Dioxins and dioxin-like
compounds in thermochemical
conversion of biomass
Formation, distribution and fingerprints
Qiuju Gao
Doctoral Thesis, Department of Chemistry
Umeå University, 2016
This work is protected by the Swedish Copyright Legislation (Act 1960:729)
ISBN: 978-91-7601-451-6
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Umeå, Sweden, 2016
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Table of Contents
Abstract
1
Sammanfattning (Summary in Swedish)
2
List of Abbreviations
4
List of Publications
5
1. Introduction
7
2. Background
10
2.1. Biomass for energy production
10
2.2. Thermal decomposition of biomass
11
2.3. Thermochemical conversion
13
2.3.1. Pyrolysis
14
2.3.2. Torrefaction
15
2.4. Dioxins and dioxin-like compounds
16
2.4.1. General description
16
2.4.2. Toxicity of dioxins
17
2.4.3. Formation mechanisms
3. Materials and methods
19
24
3.1. Feedstocks
24
3.2. Experimental setup of microwave-assisted pyrolysis and torrefaction
27
3.3. Sample extraction and cleanup
30
3.4. Instrumental analysis
4. Results and discussion
4.1. Solvent effects in pressurized liquid extraction
30
32
32
4.2. PCDDs, PCDFs and PCNs in microwave pyrolysis
37
4.2.1. Levels and relative distributions
38
4.2.2. Homologue profiles
40
4.2.3. Isomer patterns and formation pathways
42
4.3. Dioxins in torrefaction products
49
4.4. Toxicity equivalent values
54
4.5. Effects of chemical composition of feedstocks
55
4.6. MAP vs torrefaction: the importance of physical dynamics
56
5. Conclusions and future perspectives
61
Acknowledgements
64
References
65
Abstract
In the transition to a sustainable energy supply there is an increasing need to
use biomass for replacement of fossil fuel. A key challenge is to utilize biomass
conversion technologies in an environmentally sound manner. Important
aspects are to minimize potential formation of persistent organic pollutants
(POPs) such as dioxins and dioxin-like compounds.
This thesis involves studies of formation characteristics of polychlorinated
dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and naphthalenes
(PCNs) in microwave-assisted pyrolysis (MAP) and torrefaction using
biomass as feedstock. The research focuses are on their levels, distributions,
fingerprints (homologue profiles and isomer patterns) and the underlying
formation pathways. The study also included efforts to optimize methods for
extracting chlorinated aromatic compounds from thermally treated biomass.
The overall objective was to contribute better understanding on the formation
of dioxins and dioxin-like compounds in low temperature thermal processes.
The main findings include the following:
 Pressurized liquid extraction (PLE) is applicable for simultaneous
extraction of PCDDs, PCDFs, PCNs, polychlorinated phenols and benzenes
from thermally treated wood. The choice of solvent for PLE is critical, and
the extraction efficiency depends on the degrees of biomass carbonization.
 In MAP experiments PCDDs, PCDFs and PCNs were predominantly found
in pyrolysis oils, while in torrefaction experiments they were mainly
retained in solid chars with minor fractions in volatiles. In both cases,
highly chlorinated congeners with low volatility tended to retain on
particles whereas the less chlorinated congeners tended to volatize into the
gas phase.
 Isomer patterns of PCDDs, PCDFs and PCNs generated in MAP were more
selective than those reported in combustion processes. The presence of
isomers with low thermodynamic stability suggests that the pathway of
POPs formation in MAP may be governed not only by thermodynamic
stabilities but also by kinetic factors.
 Formation of PCDDs, PCDFs and PCNs depends not only on the chlorine
contents in biomass but also the presence of metal catalysts and
organic/metal-based preservatives.
Overall, the results provide information on the formation characteristics of
PCDDs, PCDFs and PCNs in MAP and torrefaction. The obtained knowledge
is useful regarding management and utilization of thermally treated biomass
with minimum environmental impact.
1
Sammanfattning (Summary in Swedish)
Omställningen till en hållbar energiförsörjning gör att det finns ett ökande
behov att använda biomassa som ersättning för fossila bränslen. Torrefiering
och pyrolys vid måttlig temperatur är förädlingstekniker som har stor
potential för detta ändamål. En viktig aspekt vid införande och
implementering av nya behandlings- och förädlingstekniker är dock att
eventuella miljörisker måste kartläggas och bildning av persistenta organiska
föroreningar (så kallade POPar) såsom dioxiner och dioxinlika föreningar
måste minimeras.
Denna avhandling fokuserar på bildning av polyklorerade dibenso-p-dioxiner
(PCDD), dibensofuraner (PCDF) och naftalener (PCN) i biomassa som
behandlats med mikrovågsassisterad pyrolys respektive torrefiering. Det
huvudsakliga syftet var att studera förekomst och bildning av PCDD, PCDF
och PCN i de produkter som genereras (kol, olja/vattenfas, och gas) med fokus
på s.k. ”fingeravtryck” (homologprofiler och isomermönster) och de
underliggande bildningsmekanismerna. I projektet har även analytiska
metoder för extraktion av klorerade aromatiska föreningar i fasta
pyrolysprodukter utvecklats. Några av de best betydelsefulla resultaten från
avhandlingen är följande:
 Extraktion av PCDD, PCDF, PCN, polyklorerade fenoler och bensener
från fasta pyrolysprodukter med hjälp av lösningsmedel vid högt tryck
och hög temperatur är en effektiv extraktionsmetod. Extraktionens
effektivitet är dock beroende av valet av lösningsmedel och graden av
karbonisering av biomassan.
 Vid mikrovågsassisterad pyrolys återfanns PCDD, PCDF och PCN
främst i pyrolysoljorna medan de vid torrefiering återfanns
huvudsakligen i de fasta pyrolysprodukterna med bara en mindre
delmängd i oljefasen.
 Isomermönstren av PCDD, PCDF och PCN vid mikrovågsassisterad
pyrolys omfattar färre isomerer än de som rapporterats från
exempelvis förbränningsprocesser. Sannolikt kan detta förklaras av
att bildningsvägar som involverar (klor)fenoler och reaktiva
fenoxiradikal-intermediat är mer aktiva i denna process.
Förekomsten av isomerer med låg termodynamisk stabilitet tyder på
att bildningsvägarna POPs i mikrovågsassisterad pyrolys inte bara
regleras av termodynamiska faktorer utan även av kinetiska faktorer
och intermediatens reaktivitet.
 Den potentiella bildningen av PCDD, PCDF och PCN beror inte bara
på klorinnehållet i biomassan utan även på förekomsten av
2
katalyserande metaller och organiska och/eller metallbaserade
impregneringsmedel.
Detta avhandlingsarbete har bidragit med kunskap kring bildning av PCDD,
PCDF och PCN vid mikrovågsassisterad pyrolys och torrefiering av biomassa
som kan bidra till produktion och nyttjande av termiskt förädlad biomassa
med minimal miljöpåverkan.
3
List of Abbreviations
CCA
Chromated copper arsenate
GC/MS
Gas chromatography/Mass spectrometry
DCM
Dichloromethane
MAP
Microwave-assisted pyrolysis
PAH
Polycyclic aromatic hydrocarbon
PCBz
Polychlorinated benzene
PCDD
Polychlorinated dibenzo-p-dioxin
PCDF
Polychlorinated dibenzofuran
PCN
Polychlorinated naphthalene
PCP
Pentachlorophenol
PCPh
Polychlorinated phenol
PIC
Product of incomplete combustion
PLE
Pressurized liquid extraction
POPs
Persistent organic pollutants
PUF
Polyurethane foam
TEF
Toxic equivalency factor
TEQ
Toxic equivalent
4
List of Publications
This thesis is based on studies described in the following four appended
papers, which are referred to in the text by the corresponding Roman
numerals.
I.
Gao, Q., Haglund, P., Pommer, L., Jansson, S., 2015. Evaluation of
solvent for pressurized liquid extraction of PCDD, PCDF, PCN, PCBz,
PCPh and PAH in torrefied woody biomass. Fuel, 154, 52-58.
II.
Gao, Q., Budarin, V.L., Cieplik, M., Gronnow, M., Jansson, S., 2015.
PCDDs, PCDFs and PCNs in products of microwave-assisted
pyrolysis of woody biomass - Distribution among solid, liquid and
gaseous phases and effects of material composition. Chemosphere,
145, 193-199.
III.
Gao, Q., Cieplik, M., Budarin, V.L., Gronnow, M., Jansson, S., 2016.
Mechanistic evaluation of polychlorinated, dibenzo-p-dioxin,
dibenzofuran and naphthalene isomer fingerprints in microwave
pyrolysis of biomass. Chemosphere, 150, 168-175.
IV.
Gao, Q., Edo, M., Larsson, S.H., Collina, E., Rudolfsson, M., Gallina
M., Jansson, S., 2016. Physical transformation and formation of
PCDDs and PCDFs in torrefaction of biomass with different chemical
composition. Manuscript
Published papers are reproduced with permission from publisher (Elservier
Science)
5
Author’s contributions
Paper I
I was involved in planning the experiment. I was responsible for the lab work,
including sample cleanup and GC-MS analysis, data evaluation and
interpretation, and writing the paper.
Paper II
I was involved in planning the experiment. I performed the pyrolysis
experiments, in co-operation with co-authors at York University. I was also
responsible for sample treatment, GC/MS analysis, evaluating the data and
writing the paper.
Paper III
I was responsible for evaluating and interpreting data obtained from
experiments of Paper II, and writing the paper.
Paper IV
I planned and performed the experiments in co-operation with my co-authors,
and I was responsible for both evaluating results and writing the paper.
6
1. Introduction
In the transition to sustainable energy supplies, there is an increasing need for
the use of biomass as a replacement of fossil fuel. Utilization of biomass as an
energy carrier can reduce the total atmospheric greenhouse gas burden and
mitigate global warming. An important role in renewable energy supplies has
been assigned to biomass in future energy scenarios and plans (McKendry,
2002). The European directive on promotion of the use of renewable energy
(2009/28/EC) sets goals that, by 2020, at least 20% of the total energy
consumption should be obtained from renewable sources including biomass
(Tanger et al., 2013).
Biomass has several drawbacks as a substitute for fossil fuel, including low
energy density, high moisture content and hydrophilicity (Yan et al., 2009).
Use of untreated biomass is also problematic due to its susceptibility to
microbial degradation, heterogeneous composition and high bulk volume,
which complicate process control and logistics management (Uslu et al.,
2008). In modern practice, biomass conversions are often necessary to
improve properties to reach appropriate characteristics and desirable quality
as fuels.
Among the technologies for biomass thermal conversion, pyrolysis and
torrefaction have becoming increasingly accessible for both pilot and industry
scale. Conventional pyrolysis and torrefaction utilize moderate temperatures
(400–550 °C and 200–350 °C, respectively) at oxygen-deficient conditions
(Demirbaş and Arin, 2002; Van der Stelt et al., 2011). Primary products
include liquid (in pyrolysis) and carbon-rich char (pyrolysis and torrefaction).
The char products provide a number of potential applications, including
energy production through coal co-firing. Biochar from pyrolysis can also be
utilized for soil amendment and long-term carbon sequestration. The
pyrolytic liquid (as a result of condensation of volatiles) can be used as a fuel
product (known as bio-oil) after further upgrading and/or as an intermediate
for synthesis of fine chemicals (Demirbaş and Arin, 2002).
Although thermal conversion of biomass is considered to be a sustainable
alternative, a key challenge in the global transition to renewable energy
supplies is to utilize biomass in an environmentally sound manner. Important
aspects are to minimize the potential formation of persistent organic
pollutants (POPs), such as polychlorinated dibenzo-p-dioxins (PCDDs) and
dibenzofurans (PCDFs) must be minimized.
7
PCDDs and PCDFs, commonly known as dioxins, are by-products of
thermochemical processes and are among the prioritized POPs included in the
Stockholm Convention, a global treaty aimed to protect human health and the
environment (UNEP, 2001). Polychlorinated naphthalenes (PCNs) may also
be formed along with PCDDs and PCDFs, and are considered to be “dioxinlike” as they have similar chemical structures and toxicological properties
(Schneider et al., 1998). The combination of an inadequate processing
temperature and insufficient oxygen favors formation and survival of
chlorinated aromatics. Studies from waste incineration have shown that, at
low temperature region (200-400 °C) dioxins and other trace chlorinated
organics can be formed via complex heterogeneous reactions catalyzed by fly
ash (Altarawneh et al., 2009). Two main mechanisms have been proposed: the
surface-mediated precursor pathway and the so-called de novo synthesis from
carbonaceous matrix (Born et al., 1993b; Stieglitz, 1998). Chlorine sources (
Cl2, Cl radicals or chlorinated precursors) and metal catalysts appear to be
essential for dioxin formation (Addink and Altwicker, 1998; Procaccini et al.,
2003).
In spite of the fact that the temperatures employed in pyrolysis and
torrefaction are within the range in which chlorinated compounds are formed,
little is known regarding the potential formation of dioxins and dioxin-like
compounds in those processes. It is unclear whether their formation is
affected by fuel type and operating conditions; and how the chlorinated
organics are distributed among the vapor, liquid and solid products. These
aspects are important from a regulation perspective, regarding utilization of
biomass products from thermochemical conversion. Lack of data on
occurrences, profiles and transformation of POPs in biomass conversion
represents a crucial knowledge gap that need to be filled.
The present project involves studies on formation characteristics of POPs
from thermochemical conversion of biomass. Thermochemical processes
underlying in this thesis include microwave-assisted pyrolysis (MAP) (Paper
II and III) and torrefaction (Paper IV). The reason to select MAP and
torrefaction as main focus is that both processes are typically operated at low
temperature regions. POPs specifically addressed in this thesis are PCDDs,
PCDFs and PCNs because all of them are considered to be by-products of
thermal processes. The project also involved the development of analytical
methods (Paper I) to enable extraction and analysis of POPs in thermally
treated biomass.
Objectives
The objectives of the work underlying this thesis were to investigate: the levels
and distributions of PCDDs, PCDFs and PCNs in MAP and torrefaction; effects
8
of biomass properties on formation of PCDDs, PCDFs and PCNs; the
fingerprints (homologue profiles and isomer patterns) of chlorinated
compounds; and the underlying formation pathways in MAP and torrefaction.
The overall aim is to contribute to a better understanding of POPs formation
and transformation in thermal conversion of biomass in moderate conditions,
thereby facilitating the development of environment-friendly conversion
strategies.
Specific aspects addressed include:
 Effects of key factors (particularly solvent choice) on the efficiency of
pressurized liquid extraction (PLE) for simultaneous extraction of
various POPs from thermally treated biomass (Paper I).

Formation and distribution of PCDDs, PCDFs and PCNs in biomass MAP
processes (Paper II), focusing on levels and relative abundances of
PCDDs, PCDFs and PCNs in gas, liquid and char products.

Fingerprints (homologue profiles and isomer patterns) of PCDDs, PCDFs
and PCNs in MAP products and the underlying formation pathways
(Paper III).

PCDDs and PCDFs in biomass torrefaction, focusing on origins of dioxins
and their physical transformations (Paper IV).
9
2. Background
2.1. Biomass for energy production
Biomass, by definition, includes all biological materials derived from living
organisms. In the context for renewable energy source, it often refers to
lignocellulosic materials such as wood, energy crops, agricultural and forest
residues (Yan et al., 2009). Use of biomass to replace fossil fuel has received
increasing interest because of its potential ability to mitigate both energy crisis
and climate change derived by anthropogenic release of CO2. Currently in
Sweden, biomass contributes about 129 TWh, a fifth of the total energy supply
(ER, 2015). The biomass for energy purpose in Sweden is mainly by-products
and residues (e.g., sawdust and bark) flow in the forest and agricultural
industry (Hoffmann and Weih, 2005). This includes biomass from both
conventional forestry (e.g., pine, spruce and other softwood species) and fastgrowing woody crops with short rotation (3-15 years) such as poplar and Salix.
Waste wood, both untreated and preservative-treated, from construction and
demolition can also be used for energy recovery (Krook et al., 2004).
One of the main concerns to use wood for energy recovery from industrial
residues is the uncertainty of its contamination history and the subsequent
environmental impact. Waste wood has often been treated with preservatives
to protect against microbial and fungal decay (Krook et al., 2008). The most
common ways in Sweden for wood treatment were creosote oils or chromated
copper arsenate (CCA), or a mixture of both until end of 1970s (Sundqvist,
2009). Pentachlorophenol (PCP) were also commonly used in Sweden until
1970s for treatment (dip or pressure-impregnation) of wood products
(Sundqvist, 2009). PCP preservatives were often contaminated by dioxins to
various degrees during manufacture. Due to their persistence, both PCP and
dioxins are still present at substantial concentrations in large proportions of
treated wood that is still in service (Piao et al., 2011). One method to destroy
the dioxins when disposing of such contaminated waste wood, and thus
minimize potential environmental risks, is controlled incineration (Sundqvist,
2009). However, it is often difficult to identify whether preservatives have
been used to treat wood, and if so which preservatives, based only on the
wood’s color and odor (Krook et al., 2004).
Despite the contribution of biomass to sustainable energy supplies, there are
several limitations to its use for energy purposes. Firstly, untreated biomass
has much higher H/C and O/C ratios, and thus lower heating value than
conventional fossil fuels (Vassilev et al., 2010). Secondly, its generally
tenacious, fibrous structure and heterogeneous composition complicate
process design and control. Thirdly, agricultural production of biomass is
10
relatively land intensive and is logistically expensive due to the generally low
energy density of biomass (Lam et al., 2015). a key challenge for biomass
based systems is to develop advanced conversion technology in order to
compete with fossil fuels.
2.2. Thermal decomposition of biomass
The major components of plant biomass include cellulose, hemicellulose,
lignin and, to lesser extent, organic extractives and inorganic minerals
(Vassilev et al., 2010). Cellulose, hemicellulose and lignin constitute the
polymeric structure of plant cell walls, and their contents vary substantially
depending on the species and parts of the plants the biomass originated from.
Generally, hardwood contains less lignin than softwood but the cellulose
content is more or less the same (McKendry, 2002).
Hemicellulose, cellulose and lignin decompose in different temperature
ranges: 150-300 °C, 300-400 °C and 280-500 °C, respectively (Basu, 2010).
Decomposition processes of these three main components of biomass are
complex, but dehydration and decarboxylation are the main reactions
involved. The main reaction pathways have been categorized into a few
reaction regimes (Bergman et al., 2005), as shown in Figure 1. These include
an initial dehydration and non-reactive temperature stage, followed by
polymer softening, depolymerization, and carbonization/mass loss stages.
The decomposition processes for each polymer are similar, although they
occur at different temperatures. Hemicelluloses have much more diverse
structures than cellulose and lower thermal stability (or less resistance) due
to their lack of crystallinity. Thermal degradation of hemicellulose can start at
temperatures as low as 200°C (Manuel and Metcalf, 2008). The species
formed in the initial depolymerisation or fragmentation reactions may
undergo additional secondary thermal decomposition reactions to form
volatile
products.
Alternatively,
they
may
be
involved
in
condensation/polymerization reactions that result in formation of high molar
mass products like char (Manuel and Metcalf, 2008).
11
Figure 1. Decomposition regimes of lignocellulosic materials during thermochemical conversion
(Bergman et al., 2005).
Low temperature dehydration and depolymerization of cellulose are
important processes in both slow pyrolysis and torrefaction. Thermal
treatment of wood at temperatures under 250°C leads to thermal degradation
of its constituents and changes in crystallinity (Lam et al., 2015). The
reduction in the degree of polymerization when heating the cellulose for hours
can be associated with the formation of free radicals, elimination of water,
formation of carbonyl and carboxyl groups, as well as the evolution of carbon
dioxide (Sandberg et al., 2013). Moreover, the presence of small amounts of
inorganic impurities (less than 0.1 %) can considerably alter the pyrolysis and
combustion characteristics of cellulose and promote the formation of
hydroxyl-acetaldehyde via fragmentation or open-ring reactions (Lam et al.,
2015).
12
The main products of the thermal decomposition of hemicelluloses found
include acetic acid, furans, and various mono- and oligo-pentoses (Lam et al.,
2015). The thermal degradation of lignin results in the formation of
monomeric phenols, guaiacols and syringols, formic acid, formaldehyde,
methanol, CO2 and water (Sandberg et al., 2013). Generally, the thermal
degradation of lignin can be described by a competitive mechanism involving
depolymerization and condensation/carbonization reactions. Chain
depolymerization is expected to occur via successive formation of the new
phenolic structure through cleaving the phenolic end structure. The phenolic
α and β-ester types of dimers became reactive at temperatures around 200250°C, and are among the main products in lignin pyrolysis (Lam et al., 2015).
Thermal decomposition of lignocellulosic biomass results in formation of,
among other substances, a complex mixture of condensable hydrocarbons,
known as tar. The mixture consists of single- to five-ring aromatics, phenolic
compounds and complex polycyclic aromatic hydrocarbons (PAHs)
(Wolfesberger et al., 2009). The amounts and nature of tar formed largely
depend on the feedstock properties, the presence of catalysts and operating
conditions. At moderate temperatures (about 500 °C), the tar-like products
are highly branched, while at high temperatures (over 700 °C) they tend to be
highly condensed and less oxygenated. Naphthalene, one of the most
abundant light tar components, can act as a precursor for PCN formation via
direct chlorination (Jansson et al., 2008). Other PAHs in tar products can also
act as precursors for formation of chlorinated aromatics, via de novo synthesis
and other pathways (Weber et al., 2001).
2.3. Thermochemical conversion
Thermochemical conversion is the controlled heating or oxidation of
feedstocks to generate energy products and/or heat (Demirbaş and Arin,
2002). It covers a range of technologies including combustion,
gasification, pyrolysis and torrefaction (Van der Stelt et al., 2011).
Gasification, pyrolysis and torrefaction are all considered to be processes in
which materials are thermally decomposed in the absence of oxygen, or
significantly less oxygen is present than required for complete combustion
(Tanger et al., 2013). Complete combustion involves the production of heat via
oxidation of carbon- and hydrogen-rich biomass to CO2 and H2O. Gasification
is the exothermic partial oxidation of biomass with operating conditions
optimized for high yields of gas products rich in CO, H 2 and CH4. Pyrolysis is
the thermal decomposition of biomass in the absence of oxygen. There are no
clear boundaries between the three processes. For example, pyrolysis can be
considered an incomplete gasification process, and torrefaction as an initial
process of both gasification and pyrolysis.
13
The yields of major products (gas, oil and char) vary depending on the
operating conditions (particularly temperature, residence time and oxygen
supply) (Basu, 2013). Fast heating rates and moderate temperature favor
generation of liquid products (pyrolysis), low temperature and long residence
times primarily result in charcoal (torrefaction), while gases (condensable and
non-condensable) are largely produced at high temperature and heating rate
(gasification). The major differences between combustion, gasification,
pyrolysis and torrefaction, in terms of operating conditions, are illustrated in
Figure 2. The studies underlying this thesis primarily focused on pyrolysis
(Papers II and III) and torrefaction (Paper IV), thus the following
description of conversion technologies mainly addresses these two processes.
Figure 2. Comparison of four biomass thermochemical conversion processes — combustion,
gasification, pyrolysis and torrefaction — showing the major products. Figure from Yin et al.
(2012), with modification by the author of this thesis.
2.3.1. Pyrolysis
Pyrolysis is the thermochemical decomposition of biomass at elevated
temperature in the absence of oxygen. The process is endothermic, and
usually carried out in the temperature range 300-650 °C (Mohan et al., 2006).
The main initial products of pyrolysis are condensable gases (bio-oil or
pyrolysis oil) and solid char. Some of the condensable gas may further
decompose into secondary products including CO 2, H2 and CH4. The yields
and chemical composition of pyrolysis products depend on feedstock
14
properties, pyrolysis temperature and heating rate. Pyrolysis can be classified
as slow, fast and flash pyrolysis depending on the heating rate, residence time
and reaction rate (Mohan et al., 2006). In fast pyrolysis, the vapor residence
time is about a few seconds and the primary products are bio-oil and gas. In
slow pyrolysis residence times are longer (up to a few minutes), and the
primary product is char.
In addition to conventional pyrolysis, the integration of microwaves and fast
or flash pyrolysis process, known as microwave-assisted pyrolysis, as a
novel conceptual design for biomass conversion, has become increasingly
accessible at both pilot and industrial scales (Yin, 2012). Unlike conventional
pyrolysis where heat is transferred from the surface towards the core of the
feedstock by conduction and convection (Mohan et al., 2006), in MAP thermal
energy is transferred from electromagnetic energy by “dielectric heating”
(Mushtaq et al., 2014). It is a process of energy conversion rather than heating.
Microwave irradiation generates in-core volumetric heating by directly
coupling microwave energy with exposed biomass (Shuttleworth et al., 2012).
It has been reported that the temperature at which biomass decomposes
during MAP was lower than in conventional pyrolysis (Budarin et al., 2010),
partly due to water evaporation, which both removes considerable amounts of
energy from the active centers (thereby cooling them) and redistributes this
energy throughout the rest of the material (Robinson et al., 2010; Fan et al.,
2013).
2.3.2. Torrefaction
Torrefaction is a thermochemical process in oxygen deficient conditions.
Biomass temperatures during torrefaction are typically between 200°C and
350°C, and its residence time ranges from a few minutes to several hours
(Nordin et al., 2013). The torrefaction process is often operated at ambient
pressure, in an inert atmosphere to avoid oxidation and combustion of the
biomass. It is also known as mild and slow pyrolysis although pyrolysis
processes are usually operated at temperatures above 350°C. The word
torrefaction originates from the French word torréfaction, meaning roasting,
typically of coffee beans, a process similar to torrefaction of biomass, but using
air at a relatively low temperature (Basu, 2013). Torrefaction is initiated with
moisture evaporation, followed by partial devolatilization. Part of the biomass
volatilizes and forms a torrefaction gas, which can be combusted and the heat
can be used for biomass drying and for heating of the torrefaction process
(Bergman et al., 2005). Two torrefaction regimes can be defined, depending
on the processing temperature: light and severe. In light torrefaction, at
temperatures below 240 °C, only hemicellulose is decomposed, while lignin
and cellulose are not affected substantially (Bilgic et al., 2016). In severe
torrefaction (at temperatures exceeding 270 °C), cellulose and lignin start to
15
decompose. One of the fundamental advantages of the torrefaction process is
energy densification (typically by a factor of 1.3) via reduction of the O/C and
H/C ratios of the biomass (Van der Stelt et al., 2011). Typically, in woody
biomass torrefaction, about 70% of the mass is retained as a solid product,
which contains 90% of the energy content of the untreated material. The
remaining 30% of the mass is converted into torrefaction gas, which normally
contains only 10% of the initial energy of the biomass (Van der Stelt et al.,
2011). Torrefied biomass has been demonstrated to be suitable as fuel for
various applications, particularly entrained flow gasification, small-scale
combustion and co-firing in pulverized fired power stations (Bergman et al.,
2005). A higher biomass to coal ratio can be used with torrefied fuels than
with untreated biomass (Basu, 2010).
2.4. Dioxins and dioxin-like compounds
2.4.1. General description
Dioxin is a collective term for PCDDs and PCDFs. They are chlorinated
aromatic hydrocarbons that are similar in structure and chemical properties.
They consist of two benzene rings that are interconnected by either two
oxygen bridges (PCDDs) or one oxygen bridge and a single C-C bond (PCDFs),
as shown in Figure 3. Chlorine substitution can occur at carbon atoms
numbered 1 – 9 in the structures shown in Figure 3, resulting in a total of 75
and 135 individual PCDD and PCDF congeners, respectively. PCDDs or
PCDFs with the same number of chlorine atoms constitute a homologue
group of isomers. The relative abundance of congeners within each
homologue group is referred to as an isomer distribution pattern. There
are 16 homologue groups in total, 8 of PCDDs and 8 of PCDFs (Table 1). For
example, within the pentachlorinated homologue group, there are 10 and 16
different PCDD and PCDF isomers, respectively.
Figure 3. PCDD, PCDF and PCN structures and substitution positions.
16
Some chlorinated compounds have been classified as “dioxin-like” due to
the similarity of their chemical structures, physicochemical properties and
toxic behaviors. These include some non-ortho- or mono-ortho-substituted
(tetra- to hepta-) polychlorinated biphenyls (PCBs). Another emerging group
with dioxin-like properties is PCNs. PCNs consist of two fused benzene rings
that share two carbon atoms. Chlorination can occur at positions 1-8 (Figure
3), resulting in a total of 75 different PCN congeners (Table 1). Only laterally
substituted tetra- to octa-CNs are considered to be dioxin-like.
Table 1. Numbers of isomers in each group of PCDD, PCDF and PCN
homologues
Homologues
MonoDiTriTetraPentaHexaHeptaOcta-
Number of Cl
Number of isomers
PCDD
PCDF
PCN
2
10
14
22
14
10
2
1
4
16
28
38
28
16
4
1
2
10
14
22
14
10
2
1
75
135
75
1
2
3
4
5
6
7
8
Total
2.4.2. Toxicity of dioxins
Dioxins in the environmental media as well as biological samples exist as
mixture of various congener. The toxicity varies substantially among the
different congeners. Of the 75 PCDDs and 135 PCDFs, only those fully
substituted in the lateral (β- or 2,3,7,8-) position are toxic (Van den Berg et
al., 2006). Many congeners are present at substantially higher concentrations
than 2,3,7,8-TeCDD.
To facilitate and simplify risk assessment of potential exposure to dioxins,
various toxicity equivalency factors (TEFs) systems have been developed
to express the relative toxicity. 2,3,7,8-TCDD which is considered to be the
most toxic congener of the PCDD/Fs is assigned a TEF value of 1 and the
remaining congeners with a lower toxicity relative to 2,3,7,8-TeCDD have
lower TEF values than 1 (Van den Berg et al., 2006). The criteria to include a
17
compound in a TEF scheme include a structural relationship to the PCDD/Fs,
binding to the aryl hydrocarbon receptor, persistence and accumulation in the
food-chain. By multiplying the mass or concentration of each dioxin congener
with its corresponding TEF-value, toxic equivalents (TEQs) of each
congener concentration are obtained. The overall toxicity of dioxins in a
sample can then be simply calculated by summing the TEQ for all of the
detected congeners. A number of TEQ/TEF systems have been developed in
recent decades. The two most commonly used are the International TEF (ITEF) system which used in the EN 1948 standard (ECS, 2006) and the WHOTEQ system developed by the World Health Organization (Van den Berg et al.,
2006). The WHO system is similar to the I-TEF scheme, except for the
inclusion of TEFs for dioxin-like coplanar PCBs and the assessment of
pentachloro- and octachloro-congeners. I-TEF values were used for TEQ
evaluations in the studies underlying this thesis since coplanar PCBs were not
analyzed. TEFs of the 17 toxic PCDD/Fs according to the I-TEF system are
summarized in Table 2.
Although TEF values have not yet been determined for most PCNs, some PCN
congeners have been suggested to be active enzyme inducers and bind to the
aryl hydrocarbon receptor (Falandysz, 1998). Two of these congeners,
1,2,3,4,6,7-HxCN and 1,2,3,5,6,7-HxCN, have been frequently identified in
human and environmental samples, and identified as highly persistent and
bioaccumulating (Hooth et al., 2012). Thus, inclusion of PCNs in the WHOTEF scheme has been proposed (Van den Berg et al., 2006).
Table 2. Toxic equivalency factors (I-TEFs) for PCDDs and PCDFs
PCDDs
2,3,7,8-TeCDD
TEF
PCDFs
1
2,3,7,8-TeCDF
TEF
0.1
1,2,3,7,8-PeCDD
0.5
1,2,3,7,8-PeCDF
0.05
1,2,3,4,7,8-HxCDD
0.1
2,3,4,7,8-PeCDF
0.5
1,2,3,6,7,8- HxCDD
0.1
1,2,3,4,7,8-HxCDF
0.1
1,2,3,7,8,9- HxCDD
0.1
1,2,3,6,7,8- HxCDF
0.1
0.01
2,3,4,6,7,8- HxCDF
0.1
0.001
1,2,3, 7,8,9- HxCDF
1,2,3,4,6,7,8-HpCDD
OCDD
0.01
1,2,3,4, 7,8,9- HpCDF
0.01
OCDF
18
0.1
1,2,3,4,6,7,8- HpCDF
0.001
2.4.3. Formation mechanisms
Thermal formation of PCDD and PCDF can occur both homogeneously (in
gas phase) and heterogeneously (on solid surfaces such as soot or ash). The
homogeneous pathway involves gas phase reactions with precursors at
temperatures of 400-800 °C. PCPh and PCBz are among the most important
precursors (Born et al., 1993). Heterogeneous formation proceeds via two
surface-catalyzed reaction pathways. One is known as de novo synthesis, a
process involving simultaneous oxidation and chlorination from a
carbonaceous matrix at temperatures between 200 and 400 °C (Stieglitz,
1998). The other is the precursor pathway, involving metal catalysts
(mainly Cu and Fe) at temperatures between 200 and 400 °C. The
homogeneous pathway in gas phase is much less important than the two
heterogeneous (de novo synthesis and catalyst-related precursor) pathways.
De novo synthesis is one of the dominant pathways for PCDF formation,
whereas PCDDs often originate from precursors with limited contributions of
de novo pathways (Hell et al., 2001; Wikström et al., 2004).
Precursor pathway involves condensation of structurally-related
precursors, particularly phenolic species. Homogeneous reactions occur via
gas phase condensation of precursors, whereas the heterogeneous pathway
involves reactions promoted by transition metals (mainly Cu and Fe).
Chlorophenols can either be formed via catalyzed reactions or released
directly from the fuel. Chlorobenzenes can also act as precursors. However,
the formation rate from chlorobenzene is much slower (two orders of
magnitude) than formation from corresponding chlorophenols (Ghorishi and
Altwicker, 1996). It is likely that the formation follows the same phenoxy
radical intermediates, which can be formed by oxidation of chlorobenzenes.
The formation pathway from precursors is mainly associated with the
coupling of molecule/molecule, molecule/radical, or radical/radical species.
An important step is formation of the phenoxy radical from a phenol molecule.
Under pyrolytic conditions, formation of chlorophenoxy radicals is mainly
initiated through thermal decomposition of chlorophenol species. O-H bond
fission or cleavage (R1) is believed to be more important than direct expulsion
of a Cl atom, based on calculated rate constants (Evans and Dellinger, 2003):
2-C6H4ClOH  2-C6H4ClO· + H
(R1)
When oxygen is present, decomposition of chlorophenol can commence at
temperatures 150 K lower. An oxygen molecule can react initially with
chlorophenol by abstraction of its phenolic H to form HO 2:
2-C6H4ClOH + O2  2-C6H4ClO· + HO2
19
(R2)
In the presence of Cl atoms, Cl radical is the dominant abstractor of the
phenolic H, resulting in HCl production:
2-C6H4ClOH + Cl  2-C6H4ClO· + HCl
(R3)
Phenoxy radical exhibits low reactivity with oxygen and does not undergo
decomposition. Phenoxy radical has a resonance structure and the radical can
be localized not only on the phenolic oxygen, but also on the para- and orthocarbons (Figure 4). Thus, self-condensation of phenoxy radical can occur via
the ortho- and para-carbon and the phenolic oxygen. The major product of
self-coupling is believed to be para-para dimers, based on molecular spin
density of orbital theory (Libit and Hoffmann, 1974). Experimental study
under pyrolysis condition showed that, at low temperature (500-850 k), the
major product of the self-dimerization of the phenoxy radicals was via orthoortho coupling (Wiater et al., 2000). The para-para coupling was less
important due to the relatively high activation energy. Instead, kinetic process
related to activation energy is a dominating factor in terms of the formation of
ortho-ortho coupling (Armstrong et al., 1983). Regarding the selfcondensation of chlorinated phenoxy radicals, pathway via ortho-carbons
substituted with chlorine atoms could be inhibited due to steric effect (Weber
and Hagenmaier, 1999).
Figure 4. Phenoxy intermediates with radical localized at different positons.
The formation of PCDFs is exclusively the result of the condensation of two
radicals, while the formation of PCDDs can involve radical/radical,
molecule/radical or molecule/molecule coupling (Evans and Dellinger,
2003). Among the different type of coupling in PCDD formations, the
radical/radical pathway is kinetically favoured. The chlorination patterns of
chlorophenols for PCDD formation also differ from those required for PCDF
formation. Formation of PCDDs requires both chlorophenols to have at least
one ortho-chlorine, while formation of PCDFs requires both chlorophenols to
20
have one available ortho-hydrogen. This explains the exclusive formation of
PCDDs from 2,3,6-trichlorophenoxy intermediate since there is not orthohydrogen available for PCDF formation (Altarawneh et al., 2009).
De novo synthesis involves oxidative decomposition of a carbonaceous
matrix (e.g., soot, activated carbon or char). Carbon with oxygen incorporated
in the macromolecule yield greater amounts of PCDDs and PCDFs than more
ordered carbon sources such as graphitic matrices (Tame et al., 2007). Two
basic reactions are involved, chlorination and oxidation: inorganic chlorine is
initially transferred to the macromolecular carbonaceous structure and form
C-Cl bond, then oxidative degradation of the structure.
The de novo synthesis pathways should not be considered separately from the
precursor pathway. Rather, the formation of dioxins proceeds through
numerous reactions, some of which can be considered as de novo synthesis by
definition, whereas others involve precursor intermediates. Figure 5 presents
a schematic of de novo formation of PCDDs and PCDFs (Tame et al., 2007)
showing a number of routes involved in de novo processes. For example, the
de novo pathway can be initiated by oxidation of a carbon matrix to form CO,
CO2, short chain molecules, benzene and macromolecules. The formation of
PCDDs and PCDFs involves: i) precursor molecules such as (chloro)phenol
intermediates; ii) direct formation from surface-bound chlorinated or nonchlorinated aromatic intermediates such as PAHs or phenolic compounds;
and iii) direct release of pre-existing PCDD and PCDF structures during
oxidation, when oxygen is incorporated in the carbon skeleton). The de novo
pathway occurs primarily in the 200-400 °C temperature range. Oxygen is
necessary for de novo formation of PCDDs and PCDFs, and their formation is
most favourable with O2 concentrations around 5-10% in the reactant stream
(Addink and Olie, 1995).
21
Figure 5. Schematic diagram of de novo formation of PCDDs and PCDFs. The red arrows
indicate dominant routes (Tame et al., 2007).
Chlorination/dechlorination mechanisms have been investigated
extensively. The chlorine in the feedstock can be released mainly (>90%) as
HCl. HCl is normally first oxidized to Cl2 gas, and subsequently chlorinate
aromatics, rather than acting directly as a chlorination agent. The Deacon
process is believed to be important for conversion of HCl to Cl2 in the presence
of O2 and a metal catalyst (Hisham and Benson, 1995). Copper chloride
species (CuCl and CuCl2) are the most important Deacon catalysts with
divalent copper chloride being most efficient. The overall reaction can be
summarized as:
HCl + 1/4O2  ½ H2O + ½ Cl2
(R4)
The process involves two steps as shown in R5 and R6. The Cl2 formed in the
reactions can subsequently participate in chlorination of aromatics. It has
22
been shown that the Cl2 yield during the Deacon process was maximal at 400
°C, while at 300 °C the yield was close to zero (Gullett et al., 1990).
CuCl2 + ½ O2 CuO + Cl2
(R5)
CuO + 2 HCl  CuCl2+ H2O
(R6)
The Cl2 yield from HCl conversion via the Deacon reaction depends on oxygen
content. The Cl2 yield is positively correlated to the O2 concentration in the
range of 0-3% (Gullett et al., 1990). The Deacon reaction is favored at
temperatures higher than 600 °C in the presence of high concentrations of
HCl (Liu et al., 2000). Interestingly, the presence of water vapor in the
reactant stream can shift the Deacon reaction equilibrium to formation of HCl,
thereby suppressing Cl2 liberation (Wikström et al., 2003a). Thus, the
chlorination capacity is reduced and the homologue profile can be shifted
towards more lightly chlorinated groups. It should be noted that the
importance of the Deacon reaction in the formation of dioxins has been
questioned, and an alternative chlorination route involving aromatic
compounds via reaction with CuCl2 has been suggested (Born et al., 1993a).
Dechlorination of PCDDs and PCDFs requires the presence of a catalyst of
copper or other metal compound (Weber et al., 2002). The most important
parameters in determining the rate of degradation were the reaction time and
temperature, but at relatively low temperatures, the temperature had a greater
influence than the reaction time. Longer reaction times and higher
temperatures both increased the degradation efficiency (Song et al., 2008).
PCN formation mechanisms have not been studied as extensively as those
of PCDDs and PCDFs. Some studies suggested that PCNs can be formed via
both de novo mechanism from carbon matrices and hydrocarbons, similar as
PCDDs and PCDFs (Ryu et al., 2013). Precursor pathways involving
chlorinated compounds have also been reported, and chlorination of non- and
less chlorinated aromatics is believed to be important in PCN formation (Ryu
et al., 2013). In addition, it has been proposed that the formation of PCNs
correlates with that of PCDFs (Oh et al., 2007). The further chlorination of low
chlorinated compounds is believed to be important in PCN formation
(Jansson et al., 2008).
23
3. Materials and methods
Formation of POPs depends on feedstock properties and operating conditions
of thermal conversions. The presence of Cl and transition metals are among
the crucial factors. To study the effect of biomass composition on the
formation of PCDDs, PCDFs and PCNs, various biomass feedstocks
containing different amounts of Cl and Cu and with different contamination
profiles was chosen in the experiment. Two types of technologies, MAP and
torrefaction were selected to represent thermal conversion at moderate
condition. MAP has a high heating rate and short reaction time; while
torrefaction has a low heating rate but with similar temperature. Both are
under oxygen deficient condition.
3.1. Feedstocks
Five biomass feedstocks (Figure 6) were selected for the studies: a very clean
conifer (Norway spruce and Scots pine) stemwood assortment; Norway spruce
bark with higher Cl and Cu contents than the stemwood; impregnated
stemwood (probably treated with organic and metal-based preservatives)
from a discarded telephone pole containing high amounts of heavy metals
(35.9 and 362 mg kg-1 of Cr and As, respectively); cassava stems with high Cl
contents; and particle board containing PCP preservatives. Information on the
five feedstocks (including their Cl, Cu and Fe contents) is summarized in Table
3. More details about their chemical properties and energy contents are
presented in the paper II-IV. The first three feedstocks (stemwood, bark and
impregnated stemwood) were used in both MAP (Papers II and III) and
torrefaction (Paper IV). The cassava stems and particle board were used only
in torrefaction (Paper IV). The biomass used in Paper I to evaluate the PLE
method included torrefied Eucalyptus and spruce wood chips, the chemical
compositions of which were not analyzed.
The term of stemwood used in this thesis was also referred to as softwood in
Paper II and III, since it was from a softwood species. However, for
consistency in terminology, the term stemwood is constantly used in the
thesis.
24
Figure 6. Feedstocks used in MAP experiments (A, B and C) and torrefaction experiments (A to
D): stemwood pellets (A), bark pellets (B), impregnated stemwood (C), cassava stems (D), and
particle board (E).
25
Table 3. Selected information on feedstocks used in experiments reported in
Papers I-IV.
Chemical composition
Feedstocks
Stemwood
(Papers II-IV)
Bark
(Papers II-IV)
Impregnated
stemwood
Description
Cl, %
Cu,
mg kg-1
Fe, %
Ash, %
<0.01
0.97
0.001
0.4
A clean assortment (pellets) made
from a mixture of bark-free
Norway spruce and Scots pine
0.02
3.39
0.04
3.9
Norway spruce bark pellets
<0.01
3.38
0.006
0.5
Wood chips from a discarded
Scots pine telephone pole,
presumably impregnated with
organic and metal-based
preservatives
0.29
3.56
0.006
3.8
Cassava stem pellets made from
stem residues after cropping the
starchy roots
0.15
11.4
0.07
2.7
Composed of wood chips, sawmill
shavings and sawdust with
synthetic resin or other binder,
collected at a waste disposal site
-
-
-
-
(Papers II-IV)
Cassava stems
(Paper IV)
Particle board
(Paper IV)
Eucalyptus
chips
Torrefied at 300 °C for 16 min
(Paper I)
Spruce chips treated by PCP
impregnation followed by
torrefaction (270 °C for 50 min)
Spruce wood
(Paper I)
26
3.2. Experimental setup of microwave-assisted pyrolysis and
torrefaction
MAP and torrefaction were performed at York University (UK) and Swedish
University of Agricultural Sciences (SLU, Umeå), respectively. Both
experiments were conducted at bench-scale (Figure 7 and 8).
The sampling setups both consisted of condensers and gas samplers. For the
MAP experiments there were two condensers connected in series with
different cooling temperatures (0 °C and room temperature), enabling
fractionation of liquid products into oil and aqueous phases during sampling.
In torrefaction experiments the liquid products were collected by a single
condenser with no fractionation of aqueous and oil phases. In both cases, the
non-condensable vapor (gas products) that escaped from the cooling traps
was collected on a glass fiber filter followed by a polyurethane foam plug
(PUF). Vacuum units were connected to the outlet of the gas samplers to
facilitate continuous extraction of volatile products. For torrefaction, nitrogen
was applied to the reactor to ensure an oxygen-deficient atmosphere. All
experiments were run in triplicate, and duplicate field blanks were prepared.
27
Figure 7. MAP experimental setup and schematic diagram of sample collection equipment: 1Controller, 2- Microwave reactor, 3- First condenser, 4- Pyrolysis liquid collector, 5- Second
condenser, 6- Pyrolysis liquid collector, 7- Gas sampling point, 8- Electromagnetic valve, 9Vacuum pump.
28
Figure 8. Torrefaction experimental setup and schematic diagram of sample collection
equipment: 1- Furnace, 2- Tubular reactor, 3- Condenser, 4- Condensables collector, 5- Gas
sampling point, 6- Electromagnetic valve, 7- Vacuum pump.
29
3.3. Sample extraction and cleanup
PCDDs, PCDFs and PCNs in gas, liquid and char products were extracted by
Soxhlet extraction, liquid-liquid extraction and PLE, respectively, using
toluene as the extraction solvent. Prior to extraction, 13C-labelled internal
standards were added to the samples. PLE was performed at 160 °C using
three extraction cycles, a flush volume of 60%, 5 min static time and 90 s purge
time.
After extraction samples were cleaned-up by multilayer silica column, then
fractionated using an AX21 carbon/celite column (Figure 9). Corresponding
recovery standards were added to the final solution before gas
chromatography/high resolution mass spectrometry (GC/MS) analysis
(Liljelind et al., 2003). 13C-labeled internal standards were added to samples
before extraction, and recovery standards were added to the final extracts
before instrumental analysis.
3.4. Instrumental analysis
PCDDs, PCDFs and PCNs were analyzed by GC/MS, using a Hewlett-Packard
5890 gas chromatograph (Agilent) coupled to an Autospec Ultima mass
spectrometer, equipped with a J&W DB5-ms fused silica capillary column (60
m × 0.25 mm i.d. × 0.25 µm film thickness). For the isomer-specific analysis
in Paper III, samples were re-injected onto a SP 2331 column (60 m × 0.25
mm i. d. × 0.25 µm film thickness) to resolve isomers co-eluting on the DB5ms column. Separation of PCN isomers was performed on a J&W fused silica
capillary column DB5 (60 m × 0.25 mm i. d. × 0.25 µm film thickness). Details
of GC/MS parameters for analyzing PCPh, PCBz and PAHs are described in
Paper I.
The analytes were quantified using the isotope dilution method. Field and
laboratory blanks were treated in the same manner as the samples. Data with
recoveries outside the acceptable range of the EN 1948 standard method (ECS,
2006) were excluded.
30
Stemwood
Cassava
stems
Impregnated
stemwood
Bark
Paper IV
Papers II &III
Microwave-assisted pyrolysis
Torrefaction
Liquid
Gas
Soxhlet
Particle
board
Liquid extraction
extraction
Char
PLE
Extracts
Multilayer silica column
Carbon/Celite column
GC/HRMS
Figure 9. Experimental procedures applied to determine PCDDs, PCDFs and PCNs from
pyrolysis and torrefaction.
31
4. Results and discussion
4.1. Solvent effects in pressurized liquid extraction
This section outlines findings from the analytical methodology evaluation, in
which assessed the applicability of PLE for simultaneous extraction of PCDDs,
PCDFs, PCN, and related precursors from thermally treated biomass (Paper
I). The optimized PLE method developed in the study was subsequently
applied to extract chlorinated compounds in char products obtained via MAP
or torrefaction in Paper II-IV.
Quantitative analysis of multiple chlorinated organics in thermally treated
biomass is challenging due to their ultra-trace levels and the highly complex
matrix. Extractions of target compounds often involve Soxhlet extraction,
which is often time- and labor- intensive, and requires large amounts of
organic solvents. PLE involves extraction with organic solvent at elevated
temperature and pressure, enabling exhaustive extraction of target
compounds with relatively short extraction time and low solvent
consumption. However, as with all other extraction techniques, the extraction
efficiency of PLE is matrix-dependent and the reported methods for
environmental samples (e.g., soils and fly ash) cannot be directly applied to
thermally treated biomass.
The PLE method development was divided into three stages: screening,
optimization and validation, as illustrated in Figure 10. In the screening
stage, a series of PLE experiments was performed to evaluate the performance
of five solvents with different polarities: n-hexane, toluene, dichloromethane
(DCM), methanol and acetone. The extraction efficiency of each solvent was
examined by comparing the recoveries of spiked internal standards and the
contents of co-extracted matrix material. In the optimization stage, the two
best solvents identified during screening were used as a binary solvent
mixture, and its performance in PLE was compared to that of the individual
solvents. In the validation stage, PLE with the best individual solvent and the
binary mixture was further evaluated. The PLE method was also compared
with Soxhlet extraction. The entire experimental design is illustrated in Figure
10.
32
Solvent screening
Untreated
eucalyptus
Torrefied
Eucalyptus
•
•
•
•
•
Torrefied
Eucalyptus
Untreated
eucalyptus
PLE
•
•
•
•
•
Method Validation
Optimization /Binary solvent
PCP-impregnated
spruce chips
(untreated)
PLE
PLE
Methanol
Acetone
Dichloromethane
Toluene
n-Hexane
PCP-impregnated
spruce chips
(torrefied)
•
•
Toluene
Toluene/n-Hexane
(1:1)
Soxhlet extraction
•
•
Toluene
Toluene/n-Hexane
(1:1)
Open column cleanup
Open column cleanup
Open column cleanup
GC/HRMS
GC/HRMS
GC/HRMS
Toluene --> Satisfied recoveries
n-Hexane --> Satisfied recoveries
Methanol --> Matrix effect
Acetone --> Matrix effect
Dichloromethane --> poor reproducibility
•
•
Toluene --> Satisfied
Toluene/n-Hexane --> Satisfied
•
•
Toluene --> Best performance
Comparable results by PLE with those
of Soxhlet extraction
Figure 10. Schematic diagram of experimental design and results of PLE method development (Paper I)
33
Solvent screening was conducted by evaluating recoveries of spiked
internal standards using five solvents with different polarities: methanol,
acetone, DCM, toluene and n-hexane. As shown in Figure 11, recoveries from
torrefied wood were poor when using solvents with high polarity such as
methanol and acetone. Extraction of untreated wood was less solventdependent and comparable results were obtained with polar and non-polar
solvents. Toluene provided the best performance of the five investigated
solvents. Amounts of co-extractives obtained indicated that the poor
recoveries from torrefied biomass provided by polar solvents were related to
matrix effects. For example, methanol gave rise to about 6-fold more coextractives from torrefied samples than toluene (ca. 1.92 and 0.32% of the
sample mass, respectively).
The high co-extractive yields are probably due to co-extraction of thermally
degraded lignocellulosic compounds. The degraded materials that are rich in
hydroxyl groups may be more soluble in polar solvents with high hydrogen
bonding capability (e.g., acetone and methanol), in accordance with the “like
dissolves like” principle. The consequently higher contents of co-extractives in
polar solvent extracts may interfere with subsequent analyses. Indeed, we
observed that residues of methanol extracts following solvent removal were
dense, highly viscous, gelatinous and (hence) difficult to re-dissolve and
transfer to the cleanup columns. Clearly, some of the analytes may be
encapsulated in such residues, leading to losses of target compounds.
The performance of a solvent mixture was evaluated after the solvent
screening. As the screening experiments revealed that polar solvents with
hydrogen-bonding potential extracted large quantities of interfering
materials, binary mixtures featuring acetone were not investigated. Instead, a
mixture of n-hexane and toluene (1:1) was tested in the hope of achieving
similar extraction efficiency to that for toluene alone while releasing less coextracted material than when using either of the individual solvents. The
results showed that combination of toluene and n-hexane (1:1) gave
comparable recoveries as those by toluene; while the co-extractive content of
the solvent mixture from the torrefied wood sample was lower (0.24%) than
that achieved using toluene alone.
Method validation was conducted using PCP-impregnated spruce wood
(both untreated and torrefied). Technical PCP normally contains relatively
large quantities of impurities including PCDDs and PCDFs, and has been
widely used as a wood preservative. Impregnation of spruce chips with
technical PCP were aiming to ensure that the torrefied material would contain
measurable quantities of PCDDs and PCDFs so that the developed PLE
34
method could be validated against low, intermediate and high levels of the
target analytes in a single sample.
Comparable results were obtained using toluene and the binary mixture for
most of the PCDDs and PCDFs. However, toluene provided higher yields of
some PCNs, PAHs, and most of the PCBzs and PCPhs in the torrefied wood
samples. The difference in performance between toluene and the binary
mixture was less pronounced for the untreated wood sample. This indicates
that analyte-matrix interactions are stronger in torrefied wood than in
untreated wood, and that toluene disrupts these interactions more efficiently
than the binary mixture. The comparison of PLE and Soxhlet results showed
that the PLE method with toluene provided similar performance to traditional
Soxhlet methodology for extracting PCDDs and PCDFs, and better
performance for extracting PCNs, PAHs, PCBz and PCPh.
In summary, our results indicated that the choice of solvent for PLE is critical
because the extraction efficiency depends on the nature of the biomass matrix
as well as properties of the target analytes. Solvents with high polarity release
high amounts of interfering co-extractives from thermally degraded
hemicellulostic biomass, while non-polar solvents such as hexane do not
efficiently extract the target analytes from torrefied wood.
35
A
1,2
1
0,8
0,6
0,4
0,2
0
Hexane
Toluene
PCDD
B
DCM
PCDF
Acetone
Methanol
PCN
1,2
1
0,8
0,6
0,4
0,2
0
Hexane
PCDD
Toluene
PCDF
DCM
PCN
PAH
Acetone
PCBz
Methanol
PCPh
Figure 11. Recoveries of PCDD, PCDF, PCN, PAH, PCBz and PCPh from untreated (A) and
torrefied wood (B) using five indicated extraction solvents (Paper I). Error bars represent ± 1
standard deviations.
36
4.2. PCDDs, PCDFs and PCNs in microwave pyrolysis
This subsection provides findings from MAP studies (Paper II and III).
Paper II reports levels and relative distributions of the chlorinated
compounds detected among gaseous, liquid and char products. In Paper III,
we investigated the homologue profiles and isomer patterns in MAP products,
and proposed possible formation pathways in MAP based on fingerprints of
the products (Paper III).
As mentioned above, three feedstocks were used in MAP experiments:
stemwood, bark and impregnated stemwood. The resulting non-condensable
gases, liquids (oil and aqueous phases) and chars were collected and analyzed
for PCDDs, PCDFs and PCNs. The entire MAP process took around 10 min,
including heating up time. The temperature profile over the course of the MAP
process varied slightly with the feedstock used (Figure 12). Pyrolysis of the
three feedstocks under these conditions yielded liquid fractions amounting to
34-40% of the original feedstock mass, in accordance with results of previous
MAP studies using wood and agricultural biomass as feedstocks at comparable
temperatures (Budarin et al., 2009; Robinson et al., 2010).
700
600
200
500
150
400
100
300
200
50
100
0
Pressure, mbar
Temperature, °C
250
0
1
3
5
7
9
Time, min
T (stemwood)
T (impregnated stemwood)
P(bark)
T (bark)
P(stemwood)
P (impregnated stemwood)
Figure 12. Temperature (T) and pressure profiles during MAP treatment of the indicated
feedstocks (Paper II).
37
4.2.1. Levels and relative distributions
Concentrations of PCDDs, PCDFs and PCNs in the MAP products varied
depending on the feedstock, phase of products (gas, liquid and char) and
compound groups (Figure 13). The highest concentrations of investigated
compounds were found in bark products – about three times higher than
those found in impregnated stemwood products. The lowest concentrations
were found in stemwood products. The high concentrations in bark pyrolysis
products could be related to the relatively high contents of chlorine and
transition metals in the feedstock. In addition, the generally high contents of
phenolic extractives in tree bark and organic contaminants originating from
atmospheric deposition could also lead to formation of chlorinated organics,
as discussed below. The pyrolysis of impregnated stemwood yielded higher
concentrations of PCDDs, PCDFs and PCNs than pyrolysis of clean stemwood.
This could potentially be associated with the presence of metal-based
preservatives, which could sustain smoldering during pyrolysis. The presence
of transition metals could also contribute to the higher concentrations of
analytes, although the Cl content of the impregnated stemwood was similar to
that of the clean stemwood. The effect of biomass chemical composition on
the formation of chlorinated compounds is discussed further in the following
text, together with the results obtained from torrefaction experiments (Paper
IV).
PCDDs, PCDFs and PCNs were most abundant in the oil fractions of the
pyrolysis products, which contained up to 68% of the total measured outputs
of these analytes in each MAP experiment. The relative abundances of these
compounds in the non-condensable gas and aqueous phases was much lower,
possibly due to a “wet scrubbing” effect, i.e. physical or chemical trapping of
gases in droplets of the liquid products (Zwart et al., 2009). The partitioning
of the bulk of the POP content into the oil fraction may be due to its
hydrophobicity and the distillation effect, together with wet scrubbing as
noted above.
The distribution of PCNs among the product fractions differed slightly from
the PCDD and PCDF distributions. For example, the relative abundances of
PCNs in char products of both bark and impregnated stemwood (32.4 and
23.9%, respectively) were substantially greater than those of PCDDs or
PCDFs, which were quite similar (26.7-28.9% in bark products and 16.018.9% in impregnated wood products). This could be related to differences in
absorptive interactions of these compound classes with the carbon matrix.
38
PCDDs
ng/kg input
70
35
0
Stemwood
Bark
Impregnated stemwood
PCDFs
ng/kg input
80
40
0
Stemwood
Bark
Impregnated stemwood
Stemwood
Bark
Impregnated stemwood
Aqueous
Char
PCNs
ng/kg input
70
35
0
Gas
Oil
Figure 13. Concentrations and distributions of PCDDs, PCDFs and PCNs in MAP products of
stemwood, bark and impregnated stemwood (ng kg -1). Error bars represent ± 1 SD (Paper II).
39
4.2.2. Homologue profiles
The evaluation of the homologue profiles primarily focused on the oil and char
products since they accounted for most of the PCDDs, PCDFs and PCNs (83±
9%). The homologue profiles (Figure 14) of all three compound groups were
dominated by the less chlorinated homologues. The homologue abundance
decreased as the degree of chlorination increased. This trend was observed for
all three feedstocks, and is consistent with results of earlier pyrolysis studies
(Weber and Sakurai, 2001). The oxygen-deficient conditions used in pyrolysis
are believed to be responsible for the trend because they probably limit the
formation of free chlorine via Deacon-type metal-catalyzed reactions. The
importance of oxygen deficiency was demonstrated by Pekarek et al. (Pekárek
et al., 2001), who observed a dramatic shift towards less chlorinated PCDDs
and PCDFs in pyrolysis products as the oxygen content was reduced from 10%
to < 0.001 %. We postulated that low process temperatures could also restrict
chlorination degrees, since chlorination is highly temperature-dependent
(Jansson et al., 2008).
In bark and impregnated stemwood MAP products, the oil fractions appeared
to have greater relative abundances of less chlorinated homologues than the
chars (Figure 14). The observed differences in homologue profiles between the
oil and char products could be related to difference in vapor pressures
between the low and high chlorinated congeners. The highly chlorinated
congeners with low volatility tend to be retained by particles whereas the less
chlorinated congeners tend to volatilize into the gas phase.
40
4
60%
3
40%
2
20%
1
0%
0
Char
Relative % of PCDF
Stemwood
Oil
Char
Bark
Oil
Char
Impregnated
stemwood
100%
5
80%
4
60%
3
40%
2
20%
1
0%
0
Oil
Char
Oil
Stemwood
Char
Bark
Oil
Char
Impregnated
stemwood
100%
5
80%
4
60%
3
40%
2
20%
1
0%
Oil
Char
Stemwood
Oil
Char
Bark
Oil
Char
0
Degree of chlorination
Oil
Relative % of PCN
Degree of chlorination
80%
Degree of chlorination
5
Relative % of PCDD
100%
Impregnated
stemwood
Mono-
Di-
Tri-
Te-
Hx-
Hp-
Octa-
# Cl
Pe-
Figure 14. Degree of chlorination (#Cl) and relative abundance of PCDD (mono-to octa-), PCDF
(mono-to octa-) and PCN (tri-to octa-) homologues in pyrolysis oils and chars. Error bars
represent ±1 SD (Paper II).
41
4.2.3. Isomer patterns and formation pathways
This subsection provides findings of Paper III on isomer patterns of PCDDs,
PCDFs and PCNs in MAP, and inferences regarding formation pathways. The
discussion mainly focuses on low chlorinated homologues (mono- to tetra-),
due to the low detected concentrations of penta- to octa-chlorinated
homologues (some are close to detection limit), which hindered identification
of clear isomer patterns. The results of isomers in stemwood MAP products
are not discussed due to their low concentrations.
PCDF isomer distributions (Figure 15) in bark and impregnated stemwood
MAP products were similar, both being dominated by 2-MoCDF, 4-MoCDF,
2,4-DiCDF, 2,4,6-TriCDF, 2,4,8-TriCDF and 1,2,4,6-TeCDF. The isomer
patterns were also less complex than those reported in wood burning and
waste incineration products (Bacher et al., 1992; Wikström and Marklund,
2000), which usually consisted of many isomers at comparable
concentrations. The MoCDF signature in MAP products, with dominance of
4- and 2-MoCDF, differed from signatures frequently reported in combustion
processes, where 2-MoCDF and 3-MoCDF were generally dominating (Ryu et
al., 2004; Ryu et al., 2006). The dominating 2-MoCDF and 3-MoCDF from
combustion processes has been reported to be mainly a result of chlorination
from unsubstituted DF (Wikström and Marklund, 2000). The differences
between the MoCDF patterns suggest that initial chlorination directly from
DF could be less prevalent in MAP. The temperature employed in our MAP
study (up to 200 °C) is probably insufficient to support extensive formation of
DF, for which a high temperature (>650 °C) is often required for condensation
of unsubstituted phenol and benzene (Wikström and Marklund, 2000).
Dominant formation of 2- and 4-MoCDF has been previously reported, in
both pyrolysis and oxidation-driven experiments, using various chlorinated
and unsubstituted phenols as precursor compounds (Yang et al., 1998; Ryu et
al., 2005). Reactions involving (chloro)phenoxyl radicals as precursors have
been proposed to be essential for the formation of particular isomers. The
relative reaction rate involving formation of radical intermediates is one of the
main factors governing the patterns of isomer distributions (Evans and
Dellinger, 2003), as further discussed in the following text.
42
12
6
0
1-
3-
2-
4-
6
3
0
4
2
Concentration, ng kg
-1
0
8
4
0
0,2
0,1
12368/
12478/
14678-
13479-
12346-
12378-
12367/
12678-
23467-
23478-
12389-
123468-
124678/
134678-
124689-
123467-
123478-
123678-
234678/
123689-
123489/
123789-
0,0
0,2
0,1
0,0
0,4
0,2
0,0
1234679-
1234689-
Bark
Impregnated
Figure 15. PCDF isomer distributions of MAP products of bark and impregnated wood. The
results are presented as total concentrations (sum of gas, liquid and char) after subtraction of
the concentrations of untreated feedstocks. Error bars represent ± 1 standard deviation (n=3)
(Paper III).
43
PCDD isomer distributions (Figure 16) were dominated by 1-MoCDD and 2MoCDD, 1,3-, 2,7-, 2,8-DiCDD, 137/138-, 136- and 124-TriCDD in both bark
and impregnated stemwood products. TeCDD was exclusively comprised by
1,3,6,8- and 1,3,7,9-TeCDD, which are known to be condensation products of
chlorinated phenol precursors, typically originating from dimerization of
2,4,6-chlorophenol (Sidhu et al., 1995). Apart from their formation in
combustion processes, such a reaction has been demonstrated to take place
readily at low temperature (< 300 °C) via metal-mediated catalysis (Manuel
and Metcalf, 2008).
The differences in patterns of MoCDD congeners observed in MAP and
combustion products are particularly notable. 1- and 2-MoCDD were present
at similar concentrations in MAP products, while 2-MoCDD is substantially
less prevalent in wood burning and waste incineration products. 1-MoCDD is
much less thermodynamically stable than 2-MoCDD (Wang et al., 2004). The
higher proportion of 1-MoCDD in the MAP products may be due to the
temperature and residence time employed in MAP being too low to reach
thermodynamic equilibrium.
44
4,00
2,00
0,00
2-
1-
6
3
0
1,3-
2,7-
2,3-
2,8-
137/138-
136-
124-
147/237-
2,0
Concentration, ng kg
-1
1,0
0,0
4
2
0
1368-
1379-
1248/1246/
1247/1249-
1378-
1269/1234-
1
0
12468/ 12469- 12368- 12478- 12379- 12369/ 12346/ 12378/ 123891247912476- 12347- 123672
1
0
123468-
123679/
123689-
123678-
123467-
123789-
2
1
0
1234679-
1234678-
Bark
Impregnated
Figure 16. PCDD isomer distributions in MAP products of bark and impregnated wood. The
results are presented as total concentrations after subtraction from concentrations in
feedstocks. Error bars represent ± 1 standard deviation (n=3) (Paper III)
45
PCN isomer distributions (Figure 17) slightly differed between bark and
impregnated stemwood MAP products. 2,3-, 1,3-DiCN and 1,2,3-TriCN were
major isomers in bark MAP products, while 15/27-, 14/16-DiCN, 1,4,6-, 1,2,7CN dominated in impregnated stemwood products. PCN isomer patterns in
bark and impregnated wood both revealed a tendency for sequential chlorine
substitution from DiCN to TriCN.
A similar successive chlorination pattern has previously been observed in a
naphthalene chlorination experiment, in which 1-MoCN, 1,4-DiCN, 1,4,6TriCN, and 1,2,4,6-TeCN were found to be the dominant isomers (Ryu et al.,
2013). Chlorination from unsubstituted naphthalene is an important PCN
formation pathway in municipal solid waste combustion (Jansson et al.,
2008). However, the isomer distributions in the combustion study were more
complex than those we observed, suggesting more complex formation
pathways occur during high temperature and oxidative processes. For
example, de novo synthesis or PAH breakdown, common PCN formation
pathways in combustion, are oxidation-driven. The oxygen-deficient
conditions in MAP could limit de novo synthesis reactions. The low process
temperatures could also restrict PCN chlorination to higher degrees, since
PCN chlorination is highly temperature-dependent (Jansson et al., 2008).
Furthermore, the short reaction time and high heating rate in MAP could also
contribute to the observed selective isomer patterns. The relative importance
of these factors for determining isomer patterns in MAP products requires
further investigation.
Another notable aspect of the findings is that the dominant isomer 1,2,3TriCN found in bark MAP products is reportedly thermodynamically
unfavorable (Zhai and Wang, 2005). Isomers with higher thermodynamic
stability, substituted at 2,3,6,7-sites that are commonly favored during waste
incineration, were not observed in our study. It appears that successive
chlorination, providing high electrophilic stability, appears to be more
important than the thermodynamic stability or steric hindrance effect (Zhai
and Wang, 2005).
46
20
10
0
13-
14/16-
15/27-
26/17-
23-
14
7
0
136- 135- 137- 146- 124- 125- 126- 127- 167- 123- 145-
2
1248-
1258/
1268-
1458-
12478-
12358/
12368-
12458-
1278-
1245-
146712468-
12456-
1246/1247
/ 125712457-
1368/
12561235/
13581237/1234
/ 1267-
1357-
0
12357/
12467-
Concentration, ng kg
-1
4
2
1
12345-
12367-
12356-
12346-
0
0,8
0,4
123456-
123578-
124568/
124578-
123568-
123457-
123467/
123567-
0,0
0,4
0,2
0,0
1234567-
1234568-
Bark
Impregnated
Figure 17. PCN isomer distributions in bark and impregnated wood MAP products. The results
are presented as total concentrations (sum of gas, liquid and char) after subtraction of
concentration in feedstock. Error bars represent ± 1 standard deviation (n=3) (Paper III).
47
In summary, the highly selective isomer patterns of PCDDs, PCDFs and PCNs
in MAP products suggest that multiple pathways involving (for instance)
formation and thermal degradation, are probably less involved in MAP than
in combustion processes. For example, de novo synthesis and dechlorination
of highly chlorinated compounds, which typically generate a “combustion”
pattern with no dominance of particular isomers, could be less important in
MAP. Incorporation of oxygen as well as a sufficiently high temperature,
which are essential for de novo formation, might be limiting factors in MAP at
low temperature (up to 200 °C).
The presence of 1,2,3-TriCN, 1-MoCDD and 4-MoCDF, all of which have low
thermodynamic stability, in the products is intriguing. We proposed that
kinetic factors rather than thermodynamic stability may be important in the
MAP process. Reactions involving (chloro)phenoxyl radicals as precursors at
low activation energy could be essential for the formation of particular
isomers (Evans and Dellinger, 2003). We proposed a PCDF formation
pathway involving cross-coupling of (chloro)phenoxyl radical intermediates
and successive chlorination, as shown in Scheme 1. Following initiated
formation of 4-MoCDF by cross-coupling of a phenoxyl and 2-MoCPh radical,
chlorine substitution is directed to the same benzene ring at the 2-position,
providing high electrophilic stability. Subsequent chlorine substitution may
then occur governed by electron density distribution.
48
Scheme 1. Postulated PCDF formation pathways involving cross-coupling of (chlorinated)
phenoxyl radical intermediates (Paper III) followed by step-wise chlorination.
4.3. Dioxins in torrefaction products
This section presents findings of the torrefaction study (Paper IV). The
temperature profile observed during the course of torrefaction of each type of
feedstock used is shown in Figure 18. The entire torrefaction process,
including heating-up time, lasted about 2.5 hours. The torrefaction
temperatures were generally lower than the furnace temperature setting (290
°C) except that of impregnated wood, for which the temperature fluctuated
from 300 °C up to 327 °C. This is likely due to the presence of high contents
of metal-based preservatives which could alter the thermal degradation
behavior by enabling sustained smoldering during heating. The higher
temperature in experiment with the impregnated stemwood led to a much
higher degree of thermal decomposition of feedstock and already a pyrolysis
occurred. This was reflected by the lower yield of char (47%), compared to
torrefaction from the other four feedstocks (85 and 91%).
49
Heating
Torrefaction
340
300
Temperature (⁰C)
260
220
Particle board
180
Cassava stems
140
Impregnated
stemwood
Bark
100
60
Stemtwood
20
0
0,5
1
1,5
2
2,5
Time (h)
Figure 18. Temperature profiles of torrefaction processes with the indicated feedstocks (Paper
IV).
As shown in Figure 19. Among the five feedstocks, particle board has the
highest concentration for both PCDDs and PCDFs, while the concentrations
of PCDDs and PCDFs in the other four feedstocks are much lower. For the
torrefied products, highest PCDFs concentration (sum of gas, liquid and
chars) were found in impregnated stemwood followed by those from particle
board. For PCDDs, the highest concentrations were found after torrefaction of
particle board. Comparison of concentrations in the total torrefied products
with those in untreated materials indicated that formation of PCDFs during
torrefaction were insubstantial.
It has been shown that in heterogeneous pathways involving metal catalysts,
de novo synthesis is one of the dominant pathways of PCDF formation,
whereas PCDDs mostly originate from precursor pathway, with limited
contributions from de novo pathways (Hell et al., 2001; Wikström et al.,
2004). Hence, a PCDD/PCDF ratio <1 is often regarded as a characteristic of
de novo synthesis. Precursor-dominated formation, in contrast, often leads to
a PCDD/PCDF ratio >1. Thus, the dominance of PCDDs over PCDFs we
observed in the products indicates that precursor pathways were most
50
important in our torrefaction process. Regarding relative distributions in the
torrefaction products, PCDDs and PCDFs were found predominantly in chars,
in contrast to MAP products, where chlorinated compounds were mainly
found in oil products. This difference in relative distributions in various
phases is further discussed in the following section.
Homologue profiles of torrefaction products varied from sample to sample.
For torrefaction of stemwood, bark and cassava stems, low chlorinated
compounds were dominant. For impreganated stemwood, MoCDF exclusively
dominated the homologue profile, accounting for 94-99% of the total PCDFs.
The homologue profile of torrefied particle board was dominated mainly by
the highly chlorinated PCDDs, with hexa- to octa-CDD accounting for 89-94%
of the total PCDDs. This could be related to high concentrations of hepta- and
octa-CDD in the feedstock, which might originate from PCP preservative (Li
et al., 2012).
The less chlorinated compounds have a higher tendency to volatize than
highly chlorinated compounds, which tend to be more preferentially retained
in char. This pattern can be clearly seen in results of bark, cassava stem and
particle board torrefaction experiments (Figure 20). The results regarding the
partitioning of highly and low chlorinated compounds between volatile and
char products were similar to those observed in our MAP study (Paper II), and
could be related to the differences in their vapor pressures.
PCDDs and PCDFs found in torrefied products could partly originate from
physical transfer directly from the input feedstock via distillation, e.g.,
evaporation followed by condensation and/or adsorption. In addition, PCDDs
and PCDFs in torrefied products appear to have partly originated from
formation during thermal processes, as their concentrations were higher in
the torrefaction products (sum of gas, liquid and solid) than in the feedstock.
Moreover, dechlorination and/or degradation of highly chlorinated
compounds could occur during torrefaction. For example, the relative
percentage of OCDD was higher in untreated particle board than in its
torrefied products, while relative abundances of HxCDD and HpCDD were
higher in torrefied particle board than in the feedstock.
51
PCDD
18000
Gas
Liquid
Char
Feedstock
10000
ng kg
-1
2000
600
400
40
200
20
0
Stemwood
Bark
Impregnated
stemwood
Cassava
stems
Particle
board
PCDF
500
gas
liquid
char
Feedstock
ng kg-1
400
300
200
100
0
Stemwood
Bark
Impregnated
stemwood
Cassava
stems
Particle
board
Figure 19. Concentrations and distributions of PCDDs and PCDFs in the five raw
feedstocks and their torrefaction products (ng kg -1). Error bars represent ± 1 SD (Paper
IV)
52
Bark
1
0,5
0
Cassava stems
1
0,5
0
Particle board
1
0,5
0
Gas
Liquid
Char
Figure 20. Relative abundance of each homologue in gas, liquid and solid torrefaction
products of bark, cassava stems and particle board (Paper IV).
53
4.4. Toxicity equivalent values
The TEQ values of the examined MAP and torrefaction products were
generally quite low, except those of particle board (Table 4). Volatiles and
chars accounted for most of the TEQ in MAP and torrefaction products,
respectively. Based on char weight, these TEQ concentrations are
substantially lower than the Swedish guideline values for dioxins in solid
materials intended for sensitive land use (50 pg TEQ g-1) (Elert et al., 1997).
Although there are no specific guidelines for TEQ concentrations in bio-oil,
they were much lower than those reported in wood combustion (7.3-22.8 ng
I-TEQ kg FUEL-1) (Lavric et al., 2004). The total TEQ of torrefaction products
of particle board (47 ng I-TEQ kg fuel-1) was the same order of magnitude as
those observed in particle board combustion products (Tame et al., 2007), and
was about 7 times higher than those of untreated feedstock (6 ng I-TEQ kg
fuel-1). For management and controlling of POPs pollution in thermal
treatment of biomass, caution should be taken on utilization of char products
especially when using contaminated biomass as feedstock.
Table 4. I-TEQ values of MAP and torrefaction products from various feedstocks (ng ITEQ kg FUEL-1).
MAP
Torrefaction
Gas
Liquid
Char
Gas
Liquid
Char
Stemwood
0.006
0.03
0.01
0.06
0.05
0.7
Bark
0.006
0.2
0.102
0.07
0.05
0.7
Impregnated
stemwood
0.009
0.05
0.07
0.2
0.1
0.4
Cassava stems
-
-
-
0.07
0.06
0.6
Particle board
-
-
-
1
1
44
54
4.5. Effects of chemical composition of feedstocks
This section summarizes the results presented in Paper II-IV concerning
effects of feedstock properties on dioxin formation. As observed in both MAP
and torrefaction experiments, the formation of chlorinated organics varied
depending on feedstock types and their composition. Chlorine contents of the
fuel is known to be a crucial factor in the formation of PCDDs, PCDFs and
PCNs in processes such as waste incineration. Although trace amounts of
chlorine were present in the stemwood, bark and impregnated wood we used,
the relatively high amounts of PCDDs and PCDFs in bark MAP products,
compared to stemwood and impregnated wood products (Paper II) suggest
that chlorine could also play a key role in their formation during MAP
However, our studies also showed that levels of chlorine inputs and amounts
of PCDDs and PCDFs formed were not always positively correlated. For
example, formation of PCDDs and PCDFs in the torrefaction of cassava stems
were low although it had higher chlorine contents than the other four terrified
feedstocks (Paper IV). These results indicate that the feedstock’s chlorine
content may be important, but it is not the only factor that governs formation
of chlorinated compounds.
The active chlorine species that may be present must be considered when
addressing the role of Cl in formation of chlorinated compounds (Gullett et
al., 2000; Wikström et al., 2003b). The chlorine in the feedstock can be
released mainly (>90%) as HCl, which rarely acts directly as a chlorination
agent. The presence of metal catalysts is often required for converting HCl to
Cl2. Such reactions involving metal catalysts have been reported at
temperatures as low as 150°C (Gullett et al., 2000). Further, the relatively high
Fe content in biomass may promote the formation of free chlorine (Cl2), which
is a very efficient chlorinating agent and may promote formation of PCDD and
PCDF (Stieglitz, 1998; Kim et al., 2007).
Generally, bark has higher contents of phenolic extractives than stemwood,
regardless of the tree species from which the wood is taken (Vassilev et al.,
2010). Furthermore, coniferous species are rich in terpenes and other light,
cyclic organic compounds that have relatively little thermal stability, and
might give rise to formation of aromatics. As already mentioned, phenolics,
especially chlorinated phenolics, are widely considered to be crucial
intermediates in the heterogeneous formation of polychlorinated aromatics.
Pyrolysis of lignin components has been shown to generate high levels of
phenolics (Fu et al., 2014). Whether or not these phenolics are single
(chloro)phenol molecules directly produced during lignin matrix breakdown,
or associated with other aromatic compounds cannot be determined from
results of Studies I-IV. However, in either case these phenolic compounds may
55
certainly play a role in the formation of dioxins and PCNs in the considered
systems. Although stemwood contains substantial amounts of lignin (26.0%),
its chlorine content is low, which may restrict the potential for formation of
chlorophenols and other chlorinated organics (and hence PCDD/PCDF/PCN
compounds) during its thermal treatment.
The presence of metal-based preservatives can alter the thermal degradation
behavior of biomass by enabling sustained smoldering during heating
(Richards and Zheng, 1991). This may enhance the formation of
polychlorinated compounds, as observed in our experiments with
impregnated wood (Paper II and IV). PCP as preservatives often contains
high amount of highly chlorinated PCDD as impurities of its synthesis. PCDD,
mainly OCDD can be readily formed under pyrolysis of PCP (Altwicker et al.,
1990). The presence of high concentration of PCDDs in the torrefaction
products of particle board (Paper IV) could partly originate from
volatilization and/or re-condensation of volatized PCDD onto the char
surfaces.
4.6. MAP vs torrefaction: the importance of physical
dynamics
This section compares our MAP and torrefaction setups from a physical
dynamics perspective, based on summarized findings of Paper II, III and
IV. To facilitate the discussion on the specifics of low-temperature
thermochemical processes occurred in MAP and torrefaction, typical
combustion condition is also included in the following comparison.
In MAP experiments, POPs were predominantly found in the pyrolysis oils,
while in the torrefaction experiments large fractions of POPs were found in
solid chars and small fractions in volatiles. We proposed that some factors
related to physical dynamics could influence the relative distribution of POPs
in gas, liquid and solid products. MAP and torrefaction both involve low
temperature regimes, but other process parameters substantially differ
between the two processes, including the heating rate, residence time and air
pressure (vacuum in MAP vs N2 flow in torrefaction), which could
substantially affect POP formation characteristics. Generally, chlorinated
organics may form either in the gas phase (vapor) homogeneously, or on solid
surfaces such as a degraded char matrix (in MAP and torrefaction) or on soot
or fly ash surfaces (in combustion) via heterogeneous reactions. In reactions
involving (chloro)phenol precursors, the formation depends strongly on
precursor concentrations and residence times, which are influenced (inter
56
alia) by the pressure in the reactor (vacuum in MAP vs N2 supply in
torrefaction). Figure 21 illustrates some of the major routes whereby PCDDs,
PCDFs and PCNs (collectively referred to as dioxins in the figure) can be
formed and transferred in the three types of thermal processes.
57
Reaction chamber
Volatiles
Combustion
+ O2
end up in flue gas
- CO2, CO, Nox, H2O
de novo
catalytic
> 600 °C
O2, Δt≈100° C/s
Post combustion zone/condensors/PUF
- PICs
Chars
Biomass
> 650 °C
chlorination /dechlorination
quenching to <400 °C
Evaporate quickly due to the vacuum
MAP
up to 200° C
<<atm, Δt≈1 ° C/s
Dioxins (Secondary formation)
condensation
Gas phase
precursor
reaction
Dioxins (Primary formation)
Volatiles
CO2, CO, Nox, H2O
quenching to <400 °C
de novo/precursor
- Non-condensables (CO2, CO etc.)
- Condensables with low vapor pressure
(precursors, H2O etc)
Degradation
Chars
Precursors(in gas)
React with carbon matrix
Evaporate quickly due to vacuum
Dioxins on char surface
- Non-condensables (CO2, CO etc.)
Volatiles
- Condensables (with high vapor pressure)
Torrefaction
200-300 °C
N2,Δt ≈ 0.1 ° C/s
Dioxins in non- or condensables
Chars
Degradation
Precursors
React with carbon matrix
Dioxins (in chars)
Lower fraction evaporated than in MAP
Dioxins
Figure 21. Schematic diagram of formation and transformation paths of dioxins in combustion, MAP and torrefaction. Lines in red represent important
routes for transformation of organic compounds.
58
In most combustion scenarios, temperatures range from 600 to 1000s °C.
During the initial stage as the temperature rises (at heating rates typically
above 100 K/sec), volatilization occurs, resulting in gas phase products
containing volatiles and water vapor. The volatile substances will continue
reacting with O2 to form CO2, CO, NOx etc. The products of incomplete
combustion (PICs), including precursor species formed in gas phase, will
undergo numerous reactions as the temperature and oxygen concentration
are sufficiently high. Dioxins are formed from primary processes in
combustion chambers. As volatile products pass through the post-combustion
zone with quenching conditions at temperatures below 400 °C, secondary
formation processes involving even more complex pathways, such as
condensation of precursors in gas phase will occur. The char residues and fly
ash obtained from primary combustion can provide an organic matrix for de
novo formation of dioxins. The presence of transition metal catalyst can also
promote their formation.
For our MAP experiments the temperature was increased to 200 °C at a
heating rate of about 1 °C/sec. The system used, with a connected vacuum
unit, provides much lower pressure (approximately a tenth of the atmospheric
pressure) than typical combustion conditions. This results in faster
evaporation of volatiles from the surface of the char matrix. Hence, most
primary precursors that were formed from biomass degradation readily
evaporated and ended up in the pyrolysis condensates. Only a minor fraction
remained on the char matrix. This could explain why we found lower amounts
of PCDD/Fs in chars than in oil condensates. In addition, in MAP with low
temperature and oxygen-deficient conditions, the gas phase reactions
involved in PCDD/F formation could be much less complex than in
combustion, where the solid carbonaceous residues generated are highly
reactive.
In the torrefaction experiments, the temperature reached 200-300 °C at a
heating rate of 0.1 °C/s, 10-fold slower than in the MAP setup. With N2
continuously introduced, and a vacuum unit connected at the outlet, the
volatile organics were constantly “scavenged”, hence concentrations of volatile
organics in gas phase inside the reactor were much lower than in combustion
processes with no inflow of N2. Concentrations of volatiles inside the
torrefaction reactor were much higher than those in the MAP system, and the
residence time of precursor compounds was longer. Moreover, the
combination of low torrefaction temperatures and pressures hindered
volatilization of some precursor compounds with relatively high boiling
points, in contrast to MAP where the precursors evaporated at temperatures
below their boiling points due to the vacuum. Consequently, during
59
torrefaction the precursor compounds had a higher tendency to remain in
solid carbon matrices, allowing PCDD and PCDF formation reactions. The
torrefaction temperature was also too low to make the PCDDs and PCDFs
present in the raw feedstock evaporate into gas phase. This could partly
explain the higher proportions of PCDDs and PCDFs in solid torrefied
products and lower proportions in condensates compared to corresponding
proportions in MAP products.
The water contents of the feedstocks also differed between our MAP and
torrefaction experiments. For example, in torrefaction, the materials were
pre-dried and degradation of relatively reactive saccharide components could
occur in early stages of the process, while in MAP experiments raw feedstocks
were directly introduced without pre-drying and water evaporated at the
beginning of the pyrolysis. Whether or not the presence of water vapor affects
PCDD and PCDF formation is unclear. Nevertheless, the solid residues
generated in both MAP and torrefaction contained unconverted organic
carbon which can adsorb organochlorines. Moreover, rates of adsorption and
desorption strongly depend on the atmospheric conditions. In MAP with
vacuum conditions desorption could be favored, whereas in torrefaction
adsorption may prevail. Regarding the carbon balance in volatiles and solid
products, the organic volatiles generated in combustion often contain more
than 90% of the initial carbon, while in our MAP experiments the char yields
were 35-46%. In torrefaction, normally more than 90% of carbon remains in
solid phase. The differences in yields of thermochemical products in different
processes will certainly influence the distribution of dioxins among phases.
60
5. Conclusions and future perspectives
The major conclusions from the studies underlying this thesis can be
summarized as follows:
 In PLE extraction of POPs from thermally treated biomass for
simultaneous determination by GC/MS, solvents with high polarity
should not be used due to matrix interference from the high coextraction of thermally degraded lignocellulosic compounds. Raw
wood is less solvent-dependent and comparable results can be
obtained with polar and non-polar solvents.

In MAP experiments, POPs were predominantly retained in pyrolysis
oils, while in torrefaction experiments large fractions of POPs were
observed in solid chars with little in volatiles. These findings suggest
that physical dynamic conditions (particularly heating rate and
pressure in the reactor) could govern distributions of POPs in gas,
liquid and solid products. The abundances of POPs in noncondensable gas products were relatively low in both MAP and
torrefaction experiments. This information could facilitate the
development of strategies for controlling gas phase emissions of
organic pollutants, and utilization of different thermally treated
products.

In both MAP and torrefaction experiments, highly chlorinated
congeners with low volatility tend to be retained in chars, whereas less
chlorinated congeners tend to volatize into the gas phase.

Isomer patterns of MAP products are more selective than patterns
observed in most combustion studies, indicating less complex
formation pathways in the low temperature and oxygen deficient
conditions used in MAP than in combustion. However, in our opinion,
the complexity of formation mechanisms involving heterogeneous
routes, as well as the uncertainties in many thermochemical and rate
parameters make it difficult to make a conclusive statement about the
contribution of each pathway. Nevertheless, the presence of isomers
with low thermodynamic stability in MAP suggests that the formation
pathways of PCDDs, PCDFs and PCNs could be governed not only by
thermodynamic factors but also kinetic factors and the reactivity of
precursor intermediates.
61

Chemical composition of feedstock, particularly its contents of
chlorine and catalytically active metals (mainly Cu and Fe), plays key
roles in PCDD, PCDF and PCN formation. Chlorine is not the only
important factor, as shown by the low concentrations of PCDD/Fs in
torrefaction products of cassava stems (which have high Cl contents).
The abundances of PCDDs, PCDFs and PCNs depend on complex
interactions between feedstock-related and process variables.
Although experiments can be designed to evaluate the effect of
individual variables, the influence on dioxin formations solely based
on knowledge of the feedstock content are difficult to apply in a
predictive manner.

Thermal conversion of biomass with high levels of chlorinated organic
contaminants such as PCP can result in high formation of dioxins
compared to clean wood. For management and recycling of
contaminated waste wood, controlled incineration still appears to be
most appropriate option for dioxin destruction. In addition, it may be
necessary to identify and investigate the particleboard still‑in‑use
and in demolition containing PCP preservatives to prevent dioxin
contamination.
Future perspectives
 The presented studies indicate that (chloro)phenoxyl intermediates
could be important precursors in POP formation. Chlorination
patterns of precursors could play key roles in final PCDD/F profiles.
Detailed information on PCPhs concentrations and isomer patterns
in feedstock and thermally treated products, could be useful for fully
elucidating the role of PCPhs as precursors. Also, formation of both
PCDFs and PCNs appeared to proceed via chlorination of non- or low
chlorinated compounds. Quantification of non-chlorinated DD, DF,
CN and MoCN would be helpful to clarify the formation mechanisms
involving chlorination of non-chlorinated compounds.

In a real scenario of torrefaction at full scale, the torrefaction gas is
generally combusted and the heat is typically used for biomass drying
and heating during the torrefaction process. This differs from the
experimental setup in our bench-scaled torrefaction where the gas
products were removed directly by partial vacuum. The torrefaction
gas generally has complex composition, consisting of a permanent gas
fraction and a condensable fraction. Whether or not the flue gas used
for pre-drying biomass affects dioxin formation requires further
investigation.
62

As speculated in Chapter 7, the vacuum applied in the MAP system
could have contributed to differences in dioxin distributions among
products observed in the MAP and torrefaction experiments.
However, more experiments are required to draw solid conclusions
regarding these differences. For example, conducting MAP under N 2
using the same feedstock or torrefaction under vacuum might be
helpful for comprehensive comparison of the results under different
operating conditions.
63
Acknowledgements
First and foremost, I would like to express my gratitude to my supervisor Stina
Jansson for giving me this opportunity, for sharing your great expertise in the
field of organic pollutants in thermochemical processes, for supporting me on
all scientific conferences and field studies. Thanks for always being available
on the helpful consultations. Thanks to my co-supervisor Peter Haglund and
Silvia Larsson for the great guidance and inspiration. Thanks also to my
external mentor Mariusz from Energy Centre of the Netherlands for the
scientific guidance and enthusiastic discussion. My coauthors are
acknowledged for all the contribution of publications. I would like to
acknowledge Bio4Energy (www.bio4energy.se), a strategic research
environment appointed by the Swedish government, for the financial support
of this work.
There are many colleagues that deserve special recognition for contributing to
this project. Big thanks go to Per Lilielind for help with the GC/MS analysis
and discussions. I would like to thank Mark and Vitaliy from York University,
Magnus from SLU for the help in numerous ways succeeding with my field
experiments. Thanks are due to Mats Tysklind for supporting me in GloCom
project. Mar Edo for being great companion and support throughout the
years. Thanks go to all the “combustion” girls Mirva, Lisa and Eva for the
fantastic accompany and strong support. Thanks go to Maria for instructing
me on sample cleanup and Staffan for helping me to get a quick start on
Masslynx.
All staff, fellow students and friends, I sincerely thank you for the fantastic
time, lunches, “fika” and laughs. Thanks go to Sandra for the most memorable
time and for your contagious smile, always shining in the northern darkness.
I wish to thank Cathrin, Christine (My punch-out partner, Chantons et buvons
!!), Kristin (hot as a fireball), Joao, Matyas, Jana, Mandana, Meldi, Lan,
Marcus, Sherry and all the people at MKL for all your support and the
wonderful time we have spent. My Chinese friends Chaojun, Yaozong, Junfang
and Jin are specially acknowledged for making me feel at homeland. Thank
all the people surrounding for making it such a nice place to work and live
with.
Finally, I would like to thank my family for the incredible support and
patience. Thanks my mother for the endless love. Thanks Ping for always
supporting my pursuits. I would like to express my deepest gratitude to my
fantastic son Linus. I wish all the success and peace that you deserve.
64
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