Scrap Tire Combustion

There are over 280 million scrap tires produced annually in the United States. Of these over 100 million are used as fuel and most of these are burnt to supplement fuel use in cement and other rotary kiln operations. The cement process is particularly convenient for tire combustion because the reinforced steel wire in the tire tread can be a source of iron for the cement chemistry. Kilns burning tires must comply with the EPA s boiler and industrial furnace act and hence are heavily regulated as a pollution source. Table 6.3 gives a typical average composition of tires supplied by the Rubber Manufacturers Association of America. 

Percent of coal energy replaced with tires 

10 Pulverized Fuel (Coal/Coke) Firing in Kilns 

Milling of coal or coke produces a powder called pulverized fuel which contains particles of a wide range of sizes. As we saw in Chapter 3, the distance at which a particle in a particle-laden jet will travel in a combustion chamber plays a role in the damping of the jet s turbulent energy. Therefore theoretical analysis of combustion must take the particle size distribution of the fuel into account. Pulverized fuel fineness is therefore an important parameter in the modeling of coal combustion. An analytical expression of particle size distribution that has found a wide application for expressing the fineness of pulverized fuel is the Rosin-Rammler relation. The relationship is given by (Field et al., 1964) 

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EPA 1997. Air Emissions from Scrap Tire Combustion. US Environmental Protection Agency, EPA-600/ R-97-115. 

The largest scrap tires combustion system is the Oxford Energy plant in Modesto, California. It consumes about 4.9 million tires per year and generates 14 MW of power. A second Oxford Energy power plant, designed to burn about 9-10 million tires per year, is under construction in Connecticut. Commercial operation is planned for 1991. 

The main noneconomic barriers to scrap tire combustion are the time required for permitting a plant and the concerns of neighbors regarding environmental, health, and safety issues. Because of the test bums required and time delays in permitting, many cement plant and pulp and paper mill operators hesitate to change their operation for the small savings realized by burning scrap tires. 

Scrap tire combustion is practiced in power plants, tire manufacturing plants, cement kilns, pulp and paper plants, and small package steam plants. 

Noneconomic barriers to scrap tire combustion include problems in siting new facilities and environmental concerns. These two types of noneconomic barriers are related since objections to siting are usually due to perceived environmental problems. These noneconomic barriers are discussed below for power plants and tire derived fuel usage. 

Ohio Air Quality Development Authority 1991. Air Emissions Associated with the Combustion of Scrap Tires for Energy Recovery. Malcolm Pirnie Inc., Columbus, OH. 

Seven cement kilns in the United States utilize about 6 million scrap tires per year to replace conventional fuels. Cement kilns appear to be ideal for scrap tires because of their high operating temperatures (2,600 F) and good conditions for complete combustion, which minimize air pollution problems. Also, there is no residue, since the ash is incorporated into the cement product. Of the 240 cement kilns in the United States, about 50 are equipped with precalciner/preheaters, making them most suitable for tire combustion. 

The two technologies with the most potential for using a major portion of scrap tires generated each year, and actually reducing the tire stockpiles, are pavements with rubber additives and combustion for energy generation. 

The book is presented in two parts. Part I covers the problems associated with scrap tires and identifies existing and potential source reduction and utilization methods that may be effective in solving the tire problem. Barriers to increased utilization and options for removing the barriers are identified and evaluated. Part II provides information on the use of whole, scrap tires and tire-derived-fuel (TDF) as combustion fuel, and on the pyrolysis of scrap tires. The use of whole tires and TDF as a primary fuel is discussed for dedicated tire-to-energy facilities. The use of whole tires and TDF as a supplemental fuel is discussed for cement manufacturing plants, electric utilities, pulp and paper mills, and other industrial processes. The focus of Part II is on the impact of burning whole tires and TDF on air emissions. The information in the book is from the following documents ... 

Scrap tires that are left in piles may catch on fire. Since tires usually burn with incomplete combustion, many dangerous and polluting gases are emitted into the environment. In addition, burning tires can leak oil and aromatic hydrocarbons into the soil. 

Since tires are made of hydrocarbons, scrap tires can be used as a fuel source. Tire derived fuel (TDF) is used in power plants, paper mills, and cement kilns. Environmentalists are concerned, however, that incomplete combustion may result in the release of toxins. 

With waste rubber goods, as with nonsegregated plastic wastes, it is possible to simply burn the rubber and regain the considerable fuel value. Whole scrap tires have been burned in cement kilns, for example, to obtain a proportionate saving in the usual oil or gas fuel supply used in these operations. At the extremely high kiln temperatures the combustion is complete and any steel belts or beading are incorporated into the cement klinker without problems. 

There are 18 facilities that use scrap tires for pulp and paper productionJ Tire-derived fuel is mixed with wood waste to produce steam. The TDF improves the combustion efficiency and displaces fossil fuels. A significant portion of Oregon s scrap tires goes to this market. The volume of scrap tires used for this application has fallen from its high in 1996 much like cement industry use. It should be noted that the decrease in TDF consumption was never attributed to increased air pollution or environmental degradation. 

The process is being used on a commercial scale on Hokkaido, Japan, but appears to be uneconomical in the United States. (Mixtures of scrap tires with waste plastics have also been pyrolyzed.247) It appears to be cheaper in the United States to burn the tires for energy at lime kilns, paper mills, and such.248 The sulfur in the tires must be trapped by the lime or by other means. The combustion conditions must be such that the usual thick black smoke of burning rubber is not present. Recycling the rubber in the tires to more items of rubber would be preferable to just burning them as fuel. The real key to fewer tires to recycle is to devise better land use systems that require fewer automobiles (see Chap. 15). 

Combustion of scrap tires and, for that matter, any pulverized fuels including coal, coke, or biomass proceeds in two phases. First, the organic solid polymer undergoes pyrolysis upon reaching a temperature of about 250-300°C to release the volatile matter and solid residue (char). Second, these volatiles and char undergo combustion (Figure 6.3). 

Processed-engineered fuels (PEF), also known as pellet fuels, are produced from a mixture of plastic with other recycled materials. The other waste can be unre-cyclable paper, tires, or other scrap materials. The amount of plastic is varied to yield a pellet fuel possessing the desired combustion characteristics. The idea is to design PEFs to provide highly predictable and uniform combustion characteristics. Essentially, the plastic that can be recovered or recycled by other means is targeted for this approach. 

The airborne particles differ significantly not only in size but also in chemical composition, depending primarily on the sources of the particles. Desert and other bare lands are sources of dust particles which are primarily made of mineral oxides. Sea salt originated from sea spray consists mainly of sodium chloride. Soil tillage in agriculture produces dust particles characterizing the land surface. In contrast, urban dust is often rich in cement powder, tire scraps, car exhaust particulate. Soot particles generated from various combustion processes are essentially elemental carbon. Combustion also produces fly ash which is rich in oxides and contains toxic metals. 

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