One plant is brand new, the other just celebrated its 20th anniversary. One’s located in Florida while the other’s in Minnesota. The Lee County (Fla.) Waste-to-Energy Expansion Project is fueled by municipal solid waste, while Great River Energy’s Elk River Station burns refuse-derived fuel (RDF), but both waste-to-energy (WTE) plants do well what they were designed to do: efficiently dispose of garbage.

“The thing about WTE is that it is primarily a method of waste disposal,” said Don Castro, P.E., HDR Engineering, and project manager on the Lee County project. Its main purpose in life is to make provisions for safe and sustainable waste disposal practices. “The energy that comes along with it is secondary,” he said.

WTE has relatively low CO 2 emissions–comparable to those of natural gas used for electricity generation–and offsets the fossil fuels that would be used to generate an equivalent amount of electricity. Less municipal waste is sent to landfills so less methane is produced–a gas with a global warming potential 21 times that of CO 2 .

“Elk River has converted almost 6 million tons of garbage into renewable electric generation over the last 20 years,” said Tim Steinbeck, plant manager at the Minnesota facility. “We recycled an older facility through many years of change and coupled with the RDF processing plant, we keep 100 people in green jobs.”

The New and the Old

When the Lee County WTE Facility Expansion Project went into commercial operation in November 2007, it was among the first new municipal waste combustion (MWC) facilities built in the U. S. in more than 10 years. The project, which is owned by the Lee County Solid Waste Division, was completed for less than its original budget of $123 million.

Two MWCs with a 39 MW turbine generator were already in operation at the site when the new unit was built. The new unit has a stand-alone 19 MW turbine generator and can convert 636 tons of waste per day into renewable power–recovering approximately 600 kWh from each ton of municipal waste.

Lee County refuse trucks take the garbage collected curbside to the plant complex for combustion. The facility uses reclaimed water for all process water needs, including boiler makeup water, and captures ferrous and non-ferrous metals from the post-combustion process for sale into the metals marketplace.

By contrast, Elk River Station has been through several iterations since it began commercial operations in 1950, including a short stint as a nuclear power plant.

At first, Elk River burned coal and oil. Construction of the nuclear reactor began in 1958 and it started producing electricity in 1963. By 1968, the reactor was decommissioned and Elk River once again burned coal and oil. In 1989, Great River undertook a $33 million conversion project to create the RDF plant. Now, Elk River Station’s three generators produce 35 to 42 MW, using approximately 300,000 tons of RDF annually.

Waste is collected from five Minnesota counties and shipped to the RDF processing facility, where recyclable and non-combustible materials are removed. What’s left is shredded. Out of 1,500 tons of municipal waste sent to the processing facility each day, about 1,250 tons of fuel stock is produced.

Great River Energy says the plant reduces the amount of waste entering the state’s landfills by more than 400,000 tons a year. Great River Energy is the second largest electric utility in Minnesota in terms of generating capacity and the fifth largest generation and transmission (G&T) cooperative in the U.S. in terms of assets.

Plenty of Fuel

Since the economic downturn there has been a slight decrease in the amount of RDF from the processing facility, said Elk River’s Steinbeck. Elk River Station burned 281,727 tons of RDF in 2008.

Seasonal blips also occur in the fuel pipeline at which times the plant cannot operate at full capacity, but Steinbeck said that is usually a small percentage of its total annual hours.

Elk River is in the process of negotiating long-term contracts with local counties and is also investigating burning some tire-derived fuel, for which the plant is already permitted.

In Florida, meanwhile, the amount of garbage Lee County’s residents were producing exceeded the combustion capacity of the existing MWCs as recently as nine years ago.

“Although they had 1,200 tons per day of combustion capacity, they were hauling garbage to their landfill, which was filling up,” said HDR Engineering’s Castro. “Since we’ve completed the project, virtually the only thing going to the landfill is ash.”

O&M Challenges

RDF at 5,500 Btu per pound is similar to low-grade coal or the lignite that Great River burns in some of its other large generating facilities, but there are challenging differences.

“RDF is a very difficult fuel to handle and meter,” said Steinbeck. “It’s been a long evolution over our 20-year history to modify our fuel handling system to improve our operations.”

A bin modification project is currently underway to improve the material handling system to get the fuel to meter into the boilers better.

“The better we can meter, the better we can control emissions and adjust our air,” said Steinbeck.

A second big difference between burning coal and RDF is the problems RDF causes in the boiler. When everything in the garbage is combusted, plant operators have to deal with corrosion, said Castro. In particular, chlorides — which come from the chlorine in the waste stream — form an acidic gas compound that will attack the boiler tubes.

“Garbage is a much tougher fuel to burn than nearly anything I can think of because it’s changing from minute to minute,” said Castro. “Over time, we have learned to deal with those high temperatures combined with acidic gases but it’s a pervasive problem and it increases your maintenance costs.”

The solutions are better metallurgy, good boiler design and carefully monitored maintenance. The economics of replacing certain boiler tubes every couple of years or paying extra to improve the metallurgy dramatically to extend that time frame have to be calculated for each project.

Boiler pressure parts at Elk River have been upgraded with appropriate alloys to withstand the chloride corrosion.

There is an economy of scale to O&M for WTE plants. “These units do enjoy economy of scale but that’s a dual-edged sword,” said Castro. “It means you have to do big plants to be cost-effective and you need a pretty good population center to do big plants.”

When smaller plants are built, key functions are still needed, like crane operators who must be on duty no matter how many tons they are loading.

At Elk River, the equipment’s age is an ongoing challenge. Steinbeck said the company is committed to making continuous safety and operations improvements.

“One of our major challenges with operating a near-60-year-old power plant is modernization and obsolescence so we continue to look at making sure we have a safe operating facility and replace equipment that can no longer be economically serviced,” he said.

Emissions

Lee County was one of the first projects of its type to be permitted and built under the U.S. Environmental Protection Agency’s new source performance standards for municipal waste combustion facilities since they were promulgated in the mid-1990s. It also was among the first since the state of Florida tightened requirements for nitrogen oxide (NO X ) emissions. Environmental compliance was achieved with a combination of flue gas recirculation and advanced selective noncatalytic reduction (SNCR) controls, using urea as the reagent.

“The NO X limits were a concern during the permitting and design phase but they have turned out to not be an issue in the operating phase,” said Castro. “Our initial NO X was 140 parts per million (PPM), but each month it drops and at 110 ppm that’s where we’ll stay.”

There are two categories of WTE plant emissions: those that are monitored on a continuous basis, like NO X , carbon monoxide (CO), opacity and sulfur dioxide (SO 2 ), and those that are checked annually, including particulates, hydrogen chlorides, dioxins, mercury and a few more exotic parameters, which are measured from the stack.

In 2008, Elk River reported a total of six CO exceedances, the lowest number in several years, and one opacity spike while isolating a baghouse compartment to replace a leaking bag. (For caption and credit information, click on this image in the gallery below.)

“Emissions are different and less than from coal,” said Steinbeck. “At Elk River, we scrub and have a bag house. We filter 100 percent of the flue gas and control the SO 2 through our dry scrubber system.”

Elk River also must control for chlorides and hydrogen chloride gas. Its scrubber system takes out almost 100 percent of the hydrogen chloride and almost 90 percent of the sulfur dioxide.

The operating costs of running the Lee County facility on a day-to-day basis are roughly offset by the energy and recovered material sales, said Castro. Electricity is sold to Seminole Electric, a generation co-op that resells it wholesale to its members.

“What that leaves you with is the debt service, which is typically paid off on a 20- or 25-year basis,” he said. The debt retirement cost remains with the generator, but it’s a stable cost, like a mortgage.

Even so, it remains cheaper to dig a hole in the ground and fill it with garbage. “The challenge is still the cost; it’s more expensive than the landfill option,” said Steinbeck.

The Renewables Premium

Elk River Station is considered a renewable energy producer so it is part of Great River Energy’s renewables portfolio.

“Great River Energy sees value in that aspect of it,” said Steinbeck. “With its older technology, it’s not as efficient as a state-of-the-art pulverized coal facility, but it’s equal to or better than comparable biomass renewable or certainly wind generation in our area.”

If a federally mandated renewable portfolio standard (RPS) becomes law, electricity from the WTE plants will become more valuable and demand a premium. Steinbeck said an RPS could push local communities to do further research into WTE plants as they work to put renewable energy into their portfolios.

The Lee County WTE is already cashing in.

“Lee is one of the first waste-to-energy units in Florida to sell renewable energy credits along with their energy and capacity,” said Castro. “They are making about $1 million a year selling RECs to Seminole Electric.”

RECs could help offset the differential between the costs of a WTE plant and the landfill option. There are other economics that could tip the scales in WTE’s favor, too. Today, in some areas of the Northeast where new landfills cannot be sited, garbage is shipped out of state and fuel costs are rising. At some point, digging a hole in the ground and filling it with garbage may no longer be the favored option.

Sidebar: Operations at the Elk River Station WTE Plant

One of the major challenges with operating a 60-year-old power plant is obsolescence. At Elk River Station, plant manager Tim Steinbeck said equipment that can no longer be serviced economically is replaced.

“With the RDF conversion 20 years ago, we installed a dry scrubber but there’s been some obsolescence in the controls and technology so we’ve recently upgraded some of the scrubber control system and mechanical slakers,” he said.

The control system on one of the units has been upgraded, too. The turbine generators are original equipment and have undergone minor modifications.

Making sure the plant is a safely operating facility is a top priority. When the utility was concerned about arc flash regulations, for instance, switch gear was upgraded and relocated.

Sidebar: Other recent projects include: