Volume II Chapter 3.0 Pages 4 of 10 page next page 5

3.5.1. CDD/CDF Contamination in Fuel as a Source of Combustion Stack Emissions 3-64

3.5.2. Formation of CDDs/CDFs from Precursor Compounds 3-67

3.5.1. CDD/CDF Contamination in Fuel as a Source of Combustion Stack Emissions

The first theory states that CDD and CDF compounds present as contaminants in the fuel or waste products that are fed into the combustion chamber are responsible for dioxin and dibenzofuran emissions out the stack of the combustion process.

Most work in this area has involved the study of municipal solid waste incineration (MSWI) in which case CDDs and CDFs have been analytically detected in the raw refuse fed into the MSWI. Tosine, et al. (1983) first reported detecting trace amounts of HpCDD and OCDD in the MSW fed into an MSWI in Canada. HpCDD ranged in concentration from 100 ppt to 1 ppb, and OCDD ranged from 400 to 600 ppt. Wilken et al. (1992) separated the various solid waste fractions of MSW collected from municipalities in Germany and analyzed them for the presence of CDDs/CDFs and other organochlorine compounds.

Total CDDs/CDFs were detected in all MSW fractions in the following range of concentrations: paper and cardboard = 3.1 to 45.5 ppb; plastics, wood, leather, textiles combined = 9.5 to 109.2 ppb; vegetable matter = 0.9 to 16.9 ppb; and "fine debris" (defined as particles < 8 mm) = 0.8 to 83.8 ppb. Ozvacic (1985) measured CDDs/CDFs in the raw MSW fed into two MSWIs operating in Canada.

In one MSWI, CDDs were detected in the refuse in a range of concentration from 10 to 30 ppb, but no CDFs were detected (detection limit: 1 pg/g). In the MSW fed to the second MSWI, CDDs were detected in a range of 75 to 439 ppb, and CDFs were detected only in one of three samples at a total concentration of 11 ppb. EPA has reported on the detection of CDDs/CDFs in refuse derived fuel (RDF) burned in a large, urban MSWI (Federal Register, 1991). From 13 MSW samples taken prior to incineration, CDDs were detected in a range of 1 to 13 ppb, and CDFs were measured in a range of 0 to 0.6 ppb. In these samples, OCDD predominated, and the lower chlorinated congeners were not detected.

Despite these findings, the conditions of thermal stress imposed by the incineration process discounts the likelihood that the total magnitude of CDDs and CDFs, as measured in the raw MSW, can explain the total magnitude of concentration as an emission from the stack of the MSWI (Clement et al., 1990; Commoner, 1990). Contamination, however, may partially contribute to the stack release. Clement and coworkers (1988) performed a mass balance involving an input versus output of dioxin at two operational MSWIs in Canada.

These mass balance calculations clearly demonstrated that the mass of CDDs and CDFs emitted at the point of the stack was much greater than the mass in the raw MSW incinerated at the MSWIs, and that the profiles of the distributions of CDD/CDF congeners were strikingly different. Primarily, higher chlorinated congeners were detected as contaminants in the waste, whereas the total array of tetra - octa CDDs/CDFs were emitted from the stack.

Commoner and coworkers (1984; 1985; 1987) evaluated the test data of a mass burn MSW incinerator for the concentration of CDDs and CDFs at multiple sampling points during the combustion process:
(1) exit to the furnace;
(2) entry to the heat exchanger;
(3) inlet to the electrostatic precipitator (ESP);
(4) exit to the ESP; and
(5) exit to the smokestack.

Lowest or nondetectable concentrations of CDDs/CDFs were found at sampling point (1), and highest concentrations were measured at sampling point (5). From these sampling data, Commoner concluded that: CDDs/CDFs were not formed within the furnace region where the waste material was combusted and that usually only OCDD and OCDF were detected in extremely low concentrations at the point of exit to the furnace (if dioxins were detected at all).

It was also concluded that the CDDs/CDFs were mostly formed as a synthesis process catalyzed by the properties of fly ash in combination with chlorine, and that this probably transpired within areas downstream of the combustion zone where the combustion offgases had cooled to less than 400 C. Commoner et al. (1984, 1987) ruled out the effectiveness of combustion as a major factor in CDD/CDF emissions from the stack; this would be expected if waste contamination was solely responsible for the emission. This phenomena was independently observed by Environment Canada in a series of tests of a modular MSW incinerator (Hay, et al., 1986; Environment Canada, 1985).

On a mass balance basis, the concentration of CDDs and CDFs measured at the stack was approximately two orders of magnitude higher as compared to the inlet to the boiler just after exiting the secondary furnace. The temperatures of the combustion gases at these two points of measurement were 130 and 740 C at the stack and boiler inlet, respectively (Environment Canada, 1985). For the most part, only OCDD was present in the hot gases exiting the furnace, whereas all the congeners were present in the stack emissions, thus giving further evidence that CDDs/CDFs are formed after the combustion zone.

Using similar protocols, EPA and Environment Canada (1991) jointly evaluated the emission of CDDs and CDFs from a refuse-derived fuel MSWI operating in the United States. It was found that approximately 5 milligrams of total CDDs and CDFs per metric ton of MSW burned by the facility were measured in the raw MSW prior to combustion, but no CDDs nor CDFs were detected at the point of exit to the furnace prior to the inlet to the economizer (i.e., the heat exchanger used to extract additional heat from the hot gases). Once heat in the combustion gas was extracted for energy purposes and the gases were further cooled to less than 400 C, the total array of tetra- through octa-CDDs and CDFs could be detected.

These series of experiments in which the mass balance of CDD/CDF was estimated within the entire combustor, beginning with the waste and ending with the stack, discount the first theory of dioxin formation (i.e., that dioxin in the feed accounts for all emissions of dioxin from the stack to the air). Moreover, it is expected that the conditions of thermal stress imposed by typical incineration and other combustion sources would destroy and reduce the CDDs and CDFs present as contaminants in the waste to levels that are 0.0001 to 10 percent of the initial concentration, depending on the performance of the combustion source and the level of combustion efficiency. Stehl et al. (1973) demonstrated that the moderate temperature of 800 C enhances the decomposition of CDDs at a rate of about 99.95 percent, but that lower temperatures result in a higher survival rate.

Theoretical modeling has shown that unimolecular destruction of CDDs/CDFs at 99.99 percent can occur at the following temperatures and retention times within the combustion zone: 977 C with a retention time of 1 second; 1000 C at a retention time of 1/2 second; 1227 C at a retention time of 4 milliseconds; and 1727 C at a retention time of 5 microseconds (Schaub and Tseng, 1983). Thus, CDDs and CDFs would have to be in parts per million concentration in the feed to the combustor to be found in the part per billion or part per trillion levels in the stack gas emission (Shaub and Tseng, 1983). However, it cannot be ruled out is that CDDs/CDFs in the waste or fuel may contribute (up to some percentage) to the overall concentration leaving the stack.

3.5.2. Formation of CDDs/CDFs from Precursor Compounds

The second theory states that the production of CDDs and CDFs is a direct result of in-situ thermal degradation of precursor compounds during or after combustion of organic materials. Present theory is mostly derived from laboratory experiments involving the heating of suspect precursors in quartz ampules under starved-air conditions, and in experiments investigating the role that combustion fly ash has in promoting the formation of CDD/CDFs from precursor compounds.

Liberti and Brocco (1982) postulated that the general reaction that may be taking place in a typical combustion process is a thermolytic synthesis and interaction between two families of precursors indicated by A and B. Precursors A are aromatic compounds having a definite phenolic structure (e.g., phenol and polychlorinated phenols), and precursors B are chemical species that can act as a chlorine donor (e.g., PVC and HCl). Esposito et al. (1980) offered a chemical basis for defining a dioxin precursor:

1. The compound is comprised of an ortho-substituted (positions 1 and 2 on the compound) benzene ring in which one of the substituents is an oxygen atom directly attached to the ring, and

2. It then must be possible for the two substituents on the benzene ring to react with each other to form a new and independent compound under the influences of heat and pressure (i.e., dioxin).

Dickson and Karasek (1987) further refined this definition to be consistent with the formation kinetics thought to occur within combustion processes. In their definition, the term "precursor" refers specifically to chlorinated aromatic compounds that are either already present on the surface of combustion fly ash, or are present in the gas phase prior to entering a critical region outside the combustion zone where the gases have cooled and where heterogeneous catalyzed reactions take place that form CDDs/CDFs. Chlorophenols and chlorobenzenes were identified as ideal precursor compounds in these reaction pathways.

Controlled laboratory combustion experiments involving the thermal degradation of aromatic compounds, either singly or in mixtures, have provided useful data in identifying ideal precursor compounds. For example, Jansson and coworkers (1977) generated CDDs through the pyrolysis of wood chips treated with tri-, tetra-, and penta-chlorophenol in a bench-scale furnace operated at 500-600 C. Stehl and Lamparski (1977) combusted grass and paper treated with the herbicide 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in a bench-scale furnace at 600-800 C and generated ppmv levels of TCDD.

Ahling and Lindskog (1982) have reported on the formation of CDDs during the combustion of tri- and tetrachlorophenol formulations at temperatures of 500-600 C. Decreases in oxygen during combustion generally increased the yield, and the addition of copper salts to the tetrachlorophenol formulation significantly enhanced the yield of CDDs. Combustion of pentachlorophenol resulted in low yields of CDDs except when burned with an insufficient supply of oxygen.

In that case, the investigators noted the formation of tetra- through octa-chlorinated congeners. Buser (1979) generated CDDs/CDFs on the order of 0.001-0.08 percent (by weight) by heating tri-, tetra-, and pentachlorobenzenes at 620 C in quartz ampules in the presence of oxygen. It was noted that chlorophenols were formed as combustion by-products, and Buser (1979) speculated that these were acting as reaction intermediates in the formation of CDDs/CDFs.

Recently it has been demonstrated that CDDs and CDFs are formed from aromatic precursor compounds adsorbed onto the reactive surface of fly ash (particulate matter) entrained in the combustion plasma. Moreover, formation occurs outside and downstream of the combustion zone of a furnace to a combustion source in regions where the temperature of the combustion offgases has cooled to between 200 and 400 C (Vogg et al., 1987; Bruce et al., 1991; Cleverly et al., 1991; Gullet et al., 1990a; Commoner et al., 1987; Dickson and Karasek, 1987; Dickson et al., 1992).

Vogg and coworkers (1987) have shown that inorganic chloride ions, such as copper chloride, present in the combustion gas may act as a catalyst to promote surface reactions on particulate matter to convert aromatic precursor compounds to chlorinated dioxins and dibenzofurans. After carefully extracting organics from MSWI fly ash, Vogg et al. (1987) added a known concentration of isotopically labeled CDDs/CDFs to the matrix. The MSWI fly ash was then heated in a laboratory furnace at varying temperatures for 2 hours. The treated fly ash was exposed to increasing temperatures in 50 C increments in a temperature range of 200 to 400 C. Table 3-21 summarizes these data.

Because the relative concentration of CDDs/CDFs increased while exposed to varying temperature, Vogg, et al. (1987, 1992) concluded that formation of CDDs and CDFs from precursor compounds on the surface of fly ash transpires during MSW incineration within a specific range of temperature, 250 to 450 C. Within this range, the concentration of CDDs/CDFs increases to some maxima, and outside this range the concentration diminishes.

Vogg et al. (1987) proposed an oxidation reaction pathway giving rise to the formation of CDDs and CDFs in the post-furnace regions of the incinerator in the following order:

Table 3-21Concentration of CDDs/CDFs on Municipal Incinerator Fly Ashat Varying Temperatures
(1) hydrogen chloride gas (HCl) is thermolytically derived as a product of the combustion of heterogeneous fuels containing abundant chlorinated organic chemicals and chlorides;
(2) oxidation of HCl, with copper chloride (CuCl2) as a catalyst, yields free gaseous chlorine;
(3) phenolic compounds (present from combustion of lignin in the waste or other sources) entrained in the combustion plasma are substituted on the ring structure by contact with the free chlorine; and
(4) the chlorinated precursor to dioxin (e.g., chlorophenol) is further oxidized (with copper chloride as a catalyst) to yield CDDs and CDFs and chlorine.
Gullett and coworkers (1990a; 1990b; 1991a; 1991b; 1992) have studied the formation mechanisms
expand table Table V2 3-21

through extensive combustion research at EPA, and have verified the observations of Vogg et al. (1987). It was proven that CDDs and CDFs could be ultimately produced from low temperature reactions (i.e., 350 C) between Cl2 and a phenolic precursor combining to form a chlorinated precursor, followed by oxidation of the

chlorinated precursors (catalyzed by a copper catalyst such as copper chloride) as in examples (1) and (2), below.

(1) The initial step in the formation of dioxin is the formation of chlorine from HCl in the presence of oxygen (the Deacon process), as follows (Vogg et al., 1987; Bruce et al., 1991):

Diagram V2 3-1

(2) Phenolic compounds adsorbed on the surface of fly ash are chlorinated to form the dioxin precursor, and the dioxin is formed as a product from the breakdown and molecular rearrangement of the precursor. The reaction is promoted by the presence of heat and copper chloride acting as a catalyst (Vogg et al., 1987; Gullett et al., 1992):

Diagram V2 3-2

The major direct source of chlorine available for participating in the formation of CDDs/CDFs is gaseous HCl, which is initially formed as a combustion by-product from the chlorine and chlorinated organic chemicals contained in the MSW (and other fuels) (Vogg et al., 1987; Bruce et al., 1991; Cleverly, 1984; Commoner et al., 1987). MSW contains approximately 0.45-0.90 percent (by weight) chlorine (Domalski et al., 1986). MSW incinerators are a major stationary combustion source of air emissions of HCl, which average between 400 to 600 ppm in the combustion gas (U.S. EPA, 1987). HCl is converted to chlorine vapor by the Deacon process, and the vapor phase chlorine directly chlorinates a dioxin precursor along the aromatic ring structure.

Oxidation of the chlorinated precursor in the presence of an inorganic chloride metal catalyst (of which copper chloride was found to be the most active) yields CDDs and CDFs. Increasing the yield of chlorine in vapor phase from the oxidation of HCl generally causes an increase in the rate of formation of CDDs/CDFs. Formation kinetics are most favored at temperatures between 200 to 350 C.

Reductions in chlorine production, either by limiting initial HCl concentration or by shortening the residence time in the Deacon process temperature window, should result in decreases in the rate and magnitude of formation of CDDs and CDFs (Bruce et al., 1991; Gullett et al., 1990b; Commoner et al., 1987). Bruce and coworkers (1991) observed a general increase in the formation of CDDs and CDFs with increases in the vapor phase concentration of chlorine. Figure 3-2 shows the apparent dependence of the extent of formation of CDDs and CDFs upon chlorine concentration in
the vapor phase. Bruce et al. (1991) verified a dependence on the concentration and availability of gaseous chlorine in the thermolytic formation of CDDs/CDFs.

In the testing of a variety of industrial stationary combustion sources during the National Dioxin Study in 1987, EPA made a series of qualitative observations on the relationship between total chlorine present in the fuel/waste and the magnitude of emissions of CDDs and CDFs from the stack of the tested facilities (U.S. EPA, 1987). In general, combustion units with the highest CDD emission concentrations had greater quantities of chlorine in the fuel, and, conversely, sites with the lowest CDD emission concentrations contained only trace quantities of chlorine in the feed. The typical chlorine content of various combustion fuels has been reported by Lustenhouwer et al. (1980) as: coal: 1,300 g/g; MSW: 2,500 g/g; leaded gasoline: 300-1,600 g/g; unleaded gasoline: 1-6 g/g.

Figure 3-2The Association Between Vapor Phase C12and the Formation of CDDs/CDFs
figure Formation of CDDs figure Formation of CDFs
expand Figure V2 3-2 i expand Figure V2 3-2 ii

The role that temperature plays in the formation kinetics has been investigated by Oberg et al. (1989) on a full-scale hazardous waste incinerator operating in Sweden. Oberg confirmed that the formation of CDDs/CDFs occurs after the furnace. Most of the formation transpired in the boiler used to extract heat for co-generation of energy. In this investigation, significant increases in total concentration of dioxin TEQ occurred between temperatures of 280-400 C, and concentrations declined at temperatures above 400 C. This is in agreement with the experimental evidence of the temperature range defined as the "window of opportunity" for catalytic formation of CDDs/CDFs on the surfaces of fly ash particles.

Dickson and Karasek (1987) have demonstrated that CDDs/CDFs can be directly formed from the thermal conversion and oxidation of chlorinated precursors, in particular chlorophenols, on the surface of MSWI fly ash while heated in a bench-scale furnace. Their experiment was designed to mimic conditions of MSW incineration; to identify the step-wise chemical reactions involved in converting a precursor compound into dioxin, and to determine if MSWI fly ash could promote these reactions. MSWI fly ash was obtained from a facility in Canada and a facility in Japan.

The MSWI fly ash was extensively solvent-extracted for any organic constituents prior to initiating the experiment. Twenty grams of fly ash were introduced into a bench-scale oven (consisting of a simple flow-tube combustion apparatus) and heated at 340 C overnight to desorb any remaining organic compounds from the matrix. 13C12 -labeled pentachlorophenol (PCP) and two trichlorophenol isotopes (13C12- 2,3,5-T and 3,4,5-T) were added to the surface of the clean fly ash matrix, and placed into the oven for 1 hour at 300 C. Pure inert nitrogen gas (flow rate of 10 ml/min) was passed through the flow tube to maintain constant temperatures. Tetra- through octa- CDDs were formed from the labeled pentachlorophenol experiment; over 100 g/g of total CDDs were produced.

The congener pattern was similar to the congener pattern found in MSWI emissions. The 2,4,5-T experiment primarily produced HxCDDs and very small amounts of tetra- and octa-CDD. The 3,4,5-T experiment mainly produced OCDD and 1,2,3,4,6,7,8-HpCDD. Dickson and Karasek (1987) proposed that the chlorinated phenol may undergo molecular rearrangement or isomerization as a result of dechlorination, dehydrogenation, and trans-chlorination before condensation occurs to ultimately form CDDs on the fly ash surface. These reactions ultimately dictate the types and amounts of CDDs that are formed.

Nestrick and coworkers (1987) reported on the thermolytic reaction between benzene (an unsubstituted precursor) and iron (III) chloride on a silicate surface to yield CDDs/CDFs at temperatures 3 150 C. The experimental protocol was to introduce 100 - 700 mg of native and 13C6-benzene into a macro-reactor system consisting of a benzene volatilization chamber connected to a glass tube furnace.

The investigators noted the relevance of this experiment to generalizations about combustion processes because benzene is the usual combustion by-product of organic fuels. Inert nitrogen gas was used to carry the benzene vapor to the furnace area. The exit to the glass tubing to the furnace was plugged with glass wool, and silica gel was introduced from the entrance end to give a bed depth of 7 cm to which the FeCl3 was added to form a FeCl3/silica reagent.

The thermolytic reaction took place in a temperature range of 150-400 C at a residence time of 20 minutes. Although di- through octa-CDD/CDF were formed by this reaction at all the temperatures studied, the percent yields were extremely small. Table 3-22 summarizes these data.