Waste energy conversion system

An on-site modular method and system for processing waste typically including food from a food services facility. The waste is processed in a size reduction unit and subsequently dried in a dryer. The dried shredded waste is processed to produce densified pellets. The food binds the non-food waste in the pellets. The pellets are burned in a thermal conversion unit to produce a gas and waste heat. The waste heat is directed to the dryer for use therein. The gas is fed to a generator or turbine to produce electricity. A portion of the electricity produced is used to energize the process.

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Description
RELATED APPLICATIONS

The subject invention claims the benefit of and priority to U.S. Provisional Application No. 60/998,153, filed Oct. 9, 2007 which is incorporated herein by reference.

GOVERNMENT RIGHTS

Certain aspects of the subject invention resulted, at least in part, from U.S. Government funding under U.S. Army Contract Nos. W911NF-06-C-0035; W911NF-06-C-0090; W56 HZV-05-C-0012; W56 HZV-05-C-0661; and W91B9472000385. The U.S. Government may have certain rights in the subject invention.

FIELD OF THE INVENTION

This subject invention relates to waste-to-energy conversion systems.

BACKGROUND OF THE INVENTION

Municipal waste is typically collected and transported to a landfill. Given the shortage of landfill sites, some effort has been made to recycle certain components of the waste. The waste is also sometimes burned. The waste is also sometimes thermally decomposed to produce electricity and/or generate heat. U.S. Pat. No. 7,252,691 incorporated herein by this reference discusses waste-to-energy systems and the numerous problems associated therewith. Typically, the waste is shredded, dried and incinerated. Sometimes, pellets or briquettes are formed which can be used as fuel in a boiler which produces steam used to drive a steam turbine which in turn drives an electrical generator.

Most waste-to-energy conversion systems are large scale and physically located at a landfill or waste treatment plant. Sorting of the waste to produce viable fuel pellets is a major problem since municipal waste includes metal, wood products, chemicals, glass, and a variety of other materials. Often, the waste-to-energy system is not cost-effective due to the sorting required and the energy draw of the various components of the system. Toxic emissions may be produced and the burning process generates undesirable ash which may be hazardous. The preceding is merely a summary of some of the many problems encountered in engineering and implementing a viable waste-to-energy conversion system.

Extensive work has been done on the development of waste-to-energy systems using densified or pelletized waste made from refuse-derived-fuel (RDF) to produce a feedstock that can be efficiently combusted in boilers, kilns, fluidized bed reactors, pyrolysis and gasification systems. Densified RDF had its beginnings with the densification of wood in 1880. Extensive work in Great Britain and the United States was carried out in the 1970's and 1980's which highlighted the problems associated with the production of densified RDF from municipal solid waste, particularly related to the pellet mill. Production rates near the rated capacity of the pellet mill were not achievable. Oversized pieces and stringy components of the shredded waste frequently jammed the mill and dies while press wheels wore out at faster rates than desired. Multiple shredding steps were necessary to reduce the size of the feedstock prior to pelletization. The size distribution of the pellets was not a crucial factor when the pellets were burned in boilers, but is important in downdraft gasification processes. Waste plastic film in the feedstock caused fracture of the pellets into shorter lengths. Physical degradation of the pellets to smaller pellets and fine particles as a result of conventional handling methods was a common problem. The addition of various inorganic and organic binders to the shredded waste produces more dense pellets with increased mechanical strength. Present day downdraft gasifiers require pellet diameters less than ½ inch, and typically ¼ inch. In addition, food waste and RDF has not been densified into fuel pellets because of the adverse effects of high moisture content on the integrity of the fuel pellets. The subject invention overcomes the obstacles associated with past methods of producing fuel pellets from RDF.

Other prior art includes U.S. Pat. No. 4,026,678 which discloses a process for producing a pelletized fuel from non-sewage sludge collected by municipalities. U.S. Pat. No. 4,445,906 discloses briquettes produced from waste products. U.S. Pat. No. 4,496,365 also discloses a method of producing fuel briquettes from waste products. U.S. Pat. No. 3,790,091 discloses how paper and plastic may be pelletized as a fuel. U.S. Pat. No. 3,910,775 discloses the production of briquettes from waste products. U.S. Pat. No. 4,225,457 discloses briquettes made of waste and crushed caking coal including coal fines. U.S. Pat. No. 4,561,860 discloses the production of pellets made from waste products. U.S. Pat. No. 6,506,223 discloses a fuel pellet produced by the combination of waste material with a binder.

U.S. Pat. No. 5,431,702 discloses the production of fuel pellets or briquettes from sewage sludge solids and municipal wastes. U.S. Pat. No. 5,562,743 discloses pellets manufactured from waste products. U.S. Pat. No. 4,859,211 discloses a system for the treatment of community waste products. Harvey Alter in the book Material Recovery from Municipal Waste, Unit Operations in Practice (Marcel Dekker, Inc., N.Y.) includes chapter 8 entitled “Refuse-Derived Fuel and Densified Refuse-Derived Fuel.” All of these references are incorporated herein by this reference.

Despite the numerous prior art regarding waste-to-energy conversion systems, the problems noted above remain. Chapter 8 of the above referenced book concludes that commercial experiences with waste-to-energy systems “have not been good.”

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a viable waste processing system and method.

It is a further object of this invention to provide such a waste processing system and method which is modular and which thus can be used on-site where the waste is generated.

It is a further object of this invention to provide such a waste processing system and method which reduces the costs associated with waste disposal.

It is a further object of this invention to provide such a waste processing system and method which is self-powered while operating.

It is a further object of this invention to provide such a waste processing system and method which generates electricity and, optionally, heat.

It is a further object of this invention to provide such a waste processing system and method which produces high quality fuel pellets in a cost efficient manner.

It is a further object of this invention to provide such a processing system and method which is not an environmental burden.

It is a further object of this invention to provide such a waste processing method which does not require an inordinate amount of sorting operations.

It is a further object of this invention to provide such a waste processing system and method which does not produce toxic emissions.

It is a further object of this invention to provide such a waste processing system and method which does not produce any hazardous ash.

The subject invention results from the realization, at least in part, that food service facilities such as restaurants, university and prison cafeterias, and food service facilities at sporting and consumer events generate large quantities of waste food which can be combined with paper and plastic waste (metal and glass is often presorted by the facility itself) to produce fuel pellets of an ideal composition for generating heat and electricity used by the on-site waste-to-energy system itself and also used by the facility resulting in a self powered, cost effective system.

This subject invention features a modular on-site method of processing RDF waste including food from a food services facility. The preferred method comprises shredding the waste in a shredder, drying the shredded waste in a dryer, and using a pelletizer to pelletize the dried shredded waste. The food binds the non-food waste in pellets. The pellets are burned in a gasifier to produce a gas and waste heat. The waste heat is directed to the dryer for use therein to dry the shredded waste. The gas is combusted in a generator to produce electricity. A portion of the electricity produced is used to energize the shredder, the dryer, the pelletizer, and the gasifier.

The pelletizer preferably includes a pre-compaction auger and the method further includes pre-compacting the dried shredded waste prior to pelletizing. Typically, the shredder, the dryer, the pelletizer, the gasifier, and the generator are linked together in a modular unit transported to the food services facility. A portion of the generated electricity may be used to energize loads of the food services facility. Also, a portion of the generated electricity can be sold. The shredder is preferably configured to shred the waste into pieces between ⅜″ and ½″. The shredded waste is preferably dried to a moisture content of between 12-15 percent. The preferred pellets are between 18 and ½ inches in diameter and between ¼ and ⅝ inches long.

An on-site food services facility waste processing system in accordance with the subject invention features a generator configured to combust gas to produce electricity. A gasifier produces gas for the generator and waste heat. A shredder is powered by the generator for shredding the waste including food from the food services facility. A dryer uses the gasifier waste heat to dry the shredded waste. A pelletizer is powered by the generator for producing pellets of the dried shredded waste with the food binding the non-food waste. The pellets are burned in the gasifier to produce gas as a fuel for the generator. An on-site method of processing waste from a facility includes feeding the waste to a shredder on-site at the facility, drying the shredded waste in a dryer, using a pelletizer to pelletize the dried shredded waste, burning the pellets in a gasifier to produce a gas and waste heat, directing the waste heat to the dryer for use therein to dry the shredded waste, burning the gas in a generator to produce electricity, and using a portion of the produced electricity to energize at least one of the shredder, the dryer, the pelletizer, and the gasifier.

In one example, waste is shredded in a shredder on-site at the facility. The shredded waste is dried in a dryer on-site at the facility. A pelletizer on-site at the facility pelletizes the dried shredded waste. The subject invention also features a method of processing waste from a facility including shredding the waste in a shredder, drying the shredded waste in a dryer, using a densifier to densify the dried shredded waste, burning the densified waste to produce gas and waste heat, directing the waste heat to the dryer for use therein to dry the shredded waste, using the gas to produce electricity, and using a portion of the electricity to energize at least one of the shredder, the dryer, the pelletizer, and the gasifier.

The densifier typically includes a pelletizer with a pre-compaction auger and the method further includes pre-compacting the dried shredded waste prior to pelletizing.

An on-site facility waste processing system in accordance with the subject invention features a shredder for shredding waste from the food services facility, a dryer for drying the shredded waste, a densifier for densifying the dried shredded waste, and means for burning the densified waste to produce electricity used by at least one of the shredder, the dryer, and the densifier.

The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:

FIG. 1 is a highly schematic conceptual three-dimensional view showing a food services facility with an on-site waste processing system in accordance with the subject invention;

FIG. 2 is a schematic block diagram showing the primary components associated with an example of a waste processing system in accordance with the subject invention;

FIG. 3 is a schematic cross-sectional view showing an example of a shredder useful in connection with a waste processing system in accordance with the subject invention;

FIG. 4 is a schematic view of the cutting rotor of the shredder shown in FIG. 3;

FIG. 5 is a schematic depiction of the uniform shredded waste produced by the shredder shown in FIG. 3;

FIG. 6 is a plot showing the electrical power consumption over time of the shredder shown in FIG. 3;

FIG. 7 is a schematic cross-sectional view showing an example of a pelletizer useful in the waste processing system shown in FIG. 2;

FIG. 8 is a schematic partially cross-sectional side view of another example of a palletizer useful in connection with the waste processing system of the subject invention;

FIG. 9 is a schematic partially cross-sectional end view of the pelletizer shown in FIG. 8;

FIG. 10 is a schematic top view of a pre-compacting auger useful in connection with the pelletizer shown in FIGS. 8-9;

FIG. 11 is a schematic three-dimensional top view showing a number of waste pellets produced from the shredded waste shown in FIG. 5 in accordance with the subject invention;

FIG. 12 is a plot showing the electrical draw of the pelletizer component of the waste processing system of the subject invention;

FIG. 13 is a schematic three-dimensional side view of a gasifier useful for processing waste in accordance with the subject invention; and

FIG. 14 is a plot showing gasifier temperature as a function of different fuel.

 DETAILED DESCRIPTION OF THE INVENTION

Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer.

FIG. 1 schematically shows food services facility 10 (e.g. a university cafeteria, a restaurant, a military encampment or the like) and modular on-site waste processing system 12 in accordance with an example of the subject invention. The prototype system 12 occupies a footprint of 8 feet by 20 feet and can be housed in a transportable container 8 feet high.

The typical waste from food services facility 10 includes food, paper, and plastic. Often, glass, and metal waste is presorted by the facility itself for recycling. Thus, in one example, little if any sorting operations are required for the operation of system 12. System 12 utilizes the waste including food products to generate electricity (and optionally heat) a portion of which is used by the system itself. The preferred system is thus self-powered except during start-up. Excess electricity and heat can be used by facility 10, by another facility, or can be sold. In this way, over time, the system pays for itself. The facility also benefits in that its waste need not be collected and transported to a landfill thus reducing the cost of operating facility 10. Instead, waste from facility 10 is directed to system 12 typically located directly adjacent to facility 10. And, facility 10 does not contribute to the landfill problem resulting in an environmentally friendly facility.

FIG. 2 depicts the primary components associated with one preferred system in accordance with the subject invention. Facility waste 20 is fed directly to shredder 22. As used herein, shredding includes chopping and/or grinding operations as well. The shredded waste is then fed to dryer 24 where heat is used to dry the shredded waste to a desired moisture content. The dried shredded waste is then densified in a densifier such as pelletizer 26. The food waste binds the non-food waste (e.g., paper and plastic) in the pellets. The pellets (or briquettes) are then burned in gasifier 28 producing a synthesis gas which is used as a fuel in a generator 30 configured to operate on such fuel to produce electricity.

Other means of thermally-converting the densified waste to produce electricity are possible. The subjection invention is thus directed to methods and/or system for densifying or pelletizing waste including Refuse-Derived Fuel (RDF) and food waste to produce a feed stock that can be efficiently combusted in boilers, kilns, fluidized bed reactors, pyrolysis and gasification systems, and/or other suitable thermal decomposition devices and systems to produce heat, electricity, liquid fuels, and/or gaseous fuels.

Preferably, gasifier 28 produces waste heat as shown at 32 which is piped to dryer 24 for use therein to dry the shredded waste. Waste heat (e.g., exhaust) from generator 30 as shown at 34 may also be utilized in the drying process. Excess heat as shown at 36 may be used by facility 10, FIG. 1 or may be used elsewhere or sold. The electricity generated by generator 30 as shown at 38a, 38b, 38c, and 38d is preferably used to power shredder 22, dryer 24, gasifier 28, and pelletizer 26, respectively, and typically meets the full energy demand of all of these components resulting in a self-powered system.

Conveyors, chutes, and other conveyance means can be used to transport the waste 20 to shredder 22, to transport the shredded waste to dryer 24, to transport the dried shredded waste to pelletizer 26, and to transport the pellets to gasifier 28. In so far as any of these transportation mechanisms require electricity, electricity from generator 30 can be used to power these subsystems as well. Excess electricity as shown at 40 can be used by facility 10, FIG. 1 or elsewhere or can be sold.

Food service facilities which typically generate large quantities of waste food and which typically also presort certain undesirable waste products enables the combination of the waste food with the paper and plastic waste in a densified pellet or briquette wherein the waste food binds the paper and plastic in the pellet. These unique fuel pellets are of an ideal composition for generating heat and electricity on-site at the facility. The system itself is self-powered and runs using the heat and electricity the system generates. The result, unlike the prior art, is a viable waste processing system and method. The system is modular and thus can be used on-site at a food services or other facility generating suitable waste. The cost of disposing of the waste is severely reduced. High quality fuel pellets are produced in a cost efficient manner. The facility is rendered environmentally friendly. Little, if any, sorting operations are typically required. Toxic emissions are reduced as is the amount of ash.

In one embodiment, the composition of the waste was 38-45 percent paper, 8-12 percent plastic, 14-40 percent food waste and 10-33 percent scrap material including metal and glass. Other suitable compositions may be employed as the present invention is not limited in this respect. In one embodiment, the moisture content of the waste is 20 to 25 percent although other suitable moisture contents may be employed. In one embodiment, the scrap material is manually or mechanically separated out from the remainder of solid waste. In one illustrative embodiment, the system handles the waste generated by a facility serving 550 persons generating approximately one ton per day of waste over an approximately 16 hour period.

The typical process begins with the introduction of waste that is fed manually or via a conveyor to shredder 22. The preferred shredder has been designed to produce a consistent size distribution of the shredded waste, for example, ½ inch or less. The shredder cuts through thin plastic film in the solid waste and produces uniform size plastic instead of long strings of plastic. The consistent size distribution and/or lack of stringy plastic aids in producing uniform and high density pellets. The maximum dimension of the shredded waste is preferably between ⅜ to ½ inch.

The shredded waste is dried in a continuous dryer 24 using heated air to a moisture content of 12-15 percent with the use of waste heat from gasifier 28. Other suitable drying arrangements may be employed. Approximately 20 percent of the waste heat is required to dry the pellets to the desired moisture content. Shredded waste with a moisture content greater than 15 percent may not be desired.

In one embodiment, the dried shredded waste is conveyed to pellet mill 26 through a feed chute. Preferably, the waste is first fed to a pre-compacting auger to increase the bulk density of the waste and then to a rotary drum pellet mill to produce pellets of a desired size and density for use in downdraft gasifier 28. Drying the pellets and use of the compacting auger allow the pellet mill to operate at its rated capacity.

In one experiment, high pellet moisture content without the compacting auger resulted in throughput rates from 5 to 8 times lower than the rated capacity. A rotary drum pelletizer uses a combination of centrifugal force and roller compaction to produce consolidated pellets. The rotating drum (ring die) includes a plurality of drilled holes through which the pre-compacted shredded waste is forced as it forms a compressed rod with the wall of the drum. The ring die can be changed to produce pellets of different size and density. In one embodiment, for example, using a typical downdraft gasifier, pellets of ¼ inch diameter by ½ inch long with a bulk density of greater than 26 lbs/ft3, results in optimum gasifier operation through control of the height of the gasification zone and air flow in the gasifier. In one example, the bulk density was approximately 30 lbs/ft3. The denser the fuel pellets, the more uniformly they will burn in the gasifier. As the dried waste is fed through the die, the moisture content is reduced from frictional heating of the pellet.

The power required for shredding and pelletizing the waste typically comes from electrical generator 30 and is equal to about 3-10 percent of the total energy (based on an energy content of 8,700 BTU/lb) in the solid waste. This is termed the parasitic energy loss of the waste-to-energy conversion system. The power required for conveying the waste to the shredder and/or the pellet mill, in one embodiment, is very small compared to the operating power of the two units. For a parasitic energy loss of 5 percent, a WEC system having a preprocessor, downdraft gasifier and electrical generator has a combined (heat and power) conversion efficiency of approximately 65 percent and a conversion efficiency of just electricity of 19 percent.

A solid waste preprocessor subsystem intensifies gasifier operations. In this regard, the gasifier may be downsized because the created fuel comprises more BTU/volume than currently available pellet fuel. This may also allow for smaller equipment dimensions. As will be discussed below, material supplied at approximately 3 lb/ft3 is processed to produce a fuel that is at least 10 times denser. Also as will be discussed below, moisture content, shred size, the use of a compacting auger, and/or pellet size help to create a higher energy content fuel. As a result, in one embodiment, the system and methods described herein allows for the combustion of 10 times more material at once than a system that does not densify its feedstock. Further, this may allow for a reduction in the size of the processing equipment.

One goal of the invention is to reduce the logistics burdens in the areas of fuel transportation and in waste removal. The waste-to-energy conversion technology provides facilities with significant reductions in these burdens as well as introducing significant cost savings against the cost of petroleum products. The waste can be converted into sufficient heat and electricity to supply all of the energy needs of the kitchens and other facilities. Other waste including wood can be converted into electrical energy sufficient to provide from 10 to 15 percent or even greater amounts of the power generation requirements for the facility.

Other applications of the invention include the conversion of Refuse-Derived Fueled (RDF) and food waste generated at educational institutions (colleges and universities), hospitals, prisons, sporting and entertainment facilities, and supermarkets, as well as food processing industries.

In accordance with the subject invention, the combination of food waste and RDF is densified into fuel pellets. Food waste is preferably used as a binder. Low parasitic energy losses for the pelletizer and maintenance of the rated throughput are achievable through the use of a compacting auger prior to pelletization. Low parasitic energy losses for the pelletizer and maintenance of the rated throughput are also achievable through control of the moisture content of the shredded waste. Generally uniform, small diameter (e.g., approximately ¼ inch), high density and mechanically strong pellets required for achieving optimum performance of the gasifier can be obtained with low parasitic energy loss by shredding the solid waste to a consistent size distribution and by eliminating stringy plastic. Methods of tailoring the properties of the fuel pellets to achieve optimum performance of the gasifier are provided. Reducing the moisture content of the pellets to 9-12 percent reduces odors and/or reduces mold and bacteria growth for a period of one to two weeks.

EXAMPLE 1

A variety of commercially-available process equipment was evaluated for use in the solid waste preprocessing unit. Experimental trials were run to assess the suitability of candidate equipment for use as-is or with modification in the preprocessing unit. A waste shredder followed by a rotary drum pellet mill was found to produce pellets of good quality and preferred size, without exceeding a parasitic energy loss target of less than 10 percent of the gross energy content of the feed waste stream. The pellets were evaluated in a small modular biopower downdraft gasifier system for their ability to make a high quality synthesis producer gas to produce heat and/or generate electricity.

Simulated waste streams, derived from Meals-Ready to Eat (MREs), were formulated according to the Force Provider Training Module (Fort Polk) with uncooked food. Table 1 shows the waste composition that was used in the solid waste pre-processing unit. Cardboard and paper were added to the MREs to meet the Force Provider requirements. Approximately 700 lbs of waste was prepared for size reduction and pelletization test trials.

TABLE 1 Composition Component (weight percent) Source Plastic 13.33 MRE packaging, UGR plastic trays, bag liners Food 44.44 MRE food waste Paper 42.22 MRE fiberboard cases, MRE packaging, Chinette trays, cardboard Total 100

Investigation of size reduction equipment focused on industrial shredders due to their ability to handle a range of materials at varying loads. Initially, a 460 VAC, 3 phase, 3 HP shredder was selected for use with the intent of reducing the waste to a size ¼″ or smaller. Two other higher horsepower shredders were investigated for size reduction. A larger unit (460 VAC, 25 HP) with a higher horsepower drive was used for primary size-reduction. Simulated waste streams of the waste composition shown in Table 1 were processed into one inch strips using this unit. The waste was then fed into a smaller shredder (230 VAC, 5 HP) with 5/16 in teeth to achieve the desired pellet dimensions of ¼ inch to ½ inch diameter. The average power requirements for the large and small shredding units were 9.5 and 2.4 HP, respectively. The maximum power usages for the primary and second size reduction units were 27 and 8 HP, respectively.

During the preliminary shredding tests, the first stage shredder had some difficulty handling mixed material of different hardness or thickness. The shredded product had an inconsistent size distribution and density from run to run. As a result, the shredded material from this first shredder was processed by a second shredder in order to achieve the desired consistency, resulting in an additional energy penalty. The equipment shredders produced long strings of plastic because of the incompletely shredded thin plastic film present in the feed waste. Stringy plastic may limit the ability to produce uniform pellets.

EXAMPLE 2

Other shredders were evaluated, specifically those used to process hospital and plastic waste. A rotary waste shredder was used to shred stimulant encampment waste.

Shredder 22, FIGS. 3-4 incorporates hydraulic ram 50 that pushes material against a rotating cutter drum 52 that is driven by an electric motor. Material that is smaller than the screen 54 apertures fall to a collection area below, where it is pushed forward with an auger to a conveyor.

Two cases of MREs (˜12 pounds, without the magnesium heater packs) were processed for evaluation. One case was loaded into a rotary waste grinder, outfitted with a 0.5″ aperture screen, while the other case was shredded with a 1.0 inch aperture screen using the same machine. Observations were that the equipment handled the material well, and the waste was more homogenous than what was produced with the two-stage shredder. Additionally, the tool cut through thin plastic film with no difficulty, and no long stringy waste was apparent in the shredded product.

Pelletization trials indicated that a smaller shred size was conducive to higher quality pellets. A ⅜ inch shred size was the target for shredding tests. A specialty, non-stick coated ⅜ inch screen was fabricated and installed on a shredder. Approximately 100 pounds of solid waste were placed in the shredder. Initially, throughput with the ⅜ inch screen was the same as the rated capacity; towards the end of the run, the throughput decreased below the rated capacity. Upon inspection of the shredder, the screen appeared clogged. The screen was behaving more like an extruder rather than a separator. Approximately 340 lbs of waste were shredded without clogging on a on a larger unit with a ½ inch screen for pelletization trials. A depiction of the ½ inch shredded military feeding waste 60 is shown in FIG. 5.

Electrical measurements were taken of the shredder to evaluate the power requirements. A power metering and acquisition system was setup to monitor the electrical consumption of the shredding machine. FIG. 6 shows the results obtained during a 20 minute period while the unit was processing the feedstock material at the rated capacity. The results indicate that the shredder had a peak power consumption of 15.75 kW, at an average of ˜5 kW. This does not include electricity usage of the auger or conveyor used to transport the material out of the shredder; however those are expected to contribute minimally to the overall consumption. The parasitic loss of the shredder as a function of run time was calculated assuming that the energy content of the pelletized product is ˜8000 BTU/lb and a batch of 1500 lbs of trash is to be shredded. The results are summarized in Table 2.

TABLE 2 Time to Shred 1500 lbs Throughput Parasitic Loss* Waste (lbs/hr) BTU consumed (percent) 3 500 50,814 1.67 4 375 67,752 2.23 5 300 84,689 2.78 6 250 101,627 3.34 7 214 118,565 3.90 8 188 135,504 4.45 9 166 152,441 5.00 10 150 169,378 5.57 11 136 186,317 6.12 12 125 203,255 6.68 13 115 220,193 7.23 *Based on 1500 lbs feedstock, an energy content of 8000 BTU/lb, 10.5 × 106 BTU bulk feed energy, and an overall conversion efficiency of 29 percent of thermal energy to electrical power.

The parasitic energy loss is about 4 percent if the shredder is run at its rated capacity of 200 lbs/hr. The run time would be dependent on the screen size in place on the shredder as well as the feedstock composition. The energy content used in this example is actually lower than previously obtained. Prior results indicate the waste has an energy content closer to 9000 BTU/lb. Therefore, the parasitic loss by the shredder could be potentially lower than presented here.

EXAMPLE 3

An initial batch of shredded waste (100 lbs) was processed for pelletization using a roller press. FIG. 7 is a schematic view of a roller press 76 that was used in the test program. Material 60 from feed hopper 62 is supplied into the roll nip with a horizontal screw 64 driven by a variable speed drive 66. Material to be processed is placed into the feed hopper 62 from where it flows into the screw at the feed inlet. The material is then compacted between two rolls 64a and 64b which are cantilevered on the end of shafts outside bearing blocks 68. The rolls are also driven by a variable speed drive. A fully adjustable hydraulic system 70 provides the force holding the rolls together.

The exit stream from the roll press is a moderately densified sheet of waste. The final densification operation involves passage through briquetting rolls to form the solid waste-based fuel product. The waste will leave these stages having the desired dimensions and density.

EXAMPLE 4

A rotary drum pellet mill 26′ as shown in FIGS. 8-9 was identified as an alternative method of producing pellets. The rotary drum pelletizer uses a combination of centrifugal force and roller compaction to produce a consolidated pellet. Most commercial units now only use one motor to drive/turn the rotating drum. The rotating drum has a number of drilled holes through which shredded waste is forced as it forms a compressed rod within the wall of the drum. Approximately 520 lbs of shredded waste were processed for pelletization trials.

EXAMPLE 5

The gasification process typically requires that a certain moisture level of the feedstock be attained in order to provide constant and optimized conditions. A batch resin drier with thermostatic control and desiccator units was used to provide drying capabilities. Using a moisture analyzer, the shredded feedstock was determined to have a moisture content of ˜21 percent. The dyer was employed to remove moisture from the half the feedstock in 50 pound batches to a target value of ˜12 percent. The dryer was run at 200° F., the contents were mixed every 15 minutes, and the moisture analyzer was employed at specific intervals until the desired moisture content was reached. Table 3 shows the properties of the materials that were pelletized.

TABLE 3 Shred Size, Moisture Bulk Density, inches Weight, lbs Content, percent lbs/ft3 ⅜ 65 12 11 ½ 165 21 8 ½ 169 12 11

Shredded waste with 12 percent moisture content was pelletized in a large industrial pelletizer and also in a small pilot sized unit, both utilizing a ¼″ diameter die with a depth of 1.5″. The large unit was utilized to generate a large amount of pellets quickly. The small unit is more desirable for the containerized pre-processor application due to footprint and weight considerations. Previous attempts to pelletize feedstock with this unit resulted in a very low throughput rate (˜30 to 50 lbs/hr) compared to the rated capacity of 250 lbs/hr. However a modification to the feed auger was made to pre-compact the shredded material and thereby increase the bulk density; it was believed that the low density of our feedstock was prohibiting adequate transfer of the material to the die. A depiction of pre-compacting auger 76 is shown in FIG. 10.

The modified auger resulted in a several fold improvement in throughput. Pellets were produced from this unit at the rated capacity of the pelletizer of 250 lb/hr, with a moisture content of 10 percent and a density of 30 lbs/ft3. The bulk calorimetric content was 8181 BTU/lb. FIG. 11 shows the pellets 80 produced by the small pelletizer with the ½ shredded waste shown in FIG. 5. Shredded waste with a moisture content of 21 percent may not be able to be processed to pellets of sufficiently high quality. Several observations regarding the moisture content of the shredded waste were made. Pellets were produced at a rate of 275 lbs/hr with a final moisture content of 8 percent.

The results demonstrated the importance of the drying process to produce a consistently dried shredded waste.

A power metering and acquisition system was setup to monitor the electrical consumption of the small pelletizing machine. FIG. 12 shows the results obtained while the unit was processing the shredded feedstock material. The results show that the pelletizer had a peak power consumption of 2.1 kW and an average of ˜1.73 kW. From this information, and assuming a certain BTU value of pelletized product, the parasitic loss of the shredder as a function of run time could be calculated. For example, if the pelletized product has an energy content of 8000 BTU/lb and there is a 1500 lbs batch of trash to be pelletized, the parasitic energy loss is about 1 percent at the rated capacity of 250 lbs/hr. These results are summarized in Table 4.

TABLE 4 Time to Pelletize 1350 lbs dried Throughput Parasitic Loss* shred (lbs/hr) BTU consumed (percent) 3 450 17,664 0.58 4 338 23,552 0.77 5 270 29,440 0.97 6 225 35,328 1.16 7 193 41,216 1.35 8 169 47,104 1.55 9 150 52,992 1.74 10 135 58,880 1.93 11 123 64,768 2.13 13 104 76,545 2.52 *Based on 1500 lbs feedstock, an energy content of 8000 BTU/lb, 10.5 × 106 BTU bulk feed energy, and an overall conversion efficiency of 29 percent of thermal energy to electrical power.

The total parasitic energy loss for the shredder and the pelletizer would be about 5 percent of the total energy content of the waste feed stream.

EXAMPLE 6

The solid fuel pellets produced from the solid field feeding waste were converted into electricity via a small modular downdraft gasifier system 28, FIG. 13. The gasifier system is used to generate producer gas from organic materials, such as, wood chips, switch grass pellets, and other agricultural waste, and plastics. The downdraft gasifier is an open top design with secondary air addition to the char bed for thermal destruction of the residual tars formed during gasification. During one run, approximately 80 lbs of pellets were tested. These pellets were made using the two stage shredder and pellet mill. During a latter run, approximately 100 lbs of pellets were tested. These pellets were made using the waste shredder and pellet mill.

The pellets were made from the waste materials given in Table 1. For the latter run, the pellets consisted of two small lots of pellets (30 lbs and 70 lbs). Table 5 lists the properties of the pellets. Ultimate, proximate and BTU analyses of the pellets were made by Hazen Research. Inc. (Golden, Colo.).

TABLE 5 70 lbs 30 lbs Test 81.6 lbs (May 2, 2006) (Jun. 18, 2007) (Jun. 18, 2007) Moisture content, % 20.4† 5.4† 12 8 Pellet diameter inch 0.383-0.425 0.370-0.379 ¼ ¼ Pellet length, inch ⅛-1 ⅛-1 ⅛-1 ⅛-1 Density, lb/ft3 26.2 28 As As As received Dry basis received Dry basis received Dry basis Proximate Moisture, % 5.3 0 5.38 0 Ash, % 5.81 6.14 5.11 5.40 Volatile, % 77.3 81.63 78.16 82.60 Fixed C, % 11.59 12.23 11.35 12.00 Ultimate Moisture, % 5.3 0 5.38 0 Carbon, % 49.82 52.61 48.94 51.72 Hydrogen, % 6.94 7.33 6.64 7.02 Nitrogen, % 0.85 0.90 0.79 0.83 Sulfur, % 0.15 0.16 0.16 0.17 Ash, % 5.81 6.14 5.11 5.40 Oxygen*, % 31.13 32.86 32.98 34.86 HHV**, BTU/lb 9178 9692 8915 9422 LHV***, BTU/lb 9004 8235 8763 †Pellets received from IST has a moisture content of 20.4%; pellets air dried for two days on floor of building *by difference **Higher heating value; ***Lower heating value

The gasifier was started on wood chips and operated for 4.5 hours prior to starting to hand feed the pellets. The pellets were fed over a period of 63 minutes. After an initially rapid rate of feed to refill the gasifier, the pellet feeding rate decreased to a steady 23.45 kg/hr. The producer gas was fed to a spark-ignited GM Vortec 8.1L V-8 engine driving a generated, in an engine/genset system. The electrical load was set at 26.5 kWe during the test; the gasifier consumed 0.88 kg dry pellets per kWehr. Shortly after the start of feeding the pellets, the amount of producer gas required to fuel the engine started to decrease from about 85 Nm3/hr to reach values around 75 Nm3/hr, reflecting an apparent 13 percent higher heating value in the gas derived from the pellets. The pressure drop through the char bed ad through the grate stayed relatively constant during the run suggesting that the pellet char did not prematurely down to form smaller particles which would have significantly increased the pressure drops through the gasifier. A sample of the filtered gas showed only 45 ppm tars and less than 1 ppm particulates. Based both on the low gasifier pressure drops and physical examination of the pellets, the hand-fed pellets had sufficient physical integrity during gasification to lead to the formation of a good char bed for downdraft gasification.

Wood chip testing showed that the gasifier consumed 0.81 kg/kWehr at a higher, more efficient engine output level of 45 kWe. Because the engine is more efficient at higher power outputs, it was concluded that the pellets are at least as good a fuel for the system as wood chips and probably better on a weight basis and is in general agreement with the higher heating value of the pellets.

EXAMPLE 7

The producer gas was manually valved to a flare for disposal by combustion or to a 60 kWe Tactical Quiet Generator (TQG) diesel engine to generate electricity. The draft to pull the producer gas through the gasifier system was supplied by an air-powered eductor to the flare or by the turbocharger of the TQG engine.

Hardwood chip char left from the previous gasifier operation was ignited to start the gasifier. Softwood chips were then fed for 98 minutes to thoroughly heat soak the gasifier to its normal operating temperatures and the producer gas was initially sent to the flare. The engine/genset was started on diesel fuel and then fumigated with the producer gas. 58 kWe was initially generated with a producer gas flow rate of 50 Nm3/hr and with an unknown amount of extra diesel fuel. The electrical load was reduced to a conservative value of 48 kWe, and the process gas flow was increased to 61 Nm3/hr.

Pellets from the 30 lb lot were then fed into the gasifier. The transition from wood chips to pellets resulted in a net lowering of the temperatures in the upper part of the gasifier, but the core of the gasifier remained at the desired temperature thought to be necessary for good quality gas production with low tar values. The gasification of the pellets proceeded smoothly with no observed problems. The engine ran smoothly on the producer gas made from the encampment waste trash derived pellets and generous amounts of pilot diesel fuel.

The 70 lb lot was started after the initial lot of waste pellets was nearly consumed. The transition between the two lots was smooth although the temperatures in the upper part of the gasifier slightly dropped due to the higher moisture content of the second lot. FIG. 14 shows the gasifier temperatures during the test run, with the times of feed changes indicated. During the transition from the pellets containing 8 percent moisture to those with 12 percent moisture, the temperatures declined a bit more, but then recovered nicely as automatic controls compensated and the pellet char accumulated.

As the last of the trash pellets were about to be gasified, the producer gas was returned to the flare. The flare burned very cleanly visually with no visible smoke or flame seen in the daylight environment. The clean combustion is typical of flare operations with the low tar and particulate levels in the producer gas from the gasifier system. A slipstream of the producer gas after the filter was taken and the tar concentration was 19 ppm by weight and particulate concentration was 1.6 ppm by weight, well below the allowable upper limits for producer gas to internal combustion engines. This indicates the superb ability of the gasification system to convert a difficult plastics-containing feedstock into a clean producer gas.

Gasification of the hand-fed trash pellets proceeded very smoothly. Because of the rounded nature of the pellets, there was not observable problem with rat-holing or bridging in the gasifier, which has been observed with raw, shredded trash. Tar and particulate measurements of the cooled and filtered producer gas shown very low levels typical of woodchip gasification, indicating the gasifier cracked the large amount of plastic vapors to permanent gases and not-condensable levels of residual hydrocarbons.

The ¼ inch diameter size of the pellets gasified quite well in the gasifier using the gasifier algorithms already developed for gasifying wood chips. A larger diameter pellet might be easier to make and its larger char might be easier to retain on the grate, but it would take longer to pyrolyze and would be expected to expand the flaming pyrolysis zone. This would release more tar vapors deeper in the gasifier where they would have less time to be destroyed by the gasifier. The current ¼ inch pellets are preferred to keep the tar levels in the producer gas at the currently low, acceptable level.

Note that specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. For example, the examples above relate to RDF waste and food. Other waste may be processed in accordance with the subject invention including but not limited to agricultural waste and the like. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments or examples disclosed in the subject application are not to be taken as the only possible embodiments. Other embodiments will occur to those skilled in the art and are within the following claims.

In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended.

Claims

1. An on-site method of processing RDF waste including food from a food services facility, the method comprising:

shredding the waste in a shredder;
drying the shredded waste in a dryer;
using a pelletizer to pelletize the dried shredded waste wherein the food binds the non-food waste in the pellets;
burning the pellets in a gasifier to produce a flammable fuel gas and waste heat;
directing the waste heat to the dryer for use therein to dry the shredded waste;
combusting the gas in a generator to produce electricity; and
using a portion of the electricity produced to energize the shredder, the dryer, the pelletizer, and the gasifier.

2. The method of claim 1 wherein the pelletizer includes a pre-compaction auger and the method further includes pre-compacting the dried shredded waste prior to pelletizing.

3. The method of claim 1 in which the shredder, the dryer, the pelletizer, the gasifier, and the generator are linked together in a modular unit transported to the food services facility.

4. The method of claim 1 in which a portion of the generated electricity is used to energize loads of the food services facility.

5. The method of claim 1 in which a portion of the generated electricity is sold.

6. The method of claim 1 in which the shredder is configured to shred the waste into pieces with a maximum dimension of between ⅜″ and ½″.

7. The method of claim 1 in which the shredded waste is dried to a moisture content of between 12-15 percent.

8. The method of claim 1 in which the pellets are between ⅛ and ½ inches in diameter and between ¼ and ⅝ inches long.

9. An on-site food services facility waste processing system comprising:

a generator configured to burn gas to produce electricity;
a gasifier producing gas for the generator and waste heat;
a shredder powered by the generator for shredding the waste including food from the food services facility;
a dryer powered by the generator which uses the gasifier waste heat to dry the shredded waste; and
a pelletizer powered by the generator for producing pellets of the dried shredded waste with the food binding the non-food waste, the pellets burned in the gasifier to produce gas as a fuel for the generator.

10. The system of claim 9 wherein the pelletizer includes a pre-compaction auger for pre-compacting the dried shredded waste prior to pelletizing.

11. The system of claim 9 in which the shredder, the dryer, the pelletizer, the gasifier, and the generator are housed in a modular unit transportable to the food services facility.

12. An on-site method of processing waste from a facility, the method comprising:

feeding the waste to a shredder on-site at the facility;
drying the shredded waste in a dryer;
using a pelletizer to pelletize the dried shredded waste;
burning the pellets in a gasifier to produce a gas and waste heat;
directing the waste heat to the dryer for use therein to dry the shredded waste;
combusting the gas in a generator to produce electricity; and
using a portion of the produced electricity to energize at least one of the shredder, the dryer, the pelletizer, and the gasifier.

13. An on-site facility waste processing system comprising:

an on-site generator configured to burn gas to produce electricity;
an on-site gasifier producing gas for the generator and waste heat;
an on-site shredder powered by the generator for shredding the waste from the facility;
a dryer which uses the gasifier waste heat to dry the shredded waste; and
a pelletizer for producing pellets of the dried shredded waste burned in the gasifier to produce gas as a fuel used by the generator.

14. An on-site method of processing waste including food from a food services facility, the method comprising:

shredding the waste in a shredder on-site at the facility;
drying the shredded waste in a dryer on-site at the facility; and
using a pelletizer on-site at the facility to pelletize the dried shredded waste wherein the food binds the non-food waste in the pellets.

15. The method of claim 14 further including:

burning the pellets in a gasifier to produce a gas and waste heat,
directing the waste heat to the dryer for use therein to dry the shredded waste,
combusting the gas in a generator to produce electricity, and
using a portion of the produced electricity to energize the shredder, the dryer, the pelletizer, and the gasifier.

16. An on-site food services facility waste processing system comprising:

an on-site shredder for shredding waste including food from the food services facility;
an on-site dryer which uses heat to dry the shredded waste; and
an on-site pelletizer for producing pellets of the dried shredded waste with the food binding the non-food waste.

17. The system of claim 16 further including:

a generator configured to combust gas to produce electricity, and
a gasifier which burns the pellets producing gas for the generator and waste heat.

18. The system of claim 17 in which the shredder, dryer, and pelletizer are powered by the generator.

19. The system of claim 17 in which the gasifier waste heat is directed to the dryer for drying the shredded waste.

20. An on-site method of processing waste from a facility, the method comprising:

shredding the waste in a shredder;
drying the shredded waste in a dryer;
using a densifier to densify the dried shredded waste;
burning the densified waste to produce gas and waste heat;
directing the waste heat to the dryer for use therein to dry the shredded waste;
using the gas to produce electricity; and
using a portion of the electricity to energize at least one of the shredder, the dryer, the pelletizer, and the gasifier.

21. The method of claim 20 wherein the densifier includes a pelletizer with a pre-compaction auger and the method further includes pre-compacting the dried shredded waste prior to pelletizing.

22. An on-site facility waste processing system comprising:

a shredder for shredding waste from the food services facility;
a dryer for drying the shredded waste;
a densifier for densifying the dried shredded waste; and
means for burning the densified waste to produce electricity used by at least one of the shredder, the dryer, and the densifier.

23. An on-site method of processing waste from a facility, the method comprising:

reducing the size of the waste on-site at the facility;
drying the waste in a dryer;
densifying the dried waste;
thermally converting the densified waste to produce a gas and waste heat;
using the waste heat to dry the shredded waste;
combusting the gas to produce electricity; and
using a portion of the produced electricity for the size reduction, drying, and/or densification processes.
Patent History
Publication number: 20090090282
Type: Application
Filed: Feb 14, 2008
Publication Date: Apr 9, 2009
Inventors: Harris Gold (Lexington, MA), Barry Michael Cushman (Hudson, MA), Leslie S. Rubin (Newton, MA), Gregory Ocnos (Southboro, MA), Robert F. Mulligan (Providence, RI), Matthew J. Young (Acton, MA)
Application Number: 12/070,032
Classifications
Current U.S. Class: Means For Liberating Gas From Solid Fuel (110/229)
International Classification: F23G 5/027 (20060101);