon, hydrogen, and trace elements) to gases. A pyrolysis process is commonly used to thermally decompose and chemically transform the waste materials. 
     The term, “pyrolysis”, can have different meanings depending on its context. For example, “pyrolysis” is defined as the “transformation of a compound into one or more substances by heat alone, i.e., without oxidation.” ( Hawley&#39;s Condensed Chemical Dictionary , 13 th  Ed. (1997).) In the Code of Federal Regulations (CFR) setting forth standards for the performance of hospital/medical/infectious waste incinerators, “pyrolysis” means “the endothermic gasification of waste materials using external energy.” (40 C.F.R. §60.51c) Typically, in commercial pyrolysis operations, waste material is loaded into a pyrolysis furnace or chamber, and there is generally some small amount of air (oxygen) present in the furnace. There can be several reasons for the presence of the air in the furnace. Some air may enter the furnace during the loading of the waste in the furnace chamber as the door to the chamber is opened and closed. Also, some air may be entrained within the waste. Further, the pyrolysis furnace may be operated at a slight negative pressure resulting in a small amount of air being drawn into the furnace through deficient seals. Thus, the term, “pyrolysis”, is commonly used in the industry and used herein to encompass processes, wherein the atmosphere in the pyrolysis furnace may at times contain a very small amount of air (oxygen) during the pyrolysis reaction, but the amount is so small as to preclude the presence of visible combustion. 
     For industrial applications, the pyrolysis of the waste materials is typically a first step in the overall destruction of the materials. The pyrolysis process volatilizes or gasifies the organic compounds found in the waste and produces exhaust gases containing volatile organic compounds. In a second step, a burner unit combusts or oxidizes the volatile organic compounds. 
     Pyrolysis furnaces should not be confused with incinerators that operate in a starved-air mode. Such incinerators typically include primary and secondary combustion chambers. In the incineration process, a burner or other ignition source produces an open flame in the primary chamber. Combustion air is supplied to the primary chamber at a rate which is less than the stoichiometric amount of oxygen required to achieve complete combustion of the volatile organic compounds evolved from the thermal decomposition of the organic waste materials. Then, in the secondary combustion chamber, excess combustion air is supplied to completely decompose and oxidize the waste exhaust gases. Lewis, U.S. Pat. Nos. 4,474,121 and 4,517,906 disclose methods and apparatus for controlling the addition of an auxiliary fuel to a two-stage combustion furnace system which is operated in a starved-air mode in the first stage and in an excess air mode in the second stage. One problem with such starved-air incinerators is that the open flame in the primary combustion chamber produces turbulence and causes the suspension of particles in the exhaust gas stream. The particulate passes through the secondary combustion chamber and is emitted as pollutants, unless additional pollution control systems (e.g., scrubbers) are employed. It is expensive to install such air pollution control systems on incinerators, but such systems are often necessary to meet emission standards. 
     As discussed above, pyrolysis processes for destroying waste materials are generally known in the industry. For example, Hansen et al., U.S. Pat. No. 5,868,085 discloses a waste treatment unit having: a main frame; an input stage through which the waste material to be treated is introduced through an arrangement of valves that can be controlled to prevent unwanted incorporation of air or oxygen into the pyrolytic process; and a pyrolytic assembly comprising a thermally-insulated outer housing coaxially surrounding an ellipsoidally-shaped pyrolytic chamber. A rotatable screw conveys waste through the retort as the pyrolysis reaction takes place. A heating chamber is defined as the space between the outer housing and the retort. Fuel gases are combusted within the heating chamber to provide a source of heat energy for the pyrolysis. According to the &#39;085 Patent, the gases liberated from the feed material during pyrolysis are processed to draw off pollutants contained therein by a combination of condensation and thermal oxidation. The gases are then either vented to the atmosphere or routed to supply energy, such as to a steam generator. 
     Keough, U.S. Pat. No. 4,648,328 discloses an apparatus and process for the pyrolysis of used vehicular tires. The apparatus includes a reaction chamber. According to the &#39;328 Patent, tire fragments are introduced into and removed from the reaction chamber through airlock mechanisms to prevent the ingress of ambient air as the fragments are conveyed through the chamber by a chain and flight conveyor. The process includes shredding the used tires, preheating the tire fragments, passing the fragments through the reaction chamber, separating solid and gaseous products, and recycling a portion of the gaseous product to the heating means. 
     Also, incinerator processes, which introduce a flame into the incinerator chamber to burn the waste, are known. Brookes, U.S. Pat. No. 4,603,644 discloses an incinerator having a receiving chamber with an opening (vent) in a rear wall. An ignition chamber is supplied with fuel and air and fires a flame down onto the biomass placed in the chamber. The opening in the receiving chamber leads to an afterburner chamber having a burner member which bums the volatile constituents in the gases from the receiving chamber. The afterburner chamber transfers the heat to ducts which occupy the space under the receiving chamber, a heat transfer chamber. 
     One problem with the foregoing processes is that firing the burner in the chamber can cause instability and turbulence leading to the emission of particulate and ash material. These materials may be emitted from the system as pollutants. Accordingly, there is a need for a pyrolysis process, wherein a flame is not introduced in the pyrolysis chamber to thermally decompose the waste. One object of the present invention is to provide such a pyrolysis process. 
     In addition, Brookes, U.S. Pat. No. 5,611,289 discloses a gasifier for gasifying biomass waste. The gasifier comprises a primary chamber for receiving the waste, a fume transfer vent, and a mixing chamber to accept the pyrolysis gases from the primary chamber. The fumes then flow to an afterburner chamber, where a burning flame oxidizes the constituents of the fumes. According to the &#39;289 Patent, a partitioning wall is disposed between the flame chamber and the primary chamber so as to preclude the heating flame from entering the chamber. A heat transfer chamber accepts the fully oxidized fumes, and heat from the fumes causes the heat transfer chamber to be heated. The primary chamber has a heat conductive floor and is superimposed on the heat transfer chamber. The heat from the heat transfer chamber rises through the floor to heat the primary chamber and biomass waste. 
     However, one disadvantage with the foregoing, conventional pyrolysis process is that transferring heat through the floor of the primary chamber is a relatively slow process. Thus, there is generally a long time period required for raising the temperature in the primary chamber and completing the pyrolysis reaction. This time-consuming process can be costly and inefficient. 
     Another disadvantage with the above-described conventional pyrolysis process is that depending upon the type of waste, it may not be possible to reach the required temperature in the primary chamber even if heat is applied through the floor for a long period of time. To overcome this limitation, the door to the primary chamber has a small air inlet allowing a small amount of air to enter the chamber. The introduction of air raises the temperature of the chamber by means of combustion of the waste material. Once combustion occurs, the process becomes exothermic and is no longer a pyrolysis process. 
     Further, the afterburner chamber is always in fluid communication with the heat transfer chamber and the hot gases always pass through the heat transfer chamber without control. Thus, heat is continuously transferred into the primary chamber so long as the auxiliary heat input burner in the afterburner chamber is burning. This results in two potential problems: 1) volatile organic compounds can be produced in the primary chamber before the afterburner chamber has reached the proper operating temperature resulting in incomplete combustion and emissions; and 2) highly volatile waste may evolve volatile organic compounds at such a high rate that the primary chamber will exceed acceptable temperatures, thus driving the volatilization rate ever faster to excess temperature limits and excess emissions. 
     In view of the foregoing problems with conventional pyrolysis processes, there is a need for a system, wherein the transfer of hot gases from the oxidation chamber to the pyrolysis chamber can be conducted in a controlled manner. If desired, the hot gases should be capable of being transferred to the pyrolysis chamber rapidly to heat the waste materials. One object of the present invention is to provide such a pyrolysis process. These and other objects, features, and advantages of the present invention are evident from the following description and figures. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a process for the pyrolysis of waste material, particularly medical waste. In general, the process comprises the following steps. The waste material is placed in a sealed pyrolysis container, and the container is inserted into a load chamber. The discharge port of the container is connected to a pyrolysis gas transfer duct so that the container is in fluid communication with an oxidation chamber. The discharge port should be connected to the pyrolysis gas transfer duct by a mechanical locking means to form a substantially air-tight seal. 
     The load chamber holding the pyrolysis container is heated so that heat is transferred into the container causing the waste materials to decompose and produce pyrolysis gases comprising volatile organic compounds. The pyrolysis gases flow from the pyrolysis container, through the pyrolysis gas transfer duct, and into the oxidation chamber. The pyrolysis gas transfer duct may contain an air inlet port for maintaining the pyrolysis container at a negative pressure and adding air flow for initial pyrolysis gas combustion at the inlet to the oxidation chamber. 
     The oxidation chamber includes a burner unit and at least one air inlet port for controlling air flow into the oxidation chamber. The burner unit is located in the upper portion of the oxidation chamber and produces a flame for preheating the oxidation chamber and maintaining the required temperature for combustion of the pyrolysis gases. The oxidation chamber typically comprises multiple air inlet ports. Particularly, the oxidation chamber may contain tangential air inlet ports for directing air tangentially into the chamber, and radial air inlet ports for directing air radially into the chamber. In the oxidation chamber, the pyrolysis gases are combusted and heat is produced. At least a portion of the heat produced in the oxidation chamber is directed through a hot gas transfer duct, and into the load chamber. 
     The hot gas transfer duct contains at least one hot gas control damper. A microprocessor may be used to control the hot gas control damper and regulate the amount of heat directed to the load chamber. The microprocessor may use an algorithm including a time/temperature profile, combustion air input rate, and burner input rate to determine the endpoint of the process. 
     Different pyrolysis containers may be used. In one embodiment, the container has an integrated structure comprising four sidewall panels, a base panel, a cover, and a discharge port. The container may be made from a high temperature-resistant metal alloy and include a high temperature-resistant gasket for sealing the cover. The container may be introduced into the load chamber by means of transport guide rails. In another embodiment, the pyrolysis container includes a rectangular-shaped recessed portion, wherein the recessed portion extends upwardly from the base panel to provide a core heating surface. Various other sealed pyrolysis containers having different geometries and designs may be used in accordance with this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features that are characteristic of the present invention are set forth in the appended claims. However, the preferred embodiments of the invention, together with further objects and attendant advantages, are best understood by reference to the following detailed description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a schematic diagram showing one embodiment of the pyrolysis process of the present invention; 
     FIG. 2 is a close-up perspective view of the oxidation chamber shown in FIG. 1; 
     FIG. 3 is a perspective view of one embodiment of a sealed pyrolysis container for use in accordance with the pyrolysis process of the present invention; 
     FIG. 4 is a close-up view of the lid region identified in FIG. 3 showing one embodiment of the lid closure mechanism of the present invention; and 
     FIG. 5 is a perspective view of another embodiment of a sealed pyrolysis container for use in accordance with the pyrolysis process of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to a process for the pyrolysis of waste materials particularly medical waste. 
     Referring to FIG. 1, a schematic illustration of the pyrolysis process of the present invention is generally illustrated. The entire system used to perform the destruction of the waste materials can be referred to as a pyrolysis unit and is generally indicated at  10  in FIG.  1 . In practice, the pyrolysis unit  10  can be first purged with air. Then, a burner unit  12  located in an oxidation chamber  14  can be ignited to produce a heating flame. A vertically disposed oxidation chamber  14  having an upper portion  16  and a lower portion  18  is shown. The burner unit  12  is situated in the upper portion  16  of the oxidation chamber  14  so that the flame is projected downwardly. The fuel source of the burner unit  12  is typically an industrial fuel such as propane gas or natural gas. Air (oxygen) is supplied to the burner unit  12  to support combustion of the fuel. Typically, ambient air is used to supply the oxygen, but any material containing a sufficient amount of oxygen can be used such as oxygen-enriched air. In one embodiment, preheated air generated from the heat of the pyrolysis process can be used to support combustion of the fuel. 
     The combustion air for pyrolysis gas destruction is supplied to the oxidation chamber  14  by means of at least one air inlet port. In a preferred embodiment, the oxidation chamber includes multiple air inlet ports  20  and  22  having the structures shown in FIG.  2 . Tangential air inlet ports  20  direct the air tangentially into the oxidation chamber, and radial air inlet ports  22  direct the air radially into the oxidation chamber. It has been found that air inlet ports having these designs are particularly advantageous, because the tangential air initiates combustion and protects the walls of the chamber in an area that can have the highest flame temperatures, and the radial air creates turbulent mixing that promotes good combustion without use of physical baffles in the chamber. 
     Referring back to FIG. 1, the hot gases initially generated in the oxidation chamber  14  by the combustion of the fuel and oxygen are vented to the atmosphere through an exhaust by-pass duct  24  containing a by-pass damper  26 . The temperature of the hot exhaust gases in the oxidation chamber  14  is measured by a temperature sensing element  28 . The temperature sensing element  28  is positioned so that it will measure the temperature of the hot exhaust gases at a point when the gases have been retained in the oxidation chamber  14  for the required time for efficient combustion. When the exhaust gases reach a pre-set temperature, a main hot gas control damper  30  in a hot gas transfer duct  32  starts to open and the by-pass damper  26  in the by-pass exhaust duct  24  starts to close. The opening of the main damper  30  diverts the hot gases to the hot gas transfer duct  32 , thereby allowing the hot gases to be transported from the oxidation chamber  14  to a load chamber  34 . 
     In this manner, the temperature within the load chamber  34  is controlled by the balance of hot gas flow between the load chamber  34  and the by-pass exhaust duct  24 . The hot gas flow is adjusted by the relationship of the positions of the main damper  30  and by-pass damper  26 . 
     The waste materials  36 , to be destroyed in the pyrolysis process, are placed in a sealed pyrolysis container  38  having a discharge connector port  39 . By the term, “waste material”, as used herein, it is meant any suitable product that can be subjected to the pyrolysis process including, but not limited to, controlled substances, pharmaceutical products, animal carcasses, chemicals, toxic substances, hazardous substances, biological agents, and medical waste. The process of the present invention is particularly suitable for destroying medical waste which includes, for example, biological cultures, human pathological tissue and organs, blood-borne products, vials, intravenous bags, needles, syringes, scalpel blades, sutures, swabs, bandages, dressings, and other hospital and infectious waste. The structure of the pyrolysis container  38  is described in further detail below. 
     As shown in FIG. 1, the sealed pyrolysis container  38  is inserted into the load chamber  34 . The pyrolysis container  38  may be inserted into the load chamber  34  by means of transport guide rails (not shown). A discharge connector port  39  is connected to a pyrolysis gas transfer duct  40  which leads to the oxidation chamber  14 . In this manner, the pyrolysis container  38  is placed in fluid communication with the oxidation chamber  14 . The pyrolysis container  38  is connected to the pyrolysis gas transfer duct  40  using any suitable mechanical fastening means. For example, the pyrolysis container  38  may be connected by a locking rod means. This locking mechanism exerts sufficient force to keep the pyrolysis container  38  and pyrolysis gas transfer duct  40  tightly connected to each other, thereby creating a substantially air-tight seal that is resistant to high temperatures. After the pyrolysis container.  38  has been placed in the load chamber  34 , the door  42  of the load chamber  34  is closed, thereby creating a tight seal that is resistant to high temperatures. 
     As described above, the heat and hot gases produced in the oxidation chamber  14  flow through the hot gas transfer duct  32  and enter the load chamber  34  so that high intensity heat is transferred into the waste materials  36  in the pyrolysis container  38 , and the waste materials  36  are thermally decomposed and transformed. Typically, pyrolysis of the waste materials begins at a temperature of about 450° F. At this temperature, the more volatile components of the waste start to gasify. In many instances, heat is continuously applied until the internal temperature of the pyrolysis container  38  is in the range of about 800° F. to about 1600° F., so that all of the organic components in the waste are gasified. The load chamber  34 , which holds the pyrolysis container  38 , is typically heated to a temperature in the range of about 1000° F. to about 1800° F. and held at a temperature necessary to ensure attainment of the desired temperature within the pyrolysis container  38 . After the hot gases in load chamber  34  have transferred heat into the pyrolysis container  38 , they are vented through load chamber exhaust duct  48 . 
     The pyrolysis gases produced in the pyrolysis container  38  contain volatile organic compounds and are vented through the pyrolysis gas transfer duct  40  to the oxidation chamber  14 . The pyrolysis gas transfer duct  40  includes an air inlet port  44 , where air is injected to cause a slight negative pressure in the transfer duct  40  by a venturi effect. This negative pressure helps prevent leakage of the pyrolysis gases from the pyrolysis container  38 . The pyrolysis gases containing the volatile organic compounds enter the oxidation chamber  14 , wherein the pyrolysis gases are combusted and the volatile organic compounds are substantially oxidized. The pyrolysis unit further includes an atmosphere exhaust vent  46  for venting the oxidized pyrolysis gases and hot gases into the atmosphere. 
     In one embodiment of the present invention, the pyrolysis unit  10  comprises multiple load chambers  34 . A pyrolysis container  38  holding waste material, as described above, is introduced into each load chamber  34 . Each pyrolysis container  38  is connected via a separate pyrolysis gas transfer duct  40  into a single oxidation chamber  14 . In this manner, multiple load chambers  34  can be integrated with one oxidation chamber  14 , and an efficient process can be maintained. 
     Conventional microprocessor controllers are used to program the process steps and issue the appropriate commands. More particularly, a microprocessor monitors the temperature and draft at several locations in the pyrolysis unit  10  and uses the data from sensors to adjust the amount of fuel being fed to the burner  12 ; the combustion air introduced into the oxidization chamber  14 ; the hot gas flow rate directed to the load chamber  34 ; the hot gas flow rate diverted to the by-pass exhaust duct  24 , and the negative draft pressure that moves the gases through the process. When the programmed sequence has been completed, the microprocessor switches the unit to a cool down mode by shutting down the burner  12  and directing cool air into the load chamber  34 . The microprocessor controls the energy balance in the system to prevent an uncontrolled thermal event from occurring in the oxidation chamber  14 . Further, the microprocessor controls an interlocking mechanism that prevents the door  42  to the load chamber  34  from being opened before the chamber  34  has been cooled to a pre-set temperature. 
     Referring to FIG. 3, the pyrolysis container  38  is shown in more detail. The pyrolysis container  38  has a box-like structure including four sidewall panels  48 ,  49 ,  50 , and  51 ; a bottom panel  52 ; and a removable cover (lid)  53 . The pyrolysis container  38  can be fabricated using any suitable high temperature resistant material such as a metal or ceramic. The material is thermally-conductive so that heat can be transferred to the interior of the pyrolysis container  38 . The pyrolysis container  38  has good mechanical strength so that it can hold a substantial amount of waste and be transported and handled easily. 
     As shown in FIG. 4, after the waste material to be processed is loaded, a sealing gasket(s)  54  is installed and the lid  53  is attached using high temperature compression hardware  56  or other suitable fasteners, thus forming a substantially air-tight seal. Then, the sealed pyrolysis container  38  can be inserted into the load chamber  34  using transport guide rails  58  (FIG.  3 ). As the pyrolysis container  38  is inserted into the load chamber  34 , the discharge connector port  39  engages and locks with the pyrolysis gas transfer duct  40  (FIG.  1 ). This locking mechanism holds the pyrolysis container  38  in place and creates a tight seal between the container and pyrolysis gas transfer duct  40 . In this manner, the pyrolysis container  38  is placed in direct fluid communication with the oxidation chamber  14 . 
     The pyrolysis container  38  may have a box-like structure as shown in FIG.  5 . In this embodiment, the bottom panel  52  of the container  38  includes a rectangular-shaped recessed portion  60 . This concave area  60  extends upwardly from the bottom panel  52  to provide a core heating surface. This channel-like portion  60  increases the amount of surface area of the container  38 , allowing more heat to be transferred into the container  38 . In accordance with the present invention, the pyrolysis container may have other geometries and designs to further enhance heat transfer performance or improve the placement of specific waste types. In addition, other heat conductive elements, such as conductive rods or high temperature heat pipes, may be connected to the outside walls and project into the pyrolysis container space to improve the heat transfer rate to areas within the load. 
     The placing of the waste material in the pyrolysis container and inserting of the container in the load chamber provides several advantages over conventional systems as discussed above, wherein the waste is placed directly in a pyrolysis furnace or chamber. These advantages include, but are not limited to, the following: (1) all surfaces of the pyrolysis container are exposed to the hot gases; (2) air leakage into the pyrolysis container is controlled; (3) the hot pyrolysis container can be quickly removed by mechanical means and replaced with a new pyrolysis container holding the waste materials to be destroyed, thereby maintaining a hot load chamber, conserving fuel which would have been consumed while heating the load chamber, and greatly increasing efficiency of the pyrolysis process; and (4) the residue produced from the pyrolysis process is contained for easy handling to a disposal point. 
     It is appreciated by those skilled in the art that various changes and modifications can be made to the description and illustrated embodiments herein without departing from the spirit of the present invention. All such modifications and changes are intended to be covered by the appended claims.