As
environmental concerns continue to mount, engineers are tasked with developing
innovative solutions to reduce landfill space. Some engineers have been
successful in reducing use of landfill space by up to 90 percent and, in the
process, combating pathogens in the trash and automatically retrieving ferrous
and non-ferrous metals for recycling.
One
such example of this landfill innovation can be found on the island of Aruba,
where landfill space is at an absolute premium. There, an innovative process
and complex control system is being used to convert unsorted household garbage
into a safe, inert material called Fluff, which is similar to cellulose
insulation used in the attic of a house.
"On
an 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 another
landfill until the problem is resolved. But on an island, there is no bypass
capability and there is a need for a facility that runs reliably and can handle
all of the garbage that they produce."
Working
in conjunction with the U.S. Army Corp. of Engineers in the 1990s to process
waste from military bases, WastAway developed a technology that could fully
process garbage loaded into the system in 20 minutes. The resulting "Fluff" can
then be used as a peat moss replacement in agricultural applications, as a fuel
source to make electricity from steam or as a material component in various
building products.
Fluff
is a valuable commodity in Aruba, because
topsoil 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 cover
the landfill. Eventually, vegetation will be planted in the Fluff to improve
the appearance of the area.
"The
current facility on Aruba employs three
WastAway lines designed to process 50 percent of the island's waste. Since the
lines have been installed, Aruban sanitation officials have learned that humans
on 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 of
the facility, with the goal now being the ability to process all of the waste
on the island."
Powering
the WastAway Process
As
advanced as the WastAway system is, at its core are time-proven technologies such
grinders, shredders, belts with magnets and Eddy currents to remove ferrous and
non-ferrous materials from the garbage input into the system. Also central to
the system is a hydrolyzer, which is similar to an autoclave that would be used
to sterilize surgical instruments.
"The
hydrolyzer in the WastAway system is unique because it operates in a continuous
flow mode rather than in a batch process. Therefore, material can constantly move
under pressure through the hydrolyzer," says Brown. "Material moves in and out
of the high-pressure steam environment without the need to constantly
pressurize and de-pressurize the vessel."
The
WastAway system uses a series of operations, from cell to cell, that gradually
breaks down the garbage. In essence, it functions as an automated processing
line with the garbage loaded into a hopper on the front end. From that point,
the garbage is automatically processed without human intervention. The
shredders, grinders and equipment used to process the garbage are
PLC-controlled; variable speed drives are used on conveyor belts to move the
garbage through the system.
"Mitsubishi
Electric's Q series PLC controls the process, conveyors and all of the I/O
throughout all three lines at the plant in Aruba,"
says Kurtis Ullein of Mitsubishi Electric Automation Inc. "All of the drives in
the system, from 300 hp down to 3 hp, are controlled by a CC-Link industrial
network connected to the PLCs.
Two
Q series PLCs and two Q Series Safety PLCs are used per line, which provide
safety monitoring for the conveyors, hoppers, door switches and grinders. All
safety operations are achieved using CC-Link Safety and remote I/O safety
blocks. Another CC-Link: IE network communicates between the Q-PLC and the QS
Safety PLCs to coordinate both functions. Ethernet connectivity in the PLCs and
CC Link: IE network enables remote monitoring and the ability to make
programming changes over the Internet.
Variable
frequency 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 to
control 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 provide
a way to automatically slow down the line upstream to balance the capacity of
the system and prevent overloads in any single area.
Design
Challenges
"Both
the scope and timing of the project, due to design delays and contracting
issues 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 seven
months from quotation to a fully operational facility including construction of
the building. The schedule only allowed eight weeks to deliver the complete
control system and electrical panels." Within this eight-week period, only five
of those weeks were allotted to commissioning and making the system fully
operational after getting live power in the facility.
Another
difficulty was designing and fabricating the control panels according to
Netherland-Antilles electrical specifications and delivering the system in
eight weeks for shipment. The complete system has more than 30 electrical panels
and enclosures.
"Part
of the challenge we encountered was creating a flexible control system that
allowed us to tune the process as we rolled out the system," says Mike King, owner
and president of
Motion Control
Express, the integrator that helped develop the system on Aruba. "We couldn't optimize the components ahead of
time, so we needed the capability to make adjustments as part of rolling the
plant out."
With
the lines spread out over a large area in a 42,000 sq. ft. building, diagnostic
tools need to identify if a section, a shredder or Eddy current drive, for
example, has a failure. The system communicates status through the plant using
the HMIs, and also provides automatic shutdown to keep garbage from piling up. The
design goal was to be able to direct maintenance personnel within one foot of
any diagnostic issue, and pinpoint specific solutions. To do this, the control
system dynamically informs the plant operators what the system needs in order
to complete the task. This approach to maintenance compresses the training
requirements for new operators and also helps point to potential issues before
a fault occurs.
Starting
with Simulation
WastAway
engineers used SolidWorks to develop a model of the complete Fluff control
system as part of the design process. These models were then imported into PCs
as bitmap files and animated to compress the timing of the software
development.
"Due
to the tight timeframe, we wanted to look on the computer screen and watch the
system cycle properly before moving onsite," says King. "We were able to
simulate the process that would appear on the HMI screen using a simulator from
Mitsubishi and GT SoftGOT products (software visualization tools)."
SolidWorks
was used to verify the dimensions and develop the system's complex process
model. Motion Control Express, working as the project's software developer,
used this input to replicate the mechanical system inside the Mitsubishi HMI
and animate it to watch the different gates move and verify the timing of the
signals to ensure proper operation before arriving at the plant location.
When
bringing the system up, the engineering team wanted to be sure it was done step
by step, carefully verifying the process before making steam vessels, for
example, operational. The entire project is programmed using Mitsubishi's GX
Developer software package as PLC ladder logic code.
From
WastAway's central location in Tennessee, the Aruba project can be fully
controlled through a remote connection to the HMI. "Anything that can be
controlled in Aruba, we will have that capability back in Tennessee to help
address any current and future issues with the system," says Brown.
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