Cleaner and Leaner
When the Clean Air Act of 1990 was passed, it forced municipal solid waste (MSW) combustor plants across the U.S. to spend more than $1 billion on required upgrades, including combustion controls and sophisticated air pollution control equipment. By 2000, the industry had achieve
d compliance, and the U.S. EPA has reported that many of the upgrades exceed CAA requirements.
Unfortunately, these power plants still have an undeserved reputation as being dirty. Paul Gilman, chief sustainability officer at Covanta Energy, which operates 44 waste-to-energy (WtE) plants across the globe, says most folks who describe these facilities as dirty are referring to the way they were operated prior to the CAA. “The act has made facilities that have survived the transition dramatically better in their emissions characteristics,” he says. “The percent reductions from emissions ranging from particulate matter, dioxins and mercury are on the order of 95 to 99 percent. NOx (nitrogen oxide) is the only pollutant that didn’t see those dramatic reductions—only about 20 to 30 percent—because it’s much more difficult to go after.”
But that hasn’t stopped Covanta from trying to better control it. “We’ve developed a proprietary technology that we believe can cut NOx emissions in half,” Gilman says. “We’ve installed it at 13 different units now, and we hope to continue to move that out across our facilities.”
Referencing a recent EPA report, Gilman says that today, the emissions per unit of energy are lower for WtE facilities than for landfill gas-to-electricity plants, about the same for natural gas-fired plants, and easily better than coal- and oil-fired plants.
Besides dramatic improvements in emissions, efficiencies in power production have also been an area of improvement at WtE plants. “When these plants were first being developed, the emphasis was on solid waste management, and secondarily on energy,” Gilman says. “Facilities today are built with boilers that are more focused on power production.”
Figures Covanta typically uses indicate that existing facilities produce about 550 kilowatt hours (kWh) for every ton of MSW. “A boiler we would build today in a community would probably be in the range of 700 to 800 kWh per ton,” Gilman says. “The technologies have improved and the newer facilities have higher efficiencies, but that’s not where we quit.”
Covanta currently has a program designed to decrease the amount of electricity used within a facility to ensure the maximum amount of power goes to the grid. The Produce More Juice effort includes anything from using variable-speed motors to installing energy efficient lighting in their facilities, Gilman says. “In that same sense, we do lots of things to improve natural resource utilization of our own facilities, such as taking them down to zero discharge and low water use,” he adds.
Inside the Plant
Waste Management-owned Wheelabrator Inc., the other dominant U.S. WtE company, also continually works to improve the operability of its 17 facilities. Recently, Wheelabrator completed major upgrades at a WtE facility and refuse-derived fuel (RDF) receiving plant in Portsmouth, Va., investing more than $20 million in capital upgrades and maintenance improvements.
The Portsmouth facility is not at a landfill, which is common for WtE plants. Rather, the public authority trucks waste in from eight transfer stations to the RDF facility, where it is processed and shredded, then sent across the road into a storage pit on the power plant side, via an underground conveyor. “That’s where we use that now-processed waste, to burn as our fuel in our four boilers,” says Portsmouth Wheelabrator Plant Manager Paul Grego.
“When we bought the plant, boiler availability was operating at around 70 percent, and today we’re around 85 percent,” Grego says. By the end of next year, he expects that to increase to 90 percent. “A lot of the cost went to boiler pressure part replacements,” he says. “They were built in 1985, so they are about 26 years old.”
A total grate replacement on the existing traveling grate system was also done, according to Grego. A traveling grate system consists of a series of rigid and movable grates that move the waste through its natural process direction. “On the reliability side, we did a lot of material handling changes, such as fly ash conveyors and bottom ash conveyors that were causing a lot of plant down time,” he says. “They were either replaced or repaired to good operating condition.”
Another key improvement was the installation of a high-efficiency over-fire air system. “This, in conjunction with the new grate system, gives us the proper air and fuel ratio for complete combustion,” Grego says. “We’ve improved the overall combustion of the units and lowered CO2 emissions, and those are the biggest improvements as far as efficiency upgrades.”
New turbine controls were also installed, he adds. “We have an inlet pressure control system that steadies out the boiler header pressures, and that helps maintain a consistent steam flow, and again helps improve the overall combustion process.”
Maintenance is Key
While upgrading and adapting, new technologies play major roles in keeping WtE plants efficient and running as long as possible, general maintenance is also important, Grego says.
Typically, a WtE plant has a 50-year life cycle, Grego says. “The upgrades made at the 26-year-old Portsmouth facility would certainly extend the life of the plant through that cycle for the next 24 years.”
In reality, the life cycle can continue indefinitely if the plant and boilers are properly maintained on an annual basis. “Across the board, our overall boiler availability is at about 93 percent, which is excellent in the industry,” Grego says. “If you maintain the units, you should expect to be able to run them year after year in the future.”
Covanta’s Fairfax, Va., facility has been operating for 20 years, according to Gilman, and soon the community will have to decide whether to purchase the facility. A third party was hired to determine the value of the plant, which amounted to about $418 million. “That’s more than what it cost to build the plant,” Gilman says. “If you run these plants well and maintain them well, even after 20 years they hold their value and they can operate for another 20 years easily.”
At Covanta’s Union County, N.J., plant, the county is planning to extend its current mortgage over 30 years. “We’re projecting a 50-year life span for that facility,” Gilman says. “It’s all about taking care of them. Our capacity factors are very high—they rival nuclear power plants—but when they go into outage we do the kind of work to make sure they’re going to last 40 or 50 years.”
Landfills, Revenues and Recycling
Along with plant maintenance, comes maintenance of the landfill. The longer a landfill stays open, the better the situation for the WtE plant.
“When a community looks at what they want to do with their post-recycled waste—whether they want to send it to a landfill or do energy recovery—the benefits of doing energy recovery include extending the life of the landfill by 10 times,” Gilman says. “They’re also looking at the production of renewable electricity, which may qualify for incentives in some states.”
Those incentives vary considerably from state to state. In those that have renewable portfolio standards, MSW is in the top tier in some, lower tier in others. “The price of electricity from a WtE plant going on the grid is less than the price wind, solar and other renewable power gets,” Gilman says. “It’s a fairly competitive price at the outset, and that’s because besides the electricity revenues, there are revenues from taking in the waste and the recycled metals.”
The amount of metal recovered during the process is significant, he adds, as after the combustion process the ash is filtered to remove all of the remaining metals.
In some instances, zero-waste advocates have asserted that the WtE industry is competing with the recycling industry, which can potentially jeopardize MSW streams. “Some say that we want to burn everything that can be recycled, and that’s just silly,” Gilman says. “The communities where we operate have higher recycling rates than average U.S. communities, and many have world-class recycling rates. For example, Marion County, Ore., has a 60 percent recycling rate and it is climbing to 70 percent, and they are often evaluating whether they need to expand their Wte facility.”
Lee County, Fla., where Covanta also has a WtE facility, has a recycling rate that is twice the national average, according to Gilman. “[The WtE industry and recycling industries] are logically compatible and aren’t in competition, but for some reason, some zero-waste folks like landfills and think of energy recovery facilities as competition for recycling.”
There isn’t much Btu value in cans, bottles or metals, Grego adds. “If they can be recycled ahead of time, that’s better.”
Though it seems as if the flow of MSW should remain steady, Grego says that volumes are slowly decreasing over time. “It is good that there is growth in areas like solid-stream recycling, because that’s waste that is typically not valuable to us.”
While the development of new, more efficient WtE facilities continues, Gilman says work to improve existing operations never ceases. That includes emissions, where major WtE upgrades first began. “The Clean Air Act has been terrific for us because our communities know we’ve made dramatic changes in emissions,” Gilman adds. “We’re committed to further reductions, and we’ve been very successful in the past few years. As a company, or even a sector in general, we’re not resting on the morals of the Clean Air Act.”
Author: Anna Austin
Associate Editor, Biomass Power & Thermal