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Waste-to-energy plants are eating it up.
By Diane McDilda
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, American’s 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 US Environmental Protection Agency (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%.
For Montgomery County, Pennsylvania, 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 (the authority) an independent, governmental organization that encompasses a total of 22 municipalities.
In 1990, landfills in the area began to reach capacity and the authority, concerned with limited space, 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 initially 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, more than 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 per day of trash come in and are incinerated every day. 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.
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, 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 used 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 times, 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.
The 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. Between 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.
All tolled the overhaul included in 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 exiting PLC.”
Polidore wrote the front-end software that provides the communication between internal boiler conditions, such as temperature and pressure, to 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 of the gates. 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 Corporation 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 that 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 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.
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 halt unless another problem or maintenance issue arose. This meant the furnace would limp along until another issue surfaced with down time 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 has been no significant increase in operational time, because previously all scheduled maintenance would be done simultaneously with grate repair meaning that separate time allowances for maintenance were rarely needed. Now, rather than replacing grates, more downtime can be directed toward safer, maintenance-oriented work.
Because of the tenacity for 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, 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 safer, 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 “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 conscious.
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 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 non-profit organization, the authority uses the income to maintain the tipping fee, or in this case, the waste generation fee (WGF) 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 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 their standards means upgrading their 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.
Diane McDilda lives in Gainesville, FL.
DE - March/April 2008
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