NAT'L INST. OF STAND & TECH NBS I I I I I I Referonce publications aiiidl osmmsi

NBSIR 84-2975

Technical Issues Concerned With

PCDD and PCDF Formation and

Destruction in MSW Fired

Incinerators

Walter M. Shaub

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MD 20899

November 1984

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U S. DEPARTMENT OF COMMERCE

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NBSIR 84-2975

TECHNICAL ISSUES CONCERNED WITH

PCDD AND PCDF FORMATION AND

DESTRUCTION IN MSW FIRED

INCINERATORS

2^.

QC/oo

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Walter M. Shaub

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MD 20899

November 1984

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

NATIONAL BUREAU OF STANDARDS, Ematt Amblar, Dkaetor
T'l « ' !W-‘‘ II
TABLE OF CONTENTS

Page

1. EXECUTIVE SUMMARY 1 2. GOAL OF THIS REPORT 4 . OBJECTIVES OF THIS REPORT 5

4. BACKGROUND 6

5. INTRODUCTION 7 . ANALYSIS OF TECHNICAL QUESTIONS 10

7. MAJOR CONCLUSIONS 33

8. REFERENCES 34

9. APPENDIX 36

1 . EXECUTIVE SUMMARY

This report has been written in response to a series of questions dealing

with the chemical and physical properties of polychlorodibenzo-p-dioxins

(PCDDs) and polychlorodibenzofurans (PCDFs) and the possibility of their being

emitted from municipal solid waste (MSW) fired incinerators. The questions

taken as a whole ask if there is reason to expect that PCDD and PCDF emissions

can be minimized when an MSW fired incinerator is properly operated.

Answering the questions that have been posed shows that:

• In terms of their implications for proper incinerator operation, one

must consider both possibilities that chlorinated precursor compounds

are found in MSW and that chlorinated precursors are being formed from

chlorination reactions that occur within the incinerator.

• No one has yet conclusivley shown by experiments or testing of pilot

or full scale MSW fired incinerators that PCDDs and PCDFs will be

formed in the post-flame combustion zone of an incinerator from

surface reactions on fly ash particles. If such surface reactions

occur:

• Catalytic PCDD and PCDF formation may take place in the

post- flame combustion zone of an incinerator at temperatures in the range of about 400“C up to temperatures at

which thermal destruction of these compounds and their

precursors occurs.

-1 -
• Combustion factors, e.g., mixing, cold surfaces, etc.,

which can perturb the uniformity of the high temperature

combustion process, may tend to increase the concentration of precursor compounds.

• The role of HCl in the formation of PCDD and PCDF in this

type of surface reaction is probably unimportant due to

the complexities of any likely chlorination mechanism and

limited reaction times. Even if HCl could play a role,

attempts at separation of chlorine from MSW prior to

Incineration would likely have no effect on this role due

to the high percentage of chlorine-containing waste that

would have to be removed.

• If a fly-ash catalyzed post-flame combustion zone process produced

PCDDs and PCDFs, Increases in precursor compounds should have no

effect on the production rate when available reaction sites are

limited by slow thermal desorption. If condensed phase (grate bed)

reactions are the principal mechanism for PCDD and PCDF production, an

increase in the level of chlorinated precursor compounds could affect

the level of PCDD and PCDF emissions if the incinerator combustion

efficiency was low.

• The key to minimizing concentrations of chlorinated precursor

compounds in the combustion zone is to assure as nearly so as possible

that they are uniformly mixed with sufficient air and unifoirmly heated

to flame temperatures.

-2 -
• If the PCDD and PCDF precursor compounds are effectively destroyed in

the combustion zone, it is not possible for them to reform at any

significant levels after the combustion zone^

• In or on the grate bed some chlorination of lignin may take place.

The nature and source of the chlorinating agents are not well known.

Chlorinated lignin material, when heated, may depolymerize to produce

chlorinated compounds, such as chlorinated phenols. The chlorinated

phenols and other compounds that may be produced could be destroyed by

further thermal stressing.

• Improper operation of the incinerator could increase the precursor

concentrations found in the stack through mass transport of these

compounds when combustion efficiency is low and control devices do not

contain these furnace emissions. In principle, this can be corrected.

However, even if precursor compounds are transported to the stack, no

significant PCDD or PCDF formation is expected to occur in the stack.

-3 -
2. GOAL OF THIS REPORT

• Have an answer, which is supported by valid scientific evidence, to each

technical question about PCDD and PCDF formation and destruction in MSW

fired incinerators posed by Mr. Casowitz.

-4-
3. OBJECTIVES OF THIS REPORT

• Extract from published material information which is pertinent to

questions that have been raised concerning PCDD and PCDF formation and

destruction in MSW fired Incinerators.

• Utilize information obtained from literature reports to analyze technical

Issues concerned with PCDD and PCDF formation and destruction in MSW fired

Incinerators.

• Relate analyses to practical implications for minimization of PCDD and

PCDF emissions from MSW fired Incinerators.

• Identify and discuss uncertainties of analyses of technical issues

concerned with PCDD and PCDF formation and destruction in MSW fired

incinerators.

• With respect to Mr. Casowltz' inquiry, achieve an understanding of

technical Issues concerned with PCDD and PCDF formation and destruction in

MSW fired incinerators.

-5-
4. BACKGROUND

Large municipalities are rapidly running out of landfill space in which

to dispose of municipal solid waste (MSW). Waste disposal in MSW fired

incinerators is an alternative. The City of New York is currently attempting

to develop a safe and economical means to dispose of its municipal waste

through incineration.

Technical Issues have been raised concerning prospects for minimizing

PCDD and PCDF emissions from incinerators. Mr. Paul Casowltz, Deputy

Sanitation Commissioner, Office of Resource Recovery and Waste Disposal

Planning, the City of New York Department of Sanitation, requested the

National Bureau of Standards (NBS) to provide technical expertise to analyze

the issues which are of concern. Subsequently, questions concerning PCDD and

PCDF formation and destruction in MSW fired incinerators were posed to NBS by

Mr. Casowitz.

-6 -
5. INTRODUCTION

This work is based upon an analysis of current literature reports and the

personal expertise of Dr. Walter M. Shaub, who has conducted extensive

analyses of the physical and chemical properties and behavior of dioxins and

related compounds in thermal environments. References [1,2] of this report

can be used as an extensive bibliography to this field. The latter reference

can be used as a sourcebook about NBS related research.

In the section which follows this introduction, each question associated

with the technical points posed by Mr. Casowitz is analyzed and answered.

- 7 -
THE CITY OF NEW YORK Department of Sanitation

September 19, 1984

61 CHAMBERS STREET, RCX)M 815

NEW YORK NY 10007

PAUL D. CASOWrrZ. P.E. DEPUTY COMMISSiONER

OFFiCE OF RESOURCE

RECOVERY AND WASTE

DiSPOSAL PLANNING

TELEPHONE (212) 566-6260

Dr. Walter Shaub

U.S. Department of Commerce

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Bldg. 224

Room: 258B

Gaithersburg, Maryland 20899

Dear Dr. Shaub:

I want to thank you again for taking time to speak with me about your

report Containment of Dioxin Emissions from Refuse Fired Thermal Processing

Units - Prospects and Technical Issues .

Your approach establishes a theoretical framework that provides an extremely useful perspective for evaluating both the available experimental

evidence and also the design of future experiments.

I had been under the false impression that Dave Lipsky of Fred C. Hart

Associates had sent you a copy of their report on the potential public health

risks from dioxin and furan emissions from the resource recovery facility proposed for the Brooklyn Navy Yard. Apparently, your copy was not sent out so I have enclosed a copy with this letter. I apologize for the late delivery. As you

are aware, I very much wanted you to be part of the expert review panel for this

report. I would still greatly appreciate any comments or suggestions that you

might care to make in regard to the Hart Study. The report has not been officially

released yet. I would ask that you treat it as an internal working document for

the next four weeks by which time the report should have been released.

One of the policy issues that Barry Commoner has raised concerns the

relationship between the PVC content of the municipal solid waste and the emissions

of PCDD's and PCDF's from an MSW fired incinerator. It would be of great assistance

to me if you would comment briefly on the following points:

(1) In a properly operated incinerator (i.e., where temperature, excess air,

mixing and residence time are set to maximize combustion efficiency)

dioxins and furans may still be formed after the combustion zone from

surface reactions on fly-ash particles.

- What temperatures would favor the formation of PCDD and

PCDF from chlorinated phenols or other likely precursors?

NewYork

LET’S CLEAN UPi

newYORK.
Dr. Walter Shaub

September 19, 1984

Page No. 2

- What combustion factors would tend to increase the con- centration of precursor compounds?

- What role, if any, would the presence of HCl be likely to

have in the formation of PCDD and PCDF in this type of

surface reaction?

(2) What effect would an increase in the level of chlorinated precursor com- pounds have on the levels of PCDD and PCDF emissions?

(3) Could improper operation of the incinerator increase the precursor con- centrations found in the stack? How?

(4) What combustion conditions would be required to effectively destroy

chlorinated precursors in the combustion zone? Could these precursor

compounds reform after the combustion zone?

(5) Could PVC or other sources of chlorine play a role in the formation of

chlorinated phenols or other organic compounds by combustion in combination with paper on the grate? Barry Commoner has stated that the lignin

in paper will give rise to phenols which will combine with the chlorine.

(6) Can the levels of chlorinated precursor compounds likely to be found

in MSW account for their concentrations in the emissions or is it more likely that these precursors are being formed from chlorination reactions

that occur within the incinerator?

(7) Are the emissions of PCDD and PCDF likely to be limited by the levels

of these precursor compounds or by the available surface area provided

by the particulate?

I hope I am not imposing too much on your time. New York City has a tremendous waste disposal problem (we generate over 20,000 tons each day and we are rapidly running out of landfills. We want to select the safest and most reliable

technology for meeting this waste disposal need. If removing PVC is likely to

further reduce PCDD or PCDF emissions, we will pursue the feasibility of this

alternative. If on the other hand the PCDD and PCDF are independent of the PVC

content of the waste or there are better control techniques, we do not want to

waste time and resources with PVC removal.

Any help you could provide would be most appreciated.

Sincerely

Paul Casowitz, P.E.

^

Deputy Sanitation Cormiissioner

PC:rl

r !
6. ANALYSIS OF TECHNICAL QUESTIONS

CAN THE LEVELS OF CHLORINATED PRECURSOR COMPOUNDS LIKELY TO BE FOUND IN MSW

ACCOUNT FOR THEIR CONCENTRATIONS IN THE EMISSIONS OR IS IT MORE LIKELY THAT

THESE PRECURSORS ARE BEING FORMED FROM CHLORINATION REACTIONS THAT OCCUR

WITHIN THE INCINERATOR?

There are insufficient amounts of PCDDs or PCDFs in uncombusted MSW waste

streams to account for their emissions from stacks [2]. Fewer data are

presently available about the sources of chlorinated precursor compounds in

uncombusted waste. The identities of all the chlorinated precursor compounds

are uncertain. Chlorophenols appear to be precursor compounds [2]. Other

chlorinated precursor compounds have been suggested [2].

The amounts of chlorinated precursor compounds found in MSW are highly

variable. Thus, it is difficult to quantitatively measure concentrations of

these compounds in a statistically representative manner [2]. Consequently,

the levels of the precursor compounds, particularly on or in the mass of

municipal waste which is burned on the grates, are not known with certainty.

Therefore, in terms of their implications for proper incinerator

operation, one must consider both possibilities that chlorinated precursor

compounds are found in MSW and that chlorinated precursors are being formed

from chlorination reactions that occur within the incinerator.

In the following questions which Mr. Casowltz has raised, these

Implications are examined.

-10-
IN A PROPERLY OPERATED INCINERATOR (I.E., WHERE TEMPERATURE, EXCESS AIR,

MIXING AND RESIDENCE TIME ARE SET TO MAXIMIZE COMBUSTION EFFICIENCY) MAY

DIOXINS AND FURANS STILL BE FORMED AFTER THE COMBUSTION ZONE FROM SURFACE

REACTIONS ON FLY ASH PARTICLES?

It has not been demonstrated that PCDDs and PCDFs "will" be formed at

significant levels in properly operated incinerators. What is known is that

under some conditions, some incinerators will at times emit detectable levels

of PCDDs and PCDFs [1]. These findings do not exclude the possibility that

proper operating conditions can be determined that will promote the minimization of PCDD and PCDF emissions.

Theoretical studies conducted at the NBS suggest that conditions may

sometimes exist in which PCDDs and PCDFs may be formed in the post-flame

combustion zone of an incinerator from surface reactions involving chlorinated

precursor compounds and fly-ash particles [2]. Catalytic formation of PCDDs

and PCDFs on surfaces can be made to occur under some laboratory conditions

[ 2-6 ].

Therefore, no one has yet conclusively shown by experiments or testing of

pilot or full scale MSW fired Incinerators that PCDDs and PCDFs will be formed

in the post-flame combustion zone of an incinerator from surface reactions on

fly-ash particles.

- 11 -
WHAT TEMPERATURES WOULD FAVOR THE [POST-FLAME COMBUSTION ZONE] FORMATION OF

PCDD AND PCDF FROM CHLORINATED PHENOLS OR OTHER LIKELY PRECURSORS?

High temperatures which can promote the thermal and oxidative destruction

of PCDDs, PCDFs and precursor compounds (e.g., chlorophenols) in the gas

phase, should also promote the destruction of these compounds on the surface

of fly-ash particles. There is no known unique thermal kinetic stability

attributable to these compounds when they are adsorbed on fly ash surfaces

[ 2 ].

The lower temperature limit for PCDD or PCDF formation from chlorophenols

or other likely precursors is set by the temperature required for effective

thermal desorption of these compounds and other competitively adsorbed species

in periods which are short relative to the post-flame combustion zone transit

time [2], At temperatures lower than about 400 °C, the rate of thermal desorption of these compounds from fly-ash becomes slow compared to the transit time

of the fly-ash particles through the incinerator. This means that at low

temperatures, sites for PCDD and PCDF formation on fly-ash are unavailable.

Under such conditions, PCDD and PCDF production is insufficient to account for

reported emissions. This is one reason, for example, why the possibility for

PCDD or PCDF formation from reactions in the stack of an incinerator can be

discounted.

Therefore, catalytic PCDD and PCDF formation may take place in the postflame combustion zone of an incinerator at temperatures in the range of about

400 “C up to temperatures at which thermal destruction of these compounds and

their precursors occurs.

- 12 -
WHAT COMBUSTION FACTORS WOULD TEND TO INCREASE THE CONCENTRATION OF PRECURSOR

COMPOUNDS?

To answer this question precursor compounds must be suggested. To do

this requires an examination of possible environments in which PCDD and PCDF

formation may take place. There are at least two possible environments to

consider: (a) reactions involving surfaces of fly-ash in the post-flame

combustion zone of an MSW fired incinerator [2]; or, (b) reactions involving

condensed phase environments, e.g., on or in the grate beds of an MSW fired

incinerator

.

Kinetic modeling of a worse case (beyond that which is reasonably likely

to happen) formation model for PCDD formation conclusively eliminates the

possibility of gas phase formation of PCDDs from any precursor compounds, with

or without HCl present [2]. The possibility that PCDFs may be formed in the

gas phase by radical abstraction of oxygen from PCDDs has not been studied by

experiment. This latter statement does not exclude PCDF formation from

precursor compounds due to non-gas phase formation processes.

(a) Post-flame combustion zone environments:

The formation of PCDDs or PCDFs from fly-ash catalyzed post-flame

combustion zone reactions may involve chlorobenzenes or chlorophenols. (In a

combustion environment, chloro-2-phenoxyphenols are probably best thought of

as short-lived reactive intermediates rather than as precursors [2]). Except

as discussed below, possibilities for other post-flame combustion zone

precursors may not be important: in terms of chemical kinetics, when reaction

times are limited, the possibility for postulating realistic PCDD or PCDF

-13 -
formation mechanisms from precursor compounds that are very much structurally

unlike PCDDs or PCDFs is small. This is because when a precursor is structurally unrelated to PCDDs or PCDFs, a very complex pathway must be postulated

to account for PCDD or PCDF formation. As the postulated mechanism necessarily becomes more and more complicated the reaction probability for PCDD or

PCDF formation becomes small [2]. These arguments can be demonstrated by

analytical and numerical techniques [2,8].

In the post- flame combustion zone, two other possibilities to consider

are that the unchlorinated phenols or unchlorinated benzenes may be precursor

compounds. If so, this would require the presence of a high density of

chlorinating agent on the surface of fly-ash particles and/or in the gas phase

of the post-flame combustion zone and that additionally, chlorination of the

unchlorinated compounds on the surface of the fly-ash be both very rapid and

very extensive - in order to account for the often reported [1,2] skewed

distribution of chlorinated PCDD emissions towards the more highly chlorinated

isomers. Note that preferential reactive destruction of the less chlorinated

PCDD isomers by hydroxyl radicals is likely to be much less extensive in the

post-flame combustion zone than in the flame combustion zone.

It is interesting to note that at least in the case of a silica surface,

HCl is a very poor compound for promoting surface chlorination by displacement

of hydroxyl groups [8,10] even at elevated temperatures. This would seem to

imply that chlorination of unchlorinated organics takes place by surface

activation of the organic species. It is not known to what extent surface

condensed heavy metal halides [11] may promote post-flame combustion zone

chlorinations or whether CCl^, which is a more effective chlorinating agent

- 14-
than HCl [10], may play some role. The conceptual complexities of these

reactions [8,11] and the limited reaction time in the post-flame combustion

zone suggest that these possibilities are not significant except under some

unusual circumstances.

Experiments have been perfoirmed [3-6] which suggest that some catalytic

chlorination of PCDDs and other organic compounds can occur under controlled

laboratory conditions. More research is necessary to conclusively show

whether or not and to what extent such reactions can occur in the post-flame

combustion zone of MSW fired incinerators.

Of the possible precursor compounds suggested above, concentration

increases in the post- flame combustion zone may be attributable to several

factors which include [2]:

(i) poor mixing efficiencies between fuel and oxidant, resulting

in heterogeneous combustion conditions and wide temperature

fluctuations or low average temperatures;

(ii) gasification of unburnt or partially combusted condensed

phase organic material which has been buoyantly transported

to the post-flame combustion zone;

(iii) boundary layer transport along cold surfaces, e.g., walls,

of unburnt or partially combusted fuel to the post- flame

combustion zone; and

- 15 -
(iv) incomplete combustion due to low average temperatures.

Research is necessary to quantify these and other possibilities. Test

plans should include a consideration of the relationship of incinerator operations and design characteristics to these conditions.

(b) Condensed phase environments:

Chemical and physical processes take place in and on material which is

burning on the grates of an MSW fired incinerator. These processes may

produce chlorinated precursor compounds or PCDDs or PCDFs. It is difficult

without conducting further research to make more than just a few qualitative

observations concerning combustion factors that could increase the concentration of these compounds on or in the material which burns on grates. This is

presently due to a general lack of useful information about this particular

aspect of the combustion process in MSW fired incinerators. A few observations which can be made will be presented in response to additional points

raised by Mr. Casowitz.

Any compounds produced on or in the material which burns on the grates of

an MSW fired Incinerator must subsequently pass through the rest of the

incinerator in order to be emitted through the stack. This implies that

combustion conditions could be established which minimize emissions of these

compounds. The combustion factors cited above are likely to contribute to

these emissions if not adequately controlled.

- 16 -
Therefore, combustion factors, e.g., mixing, cold surfaces, etc., which

can perturb the uniformity of the high temperature combustion process, may

tend to increase the concentration of precursor compounds. Research and

testing are required to optimize these combustion factors to minimize concentrations of precursor compounds.

- 17 -
WHAT ROLE IF ANY WOULD THE PRESENCE OF HCl BE LIKELY TO HAVE IN THE FORMATION

OF PCDD AND PCDF IN THIS TYPE OF SURFACE REACTION?

Some mimerical estimates are made to suggest what role if any the

presence of HCl may have in promoting the surface catalyzed formation of PCDDs

or PCDFs.

Emission levels of HCl from municipal incinerators are on the order of

100 ppm by weight [12-15]. The concentration level of HCl in the post-flame

combustion zone is about the same order of magnitude. This means that at one

atmosphere pressure and a temperature of about 600 °C there are about 6 x

10'*’^^

4*3 HCl molecules per cm in the post-flame combustion zone. (The arguments made

here are not critically sensitive to the assumed temperature.)

The maximum number of chlorine atoms that can be adsorbed on a silica

surface is reported to be between three and four chlorine atoms per 100 square

Angstroms under favorable conditions [10-16]. On fly-ash in an MSW fired

incinerator the maximum number of chlorine atoms which can be adsorbed per 100

square Angstroms is probably much less than this, e.g., due to competitive

adsorption, steric effects, variations in fly-ash surface composition, etc.

Assume that the maximum coverage of fly-ash particles is no more than about

3.5 chlorine atoms per 100 square Angstroms [10]. Assume that the average

fly-ash particle diameter is about 10 micrometers, that the fly-ash particles

are approximately spherical, and that the fly-ash loading in the post-flame

“6 +3 combustion zone is about 10 grams per cm [2]. With these assumptions the

maximum number of adsorption sites for HCl is about 8 x lO"*"^^ sites per cm"^^

of fly-ash laden gas in the post-flame combustion zone.

- 18 -
When the above assumptions are made , the ratio of HCl in the gas phase of

the post- flame combustion zone to the maximum amount of HCl that can be

surface adsorbed on fly-ash is about 750. For reasons mentioned above, this

figure is only an estimate. The actual ratio may be much larger.

In chlorination experiments reported by Eiceman and Rghei [4,5] an

attempt was made to chlorinate dioxins on fly-ash with a mixture of HCl in air

at elevated temperatures. The nominal fly ash size used was between 100 and

140 mesh (an average particle size diameter of about 63.5 micrometers).

Eiceman and Rghei [4,5] xosed a typical fly ash loading of 1.6 grams per

+3 3.76 cm . Therefore in these experiments, the maximum number of adsorption

sites for chlorine atoms was about 2.8 x lO"*”^^ sites per cm^^ of fly-ash

loaded gas volume, assximlng as before that the fly-ash particles were approximately spherical and that the maximum possible chlorine atom loading was about

3.5 chlorine atoms per 100 square Angstroms [10]. The Instantaneous gas phase

molecular number density of HCl at a reported loading of one percent by volume

of HCl in air at one atmosphere pressure and a temperature of 300 °C was about

1.3 X

10"^^^ molecules of HCl per cm"*"^. Therefore in this reported experiment,

the ratio of the gas phase HCl to surface adsorbed HCl was about five. This

is a ratio which is smaller by about a factor of 100 than the value of a

typical ratio calculated above for HCl in an MSW fired incinerator. Eiceman

and Rghei [4,5] report that even at this low ratio of five, promotion of

chlorination reactions was found to be independent of the HCl concentration.

This independence of chlorination reactions from HCl concentrations was

reported for ratios from about five to about 250 [4,5].

- 19-
Benzene and phenol are In some respects structurally similar to PCDDs and

PCDFs. This implies that the same results might be observed for HCl promotion

of fly-ash catalyzed chlorination of these precursors [3]. This supposition

needs to be verified by experiment.

Polyvinyl chloride (PVC) in municipal waste streams is reported to

account for no more than about 25% of the HCl which is produced and emitted

during the combustion process which takes place in MSW fired incinerators

[15,17]. Therefore removal of all PVC from municipal waste streams prior to

incineration would reduce the level of HCl such that the ratio of the gas

phase HCl to the maximum value for surface adsorbed HCl (actually surface

adsorbed chlorine atoms) would still be very large (about 560 based on the

above assumptions) compared to the ratios studied in the experiments reported

by Eiceman and Rghei [4,5]. The fact that the extent of chlorination is

Independent of HCl concentration even at the low ratio of five, implies that

if all PVC were removed from municipal waste streams prior to incineration, no

effect on the level of PCDD and PCDF emissions would result if PCDD and PCDF

formation critically involved surface catalyzed HCl chlorination reactions in

the post-flame combustion zone.

It has previously been suggested in this report that given the likely

complexities of a multiple chlorination mechanism and the limited reaction

time available in the particle laden gas flow stream of the incinerator,

extensive fly-ash catalyzed chlorination reactions appear to be unlikely.

However, if such reactions could occur, the above discussion Implies that

extensive removal of chlorine, by as much as a 99+% reduction of chlorine -

content from the MSW stream prior to introduction into the incinerator, would

-20-
likely have no effect on the extent of such reactions. This author is not

aware of any pre- treatment technology which could effectively remove 99+% of

the chlorine from an MSW stream prior to incineration.

Therefore, the role of HCl in the formation of PCDD and PCDF in this type

of surface reaction is probably unimportant due to the complexities of any

likely chlorination mechanism and limited reaction times. If HCl could play a

role , attempts at separation of chlorine from MSW prior to Incineration would

likely have no effect on the role due to the high percentage of chlorine that

would have to be removed.

-21 -
WHAT EFFECT WOULD AN INCREASE IN THE LEVEL OF CHLORINATED PRECURSOR COMPOUNDS

HAVE ON THE LEVELS OF PCDD AND PCDF EMISSIONS?

(a) Post-flame combustion environment:

Assume that the principal mechanism for PCDD and PCDF formation is due to

catalytic reactions which take place upon the surface of fly-ash in the postflame combustion zone. Assume that the concentration of polychlorinated

precursor compounds in the gas phase of the post- flame combustion zone is

significantly greater than the number of available surface reaction sites per

unit volume of fly-ash laden combustion gas. A typical chlorophenol loading

+3 level is about 300 micrograms per M [2]. Then the rate determining step for

PCDD and PCDF formation should depend upon the number of available surface

reaction sites, and the emissions of PCDDs and PCDFs should not be affected by

or correlated to any further increase in the level of chlorinated precursor

compounds. In this instance the level of emissions would depend upon other

factors, e.g., the rate of desorption of chemisorbed species, competitive

species adsorption, etc. Consider the following example of what the above

discussion implies.

—A +3

At a fly-ash mass loading of 3.5 x 10 grams per cm (about 0.1 gram

+3

per foot ) in the post-flame combustion zone of an MSW fired incinerator it

has been estimated that the number of surface reaction sites does not exceed

about 3 X 10 sites per cm [2] of fly-ash laden combustion gas. This is

an upper limit estimate for the reasons discussed previously, i.e., the actual

number of available reaction sites is probably much less due to heterogeneties

in particle morphology, steric effects, competitive adsorption, etc. For

example, if competitive adsorption is accounted for, an estimate for the

-22 -
available number of reaction sites is about 3 x sites per of flyash laden combustion gas [2].

In the post-flame combustion zone, a gas phase mass loading of 10

+3 micrograms per M of a trichlorophenol corresponds to a molecular gas density

of about 3 X lO"*"^^ molecules per cm'*’^. This is about equal to the number of

available reaction sites. Therefore an increase in the mass loading of

+3 trichlorophenol, e.g., to a value of 100 micrograms per M would not necessarily result in an increase in the level of, e.g., PCDD emissions. Other

factors would have to be examined such as the rate of desorption of chemisorbed species from the fly-ash surface [2]. Under these circumstances, in

which PCDD emissions are not related to precursor concentrations, the level of

PCDD emissions woixld be more sensitive to the time dependent temperature

distribution function for the incinerator than to the ambient level of

precursor compoiinds.

(b) Condensed phase environment:

If the principal driving force for PCDD and PCDF formation is attributed

to mechanisms which occur on or in the mass of waste which burns upon the

grate bed of an MSW fired incinerator, the effect of an increase in the level

of chlorinated precursor compounds on the level of PCDD or PCDF emissions is

difficult to predict or estimate. Almost no useful information is reported in

the literature from which to draw conclusions. Additional discussions of

grate bed chemistry will be presented in response to the following questions

raised by Mr. Casowitz.

- 23-
If under some conditions increases in precursor compounds promote

increased production of PCDDs and PCDFs in the condensed phase environment,

other factors may compensate for this enhancement and promote destruction of

these compounds. Potential controlling factors associated with the combustion

environment have been suggested in this report and in reference [2].

Therefore, if a fly-ash catalyzed post-flame combustion zone process

produced PCDDs and PCDFs , increases in precursor compounds should have no

effect on the production rate when available reaction sites are limited by

slow thermal desorption. If condensed phase (grate bed) reactions are the

principal driving force for PCDD and PCDF production, an increase in the level

of chlorinated precursor compounds could affect the level of PCDD and PCDF

emissions if the incinerator combustion efficiency were low.

-24-
ARE THE EMISSIONS OF PCDD AND PCDF LIKELY TO BE LIMITED BY THE LEVELS OF THESE

PRECURSOR COMPOUNDS OR BY THE AVAILABLE SURFACE AREA PROVIDED BY THE

PARTICULATE?

A response to this question is predicated on the presumption that the

principal means for PCDD and PCDF formation is by catalytic reactions due to

the interactions of fly-ash with precursor compounds in the post-flame combustion zone. This presumption has been suggested [2] but not proven at the

present time. If this presumption is true, then the preceding discussions in

this report are pertinent and should be referred to for a response to this

question. In addition, the NBS report mentioned in the references should be

referred to for further information [2].

Therefore, if a fly-ash catalyzed post-flame combustion zone process

produces PCDDs and PCDFs, Increases in precursor compounds should have no

effect on the production rate when available reaction sites are limited by

slow thermal desorption.

-25-
WHAT COMBUSTION CONDITIONS WOULD BE REQUIRED TO EFFECTIVELY DESTROY

CHLORINATED PRECURSORS IN THE COMBUSTION ZONE?

The key to minimizing concentrations of chlorinated precursor compounds

in the combustion zone is to assure as nearly so as possible that they are

uniformly (homogeneously) mixed with sufficient air and uniformly thermally

heated to flame temperatures [2]. (Actually temperatures as low as about

900 °C should be more than adequate to destroy most organic compounds

efficiently during available transit times if mixing and thermal stressing are

efficient .

)

In practice the above conditions are sometimes difficult to achieve. In

large scale combustion devices such as MSW fire incinerators, considerable

effort is required to minimize thermal fluctuations and uniformly heat a

particulate laden gas stream. Particles, e.g., fly-ash in combustion streams,

have size and composition dependent heating times [2].

Compensations can be made for these spatial and temporal heterogeneties

by use of e.g., excess air, staged combustion, increased hot gas furnace

residence times, afterburners, etc. The effectiveness of each of these

approaches to minimization of PCDD and PCDF emissions can in principle be

determined by a carefully designed testing program.

Therefore, the key to minimizing concentrations of chlorinated precursor

compounds in the combustion zone is to assure as nearly so as possible that

they are uniformly mixed with sufficient air and uniformly heated to flame

temperatures.

-26 -
COULD THESE PEIECURSOR COMPOUNDS REFORM AFTER THE COMBUSTION ZONE?

If the PCDD and PCDF precursor compounds are effectively destroyed

(completely oxidized to CO 2

, H

2O and HCl) in the combustion zone, it is not

possible for them to reform at any significant levels after the combustion

zone [2], Any apparent reformation of these compounds would be attributable

to and depend upon the extent of any incomplete combustion.

If the formation of PCDDs and PCDFs were to depend upon the reformation

of precursor compounds, the probability for PCDD and PCDF formation would be

much reduced from the case of PCDD and PCDF formation from pre-existing

precursor compounds [2], This is because the probability for formation of any

compound is generally sensitive to the complex interactions and effects of all

of the pathways of a formation mechanism. Generally, the more complex the

mechanism for compound formation and the more there are opposing characteristics in the mechanism (e.g., destruction pathways or alternate product formation pathways) , the lower the probability that the particular compound will be

formed [2].

Therefore, if the PCDD and PCDF precursor compounds are effectively

destroyed in the combustion zone, it is not possible for them to reform at any

significant levels after the combustion zone.

-27 -
COULD PVC OR OTHER SOURCES OF CHLORINE PLAY A ROLE IN THE FORMATION OF

CHLORINATED PHENOLS OR OTHER ORGANIC COMPOUNDS BY COMBUSTION IN COMBINATION

WITH PAPER ON THE GRATE? IT HAS BEEN SUGGESTED THAT THE LIGNIN IN PAPER WILL

GIVE RISE TO PHENOLS WHICH WILL COMBINE WITH THE CHLORINE.

(a) Post-flame combustion zone environment:

On undergoing incomplete combustion wood and wood components will

decompose to produce many lower weight organic compounds, including substantial amounts of alkyl- and alkoxy- substituted phenols [18-22]. The probability that these unchlorinated phenols will undergo extensive gas phase chlorination by HCl or other chlorine containing species is negligible. This statement can be substantiated by simple worse case modeling [8]. As previously

discussed in this report, the probability that these unchlorinated phenols

will undergo extensive chlorination in the post-flame combustion zone due to

catalytic processes involving fly-ash is likely to be limited by the

complexities of the chlorination reactions. Qualitatively chlorination of

organic compounds has been shown to occur under some laboratory conditions

[3j. The extent of the reported chlorination does not appear to account for

the levels of polychlorinated phenols reported in incinerator emissions.

(b) Condensed phase environment:

Sjostrom [22] has reported that lignin is readily chlorinated. For

example, extensive chlorination through substitution at C-5 and C-6 positions

(and to a lesser extent at the C-1 and C-2 positions) of the gualacyclopropane

units of lignin is possible [22]. Subsequent substitutions, and oxidative or

pyrolytic thermal stressing could then lead to side chain displacement,

cleavage of methoxyl groups and lignin depolymerization to produce lower

-28 -
molecular weight compounds, e.g., chlorophenols [22]. This is evident for

example from the extensive amount of chlorinated lignin products found in

spent bleach liquor from chlorination and alkali extraction stages in the pulp

bleaching process [23-25]. Production of some paper involves these bleaching

processes. Some discarded waste paper may therefore contain significant

amounts of chlorinated organic materials. On heating this may depolymerize

,

potentially resulting in production of chlorophenols. At the present time

this is an untested suggestion.

The nature and sources of other chlorinating agents present in the mass

of municipal waste which is burned on the grates of an MSW fired incinerator

are not precisely known. Kinetic processes which may promote PCDD and PCDF

formation in these instances have not been determined. It has not clearly

been established that HCl is the principal chlorinating agent in municipal

waste combustion. Carefully designed experiments would be required to resolve

these issues

.

As mentioned previously, it appears that PVC in municipal waste streams

accounts for only about 25% of the HCl which is produced and emitted during

the combustion process [15,17]. In municipal waste there are numerous other

possible sources of chlorine which could contribute to production of HCl:

plasticizers, plastic additives, wood preservatives, fire retardants,

solvents, propellants, cutting fluids, bleaching agents, etc. [2], In addition, it has been known since at least 1809 [27] that HCl can be produced from

inorganic chlorine sources, e.g., NaCl. Uchlda and co-workers [14] report

that production of HCl from inorganic chlorine sources in municipal waste can

be extensive. Thus, even if lignin in discarded waste paper is not already

-29 -
partially chlorinated from bleaching processes, formation of precursors, e.g.,

chlorophenols, may be promoted by a virtually ubiquitous availability of

chlorine in the waste stream. Careful studies are needed to test and quantify

these suggestions.

Based on the above observations it is probably advisable to focus upon

minimization of PCDD and PCDF emissions by combustion control and/or postcombustion control technologies rather than attempting to remove sources of

chlorinated materials by presorting municipal waste streams prior to feeding

them into an MSW fired incinerator. Theoretical studies carried out at NBS

suggest that minimization of PCDD and PCDF emissions in principle can be

achieved by combustion control and/or application of post-flame combustion

zone control technologies, without resorting to pre-sorting of municipal waste

streams [2].

Therefore, in or on the grate bed some chlorination of lignin may take

place. The nature and source of the chlorinating agents are not established.

Chlorinated lignin material, when heated, may depolymerlze to produce chlorinated compounds, such as chlorinated phenols. The chlorinated phenols and

other compounds that may be produced could be destroyed by further thermal

stressing.

-30-
COULD IMPROPER OPERATION OF THE INCINERATOR INCREASE THE PRECURSOR

CONCENTRATIONS FOUND IN THE STACK? HOW?

If an MSW fired incinerator combustion furnace does not efficiently

destroy precursor compounds they will be transported from the combustion

furnace to the next sections of the incinerator. These sections may include,

the energy recovery devices, scrubbers, air pollution control units, etc.

These devices can be analyzed in terms of a net transport of combustion

exhaust gases containing fly-ash laden with PCDDs and PCDFs. The extent to

which each of these devices affects the resulting emissions levels of PCDDs

and PCDFs can be suggested theoretically and measured reasonably quantitatively in principle

.

An increase in the mass transport of precursor concentrations to the

stack of an MSW fired incinerator can be due to a combination of incomplete

minimization of PCDD and PCDF formation in the combustion furnace and/or

subsequent failures of control devices to contain or minimize the resulting

emissions. In principle, this problem can be corrected by improving

incinerator performance.

The presence of or an increase of precursor concentrations found in the

stack of an MSW incinerator is not likely to result in significant formations

or increased formations of PCDDs or PCDFs in the stack. There are reasons for

this. First, at stack temperatures, the rate of thermal desorption of chemisorbed PCDDs and PCDFs, chlorophenols and numerous other organic compounds Is

so slow that reactive sites for formation of PCDDs and PCDFs are not available

during the exhaust gas transit time through the stack of the incinerator

[2]. For example, the desorption time for even a small fraction of cheml-

-31 -
sorbed PCDDs and PCDFs at 400 °F (about 200 °C) is many millions of times longer

than the stack transit time [2]. This means that PCDD and PCDF formation

reactions become inoperative in the stack. Second, at stack temperatures,

competitive adsorption involves not only chemisorption but also contributions

due to condensation of high molecular weight species, e.g., polycyclic

aromatic compounds, resulting in multiple layer rather than monolayer coverage

of fly-ash particles in times which are very short compared to stack gas

transit times.

The fact that PCDD and PCDF fly-ash concentrations have been found to be

significantly higher in stack emissions than in fly-ash taken from electrostatic precipitators (ESPs) is probably due to the difference in particle

sizes between ESP ash and stack ash and/or condensation isotherms which favor

a higher fly-ash to gas phase ratio for the partioning of PCDDs and PCDFs in

the stack than in the ESP unit, since the latter operates at a higher temperature.

Therefore, improper operation of the incinerator could increase the

precursor concentrations found in the stack through mass transport of these

compounds when combustion efficiency is low and control devices do not contain

these furnace emissions. In principle, this can be corrected. However, even

if precursor compounds are transported to the stack, no significant PCDD and

PCDF formation is expected to occur in the stack.

-32-
7 . MAJOR CONCLUSIONS

Technical Issues concerned with underlying factors that can potentially

affect the levels of PCDD and PCDF emissions from MSW fired incinerators have

been analyzed. This analysis of the technical issues that have been raised

suggests that:

• Removal of sources of chlorine by pre-sorting of MSW waste

streams prior to incineration is not a reasonable option

compared to attempting combustion control and/or post-combustion

control options for minimizing PCDD and PCDF emissions from MSW

fired incinerators.

® If precursor compounds are transported to the stack of an MSW

fired incinerator, no significant PCDD or PCDF formation is

expected to take place in the stack.

-33-
8. REFERENCES

[ 1 ]

[ 2 ]

[3]

[4]

[5]

[ 6 ]

[7]

[ 8 ]

[9]

[ 10 ]

[ 11 ]

[ 12 ]

[13]

[14]

[15]

[16]

[17]

[18]

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 , NY: Final Report to ASME, ASME, NY, NY, 1980.

Shaub, W.M. , Containment of Dioxin Emissions from Refuse Fired Thermal

Processing Units - Prospects and Technical Issues , NBSIR 84-2872,

National Bureau of Standards, Washington, DC, 1984. And references

contained therein.

G.A. and Rghel, H.O.

,

Envir . Sci

.

& Tech. 16

,

53-56

G.A. and Rghei, H.O., Chemosphere 1L» 833-839 (1982)

0. and Eiceman, G.A.

,

Chemosphere ii. 569-576 (1982)

0. and Eiceman, G.A.

,

Chemosphere 11» 421-426 (1984)

Shaub, W.M. and Tsang, W. , Envir. Sci. & Tech. 17 , 721-730 (1983).

Peri, J.B., J. Phys. Chem. 70, 2937-2945 (1966).

Hair, M.L. and Hertl, W. , J. Phys. Chem. 2070-2075 (1973).

McDaniel, M.P., J. Phys. Chem. 532-537 (1981)

Greenberg, R.R., Zoller, W.H. and Gordon, G.E., Envir. Sci. & Tech. 12 , 566-573 (1978).

Carotti, A. and Smith, R.A. , EPA Report No. SW-18c, 1974.

Brocco, D., Cecinato, A. and Liberti, A., "PCDDs and PCDFs in the

Environment", In: Physico-Chemical Behavior of Atmospheric Pollutants , Versino, B. and Ott, H. , Editors, D. Reidel Pub. Co., Dordrecht, Holland

(1982).

Uchida, S., Kamo, H. and Kanaya, K. , Ind. Eng. Proc. Des. Dev. 22 , 144-149 (1983).

Candreva, F. and Dams, R. , Sci. of Tot. Envir. 17 , 155-163 (1981).

Perl, J.B. and Hensley, A.L., J. Phys. Chem. 72 , 2926-2933 (1968).

Klximb, D. , Res. Rec. & Cons. 225-233 (1976).

Hileman, F.D., Wojcik, L.H. , Futrell, J.H. and Elnhorn, I.N.

,

"Comparison of the Thermal Degradation Products of Alpha-Cellulose and

Douglas Fir Under Inert and Oxidative Environments", In: Thermal Uses

and Properties of Carbohydrates and Lignins , Shafizadeh, F.,

Sarkanen, K. V. and Tillman, D.A. , Editors, Acad. Press, NY, NY (1976).

-34-
[19] Llbertl, A., Goretti, G. and Russo, M.V., Chemosphere 12 , 661-663

(1983).

[20] Harkln, J.M. , "Lignin - A Natural Polymeric Product of Phenol

Oxidation", In: Oxidative Coupling of Phenols , Taylor, W.I. and

Battersby, A.R. , Editors, Marcel Dekker Inc., NY, NY (1967).

[21] Irwin, W.J., Analytical Pyrolysis - A Comprehensive Guide , Marcel Dekker

Inc., NY, NY (1982).

[22] Sjostrom, E., Wood Chemistry - Fundamentals and Applications , Academic

Press, NY, NY (1981).

[23] Pfister, K. and Sjostrom, E. , Pap. Puu. 61 , 619-622 (1979).

[24] SSVL Environmental Care Project No. 7, "Chloride in Recovery Systems",

Final Report, p. 23 (1977).

[25] Harden, H.-L. and Lindgren, B.O. , Part I Comm. Swed. For. Prod. Res.

Lab. Ser. B No. 349 (1975).

[26] Esposito, M.P., Tiernan, T.O and Dryden, F.E. Dioxins, EPA-600/2-80-197

(1980).

[27] Gay-Lussac, J.L. and Thenard, L.J. , Arcueil. Mem. Phys. 339 (1809).

-35-
9. APPENDIX

On internal review the author was advised to consider more explicitly the

possible contribution of buoyantly transported uncombusted fly-ash to PCDD

emissions in the post-flame combustion zone:

WHAT CONTRIBUTIONS TO PCDD EMISSIONS MIGHT BE EXPECTED FROM BUOYANT TRANSPORT

OF UNCOMBUSTED OR PARTIALLY COMBUSTED FLY-ASH TO THE POST-FLAME COMBUSTION

ZONE?

Sjostrom [1] has reported that the lignin content of paper (actually pulp

used for the production of paper) is about 0,5 to 1.0 percent by weight and

can contain about ten percent by weight of chlorine if the paper has been

bleached with a chlorinating agent. Karasek and co-workers [2] have reported

that about two percent by weight of fly-ash appears to be uncombusted or

partially combusted when transported to the post-flame combustion zone.

Using the above Information, a worse case (beyond that which is likely to

occur) analysis of PCDD production from buoyantly transported uncombusted or

partially combusted fly-ash can be made:

Hutzinger and co-workers [3] have studied PCDD production by strong

chlorination of lignin with a source of chlorine (HCl or PVC) in heated quartz

ampoules. They have reported a conversion efficiency (defined as amount of

PCDD formed compared to amount of lignin burned) of lignin to PCDDs of

—9

43.6 X 10 . A typical fly-ash loading in the post-flame combustion zone is

—3 —3

about 3.5 g*M (about 0.1 g*ft ) [4], With these assumptions the worse case

total amount of PCDDs that may be produced in the post-flame combustion zone

-36-
of an incinerator is equal to the product of the fly-ash loading (3.5 g*M )

times the fraction of fly-ash which is uncombusted or partially combusted

(0.02) times the fraction of this which could be lignin (0.01) times the

conversion efficiency of lignin to PCDDs (43.6 x 10 ). Tlie resulting

-2

predicted PCDD level is about 3 x 10 ng»M . In an incinerator environment,

some lignin is lost from the fly-ash due to partial combustion on passage

through the flame combustion zone. Therefore this result is an upper limit

worse case value. Typical PCDD emission levels may range from 10^^ to about

+4 —3

10 ng*M . Therefore, it appears that PCDD production from the transport of

partially combusted fly-ash to the post-flame combustion zone is not at all

sufficient to account for observed PCDD emissions.

Therefore, the contribution to PCDD emissions resulting from buoyant

transport of partially combusted fly-ash to the post-flame combustion zone is

not at all sufficient to account for reported emission levels of PCDDs.

[1] Sjostrom, E. , Wood Chemistry - Fundamentals and Applications , Academic

Press, N.Y., N.Y. (1981).

[2] Clement, R. E. and Karasek, F. W. , J. Chrom. 234 , 395-405 (1982).

[3] Olie, K. , Berg, v. d. M. , and Hutzinger, 0., Chemosphere 12, No. 4/5,

427-636 (1983).

[4] Shaub, W. M. , Containment of Dioxin Emissions From Refuse Fired Thermal

Processing Units - Prospects and Technical Issues , NBSIR 84-2872,

National Bureau of Standards, Washington, D.C. (1984).

-37 -
NBS-114A (REV. 2-80

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBSIR 84-2975

2. Performing Organ. Report No. 3. Publication Date NBS November 1984

4. TITLE AND SUBTITLE

TECHNICAL ISSUES CONCERNED WITH PCDD AND PCDE EOKMATION AND DESTRUCTION

IN MSW FIRED INCINERATORS

5. AUTHOR(S)

DR. WALTER M. SHAUB

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

CENTER FOR FIRE RESEARCH

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

INTERNAL

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State. ZIP)

THE CITY OF NEW YORK DEPARTMENT OF SANITATION

OFFICE OF RESOURCE RECOVERY AND WASTE DISPOSAL PLANNING

51 CHAMBERS STREET, ROOM 8l5

NEW YORK, NEW YORK lOOOT

10. supplementary notes

[321 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document Includes a significant

bibliography or literature survey, mention it here) TECHNICAL ISSUES CONCERNED WITH UNDERLYING FACTORS THAT CAN POTENTIALLY

AFFECT THE LEVELS OF PCDD AND PCDF EMISSIONS FROM MSW FIRED rNCINERATORS

HAVE BEEN ANALYZED. MAJOR CONCLUSIONS OF THIS STUDY ARE THAT PRESORTING

OF MUNICIPAL WASTE PRIOR TO INCINERATION TO REMOVE CHLORINE DOES NOT APPEAR TO BE A REASONABLE OPTION AND THAT PCDD AND PCDF FORMATION IS NOT

LIKELY TO TAKE PLACE IN THE STACK OF AN INCINERATOR.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) catalysis; dioxins; fly-ash; grate bed; incinerator; mxmicipal ^raste;

PCDD; PCDF; pre-sorting; stack emissions

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