Waste-to-energy plants are eating it up.
Like food from
a horn of plenty, garbage never stops coming. People have been generating
garbage at the same rate since 2000, about 4.6 pounds per person per day. But
even with this plateau, the population continues to mount, and so does the
garbage. In 2005 alone, people in the United States produced 245.7 million tons
of rubbish.
Along with
making garbage, Americans are proficient at gobbling up electricity. On average,
every household uses approximately 1.3 kW hourly. But what does generating
garbage have to do with consuming electricity? Plenty, especially when it comes
to operating a waste-to-energy (WTE) facility. And when changes need to be made
to ensure that garbage is processed and electricity is generated, it’s paramount
to work with people and equipment that can get the job done.
The
Power of Trash
WTE plants
provide a method of disposing of municipal solid waste with the added benefit of
generating electricity. Because no other fuels, other than garbage, are used to
generate electricity, the EPA has deemed garbage to be renewable energy.
The Integrated
Waste Services Association reports that today there are 89 WTE plants in
operation in the US. Together they have the capacity to generate nearly 2,700 MW
of electricity. This renewable form of energy production operates 24 hours a
day, seven days a week, over 365 days a year to produce 17 billion kWh of
electricity. That’s enough electricity to run 2.3 million homes in the US. WTE
facilities account for nearly 20% of all renewable resource energy generation
here in the states.
While
generating electricity is a plus, reducing the mass and weight of garbage is
generally the driver for incineration. WTE facilities can’t make the waste
completely disappear, but they do reduce the amount of garbage having to be
disposed of every day by turning tons of garbage in to pounds of ash. Weight
reduction usually falls in on the order of 75%, with volume reduction near
90%.
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Photo: Veolia |
| The Conshohocken WTE facility |
For Montgomery
County, PA, WTE has proved to be a successful trash-power combo. Waste in the
area is handled through the Waste System Authority of Eastern Montgomery County,
an independent, governmental organization that encompasses a total of 22
municipalities.
In 1990,
landfills in the area began to reach capacity. Concerned with limited space, the
authority contracted with what was then called Montenay Energy Resources for the
processing of 1,200 tons per day of municipal solid waste in its Conshohocken
WTE plant.
It warrants
mention that the Montenay Energy Resources of Montgomery County is part of
Veolia Environmental Services (Veolia ES) and has recently begun being referred
to as such. Veolia ES is the second-largest waste management company in the
world and the third-largest WTE management facility in North America.
In the
Conshohocken WTE facility, fuel oil is used to ignite a flame in the furnace.
Much like a campfire started with a lighter and stoked with wood and brush, once
the fire is going the only fuel needed is the rubbish itself. Temperatures
within a furnace can reach upwards of 2,100°F. The heat produced by the burning
waste is used to generate steam, which in turn drives a turbine and generates
electricity. For every 100 tons of trash burned, over 50 MW of electricity is
generated.
Fans keep odors
within the facility and circulate air through the furnace to support the
combustion. Beyond electricity, the process produces ash and melted steel, which
fall to the bottom of the furnace. The steel is recycled, while the ash is
transported to the landfill. Any gases released by the system are treated with
air-handling equipment before being discharged from the stack.
Veolia ES owns
and operates the plant that sits on 20 acres of land leased from Montgomery
County. Ninety percent of the waste being delivered to the Conshohocken WTE
plant comes from the 22 municipalities within the authority’s jurisdiction. The
remaining 10% comes from commercial businesses outside of the boundaries of the
waste authority.
Tim Hartman is
the executive director of the authority and explains that the facility has a
permitted capacity to receive 2,520 tons per day. On average 1,200 tons of trash
come in and are incinerated daily. Because of the high startup and operational
costs for a WTE plant, there have to be ways to ensure that a certain amount of
waste will be brought to the facility to make it economically feasible.
To financially
entice haulers to deliver their loads to the Conshohocken WTE facility, a
waste-generating fee was implemented by the authority. The fee provides a
guaranteed revenue stream sufficient to support the cost of transporting waste
from the authority’s two transfer stations to the WTE plant, processing the
waste, generating electricity, and disposing of the remaining ash.
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Photo: Bosch Rexroth |
| The new pump replaces six old gear pumps at the waste-to-energy facility. |
Electricity
generated at the plant is sold to PECO, Pennsylvania’s largest electrical
utility, serving 1.6 million customers. The WTE plant generates enough
electricity to power 30,000 homes, but that power doesn’t necessarily stay near
the plant. Pennsylvania’s electricity market was deregulated in 1996, so
customers have the option to choose their electricity supplier. Electricity
generated by burning Montgomery County’s solid waste likely powers homes in five
different counties and the city of Philadelphia.
Improving Grate Operations
Like most
modern WTE plants, the Veolia ES Conshohocken facility accepts waste from local
haulers. The trucks discharge their loads into one of two hoppers that drop
waste into a pit measuring 150 feet by 50 feet. A grapple reaches 80 feet down
into the pit and lifts the waste to one of the furnaces. Waste is pushed into
the furnace using a ram feeder.
In the furnace,
hydraulically operated grates move the waste back and forth, bringing waste
through, allowing it to burn, and moving ash out. But it’s a little more
complicated than a tango across the furnace floor. Grates move the waste at a
set speed of 12-inches every 26 seconds, carrying the waste through the drying
zone, the burning zone, and the burnout zone. Unlike more standard, fuel-based
power plants, both the amount of time needed to burn the material and the amount
of energy produced varies from one load to the next.
“Trash isn’t
like coal or gas with a stable Btu value,” says John Polidore, plant operations
manager. “With trash, the Btu value changes from one load to the next. One load
could be dry cardboard with a Btu value of 8,000 to 8,500 Btu per pound. Then we
could have a load of wet restaurant waste that could be as low as 2,000 to 2,500
Btus per pound.”
Flexibility is
needed to adjust the time waste remains within the furnace to ensure that it is
completely burned. It was this need for flexibility that caused Veolia ES to
make modifications to the engine system that powered and moved the grates at the
Conshohocken plant.
Previously,
when waste needed to stay in the furnace longer, the grates would stop at one
end of the furnace so the waste could have sufficient time to burn. After a
sufficient “pause time” the grates would then begin to move the waste back
across the furnace—at least that was how it was supposed to work. Often, after
stopping, the grates would become locked, or welded, unable to move waste
through a certain section of the furnace. This was a particular problem with
loads of steel-belted tires. Burning rubber produces a high Btu value, and the
intense heat would often melt the steel belts, causing fusing and stoppage of
the grates. Aluminum also bore its brunt of high Btus wreaking havoc on the
grates.
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Photo: Bosch Rexroth |
| The previous main power unit with gear pumps |
he facility
was originally running its two boilers, each using six gear pumps designed in
Germany. Ultimately, the problematic stop-and-go motion was controlled by a
total of 24 gear pumps awkwardly stacked and submersed in a tank. What with the
grates welding shut and the awkward location of gear pumps, there was too much
effort involved with keeping the plant up and running.
Polidore had
previously worked with Keith Metz of Bosch Rexroth Hydraulics service group in
Bethlehem, PA, to solve a problem with hydraulic fluid. Because of the
successful outcome of that project and the pair’s amiable relationship, Polidore
contacted Metz for help in reconfiguring the boiler gear pumps.
Metz came up
with a solution based on operations he had been involved with in machine tool
applications. He was confident the equipment he had in mind would work as he’d
seen in applied in situations where accuracy was paramount. As a result, the six
fixed displacement pumps for each boiler were replaced with one piston pump
capable of smoothly running operations through the furnace and boiler.
The overhaul
included the installation of four top-plate pump motor groups, each with A10VSO
pressure-compensated pumps and new electric motors. Pressure controllers were
installed to maintain constant pressure within the range of the pumps to ensure
that only the necessary amount of hydraulic fluid was delivered to the flow
control valve. The Bosch Rexroth model 2 FRE proportional-flow control valve is
a two-way valve that allows an electrical signal to control the flow of fluid
independent of variations in pressure and temperature. Each valve assembly
contains a proportional solenoid with inductive positional transducer, a
metering orifice, and a pressure compensator. The valves are controlled by Bosch
Rexroth VT 5010 amplifier cards that are controlled using Montenay’s existing
programmable logic controller (PLC).
“The original
system used a stack of six fixed displacement gear pumps submerged in the
reservoir. We replaced these multiple gear pumps with a pressure-compensated
Bosch Rexroth piston pump. This gave us the ability to vary the flow from 0
cubic centimeters to 18 cubic centimeters,” explains Metz. “The output flow of
the pump could then be controlled very accurately via a Bosch Rexroth
pressure-compensated, flow-control valve. Electronic control of this valve was
accomplished using our driver card in conjunction with the customer’s PLC. When
using gear pumps, the customer only had one speed. The Bosch Rexroth system gave
them the ability to vary the speed as needed via their existing PLC.”
Polidore wrote
the front-end software that provides the communication between internal boiler
conditions, such as temperature and pressure, and the system hydraulics. In
programming the system, measurements were made in 0.1-V increments. The voltage
runs through an amplifier card that allows the valve to open proportionally.
The primary
benefit of the overhaul was the ability to vary the feed rate as the waste makes
its way back and forth through the furnace and the ash is removed. Now, rather
than stopping and allowing the waste to burn longer, the grates continue moving.
And because the grates don’t stop, they don’t bind or weld. This speed of the
grates and time needed to completely burn the waste is controlled by varying the
flow of hydraulic fluid to the pumps that manage the speed. The variation in
hydraulic fluid is determined through monitoring parameters of the boiler.
“We measure
boiler outputs and parameters,” Polidore says. Parameters in the boiler mist
dictate whether or not the waste has burned long enough.
Metz explains
that this type of equipment originated and is more common in the steel industry.
Because of its suitability to fire-prone situations that rely on hydraulic
fluids, it can easily be adapted to similar situations.
Managing Downtime
When the
decision was made to incorporate the new design, Polidore and Metz worked with
Airline Hydraulics Corp. to test it out. The new system was installed on one of
the boilers and was allowed to operate for just under a year. After that proved
successful, the remaining boiler was retrofitted. And once the changes were
complete, Polidore says, the operational improvements were noticeable
immediately.
“We used to
have a stuck grate every five weeks, but since 2005 we haven’t had any,” says
Polidore. Each boiler contains 10 grate sections. Grate sections are replaced at
a frequency of two sections a year—meaning that over the last five years all of
the grates in the furnaces have been replaced. Polidore explains the
complications experienced under the previous system when boilers were brought
back online with new grates. “The grates are made out of steel bars. Metals
adhere to clean bars until the bars adjust to the elements. The first six weeks
after a replacement were always a nightmare. We’ve done three change-outs since
replacing the gear pumps and haven’t had any sticks.” Not one hiccough with the
grates since the pump and motor change-outs were completed—an impressive
achievement.
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Photo: Bosch Rexroth |
| In tests, one new unit ran the feed system for one boiler. |
Because of
carefully scheduling repairs at the WTE, the Conshohocken facility was able to
minimize downtime even when grates had seized. To repair a grate, the boiler
would have to be shut down for 24 hours. This included a cool-down time before
the grate was even accessible in addition to the time needed to make the repair.
The price tag for each repair episode ran in the tens of thousands of dollars.
To reduce the scheduling and monetary impact of each repair, Polidore often had
to operate boilers even when a grate was inoperable, putting the burden on other
grates within the furnace. With the furnaces operating with grates in 10 zones,
one zone could go down and the furnace could still remain operational.
Operations rarely came to a halt unless another problem or maintenance issue
arose. This meant the furnace would limp along until another issue surfaced,
with downtime rarely serving just one problem.
Overall, the
plant experiences 92% operational time—a commendable statistic, especially since
this was met even before repairs were made. Polidore explains that there’s been
no significant increase in operational time, because previously all scheduled
maintenance would be done simultaneously with grate repair—meaning that separate
time allowance for maintenance was rarely needed. Now, rather than replacing
grates, more downtime can be spent on safer, maintenance-oriented work.
Because of the
tenacity of scheduling, incoming waste was never diverted to a landfill strictly
due to boiler shutdown from the grates seizing. It’s worth noting, however, that
in a similar situation another facility may not be able to accommodate waste as
it continues to come in, meaning waste would have to be diverted and a loss of
fees unavoidable.
While every
system has its pros and cons, one drawback to the hydraulic system is an
increase in maintenance time to keep the temperature controls on target.
Polidore says the previous gear pumps required little maintenance when compared
to the displacement pumps that have more moving parts. But when weighing
additional maintenance to time allotted for grate repairs, Polidore emphasizes
that the time involved is comparable and that the effort to maintain the current
system is much less dangerous than making repairs to the previous one. “Cleaning
the grates was inherently not safe. We have a great safety record, but there
were risks.” The new system is time better and more safely spent.
Incinerators Versus Landfills
Many factors
are involved when it comes to determining a cost-effective method of handling
municipal solid waste. In areas where land is readily available, tipping fees
are low, and electricity is inexpensive, the cost to operate an incinerator may
be unreasonable. Not to mention the public resistance to incinerators. Impeded
from past problems with air quality, incinerators have suffered from the BANANA
(build absolutely nothing anywhere near anything) attitude. However, air-quality
controls have become more stringent, and other power sources have come under
fire, bringing WTE back into the public consciousness.
If WTE is
determined as a viable option, there are still decisions to be made regarding
the delegation of responsibility. The relationship between Veolia ES and the
authority is unique in that Veolia ES owns the facility. A more common approach
is for the authority or the municipality to maintain ownership of the facility
and contract out operations. Veolia ES and the authority truly work as a team,
sharing costs and expenses with a goal to minimize the tipping fee for
haulers.
In the case of
the Conshohocken facility, 90% of the revenue from the sale of electricity goes
to the authority. As a nonprofit organization, the authority uses the income to
maintain the tipping fee, or in this case, the waste-generation fee at a
reasonable level.
“We started
flow control in an effort to generate enough waste and provide a market base for
commercial haulers,” Hartman says. Of the 22 municipalities within the
authority, only 18 of those have trash collection and disposal handled by the
municipalities. Waste in the remaining is handled through commercial haulers.
“In order to get the commercial haulers to come to the WTE facility, we have to
have a comparable tipping fee.”
With similar
tipping fees, there may be other aspects that make disposing at the WTE facility
more agreeable. Two considerations are driving distance and road conditions,
important subjects to haulers. The nearest landfills are located well outside
the towns and boroughs.
Delivering
loads to the landfill puts additional miles on the trucks. And when the weather
is bad, drivers tend to appreciate a concrete floor inside a building over the
wet, cold, or muddy conditions battled outside at a landfill.
All-in-all
operations at the Conshohocken WTE plant have been successful, according to
Hartman, who still refers to the facility as Montenay. “We were very confident
in Montenay as operators of the system. They always seem to trying different
things and improving their standards.”
If
improving the standards means upgrading the system, it’s true. With improvements
in handling waste and eliminating the need for dangerous repairs or carefully
scheduled downtime, operations at the Conshohocken WTE plant have been very
successful.