F-formation in boiler ash

Chemosphere,Vol. 29, No. 6, pp. 1235-1243, 1994

Pergamon 0045-6535(94)00212-6

Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All fights reserved 0045-6535/94 $7.00+0.00

TEMPERATURE DEPENDENCE OF PCDDIF-FORMATION IN BOILER ASH

Wunsch, P.*, Leichsenring, S., Schramm, K.-W., Kettrup, A. GSF-Forschungszentrum ~ r Umwelt und Gesundheit, Institut ~ r Okologische Chemie, Postfach 1129, 85758 OberschleiBheim *) To whom reprint requests should be adressed.

Keywords: Waste Incineration; PCDD/F Formation; Boiler Ash, Isomer Pattern

(Received in Germany 3 June 1994; accepted 5 July 1994)

ABSTRACT In a municipal waste incinerator boiler ash has been sampled from the horizontal superheater pass. Samples were taken at six different sites at flue gas temperatures ranging from 630 °C to 220 °C. Moreover, three samples were taken when the mechanical knocking system was working. A strong increase in concentration is observed, especially at 300-220 °C, which shows that PCDD/F are formed in the boiler outlet. The isomer pattern is consistent with that of typical combustion samples. As predicted by theoretical investigations, the isomer pattern is dominated by only a few isomers at high temperatures, while the isomer distribution at low temperatures is more even.

INTRODUCTION The formation of PCDD/F in the electrostatic precipitator (ESP) at temperatures of 300-400 °C has been investigated by many authors. Because of this synthesis, the ESP should be operated at temperatures below 250 °C. If this is made sure, flue gas is passing the temperature range for PCDD/F formation already before the ESP, i.e., in the boiler. Is there PCDD/F formation in the boiler? If so, to what extent and at which temperature PCDD/F is formed? While most of the flue gas is passing the temperature range for PCDD/F formation within seconds, part of the dust carried by the gas is deposited at the walls of the superheater pass. This is lowering the effectivity of the heat exchanger and can lead to formation ofPCDD/F. 1 In many facilities, a mechanical mechanism exists for removing deposited material from the flue gas canal. Are there differences in PCDD/F concentrations between samples taken with and without mechanical knocking? To what extent is this boiler ash contaminated by PCDD/F?

1235

1236

EXPERIMENTAL

Description of the plant

After the third pass of a multi-pass boiler, a horizontal superheater pass is following. This pass contains three vertical superheater tubes, one vertical evaporator tube and two vertical economiser tubes. The surface of every tube can be cleaned during operation from boiler ash that sticks on by a mechanical knocking system. Below the horizontal superheater pass, a conveyor is transporting the boiler ash to a loading plant. At this conveyor, it is ~ossible to take samples exactly under every superheater, evaporator and economiser (Fi~. 1). ~ Turbine

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FR:

Horizontal Pass

Fig. 1: Combustion chamber and horizontal superheater pass.

Table 1: Temperatures of flue gas and pipes at the sampling points. Flue Gas

Funnel Furnace

850 - 950 *C

Pipe ca. 255 °C

Feed Water / Saturated Vapor I

2nd Pass

720 - 760 °C

ca. 255 °C

!Feed Water / Saturated Vapor

3rd Pass

700 - 730 °C

ca. 255 °C

Superheated Vapor

SH l a

620 - 640 °C

255 - 300 °C

Superheated Vapor

II

SH l b

520- 550 °C

300- 370 °C

Superheated Vapor

III

SH 2

480 - 500 °C

340 - 400 °C

Superheated Vapor

IV

EV

390 - 400 °C

ca. 255 °C

Superheated Vapor

V

ECO 2

not measured

235 - 180 °C

Feed Water

VI

ECO 1

215 - 225 °(3

'180 - 122 "C

Feed Water

The maximum combustion bed temperatures in the furnace, measured with an IR camera are about 910 °C. The average velocity of the flue gas is less than 5 m/s. The flue gas needs more than 8 s to reach the horizontal superheater pass. There the temperature of the flue gas decreases from 630 °C to 220 °C. The temperature of the

1237 pipe lines in the superheater, evaporator, and economiser tubes decreases from 300 °C to 120 °C. The dust content of the flue gas under normal load of the municipal waste incinerator is about 1.4 g/m 3. In average, the mechanical knocking system is activated every 30 min.

Sampling and PCDDIF Analysis Under the horizontal superheater pass about 1 kg boiler ash per sampling point has been taken, without operating the mechanical knocking system, at the sampling funnels I, II, III, IV, V and VI. At the sampling funnels II, III and VI about 1 kg boiler ash per sampling point was taken while the mechanical knocking system was operating. Next, the boiler ash samples were diminished in a sample splitter to 10 g and pulverised in an achat mill. For analysis,

13C12standards were added and the samples were extracted with toluene for 24 hours. The clean-up

combines a set of established chromatographic steps with alumina, basic and silica in series. Identification and quantification were done with HRGC/HRMS using the columns Restek RT'2330, 60 m, 0.25 mm ID, 0.1~tm df and DB-5 J+W, 60m, 0.25 mm ID 0.1~tm df 2 Measurements were conducted with the mass spectrometers Finnigan MAT 95 (R=10000) or 8230 (R = 5000) (EI-SIM-Mode, tracing the M ÷, M ÷2 ions). Not for all 210 isomers a 13C12 standard was added. The isomers, for which no standard was available, were quantified using a response factor of one for all isomers of the same chlorination degree.

RESULTS AND DISCUSSION Because of self-recording temperature measurement in the pipe and of the flue gas in the horizontal superheater pass, conclusions on the temperature dependence of destruction, and formation of PCDD/F in the boiler ash are possible. Samples taken without mechanical knocking system is working give hints on destruction and formation of PCDD/F on the surface of boiler ash particles which were exposed to the measured temperatures only for a short time. During operation of the mechanical knocking system, boiler ash that sticks on the walls of the superheater pass for a longer time is struck off, so that a long-term effect of destruction and formation of PCDD/PCDF on boiler ash can be studied.

Samples Taken Without Mechanical Knocking Samples taken while the mechanical knocking system was not working show low PCDD/F concentrations of about 0.1 ng I-TE/g at the exit of the burning chamber (630 °C) (Table 2, left part); this low concentration is a consequence from the fact, that the major part of PCDD/F is destroyed in the burning chamber. In the temperature range of 630-300 °C, PCDD/F concentration on the boiler ash is increasing modestly by a factor of 1.5 (Table 2, middle part). This increase can be explained by the fact, that more and more PCDD/F are adsorbed on the surface of the boiler ash. 3 There i s a remarkable increase of concentrations between 535 °C to 490 °C. It should be noted, that the boiling points of all PCDD/F are within this temperature range (400-500 °C) 4. So the vapour pressure is dropping there very strongly, and the compounds are increasingly adsorbed on the boiler ash. In the temperature range of 300-200 °C, a strong increase in concentration is observed (Table 2, middle part). This increase cannot be explained exclusively by the fact, that more and more PCDD/F is adsorbed on boiler ash particles due to the temperature decrease, because lower chlorinated compounds show a stronger increase than the highly chlorinated ones. Instead, a "de novo" synthesis of PCDD/F seems to be the reason of the increase in concentration.

1238

According to many authors 5,6, the optimum for de novo synthesis is between 300 and 350 °C; the yield at 250 °C being 2 to 75 times lower. But the temperature of ash in the superheater pass is likely to be above the temperature of the gas, which is measured. So it seems reasonable to assume that even at (gas) temperatures of 300-200 °C, there can be "de novo" synthesis. PCDD concentrations are increasing by a factor of 22 while cooling from 630 to 220 °C; PCDF concentrations are increasing by a factor of 3 only. For PCDD and PCDF, this increase is especially strong in the temperature range of 300-200 °C. Since the vapour pressures of PCDD and PCDF of the same chlorination degree are similar 4, different vaporisation cannot be the reason. So PCDD seem to be favoured by "de novo" synthesis in this plant. The homologue profile (Figs.2-4) shows high concentrations of highly chlorinated PCDD and of low chlorinated PCDF which is consistent with the typical (waste) burning pattern. 7 It doesn't vary very much from sampling point to sampling point. Just as there is a characteristic (waste) combustion pattern for homologue profiles, there seems to be an characteristic (waste) combustion isomer pattern, too. 8 For PCDD, this pattern is dominated by only a few isomers per chlorination degree. The most abundant isomers are reported to be 1,3,6,8-, and 1,3,7,9-TCDD, 1,2,4,7,9-, 1,2,4,6,8-, 1,2,3,6,8-, and 1,2,3,7,9-PCDD; 1,2,4,6,7,9-, 1,2,4,6,8,9-, 1,2,3,4,6,8-, 1,2,3,6,7,9-, and 1,2,3,6,8,9-HxCDD (Usually, not all isomers are detected separately, because some are coeluting from the standard columns used). The same pattern is observed in all samples considered here (Table 2, right part). This isomer distribution was explained by some authors by condensation of chlorophenols. In flue gas, 2,4,6Trichlorophenol, 2,3,4,6-Tetrachlorophenol, and Pentachlorophenol are the most abundant chiorophenols, so their Ullmann condensation products are supposed to be the most abundant PCDD/F. 8,9 Others interpret the isomer pattern as the result of thermodynamic product distribution. 1o If free Gibbs energy of formation of PCDD/Fs determines the isomer distribution, the pattern should be dominated by a few isomers only while, at lower temperatures, the distribution is more even.10 This is the case for the samples considered here. E.g., the three most abundant TCDD make for 40-43 % of the TCDD sum at 630 o, compared to 60 % at 220 °C (Fig. 5) This confirms the thermodynamic explanation of isomer patterns. PCDF isomer pattern of the samples is less characteristic, which is consistent with the literature. 8,9 7Y 6 5-

[] TCDD

o

• PCDD o o

IllHxCDD 3¸

[] HpCDD [ ] OCDD

630

535

490

395

300

220

Temp. (°C)

Fig. 2: Relative content of PCDD in the boiler ash (without mechanical knocking) at the sampling points I to VI.

1239

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6 C"5

[] TCDF • PCDF

L

0 3 2 1

630

535

490 395 Temp. (°C)

300

• HxCDF [] HpCDF [] OCDF

220

Fig. 3: Relative content of PCDF in the boiler ash (without mechanical knocking) at the sampling points I to VI.

2001301500010000 ¢-

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N

o

Temp. oc

Fig. 4: PCDD/F homologue profile (boiler ash, without mechanical knocking)

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1242

Samples Taken With Mechanical Knocking Boiler ash samples taken while the mechanical knocking system was working

show considerable PCDD/F

concentrations, though lower ones than without dust mechanical knocking (Table 2, left part). Concentrations are increasing by a factor of 20, the increase between 300 and 220 °C being the most important (Table 2, middle part). Thus, for deposited boiler ash, there seems to be a de novo synthesis too, which is more effective than for not deposited boiler ash. This was expected because of longer reaction times). Remarkably, concentrations are lower than those taken while the knocking system was shut of (Table 3). This seems strange at a first glance, because one could assume, that for deposited boiler ash reaction time for "de novo" synthesis is longer. (The knocking system is activated every 30 min.) So other effects must be considered. The largest difference between samples with and without knocking (table 3) is at 535 °C and decreases to lower temperatures. At 535 °, the difference is larger for the high chlorinated compounds than for the low chlorinated. This could be a result of desorption/sublimation. At 535 °C, a big part of the compounds is" desorbed and sublimates, because the temperature is above the boiling point of PCDD/F. But there must be another effect, because the concentration of compounds with high chlorination degree, is especially low compared to the samples without knocking. It could be a dechlorination, making high chlorinated compounds disappear. So 3 effects are super positioning: a) sublimation/desorption into gas stream, b) dechlorination of the high chlorinated compounds and c) "de novo" synthesis. The homologue profile shows, compared to the samples taken without knocking, a shift to lower chlorinated PCDD/F as well as to PCDF, which seem to be dechlorinated more easily (the vapour pressures being essentially the same for PCDD and PCDF for the same chlorination degree). The PCDD/PCDF ratio is rising moderately from 0.8 to 1.03 while cooling from 535 to 220 °C. The maximum is observed at 490 °C. The PCDD isomer pattern of the samples with knocking is the typical isomer pattern of burning.

Table 3: Ratio of concentrations on the samples without mechanical knocking to those on samples with mechanical knocking. Sampling Point T[°C] Sum of TCDD Sum of PCDD Sum of HxCDD Sum of HpCDD OCDD Sum

II 535 4,33 6,29 9, 89 17,03 20, 13 14,96

III 490 2,58 2,54 3, 15 4,03 4,04 3,83

VI 220 2,07 3,07 5,00 4,97 4,31 4,44

Sum of Sum of Sum of Sum of OCDF Sum

7, 98 7,63 8,48 13, 14 13,20 9, 50

4,751 4,221 3, 63 2,83 3,18 3, 59

0,99 1,44 1,57 2, 35 2,17 i, 58

ii, 94 8, 99

3, 74 3, 76

3,04 1,95

PCDD/F I-TE

TCDF PCDF HxCDF HpCDF

1243

REFERENCES 1.

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