On-line analysis of gas-phase composition in the combustion chamber and particle emission characteristics during combustion of wood and waste in a small batch reactor

Article abstract of DOI:10.1021/es049493o

The emission of participate matter and gaseous compounds during combustion of wood and refuse-derived fuel in a small batch reactor is investigated by laser mass-spectrometric on-line measurement techniques for gas-phase analysis and simultaneous registration of physical aerosol properties (number size distribution). The gas-phase composition is addressed by a laser-based mass spectrometric method, namely, vacuum-UV single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS). Particle-size distributions are measured with a scanning mobility particle sizer. Furthermore, a photoelectric aerosol sensor is applied for detection of particle-bound polycyclic aromatic hydrocarbons. The different phases of wood combustion are distinguishable by both the chemical profiles of gas-phase components (e.g., polycyclic aromatic hydrocarbons, PAH) and the particle-size distribution. Furthermore, short disturbances of the combustion process due to air supply shortages are investigated regarding their effect on particle-size distribution and gas-phase composition, respectively. It is shown that the combustion conditions strongly influence the particle-size distribution as well as on the emission of particle-bound polycyclic aromatic hydrocarbons.

Full text of DOI:10.1021/es049493o

Research
Introduction
On-Line Analysis of Gas-Phase
Composition in the Combustion
Chamber and Particle Emission
Characteristics during Combustion
of Wood and Waste in a Small
Batch Reactor
The investigation of incineration processes is an important
task in environmental and analytical chemistry, because the
combustion of biomass and fossil fuels represents the largest
source of anthropogenic air pollution. Emissions produced
by combustion processes are composed of gaseous pollutants
as well as particulate matter. Therefore, the gas phase and
emitted particles should be considered in order to obtain a
comprehensive characterization of the emission profile of
combustion processes.
Several epidemiological studies in the past decades
establish a correlation between the concentration of par-
ticulate matter in the ambient air and adverse effects on
human health (1, 2). In this context, the fine (d < 2.5 µm) and
ultrafine (d < 0.1 µm) particles are of special concern due
to their ability to penetrate deep into the lung upon inhalation
(3). In industrialized areas, anthropogenic combustion
processes are the major source of emissions of fine and
ultrafine particles into the atmosphere. In particular, in-
dustrial incineration plants and traffic emissions (4-6) are
the major sources of the anthropogenic particle load of the
atmosphere. The new insights into particle-mediated health
effects motivate the investigation of particle emissions from
anthropogenic sources, especially focusing on the fine and
ultrafine particle-size range. In this context physical char-
acterization (i.e., particle number concentrations and size
distributions) as well as chemical characterization is neces-
sary for a better understanding of formation mechanisms as
well as for an estimate of possible adverse effects on human
health. For example, soot particles from combustion pro-
cesses are known to be inhalable carriers of highly carci-
nogenic compounds such as polycyclic aromatic hydrocar-
bons (PAH) (7, 8). To reduce aerosol emissions by means of
process-integrated primary measures, it is necessary to learn
how and why particle properties are correlated with plant
operating conditions such as air supply and boiler load.
In general, there are two main sources for generation of
particles from combustion processes: Particles can be formed
from (1) inorganic material in the fuel ash and (2) incomplete
combustion of the organic fraction of the feeding material
(9, 10). In the first case, the particles contain mainly inorganic
salts (mixture of alkali compounds, sulfates, nitrates, and
silicates), whereas the particles of the latter type are composed
predominantly of soot and condensable unburnt organic
material. These two particle types, however, can occur as
complex internal and/or external mixtures. The emission of
particulate matter is correlated to the operating conditions
of the combustion process. This is particularly true for
particles of the latter type, which are suspected to exhibit a
strong impact on human health (11).
T . F E R G E , † , ‡ J . M A G U H N , † K . H A F N E R , †
F . M U¨ H L B E R G E R , † , ‡ M . D A V I D O V I C , §
R . W A R N E C K E , | A N D
R . Z I M M E R M A N N * , † , ‡ ,
Forschungszentrum fu¨r Umwelt und Gesundheit, Institut fu¨r
O¨ kologische Chemie, Ingolsta¨dter Landstrasse 1,
D-85764 Neuherberg, Germany, Analytische Chemie, Lehrstuhl
fu¨r Festko¨rperchemie, Institut fu¨r Physik, Universita¨t
Augsburg, Universita¨tsstrasse 1, D-86159 Augsburg, Germany,
Clausthaler Umwelttechnik-Institut GmbH, Leibnizstrasse
21+23, D-38678 Clausthal-Zellerfeld, Germany,
Gemeinschaftskraftwerk Schweinfurt, Hafenstrasse 30,
97424 Schweinfurt, Germany, and Abteilung Umwelt-und
Prozesschemie, Bayerisches Institut fu¨r Angewandte
Umweltforschung und-technik, Am Mittleren Moos 46,
D-86167 Augsburg, Germany
The emission of particulate matter and gaseous compounds
during combustion of wood and refuse-derived fuel in a
small batch reactor is investigated by laser mass-
spectrometric on-line measurement techniques for gas-
phase analysis and simultaneous registration of physical
aerosol properties (number size distribution). The gas-phase
composition is addressed by a laser-based mass
spectrometric method, namely, vacuum-UV single-photon
ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS).
Particle-size distributions are measured with a scanning
mobility particle sizer. Furthermore, a photoelectric aerosol
sensor is applied for detection of particle-bound polycyclic
aromatic hydrocarbons. The different phases of wood
combustion are distinguishable by both the chemical profiles
of gas-phase components (e.g., polycyclic aromatic
hydrocarbons, PAH) and the particle-size distribution.
Furthermore, short disturbances of the combustion process
due to air supply shortages are investigated regarding
their effect on particle-size distribution and gas-phase
composition, respectively. It is shown that the combustion
conditions strongly influence the particle-size distribution
as well as on the emission of particle-bound polycyclic
aromatic hydrocarbons.
In several studies the effects of process control measures
on the emission of aerosol particles into the flue gas of
incineration processes were investigated. In particular, this
applies to the case of combustion of biomass, which
nowadays is more and more discussed as a CO2-neutral fuel.
Particles emitted from wood combustion are found to be
primarily in the submicrometer size range (12). For instance,
the number concentration of particles in the raw gas of large-
scale biomass combustion has been reported to be (1-2) ×
107 particles/cm3 and the maximum of the number-size
distribution to be 150-300 nm (13). Other studies revealed
similar results with maxima of the size distribution around
100 nm, exhibiting a shift to larger particle sizes during
decreased oxygen concentration and thus more incomplete
* Corresponding author phone: +49 89 3187 4544; fax: +49 89
3187 3510; e-mail: ralf.zimmermann@gsf.de.
† Institut fu¨r O¨ kologische Chemie.
‡ Universita¨t Augsburg.
§ Clausthaler Umwelttechnik-Institut.
| Gemeinschaftskraftwerk Schweinfurt.
Bayerisches Institut fu¨r Angewandte Umweltforschung und-
technik.
9
10.1021/es049493o CCC: $30.25
Published on Web 01/22/2005
2005 American Chemical Society
VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1393

combustion (14-16). Generally, large-scale boilers can be
operated under more favorable, that is, less nonstationary
combustion conditions. Due to larger combustion air and
feeding material volumes, the influence of local fluctuations
in combustion performance is attenuated. Hence, particles
originating from large-scale combustion facilities usually
contain a relatively small organic fraction (13, 17-19). The
submicrometer particles emitted are mainly composed of
potassium chloride and potassium sulfate, while supermi-
crometer particles contain species such as calcium, mag-
nesium, silicon, or aluminum. This chemical composition
shows that these combustion particles originate from the
fuel ash (10, 17, 18, 20, 21). In small batch-operated boilers,
the particle-number distribution shows a larger variation,
ranging from 107 to 1010 particles/cm3 (22-24). The level of
excess air seems to affect the particle-number concentration.
In automatically and permanently operated wood chip
burners, the particle-number concentration decreases when
the oxygen concentration in the flue gas is reduced (22, 23).
Furthermore, the number distribution changes according to
the different combustion phases. For example, during the
start-up phase of the combustion the particle-size distribution
is reported to exhibit a maximum at the largest particle size
around 200 nm, during the intermediate phase at lower
particle sizes in the range of 150 nm, and during burn-out
at the lowest particle size with a maximum around 50 nm
(24, 25). As in large-scale combustion, the main elemental
composition of the particles is due to potassium, chlorine,
sulfur, and oxygen. The concentration of organic carbon can
be as low as 1-10% in the fly ash under conditions of complete
combustion (26).
FIGURE 1. Schematic drawing of the combustion reactor: (A) fuel
bed; (B) sampling point for VUV-SPI-TOFMS measurements; (C)
sampling point for particle measurements; (D) combustion chamber.
Investigations on the direct effects of changes in process
parameters on the emission of particles are important to
increase knowledge of the formation mechanisms and
particle emission levels. Lillieblad et al. (15) report in a recent
publication on the influence of boiler operation on the
emission of submicrometer particles and several gaseous
compounds (e.g., PAH) from biomass combustion. They find
a dependency of the particle-number concentration on boiler
load, resulting in higher particle concentrations with in-
creasing load. Potassium and sulfur dominated the submi-
crometer particle composition for all loads, making up 69-
87% of the analyzed mass. The concentration of elemental
carbon was higher for low load compared to medium load.
This is in agreement with an increase in CO and total
hydrocarbons at low load, indicating conditions of incomplete
combustion.
were investigated regarding the size distribution with a
scanning mobility particle sizer (SMPS). Furthermore, the
PAS signal was monitored. Several measurements were
performed during stable combustion conditions as well
disturbed combustion conditions, namely, periods of in-
complete combustion due to air-supply shortages (CO peaks).
Experimental Methods
Fuel Characteristics. Wood combustion was performed with
wood chips from waste wood that had a grain size of
approximately 3 cm. The fuel had an average caloric value
of 16 MJ/kg and a humidity of 28% (45). The experiments
dealing with waste combustion were performed with a
mixture of wood chips and refuse-derived fuel (RDF). RDF
is made of domestic waste, which is first sorted to remove
incombustibles, dried, and eventually mixed and pelletized.
The pellets of the RDF used in this study had an approximate
size of 2 × 4 cm. The RDF had an average caloric value of
22 MJ/kg and a humidity of 7%. The ash content was about
18% (45).
The concentration of carbon-containing particles can be
influenced by process control measures (27). For example,
by firing the back-up oil burners, a very pronounced
additional maximum in the size distribution around 30 nm
shows up. Such newly formed particles predominantly are
due to unburnt carbon (soot) (28, 29).
For each experiment 4 kg of wood chips out of a much
larger batch of waste wood was used. The repeatability of the
experiments was therefore good from one experiment to the
next. Combustion of RDF was performed by thorough mixing
of 2 kg of wood chips with 2 kg of RDF pellets to result again
in a fuel batch of 4 kg.
Combustion Process. The measurements were carried
out in a small batch-operated combustion reactor with a
thermal capacity of 50-150 kW (Figure 1). In the described
experiments the reactor was fired with wood chips.
The advantage of the small reactor is that air supply and
boiler load can easily be adjusted. Therefore several operation
conditions as well as disturbed combustion processes can
be simulated and monitored. Figure 1 shows a schematic
drawing of the reactor. The insides of the combustion
chamber, which are covered with firebricks, are preheated
to approximately 1000 °C by a methane gas burner. Sub-
A correlation of combustion conditions with emission of
gas-phase and particulate constituents needs the application
of on-line techniques for a time-resolved analysis of gas and
particle phases. In recent papers it was demonstrated that
laser-based mass spectrometric methods are feasible for the
on-line detection of gas phase components in the flue gas
of industrial incineration plants, especially for PAH (30-40).
On the other hand, the photoelectric aerosol sensor (PAS)
can be used for the on-line detection of particle-bound PAH
(PPAH) as a sum value, giving information about the chemical
composition of combustion particles (24, 41-43).
In this paper, we report on the correlation of gas phase
and particle emissions during combustion of wood and
mixtures of wood with refuse-derived fuel in a small batch-
operated boiler. The gas-phase composition was measured
by vacuum-UV single-photon ionization time-of-flight mass
spectrometry (VUV-SPI-TOFMS) (38, 44). Particle emissions
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1394 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 6, 2005

The sampled flue gas was subsequently analyzed by
vacuum-UV single-photon ionization (VUV-SPI) mass spec-
trometry (38, 44, 46-49). In the measurements described
here, VUV photons with a wavelength of 118 nm were used
for a single-photon absorption/ionization process. Ions were
subsequently analyzed in a time-of-flight mass spectrometer
(TOFMS). The 118 nm radiation is generated via a rare-gas
cell for frequency tripling of the 355 nm radiation of a Nd:
YAG laser (50-53). As only compounds with an ionization
potential (IP) below the photon energy can be ionized,
selectivity of the ionization process is obtained by virtue of
the ionization potential. The photon energy for SPI is 10.5
eV (118 nm). VUV-SPI is a very soft (i.e., nearly free of
fragmentation) ionization method. In most cases only the
molecular ion peaks of detected compounds are present in
the mass spectra. Therefore, the method is suited to analysis
of gas mixtures. A more detailed description of the technique
is given in references 38, 40, and 44.
All gas-phase measurements were performed with a
mobile TOFMS, which is described in detail in a previous
publication (38). The instrument is equipped with a Nd:YAG
laser, which provides 355 nm radiation with a pulse energy
of 7 mJ. However, the results showed that the ionization
efficiency is highly dependent on the energy of the 355 nm
laser radiation (∼I3). Therefore, a more powerful laser (Surelite
II, Continuum, CA) was coupled to the instrument and used
for pumping of the rare-gas cell. This laser supplies 355 nm
pulses with a pulse energy of 20 mJ into the rare-gas cell.
With this setup, limits of detection of 85 parts per billion
(ppb) for benzene, 120 ppb for toluene, and 134 ppb for
xylene were obtained (S/N ) 2). Limits of detection were
determined according to literature (31). In a recent instrument
the limits of detection achieved here could be lowered due
to further improvement of the laser setup and are as low as
1.5 ppb for benzene and 2 ppb for toluene (54).
Particle Measurements. The particle measurements were
performed at the beginning of the flue gas duct above the
combustion chamber (Figure 2) at a flue-gas temperature of
about 250 °C. Particle sampling was performed isokinetically.
The outer part of the sampling line was heated to temper-
atures above 120 °C to avoid condensation of water and other
condensable gases. The aerosol passed a heated preimpactor
for precipitation of particles with diameters larger than 40
µm. The aerosol then was immediately diluted with particle-
free air prior to analysis by a factor of 100 by use of two
ejector diluters in line (model VKL-10E, Palas GmbH,
Karlsruhe, Germany). The first diluter was also heated to 120
°C to avoid condensation effects. A detailed description of
the sampling system can be found in the literature (27). The
diluted aerosol was subsequently analyzed regarding its size
distribution with a scanning mobility particle sizer (SMPS,
model 3936, consisting of a DMA model 3071 and a CPC
model 3025A, TSI Inc., St. Paul, MN). Additionally, informa-
tion about the chemical nature of the particles could be
obtained with a photoelectric aerosol sensor (PAS 2000,
EcoChem, U¨ berlingen, Germany). The PAS is working on the
principle of photoionization of particle-bound PAH. Car-
bonaceous aerosol particles carrying adsorbed PAH mol-
ecules emit electrons when irradiated with UV light and thus
are positively charged. Subsequently, the particles are
collected on a filter inside an electrometer, which detects
the charge deposited on the filter. The analyzer signal
therefore is a rough measure of total PAH adsorbed on carbon
particles (42, 43). Note that some interferences due to metal
oxides and soot particles occur and therefore the signal
cannot be considered as quantitative measure of PAH-rich
particles in combustion processes.
FIGURE 2. Schematic drawing of the sampling system for VUV-
SPI-TOFMS measurements and the particle sampling and analysis
system.
sequently, the burner is removed and the grate with the
combustion fuel is installed below the combustion chamber.
Then the fuel is heated and dried by the heat radiation from
the chamber walls. Shortly after, flaming combustion starts.
The primary air supply is located below the grate, the air
being blown through the fuel bed from below. Additionally,
secondary air is blown into the combustion chamber through
nozzles, which are located 1.0 m above the burning bed.
All measurements were started when the fuel was inserted
into the combustion chamber (t ) 0 s) and were not stopped
until all material was burnt and no smoldering could be
detected by visual inspection through a small window located
at the top of the combustion chamber.
Combustion experiments performed in the batch reactor
can exemplify the temporal behavior of a distinct grate zone
in industrial incineration plants, where feeding material is
moved continuously down the grate. In this context, one
complete combustion cycle in the batch reactor represents
a complete passage over the grate zone.
VUV-SPI Measurements. The sampling point for the SPI
measurements was located below the nozzles of the second-
ary air supply about 30 cm above the flames and therefore
in the primary combustion zone. Figure 2 shows a scheme
of the sampling system. A quartz-glass tube (i.d. 5 mm)
shielded by a stainless steel tube was inserted into the
combustion chamber and a sample flow of 5 L/min was
applied by a small sampling pump. The sampled gas passed
a heated quartz-fiber filter to prevent dust from entering the
further sampling line. Behind the filter, the tip of a quartz
capillary (i.d. 0.3 mm) was located in the center of the tube
and a small amount of the sample flow (approximately 15
mL/min) was split, flowing to the mass spectrometer for
analysis. The whole sampling line was heated to 250 °C to
prevent condensation of tarry substances and thus clogging
of the capillary. A detailed description of the inlet system is
available in the literature (35, 38).
The SMPS was operated in the size range from 28 to 430
nm; scanning times were adjusted to a scan-up time of 30
s and a retrace time of 10 s, so that a complete size distribution
9
VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1395

FIGURE 3. Typical VUV-SPI time-of-flight mass spectrum obtained
during combustion of wood chips (a) and wood/RDF (b); lower spectra
in each panel are 10-fold amplified. Major peaks are due to benzene
(78 m/z), naphthalene (128 m/z), and phenanthrene (178m/z). Complete
assignment of peaks is given in Table 1.
TABLE 1. Peak Assignment for VUV-SPI Spectra Measured
during Combustion Experiments with Wood and Wood/RDF as
Feeding Material
m/z [amu]
total molecular formula
assigned compound
17
28
NH3
ammonia
C2H4
ethylene
30
NO
nitrogen oxide
propyne
vinylacetylene
pyrrole
benzene
aniline
benzonitrile
indole
40
C3H4
52
C4H4
67
C4H5N
C6H6
78
93
C6H5NH2
C6H5CN
C8H7N
103
117
128
153
167
178
C
C
C
C
10H8
naphthalene
cyanonaphthalene
carbazole
10H7CN
12H9N
14H10
phenanthrene
could be recorded every 40 s. One PAS measurement was
performed every 10 s. For correlation with the SMPS data
these values were averaged in the appropriate time intervals
to yield comparable 40 s values.
FIGURE 4. (a) Time-intensity profiles of benzene and phenanthrene
recorded with VUV-SPI-TOFMS during a measurement with stable
combustion conditions. The different phases of wood combustion
are indicated by the vertical lines. (b) Time profiles of the number
concentration of selected particle sizes (30, 60, 100, and 200 nm).
Results and Discussion
Wood Combustion. The first experiments were performed
during burning of wood chips under stable process condi-
tions. Figure 3a shows a typical sum spectrum obtained by
VUV-SPI-TOFMS at a wavelength of 118 nm. Aromatic
compounds such as benzene (78 m/z), naphthalene (128
m/z), and phenanthrene (178 m/z) are clearly visible in the
spectrum. A tentative assignment of the observed peaks is
given in Table 1.
The VUV-SPI technique allows the simultaneous on-line
real-time detection of several target compounds with high
time resolution. In principle, 10 spectra/s can be recorded;
here a time resolution of 2 Hz was achieved as five spectra
were internally averaged on the transient recorder. The signal
intensity is directly correlated to the absolute concentration
of the target compound in the sample gas; therefore the time-
intensity profile is a direct measure of the time-dependent
concentration changes of several target compounds (35, 55,
56). However, profiles of different compounds cannot be
compared directly in terms of absolute concentration without
previous calibration due to substance-specific differences in
the ionization cross sections. Figure 4a shows the time-
intensity profiles of benzene and phenanthrene as exemplary
compounds present in the gas phase, which were detected
by VUV-SPI during a typical wood combustion experiment.
In general, the course of wood combustion can be sum-
marized as follows (57, 58): When wood is heated, its
constituents start to hydrolyze, oxidize, dehydrate, and
pyrolyze (drying/outgassing phase). With further increasing
temperature, combustible volatiles as well as tarry substances
and reactive carbonaceous char are formed. When the
ignition temperature of the volatile and tarry substances is
reached, the exothermic oxidative reactions start (flaming
combustion). During flaming combustion, char formation
continues until the flux of combustible volatiles drops below
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1396 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 6, 2005

the level needed to maintain flaming combustion. At this
time, the afterglow or smoldering combustion phase starts,
best described as solid-phase combustion of the residual
char (59). The different combustion phases are marked in
Figure 4a. During the start-up phase (drying and outgassing),
no SPI-TOFMS signal can be detected except for a short peak
of the benzene concentration right at the beginning of the
process. Flaming combustion initiates at t ) 600 s. During
this phase, the signal of benzene shows a quite high intensity.
Obviously it is released in relatively high concentrations
whereas higher mass aromatic hydrocarbons (like phenan-
threne) show only weak signals. The relatively constant level
of benzene (and naphthalene, not shown here) is typical for
the combustion experiments with stable conditions per-
formed in this study. Because the sampling point was located
below the secondary air nozzles, the data describe the primary
combustion zone. The transition from flaming combustion
to the smoldering phase takes place at t ) 1150 s (also
observed by visual inspection). Immediately, the pattern of
aromatic compounds changes completely. The profiles of
the small compounds decrease considerably, whereas the
large PAH increase in concentration (high signal intensity).
The emission profile gets dominated by the aromatic
hydrocarbons of higher mass. Pyrolysis of the remaining fuel
is the most prominent decomposition reaction during the
smoldering combustion phase. The primary products of the
pyrolytic decomposition of wood are cracked. Under the still
high temperature conditions, the growth of polynuclear
aromatics is due to low molecular weight hydrocarbons,
which are formed directly from the gas-phase cracking of
remaining wood compounds (60). The shift observed in the
hydrocarbon pattern, which is indicating the transition from
flaming to smoldering (that was also observed by visual
inspection), was consistently found in all combustion
experiments performed in this study.
Figure 4b shows the corresponding SMPS time profiles of
particle-number concentrations of particles of a selected size
(30, 60, 100, and 200 nm). No significant change in number
concentrations can be observed at the transition from the
start-up phase to flaming combustion. Particles with a
diameter of 30 nm exhibit high number concentrations of
(2-5) × 109 particles/cm3. Not until the transition from
flaming to smoldering combustion do particles of larger
diameters start to increase in number concentration. The
30-nm particle concentration starts to decrease immediately
after the transition; larger particles (60 and 100 nm) peak at
the beginning of the smoldering phase and decrease again
in concentration subsequently.
Figure 5 shows a comprehensive view of the particle-size
distribution during this combustion experiment measured
with the SMPS. In the contour plot and the three-dimensional
size distribution plot, the boundaries of the different
combustion phases are plotted as white lines. During the
outgassing phase, the particle-size distribution exhibits a
maximum around 30 nm. With the onset of the flaming
combustion, the particle-size distribution still exhibits a clear
maximum at 30 nm with a higher number concentration of
7 × 109 particles/cm3. This corresponds to the concentration
range stated in the literature (10). Our finding of larger particle
diameters during the smoldering phase of combustion
appears to contradict literature data, since we observe an
increase in particle size as well as particle-number concen-
tration. As is shown in Figure 5, the maximum of the particle-
size distribution during the smoldering phase is around 60
nm with a concentration of 1.1 × 1010 particles/cm3. This
effect of increasing particle size with transition to smoldering
combustion was consistent throughout all measurements
performed. The chemical nature of the particles was inves-
tigated with the aid of the PAS with respect to particle-bound
PAH (PPAH), which can be used as indicators of incomplete
FIGURE 5. (a) Particle number size distribution for a wood
combustion experiment during stable combustion conditions. (b)
Contour plot of the particle-size distribution during wood combustion
with stable combustion conditions. (c) Three-dimensional repre-
sentation of the particle-size distribution during wood combustion
with stable combustion conditions.
combustion. However, in experiments with stable combus-
tion conditions, the PAS showed no relevant photoemission
(PE) signal. This indicates that the particles emitted from the
combustion chamber are predominantly of inorganic nature
(mainly potassium salts), as was typically observed for
biomass combustion (10, 19, 22, 23). The fuel-inherent
inorganic material is released as vapor and subsequently
nucleates in the upper region of the burning chamber forming
particles in the ultrafine particle-size range. During the
smoldering phase, the temperature at the top of the
combustion chamber drops from 750 to 600 °C. The
temperature in the subsequent flue gas duct is consequently
reduced even further, causing semivolatile gas-phase con-
stituents to condense on particles, thus increasing their size.
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VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1397

FIGURE 6. Temperature and PE-signal profile during a measurement
with hot CO peak. The period of the CO peak (410-510 s) is indicated
by shading.
Note that due to technical reasons, the temperature refers
to the top of the combustion chamber and not to the actual
particle sampling point in the flue gas channel. Also the
formation rate of PAH in the gas phase in the primary
combustion zone is increased. This agrees well with studies
on pyrolysis of wood and formation of PAH in the gas phase
(60). Despite an increase in PAH formation, no significant
increase of PAH-containing soot particles could be observed
by photoemission. The increase in size and number con-
centration of inorganic particles containing chloride and
sulfate is of particular importance in large industrial scale
incineration plants as well as in coal and biomass combustion.
Alkali species often serve as carriers of chlorine and sulfates,
whose deposition is a persistent cause for intermittent fouling
and corrosion problems (61, 62). The growth in particle size
enhances the deposition probability inside boiler sections
such as, for example, the heat exchanger tubes. These particle
deposits are therefore suspected to play a major role in
corrosion, which constitutes a severe problem, especially in
large-scale waste incineration plants.
Influence of Combustion Conditions: Impact of Short-
Time Disturbances. One objective of this work is to
investigate the influence of combustion conditions on the
emission profile in the gas and particle phase. Due to
increasing importance of biomass combustion in energy
production as well as domestic heating, profound knowledge
of the influence of combustion conditions on the emission
profile is needed. Likewise, the investigation of combustion
processes in reactors with well-defined conditions can be
used as model experiments for large-scale industrial incin-
erators. Conditions of disturbed combustion can be easily
simulated by decreasing the air flow through the secondary
air nozzles in the upper part of the burning chamber (see
Figure 1), causing an insufficient oxygen supply in the
secondary burning zone. This is referred to as “hot CO peak”
due to the constant high-temperature level in the primary
combustion zone.
FIGURE 7. (a) Time-intensity profiles of benzene and phenanthrene
recorded with VUVSPI-TOFMS during a measurement of wood
combustion with hot CO peak. (b) Time profiles of the number
concentration of selected particle sizes during a measurement of
wood combustion with hot CO peak.
During a wood combustion experiment, a CO peak was
induced by reducing the secondary air supply. The resulting
temperature profile in the secondary combustion zone of
the combustion chamber along with the observed PE signal
is shown in Figure 6. The period of the hot CO peak is marked.
During this period, the temperature at the top of the
combustion chamber drops from 850 to 750 °C. At the same
time the PE signal increases to values of up to 16 pA, indicating
an emission of large amounts of PAH-rich particles (again,
note that there can be interference by metal oxides). Figure
7 panel a shows the gas-phase profiles of benzene and
phenanthrene and panel b shows the time profiles of the
particle-number concentrations. Again, the combustion
phases are indicated as well as the period of the CO peak.
As was observed for stable combustion conditions, a shift in
the emission pattern of gas-phase compounds occurs at the
transition from flaming to smoldering combustion. In the
same way, the particle concentrations show similar behavior;
that is, the 30-nm particles exhibit high number concentra-
tions of 3 × 109 particles/cm3 during flaming combustion
and a decrease at the transition to smoldering, whereas the
particles of larger diameters show an increasing number
concentration at this time. Note that the number concentra-
tion for flaming combustion refers to the period before the
CO peak, as particle emissions stay affected also after the CO
peak has vanished (see below).
9
1398 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 6, 2005

by gas to particle conversion. Furthermore, these high
number concentrations lead to coagulation of particles,
resulting in a second maximum around 130 nm (dN/d log
D ) 4 × 1010 particles/cm3). Additionally, the chemical nature
(and thus the photoelectric properties) of the particles is
influenced as is shown in Figure 6. Obviously, reduced
postcombustion of volatiles and particulate matter produces
large amounts of photoelectric active particles. It is likely
that these particles generated by incomplete combustion
are predominantly soot particles containing large amounts
of PAH.
The time-intensity profiles of gas-phase constituents
measured with VUV-SPI are influenced (e.g., in the case of
benzene, Figure 7a), although the sampling point of the VUV-
SPI measurement was located below the secondary air nozzles
(Figure 1). This effect can be explained by the reduced heat
radiation from the secondary combustion zone during the
period of secondary air shortage and demonstrates that
changes in the secondary combustion zone conditions affect
the chemical reactions in the primary zone (i.e., by thermal
coupling of primary and secondary combustion).
After a return to the normal operation mode, the PE signal
shows a steep decrease, but stays at elevated levels at 3-4
pA. Similarly, the particle-size distribution still exhibits its
bimodal shape (admittedly less pronounced) in the early
post-CO peak period. Obviously, more photoelectrically active
particles are emitted after the disturbance compared to
normal flaming wood combustion. Therefore, it has to be
assumed that the emission characteristics of combustion
processes depend not only on the actual state of the
combustion process but also on the “process history” (63).
The transition to smoldering combustion occurs at 1050
s. This is obvious from the change in the emission profiles
of the gas-phase constituents. At the same time, the particle-
size distribution shows the characteristic, pronounced shift
to larger particle diameters with a new maximum at 60 nm
and a particle-number concentration of 1.0 × 1010 particles/
cm3. The PE signal is still slowly decreasing. This shows that
photoelectrically active particles are still emitted.
One interesting feature of the transition from flaming to
smoldering is seen in Figure 6. At t ) 1040 s the PE signal
as well as the temperature shows a short peak. At this moment
a second ignition could be observed of some feeding material
lasting only for several seconds. Therefore the temperature
rises again in the combustion chamber. The PE signal shows
that this second flaming takes place under conditions of
incomplete combustion, which becomes obvious when one
compares the gas-phase signals (Figure 7a) of this time period
(t ) 1040 s) with the CO peak. A similar secondary ignition
event was described in a study on biomass ignition and
explained by inhomogeneities in the sample so that a
coexistence of different combustion zones is possible (64).
As the ignition depends on the diffusion of oxygen to the fuel
surface and reactions with volatiles, it is imaginable that in
some region of the grate unburnt material remained and a
second ignition could be observed. However, this effect was
observed only once, so that a more complete explanation of
this behavior still needs further investigation.
Combustion of Refuse-Derived Fuel. In Figure 3b a typical
mass spectrum of an experiment with RDF/wood mixture as
burning material is depicted. Besides the pure hydrocarbon
compounds, also nitrogenous compounds can be detected
by VUV, namely, benzonitrile (103 m/z), indole (117 m/z),
naphthalenecarbonitrile (153 m/z), and carbazole (167 m/z).
A complete assignment of observed peaks is given in Table
1. Nitrogenous compounds play a role in the formation of
NOx and are a useful indicator for flame chemistry. A detailed
discussion on studies of nitrogen-containing compounds
in combustion processes will be given in a forthcoming
publication (65).
FIGURE 8. (a) Particle-size distribution for a wood combustion
experiment with hot CO peak. (b) Contour plot of the particle-size
distribution during wood combustion with hot CO peak. (c) Three-
dimensional representation of the particle-size distribution during
wood combustion with hot CO peak.
The particle-size distributions as well as contour and
three-dimensional plots are shown in Figure 8. The CO peak
itself occurred between 410 and 510 s. At this time the particle-
size distribution shows a considerable increase in number
concentration as well as the rise of a second maximum.
During stable flaming combustion, before the secondary air
supply was decreased, the particle-size distribution exhibits
a maximum of 30 nm with a particle-number concentration
of 3 × 109 particles/cm3. During the CO peak the maximum
of the ultrafine mode is slightly shifted to 35 nm and the
number concentration increases to 1.5 × 1011 particles/cm3.
Due to the lower temperature, nucleation and condensation
reactions take place, leading to higher particle concentrations
9
VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1399

FIGURE 9. Temperature and PE-signal profile during a measurement
with wood/RDF as burning material. The period of the occurring
cold CO peak (350-410 s) is indicated by shading.
In combustion processes, disturbances arising from the
primary combustion zone can be observed accidentally. The
occurrence of this so-called cold CO peak (contrary to hot
CO peaks, see above) is caused by fuel inhomogeneities (like
agglomeration of wet feeding material) leading to locally poor
oxygen supply in the primary combustion zone and resulting
in decreased temperature in the fuel bed. Due to such
inhomogeneities in the feeding material, a cold CO peak
could be observed during a measurement.
In Figure 9 the temperature profile and the PE signal
during a measurement with a cold CO peak is depicted. Again,
the period of the CO peak is marked. During the CO peak the
temperature at the top of the combustion chamber drops
from 890 to 790 °C, resembling the temperature drop during
the hot CO peak (i.e., the distinction between the two types
of CO peaks refers to the temperature in the fuel bed and not
to the temperature in the secondary combustion zone). The
PE signal again shows large values of up to 17 pA. Figure 10
panel a depicts the gas-phase profiles of benzene and
phenanthrene, whereas in panel b the time profiles of the
particle-number concentrations of particles with diameters
of 30, 60, 100, and 200 nm are shown. Again, the combustion
phases are indicated as well as the period of the CO peak.
In accordance with the experiments on wood combustion,
a shift in the emission profile of gas-phase compounds occurs
at the transition from flaming to smoldering combustion
(Figure 10). However, in comparison with wood combustion
(Figures 4 and 7), some differences can be observed. The
decrease of particles with 30 nm diameter exhibits a steady
decrease over the whole sampling period. The increase of
larger particles during smoldering combustion is most
obvious for particles with 200 nm diameter, whereas particles
around 60-100 nm show a short-term decline shortly after
the transition from flaming to smoldering and a new rise in
concentration thereafter. Obviously, the shift in the particle-
size distribution is not as incisive as in the case of combustion
of pure wood chips.
FIGURE 10. (a) Time-intensity profiles of benzene and phenanthrene
recorded with VUV-SPI-TOFMS during a measurement with wood/
RDF as burning material with cold CO peak. (b) Time profiles of
the number concentration of selected particle sizes during a
measurement with wood/RDF as burning material with cold CO
peak.
During the period of disturbed combustion conditions,
one difference from the so-called hot CO peak becomes
obvious: The emission of gaseous compounds in the primary
combustion zone exhibits changed behavior. While the
benzene signal is reduced, phenanthrene becomes a promi-
nent compound in the gas phase of the primary combustion
zone. In this context, the cold CO peak resembles the
smoldering combustion phase. This finding is plausible due
to the mechanism responsible for the appearance of a cold
CO peak due to locally insufficient oxygen supply in the fuel
bed and thus emerging conditions of incomplete combustion.
Figure 11 depicts the particle-size distributions along with
contour and three-dimensional plots of the temporal varia-
tion of the particle-number distribution. Before the CO peak
(during normal flaming combustion) the size distribution
exhibits a maximum at around 50 nm with a number
concentration of 2.5 ×1010 particles/cm3. These larger particle
diameters are due to the use of RDF, the combustion of which
generally exhibits larger particle diameters compared to wood
combustion. Again, a slight shift in the particle-size distribu-
tion can be observed at the transition from flaming to
smoldering combustion. The maximum of the particle-size
distribution during smoldering is around 80 nm; in contrast
to the measurements performed with wood as feeding
material, the number concentration is decreased to 1.5 × 108
particle/cm3.
9
1400 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 6, 2005

Therefore, conditions of incomplete combustion are present
in the primary combustion zone, resulting in the observed
change in the emission pattern detected with VUV-SPI right
above the flame front.
In the post-CO phase, the particle-size distribution
immediately returns to a unimodal size distribution with a
maximum at 64 nm, which is a slightly larger value compared
to the maximum found before the disturbance (53 nm).
Besides this small shift of the size distribution, there is no
strong influence of the disturbance on the particle emission
(regarding particle size) after return to normal operation.
For both measurements of disturbed combustion condi-
tions (wood as well as wood/RDF mixture) a different
emission characteristic could be observed for the post-CO
period compared to combustion under “normal” combustion
conditions. As was pointed out above, it has to be assumed
that the emission characteristics of combustion processes
depend not only on the actual state of the combustion process
but also on the “process history”. The observation of a
changed emission characteristic after CO peaks (here the
particle-size distribution and photoelectric activity) resembles
the emission memory effects, which were observed in an
incineration plant after periods of disturbed combustion.
After unstable combustion conditions, high emission levels
of PAH and PCDD/F (gas phase and particle phase combined)
typical for incomplete combustion were observed for several
hours (63, 66) even though the actual combustion conditions
were again stable. A possible explanation for memory
phenomena is the formation of surface deposits on the
internal surface of the boiler, most probably an effect of ash
deposition. After return to normal combustion conditions,
that is, high temperatures in the postcombustion zone, these
wall deposits start to pyrolyze and evaporate, causing
sustained release of PAH and soot particles. In the early post-
CO peak phase (in the case of the hot CO peak), the number
concentration still shows a bimodal distribution. At t ) 800
s (i.e., approximately 300 s after the end of the hot CO peak),
the maximum at 130 nm has vanished and the particle-size
distribution again resembles the unimodal distribution as
observed before the CO peak (single maximum around 30
nm). However, the particle-number concentration still is
increased (8 × 109 instead of 3 × 109 particles/cm-3) at t )
800 s (Figure 8). Additionally, the PE signal also shows higher
levels than before the hot CO peak (Figure 6). In the case of
the cold CO peak, an immediate return to the unimodal size
distribution but with a larger maximum can be observed
(Figure 11). Similarly, the PE signal shows a slow decrease
after the return to normal operating conditions (Figure 9).
When compared to combustion disturbances in larger-scale
waste incineration processes (63), these results might be an
indication for similar processes resulting in memory emis-
sions. Upon comparison of the PE signals of both types of
CO peaks, a slower decline and therefore an extended
memory was observed for the cold CO peak. However, more
work is required to address these issues.
FIGURE 11. (a) Particle-size distribution for a wood/RDF combustion
experiment with cold CO peak. (b) Contour plot of the particle-size
distribution during wood/RDF combustion with cold CO peak. (c)
Three-dimensional representation of the particle-size distribution
during wood/RDF combustion with cold CO peak.
During the disturbance (lasting from 350 to 410 s), a
bimodal particle-size distribution is observed similar to the
bimodal distribution of the hot CO peak. The maximum in
the ultrafine range is slightly shifted to smaller particle sizes
from 50 to 30 nm. The second maximum appears at 160 nm.
Here again, the decreased temperature leads to nucleation
and condensation effects, which cause the bimodal particle-
size distribution. Likewise, a high PE signal (up to 18 pA) is
observed (Figure 9). However, some differences are visible
in the emission profile of the gaseous species (Figure 10a)
when compared to the hot CO peak (Figure 7a). In the period
of the CO peak, a short-term increase of phenanthrene as
well as a considerable decrease in the benzene signal is
apparent. The disturbance is caused by inhomogeneities in
the fuel bed, leading to decreased oxygen supply in the fuel.
Acknowledgments
This study was carried out within the scope of the GSF-Focus
“Health relevance of aerosols”, which coordinates aerosol-
related research within the GSF Research Center. Funding
from the BMBF, Germany, and the GSF Research Center as
well as from the BStMUGV and the European Union (Grant
UGV08030604128) is gratefully acknowledged. We thank M.
Blumenstock, K. Neuer-Etscheid, and T. Hauler for contri-
butions during the measurement campaign as well as T.
Adam, S. Mitschke, T. Streibel, W. Welthagen, and S.
Gallavardin for support and discussions. T.F. thanks the
Deutsche Bundesstiftung Umwelt for a Ph.D. scholarship.
9
VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1401

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Received for review April 2, 2004. Revised manuscript re-
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