Dibenzofuran (PCDD

Dibenzo-p-dioxin/Dibenzofuran. (PCDD/F) Emission in Municipal. Solid Waste Incinerators. DONGHOON SHIN, †. SANGMIN CHOI,* , †. JEONG-EUN OH, ‡...
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Environ. Sci. Technol. 1999, 33, 2657-2666

Evaluation of Polychlorinated Dibenzo-p-dioxin/Dibenzofuran (PCDD/F) Emission in Municipal Solid Waste Incinerators D O N G H O O N S H I N , † S A N G M I N C H O I , * ,† JEONG-EUN OH,‡ AND YOON-SEOK CHANG‡ Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-Dong, Yusong-Gu, Taejon 305-701, Korea, and School of Environmental Engineering, Pohang University of Science and Technology, San 31, Hyoja-Dong, Nam-Gu, Pohang, Korea

This paper summarizes an engineering approach taken for 10 commercial MSW incinerators to formulate a basis for PCDD/F emission reduction schemes and to interpret the measurement results before and after the plant modifications of moderate scale. Control strategy of PCDD/F emission from the operating incinerators can be established on the basis of the interpretation of reliable measurements as well as understanding of the formation/destruction processes. The fate of PCDD/F is known to be influenced by the flue gas subprocesses: the initial formation/ oxidation in the combustion chamber, reformation through synthesis, removal by adsorption, and catalytic destruction. A simplified model is proposed for PCDD/F level at various stages of the flue gas, basically integrating the reaction kinetic rates at the corresponding state. Since the kinetic rate is dependent on temperature, time-temperature history of the flue gas is considered as the important input data. Using the plant design and operating conditions, this global prediction model calculates the PCDD/F level not only at the stack but also at the process midstream. The model predictions are discussed along with measurement results from the different design of incinerator plants.

Introduction Incineration has become an economical method of municipal solid waste treatment; reducing its volume and weight and producing thermal energy. However, incineration is seen to the general public as the secondary pollution generation, and among others, emission of PCDD/F is the subject that the incineration industry confronts. Various research has been carried out to uncover the formation mechanism of PCDD/F. They include measurement experiences in actual incineration systems (1-16) and lab-scale apparatus (17-36) and several theoretical studies (20, 37, 38). Various design and operation parameters that influence the emission of PCDD/F have been identified. Among them, process temperature is believed to be the most influential parameter (21-36). Temperature in an incineration chamber is recommended to be high enough to destroy * Corresponding author telephone: +82-42-869-3030; fax: +8242-862-1284; e-mail: [email protected]. † Korea Advanced Institute of Science and Technology. ‡ Pohang University of Science and Technology. 10.1021/es980932r CCC: $18.00 Published on Web 06/25/1999

 1999 American Chemical Society

unburned hydrocarbons (39). Meanwhile, the temperature range of 250-450 °C in off-gas treatment systems such as electrostatic precipitator (ESP) or waste heat recovery boiler is adequate for the formation of PCDD/F (2, 5, 10-13). Metals in dust and particulates can catalyze the formation reaction of PCDD/F (19, 22, 27, 34-36), and the dust concentration in the flue gas has a close relationship with the PCDD/F emission (14, 40). Chlorine is another important reactant of PCDD/F formation (17, 19, 27, 29). Several measurement studies showed that the gaseous chemical species such as oxygen, CO, and HCl represented weak correlation with PCDD/F emission in incineration systems, which is still controversial (29, 40-42). Bag filter and selective catalytic reactor (SCR) are efficient facilities for removal of PCDD/F (2, 3, 11, 14), and other methods were also recently introduced (43-48). In the present study, the formation and removal mechanisms of PCDD/F in MSW incineration processes are discussed in relation with emission measurement results. The emission measurements have been performed in 10 MSW incineration systems, which are in commercial operation and have various combustor types and flue gas cleaning processes, and operating conditions were monitored to evaluate the incineration performance. In the meantime, the published literature on PCDD/F formation and removal mechanisms is reviewed to uncover the influential parameters and their effectiveness. On the basis of this review, empirical models on the formation and removal of PCDD/F are introduced and adopted to estimate the PCDD/F emission in MSW incinerators.

PCDD/F Emission Measurements In Korea, municipal solid waste incinerators have been built since the late 1980s, and 10 plants are currently in normal operation, as shown in Table 1. Most of the incinerators are of stoker grate type except one roller grate type. Six incineration systems adopted the flue gas cleaning system with electrostatic precipitator (ESP) and wet scrubber, and three of them are with a selective catalytic reactor (SCR). Four other incineration plants have flue gas cleaning systems of spray-drying absorber (SDA) and bag filter (two with SCR). After the measurement results were made known to the public in 1997, incinerators with emission levels of PCDD/F higher than 0.1 ng of TEQ/Nm3 were modified to reduce the emission at the stack. Four incinerators added dry injection (DI) of activated carbon (AC) in front of dust collectors (nos. 1, 2, 6, and 9), and four incinerators added activated carbon to scrubbing solutions (nos. 1, 3, 5, and 7). One incinerator added SCR at the end of the stream (no. 3). The different flue gas treatment equipment results in different flue gas time history, which determines the formation and destruction of PCDD/F in the flue gas stream. In the systems with ESP and wet scrubber, dusts are removed previously to acidic gases, and the exit gas temperature of the wet scrubber is below 100 °C. Meanwhile, acidic gases in the systems with SDA/DI and bag filter are removed prior to dust, and the exit gas temperature is generally around 150 °C at the filter bag house. Hence, process temperature and pollutants composition of the flue gas should be integrated to determine the emission of PCDD/F at the stack. The operating conditions of each incinerator are summarized in Table 2. Calculation of the residence time in each equipment is based on the plant basic design data. PCDD/F concentration in the flue gas is measured following the Korean standard measurement procedure (a modified version of EPA MM5, Figure 1): Flue gas is sucked VOL. 33, NO. 15, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY



TABLE 1. Operating Municipal Solid Waste Incinerators in Korea off-gas treatment systems



type (ton/day)

before modification (1997)

after modification (1998)

1 2 3 4 5 6 7 8 9 10b

stoker grate (200) stoker grate (200) stoker grate (200) roller grate (400) stoker grate (300) stoker grate (200) stoker grate (200) stoker grate (200) stoker grate (200) stoker grate (200)

ESP, wet scrubber ESP, wet scrubber DI(AC), ESP, wet scrubber ESP, wet scrubber, SCR ESP, SCR, wet scrubber ESP, wet scrubber, SCR SDA, bag filter SDA (AC), bag filter, SCR SDA, bag filter SDA, bag filter, SCR

DI(AC), ESP, wet scrubber (AC) DI(AC), ESP, wet scrubber DI(AC), ESP, wet scrubber (AC), SCR nma ESP, SCR, wet scrubber (AC) DI(AC), ESP, wet scrubber, SCR SDA (AC), bag filter nm SDA, DI(AC), bag filter nm

nm, not modified.


Commission in 1998.

TABLE 2. Operating Conditions of the Incineratorsa furnace exit temp (°C)

no. 1 2 3 4 5 6 7 8 9 10 a

850 900 1100 910 900 850 850 900 925 950

boiler res. time (s) 6 6 6 6 6 6 6 6 6 6

dust collector exit temp (°C)

260 270 170 250 (212) 290 (271) 240 (226) 230 206 260 185

activated carbon (mg/Nm3) (80) (80) 80 0 0 (80) (50) 50 (50) 50

Values after modification (1998) are given in parentheses.


res. time (s) 15 15 15 15 15 15 12 12 12 12

SCR exit temp (°C)

area velocity (Nm3 m-2 h-1)

V2O5 (%)

temp (°C)

250 (230) 260 (230) 160 240 (210) 290 (259) 230 (220) 170 140 160 145


na na (3) 3 1.5 3 na 3 na 3

na na (274) 310 290 (259) 310 na 220 (195) na 280

na (7.5) 4 (3) 3.45 7.34 na 4 (3) na 7.5

na, not available.

FIGURE 1. Schematic of Korean standard method for dioxin sampling in stack. isokinetically, and PCDD/F is collected while passing through a circular filter, two water bottles, an adsorption tube (XAD2), and a diethylene glycol bottle. At each measurement, more than 3 Nm3 of stack gas was sampled for about 3 h at the condition of isokinetic factor to be within 95-110%. The PCDD/F concentrations of the collected samples were analyzed by HRGC/HRMS. In 1998, the concentrations of PCDD/F between system elements in several incinerators were also measured. The measurement results are shown in Table 3 for the series executed in 1997 (before the modification) and 1998 (after the modification). In 1997, eight incinerators emitted PCDD/F over the regulation level (0.1 ng of TEQ/Nm3), and 2658



two systems emitted hundreds times of the regulation level (nos. 1 and 2). The average emission was 5 ng of TEQ/Nm3 in 1997. After the plant system improvement, the emission level decreased significantly, and the average was 0.73 ng of TEQ/Nm3. However, six plants need further improvement to satisfy the regulation.

Prediction Model Modeling Strategy. Flue gas conditions, which are believed to be related with PCDD/F formation destruction such as temperature, dust, and unburned hydrocarbon concentration, can (and does) change along the flue gas passage. Temperature of the flue gas changes significantly while it

TABLE 3. PCDD/F Emission Levels of the Incineration Plants (12% O2 Basis) PCDD/F concentration (ng of TEQ/Nm3) 1997 no. 1 2 3



6 7 8

9 10




stack boiler exit ESP exit stack stack boiler exit ESP exit wet scrubber exit stack boiler exit ESP exit wet scrubber exit stack boiler exit ESP exit SCR exit stack stack SDA exit stack boiler exit SDA exit bag filter exit stack stack boiler exit bag filter exit



1998 3rd








23.13 0.334

3.283 0.058



4.548 0.07 1.892 0.444 0.439 0.088 5.732 0.819 1.211 0.82 2.771 3.991 0.459 0.198 0.193 1.8 0.021

1.942 4.713 1.346 4.963

36.5 0.517








0.83 1.158

1.017 0.909


0.757 1.034





0.359 0.029 0.7 0.026 2.473 0.324 0.077 0.434 0.027 0.51 0.079



passes through the waste heat boiler, water cooler, SDA, and wet scrubber. Particulates are removed in the dust collectors (ESP and bag filter). Acidic gas (SOx and HCl) is removed in scrubbers (DI, SDA, and wet scrubber). Hence, prudent consideration of these factors along the flue gas passage is indispensable in predicting the PCDD/F concentration. In the simplest possible manner, integration of the formation and removal reaction kinetics along the flue gas time history produces the PCDD/F level at the corresponding state of the flue gas:


5.016 0.15

avg stack emission



(rformation - rremoval) dt

where rformation is the formation kinetics and rremoval is the removal kinetics. Integration must be performed along the time period where a flue gas volume element experiences from the incineration chamber to the appropriate flue gas location, ultimately leading to the stack. This simple model would then require initial conditions, kinetic rates, and its controlling parameters and operation conditions, which would affect the kinetic rates. As an engineering approach, the above strategy can be applied by employing appropriate sets of kinetic rates and supplying the corresponding flue gas conditions. The following is a description of this strategy focusing on the formation by synthesis, the removal by adsorption to the particulate surface, and the destruction in the catalyst. Formation of PCDD/F. A review of the recent publications regarding effective parameters on PCDD/F formation appears in the Appendix. The general consensus is that the formation rate would reach its maximum at approximately 400 °C for the flue gas condition of the actual incineration system, and loads of dust, chlorine, and precursors can be considered as major influential parameters of PCDD/F formation. To predict the formation of PCDD/F in the incineration process, the data including the reaction of gas-phase


0.283 0.011 0.755 0.037


avg 1.409 4.265 0.064




0.279 0.111 1.2 0.024






0.567 0.019


precursors is adopted (nos. 12-16 in Table 4). Unfortunately, the reported data were not suitable to determine realistic reaction kinetics, as the flue gas condition of actual plants is different from that of lab experiments. To extract the effect of temperature, the data are normalized with the maximum formation rate (at 400 °C), which is shown in Figure 2 and appears to be of Gaussian function in temperature; hence, the effect of temperature on the formation reaction is modeled as

dCPCDD/F ∝ exp[(T - 673)2/4500] dt Loads of dust, precursor, and chlorine source should join the formation kinetics as

dCPCDD/F ) k1CdustCprecursorCchlorine exp[(T - 400)2/4500] dt where CA is the concentration of A (ng/Nm3) in the flue gas, and k1 is the reaction coefficient. Cchlorine can be calculated by elemental analysis of the raw waste. Measurement data of dust and precursor load are rare. Several studies reported that the dust concentration of raw flue gas is several g/Nm3 (1, 11, 14, 20). Heeb et al. (7), Theodore et al. (4), and Hunsinger et al. (1) showed that the concentration of precursors such as chlorophenol and chlorobenzene varied in the range of 5-40 µg/Nm3 depending on the operating conditions. However, these parameters are strongly affected by the design and operating conditions of the incineration system as well as the raw waste composition. Hence, a drastic assumption is necessary to reach a realistic prediction model of PCDD/F emission: The concentrations of dust, precursors, and chlorine, which are difficult to measure properly (particularly at incineration chambers), are assumed to be independent of incinerators based on the VOL. 33, NO. 15, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY



TABLE 4. Related Studies on Effect of Temperature on PCDD/Fs Formation Rate peak temp of formation (°C)

experiment and analysis no.



1 Karasek (21) fly ash, fire brick 2 Stieglitz (22) 1) carbon-free fly ash 2) Mg-Al-silicate with charcoal, potasium chloride, CuCl2 3 Stieglitz (23) fly ash 4 Dickson (24) silica gel, CuCl2, activated carbon, chlorophenol 5 Addink (25) fly ash, activated carbon 6 Milligan (26) fly ash 7 Stieglitz (27) 1) fly ash

8 Stieglitz (28) 9 Addink (29) 10 Milligan (30) 11 Schoonenboom (31) 12 Born (32) 13 Milligan (33) 14 Froese (34)

15 Gullett (35) 16 Gullett (36)

2) extracted ash, charcoal 3) mixture of KCl, CuCl2, and FeCl3 (or Zn) carbon-free fly ash, carbon Fly ash, activated carbon fly ash, carbon KCl, CuCl2, activated carbon, alumina carbon-free fly ash fly ash

gas flow

gas-phase absorber

temp (°C)



formation source de novo precursor synthesis reaction

N2, chlorophenol 250, 300, 350 air + H2O 250, 300, 350

no no

300 300

na 300

yes yes

yes no

air dry air

250, 300, 350 250,300,350

no silica gel

300 300

300 na

yes yes

no yes


200, 250, 300, 350, 400 water





10% O2 in N2

275-350, step: 25



air, SO2, HCl, Cl2, H2O

300, 400, 500, 600



methylene- 300, 325 300, 325 chloride 3001,2 toluene 3001,2 3503


air, H2O

300, 350






10% O2 in N2, Cl2 or HCl 10% O2 in N2 air

298, 323, 348, 373, 398 toluene





300, 325, 350 300, 350, 400

hexane hexane

325 350


yes yes

no no







350, 400 na














O2, N2, H2O, 200-450, step: 20 HCl, phenol 10% O2 in N2, 250-400, step: 25 chlorophenol 20% O2 in N2, 300, 400, 500, 600 acetylene

1) carbon-free fly ash 2) metal oxides mixture various metal 10% O2 in N2 200, 300, 400, 500 oxides, chlorophenol CuO, or CuSO4, 10% O2 in N2, 300, 400, 500 chlorophenol HCl, or Cl2 or SO2 + phenol

(carbon) toluene, 400 charcoal toluene, 400 charcoal

phase depending on temperature (1, 23, 28, 29, 33). Husinger et al. (1) measured PCDD/F at the boiler exit and reported that most of PCDD/F stayed in the gas phase although the flue gas temperature was low at 220 °C, which meant that the gas-phase PCDD/F was hardly adsorbed to fly ash while in the waste heat boiler. The formation rate can be calculated by integrating the kinetics along the flue gas time history:


FIGURE 2. Relative formation rates of PCDD/F (relative to 400 °C case). An asterisk (*) indicates data for the case of 700 mg/L of chlorophenol. assumption that the flue gas condition may be similar. Then the kinetics is simplified as a function of temperature only:

dCPCDD/F ) k2 exp[(T - 673)2/4500] dt This global kinetics model represents total PCDD/F, a portion of which may stay in the gas phase or in the solid 2660





k2 exp[(T - 673)2/4500] dt

The formation rate of each congener tends to be arbitrary, and it is difficult to find out any rule for the emission pattern even at one system as shown in Table 3 and in other publications (3, 5, 12, 39). To simplify the procedure, the TEQ value is modeled. As shown in Figure 3, the total PCDD/F amount (summation of the toxic congeners of from tetra- to octa-CDD/F) can be calculated by multiplying the TEQ value by a factor ()12 in the present study), although uncertainty still exists. To derive the coefficient k2, several published data and the measurement results of the present study are adopted as shown in Figure 4, and the value of k2 is selected to be 4 ng of TEQ s-1 Nm-3. This simplified PCDD/F formation model contains serious limitations that should be discussed in the future studies: no theory on the Gaussian expression of the temperature effect, drastic assumptions on the effect of other

FIGURE 3. Relationship of TEQ and total PCDD/F (>Tri-CDD/F): r 2 ) 0.937.

FIGURE 5. Effect of temperature on adsorption site (r 2 ) 0.973).

Aa ) ηaAtotal ηa ) exp (-0.0136T)

FIGURE 4. PCDD/F formation in waste heat boiler depending on boiler exit temperature (r 2 ) 0.758). parameters (dust, chlorine, and precursors). And the assumption of the ratio of the TEQ to total PCDD/F, which is modeled to be 12 in the present study, should be discussed with other plants. Possible influential phenomena such as memory effect, corona effect in ESP, etc. need to be understood for the correct prediction of PCDD/F generation at MSW incinerators. Removal of PCDD/F by Adsorption. Removal of pollutants by adsorption is one of the well-known processes. Fabric bag filter, activated carbon tower, and, recently, wet adsorption techniques have been applied in the flue gas treatment facilities of municipal solid waste incinerators (10, 11, 14, 49, 50). In particular, a fabric bag filter house with activated carbon injection is popular for the removal of PCDD/F (2, 11, 14). Fujii et al. showed that fly ash cake on the bag filter surface was effective to adsorb PCDD/F from the flue gas and that the injection of activated carbon upgraded the removal efficiency (14). Meanwhile, an electrostatic precipitator (ESP) is known to have low removal efficiency of PCDD/F because of low contact probability of particulate matter with the flue gas (49). Milligan et al. reported that the fly ash from an incinerator has about 1 × 10-5 mol/g adsorption sites (51). Meanwhile, the number of adsorption sites (or surface area) of activated carbon is 2 orders of magnitude higher than that of fly ash (52-54). As temperature increases, the fractional surface coverage decreases due to the adsorption/desorption equilibrium (Figure 5). On the basis of the experimental results (51, 54), the temperature effect on the adsorption equilibrium is modeled as

where Aa, Atotal, ηa, and T are the number of available adsorption sites (mol/Nm3), the total adsorption sites of particulate matter (mol/Nm3), the coefficient concerning temperature effect, and the temperature (°C), respectively. The present expression is from the curve fitting of limited data from refs 51 and 54 that could not be asserted for all other adsorption kinetics. Hence, careful consideration is necessary to expand this model to other cases. Unfortunately, the contribution of lime on the adsorption in flue gas is not well understood. From measurement studies in the incineration system (10, 11, 14), the contributions of activated carbon and lime injection are estimated to be 60 and 1 times that of fly ash, respectively. When activated carbon or lime is injected, the total adsorption site number is modeled as

Atotal ) 1 × 10-5(Cdust + Clime) + 6 × 10-4CAC (mol/g) where the subscript AC means activated carbon. Adsorption mechanisms of PCDD/F occurring in dust collectors (filter bag house and ESP) and solvent injection equipment (DI and SDA) involve adsorption in flight and adsorption in particulate layer (2, 11, 14). While only the adsorption in flight occurs in ESP, SDA, and DI, both occur in a bag filter. A simplified adsorption model based on the experimental results is proposed:

dCPCDD/F ) -k3AaCPCDD/F dt where k3 is an adsorption coefficient (9200 Nm3 s-1 mol-1), which is derived by fitting the experimental results (10, 11, 14, 55). The adsorption efficiency of PCDD/F is derived as

ηi ) 1 - exp(-k3Aatf) where tf indicates the flue gas residence time (s). For the adsorption in the particulate layer, a different treatment is necessary as the flue gas contacts the particle in a different manner from the in-flight adsorption. In general, the depth of the dust cake on the bag filter can be considered to be constant and independent to the dust loading it is usually controlled by the dust beat-off program by checking pressure difference. Then the contact time of the dust cake and the flue gas is proportional to the inverse of filtration velocity vf:

tf ∝ 1/νf And the adsorption kinetics is proportional to the adsorption VOL. 33, NO. 15, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY



FIGURE 7. Effect of temperature and content of V2O5 on PCDD/F destruction efficiency (r 2 ) 0.950).

FIGURE 6. Comparison of prediction and measurement: (a) at SDA + ESP (r 2 ) 0.788) and (b) at bag filter (r 2 ) 0.630). site density when the depth of the filter cake is constant:

FIGURE 8. Comparison of measurement and prediction of SCR destruction efficiency (r 2 ) 0.851).

Aa dCPCDD/F Cdust + Clime + 60CAC η ) ∝ dt Cdust + Clime + CAC a Cdust + Clime + CAC

efficiency of organic compounds in SCR and showed that area velocity Av is one of the major parameters determining the destruction efficiency, which is defined as (56)

Then, the adsorption efficiency for the filter cake is

ηf ) 1 - exp[k4Aa/(Cdust + Clime + CAC)νf] where k4 is 1.5 × 106 g m/mol s-1 derived by fitting the experiment results (11, 14). The total adsorption efficiency is calculated by

ηt ) 1 - (1 - ηi)(1 - ηf) A rough comparison of the prediction and the measurement results for the published data are shown in Figure 6 (10, 11, 14, 55). The experiment with SDA and ESP shows good agreement and that of bag filter shows varied agreement. In the comparison, the flue gas residence times in SDA, ESP, and bag filter are assumed to be 10, 15, and 15 s, respectively, which are the normal designs for MSW incineration system. This empirical model is meaningful at the dust collectors for MSW incinerators, and modification may be nessasary for it to be used for other adsorption equipments with different operating conditions. Destruction of PCDD/F in SCR (Selective Catalytic Reactor). SCR has been applied to reduce NOx in combustors and boilers. Since the ability of SCR to destroy gaseous organic materials was noticed (56, 57), waste incineration systems adopted SCR to reduce PCDD/F emission from the exit flue gas. In general, the catalysts for incinerators consist of TiO2, WO3, and V2O5, while V2O5 is active in the destruction of the organic materials. Ide et al. investigated the destruction 2662



Av ) G/Ac where G is the total gas flow rate (Nm3/h) and Ac is surface area of the catalysts (m2). Hiraoka et al. showed that the operating temperature and the structural shape of catalysts are also important parameters on the destruction rate (57). On the basis of several experimental studies, temperature, area velocity, and content of effective catalyst material (generally V2O5) are considered to be the relevant parameters. The experiment results are fitted to generate a destruction efficiency model of PCDD/F for SCR, which is applicable for gas temperature above 175 °C (58):

η2 ) 1 - exp(-k5) 1 k5 ) 0.04fV2O50.5(T - 175)1.2 Av Figure 7 shows the comparison of the experimental and the predicted results, in which the area velocity is assumed to be constant (15 Nm3 m-2 h-1). As this equation is empirical, which is meaningful only for the limited cases, the reliability of this reaction kinetics remain to be evaluated in a future study. Comparison of this model with other published data is shown in Figure 8.

Flue Gas Time History and PCDD/F Emission Incineration plants 1-3 have similar configuration of the flue gas treatment process (boiler, ESP, and wet scrubber)

FIGURE 9. Flue gas time history of incinerator 1 (in 1998).

FIGURE 11. Flue gas time history of incinerator 4 (1998).

FIGURE 10. Flue gas time history of incinerator 3 (in 1998).

FIGURE 12. Flue gas time history of incinerator 5 (in 1998).

that results in similar flue gas time history of temperature and pollutants. However, the PCDD/F emission of incinerator 3 showed a big difference (Tables 3 and 4). As shown in Table 2, the flue gas temperatures at the boiler exit and ESP of no. 3 are significantly lower than those of other plants with ESP. Figure 9 shows the flue gas time history of incinerator 1. As the flue gas experiences the formation temperature range (250-450 °C) in the waste heat boiler, the PCDD/F concentration increases at the exit of the boiler. The temperature of the following ESP is also in the formation range so that the PCDD/F concentration further increases. Incinerator 2 is similar to incinerator 1. However, incinerator 3, of which flue gas time history is shown in Figure 10, is different from nos. 1 and 2 by the high furnace temperature (∼1100 °C), the low ESP temperature (∼160 °C), and the activated carbon injection before ESP. Hence, PCDD/F in the flue gas are efficiently adsorbed to the particulate matter, and the formation reaction is suppressed because of low temperature. Time history of the flue gas temperature of incinerator 4 that has the same element composition with nos. 6 and 3 after the modification (boiler, ESP, wet scrubber, and SCR) is shown in Figure 11. The PCDD/F concentration of incinerator 4 is expected to be high due to the high boiler exit temperature as well as the high ESP temperature until the flue gas enters the SCR. Incinerator 5 has SCR between ESP and wet scrubber, of which the flue gas time history is shown in Figure 12. High PCDD/F formation in ESP is expected from the high operating temperature of ESP. However, most of the PCDD/F could be destroyed in the following SCR. The flue gas of the incinerators with bag filter experiences similar time history except for incinerators 8 and 10, which have SCR at the end of the pipe. The flue gas time history of incinerator 8 is shown in Figure 13. Also the measurement results at each element are shown in the figure. Agreement with the prediction is remarkable. Due to the optimized process temperatures and well-organized off-gas treatment

FIGURE 13. Flue gas time history of incinerator 8. systems, the PCDD/F emissions of incinerators 8 and 10 satisfy the regulation. The summary of the PCDD/F emission of the incinerators is shown in Figures 14 and 15. The emission levels in 1998 are approximately an order of magnitude lower than those of 1997, as a result of the modification of facilities and the improvement of operation conditions. Still in several incinerators, the emissions of PCDD/F are over the regulation (0.1 ng of TEQ/Nm3). The prediction model shows reasonable agreement with the measurement results. Most of the predicted values are in the range of the measurement variance, and the reductions of the emission level by system modification are also predicted accordingly. PCDD/F concentration between element equipment shown in Figure 15 also supports the reliability of the prediction model, which is checked in Figure 16. Although the model prediction shows good agreement with the measurement results, several aspects should not be ignored. The first is that the kinetics of formation as well as removal can be interfered with by changing operation VOL. 33, NO. 15, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY



FIGURE 16. Reliability of the prediction model (r 2 ) 0.987).


FIGURE 14. Prediction and measurement results of PCDD/F emission: (a) in 1997 and (b) in 1998 after improvement.

FIGURE 15. Measurement and prediction of PCDD/F between system elements. conditions, which are influenced by design concept, operation manner, and waste characteristics. The second is that the model has several critical assumptions on the kinetics models, which include assumptions that the loads of dust, precursors, and chlorine in the flue gas is similar for the considered incinerators. The efficiencies of adsorption and catalytic destruction are based on several published data, and further improvement is necessary. However, this prediction work shows that the PCDD/F emission of MSW incinerators is strongly related with flue gas time history in the process and that integration of the kinetics along the flue gas passage is necessary to estimate the emission rate properly. 2664



Formation of PCDD/F and Parameters in MSW Incinerators. In the MSW incineration process, PCDD/F are possibly formed in waste heat boilers and dust collectors (11-14). The first to consider is that production of PCDD/F may be related to the combustion condition (15, 39). When the good combustion condition in the incinerator is maintained, destruction efficiency is high and the emission of PCDD/F at the exit of incineration chamber is expected to be safely low (39). However, PCDD/F may be produced at the flue gas passage as long as the flue gas contains fly ash, unburned hydrocarbons, and heavy metals, and gas temperature is adequate for formation. Since significant emission of PCDD/F from the incineration system was reported (59), many researchers had tried to uncover the detailed mechanisms of PCDD/F formation. Most of all, temperature has been accepted as one of the major influential parameters for PCDD/F formation (2136). Although numerous studies were reported, the optimum formation temperature is still in controversial arguments. Several references, which investigated the temperature effect on PCDD/F formation, are listed in Table 4. Various solidphase and gas-phase reactants were used in various environments, and the reported optimum temperature spreads over from 300 to 500 °C. Experiments 1-3 in Table 4 are the pioneering works in this area, of which research focuses were de novo synthesis with carbon in fly ash or model catalyst mixture. They did not concern gas-phase precursors and PCDD/F and analyzed only the solid residue after the formation reaction. According to Milligan et al. (33) and Altwicker et al. (60), a major part of formed PCDD/F evaporates rather than remains in the solid phase at high temperatures above 350 °C, and this is why the detected optimum temperature in their experiments appeared to be 300 °C. The successive studies of experiments 4-6 tried to absorb the evaporated PCDD/F using silica gel, water, and methylene chloride, respectively, which are not effective absorbers of PCDD/F at all. Recent reports used reasonable solvents to extract the PCDD/F from flue gas and showed the optimum temperature for PCDD/F formation to be higher than at least 300 °C, except for Stieglitz et al. (27). Milligan et al. (30) showed that de novo synthesis in the existence of a chlorine source in the gas stream had a peak rate at 373 and 398 °C for PCDDs and PCDFs, respectively. However, experiments 1-11 in Table 4 did not consider the reaction of precursors such as chlorophenols, from which formation rate of PCDD/F is 1 or 2 orders of magnitude higher than de novo synthesis (33-36). The maximum formation rate of de novo synthesis did not exceed over 200 ng min-1 (g of fly ash)-1 (21-31), while the reported formation rate by Milligan et al. (33) at 150 µg/m3 of chlorophenol concentration was 3300 ng min-1 (g of fly ash)-1. This is a reasonable value

to account for the emission rate at real incinerators. The measurement studies of possible precursors in the flue gas showed significant amounts of chlorophenols, chlorobenzens, polycyclic aromatic hydrocarbons, etc. (1, 4, 7). Hence, the formation from the precursor reaction must be considered to predict the formation rate correctly. All of the papers reporting the reaction of chlorophenol or other precursors showed the peak formation temperature of 400 °C. Milligan et al. (33) showed that the total formation rate at low chlorophenol concentration condition had a peak at 350 °C, but the formation rate at 400 °C was also near that of 350 °C and higher than that of 300 °C. As the concentration of chlorophenol increased, the peak formation temperature shifted toward 400 °C. They also showed that most of PCDDs stayed in gas phase at temperatures above 325 °C. Other studies by Born et al. (32), Froese et al. (34), and Gullett et al. (35) showed that the peak temperature of PCDD/F formation rate from the precursor reaction was 400 °C. To summarize, most of the experiments with a reasonable absorber for gas-phase PCDD/F showed that de novo synthesis exhibits a peak temperature of higher than 300 °C. And the precursor reaction, of which the kinetic rate was 1 or 2 orders of magnitude higher than de novo synthesis, appeared to have the peak formation temperature of 400 °C. Hence, it is considered reasonable to accept 400 °C as the peak temperature of PCDD/F formation in an incineration system. Existence of chlorine in the raw municipal solid waste is usually pointed out as the most important reason that incineration systems are considered to be a major source of PCDD/F emission. Many studies suggested that the Deacon reaction might be the governing reaction of PCDD/F formation. Gullett et al. showed that Cl2 was a more reactive source to chlorinate the organic materials than HCl (35, 36), and various research showed that sulfur injection in the reacting area depressed the Deacon reaction and resulted in efficient reduction of PCDD/F formation (38, 44, 46). Meanwhile, Addink et al. concluded that there was no significant contribution of Cl2 on PCDD/F formation rate prior to HCl (29). The role of chlorine sources on PCDD/F formation is still unclear and needs further study. A parametric experiment varying Cl2 concentration showed that the increased Cl2 concentration resulted in an increased PCDD/F emission, and the formation rate appeared be linear to the concentration, while the results of each congener showed a different pattern (17). Luijk et al. showed that the CuCl2 load in reacting particles could influence the PCDD/F formation (19). The formation rate of each congener showed a different reaction on the CuCl2 load, and the total formation rate seemed to increase as the CuCl2 load increased but not a linear relationship (19). The precursors in flue gas are important sources for PCDD/F formation, and the kinetic rates appear to be linear to the precursor concentration (33). Kaune et al. showed that PCDD/F emission rate had almost a linear relationship with precursor concentration (15). Dust surface in flue gas is the reaction area of PCDD/F formation. Fujii et al. (14) and Kilgroe et al. (39) showed that the dust emission rate had a very close relationship with the PCDD/F emission rate. Gullett et al. investigated the effectiveness of various metals on PCDD/F formation (35) and concluded that copper-based catalysts were most effective (especially CuCl2) (17, 35). Oxygen plays an important role for PCDD/F formation (18); however, the formation rate is not strongly dependent on the oxygen concentration.

Acknowledgments The authors are grateful for the support of Energy and Environment Research Center of KAIST, and Institute of Environmental and Energy Technology of POSTECH.

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Received for review September 9, 1998. Revised manuscript received April 26, 1999. Accepted May 13, 1999. ES980932R