Shredding old tires before burning has been par for the course for energy seekers in the past. Now, however, the cement industry, the nation's third-largest energy user, has begun using a scrap tire technology that skips the shredding and gets right to the burning.
Used tires can reduce virgin fuel amounts by supplying about 25 percent of the energy required to produce cement in kilns, a discovery made by more than 100 U.S. cement facilities and electric powerplants by the end of 1996. Additionally, pound-for-pound, tires have both a higher energy content and burn with less emissions than coal, says the Scrap Tire Management Council, Washington, D.C.
Twenty-four of those 100 facilities using scrap tires for energy currently are testing a recycling system that burns whole, rather than shredded, tires. With overall lengths from 450 to 1,000 feet, diameters of 12 to 25 feet and rotation speeds of 20 to 80 revolutions per minute, cement kilns reportedly are the world's largest moving manufacturing machines. By using whole tires, the kilns can reduce the time, energy and cost involved in shredding tires before consumption.
Here's how it works: Whole tires are placed in a specially-designed charging apparatus from which they are dropped into the calcining zone at the long dry cement kiln's center where the most energy is needed (see chart above). Here temperatures are 1,800 degrees F or more. In minutes, the tires are consumed completely and their energy-bearing constituents, including their metal beads, are used productively.
One advantage to the system is that the tires tend to even out the temperature of the kiln's burning zone, thus increasing the life of its refractory.
Besides using smaller passenger car and light truck tires, the system can handle whole medium-commercial tires such as those found on 18-wheelers. Energy from these larger tires, in addition to regular tires, is being used to produce cement at kilns in San Antonio; Blandon, Pa. and Joliette, Quebec, Canada.
While tires are the major waste fueling cement kilns, just about any energy containing material that fits through the opening in the system can be used. For example, pharmaceutical product returns and wastes are largely organic compounds which can be combusted for energy. Because of the very high temperatures in the kilns, up to 3,000 degrees F in the burning zone at the kiln's end, 99.9999 percent of the materials are destroyed.
Besides becoming twice as hot as they would in incinerators, the materials also stays in the kiln up to twice as long. The 3.5 tons of wastes generated annually in the healthcare industry can be used as fuel rather than incinerated or landfilled.
Healthcare wastes are mainly carbon-based and thus are a good energy source. With either pharmaceutical or healthcare wastes, residues from inorganic compounds become raw materials that wind up as part of the cement.
The system also can be used to recycle properly prepared refinery and hazardous waste into energy to produce cement. For more information, contact: Cadence Environmental Energy Inc., Cadence Park Plaza, Michigan City, Ind. 46360. (219) 879-0371: Fax: (219) 873-3365. E-mail: [email protected]
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