On the mechanisms of dioxin formation in combustion processes

Ckmosphere,

Vol. 31, No. 9, pp. 4099~117, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0045-6535/95 $9.50+0.00

Pergamon 0045-6535(95)00287-l

ON THE

MECHANISMS IN

H.Huang+ Department

of Chemical

Free University

OF

DIOXIN

COMBUSTION

FORMATION

PROCESSES

and A.Buekens*

Engineering

of Brussels, Pleinlaan

and Industrial

2, 1050 Brussels, Belgium

(Received in Germany 31 July 1995; accepted 4

The experimental detailed

incinerator

synthesis

observations

and precursor

synthesis

structure

sources

combustion

reactions

synthesis

described

are

combustion

zone;

compounds

including

consisting

it appears

of degenerated

from all combustion

sources

experiments

and incinerator

structure

zone.

As de graphitic

emissions formed

from actual

in gas phase source for de

in combustion

including

of soot particles

New explanations

systems

is

between

to aromatic

are given for some

the high dioxin emissions

from coal combustion,

and the discrepancy

systems.

degenerated

are the plausible

formation

from

de nova

of the graphitic structure of soot particles in the

of the graphitic

observations

particles

structures

dioxin

in the postcombustion

the low dioxin emissions

of certain

soot

sources,

including

The de nova synthesis

of the particulate

that

graphitic

process: (1) the formation

experimental

in this paper.

the morphologies

and

combustion studies

in actual combustion

morphology

With this understanding

PCDD/Fs

simulation

are discussed

that carbon

and (2) the conversion

unrelated

waste incineration, laboratory

examined

of dioxins.

as a two-stage

seemingly

indicate

September ,995)

from various

laboratory

of dioxin formation

for dioxin formation,

combustion nova

pathways

mechanism

experiments

is essential

and from

formation

seems to be the dominant nova

on dioxin formation

measurements

Chemistry

from municipal

the similar dioxin

the dioxin formation

“fingerprint”

rates observed

in

They are emitted

in

measurements.

1.

Dioxins

are a group of chlorinated

trace quantities some isomers control

from combustion of dioxins

of dioxin

mechanisms

polynuclear

sources, especially

aromatic

compounds.

the incineration

of municipal

are highly toxic and may have carcinogenic

emissions

of dioxin formation

from have

combustion been

sources

studied

has

extensively.

received

wastes.

and mutagenic great

attention

It is now believed

Because

effects, the and the

that dioxins

*Correspondence concerning this paper should be addressed to Prof. A. Buekens. +On leave from Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, P.O. Box 1254, Guangzhou 5 10070, China.

4099

4100 are formed

in the low-temperature

postcombustion

zone

of incinerators

heterogeneous catalytic reactions occurring in the flue gas - fly ash environment.

through

some

The detailed

chemical reactions are however not well understood. Recent reviews on the formation mechanisms and models of dioxins are given by Altwicker et al.lm3 and Luijk4. In an effort to incorporate the kinetics of dioxin formation into an incinerator mathematical model for process simulation and optimization purposes, the findings of the formation mechanisms of dioxins have been studied in our department.

The results especially those concerning the connection of laboratory findings to

incinerator observations are presented in this paper. 3. The Fxoerimental Fvidence of Dioxin Formation 2.1 Dioxin Fmrssions from Various Combustron Sources Some data of dioxin emissions from various combustion sources are shown in Table 1. Several observations can be made: (1) Dioxins have been detected in a variety of combustion sources. The prediction from the “trace chemistry of fire” hypothesis that “emissions of PCDDs and PCDFs are general phenomena related to all combustion processes **c,*l has been verified to a large extent. (2) Municipal solid waste incineration has very high dioxin emissions while coal combustion generates the lowest dioxin emissions. This fact has been recognized in many previous studies.**23

(3) Dioxins emitted from these combustion processes have similar “fingerprints”. refer to dioxin “fingerprints” collectively.

as the furan/dioxin

Here we

ratio, the homologue profile and isomer pattern

The typical dioxin “fingerprints” from combustion sources are: furan/dioxin ratio (w/w) is

larger than 1, the weight distribution

of the homologues

increases with increasing degree of

chlorination for PCDDs, but shows a maximum at PsCDF or HsCDF for PCDFs, and the isomer pattern contains almost every isomer. 24,25 Dioxins from other sources, e.g. pulp bleaching and chloralkali

electrolysis

have different

dioxin

“fingerprints”.25

Because of the similar dioxin

“fingerprint” from all combustion processes, especially the similar furanjdioxin ratio, it is very likely that the mechanisms of dioxin formation in all these combustion processes are similar or at least their controlling steps are similar. _.2 3

Comprehensive

measurements

of dioxin emissions from incineration

plants have been

conducted for a number of years. The data from a recent German report are shown in Tables 2 to 4. Several observations can be made: (1) Dioxins already present in MSW are destroyed in the combustion zone of incinerators as shown in Table 2, but dioxins are regenerated in the postcombustion zone as indicated in Table 3; overall dioxins are formed in the incineration process as shown in Table 4. This phenomenon of dioxin regeneration in the low-temperature postcombustion zone has been known since 1980s and confirmed by a large number of studies. (2) Table 4 shows that the important streams of dioxin emissions from incinerators are ESP ash, filter cake and stack gas accounting for about 56.7%, 22.7% and 11.8% of the total TE output respectively.

4101

Table 1. Dioxin emissions

Combustion

from various combustion

sources

sourcesa.b

Dioxins in fly ash (ng-TE/g)

Ref.

0.2-63

l-28

PI

0.1-0.5

n.a.

161

Wood combustion: Natural wood Waste wood

0.02-l .8 2.7-l 4

K-8

Coal combustion

nd.

n.d.-10

[12-141

Plastics pyrolysis

detected

detected

[15,161

Oil combustion: Oil furnace Leaded gasoline in vehicle Leadfree aasoline in vehicle

0.03-0.3 1O-60 (pg/L-oil) 3.5 (Da/L-oil)

n.a.

1171

Dioxins in flue gas (ng-TE/Nm3)

Municipal

waste incineration

Hazardous

waste incineration

Gas combustion

(7-l 1]

I n.a.

I 0.07-l 00

“TE is referred to I-TEQ.

bn.a.=not available,

[I81

n.d.=not detected.

Table 2. Balancing of dioxin flows in the combustion values of 11 incineration plants, adapted from (51)

Dioxin concentration

Input/output Input: municipal

waste

output: bottom ash slag water raw gas

zone of incinerators

Mass/volume flow per tonne waste

0.09 ng-TE/g

0.03 ng-TEIg 9 ng-TEIL 1.4 ng-TE/Nm3

I u 91

Dioxin flow (ugTE/tonne-waste)

90

300 kg 350 L 5000 Nm3

(mean

9 3.2 7

total

19.2

Balance

-70.8

I

4102

Table 3. Balancing of dioxin flows in the postcombustion values of 11 incineration plants, adapted‘from [5])

zone of incinerators

input/output

(mean

Dioxin flow (ugTE/tonne-waste)

Input: raw aas output: boiler ash ESP ash scrubber water filter cake stack gas

0.2 ng-TE/g 4 ng-TE/g 7 ng-TEIL 6 ng-TElg 5 ng-TE/Nm3

1.4 120 5 48 25

7 kg 30 kg 750 L 6 kg 5000 Nm3

total Balance

Table 4. Balancing of dioxin flows in incinerators plants, adapted from [5])

Input/output

Input: municipal

Dioxin concentration

waste

output: bottom ash slag water boiler ash ESP ash scrubber water filter cake stack gas

Mass/volume flow per tonne waste

0.09 ng-TE/g

0.03 ng-TElg 9 ng-TEIL 0.2 ng-TElg 4 ng-TElg 7 ng-TEIL 6 ng-TElg 5 ng-TE/Nm3

(mean values

90

300 kg 350 L 7 kg 30 kg 750 L 6 kg 5000 Nm3

9 3.2 1.4 120 5 48 25

total

211.6

Balance

121.6

of 11 incineration

4103

(3) Table 4 also shows that the dioxin concentration

of solid discharges

from incinerators

is in

the increasing

order of bottom

ash c boiler ash < ESP ash < filter cake < fly dust (for the dioxin

concentration

of 5 ng-TE/Nm3

and dust loading

dioxin concentration gas are present the upstream

of 0.02 g/Nms in stack gas,5 the corresponding

in fly dust is about 25 ng-TE/g assuming that only 10% of the PCDD/Fs in stack

in particulate

system),

phase considering

therefore

it appears

the possible

desorption

of gaseous

that the finer the particles,

PCDD/Fs

in

the higher

are the dioxin

An optimum

02 level with

concentrations. (4) The effects of flue gas 02, HCI and SO2 levels minimum chlorine

dioxin

formation

can be found

and dioxin formation

been suggested

on dioxins:

incinerators. ,26,27

for some

seems still unclear as conflicting

to be able to suppress

the relationship

between

results are reported;zs-sc

dioxin formation, 31 but laboratory

SO2 has

and field testings seem

inconcIusive.a2Ja

2.3 Laboratorv

Study

Laboratory

experiments

simulating

mechanisms

of dioxin formation

others4s-5s.

Two formation

novo synthesis

formation these

have

respectively. phase material

material

In precursor

to operate

are summarized

simultaneously

have all been converted

and solid phase residence

experiments

the differences

in testing

dependence good

of dioxin

agreement.

acceptable synthesis

material

formation

produced

into ug/g.min

laboratories

and analytical

on temperature

Additional

of dioxins are summarized

(1) Gas phase material:

metallic catalyst possible

procedure.

observations

concentration

the total

given

experiments about

of solid phase

ratio in de

agreement

observations

considering such as the

in the various

reports

can be regarded

the influencing

in

comparison.

rate and furan/dioxin

Other

and oxygen

downstream

are in

to have an

factors

in de nova

as follows:

02 is essential

for de nova synthesis44 469

Hz0 may affect the homologue

material:

are essential

catalyst;3s

pathway

PCDD/F formation

based on the weight

are in reasonable

rate is about 0.5 order with respect to 02 concentration; (2) Solid phase

of dioxin

In Tables 5 and 6

are captured

The various units reporting

the de nova synthesis

Therefore,

have little influence,s*,s9,47,4*

the de

the carbon

formation

time in Tables 5 and 6 to allow for a quantitative

from different

reproducibility.

the

are formed from

The two pathways

and precursor

From Table 5 it can be seen that the total PCDD/F formation nova synthesis

dioxins

in de nova synthesis

and independently.34

synthesis

it. Gas phase PCDD/Fs

with solid phase PCDD/Fs.

literature

have been identified:

pathway

and chlorophenols;

for de nova

study

et al.ss-44, Dickson et al.34 and

conditions

formation

to

solid phase material was placed inside a fixed bed reactor and gas

was passed through together

conditions

is the carbon present in solid phase material.

In all experiments

and analyzed the original

formation.

been shown

experiments

relevant to incinerator

such as chlorobenzenes

source of dioxin formation

postfurnace

have been carried out by Stieglitz

pathways

and precursor

gas phase precursors

incinerator

Carbon

of certain

PCDD/F

formation

the presence

of HCI, SO2, CO and H2

profile.s*+t

degenerated

for de nova synthesis;“”

the total PCDD/F formation

graphitic

structure,

Cu2+ is a strong catalyst,44

rate is about

and 1.5 order to Cu*+,44 but is independent

chlorine

and

Fes+ is another

first order with respect

of the solid surface areado.

to carbon

4104

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¢0

~-

0

o el-

¢-. 0

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O~

o

Q

0

0

,-

6,,~

~-o

d

~

o

o

o

..Q

v

¢-

._o mE ,-- m 0 ~ 0 -i ~

o

0 I

0

0 O9

0 C~

0 ,~

C~

0 c',J

•

IJ -

o 0

-c/ :

P'CO

0 CO

"

E 0 I--

m O~

tO

¢-

=

-~ o~

0

-

~

~

~'F= t-

0

v

¢0 LL

U_

o (L

A

"0 ffl

tO

q

r-

-~

o

A

E~ E2

c-

.l__" 0

OE


_.

~z ~

-~ -

o~ ~

< .=

< ._~

ro

O.

~'1

r-

O9

.,.2" e-

el ~ ¢-

<

g~

N .--

0

~~

¢"

-~--~ 1

Q.-~_

~E

4105

e-

.c x 0

0

C~ 0 CD

o.

o 0

0 I,

0

C~

c~

~-

~D r-

v-

o

v-

6 z~ ~E

v

~E ~D 0

~5

.°

0

0 0

~0 C~

0 0

0

0

~D O

0 CD

E

~D

%g 0

~8 O9

w~

L~

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O :

[..3

0.

0

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"6 0

~T ~E

t~ CO

~

Do~:

er. .,~

N

4106

(3) Temperature: observed

Maximum

dioxin

formation

occurs at about

can be

below 250°C and above 400°C.s4-sc

(4) Reaction time: In most of the experiments phase residence solid

3OO”C, little dioxins

phase

gas phase residence

time is not important,

solid

time is the relevant time scale; in the course of the reaction from 5 to 30 minutes of

residence

time,

the

PCDD/F

formation

rate

remains

constant

shown

in one

experiment.4g (5) Product distribution:

The typical combustion

ratio of organochlorocompound

products

“fingerprint”

of dioxins is produced,

the molar

PCDD : PCDF : PCBz : PCPh : PCNP :

is approximately

PCB = 1 : 1.6 : 75 : 6 : 0.6 : 0.3; 36-41 the phase distribution

of PCDD/Fs

is about 0.7%, 37% and

94% in the gas phase at 250, 300 and 350°C respectively.48 In the experiments formation

rates vary

adopted.

Metallic

furan/dioxin

of precursor

formation

significantly

probably

more

catalyst

pathway

such as Cu2+ is no longer

ratio is much smaller

than 1.

shown

in Table

due to the different essential

Other observations

6 the total

precursor

for dioxin are similar

PCDD/F

concentrations

formation.s48ss to de nova

The

synthesis

experiments. 3. Analvzing

the Experimental

3.1 De Nova Synthesis

vs. Precursor

Under laboratory novo

synthesis

experiments

Evidence

conditions

comparing

is several

orders

(3.75X1 O5 and 4.1X106 boiler

and stack

The question

experiments

arises

which

stoichiometry

n is the reaction

has been proposed

formation

rate

kinetic

when the precursor

assuming

that two precursor

rate is expected

reactions

inferred

from the

molecules

are coupled

to form a

the kinetics do not necessarily

these

concentration concentration

data

by Kolluri

and Altwicker:56

is increased

we can estimate is primarily

influenced

order

700 ng/mL51 a first-order

dependence

first order for the PCDD/F

by the

formation

formed

time of 3

amount

to 4.5

of n = In(rl/r2)/ln(Pl/P2)

precursor

concentration

and experimental

=

rate with respect to the precursor

we estimate

of

of 150, 350 and

on T4CPh concentration evidence

in these

= 1.4. In an experiment

gas phase T4CPh concentration

of PCDD formation

Based on these theoretical

pathway

listed in Table 6, assuming that

order is n = ln(50.1/1.6)/ln(4.1X106/3.75X105)

from T4CPh for three different

follow the

to 3.27 ug/Nm3, the dioxins formed are 200

a reaction

rate

formation

For a reaction

is 0.327 ug/Nm 3, the dioxins

the

the reaction

from precursor

results based on this model with

= 1.6. Also, from the first two experiments

from the data.

is actually

to

Canada

to be second order with respect to the

ln(200/4.5)/ln(3.27/0.327)

PCDD formation

,45 comparing

in the Environment

reaction model for dioxin formation

data have been presented

ng/Nm3; when the precursor

PCDD/F

in these postfurnace

pathway.

by Shaub and Tsang and some calculation

modified

experiments,

incinerator

experiment@’

IO ug/Nm3

is faster than de

concentration

in actual

in two laboratory

But for heterogeneous A heterogeneous

seconds,

From

pathway

or r = k.P”, (n = 2) where r is the reaction rate, k is the kinetic constant and

order.

“law of mass action”.

precursor

the precursor

than those

of about PCDD/F

or P + P + D, the reaction

precursor concentration,

ng/Nm3.

ug/Nm3

measurements

through

However,

higher

for precursor formation

From a reaction dioxin structure,

formation

5 and 6.

of magnitude

incinerator laboratory

dioxin

Tables

conditions study54m55).

Formation

can be seen that it is at least

concentration

although

we

4107 note that for a wide range of precursor concentration the reaction order may not be constant. Thus we calculate approximately 2.67XlO05.

that rac t ua t/rexperimental = PactuaPexperimental)” = (10/3.75X105)’ = This implies that the experimental PCDD/F formation rates in Table 6 should be

reduced by 5 orders of magnitude when compared with incinerator postfurnace conditions and the actual PCDD/F formation rate from precursor pathway would range from 9X10-’ to 5X10m4ug/g.min. Comparing de nova synthesis with precursor formation we draw the following points: (1) The rate of dioxin formation in de nova synthesis ranges from 0.014 to 0.128 ug/g-min in Table 5 and in precursor formation is from 9X10-’ to 5X1c4 ug/g.min as discussed above, so de nova synthesis is likely to be several orders of magnitude faster than precursor formation in actual combustion systems. (2) De nova synthesis

can produce the correct combustion

“fingerprint”

of dioxins, i.e.

furan/dioxin ratio > 1 and the typical homologue profile, which agree with actual incinerator findings. But in precursor formation little furans are produced and the furan/dioxin ratio is much smaller than I.

(3) Not only the dioxin “fingerprint”, but also the distribution of other organochlorocompounds from de nova synthesis resembles quite well the actual incinerator data which typically show a decreasing order of PCBz > PCPh > PCDF > PCDD z PCNP > PCB.41 (4) In most measurements PCBz and PCPh together with PCDD/F are found to increase when the flue gas passes the postcombustion region of incinerators, but the ratio PCDD / PCDF / PCBz / PCPh remains approximately constant, e.g. when measured at the inlet, middle passes and outlet of a waste heat boiler. 57 This indicates that PCDD, PCDF, PCBz and PCPh are formed in parallel reactions, not in serial reactions. The above evidence suggests that de nova synthesis is the dominant mechanism of dioxin formation in actual combustion processes. Certainly from basic chemistry the precursor formation pathway is much easier than de nova synthesis. However, in actual combustion systems dioxin formation from precursors is limited by the low precursor concentration (about 10 ug/Nm3 in municipal waste incinerators), on the other hand, the starting material for de nova synthesis, i.e. 02 (about 10% of the flue gas) and carbon in fly ash (the carbon concentration of fly ash is about 5%, and the ash loading of raw gas is about 1 g/Nm3) is relatively abundant. 3.2 I aboratory Study vs. Incinerator Measurement Another question arising when comparing laboratory study with incinerator measurement is whether the experimental

PCDD/F formation

rate from de nova synthesis is able to explain

quantitatively the observed PCDD/F emission level of incinerators.

We analyze this question by an

order of magnitude estimation as follows: From Table 5 we choose Milligan and Altwicker’s data because in this experiment true fly ash was used and the 02 concentration

was 10% which

resembles incinerator postfurnace conditions, thus assume the total PCDD/F formation rate as 34 rig/g min and neglect the influence of other factors; from Table 4 we have the dioxin concentration of fly ashes (to estimate ng-total PCDD/Fs from ng-TE, multiply by 50, see [24]); then assuming 50% of the dioxins generated in de nova synthesis are present in solid phase we calculate the required reaction time to reach the measured dioxin level as shown below:

4108

Dioxin concentration, nglg Boiler ash ESP ash Filter cake Fly dust

Required reaction time, min

10 200 300 1250

lOI.” 17:6 73.5

It appears that the experimental PCDD/F formation rate is sufficient to explain the dioxin level of collected particles in APCDs, but is not sufficient for uncollected particles or fly dust in stack emissions. This analysis agrees with the discussions by Altwicker et al.1-3g4g We note that in the above analysis the possible adsorption of gaseous PCDD/Fs from the flue gas has been neglected and that all ashes tested in de nova synthesis experiments shown in Table 5 are in fact ESP ash, so the PCDD/F formation rate obtained may only be applicable to ESP ash. 3.3 The Relationship Between Carbon Morpholoav and Dioxin Formation The influence of carbon morphology on dioxin formation was firstly recognized by Stieglitz et al.42*44 In their de nova synthesis experiments when active charcoal, sugar coal (from pyrolysis of glucose) or soot (from a domestic oil burner) were tested, dioxins were formed, but when graphite was tested, little dioxins were found. They suggested that carbon of certain degenerated graphitic structure is the primary source for de nova synthesis of dioxins. Milligan

and Altwicker

also reveals a relationship

between

Similar experimental work by

carbon morphology

and dioxin

formation.4ge5s From this evidence and the discussions in section 2.3 it seems that a condition for de nova synthesis of dioxins to occur is the presence of carbon of certain degenerated graphitic structure together with oxygen and minute quantities of chlorine and metal ion at 250-4OO“C (MSWI fly ash contains organic chlorine ca. 500 ug/g and copper 500-1500 ug/g.ss-54) In most combustion processes oxygen and minute quantities of chlorine and metal ion are present, a crucial factor of dioxin formation is therefore likely to be the carbon morphology. To relate this laboratory finding to practical combustion systems the carbon morphology of the particulate emissions from different combustion sources thus appears to be important.

Morphological descriptions of the numerous

crystalline and amorphous forms of carbon have been given in some handbooks.5gs0

In what

follows we examine in detail the carbon morphology of the particulate emissions from gas, liquid and solid combustion and the possible connections to dioxin formation. In gas combustion the particulate emissions consist of soot particles formed in homogeneous combustion reactions61-ss (see also the biennial Symp. Combust.) Soot formation can be observed in flames of almost all organic gases and vapours including methane, ethane, propane, ethylene, acetylene, benzene and alcohols. soot at combustion temperature

The yellowish luminosity of flames is due to soot formation as radiates strongly in the yellow region of the electromagnetic

spectrum. Soot formation in gas phase reactions is believed to proceed via three steps: nucleation, surface condensation and particle growth, and coagulation.

Soot particles have diameters in the

submicrometer range and are usually clustered together in chains and thus appear as a carbon filament in electron micrographs.

The elemental composition of soot is 97-99% by weight carbon

and l-3% hydrogen, so sometimes soot is also termed as carbon particles or black carbon. The carbon in soot particles is basically crystalline with a graphitic structure in which the graphitic sheets are stacked turbostratically.

Soot from all combustion sources has similar graphitic structures

4109

irrespective

of the type of flame, the nature of the fuel being burned and other combustion

conditions although the extent of sooting does vary considerably with these factors. Generally the sooting tendency of fuels is in the decreasing order of aromatics > alkynes > alkenes > alkanes, and a diffusion flame produces more soot than a premixed flame for the same fuel. The half-life for soot burnout is about 7X10”

seconds, thus soot burnout is usually diffusion-controlled.

The emissions

of soot particles from a gas burning system depend on soot formation and burnout, but in most cases due to incomplete mixing and combustion, some soot particles will leave the combustion zone and deposit on the chimney as chimney black or discharge with the exhaust as black smoke. Dioxins have been detected in exhausts from gas combustion including

methane,

thermodynamically

propane

and

ethylene

flames.’ g

in the presence of HCI

Because

(1) dioxins

are not

stable at combustion temperature: (2) the homogeneous mechanism is unlikely

to explain dioxin formation; and (3) in the postcombustion

zone of a gas burning system the

degenerated graphitic structure of soot particles combined with 02, trace quantities of chlorine and metal ion will suffice to create a condition for de nova synthesis of dioxins, therefore it is very likely that dioxins are formed via a de nova synthesis mechanism occurring on the soot particles and that dioxin formation is related to the sooting tendency of the flame. This assumption is in agreement with the experimental observation by De Fre and Rymen that dioxin formation in a yellow, slightly sooting ethylene flame is much higher than in a blue, carbon-lean ethylene flame.lg In liquid fuel combustion the fuel undergoes combined vaporization/pyrolysis phase combustion

followed by gas

reactions, the particulate emissions consist of soot particles formed in the

process and mineral matter or ash present in the fuel (and coke from liquid phase cracking for heavy oils). 64 The proportion of soot and ash particles depends on the fuel type and combustion conditions.

For spray combustion in an internal combustion engine “soot is always produced and

the flame zone always exhibits a yellow luminosity”.64

Soot from a domestic oil burner has been

shown to be active in de nova synthesis experiments,42v44 so that dioxin formation through a de nova synthesis mechanism occurring on soot particles in actual combustion system of liquid fuels can be expected to occur. This explains the findings that dioxins are emitted from gasoline fueled cars and a waste oil furnace.17n1e In solid fuel combustion the fuel undergoes pyrolysis at first and decomposes into volatiles and char, then the former are burned in gas phase and the later in solid phase, the particulate emissions consist of soot particles formed in gas phase reactions and ash with residual carbon formed in solid phase combustion.

The proportion of soot and ash particles depends on the

volatiles and ash content of the fuel and combustion conditions. The higher the volatile content, the higher is the sooting tendency. Coal usually burns with low to non-sooting

flames depending

on the coal ranks.65z66

Assuming that dioxin formation is related to the sooting tendency as discussed above we list the volatile

contentrs7, the sooting tendency65 and the available dioxin emission data from coal combustion12*‘4 according to the type of coal as below (the type of coal tested in [14] is found in

WI):

Coal type

Volatiles, %

Sooting

Dioxins in ash, ng-TE/g

Bituminous Subbituminous Anthracite

18-45 15-20 8-15

Yes No No

l-10 (3 samples) nd. (1 sample) 0.04 (1 sample)

4110

These very limited data do not reject the assumption.

Probably, in a sooting flame the soot

particles serve as a source for de nova synthesis of dioxins because of their degenerated graphitic structures, whereas in a non-sooting flame the particulate emissions are mainly mineral matter or ash particles originated from the fragmentation and entrainment of char as well as the vaporization and condensation of mineral matter, 6g,70 the residual carbon in ash is in an activated state,71 but most of the carbon is likely to be bonded to metal or oxygen or in aliphatic hydrocarbons and does not have the degenerated graphitic structure as a source for de nova synthesis of dioxins.

This

gives another possible explanation for the low dioxin emissions from coal combustion. Wood combustion and plastics combustion have slightly to medium sooting flames.66~72~73 PVC and PS fires are the most sooting flames. 72,73 Dioxin emissions from these combustion sources have been reported.7-11115*16 In municipal waste combustion luminous yellowish sooting flames are dominant74 mainly due to the high volatile content, the particulate emissions consist essentially particles75.

of soot and ash

Usually we refer to the particulate emissions as fly ash and there is no need to

distinguish between soot and ash particles. But in fact soot and ash particles have different origins, morphologies

and compositions.

Soot is from gas phase combustion

reactions and roughly

spherical with a diameter ranging from 0.005 to 0.25 urn and has a very high carbon content.61-63 Ash is from the high-temperature transformation of incombustible matter present in the fuel and has a particle size ranging from about 1 to 100 urn and a high mineral matter content.6g*70 An electron micrograph of MSWI fly ash shows that there are some “carbon fibers” with a high carbon content among other large ash particles. 35 These “carbon fibers’ resemble well the electron microscopic view of soot particles strung together in chains as shown in another electron micrograph for soot particles from a gas flame61, so these “carbon fibers” are probably soot particles.

In actual

combustion systems most of the soot particles are too small to be captured by APCDs and will be emitted with stack gas, in other words, most of the uncollected particles in stack emissions are soot particles,

This accounts for the observation

in section 3.2 that the fine particulates

in stack

emissions have a very high dioxin concentration noticing the assumption that de nova synthesis of dioxins occurs on soot particles. In the above discussions beginning with an observation originated from de novo synthesis experiments that some degenerated dioxins to occur, an examination

graphitic structure is necessary for de novo synthesis of

of the morphology

of the particulate

emissions from actual

combustion processes directs us to believe that the small soot particles from gas phase combustion have a very high graphitization

degree and are thus a primary source for de novo synthesis of

dioxins, whereas the large ash particles from solid phase combustion have a low graphitization degree and are thus not an important source for de novo synthesis of dioxins. With this assumption many experimental observations can be explained. 54 The Nature of De Novo Synthesis of Dioxins The de novo synthesis of dioxins occurs essentially by a process of the oxidative degradation of the degenerated graphitic structures; the major oxidation products are CO and CO*; the byproducts are a variety of organic compounds including PCDD, PCDF, PCBz, PCPh, PCNP and PCB se,4294

4111

Probably, for amorphous carbon the oxidation products are only CO and CO2. For graphite, because the graphitic sheets are stacked orderly and strengthened by bonds of the van der Waals type between them,5g at the low reaction temperatures considered the oxidative attack occurs only on the periphery sites of the graphitic sheets, but not on their surfaces.67,76,77 This peripheral attack opens the hexagonal carbon rings of the graphitic sheets.,”

so that the oxidation products

are also CO and Con. For carbon with degenerated graphitic structures including activated carbon, charcoal,

carbon black and soot, because the graphitic

sheets are stacked disorderly

and

disorientedly and surface imperfections such as vacancies and dislocations are present, not only the periphery but also the surface of the graphitic sheets can be attacked especially when metallic catalysts are present.67s76,77 In the basal plane attack some of the hexagonal carbon rings of the graphitic sheets may not be opened and hence serve as a source for the formation of aromatic compound classes including PCDD/Fs. This explains why the carbon morphology is important for dioxin formation. The similar dioxin “fingerprint” from all combustion sources may originate from the similar graphitic structure of soot particles from all combustion sources.

Hydrogen atoms are present in

soot particles and distributed between the graphite layers, 62 chlorine atoms may also be present and take the form as organic chlorines.

The role of metallic catalysts in de nova synthesis is

probably to facilitate the oxidative attack on the surface of the graphitic layers which causes a drastic increase of the carbon gasification rate at low temperature@’ and lead to the transformation of graphitic structures into organic compounds. 4. A Scheme of Dioxin Formation in Combustion Processes Following the above discussions it is possible to describe dioxin formation in combustion systems as a two-stage process: (1) carbon formation: carbon particles consisting of degenerated graphitic structures are formed in the combustion zone; (2) carbon oxidation: the unburnt carbon particles continue to be oxidized in the low-temperature postcombustion zone and PCDD/Fs are formed as by-products of the oxidative degradation of the graphitic structure of the carbon particles. Many steps and chemical reactions are involved in each of the two stages.

For carbon formation

there are at least three steps: nucleation, particle growth and agglomeration; for carbon oxidation four steps: oxidant interaction

adsorption,

with graphitic

formation

of complex

intermediate

carbon structure and products desorption.

involved are extremely complex and heterogeneous.

with metal ion catalysts, The chemical reactions

The details need to be further elucidated in

more experimental and theoretical studies. 5. Use of the Theorv to Exolain the Fxoerimental Observations Based on the above two sections new explanations

of the observations

in section 2 are

given. We note that these explanations are preliminary. For section 2.1: (1) Dioxin formation in the combustion of all organic fuels can be described using the above general scheme of dioxin formation.

For example, in a forest fire carbon particles

are present in the dense black smoke and will undergo low-temperature oxidation in the open air and hence may generate dioxins in the process.

(2) The very high dioxin formation in municipal

4112

waste incineration

is due to the highly sooting flame, high metal content and unsteady-state

combustion of municipal wastes.

In MSWI the feed is extremely heterogeneous and non-uniform,

so unsteady-state operation occurs often. This may set free more unburnt carbon particles to the postcombustion zone and increase significantly the de nova synthesis of dioxins. formation in coal combustion is mainly due to its low to non-sooting flames.

The low dioxin

(3) Soot from all

combustion processes has similar graphitic structure. 63 The oxidative degradation of the similar graphitic structure should give similar product distribution including the similar dioxin “fingerprint” from all combustion sources. For section 2.2: (1) At combustion temperature dioxins are not thermodynamically stable and decomposition is favoured over formation.

(2) Dioxin formation occurs on particulate matter and

desorption is not favoured at low temperatures, so that most of the dioxins found are in particulate phase. (3) “The finer the particles, the higher are the dioxin concentrations” is due to that the finer particulate matter contains a higher proportion of soot particles and de nova synthesis of dioxins occurs on the small soot particles.

(4) Flue gas 02 level can affect both carbon burnout in the

combustion zone and de nova synthesis in the postcombustion

zone; at a low 02 level more

unburnt carbon particles are present so dioxin formation is increased; at a high 02 level, however, the de novo synthesis of dioxins can also be increased because of the higher oxidant concentration in flue gas; depending on the structure and operating conditions of incinerators

sometimes an

optimum flue gas 02 level with minimum dioxin formation can be reached when these competing effects are balanced. Gas phase chlorine is not involved in de nova synthesis of dioxins as shown in laboratory experiments3*

on the other hand, the amount of chlorine in municipal waste is about

six orders of magnitude in excess of the chlorine present in dioxins emitted from incinerators7* therefore the controlling factor in dioxin formation may not be the chlorine supply but rather the formation of the biaryl structure of dioxins.

Chlorine is however a well-known flame inhibitor, its

presence in municipal waste may increase carbon formation in flames and hence the dioxin formation to some extent.

SO2 addition reduces dioxin emissions in some cases.

However,

because gas phase chlorine is not involved in de nova synthesis, its action may not be by reducing gas phase Cl;! through the Deacon reaction but by masking the catalytic dust surface due to sulphatization of SO3 which is in equilibrium with SO2.79 Most of the discussions in this paper are inferential. Experimental work to further clarify these points would include: comparison of dioxin formation in de nova synthesis experiments with and without gas phase precursors in the concentration range of IO-100 ug/Nms at both a short and a long time scale, as well as with MSWI boiler ash, ESP ash and filter cake as solid phase material respectively; characterization of the morphology and chemical state of carbon in MSWI and coal fly ashes; laboratory study of dioxin formation in gas combustion with sooting and non-sooting flames; laboratory and field studies of dioxin emissions from coal combustion for different ranks of coal; field study of the effects of optimal waste charge to incinerators on dioxin emissions. In actual combustion systems whether they are gas, liquid or solid combustion sooting flames can be recognized from the bright yellow coloration of the flame and the dark black coloration of the ash and smoke.

Technical measures that can maintain stable operation of incinerators, reduce

carbon formation in the combustion zone, enhance carbon burnout in the freeboard and inhibit Iowtemperature carbon oxidation in the postcombustion zone are expected to reduce dioxin emissions.

4113

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