Formation and degradation of PCDD/F in

waste incineration ashes

Lisa Lundin

Akademisk avhandling

som med vederbörligt tillstånd av rektorsämbetet vid Umeå Universitet för avläggande av Filosofie Doktorsexamen vid Teknisk-Naturvetenskapliga fakulteten i Umeå, framlägges till offentlig granskning vid Kemiska Institutionen, hörsal N320 i Naturvetarhuset fredagen den 9 november 2007, klockan 13.00. Avhandlingen kommer att försvaras på engelska. Fakultetsopponent: Prof. James A. Mulholland, Georgia Institute of Technology, Atlanta, USA

Department of Chemistry Umeå University

Umeå 2007

UMEÅ UNIVERSITY DOCTORAL DISSERTATION Department of Chemistry SE-901 87 Umeå November 2007 Lisa Lundin

Formation and degradation of PCDD/F in waste incineration ashes ABSTRACT The disposal of combustible wastes by incineration is a controversial issue that is strongly debated by both scientists and environmental activists due to the resulting emissions of noxious compounds, including (inter alia) polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), heavy metals and acid gases like sulfur dioxide. Currently available air pollution control devices are capable of effectively cleaning flue gases, and PCDD/F emissions to air from modern municipal solid waste (MSW) incinerators are low. However, the PCDD and PCDF end up in ash fractions that, in Sweden, are usually deposited in landfills. The European Union has recently set a maximum permitted total concentration of 15 μg TEQ/kg for PCDD/F species in waste. Fly ash from municipal solid waste (MSW) incineration containing PCDD/Fs at concentrations above this limit will have to be remediated to avoid disposing of them in landfills; an expensive and environmentally unfriendly option. Therefore, effective, reliable and cost-effective methods for degrading PCDD/F in fly ash are required, and a better understanding of the behavior of PCDDs and PCDFs during thermal treatment will be needed to develop them. In the studies this thesis is based upon both the formation and degradation of PCDDs and PCDFs in ashes from MSW incineration were studied. The main findings of the investigations regarding PCCD/F formation were: The concentrations of PCDD and PCDF in fly ash increased with reductions in the temperature in

the post-combustion zone. The homologue profile in the ash changed when the temperature in the post-combustion zone

changed. The final amounts of PCDD and PCDF present were affected by their rates of both formation and

degradation, and the mechanisms involved differ between PCDDs and PCDFs. The main findings from the degradation studies were: The chemical composition of ash has a major impact on the degradation potential of PCDD and

PCDF. The presence of oxygen during thermal treatment can enhance the degradation of PCDD and

PCDF. Thermal treatment is a viable option for degrading PCDD and PCDF in ashes from MSW. Shifts in chlorination degree occur during thermal treatment. Rapid heat transfer into the ash is a key factor for ensuring fast degradation of PCDD and PCDF. Degradation of other chlorinated organic compounds, e.g. PCB and HCB, also occurs during

thermal treatment of ash. Reductions in levels of PCDD and PCDF were not solely due to their desorption to the gas phase. Differences between the behavior of 2378-substituted congeners of PCDD and PCDF and the

other congeners during thermal treatment were observed. Differences in isomer patterns of both PCDD and PCDF were observed between the ash and gas

phases after thermal treatment at both 300 and 500 oC. Overall, the results show that the formation and degradation mechanisms of PCDDs differ substantially from those of PCDFs. Thus these groups of compounds should be separately considered in attempts to identify ways to reduce their concentrations. Keywords: MSW, fly ash, ash, PCDD, PCDF, formation, degradation, destruction, dechlorination, thermal treatment, remediation

Language: English ISBN: 978-91-7264-411-3 Number of pages: 59 + 4 Papers

Formation and degradation of PCDD/F in

waste incineration ashes

Lisa Lundin

UMEÅ UNIVERSITY

Department of Chemistry Umeå 2007

Department of Chemistry UMEÅ UNIVERSITY SE-901 87 Umeå, SWEDEN

Copyright © 2007 by Lisa Lundin ISBN 978-91-7264-411-3 Printed in Sweden by VMC, KBC, Umeå University, Umeå 2007

i

CONTENTS

LIST OF PAPERS .......................................................................................... ii

ABBREVIATIONS......................................................................................... iii

SUMMARY IN SWEDISH...............................................................................iv 1. Introduction ......................................................................1

Objectives..............................................................................................3 2. Combustion.......................................................................4

Combustion techniques for MSW...........................................................6 Residues after MSW incineration...........................................................8

3. Fly Ash..............................................................................9 4. Experimental ...................................................................13

Analysis of PCDD/F, PCB and HCB.....................................................24 5. Formation of PCDD and PCDF.........................................26

Formation in the combustion process..................................................27 Formation during thermal treatment of ash.........................................32

6. Degradation of PCDD and PCDF ......................................34

The effects of ash composition during thermal treatment.....................34 PCDD vs PCDF....................................................................................37 Degradation experiments with large-volume samples...........................40 Dechlorination or destruction? ............................................................42 Degradation of other chlorinated organic compounds ..........................43 Desorption of PCDD and PCDF............................................................44

7. Concluding remarks ........................................................49

ACKNOWLEDGMENTS ............................................................................... 51 8. References.......................................................................53

ii

LIST OF PAPERS

This thesis is based on the following Papers, which are referred to in the text by their respective Roman numerals. In addition, for convenience, the experiments and analyses described in Papers I-IV are sometimes described as Studies I-IV, respectively. I. Lisa Lundin, Stellan Marklund. (2007). Distribution of mono-

to octa-chlorinated PCDD/F in fly ashes from a municipal solid waste incinerator. Submitted to Environ. Sci. Technol.

II. Lisa Lundin, Stellan Marklund. (2005). Thermal degradation of

PCDD/F in municipal solid waste ashes in sealed glass ampules. Environ. Sci. Technol. 39:3872-3877

III. Lisa Lundin, Stellan Marklund. (2007). Thermal degradation of

PCDD/F, PCB and HCB in municipal solid waste ash. Chemosphere. 67:474-481

IV. Lisa Lundin, Johanna Aurell, Stellan Marklund. (2007). Fate of

PCDD and PCDF during thermal treatment of waste incineration ash. Manuscript.

Reprinted Papers are presented with the kind permission of the publishers.

iii

ABBREVIATIONS APC Air pollution control ASE Accelerated solvent extraction BFB Bubbling fluidized bed CFB Circulating fluidized bed DD Dibenzo-p-dioxin (non-chlorinated) DF Dibenzofuran (non-chlorinated) D.S. Dry substance FBC Fluidized bed combustor GC Gas chromatography HCB Hexa-chlorinated benzene IS Internal standard I-TEQ International toxic equivalents LOI Loss on ignition MS Mass spectrometry MSW Municipal solid waste MSWI Municipal solid waste incineration PAH Polycyclic aromatic hydrocarbon PCB Polychlorinated biphenyl PCBz Polychlorinated benzene PCDD Polychlorinated dibenzo-p-dioxin PCDF Polychlorinated dibenzofurans PIC Products of incomplete combustion PLE Pressurized liquid extraction RS Recovery standard SEM Scanning electron microscopy WHO-TEQ Toxic equivalence according to the World Health

Organization Congener A general term used to describe individual members

of a class of a compound Isomers Compounds with identical molecular formula but

differing in structural formula Homologue A group of organic compounds in which the

members differ in position; herein, the number of chlorines.

Pattern The relative abundance of the individual isomers in a group of homologues

Profile The relative abundance of the different homologue groups in a series.

iv

SUMMARY IN SWEDISH Förbränning av hushållsavfall är ett kontroversiellt ämne som engagerat både forskare och miljöaktivister sedan forskare i slutet av 1970-talet hittade två grupper av organiska föreningar, polyklorerade dibenso-p-dioxiner (PCDD) och polyklorerade dibensofuraner (PCDF), i både rökgaser och aska från förbränningsanläggningar där hushållsavfall förbrändes. PCDD och PCDF benämns i vardagligt tal som dioxiner. Avfallsförbränning är idag en viktig del i kretsloppet då den kan avlägsna alla organiska miljögifter som finns lagrade i våra konsumtionsprodukter. Dagens materialåtervinnings metoder klarar inte av att avlägsna dessa miljöstörande ämnen. Genom att förbränna avfall tar vi dessutom tillvara en energiresurs samtidigt som volymen av avfallet minskas. När avfall förbränns bildas det en mängd olika organiska föreningar när rökgaserna kyls av. Dessa ämnen, där ibland dioxiner, renas bort från rökgaserna väldigt effektivt och hamnar i de restprodukter som uppstår vid avfallsförbränning istället. Restprodukterna som är bottenaska, flyaska och rökgasreningsprodukter deponeras i stor utsträckning idag på grund av höga halter av olika salter, tungmetaller och dioxiner. Augusti 2006 infördes nya regler i EU för hur mycket dioxiner avfall får innehålla, dit räknas askor från förbränning av hushållsavfall. Gränsvärdet är 15 μg TEQ/kg och avfall som innehåller en större mängd dioxiner än det måste behandlas för att minska dioxin mängden. Det här innebär att behovet av en enkel och kostnadseffektiv behandlingsmetod behöver tas fram och för att kunna göra det behöver dioxinernas beteende vid termisk behandling studeras, vilket har gjorts i denna avhandling. En kartläggning av dioxiner i flygaska från en fullskaleanläggning visade att halten dioxiner i flygaskan ökar när rökgaserna kyls av plus att de lågklorerade (mono- till tri-) dioxinerna var de dominerande kongenerna i flygaskan oavsett temperatur. Flera olika bildningsmekanismer (de novo syntes, bildning från precursorer, klorering och även deklorering av dioxiner) sker parallellt och ger upphov till det specifika kongenmöster som återfinns i aska från förbränning av hushållsavfall. Termisk behandling av aska har visat sig vara en effektiv metod att reducera mängden dioxiner men för att kunna utveckla denna teknik så krävs mer kunskap om hur dioxiner beter sig vid termisk behandling. Resultaten som redovisas i denna avhandling visar att askans sammansättning spelar en avgörande roll för vilka förhållanden som krävs för att få en reducering av mängden dioxiner. Ett högt innehåll av kalk (som ger askan ett högt pH-värde), koppar, järn och svavel visade sig vara gynnsamt för en hög nedbrytning av dioxiner. Det visade sig också att dioxiner och furaner beter sig olika vid termisk behandling. De toxiska kongenerna visade sig vara mer eller mindre utsatta för nedbrytning beroende på askan sammansättning. Det har också visats att reduktionen av mängden dioxiner i aska som behandlas termiskt beror på nedbrytning och/eller av att molekylerna tappar klor och inte av att dioxinerna avdunstar till omgivande atmosfär.

1

1. Introduction Waste combustion is not a new activity; plants for incinerating waste were

built in England and Germany at the end of the 19th century. The first

Swedish waste incineration plant was constructed at the beginning of the

20th century, but it was not until the 1960s that incineration became more

commonly used for waste disposal. Today, incineration is a very important

method of treating municipal solid waste (MSW) in Sweden. In 2006,

46.8% of the total amount of MSW produced in Sweden was incinerated

(1).

The pros and cons of thermal waste utilization have been, and still are,

strongly debated by both scientists and environmental activists, due to the undesirable emissions it generates of compounds like polychlorinated

dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), heavy metals and

acid gases like sulfur dioxide. The volume and mass of MSW can be

reduced by combustion by up to 90% and 80%, respectively. However,

residues in the form of ashes are produced that are considered to be of

environmental concern. Emissions of acidic gases and heavy metals, as

well as hazardous volatile organics, to the atmosphere are also drawbacks

of the incineration technique if the flue gases are not well cleaned.

However, the incineration of MSW also has several advantages (in addition

to reducing its mass and volume) including reducing its toxicity and the

scope it provides for recovering exothermic energy.

Two groups of organic compounds of greatest concern are the

polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated

dibenzofurans (PCDF), commonly referred to collectively as “dioxins”,

which were first shown to be both released in flue gases and retained in

the solid residues of MSW incinerators in 1977 (2). Dioxins have never

been commercially manufactured or found to be of any benefit or known

2

use, but they can still be found almost everywhere in the global environment (3). According to Rappe (4), profiles found in environmental

samples are typical combustion profiles, indicating that various

combustion processes are the major ultimate sources for this worldwide

contamination. Incineration of MSW has been suggested to be the most

serious contributor to dioxin emissions, judging from the high dioxin

concentrations in flue gases and fly ash discharged from incinerators. With

better knowledge on the formation of dioxins, the highest concentrations

are now found in the ashes and the products from the air pollution control

devices (APCD). Following the acquisition and application of better

knowledge regarding the formation of dioxins, the amounts emitted to the

atmosphere from MSW incinerators have declined, and the highest

concentrations are now found in the ashes and products from the APCD.

In the past the major pathway for the dioxins into the environment was

their emission to the air. However, since cleaning of flue gases has

improved over the years, emissions to terrestrial and aqueous

environments via leaching from uncontrolled landfills containing fly ash

and APC residues have become the major sources to the environment, and

to some extent even from controlled landfills. Several studies have shown

that dioxins can be transported from landfills in a colloidal form following

mobilization by surfactants or water-soluble organic solvents (5-9).

3

Objectives Briefly, the objectives of the work outlined in this thesis were to investigate

the formation and destruction of PCDDs and PCDFs in ashes produced

during the incineration of municipal solid waste and to find a viable, cost-

effective method for detoxifying ashes to avoid expensive landfilling. The

following aspects were investigated in detail:

The distribution of mono- to octa-chlorinated PCDD/F in fly ashes at

different temperatures in an incineration plant (Paper I)

The importance of the temperature, treatment time, atmosphere and

ash composition for the destruction of tetra- to octa-chlorinated

PCDD/F in a closed system (Paper II) The destruction of mono- to octa-chlorinated PCDD/F in larger

volumes of fly ash than those previously tested in a closed system

(Paper III) The fate of mono- to octa-chlorinated PCDD/F in waste incineration

ashes during thermal treatment (Paper IV)

4

2. Combustion Combustion is an exothermic process in which combustible substances

(e.g. carbon and hydrogen) is oxidized to CO2 and H2O. In complete

combustion all the combustible material is converted to CO2 and H2O. For

this, there are four basic requirements:

a supply of air for complete oxidation

sufficiently high temperatures for the reactions to proceed

sufficiently quickly

a sufficiently long residence time at high temperature

sufficient mixing (turbulence) of fuel components and air

Combustion is a complex chemical process involving radical reaction

sequences in which carbon, hydrogen, sulfur and fuel-bound nitrogen are

oxidized to CO2, H2O, SO2 and various oxides of nitrogen. MSW contains

highly volatile, reactive hydrocarbon solids, which are first dried and

heated to reaction temperature. In combustion processes, two important

steps occur in the solid phase (10): pyrolysis, in which the solids undergo

devolatilization reactions to yield volatiles (gases and tars) and a solid

fraction (char); followed by combustion per se, in which the char

undergoes heterogeneous reactions to yield gaseous products and an inert

residue (ash).

Since municipal solid waste (MSW) is a very heterogeneous fuel and

consists of a lot of other elements than carbon and hydrogen, e.g.

incombustible ash forming matter and trace elements, which disturbs the

combustion, complete combustion is impossible to achieve. In addition,

accumulation of ash on boiler tubes causes additional formation of organic

compounds like PCDD/F. These “memory effects” are thought to be the

major reason for formation of PCDD/F in full scale incineration plants

(11).

5

In municipal waste combustion luminous, yellowish, sooting flames

predominate, mainly due to the high volatile content; the particulate

emissions consist essentially of soot and ash particles. The very high rates

of dioxin formation in MSW incineration are due to the highly sooting

flames, high metal contents and unsteady-state combustion of MSW (a

common feature of its incineration, due to the extreme heterogeneity of the

feed).

Combustion of MSW results in flue gases and solid residues that contain

inorganic and organic pollutants (for example: Hg, SOx, NOx, HCl,

PCDD/F, PCB, PCBz, PCPh and PAH) because the fuel contains sulfur,

nitrogen and, chlorine, leading in conjunction with the incomplete

combustion to the production of new compounds, products of incomplete

combustion (PIC). Flue gases must therefore be cleaned, and the solid

residues deposited in landfills. The control of dioxin emissions from the

combustion of MSW has focused on the flue gases, and the solid residues

have not been considered. However, measures to decrease dioxin contents

in the residue are also essential for reducing the total release of dioxin

from MSW incinerators. Reductions in residues will greatly lower the total

amounts released because dioxins are more concentrated in the fly ash

than in any other stream from a MSW incinerator.

Techniques for pre-treating MSW to improve its combustion parameters

have been introduced in Japan, involving crushing, drying, screening and

molding it, to produce “refuse-derived fuel” (RDF), which is a more

uniform, transportable and storable fuel with a higher volatile fraction

than the initial MSW. CaO or Ca(OH)2 are added to the RDF during its

processing, as a binder, and it has been subsequently shown that the

calcium compounds in RDF can reduce the emissions of HCl in the flue

gas. However, despite these advances, further improvements, and greater

knowledge of the processes involved in MSW combustion, are still required.

6

Combustion techniques for MSW Two main types of plants used for incinerating MSW are fluidized bed

combustors (FBCs) and mass burn combustors. The latter, which are most

the commonly used type in Europe, are generally grate-fired (Figure 1). In

these combustors fuel is fed onto grates, sometimes movable, combustion

air is introduced in excess from below the grate, and the incineration

temperature is usually between 1000 and 1100oC. Grate-fired incinerators

are highly tolerant of variations in the quality of the fuel, which is probably

one of the main reasons for their popularity in the industry.

Figure 1. Schematic diagram of a grate-fired Mass burn combustor

7

Figure 2. Schematic diagram of a fluidized bed combustor with a circulating bed (CFB) In FBCs the fuel is introduced into and combusted in a bed of inert

material, such as quartz sand or limestone particles. The heat is

transferred directly through conduction between the sand and the walls of

the combustion zone. This is a more effective way to transfer heat,

compared to the convection used in grate-fired combustors, and thus a

lower combustion temperature, 850oC, is sufficient for fuel burn out. There

are two main kinds of FBCs; bubbling bed (BFBs) and circulating bed

(CFBs) (Figure 2). In both cases air is introduced at the bottom of the

vessel and the sand then acts as a fluid. However, in a CFB the velocity of

the air is high enough to cause some of the sand to be carried by the flue

gases up in the incinerator, then the sand and other heavy particles are

subsequently separated in cyclones, while in a BFB the velocity of the air

is lower, so practically no sand is carried up with the flue gases.

8

The advantages of fluidized bed combustion are their very even and low

combustion temperatures, low pollution-generating combustion and

flexibility for handling many kinds of wastes with low calorific values as

fuels.

Residues after MSW incineration As mentioned above, incineration of MSW reduces its volume and weight

by up to 90% and 80%, respectively, leaving residues in the form of ashes

and flue gas cleaning products. Bottom ash is the main ash fraction in a

mass burn combustor, accounting for about 80% of the total ash flow,

whereas fly ash is the main ash fraction in fluidized bed combustors,

accounting for about 90% of the total ash flow (12). In Sweden, mass burn

combustors are the most commonly used types of plants for incinerating

MSW, and bottom ash is, therefore, the predominant fraction. The

remainder consists of fly ash, flue gas cleaning products and mixtures of

various ash fractions. Bottom ash is almost free of organic compounds (13)

even in old combustors. The small amounts of PCDD/F present in bottom

ash have been shown to be associated with particles with <0.21mm

diameters (14) and it is considered to be of minor environmental concern.

However, fly ash and flue gas cleaning products contain significant

quantities of toxic organic compounds (13), collectively known as

persistent organic pollutants (POPs), which include polychlorinated

dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). The

concentrations of PCDD/F in fly ash and APC-residues vary with the fuel,

combustion conditions and design of the combustion plant. Between 80%

and 95% of the total amount of PCDD/F can be adsorbed on the fly ashes

(15,16) , depending on the temperature and air pollution control devices

used. The generation of fly ash depends on the amount of dust originally

released from the fuel bed, the velocity of flue gas inside the boiler and the

combustion mode. Air pollution control residues consist of various kinds of

ash depending on the system used, and can include fly ash collected in a

9

cyclone, dry and/or semi-dry scrubber residues, and particulates from

fabric filters.

3. Fly Ash Definition: fly ash is the collective term for the inorganic particulates (with

a small content of organic matter) entrained in the combustion gases,

which are either carried over from the primary combustion chamber or

formed during cooling of combustion gases.

Definition: soot is the collective term for the organic particulates (with a

small content of inorganic matter) entrained in the combustion gases,

which are formed and carried over from the primary combustion chamber.

Further coagulation can occur during cooling of combustion gases.

The physical and chemical characteristics of fly ash (and solid residues in

general) depends, inter alia, on the composition of the fuel, types of

incinerator and air pollution control devices (APCD) used, and operating

conditions.

Further complications are that soot particles are present in the flue gases

in addition to the fly ash, and aerosols (defined as solid particles or liquid

droplets, ranging in particle size from 0.001 to over 100 μm suspended in a

gas). However, aerosols will not be considered in this thesis.

Usually fly ash and soot are jointly denoted fly ash, but strictly they have

different origins. Soot mainly consists of organic particles with very high

carbon contents, formed from gas phase combustion reactions, which are

roughly spherical with diameters ranging from 0.005 to 0.25 μm. Soot

formation in gas phase reactions is a complex process, but is believed to

10

proceed via three steps: nucleation, surface condensation and particle

growth, and coagulation (17). Fly ash is formed from the high-temperature

transformation of incombustible matter present in the fuel and has particle

sizes ranging from about 1 to 100 μm and high mineral matter contents.

The processes involved in fly ash formation are: vaporization, melting,

crystallization, virtification, condensation and precipitation, which occur

during combustion and treatment of the flue gases (18). The proportions of

soot and ash particles depend on the fuel type and combustion conditions.

The higher the volatile content is in the fuel, the higher is the sooting

tendency. In a sooting flame the soot particles can serve as sources for de

novo synthesis of dioxins due to their degenerated graphitic structures,

whereas in a non-sooting flame the particulate emissions mainly consist of mineral matter or ash particles originating from the fragmentation and

entrainment of char and the vaporization and condensation of mineral

matter. The residual carbon in ash is in an activated state, but most of the

carbon is likely to be bonded to metal or oxygen, or in aliphatic

hydrocarbons, and does not have the degenerated graphitic structure that

provides a source for de novo synthesis of dioxins (19).

Analyses of MSWI solid residues from APCDs have shown that they consist

of calcium, chlorine, silicon, and aluminum as major elements in addition

to minor amounts of carbon, sulfur, sodium, potassium, iron, titanium,

magnesium, fluorine, and phosphorous (20). The inorganic elements like

metals are found in different ashes due to their volatility. The fate of

inorganic elements present in MSW varies substantially depending on

various factors. The fate of metals is of particular significance because they

can strongly affect both the formation and degradation/destruction of

PCDD/F in ashes, since they can act as catalysts (16,21-23). Metals can be

divided into several classes depending on their volatility, as illustrated in

(Figure 3), which is based on data acquired in both theoretical modeling

11

and experimental studies (24,25). Generally, chloride-forming elements are

mostly found in the fly ash, whereas oxide-forming elements remain in the

bottom ash. According to chemical equilibrium model calculations the

presence of both S and Cl influences the volatility of several heavy metals

(26), which could partly explain why varying amounts of PCDD/F are

formed during the incineration of MSW.

Figure 3. Distribution of elements from MSW incineration depending on their volatility. Adapted from (27). Elements with high boiling points and low volatility tend to remain in the

bottom ash and grate siftings, while more volatile elements with low boiling

points are generally evaporated, and when the temperature of the flue

gases decreases they condense on fly ash particles and are captured as

residues in the APCDs, such as Electrostatic Precipitator (ESP) ashes and

filter ashes (28). Si and Fe tend to be predominantly found in the bottom

ash and grate siftings, while Na and K largely occur in the fly ash.

Morphological analysis of various MSW ashes by SEM has found a wide

range of shapes in the particles, including flakes, prisms, needles, spheres

Class IV Volatile elements emitted in vapour form Class III Elements enriched in fly ash Class II Elements distributed equally between fly ash and bottom ash Class I Elements that are mainly retained in bottom ash

Al Ca Fe K Mg Na Si

Hg Br Cl F

B Se I

As Cd Ge K Na Pb Sb Sn Ti Zn

Ba Co Cs Cu Mo Sr W Ni

Ca Cr Fe Mg Mn Zr

Incr

easi

ng v

olat

ility

12

and sintered agglomerations of dust (20). Char and agglomerated spheres

have also been observed (29).

The speciation of metals has been investigated by a number of research

groups (27,30). However the relationship between metals and the

formation of PCDD and PCDF has not been intensively investigated,

although attempts have been made to correlate variations in the speciation

of copper in fly ashes and the amounts of PCDD and PCDF formed.

13

4. Experimental

In all of the studies that this thesis is based upon, fly ashes from full-scale

municipal waste incinerators in Sweden were used. Their origin and

characteristics are presented below together with a description of the

experimental set up.

Paper I: Distribution of Mono- to Octa-chlorinated PCDD/Fs in Fly Ash from a Municipal Solid Waste Incinerator

The fly ash used in Study I was from the Dåva combined heat and power

plant, Umeå Energi, Umeå, Sweden, sampled at three points (Figure 4)

under normal combustion conditions. Ash I-A was obtained from a point

where the temperature of the flue gases exceeded 700 oC, ash I-B from a

cooler zone (300 - 400 oC), and ash I-C from a textile filter with a working

temperature of approximately 170 oC.

Figure 4. Schematic diagram showing ash sampling locations (Paper I)

14

Grab samples were taken at the locations marked in Figure 4 and analyzed

for mono- to octa-chlorinated PCDD/F in order to map the concentrations

and distributions of mono- to octa-chlorinated PCDD/F isomers in fly

ashes collected from three points in a full-scale MSW incinerator as they

cooled from 700 oC to 170 oC. The three ashes were analyzed in triplicate.

Two samples from each ash were extracted by Soxhlet and the third by

PLE.

The PCDD/F profiles of the three ashes are presented in Figure 5 and their

characteristics in Table 1.

0

2

4

6

8

10

12

14

16

18

20

MoCDD

DiCDD

TriCDD

TeCDD

PeCDD

HxCDD

HpCDD

OCDD

MoCDF

DiCDF

TriCDF

TeCDF

PeCDF

HxCDF

HpCDF

OCDF

Con

cent

ratio

n of

PC

DD

[ng/

g]

0

10

20

30

40

50

60

Con

cent

ratio

n of

PC

DF

[ng/

g]

PCDD I-A PCDD I-B PCDD I-C PCDF I-A PCDF I-B PCDF I-C Figure 5. Homologue profiles of PCDD and PCDF for ashes I-A, I-B and I-C used in Paper I.

15

Table 1. Characteristics of the three ashes used in Paper I. Ash I-A Ash I-B Ash I-C T (oC) > 700 400-300 ~170 Dry substancea 99.8 99.5 98.8 CaOb 19 22.1 28.2 Na2Ob 3.18 2.87 3.31 Fe2O3

b 5.56 4.74 2.88 SiO2

b 39.4 33.6 21.1 LOI 3.8 8.2 15 Cuc 1160 792 917 Cdc 25.4 43.6 142 Snc 114 75.8 181 Nic 259 162 113 Pbc 1170 1180 2540 Znc 9940 1170 21300 Sc 26500 21300 28100 a %, b % of dry substance, c mg/kg of dry substance

Paper II: Thermal degradation of PCDD/F in municipal solid waste ashes in sealed glass ampules.

The three ashes used in Paper II originated from two different plants, one

with a fluidized bed (ashes II-A and II-C) and one with an old grate

incinerator (ash II-B). The fluidized bed was a 15 MW unit of the bubbling

type (BFB), built in the middle of the 1980s. The flue gas cleaning system

consisted of a cyclone and a textile filter. Ashes II-A and II-C were obtained

in October 2000. Ash II-A is a filter ash and was taken from the apparatus

after filtration, while ash II-C was taken from the cyclone prior to filtration.

Ash II-B was obtained in January 1986 from a 20 MW grate incinerator

built in 1984 and was taken from the electrostatic precipitator. The only

flue gas-cleaning used at this time was an electrostatic precipitator

working at 150 oC, and the process used no additives. The purpose was to

investigate the degradation behavior of PCDD/F during thermal treatment

in ashes with different composition.

16

Profiles of the PCDD/F content of all three ashes (II-A, II-B and II-C) can be

seen in Figure 6 and the characteristics of the ashes in Table 2.

Two of the most important differences between the ashes were in the

amounts and types of carbon present in them. Carbon present in the form

carbonate is not available for formation of PCDD/F. Another parameter

that has been shown to be important is the pH, which also differed

between the ashes. Other parameters that differed substantially between

the ashes were their S and Cl contents.

0

10

20

30

40

50

60

70

80

TeCDD PeCDD HxCDD HpCDD OCDD TeCDF PeCDF HxCDF HpCDF OCDF

Con

cent

ratio

n of

Ash

II-B

[ng/

g]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Con

cent

ratio

n of

Ash

II-A

and

II-C

[ng/

g]

Ash II-B Ash II-A Ash II-C Figure 6. PCDD/F profiles of untreated ashes II-B (left y-axis), II-A and II-C (right y-axis). The concentration of OCDD in ash B was 11 ng/g ash. (Paper II)

17

Table 2. Characteristics of the three ashes used in Study II. Ash II-A Ash II-B Ash II-C Facility BFB Grate BFB Type of ash Filter ash ESP ash Cyclone ash Dry substance (%) 99.5 98.1 99.8 Ca 1.9 11.1 0.58 Carbonatea 1.9 0.86 0.52 CaOb 10.4 11.7 17 Na2Ob 30.6 3.8 5.6 Fe2O3

b 2.0 5.5 4.3 SiO2

b 6.6 27.7 41.4 Clb 22 5.8 1.7 Cuc 9470 1010 3840 Cdc 98.3 184 14.3 Snc 207 164 73 Nic 60 158 159 Pbc 6800 7720 1140 Znc 8930 23600 5930 Sc 35500 11700 7930 pH 11.5 7.5 11.5 a % by weight, b % of dry substance, c mg/kg of dry substance The investigations reported in Paper II were divided into three parts: a

screening study, the main study and finally four additional experiments

(Table 3) in which the three fly ashes were used. In the screening study

(Experiments 1-6, single samples) ash II-A was used, and the effects of

temperature (150, 300 and 450oC) and atmosphere (nitrogen or air), with a

constant residence time of 2h were investigated. Based on the screening

study, in the main study (Experiments 7-25, mean values from two

samples presented, with one exception), ashes II-B and II-C were used

where the influence of temperature (150 and 300oC), residence time (1 and

2 h) and atmosphere (nitrogen or air) was investigated. Additional

experiments (Nos. 26-29, single samples) were performed on ash II-B,

which was heated to 450 and 600oC under atmospheres of nitrogen and air

for 1 h (Table 3).

18

Approximately 3 g of ash in soda glass ampoules (volume, 25 ml) were

used in all experiments except those at 600oC, in which quartz glass

ampoules were used. The ampoules were sealed with atmospheres of either

nitrogen or air. Once the predetermined temperature (150, 300, 450 or

600oC) was reached in the furnace, a sealed glass ampoule was placed in

the furnace separately, and treated under conditions shown in Table 3.

After the thermal treatment, the glass ampoule was left to cool down to

room temperature without any quenching.

19

Table 3. Experimental matrix showing the ashes and conditions - temperature (T, oC), residence time (t, hours) and atmosphere (N2 or air) – used in each experiment (Paper II). Study Ash T [oC] t [h] atmosphere No of samples 1 Screening II-A 150 2 N2 1 2 Screening II-A 150 2 Air 1 3 Screening II-A 300 2 N2 1 4 Screening II-A 300 2 Air 1 5 Screening II-A 450 2 N2 1 6 Screening II-A 450 2 Air 1 7 Main II-B 150 1 N2 2 8 Main II-B 300 1 N2 2 9 Main II-B 150 2 N2 2 10 Main II-B 300 2 N2 2 11 Main II-B 150 1 Air 2 12 Main II-B 300 1 Air 2 13 Main II-B 150 2 Air 2 14 Main II-B 300 2 Air 2 15 Main II-B 225 1.5 N2 2 16 Main II-B 225 1.5 N2 2 17 Main II-B 225 1.5 N2 2 18 Main II-C 150 1 Air 2 19 Main II-C 300 1 Air 2 20 Main II-C 150 2 Air 2 21 Main II-C 300 2 Air 2 22 Main II-C 150 1 N2 2 23 Main II-C 300 1 N2 2 24 Main II-C 150 2 N2 2 25 Main II-C 300 2 N2 2 26 Additional II-B 450 1 N2 1 27 Additional II-B 450 1 Air 1 28 Additional II-B 600 1 N2 1 29 Additional II-B 600 1 Air 1

20

Paper III: Thermal degradation of PCDD/F, PCB and HCB in municipal solid waste ash A filter ash from a MSW mass burning plant in Sweden was used in Study

III. The plant has an incinerator with an effect of 65 MW and capacity of

20 tonnes h-1. NH3 is added to decrease NOx levels, and active carbon and

Ca(OH)2 are added before the fabric filter. The profiles of PCDD/F in the

filter ashes investigated in Study III are shown in Figure 7.

0

1

2

3

4

5

6

7

MoCDD

DiCDD

TriCDD

TeCDD

PeCDD

HxCDD

HpCDD

OCDD

MoCDF

DiCDF

TriCDF

TeCDF

PeCDF

HxCDF

HpCDF

OCDF

Con

cent

ratio

n of

PC

DD

[ng/

g]

0

10

20

30

40

50

60

70

80

90

Con

cent

ratio

n of

PC

DF

[ng/

g]

PCDD PCDF Figure 7. PCDD and PCDF profiles in the filter ash examined in Study III. The filter ash used in Study III was also analyzed for its concentrations of

inorganic compounds, which are presented in Table 4. Table 4. Characteristics of the filter ash used in Study III. Conc in Filter ash Conc in Filter ash DSa 99.2 Cuc 1270 Cb 4.69 Cdc 191 CaOb 25.2 Snc 147 Na2Ob 4.61 Nic 97.6 Fe2O3

b 3.08 Pbc 3420 SiO2

b 18.2 Znc 37800 LOI 16.3 Sc 44000 a % by weight, b % of dry substance, c mg/kg of dry substance

21

In this study, the temperature required for optimal destruction of

PCDD/Fs in the filter ash used in the experiments was initially determined, by placing a series of samples from the same batch of filter

ash, each weighing approximately 3 g, in sealed glass ampoules, then

thermally treating them for 1 hour at the following temperatures: 300, 365

and 450 oC.

From the results of these experiments an average temperature of 500 oC

(range 470-530 oC) was selected for a series of thermal degradation

experiments, in which ash samples weighing approximately 130 g were

thermally treated in a stainless steel pipe (Figure 8) with a maximum

volume capacity of 200 ml. A Lyngbyovnen furnace of type S 90 (3x380

volts, 9 KW) was used for these experiments. For each of the thermal

treatment times selected (60, 90, 120, 240 and 480 min), two experimental

runs were carried out. At the end of each treatment, the pipe containing

the ash sample was cooled by immersion in water. One sample subjected to thermal treatment for each selected time was quenched by pouring it

into in a glass jar containing water, to minimize the time the temperature

of the ash was in the 400 - 200 oC interval as it cooled and thus minimize

formation of PCDD/F. The quenched ashes were dried overnight.

Figure 8. Stainless steel pipe used in Study III.

22

Paper IV: Desorption of non- to octa-chlorinated PCDD/F from municipal solid waste ash during thermal treatment The ash used in Study IV came from the same batch of ash used in Study III. The profiles of PCDD/F in the filter ashes investigated in Study IV are

shown in Figure 9.

0

1

2

3

4

5

6

7

8

MoCDD

DiCDD

TriCDD

TeCDD

PeCDD

HxCDD

HpCDD

OCDD

MoCDF

DiCDF

TriCDF

TeCDF

PeCDF

HxCDF

HpCDF

OCDF

Con

cent

ratio

n of

PC

DD

[ng/

g]

0

10

20

30

40

50

60

70

Con

cent

ratio

n of

PC

DF

[ng/

g]

PCDD PCDF Figure 9. PCDD and PCDF profiles in the filter ash used in Study IV.

Fly ash was heated in a stainless steel reactor with a diameter of 75 mm

and length of 750 mm (Figure 10). A crucible with approximately 10 g ash,

supported by quartz glass wool, was placed in the middle and heated to

300 or 500 oC in air. When the desired temperature, 300 or 500 oC, was

reached (after approximately 10 and 15 min respectively), it was

maintained for a further 30 min. Measurements of the temperature in the

gas phase during the experiments showed that the desired temperature

was maintained within ±5 oC.

23

Figure 10. Schematic diagram of the experimental rig. 1, Glass bottom; 2, Stainless steel tube; 3, Gas

inlet; 4, Glass top; 5, Heating jacket; 6, Crucible; 7, Thermocouple; 8, Gas out let to sampling

equipment. Following each thermal treatment the gas phase was sampled by purging

the reactor with preheated, instrument-quality argon (to avoid formation of

PCDD/F during the sampling) at a flow rate of ca. 15 l/min, which was

sampled with the cooled probe polyurethane foam plug sampling technique

according to the standard method EN 1948:1-3. Following this sampling

operation, the crucible was quickly removed from the reactor and left to

cool to room temperature. The reactor and crucible were rinsed with water

24

to remove any PCDD/F that may have adsorbed to the walls, and the

washings were combined with the gas phase sample by adding it to the

condensate collection flask. The reactor and crucible were heated to 550 oC

for 2h between experiments and rinsed with toluene and acetone to remove

all traces of PCCD/Fs formed in previous experiments. Experiments were

performed in triplicate with one blank for each temperature treatment, but

data from only two samples subjected to the 500 oC treatment were used,

due to analytical problems. The ash quantities and sampled gas volumes

are shown in Table 5.

Table 5. Experimental variations in Study IV. Experiment T (oC) Amount of ash (g) Gas phase volume (m3) 1 300 10.34 0.153 2 300 9.97 0.139 3 300 10.68 0.160 4 500 10.46 0.146 5 500 10.51 0.154 6 500 10.63 0.161

Analysis of PCDD/F, PCB and HCB For a more detailed description see Liljelind et al. (31).

Analysis of PCDD/F followed EU standard methods EN 1948:1-3 with

small differences in the labeled standards used and the use of an SPE disc

(for flue gas sample extraction) (32).

The organic compounds of interest were extracted from the ash samples

using a Soxhlet-Dean-Stark extractor. For this, approximately 3 g of ash

was mixed with 20 g of toluene-washed sand, treated with acetic acid, and

then extracted over 48 h with two portions of toluene (400 ml in total).

Labeled internal standards for the organic compounds of interest were

added before the addition of acetic acid. In Studies I, III and IV two

additional congeners for the PCDD/F RS were used; 1,2,3,7,8,9-HxCDF

25

and 1,2,3,4,7,8,9-HpCDF. The extract was concentrated and divided into

three portions (50:25:25), with the exception for Study IV, where the hole

extract was used.

In Paper I extractions of the ashes was also performed with PLE using an

ASE 200® system (Dionex, Sunnyvale, CA, USA) equipped with 10 ml

stainless steel cells according to EPA standard method 3545A). The

procedure for the ASE extractions was as follows: each cell was filled with

approximately 4 g of ash and topped up with Na2SO4 before being sealed.

The cell was then heated to a pre-set temperature of 150 oC for 7 minutes

while a solvent mixture of glacial acetic acid in toluene (5% v/v) was

pumped through it. After three of these extraction cycles, the extract

obtained was concentrated and 13C-labelled internal standard was added,

and it was divided into three portions (50:25:25).

Subsequent analysis revealed that the ASE system is as effective as

extraction with Soxhlet to extract all the chlorination degrees of PCDD and

PCDF.

The PCDD/F and PCBs were purified by applying 50% of the extract to a

multilayer silica column and eluting with n-hexane directly onto an

alumina oxide column, which was eluted with

cyclohexane:dichloromethane (49:1) followed by

cyclohexane:dichloromethane (1:1). In Studies I and IV the alumina oxide

column was only eluted with cyclohexane:dichloromethane (1:1) and in

Study III the alumina oxide column was excluded. The

cyclohexane:dichloromethane fraction, which contained the PCDD/F and

PCB was concentrated then further purified, either on a carbon:Celite

column (Paper II), or a column with a mixture of AX21-carbon and Celite

(7.9:92.1) (Papers I, III and IV). The carbon columns were eluted with n-

hexane, cyclohexane:dichloromethane and toluene. The AX21-carbon

column was back-eluted with the toluene. The n-hexane and

26

cyclohexane:dichloromethane fractions were combined and used for

determination of the non-planar PCBs, whilst the toluene fraction was

used for determination of the PCDD/Fs and the planar PCBs. Both

fractions were concentrated to 40 μL tetradecane after addition of 13C-

labelled recovery standard. The congeners in the PCDD/F recovery

standard were 1,2,3,4-TeCDD, 1,2,3,4,6-PeCDF, 1,2,3,4,6,9-HxCDF and

1,2,3,4,6,8,9-HpCDF and for the PCB a penta-chlorinated congener, #97

and a hepta-chlorinated congener, #188, were used.

The PCDD/F and PCB were analyzed by GC/MS, using

a Waters AutoSpec ULTIMA NT 2000D in Studies I and IV

a VG 70 250 S instrument in Study II an Autospec system (Fisons Instruments) in Study III

The HCB was analyzed by transferring 25 % of the extract to a silica

column and eluting with 60 mL cyclopentane. The resulting fraction was

concentrated to 1 ml in toluene and a 13C-labelled recovery standard was

added, before HCB determination was carried out by GC-LRMS, using an

Agilent Technologies system with a 6890N GC and a 5975 MSD mass

spectrometer.

5. Formation of PCDD and PCDF When municipal solid waste (MSW) is combusted various organic

compounds are formed, due to incomplete combustion, one cause of which

is the ash-forming substances that are minor constituents of MSW and

can disturb the combustion process. Other possible causes include (inter

alia) variations in the temperature in the combustion zone, changes in the

air supply, and insufficient mixing. These have long been thought to be the

main reasons for the formation of unwanted organic compounds, products

of incomplete combustion (PIC). However, new results indicate that the

formation of PICs in full-scale waste incineration plants is almost

27

exclusively due to particles deposited on ductwork in the region where the

temperature is between 450 and 200 oC (11,33-36).

PCDD and PCDF are only two of many groups of organic compounds that

form during the combustion of MSW. Other persistent organic pollutants

(POPs) include PCBs, chlorobenzenes, chlorophenols, brominated DD/Fs

and PAHs, to list just a few types. They are all PICs and some of them pose

threats to the environment since they are persistent, toxic and may

bioaccumulate. This thesis concentrates on the formation of PCDD and

PCDF in fly ash generated during the combustion of MSW and the

degradation of PCDD and PCDF in thermal treatments designed to

eliminate these compounds from the fly ash.

Formation in the combustion process The objective of the study described in Paper I was to the map the

concentrations and distributions of mono- to octa-chlorinated PCDD/F

isomers in fly ashes collected from various points in a full-scale MSW

incinerator as the flue gases cooled from 700 oC to 170 oC.

It is well established that most formation of PCDD and PCDF occurs in the

post-combustion zone. The total concentration of PCDD and PCDF in flue

gases and the fly ash has been shown to increase as they progress though

this zone in several studies (31,37-43), and this was confirmed for fly ashes

in Study I. Clear increases in the concentrations of PCDDs and PCDFs, of

all chlorination degrees, were observed in fly ash as the temperature

decreased in the post-combustion zone (Figure 5 and Table 6).

28

Table 6. Concentrations of Mo-OCDD/Fs [ng/g ash] in Ashes A, B and C, and corresponding ng WHO-TEQ /g values (Study I). Ash I-A Ash I-B Ash I-C T [oC] >700 400-300 ~170 Sum of 1-8 PCDD/F 8.6 65 193 Sum of 1-8 PCDD 1.5 20 44 Sum of 1-8 PCDF 7.5 46 150 WHO-TEQ 0.1 0.6 1.8 Sum of 2,3,7,8-substituted PCDD and

PCDF congeners 1.8 28 61 Relative fraction of 2,3,7,8-substituted

PCDD and PCDF congeners 7 24 15 Sum of 1-3 PCDD 0.9 2.1 4.9 Sum of 1-3 PCDF 4.1 24 85 Sum of 4-6 PCDD 0.4 3.9 10 Sum of 4-6 PCDF 3.1 16 55 Sum of 7-8 PCDD 0.2 14 29 Sum of 7-8 PCDF 0.3 5.5 9.3

PCDD and PCDF can theoretically be formed via two types of reactions:

homogeneous (gas phase) reactions and heterogeneous (interactions

between gas and solid phase) reactions. Homogeneous reactions mainly

occur at high temperatures, but the probability that they will occur is very

low according to calculations (44). A widely held belief is that the main

route for their formation is via heterogeneous reactions in the low

temperature interval (450 – 200oC) in the post-combustion zone.

In an early study of these phenomena, Lustenhouwer et al.(45) postulated

three possible origins for the PCDD and PCDFs found in flue gases and fly

ashes from combustion of MSW. One hypothesis was that PCDD and PCDF

could be present in the feedstock and survive the combustion process

intact. Alternatively, the PCDD and PCDF could be formed either from

smaller organic molecules, so-called precursors that adsorb to the surface

29

of particles and react under the influence of metal catalysts or de novo

from their constituent elements (C, H, O, Cl) in processes involving fly ash

particles. Postulated mechanisms to account for such de novo synthesis

have varied over the years, but according to a recent explanation by Iino et

al (46) breakdown reactions of residual carbon matrices in the ash or soot

are involved.

Today, almost three decades after Lustenhouwer et al.’s (45) contribution,

the mechanisms responsible for the formation of PCCD/F have still not

been fully elucidated, indicating that several, complex interacting

processes are probably involved. Hunsinger et al (11) found that PCDD and

PCDF formed during fuel bed burnout on the grate of a system they

investigated had been almost completely destroyed in efficiently controlled

combustion when the flue gases left the combustion chamber. In addition,

if the combustion was disturbed high amounts of PCDD and PCDF could

leave the combustion chamber. Therefore, a possible formation mechanism

has been proposed, chlorination of carbon skeletons. There are also

indications that the de novo and precursor pathway may be closely related

(47,48), and the traditional distinction between the two is becoming

blurred (49,50).

Dechlorination also strongly influences the final congener patterns in both

flue gases and fly ashes since formation and degradation reactions of

PCDD and PCDF occur simultaneously in flue gas under normal

combustion conditions at temperatures over 200 oC. At high temperatures

(>600 oC) the degradation reactions will dominate over the formation

reactions, whereas formation reactions dominate at temperatures below

500 oC. The combination of these processes explains the congener patterns

that are characteristic signatures of specific combustion sources (51) and

is denoted the dualistic principle. This concept has been supported by

Weber et al (52), amongst others.

30

Over the years it has become evident that the kinetics and importance of

formation via precursors, de novo synthesis and chlorination differ

between PCDDs and PCDFs, in ways which probably explain why PCDFs

are generally present at high concentrations in MSW ashes than PCDDs,

as found for all three ashes examined in Study I, in which PCDFs

accounted for more than 75% of the total PCDD/F concentration (Table 6).

One reason for this may be that PCDF can be formed via several pathways

while the formation pathways to PCDD are more limited. PCDF can be

formed from diverse precursors like biphenyls and phenols, through de

novo synthesis and chlorination of dibenzofurans. It has also been shown

that they can form via the incorporation of oxygen into PAH molecules (46).

Formation of PCDDs requires the presence of at least one ortho-

substituted chlorophenol, and the concentration of unsubstituted phenol

in raw gas is generally more than 100 times higher than that of

chlorophenols (53), which explains the higher concentration of PCDFs

compared to PCDDs.

In Study I, the homolog profile was found to be less temperature-

dependent for PCDF than for PCDD (Figure 7), indicating that the

mechanisms of formation for these species differ. This could be due to that

the PCDF molecule is more stable than the PCDD molecule, since the

PCDF molecule is defined as an aromatic molecule and fulfils the Hückels

rule with (4n+2) π electrons. Differences in formation mechanisms of PCDD

and PCDF have also been reported by other authors (54-56). The greater

between-temperature differences in the homologue profile of PCDD than

the PCDF homologue profiles may also reflect the fact that PCDFs were

present at much higher concentrations than PCDDs. Even though the

homologue profile differed between the ashes, the isomer pattern within

each homologue group showed very little variation.

Formation from lightly or nonchlorinated precursors, followed by further

chlorination, is believed to be a major route for PCDF formation, while the

31

main formation route for PCDD is believed to be chlorophenol

condensation (57). Ryu et al. (58) concluded that DF chlorination can play

an important role in the formation of PCDF, while DD chlorination is not a

major contributor to PCDD formation. However, as mentioned above, a

combination of formation and degradation reactions are responsible for the

final congener pattern found in both flue gases and ashes. Evidence for

this was presented in Paper I, since no single formation route could

explain the congener pattern observed in the fly ash, in accordance with

reports by other authors (59,60). However, there were indications that

condensation of chlorinated phenols and chlorination occurred. It is also

known that dechlorination can occur at low temperatures, so this

mechanism probably contributed to the PCDD and PCDF congener

patterns observed in our ash samples from the post-combustion zone.

The degree of chlorination (Table 7) varied between the ashes examined in

Study I for the PCDD, and increased with decreasing temperature in the

post-combustion zone. For the PCDF the degree of chlorination did not

vary between the ashes.

Table 7. Degrees of chlorination of PCDD and PCDF in the ashes examined in Study I. Ash I-A Ash I-B Ash I-C T [oC] >700 400-300 ~170 PCDF 3.2 3.6 3.3 PCDD 2.6 5.3 5.1

Figure 11 shows the proportions of mono-tri, tetra-hexa and hepta-octa

species in the total amounts of PCDD and PCDF in the ashes examined in

Study I. PCDD was dominated by mono- to tri-chlorinated species at high

temperature, while hepta- and octa-chlorinated species dominated at the

lower temperatures (Figure 11). The contribution of the mono- to tri-

chlorinated PCDF to the total amount of PCDF was unchanged, but the

contribution of the hepta- and octa-chlorinated congeners increased

32

(Figure 11). The major changes in the PCDD homologue profiles were

observed between the high and medium temperatures. However, no

previous study has focused on the lightly chlorinated (mono-, di- and tri-

chlorinated) congeners of PCDD and PCDF in fly ash from a full-scale MSW

incinerator, or PCDD and PCDF in fly ash from such a plant sampled at

temperatures exceeding 500 oC. Analyses of PCDDs and PCDFs in deposits

have been reported (40) , but no published studies have analyzed the

lightly chlorinated PCDD and PCDF congeners in them.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

PCDD PCDF PCDD PCDF PCDD PCDF

Ash A (>700 C) Ash B (400-300 C) Ash C (~170 C)

Rel

ativ

e co

ntrib

utio

n to

am

ount

of P

CD

D o

r PC

DF

Mono-Tri PCDD Mono-Tri PCDF Tetra-Hexa PCDD Tetra-Hexa PCDF Hepta-Octa PCDD Hepta-Octa PCDF Figure 11. Relative contributions of mono-tri, tetra-hexa and hepta-octa congeners to the total amounts of PCDD and PCDF in the ash examined in Study I.

Formation during thermal treatment of ash In Study IV formation of PCDD and PCDF was observed when ash was

thermally treated at 300 oC in a closed system with air present (for

experimental details see chapter 4). The total amount of PCDD/F was

almost 40 times higher than in the untreated ash, and the increase was

greater for the PCDF than the PCDD (Table 8). The PCDD:PCDF ratio

decreased from 0.2 in the untreated ash to 0.05. These findings clearly

33

show that PCDD and PCDF have different formation mechanisms. Other

authors have also seen that more PCDF is formed than PCDD at 300 oC

(61) , and Stieglitz et al. (48) have noted that PCDDs (but not PCDFs) are

partly formed via the condensation of phenyl rings. In addition, Wikström

et al. (62) have observed a difference in the optimum net formation

temperature for PCDD and PCDF, which also could be related to

differences in their formation and/or destruction mechanisms.

Since the total amount of PCDD/F increased 10 times more than the

WHO-TEQ concentration in the ash treated at 300 oC, it can be concluded

that the formation of 2,3,7,8-chlorinated congeners was not favored over

other congeners. In fact, the relative contribution of 2,3,7,8-chlorinated

congeners to the total amount of PCDD/F decreased from 17% to 1%, and

of the 2,3,7,8-chlorinated congeners present in the ash 87% were PCDF-

congeners. Table 8: Results from thermal treatment of ash at 300 oC in Study IV. All concentrations are in ng/g for the ashes and ng/g treated ash for the gas phases. Ash Ash Gas phase T [oC] --- 300 300 Sum of PCDD/F 194 7109 5451 Sum of 1-8 PCDD 33 309 206 Sum of 1-8 PCDF 161 6800 5185 WHO-TEQ 3.7 15 34 PCDD:PCDF 0.2 0.05 0.05 Sum of 2,3,7,8-subsituted PCDD and

PCDF congeners

32 92 193

Contribution of 2,3,7,8-substituted

congeners to the total PCDD/F [%]

17 1 4

Degree of chlorination, PCDD 5.6 1.9 2.9 Degree of chlorination, PCDF 3.8 1.6 3.8

34

6. Degradation of PCDD and PCDF Degradation can occur via dechlorination/hydrogenation reactions, in

which one or more chlorine atoms in the molecules are replaced by

hydrogen atoms. Water has been shown to be able to supply the hydrogen

required for this reaction (63). Oxidation reactions in which the carbon-

oxygen bond is broken and the PCDD or PCDF molecules are degraded to

smaller chlorinated molecules like PCBzs (64), and ultimately CO2,

comprise another degradation pathway.

The effects of ash composition during thermal treatment Degradation/destruction of PCDD and PCDF in fly ash seems to be highly

dependent on the composition of the fly ash (Paper II and (16,21,23,52,63,65-69). The objective of the study in Paper II was to

evaluate how different elements and compounds affect the degradation of

PCDD and PCDF. In Paper II the degradation potential of three different

ashes was evaluated with respect to temperature, treatment time and

atmosphere. The ashes differed in amounts of carbon, chlorine, metals (Cu

and Zn), Na2O, sulfur and pH (Table 2 in the experimental section). The

results showed that the three ashes clearly had different degradation

potentials for PCDD and PCDF during thermal treatment and that the

composition of the ash affects the conditions required to ensure their

degradation in a specific ash.

In Paper II it was shown that fly ash clearly plays an important role in

degradation reactions of PCDD and PCDF, acting as a catalytic surface in

degradation reactions. Temperatures below 200 oC were too low to achieve

degradation of the total amount of PCDD and PCDF during thermal

treatment of fly ash, regardless of the composition of the fly ash, and at

35

temperatures over 600 oC degradation reactions were strongly favored

(Figure 12).

0

50

100

150

200

250

untre

ated

150,1

,nitro

gen

150,1

,air

150,2

,nitro

gen

150,2

,air

300,1

,nitro

gen

300,1

air

300,2

,nitro

gen

300,2

,air

450,1

,nitro

gen

450,1

,air

450,2

,nitro

gen

450,2

,air

600,1

,nitro

gen

600,1

,air

T [C], t [h], atmosphere

Con

cent

ratio

n to

tal P

CD

D/F

in A

sh B

[n

g/g]

0

1

2

3

4

5

6

7

8

9

Con

cent

ratio

n to

tal P

CD

D/F

in A

sh A

an

d C

[ng/

g]

Ash II-B Ash II-A Ash II-C Figure 12. Total concentrations of PCDD/F after treatment in ash B (left y-axis) and ashes A and C (right y-axis): ash A, no experiments with t =1 h or at T = 600 oC; ash B, no experiments at T = 450 oC when t = 2 h; ash C, no experiments at T = 450 or 600 oC (Study II) Temperature appears to be the most important factor determining whether

formation or degradation was favored, but variables such as fly ash

composition, treatment time, and atmosphere have also been found to be

very important (21,63,67,68). In short: the temperature required to ensure

high levels of degradation of PCDD and PCDF varies with the composition

of the fly ash to be treated. High temperatures were previously thought to

be needed to achieve complete degradation of PCDD and PCDF in fly ash in

the presence of oxygen and/or with high concentrations of unburned

carbon since formation via de novo synthesis occurs at temperatures

between 250 and 400 oC. However, the results presented in Paper II indicate that other components in the fly ash are more important than

other factors like temperature and atmosphere to achieve high degradation

of PCDD and PCDF. In addition, it was shown in Paper II that oxygen can

enhance the degradation of PCDD and PCDF in some cases. The oxygen

36

can oxidize the carbon and other compounds present in the ash, and then

degradation under oxygen-deficient conditions can take place. However,

the ash:air ratio was shown to be important. In the experiments reported

in Paper IV, where the ash:air ratio was 0.008 g/mL, smaller proportions

of the total amounts of PCDD and PCDF were degraded than in Study III, where the ash:air ratio was 0.12 or 0.65 g/mL (Table 9). Variations in the

ash:air ratio had no apparently significant effects on differences between

the proportions of PCDD and PCDF that were degraded.

Table 9. Variations in temperature, residence time and ash:air ratios in Studies III and IV, and resulting reductions in total amounts of PCDD/F. Paper III Paper III Paper IV T [oC] 450 500 500 t [min] 60 60 45 Ash:air ratio [g/ml] 0.12 0.65 0.008 Reduction in total amount of PCDD/F [%] 99 96 74

Concentrations of carbon, metals (Cu, Fe, Sn), calcium and potassium

(which gives the ash a high pH) have been shown to affect the degradation,

as well as the formation, of PCDD and PCDF (16,23,52,61).

Very low or negligible degradation was observed at 150 oC in Study II (Figure 12), in accordance with a previous study (68,70). However, when

the ashes were treated at 300 oC differences appeared (Figure 12). In ashes

II-A and II-B the total concentrations of tetra- to octa-chlorinated PCDD/F

were reduced by 95 and 70%, respectively. However, an increase in the total concentration of PCDD/F in ash II-C was observed, from 0.9 to 5.6

ng/g of ash. When the temperature was increased to 450 or 600 oC highly efficient degradation of the PCDD/Fs in ash II-B occurred; both the total

amounts and I-TEQ value decreased by >99%.

The differences in degradation potential in these three ashes appeared to

be due to differences in their composition. The reasons for the difference

37

between ash II-A and II-C in degradation behavior could be that ash II-A

had more Cu, Cl, C, and S and higher amounts of PCDD and PCDF than

ash II-C. Ash II-A was richer in copper, implying that it should

theoretically have had a higher potential for formation of PCDD and PCDF,

but its concentration of sulfur was extremely high, which may have

inhibited the Deacon reaction, allowing degradation to dominate. Sulfur is

known to inhibit the formation of PCDD/F in flue gases by reacting with

the copper catalytic sites (71), which could also occur during thermal

treatment. The high concentration of unburned carbon in ash II-B

contributed to the degradation behavior of PCDD and PCDF in this ash,

but it is unlikely to be the main reason for the low degradation of PCDD/F

in this ash. The fact that no obvious difference was observed between

cases when oxygen was present and cases when it was absent in the

atmosphere of the sealed glass ampoules in Study II indicates that the

presence of oxygen may enhance the degradation of PCDD and PCDF.

However, the different combinations of pH and amounts of PCDD and

PCDF, unburned carbon, metals (Cu, Fe, Sn and Na), and sulfur in the

ashes examined in Study II made it difficult, or even impossible, to

conclude that any single parameter was responsible for the degradation of

PCDD/F in these thermal treatment experiments. The reduction in the

total amount of PCDD/F observed in the study was due to dechlorination

and/or destruction of PCDD/F; depending on the temperature and ash

composition, either of these processes may be the more important.

PCDD vs PCDF Study II showed that the composition of the ash influences the relative

extent of degradation of PCDD and PCDF. In the experiments with ash II-A

(and ash II-B at 300 oC), PCDFs were degraded to a greater extent than

PCDDs. However, for ash II-C, in the T = 300 oC, t = 2 h in nitrogen

treatment the PCDF decreased more than the PCDD, but the PCDF

38

congeners made a greater contribution to the increase in PCDD/F levels

observed following treatment at T = 300 oC, t = 1 h in air. In Study III (and

Study IV when the ash was treated at 500 oC), the PCDDs were more

prone to degradation than the PCDFs.

The results from Study II also indicated that the 2,3,7,8-positions were

more prone to dechlorination than the others, since the relative

contribution of the 2,3,7,8-substituted congeners to the total amount of

PCDD/F decreased when ash II-B was thermally treated at 300 oC (Table

10). When the 2,3,7,8-substituted PCDD and PCDF data were separated, it could be seen that the 2,3,7,8-substituted PCDD were more prone to

degradation. Their relative contribution to the total amount of PCDD

decreased, while the relative contribution of the 2,3,7,8-substituted PCDF

was unchanged compared to the untreated ash. For the other two ashes,

II-A and II-C, used in Study II no clear trend was discernible. However, for

ash II-A the results indicate that the 2,3,7,8-substituted PCDF require

higher temperature to degrade than the 2,3,7,8-substituted PCDD

congeners did. For ash II-C the results indicate that the 2,3,7,8-

substituted PCDF congeners were more prone to degradation than the

2,3,7,8-substituted PCDD congeners, a treatment time of 1 h sufficing for

the former, compared to 2 h for the latter. In Study III the 2,3,7,8-

substituted PCDD and PCDF congeners were degraded to the same extent

as the other congeners (Table 10). The contribution of 2,3,7,8-substituted

PCDD and PCDF congeners increased in the ash after treatment at 500 oC

and the increase was greater for the PCDF in Study IV (Table 10).

However, reductions in the contents of the 2,3,7,8-substituted congeners

in the ash treated at 300 oC in were observed in Study IV (Table 10).

39

Table 10. Relative contribution of the 2,3,7,8-substituted congeners to the total amount PCDD/F.

Pape

rA

shΣ

2,3

,7,8

-PC

DD

co

ngen

ers

Con

trib

utio

n to

the

tota

l am

ount

PC

DD

Σ 2

,3,7

,8-P

CD

F co

ngen

ers

Con

trib

utio

n to

the

tota

l am

ount

PC

DF

IIU

ntre

ated

A1.

452

2.4

50II

A 3

00 o C

(nitr

ogen

)0.

0419

0.02

42II

A 4

50 o C

(nitr

ogen

)0.

0518

0.02

21II

Unt

reat

ed B

3946

2822

IIB

(30

0 o C

, 2h,

air)

512

417

IIU

ntre

ated

C0.

4173

0.24

70II

C (3

00 o C

, 1h,

air)

0.29

270.

4410

IIC

(300

o C, 2

h, a

ir)0.

2240

0.60

34III

Unt

reat

ed11

3616

9III

500

o C, 6

0 m

in0.

1759

0.26

10IV

Unt

reat

ed12

3620

10IV

300

o C12

480

1IV

500

o C3

4315

29

40

The differences between the degradations of 2,3,7,8-substituted congeners

could be dependent on the ash composition. However, Wehrmaier et al (51)

have calculated the relative reactivity for all PCDD congeners and found

that PCDD congeners with chlorine in 2,3,7,8-positions are more reactive

than others, and thus the destruction of PCDD should result in PCDD

congener patterns in which congeners with chlorines at 1,4,6,9-positions

predominate. In addition, Weber et al did not observe any preferential

degradation of the 2,3,7,8-substituted PCDDs and PCDFs in another study

(16,23). These findings strongly indicate that degradation patterns depend

on the ash composition.

Degradation experiments with large-volume samples Since data from most previous studies on the degradation of PCDD/F in fly

ash have been derived from experiments with small amounts of fly ash (< 3

g), the aims of the study in Paper III were to investigate the degradation of

native compounds, mono- to octa- chlorinated PCDD/Fs, PCBs and HCB

in larger quantities of ash (200 mL or ~130 g), in a closed system with low

oxygen conditions. Regardless of the treatment time the I-TEQ

concentration was <0.05 ng TEQ/g when the treatment temperature was

500 oC (Table 11). However, the total concentrations of PCDD and PCDF

decreased with increases in the treatment time (Table 11). Thermal

treatment of filter ash samples at 500 oC for 60 min in a closed system

providing low oxygen conditions resulted in 96.5% and 99% reductions in

the total and I-TEQ concentrations of PCDD and PCDFs, to 8.5 ng/g ash

and <0.05 ng I-TEQ/g ash, respectively. Increasing the thermal treatment

time to 480 min, at the same temperature, yielded 99% reductions in both

total and I-TEQ concentrations of the mono- to octa-chlorinated PCDD/Fs.

Similar effects were observed for HCB and PCBs.

41

Table 11. Concentrations of PCCD/Fs [ng/g] after treatment at 500 oC for varying times (Study II). Treatment time [min] --- 60 90 120 240 480 No. of samples 3 2 2 2 2 2 Sum of 1-8 PCDD/F 218 8.5 4.5 6.3 4.2 1.8 I-TEQ 3.2 <0.05 <0.05 <0.05 <0.05 <0.05 Sum of 1-3 PCDD/F 71 3.9 2.9 4.4 2.1 1.5 Sum of 4-8 PCDD/F 147 4.6 1.6 1.9 2.1 0.3 Sum of 1-3 PCDF 68 3.9 2.9 4.3 2.0 1.4 Sum of 1-3 PCDD 3.4 0.02 0.02 0.1 0.1 0.05 Sum of 4-8 PCDF 121 4.3 1.6 1.8 2.0 0.3 Sum of 4-8 PCDD 26 0.3 0.07 0.1 0.2 0.07

Shifts in chlorination degree were observed in both the PCDD and PCDF.

The untreated ash was dominated by tetra- to octa-chlorinated isomers

while the treated ashes had approximately the same or higher

concentrations of mono- to tri-chlorinated isomers (Figure 13).

0%

20%

40%

60%

80%

100%

PCDD PCDF PCDD PCDF PCDD PCDF PCDD PCDF PCDD PCDF PCDD PCDF

Untreated 60 90 120 240 480

Rel

ativ

e co

ntrib

utio

n to

tota

l am

ount

PC

DD

or P

CD

F

Mono-Tri PCDD Mono-Tri PCDF Tetra-Octa PCDD Tetra-Octa PCDF Figure 13. Relative contribution of mono-tri and tetra-octa congeners to the total amount of PCDD or PCDF for ash treated at 500 oC for 60, 90, 120, 240 and 480 min. The results from the study described in Paper III indicate that PCDD,

PCDF and other toxic organic compounds in ash from incinerated MSW

42

can be effectively degraded by this procedure, which combines relatively

low temperatures, short treatment times, and low oxygen conditions. The

importance of heat transfer for fast, complete degradation of organic

compounds in the ash was established, and shown to enhance the

degradation and/or allow shorter treatment times. A closed system with a

pipe externally heated using heat generated from the incineration process

as the source of thermal energy for the degradation procedure, with an

internally heated screw to enhance heat transfer and thus reduce the

thermal treatment time, could provide a viable means for degrading PCDD,

PCDF, PCB and HCB in waste incineration ash.

Dechlorination or destruction? Differences between the degradation processes of dioxins and furans have

been observed (16,23,70). The mechanisms of degradation reactions

obviously depend on the composition of the fly ash, and since this is

seldom reported it is difficult to determine the causes of differences in the degradation patterns observed. It has been proposed that iron and copper

are involved in dechlorination reactions and that other degradation

reactions are facilitated by the alkalinity of the ash (23). The question of

whether dechlorination precedes degradation of PCDD/F or whether the

two processes occur simultaneously has also been considered (67) , and it

has been shown that either of these two possibilities may occur (16).

Experiments on model and real fly ash have yielded different results

regarding relative rates of degradation of PCDF vs PCDD (23,70). In the

model ash OCDFs were dechlorinated more rapidly than OCDDs (23) , but

in real fly ash PCDDs were more prone to degradation than PCDFs (70). In

Study II we found that the PCDFs were degraded to a greater extent than

PCDDs in ash A, while in ash C the opposite was true. The cause of this

difference is not yet known. In Study III the PCDD/Fs were dominated by

PCDFs, which accounted for 94-98% of the total content, in the ashes

treated at 500 oC. This indicates that for PCDFs, the degradation process

43

involved a combination of dechlorination and destruction. However, for

PCDDs, no firm conclusion can be drawn as to whether degradation was

due to dechlorination and/or destruction, particularly given the very low

concentrations of all the PCDD congeners (mono- to octa-).

Degradation of other chlorinated organic compounds There have been very few studies of possible degradation products of

PCDD and PCDF, although Visez and Sawerysyn (64) have shown that

PCBz can form via the degradation of PCDF (and to a lesser extent

degradation of PCDD). In Study III we showed that PCDD and PCDF are

not the only organic compounds that are degraded during thermal

treatment. Treatment at 500 oC for 60 min resulted in more than 90%

degradation of the majority of the PCBs, while treatment for 480 min

increased the extent of degradation to more than 97%. Degradation of

PCBs has also been reported by Stach et al (72) at even lower

temperatures. The study reported in Paper III also showed that 99% of the

HCB was degraded by the 60 min treatment. Stach et al (73) have also

shown that HCB is dechlorinated when thermally treated and that the

higher the temperature was the lower the degree of chlorination of the

resulting chlorobenzenes was. So, if HCB is a degradation product of

PCDD and PCDF it is not a problem since it also degrades. Results from a

bio assay study on thermally treated fly ash indicate that toxic dioxin-like

degradation products is not likely to form during the treatment (74).

44

Desorption of PCDD and PCDF In Study IV the behavior of PCDD and PCDF during thermal treatment

was investigated to evaluate whether the reductions observed in Studies II and III were really due to degradation, rather than merely

volatilization/desorption of the PCDD and PCDF.

Ash was thermally treated at 500 and 300 oC for 30 min, then the gas

phase (air) was sampled, preheated argon was passed through the

experimental apparatus (Figure 10). There were substantial differences in

total concentrations of PCDD and PCDF in the ashes after treatment, and

also between the gas phases. Reduction was achieved at 500 oC, whereas

formation of PCDD and PCDF was promoted at 300 oC. The reduction at

500 oC could not be explained by desorption of PCDD and PCDF, as

concluded by Hagenmaier (67), since the amounts of PCDD and PCDF

detected in the gas phase were lower than the corresponding reductions in

the ash, as can be seen in Table 12 and Figures 14-15.

Table 12. Results from thermal treatment of ash at 500 oC in Study IV. All concentrations are in ng/g for

the ashes and ng/g treated ash for the gas phases.

Ash Ash Gas phase T [oC] --- 500 500 Total PCDD/F 194 51 13

Σ PCDD 33 7 0.7

Σ PCDF 161 44 12 WHO-TEQ 3.7 1.9 0.03 PCDD:PCDF 0.2 0.16 0.06 Degree of chlorination PCDD 5.6 1.9 2.9 Degree of chlorination PCDF 3.8 1.6 3.8

45

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

MoCDD

DiCDD

TriCDD

TeCDD

PeCDD

HxCDD

HpCDD

OCDD

MoCDF

DiCDF

TriCDF

TeCDF

PeCDF

HxCDF

HpCDF

OCDF

Con

cent

ratio

n of

PC

DD

[ng/

g]

0

2

4

6

8

10

12

Con

cent

ratio

n of

PC

DF

[ng/

g]

PCDD PCDF Figure 14 Homologue profile of ash treated at 500 oC in Study IV.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

MoCDD

DiCDD

TriCDD

TeCDD

PeCDD

HxCDD

HpCDD

OCDD

MoCDF

DiCDF

TriCDF

TeCDF

PeCDF

HxCDF

HpCDF

OCDF

Con

cent

ratio

n of

PC

DD

[ng/

g tr

eate

d as

h]

0

1

2

3

4

5

6

7

Con

cent

ratio

n of

PC

DF

[ng/

g tr

eate

d as

h]

PCDD PCDF Figure 15. Homologue profiles of gas phase samples treated at 500 oC in Study IV.

46

0

20

40

60

80

100

120

140

160

MoCDD

DiCDD

TriCDD

TeCDD

PeCDD

HxCDD

HpCDD

OCDD

MoCDF

DiCDF

TriCDF

TeCDF

PeCDF

HxCDF

HpCDF

OCDF

Con

cent

ratio

n of

PC

DD

[ng/

g]

0

500

1000

1500

2000

2500

3000

3500

4000

Con

cent

ratio

n of

PC

DF

[ng/

g]

PCDD PCDF Figure 16. Homologue profile of ash treated at 300oC in Study IV.

0

10

20

30

40

50

60

70

80

90

MoCDD

DiCDD

TriCDD

TeCDD

PeCDD

HxCDD

HpCDD

OCDD

MoCDF

DiCDF

TriCDF

TeCDF

PeCDF

HxCDF

HpCDF

OCDF

Con

cent

ratio

n of

PC

DD

[ng/

g tr

eate

d as

h]

0

200

400

600

800

1000

1200

1400

1600

1800

Con

cent

ratio

n of

PC

DF

[ng/

g tr

eate

d as

h]

PCDD PCDF Figure 17. Homologue profile of the gas phase samples treated at 300 oC in Study IV.

47

In addition, following the 300 oC treatment the amount of PCDD/F present

in the gas phase amounted to over 40% of the total amount of PCDD/F in

the ash and gas phases, but the corresponding proportion for the 500 oC

treatment was only 20% (Table 8 and 12). Altwicker et al (75) have

investigated the phase distribution at temperatures between 250 and 350 oC with a low gas flow (0.1 – 1 l/min) over fly ash, and they concluded that

both desorption and destruction rates increased with increasing

temperatures. These results imply that at high temperatures where

degradation dominates over formation of PCDD and PCDF, desorption of

PCDD and PCDF will be low and cannot explain the low concentrations in

the treated ash, as also concluded in Paper IV. It has also been concluded

by Mätzing et al (76) that the temperature and the chemical composition of

the fly ash particles were important parameters for the adsorption of PCDD

and PCDF.

When comparing the isomer pattern within each homologue group for the

treated ash and corresponding gas phase in Study IV, it could be

concluded that the PCDD and PCDF congeners behaved differently

depending on the temperature. The isomer distribution on fly ash was

reported to remain constant during thermal treatment at 400 oC by Addink

et al (70). In addition, the isomer distribution was the same in both gas

phase and treated ash. This was not found in Study IV. Differences in

isomer patterns (Figure 18) and homologue profiles (Figure 14-17) were

found both between the treated ashes and between the ash and

corresponding gas phase. When comparing the ash treated at 300 oC with

the ash treated at 500 oC more differences were found in the PCDFs than

in the PCDDs, implying that the PCDF congener pattern was more

sensitive to temperature changes than the PCDD pattern. There were also

differences between the gas phase samples at 300 and 500 oC, however,

here the PCDD congener pattern was more sensitive to temperature

changes than the PCDF pattern.

48

There were differences for most of the homologue groups when comparing

the congener patterns between the ash and corresponding gas phase

sample at 300 and 500 oC and when there was a difference it was more

pronounced for the 500 oC samples (Figure 18).

A

B

C

D

Figure 18. GC-HRMS chromatograms of hexa-chlorinated PCDDs found in ash treated at 300 oC (A),

the corresponding gas phase (B), ash treated at 500 oC (C) and the corresponding gas phase (D).

49

7. Concluding remarks

From the results in the four papers that this thesis is based upon the

following conclusions can be drawn:

The concentration of PCDD and PCDF increases in the post-

combustion zone when the temperature decreases (Paper I). Moreover, PCDF species predominated over PCDDD species in a

temperature-independent fashion.

The homologue profile in the ash changed when the temperature in

the post-combustion zone decreased (Paper I). The PCDD

homologue profile was found to vary with temperature while it was

unchanged for the PCDF. The mono- to tri-chlorinated PCDF

predominated over the whole temperature range, while a change

from predomination of the lightly chlorinated species to the highly

chlorinated was seen for the PCDD.

Since PCDDs and PCDFs have different formation and degradation

mechanisms they should be considered as the two separate

compound classes that they are (all the Papers). De novo

synthesis, formation via precursors and chlorination are not

equally important for PCDD and PCDF species.

The chemical composition of ash has a major impact on the

degradation potential of PCDD and PCDF in ash (Paper II).

The presence of oxygen during thermal treatment can enhance the degradation of PCDD and PCDF (Paper II).

Shifts in chlorination degree occurred during thermal treatment

(Paper III and IV). Depending on the temperature both increase

and decrease in chlorination degree was observed.

50

Degradation of other chlorinated organic compounds, like PCB and

hexa-chlorinated benzene, also occurred during thermal treatment

of ash (Paper III). Reductions of 90% for the majority of the PCBs

was observed and a 99% reduction for hexa-chlorinated benzene.

Reductions in PCDD and PCDF levels were shown not to be solely

due to their desorption to the gas phase (Paper IV).

Differences in the behavior of the 2,3,7,8-substituted congeners of

PCDD and PCDF during thermal treatment were observed (Papers II, III and IV). In some cases they were more prone to degradation

than the other PCDD and PCDF congeners and in some the

opposite was true.

Differences in isomer patterns were observed between the ash and

gas phase after thermal treatment at both 300 and 500 oC (Paper

IV).

Thermal treatment is a viable method to degrade PCDD and PCDF

in ashes from MSW in small scale systems (Paper III). Efficient heat transfer into the ash is a key factor to promote fast

degradation of PCDD and PCDF (Paper III).

51

ACKNOWLEDGMENTS

Jag vill börja med att tacka Gunilla Söderström och Lisa Redin (då Börjesson) för att ni väckte mitt intresse för förbränning när jag läste Miljökemi C för en herrans massa år sedan. Ett stort tack till Statens energimyndighet (STEM), Avfall Sverige (fd RVF), Kvaerner Power AB, Ångpanneföreningens forskningsstiftelse, Umeå Energi, JC Kempes minnes akademiska forskningsfond, Knut och Alice Wallenbergs stifelse för att ni har varit med och finansierat min forskning i olika utsträckningar. I would also like to thank John Blackwell and his team for doing such a great work with the English language in my papers and thesis. Stellan Marklund, min handledare och följeslagare i askans värld, tack för alla glad tillrop och din positiva inställning till det jag ägnat mig åt de senaste åren av mitt liv. Det har inte alltid gått som jag tänkt mig men vi har lyckats rädda situationen de flesta gånger. Du har alltid trott på det vi har företagit oss och det är jag tacksam för. Jag vill också tacka dig för ditt engagemang i vad jag ska ta mig till efter att detta är slut. Tack till alla som funnits med i förbränningsgruppen under de år jag har doktorerat. Johanna, tack för all hjälp det sista halvåret och för alla givande diskussioner vi haft om bildning och nedbrytning av dioxiner. Utan dig vore mitt liv fullt av läckor... Stina, trevligare rumskompis hade jag inte kunnat ha. Vi har avhandlat många ämnen förutom dioxiner och förbränning när vi har suttit och tryckt framför våra datorer. Gunilla L och Maria, ni har varit klippor på lab och vid massen! Tack för att ni har stått ut med mina återkommande frågor... Per för ovärderlig hjälp med massen när den inte gjort som jag velat och för de svar du formulerat till mina kluriga frågor om detektionsgränser. Jerker, för att du egentligen inte är en förbränningsnisse utan fått mig att se saker i ett litet annat perspektiv. Till alla de medarbetare som funnits på miljökemi den här tiden, som gjort det roligt att gå till jobbet till och med på måndagarna! Det har varit ett härligt gäng doktorander som jag har fått glädjen att lära känna. Vi har kämpat tillsammans mot möjliga och omöjliga mål. Men vi har lyckats väldigt bra tycker jag. Lena, det har sagts förut, men tål att sägas igen: du är en klippa! Vad skulle jag ha gjort utan din hjälp med sådant som en stackars miljökemidoktorand inte får in i sitt huvud. Sture, tack för alla dina kloka kommentarer. Det får mig alltid att tänka till lite extra när du ställer en klurig fråga!

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Till alla som har suttit med i doktorandsektionens styrelse under dessa år men speciellt Linda, Tuss, Ola och Anna. Jag vill tro att vi har förbrättrat vilkoren för doktoranderna på TekNat och inte minst så har det varit roligt att lära känna andra doktorander vars väg jag annars inte korsat! Mamma och pappa, tack för all hjälp jag har fått under mina år som student. Jag hoppas att ni förstår att jag är er evigt tacksam för den utbildning som ni har givit mig! Bror Niklas: du hann inte disputera för mig, hahahaha... Jag hoppas att du förstår mer av det som jag gjort än jag förstår av det som du ägnat dig åt de senaste åren! Till Stefan för att du hjälpt mig med backuper och andra datorrelaterade problem, för att du suttit uppe sena kvällar den sista veckan och hjälpt mig med med diverse formateringar som jag inte förstår mig på och för de fina figurerna som du gjort åt mig! Men mest för att du står vid min sida i vått och torrt! Mina älsklingar Linnea och Emma, som gör livet värt att leva. Ni sätter allt det jag ägnar mig åt på dagarna i ett perspektiv som berikar min forskning på ert alldeles speciella sätt.

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