POWER Magazine

The Asbury Power Plant in rural southwest Missouri is off the beaten path in more ways than one. Three years ago, Empire District Electric Co., the plant's owner/operator, began mixing pieces of discarded tires into its coal fuel supply. Each ensuing year, without compromising local air quality, the plant has rid the area of millions of tires that otherwise would have ended up in a landfill. For demonstrating that a blight can be made right, Asbury is one of POWER's 2005 Top Plants.

From the outside, Empire District Electric Co.'s 35-year-old, 212-MW Asbury Power Plant (Figure 1) looks like your typical coal-fired generating station. But looks can be deceiving. In July 2002, the plant's operators began mixing tire-derived fuel (TDF) into its supply of Powder River Basin (PRB) and local bituminous coals.


1. Tire-fired. The 212-MW Asbury Power Plant in southwest Missouri has been burning tire-derived fuel as well as coal for the past three years. Courtesy: Missouri Department of Natural Resources

For finding a way to burn TDF with only minor modifications to the plant's fuel conveyors, and for demonstrating an economic and environmentally friendly solution to waste tire disposal, Asbury earns a 2005 Top Plant award from POWER.

Empire District Electric Co. (Joplin, Mo.) is an investor-owned utility that serves 157,000 customers in southwest Missouri, southeast Kansas, northeast Oklahoma, and northwest Arkansas. The Asbury Power Plant, about 20 miles north of Joplin, has two units. Unit 1, rated at 192 MW, came on-line in June 1970. It is powered by a Westinghouse turbine with a steam flow rate of 1,375,000 lb/hr. The much-smaller Unit 2, rated at 20 MW, started up in June 1986. It also has a Westinghouse turbine whose steam conditions are 200,000 lb/hr at 850 psi and 900F. Notably, the two units share one boiler, which has cyclone burners and operates at a pressure of 1,850 psi and a temperature of 1,005F.

Asbury originally was a mine-mouth plant that got its supply from Pittsburg & Midway Coal Mining Co. In 1989, Empire began mixing the local bituminous coal with PRB coal. Not much changed until 2002, when—more for environmental than economic reasons—the utility decided to see if adding TDF to the plant's fuel mix was doable and made sense.

What Empire's managers learned is that replacing even a small percentage (1% to 2% by weight) of the coal supply with TDF yields disproportionately large benefits. Among them:

• According to EPA studies, burning TDF emits about the same level of heavy metals (zinc, thallium, cadmium, lead, nickel, and chromium) and air pollutants (NOx, SOx, CO, HCl, dioxin, and fluoride) as burning coal. In fact, many characteristics of TDF that are important to plant operators are superior to those of coal (see table).


Several characteristics of tire-derived fuel compare quite favorably to those of coal Sources: Empire District Electric Co., Missouri Department of Natural Resources

• The ash residues from burned TDF can have a lower concentration of heavy metals than the ash from certain coals.

• Burning tires makes use of an energy resource that would be wasted if the tires were to end up in a landfill.

• Fewer trashed tires end up on the side of the road, where they can collect stagnant rainwater and serve as an ideal breeding ground for mosquitoes.

According to the U.S EPA, about 24 million tires per year are used as fuel in utility boilers nationwide. Jared Wicklund, Empire's fuel contracts manager, says that since Asbury began burning TDF three years ago the plant has consumed 50 million pounds of the stuff, or the equivalent of 2.5 million tires. "For the last two years, 1% to 2% of Asbury's total fuel, by weight, has been TDF," he explains. Last year alone, the plant generated an estimated 25,000 MWh from TDF.

Empire was encouraged to pursue the project by the Missouri Department of Natural Resources (MDNR). "The MDNR has authorized us to burn up to 20,000 tons of TDF—the equivalent of two million tires—annually. Asbury used just over 11,000 tons of TDF in 2003 and 9,500 tons in 2004," Wicklund says. MDNR also places a ceiling—2%—on the share of Asbury's fuel mix that TDF can represent.

The main environmental benefit of the project—the removal of nearly a million discarded tires from view and landfills per year—is even more significant when you consider the rural nature of Empire's service territory. According to Richard Allen, an environmental specialist at MDNR, "Missouri scraps five million scrap tires a year, and Asbury takes in a fifth of them. So we support their burning of TDF wholeheartedly." But does doing so benefit Empire economically? "Including the expense of transportation and processing, the cost of TDF is comparable to the cost of coal," Wicklund says.

Empire has even found a way to leverage the process of acquiring TDF into a public relations benefit. Twice a year the utility conducts a tire-recycling drive in partnership with one of the cities it serves. The drives raise awareness of the benefits of using TDF and improve Empire's environmental standing among its customers. The most recent campaign, a two-day event held this past April, resulted in a record-setting collection of just over 2,000 tires. Empire also has a contract with Kansas City–based MA Associates, under which the company supplies Asbury with any additional tires needed to maintain the 1% TDF/coal ratio.

The Babcock & Wilcox (Barberton, Ohio) cyclone burners located on the front of the Asbury plant's boiler made switching to TDF easy. Cyclone burners are the most popular choice for burning tire-derived fuel. According to Harold Colgin, Asbury plant manager, "Because cyclone burners require no changes to burn TDF, our project had a lower capital cost."

Naturally, the cyclone burners cannot be fed whole tires. But they can accept shredded tire pieces, which arrive at Asbury as chips measuring roughly 1.5 x 1.5 inches. The chips flow freely through the plant's existing feeder tubes and then mix with the crushed coal. The small surface area of the chips improves combustion efficiency.

The only piece of equipment that Empire had to modify for the project was Asbury's coal conveyor system. The PRB/bituminous coal blend used by the plant is crushed to a size that can pass through a No. 4 sieve and then is mixed with the TDF. Asbury's boiler has five main cyclone burners equipped with horizontal cylinders 10 feet in diameter. "These are well suited for burning TDF because the fuel mixture spins around the inside of the burner, allowing for a good burn," Colgin explains.

You might think that using fuel made up of particles so different in size would cause combustion problems. But Wicklund says that that's not the case. "Air enters perpendicular to the cyclones at a speed of 300 mph. This creates cyclonic action and increases retention time sufficiently to consume the fuel," he explains. Since it began burning TDF, Asbury has not experienced any significant problems. "The plant's availability was 90% in 2002, 88.5% in 2003, and 89% in 2004," Colgin proudly reports.

Asbury uses two units from Pennsylvania Crusher Corp. (Broomall, Pa.) to reduce 95% of the coal to a size of less than 1/8 inch. According to Wicklund, "Two issues prevented us from introducing TDF into the crushers. The first is that each unit is equipped with a magnetic separator that would reject scraps containing steel, from the tires' steel belts. The second is a consequence of the resiliency of rubber, which would reduce the feeder's reliability." To solve these problems, Empire added a new hopper and conveyor after the crushers to facilitate introduction of the TDF into the fuel stream (Figure 2).


2. Coal conveyor mod. Because the Asbury plant's coal crushers proved unable to handle rubber, or tire chips containing steel, Empire District Electric Co. had to install a new conveyor and hopper downstream of the crushers. The tire-derived fuel and the coal now are mixed prior to entering the cyclone burners of the plant's single boiler. Courtesy: Empire District Electric Co.

Likewise, Asbury's steam generator needed no modifications to its 86,000-square-foot furnace to handle TDF. The furnace floor slopes 7.5 degrees toward the cyclone outlets and is equipped with one slag tap. The superheater section is not exposed to any radiant heat. There are 20 wall blowers, 28 retract steam blowers, and nine water blowers. Four hoppers under the unit's convection pass are used for ash collection. As currently configured, the boiler is followed by two electrostatic precipitators (ESPs)—an original unit from UOP LL, and one from Lodge Cottrell that was added in 1977 to remove particulate material from the gas stream.

Asbury's ash-handling systems were supplied by United Conveyor Corp. (Waukegan, Ill.). Neither had to be modified to handle ash from burned TDF. The bottom ash system features one slag tank with dual outlet doors, a clinker grinder, and dual 10-inch sluice lines from the slag tank. The four hoppers under the convection pass are sluiced continually to the slag tanks. The flyash system comprises a hydrovac pump, an air separator, and 12 hoppers with valving under the ESPs.

Although the Asbury Power Plant is 35 years old, it continues to set continuous-run records. The most recent record of 190 days ended on January 3, 2000, when the plant was taken off-line to clear a pluggage in the superheater.

Harold Colgin attributes the reliability of Asbury to its employees' dedication and expertise. "They really know what they're doing, and they're constantly thinking about what they need to do to keep the plant on-line."

In 2003, to acknowledge the success of the TDF project, Empire District Electric Co. was honored by then-Missouri Governor Bob Holden with three awards: the Pollution Prevention Award, the Recycling Award, and the Statewide Environmental Excellence/Pollution Prevention Award.