Energy & Fuels 2003, 17, 1277-1282
1277
Combustion Characteristics of Coal Briquettes. 2. Reaction Kinetics N. Emre Altun,† Cahit Hicyilmaz,*,† and A. Suat Bagci‡ Departments of Mining Engineering and Petroleum and Natural Gas Engineering, Middle East Technical University, 06531, Ankara, Turkey Received December 13, 2002. Revised Manuscript Received April 28, 2003
This study comprises the influence of the major briquetting parameters, such as binder type and amount of binder and water addition, on the combustion kinetics of the coal briquettes. In this manner, briquettes that have been prepared with different organic agents (molasses, carboxyl methyl cellulose, Peridur XC3, Peridur C10, and sulfide liquor) and inorganic agents (cement and bentonite) were combusted in a reaction cell assembly that operated in coordination with a continuous gas analyzer. Moreover, not only was the addition amount of the binder providing the most favorable reaction kinetics with the lowest activation energy varied, but the quantity of the water added was also varied with fractions of 5%, 10%, and 15% (by mass), to determine the possible effect of variations in binder and water quantities. The influence of the parameters of concern on the combustion kinetics of the coal briquettes was investigated using the effluent gas analysis method and was interpreted by an Arrhenius kinetic model that operated on the basis of the changes in the amounts of CO and CO2 that were evolved and the amount of O2 gases that were consumed, as a function of temperature and time at three different pressure levels: 25, 50, and 75 psig. At the end of experiments, the activation energy and Arrhenius constant for each run were calculated, and it was observed that the liability of the coal briquettes to ignite and the efficiency and effectiveness of the combustion reaction were considerably affected both by the binder type used and by the amount of binding agent and water addition.
Introduction The combustion of fossil fuels is one of the most complex reactions on which several factors are effective. The parameters related to the combustion reaction and the natural characteristics of the fuel dictate the way that the reaction will proceed. Determination of the reaction kinetics through the activation energy concept is a common and reliable way of expressing the efficiency and effectiveness of the combustion reaction, the liability of the fuel to ignite and combust, and the situation of various phases involved within the overall reaction sequence. Because the concept of activation energy can be adapted very well to the combustion phenomena, many studies that have attempted to characterize various fuels (such as coal, crude oil, oil shale, and asphaltite) and elucidate the influence of various parameters (pressure, catalysts, heating rate, particle size, etc.) have been performed. One instance of outstanding research was reported by Weijdema;1 that research involved the oxidation kinetics of an in situ combustion process. In this study, the manner in which the in situ combustion process (which is an enhanced oil recovery method) proceeded was interpreted by developing a kinetics model, employing the * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Department of Mining Engineering. ‡ Department of Petroleum and Natural Gas Engineering. (1) Weijdema, J. Determination of Oxidation Kinetics of the In Situ Combustion Process; Technical Report; Koninklijke/Shell E&P Laboratorium: Rijswijk, The Netherlands, 1968; pp 1-21.
data obtained by effluent gas analysis. Indeed, this work has shown that using the reaction kinetics and activation energy concept was the most precise way to characterize fossil fuel combustion, reflecting the very complex subsequent oxidation phases in a considerably simple and reliable way. Being encouraged by the results derived in Weijdema’s study, a vast number of scientists, if not all, contributed to and developed the activation energy concept and made it applicable with various methods and to all fossil fuels. Fassihi et al.2 showed that the combustion of crude oil in porous media follows several consecutive reactions; this determination was made by assessing the reaction kinetics of the experimental data. C¸ elebiogˇlu and Bagci3 evaluated the effect of various metallic catalysts on the oxidation of light crude oil in a limestone environment. Through the application of a computerized kinetic model to the realized combustion experiments, the reaction kinetics of light crude was observed to have been affected considerably by the addition of catalysts. Another detailed study, which concerned the oxidation kinetics of coal, was performed by Smith et al.,4 who investigated (2) Fassihi, M. R.; Brigham, W. E.; Ramey, H. J., Jr. The Reaction Kinetics of In Situ Combustion. 55th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME; Society of Petroleum Engineers, Richardson, TX, 1980; pp 21-24. (3) C¸ elebiogˇlu, D.; Bagci, S. The Effects of Metallic Catalysts on Light Crude Oil Oxidation in Limestone Medium. Fuel Process. Technol. 2002, 79, 29-49. (4) Smith, S. E.; Neavel, R. C.; Hippo, E. J.; Miller R. N. DTGA Combustion of Coals in the Exxon Coal Library. Fuel 1981, 60, 458462.
10.1021/ef0202900 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/21/2003
1278 Energy & Fuels, Vol. 17, No. 5, 2003
the distinct and specific combustion phases, as well as characteristic activation energies, for 66 different coals. Smith’s group utilized a thermal analysis procedure and successfully adapted the obtained thermogravimetry/ differential thermal gravimetric analysis (TG/DTG) data to the Arrhenius equation, producing a relative scale of combustion liability among the coal types involved. The significant changes that occurred in the combustion characteristics and oxidation kinetics of three lignites from Turkey after cleaning were evaluated by Ozbas et al.5 The cleaning process was observed to have resulted in easier ignition and combustion of the coal samples, because of the removal of inorganic matter content of the lignites. The less-combustible fraction that was involved within three coals of different origins was characterized, from the viewpoint of liability, to react with oxygen by Shu et al.6 A multistep procedure was applied that involved the centrifugal separation and thermogravimetric analysis for the thermal characterization of the less-combustible content of coals. As well as combustion reactions, reaction kinetics also proved to be one of the most useful tools in the characterization of pyrolysis reactions and devolatilization kinetics. Arenilla et al.7 utilized a combination of a thermogravimetric analyzer and a mass spectrometer to determine the Arrhenius parameters of coal devolatilization kinetics. Determination of the combustion efficiency and characteristics via reaction kinetics and activation energy is also extensively used for the investigation of related features of petroleum-based solid fuels. Altun et al.8,9 examined the combustion and pyrolysis kinetics of asphaltite, to determine the specific features in the reaction patterns. Altun et al.8,9 also showed that the influence of the major parameters of fossil fuel combustion, such as particle size and heating rate, can be reliably determined by the assessment of reaction kinetics as a function of those parameters. The oxidation behavior of a Turkish oil shale was studied by Tugˇluhan et al.10 via combustion that was performed in a reaction cell assembly. The very characteristic combustion behavior of the oil shale was demonstrated by complementary O2 consumption versus time and temperature curves, leading to determination of the succeeding steps in the overall combustion scheme (low-temperature oxidation, high-temperature oxidation, transition zone) at different pressure levels and their driving mechanisms (i.e., whether the reactions of the determined phases were chemically controlled or diffusion-controlled). The consequences obtained in these and similar studies clearly claim that investigation of the activation (5) Ozbas, K. E.; Hicyilmaz, C.; Kok, M. V.; Bilgen, S. Effect of Cleaning Process on Combustion Characteristics of Lignite. Fuel Process. Technol. 2000, 64, 211-220. (6) Shu, X.; Xu, X.; Fan, H.; Wang, S.; Yan, D. Application of TGDTG Analysis and Centrifugal Separation in the Investigation of Less Combustible Constituents in Coals. Thermochim. Acta 2002, 381, (1), 73-81. (7) Arenillas, A.; Rubiera, F.; Pevida, C.; Pis, J. J. A Comparison of Different Methods for Predicting Coal Devolatilisation Kinetics. J. Anal. Appl. Pyrol. 2001, 58-59, 685-701. (8) Altun, N. E.; Kok, M. V.; Hicyilmaz, C. Effect of Particle Size and Heating Rate on the Combustion of Silopi Asphaltite. Energy Fuels 2002, 16, 785-790. (9) Altun, N. E.; Hicyilmaz, C.; Kok, M. V. Effect of Particle Size and Heating Rate on the Pyrolysis of Silopi Asphaltite. J. Anal. Appl. Pyrol. 2003, 67, (2), 399-379. (10) Tugˇluhan, A. M.; Mehmetogˇlu, M. T.; Bagci, S. Oxidation Kinetics of a Turkish Oil Shale. Fuel Process. Technol. 1991, 29, 231240.
Altun et al.
energy and reaction kinetics proved to be the most useful tool for the scientific studies that involve the characterization of the fossil fuels from many points for approximately three decades. Being a considerable alternative for fine coal utilization, briquetting is understood to be one of the most important areas of interest in the coal industry, and the combustion characteristics of the coal briquettes is an important matter of discussion for evaluating the quality of the briquettes as a fuel. Therefore, the assessment of the reaction kinetics of the coal briquettes is also a critical issue and is complementary to the thermal features of the briquettes for the qualitative and quantitative investigation of the efficiency and effectiveness of coal briquette combustion. In this study, combustion characteristics of the coal briquettes and the influence of the major briquetting parameters (type of binder, amount of binder, and amount of water addition) on the combustion behavior of the briquettes were determined from the viewpoint of reaction kinetics. Therefore, the previous research in which the thermal features of the coal briquettes were evaluated is complemented and made more meaningful by the determination of the liability of the briquettes to ignite and combust via the activation energy concept. Experimental Section Reaction kinetics runs were performed using the briquettes that were made using C¸ orum-Alpagut lignite, as in the previous part of this study. The related proximate and ultimate analyses, and particle size distribution results of the sample, are, thus, the same as those given in the previous part. The procedure that was followed in the preparation of the coal briquettes for the commencement of the reaction kinetics runs was as described previously. Again, briquettes, with dimensions of 2.5 cm × 8.0 cm, were prepared, whose reaction kinetics features served as the base point for the assessment of the influence, because the parameters of concern were varied. The drying temperature of the briquettes was limited to 40 °C, to prevent spontaneous combustion, and left for 3 days in the laboratory furnace. For the production of the briquettes, binders commonly used in the industry were selected: molasses, carboxyl methyl cellulose (CMC), Peridur XC3, Peridur C10, and sulfide liquor were used as organic agents, whereas cement and bentonite represented the inorganic agents category. Each binder was added at a fraction of 10%, with 10% water addition (by mass). In the second phase of the study, by which the influence of the added binder amount was investigated, 5%, 10%, and 15% (by mass) of the most favorable binder, from the viewpoint of reaction kinetics, were involved in the briquettes. Also, the effect of water addition was investigated by preparing coal briquettes with a water content of 5%, 10%, and 15% (by mass). Combustion kinetics experiments were conducted with the reaction cell assembly,11 whose detailed description and schematic form was given in the first part of this study. In the literature, similar systems have been arranged and used successively by many scientists to obtain the data required for the proceeding kinetic analysis from the changes in consumed and evolved gas concentrations throughout the reaction period of the concerned fuel.12-15 Both the procedure that has been described for the combustion of briquettes in (11) Bagci, A. S. The Application of Dry and Wet Combustion on Limestones Containing Heavy Oils with the Analysis of Combustion Reaction Kinetics. Ph.D. Thesis, Institute of Natural and Applied Sciences, Middel East Technical University (METU), Ankara, Turkey, 1986, pp 1-433.
Combustion Characteristics of Coal Briquettes. 2.
Energy & Fuels, Vol. 17, No. 5, 2003 1279
the first part and the temperature regime that was followed previously were applied for the kinetic runs. Hence, the interpretation of the reaction kinetics and determination of the activation energies through the kinetic model that was used relied on the variations in the core temperature of combustion, as well as the amount of O2 that was consumed and the amount of carbon oxide gases that were evolved as the reaction proceeded.
Results and Discussion Kinetic Model. For the kinetic modeling of the combustion reactions and the evaluation of the effluent gas data, a model that involved the typical parameters of Arrhenius kinetics is utilized. This model was first derived by Weijdema1 for the determination of the oxidation kinetics of the in situ combustion process and is based on the measurement of the rate of O2 consumption as a function of time. Hence, the equations utilized in the interpretative relations for nonisothermal experiments are generalizations of those originally derived by Weijdema.1 This model was developed and improved by Fassihi et al.2,13 and was successively adopted for the determination of oxidation kinetics. Because of the reliable results that are obtained with this model, it has been used in several oxidation kinetics studies that have been performed with oils and oil shales and for the determination of the kinetics of in situ combustion processes.2 The rate of O2 consumption, in moles per unit time per unit volume of the sample, is equal to
Rate of O2 consumption )
q∆O2 AL
(
)
q∆O2 dCf ) -R AL dt
(3)
in which R is the proportionality factor, which is equal to the amount of oxygen that reacts with 1 g of fuel. The rate of O2 consumption at any instant then can be obtained by combining eqs 2 and 3 in the following form:
dCf q∆O2 E ) ArPO2mCfn exp ) -R AL RT dt
(
)
(4)
Integration of eq 4 between t ) t1 and t ) ∞ yields
RCf(t) )
∫t∞ 1
q∆O2 dt AL
(5)
where Cf ) 0 at t ) ∞. Again, from eq 4,
(
Cfn(t) ) (q∆O2/AL) 1/ArPO2mCfn exp -
E RT
)
(6)
When eq 6 is substituted into eq 5, the following expression is derived:
∆O2 [
∫t
∞
1
n
(
) β exp -
O2 dt]
E RT
)
(7)
where
(1)
where q is the constant gas flowrate (in moles per unit time), ∆O2 the changing O2 concentration in the exit gas, A the cross-sectional area of the sample pack, and L the length of the sample pack. However, eq 1 is generally expressed in terms of more-detailed figures:
q∆O2 E ) ArPO2mCfn exp AL RT
The rate of O2 consumption is also equal to the rate of decrease of the fuel amount, which provides the relationship
(2)
where Ar is the Arrhenius constant; PO2 is the partial pressure of oxygen (given in pascals); m is the reaction order, with respect to the partial pressure of oxygen; Cf is the fuel concentration (in units of 1/s); n is the reaction order, with respect to fuel concentration; E is the activation energy (in units of kJ/mol); R is the universal gas constant (given in units of J‚(mol‚K)-1); and T is the absolute temperature (in Kelvin). (12) Burger, J. G.; Sahuquet, B. C. Chemical Aspects of In Situ CombustionsHeat of Combustion and Kinetics. Soc. Pet. Eng. J. 1972, (October), 410-422. (13) Fassihi, M. R.; Ramey, H. J., Jr.; Brigham, E. W. Reaction Kinetics of In Situ Combustion: Part 1sObservation. Soc. Pet. Eng. J. 1984, (August), 399-408. (14) Fassihi, M. R.; Ramey, H. J., Jr.; Brigham, E. W. Reaction Kinetics of In Situ Combustion: Part 2sModeling. Soc. Pet. Eng. J. 1984, (August), 408-416. (15) Dubdub, I.; Hughes, R.; Price, D. Kinetics of In Situ Combustion of Athabasca Tar Sands Studied in a Differential Flow Reactor. Chem. Eng. Res. Des. 1990, 68, 342-349. (16) Kok, M. V.; Hicyilmaz, C.; Ozbas, K. E. Effect of Cleaning Process on the Combustion Characteristics of Two Different Rank Coals. Energy Fuels 2001, 15, 1461-1468. (17) Ceylan, K.; Karaca, H.; O ¨ nal, Y. Thermogravimetric Analysis of Pretreated Turkish Lignites. Fuel 1999, 78, 1109-1116.
m
q n-1 ArPO2 β) AL Rn
( )
(8)
In this model, the temperature is gradually increased at a constant rate. The relative reaction rate can be calculated on the basis of the amount of O2 consumption, with respect to temperature changes as the combustion proceeds. The O2 consumption, which corresponds to different temperature values, is obtained by the graphical integration of the values on the left-hand side of eq 7 (i.e., by the graphical integration of the ∆O2 ) f(t) curve). The logarithm of these values then can be graphed versus 1/T to obtain a slope of -E/(2.303R) and an intercept of log β.1 Because of the huge series of data that were related to the variations in temperature and the concentrations of the effluent gases during one combustion run, a computer program written by Bagci,11 following the previously described model, was used to acquire the relative reaction rates. The output of the so-called computer program formed a base for the calculation of activation energies: The calculated relative reaction rates were plotted against inverse temperature values. After the outliers were discarded, a straight line was obtained whose slope was equal to the activation energy (EA) of the related experiment. The log-log plot of the true intercept of the straight line previously mentioned, against the partial pressure of O2, provides the Arrhenius constant Ar, which is given by the intercept of the curve obtained in this plot.14 After the activation energy and the Arrhenius constant values were retrieved, the kinetic parameters required were completed and the combustion of
1280 Energy & Fuels, Vol. 17, No. 5, 2003
Altun et al.
Figure 1. Plot of relative reaction rate versus inverse temperature for the briquette composed of 10% CMC and 10% water at 25 psig.
the coal briquettes was expressed from the viewpoint of reaction kinetics. Reaction Kinetics of the Coal Briquettes. In the first phase of the study, the possible contribution of different binder types on the combustion behavior of coal briquettes were evaluated, for the purpose of determining if these agents assisted in providing an easier combustion or if their addition brought about an adverse effect, resulting in a more difficult reaction. A representative reaction kinetics plot is illustrated in Figure 1 for the combustion of the coal briquette that was composed of 10% CMC and 10% water addition at a pressure of 25 psig, and the calculated activation energies, with respect to these plots, are given in Table 1. After the experiments, the activation energy of the binderless briquette was determined to be 42.80 kJ/mol (see Table 1). At this point, we note that the effluent gas analysis technique and the complementary activation energy concept applied in this study produced reliable results, in comparison with the activation energy values found in the literature (see Table 2). The observed activation energy value clearly reflects the oxidation kinetics of the studied Alpagut lignite reasonably, because the variations observed in the activation energies of various coal types are attributable to the difference in origin, rather than being undefinitive extremes. From the table, it is obvious that the type of binding agent proved to have a considerable effect on the combustion behavior of coal briquettes. The addition of organic-type binders decreased the activation energies of the coal briquettes. The lowest activation energy was obtained with the CMC-added briquettes (35.69 kJ/mol) and sulfide liquor was the agent that produced the highest activation energy among the organic binders (see Table 1). In contrast, the activation energies for the
Table 1. Activation Energy Values, According to Different Binders and Pressure Levels activation energy (kJ/mol)
run No.
air pressure (psig)
1 A (001) 1 B (002) 1 C (003)
25 50 75
no additive no additive no additive MEAN
46.10 43.52 38.79 42.80
2 A (004) 2 B (005) 2 C (006)
25 50 75
10% molasses 10% molasses 10% molasses MEAN
43.64 39.28 35.60 39.51
3 A (007) 3 B (008) 3 C (009)
25 50 75
10% sulfide liquor 10% sulfide liquor 10% sulfide liquor MEAN
42.66 38.35 37.85 39.62
4 A (010) 4 B (011) 4 C (012)
25 50 75
10% Peridur C10 10% Peridur C10 10% Peridur C10 MEAN
41.50 39.74 32.93 38.06
5 A (013) 5 B (014) 5 C (015)
25 50 75
10% Peridur XC3 10% Peridur XC3 10% Peridur XC3 MEAN
40.09 39.02 32.23 37.11
6 A (016) 6 B (017) 6 C (018)
25 50 75
10% CMC 10% CMC 10% CMC MEAN
38.75 36.47 31.86 35.69
7 A (019) 7 B (020) 7 C (021)
25 50 75
10% bentonite 10% bentonite 10% bentonite MEAN
47.08 45.36 42.64 45.03
8 A (022) 8 B (023) 8 C (024)
25 50 75
10% cement 10% cement 10% cement MEAN
49.28 45.74 43.34 46.12
binder type
combustion of briquettes that have been bound with cement and bentonite were higher than that of the briquette with no additive, with corresponding values
Combustion Characteristics of Coal Briquettes. 2. Table 2. Activation Energy Values Obtained for Different Coals
researcher
name/type of coal
method used
activation energy (kJ/mol)
Ozbas et al.5 Kok et al.16 Smith et al.4 Ceylan et al.17
Soma lignite Tunc¸ bilek lignite 66 different coals Golbasi lignite
TG-DTG TG-DTG DTG-DTA TG-DTA-DSC
∼26 ∼36 4-39 ∼47.5
Table 3. Arrhenius Constant Values, According to Different Binders binder type
Arrhenius constant, Ar
no additive molasses sulfide liquor Peridur C10 Peridur XC3 CMC bentonite cement
7.46 × 10-3 4.50 × 10-3 1.27 × 10-3 1.53 × 10-3 2.41 × 10-3 6.10 × 10-3 4.36 × 10-2 1.90 × 10-2
of 46.12 and 45.03 kJ/mol, respectively. Depending on the decreases in the activation energies, it can be stated that the addition of all organic binders provided easier combustion. However, combustion was more difficult with the inorganic agents (cement and bentonite), and this drawback was reflected by the increased activation energies. The organic constituents of the organic binders assisted in the combustion of coal briquettes, by increasing the combustible content, and, therefore, also increased the liability of the coal briquettes to ignite and combust. CMC seemed to be the best agent, from the viewpoint of reaction kinetics. In contrast, the inorganic constituents (cement and bentonite) increased the amount of noncombustible content of the coal briquettes, resulting in a deficiency in the combustion kinetics. This behavior was in full agreement with the common observations in the literature that the activation energy was directly related to the combustible content of the fuels. Ozbas¸ et al.5 found out that the activation energies of the washed samples were lower than those of the R. O. M. coals, depending on the removal of inorganic species via cleaning. Tugˇluhan et al.10 reported that activation energies increased as the organic matter content decreased. In addition to the activation energies, the Arrhenius constant Ar was also determined for each run, according to the procedure that was described previously; the Arrhenius constant values are given in Table 3 for each type of binder. Second, the influence of the variations in the addition amount of the binding agent and water was studied from the viewpoint of reaction kinetics, using CMC, which was previously determined to be the most favorable agent, resulting in the lowest activation energy among all other binder types. The calculated activation energies according to binder and water addition quantities of 5%, 10%, and 15% are given in Tables 4 and 5. The variations in binder and water amounts are observed to have resulted in significant changes in the kinetics behavior of the combustion runs and mean activation energies. The activation energy decreased from 40.60 kJ/mol to 34.90 kJ/mol as the CMC addition was shifted from 5% to 15% (see Table 4). However, the activation energy was negatively affected by the increase in the water addition; the lowest value (34.43 kJ/
Energy & Fuels, Vol. 17, No. 5, 2003 1281 Table 4. Activation Energy Values, According to Different Binder Amounts and Pressure Levels
binder amount
activation energy (kJ/mol)
25 50 75
no additive no additive no additive MEAN
46.10 43.52 38.79 42.80
9 A (025) 9 B (026) 9 C (027)
25 50 75
5% CMC 5% CMC 5% CMC MEAN
44.08 41.30 36.44 40.60
6 A (016) 6 B (017) 6 C (018)
25 50 75
10% CMC 10% CMC 10% CMC MEAN
38.75 36.47 31.86 35.69
10 A (028) 10 B (029) 10 C (030)
25 50 75
15% CMC 15% CMC 15% CMC MEAN
37.17 34.93 32.61 34.90
run No.
air pressure (psig)
1 A (001) 1 B (002) 1 C (003)
Table 5. Activation Energy Values, According to Different Water Amounts and Pressure Levels
run No.
air pressure (psig)
water amount (%)
activation energy (kJ/mol)
11 A (031) 11 B (032) 11 C (033)
25 50 75
5 5 5 MEAN
37.42 35.62 30.25 34.43
6 A (016) 6 B (017) 6 C (018)
25 50 75
10 10 10 MEAN
38.75 36.47 31.86 35.69
12 A (034) 12 B (035) 12 C (036)
25 50 75
15 15 15 MEAN
42.46 41.36 36.00 39.94
mol) was obtained when the water addition was 5% (by mass), whereas a water addition of 15% resulted in the highest activation energy (39.94 kJ/mol; see Table 5). Also, the increase in the activation energy was much higher when the water amount was increased from 10% to 15%. Because of these results, it can be stated that an increase in the amount of an organic binder contributes to the combustion of the coal briquettes. The successive decreases observed in the activation energies indicated that the efficiency of the combustion process was improved and a more effective activation and easier ignition were achieved by the increases in the binder addition. In contrast, the increase in the activation energies of briquettes with higher water contents indicated the deficiency and difficulty during the ignition and combustion of the molecules. This situation was probably due to the increase in the portion of heat wasted for the evaporation of greater amounts of water, and the remaining amount might have been insufficient for reaching the threshold heat value that is necessary for the ignition of the combustible species easily. The Ar values that are associated with the combustion of coal briquettes with various binder and water ratios are given in Tables 6 and 7, respectively. Conclusions With respect to the changes in the reaction kinetics and activation energies of coal briquettes with varying
1282 Energy & Fuels, Vol. 17, No. 5, 2003 Table 6. Arrhenius Constant Values, According to Different Binder Amounts binder amount
Arrhenius constant, Ar
5% CMC 10% CMC 15% CMC
4.98 × 10-3 6.10 × 10-3 3.84 × 10-3
Table 7. Arrhenius Constant Values, According to Different Water Amounts water amount (%)
Arrhenius constant, Ar
5 10 15
3.10 × 10-3 6.10 × 10-3 1.45 × 10-2
binder types, binder amounts, and water addition amounts, the following conclusions can be made: (1) Binder type was determined to be one of the major factors that influence the combustion kinetics of coal briquettes. The combustion quality of the coal briquettes was upgraded by organic agents, from the viewpoint of combustion kinetics, which was indicated by noticeable decreases in the activation energies. This trend of decline in the activation energies proved that the utilization of organic binders increased the liability of the briquettes to ignite and combust; it also showed that the use of organic binders results in an easier combustion process. (2) Both of the inorganic binderssbentonite and cementsinfluenced the combustion of coal briquettes adversely. The activation energies of the briquettes with
Altun et al.
added bentonite and cement were noticeably higher, in comparison to that of the briquette with no additive. This finding claims that the utilization of inorganic agents acted as an obstacle resulting in a more difficult combustion. Consequently, inorganic binders decreased the quality and extent of combustion of coal briquettes, in terms of reaction kinetics, and their usage proved to be disadvantageous. (3) Among the binders involved in this study, the addition of CMC generated the greatest reduction in the activation energy value for all pressure levels. Accordingly, CMC was observed to be the best agent, resulting in the greatest improvement in the kinetics characteristics of the coal briquettes. (4) The combustion kinetics of the coal briquettes was influenced by the variations both in the binder amount and in water addition amount. The combustion kinetics was favored by higher binder ratios, where lower activation energies were achieved as the binder addition was shifted from 5% to 15%. On the other hand, the liability of the briquettes to combust declined as the water addition ratio was increased to 15%, and this situation was directly reflected by the increases in the activation energy. Given these observations, it can be stated that the efficiency and quality of the coal briquettes were influenced negatively by increases in the water quantity, whereas the addition of higher amounts of an organic binder, carboxyl methyl cellulose, improved the manner in which combustion proceeded. EF0202900
