the factors controlling combustion and gasification ... · the factors controlling combustion ......

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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

[email protected]

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

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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.

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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]:

, (3)

where is the particle diameter and is the Reynolds number. The value of the

constant 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)

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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

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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 Stefan-

Boltzmann’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

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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.

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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.

Figure 1: 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

0

200

400

600

800

1000

0 0.1 0.2 0.3 0.4 0.5

Tem

pe

ratu

re [C

]

Reactor Length [m]

Average thermocouple reading ˚C

Temperature fit for gas

Feeding silo

Adjustable

feeding probe

Heating elements

Measuring windows

Liquid nitrogen

collecting

65 cm

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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 Unit

Ash content ISO 1171:1997 13.7 m-% (dm)

Sulphur ASTM D 4239 0.33 m-% (dm)

Volatile matter CEN/TS 15148, ISO 562 34.5 m-% (dm)

Calorimetric heat value CEN/TS 14918, ISO 1928 (mod.) 28.1 MJ/kg

C CEN/TS 15104, ISO/TS 12902 67.8 m-% (dm)

H CEN/TS 15104, ISO/TS 12902 4.6 m-% (dm)

N CEN/TS 15104, ISO/TS 12902 2.04 m-% (dm)

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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: The density of the coal and char samples. The char was produced in the drop-tube

reactor in nitrogen atmosphere during the experiments.

Sample Temperature [˚C] Volatile matter Density [kg/m3]

Coal - - 1276.9

Char 850 0.439 716.4

Char 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

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.

Figure 3: 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.

0 %

20 %

40 %

60 %

80 %

100 %

0 0.5 1 1.5 2

Dry

ash

fre

e c

on

vers

ion

Residence time [s]

2%O2 98%N2 Data

3%O2 97%N2 Data

6%O2 94%N2 Data

8%O2 92%N2 Data

2%O2 98%N2 Model

3%O2 97%N2 Model

6%O2 94%N2 Model

8%O2 92%N2 Model

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Figure 4: 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.

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: 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.

0 %

20 %

40 %

60 %

80 %

100 %

0 0.5 1 1.5 2

Dry

ash

fre

e c

on

vers

ion

Residence time [s]

2%O2 98%CO2 Data

3%O2 97%CO2 Data

6%O2 94%CO2 Data

8%O2 92%CO2 Data

2%O2 98%CO2 Model

3%O2 97%CO2 Model

6%O2 94%CO2 Model

8%O2 92%CO2 Model

0 %

5 %

0 0.5 1 1.5Dry

ash

fre

e c

on

vers

ion

Residence time [s]

Data

Model

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Figure 6: 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:

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.

1000

1100

1200

1300

1400

0 0.5 1 1.5 2

Tem

pe

ratu

re [K

]

Residence time [s]

2%O2 98%N2 Data

3%O2 97%N2 Data

6%O2 94%N2 Data

8%O2 92%N2 Data

2%O2 98%N2 Model

3%O2 97%N2 Model

6%O2 94%N2 Model

8%O2 92%N2 Model

1000

1100

1200

1300

1400

0 0.5 1 1.5 2

Tem

pe

ratu

re [K

]

Residence time [s]

2%O2 98%CO2 Data

3%O2 97%CO2 Data

6%O2 94%CO2 Data

8%O2 92%CO2 Data

2%O2 98%CO2 Model

3%O2 97%CO2 Model

6%O2 94%CO2 Model

8%O2 92%CO2 Model

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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) 7.75×104 m s

-1

Activation energy (Ea) 103 kJ mol-1

Partial reaction order (m) 0.244 -

Char gasification in carbon dioxide


-1


Partial reaction order (m) 0.935 -

Char oxidation in carbon dioxide


-1


Partial reaction order (m) 7.52×10-6

-

-13-




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.

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6. REFERENCES

[1] 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.

[2] Carpenter, A.M. and Skorupska, N.M. (1993). Coal combustion - analysis and

testing, IEACR, 64.

[3] 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.

[4] 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.

[5] Grönvall, J. (1989). Kiinteän polttoainehiukkasen palamismalli. MSc. thesis.

Finland. Tampere University of Technology.

[6] 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.

[7] Laurendeau, N.M. (1978). Heterogeneous kinetics of coal char gasification and

combustion. Progress in Energy and Combustion Science , 4 (4), pp. 221-270.

[8] Paananen, M. (2008). Kaksiväripyrometrian soveltaminen eri happipitoisuuksissa

poltettavien kivihiilihiukkasten lämpötilan ja koon mittaamiseen. MSc. thesis.

Finland. Tampere University of Technology.

[9] Raiko, R. (1986). Factors Affecting Combustion of Milled Peat in Fluidized Bed.

PhD thesis. Finland. Tampere University of Technology.

[10] Reid, R.C., Prausnitz, J.M., and Poling, B.E. (1987). The Properties of Gases &

Liquids (4th ed.). McGraw-Hill, Inc. New York.

[11] 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.

[12] 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.

the factors controlling combustion and gasification ... · the factors controlling combustion ......

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