Controlled burning of tires or TDF for fuel value occurs most frequently in two types of process units - kilns and boilers. This chapter will describe the general process operation of cement kilns and boilers. The various types of boiler configurations will be described with attention to the implications for burning tires or TDF. Kilns in two industries have burned tires or TDF supplementally - lime manufacturing and, more commonly, cement manufacturing.
Currently, in the U.S., a few boilers operate by burning solely whole tires or TDF, all in the electric utility industry. These are discussed in Chapter 3, Dedicated Tires-to-Energy Facilities. Chapter 4, Tire and TDF Use in Portland Cement Kilns, discusses in more detail the use of TDF in lime and cement kilns.
Most often, boilers burn tires or TDF as a supplemental fuel for either coal, gas, refuse-derived-fuel (RDF), or wood waste. The two industries where supplemental use of TDF is most prevalent are electric utilities, where the primary fuel is most often coal, and pulp and paper mills, where the primary fuel is most often wood waste, also known as hog fuel. These industries are discussed further in Chapter 5, TDF Use in Waste Wood Boilers, and Chapter 6, Tires as Supplemental Fuel in Electric Utility Boilers.
Finally, several other industrial processes have tested or used TDF as a supplemental boiler fuel to coal or RDF. These include plants that manufacture chemicals, glass, grain, steering and gear manufacturing, and tractors. These other industrial processes are grouped together, and are discussed in Chapter 7, Supplemental TDF Use in Other Boiler Applications.
Rotary portland cement kilns cam use TDF or whole tires as supplemental fuel. Kilns are large cylinders that tilt slightly downward to one end and rotate slowly, so that feed materials travel to the far end by gravity.1 Fuel is generally fired at the lower end, so that the hot gases rise upward through the kiln, passing countercurrent to the descending raw feed material.1 As feed travels down the kiln, water is evaporated, and a chemical reaction occurs by which the feed changes to a rock-like substance called clinker. Clinker is cooled after exiting the kiln, and then ground with gypsum to make cement.1 Under normal operation, no solid waste such as ash or slag exits the kiln; all raw feed and fuel components are incorporated into the clinker. Even if the kiln is upset, the out-of-specification clinker that results can often be reground and recycled to the kiln. Details of the cement process and environmental impacts are presented in Chapter 4.
When whole tires are used as supplemental fuel in cement manufacture, they generally enter the process at the upper feed end of the kiln. Depending on the specific process flow at a facility, TDF can be added at the feed end, at the lower (firing) end, or in a raw feed preheater/precalciner that is located before the raw feed entrance. These options are described in more detail in Chapter 4, Tire and TDF Use in Portland Cement Kilns.
The type of boiler configuration and firing method significantly affect the success of burning tires or TDF. This section serves to summarize the implications of burning TDF in several boiler configurations most common in the industry at this time.
Coal fuel in boilers is primarily combusted by suspension firing or by grate firing. Boiler configurations that combust fuel in suspension include the fluidized bed and the cyclone types. Combustion occurs primarily on the grate in underfed stoker boilers. Combustion happens both in suspension and on the grates in spreader stoker type boilers, depending on the fuel size and the grate type, i.e., traveling, reciprocating, or chain.
TDF is difficult to burn in suspension because of its size and weight. Some industrial experience exists burning TDF in pulverized, cyclone, and spreader/stoker boilers. One utility tested whole tires in a pulverized boiler. Recently, much interest and some TDF testing has focused on TDF use in fluidized bed boilers, where fuel is suspended in a hot bed of inert material.
Metal contained in tires can cause operational difficulties. If whole tires or TDF, wire-in, is used, the wire must be removed from the grate or bed. Wire that becomes trapped on the grate can become molten and plug grate holes vital to incoming combustion air.2 Small pieces of radial mat-type wire can form "bird-nest" shaped accumulations that block conveyor joints, slag exit points, and augers.2 Further, facilities selling the slag that results from combustion may need to separate the metal from the slag to maintain a salable product. One facility quenches their slag into small beads, which they sell. Because buyers could not tolerate the heavy sharp bead wire, the company installed a magnetic separator to remove the wire. Other facilities have decided that wire-free TDF is mandatory.3,4
Zinc content of the tires may be an issue, also. Boilers that combust fuel in suspension typically maintain a higher chamber temperature (2000'F) than those that combust on a grate (1600-1650'F). At 2000'F, zinc compounds from the TDF
may be fairly volatile.5 Zinc oxide crystals could condense onto the slag or ash surface in cooler areas, in which case the zinc could leach later from a landfill and cause the groundwater to exceed health standards.5 Zinc, however, could also be trapped in the glassy melt, from which it would not be leachable.5
The following sections describe each boiler type and summarize its operation with and without TDF.
In a pulverized boiler, the coal is ground to the consistency of talcum powder in a mill, and then entrained in an air stream that is fed through the burners to the boiler combustion chamber.6 Firing, therefore, occurs in suspension. Pulverized boilers can be wet-bottom, which means that coals with low ash fusion temperatures are used, and molten ash is drained from the bottom of the furnace, or can be dry bottom, which means that coals with high ash fusion temperatures are used, and dry ash removal techniques can occur.6
The ash fusion temperature is the temperature the ash particles begin to melt and agglomerate; fused ash causes plugging of the holes in the grate, and can cause significant damage to the boiler. Therefore, a higher ash fusion temperature means fewer ash problems. However, the iron content in TDF tends to lower the fusion temperature of the ash. In some cases, therefore, a higher quality coal with a higher fusion temperature may be required to counteract the effect of the TDF.
Because pulverized coal boilers are designed to bum fuel in suspension, small TDF are typically used.7 TDF is often a maximum of 1-inch in diameter, but can be as small as 1/4-
inch.7 Even so, pulverized coal boilers must often be modified with a bottom dump grate, so that the TDF that falls to the bottom can combust.7 One utility is testing whole tires in a pulverized coal boiler.8 This is described in more detail in Chapter 6.
The Electrical Power Research Institute (EPRI) created a computer model to evaluate co-firing three alternate fuels with coal in a 50 MW pulverized unit, retrofitted to accommodate feeding of the alternate fuels.7 The particulate emissions from the boiler were assumed to be controlled by an ESP. The model assumed that TDF were 1-inch maximum in size, wire-free, and that the percent TDF varied from 0 to 100 percent. The boiler was assumed to require modification of receiving, storage, and pneumatic transport equipment, and installation of a bottom dump grate to ensure complete combustion of larger pieces.7 The results showed that TDF, co-fired with coal, does not significantly affect boiler performance.7 Boiler efficiency did decrease and net heat rate did increase with increased percent TDF, because the higher excess air that was required more than offset benefits of higher heat and lower moisture of the TDF as compared to coal.7 Although EPRI did model TDF input up to 100 percent, the paper noted that, in reality, 20 percent TDF might be the limit in most boiler configurations because of boiler limitations on fuel or performance.7
Cyclone boilers, like wet-bottom pulverized coal units, burn low ash fusion temperature coal, but the coal is crushed so that 95 percent is smaller than 1/4 inch.9 The coal is fed tangentially to the cyclone burners, which are mounted horizontally on the outside of the boiler and are cylindrical in shape.9 A typical cyclone burner is shown in
Figure 2-1.10 Small coal particles are burned in suspension, but larger particles are forced against the outer wall. The resulting slag is mostly liquid because of the high radiant temperature and low fusion temperature, and is drained from the bottom of the furnace through a tap.6 Cyclone furnaces are most common in utility and large industrial applications.
Because most of the ash is removed as molten slag, addition of a bottom grate is not necessary.7 However, small TDF is required, because much of the combustion must occur in suspension.7 TDF that is too large to combust completely can get carried over into the boiler or dust collection system, and cause blockage problems.9 Therefore, particle size may inversely determine the amount of TDF that can be used in a cyclone boiler.11 Three cyclone-fired boilers at utilities have burned 1" x 1" TDF in test operation, one at the 2 percent, one at the 5 percent, and one at up to a 10 percent level.3«9,12 One pulp and paper mill plans the use of TDF in a cyclone-fired hog-fuel boiler.13
In stoker boilers, fuel is either dropped or rammed onto a grate. Stoker boilers are identified by the type of feed mechanism and the type of grate. Feed may be by spreader, overfeed, or underfeed. Grates may be travelling, reciprocating, chain, or dump type.
Approximately 12 stoker boilers are burning TDF supplementally on a commercial basis, all in the pulp and paper industry (see Chapter 5). One industrial stoker boiler at a tractor factory is testing TDF use. Five of these 13 are underfeed stokers, and 8 are spreader stokers. Of the spreader stoked boilers, 2 are reciprocating grates, 2 are travelling grates, and 4 are of unknown grate type.
Figure 2-1. Typical cyclone coal burner.10
22.214.171.124 Spreader Stoker Boilers. The large majority of boilers used to combust waste wood, or hog-fuel, are of the spreader stoker type. The term "spreader" refers to the type of fuel feeder used. A typical mechanical feeder on a spreader stoker is illustrated in Figure 2-2. A spreader stoker feeder imparts energy to a stream of crushed coal being fed to the furnace.6 Fuel drops from a hopper through a slot onto a flipping mechanism, often a wheel.2 Material hitting the wheel is propelled onto the grate.2 Because size of the fuel pieces affects how far the piece is thrown by the wheel (larger pieces are propelled further than smaller pieces), uniform coverage of the grate by the fuel occurs.12 Some combustion occurs in suspension, and some occurs on the grate. This type of combustion produces ash that retains significant carbon content, and flyash reinjection is common.
Spreader stoker boilers can have traveling grates, reciprocating grates, or dump grates.6 A traveling grate travels toward the feeder, and fuel on the grate is burned with air coming through the grate. Large fuel pieces fall quickly to the grate. Mid-sized pieces fall more slowly and often land on top of larger pieces. The fines are caught in the air up-draft, and are burned while suspended in air. Ash is dumped at the end of the hearth, and is collected in an ash pit below the grate.6 A reciprocating, or vibrating, grate is comprised of bars that resemble a series of steps sloping downward that move back and forth, pushing the burning material through the boiler. This provides air flow above and below the hearth. Ash and other materials may fall through the grate to hoppers or be dropped in hoppers at the end of the grate. Reciprocating and traveling grates are continuously cleaned of ash. A dump grate does not have continuously moving parts, and simply dumps ash at intermittent intervals to a hopper. All these grates must maintain a constant covering of ash or fuel, because exposed
Deflector plate with tuyères
Reciprocating feed plate
Figure 2-2. Typical mechanical feeder on a spreader stoker.10
grate metal can be damaged by direct contact with the heat.6 Therefore, proper fuel sizing is imperative so that good distribution of coal and ash on the grate results. Cooling from the combustion air passing through the grate protects the grate as does the insulating effect of the coal/ash layer on top.6
To burn TDF successfully in a spreader/stoker furnace, the particle size of the chipped tires must be slightly smaller than the largest coal or wood size permitted so that the TDF falls on top of a layer of primary fuel. Theoretically, a bed of large fuel pieces is created on the grate, covered with a layer of mixed TDF and smaller fuel pieces. If TDF is in direct contact with the grate, oils from the rubber would flow into the grate openings, carbonize, and plug the grate. The size of TDF can be 2 to 4 inches in diameter.
126.96.36.199 Overfeed Stoker Boilers. Coal combusted in overfeed stoker boilers is fed from above onto a traveling or chain grate, and bums on the fuel bed as it progresses through the furnace. Ash falls into a pit at the rear of the stoker.6 The same TDF issues apply as were mentioned under spreader stoker boilers.
188.8.131.52 Underfeed Stoker Boilers. In underfeed boilers, fuel is pushed by rams or screw conveyors from underneath the grate into the furnace through a channel, or retort, and spills out of the channel onto the grate to feed the fuel bed. As the fuel is pushed further from the center channel, it combusts, and ash falls over the peripheral sides of the grate into shallow pits.6 Some underfeed stokers have only one retort, but double retorts exist with side ash dump, as do multiple retort units with rear sh discharge. Heat loss and maintenance costs are higher for this type of stoker.
A fluidized bed combustion system (FBC) is one that has a high temperature (1500'F to 1600'F) inert material, such as sand, ash, or limestone, occupying the bottom of the chamber.14 Figure 2-3 illustrates a typical fluidized bed boiler. Limestone, either as primary bed material, or as an addition, provides the additional advantage of S02 scrubbing.14,15 The advantage of fluidized bed combustion over the other 3 boiler types is that the fluidization of the inert bed material allows fuels with higher moisture and ash content to be burned, and still yield nearly complete combustion. Further, SOx control is easily and efficiently accomplished. The bed material is fluidized by one of two methods as described below.
In a bubbling FBC, incoming combustion air enters the chamber through nozzles located a couple of feet below the surface of the bed, producing a violent boiling action.14 Fuel is pneumatically injected into the chamber and is suspended by this action.14 Combustion occurs partially in suspension and partially in the bed. The bed material continually scrubs the outside layer of ash from the fuel, exposing fresh combustible material for burning.14 Dense materials, like rocks and metal sink to the bottom of the sand, where a line-bed changeout system continually pulls this bottom layer out.14 The removed material is cooled, magnets pull out the metal, and screens retain rocks or other tramp debris. Bed material is then returned to the combustor.14
In a circulating FBC system, the bed is fluidized by air passing through a wall-mounted distributor.15 Combustion occurs in the same way as in the bubbling FBC. Bed material is gravity fed down into the bed.15 Fuel is fed into the
Figure 2-3. Typical fluidized bed boiler.10
combustion chamber by an air-swept spout.15 The bed material, containing fuel and ash, is then circulated through a cyclone, where the lighter bed material and unspent fuel are separated from the heavier ash, metal, and other tramp material, and are recirculated back to the bed.15
Wire removal from the fluidized bed in both systems has been a design challenge. Wire can compose up to 10 percent of a tire's weight.16 Thi3 wire does not change physical form in a fluidized bed boiler, and accumulates, inhibiting or even eliminating fluidization in the bed.16 Poor air/fuel distribution results, eventually causing the system to shut down.16
One FBC currently operating in Japan uses a revolving-type fluidized bed that allows relatively large tire chunks (up to 10 inches) to be fed to the chamber.4 The central portion of this bed is more fluidized than the outer portions, so solids flow to the center, where fuel is injected.* Deflectors above the outer bed area "lap" waves of material back to the center.4 An air distributor directs non-combustibles to drain chutes on each side of the bed.4 The amount of fluidizing air and overfire air is automatically proportioned by optical devices that measure furnace luminosity.4
One utility unsuccessfully tested TDF in a circulating FBC boiler that had been retrofitted from a spreader/stoker design.4 Problems involved wire clogging the boiler grate openings and ash drawdown, and overload of the particulate control device. Two other FBC boilers are in the planning stages, both at utilities, and both are designed for supplemental TDF use. One is a circulating FBC design, and one is a bubbling design.14,15
Three pilot tests burning TDF have been performed on FBC boilers, one of a bubbling FBC boiler, and two of circulating FBC boilers. First, Energy Products of Idaho, Inc. (EPI), tested a pilot 3 f t x 3 ft. bubbling bed FBC. The test was in response to problems resulting from TDF burning in a FBC boiler retrofitted from a spreader/stoker design, and located at a Wisconsin power plant.16 Problems during the commercial test indicated that better tramp metal removal was necessary, combustion was not adequate, and that the particulate control device, an electrified filter bed, was not commensurate with the ash levels generated.16
Because the utility test showed that the tramp material exit from the bed, a perforated "draw-down" cone, became clogged, EPI designed an on-line bed changeout system, which continually pulls the bottom layer of sand and wire out of the bed, cleanses it, and returns it.u Emission results of the pilot test burning 100 percent tires are shown in Table 2-1.u
A second pilot test has been performed by Pyropower, Inc., in preparation for construction of a 52 MW, 468,000 lb/hr circulating bed FBC in Niagara Falls, NY, for United Development Group.5 Design is for the plant to burn up to 20 percent TDF, wire-free.5 The pilot test was run on a 0.6 MW plant using from 16 to 50 percent TDF, wire-in, on a weight basis.5 The test experienced problems with uneven tire feed and wire accumulation at ash discharge points. Lime was added to the bed to reduce sulfur emissions.16 Calcium to sulfur ratio was about 1.7 to 2.0, and resulted in 90 percent sulfur capture.5 Emissions of the pilot test are summarized in Table 2-1.5
Third, a pilot test was performed by Foster-Wheeler Development Corp., in preparation for the construction of a
Table 2-1. Emission test results of three gilot FBC boiler? burning supplemental TDF5'U,TS
EPI, bulling bad FBC 100% TDF*
Pyropower, circulating bad FBC
Foster-Wheeler circulating bed FBC
20X TDF, Mire-in
* Fuel consmpt 1 on and aui flow rate were not available; therefore, pounds per Million Btu's could not be determined.
With aaaonia spray for MO, reduction.
* With lis» injected into bad for SO, reduction. ' Hot detected.
20 MW, 200,000 lb/hr circulating bed FBC in Manitowoc, WI, for Manitowoc Public Utility.15 The plant will be designed to accommodate cpal, petroleum coke, and limited amounts of municipal waste water sludge, refuse-derived-fuel, and TDF, wire-in. The pilot test burned 20 percent (by weight), 2-inch, wire-in TDF.15 Two parallel baghouses controlled the pilot unit.15 Emission results of the pilot test are summarized in Table 2-1.15
1. U.S. Environmental Protection Agency. Office of Air Quality Planning and Standards. Portland Cement Plants — Background Information for Proposed Revisions to Standards. EPA-450/3-85-003a. May 1985.
2. Schwartz, J.W., Jr. Engineering for Success in the TDF Market. Presented at the Recycling Research Institute's Scrap Tire Processing and Recycling Seminar, West Palm Beach, FL. April 27, 1989.
3. Schreurs, S.T. Tire-Derived Fuel and Lignite Co-Firing Test in a Cyclone-Fired Utility Boiler. Presented at EPRI Conference: Waste Fuels in Utility Boilers. San Jose, CA. January 28, 1991.
4. Howe, W.C. Fluidized Bed Combustion Experience with Waste Tires and Other Alternate Fuels. Presented at EPRI Conference: Waste Tires as A Utility Fuel. San Jose, CA. January 28, 1991.
5. Gaglia, N., R. Lundquist, R. Benfield, and J. Fair. Design of a 470,000 lb/hr Coal/Tire-Fired Circulating Fluidized Bed Boiler for United Development Group. Presented at EPRI Conference: Waste Fuels in Utility Boilers. San Jose, CA. January 28, 1991.
6. U.S. Environmental Protection Agency. Compilation of. Air Pollution Emission Factors, Fourth Edition, AP-42.
7. McGowin, C.R. Alternate Fuel Co-Firing with Coal in Utility Boilers. Presented at the EPRI Conference: Waste Tires as a Utility Fuel. San Jose, CA. January 28, 1991.
8. Horvath, M. Whole Tire and Coal CoFiring Test in a Pulverized Coal-Fired Boiler. Presented at EPRI Conference: Waste Fuels in Utility Boilers. San Jose, CA. January 28, 1991.
9. Stopek, D.J., A.K. Millis, J.A. Stumbaugh, and D.J. Diewald. Testing of Tire-Derived Fuel at a 560 MW Cyclone Boiler. Presented at the EPRI Conference: Waste Tires as a Utility Fuel. San Jose, CA. January 28, 1991.
10. U.S. Environmental Protection Agency. APTI Course SI:428A, Introduction to Boiler Operation. Self-Instructional Guidebook. EPA-450/2-84-010. December 1984.
11. Granger, John E. Fuel Characterization of coal/Shredded Tire Blends. Presented at EPRI Conference: Waste Fuels in Utility Boilers. San Jose, CA. January 28, 1991.
12. Hutchinson, W., G. Eirschele, and R. Newell. Experience with Tire-Derived Fuel in a Cyclone-Fired Utility Boiler. Presented at EPRI Conference: Waste Fuels in Utility Boilers. San Jose, CA. January 28, 1991.
13. Telecon. Clark, C., Pacific Environmental Services, Inc. (PES), with Bosar, L., Fort Howard Corporation, Green Bay, WI. February 27, 1991. TDF used at Fort Howard.
14. Pope, Kent M. Tires to Energy in a Fluidized Bed Combustion System. Presented at EPRI Conference: Waste Fuels in Utility Boilers. San Jose, CA. January 28, 1991.
15. Phalen, J., A.S. Libal, and T. Taylor. Manitowoc Coal/Tire Chip-Cofired Circulating Fluidized Bed Combustion Project. Presented at EPRX Conference: Waste Fuels in Utility Boilers. San Jose, CA. -January 28, 1991.
16. Murphy, M.L. "Fluidized Bed Combustion of Rubber Tire Chips: Demonstration of the Technical and Environmental Feasibility." Energy Biomass Wastes. 1988 11:371-380
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