Volume II Chapter 3.0 Pages 7 of 10 page next page 8

3.6.4. Kraft Black Liquor Recovery Boilers 3-119

3.6.5. Sewage Sludge Incineration 3-120

3.6.6. Primary Nonferrous Metal Smelting/Refining 3-122

3.6.7. Secondary Nonferrous Metal Smelting/Refining 3-123 Secondary Aluminum Smelters and Refiners 3-124 Secondary Copper Smelters and Refiners 3-124 Secondary Lead Smelters and Refiners 3-125

3.6.8. Primary Ferrous Metal Smelting/Refining 3-128

3.6.9. Secondary Ferrous Metal Smelting/Refining 3-129

3.6.10. Scrap Electric Wire Recovery 3-129

3.6.11. Drum and Barrel Reclamation and Incineration 3-131

3.6.12. Tire Combustion 3-132

3.6.4. Kraft Black Liquor Recovery Boilers

In 1987, the U.S. EPA stack tested three kraft black liquor recovery boilers for the emission of dioxin in conjunction with the National Dioxin Study (U.S. EPA, 1987). These boilers are associated with the production of pulp in the making of paper using the Kraft process. In this process, wood chips are cooked in large vertical vessels called digesters at elevated temperatures and pressures in an aqueous solution of sodium hydroxide and sodium sulfide (Someshwar and Pinkerton, 1992).

Wood is broken down into two phases: a soluble phase containing primarily lignin, and an insoluble phases containing the pulp. The spent liquor (called black liquor) from the digester contains sodium sulfate and sodium sulfide that the industry finds beneficial in recovering for reuse in the Kraft process. In the recovery of black liquor chemicals, weak black liquor is first concentrated in multiple-effect evaporators to about 65 percent solids.

The concentrated black liquor also contains 0.5 to 4 percent chlorides by weight (U.S. EPA, 1987). Recovery of beneficial chemicals is accomplished through combustion in a Kraft black liquor recovery furnace. The concentrated black liquor is sprayed into a furnace equipped with a heat recovery boiler. The bulk of the inorganic molten smelt that forms in the bottom of the furnace contains sodium carbonate and sodium sulfide in a ratio of about 3:1 (Someshwar and Pinkerton, 1992). The combustion gas is usually passed through an electrostatic precipitator that collects particulate matter prior to being vented out the stack. The particulate matter can be processed to further recover and recycle sodium sulfate.

The three sites that were stack tested by EPA (U.S. EPA, 1987) were judged to be typical of Kraft black liquor recovery boilers. The following emission factors of dioxin were derived from the stack emissions data: average CDD/CDF = 9.34E-03 g/kg (range: 4.88E-03 to 1.67 E-02 g/kg), and average TEQ = 9.71E-05 g/kg (range: 3.33E-05 to 2.06E-04 g/kg). A "medium" confidence rating is ascribed to these emission factors because the emission factors were derived from the stack testing of three Kraft black liquor recovery boilers that were judged to be fairly representative of technologies used at Kraft pulp mills in the U.S.

In 1989, EPA estimated that approximately 28.2 million metric tons of black liquor solids were burned in Kraft black liquor recovery boilers in the U.S. (U.S. EPA, 1992g). This production estimate was given a confidence rating of "high" because it is based on a recent industry-wide survey conducted by EPA. Assuming this is the quantity of black liquor that is combusted each year, then it is estimated that 264 grams of CDD/CDF and 2.7 grams of TEQ are emitted to the U.S. atmosphere annually. Based on the confidence ratings, the estimated range of potential annual emissions is assumed to vary by a factor of 5 between the low and high ends of the range. Assuming that the best estimate of annual TEQ emissions (2.7 g TEQ/yr) is the geometric mean of this range, then the range is calculated to be 0.9 to 4.3 g TEQ/yr.

As discussed in Section 3.2, approximately 500 million dry kg of pulp and paper mill wastewater sludge were incinerated in 1990 by facilities employing chlorine bleaching of pulp (U.S. EPA, 1993e). However, insufficient data are currently available to estimate emission factors for dioxin-like compounds from pulp and paper mill incinerators.

As discussed in Section 3.2, EPA proposed control technology standards that address CDD/CDF emissions for non-combustion pulp and paper mill sources in December 1993 (Federal Register, 1993a) and will propose control technology standards for combustion sources by October 1994 (U.S. EPA, 1992d).

3.6.5. Sewage Sludge Incineration

Brunner (1992) reviewed the four principal combustion technologies used to incinerate sewage sludge in the U.S.: multiple-hearth incinerator, fluidized-bed incinerator, electric furnace, and cyclone furnace. All of these technologies are "excess-air" processes (i.e., they combust sewage sludge with oxygen in excess of theoretical requirements). Of the four types of technologies, multiple-hearth incinerators are the most common.

They constitute approximately 60 percent of the 199 existing sewage sludge incineration facilities operational in the U.S. (Federal Register, 1993b). The furnace consists of refractory hearths arranged vertically in series, one on top of the other. Dried sludge cake is fed to the top hearth of the furnace. The sludge is mechanically moved from one hearth to another through the length of the furnace. Moisture is evaporated from the sludge cake in the upper hearths of the furnace.

The center hearths are the burning zone to the furnace where gas temperatures reach 871 C. The bottom hearths are the burn-out zone where the sludge solids become ash. A waste-heat boiler is usually included in the burning zone where steam is produced to provide supplemental energy at the sewage treatment plant. Air pollution control measures typically include a wet scrubber system for particulate matter control (U.S. EPA, 1987).

The fluidized-bed incinerator is a cylindrical refractory-lined shell with a steel plate structure that supports a sand bed near the bottom of the furnace (Brunner, 1992). Air is introduced through openings in the bed plate supporting the sand. This causes the sand bed to undulate in a turbulent air flow, hence the sand appears to have a fluid motion when observed through furnace portals. Sludge cake is added to the furnace at a position just above this fluid motion of the sand bed. The fluid motion promotes mixing in the combustion zone. Sludge ash exists the furnace with the combustion gases, therefore air pollution control systems typically consist of high-energy venturi scrubbers.

Electric furnaces are sometimes called infrared furnaces (Brunner, 1992). This incineration system consists of a long rectangular refractory-lined chamber. A belt conveyer system moves the sludge cake through the length of the furnace. To promote combustion of the sludge, supplemental heat is added by electric infrared heating elements within the furnace that are located just above the travelling belt. Electric power is required to initiate and sustain combustion.

Cyclonic furnaces consist of a refractory-lined cylindrical shell with a domed top (Brunner, 1992). Air is blown in at tangential burner ports on the furnace shell which causes a violent swirling pattern. This motion promotes good mixing of combustion air with the sludge feed. Sludge is fed into the furnace chamber by screw conveyor. Combustion gases exit at the top of the swirling vortex at the top of the furnace dome.

EPA has confirmed that dioxin can be emitted from sewage sludge incineration based on the testing of three multiple-hearth sewage sludge incinerators (U.S. EPA, 1987). Emission factors for dioxin were developed from these data. The average emission factor of CDD/CDF was estimated to be 1.26E+00 g/kg of dry sewage sludge (range: 8.80E-02 to 3.37E+00 g/kg). The average emission factor for TEQ was estimated to be 2.69E-02 g/kg of dry sewage sludge (range: 1.17E-03 to 3.04E-02 g/kg) assuming perfect congener distribution within the total CDD/CDFs measured.

In 1992, approximately 199 sewage sludge incineration facilities combusted about 0.865 million metric tons of dry sewage sludge (Federal Register, 1993b). Given this mass of sewage sludge incinerated/yr, the best estimates of emissions to air are 1,090 grams of CDD/CDF per year and 23 grams TEQ per year from all sewage sludge incineration facilities.

A "medium" confidence rating is ascribed to the emission factors because they were developed from the stack testing of three multiple hearth incinerators. Although multiple hearth incinerators are the dominant technology in use in the U.S. today, some uncertainty exists as to the representativeness of these derived emission estimates to possible emissions from other sewage sludge incineration technologies.

The production estimate is assigned a "high" confidence rating because it is based on an extensive EPA survey to support rulemaking activities. Based on these confidence ratings, the estimated range of potential annual emissions is assumed to vary by a factor of 5 between the low and high ends of the range. Assuming that the best estimate of annual emissions (23 g TEQ/yr) is the geometric mean of this range, then the range is calculated to be 10 to 52 g TEQ/yr.

3.6.6. Primary Nonferrous Metal Smelting/Refining

Nonferrous metals include aluminum, copper, nickel and magnesium. Insufficient information is available for evaluating CDD/CDF emissions, if any, from primary smelting/refining of nonferrous metals in the United States. However, several European investigators have investigated the presence of CDD/CDFs at some facilities in this industry.

Oehme et al. (1989) reported that the production of magnesium leads to the formation of CDDs and CDFs. Oehme et al. (1989) estimated that 500 g of TEQ are released in wastewater to the environment and 6 g TEQ are released to air annually from a magnesium production facility studied in Norway; CDFs predominated with a CDF to CDD concentration ratio of 10 to one.

The magnesium production process involves a step in which MgCl2 is produced by heating MgO/coke pellets in a pure chlorine atmosphere to about 700 to 800 C. The MgCl2 is then electrolyzed to metallic magnesium and Cl2. The Cl2 excess from the MgCl2 process and the Cl2 formed during electrolysis is collected by water scrubbers and discharged to the environment.

Oehme et al. (1989) also report that certain primary nickel refining processes generate CDDs and CDFs, primarily CDFs. Although the current low temperature process used at the Norwegian facility studied is estimated to release only 1 g TEQ per year, a high temperature NiCl2/NiO conversion process that had been used for 17 years at the facility is believed to have resulted in much more significant releases based on the ppb levels of CDFs detected in aquatic sediments downstream of the facility (Oehme et al., 1989).

Lexen et al. (1993) reported that samples of filter powder and sludge from a lagoon at the only primary aluminum production plant in Sweden showed no or little CDD/CDF.

3.6.7. Secondary Nonferrous Metal Smelting/Refining

Secondary smelters/refiners are establishments primarily engaged in the recovery of nonferrous metals and alloys from new and used scrap and dross. The principal metals of this industry both in terms of volume and value of product shipments are aluminum, copper, lead, zinc, and precious metals (U.S. DOC, 1990a). Scrap metal and metal wastes may contain organic impurities such as plastics, paints, and solvents.

Secondary smelting/refining processes for some metals (e.g., aluminum, copper, and magnesium) utilize chemicals such as NaCl, KCl, and other salts. The combustion of these impurities and chlorine salts in the presence of various types of metal during reclamation processes can result in the formation of CDDs and CDFs as evidenced by the detection of CDDs and CDFs in the stack emissions of secondary aluminum, copper, and lead smelters (Aittola et al., 1992; U.S. EPA, 1987; 1994b; 1994c; 1994d). Secondary Aluminum Smelters and Refiners

Levels of 2,3,7,8-TCDF in stack gas from an aluminum reclamation facility in the Finnish city of Vyborg have been measured at approximately 43 ng/m3 (Aittola et al., 1992). However, no studies of CDD/CDF emissions from secondary aluminum smelters located in the United States have been reported.

Aluminum is processed at more smelters than any other nonferrous metal in the United States. Also more aluminum undergoes secondary smelting than any other nonferrous metal. An estimated 1.7 million metric tons of aluminum were produced by secondary smelters in 1987 in the United States (U.S. DOC, 1990a). Secondary Copper Smelters and Refiners

Stack emissions of CDD/CDFs from a secondary copper smelter were measured by EPA during the National Dioxin Study (U.S. EPA, 1987). The tested facility recovers copper and precious metals from copper and iron-bearing scrap. The copper and iron-bearing scrap are fed in batches to a cupola blast furnace, which produces a mixture of slag and black copper. Four to five tons of metal-bearing scrap were fed to the furnace per charge, with materials typically being charged 10 to 12 times per hour.

Coke was used to fuel the furnace, and represented approximately 14 percent by weight of the total feed. During the stack tests, the feed consisted of electronic telephone scrap and other plastic scrap, brass and copper shot, iron-bearing copper scrap, precious metals, copper bearing residues, refinery by-products, converter furnace slag, anode furnace slag, and metallic floor cleaning material. Oxygen enriched combustion air for combustion of the coke was blown through tuyeres at the bottom of the furnace.

At the top of the blast furnace were four natural gas-fired afterburners to aid in completing combustion of the exhaust gases. Particulate emissions were controlled by fabric filters, and the flue gas then was discharged into a common stack. The estimated emission factors derived for this one site are: CDD/CDF = 3.89E+04 ng/kg of scrap metal smelted (range: 3.31E+04 to 4.05E+04 ng/kg); TEQ = 7.79E+02 ng/kg of scrap metal smelted (range: 7.64E+02 to 1.04E+03 ng/kg).

More than 0.3 million metric tons of copper were produced by the 24 secondary copper smelters operating in the United States in 1987 (U.S. DOC, 1990a). If the emission rates derived above are assumed to be representative of all secondary copper smelters, then the best estimate of annual air emission of CDD/CDF released by secondary copper smelting operations in the United States is 1.17E+04 grams per year and the best estimate of TEQ emission is 2.34E+02 grams per year.

A "high" confidence rating is given to the production estimate because it is based on reliable data from the U.S. 1987 Census of Manufactures. A "low" confidence rating is given to the emission estimates since they are based on direct measurements at only one U.S. copper smelter.

Based on these confidence ratings, the estimated range of potential annual emissions is assumed to vary by a factor of 10 between the low and high ends of the range. Assuming that the best estimate of annual emissions (234 g TEQ/yr) is the geometric mean of this range, then the range is calculated to be 74 to 740 g TEQ/yr. Secondary Lead Smelters and Refiners

The secondary lead smelting industry produces elemental lead through the chemical reduction of lead compounds (obtained primarily from scrap motor vehicle lead-acid batteries) in a high temperature furnace (1,200 to 1,260 degrees C).

Smelting is performed in reverberatory, blast, rotary, or electric furnaces. Blast and reverberatory furnaces are the most common types of smelting furnaces used by the 23 facilities that comprise the current secondary lead smelting industry in the United States. Of the 45 operating furnaces at these 23 facilities, 15 are reverberatory furnaces, 24 are blast furnaces, 5 are rotary furnaces, and 1 is an electric furnace.

The one electric furnace and 11 of the 24 blast furnaces are co-located with reverberatory furnaces and most share a common exhaust and emissions control system (U.S. EPA, 1994a). Furnace charge materials consist of lead-bearing raw materials, lead-bearing slag and drosses, fluxing agents (blast and rotary furnaces only), and coke.

Fluxing agents consist of iron, silica sand, and limestone or soda ash. Coke is used as fuel in blast furnaces and as a reducing agent in reverberatory and rotary furnaces. The PVC plastic seperators in the batteries are the primary source for HCl emissions from the smelters. However, the fluxing agents used at blast and rotary furnaces also react with chlorine to form calcium chloride or sodium chloride therby reducing HCl emissions from these furnaces relative to reverberatory furnaces.

Organic emissions from co-located blast and reverbertory furnaces are more similar to the emissions of a reverberatory furnace than the emissions of a blast furnace (U.S. EPA, 1994a).

The total annual production capacity of the U.S. lead smelting industry is 1.36 million metric tons. Blast furnaces not co-located with reverberatory furnaces account for 21 percent of capacity (or 0.28 million metric tons). Reverberatory furnaces and blast and electric furnaces co-located with reverberatory furnaces account for 74 percent of capacity (or 1.01 million metric tons).

Rotary furnaces account for the remaining 5 percent of capacity (or 0.07 million metric tons). Actual production volume statistics by furnace type are not available. However, if it is assumed that the total actual production volume of the industry (0.86 million metric tons in 1990) is reflective of the production capacity breakdown by furnace type, then the estimated actual production volumes of blast furnaces (not co-located), reverberatory and co-located blast/electric and reverberatory furnaces, and rotary furnaces are 180, 637, and 43 thousand metric tons, respectively (U.S. EPA, 1994a).

CDD/CDF and TEQ emission factors can be estimated for lead smelters based on the results of emission tests recently performed by EPA at three smelters (a blast furnace, a co-located blast/reverberatory furnace, and a rotary kiln furnace) (U.S. EPA, 1992i; 1993g; 1993h). The air pollution control systems at the three tested facilities consisted of both baghouses and scrubbers.

Congener-specific measurements were made at the exit points of both APCD exit points at each facility. Although all 23 active smelters employ baghouses, only 9 employ scrubber technology. Facilities that employ scrubbers account for 14 percent of the blast furnace (not co-located) production capacity, 52 percent of the reverberatory and co-located furnace production capacity, and 57 percent of the rotary furnace production capacity. From the reported data, TEQ emission factors (ng TEQ/kg lead recovered) for each of the three furnace configurations are presented below as a range reflecting the presence or absence of a scrubber.

Blast furnace: 0.63 to 8.31 ng TEQ/kg lead

Reverberatory/co-located furnace: 0.10 to 0.77 ng TEQ/kg lead

Rotary furnace: 0.28 to 0.21 ng TEQ/kg lead

If it is assumed that these emission rate ranges are representative of the range of emission rates at the non-tested facilities with the same basic furnace configuration and presence or absence of scrubbers, then combining these emission rate ranges with the estimates derived above for annual secondary lead production volumes yields a total industry-wide estimated emission to air of 1.6 g of TEQ. The estimated contributions to this total for each furnace configuration are:

Blast furnaces with scrubbers: 0.02 g TEQ/yr

Blast furnaces without scrubbers: 1.29 g TEQ/yr

Reverberatory/co-located furnaces with scrubbers: 0.03 g TEQ/yr

Reverberatory/co-located furnaces without scrubbers: 0.24 g TEQ/yr

Rotary furnaces with scrubbers: 0.01 g TEQ/yr

Rotary furnaces without scrubbers: 0.01 g TEQ/yr

A "medium" confidence rating is ascribed to the emission factors derived above because stack test data were available for 3 of the 23 active smelters in the United States and the stack test data used represent the three major furnace configurations. The "production" estimate has been assigned a "medium" confidence rating because, although it is based on a U.S. Department of Commerce estimate of total U.S. production, no production data were available on a furnace type or furnace configuration basis.

Based on these confidence ratings, the estimated range of potential annual emissions is assumed to vary by a factor of 5 between the low and high ends of the range. Assuming that the best estimate of annual emissions (1.6 g TEQ/yr) is the geometric mean of this range, then the range is calculated to be 0.7 to 3.5 g TEQ/yr.

3.6.8. Primary Ferrous Metal Smelting/Refining

Several European investigators have reported that iron ore sinter plants are sources of CDD/CDFs (Rappe et al., 1992b; Lexen et al., 1993; Lahl, 1993). However, insufficient information is available for evaluating CDD/CDF emissions from primary smelting/refining of ferrous metal in the United States.

Iron is manufactured from its ores (i.e., magnetic pyrites, magnetite, hematite, and carbonates of iron) in a blast furnace, and the iron obtained from this process is further refined in steel plants to make steel. During iron manufacturing, iron ores undergo sintering to enable better processing in the blast furnace. In the sintering process, iron ore fines are mixed with coke fines and the mixture is placed on a grate which is then heated to a temperature of 1093-1371oC.

The heat generated during combustion sinters the small particles. Also, iron-bearing dusts and slags from other processes in the steel plant are recycled as a feed mix for the sinter plant (Knepper, 1981; Capes, 1983). Lahl (1993) reported that the this management practice introduces traces of chlorine and organic compounds which are responsible for the generation of the CDD/CDFs found in these plants.

Sinter plants in Sweden and the Netherlands were reported to emit up to 3 ng TEQ/m3 stack gas or 2 to 4 g TEQ/yr per plant to the air (Rappe et al., 1992b; Lexen et al., 1993). Lahl (1993) report that emission data from plants in Germany indicate TEQ concentrations in stack gas after passage through mechanical filters and electrostatic precipitators ranging from 3 to 10 ng TEQ/nm3. Lahl (1993) estimated that, if all European sinter plants have stack concentrations of the same order of magnitude, then the total emission from sintering plants would be greater than 1 kg TEQ. This total is greater than the sum of all other identified European thermal sources of CDD/CDFs.

3.6.9. Secondary Ferrous Metal Smelting/Refining

Tysklind et al. (1989) found scrap ferrous metal processing to be a source of CDDs and CDFs at a steel mill in Sweden. Analyses showed the presence of CDDs and CDFs in the range of 0.1 to 1.5 ng TEQ/Nm3 dry gas in a plant with a 10-ton electric furnace. The higher values reportedly were obtained during the melting of metal with chlorinated materials (e.g., PVC plastics).

Raw gases collected over an open-furnace during batch jobs contained 110 ng TEQ/Nm3 dry gas when cutting oils containing chlorine were added to the scrap metal. The congener profiles of all flue gas samples showed that CDFs were predominant. The congener profiles also showed higher chlorine content when PVC was used.
Insufficient data exist to estimate emission factors for the U.S.

3.6.10. Scrap Electric Wire Recovery

The objective of wire recovery is to remove the insulating material and reclaim the metal (e.g., copper, silver, and gold) comprising the electric wire. The reclaimed metal is then sold to a secondary metal smelter. Wire insulation commonly consists of a variety of plastics, asphalt-impregnated fabrics or burlap. In ground cables, chlorinated organics are used to preserve the cable casing.

In the past, scrap electric wire was thermally treated in the United States to burn off the insulating material. However, according to industry and trade association representatives, current recovery operations typically no longer involve thermal treatment but instead involve mechanical chopping into fine particles from which the insulating material is removed by air blowing and gravitational settling of the heavier metal fraction (telephone conversation between R. Garino, Institute of Scrap Recycling Industries, and T. Leighton, Versar, Inc. on March 2, 1993; telephone conversation between J. Sullivan, Triple F. Dynamics, and T. Leighton, Versar, Inc., on March 8, 1993). No independent confirmation of this technology switch could be obtained from EPA program office representatives.

The combustion of chlorinated organic compounds catalyzed by the presence of wire metals such as copper and iron can lead to the formation of CDDs and CDFs (Van Wijnen et al., 1992). CDDs and CDFs have been detected in fly ash and bottom residues from the open-air incineration of wire scraps, and in stack samples of a wire reclamation incinerator (Chen et al., 1986; Southerland et al., 1987). Huang et al. (1992b) detected CDDs and CDFs in soils collected near electronic wire scrap incinerators used for the recovery of metals.

In these studies, the chlorinated compounds were considered to have been generated thermochemically from plastics covering the wires. Small-scale (and unpermitted) activities involving the incineration of scrap electrical wires have resulted in increased levels of CDDs and CDFs in soil samples collected from former scrap wire and car incineration sites within the vicinity of Amsterdam (Van Wijnen et al., 1992). Analysis of these soil samples showed CDD and CDF levels ranging between 60 and 98,000 ng/kg dry weight, with nine of fifteen soil samples having levels above 1,000 ng/kg dry weight.

Dioxin-like compounds emitted to the air from scrap electric wire incineration were measured from a facility during EPAs National Dioxin Study of combustion sources (U.S. EPA, 1987). The tested facility was determined to be typical of this industrial source category at that time. Insulated wire and other metal-bearing scrap material were fed to the incinerator on a steel pallet. The incinerator was operated in a batch mode, with the combustion cycles for each batch of scrap feed lasting between 1 and 3 hours. Incineration of the material occurred by burning natural gas.

Most of the wire had a tar-based insulation that was thermally removed; however, PVC coated wire was also fed to the incinerator. The estimated temperature during combustion was 650 C, and combustion preceded in a primary and secondary chamber. The tested facility was equipped with a high temperature afterburner to further destroy organic compounds entrained in the combustion gases prior to discharge to the air from the stack.

Emission factors estimated for this one facility include an average emission factor for TEQ of 1.18E-02 g/kg of scrap wire (range = 6.74E-03 to 1.69E-02 g/kg), and an average emission for total CDD/CDF of 9.89E-01 g/kg of scrap wire (range = 9.89E-01 to 3.28E+00 g/kg).

These emission factors are assigned a "low" confidence rating because the factors were derived from measurements at only one facility operating in the U.S. and it is not known how representative these test results are of other scrap electric wire incinerators. Although it is uncertain how many facilities still combust scrap wire, for purposes of this assessment, it is assumed that only minimal quantities of scrap wire are currently burned in the United States.

3.6.11. Drum and Barrel Reclamation and Incineration

Hutzinger and Fiedler (1991b) reported that the CDDs and CDFs are emitted in stack gases from drum and barrel reclamation facilities and that the concentration of CDDs/Fs found in those emissions range from 5 to 27 ng/m3. Dioxin-like compounds were measured by EPA in the stack gas emissions of a drum and barrel reclamation furnace as part of the National Dioxin Study (U.S. EPA, 1987).

These plants operate a burning furnace to prepare used steel 55-gallon drums for cleaning to base metal. The drums processed at these facilities come from a variety of sources in the petroleum and chemical industries. The cleaned drums are repaired, repainted, relined and sold for reuse. The drum burning process subjects used drums to an elevated temperature in a tunnel furnace for a sufficient time so that the paint, interior linings, and previous contents are burned or disintegrated.

The furnace is fired by auxiliary fuel. Used drums are loaded onto a conveyor that moves at a fixed speed. As the drums pass through the preheat and ignition zone of the furnace, additional contents of the drums drain into the furnace ash trough. A drag conveyor moves these sludges and ashes to a collection pit. The drums are air cooled as they exit they furnace. Exhaust gases from the burning furnace are drawn through a breeching fan to a high-temperature afterburner.

Emission factors of dioxin-like compounds were developed from EPA stack tests of a prototypical operation (U.S. EPA, 1987) yielding the following emission factors in units of g/kg: minimum TEQ = 1.12E-02; mean TEQ = 1.65E-02; maximum TEQ = 2.69E-02; mean CDD/CDF = 1.30E+00; minimum CDD/CDF = 1.16E+00; maximum CDD/CDF=1.85E+00.

Approximately 2.8 to 6.4 million 55-gallon drums are incinerated annually in the United States (telephone conversation between P. Rankin, Association of Container Reconditioners, and C. D'Ruiz, Versar, Inc., December 21, 1992).

This estimate is based on the following assumptions:
1) 23 to 26 incinerators are currently in operation;
2) each incinerator, on average, handles 500 to 1,000 drums per day; and
3) on average, each incinerator operates 5 days per week with 14 days downtime per year for maintenance activities.

The weight of 55-gallon drums varies considerably; however, on average, a drum weighs 38 lbs (or 17 kg). Therefore, an estimated 48 to 109 million kg of drums are estimated to be incinerated annually. Assuming that 109 million kg of drums are burned each year and applying the mean emission factors developed above, the best estimates of annual emissions are 140 grams per year of total CDD/CDF and 1.7 grams per year of TEQ.

A "low" confidence rating is assigned to the production estimate since it is based an expert judgement rather than a published reference. A "low" confidence rating is ascribed to the emission factor since it is developed from stack tests conducted by EPA on just one U.S. drum and barrel furnace and this one facility may not represent emissions from all current operations in the U.S.

Based on these confidence ratings, the estimated range of potential annual emissions is assumed to vary by a factor of 10 between the low and high ends of the range. Assuming that the best estimate of annual emissions (1.7 g TEQ/yr) is the geometric mean of this range, then the range is calculated to be 0.5 to 5.4 g TEQ/yr.

3.6.12. Tire Combustion

Emissions of dioxin-like compounds from the incineration of automobile tires were measured from a tire incinerator stack tested by the State of California Air Resources Board (CARB, 1991). The facility consists of two excess air furnaces equipped with steam boilers to recovery the energy from the heat of combustion. Discarded whole tires are fed to the incineration units at a rate of 3000 kg/hr.

The furnaces are equipped to burn natural gas as auxiliary fuel. The steam produced from the boilers is used to drive electrical turbine generators that produce 14.4 megawatts of electricity. The facility is equipped with a dry acid gas scrubber and fabric filter for the control of emissions prior to exiting the stack.

Emission factors for total CDD/CDF and TEQ in units of g/kg of tires combusted were derived as average values from the one tested facility stack tested in California (CARB, 1991). From these data, an average emission factor of CDD/CDF was estimated to be 1.39E-02 g/kg of tires incinerated (range: 4.28E-03 to 3.05E-02 g/kg), and average emission factor of TEQ was estimated to be 5.42E-04 g/kg (range: 1.91E-04 to 1.02E-03 g/kg).

EPA's Office of Solid Waste estimates that approximately 0.50 million metric tons of tires are incinerated in the United States annually (U.S. EPA, 1992a). This production estimate is given a "high" confidence rating since it is based on detailed study specific to the United States. The use of scrap tires as a fuel increased significantly during the late 1980s. In 1990, 10.7 percent of the 242 million scrap tires generated were burned for fuel. This percentage is expected to continue to increase (U.S. EPA, 1992a).

If it is assumed that 500 million kilograms of discarded tires are incinerated annually in the United States, then, using the emission factors derived from stack data from the one tested facility, an average of 6.9 grams of total CDD/CDF per year and an average of 0.3 grams of TEQ per year are estimated to be emitted to the air. It must be noted that these may be underestimates of emissions from this source category because the one facility tested in California is equipped with a dry-scrubber combined with a fabric filter for air pollution control.

These devices are capable of greater than 95 percent reduction and control of dioxin-like compounds prior to discharge from the stack. It is not know to what extent other tire incineration facilities operating in the U.S. are similarly controlled. If such facilities are not so equipped, then the uncontrolled emission of CDD/CDF and TEQ could be much greater than the estimates developed above.

Therefore, the estimated emission factor of dioxin from tire incineration is given a "low" confidence rating. Based on these confidence ratings, the estimated range of potential annual emissions is assumed to vary by a factor of 10 between the low and high ends of the range. Assuming that the best estimate of annual emissions (0.3 g TEQ/yr) is the geometric mean of this range, then the range is calculated to be 0.1 to 1.0 g TEQ/yr.

Buser et al. (1991) indicated that PCDTs (polychlorinated dibenzothiophenes, possibly a dioxin-like compound) can be formed in situations where large amounts of sulfur and chlorine-containing compounds are incinerated or accidentally burned. Automobile tires are known to contain sulfurous (vulcanization) compounds and certain types of chloro compounds (e.g., chloroprene). Thus, it is possible that the burning of used automobile tires could result in the formation of PCDTs.