System and method of processing solid waste

A system and associated method of processing solid waste via a waste gasification process results in usable by-products. The method utilizes a rotary kiln heated to a temperature of at least 800° F. and a reduction chamber, heated to a temperature of at least 1800° F. The solid waste is slowly rotated in the kiln for six to eight hours. The solid material is passed through screens to separate the ash from other items. Gases are monitored and transported to the reduction chamber to generate power. The gases are then transported to at least one air pollution control unit to remove contaminants before being vented into the atmosphere.

FIELD OF THE INVENTION

This invention relates to the disposal of solid waste and, more particularly, to a system and associated method of processing such waste into usable by-products in an environmentally friendly manner.

BACKGROUND OF THE INVENTION

Municipal solid waste is commonly incinerated in a combustion process at high temperatures such as 1600 degrees Fahrenheit. This incineration process seeks to destroy the waste by burning it, usually at high temperatures with excess air. The ultimate purpose of the process is to burn as much waste as possible as quickly as possible. One potential problem with such incineration is the emissions of the incinerator may contain toxic and other unwanted pollutants dangerous to human health and the environment. Another potential problem with conventional incineration is that the process destructs all the municipal solid waste and does not recycle any of it. Another problem with incinerating municipal solid waste is that the resultant ash must be sent to a particular type of landfill subject to restrictive environmental regulations.

Therefore, there is a need in the industry for a process of treating municipal solid waste in an environmentally friendly manner which uses all of the residual by-products of the process.

There is also a need for a process of treating municipal solid waste in an environmentally friendly manner which is capable of generating electricity.

SUMMARY OF THE INVENTION

The method of the present invention in one embodiment comprises a method of processing solid waste in an environmentally friendly manner using waste gasification. The waste gasification process of the present invention gasifies and reduces solid waste, most often municipal solid waste. Waste gasification converts the incoming solid waste or feedstock into combustible gases such as carbon monoxide, methane and hydrogen which contain the energy originally present in the feedstock. This waste gasification process degrades the feedstock in a rotary kiln in the absence of adequate air to support full combustion. The gases generated in the rotary kiln are not burned in the kiln, but rather transported to a reduction chamber where they may be burned to generate steam for purposes of generating electricity. The temperature inside the rotary kiln is at least 800 degrees Fahrenheit and preferably between 800-1000 degrees Fahrenheit to carry out this waste gasification process.

The waste gasification process converts the feedstock waste materials such as municipal solid waste, tires, coal gob, wood construction debris and/or lawn waste into a BTU-rich gaseous fuel. This fuel may be used “on site” as a non-fossil fuel source for various industrial processes, such as the production of hot water or steam-powering internal combustion engines as examples. This process combusts the waste in a “starved air” combuster, which degrades the municipal solid waste with less oxygen, thereby improving the quality of the resultant air emissions. This process, like the incineration process, vents the flue gases to the atmosphere after they have been sufficiently cleaned.

Nothing generated by the waste gasification process of the present invention needs to be placed in a landfill. One hundred percent of the incoming waste stream is either recovered as an alternative energy source, recycled or otherwise utilized. All of the ferrous and non-ferrous metals, aluminum and glass are recovered at the end of the process. The only other product remaining is a fine, mineral residual (primarily glass containing less than five percent carbon) that is an acceptable concrete additive.

The method comprises transporting the waste to a receiving hopper where it is temporarily stored. A rotary kiln is located downstream of the receiving hopper. The municipal solid waste or other feedstock is fed into the rotary kiln via a hydraulic ram preferably. However, any other means of introducing or feeding the municipal solid waste into the rotary kiln may be used.

The rotary kiln is heated to a temperature of at least 800 degrees Fahrenheit and preferably between 800-1000 degrees Fahrenheit. A driver such as a motorized drive system rotates the rotary kiln at a rate of approximately one revolution per minute. However, the driver may rotate the rotary kiln at any desired speed.

After the kiln has been rotated for six to eight hours preferably, gases are transported to a reduction chamber. The reduction chamber functions as a destruction mechanism for the elimination or reduction of volatile organic compounds (VOCs), hydrocarbons, dioxins, furans and various other gases and compounds. The reduction chamber is heated to a temperature of at least 1800 degrees Fahrenheit before the process begins. The reduction chamber is maintained at a temperature of between 1800 and 2400 degrees Fahrenheit by at least one gas burner. However, any other means of heating the reduction chamber may be used as well without departing from the spirit of this invention.

To maintain the temperature of the gases exiting the rotary kiln and going to the reduction chamber at a temperature of 800-1200 degrees Fahrenheit, at least one of the following may be adjusted by a programable logic controller: the feed rate of the waste going into the rotary kiln; the rotation speed of the rotary kiln; and/or the amount of air flow into the interior of the rotary kiln. The gases are retained or held in the reduction chamber for at least one second.

Upon exiting the reduction chamber the gases are cooled by a spray of water in a duct to reduce the temperature of the gases to approximately 500 degrees Fahrenheit. The gases are then passed through at least one air pollution control unit in which contaminants such as SO2, NOx, hydrogen chloride, mercury, dioxins and furans, along with particulate matter are removed. In the present invention there are preferably three air pollution control units: a reduction chamber, a spray dryer absorber and a baghouse. However, there may be more or fewer air pollution control units incorporated into the system.

In the spray dryer absorber, the gases are treated to remove any remaining acids and metals that might be present. A slurry of lime and activated carbon is injected into the top of the spray dryer absorber unit for this treatment. Spent carbon and other particulate matter that may be generated at the spray dryer absorber are collected at the bottom of the spray dryer absorber and transferred via a conveyor to a fly ash bin.

From the spray dryer absorber the gases are ducted to a multi chamber baghouse for final removal of any remaining contaminates. Although the baghouse preferably has four chambers, it may contain any number of chambers in accordance with the present invention. In the baghouse, the gases pass through filters to remove particulate matter. Particulates including ash generated in the baghouse are collected and transported on a conveyor belt to the fly ash bin.

From the baghouse the gases are discharged into the atmosphere and continuous emission monitors record the air quality of the emissions.

Thus, with the present invention all of the solid waste is used or recycled in an environmentally-friendly manner. These and other objects and advantages of the present invention will be evident from the following detailed description and the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to the drawings, and particularly toFIG. 1, there is illustrated one embodiment of a solid waste gasification facility or system10designed to process solid waste into usable by-products. Although the present invention preferably processes municipal solid waste, it may be used to process any solid waste. Municipal solid waste12is transported via trucks14into a building16. The building16protects the municipal solid waste from the elements and may be any desired size, configuration or construction. Although trucks are illustrated, any other means of transporting municipal solid waste to the building16may be used, such as rail, for example. Once inside the building16, the municipal solid waste12is moved into an enclosed receiving hopper18. Although one configuration and location of receiving hopper18is illustrated, the receiving hopper may assume numerous other configurations and may be located at other locations such as outside the building16.

Preferably, the building16is large enough (100 feet by 70 feet) to accommodate approximately 800 tons of municipal solid waste so that if a need arises to store a large quantity of waste, the facility will be able to handle it. Inside the building16, the municipal solid waste12is preferably transferred from the trucks14via one or more conveyors (not shown) to the receiving hopper18. The building16preferably has a reinforced concrete floor. Although the conveyor and receiving hopper18are preferably enclosed to keep the municipal solid waste12from getting wet from precipitation, they need not necessarily be so covered.

The receiving hopper18is positioned directly above a feeder20in the form of a hydraulic ram which pushes the municipal solid waste into a rotary kiln22, shown in greater detail in FIG.2. The hydraulic ram20is preferably able to push up to 42 tons of municipal solid waste into the rotary kiln22in a 24-hour period. However, any size ram may be used. The hydraulic ram20may assume the form of augers or any other suitable structure. Any other means of pushing the municipal solid waste12into the rotary kiln22may be used in accordance with the present invention.

As best shown inFIG. 2, the rotary kiln22has an outer shell24which is generally cylindrical in shape. It is preferably 120 feet in length. It preferably has an inner diameter of nine feet inside its shell24and an outer diameter of 10 feet. It rotates about a rotary axis A which is positioned with a 1° slope relative to horizontal. The rotary kiln22is preferably refractory lined and sealed at both ends with seal systems25. The outer shell24and seal systems25define the interior21of the rotary kiln. However, any size or configuration of rotary kiln may be used in accordance with the present invention.

As best shown inFIG. 2, the rotary kiln22is positioned on trunion frames26installed on reinforced concrete28. The rotary kiln22has a feed end30, an exit end32, and an exit chute34at exit end32. The rotary kiln22further has exit gas duct36located on the high or feed end30of the kiln22.

Each end30,32of the rotary kiln22has a double seal system25which comprises a plurality of spring steel overlapping leafs (not shown) attached to a stationary hood (not shown). The purpose of the overlapping leaf seal is to keep any air entrained particulate (dust) from escaping the rotary kiln. The second component of each double seal25is a high temperature fabric belt (not shown) that keeps outside air from entering the interior of the rotary kiln22. This fabric belt is secured to the stationary hood and fits tightly against a wear band (not shown) attached to the outer shell24of the rotary kiln22.

The rotary kiln22is driven by a drive means3including a motor4which rotates a drive shaft5which turns a first gear6. The first gear6engages a second gear7. Rotation of the first gear6causes the second gear7, which extends around the shell24of the rotary kiln24, to rotate. The second gear7is preferably welded to the shell24of the rotary kiln22but may be secured any other way. The rotary kiln22preferably rotates at a rate of one revolution per minute, but may rotate at any desired speed. Any other drive means may be used to rotate the kiln22.

The rotary kiln22is heated by a heating system35including burners37located at the feed and exit ends30,32of the kiln22and fuel lines38leading from one or more natural fuel supplies39to burners37. Although natural gas is the fuel of choice, any other fuel other than natural gas such as propane may be used in accordance with the present invention. Although the burners37, fuel lines38and fuel supply39are illustrated in certain locations, they may be located elsewhere without departing from the present invention. The burners37may be any type of burners. However, one burner known to work in accordance with the present invention is manufactured by Maxon Corporation (www.maxoncorp.com), generates 18 million BTUs and sold as a model 18 M Maxon KINEDIZER®.

Activation of the heating system35allows the operating temperature of the interior21of the rotary kiln22to be at least 800 degrees Fahrenheit and preferably between 800-1000 degrees Fahrenheit. Once the appropriate temperature has been reached in the interior21of the rotary kiln22, the flame will be extinguished in the burners37, and the municipal solid waste12inside the interior21of the rotary kiln22is able to sustain the temperature during the rest of the processing time. The high temperature inside the rotary kiln22vaporizes all moisture that is part of the municipal solid waste12. After the burners37are turned off, the municipal solid waste12serves as its own fuel source inside the interior21of the rotary kiln22.

As seen inFIG. 1, duct36extends from the upper portion of the feed end30of the rotary kiln22into a reduction chamber40. The duct36is preferably circular in cross section having a diameter of five feet. However, any other size or configuration of duct may be used in accordance with the present invention. A temperature probe42is located in the duct36. The temperature probe42is connected to a combustion control system2, shown in FIG.3.

The combustion control system2, shown inFIG. 3, includes a programable logic controller2a. The combustion control system2may add or reduce airflow into the rotary kiln22and/or the reduction chamber40, change the speed of rotation of the rotary kiln22, or change the feed rate of municipal solid waste12into the rotary kiln22.

The reduction chamber (“R.C.”)40is placed on a reinforced concrete pad (not shown) and receives gases from the rotary kiln22. The reduction chamber40is preferably cylindrical in configuration having a length of 45 feet and a diameter of 11 feet. Inside the reduction chamber40are three ceramic baffles41, each in the shape of a semi-circle. Each baffle41is constructed of firebrick and covers approximately half of the interior of the reduction chamber40. The baffles41are offset from one another to increase the retention time of the gases inside the reduction chamber40. The baffles41, once heated, retain heat and help ensure complete combustion of all volatile organic compounds (VOCs), hydrocarbons, and other pollutants. Although three baffles are illustrated and described, any number of baffles of any configuration may be incorporated into the reduction chamber. Any other configuration or size of reduction chamber may be used in accordance with the present invention.

The gases passing through the duct36to the reduction chamber40from the rotary kiln22and measured at temperature probe42are preferably at 800 degrees Fahrenheit. A heating system49including a start-up burner50, a fuel supply (“FS”)8which is preferably natural gas and a gas supply line52extending from the fuel supply to the start-up burner50is located at the first end48of the reduction chamber40. The burner50may be any type of burner. However, one burner known to work in accordance with the present invention is manufactured by Maxon Corporation (www.maxoncorp.com), generates 9 million BTUs and sold as a model 9 M Maxon KINEDIZER®. The temperature in the reduction chamber40is maintained at a minimum of 1800 degrees Fahrenheit and preferably between 1800 to 2400 degrees Fahrenheit. Therefore, the gases exiting the reduction chamber are preferably at least at 1800 degrees Fahrenheit. Once the gases brought to the reduction chamber40via duct36are self-sustaining, the start-up burner50may be shut off with the gases able to maintain the temperature. The gases are retained in the reduction chamber40for at least one second.

Duct56extends from a second or exit end58of the reduction chamber40to air pollution control equipment which includes two units: a spray dryer absorber60and a baghouse62. The duct56is preferably circular in cross section having a diameter of five feet. However, any other size or configuration of duct may be used in accordance with the present invention. The duct56has a series of misting nozzles64and a series of temperature probes66to control and monitor the temperature of the gases going from the reduction chamber40to the spray dryer absorber60. The temperature of gases going into the spray dryer absorber60is approximately 500° F.

The spray dryer absorber (“SDA”)60receives the gases from the reduction chamber40. The purpose of the spray dryer absorber60is to treat the gases for any remaining metals and acids that may be present. An eighty ton lime silo (not shown) and a thirty ton carbon silo (not shown) are installed on site. Any other size or storage means may be used to store those items. A holding tank68contains a lime slurry70stirred by a power driven stirrer72. The lime slurry70is pumped via pump74through a line76to a head tank78. Carbon is introduced into the top of the head tank78in the direction of arrow80to create a mixed slurry82in the head tank78. An overflow line84extends from the head tank78to another holding tank86. The mixed slurry82is stirred by a power driven stirrer83and is pumped via pump88through a line90to the head tank78. The mixed slurry82is passed through a line91and sprayed into an upper portion92of the spray dryer absorber60. The mixed slurry82reduces the temperature of the gases and removes SO2, NOxand certain metals from the gases. The spray dryer absorber60has a lower portion94having an exit96through which passes spent carbon and other particulate matter generated in the spray dryer absorber60. The spent carbon and other particulate matter generated in the spray dryer absorber60after having passed through the exit96are carried via an endless conveyor98to a fly ash bin100.

The fly ash bin100has an exit101through which the fly ash passes. The fly ash then is carried via endless conveyors102,103to a final ash bin104. Ash in the final ash bin may be used in concrete as filler.

The gases are ducted from the spray dryer absorber60via duct106to the baghouse62. The baghouse62has four chambers107, each chamber107having a filter108. A temperature probe109is inserted into the duct106to monitor the temperature of the gases going to the baghouse62. The temperature of the gases exiting the spray dryer absorber before going to the baghouse62is preferably 290 degrees Fahrenheit. If this temperature reaches 350 degrees Fahrenheit, the combustion control system2may shut down the operation of the facility10. Exits110are located at the bottom of the baghouse62. Ash generated in the baghouse62passes through these exits110onto an endless conveyor112which carries it to the fly ash bin100.

From the baghouse62, gases are pulled by an induced draft (“ID”) fan126through a duct112. After the gases pass through the fan126they are released to the atmosphere through an exit stack114. Gases exiting the exit stack114are preferably at 260 degrees Fahrenheit but may be at any other desired temperature.

The combustion control system2is operational such that if any of the temperature probes detect a temperature above/below a predetermined value, the operation of the facility10will be terminated or shut down. For example, no waste will be introduced into the rotary kiln22until the temperature in the kiln22is at least 800 degrees Fahrenheit. If the gases entering the baghouse62are above 350 degrees Fahrenheit, the combustion control system will cut off or shut down the facility10. In addition, if an operator observes deviations from the operating standards, the operator may manually shut down the operation of the facility10.

Referring toFIG. 2, byproducts generated in the rotary kiln22other than gases pass through out the exit or discharge chute34into a holding hopper116. The holding hopper116has an exit118through which the byproducts pass onto a conveyor120(see FIG.1). At the end of the conveyor120is an inclined vibrating screen122. Although only one inclined vibrating screen122is shown, more than one screen may be used in accordance with the present invention. The mesh of the screen122is such that ash may pass therethrough but larger particles such as pieces of metal, glass or aluminum may not pass through the screen(s). Below the screen122is conveyor103which takes the ash which has passed through the screen or screens to the final ashbin104. The larger pieces of byproduct or nonash materials caught on top of the screen122are passed along another conveyor124past a magnet126in order to separate the metals from the non-metals. The metals are taken to a recycling facility. The non-metallic materials are pulverized and may be added to the ash for use in an on-site concrete plant.

In use, the method of processing municipal solid waste using the facility10described above includes heating the interior21of the rotary kiln22to a temperature of at least 800 degrees Fahrenheit and heating the reduction chamber to a temperature of at least 1,800 degrees Fahrenheit. Untreated municipal solid waste is fed into the rotary kiln22via the feed ram20. The kiln22is rotated at approximately one revolution per minute by the drive means3.

After the waste has been in the interior21of the rotary kiln22for a predetermined time period, preferably six to eight hours, the processed municipal solid waste, residual byproduct, or residual solids are removed from the interior21of the rotary kiln22via discharge or exit chute34. These residual byproducts are transported to a holding hopper116for a time before being transported via a conveyor120to the inclined vibrating screen122. The residual byproducts are then separated into components; the ash passing through the vibrating screen122and the other residual byproducts passing along on top of the vibrating screen122to another conveyor124. The metals are then separated from the non-metals as the residual byproducts pass along endless conveyor124. The metals are recycled. The non-metals are pulverized and used as filler in concrete.

The gases generated in the interior21of the rotary kiln22are transported to the reduction chamber40. In the reduction chamber40these gases are held for at least one second while they are burned. This process may be used to generate hot water or steam which may be used to generate power.

The gases exiting the reduction chamber40are cooled in duct56with water sprayed from misting nozzles64while being transported to the spray dryer absorber60. The gases are treated to remove contaminants in the spray dryer absorber60by exposing them to a lime and carbon water mist. Particulate generated in the spray dryer absorber60is passed via a conveyor98to an ashbin100where it is collected for disposal. The gases are then passed from the spray dryer absorber60to the baghouse62via duct106. In the baghouse62the gases are further treated to remove any remaining contaminants. Particulate generated in the baghouse62is passed via an endless conveyor112to ashbin100where it is collected for disposal. The gases are then discharged to the atmosphere via a stack114.

While we have described one preferred embodiment of the present invention, those skilled in the art will appreciate changes and modifications which may be made to the present invention without departing from the scope of the present invention. For example, the baghouse may have more or less than four chambers or more than one screen may be used to separate the solids generated in the rotary kiln. Therefore, we intend to be limited only by the scope of the following claims.