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Thermogravimetric and mass spectrometric (TG-MS) analysis and kinetics of coal-biomass blends
Renewable Energy 101 (2017) 293e300
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Thermogravimetric and mass spectrometric (TG-MS) analysis and kinetics of coal-biomass blends Kandasamy Jayaraman a, Mustafa Versan Kok b, *, Iskender Gokalp a a b
Institut de Combustion, A erothermique, R eactivit e et Environnement' ICARE-CNRS, Orleans Cedex 2, France Department of Petroleum and Natural Gas Engineering, Middle East Technical University, Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2015 Received in revised form 10 May 2016 Accepted 31 August 2016
In this research, thermogravimetric and mass-spectrometric (TG-MS) analysis and kinetics of coalbiomass blends (25, 50 and 75 wt%) was studied. All the experiments were performed at 20 C/min. heating rate and under air atmosphere. The reaction regions, peak and burn-out temperatures, mass loss, maximum mass loss rate, combustion index and residue of the samples was determined. This research also focused on the main volatile products release, such as H2, O2, CO, CO2 and hydrocarbons from coalbiomass blends combustion on the basis of both their relative intensities across the temperature range 150e750 C and on their relevancy. The major release of COS is observed in decomposition stage, whereas significant SO2 release is noticed from combustion. When the percentage of biomass is increases in the coal-biomass blends, maximum rate of mass loss increases indicating the higher reactivity of the samples. The kinetic parameters of the coal-biomass blends were calculated using two different methods (Arrhenius and Coats & Redfern). The activation energy and Arrhenius constant values were increased with the increasing biomass ratio in the blends. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Coal Biomass Combustion Thermal analysis Kinetics
1. Introduction In recent years, combustion and re-combustion of renewable energy gained attention because of their fuel flexibility, high combustion efficiency, high heat transfer, and low emission of NOx, SOx and CO2 neutral. Biomass can be transformed into energy, which is considered as potential renewable energy source. The main application is the use of biomass in utility boilers alone or cofired with coal. Hence, coal-biomass blends are regarded as another alternative for co-combustion process. Also, the co-combustion of biomass with coal could potentially be good solution to overcome the specific drawbacks of individual samples, such as high volatile matter in biomass and relatively more sulphur fractions and undesirable ash content in coal. In addition to that, there is a possibility of synergetic effect in blends, which is due to the presence ash from both the samples. The industrial development of thermochemical installations for carbonaceous materials conversion majorly requires the thorough knowledge of the governing combustion parameters and their effects on process kinetics. In this scenario, thermo-analytical methods such as, thermogravimetry
* Corresponding author. E-mail address: [email protected] (M.V. Kok). http://dx.doi.org/10.1016/j.renene.2016.08.072 0960-1481/© 2016 Elsevier Ltd. All rights reserved.
(TG), differential thermogravimetry (DTG), differential thermal analysis (DTA), differential scanning calorimetry (DSC), thermogravimetry-Fourier transform infrared spectroscopy (TGFTIR) and thermogravimetry-mass spectrometry (TG-MS) techniques were employed at a growing rate in the evaluation and characterization of fossil fuels and renewable energy sources as a means of determining the combustion characteristics and kinetic parameters. Ignition and combustion behavior of different biomass and biomass blends were studied by thermal analysis techniques [1e12]. The results of the ignition study show a decrease in ignition temperature as the particle size decreases. Different coal, biomass and sewage sludge samples showed two different stages of temperature zones during combustion and gasification stages. In coalbiomass blends three different stages were also observed, known as moisture loss, pyrolysis and combustion. It was observed that the composition of major gaseous pollutants released from the cocombustion process was CO, CO2, CH4, NO, and SO2 as studied in real-time using TG-FTIR and TG-MS techniques. Also kinetic studies were performed using different approaches from the point view of activation energy determinations. In this research, combustion characteristics of coal (original and clean) and two biomass samples (poplar wood and hazelnut shell)
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and their blends is studied under air atmosphere using thermogravimetry-mass spectrometry (TG-MS) technique. The main purpose of the current study is to investigate the combustion characteristics and kinetics of coal-biomass blends. This research also generates the data on those coal-biomass blend fuels applicability for combustion. In addition to that, these results can contribute to better understanding of the coal-biomass combustion features for system level requirements.
2. Experimental In this research, two different biomass samples (poplar wood and hazelnut shell) and Saray (Thrace basin, Turkey) bituminous coal (original and clean) was used. All the samples were prepared according to the ASTM standards (D 2013-72). Coal and biomass were blended in different mass weights (0, 25, 50, 75 and 100 wt%), respectively. The proximate-ultimate analysis of the coal and biomass samples was listed in Table 1. Experiments were performed using Netzsch STA 429 thermal analyzer (TG-DTG) coupled with a quadruple QMG 511 mass spectrometer (MS), detailed elsewhere [2,3,13]. The output of the TG-DTG system was coupled to the mass spectrometer (MS) through a heated line with quartz capillary tube. The MS system was operated under a vacuum and detects the characteristic fragment ion intensity of the volatiles according to their respective mass to charge ratios (m/z). In all the experiments performed, 50 mL/min of air flow rate and 20 C/min of heating rate was used from ambient temperature to 950 C. Around 10 mg of sample was dispersed flatly on a crucible prior to experiments. A small amount of sample was used to avoid heat transfer limitations and to minimize mass transfer effects. Some of the experiments was performed twice to test the repeatability and a good consistency with the standard errors of ±1 C, is observed.
3. Results and discussion The coal-biomass combustion is one of the promising short term options for the use of renewable fuels, which offers additional environmental advantages. Theoretically, when coal-biomass blends were subjected to heat, they follow parallel and consecutive reactions and undergo permanent molecular change. The extent of this change depends on the complexity of the molecular structure of the reaction environment.
3.1. TG-DTG analysis The burning characteristics of the samples obtained from thermogravimetry (TG-DTG) may be used to effectively compare the reactivity's and combustion characteristics of the coal, biomass and their blends in combustors. Proximate-ultimate analysis results of the fuels were given in Table 1. As can be seen, biomass samples present higher concentrations of carbon, hydrogen and oxygen as compared to original coal, which also results in higher heating value. On the other hand, in addition to heating value; carbon, hydrogen and oxygen concentrations of clean coal are remarkably varied when compared to biomass samples. Sulphur contents of all samples was very low; indicating that SOx emissions are not of concern during combustion process [14]. On the other hand, higher oxygen content in biomass samples indicated higher thermal reactivity than original and clean coal [15]. Figs. 1 and 2 presented the mass loss and derivative characteristics of coal; biomass and their blends under air atmosphere at 20 C/min., respectively. In all of the samples studied, initial mass loss stage was occurred between room temperature and 110 C, due to the moisture evaporation depending on the sample properties. After this initial mass loss stage, two stages of mass losses were observed in biomass samples compared to one stage for original and clean coal sample. The second stage in biomass samples was due to decomposition of hemi-cellulose, cellulose and lignin, whereas third stage was for combustion of more complex and thermally stable structures and char oxidation. The main mass loss stage observed both in original and clean coal samples was primary carbonization stage with the release of carbon-dioxide and hydrogen. In the case of coal-biomass blends, three different stage of mass loss was observed. As expected, after moisture operation stage, two successive stages were occurred depending on the blend ratio (25, 50 and 75 wt%), due to the biomass burning, whereas the last stage was mostly due to the combustion of coal. Depending on the blend concentration, some of the mass loss at this stage was due to the biomass content. In general, the curve peaks of blends were situated between the individual fuels (original and clean coal and biomass samples). Table 2 shows the temperature ranges, peak temperatures, mass loss, residue and mass loss rate for the coal and biomass samples. The main characteristics of the samples derived from TG-DTG curves such as To (initial temperature), Tf (final or burn-out temperature), Tp (peak temperature), reaction regions and corresponding mass loss values was used to define the thermal behavior and combustion characteristics of coal, biomass and the blends. It was observed that the reaction region, peak temperature and mass
Table 1 Proximate and ultimate analysis of coal and biomass samples (as received). Sample
Moisture cont. (%)
Volatile matter (%)
Fixed carbon (%)
Ash content (%)
Heating valuea (MJ/kg)
Proximate analysis Coal (original) Coal (clean) Poplar wood Hazelnut shell
42.01 24.95 1.00 1.50
23.30 35.54 74.00 69.50
13.39 26.48 24.90 28.90
21.31 13.03 10.00 10.02
10.56 17.38 18.69 20.39
Sample
C (%)
H (%)
N (%)
S (%)
O (%)b
Ultimate analysis Coal (original) Coal (clean) Poplar wood Hazelnut shell
42.90 58.03 46.60 50.50
3.38 4.16 5.71 5.63
0.96 1.18 0.45 0.18
4.15 4.56 0.09 0.08
27.30 19.04 37.15 33.59
a b
HHV - high heating value. By difference.
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Fig. 1. TG and DTG of coal and biomass samples.
Fig. 2. TG-DTG curves of original coal and poplar wood blends in air atmosphere.
Table 2 Combustion properties of coal and biomass samples. TG/DTG data
Original coal
Clean coal
Poplar wood
Hazelnut shell
Stage-II, ( C) Peak temperature, ( C) Mass loss (%) Stage-III, ( C) Peak temperature, ( C) Mass loss (%) Burn-out temp., ( C) Residue left, (%) Mass loss rate (mg/min.) Combustion index
171e650 433 40 e e e 660 43 0.5 8.39E-09
163e670 482 65 e e e 700 23 0.6 1.29E-08
195e389 332 66.0 389e535 473 27.2 540 6.8 1.5 4.61E-08
207e371 321 57.0 371e532 458 34.0 535 9.0 1.35 4.04E-08
loss values are higher for clean coal on the main combustion stage, whereas, the residue left was lower as expected. For hazelnut shell
mass loss in each stage was higher than poplar wood. Tables 3 and 4 shows the temperature ranges, peak
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Table 3 Combustion properties of poplar wood-coal blends.
Poplar wood-original coal Stage-II, ( C) Peak temperature, ( C) Mass loss (%) Stage-III, ( C) Peak temperature, ( C) Mass loss (%) Burn-out temp., ( C) Residue left, (%) Mass loss rate (mg/min.) Combustion index Poplar wood-clean coal Stage-II, ( C) Peak temperature, ( C) Mass loss (%) Stage-III, ( C) Peak temperature, ( C) Mass loss (%) Burn-out temp., ( C) Residue left, (%) Mass loss rate (mg/min.) Combustion index
25e75 wt%
50e50 wt%
75e25 wt%
183e353 326 24 353e628 423 33 628 36 0.72 1.33E-08
180e363 328 35 366e590 476 31 590 29 1.0 2.36E-08
178e368 329 41 368e570 470 33 570 22 1.35 4.02E-08
185e364 333 22 364e675 494 55 675 17 0.54 9.78E-09
180e373 327 35 373e633 464 44 633 16 0.95 2.26E-08
173e378 330 49 378e625 462 35 625 12 1.3 3.69E-08
Table 4 Combustion properties of hazelnut shell-coal blends.
Hazelnut shell-original coal Stage-II, ( C) Peak temperature, ( C) Mass loss (%) Stage-III, ( C) Peak temperature, ( C) Mass loss (%) Burn-out temp., ( C) Residue left, (%) Mass loss rate (mg/min.) Combustion index Hazelnut shell-clean coal Stage-II, ( C) Peak temperature, ( C) Mass loss (%) Stage-III, ( C) Peak temperature, ( C) Mass loss (%) Burn-out temp., ( C) Residue left, (%) Mass loss rate (mg/min.) Combustion index
25e75 wt%
50e50 wt%
75e25 wt%
195e349 323 20 349e635 425 33 635 38 0.55 6.95E-09
185e363 324 36 363e633 416 32 633 26 0.85 1.75E-08
180e376 315 42 376e602 415 34 602 19 1.1 3.03E-08
200e353 325 22 353e660 488 55 660 19 0.62 1.05E-08
190e368 330 34 368e650 490 48 650 13 0.9 1.86E-08
215e375 316 45 375e633 455 38 633 11 1.15 2.24E-08
temperatures, mass loss, residue left and mass loss rate for the coal and biomass blends (25, 50 and 75 wt%). In the case of biomass samples and original-clean coal blends, the main observation was that the mass loss was increased in the first stage as the biomass content was increased, whereas in the second stage the mass loss was in lower values with the increase of biomass content in the blends. The increase in mass loss with an increase of biomass content in the first stage was due to the higher volatile content of the biomass samples [16,17]. Similarly, higher mass loss in the second stage with an increase in coal content was possibly due to the higher amount of char in the coal samples. It was also observed that, as the biomass content increased in the blends the corresponding burn-out temperatures are decreased due to the higher burn-out temperatures of the coal sample. Another parameter for lower burn-out temperatures of the blends was the higher volatile content of the biomass samples.
Another parameter derived from TG-DTG curves is the maximum rate of mass loss which was directly proportional to the reactivity of the samples. In the coal and biomass blends (25, 50 and 75 wt%), it was observed that as the percentage of biomass is increases in the coal-biomass blends, maximum rate of mass loss increases indicating the higher reactivity of the samples. On the other hand, higher rate of mass loss for clean coal and poplar wood indicates advance and complete combustion at lower temperatures. From the point view of the residue, it was observed that as the biomass content increased in the blends, the residue amount decreased at the final stage of the combustion process. Finally, in order to evaluate ignition and combustion performance of the coal samples, an ignition index (D), combustion index (S) and reactivity (R) is also calculated using the below mentioned equations [18,19].
D ¼ Rmax =tm ti . S ¼ Rmax Ra Ti2 Tb R ¼ ð1=Wo ÞðdW=dtÞ where; Rmax is the maximum combustion rate, Ra is the average mass loss rate, Ti is the ignition temperature, Tb is the burn-out temperature, tm is the time which corresponds to the maximum combustion rate, ti is the ignition time which corresponds to ignition temperature, Wo is the initial mass of sample and dW/dt is the maximum rate of mass loss. It was also observed that the combustion index values of the blends increased slightly with the increasing biomass ratio in the blends which is more observable with the blends of clean coal (Tables 2e4). In other words, it can be concluded that the combustion behavior of coal samples (low rank coal) could be improved with different biomass addition. 3.2. TG-MS analysis The release of gaseous species and products as a result of combustion of the coal, biomass and biomass-coal blended samples were simultaneously monitored by quadruple mass spectrometry during the TGA test. The present study was focused on the main volatile and combustion products of biomass and coal combustion on the basis of both their relative intensities across the temperature range 150e750 C and on their relevancy. It can be attributed that gaseous products was mainly composed of light volatile as H2 and H2O ((m/z) ¼ 2 and 18), hydrocarbons as CH4 and C2Hþ 4 ((m/z) ¼ 16 and 28); carbon oxides (CeO) as CO and CO2 ((m/z) ¼ 28 and 44); alcohols as CH2OH and C2H5OH ((m/z) ¼ 31 and 46); iso-prene as C5Hþ 8 ((m/z) ¼ 68); nitrogen compounds such as NO2 ((m/z) ¼ 46); aromatic compounds such as C5H4 þ, C6H6 and C7H8 ((m/z) ¼ 64, 78 and 92) and sulphur compounds (S) such as, H2S, COS and SO2 ((m/ z) ¼ 34, 60 and 66). During the analysis, mass spectrometer spectrum of mass 17 represents OH fragment of H2O in addition to the NH3. On the other hand, 46 represents both C2H5OH and COOH2, whereas 48 represents both CH4S and SO, respectively. The thermal decomposition and combustion of both the coal samples continuously took place in which majorly H2O, CO2 and H2 were released. Small amount of CO and CH4 were also observed which is not shown here, due to the coincidence of N2 and O ions. Smaller level of sulphur compounds is released, in particular COS is released during decomposition phase, whereas SO2 is released in later combustion phase. The subsequent emission of SO2 gas can be characterized that part of the sulphur is remained in the char might be later oxidized turning into the production of gases. Lower
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Fig. 3. Mass spectrum of gas release of original coal (50%) and poplar wood (50%) blends.
amount of gaseous products generated during the combustion of coals were composed by light-weight and condensable gases such as primary alcohols of CH2OH and C2H5OH and aromatic compounds of benzene (C6H6), toluene (C7H8), and isoprene (C5H8). It can be seen that there is continuous release of H2, H2O, C2H4, C2H5 and CH2OH during thermal decomposition and combustion process; whereas the release is significant in earlier stage. Similar trend is observed for the traceable amounts of C6H6, C7H8 and C5H8 gases. The CO2 is released significantly in the combustion phase as expected. Thermal decomposition and fixed carbon combustion processes are continuously taken place for original coal and biomass blend samples which can be observed from continuous releases of primary gases (Fig. 3). In the second part of mass spectrometry analysis, gaseous
emissions of different blends of coal and biomass samples upon temperature-programmed pyrolysis and combustion in the case of H2, CH4, H2O, CO and CO2 was determined at each blend compositions (Fig. 4). In the original coal with biomass blends, the H2, CH4, H2O and CO gas release rate during pyrolysis was increased with higher level of biomass content, which is due to higher amount of volatiles present in the biomass samples, which is well correlated to the high volatile matter present in the biomass samples. In the combustion phase, the CO2 shows high release rate which is caused by the difference between the fixed carbon and ash compositions in biomass-char and coal-char particles. In addition, the H2 and CH4 gases were significantly released in the clean coal combustion process, whereas these gases release are marginal during the original coal combustion. This was caused due to the low moisture
Fig. 4. Gas release profiles of original coal and poplar wood blends.
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Fig. 5. Arrhenius plots of original coal and biomass blends.
and high volatile contents in the clean coal. 3.3. Kinetic analysis The thermogravimetric non-isothermal kinetic study of mass loss during a combustion process is extremely complex, because of the presence of numerous components and their parallel and consecutive reactions. In this research, two different methods, known as Arrhenius and Coats & Redfern, were used to study the activation energy and Arrhenius constants of the samples studied. In non-isothermal
kinetics estimation by thermogravimetry, the reaction process has been done at a low linear heating rate, so that temperature resolved measurements can easily be achieved over a long time period. 3.3.1. Arrhenius method In Arrhenius method [20,21], the measured rate of mass loss accounts for gross changes in the system, the reaction model assumes that the oxidation rate of mass loss of the total sample was dependent only on the rate constant, the mass of sample remaining (W) and the temperature. The final form of the equation assuming first-order kinetics is as follows.
Fig. 6. Coat & Redfern plots of original coal and biomass blends.
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Table 5 Activation energy (kJ/mol) values of biomass, coal and biomass - coal blends. Arrhenius method
Poplar wood-original coal 0e100 wt% 25e75 wt% 50e50 wt% 75e25 wt% 100e0 wt% Poplar wood-clean coal 0e100 wt% 25e75 wt% 50e50 wt% 75e25 wt% 100e0 wt% Hazelnut shell-original coal 0e100 wt% 25e75 wt% 50e50 wt% 75e25 wt% 100e0 wt% Hazelnut shell-clean coal 0e100 wt% 25e75 wt% 50e50 wt% 75e25 wt% 100e0 wt%
Coats- Redfern method
Temperature range, C
E (kJ/mol)
Temperature range, C
E (kJ/mol)
200e430 200e330 200e325 200e325 200e325
31.36 38.39 48.10 49.88 50.89
200e580 200e580 200e580 200e580 200e530
33.45 39.02 59.29 52.29 74.99
200e450 200e345 200e330 200e325 200e325
28.30 34.81 44.50 50.38 50.89
200e650 200e600 200e600 200e600 200e530
18.74 47.08 65.11 54.22 74.99
200e430 200e320 200e325 200e320 200e325
31.36 43.81 56.46 65.52 104.79
200e580 200e580 200e580 200e580 200e530
33.45 34.88 49.26 61.21 80.97
200e450 200e320 200e330 200e320 200e325
28.30 45.01 58.07 76.05 104.79
200e650 200e600 200e600 200e600 200e530
18.74 48.55 58.03 69.58 80.97
E log dW=dt W ¼ logA 2:303*R*T
(1)
where; dW/dt is the rate of weight change, E is the activation energy (J/mol), T is the temperature (K), R is the gas constant (J/kg K) and A is the Arrhenius constant (min1). Thus a plot of log[(dW/dt)] vs. 1/T, should result in a straight line in which the slope is equal to E/(2.303*R). 3.3.2. Coats & Redfern method In Coats & Redfern [22] method, the determination of kinetic parameters for a reaction requires a three-parameter search:
Table 6 Arrhenius constant (min1) values of biomass, coal and biomass - coal blends. Arrhenius method Poplar wood-original coal 0e100 wt% 1.23Eþ03 25e75 wt% 1.35Eþ04 50e50 wt% 1.55Eþ05 75e25 wt% 2.88Eþ05 100e0 wt% 3.96Eþ05 Poplar wood-clean coal 0e100 wt% 6.46Eþ02 25e75 wt% 5.01Eþ03 50e50 wt% 6.31Eþ04 75e25 wt% 3.80Eþ05 100e0 wt% 3.96Eþ05 Hazelnut shell-original coal 0e100 wt% 1.23Eþ03 25e75 wt% 3.16Eþ04 50e50 wt% 9.12Eþ05 75e25 wt% 7.94Eþ06 100e0 wt% 2.51Eþ10 Hazelnut shell-clean coal 0e100 wt% 6.46Eþ02 25e75 wt% 5.01Eþ04 50e50 wt% 1.23Eþ06 75e25 wt% 9.77Eþ07 100e0 wt% 2.51Eþ10
Coats- Redfern method 2.12Eþ06 3.76Eþ06 5.62Eþ05 1.44Eþ06 1.97Eþ05 9.9Eþ06 2.37Eþ06 5.3 Eþ05 1.05Eþ06 1.97Eþ05 2.12Eþ06 4.9Eþ06 1.9Eþ06 7.2Eþ05 1.4Eþ05 9.9Eþ06 2.1Eþ06 1.02Eþ06 4.04Eþ05 1.4Eþ05
reaction order, activation energy, and Arrhenius constant. In this research, a reaction order (n) is assumed to be unity (n ¼ 1) to calculate the kinetic parameters, since a first order reaction model matches well with the experimental data. When reaction order equals to one, the final form of the equation takes the following form;
h . i AR *ð1 2RT=EÞ log logð1 aÞ T 2 ¼ log bE E 2:303*R*T
(2)
where; a is the amount of sample undergoing reaction, E is the activation energy, b is the heating rate, T is the temperature, R is the gas constant and A is the Arrhenius constant. Thus a plot of log[log(1a)/T2] vs. 1/T, should result in a straight line in which the slope is equal to E/2.303R. In both of the kinetic methods studied (Figs. 5 and 6 & Tables 5 and 6), the activation energy (related to the reactivity of the sample) values of the original and clean coal samples were in the range 28.3e31.36 kJ/mol, whereas the poplar wood and hazelnut shell biomass samples were in the range of 50.89e104.79 kJ/mol, respectively. On the other hand, the Arrhenius constant (related to the material structure) values were in the range 6.46 Eþ02e1.82 Eþ03 min1 for original and clean coal and 1.09 Eþ05e2.51 Eþ10 min1 for poplar wood and hazelnut shell biomass samples, respectively. The correlation coefficients of the straight lines were in the range of 0.9461e0.9974, where the activation energy and Arrhenius constant was derived. The relatively high activation energy values for biomass samples may reflect to some extent dependence of pore structures on temperature. This implies that for the burning stage, higher temperature is required, which implies that reactions with higher activation energies were more temperature dependence [23,24]. In other words, activation energy and Arrhenius constant values corresponding to coal samples were lower than all other values. However, activation energy and Arrhenius constant values was increased with the increase in biomass content in the blends. This
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trend was observed in both of the kinetic methods studied. In the case of 25 wt% blend, the values of activation energy and Arrhenius constant were slightly higher than that of coal. When the blend ratio increased to 50 and 75 wt% higher values of activation energy and Arrhenius constant are observed (Tables 5 and 6). The activation energy values of the coal and biomass blends obtained from this research were in agreement with other studies. Munir et al. [25] has reported the activation energy of different biomass samples (cotton stalk, sugar cane bagasse and shea meal) during combustion in the range of 108e116 kJ/mol. Oteo et al. [26] reported relatively 99 kJ/mol of activation energy for sewage sludge. Sanchez et al. [27] reported activation energy of 140, 143 and 173 kJ/ mol on animal manure, sewage sludge and municipal solid waste. Idris et al. [28] reported an activation energy value of 65 kJ/mol for coal and higher values in coal-biomass blends.
[3] [4]
[5] [6]
[7]
[8] [9] [10]
4. Conclusions
[11]
In air atmosphere, after the moisture loss stage, TG-DTG curves represented two stages of mass losses in biomass when compared to one stage for original and clean coal samples, respectively. In coal-biomass blends, three different stage of mass loss was observed. Also, the TG-DTG curves of blends were situated between the original-clean coal and biomass samples. The reaction region, peak temperature and mass loss values are higher for clean coal samples. The hazelnut shell mass loss in each stage was higher than poplar wood. On the other hand, combustion index values of the blends increased slightly with the increasing biomass ratio in blends. There is continuous release of H2, H2O, C2H4, C2H5 and CH2OH was observed during thermal decomposition and combustion process. Smaller amount of primary alcohols and aromatic compounds were mainly released in decomposition stage. The H2 and CH4 gases were significantly released in the clean coal combustion process, but these were marginal during the original coal combustion. Smaller level of sulphur compounds is released, in particular COS is released during decomposition phase, whereas SO2 is released in later combustion phase. The activation energy values of the original and clean coal samples were in the range 28.3e31.36 kJ/mol, whereas the poplar wood and hazelnut shell biomass samples were in the range of 50.89e104.79 kJ/mol. The relatively high activation energy values for biomass samples are due to the dependence of pore structures on temperature. Also, the activation energy and Arrhenius constant values were increased with the increasing biomass ratio in blends.
[12]
[13]
[14] [15] [16]
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[22] [23]
[24]
[25]
Acknowledgement The authors would like to express their appreciation for the d’Orle ans-France. support of Labex CAPRYSSES, Universite References [1] I. Jiricek, P. Rudasov, A. Zemlov, A thermogravimetric study of the behavior of biomass blends during combustion, Acta Polytech. 52 (2012) 39e42. [2] K. Jayaraman, I. Gokalp, Thermal characterization, gasification and kinetic
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Thermogravimetric and mass spectrometric (TG-MS) analysis and kinetics of coal-biomass blends Kandasamy Jayaraman a, Mustafa Versan Kok b, *, Iskender Gokalp a a b
Institut de Combustion, A erothermique, R eactivit e et Environnement' ICARE-CNRS, Orleans Cedex 2, France Department of Petroleum and Natural Gas Engineering, Middle East Technical University, Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2015 Received in revised form 10 May 2016 Accepted 31 August 2016
In this research, thermogravimetric and mass-spectrometric (TG-MS) analysis and kinetics of coalbiomass blends (25, 50 and 75 wt%) was studied. All the experiments were performed at 20 C/min. heating rate and under air atmosphere. The reaction regions, peak and burn-out temperatures, mass loss, maximum mass loss rate, combustion index and residue of the samples was determined. This research also focused on the main volatile products release, such as H2, O2, CO, CO2 and hydrocarbons from coalbiomass blends combustion on the basis of both their relative intensities across the temperature range 150e750 C and on their relevancy. The major release of COS is observed in decomposition stage, whereas significant SO2 release is noticed from combustion. When the percentage of biomass is increases in the coal-biomass blends, maximum rate of mass loss increases indicating the higher reactivity of the samples. The kinetic parameters of the coal-biomass blends were calculated using two different methods (Arrhenius and Coats & Redfern). The activation energy and Arrhenius constant values were increased with the increasing biomass ratio in the blends. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Coal Biomass Combustion Thermal analysis Kinetics
1. Introduction In recent years, combustion and re-combustion of renewable energy gained attention because of their fuel flexibility, high combustion efficiency, high heat transfer, and low emission of NOx, SOx and CO2 neutral. Biomass can be transformed into energy, which is considered as potential renewable energy source. The main application is the use of biomass in utility boilers alone or cofired with coal. Hence, coal-biomass blends are regarded as another alternative for co-combustion process. Also, the co-combustion of biomass with coal could potentially be good solution to overcome the specific drawbacks of individual samples, such as high volatile matter in biomass and relatively more sulphur fractions and undesirable ash content in coal. In addition to that, there is a possibility of synergetic effect in blends, which is due to the presence ash from both the samples. The industrial development of thermochemical installations for carbonaceous materials conversion majorly requires the thorough knowledge of the governing combustion parameters and their effects on process kinetics. In this scenario, thermo-analytical methods such as, thermogravimetry
* Corresponding author. E-mail address: [email protected] (M.V. Kok). http://dx.doi.org/10.1016/j.renene.2016.08.072 0960-1481/© 2016 Elsevier Ltd. All rights reserved.
(TG), differential thermogravimetry (DTG), differential thermal analysis (DTA), differential scanning calorimetry (DSC), thermogravimetry-Fourier transform infrared spectroscopy (TGFTIR) and thermogravimetry-mass spectrometry (TG-MS) techniques were employed at a growing rate in the evaluation and characterization of fossil fuels and renewable energy sources as a means of determining the combustion characteristics and kinetic parameters. Ignition and combustion behavior of different biomass and biomass blends were studied by thermal analysis techniques [1e12]. The results of the ignition study show a decrease in ignition temperature as the particle size decreases. Different coal, biomass and sewage sludge samples showed two different stages of temperature zones during combustion and gasification stages. In coalbiomass blends three different stages were also observed, known as moisture loss, pyrolysis and combustion. It was observed that the composition of major gaseous pollutants released from the cocombustion process was CO, CO2, CH4, NO, and SO2 as studied in real-time using TG-FTIR and TG-MS techniques. Also kinetic studies were performed using different approaches from the point view of activation energy determinations. In this research, combustion characteristics of coal (original and clean) and two biomass samples (poplar wood and hazelnut shell)
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and their blends is studied under air atmosphere using thermogravimetry-mass spectrometry (TG-MS) technique. The main purpose of the current study is to investigate the combustion characteristics and kinetics of coal-biomass blends. This research also generates the data on those coal-biomass blend fuels applicability for combustion. In addition to that, these results can contribute to better understanding of the coal-biomass combustion features for system level requirements.
2. Experimental In this research, two different biomass samples (poplar wood and hazelnut shell) and Saray (Thrace basin, Turkey) bituminous coal (original and clean) was used. All the samples were prepared according to the ASTM standards (D 2013-72). Coal and biomass were blended in different mass weights (0, 25, 50, 75 and 100 wt%), respectively. The proximate-ultimate analysis of the coal and biomass samples was listed in Table 1. Experiments were performed using Netzsch STA 429 thermal analyzer (TG-DTG) coupled with a quadruple QMG 511 mass spectrometer (MS), detailed elsewhere [2,3,13]. The output of the TG-DTG system was coupled to the mass spectrometer (MS) through a heated line with quartz capillary tube. The MS system was operated under a vacuum and detects the characteristic fragment ion intensity of the volatiles according to their respective mass to charge ratios (m/z). In all the experiments performed, 50 mL/min of air flow rate and 20 C/min of heating rate was used from ambient temperature to 950 C. Around 10 mg of sample was dispersed flatly on a crucible prior to experiments. A small amount of sample was used to avoid heat transfer limitations and to minimize mass transfer effects. Some of the experiments was performed twice to test the repeatability and a good consistency with the standard errors of ±1 C, is observed.
3. Results and discussion The coal-biomass combustion is one of the promising short term options for the use of renewable fuels, which offers additional environmental advantages. Theoretically, when coal-biomass blends were subjected to heat, they follow parallel and consecutive reactions and undergo permanent molecular change. The extent of this change depends on the complexity of the molecular structure of the reaction environment.
3.1. TG-DTG analysis The burning characteristics of the samples obtained from thermogravimetry (TG-DTG) may be used to effectively compare the reactivity's and combustion characteristics of the coal, biomass and their blends in combustors. Proximate-ultimate analysis results of the fuels were given in Table 1. As can be seen, biomass samples present higher concentrations of carbon, hydrogen and oxygen as compared to original coal, which also results in higher heating value. On the other hand, in addition to heating value; carbon, hydrogen and oxygen concentrations of clean coal are remarkably varied when compared to biomass samples. Sulphur contents of all samples was very low; indicating that SOx emissions are not of concern during combustion process [14]. On the other hand, higher oxygen content in biomass samples indicated higher thermal reactivity than original and clean coal [15]. Figs. 1 and 2 presented the mass loss and derivative characteristics of coal; biomass and their blends under air atmosphere at 20 C/min., respectively. In all of the samples studied, initial mass loss stage was occurred between room temperature and 110 C, due to the moisture evaporation depending on the sample properties. After this initial mass loss stage, two stages of mass losses were observed in biomass samples compared to one stage for original and clean coal sample. The second stage in biomass samples was due to decomposition of hemi-cellulose, cellulose and lignin, whereas third stage was for combustion of more complex and thermally stable structures and char oxidation. The main mass loss stage observed both in original and clean coal samples was primary carbonization stage with the release of carbon-dioxide and hydrogen. In the case of coal-biomass blends, three different stage of mass loss was observed. As expected, after moisture operation stage, two successive stages were occurred depending on the blend ratio (25, 50 and 75 wt%), due to the biomass burning, whereas the last stage was mostly due to the combustion of coal. Depending on the blend concentration, some of the mass loss at this stage was due to the biomass content. In general, the curve peaks of blends were situated between the individual fuels (original and clean coal and biomass samples). Table 2 shows the temperature ranges, peak temperatures, mass loss, residue and mass loss rate for the coal and biomass samples. The main characteristics of the samples derived from TG-DTG curves such as To (initial temperature), Tf (final or burn-out temperature), Tp (peak temperature), reaction regions and corresponding mass loss values was used to define the thermal behavior and combustion characteristics of coal, biomass and the blends. It was observed that the reaction region, peak temperature and mass
Table 1 Proximate and ultimate analysis of coal and biomass samples (as received). Sample
Moisture cont. (%)
Volatile matter (%)
Fixed carbon (%)
Ash content (%)
Heating valuea (MJ/kg)
Proximate analysis Coal (original) Coal (clean) Poplar wood Hazelnut shell
42.01 24.95 1.00 1.50
23.30 35.54 74.00 69.50
13.39 26.48 24.90 28.90
21.31 13.03 10.00 10.02
10.56 17.38 18.69 20.39
Sample
C (%)
H (%)
N (%)
S (%)
O (%)b
Ultimate analysis Coal (original) Coal (clean) Poplar wood Hazelnut shell
42.90 58.03 46.60 50.50
3.38 4.16 5.71 5.63
0.96 1.18 0.45 0.18
4.15 4.56 0.09 0.08
27.30 19.04 37.15 33.59
a b
HHV - high heating value. By difference.
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Fig. 1. TG and DTG of coal and biomass samples.
Fig. 2. TG-DTG curves of original coal and poplar wood blends in air atmosphere.
Table 2 Combustion properties of coal and biomass samples. TG/DTG data
Original coal
Clean coal
Poplar wood
Hazelnut shell
Stage-II, ( C) Peak temperature, ( C) Mass loss (%) Stage-III, ( C) Peak temperature, ( C) Mass loss (%) Burn-out temp., ( C) Residue left, (%) Mass loss rate (mg/min.) Combustion index
171e650 433 40 e e e 660 43 0.5 8.39E-09
163e670 482 65 e e e 700 23 0.6 1.29E-08
195e389 332 66.0 389e535 473 27.2 540 6.8 1.5 4.61E-08
207e371 321 57.0 371e532 458 34.0 535 9.0 1.35 4.04E-08
loss values are higher for clean coal on the main combustion stage, whereas, the residue left was lower as expected. For hazelnut shell
mass loss in each stage was higher than poplar wood. Tables 3 and 4 shows the temperature ranges, peak
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Table 3 Combustion properties of poplar wood-coal blends.
Poplar wood-original coal Stage-II, ( C) Peak temperature, ( C) Mass loss (%) Stage-III, ( C) Peak temperature, ( C) Mass loss (%) Burn-out temp., ( C) Residue left, (%) Mass loss rate (mg/min.) Combustion index Poplar wood-clean coal Stage-II, ( C) Peak temperature, ( C) Mass loss (%) Stage-III, ( C) Peak temperature, ( C) Mass loss (%) Burn-out temp., ( C) Residue left, (%) Mass loss rate (mg/min.) Combustion index
25e75 wt%
50e50 wt%
75e25 wt%
183e353 326 24 353e628 423 33 628 36 0.72 1.33E-08
180e363 328 35 366e590 476 31 590 29 1.0 2.36E-08
178e368 329 41 368e570 470 33 570 22 1.35 4.02E-08
185e364 333 22 364e675 494 55 675 17 0.54 9.78E-09
180e373 327 35 373e633 464 44 633 16 0.95 2.26E-08
173e378 330 49 378e625 462 35 625 12 1.3 3.69E-08
Table 4 Combustion properties of hazelnut shell-coal blends.
Hazelnut shell-original coal Stage-II, ( C) Peak temperature, ( C) Mass loss (%) Stage-III, ( C) Peak temperature, ( C) Mass loss (%) Burn-out temp., ( C) Residue left, (%) Mass loss rate (mg/min.) Combustion index Hazelnut shell-clean coal Stage-II, ( C) Peak temperature, ( C) Mass loss (%) Stage-III, ( C) Peak temperature, ( C) Mass loss (%) Burn-out temp., ( C) Residue left, (%) Mass loss rate (mg/min.) Combustion index
25e75 wt%
50e50 wt%
75e25 wt%
195e349 323 20 349e635 425 33 635 38 0.55 6.95E-09
185e363 324 36 363e633 416 32 633 26 0.85 1.75E-08
180e376 315 42 376e602 415 34 602 19 1.1 3.03E-08
200e353 325 22 353e660 488 55 660 19 0.62 1.05E-08
190e368 330 34 368e650 490 48 650 13 0.9 1.86E-08
215e375 316 45 375e633 455 38 633 11 1.15 2.24E-08
temperatures, mass loss, residue left and mass loss rate for the coal and biomass blends (25, 50 and 75 wt%). In the case of biomass samples and original-clean coal blends, the main observation was that the mass loss was increased in the first stage as the biomass content was increased, whereas in the second stage the mass loss was in lower values with the increase of biomass content in the blends. The increase in mass loss with an increase of biomass content in the first stage was due to the higher volatile content of the biomass samples [16,17]. Similarly, higher mass loss in the second stage with an increase in coal content was possibly due to the higher amount of char in the coal samples. It was also observed that, as the biomass content increased in the blends the corresponding burn-out temperatures are decreased due to the higher burn-out temperatures of the coal sample. Another parameter for lower burn-out temperatures of the blends was the higher volatile content of the biomass samples.
Another parameter derived from TG-DTG curves is the maximum rate of mass loss which was directly proportional to the reactivity of the samples. In the coal and biomass blends (25, 50 and 75 wt%), it was observed that as the percentage of biomass is increases in the coal-biomass blends, maximum rate of mass loss increases indicating the higher reactivity of the samples. On the other hand, higher rate of mass loss for clean coal and poplar wood indicates advance and complete combustion at lower temperatures. From the point view of the residue, it was observed that as the biomass content increased in the blends, the residue amount decreased at the final stage of the combustion process. Finally, in order to evaluate ignition and combustion performance of the coal samples, an ignition index (D), combustion index (S) and reactivity (R) is also calculated using the below mentioned equations [18,19].
D ¼ Rmax =tm ti . S ¼ Rmax Ra Ti2 Tb R ¼ ð1=Wo ÞðdW=dtÞ where; Rmax is the maximum combustion rate, Ra is the average mass loss rate, Ti is the ignition temperature, Tb is the burn-out temperature, tm is the time which corresponds to the maximum combustion rate, ti is the ignition time which corresponds to ignition temperature, Wo is the initial mass of sample and dW/dt is the maximum rate of mass loss. It was also observed that the combustion index values of the blends increased slightly with the increasing biomass ratio in the blends which is more observable with the blends of clean coal (Tables 2e4). In other words, it can be concluded that the combustion behavior of coal samples (low rank coal) could be improved with different biomass addition. 3.2. TG-MS analysis The release of gaseous species and products as a result of combustion of the coal, biomass and biomass-coal blended samples were simultaneously monitored by quadruple mass spectrometry during the TGA test. The present study was focused on the main volatile and combustion products of biomass and coal combustion on the basis of both their relative intensities across the temperature range 150e750 C and on their relevancy. It can be attributed that gaseous products was mainly composed of light volatile as H2 and H2O ((m/z) ¼ 2 and 18), hydrocarbons as CH4 and C2Hþ 4 ((m/z) ¼ 16 and 28); carbon oxides (CeO) as CO and CO2 ((m/z) ¼ 28 and 44); alcohols as CH2OH and C2H5OH ((m/z) ¼ 31 and 46); iso-prene as C5Hþ 8 ((m/z) ¼ 68); nitrogen compounds such as NO2 ((m/z) ¼ 46); aromatic compounds such as C5H4 þ, C6H6 and C7H8 ((m/z) ¼ 64, 78 and 92) and sulphur compounds (S) such as, H2S, COS and SO2 ((m/ z) ¼ 34, 60 and 66). During the analysis, mass spectrometer spectrum of mass 17 represents OH fragment of H2O in addition to the NH3. On the other hand, 46 represents both C2H5OH and COOH2, whereas 48 represents both CH4S and SO, respectively. The thermal decomposition and combustion of both the coal samples continuously took place in which majorly H2O, CO2 and H2 were released. Small amount of CO and CH4 were also observed which is not shown here, due to the coincidence of N2 and O ions. Smaller level of sulphur compounds is released, in particular COS is released during decomposition phase, whereas SO2 is released in later combustion phase. The subsequent emission of SO2 gas can be characterized that part of the sulphur is remained in the char might be later oxidized turning into the production of gases. Lower
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297
Fig. 3. Mass spectrum of gas release of original coal (50%) and poplar wood (50%) blends.
amount of gaseous products generated during the combustion of coals were composed by light-weight and condensable gases such as primary alcohols of CH2OH and C2H5OH and aromatic compounds of benzene (C6H6), toluene (C7H8), and isoprene (C5H8). It can be seen that there is continuous release of H2, H2O, C2H4, C2H5 and CH2OH during thermal decomposition and combustion process; whereas the release is significant in earlier stage. Similar trend is observed for the traceable amounts of C6H6, C7H8 and C5H8 gases. The CO2 is released significantly in the combustion phase as expected. Thermal decomposition and fixed carbon combustion processes are continuously taken place for original coal and biomass blend samples which can be observed from continuous releases of primary gases (Fig. 3). In the second part of mass spectrometry analysis, gaseous
emissions of different blends of coal and biomass samples upon temperature-programmed pyrolysis and combustion in the case of H2, CH4, H2O, CO and CO2 was determined at each blend compositions (Fig. 4). In the original coal with biomass blends, the H2, CH4, H2O and CO gas release rate during pyrolysis was increased with higher level of biomass content, which is due to higher amount of volatiles present in the biomass samples, which is well correlated to the high volatile matter present in the biomass samples. In the combustion phase, the CO2 shows high release rate which is caused by the difference between the fixed carbon and ash compositions in biomass-char and coal-char particles. In addition, the H2 and CH4 gases were significantly released in the clean coal combustion process, whereas these gases release are marginal during the original coal combustion. This was caused due to the low moisture
Fig. 4. Gas release profiles of original coal and poplar wood blends.
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Fig. 5. Arrhenius plots of original coal and biomass blends.
and high volatile contents in the clean coal. 3.3. Kinetic analysis The thermogravimetric non-isothermal kinetic study of mass loss during a combustion process is extremely complex, because of the presence of numerous components and their parallel and consecutive reactions. In this research, two different methods, known as Arrhenius and Coats & Redfern, were used to study the activation energy and Arrhenius constants of the samples studied. In non-isothermal
kinetics estimation by thermogravimetry, the reaction process has been done at a low linear heating rate, so that temperature resolved measurements can easily be achieved over a long time period. 3.3.1. Arrhenius method In Arrhenius method [20,21], the measured rate of mass loss accounts for gross changes in the system, the reaction model assumes that the oxidation rate of mass loss of the total sample was dependent only on the rate constant, the mass of sample remaining (W) and the temperature. The final form of the equation assuming first-order kinetics is as follows.
Fig. 6. Coat & Redfern plots of original coal and biomass blends.
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Table 5 Activation energy (kJ/mol) values of biomass, coal and biomass - coal blends. Arrhenius method
Poplar wood-original coal 0e100 wt% 25e75 wt% 50e50 wt% 75e25 wt% 100e0 wt% Poplar wood-clean coal 0e100 wt% 25e75 wt% 50e50 wt% 75e25 wt% 100e0 wt% Hazelnut shell-original coal 0e100 wt% 25e75 wt% 50e50 wt% 75e25 wt% 100e0 wt% Hazelnut shell-clean coal 0e100 wt% 25e75 wt% 50e50 wt% 75e25 wt% 100e0 wt%
Coats- Redfern method
Temperature range, C
E (kJ/mol)
Temperature range, C
E (kJ/mol)
200e430 200e330 200e325 200e325 200e325
31.36 38.39 48.10 49.88 50.89
200e580 200e580 200e580 200e580 200e530
33.45 39.02 59.29 52.29 74.99
200e450 200e345 200e330 200e325 200e325
28.30 34.81 44.50 50.38 50.89
200e650 200e600 200e600 200e600 200e530
18.74 47.08 65.11 54.22 74.99
200e430 200e320 200e325 200e320 200e325
31.36 43.81 56.46 65.52 104.79
200e580 200e580 200e580 200e580 200e530
33.45 34.88 49.26 61.21 80.97
200e450 200e320 200e330 200e320 200e325
28.30 45.01 58.07 76.05 104.79
200e650 200e600 200e600 200e600 200e530
18.74 48.55 58.03 69.58 80.97
E log dW=dt W ¼ logA 2:303*R*T
(1)
where; dW/dt is the rate of weight change, E is the activation energy (J/mol), T is the temperature (K), R is the gas constant (J/kg K) and A is the Arrhenius constant (min1). Thus a plot of log[(dW/dt)] vs. 1/T, should result in a straight line in which the slope is equal to E/(2.303*R). 3.3.2. Coats & Redfern method In Coats & Redfern [22] method, the determination of kinetic parameters for a reaction requires a three-parameter search:
Table 6 Arrhenius constant (min1) values of biomass, coal and biomass - coal blends. Arrhenius method Poplar wood-original coal 0e100 wt% 1.23Eþ03 25e75 wt% 1.35Eþ04 50e50 wt% 1.55Eþ05 75e25 wt% 2.88Eþ05 100e0 wt% 3.96Eþ05 Poplar wood-clean coal 0e100 wt% 6.46Eþ02 25e75 wt% 5.01Eþ03 50e50 wt% 6.31Eþ04 75e25 wt% 3.80Eþ05 100e0 wt% 3.96Eþ05 Hazelnut shell-original coal 0e100 wt% 1.23Eþ03 25e75 wt% 3.16Eþ04 50e50 wt% 9.12Eþ05 75e25 wt% 7.94Eþ06 100e0 wt% 2.51Eþ10 Hazelnut shell-clean coal 0e100 wt% 6.46Eþ02 25e75 wt% 5.01Eþ04 50e50 wt% 1.23Eþ06 75e25 wt% 9.77Eþ07 100e0 wt% 2.51Eþ10
Coats- Redfern method 2.12Eþ06 3.76Eþ06 5.62Eþ05 1.44Eþ06 1.97Eþ05 9.9Eþ06 2.37Eþ06 5.3 Eþ05 1.05Eþ06 1.97Eþ05 2.12Eþ06 4.9Eþ06 1.9Eþ06 7.2Eþ05 1.4Eþ05 9.9Eþ06 2.1Eþ06 1.02Eþ06 4.04Eþ05 1.4Eþ05
reaction order, activation energy, and Arrhenius constant. In this research, a reaction order (n) is assumed to be unity (n ¼ 1) to calculate the kinetic parameters, since a first order reaction model matches well with the experimental data. When reaction order equals to one, the final form of the equation takes the following form;
h . i AR *ð1 2RT=EÞ log logð1 aÞ T 2 ¼ log bE E 2:303*R*T
(2)
where; a is the amount of sample undergoing reaction, E is the activation energy, b is the heating rate, T is the temperature, R is the gas constant and A is the Arrhenius constant. Thus a plot of log[log(1a)/T2] vs. 1/T, should result in a straight line in which the slope is equal to E/2.303R. In both of the kinetic methods studied (Figs. 5 and 6 & Tables 5 and 6), the activation energy (related to the reactivity of the sample) values of the original and clean coal samples were in the range 28.3e31.36 kJ/mol, whereas the poplar wood and hazelnut shell biomass samples were in the range of 50.89e104.79 kJ/mol, respectively. On the other hand, the Arrhenius constant (related to the material structure) values were in the range 6.46 Eþ02e1.82 Eþ03 min1 for original and clean coal and 1.09 Eþ05e2.51 Eþ10 min1 for poplar wood and hazelnut shell biomass samples, respectively. The correlation coefficients of the straight lines were in the range of 0.9461e0.9974, where the activation energy and Arrhenius constant was derived. The relatively high activation energy values for biomass samples may reflect to some extent dependence of pore structures on temperature. This implies that for the burning stage, higher temperature is required, which implies that reactions with higher activation energies were more temperature dependence [23,24]. In other words, activation energy and Arrhenius constant values corresponding to coal samples were lower than all other values. However, activation energy and Arrhenius constant values was increased with the increase in biomass content in the blends. This
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trend was observed in both of the kinetic methods studied. In the case of 25 wt% blend, the values of activation energy and Arrhenius constant were slightly higher than that of coal. When the blend ratio increased to 50 and 75 wt% higher values of activation energy and Arrhenius constant are observed (Tables 5 and 6). The activation energy values of the coal and biomass blends obtained from this research were in agreement with other studies. Munir et al. [25] has reported the activation energy of different biomass samples (cotton stalk, sugar cane bagasse and shea meal) during combustion in the range of 108e116 kJ/mol. Oteo et al. [26] reported relatively 99 kJ/mol of activation energy for sewage sludge. Sanchez et al. [27] reported activation energy of 140, 143 and 173 kJ/ mol on animal manure, sewage sludge and municipal solid waste. Idris et al. [28] reported an activation energy value of 65 kJ/mol for coal and higher values in coal-biomass blends.
[3] [4]
[5] [6]
[7]
[8] [9] [10]
4. Conclusions
[11]
In air atmosphere, after the moisture loss stage, TG-DTG curves represented two stages of mass losses in biomass when compared to one stage for original and clean coal samples, respectively. In coal-biomass blends, three different stage of mass loss was observed. Also, the TG-DTG curves of blends were situated between the original-clean coal and biomass samples. The reaction region, peak temperature and mass loss values are higher for clean coal samples. The hazelnut shell mass loss in each stage was higher than poplar wood. On the other hand, combustion index values of the blends increased slightly with the increasing biomass ratio in blends. There is continuous release of H2, H2O, C2H4, C2H5 and CH2OH was observed during thermal decomposition and combustion process. Smaller amount of primary alcohols and aromatic compounds were mainly released in decomposition stage. The H2 and CH4 gases were significantly released in the clean coal combustion process, but these were marginal during the original coal combustion. Smaller level of sulphur compounds is released, in particular COS is released during decomposition phase, whereas SO2 is released in later combustion phase. The activation energy values of the original and clean coal samples were in the range 28.3e31.36 kJ/mol, whereas the poplar wood and hazelnut shell biomass samples were in the range of 50.89e104.79 kJ/mol. The relatively high activation energy values for biomass samples are due to the dependence of pore structures on temperature. Also, the activation energy and Arrhenius constant values were increased with the increasing biomass ratio in blends.
[12]
[13]
[14] [15] [16]
[17]
[18]
[19] [20] [21]
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[25]
Acknowledgement The authors would like to express their appreciation for the d’Orle ans-France. support of Labex CAPRYSSES, Universite References [1] I. Jiricek, P. Rudasov, A. Zemlov, A thermogravimetric study of the behavior of biomass blends during combustion, Acta Polytech. 52 (2012) 39e42. [2] K. Jayaraman, I. Gokalp, Thermal characterization, gasification and kinetic
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