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Aerosol and Air Quality Research, 16: 2560–2569, 2016 Copyright
© Taiwan Association for Aerosol Research ISSN: 1680-8584 print /
2071-1409 online doi: 10.4209/aaqr.2016.07.0329 Pyrolysis
Characteristic Analysis of Particulate Matter from Diesel Engine
Run on Diesel/Polyoxymethylene Dimethyl Ethers Blends Based on
Nanostructure and Thermogravimetry Hao Yang, Xinghu Li, Yan Wang*,
Mingfei Mu, Xuehao Li, Guiyue Kou School of Transportation Science
and Engineering, Beihang University, Beijing 100191, China
ABSTRACT
This paper focuses on the nanostructural and pyrolysis
characteristics of particulate matter (PM) emitted from a diesel
engine fueled by three diesel/polyoxymethylene dimethyl ethers
(PODEn) blends. PM was collected using a metal filter from the
exhaust manifold. The collected PM samples were characterized using
scanning electron microscopy (SEM), transmission electron
microscopy (TEM) and thermogravimetric analysis (TGA). SEM and TEM
analysis showed that PM generated by the 20%-PODEn blend was looser
and had a smaller average diameter than that emitted when a lower
proportion of PODEn was used. Additionally, fringe length was
reduced, and separation distance and tortuosity were increased,
when the 20%-PODEn blend was used instead of the other blends.
These changes improved the oxidation reactivity of the PM. TGA
demonstrated that the PM pyrolysis process was divided into low
temperature (characterized by moisture and volatile components) and
high temperature (the combustion of solid carbon) stages. When the
blending ratio was increased, the moisture and volatile components
of the PM showed no obvious change, but the ignition temperature
and activation energy were reduced. Additionally, the pyrolysis
performance was enhanced; the maximum weight loss rate was higher;
the combustion and burnout characteristic indices were higher; and
the combustion efficiency of the PM was improved. These results
show that the use of diesel blended with oxygenated fuel (PODEn)
affects the nanostructure and pyrolysis of PM, and this PM is
easier to oxidize and advantageous for diesel particulate filter
regeneration. Keywords: Polyoxymethylene dimethyl ethers;
Particulate matter; Nanostructure; Thermogravimetry; Pyrolysis.
INTRODUCTION
Diesel engines are widely used in the transportation, industry,
commerce, and power generation sectors. Diesel engines have the
advantages including high efficiency, a high level of fuel economy
with great torque output, high durability and reliability, and low
operating costs. Diesel engines are major source of emissions
including particulate matter (PM), nitrogen oxides, sulfur oxides,
total hydrocarbons, carbon dioxide, carbon monoxide as well as
toxic chemicals such as polycyclic aromatic hydrocarbons (Mwangi et
al., 2015; Tsai et al., 2015). However, it is well known that PM
emissions from diesel engines are are relatively high, due to the
combustion of heterogeneous mixture in the cylinder at higher
combustion temperatures (Saxena and Maurya, 2016). PM, also known
as soot, is primarily composed of carbon but also includes some
other organic and inorganic compounds (ash), some sulfur *
Corresponding author.
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compounds and some metal traces from unburnt fuel and
lubricating oil (Li, 2011; Sharma et al., 2012). But the
composition of PM from diesel engines may vary widely, because it
depends on lots of variables such as operating conditions, fuel
composition, lubricating oil type etc. (Popovicheva et al., 2014;
Shukla et al., 2014).
PM is microscopic with more than 90% of PM smaller than 1 µm. PM
enters the human body easily through the respiratory system and can
induce a variety of diseases (Li et al., 2003; Gonzalez-Flecha,
2004; Kaiser, 2005). It has been a concern issue in many countries
owing to its adverse health effects (Tsai et al., 2016).
Consequently, diesel PM emissions are strictly monitored around the
world. In China, fifth phase of an emission standard formulated to
implement environmental protection law “Limits and measurement
methods for emissions from light-duty vehicles” limits the PM
concentration for diesel vehicle emission to 0.0045 g km–1,
representing an 82% reduction (0.025 g km–1) compared with the
fourth phase.
Two possible methods to reduce PM emissions from diesel engines
are to use oxygenated fuel and a diesel particulate filter (DPF)
(Cheruyiot et al., 2015). As a new type of oxygenated fuel,
polyoxymethylene dimethyl ethers (PODEn) are effective at reducing
diesel PM emissions because
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PODEn have no C–C bonds in their molecular structure, a
substantial carbon/hydrogen ratio and high oxygen content with a
large cetane number (Burger et al., 2010; Zhao et al., 2011; Zhao
et al., 2013; Yang et al., 2015). PODEn can be synthesized through
a polymerization reaction using methanol as a raw material, and
many crude materials can be used to produce them, including coal
and biomass. Wall flow DPFs undergo a regeneration process to
remove PM and are one of the most common technologies used in
diesel vehicles to meet emissions standards in China, the United
States, Europe and elsewhere.
Scanning electron microscopy (SEM), transmission electron
microscopy (TEM) and thermogravimetric analysis (TGA) are used to
estimate PM emissions. SEM has been widely applied in the
observation of PM morphology, whereas TEM has been routinely used
to investigate the microstructures of diesel PM. Ishiguro et al.
(1997) reported that primary PM had two distinct parts: an inner
core and an outer shell. The inner core had a central region with a
diameter of 10 nm and consisted of several fine particles of 3–4 nm
in diameter. The nucleus exhibited several distorted structural
carbon layers. The outer shell was composed of microcrystallites
with the periodic orientation of carbon sheets. Almost all of the
crystallites were oriented perpendicular to the radius of the
primary PM with a thickness of 1 nm and width of 3.5 nm. Vander Wal
et al. (2007) demonstrated that some primary soot particles had a
hollow interior and an outer shell exhibiting evidence of
graphitization, with a more crystalline structure than that of
nonhollow particles. Su et al. (2004) reported that PM emitted from
a Euro IV heavy-duty diesel engine consisted of fullerenoid- or
onion-like particles agglomerated in a chain-like secondary
structure and They also observed that PM with different
microstructures had different oxidation reactivities.
TGA is the most widely employed method of thermal analysis and
involves monitoring the weight loss of specimens as they are heated
at a controlled rate. TGA is used in the analysis of PM stability,
kinetic parameters and volatile component ratios. Sampling is
performed in the presence of an inert (He, Ar, or N2) or oxygenated
environment. The main characteristic of TGA is its ability to
measure mass and the PM rate of change (Lapuerta et al., 2007;
Chuepeng et al., 2008). Karin et al. (2015) observed that the
chemical contents of PM could be divided into three main regions
according to oxidation temperature; moisture was vaporized at low
temperatures (25–200°C), unburnt hydrocarbons were oxidized at
temperatures of 200–500°C and finally, carbon inside the PM was
oxidized at temperatures between 500–600°C. Yehliu et al. (2012)
used TGA to show that PM derived from pure biodiesel had the
highest oxidation rate compared with ultra-low-sulfur diesel and
synthetic Fischer-Tropsch fuel. Sukjit et al. (2013) demonstrated
that the oxidation temperature of PM decreased when rapeseed
biodiesel (RME) or oxygenated additives (butanol and castor oil
methyl ester) were used. Gill et al. (2012) reported the similar
findings in their comparison of the oxidation reactivity of PM
generated from conventional diesel fuel, RME-diesel and
diglyme-diesel blends.
At present, few studies on PM emitted from diesel
engines fueled by diesel/PODEn blends have been reported. In
this study, the nanostructures and pyrolysis of PM derived from
PODEn/diesel blends were investigated using SEM, TEM, and TGA.
PODEn and diesel in volume ratios of 0/100, 10/90 and 20/80 were
used, with blends denoted P0, P10, and P20 respectively. The
purpose was to investigate the effect of the PODEn/diesel blend
ratio on PM emissions, and this paper provides a theoretical basis
on which to select appropriate regeneration techniques and
conditions. EXPERIMENTAL METHOD PM Sampling Procedure
Experiments were performed using a single-cylinder four-stroke
R180 diesel engine produced by Changchai Co., Ltd. The main engine
specifications are listed in Table 1. Diesel used in the
experiments was purchased locally and PODEn were provided by
Shandong Yuhuang Chemical Co., Ltd. Diesel was used as the base
fuel and PODEn were used as an oxygenating additive. Sampling was
performed at 1800 rpm and under 100% operating load conditions of
the diesel engine. Samples were collected from an exhaust pipe 1.2
m away from the engine exhaust manifold. The direct sampling method
used a multilayer metal wire mesh filter, which was stacked and
arranged in the exhaust pipe during the test. PM was filtered off
by the metal filter to obtain similar PM to that obtained in
real-world conditions with a DPF. PM derived from the various
diesel blends was placed into sealed sample bottles for storage. In
each experiment, the engine was warmed up for 20 minutes before PM
was collected. Analytical Techniques Scanning Electron
Microscopy
The morphology of the agglomerates was obtained using SEM. PM
samples were covered with a gold film prior to SEM in order to meet
the minimum electrical conductivity requirements. Each sample was
viewed in secondary electron mode using a Hitachi SU8010 microscope
operated at an accelerating voltage of 15 kV with a resolution of
1.00 nm. Particle size analysis was performed on SEM images using
the commercial graphic processing software package Image-Pro Plus
6.0. The corresponding column distribution map was generated in
this manner.
Transmission Electron Microscopy
To investigate the soot nanostructure, a Tecnai G2 20 was used
to capture high-resolution bright field images. The
Table 1. Diesel engine main specifications. Parameter Value
Number of cylinders 1 Rated power / kW 5.67
Rated speed / (r min–1) 2600 Bore / mm × Stroke / mm 80 × 80
Displacement volume / mL 402 Compression ratio 21:1
Injection pressure / MPa 13.72 ± 0.5
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instrument was operated at 200 kV using a LaB6 filament. The
applied magnification was up to 1,030,000× with a resolution of
0.24 nm. TEM samples were prepared as follows: a small amount of PM
was ultrasonically dispersed in methanol for 30 minutes; five drops
of the solution were then placed on a 200 mesh Lacey C/Cu TEM grid;
the grid and sample were air-dried; and TEM analysis was
performed.
The PM nanostructure was observed to be in the form of parallel
or distorted carbon lamellae. Image-Pro Plus 6.0 software was
employed for the analysis of high resolution TEM (HRTEM) images.
Fringe length (La), fringe separation (Ds) and tortuosity (Tf) were
used to describe the PM nanostructure (Vander Wal et al., 2005). La
is a measure of the physical extent of the atomic carbon layer
planes seen in the HRTEM image. Ds is the mean distance between
adjacent carbon layer planes. Tf is a measure of the curvature of
the fringes and is defined here as the ratio of the fringe length
to the distance between the two endpoints. If La was less than 0.40
nm and Ds was greater than 0.45 nm or less than 0.30 nm, the fringe
was discarded as an artifact.
Thermogravimetric Analysis
A STA-449 F3 simultaneous thermogravimetric analyzer from
NETZSCH with a resolution of 1 µg was used for TGA. The heating
rate of the apparatus was 0.1–50 °C min–1, and the maximum
temperature was set at 1500°C. To simulate the pyrolysis and
combustion that occur in an engine’s exhaust system, a mixture of
oxidizer gas agents with 10% O2 in N2 from Airgas, and N2 as a
purge gas was used. The flow rate was limited to 100 mL min–1.
Specimens with an initial mass of 2 mg were heated from room
temperature to 900°C at a rate of 10 °C min–1. An inert ceramic
crucible was used to avoid the catalytic effects a
metal crucible has on the PM; therefore, controlled PM pyrolysis
was achieved. RESULTS AND DISCUSSION SEM and TEM Image Analysis
Morphology of PM Samples
The PM samples demonstrated a certain level of agglomeration, as
illustrated by Figs. 1 and 2 (SEM at 100,000 times magnification
and TEM, respectively). Numerous quasi-spherical primary particles
of varying sizes under the action of Van der Waals, electrostatic
and surface adhesion forces formed agglomerate particles through
accumulation. Primary particles appear stacked together in the dark
regions of the TEM images, which obscures the outlines of the
primary particles. Additionally, different densities were observed.
PM samples generated from the P0 blend were densely packed but when
more PODEn was used in the blend, the packing was seen to become
looser. Similarly to oxygenated fuel, PODEn/diesel blends combust
more completely and reduce PM emissions. Therefore, the probability
of collision between primary particles decreases and large
agglomerates are not easily formed.
Size Distribution of PM Samples
According to Fig. 1, the size distribution of PM was determined
using statistical analysis in a unit area, and the mean diameters
of PMs (DP0, DP10 and DP20) were obtained (Fig. 3). Particle size
had an approximately normal distribution over the range 20–60 nm.
The size and the mean diameter of the PM decreased as blends with a
higher proportion of PODEn were used, with the mean diameters being
44.23 nm for the P0 blend and 35.4 nm for the P20
(a) P0 (b) P10 (c) P20
Fig. 1. SEM images of PM samples.
(a) P0 (b) P10 (c) P20
Fig. 2. TEM images of PM samples.
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Fig. 3. Particle size distributions of PM samples.
blend, representing a decrease of 20%. This is mainly because
the oxygen content and cetane number are higher in the blended
fuel, which promotes more efficient fuel burning. Smaller and
looser PM oxidizes more easily and is advantageous for DPF
regeneration.
Nanostructure Parameters Analysis of PM Samples Because SEM
images could not completely characterize
the nanostructure of the primary particles, HRTEM was used.
HRTEM images were digitized and analyzed using image processing
software to find the nanostructure parameters La, Ds and Tf. A
region of the HRTEM image showing clear carbon layers was selected
and converted into black and white. From this binary image, a
skeletonizing process was used and each fringe was assigned a
1-pixel width for further statistical analysis. The original HRTEM
images and final skeleton images are depicted in Fig. 4.
Fig. 5 presents histograms of the fringe length La for PM
samples collected from the three blends, and it showed that it was
concentrated in the range 0.5–1.0 nm (70%, 76%, and 88% of La for
the P0, P10, and P20 blends, respectively). The average La
decreased when a higher proportion of PODEn was used, from 1.041 nm
(P0), to 0.950 nm (P10), and then 0.819 nm (P20). The smaller the
La, the higher the degree of carbon structural disorder was, and a
previous study revealed that disordered amorphous carbon has higher
oxidation reactivity than that of highly ordered graphitic carbon
(Lu et al., 2012). Additionally, Vander Wal and Mueller (2006)
observed that carbon atoms at edge-site
(a) P0 (b) P10 (c) P20
Fig. 4. HRTEM images and the corresponding skeleton images.
(a) P0 (b) P10 (c) P20
Fig. 5. Histograms of fringe length.
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positions have a greater activity than those in the basal plane.
P20 PM was more likely to be oxidized because it contained a larger
number of edge-site carbon atoms.
The fringe separation Ds of primary particles produced when the
P0 blend was used was mostly distributed in the range 0.33–0.44 nm
with a median of 0.378 nm (Fig. 6). Separation distance increased
with the PODEn concentration, with the mean values being 0.393 nm
and 0.404 nm for the P10 and P20 blends, respectively. Higher Ds
enhances the oxidation reactivity of the primary particles, because
it facilitates oxygen access to carbon layers (Al-Qurashi and
Boehman, 2008). Therefore, the oxidation reactivity of the PM
samples derived from P10 and P20 was enhanced.
The distribution of the tortuosity Tf was mostly concentrated in
the range 1.5–1.7 (Fig. 7). The mean tortuosity was higher for
blends with a higher proportion of PODEn, ranging from 1.566 (P0),
to 1.582 (P10), and then 1.613 (P20). Tortuosity is attributed to
the existence of 5- or 7-member ring structures in the layer plane
and relates to the degree of carbon layer disorder (Yehliu et al.,
2011). When the tortuosity is increased, the proportion of carbon
layers with a graphite structure is decreased; hence, PM samples
emitted from P10 and P20 were more easily oxidized than those from
P0 TG Analysis Pyrolysis Process Analysis
Pyrolysis is a basic thermo-chemical conversion, and is the
initial and associated reaction involved in gasification,
liquefaction and combustion. Fig. 8 depicts the thermogravimetric
(TG) and derivative thermogravimetric (DTG) curves of PM
Samples.
Fig. 8(a) shows the TG curves of moisture evaporation and
volatile matter desorption in the temperature region 60–400°C. At
low temperatures, no intensive chemical reactions were found. The
TG curves remain relatively constant. Mass loss was insignificant
at approximately 1%, 3% and 5% of the total mass for the P0, P10
and P20 blends, respectively. It can be deduced that the moisture
and volatile matter content were very low in the PM samples, which
may be related to the sampling method used. In the exhaust pipe,
where temperatures are high, most of the volatile materials are
present in the gas phase and it is difficult for them to be
adsorbed or coagulate into existing PM (Stratakis and Stamatelos,
2003). The oxidation reaction of solid carbon in the PM occurs at
high temperatures (400–750°C). The PM oxidation process occurred at
lower temperatures as the PODEn proportion was increased. After
reaching ignition temperature, the TG curves decline rapidly, and
the PM sample mass loss exceeded 95% at this stage. The PM was
burnt out at 750°C, after which the TG curves tend to
stabilize.
From Fig. 8(b), weightlessness peak was present in the high
temperature regions on the DTG curve. Weightlessness peak become
larger when P10 and P20 were used. As PODEn are oxygenated fuels,
more oxygen content may be present in the PM, which promotes
oxidation. P10 and P20 demonstrated a higher oxidation rate
compared with P0 (Karin et al., 2015). Characteristic Parameters
Analysis
In the present study, the maximum weight loss rate, peak
temperature and half-life temperature were analyzed.
(a) P0 (b) P10 (c) P20
Fig. 6. Histograms of fringe separation.
(a) P0 (b) P10 (c) P20
Fig. 7. Histograms of fringe tortuosity.
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Fig. 8. TG curves and DTG curves of PM samples.
The maximum weight loss rate reflects the combustion of PM after
ignition. It corresponds to the highest reaction rate and has a
minimum value that can be found from the DTG curve. The peak
temperature (TP) refers to the temperature at which the maximum
rate of weight loss occurs. The half-life temperature (T50) is
defined as the temperature at which 50% of PM mass has been
converted. From the TG and DTG curves, the pyrolysis
characteristics of the PM samples were obtained (Table 2). The
maximum rate of weight loss was higher for higher PODEn blending
ratios, as was the PM burn rate. Additionally, TP and T50
diminished, the temperature at which the PM's initial oxidation
reaction occurred decreased, and the activation energy E
decreased.
The ignition temperature Ti is the minimum temperature of
continuous combustion in air or oxygen environments, and represents
the difficulty index of PM ignition. Numerous methods for
calculating Ti using TG and DTG curves exist, but this study used
the method described in Fig. 9. An initial three instants on the TG
and DTG curves should be identified. Point B is a perpendicular
line from the minima of DTG peak A (the point of the maximum rate
of weight loss), which transects the TG curve. Point C is the
beginning of volatilization. A tangent to the TG curve at point B
and another horizontal tangent at point C are drawn. The point at
which these lines crosses is point D, corresponding to Ti. When a
higher PODEn blending ratio was used, the PM sample Ti decreased by
2.16% (P10) and 5.97% (P20), compared with P0 (Table 3). Less
graphitization and more graphite structure disorder corresponds to
a lower PM Ti.
Combustibility Index
Combustion at low heating rates is determined by the chemical
reaction dynamics in the initial stage of the reaction. These
chemical reaction dynamic factors may also control the reaction
rate. The Arrhenius equation expresses
the combustion rate as follows (Rodríguez-Fernández et al.,
2011; Wang et al., 2014): d expdm EAt RT
(1)
where m is mass (mg), t is time (min), dm/dt is the combustion
rate (mg min–1), A is the Arrhenius pre-exponential factor (s–1), E
is the activation energy (kJ mol–1), R is the molar gas constant
(8.31 J mol–1 K–1) and T is the absolute temperature (K).
Derivation of Eq. (1) gives the following equation:
2
d d d 1d d d
R m mE T t t T
(2)
At Ti, Eq. (2) becomes
i i
2i
d d d 1( ) ( )d d dT T T T
R m mE T t t T
(3)
Multiplying
i
max mean
h
d / d d / dd / d
T T
m t m tm t T
by both sides of
Eq. (3) gives
i i
max mean
h
max mean2
i h
(d / d ) (d / d )d d( )d d (d / d )
(d / d ) (d / d )T T T T
m t m tR mE T t m t T
m t m tT T
(4)
where (dm/dt)max is the maximum combustion rate (mg min–1),
(dm/dt)mean is the average combustion rate (mg min–1),
Table 2. Pyrolysis characteristic parameters of PM samples.
PM samples Maximum weight loss rate / (% °C–1) TP / °C T50 / °C
P0 0.95 662 649 P10 0.98 651 629 P20 1.00 634 603
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Fig. 9. Definition diagram of ignition temperature
Table 3. Combustibility index of PM samples.
PM samples
Ignition temperature Ti /
K
Burnout temperature Th /
K
Maximum combustion rate (dm/dt)max / (mg min–1)
Average combustion rate (dm/dt)mean /
(mg min–1)
Combustibility index S/ (mg2 min–2 K–3)
P0 876 993 0.18 2.03 × 10–2 4.73 × 10–12 P10 863 968 0.20 2.21 ×
10–2 6.10 × 10–12 P20 840 943 0.21 2.34 × 10–2 7.25 × 10–12
(dm/dt)|T=Ti is the combustion rate at Ti (mg min
–1), Th is the PM burnout temperature (K), Th is defined as the
temperature at which a conversion of 98% is achieved; it can reduce
errors and guarantee complete PM combustion. R/E represents the
reactivity of the PM. (d(dm/dt)/dT)|T=Ti indicates the conversion
percentage of the combustion rate at Ti; ignition is quicker when
this is maximized. (dm/dt)max/(dm/dt)|T=Ti is the ratio of the
maximum combustion rate to the rate of combustion at Ti.
(dm/dt)mean/Th denotes the ratio of the mean combustion rate to Th,
and at high values PM burns rapidly. The product of these terms
defines the combustion characteristic of the PM.
The combustibility index S is defined as (Wang et al., 2009; Xiu
et al., 2011)
max mean2
1
d dd d
h
m mt tS
T T
(5)
Several crucial parameters such as Ti, Th, the maximum
rate of combustion, and the average rate of combustion are used
in a comprehensive evaluation. It can be deduced from Eq. (5) that
a higher S denotes more efficient PM combustion.
The combustibility index and other related parameters of the PM
samples are presented in Table 3. As Ti and Th were lowered, the
combustion time of the PM samples decreased. Compared with P0, the
maximum rates of combustion for the P10 and P20 blends were 11.11%
and 16.67% higher, respectively, and the average rates were 8.87%
and 15.27% higher. The index S improved as graphite structure
disorder increased. Thus, high graphite structure disorder can
improve
the combustion characteristics of P10 and P20, resulting in an
improvement in PM oxidation ability. Burnout Characteristic
Index
The burnout characteristic index Cb is the main signifier of PM
combustion performance, and influences a PM’s regeneration time and
regeneration efficiency. Cb was used to evaluate the burnout
performance of the PM samples, and was defined as follows (Zhao et
al., 2015):
1 2b
0
f fC
(6)
where 0 is the burnout time (min), equal to the time taken to
burn off 98% of the sample’s initial mass. f1 is initial burnout
rate, equal to the ratio of PM weight loss on ignition to total PM
mass (%),and reflects the effect of volatile components on the PM
ignition characteristics. f2 = f – f1 is the change in the burnout
rate (%),which reflects the carbon burnout performance of the PM
and is related to the content and form of the carbon present in the
PM. f is the total burnout rate, defined as the ratio of PM weight
loss at burnout time to the PM total mass, and was set as 98%. Cb
and other related parameters of the PM samples are listed in Table
4. The burnout characteristic index increased gradually when a
higher proportion of PODEn in the blend was used, which reveals
that if the degree of graphitization decreases or the degree of
graphite structure disorder increases, the burnout performance of
the PM improves. Calculation of Pyrolysis Kinetic Parameters
Pyrolysis kinetics expresses the influence of various
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Table 4. Burnout characteristic index of PM samples.
PM samples Burnout time 0 / min Initial burnout rate f1 /
% Burnout rate of change f2 / %
Burnout characteristic index Cb / min–1
P0 67.1 19.9 78.1 2.3 × 10–3 P10 64.5 25.7 72.3 2.9 × 10–3 P20
62.6 30.7 67.3 3.3 × 10–3
parameters on the conversion rate of PM during pyrolysis, and
its purpose is to analyze reaction processes quantitatively and to
determine reaction mechanisms. Pyrolysis kinetic parameters are
commonly calculated using the TG curve. This study adopted the
method of Stratakis and Stamatelos (2003). Using the Arrhenius
formula, the kinetic parameters of the PM samples were calculated
from the mass reduction during PM pyrolysis. The thermal kinetic
equation can be written as
/dd
E RT nm Ae mt
(7)
where m is the initial mass of the sample undergoing the
reaction (mg), t is the time (s) and mn is the mass function. n is
the reaction order, which for diesel PM in an oxidation environment
is assumed to be equal to 1 (Neeft et al., 1997). If the time step
is sufficiently small, it can be assumed that the TG curve is
composed of very small linear segments of length Δt during which
the reaction rate is constant, giving dm/dt ≈ Δm/Δt. Eq. (7) can be
replaced with the differential equation
/E RTm Ae mt
(8)
The logarithm of Eq. (8) gives
1 1ln( ) ln ( )m Em At R T
(9)
Thus, ln[(–Δm/Δt) m–1] varies linearly with 1/T. The line
intercepts the y-axis at lnA, as illustrated in Fig. 10. Based
on the experimental results of PM samples, the activation energy E
and pre-exponential factor A can be determined from the slope and
this line intercept. The linear regression is of an acceptable
quality because R2 is > 0.98 (Table 5).
During combustion, chemical reactions were accomplished by
activated molecules. The kinetic energy was sufficiently high (>
E) to destroy the original molecule structure so that reactions
could begin. Therefore, activation energy is a standard for
determining the chemical reaction rate. It is generally believed
that lower activation energies lead to faster chemical reaction
rates. The data of Table 5 reveal that E and A decreased when a
higher proportion of PODEn in the blend was used. It is clear that
PM with less graphitization or more graphite structure disorder
more easily oxidizes at low temperatures. As the Ti of the PM
samples decreased, pyrolysis performance improved, which is
advantageous for DPF regeneration.
Fig. 10. Fitting curves.
Table 5. Pyrolysis kinetic parameters of PM samples.
PM Samples Temperature / °C Activation energy E / (kJ mol–1)
Pre-exponential factor A / s–1P0 602–662 134 7.36 × 104
P10 590–651 128 5.28 × 104 P20 567–634 120 2.12 × 104
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CONCLUSIONS
This study investigated the impact of fuel blends on the
nanostructure and pyrolysis of PM. Three PODEn/Diesel fuel blends
were used by a single-cylinder diesel engine, and the PM emissions
collected through direct sampling were characterized using SEM,
TEM, and TGA.
PM generated by the P20 blend was the loosest collected, and had
a smaller average diameter than that of PM generated by the P10 and
P0 blends, which led to improved oxidative combustion.
When a higher proportion of PODEn in the blend was used, fringe
lengths were reduced, and the separation distance and tortuosity
increased, all of which could enhance the oxidation reactivity of
the PM.
Pyrolysis characteristics and kinetic parameter analysis
demonstrated that as the ignition temperature, activation energy
and pre-exponential factor decreased, the combustibility and
burnout characteristic indices were enhanced when a higher
proportion of PODEn in the blend was used. This indicates that PM
with little graphitization or high graphite structure disorder has
higher temperature sensitivity, improved combustion efficiency and
burning performance, and enhanced pyrolysis performance.
ACKNOWLEDGMENTS
The authors would like to acknowledge financial support provided
by State Key Laboratory of Automotive Safety and Energy (KF14112),
State Key Laboratory of Engines (K2014-6) and National Natural
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Received for review, July 26, 2016 Revised, September 26,
2016
Accepted, September 26, 2016