A 11107 3

C]0707

NBS

PUBLICATIONS

NBSIR 84-2872

Containment of Dioxin Emissions

from Refuse Fired Thermal

Processing Units - Prospects and

Technical Issues

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Washington, DC 20234

February 1984

Issued May 1984

OF

S' V AX*9*

Q(T

100

U S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

,U56

34-2372

1934

c . 3

NBSIR 84-2872

CONTAINMENT OF DIOXIN EMISSIONS

FROM REFUSE FIRED THERMAL

PROCESSING UNITS - PROSPECTS AND

TECHNICAL ISSUES

' national bureau

OF STANDARDS

I '"HAT? C I r- c.

O

. <

Walter M. Shaub

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Washington, DC 20234

February 1984

Issued May 1984

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige. Secretary

NATIONAL BUREAU OF STANDARDS. Emast Ambtor. Director

TABLE OF CONTENTS

Page

1. EXECUTIVE SUMMARY 1

2. GOALS OF THIS REPORT 2

3. OBJECTIVES OF THIS REPORT 3

4. BACKGROUND 4

5. MODEL DEVELOPMENT 8

6. MODEL INPUT PARAMETERS 16

7. RESULTS OF MODEL CALCULATIONS 20

8. CATALYTIC FORMATION OF DIOXINS 30

9. SUMMARY AND CONCLUSIONS 34

10. REFERENCES 37

11. APPENDIX A A1

12. APPENDIX B B1

13. APPENDIX C Cl
LIST OF FIGURES

Page

1. Possible Mechanism for Dioxin Chemisorption on Fly Ash 6

2. Parametric Dependence of Dioxin Chemisorption on Input

Parameters 26

3. Schematics of Chemisorption Control Technologies 27

4. Suggested Dependence of Dioxin Emissions on Feed Rate

at Constant Excess Air 28

5. Suggested Dependence of Dioxin Emissions on Excess Air

at Constant Feed Rate 29
LIST OF TABLES

Page

1. Summary of Assigned Input Values 19

2. Chemisorption Times for Dioxins and Other Species 25

1. EXECUTIVE SUMMARY

There are reports [1-5] that dioxins are emitted from some refuse-fired

thermal processing units. The extent to which such emissions occur and the

conditions that lead to such emissions have not been clearly defined at the

present time. If it is determined that dioxin emissions should be contained

or minimized, then it will be necessary to examine strategies for achieving

goals of containment or minimization of these emissions.

This report addresses the prospects for and technical issues concerned with

the utilization of chemisorption as a technique for containment of dioxin

emissions from refuse fired thermal processing units. The results developed

in this report suggest that containment, through chemisorption of dioxin

emissions from refuse fired thermal processing units, may be technically

feasible. Suggestions for research objectives and full scale tests are

outlined. Refuse fired thermal processing units equipped for energy recovery

may be more preferable than those units which are not equipped for energy

recovery, if chemisorption occurs more efficiently at temperatures which are

substantially lower than furnace gas exit temperatures.

1
2. GOALS OF THIS REPORT

o Achieve an understanding of a possible strategy for containment of dioxin

emissions from thermal processing units.

o Delineate technical uncertainties which should be addressed to refine the

above strategy for containment of dioxin emissions from thermal

processing units.

o Delineate experiments to resolve technical uncertainties which may be

carried out in laboratories and in full scale thermal processing units.

o Provide a simple model for engineers to use to estimate potential dioxin

containment by chemisorption.

2
3. OBJECTIVES OF THIS REPORT

o Review literature reports of dioxin emissions from thermal processing

units and assess long term dioxin containment possibilities.

o Construct an analytical model for chemisorption of dioxins in thermal

processing units.

o Utilize literature reports to estimate numerical values of input

parameters for the model.

o Utilize the model and input parameters to suggest the feasibility of

dioxin containment by chemisorption.

o Evaluate the model test results to suggest technical uncertainties for

application of chemisorption containment methods.

o Suggest laboratory-scale and full-scale tests to resolve technical

uncertainties for application of chemisorption containment methods.

3
4. BACKGROUND

There are reports [1-5] that dioxins (in this report "dioxins" means any of

the various chlorinated dibenzo-p-dioxins) and related compounds, e.g. furans

(in this report "furans" means any of the various chlorinated dibenzofurans)

are emitted from some refuse-fired thermal processing units (in this report

"thermal processing units" means incinerators and incinerators equipped for

energy recovery, i.e. boilers). The extent to which such emissions occur and

the conditions that lead to such emissions have not been clearly defined at

the present time. If it is determined that dioxin or furan emissions from

refuse-fired thermal processing units should be contained or minimized, then

it will be necessary to examine strategies for achieving goals of containment

or minimization of these emissions.

This report addresses the prospects for and technical issues concerned with

the utilization of chemisorption as a technique for containment of dioxin or

furan emissions from refuse- fired thermal processing units. There is some

preliminary evidence [6] that desorption energies of chemisorbed dioxins on

fly ash are large. Values for desorption energies, Ep, of dioxins may be as

large as 40-50+ kcal*mol ^[6]. Furans, being in some respects chemically

similar to dioxins (see Figure 1) may have comparable desorption energies. As

shown in Figure 1. it is assumed that unoccupied adsorption sites on the fly

ash are either hydroxyl groups or oxygen atoms. The basis for this suggestion

is indicated in the APPENDICES and references contained therein. If it is true

that desorption energies are substantial for these compounds, then successful

chemisorption of these compounds on some inexpensive adsorbent, e.g. fly ash,

may provide a basis for containment of these materials. If fly ash containing

chemisorbed dioxins or furans is disposed of into the environment at an ambient

4
temperature of 25 °C and if the rate constant, kp, for thermal desorption of

dioxins or furans is estimated to be [6]

k

0

< i0

13

e

- 4°000/( RT) sec-l

(!)

then the time required for one percent of the dioxins or furans to leach from

the fly ash due to thermal desorption is about 6.9 million years. Thus,

unless it can be demonstrated that chemisorbed dioxins can be leached in the

environment by other than ambient thermal stresses, the above construct

implies that chemisorbed dioxins on fly ash may remain chemically bound for

extremely long periods of time. If this is demonstrated to be a valid

finding, then chemisorption may offer a means for safe containment of dioxins

or furans, if such a containment is determined to be desirable.

This report provides a feasibility analysis for dioxin or furan containment by

chemisorption on a suitable adsorbent. In the following discussion,

chemisorption of dioxins on fly ash is modelled under conditions which may be

achieved in thermal processing units. The possibilities for examining and

testing adsorbents other than fly ash are discussed in the SUMMARY AND

CONCLUSIONS section. It is suggested that furans may be considered (see

Figure 1) as being amenable to an analysis similar to that presented in this

report for dioxins. Therefore an analysis of furan chemisorption is not

explicitly presented.

Research objectives and field testing of concepts outlined in this report are

recommended in the SUMMARY AND CONCLUSIONS section of this report. It is

suggested that the recommended research, development and testing may help

optimize understanding of chemisorption of dioxins and furans as a containment

technology.

5
exanple 91 ;®oc

2,3,T ,3-TCDO

example 12

^77^777

rurf tee X.C * WW

2,3,7,8-TCDD surface

730K u

^777^7777 - •*osc * HC2

Ci.

777777TTTT

exannie #3

“£0°C * rrfrfrn

2,3,7, 8-7C3F

surfac

s

.. 7rrr7rfrrn

C£

O Cl

example #U H r 0 . ^ f? + ^777^777^^0^ +HU

r? '-* surface 1 2,3,7, 8-TCDF 77777^777

Figure 1. Possible MechanismC s ) for Dioxia tFuran also shown)

Chemisorption on Fly Ash.

The examples shown above Include reaction of 2,2,7,8-7C3D

or 2, 3, 7, 8-TCDF with surfaces watch are either sparsely

or largely covered with hydroxyl groups. The reader Is referred to the APPIXDECZS for more details.

6
Some useful background documents are provided in the APPENDICES. These

reports are intended to provide a basis for supporting the models for

chemisorption that will be developed and to provide a basis for understanding

physical chemical aspects of dioxin emissions from thermal processing units.

In the discussions which follow, numerous assumptions and approximations are

utilized. Most of these are described in detail in APPENDIX C. Since the

intent of this report is only to suggest concept feasibility, the assumptions

and approximations should be acceptable. The reader can refine calculations

as desired. However, in view of many technical uncertainties, substantial

further refinements are probably not warranted until some of the research,

development and testing, which is recommended in the SUMMARY AND CONCLUSIONS

section of this report, is carried out.

7
5. MODEL DEVELOPMENT

In the following discussion, it is assumed that chemisorption of competitively

adsorbed species is to be modeled in which the chemisorption is described by

a Langmuir isotherm [7]. The model is constructed to describe the process of

competitive chemisorption in thermal processing units. It is assumed that

chemisorption of dioxins and other gaseous species onto entrained fly ash in a

hot flowing gas stream may be modeled by treating all species other than

dioxins in the gas stream as being approximately equivalent in terms of

chemisorption properties. The purpose of this assumption is to simplify

calculations and demonstrate the effect of a difference in the adsorption

energy, E^, between dioxins and other species, upon the chemisorption

properties of the gas/fly ash mixture. Another advantage of this

approximation (and other approximations discussed below) is that a closed-form

analytic solution may be developed which may permit the reader to easily use

the model to test input parameter variations with a pocket calculator. It is

left to the reader to construct whatever refinements are desired, using the

model to be developed, as a starting basis.

The reader is instructed to review the material presented in APPENDIX C,

particularly the material which deals with modelling catalytic dioxin

formation, prior to reading the following discussion. The material presented

in the APPENDIX provides a basis for the terminology, assumptions and

discussions which are incorporated into the discussion and model development

which follows. Where possible, the symbols used in the report which is

presented in the APPENDIX, are also used in the discussion which follows.

8
Dioxins are denoted by the symbol, d. All other competitively chemisorbed

species are denoted by the symbol, a. If the dioxins and other species are

denoted as being in the gas phase, their respective symbols are not

subscripted. If the dioxins and other species are denoted as being

chemisorbed, they are symbolized respectively as d

g and a

g

, i.e. the symbols

are subscripted to denote chemisorption. Adsorption sites for chemisorption

are denoted by the symbol, S. For simplicity all adsorption sites are

considered to be identical.

Competitive chemisorption can be described by the following representation:

d + S

a + S

( 2 )

(3)

The time dependent rates of chemisorption of d and a respectively can then be

represented by the following equations (notations follow APPENDIX C and Laidler,

in reference [7]:

d

s s 3t (d) kl

d (1- e

a

- V k

-l

d

s

*s s 3t (a) = k2

a (1 ' 0

a

* V ’ k-2 a

s

(4)

(5)

The following definitions and assumptions are made (refer to APPENDIX C for

details and an explanation of variables):

d = d - d

o s

a = a - a

o s

( 6 )

(7)

9
0

a » 0d (10)

The subscript, o, used in equations (6) and (7) above denotes initial values of

the concentrations of a and d prior to the onset of chemisorption. If the

following additional definitions are made:

al

H k

2

ao

( 11 )

Equations (6) through (13) can be used to write equation (5) in the

form

( 12 )

(13)

a = a, + a„a + a~a

s 1 2 s 3 s (14)

The physically meaningful solution to this equation can be shown to be [8]:

C 1 \ 1 - e 'V 1

C

°[e'

taJ

_4 ]

( 15 )

Below, j, C , C-. and C 9 are defined. The symbol, t , denotes time for

0 1 c 3

chemisorption of a.

10
(16)

2

j = (a

£

- 48^)

Co

E 2a

3 (17)

C

1

S a

2

' j (18)

+

CM <U

III CM O (19)

At equilibrium (t -* + ») , equation (15) has the solution:

3

^s^EQ ( 20 )

At equilibrium the forward and reverse rate components of the R.H.S. of

equation (4) can be equated to each other, and it follows from equation (10)

that the following equilibrium expression can be developed:

(a

5

)

eQ - (r,)^ (Od

This can also be written as:

(d

S)

EQ

55 (k^) f

1' 4^ (o*)

(

a

s) Eq]

Rearrangement of equation (15) provides the following solution:

t

a = -

J

a,n r^i

( 21 )

( 22 )

(23)

The time required to achieve 99% of the equilibrium value of chemisorbed is

found from use of equations (20) and (24) shown below to be:

11
a (24)

s =0.99 (a

s

) EQ

1 C9

t (99%) = 4£n [10 j 1-0. 99 ~ }] a j

(25)

using the definition:

y

- jl/!A 2

y 4NpW/

(26)

The expressions which have been developed above can be used to show that

Near equilibrium,

- 0-! + 4 d-yas )]d

s (27)

(28)

If the time required for a

& to reach an equilibrium value is significantly

shorter than the time required for d

g to reach an equilibrium value, equation

(28) above may be regarded to be exact.

Def i ni ng

Q , k

xd

o tl-y(a

s

) EQ ] (29)

p E

C

k-i

+ k

i i ^Veq!] (30)

It can be shown that near the equilibrium value of d^ that:

d

s s 2 Cl - e

" Pt

) (31)

12
This equation can be rearranged to show that near the equilibrium value of d

g

:

t = - \

in (1 - £ d

s

) (32)

To determine the time required to achieve 99% of the equilibrium value of

chemisorbed d

g

, proceed as follows. First calculate a value for ( a

s )^g*

this value for ( a

s

) Eg

and equations (6) and (22) to calculate a value for

(d

s

) EQ

.Set:

Use

d

s = 0.99 (d

s

) EQ (33)

Then use this value of d

g to calculate t^ (99%) in equation (32)

A more approximate value for t^ (99%) may be developed using equation (41) of

the report presented in APPENDIX C. Then if the activation energy for chemisorption, E. of d is non-zero [6,9], it follows that

n S

approx

t

d (99%) =

- 4N /ct \ 2 r a fe - -

P o

0.99(d

$)

4N

Is < a°\ /°a\

2

1 1 W J

e +EA

/(RT > (34)

The utility of this equation is that it explicitly illustrates the dependence

of the time to achieve near-equilibrium chemisorption upon the activation

energy for adsorption.

In performing calculations of ( a

s )^g

and (d

s )^g utilizing the above equations,

it is important to determine that in spite of approximations used, the results

obtained are physically meaningful.

13
For example, since the available number of surface adsorption sites is

limited, the calculated values for (a

s

) Eg

and (dg)^ should be reset to the

values (a ) and (d ) (these are upper limit values for a and d ) s max s max s s respectively, if:

^s^EQ > ^Vmax

^s^EQ > ( Vmax

(35)

(36)

The values for (a ) and (d ) av may be calculated from the following

s max s max relationships which are valid assuming equilibrium chemisorption [7] and

a > (a ) and d > (d ) : o

v s'max ov s'max

a /max

6d\ .

K

d

P

d

k

l

k-2d

oma (37) 0a/ EQ

K

a

Pa

k

-lk2aomd

"

0d)max

+ ^^0a)max ~ * (38)

the results:

^ 0d^max h(l+h)

_1

(39)

^ 0a^max 1 - (Vmax (40)

These equations can be used with equations (8) and (9) to show that

(dj_ £ 4N (-&) (e

s' max p \a

d /

\ d/max (41)

/a \ 2

(a ) s 4N [ -& ) (6 ) s'max p \o

a / a'max

(42)

14
In equation (37) above, p

d

, p

a

, m

d

and m

a are respectively the initial (prior

to chemisorption) partial pressures of d and a and the molecular weights of d

and a.

In addition, if there are more adsorption sites for a and d than initial gas

phase species, i.e., if

(dv ) s max

> d (43)

(a ) >

s max

(44)

Then if (a ) cr. > a and (d ).-,, > d , the calculated values for (a and sEQ o

v sEQ o’ sEQ

(d )„ should be reset to a and d respectively, instead of to (a ) and sEQ o o

r

s max

(d ) respectively,

s m&x

The rate constants for adsorption and desorption of dioxins and other

competitively adsorbed species are determined as described in APPENDIX C.

However, note that the calculations which will be described in the following

section are based on the presumption that there is a non-zero activation energy

E-^, for adsorption of dioxins on fly ash. Therefore is represented by the

analytic form:

e-E

x/(RT)

( 45 )

The superscript, d, denotes collisions of dioxins with particulates.
6. MODEL INPUT PARAMETERS

Values for input parameters have been assigned to test the model that has been

constructed. The values are reported either in Table I of this report, or in

APPENDIX C. The reader is referred to APPENDIX C for a discussion of how the

values assigned to parameters which appear in equations presented there have

been established.

The values assigned to parameters reported in Table I have been chosen based

upon information inferred from recent literature and to illustrate the

dependence of the adsorption process upon key variables. Some comments

pertaining to parameter assignment are presented in Table I. Other comments

are presented below.

In general, it should be understood that the assigned values for parameters

are approximations, just as the model itself is an approximation. As

mentioned previously, the reader can refine the model as desired and can test

other input values on this or a refined model. Notwithstanding approximations

that are made, the model serves to illustrate how the employment of

chemisorption methods may be used to contain dioxin emissions.

E^, the activation energy for chemisorption of a dioxin molecule, has been

assigned a value of 10 kcal*mol This value is assigned based on the

observation reported in the literature [9] that significant chemisorption of

dioxins on fly ash occurs at temperatures above about 100 °C. In the same

literature, it is reported that thermal desorption of dioxins is apparently

irreversible even at temperatures up to about 300 °C. Additionally, estimates

16
of the activation energy for thermal desorption of dioxins from fly ash based

upon models for catalytic dioxin formation have been suggested [6]. It

appears that the activation energy for thermal desorption of chemisorbed

dioxins may be large, perhaps in excess of 40 kcal*mol The value E_^ = 40

kcal-mol 1

is used in calculations presented in this report.

Other species besides dioxins are known to be present in the hot gas streams

of thermal processing units [1-6]. Many of these species may be capable of

being chemisorbed on fly ash [1-9]. In the calculations presented in this

report, it is assumed that all chemical species undergoing competitive

chemisorption have the same molecular diameter. This is equivalent to stating

that the total number of adsorption sites is fixed and that adsorption at any

site is non-specific to species identity. The reader can test other

assumptions. This assumption also implies the mathematical simplification of

grouping all competitively adsorbed species together and treating them as one

species with an arbitrarily assigned molecular weight. In this report, the

molecular weight for the competitively adsorbed species is assigned a value

typical of a three-ringed polycyclic aromatic. Inspection of the model

equations suggests that molecular weight dependence is slight in modeling

competitive chemisorption. The relative ratio of chemisorbed species to

dioxins is assigned a value of one hundred. Other values may be tested by the

reader. The dependence of competitive chemisorption of dioxins upon this

ratio is intuitively obvious from an inspection of the model equations.

The particulate mass loading values and particle sizes which have been assumed

for the fly ash are representative of the range of values for these parameters

during thermal processing. Unique rather than distributed values for these

17
parameters, have been assumed to avoid mathematical complications. These

approximations make it easier to recognize the dependence of chemisorption of

dioxins upon parametric variations of these variables.

The values of the activation energies for chemisorption and desorption of

competitively adsorbed species have been assigned corresponding closely to

values that have been assigned to polycyclic organic molecules, as reported

in the literature [8]. In particular, the zero value assigned to the chemisorption activation energy provides strong contrast to the value of 10 kcal •

mol ^ assigned to the chemisorption energy for dioxins. This provides a useful

test of the dependence of competitive chemisorption of dioxins upon differences

in chemisorption activation energies between dioxins and other chemisorbed

species.

18
Table I - Summary of Assigned Input Values (a)

Vari able Units Values assigned Comments

T °C 50; 150; 250; 350; 450 The assigned values span the

°K 323;423;523 ;623 ;723 range over which effects on adsorption of activation energy

mass loading, adsorption time,

etc. can be demonstrated.

E i

kcal *mol

-1

10 See discussion, this section.

E_, kcal -mol

-1

40 See APPENDIX C and discussion,

this section.

E 2

kcal *mol

-1

0 See discussion, this section.

CM 1

1

LU kcal ‘mol

-1

30 See discussion, this section.

a

P cm 5x10‘ 5

,5x10‘

4

, 5x10*

3

Corresponds to particle diameters of 1,10, and 100 micrometers.

M

-3

g*cm 10" 7 ;10‘ 6 ;10‘ 5 Corresponds to fly ash mass

P loadings of 0.1,1, and 10 grams

per cubic meter.

d m

-3

g*M 10- 5

Must be converted to units of

U molecules per cubic centimeter

for use in equations appearing

in this report.

a

0

m"3 g*M 10

2

d

o See discussion, this section.

CTd cm =4.24x10 3 cm See APPENDIX C.

aa cm CT d Arbitrary assignment; see discussion, this section.

m

d

g 321.8 See APPENDIX C.

m

a

g 178.2 See discussion, this section.

(a) All other variables requiring assignment of input values, are assigned

values in APPENDIX C and are assigned the same values in this report.

19
7. RESULTS OF MODEL CALCULATIONS

Calculations, based on the model which has been constructed and the input data

presented in this report and in APPENDIX C, have been made to test the

dependence of chemisorption upon variations in several of the input

parameters. The results of these calculations are presented in Figures 2a

through 2c and in Table II.

In Figures 2a through 2c, the y-axis variable is the base ten logarithm of the

ratio of chemisorbed dioxins to dioxins which are present in the gas phase.

The larger the numerical value of this y-axis variable, the greater the amount

of dioxins which are adsorbed on the fly ash compared to the amount of dioxins

which are present in the gas phase. To assure successful containment of

dioxins by chemisorption of dioxins on fly ash, it is necessary to determine

conditions which will maximize the value of this y-axis parameter. Since it

is presumed that it may be desirable to collect particulates laden with

chemisorbed dioxins, no modeling of chemisorption onto particles smaller in

size than one micrometer is carried out. Particles smaller in size than this

are more difficult to contain with air pollution control equipment than are

larger particles.

All of the results presented in Figures 2a through 2c are based on the

assumption of passage of sufficient time for development of dynamic

equilibrium partitioning of dioxins between the gas and entrained fly ash.

Table II suggests approximate transit times required for development of

dynamic equilibrium between gas and fly ash at typical load conditions. The

reader may test other load conditions. If the configuration of the flow in a

20
thermal processing unit is insufficient to allow time for development of

dynamic equilibrium, the reader can recalculate the partitioning using the

time dependent equations which have been presented in the model development

section of this report. If the existing gas flow configuration is found

insufficient to achieve substantial dioxin containment by entrained fly ash

chemisorption, an alternative may be to attempt reconfiguring the flow stream

to allow for packed bed chemisorption, as discussed in this report. However,

the reader is cautioned that uncertainties in approximations made in this

report dictate necessary testing to select materials, configurations and

operating conditions for optimal chemisorption.

Figures 3a through 3d illustrate possible configurations for operational

control of dioxin chemisorption on fly ash (or other adsorbing materials on

which dioxins may be chemisorbed). Figure 3a suggests that mass loading of

entrained fly ash may be controlled through variation of gas flow velocities

in the thermal processing unit furnace (combustor). If the thermal processing

unit is equipped for energy recovery, this may be undesirable if significant

fouling or slagging on the energy recovery components is likely to occur.

Figures 3b through 3d suggest alternatives if this is a problem.

Respectively, these include: (a) injection of supplemental fly ash ahead of

air pollution control equipment; (b) use of a packed bed of fly ash or other

material or material mixtures; and (c) re-injection of fly ash from the air

pollution control eqipment. The temperature of the adsorbing medium may be

controlled by variation of gas flow rates, supplemental heating, energy

recovery equipment operating conditions or other means.

21
The effectiveness of chemisorption and thermal destruction processes for

containment of dioxin emissions from a thermal processing unit can be considered

in terms of (adjustable) thermal processing unit operating parameters - the

excess air level and the waste mass feed rate to the unit. Consider the following

two cases:

(i) maintenance of a constant excess air level in the thermal processing

unit while increasing the mass feed rate, and

(ii) maintenance of a constant feed rate in a thermal processing unit

while increasing the excess air level.

Dioxin emissions from thermal processing units may be related to these two

operating parameters according to the curves suggested in Figures 4 and 5. The

reasons for suggesting this behavior are discussed below for each of the above

two cases.

Case (i): The maintenance of a constant excess air level requires that the air

flow rate should be increased when the mass feed rate is increased. Increasing

the air flow rate (at constant volume) results in a higher flow velocity. This

may result in increased particulate (fly ash) entrainment in the flow stream.

Increased particulate entrainment may provide a greater surface area for dioxin

chemisorption and also (if there is vapor depletion of the dioxin precursors)

may drop the size of 0 . These results may suppress the extent of dioxin

d

formation. This is suggested in region "A" of Figure 4.

Since the volume of the thermal processing unit is constant, increasing the air

flow rate can decrease the process gas residence time. This may shorten the time

for effective chemisorption of dioxins (particularly if there is a non-zero

activation energy for chemisorption of dioxins) and may reduce the mixing

efficiency of air and fuel (due to a decreased mixing time). Mixing problems

22
are discussed in APPENDIX B. When the mass feed rate exceeds some particular

value, these effects, which may tend to promote dioxin emissions, may dominate

over the effects which are suggested in region "A" of Figure 4. This dominating

promotion of dioxin emissions is suggested in region "B" of Figure 4.

Case (ii): The maintenance of a constant feed rate while increasing the excess

air level in a thermal processing unit may result in an increased destruction of

dioxin precursor species due to an enhancement of oxidation reactions. This is

discussed in APPENDIX B. In addition, below some value, increasing the amount

of excess air at a constant feed rate may also raise the thermal processing

unit operating temperature(s) , which also may promote more efficient destruction

of dioxins and precursors (see APPENDICES B and C). These effects are suggested

in region "A" of Figure 5.

Increasing the excess air can result in an increase in the gas flow rate in a

constant volume thermal processing unit. This may result in reduced gas

residence times in the thermal processing unit. This shortened residence time

may result in poorer mixing efficiencies (see APPENDIX B) and may reduce the

extent of effective chemisorption of dioxins, which can be time dependent if

the activation energy for dioxin chemisorption is non-zero. Both of these

effects may tend to raise the dioxin emission level from the thermal processing

unit. In addition, beyond some upper limit of the excess air level, the temperatures in the thermal processing unit may decrease (e.g., due to air dilution

effects) and this may decrease the efficiency of thermal destruction of dioxins

and precursors (see APPENDICES B and C). When the excess air level exceeds some

particular value, these effects which tend to promote dioxin emissions, may

dominate over the effects which are suggested in region "A" of Figure 5. This

dominating promotion of dioxin emissions is suggested in region "B" of Figure 5

23
The inference of the preceding discussion is that regardless of whether or not

chemisorption is employed as a dioxin emission control technology in thermal

processing units, there may, as evident from a consideration of Figures

4 and 5, be an operating parameter window for assuring effective minimization

of dioxin emissions. This implies that fine tuning of operating parameters may

be required to ensure that thermal processing units are operating within this

suggested dioxin emission minimization window. If it is not possible to

operate the unit under these dioxin emission minimization window operating

conditions, it may be necessary to employ a chemisorption technology such as

illustrated in Figure 3. It is suggested that careful research, testing and

development may lead to a definition of thermal processing unit dioxin emission

minimization window operating conditions. This may enhance our understanding of

how to minimize dioxin emissions from thermal processing units and perhaps result

in improvements of thermal processing unit designs. Further discussions of the

implications of the results presented in this report are to be found in the

SUMMARY AND CONCLUSIONS section.

24
Table II - Chemisorption Times for Dioxins and Other Species^

T

°C

t (99%)

seconds

t

d (99%)

seconds

50 2.04x10"

2 +4

7.06x10

150 1. 99xl0~ 21. 69xl0

+3

250 1. 20xl0 2

1. 59xl0

+2

350 5.82x10" 34. 00xl0

+1

450 4. 77x10 8.90X10" 1

C 3 ^ Pocket calculator results based on equations in the text and using the

-4 -5 -3

values: a = 5x10 cm, M = 10 g*cm . P P

a

These results illustrate the sensitivity of adsorption times to activation

energies for chemisorption.

25
350 °C

/

i —a .3

L° 8io(M

p g

’ cm )

350 °C

T °C

26
Figure 3a

out

Figure 3b uppleaeatai

particle * injection ia „ _

s

I

-CHS

Air pollution

control equip.

Energy recovery

I 1

1 1

Furnace

Air pollution

control equip.

Energy recovery

Furnace

vaste ,in boosting

air flow rate

Figure

packed

bed

filter

-CHS

Air pollution

control equip.

Energy recovery

Furnace

waste l in

Air pollution

control equip.

Energy recovery

Furnace

Figure 3. Schematics of Chemisorption Control Technologies.

27
Feed rate (arbitrary units)

at constant excess air

Figure 4. Suggested Dependence of Dioxin Emissions on Feed Rate at Constant

Excess Air.

28
Excess air (arbitrary units)

at constant feed rate

Figure 5. Suggested Dependence of Dioxin Emissions on Excess Air at Constant

Feed Rate29
8. CATALYTIC FORMATION OF DIOXINS

Recent literature reports have suggested that dioxin formation in thermal

processing units may occur due to chemical mechanisms which involve condensed

phase reaction steps [6,10,11], (these references are also respectively

APPENDICES C, B and A of this report). Detailed mechanisms which may

characterize condensed phase dioxin formation processes have not been tested.

The origins of such processes, e.g., condensed phase chemical reactions on

grate beds, or chemical reactions in or on fly ash (catalytic or

non-catalytic) , etc., have not clearly been determined. Carefully designed

experiments are necessary to resolve these technical uncertainties.

The methodology proposed in this report for containment of dioxin emissions

may involve the use of materials which either themselves may contain dioxin

precursors or which may provide reactive surfaces which promote catalytic

processes. In view of the brief discussion presented in the paragraph above,

it is instructive to consider if the proposed dioxin containment methodology

may enhance dioxin formation in thermal processing units.

Figure 3a illustrates a method for increasing fly ash mass loading by

increasing gas flow rates through the furnace of a thermal processing unit.

Increased gas flow rates may result in increased entrainment of particulate

matter which has only been partially combusted. This results from entrainment

of larger diameter particles which may have longer burnout times. This

problem would be typical of grate bed burning units. Increased transport of

long burning time material may result in increased emissions of unburned,

partially burned or chemically transformed organic matter from the thermal

30
processing unit. If dioxins were produced by condensed phase reactions,

higher mass flow rates of organic emissions could be promoting higher emissions

of dioxins, if other operating conditions remained unchanged. One way to

avoid this problem would be to use the methodology suggested in Figures 3b

through 3d. These approaches could permit the use of lower organic content

adsorbent material or the use of material having a more precisely known

organic content. These approaches would therefore be less likely to promote

increased dioxin emissions through condensed phase reactions. However, the

possibility that catalytic dioxin formation could occur on the surfaces of

these materials must still be considered. This is discussed below.

One way for catalytic dioxin formation to occur would be through reaction

between a gas molecule and an adsorbed molecule. These reaction mechanisms

are known as Langmuir-Rideal mechanisms [7]. Such a dioxin formation

mechanism involving likely dioxin precursor molecules is discussed in detail

in APPENDIX C. The implications of increased mass loading (increased

adsorbent surface area) with respect to the possibility that such a dioxin

formation mechanism may occur can be developed from arguments based on the

model and inputs suggested in this report as follows.

The results of the model calculations in this report suggest that dioxin

containment is most efficient at high mass loadings and at temperatures below

about 350 °C. Under these conditions, unless there is an activation energy

barrier for chemisorption of competitively adsorbed species, the elapsed time

for chemisorption will be short, on the order of a millisecond or less. This

sets an upper limit on the time window for dioxin formation by a

Langmuir-Rideal mechanism, since the gas phase concentration of precursor

species falls to zero once this time is exceeded, except in the case that

31
temperatures are sufficiently high to promote thermal desorption. If the

competitively adsorbed species are all considered to be dioxin precursors,

e.g., chlorophenol s , and the rate constant for the reaction

a + a d

s s (45)

is represented by the expression

k, = 2 e-V<R/T)

3 p

(46)

it can be shown using the formalisms developed in this report that the amount

of dioxins produced, at a mass particulate loading of 10 grams per cubic meter

and a particle diameter of 10 micron-meters, is small and is totally adsorbed

on the fly ash at temperatures below about 350 °C. This assumes an activation

energy, E^, comparable in value to that suggested in APPENDIX C.

Alternatively, dioxin formation on a catalytic surface may be postulated to

proceed by reaction between two neighboring adsorbed precursor molecules.

These reaction mechanisms are known a Langmuir-Hinshelwood mechanisms [7]. A

mechanism of this type is illustrated by:

If such a mechanism were operative, a large value of the rate constant, k^,

would imply significant dioxin production by reactions such as illustrated by

equation (47), may be possible. This assumes that the time to achieve

equilibrium coverage of an adsorbent by competitively adsorbed species is

considerably shorter than typical thermal processing unit transit times, in

the absence of significant adsorption activation energies for these species.

k

4

a + a -> d

s s s (47)

32
However, there is evidence that suggests either the value of k^ may not be

large or there may be a non-zero activation energy for adsorption of dioxin

precursors [1-6,10,11]. This evidence is derived from reports which consistently

indicate that emissions of potential dioxin precursors from thermal processing

units are generally found to be greatly in excess of concurrent dioxin

emissions. If the value of were large, this would not be found.

Therefore, in view of the above observations, it is unlikely that the reaction

step shown in equation (47) will produce large amounts of dioxins at high

adsorbent mass loading. In addition, increasing the adsorbent mass loading

will decrease the fraction of neighboring precursor molecules if vapor

depletion takes place, and this will also decrease the rate of dioxin

production by Langmuir-Hinshelwood mechanisms. Again, any dioxins which may

be produced, would be adsorbed (and therefore contained) at temperatures below

about 350 °C based on the model constructed in this report.

33
9. SUMMARY AND CONCLUSIONS

The results developed in this report suggest that containment, through

chemisorption of dioxin emissions from refuse fired thermal processing units,

may be technically feasible. Technical issues which may necessitate further

research can be identified. This research may require both laboratory scale

investigations and full scale tests of concepts developed in this report.

Suggestions for research objectives and full scale tests are outlined in the

bullets presented below. Refuse fired thermal processing units equipped for

energy recovery may be more preferable than those units which are not equipped

for energy recovery.

o As determined from the discussions presented in this report, the most

critical factor affecting the required contact time for efficient

adsorption of dioxins at any temperature is the energy of adsorption, E^.

The lower is, the shorter the required contact time for efficient

adsorption. The higher the temperature of the adsorbent, the shorter the

contact time required for efficient adsorption. If the value of EA

is

greater than zero, the adsorbent must be heated to effect efficient

dioxin containment.

o Laboratory investigations should be carried out to determine E^ for

candidate adsorbents, e.g. by type (and composition): carbonaceous

(carbon black, graphite, charcoal), ferromagnetic, mineral (sand, fly

ash, Zeolites), etc. Mixtures of adsorbents should be similarly tested.

3h
o Different adsorbents can be field tested. Fly ash may be a desirable

adsorbent since it is a by-product of refuse fired thermal processing

operations and is therefore readily available on site.

o There may be a thermal "window" for efficient chemisorption of dioxins,

which is principally defined by the energy of desorption, Ep on the high

temperature side and by on the low temperature side. The larger Ep

is, the higher the upper limit cutoff temperature for efficient dioxin

adsorption will be. The smaller E^ is, the smaller the lower limit

cutoff temperature for efficient adsorption of dioxins will be.

o Since adsorption is likely to be inefficient at furnace gas exit

temperatures, it will be necessary to drop the gas temperature after the

gas exits from the furnace of the thermal processing unit. In the

example presented in this report, the temperature should be dropped to

below about 350 °C. This implies that incinerators equipped for energy

recovery may be desirable, since the process of energy recovery can be

used to control the gas temperature and assure that the required gas

temperature drop is achieved.

o Increasing the mass loading of fly ash or other adsorbent with which the

thermal processing unit gas stream is in contact, may significantly

increase the amount of dioxins that are irreversibly adsorbed by the

adsorbent.

35
o Decreasing the particle size of a given adsorbent, or replacing low

surface area adsorbents with high surface adsorbents may significantly

increase the amount of dioxins that are irreversibly adsorbed on the

adsorbent.

o If for a given adsorbent is large, the contact time for efficient

dioxin adsorption may be long. In some instances, this may necessitate

usage of packed bed adsorbers - for providing longer contact times and

higher surface area (mass loading of adsorbent) per unit volume. In

addition adsorbent beds would likely have greater thermal inertia and

therefore may afford more uniform control of the adsorbent temperature.

Adsorbent beds may also provide better particle size control.

o Competitive (multicomponent) adsorption may limit the mass of dioxins

that is adsorbed. It is suggested that research be conducted to identify

adsorbents which are inexpensive and which preferentially (selectively)

adsorb dioxins and similar compounds, e.g. furans.

o The logic presented here for potential control of dioxin emissions should

also be applicable to control (containment) of other toxic substances.

Conditions for optimal containment may be determined by research,

development and testing as suggested above.

o The economics of applications of the suggested methodologies for dioxin

containment have not been determined. However, sorbent cleanup of

polycyclic organic matter emissions through filtering of flue gas at low

temperature through a bed of activated charcoal is a common practice in a

number of coal fired power plants in the western United States[8].

36
10. REFERENCES

(1) Harris, J. C. , Anderson, P. C. , Goodwin, B. E. , and Rechsteiner, C. E.

,

Dioxin Emissions from Combustion Sources: A Review of the Current State

of Knowledge , N.Y.: Final Report to ASME, ASME, N. Y. , N.Y. , 1980.

(2) Esposito, M. P. , Tiernan, T. 0. and Dryden, F. E. , "Dioxins" Washington,

DC: EPA-600/2-80-197, 1980.

(3) Bumb, R. R. , Crummett, W. G. , Cutie, S. S. , Gledhill, J. R. , Hummell,

R. H. , Kagel , R. 0, Lamparski, L. L. , Luoma, E. V., Miller, D. L.

,

Nestrick, T. J. , Shadoff, L. A. Stehl, R. H. , and Woods, J. S. Science

1980, 210, 385-390.

(4) Janssens, J. , Van Vaeck, L. , Schepens, P. and Adams E. , "Qualitative and

Quantitative Emissions of a Municipal Incinerator Installations", in

Physico-Chemical Behavior of Atmospheric Pollutants , Versino, B. and

Ott, H. , Eds. (Boston, MA; Reidel Pub. Co., 1982), pp. 28-38.

(5) Tiernan, T. 0., Taylor, M. L. , Garrett, J. H. , Van Ness, G. F. ,

Solch, J. G. , Deis, D. A. and Wagel D. J. , "Chlorodibenzofurans and

Related Compounds in the Effluents from Combustion Processes", presented

at the 3rd International Symposium on Chlorinated Dioxins and Related

Compounds, Salzburg, Austria, 13 Oct. 1983.

3T
(6) Shaub, W. M. and Tsang, W. , "Overview of Dioxin Formation in Gas and

Solid Phases Under Municipal Incinerator Conditions", To appear in the

Proceedings of the Symposium on the Origins and Fate of Dioxins and

Dibenzofurans in the Total Environment II , Keith, L. H. , Choudhary, G.

and Rappe, C. , Eds., to be published in 1984 by Ann Arbor Science Pub. Inc.

(7) laidler, K. J. , (and references contained therein) Chemical Kinetics ,

McGraw-Hill; New York, 1965.

(8) Schure, M. R. , Ph.D. Thesis, Colorado State University, Ft. Collins,

Colorado, 1981.

(9) Rghei, H. 0. and Eiceman, G. A., Chemosphere, 1982, YL, 569-576; 1982, 11 ,

833-839.

(10) Shaub, W. M. and Tsang, W. , Environmental Science and Technology, 1983,

17, 721-730.

(11) Shaub, W. M. and Tsang, W. , "Physical and Chemical Properties of Dioxins

in Relation Their Disposal", in Human and Environmental Risks of

Chlorinated Dioxins and Related Compounds , Tucker, R. E. , Young, A. L.

and A. P. Gray, Eds., Plenum Publishing Corporation, N.Y., 1983) pp.

731-748.

38
APPENDIX A

PHYSICAL AND CHEMICAL PROPERTIES OF DIOXINS

IN RELATION TO THEIR DISPOSAL

The physical and chemical properties of polychlorinated dibenzo-pdioxins have been considered in relation to prospects for their formation

and destruction in incinerator environments. Detailed equilibrium and

chemical kinetic considerations have been used in performing qualitative

assessments. It is concluded that there are no apparent thermodynamic

barriers to their destruction and that kinetic control is a dominating

factor in practical incinerator environments. This analysis as well as

a consideration of some existing experimental data are used to suggest

some useful guidelines and to indicate research which should be carried

out in the future regarding dioxin disposal.

A 1
INTRODUCTION

Although there is inadequate data to explicitly describe the complete

destruction or formation mechanisms of polychlorinated dibenzo-p-dioxins

(PCDDs) under thermal stress, enough information is known or can be

inferred such that important characteristic features of these processes

can be elucidated. We focus attention upon this class of chenrcals in

view of the fact that there have recently been reported several instances

in which dioxins have been associated with gaseous and particulate

emissions of incinerators. Two extensive reviews are available (Harris

et al . 1981, Esposito et al . 1980). There has been a recent suggestion

by some scientists (Bumb et al. 1980) that PCDDs may be ubiquitous as a

consequence of trace chemical processes that occur during normal

combustion. This hypothesis has been questioned (Kriebel 1981). It

appears at the present time that this issue has not been resolved.

What is known is that several dioxins, particularly 2,3,7,8-tetrachlorodibenzo-p-dioxin, are extremely toxic substances (Poland and Kende

1976). It is therefore desirable to determine what if any conditions

for incineration are likely to minimize formation and maximize destruction

of these species.

This paper, will begin with a brief review of the molecular processes

which are responsible for destruction of a large polyatomic molecule.

We next discuss the limitations imposed by equilibrium thermodynamics.

This will be followed by an assessment of kinetic control factors. Some

considerations regarding the role of non-gas phase and/or heterogeneous

A 2
processes will be presented. These factors will be utilized to interpret

some of the experimental research that has recently been reported in the

literature. Recommendations shall be presented regarding future research

as well as practical guidelines pertinent to incinerator operation and

possibly risk assessment.

The work presented here is in response to a congressional mandate,

and as directed under Section 5002 of the Resource Conservation

Recovery Act of 1976 [RCRA], and through support of the NBS Office

of Recycled Materials, the DOE Office of Energy for Municipal

Waste, and the U. S. Air Force Environics Division, Tyndall AFB.

REACTION MECHANISMS

The kinetic processes which control the decomposition of a complex

organic molecule such as a PCDD in a practical incinerator environment

may be described by the specification of reaction mechanisms. These

reaction mechanisms in turn are composed a series of elementary processes.

The rates of these elementary steps in combination with appropriate

computer programs can describe the temporal evolution of the chemical

specie during the combustion process in an incinerator. Regardless

of the detailed mechanism, the fate of the molecule is either a stepwise

breakdown to smaller, often more oxygenated species or alternatively a

transformation in concert with other species to a higher molecular

weight material (eg. soot formation)

.

A - 3
It is appropriate to first consider those kinetic processes which are

operative in the gas phase. Historically, it is these processes which

have been studied most and are best understood. Furthermore, we

shall show that an understanding of the general features which

characterize gas phase decomposition processes provides in turn,

insight into the nature of reactions operative in other phases.

As we have discussed previously (Tsang and Shaub 1981), the

important elementary processes operative in the gas phase in an

incinerator environment may be categorized as either unimolecular

or bimolecular reaction steps. The former are represented by

reactions of the type

where A represents a complex organic molecule and P denotes a

product (or products) which may not necessarily be a stable specie

(or species). The rate of destruction of A by this unimolecular

reaction step is dA/dt = -k

uA. These reactions for the present

purposes can be classified as those involving either bond rupture

or complex fragmentation. Bimolecular reactions may either involve

addition or metathesis.

A + R P + (C)

where A and P are as previously defined before, R denotes a specie

capable of undergoing reaction with A and C is the other product in

metathesis reactions. The rate of destruction of A by bimolecular

steps is

A - h
dA/dt = - (Skbi R

i )A

i

where the summation over the subscript, i, denotes the fact that

in a practical incinerator environment, there may be many reactive

species capable of undergoing chemical reaction with A.

THERMODYNAMICS CONSTRAINTS

In order to completely describe the gas phase decomposition of a

large organic molecule, it would be necessary to specify exactly all

of the reaction steps in a mechanism. In practice, this has not

been done to date even for so simple a molecule as methane! Approximations are necessary. In particular, thermodynamics imposes important

constraints on the system and in view of the lack of knowledge of

detailed mechanism, the first step must be an investigation of the

consequences if equilibrium is attained.

A complete thermodynamic characterization of the equilibrium distribution of species present in a gas phase mixture is possible

providing appropriate hydrodynamic variables, eg., temperature and

pressure, thermodynamic data for all pertinent species, and initial

fuel/oxidizer equivalence ratios (a measure of the amount of fuel and

oxidizer available) are specified. Up to the present time, the fundamental

limitation to complete equilibrium characterization of many systems is

due to the lack of thermodynamic data for numerous large organic molecules.

This has been particularly true in the case of chlorinated hazardous

waste chemicals such as benzenes, phenols, biphenyls, biphenyl ethers.

A - 5
dibenzofurans and dibenzo-p-dioxins. Recently we have utilized estimation

techniques (Shaub 1981a, 1981b, Tsang and Shaub 1981) to construct a

thermodynamic data base for these molecules. Table I for example,

summarizes ranges of gas phase heats of formation estimated for polychlorinated biphenyls (PCBs), polychlorinated biphenyl ethers (PCBEs), polychlorinated dibenzofurans (PCDFs), and polychlorinated dibenzo-p-dioxins. In

general, heats of formation decrease with increased degree of chlorination

and number of oxygen atoms present in the molecule. Variations

within each class are attributable to nearest neighbor interactions

when utilizing our estimation techniques. The extent to which

these estimates are reliable has been discussed previously

(Shaub 1981a). These estimated heats of formation as well as

estimates of heat capacities and entropies (Shaub 1981b) can be

utilized to construct the necessary thermodynamic data input for

equilibrium computations. A typical data sheet for a chlorinated

dioxin is presented in Table II. We have used this information

together with pre-existing thermodynamic data for smaller polyatomic

as well as atomic and diatomic species (Stull and Prophet 1971,

Cox and Pilcher 1970, Stull and Westrum 1969) to simulate equilibrium

chemical thermodynamic distributions of polychlorinated aromatic

species for a range of specifications of initial fuel/oxidizer

equivalence ratios, temperatures and pressures characteristic of a

practical incinerator environment. We are able to conclude that

in terms of equilibrium thermodynamics, at temperatures in excess

of 500°C, destruction of polychlorinated aromatics is essentially

complete. That is, there is no single compound with any special

thermodynamic stability characteristics. Within any particular

A - 6
class of chlorinated compounds, eg. dioxins, there is a trend

toward greater stability with increased chlorination if there are

no other sources of hydrogen available in the waste feedstock.

It is often found in practice that temperatures much in excess of

500 C are required to effect complete destruction. For example Duvall

and Rubey (1977) report that commercial PCBs and some dioxins undergo

decomposition in air between 640-740 C at a residence time of one

second. A 99.995% destruction efficiency is reported for some

PCBs at a temperature of 1000 C and the same residence time.

This is an indication that at least at the lower temperatures,

(T < 900 C) reactions may be kinetically controlled. It is important

to recognize, however, that while complete gas phase thermodynamic

equilibrium may not obtain in an incinerator environment, a bimolecularly controlled local (psuedo) thermodynamic equilibrium may be

expected to develop in practical incinerator environments for small

inorganic radicals, particularly those associated with hydrogen/oxygen

reactions, (e.g. H, 0, OH, H£0) by virtue of the fact that they

undergo fast bimolecular reactions with each other. This means

that to a good approximation, equilibrium thermodynamic calculations

should serve adequately to "track" concentration of these small

inorganic species with variations of temperature and pressure

over time scales that are characteristic of an incineration process.

Gas Phase Kinetic Control Factors

It is evident that gas phase processes for destruction of chlorinated

dioxins in practical incinerators are kinetically controlled. The

A - T
destruction of chlorinated dioxins may then be described by the

relation,

-dA/dt = k yA + (Zk

bi

R.)A

It is instructive to consider first the situation in which destruction is governed predominantly by a unimolecular mode of decomposition. It can be shown that under this condition, and subject to

a requirement of 99.99% destruction efficiency, the minimum transit

time required for achievement of this extent of decomposition is

related to the unimolecular rate constant by the expression

t(99. 99%) = 9.212 k"

1

Similarly, for 99.9999% destruction efficiency,

t ( 99 . 9999%) £

1 . 5t(99. 99%.

)

The unimolecular rate constant, k y

, may be written as (pressure

dependence is assumed negligible under conditions of incineration)

k

u * A

=o expt-E^RT).

Above, R has the value 1.987 cal/(mol*deg K). Values for the

pre-exponential factor (units of sec’^ ) and the activation energy

(units of cal/mol) may be conveniently estimated for a dioxin

molecule undergoing decomposition via simple bond rupture - a

process which sets an upper limit range upon the decomposition

time. A reasonable range for the value of the pre-exponential

factor is ^

10^ 5 '^ - sec"^ (Tsang and Shaub 1981). Substituent

chlorine atom effects (increasing degree of chlorination) should

A - 8
not be expected to result in values outside of this spectrum

of values. The activation energy for unimolecular decomposition of

a dioxin molecule via simple bond rupture should have a value

approximately equal to the bond dissociation energy of the weakest

bond. Carbon-carbon bond rupture in the aromatic rings is ruled

out as being prohibitive due to resonance stabilization in the

aromatic rings. The C-H bonds and the C-Cl bonds in benzene and

benzyl chloride have dissociation energies of 111 kcal*mol"^ and 95

kcal*mol~^ respectively. An estimate of the bond dissociation energy of

the CO bond in dioxins is obtained as follows. The heat of formation of

CgH^O* (phenoxy radical) is AH°(g,298) = 11 . 9 kcal 'mol as determined

from the reported heat of formation of phenol, AH°(g,298) =

-23.0 kcal*mol (Cox and Pilcher 1970), the bond dissociation energy

of the C^H^O-H bond, 87 kcal«mol”^ (Colussi et al . 1977), and the

heat of formation of atomic hydrogen, AH°(g,298) = 52.1 kcal*mol”^

(Stull and Prophet 1971). An estimate of the C-0 bond dissociation

energy in dioxins is that it should be comparable to the C-0 bond

dissociation energy in diphenyl ether. Since the heat of formation of

the phenyl radical is AH°(g,298) = 78.5 kcal-mol’ 1 (Benson 1976), and

the heat of formation of diphenyl ether is AH°(g, 298) = 11.94

kcal*mol~^ (Cox and Pilcher 1970), it follows that the bond dissociation

energy of the C-0 bond in diphenyl ether is

BDE % AH

°f (9 ’ 298)C

6

H

5

. + 298

>C

6

H

5

0- - W

°f<3’

298

>C

12 H 100

y 78.5 kcal«mol“^

It is unlikely that there is any significant resonance energy

associated with the central ring in dioxin. In fact, an examinaA - 9
tion of the central rings in dibenzopyran and 9,10-dihydroanthracene

utilizing thermochemical estimates (Benson 1976) and reported

values for the heats of formation of these compounds (Cox and

Pilcher 1970), leads to the conclusions that there is no

significant resonance strengthening of the bonds to the central

ring bridge atoms in these compounds and that further, there is a

slight central ring destabilization of about 1-4 kcal mol due to

ring strain. Thus by analogy, it is reasonable to assume that it

is unlikely that the bond dissociation energy of the C-0 bond

in dioxin exceeds 80 kcal mol"\ In fact, it may be lower if

partial resonance stabilization of the resulting phenoxy-like

radical structure occurs during the bond breaking process.

It follows from the above discussion that in the case of unimolecular dissociation of dioxins, a reasonable estimate for the

unimolecular rate constant is

k u £

lO

15 ’ 5 - °* 5 exp(-80000/RT)sec

_1

This value for k

y can be utilized in the expression for t ( 99 . 99%

)

to determine the time required for efficient destruction at various

temperatures. In Table III we present a summary of these times

for A = 10

15

and also A = 10^ to indicate the effect of the OO CO variation of this parameter. Clearly, effective removal of dioxins

over short periods of time can only occur at elevated temperatures,

under conditions of unimolecular kinetic control.

The contribution of bimolecular reactions to the destruction of

dioxins may be considered as follows. As mentioned previously, the

A 10
significance of the existence of local thermodynamic equilibrium

in an incinerator evironment is that we can, via computer simulation,

determine the concentrations of small inorganic radicals. We have

concluded from our equilibrium thermodynamics analysis (Tsang and Shaub

1981) that with respect to bimolecular removal, the major reactive

species in the case of fuel rich combustion are hydrogen atoms, and in

the case of stoichiometric combustion or excess air combustion, hydroxyl

radicals (OH) and to a lesser extent, oxygen atoms. Larger radicals, at

elevated temperatures characteristic of an incinerator combustion

environment are generally present at lower concentrations due to their

instability. In most cases they are also less reactive. Consider

stoichiometric or excess air combustion of dioxins. We have determined

from our previous analysis of kinetic rate constant data for the rates

of attack of OH upon organic substrates that at incinerator conditions

9 + 1 bimolecular rate constants will be of the order of k^ ^ 10 -

L«mol”^ ‘Sec

-

^ (Tsang and Shaub 1981). We have estimated the

concentration of OH radicals as a function of temperature under

the assumption of local thermodynamic equilibrium of small inorganic

radicals. The validity of this assumption has been demonstrated

(Ernst et al . 1978, Muller et al. 1980, Creighton 1980). It is

therefore possible to infer that a residence time of about 1

second and a temperature of the order of 1230 K(957 C) would be

required to insure 99.99% destruction efficiency of dioxins via

attack of OH. Thus, it can be seen by comparison with Table III,

both unimolecular and bimolecular reactions may contribute

significantly to dioxin removal at the temperatures where 99.99'

removal occurs over time scales typical of incinerator operation.

A 11
As we have mentioned previously, OH attack upon unsaturates tends

to be electrophilic in nature, leading us to expect a lowering of

the rates of bimolecular attack upon chlorinated dioxins and other

such as chlorinated biphenyls with increasing degrees of chlorination.

Thus, effluent emissions from incinerators should tend to contain

predominantly more highly chlorinated dioxins if in fact they do

contain dioxins at all. In the case of PCBs, this effect should

be more marked as C-Cl bonds are the weakest bonds in these molecules

and therefore effects associated with bimolecular attack should be

significantly enhanced as under these circumstances (ie high bond

energies) unimolecular reactions would be less predominant. These

predictions are supported by experimental results reported in recent

literature (Harrison et al. 1980, Duvall and Rubey 1977). In the case

of fuel rich systems, H atom concentrations tend to be lower than the

corresponding OH concentrations in stoichiometric or excess air combustion.

This means that since rate constants for H atom attack at incineration

temperatures are comparable to k

b

for OH attack, unimolecular decomposition

of dioxins may tend to be more important in fuel rich combustion.

It is useful to make another observation regarding the incineration

of hazardous materials such as dioxins. A distinction should be

made between mean furnace operating temperatures which may tend to

be low relative to destruction of dioxins (750 C is not atypical

in municipal incinerators), and the much higher temperatures which

are obtained in the flame regions of the furnace. Thermal profiles

in the region of flames are commonly in the range 1250 C to 1750 C over

A 12
a distance in excess of 15-20 cm with peak temperatures (1750 C and

higher) extending oyer distances of 5-10 millimeters or more (McDonald

1980, Bechtel 1980, Switzer et al . 1980, Oran et al 1980, Dibble et al

.

1980). In a practical incinerator environment, these distances may be

greatly extended. As is evident from Table III, it is these high

temperature regions which are primarily responsible for destruction of

dioxins and other similar hazardous waste stream materials. At flow

velocities of 10-50 ft*s~\ 99.99% (or even 99.9999%) gas phase destruction is to be expected if all the dioxin material is successfully heated

to these temperatures. Thus while average furnace temperatures may

be lower than what is required for dioxin disposal, local flame

temperatures if felt by all molecules in the system can provide the

impetus for successful decomposition.

With regards to the formation of chlorinated dioxins in the gas phase

once temperatures for destruction of chlorinated dioxins are achieved,

the same driving forces which are available to destroy dioxins will also

lead to decomposition of dioxin precursors such as chlorinated phenols.

Additionally, in a municipal incinerator, concentrations of hazardous

wastes tend to be low relative to the bulk of the waste stream. Formation of dioxins in the gas phase would require a minimum occurence

of a bimolecular step, characterized by a very slow rate relative

to the overall process of decomposition, due to low values associated

with bimolecular rate constants for highly sterically hindered

reactions (Gardiner 1972). There are reports of a few dioxin precursors

(predioxins - eg. polychlorinated 2-phenoxyphenol s) apparently

directly producing high yields of chlorinated dioxins under mild

A 13
(ca. 275 C) thermal stress (Esposito et al . 1980, Nilsson et al

.

1974). It should be noted that there is no indication that these

materials are especially prevalent in municipal waste streams. In

addition, there has been no demonstration that conversion of these

predioxins to dioxins occurs in the gas phase. Such a reaction would be

expected to be highly sterically hindered and possibly retarded by

resonance stabilization of an intermediate phenoxy radical structure.

The previous discussion has assumed an model in which all hazardous

waste materials such as chlorinated dioxins are in a homogenous gas

phase mixture, ie. pre-mixed burning, and that further, all of the gas

is successfully elevated to the high flame temperatures required for

successful decomposition. In practical incineration this is not the

case. Fuel and air are not pre-mixed, although a highly efficient spray

injection liquid hazardous waste incinerator may approach this ideal.

When improper mixing occurs, extremely fuel rich or fuel lean pockets of

gas may pass through some parts of the incinerator and may possibly

escape from the furnace area particularly if loading is excessive. If

the mixture is exceedingly fuel rich or fuel lean the gas pockets may

not support ignition, and consequently material destruction may not

occur due to formation of local cold spots or lack of reactive species.

Additionally, extremely fuel rich combustion may promote undesirable

condensation reactions and lead to soot formation which can perturb

energy transport to an from regions in the incinerator in addition to

other undesirable effects. Recently, liquid injection incinerators were

utilized onboard the M/T VULCANUS incinerator ship to destroy organochlorine

A - ih
wastes containing 2,3,7,8-tetrachlorodibenzo-p-dioxin (TC.DP). as an

impurity to the extent of an average 2 ppm (Ackerman et al . 1978). The

flame temperature in the liquid injection incinerators was maintained at

all times above 1250 C during the incineration of organochlorine wastes.

Residence times normally ranged from 0.5 to 2 seconds. Combined TCDD

destruction efficiencies of > 99.93% were reported for three burns. An

efficiency > 99.99% was reported for one burn. It is clear from these

results that the predicted efficient destruction of dioxins in the gas

phase is realizable in practical excess air incineration. Slight

departures from expected theoretical efficiencies may have been due to

some of the problems discussed previously.

Non Gas Phase Effects

In the last few years, there have been several reports of dioxin

emissions from full-scale combustion facilities (01 i e et al

.

1977, Buser and Bosshardt 1978, Buser et al. 1978, Eiceman et al

.

1979, 1980). In general emissions from municipal waste incinerators

have been consistently lower than those reported for industrial waste

incinerator facilities (Harris et al . 1980). The majority of

reports of dioxin emissions have been concerned with analysis of fly

ash samples, particularly those collected from electrostatic

precipitators. Although there are problems with the analytical methodology

these observations make necessary the consideration of non gas phase

effects.

A 15
In a typical incinerator equipped with an electrostatic precipitator

the principal areas for loss of solid which are of concern are the

following:

a) Fly ash and other particulates < 10 vim in diameter may pass

through the settling chambers and centrifugal separators into

the electrostatic precipitator. Here, assuming efficient operation,

particles as small as 0.1 pm to 0.01 ym will be collected. This

material is highly oxidized, with carbon or soot contents (organic

fraction) commonly as low as 5% by weight and ranging up to 40%

by weight (Domino 1979) in poorly designed or inefficiently

operated incinerators. (It is possible that real time optical

probing for soot deposited on fly ash may prove to be a useful indicator of efficient incinerator operation). Glassy particulates may

be harder to trap due to the difficulty associated with ionization

of this material. Fly ash in hoppers from electrostatic precipitators is subject to management by containment. However, due to the size

of this particulate matter great care in handling may be essential,

particularly with dry fly ash, to avoid raising a cloud of this

material

.

b) Particles smaller in size than 0.01 ym will readily escape

through an electrostatic precipitator and pass out the stacks.

This fly ash is highly oxidized and has been thoroughly thermally

stressed. This material is not subject to management by containment

and represents a direct emission into the environment. Dioxin

transport, if it occurs to an appreciable extent on this material

A 16
could be particularly serious as deposition efficiencies for

inhaled particles of this size range strongly favor pulmonary and

tracheobronchial regions as opposed to nasopharyngeal regions of

the respiratory tract (Natusch and Wallace 1974). In this respect,

it is also particularly important to control soot emissions

(organics) as for example, benzo [a] pyrene, a common soot

constituent, has been found to predominate in small pulmonary

depositing particles (Natusch and Wallace 1974).

The inorganic content of fly ash is present as metal oxides. The

relative amounts of the metal oxides present in fly ash vary widely from

one sample to another in different incinerators. On the average Si^ is

the principle inorganic oxide (^ 55%). Fly ash from municipal incinerators

tends to also have significantly higher ratios (Domino 1979, Cheremisinoff

and Morresi 1976, Hardesty and Pahl 1979) of Si 02/AT 2^3 anc* CaO/FegOs

than fly ash from lignite coal. The Al^O^ fraction in municipal

incinerator fly ash is often low due to aluminum can recycling.

Generally, fly ash will be basic when there are high amounts of

calcium and/or magnesium and low amounts of irons and/or aluminum.

Quartz (SiOg) and many types of glass also tend to be basic. That

is, there may be many hydroxyl (OH) groups on the surface of these

materials. The ash fusion temperatures (Domino 1979) of most

municipal incinerator fly ash are comparable to those reported for

fly ash from various coals (Hardesty and Pohl 1979), and typically

range from about 2000 F (1093 C, 1366 K) to about 2500 F (1371 C,

1644 K). Thermal conductivities of fly ash tend to be somewhat larger

than those for most coals.

A IT
The above information about fly ash is helpful towards development

of an understanding of the conditions under which one might

expect dioxins would be present in this material. That this is

so, may be seen from the discussion presented below. At one atmosphere

pressure and a temperature of 600 C, energy transport from the surface

of a 1000 ym diameter particle of coal to its center takes place during

a time scale of the order of one second. Under similar conditions, a

100 ym coal particle will be heated during a time scale of ^ 10 ms

(Russel et al . 1979). These figures are approximate time scales, but

nonetheless they serve to bring out the point that sub-micron size coal

particles will heat to the core in times much shorter than a millisecond.

Since fly ash particles have similar thermal conductivities, it is

reasonable to expect that during incineration submicron size particles

should be uniformly heated to high temperatures on time scales that are

short relative to transit times through high temperature zones. Fusion,

which is known to commonly occur, may further promote heating of

particle cores if the particles are transformed to a liquid state due

to convective transport of heat and oxidants. Under these conditions,

dioxins present inside sub-micron size fly ash particles may be subject

to the same unimolecular processes as in the gas phase. Additionally,

oxidative attack of hydroxyl or other small radicals on particles may

also contribute towards decomposition. For example it has recently been

demonstrated that OH radicals rapidly attack the surface of carbonaceous

materials even at room temperatures to produce approximately equal

amounts of CO and CO^ (Mulcahy and Young 1975). Since the surface of

fly ash in incinerators may be basic, surface bound hydroxyl radicals

may also play a role. Other surface effects may also be operative. The

A 18
implication of this discussion is that proyiding these particles are

subjected to the same high temperature oxidatiye conditions as is the

gas in an incinerator, high destruction efficiencies are to be expected.

In fact, the completeness of oxidation of inorganic material in submicron size fly ash may imply that dioxin and other organic component

levels in these particles should be lower than in the gas phase under

conditions of fuel lean, high temperature incineration.

Ash which collects in hoppers in the region of the electrostatic precipitator may commonly range in size up to about 10 ym. The uncertainties

regarding time scales for heating could be as large as an order of

magnitude. This does not affect the nature of our analysis regarding

submicron sized particles. However, as regards particles with diameters

on the order of 10 my, if particle heating time scales due to uncertainties

were actually higher by an order of magnitude, the time to heat the core

of these particles could be a significant fraction of the particle

transit times through the highest temperature zones. In this respect

more experimental work in determining actual particle heating times

would be very helpful. As can be determined from reports of fly ash

collected from some electrostatic precipitators, levels of dioxins have

been found to be in the ppb range. Note that if the input waste stream

contained ppm levels of dioxins, or of material that may be converted to

dioxins (to the extent of ppm dioxin levels during time scales similar

to those in incinerators), a 99.99% combustion destruction efficiency

due to rapid particle heat up relative to transit times would imply ppt

to ppb levels of dioxins will be present in electrostatic precipitator

fly ash consistent with what has been reported in some instances.

A 19
This would imply electrostatic precipitator fly ash has probably

been uniformly heated. Whether or not this analysis has validity in

an actual incinerator requires statistically representative information

about input feed streams that has not presently been reported. This

information would also provide in part, a useful guide in determining

the effectiveness of varying furnace flame temperatures and residence

times.

Recently, formation of polychlorinated dibenzofurans (PCDFs) at

levels approaching 1% has been reported from the pyrolysis of

polychlorinated biphenyls (PCBs) in sealed quartz mini-ampoules

at temperatures of 550-850 C (Buser et al . 1978b). Molar ratios

of PCB to oxygen in an ampoule were varied from 1:7.5 to about

1:7.5. The maximum amounts of PCDFs reported were found at 550 C.

At 850 C levels found are reported as < 0.01%. The important point

associated with this work, although not brought out by these

authors, is the fact that at the lower temperatures (550 C) where

highest formations occurred, their results clearly show the reaction

to be self inhibiting. That is, decreasing the amount of PCBs in

a quartz ampoule by a factor of ten actually results in more than

a six-fold increase in decomposition. Since gas phase reactions

involving oxygen are not important at these temperatures, the self

inhibiting nature of this reaction clearly rules out any chain branching

mechanism and suggests that the reactions are taking place on the surface

of the quartz ampoules.

Synthesis of chlorinated dioxins and PCDFs have also been reported

under similar conditions in quartz ampoules from chlorinated benzenes

A 20
(Buser 1979) and chlorophenates, (Rappe and Marklund 1978), It is

highly likely that again, surface effects are also operatiye here

as no kinetically plausible gas phase processes are conceivable

at these temperatures for formation of these compounds. Additionally,

it should be noted that there is a long history of problems due

to surface effects in quartz and glass vessels that have been

encountered in the study of gas phase reactions of halogen containing organic compounds. As quartz tends to be basic, it is possible

that hydroxyl groups on the surface of the quartz ampoules may be

a reactant source for promoting formation of dioxins from PCBs

and other precursors. As mentioned previously, fly ash may also

tend to be basic and usually has a high SiC>2 content. Thus, in a

manner similar to quartz, fly ash may possibly act as a promoter for

surface reactions leading to formation of dioxins and similar

materials in an incinerator environment. The relative surface of glass

ampoules compared to that of fly ash in an incinerator has not been

established. Thus the validity of extrpolating the latter results

is uncertain. However, the observation of similar distribution patterns

in chromatograms of dioxins obtained from municipal incinerator, and

controlled experiments involving chlorophenates in quartz ampoules

(Buser et al . 1978) and on leaves or wood (Rappe et al .

)

supports this

suggestion. It is important to note that in a study of production of

PCDFs from specific PCBs that was carried out in quartz ampoules (Buser

et al. 1978b), the PCBs were destroyed with an efficiency > 99.99; at a

temperature of 850 C. Since PCBs are kinetically more stable than

dioxins, comparable destruction efficiencies are to be expected for the

latter. Thus, while fly ash may possibly promote dioxins formation at

A 21
lower temperatures around 550. C, high destruction efficiencies svtill

appear to be possible if the fly ash is subsequently subjected to

elevated temperatures for times comparable to those required for gas

phase destruction at elevated temperatures, providing the ash is

uniformly stressed. This latter point is important, since if dioxins or

precursors are trapped inside particles and the cores of particles are

not thermally stressed, destruction efficiencies may be lower than

expected, due to a thermal insulating effect.

CONCLUSIONS AND SUGGESTION FOR FUTURE RESEARCH

Below, we enumerate a number of tentative conclusions which may

be drawn from the above discussions, in the hope that they will

provide some useful guidance to enable persons concerned with the

disposal of dioxins and similar materials to understand to some extent

what is possible via incineration. It seems reasonable to us to

expect that 99.99% or higher destruction efficiencies are well

within the capabilities of modern municipal waste and hazardous

waste incinerators. Our basic conclusion is that destruction

efficiencies are not likely to be limited by thermodynamic or

kinetic control factors at high enough temperatures and sufficient

residence times. However, in light of our discussions above,

the following points should be considered:

a) Mixing is very important and should be monitored closely,

especially to avoid cold spots which may result in insufficient

thermal stressing.

A 22
b) As particle size increases., core heating time scales may be

lengthened. This must be considered an important factor in terms

of particle transit times through the furnace and in fly ash disposal.

c) A careful statistically representative characterization of the

input waste feed stream is necessary to properly further assess

actual destruction efficiencies.

d) Providing stack (uncontained) emissions of dioxins and related

materials are reduced to acceptable levels, incineration of dioxins

should in principle be a manageable problem since ash in hoppers

is contained ash and therefore subject to further treatment if

needed.

As regards future research, it would be most helpful if physical

properties of various fly ash, eg. thermal conductivities, fusion

temperature, alkalinity, surface reactivity, porosity, etc. were

more thoroughly determined. Rate processes of chemical reactions

are readily subject to scientific scrutiny, and more carefully

designed experiments are needed in the future, particularly with

regards to kinetics of reactions involving materials such as dioxins

and precursors on various surfaces. It should also be noted that

since dioxins and similar chemical substances may be found in some

fly ash, the prior history of disposal practices of fly ash would

dictate that there may exist a need to investigate the chemical,

photochemical and biological fate of these substances as they may

exist in some ash dump sites. Certainly, some monitoring of historical

sites would be instructive.

A 23
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A - 2h
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Esposito, M. P. Tiernan, T. 0. and Dryden, F. E. Dioxins. Washington, D.C.:

EPA-600/2-80-1 97 , 1980.

Gardiner, W. C. Jr., Rates and Mechanisms of Chemical Reactions . Menlo Park,

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A 25
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A 26
Table I

Gas Phase Heat of Formation for Chlorinated Hazardous Waste Compounds

Class AH°(g, 298 K) kcal*mole

Polychlorinated Biphenyls + 35. 93 + 0.07

Polychlorinated Biphenyl Ethers + 10.3 + - 54.25

Polychlorinated Di benzofurans + 18.26 + - 48.12

Polychlorinated Di benzo-p-dioxins - 22.7 -> -100.2

A 27
Table II

octachl orodi benzo--p-dioxin C 12°2 C1

8

( Ideal Gas State) Mol. Wt. 459.,744

T°K

cal mol”^

S°

i

1C

1

- ( Gj-Hq ) /T

)

Hr

H

o

kcal mol”

AH°

1

AG° Log K

P

200 54.54 111.60 81.45 6.03 -100.72 -71.94 78.602

298 70.86 136.58 95.58 12.23 -100.21 -57.93 42.454

300 71.12 137.02 95.84 12.36 -100.20 -57.65 41.989

400 83.45 159.25 108.97 20.11 -99.44 -43.57 23.799

500 92.81 178.93 121.03 28.95 -98.57 -29.70 12.979

600 99.88 196.50 132.17 38.60 -97.68 -16.01 5.829

700 105.20 212.32 142.51 48.86 -96.79 -2.46 0.768

800 109.23 226.64 152.15 59.60 -95.87 10.95 -2.992

900 112.32 239.69 161.16 70.68 -95.06 24.26 -5.890

1000 114.72 251 .66 169.62 82.04 -94.17 37.46 8.186

1100 116.61 262.68 177.59 93.61 -93.78 50.58 -10.047

1200 1 18.11 272.90 185.11 105.35 -92.39 63.62 -11.584

1300 119.33 282.40 192.23 117.22 -91.59 76.59 -12.874

1400 120.32 291.28 198.99 129.20 -90.83 89.50 -13.969

1500 121 .14 299.61 205.43 141.28 -90.09 102.34 -14.908

A - 28
Table III 99.99%

Dioxin Decomposition Times, Gas Phase

k u

. 10

+15

e -80000/RT

k

u

= 10

+16

e

- 8000°/ RT

T = °C °K T

=

2 billion years 227 500 200 mi 11 ion years

6 years 477 750 0.6 years

46.5 minutes 727 1000 4.65 minutes

1 second 977 1250 1/10 second

1/2 second 1000 1273 5/100 second

4 milliseconds 1227 1500 400 microseconds

5 microseconds 1727 2000 1/2 microsecond

A. 29

APPENDIX B

DIOXIN FORMATION IN INCINERATORS

Abstract

Processes which may contribute to the formation of polychlorinated

dibenzo-p-dioxins (PCDDs) in incinerators are examined. A model mechanism

has been constructed to investigate the possibility of homogeneous gas

phase formation of PCDDs from chlorophenol s in an incinerator environment.

Numerical calculations have been made. The results lead to the conclusion

that the probability of gas phase formation of PCDDs is likely to be

very low at high temperatures, £

1200 K, if mixing between fuel and air

is efficient. In addition, production of PCDDs from chlorophenols is

found to depend upon the square of the chlorophenol concentration. Probable

sources of non-idealities in practical incinerators are examined.

Effects of use of auxiliary hydrocarbon fuel and excess air are examined.

The potential role of non-gas phase effects is considered. A discussion

of some of the significant problems which complicate a further understanding

of PCDD formation processes in incinerators has been presented in a

manner that highlights future research needs.

B 1
Introduction

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated

dibenzofurans (PCDFs) have been reported to have been found in incinerator

emissions and other combustion sources [1, 2]. It has been suggested

that these chemicals may be ubiquitous as a consequence of trace chemical

processes that occur during combustion [3]. This is a controversial

question (4a, b). With respect to their formation in municipal incinerators,

it has been suggested that likely major routes of formation are from

chemically related compounds such as chlorobenzenes, chlorophenol s and

PCBs [5]. There appears to be uncertainty as to the relative importance

of formation from non-chlorinated precursors and inorganic forms of

chlorine or from chemically unrelated chloro-organic compounds such as

polyvinyl chlorides [5]. Laboratory experiments which deal with the

formation of PCDDs and PCDFs from chlorophenols, chlorophenates , chlorobenzenes and PCBs have been reported [1, 2, 4b, 6, 7]. Most observations

concerning the formation of hazardous waste compounds such as PCDDs

during thermal incineration are fundamentally qualitative in nature in

the sense that they do not provide detailed answers which can lead to

process modifications that may minimize the formation of PCDDs. None of

the experiments [1,2, 6, 7] which have been reported to date can be

considered to be adequate to demonstrate that the compounds (PCDDs,

PCDFs, etc.) which have been observed to form are attributable to gas

phase processes [8,9]. In addition, it has previously been suggested

that in some instances there seems to be evidence that formations

reported in some laboratory experiments may in fact be due at least in

part to surface reactions [10,11]. There is a substantial body of

literature in which it has been explicitly demonstrated that reactions

B 2
involving chlorinated organic species are frequently affected or controlled

in whole or in part by the presence of reactive surfaces [8, 12-27].

In the following discussion some aspects of these problems are examined

by first considering the potential for gas phase formation of PCDDs under

strictly homogeneous conditions in order to establish a frame of reference.

Existing information about gas phase reaction processes is drawn upon to

develop a reasonable mechanism for gas phase formation of PCDDs from a

suggested precursor, chlorophenols [2]. Next, some possible gas phase

non- idealities which may develop during thermal incineration are examined.

This will be followed by a consideration of potential non-gas phase

contributions. Some general conclusions will be drawn regarding the

problem of understanding PCDD formation in incinerators.

Mechanism Construction

The reaction steps which make up the proposed gas phase mechanism,

discussed below, are shown in Table I. The species denoted by P, P*,

PD and D respectively represent polychlorinated phenols, polychlorinated

phenoxy radicals, polychlorinated 2-phenoxyphenols (dioxin precursor

[2]) and PCDDs. These compounds are shown in Figure 1, where Cl and

A

Cly denote the varying extent of chlorination of the compounds. In

Table I, the species denoted by the symbol, R, represent any other

organic fuel compounds in the gas phase mixture. R* and Pr denote a

fuel molecule from which a hydrogen atom has been abstracted and unspecified

products respectively. In Figure 2, the pathways of the proposed mechanism

are shown schematically.

The rate constants assigned to the steps in the mechanism have been

given values similar to rate constants which have been reported in the

literature for analogous reactions. However, where there are uncertainties.

B 3
the numerical values have been adjusted somewhat to bias the mechanism

in favor of PCDD formation i.e., worse case modeling .

As constructed below, the mechanism is of a general nature. That is,

isomer specific kinetic rate constants are not estimated. In this

regard note the following points. Unimolecular rate constants, except

for resonance stabilization effects, are not sensitive to substitutions

to the bond being broken at positions greater than those corresponding

to a (adjacent) substition (30). In addition it is expected that an

increasing degree of chlorine substitution of the precursor chlorophenol

s

will result in markedly slower rates of hydroxyl radical attack by

addition reactions (30). Worse case modeling implicitly accounts for

these effects in determining the overall (worse case) upper limit probability

for PCDD formation as a class of molecules. The latter observation with

respect to hydroxyl radical attack implies qualitatively that for gas

phase processes, increasingly chlorinated PCDDs should be increasingly

recalcitrant to bimolecular decomposition. That is, isomer distributions

should be skewed toward higher chlorinated species. More quantitative

statements regarding isomer specific reactions requires further experiments

and more refined theoretical calculations. As the intent of this report

is to examine the overall importance of gas phase mechanisms for PCDD formation

rather than isomer specific differences, further refinements are not necessary

here.

Each of the reaction steps as given in Table I is discussed below.

The assigned rate constants have been biased to favor PCDD formation.

Therefore the reader is strongly cautioned not to use these estimates

out of the context of this paper, as actual values for these rates will

be quite different in some cases.

The reaction,

B U
P - P- + H (Rl)

represents the simple unimolecular bond rupture of the weakest bond in

a polychlorinated phenol. The activation energy for this step is taken

as the 0-H bond energy in phenol, which has been determined experimentally

[28]. Chlorine substitution is not expected to affect substantially

this value [10]. The pre-exponential factor is assigned a value comparable

to that associated with the formation of benzyl radical from dissociation

of toluene [29] as a reasonable estimate. Thus, k(Rl) ^ 10^'^ exp(-86000/

RT) s"

1 is assigned.

The reaction,

OH + P - P- + H

2

0 (R2)

represents the process of hydrogen atom abstraction by hydroxyl radicals.

In high temperature combustion such as incineration, the hydroxyl radical

is the most important small inorganic radical present under typical

incinerator operating conditions [10, 11, 30-34]. Furthermore, there is

evidence that a local thermodynamic equilibrium between small inorganic

radicals rapidly develops in the combustion flame zone [10, 11, 34].

A temperature dependent hydroxyl radical concentration can therefore be

estimated from the local equilibrium assumption. In addition, a

9 -1 -1

bimolecular rate constant of £

10 L*mol *s for hydroxyl radical

attack on PCDDs has been estimated [10]. The mechanism uses the value

9 -1-1

of ^ 10 LmtioI *s for this reaction, and is biased in favor of PCDD

formation by ignoring the addition reaction of OH to polychlorinated

phenols, even though OH addition to aromatic compounds is known to

occur [30, 36-38]. The adduct from OH addition may simply decompose

back to the original reactants, or undergo further reaction with molecular

oxygen or other species present in the combustion system, leading eventually

B - 5
to ring opening [10, 28, 35, 36], a process which would not be expected

to favor PCDD formation. Note also that the electrophilic nature of OH

attack in addition reactions leads to a drastic lowering of the rates

with increasing halogenation and that addition reactions usually decrease

in rate as the temperature is increased [30].

The reaction,

P- - Pr (R3)

represents the decomposition of the phenoxy radical, resulting in

elimination of CO, which has been observed to occur [28, 39]. The

assigned rate constant, k(R3) ^ 10^*^ exp(-57654/RT) L«mol ^*s ^ follows

from analogy to the results of Colussi, Zabel and Benson [28].

The reaction,

P + P. - PD + Cl (R4)

represents the formation of a PCDD precursor, a polychlorinated 2-

phenoxyphenol . In order to bias the mechanism in favor of PCDD formation,

the possibility of reaction at other than the 2 position is ignored.

Reaction at other than the 2 position has been reported, although not

explicitly for the gas phase [40, 41] and may ultimately lead to polymerization

in some instances. The rate of this bimolecular reaction, (R4), is

probably very slow with a very small steric factor and possibly a negative

temperature dependence [10, 42]. It is probably slower than the hydrogen

abstraction reaction, (R7),

P* + R - P + R- (R7)

Mulcahy and co-workers [43] have reported (for the unsubstituted phenol)

that the reaction

B - 6
CH

3 + P - CH

4 + P*

has an activation energy barrier of £

8 +_ 2 kcal*mol \ Since

AH%no = 17.8 kcalTiiol"

1 for the reverse of this reaction [10, 44, 45],

the activation energy for the reverse of this reaction may be estimated

to be about 26 kcal^mol"

1

. Therefore, by analogy, to bias the mechanism

in favor of PCDD formation, a value of k(R7) %

10^ exp(-26000/RT)

L*mol“

1 *s“'

1 is assigned to the rate constant for reaction (R7). (In

all rate equations that appear in this paper the gas constant* R, has

the value 1.987 cal *mol

_1

)

.

A value of k(R4) ^

10xk(R7) is assigned

to the rate constant for the reaction, (R4).

Reaction, (R5),

PD - D + HC1 (R5)

is a molecular elimination reaction leading to PCDD formation. Rate

expressions for the molecular elimination of HC1 from alkyl chlorides

have been reported [29, 46], with pre-exponential factors in the range typically

13 5 + 0 5 ^

10 * - * and activation energies of typically %

50 +_ 5 kcal

mol""*. Reaction rates for gas phase molecular elimination of HC1 from

aromatics are not reported in the literature. However, the reaction is

not likely to be faster than above, due to geometric requirements on the

orientation of the hydrogen and chlorine atoms on the two rings and the

strengths of the bonds involved [10, 20]. The closest analogue for

reaction (R5) that is reported in the literature [47a, b] is the unimolecular

elimination of HC1 from l-(l-chloroethyl ) -2-methyl benzene for which a 13 -1

rate constant of k = 3.55 x 10 exp(-44,830/RT) s~ is reported [47a].

The value k(R5) ^ 10^ exp(-45Q00/RT) s~^ is asssigned to the rate

constant for reaction, (R5) in order to bias the mechanism in favor of

PCDD formation. In fact, by analogy to other aromatic systems [48, 49]

there is reason to suspect a substantially higher activation energy as

B - T
well as possible competition between this molecular elimination channel

and other simple unimolecular bond fission reactions. It is also

likely that there is a small steric factor to be associated with reaction,

(R5) [10].

Reaction (R6),

PD + OH - D + H

2 0 (R6)

is similar to reaction (R2), for which we have previously estimated a

9 -1 -1

bimolecular rate constant of k(R2) ^10 L*mol *s . Here reaction,

R(6), is actually a combination of a rate limiting abstraction reaction

PD + OH ^ PD- + H

2

0 (R6)

leading to production of a polychlorinated 2-phenoxyphenoxy radical

which then rapidly reacts unimolecularly to form a PCDO (although not

explicity noted, the fast reaction also results in elimination of either

a Cl or an H atom)

PD* -* D (fast)

and ignore the possibility of decomposition, in order to bias the

mechanism in favor of PCDD formation. For the same reason, the addition

reaction of OH is ignored.

Reaction (R8),

P* + OH - Pr (R8)

is expected to be faster than reaction (R2), since both reactants are

radicals. However, to bias the mechanism in favor of PCDD formation, the rates

are assumed comparable and set k(R8)= k(R2).

Reaction (R9),

D - Pr (R9)

represents the unimolecular dissociation of a PCDD to form products

which will, at the temperatures where reaction (R9) becomes important,

subsequently undergo further decomposition. A rate constant of

B - 8
k(R9) ^ lo

"15 ' 5 exp(-80000/RT) s""

1 [10] has previously been estimated

for this reaction.

Reaction (RIO),

OH + D - Pr (RIO)

is the bimolecular reaction of a PCDD with a hydroxyl radical. A value

k(R10) £

10

9 L*mol”^ «s’^ has previously been estimated for this reaction

[10,11]. The mechanism is biased in favor of PCDD accumulation by

8 -1 -1

reducing this value to k(R10) ^

10 L*mol *s

Reaction (Rll),

0

2 + P- Pr (Rll)

is the bimolecular reaction of a polychlorinated phenoxy radical with

molecular oxygen. The reaction of the unsubstituted phenoxy radical

with molecular oxygen has been estimated to have a rate constant of

8 - 1-1

^

10 L*mol” *s [48-51], and this value has been assigned here.

Reaction (R12),

R + OH - R. + h

2

0 (R12)

is expected to be slower in general than reaction (R8). However, to

9 -1 -1

bias the mechanism in favor of PCDD formation, k(R12) ^

10 L*mol »s

is assigned, since the availability of the species, R, will decrease the

concentration of P* through reaction (R7). The reverse of reaction

(R12), is neglected since in combustion typical of incineration, the

radical R*, resulting from hydrogen atom abstraction is highly labile

and can react further with molecular oxygen, other fuel molecules and

hydroxyl radicals, which will be present at substantially higher concentrations

than the phenoxy radical [8-10, 30].

Reaction (R13),

R -*• Pr (R13

)

B - 9
like reaction (R12), actually represents a broad spectrum of reactions,

and a typical value of k(R13) ^ 10^ exp(-90000/RT) s

^ has been assigned

[29, 46]. These reactions are strongly temperature dependent, and will

tend to become important at temperatures at which PCDD destruction, as

opposed to formation is already strongly favored.

Computer Simulation

The mechanism, as given in Table I has been used to study the

possibility of PCDD formation under typical incinerator combustion

conditions (below) through computer simulation via a standard Gear type

[52] ordinary differential equation system solver. Various initial

conditions were tested. A typical trichlorophenol has been considered

although this choice is arbitrary in view of the generalities of the

mechanism. Initial mole fractions of fuel reactant and oxygen are

assigned based upon assumption of stoichiometric combustion (equivalence

ratio equal to one) and conversion to carbon dioxide, water and HC1 as

major products. Atmospheric pressure was assumed and reaction at a

range of temperatures from 500 K to 1500 K was investigated. Four cases

(i-iv) were examined, for which initial mole fractions are as given in

Table II. In all cases, the concentration of hydroxyl radical was

defined through the assumption of local equilibrium to be (OH)^

£

10" 1 * 7 exp(-35000/RT) mol-L" 1 [10,30].

Case (i) is used as a base against which to test the role of molecular

oxygen and auxilliary hydrocarbon fuel in the destruction of polychlorinated

phenols and in the possible formation of PCDDs. In actual stoichiometric

or fuel lean combustion, there is (except in the case of mixing inhomogeneities

between fuel and air) considerable molecular oxygen available up to the

B 10
point of substantial fuel conversion. Therefore, this case is actually

worse than the true situation and probably greatly exaggerates gas phase

PCDD formation beyond what is realistically to be expected in ordinary

combustion in the absence of significant mixing inhomogeneities between

fuel and air. It is also not representative of a pyrolysis process,

where [OH] « [OH]^. In fact, for pyrolysis the mechanism would have

to be developed differently, with inclusion of unimolecular initiation

reactions such as reaction, (Rl), and exclusion of emphasis on reactions

involving hydroxyl radical attack. Note, however, that in the gas phase

these reactions are likely to be characterized by large activation

energies. Thus gas phase pyrolysis may not necessarily be expected to

produce substantial amounts of PCDDs, as initiation reactions may have

rate constants comparable to those for decomposition of PCDDs etc. In

addition, it is possible that polymerization reactions would have to be

included as part of the pyrolysis process [40, 41], With great care,

since the mechanism is biased in favor of PCDD formation, this case may

possibly be regarded as suggestive of the situation in which an unburned

fuel pocket, due to mixing inhomogeneities, passes into a post combustion

zone where oxygen has already been depleted, but the [OH] radical concentration

still tracks the local temperature in the incinerator. However, to

determine fully the importance of this case one would have to analyze

carefully the statistical character of variations of fuel/air inhomogeneities

throughout a combustor and in addition develop refined estimates of the

rate constants to eliminate the bias. In the case of a well designed

incinerator we suspect the importance of fuel /air inhomogeneities is

probably small relative to the bulk combustion process. This case in

which the fuel is a pure phenol in no way corresponds to municipal

incineration where such a chlorinated compound would only be present as

B 11
a trace substituent in the input feed stream and not as a major component.

In any event, note that in fact, if the phenol were present as a nearly

pure compound in a waste stream, it would be most likely recovered for

further use rather than destroyed thermally.

Case (ii) is more suggestive of combustion of a chlorophenol under

hazardous waste incinerator conditions. Normally, most incinerators are

run under stoichiometric or excess air conditions. However, to bias the

mechanism in favor of PCDD formation we have adjusted the initial mole

fraction of molecular oxygen to a level of about 60 percent of its value

in stoichiometric combustion. This has the effect of de-emphasizing the

rate of loss of the chlorophenoxy radical, P*, via reaction Rll

.

Case (iii) corresponds to a fuel rich pocket in a post combustion

zone environment in the combustion of a mixture of a phenol and an

excess of an auxiliary hydrocarbon fuel. This case is intermediate

between combustion in a liquid hazardous waste incinerator where the

percent of chloro-organic could be higher and the case of municipal

incineration where the chloro-organic content would be lower. Again,

molecular oxygen is treated as though it were unavailable.

Case (iv) is similar to case (iii) but has molecular oxygen present

during the combustion stage. Case (iv) is to be compared to case (iii)

as case (ii) is to case (i).

Pi scussion

The results of modeling these four cases for combustion during an

elapsed time period of one second, which is a residence time typical of

incineration are presented in Table III. The symbols P, P-, D and PD

used in Table III are defined in Figure 1. Below, the results are

B 12
analyzed. Concentrations for all four cases were computed for a wide

range of reaction times other than t = 1 second but are not presented

here to save page space.

Under the conditions of case (i) - i.e. post-combustion mixing and

reaction of unburned chlorophenol with hydroxyl radicals, some substantial

PCDD formation is indicated at intermediate temperatures. This is the

only case where that is true. However, this case is not realistic. It

assumes the entire initial mole fraction of phenol makes it into the

post combustion zone of an incinerator. This is not likely to happen.

In fact under normal operating conditions in a well designed incinerator,

only a very small amount of unburned fuel would be likely to enter the

combustion zone. Therefore, realistically, these mole fractions of PCDD

produced under case (i) would have to be weighted by the fraction of

fuel which actually makes it to the post combustion zone. On the basis

of such a weighting process, one would expect a much lower PCDD formation

than indicated in case (i).

In all the other cases it is quite clear that PCDD production is

very low. The use of auxiliary fuel, stoichiometric or excess air and

high temperatures (all of which are common operating conditions) promote

phenol destruction to products other than PCDDs. At flame temperatures,

gas phase PCDD formation would be below detection limits. Note also

from Table III that in all cases the level of chloro-2-.phenoxyphenol

formation is predicted by the mechanism which has been utilized, to be

much lower than PCDD formation. In part this may possibly explain why

chlorophenoxyphenols are not commonly reported as products of combustion.

As can be seen by comparing results in Tables Ilia with II Ic or

Table 1 1 1 b with 1 1 Id , it is significant to note that production of PCDDs

from the precursor, chlorophenol s , depends upon the square of the precursor

concentration.

B 13
In actual practice, it is found that PCDD emissions sometimes

exceed the levels predicted by the above results. This requires consideration

in some detail as to the nature of these circumstances. Some additional

possibilities may be:

a) PCDDs were originally present in the input feedstreams and

survive thermal destruction in significant amounts.

b) Due to mixing inhomogeneities some unburned chloroorganics

survive the main combustion environment and make it into the

post-combustion zone.

c) Temperature fluctuations (thermal transients) result in

incomplete thermal stressing due to gas entrapment in cold

pockets, promoting PCDD production at above ambient but below

flame temperature conditions.

d) Molecules other than chlorophenols are being converted to

PCDDs in the gas phase.

e) Inorganic chlorine sources in municipal waste input feed

streams lead to chlorination of organics and ultimate formation

of PCDDs in the gas phase.

f) Non-gas phase, or a combination of gas and non-gas phase

reactions are contributing to the production of PCDDs during

the combustion process.

With regard to (a), it has been stated that at least for municipal

waste streams, PCDDs are not initially present in input feedstreams in

significant amounts [5], to the extent that would be required to account

for observed levels of emissions. In the case of hazardous waste incineration

streams, the situation would depend on the feedstock. By way of a

hypothetical example, note that if it were the case that a municipal

waste stream contained 1 part per thousand of chlorophenols which in

B - It
turn contained 100 ppm of a PCDD impurity, then the expected PCDD level

in the effluent stream (gas plus fly ash combined) would be on the order

of tens of ppb if the PCDD destruction efficiency were zero (due to

dilution) and would readily fall to a few parts per trillion once the

destruction efficiency exceeds 99% for the PCDDs. We have previously

described an estimation procedure for calculating the expected destruction

efficiencies if homogeneous mixing is developed in the gas phase of an

incinerator [10,11]. This procedure was applied to the case of PCDD

destruction. We could conclude from these estimates that the gas phase

thermal destruction efficiency for PCDDs is extremely high at flame

temperatures typically achieved in an incinerator, T a. 1500 K [10,11].

With regard to (b), further investigation is warranted as there is

not much data presently available about statistically representative

temporal or spatial frequencies of unburned fuel pockets of varying

fuel/air composition in practical incinerators. Mixing inhomogeneities

may develop for various reasons, e.g., poor atomization may sometimes be

a problem in liquid hazardous waste incinerators. Samuelsen [53] has

discussed the formation of eddies or pockets of varying fuel air mixtures

in non-premixed flames. Breen [54] has estimated that eddy lifetimes

may range anywhere from 10 to 50 milliseconds. Whether or not these

eddies will ignite depends significantly upon the fuel/air ratio in an

eddy and upon the extent to which an eddy may mix together with other

eddies [53]. Appelton and Heywood [55] have discussed how inhomogeneities

due to eddies or pockets of varying fuel /air mixtures may affect pollutant

formation in continuous flow combustors burning atomized liquid fuels.

In addition, in the case of liquid injection, peculiarities of the

liquid mixture being injected may sometimes present problems. Wood and

B 15
others [56] have demonstrated that during the combustion of a multicomponent fuel drop, the composition of the drop and its surrounding

vapor change by a process of simple batch distillation. The inference

here is that one must sometimes be careful as to which fuels are to be

mixed together if fuel properties are such that substantial distillation

may be likely to occur. As an example, consider the injection of a

mixture consisting of small amounts of chloroform and chlorophenol

diluted in an auxilliary hydrocarbon fuel such as n-octane. To begin

with, if atomization and/or mixing are poor, the maximum temperature

will occur, not at the stoichiometric equivalence point, but at a somewhat

lower equivalence ratio (leaner fuel mix than theoretical stoichiometric

composition) corresponding to introduction of a substantial amount of

excess air [57]. At this maximum temperature point, the mixture will be

efficiently thermally decomposed. On movement into the fuel lean and

fuel rich sides of this maximum temperature point, temperatures may be

increasingly substantially lower and destruction efficiencies may be

component specific. In apparent contradiction to expectations based

upon a kinetic scale of incinerability [30], one would observe in some

off maximum temperature circumstances, due to the batch distillation

process, that much more chloroform than chlorophenol could persist

through the combustion zone. This is because the selective evaporation

of chloroform into the vapor phase rapidly enriches the vapor phase in

this component as the droplet begins to evaporate [58]. Depending on

the amount of enrichment this can significantly reduce the extent of

reaction in the chloroform enriched vapor phase due to the flame inhibiting

characteristics of the resulting mixture [59, 60]. If the combustion

zone is small, subsequent fluid hydrodynamic processes may remove some

of this self inhibiting mixture from the region of the combustion zone

B 16
before more substantial mixing and enhanced combustion can occur. At

the same time, the liquid droplet is becoming enriched in chlorophenol

and auxiliary hydrocarbon fuel (increasing mole fraction) by the distillation

process. This mixture may have less flame inhibiting character (due to

chloroform removal) and as such will subsequently burn more completely

on evaporation. Thus more chlorophenol than chloroform is destroyed in

these circumstances. It may be the case that as the chlorophenol is

mostly contained in a droplet which is burning at or near its boiling

point [ 61 ], some conversion to secondary products of formation may occur

within the droplet prior to complete evaporation. However, this in

itself (and similarly for the case of secondary product formation due to

pyrolysis reactions which might occur during the short period of warm-up

of the escaping vapor to its ignition point [ 62 ]) would not necessarily

create a hazardous secondary product emission problem if the chlorophenol

enriched droplet subsequently totally evaporates within the high temperature

combustion zone and providing the off-maximum temperature is still

substantially high enough to thermally decompose the resulting gas phase

mixture. Similar arguments would apply to the small amounts of chlorophenol

lost to the chloroform enriched vapor phase. A complication could arise

if flame inhibition occurs to such a substantial extent that it results

in flame extinguishment in the region of the chlorophenol enriched

drops. This problem could possibly be solved by allowing for a large

enough combustion zone such that the droplet could subsequently be reignited. In most cases, some approaches to these problems would be to

improve atomization, improve mixing, adjust the initial fuel mixture

composition to avoid undesirable distillations, provide for a high

temperature combustion zone transit time which significantly exceeds the

B IT
droplet burnout and mixing times, or to employ afterburners (staged

combustion)

.

In any event, predictions of incinerability based upon chemical

kinetics [30] are still possible. The problem of the apparent breakdown

of a kinetic scale of incinerability is solved by recognizing that the

incinerability scale is designed to be applied to homogeneous gas phase

mixtures. Thus, once a distillation process occurs the kinetic scale of

incinerability must be adjusted to account for formation of enriched and

depleted vapor mixtures and their local environment temperatures. In

such a case, a weighting must be employed to account for these physical

mass transfer and associated effects which are a consequence of the

distillation process.

It is possible to design experiments for measuring directly fast

transient fuel burning fluctuations [63,64], but these methods have not

been comr.only applied to the process of incineration. Note that if in

fact mixing inhomogeneities are a problem, it is not a problem without

possible practical solution for reasons mentioned previously.

The possibility (c), is to an extent, related to the degree of

mixing, although other parameters are also operative in controlling

thermal fluctuations [65-67]. Detection of fast (100 Hz and higher)

thermal fluctuations at a point in space in turbocombustors has been

made optically [63] and via physical measurement with fast time constant

thermocouples [68-70]. Application of optical detection to an incinerator

environment will require correction for substantial particulate entrainment

in the flow stream. The application of fast time constant thermal

measuring devices will require innovative technology such as R. R. Dils

new high temperature optical fiber thermometer [71] to avoid large error

B 18
measurements, since at flow speeds and temperatures typical of an incinerator,

convective transport is greatly diminished relative to the situation in

a turbocombustor [68-71]. Under incinerator conditions of low flow

rates, thermocouple fast transient temperature measurements are substantially

based upon radiative transport, and this dependence is the source of the

large errors in these thermal measurements when thermal fluctuations are

rapid. Numerical simulations of scales of incinerability or formation

in the presence of temperature fluctuations are possible [10,11,30,65-

67]. It is important to stress that in numerical simulations of destruction

or formation of organic species in non ideally mixed reactive flows with

temperature fluctuations, one must as a minimum, account for not only

the effects of unimolecular but also of bimolecular processes such as

those involving hydroxyl radicals [10,11]. If this is not done, results

can be misleading. The extent to which a typical thermal destruction

process as characterized by the rate constant expression

k - k T OHl + k destruction bimolecularL J unimolecular

becomes dominated by the unimolecular character may possibly provide

useful information about deviations from the ideal case of well mixed

fuel /air combustion at flame temperatures. Another important question

for case (c) that will have to be addressed is the question as to what

extent or not unburned or partially combusted fuel trapped in a relatively

cool pocket can be easily transported to the post combustion zone where

PCDD formation might possibly take place. While it is known that transport

of unburned or partially combusted fuel does occur in some specific

instances, for example in a diffusion flame [53-55], a detailed and

quantitative assessment of this problem in practical incinerators has

not yet been developed. More work needs to be done in this respect,

B 19
since at least in the case of turbopropulsion, thermal fluctuations can

be enormous [68] and fluctuations in turbulent flames in general have

been known sometimes to be substantial [53-55,67,71]. We suspect based

on observed combustion efficiencies that the statistical probability for

transport of unburned fuel in a cold pocket to the post combustion zone

is probably low in a well designed incinerator [10].

The possibility (d), that molecules other than chlorophenol s lead

to gas phase formation of PCDDs under typical incinerator conditions is

amenable to the same type of numerical analysis as presented in this

paper. It is reasonable to expect that the fewer the features a precursor

molecule has in common with PCDDs or PCDFs (e.g. aromatic character, and

chlorine or hydroxyl functional groups), the more complex the mechanism

one would have to propose for PCDD or PCDF formation. Under these

circumstances, at high temperatures, T ^1200 K, there will probably be

more competitive destruction pathways (vs. formation pathways) than in

the mechanism proposed in this report for PCDD formation from chlorophenols

(or polychlorinated 2-phenoxyphenols) . Thus, the Drobability for gas phase

formation of PCDDs or PCDFs would be expected to decrease under these

circumstances. As mentioned previously in this report, the author feel

there have been no experiments made to date which unambiguously demonstrate

that gas phase formation of PCDDs and PCDFs occurs under typical incinerator

combustion conditions [1, 2, 4b], However, the authors feel confident

that such experiments can be designed and used to test for non gas phase

and inhomogeneous mixing effects [8,9,63-71].

The possibility, (e), that of de novo synthesis from inorganic

sources of chlorine in the gas phase has not been extensively examined

[1,2]. The authors have briefly investigated the presence of FeCl

2

, FeCl

3

and

NaCl in the gas phase through numerical equilibrium modeling under

B 20
conditions typical of incineration, for example, as outlined in this

paper. This modeling indicates that in incineration, temperatures in

excess of 1000 K are required for low levels of NaCl to build up in the

gas phase and that temperatures in excess of about 1200 K are required

for low levels of iron chlorides to develop in the gas phase. Equilibrium

conditions at lower temperatures overwhelmingly favor distribution of

these metals among non gas phase species.

The quantitative reactivity of NaCl or iron chlorides as a source

of chlorine for gas phase chlorination of organics at high temperatures

has not been reported. In addition the time required for these inorganic

chloride species to develop to equilibrium concentrations is not reported

under typical incinerator combustion conditions. In the event that

aluminum recycling is not practiced, aluminum chlorides would also have

to be considered in a similar manner. Heavy metal chlorides, e.g.

ZnC^, CdCl

2

and PbCl^ are much more volatile at these temperatures [72],

but except for fairly specific waste streams the amounts of these metals

initially present is usually low.

The above discussion implies that even if these materials are

present and reactive in the gas phase at incinerator conditions, there

are not likely to be any simple mechanisms in which through only a few

steps, the production of PCDDs would occur to any significant extent.

This is because the bimolecular rates associated with the reaction steps

of any proposed mechanism would be low as a consequence of the low gas

phase concentrations of these species at incinerator operating temperatures.

As seen from modeling of chlorophenol combustion, at high temperatures

PCDD formation involving multistep processes consisting of several

reaction steps where reactant concentrations are low to begin with

cannot easily compete with other routes which lead to destruction

B 21
during the combustion process. For these reasons the authors suspect

de novo synthesis from inorganic precursors is unimportant in the gas

phase. This discussion does not imply, however, that de novo synthesis

from inorganic sources of chlorine via non-gas phase processes is to be

discounted. Quite the contrary, multiphase reactions involving inorganic

chlorides have been reported, including those which produce HC1 and Cl

2

from metal chlorides [73] and those which result in the production of

PCDDs [74] and other chlorinated organics [75].

The last and most intriguing possibility, (f), is the assumption

that non-gas phase or a combination of gas and non gas phase reactions

may be contributing to the production of PCDDs during the combustion

process. Allusions have previously been made in this paper to the

extensive literature which reports the enhanced reaction sensitivity of

organic species to the presence of reactive surfaces. Thus, reactions

involving suspended fly ash or reactive processes within or on the grate

beds of incinerators may have to be considered.

Fly ash itself is principally a mixture of silica and metal oxides

[10] and should have many reactive Lewis and Bronsted acid and base

sites which could presumably promote reactions [23-25] at temperatures

considerably below those at which the previously discussed destruction

processes dominate. In fact, chloro-organics formed from reactions in

the presence of chlorine and fly ash have recently been reported [21].

The authors have also indicated that there is other research which

appears to suggest oxygenation of chloro-organics to form PCDFs may be

due to surface reactions [10]. One must also consider the work of

Nestrick and co-workers [27] in synthesizing PCDDs from chlorophenates

which must be due to surface reactions in view of the low temperatures

B 22
in which reactions were carried out. The highly polar nature of the

metal to oxygen bond in the chlorophenates that were used and the surface

reactivity of the silica support they used probably favor ionic reactions

on the surface [23-25], possibly involving surface bound hydroxyl groups

and other reactive sites. At these temperatures [27] radical reactions

in the gas phase cannot compete. Surface reactions and/or reactions

inside solids (such as fly ash and other materials) therefore seem to be

strong candidates as potential sources for PCDD and PCDF formation.

Transport of these compounds into effluent emissions could then possibly

be due to insulation inside particles that have not been sufficiently

heated to the core. Presumably, vapor emissions of toxics would in part

then be associated with particle outgassing in the post combustion zone

[10]. In addition, particles, like unburned air/fuel pockets, may possibly

also act as transport agents for passage of unburned toxic precursors to

the post combustion zone, where formation may be favored. Alternatively,

the presence of particulates among unburned air/fuel pockets in the

post-combustion zone may promote surface reactions leading to PCDD formation

followed by thermal desorption into the gas phase. A correlation of

PCDD emissions with particle size may be informative in understanding

the relative importance of the above possibilities although care in

interpreting results may have to be exercised if data is taken from

units equipped with electrostatic precipitators due to differences in

the ease of ionization of particles of varying material composition.

Other complicating effects associated with particle emissions may

at times potentially be encountered. For example: With respect to fly

ash particle size as a function of distance measured movi ng away from

an incinerator point source, fly ash particle size distributions taken

from soil samples (surrounding the incinerator point source) may shift

B 23
to deposition of particles of smaller sizes onto the soil. This is

anticipated from differences in the terminal gravitational settling

velocity between particles of different sizes [76,77]. If the smaller

particles have been more thermally stressed in the incinerator due to

the strong dependence of particle heat up times with particle diameter

[10,78], then isomer distribution ratios of PCDDs/octa-CDD (e.g. hepta-CDD/

octa-CDD) formed on the fly ash particles should be observed to asymptotically

shift towards lower ratios as one moves away from the incinerator point

source, i.e., particle size and PCDD/octa-DD ratios reflect their

thermal history inside the incinerator and should track each other.

That the shift should occur in the direction of lower ratios on moving

away from the incinerator point source is a reflection of the increasing

thermal [30,79] and kinetic [10,11] stability of PCDDs with increasing

degree of chlorination, i.e., on the more thermally stressed particles

the more kinetically and thermally stable PCDDs should predominate. The

asymptotic nature of the shift reflects the fact that below a certain

particle size, essentially complete (to the core) thermal heating of all

fly ash particles which have been exposed to high temperatures inside

the incinerator has occurred. Whether or not this trend could be

interpreted to demonstrate that volumetric as opposed to surface formation

(or deposits) of PCDDs are occurring in the incinerator requires further

analysis, particularly with respect to heat sinking and particle porosity

during the lifetime of the fly ash particle's tranversal through the

incinerator. This analysis probably applies to municipal waste incineration

where there are not likely to be any significant amounts of PCDDs to

begin with. In the case of hazardous waste incinerators the analysis

may be more complicated depending upon which if any PCDDs are initially

present in the input feedstream.

B - 2h
In addition to reactions inside of or on the surfaces of particles,

the possibility for liquid phase transformations may have to be considered

in some instances. The literature is replete with reports of organic

synthesis of PCDDs in various solutions with and without catalysts (see

reference [2] and references contained therein). Liquid phase reactions

may possibly occur, e.g., in liquid injection hazardous waste incinerators

where atomization is poor and droplet evaporation times approach or

exceed combustion zone transit times. In these instances it may be

worthwhile to investigate alteration of droplet evaporation times,

improving atomization, use of afterburners (staging), etc., as mentioned

previously.

A further source of precursors may be from compounds entrained in

the input combustion air feedstream itself. For example, Junk and coworkers [80-83] have discussed in careful detail the presence of PCBs in

combustion air. Therefore, in addition to waste feedstream sources [1-

5, 84] additional consideration may have to be given to the history of

combustion air as it passes through an incinerator. However, with

respect to gas phase formation processes, arguments similar to those

presented in this paper probably apply, and PCDD formation from trace

organics in input combustion air is therefore probably unimportant at

the same conditions and high temperatures that would ordinarily promote

PCDD destruction.

Conclusions

Clearly, to more fully understand details of the processes for

formation and destruction of toxics such as PCDDs and PCDFs during the

process of high temperature combustion, more research is required for

reasons and in directions indicated above. On the other hand,

B 25
it is important to recognize that there is no unique thermal or kinetic

stability attributable to these compounds [30] that would prohibit their

efficient destruction at high temperatures. As seen from Tables III (ad), the use of an auxiliary fuel and the availability of molecular

oxygen reduces dioxin levels substantially. This can be attributed in

part to reaction steps R7 and Rll shown in Table I. Thus, as one possibility,

inhomogeneities in a practical sense, may to some extent be compensated

for by the use of chlorine free auxiliary fuels in after burners where

economically feasible.

As regards gas phase processes, note that the foregoing discussions

suggests gas phase PCDD formation is likely to be of low probability if

precursors are present only at low concentrations. This is a natural

consequence of the bimolecular nature of the formation reaction step, R4.

Note, however, that if PCDDs are already present in the gas phase, there is

likely to be a high probability of formation of PCDFs from PCDDs in the

gas phase, if the reaction is postulated to proceed via radical attack

on a central ring oxygen atom of a PCDD molecule resulting in abstraction of

the oxygen atom, followed by rapid ring closure. In fact, PCDFs have

been reported present in municipal incinerator emissions at high loading

levels (1, 2, 4b). The problem is to fully account for the dioxin

emission levels.

In order to further understand these problems it is appropriate that

several research objectives should be set for the scientific community as

enumerated below. This listing is not all inclusive but does serve to

highlight some current research needs with respect to understanding the

processes by which PCDDs, PCDFs and similar toxic compounds may be

formed and destroyed during practical thermal incineration:

(a) Analytical methods should be further developed and applied to

quantify types and amounts of specific organic compounds and classes of

B 26
organic compounds not only as a function of fly ash (or bottom ash, ESP

ash, etc.) particle size but also as a function of their distribution on

the surface of or inside (volumetric and internal surface due to pores)

particles. The same comments should be understood to apply to inorganic

species associated with fly ash emissions (or bottom ash, ESP ash,

etc.). In addition the distribution of species among the various phases

(solid/fly ash, soot; liquid/water aerosol, organic aerosol; gas/evolved

gas) needs to be characterized.

(b) Efforts should be made to study and understand fast transient

phenomena in incinerators, e.g. cold pockets, pre-ignition pyrolysis,

mixing inhomogeneities, etc.

(c) Much more effort should be directed toward understanding the

nature of condensed phase reactions (catalyzed and uncatalyzed), e.g.,

surface reactions, reactions in solids, etc.

(d) Predictive, reactive flow multidimensional fluid hydrodynamic

numerical simulations should be performed to study the dynamic character

of the incineration process.

(e) Laboratory scale experiments need to be performed as an aid

to refining rate constants associated with reaction steps in any proposed

mechanisms. These should include both gas phase and condensed phase

kinetics. With more refined mechanisms sensitivity analysis should be

undertaken. Similarly more thermodynamic data is needed for species

such as chlorophenols, PCDDs, PCDFs, etc., which is more reliable than

what is currently reported, if predictions of equilibrium isomer distributions

are to be made with greater accuracy.

(f) Complicating effects, e.g. selective compound distillations,

particle size separations, etc., should be examined in more detail.

B 27
(g) Optical methods should be applied to real time detection of

transient species in incinerator environments; especially hydroxyl

radical due to its importance in the combustion process. Continuous

rather than intermittent (e.g. as with a 10 Hz pulsed laser) measurements

would be desirable in order to understand perturbations in concentration

measurements due to effects such as vortex shedding.

(h) Interferences in analysis, e.g. of organic compounds in fly

ash due to naturally occurring particulates [85], should be very carefully

assessed.

B 28
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B 35
TABLE I - PROPOSED PCDD FORMATION MECHANISM

(a)

# Reaction Step Rate Constant

R1 P + P* + H 1015,5 exp(-86500/RT) s1

R2 P + OH - p* + h

2 o 10

9

L-M"

1 -S"

1

R3 P- Pr 10

13 * 6 exp(-57654/RT) S'

1

R4 P + P- - PD + Cl 10

9 exp(-26000/RT) L-M^-S' 1

R5 PD - D + HC1 10

14 exp(-45000/RT) S"

1

R6 PD + OH D + H

2

0 10

9

L-M’

1 -S'

1

R7 P* + R -v p + R- 10

8 exp(-26000/RT) L-M^-S' 1

R8 P- + OH - Pr 10

9 L-M^-S' 1

R9 D -* Pr 10

15 ' 5 exp(-80000/RT) S'

1

RIO D + OH ->• Pr 10

8 L-M^-S"1

Rll P- + o

2

-*> Pr 10

8

L-M"

1 ^'1

R12 R + OH - R- + H

2

0 10

9 L-M'^S’1

R13 R Pr 10

16 exp(-90000/RT) s1

(a) The rate constants have been biased to favor PCDD formations, therefore, the

reader is strongly cautioned not to use these estimates out of the context

of this paper, as actual values for these rates will be quite different

in some cases.

B - 36
Table II. Initial Mole Fractions^

CASE (i) CASE (ii) CASE (iii) CASE (iv)

0.1 0 . 0.1

X R 0 . 0 . 99X

p

99X

P

X

p 3.16 x 10

-2

3.16 x 10

-2

3.16 x 10’ 4

3.16 x 10

-4

(a) (OH) % (0H)

eq ^

IQ’

1 - 7

expf-35000/RT) [30]. Total pressure =

one atmosphere (760 torr). Other species, e.g. Ng* H^O, HC1

,

Pr, etc., which do not participate as reactants in the

mechanism shown in Table I make up the balance of the mole

fractions not explicitly detailed in Table II.

(b) In cases (ii) and (iv), the mole fraction of molecular oxygen

has been set to a level of about 60 percent of the stoichiometric combustion value as follows from the discussion

presented in the body of this paper.

B - 37
Table Ilia. Concentration (a) of species after 1 second: case(i

)

T(k) P P- D PD

500 7.70-4 2.31-16 6.58-28 7.10-22

600 6.42-4 2.03-9 2.34-16 2.01-13

700 5.50-4 1.20-7 5.31-11 1.86-10

800 4.79-4 2.41-6 4.21-8 1.83-9

900 4.01-4 1.85-5 1.97-6 3.16-9

1000 2.46-4 1.09-5 7.18-6 1.73-12

1100 4.11-5 6.83-7 1.91-6 1.73-12

1200 6.21-8 4.57-10 3.43-10 8.42-19

1300 9.34-19 4.12-21 5.43-32 1.49-36

1400 5.26-79 <1.00-70 <1.00-70 <1.00-70

1500 <1.00-70 <1.00-70 <1.00-70 <1.00-70

(a)

-1 ? e.g.: The number: 1.53-12 is 1.53 x 10 moles/liter.

P, P*, D, and PD are defined in Figure 1.

The symbols

B - 38
T(k)

500

600

700

800

900

1000

1100

1200

1300

1400

1500

Concentration of species after 1 second: case(ii)

p P* D PD

7.70-4 2.79-17 7.95-29 8.58-23

6.42-4 1.01-14 3.69-21 2.05-18

5.50-4 6.80-13 8.43-16 1.85-15

4.79-4 1.59-11 5.64-13 1.23-14

4.03-4 1.75-10 3.48-11 3.02-14

2.53-4 8.71-10 6.68-10 3.23-14

4.04-5 7.69-10 2.29-9 1.91-15

5.74-8 4.75-12 3.61-12 8.10-21

2.07-18 6.76-22 7.97-33 2.19-37

3.11-70 <1.00-70 <1.00-70 <1.00-70

<1.00-70 <1.00-70 <1.00-70 <1.00-70

B - 39
Table IIIc - Concentration of Species After 1 Second: Case (iii)

T(k) P P- D PD

500 7.70-6 2.31-18 6.58-32 7.10-26

600 6.42-6 2.08-11 2.55-20 2.12-17

700 5.50-6 1.20-9 5.31-15 1.86-14

800 4.79-6 2.45-8 4.26-12 1.86-13

900 4.03-6 2.00-7 2.09-10 3.45-13

1000 2.52-6 1.17-7 7.73-10 4.34-14

1100 4.12-7 6.85-9 1.95-10 1.73-16

1200 6.52-10 4.80-12 3.61-14 9.29-23

1300 9.92-21 4.38-23 2.82-36 7.73-41

1400 <1.00-70 <1.00-70 <1.00-70 <1.00-70

1500 <1.00-70 <1.00-70 <1.00-70 <1.00-70

B - 1+0
Table 1 1 Id - Concentration of Species After 1 Second: Case (iv)

T(k) P P- D PD

500 7.70-6 2.79-19 7.93-33 8.57-27

600 6.42-6 1.01-16 3.70-25 2.06-22

700 5.50-6 6.80-15 8.46-20 1.86-19

800 4.79-6 1.59-13 5.64-17 1.23-18

900 4.03-6 1.75-12 3.49-15 3.02-18

1000 2.52-6 8.68-12 6.71-14 3.21-18

1100 3.16-7 7.92-12 2.31-13 2.02-19

1200 6.29-10 5.21-14 3.95-16 9.74-25

1300 9.03-21 2.96-24 1.57-37 4.31-42

1400 <1.00-70 <1.00-70 <1.00-70 <1.00-70

1500 <1.00-70 <1.00-70 <1.00-70 <1.00-70

B - Ul
Legends for Figures

Figure 1. Species contained in the model mechanism.

Figure 2. Biased mechanism schematic.

B - 1+2
Figure 1

.

P

OH

PD

Symbolic representation of species in proposed mechanism.

B - 43
B - bk
APPENDIX C

OVERVIEW OF DIOXIN FORMATION IN GAS AND SOLID PHASES

UNDER MUNICIPAL INCINERATOR CONDITIONS

ABSTRACT

Mechanisms for the formation of dioxins during incineration via

gas and gas-fly ash interactions are presented and discussed. On the

basis of simplifying assumptions analytical expressions which relate dioxin

formation with elementary reactions are derived. Based on a postulated

mechanism, it is concluded that gas phase formation proceses do not account

for observed dioxin emissions from municipal incinerators. For gas-fly

ash interactions, observations are used to develop a model for catalytic

dioxin formation. It is concluded that catalytic and other non gas phase

processes need to be considered in order to account for observed dioxin

emissions from municipal incinerators.

c 1
INTRODUCTION

The results of quantitative analyses of effluent emissions (gas and

particulate fly ash) from municipal incinerators suggest that levels

ranging from parts per trillion to parts per million of polychlorinated

dibenzo-p-dioxins (hereafter referred to in this paper as, dioxin[s])

may sometimes be emitted to the environment [1-4].

It has been shown that [5,6] dioxins cannot be formed at the

reported levels on the basis of gas phase formation processes. Instead

it has been suggested that [5,6] dioxin formation may be associated with

catalytic processes involving suspended fly ash in the hot flowing gas

stream and/or via some other condensed phase processes. The purpose of

this report is to develop a simple model for gas-solid dioxin formation

from fly ash. First, some concepts dealing with gas phase dioxin formation

and destruction mechanisms will be reviewed. Through simplifying assumptions,

an analytical form for a previously proposed mechanism will be derived to

explicitly demonstrate the chemical processes which limit dioxin formation

in the gas phase. Second, it will be shown that a mechanism for dioxin

formation from fly ash catalysis can be constructed. When calibrated

against reported observations this leads to prediction of the expected

range of rate parameters for reactive catalytic processes.

GAS PHASE PROCESSES FOR DIOXIN FORMATION

It has been shown [6] through construction of a "worse" case

model, that it is unlikely dioxin formation may be occurring via gas

phase processes. This was demonstrated through a computer solution of

coupled rate relations assuming typical incinerator operating conditions.

C - 2
However, in developing the computer based numeric solution for predicted

levels of dioxin formation certain significant features were not brought

out. To rectify this problem an analytic solution of the previously

proposed dioxin formation model under certain simplifying conditions is

presented. The outline in a previous report is followed [6]. The

pertinent chemical species are symbolically noted in Figure 1. The

proposed "worse" case (i.e. biased towards dioxin formation beyond what

is realistically likely to occur) mechanism for dioxin formation is

shown in Table I. The species denoted by the symbol, R, denotes all

other hydrocarbon fuel molecules. R* is the corresponding radical.

Use of the reported rate constants beyond the intent of these reports is

not advised. They are biased away from likely actual numerical values

one would expect to determine experimentally.

On the basis of the "worse" case hypothesis it is assumed that

P ^ P Q

and R ^ R

q

, i.e., constant levels of precursor and hydrocarbon

fuel molecules are maintained. The mechanism is further based in favor

of dioxin formation by assuming no free molecular oxygen is present in

the post-flame combustion zone. This is equivalent to stating that reaction

step Rll shown in Table I may be ignored. It is then possible to apply

a steady state treatment to determine the concentration of P- and PD

from the proposed mechanism. For (P*)

ss

[k, + k

2 (0H)„]P

(P,) SS^ k

3

k

8

(0H)

EQ

Tk

4

P o

k

? R 0

C

Inspection of the assigned rate constants in Table I, for the temperature

range of interest in the post-flame combustion zone, leads to the result that

c 3
C(2)

k£ ( OH

) £Q ^ ^ss 'v ^3 + ^8^ ^EQ

In this and subsequent expressions, the hydroxyl radical concentration is

determined by a local equilibrium assumption [6]. Similarly,

(PD)

ss ^

k

4

Po

k

5 + k 6

(P.)

ss 'X,

k

4

k 2^^EQ

k 5^ k

3 + k 8^ 0H

^E(p

C(3)

It follows that.

d(D)/dt £

[k

5 + k6

(0H)

£Q] (PD)

$S - [kg + k1Q

(OH)

EQ

]D C(4)

For post-flame combustion zone temperatures, this reduces to,

d(D)/dt £

k

5

(PD)

ss

- k

1Q

(OH)

EQ

D C(5)

From the boundary condition that at t = 0, the dioxin level D(t = 0) = 0, it

follows that,

D ^

%

k 5( PD

>ss

k

i o^

oh

^eq

[1 - exp(-k

10

(OH)

Egt)]

and using equation (3), this may be written as,

C(6)

D >

k

4

k

2 (OH),

EQ

k

l 0^ 0H

^ EQ^- k 3 + k 8^ 0H

EQ 'E(p L 3

(1 - exp [

' k 10^ 0H

^EQ

t^ C(7)

In equation (7) the dependence of dioxin formation upon the square of

the precursor polychlorinated phenol molecule concentration is demonstrated.

It has previously been shown that at flame combustion temperatures,

dioxin production cannot compete with dioxin destruction [5,6]. Therefore

c - k
it is of interest to consider the production of dioxin in the post-flame

combustion zone where temperatures are considerably below flame combustion

temperatures.

Consider municipal solid waste burning which results in the production

-3

of post-flame combustion zone levels of ^

300 ug*M of polychlorinated

phenols. This corresponds to a level of ^

0.01% (the actual value

depends upon fuel composition and fuel air stoichiometry) of initial

fuel being polychlorinated phenols that are subsequently destroyed at a

destruction efficiency of ^99% on passage through the flame combustion

zone. The other 1% is transported to the post-flame combustion zone due

to mixing heterogeneities. Alternatively , this could be thought of as

corresponding to conversion of ^

0.01% of fuel on the grate bed of a

municipal incinerator to polychlorinated phenols which are then as in the

former case transported with a comparable destruction efficiency through

the flame combustion zone to the post-flame combustion zone.

Assume all polychlorinated phenols are present as a trichlorophenol

.

This assumption has no serious bearing upon any of the conclusions which

will be drawn. It is made in order to simplify the analysis by enabling

assumption of a specific molecular weight for the polychlorinated

phenols. The municipal incinerator is assumed to be operating at one

atmosphere total pressure. A post-flame combustion zone transit time of

one second [5,6,9] is assumed. Several solutions are summarized for a

range of post-flame combustion zone temperatures in Table 1 1 a , using

equation (7). It is clear from these results that even for a "worse"

case model, if dioxins are presumed formed by gas phase processes alone,

predicted dioxin emission levels cannot account for those which have

c 5
been reported for comparable loading levels of polychlorinated phenols

in municipal incinerators [1-4]. This is due to the dependence of the

dioxin emission level upon the square of the polychlorinated phenol

loading level.

It is not possible to account for observed dioxin emission levels

based upon a presumption that precursors of less complexity than polychlorinated

phenols are involved in a gas phase formation process. The minimum

number of critical bimolecular reaction steps is greater than one. Consider

the case in which the mechanism to produce dioxins involves a polychlorinated

benzene and polychlorinated phenol. The critical bimolecular reaction

steps will be two in number: e.g., i) either a polychlorophenoxy radical

must attack a polychlorobenzene molecule or a polychlorophenyl radical

must attack a polychlorophenol molecule, and ii) a hydroxyl radical must

attack the resulting product. Alternatively, in the case of dioxin

production from polychlorinated benzenes alone, there will be at least

three critical bimolecular reaction steps in any proposed gas phase

mechanism. This increased dependence upon bimolecular reaction steps

renders gas phase production of dioxins from other precursors than

polychlorinated phenols less likely than their production from polychlorinated

phenol precursors.

GAS-FLY ASH REACTION MECHANISMS

Assumptions

In practical MUNICIPAL incineration, the situation with respect to

multiple phase (gas plus fly ash) post-flame combustion zone processes

is complicated. It is necessary to impose simplifying assumptions in

c - 6
order to develop an approximate model for these complex processes.

The assumptions used in this paper are as follows:

1) Assume the post-flame combustion zone in a municipal incinerator

is characterized by an average temperature of 700 °C [7,8]. In an

actual incinerator there are both spatial and temporal thermal fluctuations

in the post-flame combustion zone. Since the intent of this report is only

to demonstrate that there may be a basis to suspect catalytic dioxin

formation, the assumption is not critical. Once a model has been developed

it may be tested at other temperatures as a check for sensitivity to substantial

temperature variations.

2) Assume, due to mixing heterogeneities, that polychlorinated phenols

are present in the post-flame combustion zone gas phase at a level of

_3

300 yg*M . This is a typical loading level to assume based upon reported

field observations [9], Higher and lower loading levels will also be examined.

3) Assume that the gas and fly ash both travel with the same average

flow velocity, and that the residence time of the material in the postflame combustion zone is one second. A residence time of this order is

representative of municipal incinerators [5,6,10].

4) Assume a fly ash loading of 3.5 x 10 g*cm‘ (about 0.1 g«ft ) in

the post-flame combustion zone. This would be deduced, e.g., from a stack

- fi -3

emission of 0.175 x 10 g*cm from pollution control equipment operating

at 95 % efficiency with respect to particles of this size. Fly ash

loading has been reported [11] to range from 2.8 x 10 g*cm to

-5-3

1.15 x 10 g*cm in some coal fired power plants.

5) Assume the average radius of fly ash particles in the post-flame

combustion zone is a ^

5 x 10 ^ cm (10 pm diameter). It is known that

there is a particle size distribution [12-15]. Since surface catalysis

c - 7
is being tested, larger particles having lower surface area are ignored.

Smaller particles are ignored. In coal fired power plants it has been

reported that maximal surface area is associated with particles having a

diameter comparable to that assumed here [12a]. Differences in particle

composition, (e.g., mineral, carbonaceous, ferromagnetic), while potentially

significant [12b, c] are not considered in this report.

6) Assume the fly ash is approximately spherical in shape and has

low porosity with respect to high molecular weight organics (i.e., it

is treated as a solid). Field observations [16] have shown that most

fly ash is essentially spherical for small particulates due to fusion

and surface tension effects. It frequently appears to be approximately

solid throughout when examined by microprobe techniques [16c]. The

presence of cenospheres [17] and a consideration of their potential role

as microreactors for dioxin and other compound formation or destruction

is not discussed. Cenosphere production is correlated to slag viscosity

[17]

. The variability of cenosphere concentration in municipal incinerators

is not quantified at the present time. Similarly, particle porosity

[18] has not been quantified with respect to organic compound formation

and destruction in municipal incinerators.

7) Assume an approximate surrogate for fly ash in terms of catalytic

effects is silica. This assumption is based upon reports [19-20] that

there are similarities between distribution patterns of chloroarOmatic

species produced in quartz ampoule experiments and those which have been

reported in municipal incinerator emissions. Each particle will be

assigned a density approximately that of glass [21], about 2.5 g*cm" .

C - 8
8) Assume the effective area of a surface site per adsorbed trichlorophenol

molecule (including excluded adjacent space) on a fly ash particle is

-8

equivalent to an area described by a circle having a radius of o ^

3 x 10 cm.

This is equivalent to having 3 x 10

+14

sites*cm~ 2

of surface area. This value

compares well with a previously assumed value [22]. This assumption

does not imply that a molecule (plus excluded area) actually occupies an

-8 area described by a circle having a radius of 3 x 10 cm. Rather, the

radius is assigned in order to effectively limit the number of available

+1 4 -2

reactive sites to about 3 x 10 sites*cm . 14 -2

The assumption of 3 x 10 sites«cm of fly ash surface area is

12 -3

equivalent to assuming there are 3 x 10 sites*cm of gas volume in the

post-flame combustion zone. This compares favorably with an estimate of

1 2 -3

7 x 10 adsorption sites*cm reported for polycyclic aromatic hydrocarbons

on fly ash in a coal fired power plant [12a]. In addition, in comparison to

estimated molecular surface areas for polycyclic aromatic molecules [23], the

assumed effective surface area for a trichlorophenol molecule implies that

the molecule does not lie flat on the surface of the fly ash. The nature of

the surface bonding remains to be determined. This requires a consideration

of the types of reactive sites [24] which may be present under municipal incinerator

combustion conditions.

9) Assume the total gas pressure in the post-flame combustion zone

is one standard atmosphere at the specified operating temperature.

10) Assume the molar ratio of polychlorinated phenols to other molecules

capable of undergoing competitive surface adsorption (chemisorption), is

_2

10 in the post-flame combustion zone. While an assumed mass loading

_3

of 300 pg*M of polychlorinated phenols in the post-flame combustion

zone implies the ratio in the input waste feed stream is much lower than

c - 9
this, the assumption is made here that the kinetic incinerability characteristics

of polychlorinated phenols boosts the ratio on passage through the flame

combustion zone. Alternatively, one can consider that not all molecular species

will successfully compete with polychlorinated phenols for available

adsorption sites [12a, b] at post-flame combustion zone temperatures.

11) Assume molecular species in the post-flame combustion zone

that do compete with polychlorinated phenols for adsorption (chemisorption)

sites on the surrogate fly ash have adsorption and desorption rates [12a]

approximately comparable to those for polychlorinated phenols. This

implies the relative fractional surface area coverage at any given time

will be proportional to the mole fraction ratio of adsorbing species

present.

12) Langmuir adsorption isotherms for the adsorbing (chemisorption)

species are assumed.

Mechanism for Catalytic Dioxin Formation

On the basis of the above assumptions, models for catalytic dioxin

formation via gas-fly ash interactions may be hypothesized. As an example,

one particular model involves the following reaction steps

k

a g + s * g

s C(8)

g + g

s

1—- d

s C(9)

^2

d

s * d C( 10)

Above, s represents a surface site for chemisorption; g and g

g respectively

represent gas phase and chemisorbed polychlorinated phenols; and d and d

$

respectively represent gas phase and chemisorbed dioxins. Molecular species

c 10
other than polychlorinated phenols which may also be adsorbed are represented

by the symbol, "a". The equivalent adsorption for species "a" analogous

to reaction step (8) above is represented as

a + s a

s C(ll)

Calculation of Input Parameters for the Model

If the fly ash mass loading in the post-flame combustion zone is

If the fly ash particles are assumed uniformly distributed, (more likely,

they are randomly distributed, however, the distinction is not critical)

the average distance between particles is

This implies that if the particles are uniformly distributed and move with

about the same average flow velocity as the gas in the post-flame combustion

zone (a valid assumption for 10 pm diameter particles), time scales for

diffusion of gas species to the fly ash particles will be short compared

to post-flame combustion zone transit times. This is a consequence of short

distances between fly ash particles. This means that with respect to

gas-particulate reactions, it is possible, as an approximation, to

ignore spatial mixing heterogeneities except when mixing scales are very

N C(1 2)

d 3- (Nj' 1/3

- 2cl -v 7.1 x 10' 2 cm

p % P p

Oj C(13)

c li
large and the fly ash particles cannot be treated as though uniformly

distributed.

The average mass per particle, m , is

3

9

n

p

= (4/3)ito

p pp

a, 1.3 x 10

a

g C(14)

At P * 1 atmosphere total operating pressure and a temperature of

3

700 C (973 K), the total number of molecules per cm is

N T £N (PV)/(RT) £

7.5 x 10

18 molec-cnf 3

C(15)

3

In equation (15), P = 1 atmosphere, V = 1 cm , T = 973 K, R is the universal

3 -1-1

gas constant in units of cm • atm •mol • K , and N

Q

is Avogadro's number,

23

6.02 x 10 molecules per mol.

Compared to molecular velocities (as distinguished from average gas

flow velocities) the fly ash particles can be treated as being stationary.

Therefore the relative velocity, u, can be computed from the gas molecular

velocity alone [25]

u £1.46 x 10

4 (—jjjj ) 1/2

£

3.24 x 10

4

cm-s"

1 C(16)

9

In equation (16), MW is taken as the molecular weight of a trichlorophenol

molecule, 197.35 g*mol’^ and T = 973 K (700 C). Assume the competitively

adsorbed species have the same molecular weight and therefore move at

the same molecular velocity. This assumption has no serious effect upon

the model development.

Since

c 12
C(17)

and

«

°p

« 0 . C(1 8}

the collision frequency, Zp per gas phase molecule per cm per second in a

3

gas that contains Np particles per cm is

Z_ %

irN (2o ) u

p ^ p p

C(19)

It follows from equation (16) that for both the trichlorophenol and

competitively adsorbed species.

Z n % 3.24 x 10

4

ttN (2a ) 2

y 275 coll isions*cm‘ 3 -s

_1

C(20)

p

'Xi p p

'

If the fly ash particles are completely covered by polychlorinated

phenols and competitively adsorbed species, then under assumptions made

previously, maximum fractional coverage, 0 by polychlorinated phenols

^max

is found by utilization of the relations

0 /e *5 3 max rnax C(21)

0 +0 ^1

^max amax ^

C(22)

« 5 Vx

a %

10

' 2

C(23)

In equations (21 ) - ( 23 ) » 0 is the maximum fractional

9max

polychlorinated phenol molecules. 0a is the maximum

a

coverage by the

fractional

max

coverage by the competitively adsorbed gas phase species. X and X are

9 a

c 13
respectively the gas phase mole fractions of polychlorinated phenols and

competitively adsorbed species. The above relationships are set in

terms of the total available number of sites. Contributions to surface

coverage by dioxins are ignored assuming the number of molecules of

dioxins adsorbed on the fly ash surface is always considerably less than

the number of adsorbed polychlorinated phenol molecules. The validity

of this assumption will be demonstrated. Implicit in equation (21) is

the approximation that equilibrium constants (ratio of adsorption rate

constant to desorption rate constant) for polychlorinated phenols and

competitively adsorbed species are equivalent. This approximation may

affect quantitatively the numerical solution of the model to be developed,

but does not affect the substance (conceptualization) of the model.

Using equations (21) - (23)

®n % 5/( 1 + 0 %

10' 2 maxv C(24)

C(25)

If the initial mass loading, M , of polychlorinated phenols (as

_3 trichlorophenol s) in the post-flame combustion zone is 300 pg*M , 3 then the initial molecular number density per cm , g Q

, of trichlorophenol

s

in the post-flame combustion zone is

C(26)

In equation (26), NQ

is Avogadro's number, 6.02 x 10

33

molecules *mol and MW

g is

-1 -12

the molecular weight of trichlorophenol, 197.35 g*mol . The factor 10

-3 -3

converts yg»M to units of g*cm . Since the ratio of polychlorinated

c - ik
phenols to other competitively adsorbed gas phase species has been taken to

_2

be 10 , the initial molecular number density of competitively adsorbed

species, a , is

2 13 -3

a 'u 10 g y 9.15 x 10 molecules*cm

O 'V o ^

C( 27

)

The maximum number density of adsorbed trichlorophenol molecules per

3

Np particles per cm is

max

( —EL_ )2© a, 3 x 10

13molecules*N particles" 1 *cm’

3

C(28)

K g3max

K

If the adsorbed dioxin molecules are assumed to occupy an effective area twice

that of the adsorbed trichlorophenol molecules, then

dTM

ax £

(l/2)gTM

ax £

1.5 x 10

10 molecules«N

p particles' 1 -cm’

3

C(29)

The maximum number of competitively adsorbed species per N

p

particles

3 . per cm is

o 2

a*

ax

n, 4N ( — ^

3 x 10

12

molecules»N particl es"

1 «cm"

3

p °a

amax

C(30)

This means

g^

ax ^ 3

s ^

0.03 g no C(31)

a"

ax % 0.03 a

s 'V 0 C( 32

)

The parameters developed above are tabulated in Table 3.

c 15
Model Development

The assumptions can be used together with parameters developed above

to construct a model consistent with the mechanism symbolically represented

by equatipns (8) - (1 1

)

.

Assume adsorption occurs for every gas-fly ash collision involving a

trichlorophenol molecule or competitively adsorbed molecule. Assume the

rate of desorption is much slower than the rate of adsorption. The

mechanism of competitive adsorption may be represented by

From a previous assumption it may be inferred that e

g

« ©

a

. Therefore an

equation for the reaction step represented by equation (33) may be derived

independent of the contribution from the reaction step represented by

equation (34). The assumption that every collision of a competitively

adsorbed molecule with a fly ash particle results in adsorption can then

be used to show that

a + s - a

s C(33)

g + s - g

s C( 34

)

d(a

s )/dt £

Z

p

a(l-0

a

) C(35)

Since a,SO« a„ it follows that a y a. From equation (25) % 0

-i \ /

C(36)

C - 16
Subject to the boundary condition that a

$ = 0 at t = 0, (defined as the

time at the point of transition from the flame combustion zone to the

post-flame combustion zone) equation (35) has the solution

a

s *

4Np(—JMWp (-^)

2

« C(37)

a p p

This result may be used to develop a solution for the temporal evolution

of the surface coverage, g , represented by the reaction step shown by

equation (34) for which

d (g

s

) /dt £

Z

p

g (l-©

a -0

g

) C(38)

Since gTM ax ^0.03 g Q

and 0^

« 0

a

, it follows, using equations (36) and

(37), that

d (g

s )/dt %

Z

pg

0 exp {- -jg-2 ) 2

t) C(39)

Subject to the boundary condition that g

$ = 0 at t = 0 (with t being defined

as before), equation (39) has the solution

9

S %

4N (-A-K-^)2[l-exp {- C-^)2

t)]

o a p p

C(40)

This result may be rewritten as

4N

t ^ - —S

'u Z a 2— (-5

£-) 2

ln (1 - p"o a -ll (—9 )( ) 2

} 4N

p

g

o a

p

CC 41

)

C - 17
When g. ^

0.99 gTMax and values for parameters that appear in the above

equation and as reported in Table 3 are used, it is found that

t ^

5.5 x 10“ 4 seconds C(42)

This result implies nearly complete coverage of the fly ash will occur

in times which are short compared to an assumed post-flame combustion

zone transit time of one second. Therefore the model mechanism can be examined

in terms of an assumption of steady surface coverage during the postflame combustion zone transit time. The assumption is made that the

number of dioxin molecules adsorbed on the surface is significantly less

than the number density of adsorbed trichlorophenol molecules on the

surface. The assumption of a steady level of coverage implies further

development of the model will require examination of equations (9) and

(10) and associated rate constants

k

l g + g

s

1 d

s C(9)

k

2

d

s * * d C(10)

Rate expressions for the reaction steps given by equations (9) and (10)

are

d(d

s )/dt = k-jg (0

g

- 0

d

) - k

2

d

$ C(43)

c 18
d(d)/dt = k

2

d

s C(44)

Equation (43) nay be rewritten using

) 2

<* s c< 45 > P P

as

d(d )/dt = k,ge - {-ji— {—

^3—

)

2

k

1 g + k

2

>d

$ C(46)

y

p p

It follows from equation (31) that g ^ g Q

. If the parameters Q and P are

defined as

CD ©

O

dC CT7 III O' C(47)

P "

^ 2N ^ a

^ k

l 9 o

+ k

2

P P C(4S)

equation (46) has the solution

d

s

= ( Q/P ) (l-e'

Pt

) C( 49

)

Then equation (44) has the solution

k

2 Qt k ? Q p. l - p

l

(l-ePt

) C(50)

_Pt

If e << 1 , equations (49) and (50) reduce to

d 'v, — s ^ P C(51

)

c 19
(t - 1/P) C( 52

)

The results from some reported field measurements indicate that [26]

d ^ 9 d,_ ^ s C(53)

The gas-solid dioxin partition ratio in municipal incinerators is not clearly

established at the present time. The location of sampling devices relative

to temperature gradients in thermal processing units will affect the outcome

of gas-solid organic component partition ratio measurements [27]. Other

gas-solid dioxin partition ratios have been reported [28,29]. The development

which follows may be generally applied to any reported gas-solid dioxin partition

ratios.

When equations (51) - (53) are used, and when t ^

1 second, (the

post-flame combustion zone transit time) P may be evaluated from equation

(48) to be (at T = 973 K)

P £

0.61 k

] + k2

C(54)

If 0.61 k-j « k

2

, then

k 9 % 10

2 'v C( 55

)

This validates the assumption that for a post-flame combustion zone

transit time of one second and an average temperature of 973 K (700 °C)

e-Pt « 1

C - 20
If it is assumed that

4 %

and a preexponential value of

13 -1

A0 * 10

IJ sec

1 2 ^

is assigned, it is found that at 973 K (700 °C)

-1 E

2 £

-RTln ( ^ ^

53.4 kcal-mol 1

If the average post-flame combustion zone temperature were 773 °K (500 °C

a value of 42.4 kcal*mol”"* would be determined. The value for E

2

is

significantly more sensitive to temperature than to municipal incinerator

transit time variations over a range of one half to two seconds. If

d y d c , E 0 y 56.5 kcal*mol"'* at 973 K. If d y 100 d . E 0 y 49.0 kcal -mol

at 973 K.

A representative amount of dioxins associated with fly ash [1-4] (on

a weight basis) is

d

$ £

100 ppb

If, for simplicity, it is assumed that the averaged molecular weight of

adsorbed dioxins corresponds to that for tetrachlorinated dioxins,

MW^ % 321.9 g*mol”"*, the molecular number density of adsorbed dioxins per

Np particles of fly ash per cm is

c - 21

0(56)

C(57

)

C(58)

1

C(59)
•) £

6.6 x 10® molecules*!! particles'^ *cm"® C( 60

)

d >

10‘ 7

(- s 'V* HlNN

P P 0

MW.

Equations (47), (48), (51) and (60), the relationship 0 ^

0TMax ^

10“^,

y y a post-flame combustion zone transit time of one second, and the previously

determined values for other parameters may be used to show that

k^ ^

0.73 (same units as Z ) C(61)

The value for k*j of 0.73 validates the approximation, 0.61 k-j << If

-E,/RT

*1 %

Z

p

e C(62)

and T = 973 °K (700 °C), it is found that

ki 1 E, n, -RTln(

j

-—) ^

11,500 cal -mol

1 C(63)

P

The low activation energy determined for the reaction represented by equation

(9) suggests the chemisorbed molecule is very reactive. Possibly this may be

due to removal of a hydrogen or chlorine atom from the surface bound molecule

via fast abstraction by gas phase radicals or by neighboring reactive sites

on the fly ash particulate.

CONCLUSIONS

Dioxin formation is attributed in this particular model to reaction

of gas phase and adsorbed chlorophenols. Desorption of dioxins and

other adsorbed species makes new sites available. Competitive adsorption

of species other than chlorophenols limits dioxin formation. In addition,

dioxin formation is dependent on reaction conditions. The formalism

c 22
that has been set up can be used to assess other potential scenarios.

Carefully designed laboratory experiments can be used to test the models.

Possible variations in dioxin formation as a function of chlorophenol

loading can be found in Table lib. The calculations carried out here

are based on representative reported results [1-3, 9, 19, 20, 26, 30].

The end results obtained, rate expressions for desorption and catalyzed

dioxin formation are reasonable [22,3]. The observation that a substantial

activation energy for desorption may be expected is consistent with a

report that dioxins are apparently irreversibly adsorbed on fly ash at

moderate temperatures [32]. However, unlike the situation in the gas

phase, activation energy limits for surface induced processes are

rather wide.

The squared dependence upon the precursor concentration with respect to

dioxin formation is important. This squared dependence has previously

been mentioned in the context of the gas phase process and is apparent

in Table 2b for the gas-solid process based on the model examined in

this report. This dependence sets strong limits on the extent of dioxin

formation and implies that experiments with neat samples or under conditions

of high concentration can be extremely deceptive unless properly interpreted.

In addition to the above considerations, there are reports [9, 30] that

the degree of chlorination of dioxins is sometimes more extensive than for

phenols found in the same municipal waste post-combustion streams. These

observations considered jointly with reports of chlorination reactions

[30, 33] indicate that it may be necessary to assess the potential role of

solid and gas-solid reactions on and in grate beds. Grate bed residence times

are considerably longer than flame and post-flame combustion zone transit tines

in the gas phase in municipal incinerators. In such a case, additional

c 23
possibilities may have to be considered. The convolution of these processes

with mixing inhomogeneities will have to be examined.

The rationale of the present and earlier work [5, 6] is that an

understanding of how dioxins can be formed in municipal incinerators may

lead to identification of possibilities for process modifications which may

minimize emissions.

c 2h
TABLE I - PROPOSED DIOXIN FORMATION MECHANISM^

* Reaction Step Rate Constant Units

R1 P - P- + H 10

15 * 5 exp(-86500/RT) S'

1

R2 P + OH -*• P- + H

2

0 10

9 L-M'

1 -S'1

R3 P- Pr 10 13,6 exp(-57654/RT) S'

1

R4 P + P* -* PD + Cl 10

9

exp ( -26000/ RT) L-M'

1 -S'

1

R5 PD - D + HC1 10

14 exp(-45000/RT) S’

1

R6 PD + OH -* D + H

2

0 10

9

L-M'

1 -S'

1

R7 P- + R

-* P + R- 10

8

exp ( -26000/ RT) L-M"

1 -S'1

R8 P- + OH - Pr 10

9 L-M^-S"1

R9 D Pr TO15,5

exp ( -80000/ RT) S"

1

RIO D + OH - p r 10

8

L-M'

1 -S'

1

Rll P- + 0

2

-+ Pr 10

8

L-M'

1 -S'1

R12 R + OH -* R- + H

2

0 10

9

L-M'

1 -S'1

R13 R - Pr 10

16 exp(-90000/RT) S'

1

(a) The rate constants have been biased to favor PCDD formations. The reader

is strongly cautioned not to use these estimates out of the context

of this paper, as actual values for these rates will be quite different

in some cases.

c 25
TABLE

I

la

r^.

i m1 co o

i O1 o o

o rH i—

i

rH o X x X

o vD vD v£> oo O O O • • • r^ O'.

m co o

1 Oi o1 O

o rH 1“

1

rH o X X X

o o o o CM CM CM

• • • vO i£> vO

i mi co1 u

i o

i o1 o

o rH »H »H o X X X

o m m m

vD co co co

• • •

•>3'

m co

CJ

1 o o

i o

o rH tH tH o X X X

o m m m

m 00 00 oo

• • • rH rH rH

co

t 2

00 o O O

n. m o o CO O

c co

P-.

'd

<De

d

co

CO

o3

co

•rl

d

o

o

CD

co

<L> ao

<Ho

<u

£3 •H

-p

0) o

a

0)

•H

CO<U <

C - 26
PREDICTED

DIOXIN

LEVELS

BASED

ON

GAS-SOLID

CATALYSIS

CO

id

'—

r

w 03c

o

o

<

u

CO ro

S'! CM CO rH o CM CO

• • c o O m •H

i—i COc

•HX

CO MO vO n- o

T3 ON O CM •H

o a • . TO

'O 00 ON ON O

AJ

CO rHo

in G

CO (1) "O co o St .C + I • • Or

CO s o CO ON o

T3 CM Is DO CM o W G. rHrG

i—i

t

o

CO

1 DO CM

-O o

• rH rs St

DO on 00 C

CO o

vO •H CO CO

1 G

O. s CJ o O O >

DO ro o C

P. co o

CO CJ 4Jc

(U ro CM CM o

DO 1 1 1 G

CD O o o (U rH r—

H

H D. K

ON CO •H CO CM rH *

CsP 1 1 1 S'! O o o

rH rH 1—

1

C 27
TABLE III. TABULATED VALUES FOR PARAMETERS USED “

Tn tIhe analytical method ~

Variable Definition and units value

t transit time through postflame combustion zone (second)

1

p

P

density of a fly ash particle

(g*cm°)

2.5

M

9 initial mass loading of polychlorinated phenols in post*

flame combustion zone (pg*M"'

3

)

300

H

P mass loading of fly ash

particles (g«cm"'

3

) 3.5 x

10“'

°P

radius of fly ash particle

(cm)

5 x 10‘ 4

V °a

"radius" of polychlorinated

phenol, g, and competitively

adsorbed species, a (cm)

3 x 10" 8

°d "radius" of dioxins (cm) 3/T x 10

c gas phase mole fraction ratio

of polychlorinated phenols to

other competitively adsorbed

species

10‘ 2

p post-flame combustion zone pressure (atm)

1

T post-flame combustion zone temperature (°C or °K)

700 (°C)

973 (°K)

N o Avogadro's number (molec*mol~''

)

6.02 x 10

MW

g assumed molecular weight for

polychlorinated phenols

(g*mol~ )

197.35

MW,

a assumed molecular weight for

dioxins (g*mol~ ) 321.8

n

p number density for fly a^h

particles (particles*cm’>3 )

2.7 x 10

3

Equation

in text

( 12 )
C7»

d

p

distance between particles

(cm)

7.1 x 10“^ (13)

111 p

mass per fly^ash particle

(g*particle )

1.3 x 10” 9

(14)

n

t1 total gumber of gas molecules

per crn at^specified (T, P)

(molec*cni )

7.5 x 10

18

(15)

u average molecular velocity of

gas molecules (cm*sec~ ) 3.24 x 10

4

(16)

“p collision frequency of a gas

phase ^olecule per N particles

per citi per second (collision*N

particles" *cm" *sec~ ) p

275 (20)

> g-max maximum possible fractional

coverage of fly ash by

polychlorinated phenols

10‘ 2

(24)

v a maxmaximum possible fractional

coverage of other competitively

adsorbed species

0.99 (25)

%

initial molecular number density

in the gas phase of the postflame combustion zone of poly-.,

chlorinated phenols (molec*cm“ J

)

9.15 x 10

11 (26)

a 0

initial molecular number density

analogous to g but for other

competitively adsorbed species

9.15 x 10

13

(27)

max

?s

maximum number density of

adsorbed tri-chlorophenol

molecules per N particles per

cm (molecN particles" *cm~ )

3 x 10

10

(28)

max

a

s maximum number density of

the competitively adsorbed

3 x 10

12

(30)

species per N particles per - cm"'

5 (molec*Np particles" *cm~ )

c 29
REFERENCES

[1] Harris, 0. C. Anderson, P. C., Goodwin, B. E., and C. E. Rechsteiner

Dioxin Emissions from Combustion Sources: A Review of the Current State

of Knowledge (N. Y . : Final Report to ASME, 1980).

[2] Esposito, M. P., Tiernan, T. 0. and F. E. Dryden. Dioxins (Washington,

DC: EPA-600/2-80-1 97 , 1980).

[3] Hutzinger, 0., Frei, R. W. , Merian, E. and F. Pocchiari, Eds.

Chlorinated Dioxins and Related Compounds (Pergamon Press, N.Y.,

1982).

[4] Frank, R. (Chairman, Associate Coimittee on Scientific Criteria for

Environmental Quality) et al , Polychlorinated Dibenzo-p-Dioxins : Criteria

for their Effects on Man and His Environment (Ottawa, Canada National

Research Council of Canada, NRCC No. 18574, 1981).

[5] Shaub, W. M. and W. Tsang. "Physical and Chemical Properties of

Dioxins In Relation Their Disposal", in Human and Environmental

Risks of Chlorinated Dioxins and Related Compounds , Tucker, R. E.,

Young, A. L. and A. P. Gray, Eds., Plenum Publishing Corporation,

N.Y., 1983) pp. 731-748.

[6] Shaub, W. M. and W. Tsang. "Dioxin Formation In Incinerators", accepted by

Environmental Science and Technology for publication.

[7] Sengupta, S. and K-F. V. Wong, Eds. Resource Recovery from Solid Wastes

(Pergamon Press, N.Y., 1982).

[8] General Electric Company. Solid Waste Management Technology Assessment

(Van Nostrand Reinhold, N.Y., 1975).

[9] Ahling, B. and A. Lindskog. "Emission of Chlorinated Organic Substances

From Combustion", in Chlorinated Dioxins and Related Compounds ,

C - 30
Hutzinger, 0., Frei , R. W. Merian, E. and F. Pocchiari, Eds.

(N.U., Pergamon Press, 1982) pp. 215-225.

[1C] Tsang, W. and W. M. Shaub. "Chemical Processes in the Incineration of

Hazardous Materials", in J. H. Exner, Ed., Detoxication of Hazardous

Waste (Ann Arbor, MI, Ann Arbor Science Pub., 1982), pp. 41-60.

[11] Hangebrauk, R. P., Von Lehmaen, D. J. and J. E. Meeker. J. Air

Pol 1 . Control Assoc . 14: 267-275 (1964).

[1 2a]Schure, M. R. , Ph.D. Thesis, Colorado State University, Ft. Collins,

Colorado, 1981; [b] Schure, M. R. and D. F. S. Natusch. "The Effect

of Temperature on the Association of POM with Airborne Particles", in

Polynuclear Aromatic Hydrocarbons: Physical and Biological Chemistry ,

Cooke, M., Dennis, A. J. and G. L. Fisher, Eds. (N.Y. Springer-Verlag

,

1982) pp. 713-724; [c] Clement, R. E. and F. W. Karasek. J. Chrom

234L 395-504 (1982).

[13] Raask, E. Trans. ASME . 104: 858-866 (1982).

[14] Cowherd, C. Jr., Marcus, M. , Guenther, C. M. and J. L. Spigarelli.

Hazardous Emission Characterization of Utility Boilers , (Wash., DC,

NTIS , EPA Report No. EPA-650/2-75-006, 1975).

[15] Quann, R. H., Neville, M., Janghorbani, M., Mims, C. A. and A. F. Sarofin.

Envir. Sci. Techno! . 16: 776-781 (1982).

[1 6a]Grasserbauer, M. "Characterization of Individual Airborne Particles

By Light Microscopy, Electron and Ion Probe Microanalysis, and Electron

Microscopy", in Analysis of Airborne Particle by Physical Methods ,

Malissa, H. and J. W. Robinson, Eds. (West Palm Beach, FL, CRC Press,

Inc., 1978) pp. 125-172; [b] Cheng, R. J., Mohnen, V. A., Shen, T. T.,

Current, M. and J. B. Hudson. J. Air Poll. Control Assoc . 26: 787

(1976); Yakowitz, H., Jacobs, M. H. and P. D. Hunneyball. Micron

3: 498-505 (1972).

c 31
[17] Lauf, R, J., Fuel 60: 1177-1179 (1981).

[18] Ditl , P. and R. W. Coughlin. Envir. Sci. & Tech . 11: 701-706 (1977).

[19] Buser, H. R. , Bosshardt, H.-P., and C. Rappe. Chemosphere 7: 109-119

(1978).

[20] Rappe, C., Marklund, S., Buser, H. R., and H.-P. Bosshardt. Chemosphere .

7: 269-281 (1978).

[21] Weast, R. C., Ed., Handbook of Chemistry and Physics , 47th Edition.

(CRC Co., Cleveland, OH, 1966), p. FI.

[22] Laidler, K. J., Chemical Kinetics . (McGraw-Hill Book Co., N.Y.

1965), pp 294-296.

[23] Snyder, L. R. Principles of Adsorption Chromatography (N.Y., Marcel

Dekker Inc. , 1968).

[24] Heftmann, E., Chromatography (N.Y., Reinhold Pub. Corp., 1967), p. 52.

[25] Benson, S. W. , The Foundations of Chemical Kinetics (N.Y., McGraw-Hill

Book Co. , 1960) , p. 155.

[26] Janssens, J., Van Vaeck, L., Schepens, P. and E. Adams. “Qualitative

and Quantitative Emissions of a Municipal Incineration Installation",

in Physico-Chemical Behavior of Atmospheric Pollutants , Versino, B.

and H. Ott, Eds. (Boston, MA, Reidel Pub. Co., 1982), pp. 28-38.

[27] Eiceman, G. A. and V. J. Vandiver. Atmos. Envir . 17: 461-5 (1983).

[28] Cavallaro, A., Luciani, L., Ceroni, G., Rocchi, I., Invernizzi, G.

and A. Gorni . Chemosphere 11: 859-68 (1982).

[29] Olie, K. , Lustenhower J. W. A. and 0. Hutzinger. "Polychlorinated

Di benzo-p-dioxins and Related Compounds in Incinerator Effluents",

in Chlorinated Dioxins and Related Compounds- Impact on the Environment ,

Hutzinger, 0., Frei, R. W. , Merian, E. and F. Pocchiari, Eds., (N.Y.,

Pergamon, 1982), pp. 227-244.

c 32
[30] Tiernan, T. 0., Taylor, M. L., Garrett, J. H., Van Ness, G. F., Solch,

J. G., Deis, D. A. and D. J. Wage!. "Chlorodibenzofurans and Related

Compounds in the Effluents from Combustion Processes", presented at

the 3rd International Symposium on Chlorinated Dioxins and Related

Compounds, Salzburg, Austria, 13 Oct. 1983.

[31] Hayward, D. 0. and B. M. W. Trapnell . Chemisorption , (London

Butterworths , 1964).

c 33
NBS-H4A (REV. 2*80 ) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 84-2872 April 1984

4. TITLE AND SUBTITLE

Containment of Dioxin Emissions From Refuse Fired Thermal Processing

Units - Prospects and Technical Issues

5. AUTHOR(S)

Walter M. Shaub

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE S. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS ( Street, City. State. ZIP)

10. SUPPLEMENTARY NOTES

Note to Center and WERB Reviewers: Appendices A,B, and C have already

Been through the WERB review process, do not review.

Qd Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant in formation. „ bibliography or literature, survey, mention it here) . , , This report reviews and addresses the prospects

cone e r ne d w i th the ut il i z: t ainme nt of di oxin em i s s i

The re suits de veloped i n chem i s orpt i on of diox in e: unit s , may be techni c ally

and fu 11 sc ale tests ar e equi pp e d f o r e nergy r e c o V

whi c h are n ot equippe d fo

ef fi c i ent ly at temper at ur

gas e x it t

mpe ratures .

If document includes a significant

for and technical issues

as a technique for conons from refuse fired thermal processing units,

this report suggest that containment, through

missions from refuse fired thermal processing

s for research objectives

d thermal processing units

able than those units

chemisorption occurs more

ally lower than furnace

outlined.

Suggestion:

Refuse fire*

if

are substantii

recovery

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

boilers, catalysis , chemisorption , containment, dioxins, furans

incinerator emissions, incinerators, thermal processing units

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