EPRI Journal, Sep-Oct 1995 v20 n5 p28(7) Tapping the tire pile. Leslie Lamarre. Abstract: Many electric utilities have realized the value of scrap tires as a low-cost and less environmentally-harmful feed stock. However, utilities must select the proper type of boiler to ensure the optimal burning of scrap tires. In addition, they must develop strategies and infrastructure to collect and process scrap tires to reduce costs. Full Text: COPYRIGHT 1995 Electric Power Research Institute What happens to car tires after they've exhausted their life on the road? Whether they are handed over to a tire dealer or tossed into the trash, many tires get a second life, reincarnated into products like doormats, park benches, and playground equipment. But the biggest single market for scrap tires is fuel--fuel that supplements the feedstock of paper mills, cement kilos, and even electric utility boilers. As well as offering a higher heating value than coal, tires can lower utilities' fuel costs and reduce pollutants like nitrogen oxides and ash. But it's the rare utility boiler that is amenable to burning tires successfully. Old tires are a problem. Most landfills don't allow them because they tend to capture methane gas and float to the top of the garbage heap. And when discarded in tire piles, they collect rainwater and provide an ideal breeding ground for disease--carrying mosquitoes. If tire piles catch fire, they can burn uncontrollably for days--even months on end, spewing billows of acrid black smoke into the air and oozing oil into the ground. Our nation has become obsessed with finding things to do with scrap tires. Spent tires have found their way into just about everything, from park benches to designer clothing. But these niche markets are nothing in comparison to the broad market for tires as fuel. The country's pulp and paper mills were the first to catch on to this idea. Cement kilns soon followed suit, and today they account for the largest percentage of scrap tire use. A newer entrant to the tire-derived-fuel market is the electric utility industry. Starting in the late 1980s, utilities began to burn tires as a supplemental fuel in their coal-fired boilers. According to Michael Blumenthal, executive director of the Scrap Tire Management Council, the industry's interest in this practice is on the rise. "Utilities are fairly new to this game," says Blumenthal, "but there are more utilities interested than ever before, and the inquiries are far more serious than they were in the past." As of July of this year, eight utilities were burning tires in their power plants on a regular basis, three were conducting test burns, and four others were investigating the idea. One incentive for using tires as a power plant feedstock is that some of the cost can be offset with money from state programs or through "tipping fees" received from the entity disposing of the tires. This can result in very low fuel costs. "Before competition increased in the industry, savings on fuel costs didn't mean as much, since fuel was an expense that could be passed on to consumers," says Chuck McGowin, who oversees EPRI's research on tire-derived fuel. "But now that utilities are competing with each other for customers, they want to keep their rates as low as possible. Cheaper fuel can help them do that." In most cases, tires represent a small percentage of the fuel burned in a utility boiler--usually less than 10%. But even 3% of the fuel feeding a 365-MW plant is the equivalent of 6 million to 7 million tires annually. And that, Blumenthal says, "is more than just a drop in the bucket." According to the Scrap Tire Management Council, U.S. tire piles now hold some 850 million tires. And every year we toss another 250 million--about one for every man, woman, and child in the United States. In 1994, 138 million scrap tires were put to productive use; 27 million of them were consumed by power companies. The Scrap Tire Management Council predicts that consumption by power companies will increase to 40 million in 1995 and to 52 million in 1996. Putting scrap tires to good use obviously helps resolve a potentially major health problem while eliminating a significant fire hazard and an eyesore. From a utility's perspective there are other advantages: tires have a high heating value (about 15,000 Btu per pound, compared with about 12,000 for bituminous coal and 5000 for wood), they are often cheaper and sometimes cleaner than coal (they contain less nitrogen than coal in general, as well as less ash than most coals and less sulfur than higher-sulfur coals), and using them provides an opportunity for positive publicity. "Burning tires in coal plants is a great way to tap the resource of an old plant that might otherwise be replaced simply because it can't compete," says McGowin. "The practice really strengthens the economics of these plants and helps utilities get more life out of them while providing a public service." But as many utilities that have investigated the practice can attest, making it work is not always easy. Challenges, challenges Most important, a utility has to have precisely the right type of boiler in order to consider burning tires. Experience to date has shown that cyclones and stokers work well. Unfortunately, they are also among the least common types of coal plants in the industry, with cyclone-fired boilers representing about 9% of all coal-fired capacity and stoker-fired units even less. According to McGowin, most of the electric utility industry's experience with tire burning has involved cyclone-fired boilers. One great advantage of this boiler type for burning tires is that, in general, no modifications to the boiler itself are necessary. Typically, the only hardware needed is a conveyor system for feeding tire-derived fuel to the boiler. And since tire-derived fuel has a lower nitrogen content than coal, emissions of nitrogen oxides from cyclone-fired plants (known to be high [NO.sub.x] emitters) are reduced. In order for tires to be used in cyclone boilers, the wire around the rim of a tire should be removed. Called bead wire, this material is much stiffer than the wire mesh beneath the tread of steel-belted tires, and it it winds up in the ash it can be problematic, since ash is often sold as a traction agent for use on winter roads. Says Bob Newell, manager for strategic energy options at Wisconsin Power and Light Company (WP&L), "The last thing you want to put on the roads is needle-like pieces of metal." Any metal left over from the combustion process can be removed with a magnetic separation device. WP&L has been burning shredded tires in cyclone-fired boilers since 1989. Tests there, like tests in cyclones at other utilities, found that emissions from cofiring tires with coal were essentially equivalent to those from burning coal alone, although trace metals were reduced somewhat when tires were used and sulfur dioxide could vary, depending on whether the coal used had a low or medium sulfur content. Today WP&L uses shredded tires for up to 10% of the fuel in six cyclone boilers, eating up about 20,000 tons of tire fuel annually, or 2 million tires. The utility has found that tire chips measuring about 1 square inch are an ideal size for a cyclone boiler. If the chips are any bigger, it reports, there often is not enough residence time in the cyclones to ensure that the pieces are burned completely. Illinois Power Company, which has been using tires for up to 2% of the fuel in two 560-MW cyclone units since early this year, has also found that 1-square-inch tire chips are ideal. According to David Stopek, coordinator of research and development for Illinois Power, such chips handled well in the coal feed system. "Even at 1 inch, there is some material that does not burn completely," he says, "but it's a very tiny fraction." He notes that his utility has installed a special system to remove this unburned material from the ash. Stoker-fired units are also well suited for burning tire fuel, since the fuel sits on a moving grate near the bottom of the boiler, as wood sits on the floor of a fireplace. This results in a longer combustion period, enabling the fuel to burn completely. Utilities that have burned tire chips in stoker-fired boilers have found that chips measuring up to 2 square inches work well. Among them is New York State Electric & Gas Corporation, which has been burning shredded tires at Jennison station, a 74-MW plant with four stoker-fired coal boilers, since the spring of 1991. "We'll continue it as long as our stokers are around," says Wally Benjamin, a senior technical associate with the utility, adding that no major modifications were required for material handling--not even a conveyor system. Jennison station can burn fuel containing up to 25% tire chips by weight. As of July of this year, the plant had consumed the equivalent of more than 2.3 million car tires. Other approaches Pulverized-coal boilers, the most common type in the industry, are generally much more difficult to adapt to tire burning than are cyclone and stoker boilers. Before the coal enters the boiler, it is literally pulverized into a fine powder so that it can be burned in suspension. Although tires can be shredded into fine granules, such pieces are still larger than the coal particles, and the challenge is to get them to burn completely. A recent EPRI study of finely shredded tire particles in a 65-MW pulverized-coal boiler showed that the particles burned rapidly but that the larger pieces, the char, and the carbon black in the rubber fell into the water-filled bottom of the ash hopper and burned on the surface of the water. EPRI's researchers concluded that in order to burn completely, this debris required a longer residence time than was available in the test unit. Only further testing can determine whether a larger pulverized-coal boiler would provide sufficient residence time to burn the tire material completely. Ohio Edison Company took an entirely different approach to burning tires in a pulverized-coal plant by using whole tires. This enabled the utility to avoid the costly process of shredding tires for burning. Ohio Edison burned whole tires successfully in its 42-MW pulverized-coal boiler in Toronto, Ohio, for 18 months until the plant was closed in 1993 for unrelated reasons. Augie Szempruch, director of project development for Ohio Edison, notes that burning whole tires is not a practice that can be carried out successfully at just any pulverized-coal plant. Characteristics making the Toronto plant amenable to whole-tire burning included the boiler's wet-bottom design and its high operating temperature (about 3200 [degrees] F). Because of the wet-bottom design, unburned tires were allowed to fall into the pool of molten slag in the bottom of the furnace, where they stayed as long as necessary to ensure complete combustion. Getting the pulverized-coal boiler to accept whole tires took some significant modifications to the fuel-feeding system. The utility designed a tire delivery system complete with a conveyor and a lock hopper to drop the tires into the boiler at precalculated intervals. In a four-day test in the spring of 1990, the utility burned a fuel mix containing up to 20% tires--one tire every 10 seconds. At this mix, researchers recorded a 36% reduction in [NO.sub.x] emissions, a 28% reduction in particulates, and a 14% reduction in [SO.sub.2]. To top it off, says Szempruch, "we found that cofiring 20% tires in a nonreheat unit makes the heat rate about as good as that in a reheat unit." Another successful approach to burning tires in coal-fired power plants is the use of fluidized-bed combustion (FBC) technology. Independent power producers, electric utilities, and others use FBC boilers to incinerate low-grade liquid- and solidwaste fuels; typically coal is the primary fuel. In this type of boiler, hot air causes particles of fuel and limestone to mix together in a turbulent, burning bed. As with most other coal-fired plants, bead wire from the tires must be removed before firing in an FBC unit, since the wire can accumulate in the lower portion of the bed and cause problems like bed defluidization or even a lengthy outage. According to an EPRI report released in 1993, scrap tires have the highest energy content of all the alternative fuels considered for FBC boilers, including municipal solid waste, biomass, and sewage sludge. The report notes that for tire burning, FBC units should be designed with long furnace gas residence times, an overfire or secondary air system, and fly ash injection to ensure complete combustion. McGowin of EPRI notes that only a few fluidized-bed units have been built with tires in mind; one is a 20-MW unit run by Manitowoc Public Utilities in Wisconsin, which cofires tire chips and petroleum coke. Infrastructure issues The technical challenge of getting tires to burn well in coal-fired boilers is just half the battle. The costs related to fuel supply and preparation can either make or break a project. Utilities can opt to accept whole tires (and in many cases receive a tipping fee) and shred the tires themselves, or they can hire a contractor to supply them with shredded tires, ready for firing. McGowin notes that utilities that choose to hire a contractor should be sure to negotiate a low delivered cost on the fuel, since the contractor collecting the tires receives the tipping fee. Most of the utilities cofiring tires opt to have chipped fuel delivered to their boilers. Illinois Power has hired a vendor, Waste Recovery Inc., to collect the tires, shred them to the utility's specifications, and deliver them to the tire-burning plant. Going one step further, the utility's five-year agreement with Waste Recovery specifies that the vendor is responsible for keeping the on-site storage, delivery, and feed systems running smoothly. "This plant is his cash register," says David Stopek of Illinois Power. "For him to get paid, this thing has to keep running. If something goes wrong, he has to fix it. We provide no extra people." Waste Recovery also provides the hardware needed for on-site fuel handling and feeding. Illinois Power estimates that it will save about $670,000 annually by burning roughly 7.5 million tires in its two cyclone boilers, reducing annual coal consumption by about 80,000 tons and [SO.sub.2] emissions by about 3200 tons. WP&L uses different approaches at different facilities. One plant (the first one to cofire tire chips) has its own shredding facility, while a vendor delivers ready-to-fire tire chips to the other two tire-burning plants. Bob Newell says state-sponsored financial incentives played an important role in WP&L's decision to pursue the use of tires. In fact, a grant from the Wisconsin Department of Natural Resources paid for temporary tire-derived-fuel-handling facilities and flue gas testing during the initial test firing. Northern States Power Company and Manitowoc Public Utilities have received similar grants. In addition, since 1990, Wisconsin has offered $20 per ton for waste tires used in boilers for energy recovery. And just this year the state adopted a program that offers individuals or businesses that process tires an additional $20 per ton. Now a utility that both processes tires and uses them in its boilers can receive $40 per ton. Wisconsin's incentive program, which was adopted to rid the state of its waste tire overload, ends in 1996. According to Paul Koziar, manager of the waste tire program for the Wisconsin Department of Natural Resources, the incentives are intended only as seed money to help cover the initial capital investments and early operating expenses related to tire burning. Before Wisconsin instituted its incentive program, only about 15% of the state's scrap tires were used, and only a small amount went for energy production. Today, however, all of the 4 million to 5 million scrap tires generated annually are being put to productive use, and another 1 million to 2 million are being pulled from the scrap heap. In fact, more than 90% of the state's tire stockpiles are now cleaned up. According to Koziar, roughly 95% of the scrap tires used every year become fuel. Wisconsin is not the only state encouraging the use of scrap tires. According to the Scrap Tire Management Council, 48 states have scrap tire regulations and 30-35 of them offer some sort of financial incentive. The program offered through the Illinois Department of Commerce and Community Affairs is widely recognized--along with Wisconsin's--as among the most aggressive in the country. The state of Illinois offers grants and loans to encourage the use of tires for fuel, including funding to help suppliers of tire-derived fuels establish their businesses. Utilities and others have relied on this money to conduct test burns and to purchase equipment for tire-burning plants. For instance, Illinois Power received $457,000 from the state, which went toward the purchase of a fuel-handling system and slag-cleaning equipment. According to Alan Justice, manager of the state's Used Tire Recovery Program, by the end of the year Illinois will have enough fuel-burning capacity on-line to use more than the 12 million tires generated annually in the state, making the state a net importer of scrap tires. (Each year a small number of tires from the state's stockpile of several million are also used.) The state's other tire consumers include cement kilns and a new dedicated tire-burning facility to be operated by an independent power producer--the third such plant in the country. Such high levels of tire consumption raise the question of whether there will be enough tires to go around if the utility industry takes full advantage of its opportunities for cofiring tire-derived fuel. "Supply is one of the major concerns for utilities, and rightly so," acknowledges Blumenthal of the Scrap Tire Management Council. "But so far, the regional supply has been more than sufficient to meet demand, and there are many areas of the country still ripe for tire use." Besides, since the use of tire-derived fuel is restricted by boiler type, there are a limited number of facilities capable of tapping this resource. The sense among the utilities using tire-derived fuel is that those who manage to jump on the bandwagon quickly will benefit the most. "Using tire-derived fuel is a great way to make lots of tires go away--cost-effectively," says Stopek of Illinois Power, noting that his utility alone will be using up about 7.5 million scrap tires annually, or about half of those generated each year in the state. "After all, you can only make so many rubber doormats." Further reading Tire-Derived-Fuel Cofiring Test in a Pulverized-Coal Utility Boiler Final report for RP2190-8, prepared by Iowa State University. December 1994 EPRI TR-103851. Fluidized-Bed Combustion of Alternate Fuels Final report for RP2190-6, prepared by Combustion Systems, Inc. December 1993 EPRI TR-100547. Proceedings: Strategic Benefits of Biomass and Waste Fuels. December 1993 EPRI TR-103146. Strategic Analysis of Biomass and Waste Fuels for Electric Power Generation Final report for RP3295-2, prepared by Appel Consultants, Inc. December 1993 EPRI TR-102773. Proceedings: 1991 Conference on Waste Tires as a Utility Fuel September 1991 EPRI GS-7538. RELATED ARTICLE: POWER FROM TIRES Electric utilities have been turning tires into power since the late 1980s, mostly in coal-fired boilers. Here are just some examples of the industry's tire-burning experiences. 1 Wisconsin Power and Light uses scrap tires for up to 10% of the fuel in six cyclone boilers, including two units at the Rock River station, shown here. 2 Ohio Edison burned whole tires successfully for 18 months in its 42-MW pulverized-coal boiler in Toronto, Ohio. 3 Shredded tires are ready for burning at the Big Stone plant in Milbank, South Dakota. Jointly owned by Otter Tail Power, Northwestern Public Service Company, and Montana-Dakota Utilities, the 41 5-MW facility has been using tires as a supplemental fuel since 1990. 4 Shredded tires are conveyed into Illinois Power's Baldwin plant, where they will feed two 560-MW cyclone boilers. The utility has used tires for up to 2% of the fuel at these units since early this year. 5 This 26-MW plant in Sterling, Connecticut, is totally fueled by scrap tires, consuming about 10 million tires annually. Power from the plant, one of only three dedicated tire-burning plants in the country, is sold to Connecticut Light & Power. CMS Generation--a subsidiary of CMS Energy, the holding company of Consumers Power Company--owns a 50% share in the plant. Background information for this article was provided by Chuck McGowin of the Generation Group's Renewables & Hydro Business Unit. Article A17492290