Recycling Garbage with High-Powered DrivesRecycling Garbage with High-Powered Drives
August 20, 2010
Asenvironmental concerns continue to mount, engineers are tasked with developinginnovative solutions to reduce landfill space. Some engineers have beensuccessful in reducing use of landfill space by up to 90 percent and, in theprocess, combating pathogens in the trash and automatically retrieving ferrousand non-ferrous metals for recycling.
Onesuch example of this landfill innovation can be found on the island of Aruba,where landfill space is at an absolute premium. There, an innovative processand complex control system is being used to convert unsorted household garbageinto a safe, inert material called Fluff, which is similar to celluloseinsulation used in the attic of a house.
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"Onan island, solutions that may work on the mainland are not always workable,"says Mark Brown, president of WastAway."If you are on the continental U.S.and have a unique technology that fails, you can truck the refuse to anotherlandfill until the problem is resolved. But on an island, there is no bypasscapability and there is a need for a facility that runs reliably and can handleall of the garbage that they produce."
Workingin conjunction with the U.S. Army Corp. of Engineers in the 1990s to processwaste from military bases, WastAway developed a technology that could fullyprocess garbage loaded into the system in 20 minutes. The resulting "Fluff" canthen be used as a peat moss replacement in agricultural applications, as a fuelsource to make electricity from steam or as a material component in variousbuilding products.
Fluffis a valuable commodity in Aruba, becausetopsoil is a resource in short supply. The island currently has a large,uncovered landfill which is the target for the initial use of Fluff - to coverthe landfill. Eventually, vegetation will be planted in the Fluff to improvethe appearance of the area.
"Thecurrent facility on Aruba employs threeWastAway lines designed to process 50 percent of the island's waste. Since thelines have been installed, Aruban sanitation officials have learned that humanson the island may not be generating quite as much waste as originally thought,"says Brown. "As a result, phase two of the project will increase the size ofthe facility, with the goal now being the ability to process all of the wasteon the island."
Poweringthe WastAway Process
Asadvanced as the WastAway system is, at its core are time-proven technologies suchgrinders, shredders, belts with magnets and Eddy currents to remove ferrous andnon-ferrous materials from the garbage input into the system. Also central tothe system is a hydrolyzer, which is similar to an autoclave that would be usedto sterilize surgical instruments.
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"Thehydrolyzer in the WastAway system is unique because it operates in a continuousflow mode rather than in a batch process. Therefore, material can constantly moveunder pressure through the hydrolyzer," says Brown. "Material moves in and outof the high-pressure steam environment without the need to constantlypressurize and de-pressurize the vessel."
TheWastAway system uses a series of operations, from cell to cell, that graduallybreaks down the garbage. In essence, it functions as an automated processingline with the garbage loaded into a hopper on the front end. From that point,the garbage is automatically processed without human intervention. Theshredders, grinders and equipment used to process the garbage arePLC-controlled; variable speed drives are used on conveyor belts to move thegarbage through the system.
"MitsubishiElectric's Q series PLC controls the process, conveyors and all of the I/Othroughout all three lines at the plant in Aruba,"says Kurtis Ullein of Mitsubishi Electric Automation Inc. "All of the drives inthe system, from 300 hp down to 3 hp, are controlled by a CC-Link industrialnetwork connected to the PLCs.
TwoQ series PLCs and two Q Series Safety PLCs are used per line, which providesafety monitoring for the conveyors, hoppers, door switches and grinders. Allsafety operations are achieved using CC-Link Safety and remote I/O safetyblocks. Another CC-Link: IE network communicates between the Q-PLC and the QSSafety PLCs to coordinate both functions. Ethernet connectivity in the PLCs andCC Link: IE network enables remote monitoring and the ability to makeprogramming changes over the Internet.
Variablefrequency drives were selected for use in the project because of the shredders'and grinders' high power requirements - which can run up to 300 hp. The need tocontrol the speed of the conveyors was also a determining factor. For example,if the garbage input starts to back up at a downstream hopper, the VFDs providea way to automatically slow down the line upstream to balance the capacity ofthe system and prevent overloads in any single area.
"Boththe scope and timing of the project, due to design delays and contractingissues on the island of Aruba, provided the major challenge," according to Brown."Once the team started on the project, we had to be operational within sevenmonths from quotation to a fully operational facility including construction ofthe building. The schedule only allowed eight weeks to deliver the completecontrol system and electrical panels." Within this eight-week period, only fiveof those weeks were allotted to commissioning and making the system fullyoperational after getting live power in the facility.
Anotherdifficulty was designing and fabricating the control panels according toNetherland-Antilles electrical specifications and delivering the system ineight weeks for shipment. The complete system has more than 30 electrical panelsand enclosures.
"Partof the challenge we encountered was creating a flexible control system thatallowed us to tune the process as we rolled out the system," says Mike King, ownerand president of Motion ControlExpress, the integrator that helped develop the system on Aruba. "We couldn't optimize the components ahead oftime, so we needed the capability to make adjustments as part of rolling theplant out."
Withthe lines spread out over a large area in a 42,000 sq. ft. building, diagnostictools need to identify if a section, a shredder or Eddy current drive, forexample, has a failure. The system communicates status through the plant usingthe HMIs, and also provides automatic shutdown to keep garbage from piling up. Thedesign goal was to be able to direct maintenance personnel within one foot ofany diagnostic issue, and pinpoint specific solutions. To do this, the controlsystem dynamically informs the plant operators what the system needs in orderto complete the task. This approach to maintenance compresses the trainingrequirements for new operators and also helps point to potential issues beforea fault occurs.
WastAwayengineers used SolidWorks to develop a model of the complete Fluff controlsystem as part of the design process. These models were then imported into PCsas bitmap files and animated to compress the timing of the softwaredevelopment.
"Dueto the tight timeframe, we wanted to look on the computer screen and watch thesystem cycle properly before moving onsite," says King. "We were able tosimulate the process that would appear on the HMI screen using a simulator fromMitsubishi and GT SoftGOT products (software visualization tools)."
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SolidWorkswas used to verify the dimensions and develop the system's complex processmodel. Motion Control Express, working as the project's software developer,used this input to replicate the mechanical system inside the Mitsubishi HMIand animate it to watch the different gates move and verify the timing of thesignals to ensure proper operation before arriving at the plant location.
Whenbringing the system up, the engineering team wanted to be sure it was done stepby step, carefully verifying the process before making steam vessels, forexample, operational. The entire project is programmed using Mitsubishi's GXDeveloper software package as PLC ladder logic code.
FromWastAway's central location in Tennessee, the Aruba project can be fullycontrolled through a remote connection to the HMI. "Anything that can becontrolled in Aruba, we will have that capability back in Tennessee to helpaddress any current and future issues with the system," says Brown.
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