The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
The Factors Controlling Combustion and Gasification Kinetics
of Solid Fuels
Tolvanen H.M.1, Kokko L.I.2, Raiko R.3
Tampere University of Tampere
Korkeakoulunkatu 6, P.O. BOX 589
Tampere
Finland
henrik.tolvanen@tut.fi
ABSTRACT
This article presents the ways to model and measure factors controlling combustion and
gasification kinetics of solid fuels. When modelling solid fuel combustion and
gasification, four phenomena controlling conversion rate are often mentioned: boundary
layer diffusion, chemical kinetics, pore diffusion and ash layer diffusion. In this study,
experiments related to the chemical kinetics of a specific coal char have already been
conducted. The experimental results and the modelling parameters determined are
presented in this article. In addition to this, other rate controlling phenomena and methods
to study them are also discussed.
In this research article, the chemical kinetics of coal char combustion and gasification
have been studied under low temperature levels and at high heating rates. The
measurements consisted of weight loss experiments with 100-125 µm sized char particles
in a laminar drop-tube reactor (DTR) in various atmospheres. Char oxidation and
gasification were studied in a mixture of oxygen in nitrogen, and oxygen in carbon
dioxide at a gas temperature of 1123 K. The oxygen concentrations used in the
experiments were 2, 3, 6, and 8-vol %. Char gasification by carbon dioxide was studied
separately at a gas temperature of 1173 K. In addition to weight loss the fuel particle
diameter, surface temperature, and velocity were also measured during combustion.
These four variables are of foremost importance in combustion and gasification
modelling. Particle diameter and velocity in the reactor were measured with a high-speed
charge-coupled device (CCD) camera, whereas the surface temperature of the particle
was measured with a two-color pyrometer.
The results show that with the oxygen concentrations used, replacing nitrogen with
carbon dioxide in the reactor atmosphere has a notable decreasing effect on the surface
temperature of the char particle. The kinetic parameters of the char studied were
determined by using the data from the temperature and conversion measurements. The
parameters were determined by minimizing the sum of square errors between the
measured points and the model prediction with the Simplex algorithm. After this, the
kinetic parameters determined can be used as input values in computational fluid
dynamics (CFD) calculations.
The next step in this study is to concentrate on other reaction rate controlling factors.
When it comes to combustion, boundary layer diffusion has already been widely studied,
and the diffusion coefficients of various gases as well as the mathematical correlations for
-1-
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
them can be found from the literature of the field. Therefore, in the future, the emphasis
of this research project will be on char porosity and ash layer diffusion. Ash layer
diffusion becomes a significant factor when the fuel ash melts and limits oxygen transfer
to the particles’ active sites. The ultimate goal after the experimental work is to construct
a comprehensive model, which takes into account all the rate controlling factors in char
combustion and gasification.
Keywords: Char combustion, drop-tube reactor, carbon dioxide, chemical kinetics,
two-color pyrometer, gasification
1. INTRODUCTION
Fossil fuels play still a significant role in the world’s energy production. Today, more
than 80% of the energy used in the world is produced by combusting fossil fuels because
they are cheap and can provide energy regardless of weather conditions, unlike wind and
solar power, for example. Coal as an energy source is relatively abundant and it is easy to
use. However, producing energy by using coal as much as recently leaves an enormous
effect on the environment. Climate change especially has raised questions on coal usage.
In 2008 alone, the world scale consumption of coal was 6,566,392 thousand tons [6].
Laboratory scale testing provides useful and necessary information on solid fuel behavior
during combustion and gasification. This information can be used when designing larger
power plants and burning facilities. Plenty of laboratory scale equipment has already been
developed for combustion research. A drop-tube reactor (DTR) is one that can be used to
simulate the temperature level, atmosphere, and heating rate in a similar way to fluidized
bed combustion, or pulverized fuel firing [2].
So far, the aim of this research project has been to study and model coal combustion
chemical kinetics under fluidized bed conditions. In this article, fluidized bed conditions
refer to a furnace temperature level of 1123 K, a low oxygen concentration (less than 10
vol-%), and a high carbon dioxide concentration. Coal char chemical kinetics in a high
carbon dioxide concentration have been studied extensively. One of the most recent
studies is done by Everson [3]. However, the existing theoretical combustion models
cannot always accurately predict all the effects of the phenomena taking place during
combustion, which is why real-life experimentation is needed.
2. FACTORS CONTROLLING THE RATE OF SOLID FUEL
PARTICLE COMBUSTION AND GASIFICATION
In combustion and gasification, exterior gas molecules diffuse to the particle surface and
into its interior parts, where they react heterogeneously with residual char. High
temperature speeds up these reactions. In the case of small particles, the reaction rate is
controlled by chemical kinetics. The reaction rate of large particles in turn is controlled
by diffusion of the reacting gas through the boundary layer to the particle surface. Pore
diffusion or ash layer on the particle surface can also have an effect on the reaction rate.
-2-
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
2.1
Boundary layer diffusion
With high temperature levels and large particle sizes, the combustion or gasification
process of a char particle is mainly controlled by diffusion of the reacting gas through the
particle boundary layer to its surface. Boundary layer thickness is affected by the particle
size. The diffusion flow of the reacting gas per surface area
can be obtained from
Fick’s law as follows:
,
(1)
when << , and where
is the binary diffusion coefficient of gas in gas , and is
the concentration of the gas in the atmosphere. Fick’s law can then be integrated into the
following form:
,
(2)
where subscripts
and stand for the outside of the boundary layer and the particle
surface respectively, and
is the mass transfer coefficient. The coefficient
for a
spherical particle can be obtained from the Sherwood number correlation [7]:
,
where
constant
(3)
is the particle diameter and
is the Reynolds number. The value of the
is 0.3…0.35, and at high temperatures the Schmidt number can be written as:
,
(4)
where is the kinematic viscosity of the gas. The binary diffusion coefficient can be
estimated by using the theory related to molecular diffusion. According to Reid, the
binary diffusion coefficient can be written as [10]:
,
(5)
where is the absolute temperature, is the Boltzmann’s constant, is the number
density of molecules in the mixture,
is a characteristic length,
is the order of unity,
is the collision integral for diffusion, and
is a coefficient that can be written as:
,
(6)
where
and
are the molecular weights of substances A and B. If the mass transfer
coefficient
and the oxygen concentration on the spherical particle surface are known,
the molar reaction rate of carbon per surface area can be written as [5], [9]:
.
(7)
-3-
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
However, this approach cannot be used, since it is not possible to determine the oxygen
concentration on the particle surface. Therefore, the other factors controlling reaction rate
must be known.
2.2
Chemical kinetics
When determining the products of the char combustion process in this study, the four
following chemical reactions were taken into account. The reaction enthalpies of these
reactions are presented in molar form.
1.
2.
3.
4.
Other important reactions during char combustion are the reactions between water and
char, and the oxidation of sulphur. Reactions 1 and 2 are the most important
heterogeneous oxidation reactions, reaction 3 is a homogeneous reaction that takes place
in the boundary layer, and reaction 4 is the carbon dioxide gasification reaction.
Reactions 1, 2, and 3 are exothermic, whereas the gasification reaction 4 is endothermic.
Chemical kinetics is the limiting factor of char combustion at low temperatures and with
small particle sizes. The reaction between solid coal and the reactive gas, in this case
oxygen (reactions 1 and 2), has been noted to obey the following equation [9]:
,
(8)
where is the reaction rate coefficient, and is the order of the reaction. The subscript
in the concentration refers to the particle surface. The reaction rate coefficient
can be
written with the Arrhenius equation as follows:
,
(9)
where is the pre-exponential factor,
is the activation energy,
is the universal gas
constant, and
is the particle temperature. Factors and
are the so called kinetic
parameters. The reaction between the oxidizer and solid char can be divided into different
stages: gas adsorption to the particle surface, desorption of the products from the surface,
and possible adsorption of the gaseous products back to the surface.
When it comes to reaction product modelling, reaction 3 is problematic because it does
not affect coal conversion directly. However, reaction 3 may increase the particle
boundary layer temperature, which in turn increases the particle surface temperature. If
the rate of reaction 3 is high, it can change the oxygen concentration in the boundary
layer of the char particle. In this article, reaction 3 is only indirectly included in the
model. The reaction enthalpy of the overall reaction on the particle surface can be
correlated with a temperature dependent equation. If reaction 3 is somehow altering the
product ratio used, it can be seen in the apparent kinetic parameters. The production ratio
-4-
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
between carbon monoxide and carbon dioxide can be correlated according to the
following equation [4]:
.
(10)
The average stoichiometric coefficient of the carbon combustion reactions is then [5]:
(11)
The total reaction enthalpy of char combustion can be determined from the reaction
product ratio. However, equation (11) only describes the product ratio from the reactions
taking place on the particle surface. The homogenous reaction 3 presented above can
generate an error to the correlation. Thus, as a result of equation (10), the kinetic
parameters presented in this article do not apply if the temperature is significantly
different from the measuring conditions already described.
2.3
Combined chemical kinetic and boundary layer diffusion model
In the case of char combustion, the energy balance equation can be written as:
,
(12)
where is a coefficient related to Stefan flow,
is the area of the particle,
is the
convective heat transfer coefficient,
is the particle emissivity,
is the StefanBoltzmann’s coefficient,
is the pass of the particle, and is the heat capacity of the
particle. The total reaction enthalpy
can be determined with the help of equation
11. In nitrogen atmosphere, if reactions and are taken into account, the reaction rate
equation can be expressed as follows:
.
(13)
where
is the total consumption rate of carbon in the particle,
is the carbon
conversion,
is the partial reaction order of the amount of carbon related to reactions
and in nitrogen atmosphere,
is the reaction rate coefficient related reactions and
in nitrogen atmosphere, and
is the diffusion coefficient of oxygen in nitrogen. When
nitrogen in the reactor atmosphere is replaced with carbon dioxide, the gasification
reaction rate has to be added to the oxidizer reaction, and the total reaction rate equation
can be written as follows:
,
(14)
where
is the reaction rate coefficient related to reactions and in carbon dioxide
atmosphere,
is the reaction rate coefficient related to the gasification reaction, and
is the diffusion coefficient of oxygen in carbon dioxide. In this study, the boundary
-5-
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
layer diffusion of carbon dioxide was not taken into account in the gasification reaction
rate, since the rate of the gasification reaction is slow, and there is an abundance of
carbon dioxide in the boundary layer. The partial reaction order
for the amount of
carbon in this case is different than in nitrogen atmosphere.
The diameter decrease of the particles was modelled with the help of the following
equation:
,
(15)
where
is the initial diameter, indicates the final size in proportion to the initial size,
and
is the ash mass fraction of the fuel. Factor was defined with the data from the
conversion and diameter measurements. The heat capacity of the char particle was
calculated with the correlation presented by Tomeczek in his article [12].
2.4
Ash layer and pore diffusion
Coal contains always a certain amount of ash. At high combustion temperatures this ash,
or mineral matter, can deform and melt forming an ash layer on the surface of the coal
particle. The molten ash adds resistance to the gas diffusion to and from the particle
surface. The effects of the ash layer can be studied in a drop-tube reactor by comparing
the reaction rates and temperatures of the original char particles to those of particles that
have been partially burned at high temperatures and formed a molten ash layer on their
surface.
The porosity of a coal species can also have a remarkable effect on the reaction rate when
the rate of combustion is controlled by chemical kinetics, i.e. low combustion
temperatures and small particle sizes. Pore evolution is especially affected by the
particle’s temperature history. Porosity has an effect on the rate of the reactant gas
diffusion to the inner parts of the particle, and to the diffusion of the product gases out of
the particle. After the reactant gas molecule has diffused through the boundary layer, it
then has to travel through the pores to reach the reactive surface in the particle. The
particle pores can be subdivided into three categories according to their dimensions:
micropores, mesopores and macropores. The dimension boundaries of these groups,
however, are not precise; the maximum diameter for micropores ranges from 1.2 nm to
3nm, and for mesopores from 20 nm to 50 nm [1]. In general, porosity increases the
reactive surface area of the particle.
3. EXPERIMENTAL SETUP AND PROCEDURE
The experimental setup used in this study enabled measuring the sample char particles’
conversion, diameter, surface temperature, and velocity during the same measurement
run. A laminar drop-tube reactor (DTR) was constructed for the measurements. The
reactor was coupled with a high-speed camera and a two-color pyrometer for optical
measurements. The measurements were done to determine the kinetic parameters of the
char while using the combined boundary layer diffusion and chemical kinetics model.
-6-
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
3.1
Laminar drop-tube reactor
The DTR consisted of three modular parts: an adjustable feeding probe, a reactor part,
and a collecting system. The reactor itself was an austenitic stainless steel tube with an
inside diameter of 26.7 mm, and with a temperature resistance up to 1300 K. The reactor
was covered with separately adjustable heating elements. Windows for measurements
were also built into the reactor, and they were placed in the lower end. The center point of
the windows was 53.5 cm below the beginning of the heating zone. The maximum weight
loss measuring distance was 65 cm. Due to the placement of the windows, the
corresponding distance for the two-color pyrometer and the high-speed camera
measurements was 53.5 cm.
The feeding probe for the reactor was assembled from three tubes: a particle feeding tube,
a water jacket, and a smaller tube that fed water into the bottom level of the probe. The
main function of this adjustable probe was to carry the particles to a wanted level inside
the reactor, and maintain them at a low temperature before entering the heating zone. The
particles were inserted to the probe from a silo. A water-cooling jacket around the particle
feeding tube kept the inside temperature of the probe at less than 100ºC. This instalment
made sure that the combustion processes of the particles started only after they entered
the reactor itself.
Feeding silo
65 cm
Heating elements
Measuring windows
Liquid nitrogen
collecting
Figure 1:
Temperature [C]
Adjustable
feeding probe
1000
800
Average thermocouple
reading ˚C
600
400
200
Temperature fit for gas
0
0
0.1
0.2
0.3
0.4
0.5
Reactor Length [m]
A schematic figure of the laminar DTR, and the temperature profile measured from
within it.
The volume flow of the gas mixture at 273 K was 1.585 l/min, which corresponded with
average gas velocities of 0.1735 m/s, 0.200 m/s, and 0.209 m/s at furnace temperatures of
973 K, 1123 K, and 1173 K respectively.
3.2
High speed camera
A high-speed camera was employed to take pictures from the particle stream inside the
reactor through the measuring window. These pictures were then analyzed with a
computer program in order to determine the velocity and diameter of the combusting
particles. The program used for analyzing the particle diameter had been developed by
PhD. Markus Honkanen. The particle velocity profile in the reactor was needed for
calculating the particle residence time in the reactor.
The high-speed camera in question was an AVT Marlin 145-B2 with a 1380×1090
resolution, and a black and white CCD-cell. A pulse LED-light provided illumination in
the reactor, and gave the falling particles a double shadow in the images. By using the
information regarding the distance of the shadows and the time delay between the two
-7-
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
pulses, the analysis program can determine the velocity of the particles. The diameter of
the particles was measured separately after combustion by scattering the particles on a
glass plate, where they were again photographed. After this, these pictures were analyzed
as well. On top of the plate the particles were easier to get into focus, and thus they
appeared sharper in the latter images.
Figure 2:
The high speed camera and the LED light placed on opposite sides of the measuring
window on the side of the reactor. The LED light provided background illumination
for the falling particles.
In figure 2, the high speed camera can be seen on the right side of the reactor. The LED
light on the left side provided background illumination for the particles.
3.3
Two-color pyrometer
The surface temperature of the particles was measured with a two-color pyrometer.
During the measurements, the pyrometer’s optics were exposed to the combusting
particles’ radiation. For each measurement run, the minimum amount of particles
detected was set to be 100.
In this study, the two-color pyrometer allowed measuring the particles’ radiation with two
narrow wavelength bands. The temperature of the combusting particle could then be
determined from the ratio of these wavelength measurements. The selection of the
wavelengths is mainly dependent on the following factors: there has to be enough spectral
radiation at the selected wavelengths and at the concerned temperatures, and absorption
of thermal radiation into the gas atmosphere has to be minimized. The wavelength bands
used were 1.0 and 1.6 µm for the main signals, and 1.25 µm for the reference signal.
Paananen, who constructed the pyrometer, presents it and the measuring procedure with
more detail in his thesis. [8].
3.4
Fuel composition and density
Fortum Oyj, an energy company operating in northern Europe, provided the Russian coal
fuel used in the experiments. The fixed carbon amount (total amount minus moisture, ash,
and volatile matter) of the coal in question was calculated to be approximately 45%.
According to Smoot [11], this coal falls under the classification of high volatile C
bituminous. The ultimate and proximate analyses of the coal are presented in Table 1.
Table 1:
The ultimate and proximate analyses of the coal.
Analysis
Method
Result
Ash content
ISO 1171:1997
13.7
Sulphur
ASTM D 4239
0.33
Volatile matter
CEN/TS 15148, ISO 562
34.5
Calorimetric heat value
CEN/TS 14918, ISO 1928 (mod.)
28.1
C
CEN/TS 15104, ISO/TS 12902
67.8
H
CEN/TS 15104, ISO/TS 12902
4.6
N
CEN/TS 15104, ISO/TS 12902
2.04
-8-
Unit
m-% (dm)
m-% (dm)
m-% (dm)
MJ/kg
m-% (dm)
m-% (dm)
m-% (dm)
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
The density of the coal used in these experiments was measured by sinking a sample of
the uncrushed coal into a container filled with water. This method provided a density
value close to coal’s true density. The measured apparent density value of the coal and
the calculated density values of the char are shown in Table 2.
Table 2:
Sample
Coal
Char
Char
The density of the coal and char samples. The char was produced in the drop-tube
reactor in nitrogen atmosphere during the experiments.
Temperature [˚C] Volatile matter Density [kg/m3]
1276.9
850
0.439
716.4
900
0.447
706.1
The density of the coal used in these experiments was measured by sinking a sample of
uncrushed coal into a container filled with water. This method provided a density value
close to coal’s true density. The measured apparent density value of the coal and the
calculated density values of the char are shown in Table 2.
4. RESULTS
Dry ash free conversion
The char combustion measurements in nitrogen atmosphere were conducted with 2, 3, 6,
and 8 vol-% of oxygen in nitrogen. The furnace temperature was set to 1123 K. Figure 3
shows the measured conversion values as a function of residence time.
100 %
2%O2 98%N2 Data
80 %
3%O2 97%N2 Data
60 %
6%O2 94%N2 Data
40 %
8%O2 92%N2 Data
20 %
2%O2 98%N2 Model
0%
3%O2 97%N2 Model
0
0.5
1
1.5
Residence time [s]
Figure 3:
2
6%O2 94%N2 Model
8%O2 92%N2 Model
Char conversion (dry ash free) with 2, 3, 6, and 8 vol-% of oxygen in nitrogen at a
furnace temperature of 1123 K. The points represent the average of the measured
values and the lines show the model prediction.
In Figure 3, the effect of increasing the oxygen concentration can be clearly seen as an
increase in the conversion rate. Another notable observation is that especially with lower
oxygen concentrations, 2 and 3 %, the conversion starts with a significant delay. This
reaction initiation delay could be explained with a closer examination to how the char is
produced. Since the char particles were in contact with air when they were stored, they
could have absorbed a number of impurities, which would then block the reaction at the
beginning of the combustion process.
The char combustion measurements in carbon dioxide were also conducted with 2, 3, 6,
and 8 vol-% of oxygen in nitrogen. The furnace temperature was again set to 1123 K.
Figure 4 presents the dry ash free conversion of char in these aforementioned conditions.
-9-
Dry ash free conversion
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
100 %
2%O2 98%CO2 Data
80 %
3%O2 97%CO2 Data
60 %
6%O2 94%CO2 Data
40 %
8%O2 92%CO2 Data
20 %
2%O2 98%CO2 Model
0%
3%O2 97%CO2 Model
0
0.5
1
1.5
2
6%O2 94%CO2 Model
Residence time [s]
Figure 4:
8%O2 92%CO2 Model
Char conversion (dry ash free) with 2, 3, 6, and 8 vol-% of oxygen in carbon dioxide
at a furnace temperature of 1123 K.
According to the results shown in Figure 4, when nitrogen was replaced with carbon
dioxide with 6 and 8 % oxygen concentrations, the conversion behaved quite linearly.
With lower oxygen concentrations, the conversion rate seems to be similar to the nitrogen
measurements at the beginning of the combustion process. At the beginning of the char
combustion process, with 8 % oxygen in carbon dioxide, the conversion rate is lower than
in nitrogen, and it seems to stay constant throughout the process.
Dry ash free conversion
Char gasification by carbon dioxide was studied at a furnace temperature of 1173 K.
Figure 5 illustrates the dry ash free weight loss results of char gasification. The relative
variation in the conversion measurements was somewhat more substantial than in the
other cases due to a very minor weight loss.
Figure 5:
5%
Data
Model
0%
0
0.5
1
Residence time [s]
1.5
Char conversion (dry ash free) in carbon dioxide at a temperature of 1123 K.
A notable fact to be seen from Figure 5 is that even with the maximum combustion length
the conversion only reached a final average value of 3.5 %. This means that at 1173 K,
the heterogeneous reaction rate between char and carbon dioxide was significantly lower
than the reaction rate between char and oxygen at the same temperature. This indicates
that the endothermic reaction enthalpy of the gasification reaction does not have a
substantial impact on the combustion temperature of the char particle.
The surface temperatures of the combusting particles were measured with the two-color
pyrometer, and they are presented in Figure 6 and 7.
-10-
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
2%O2 98%N2 Data
Temperature [K]
1400
3%O2 97%N2 Data
1300
6%O2 94%N2 Data
1200
8%O2 92%N2 Data
1100
2%O2 98%N2 Model
1000
3%O2 97%N2 Model
0
0.5
1
1.5
2
8%O2 92%N2 Model
Residence time [s]
Figure 6:
6%O2 94%N2 Model
Char particle temperature for 2, 3, 6, and 8 vol-% of oxygen in nitrogen at a furnace
temperature of 1123 K.
Figure 6 shows that the oxygen concentration clearly had an effect on the particle surface
temperatures. A minor difference in the temperatures between the concentrations can be
seen already at 0.2 s. The 2 and 3 % oxygen concentrations behaved in a similar way, but
especially with the 8 % concentration the temperature peak was much higher, and it was
reached sooner in comparison with the other cases. The water-cooled probe of the reactor
ensured that the particles entered the reactor at room temperature. Therefore, the particle
temperature was assumed to be 293 K at the initial point.
When nitrogen was replaced with carbon dioxide in the reactor atmosphere, the particle
surface temperature decreased. This decrease could be seen in all measurements with
carbon dioxide. The temperature profile of the particle also seemed more even, which
explains the linear conversion behavior in Figure 4. Figure 7 shows the particle
temperatures measured with carbon dioxide as follows:
2%O2 98%CO2 Data
Temperature [K]
1400
3%O2 97%CO2 Data
1300
6%O2 94%CO2 Data
1200
8%O2 92%CO2 Data
1100
2%O2 98%CO2 Model
1000
0
0.5
1
1.5
2
3%O2 97%CO2 Model
6%O2 94%CO2 Model
Residence time [s]
8%O2 92%CO2 Model
Figure 7:
Char particle temperature for 2, 3, 6, and 8 vol-% of oxygen in carbon dioxide at a
furnace temperature of 1123 K.
One reason for the drop in the particle temperatures could be the endothermic gasification
reaction 4. However, the gasification reaction was slow compared to the oxidizing
reactions (Figure 5), and therefore it cannot be the only reason. A part of the temperature
decrease can be explained with the difference in the heat capacity and the diffusivity
between nitrogen and carbon dioxide. The boundary layer diffusion of oxygen into the
particle is slower in carbon dioxide than in nitrogen. In addition to this, carbon dioxide
has a greater molar heat capacity than nitrogen, and therefore it can store more energy in
its boundary layer. A major factor in the temperature decrease of the particles may also
be that the excess amount of carbon dioxide in the boundary layer decreases the rate of
reaction 3.
-11-
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
Char oxidation was modelled both in nitrogen and in carbon dioxide. The lines in figures
3-6 represent the combined boundary layer diffusion and chemical kinetics model
prediction of char particle conversion and temperature. Due to the small size of the char
particles, the temperature was assumed to be uniform throughout them.
In nitrogen, two heterogeneous reactions (Reactions 1 and 2) between char and oxygen
were taken into account. In the case of carbon dioxide, the effect of the gasification
reaction was also considered. The temperature dependent production ratio of carbon
dioxide and carbon monoxide was determined according to Equation 10. As a result,
three different sets of kinetic parameters were determined: one set to describe the
heterogeneous reactions between char and oxygen in nitrogen, one set to describe the
same reactions in carbon dioxide, and one set to describe the gasification reaction. These
determined kinetic parameters are so called apparent kinetic parameters, which means
that in addition to chemical kinetics, they also take into account other phenomena, such
as pore diffusion.
To determine the kinetic parameters, the following steps were taken:
1. Setting the initial guesses for the kinetic parameters for Equations 13 and 12:
90.0 kJ mol-1, 2.0×104 m s-1, and 1.
2. Calculating the particle surface temperature that realized Equation 12.
3. Calculating the conversion from the reaction rate equation by using the
temperature.
4. Comparing the calculated conversion and temperature with the measured ones,
and determining the squared error between them.
5. Searching the kinetic parameter set with the Simplex algorithm by choosing the
parameters that gave the least square error.
The previous procedure could be conducted separately for each oxygen concentration, or
for all four of them. In this article, the presented kinetic parameters are fitted to all four
different datasets. The calculated kinetic parameters are presented in Table 3.
Table 3:
The chemical kinetic parameters for char oxidation in nitrogen, char gasification in
carbon dioxide, and char oxidation in carbon dioxide.
Char oxidation in nitrogen
Pre-exponential factor (A)
m s-1
7.75×104
Activation energy (Ea)
kJ mol-1
103
Partial reaction order (m)
0.244
Char gasification in carbon dioxide
Pre-exponential factor (A)
m s-1
18.6×101
Activation energy (Ea)
kJ mol-1
103
Partial reaction order (m)
0.935
Char oxidation in carbon dioxide
Pre-exponential factor (A)
m s-1
3.18×104
Activation energy (Ea)
kJ mol-1
143
Partial reaction order (m)
7.52×10-6
-12-
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
The low partial reaction order (m) in the char oxidation case in carbon dioxide means that
the conversion rate is practically independent of the char amount left in the particle.
5. CONCLUSIONS
Carbon conversion rate during char combustion in a mixture of nitrogen and oxygen was
noted to be dependent on the oxygen concentration of the combustion environment. The
carbon conversion increased steadily along with the growing oxygen concentration. The
particle temperature was also strongly affected by the oxygen concentration, and the
temperature increased significantly in the case of 8 vol-% of oxygen. When nitrogen was
replaced with carbon dioxide in the DTR atmosphere, carbon conversion as a function of
residence time was more linear and showed a minor decrease in the beginning compared
with the nitrogen counterpart. The measured particle temperatures showed a clear
decrease in all four cases when nitrogen was replaced with carbon dioxide in the DTR.
This phenomenon was the strongest with 8 vol-% of oxygen.
Compared to the oxidation reaction, the char gasification by carbon dioxide was noted to
be very slow at 1173 K. Therefore, it can be stated that the gasification reaction itself had
little to do with the changes in the reaction rate and the temperature decrease when
nitrogen was replaced with carbon dioxide. Possible reasons for the temperature decrease
may be the differences in the gas properties (heat capacity and diffusivity) between
nitrogen and carbon dioxide, or that carbon dioxide was occupying a larger share of the
active sites on the particle surface, and thus blocking the oxidation reaction. Carbon
dioxide might have also changed the reaction balance between reactions 1 and 2.
In the char combustion model, both boundary layer diffusion and chemical kinetics were
taken into account. The determined kinetic parameters and the boundary layer diffusion
correlations predicted the conversion and temperature behavior of char combustion in
nitrogen fairly accurately. The results were better with higher oxygen concentrations.
However, the model was not able to predict the reaction initiation delay at the beginning
of the combustion. The reason for this delay remains unknown; it may be caused by
adsorption of impurities to the particle surface during char storage, or by moisture in the
particles. The gasification reaction chemical kinetic parameters were determined with the
information from measurements conducted at one temperature only.
The combined oxidation and gasification model was used to describe char combustion in
high carbon dioxide concentrations. Another set of chemical kinetic parameters were
determined for the oxidizing reactions under these conditions. The model was again able
to predict the tendency of the measured conversion, but it lacked in accuracy, especially
with lower oxygen concentrations. The results presented in this article can be directly
used to estimate the behavior of the studied coal in pulverized fuel firing, where the
particle size is the same as in the measurements. They can also be used in the chemical
kinetic sub-model in fluidized bed reactor designs, and as input values in CFD
calculations. A future recommendation regarding the modelling is that reactions 1 and 2
should be modelled as separate reactions with their own kinetic parameters and reaction
enthalpies. This would increase the accuracy of the model also outside the measured
temperature range.
-13-
The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
6. REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
Bar-Ziv, E. and Kantorovich, I.I. (2001). Mutual effects of porosity and reactivity
in char oxidation. Progress in Energy and Combustion Science, 27, pp. 667-697.
Carpenter, A.M. and Skorupska, N.M. (1993). Coal combustion - analysis and
testing, IEACR, 64.
Everson, R.C., Hein, W.P.J., Kaitano, R., Falcon, R., and Cann, V.M. (2008).
Properties of high ash coal-char particles derived from inertinite-rich coal:II.
Gasification kinetics with carbon dioxide. Fuel, 87, pp. 3403-3408.
Field, M.A., Gill D.W., Morgan B.B., and Hawskley P.G.W. (1967). Combustion
of Pulverized Coal. The British Coal Utilization Research Assoc. Leatherhead. p.
413.
Grönvall, J. (1989). Kiinteän polttoainehiukkasen palamismalli. MSc. thesis.
Finland. Tampere University of Technology.
International Energy Statistics. (2008). Retrieved 4.30.2010, from U.S. Energy
Information Administration:
http://tonto.eia.doe.gov/cfapps/ipdbproject/iedindex3.cfm?tid=1&pid=1&aid=2&c
id=&syid=2008&eyid=2008&unit=TST.
Laurendeau, N.M. (1978). Heterogeneous kinetics of coal char gasification and
combustion. Progress in Energy and Combustion Science , 4 (4), pp. 221-270.
Paananen, M. (2008). Kaksiväripyrometrian soveltaminen eri happipitoisuuksissa
poltettavien kivihiilihiukkasten lämpötilan ja koon mittaamiseen. MSc. thesis.
Finland. Tampere University of Technology.
Raiko, R. (1986). Factors Affecting Combustion of Milled Peat in Fluidized Bed.
PhD thesis. Finland. Tampere University of Technology.
Reid, R.C., Prausnitz, J.M., and Poling, B.E. (1987). The Properties of Gases &
Liquids (4th ed.). McGraw-Hill, Inc. New York.
Smoot, D.L. (1991). Coal and Char Combustion. In Fossil Fuel Combustion: A
source Book (Editor William B, Sarofim A.F.). John Wiley and Sons, Inc. New
York. pp. 653-781.
Tomeczek, J. and Palugniok, H. (1996). Specific heat capacity and enthalpy of
coal pyrolysis at elevated temperatures. Fuel, 75 (9), pp. 1089-1093.
7. ACKNOWLEDGEMENTS
We would like to thank Metso Power Oy and Fortum Oyj for their financial support
under the FOXYMET project, and for their permission to publish this article. The authors
also acknowledge the help of Ph.D. Markus Honkanen, M.Sc. Matti Paananen, B.Sc. Kai
Hämäläinen, B.Sc. Taru Siitonen, and laboratory technicians Matti Savela and Jarmo
Ruusila.
-14-