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REACTIVITY AND STRUCTURAL CHANGES OF COAL DURING ITS COMBUSTION IN A LOW-OXYGEN ENVIRONMENT Pedro N. Alvarado, Francisco J. Cadavid, Alexander Santamaria, and Wilson Ruiz Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01913 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 16, 2016

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REACTIVITY AND STRUCTURAL CHANGES OF COAL DURING ITS COMBUSTION IN A LOW-OXYGEN ENVIRONMENT

Pedro N. Alvaradoa, Francisco J. Cadavidb, Alexander Santamaríaa*, Wilson Ruiza.

a

Química de Recursos Energéticos y Medio Ambiente, Instituto de Química, Facultad de

Ciencias Exactas y Naturales, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín, Colombia b

Grupo de Ciencia y Tecnología del Gas y Uso Eficiente y Racional de la Energía, Facultad de Ingeniería, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín, Colombia

*Corresponding author Alexander Santamaría Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín, Telefono: + 57 4 2196654 Correo: [email protected], [email protected]

ACS Paragon Plus Environment

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KEYWORDS. MILD coal combustion, low oxygen content, coal reactivity, Raman spectroscopy, unburned coal particles.

ABSTRACT. The aim of this study is to improve the understanding of phenomena that occur when a solid carbonaceous material is burned in a high temperature with low-oxygen content (< 10 % v/v) environment similar to that found in the Moderate and Intense Low Oxygen Dilution (MILD) combustion. The morphology, reactivity and physicochemical properties of partially-reacted coal samples extracted from an environment emulating MILD combustion conditions were investigated through different analytical techniques including elemental and thermogravimetric analysis, scanning electron microscopy, surface area and Raman spectroscopy. The early stage of coal burn-out was characterized by some changes in the organic constituents of coal since the low molecular weight compounds reacts quickly and are the first to be removed. After devolatilization process under low oxygen combustion, an increase in surface area and porosity was observed simultaneously with a reduction in carbonaceous material reactivity due to the gradually increase of crystalline order and structural rearrangement of the carbonaceous network making it more resistant to oxidation as it was evidenced by Raman spectroscopy. The low reactivity of the carbonaceous material during the last stage of heterogeneous oxidation could explain the high level of unburned carbon that can be found in combustion systems that use flue gas recirculation, as occurs in MILD combustion.


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INTRODUCTION. Approximately, 85% of energy demand worldwide is supplied by fossil fuels1. However, despite the advantages of their use, emissions of pollutants and greenhouse gases generated in combustion processes have become a global problem especially when coal is used as primary source of energy. Therefore, the opportunity of improving combustion efficiency along with the reduction of pollutant emissions represents one of the major goals of many researches in the scientific community. In the last two decades, the implementation of new technologies for burning coal such as fluidized bed combustion, combustion with oxygen enrichment, chemical looping combustion and moderate and intense low-oxygen dilution (MILD) combustion has made remarkable progress in terms of both efficiency and emissions. In particular, MILD combustion technology is an innovative approach that offers ultra-low pollutants, enhanced combustion stability, thermal field uniformity and broad fuel flexibility2–7. Essentially, the concept of this technology relies on the exhaust gas and heat recirculation to create a volumetric reaction zone. The exhaust recirculation causes an intense dilution inside the combustion chamber, so the oxygen concentration is significantly decreased. On the other hand, the heat recirculation maintains the chemical reactions even when the oxygen level is sustained low. The application of these principles to practical systems has taken different routes and received different names to describe the process. Some of them relied on a descriptive form of the resulting combustion process, i.e. flameless oxidation2, whereas others described the features based on the reactants streams, i.e., high-temperature air combustion 3,4. Although MILD combustion has been extensively studied and implemented in various industrial sectors for combustion of gaseous and liquid fuels8–11, and its implementation in burning solid fuels is still in the initial stage of development, in particular for pulverized coal combustion12–18 . It is important to recall that coal combustion can be described in two stages


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starting with the pyrolysis and decomposition of volatile material, followed by the oxidation of char. In fact, the last stage is considered the limiting step of the reaction rate and it has a large impact in the process efficiency when the combustion is not complete. As a result, a high carbon levels in the ashes can be obtained, which has a number of adverse effects in terms of a significant decrease in efficiency and heat transfer; besides the economic costs for the use or proper disposal of fly ash with high carbon content 19. This is a major concern in conventional combustion systems and can be even worst in MILD combustion, where the intense recirculation of flue gases causes a decrease in the oxygen level inside the combustion furnace. Previous work20 have pointed out that the combustion of coal particles exhibit a range of reactivity that decreases by increasing burn-out. Also, it has been proposed that the low reactivity observed in the residual carbonaceous material coming from utility boilers operated with different oxygen content was caused mainly by thermal deactivation 19. Something similar was suggested by Rusell and Gibbins

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through the analysis of char samples prepared under

realistic heating conditions of pulverized coal combustion in commercial furnace. On the other hand, Davis et al.22 observed that the reactivity in the latter stages of burn-out of chars generated by direct injection of pulverized coal was significantly impacted by the degree of structural ordering during the evolution of the carbonaceous matrix, as it was confirmed by high–resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) analysis. In contrast, on MILD combustion technology, a great volume of work have been carried out in the applicable and fundamental side23–27, Many of these studies were focused in furnace environment characterization, NOx emissions and coal burnout through experimental and numerical approaches. However, although it is been demonstrated that parameters such as


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furnace temperature, coal rank, particle size and reaction environment can affect the coal burnout level6,13,16, a deeper understanding about the underlying physical and chemical phenomena that may be responsible for the deactivation of pulverized coal particles under MILD combustion needs to be addressed. In fact, according to Davis et al., the deactivation of pulverized coal at higher burn-out levels are related to (1) particle morphology and macroporosity, (2) carbon/mineral interactions, and (3) chemistry, crystallinity, and ultrafine structure of the organic material. In this paper, we are going to be focused on evaluate the effect of high temperature and low oxygen concentration in the physicochemical characteristics of unburned coal particles obtained under simulated MILD combustion conditions using a natural gas premixed flame as reactor. . METHODOLOGY Combustion reactor. To simulate the controlled conditions found in MILD combustion environment, the experiments of pulverized coal combustion were carried out in a low-power premixed flame of natural gas (Figure 1) operated at equivalent ratios of 0.65 and 0.85. This flame provides the possibility of generating a vitiated and hot environment for MILD combustion since the temperature and dilution level can be controlled by changing the equivalence ratio to obtain a required low oxygen concentration in the combustion products. The natural gas burner designed and constructed in our laboratory consists of three concentric stainless tubes. The central tube (360 mm length and 84 mm id) where natural gas and air were mixed and stabilized through a honeycomb-type ceramic cylinder to produce a flat laminar velocity profile at the burner exit, the intermediate annular tube (100 mm annulus) used as water cooling system and the outer annular tube through which an argon stream is supplied to


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stabilize the flame. Finally, to avoid the disturbance caused by the entrance of the ambient air into the system, a glass chimney (350 mm length and 160 mm id) was used. A side wall slot was machined on glass chimney to allow the access of the flame igniter, the sampling probe, and thermocouple.

Figure 1. Experimental setup for coal combustion under simulated MILD conditions Coal samples and combustion experiments. A Colombian coal coming from “Sinifana Basin” in Amagá Antioquia has been selected for this study28 since it has been widely used in electricity generation and local industries. According to proximate and ultimate analysis reported in Table 1, this coal can be classified as sub-bituminous A.

Table 1. Proximate and ultimate analysis of pulverized coal sample For combustion experiments, 300 mg of pulverized coal with particle diameters between 75 to 150 µm were placed in a sample holder made of stainless steel mesh (Tyler series reference 200), and then suspended into the flame with the help of a servomotor at heights of 25 mm and 30 mm above burner surface where the oxygen concentration is low and coal particles can react with combustion products of natural gas at high temperature (Table 2). Standards gases released from the reference flame were also sampled using a water-cooled probe (225 mm length and 3.18mm id) that was inserted through a small slot on the glass chimney at the height of sampling position. Measurements of CO, CO2 and O2 concentrations were obtained using a SICK MAIHAK S710 analyzer which is provided by a paramagnetic detector for O2 and a nondispersive infrared detector (NDIR) for CO and CO2, the results are shown in Table 2. The


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analyzer measurement accuracies were estimated to be: 6% for CO, 3% for CO2, and 3% for O2 of measured value.

Table 2. Flame environment conditions for pulverized coal combustion The temperature at the sampling location was monitored with the help of an R-type thermocouple using a rapid insertion method to avoid the soot deposition on it. All the experiments were performed in such a way that partially-reacted coal particles were rapidly withdrawn from the flame at different burn-out levels between 0 to 900 s, and then it was put in contact with liquid nitrogen vapors to quench chemical reactions. Finally, those particles were stored for further analysis to evaluate the evolution of carbonaceous material with the oxygen environment. Carbonaceous material reactivity test. Char reactivity was performed in air at 400 °C using a thermogravimetric analyzer (TGA Q 500). This temperature was chosen to minimize the changes in the carbonaceous structure due to thermal "annealing", which could take place at higher temperatures29. The experimental procedure can be found else where30 and it is briefly described here: approximately, 3.0 to 4.0 mg of carbonaceous material extracted from the flame at different burn-out levels (42, 52 and 62 %), were heated in a platinum holder under nitrogen atmosphere up to 105 °C to remove the moisture. Subsequently, the temperature was increased at a heating rate of 10 °C/min up to 400 °C where it was kept constant for 10 minutes. Then, the N2 atmosphere was changed to air to perform the reactivity measurements during 6 hours. Finally, the temperature was increased again to 600 °C to completely burn the


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residual carbon. The specific reactivity of carbonaceous material was calculated on dry and ash-free basis according to equation 1:

=−

1 (1)

Where W is the initial weight of dry ash-free sample (d.a.f.). Chemical and morphological characterization of the unburned carbonaceous material. The chemical characterization of partially-reacted coal samples obtained from the flame was performed by Raman spectroscopy to provide qualitative information on the structural organization degree present on particles. Raman spectra of samples taken at different burn-out were recorded with a Raman microscope system (lab Raman HR Horiba) using a 632.8 nm (1.96 eV) laser line as an excitation source. Spectra of samples were recorded in the range of 50 cm-1-3500 cm-1 using a 50X magnification objective and 20 s exposition times. The spectra were processed with the free software Lab Plot. After a multipoint baseline correction, spectra were fitted by combination of four Lorentzians curves to characterize the graphitic component (G-band) and non-graphitic component (D1, D3 and D4 bands) found in the wavenumber range between 1000 cm-1 and 1800 cm-1, respectively. Seven different spots in each sample were analyzed and averaged in order to improve the statistical significance. Three measurements of each sample were taken in order to estimate the method reproducibility. The uncertainty of Raman measurements was less than 5%. The structural characterization of the partially-reacted coal samples was complemented by SEM and gas adsorption measurements. SEM images of the unburned samples deposited onto a graphite tape and coated with a nano-film of cadmium– gold were recorded using a JEOL JSM-5410 instrument operated with an applied voltage of 20 kV. The specific BET surface area of the carbonaceous materials was determined by means of


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adsorption–desorption isotherms of nitrogen at –196 °C using a Micromeritics ASAP 2020 instrument.

RESULTS AND DISCUSSION. It is important to mention that the experimental setup used in this study does not allow the flue-gas recirculation that is normally achieved inside the MILD combustion furnaces. So, instead of recirculation, the high dilution level required for the experiments carried out here was the result of a vitiated environment caused by the combustion products coming from the natural gas flame (see Table 2). Figure 2 shows a sequence of images of the pulverized coal combustion performed at low oxygen concentration (4.0 % and 8.0 % by vol.). The first stage of the combustion process was the ignition. It is proposed that the ignition occurs through chemical reactions in the homogeneous phase involving the volatile matter released from coal without producing any spark. Then, the flame propagates around the pan containing particles until a stable yellow flame is formed due to the combustion of volatiles. The stage of volatile matter combustion is faster than the stage of char oxidation which is characterized under our experimental conditions by the absence of any visible flame.

Figure 2. Image sequences of coal combustion under two oxygen diluted conditions It can be note that the evolution of volatiles from solid fuel particles completely separates the particle pyrolysis process from char oxidation. The volatiles evolved are combustible and they consume any oxygen being transported toward the surface of fuel particles. The intense oxidation of the volatiles around the particles keeps the oxygen concentration at a value near


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zero at the surface of the char. Consequently, the flame attachment to the particle does not occur until the very end of the pyrolysis sequence, and the char oxidation does not proceed until volatile evolution is essentially complete 31, as it was observed in Figure 2. Burn-out profiles of pulverized coal. The combustion experiments were performed by duplicated over 30 samples of partially-reacted coal particles extracted at different burn-out levels. Figure 3 shows the burn-out profiles of pulverized coal as a function of time for the two oxygen concentrations (4.0 and 8.0% v/v) evaluated in this study at the temperature around 1150 K. The dashed line introduced as reference in Figure 4 corresponds to the dry and ashfree fraction of volatiles obtained from the thermogravimetric analysis of raw pulverized coal. This dashed line divides the process in two stages, 1) the devolatilization stage (d.a.f. burn-out < 0.46) and 2) the heterogeneous char oxidation of (d.a.f. burn-out > 0.46). However, it should be mention that this separation is just a numerical operation since the more extreme the conditions are, the less distinguishable in time and space are these two processes32,33.

Figure 3. Coal burn-out at two diluted oxygen contents as a function of time In general, no significant differences were observed in coal burn-out during the devolatilization stage at the two oxygen environments. Thus, the devolatilization of coal depends weakly on oxygen concentration and it is mainly controlled by the rate of convective heat transfer. On the other hand, during the heterogeneous oxidation stage, the coal burn-out increased slightly when the oxygen content goes from 4.0 % to 8.0 %. Particularly, a difference of burn-out near to 10% between both oxygen environments was obtained when the coal was burned in a window time between 500-900 s. It can be assumed that the char heterogeneous oxidation is controlled


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by the combination of pore diffusion and chemical reaction (regime II). For a reaction occurring in this regime, its reaction rate depends on morphological characteristics like specific surface area, as well as, oxygen partial pressure at the outer surface of char particle, and the intrinsic reactivity. In the next section, we will get some insights about the factors affecting coal burn-out through the characterization of carbonaceous particles in terms of reactivity, morphology, surface area and Raman spectroscopy. Char reactivity analysis. Isothermal reactivity at low temperature is widely used to characterize the char reactivity to predict the behavior occurring at higher temperatures. Nonetheless, even while operating at lower temperatures and establishing a kinetic controlled zone in a TGA, in most of the reactivity plots, a maximum can be observed. In this study, the char reactivity analysis was performed by TGA at 400 °C on three partially-reacted coal samples extracted at different burn-out levels (44 %, 52 %, 62 %) from flame reactor at low O2 concentration. It is worth mentioning that the samples evaluated here are representative of the initial and intermediate stages of char oxidation since all of them were obtained after the devolatilization process.

Figure 4. Reactivity of carbonaceous material as a function of d.a.f conversion Figure 4 shows the reactivity profiles obtained from TGA for three char samples with different burn-out levels. In general, each char sample reached its reactivity peak value at low TGA conversion. However, it was observed that the reactivity decreases as the burn-out level of char increases from 42% to 62%. The behavior observed in reactivity before reaching the maximum value in the conversion curve has been associated to the building up of partial pressure,


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opening of the previously closed pores, or a balance between the mass gain because of stable complex formation and the mass loss because of carbon gasification. Additionally, it can be noted that the intensity of the specific reactivity peak decreased faster as the burn-out level of the carbonaceous samples coming from a low-oxygen environment (4.0 % v/v) goes from 44% to 62%. This behavior was simultaneously followed by a displacement of its maximum toward lower conversion values due to intraparticle diffusion limitation. This information may imply not only a variation in the amount and/or accessibility of active sites (heteroatoms, edges and imperfections in the carbon structure) that participate in the combustion reactions, but also, major changes of the turbostatic carbon network leading to a more order structure. This can have important practical implications in coal MILD combustion technology; where the environments are characterized by a low oxygen concentration ( 300s) the generation of pores is completely achieved and the D band broadening gets constant. A similar analysis can be obtained for the partially-reacted coal samples coming from the combustion environment with 8.0% of oxygen. However, it is possible to see that for a particular burn-out, the development of the carbonaceous network arrangement occurs much faster as the oxygen content increases. These results agree with those obtained by scanning electron microcopy (SEM) and surface area analysis. It is interesting to observe that the D band was quite dispersive toward smaller wavenumbers for the partially-reacted coal samples under the two O2 environments. A similar behavior was observed for the G band that shifted toward larger wavenumber but to a lesser extent (see Table 3). This simultaneous behavior observed on dispersion can be associated to a reduction in crystalline size domain due to oxidation effect and porosity generation41. So, the combined effect of both bands dispersion and bandwidth reduction indicates that although the carbonaceous material is consumed by oxidation, the material has still a high degree of threedimensional structural organization.

Table 3. Raman parameters of unburned coal particles coming from two different oxygen environments Notwithstanding, due to the overlap observed between D and G bands, it has been suggested other peak analysis for qualitative correlations between Raman spectra and structural parameters, as the one performed for carbonized materials obtained from low rank coals such


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as lignite

42

. Therefore, it is essential to perform spectra deconvolution in order to obtain

detailed information about the structural organization of the carbonaceous network. Figure 9 shows the Raman spectra deconvolution of carbonaceous material obtained at different burnout when the oxygen concentration was 4.0 %. The fitting procedure was performed by duplicate over each average spectrum obtained from 5 measurements done on sample to improve the uncertainty caused by the large sample heterogeneity as it was observed by SEM.

Figure 9. Deconvoluted Raman spectra of partially-reacted coal particle obtained from coal combustion at 4.0% v/v O2 concentration. The D1 band located around 1360 cm-1 refers to highly disordered carbonaceous materials composed of carbon-carbon bonds in aromatic ring structures. In addition, this band is also very sensitive to crystal size

43–45

. The D3 band centered at 1500 cm-1 has been assigned to

amorphous carbon structures, five-membered ring structures or methyl or methylene groups on sp3 or sp2 electron configuration

34,46,47

; The D4 band found around 1200 cm-1 has been

suggested for aromatic-aliphatic carbon-carbon bonds and aromatic and aliphatic ethers and it has also been proposed as an indicator of active sites on activated carbon 48. Finally, the G band located around 1580 cm-1 has been assigned to the graphitic component of carbonaceous materials29. At first glance, a decreased in the D3 band intensity was observed when carbonaceous material burn-out varied from 22% to 44% which corresponds to the coal devolatilization step. This is an indication that part of the amorphous carbon reacts quickly (molecular aromatic species, aliphatic species, etc.) so that the graphite structure gradually rearranged. This is also reflected


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in the bandwidth reduction of D band on the non-deconvoluted spectrum (or D1 and D4 combined) and the hyperfine development of the G band which becomes sharper and narrower with increasing burn-out. Separate analysis of the D1 band intensity indicates that the crystalline size domain decreases slightly during volatilization (between 22-44% burn-out) since this parameter is inversely proportional to the intensity 37. However, at burn-out level higher than 44% the crystal size remains almost constant. Therefore, considering the above mentioned, it is expected that at higher burn-out the reactivity decreases as the three-dimensional order increases. A similar behavior was observed for the samples extracted from the combustion environment with oxygen content of 8.0% v/v (see the results in the Figure 2 of the supplemental material). Figure 10 shows the variation of intensities ratio corresponding to G and D1 bands as function of carbonaceous burn-out. It is possible to see two different behaviors depending on the conversion step. Several authors have suggested that the ID1/IG ratio decreases with time when a pyrolysis treatment is performed in a non-oxidative atmosphere

37,49

. During this process, a

reorganization of small aromatic clusters can lead to the formation of larger-size crystalline structures. However, in an oxidative atmosphere, as it was performed in this study, an opposite behavior was observed. In this case, the increment in the ID1/IG ratio may indicate that the crystalline domain size of the carbonaceous network decreases due to generation of porosity that can be seen as defects that cause an interruption of the symmetry of the carbonaceous network structure.

Figure 10. ID/IG ratio of the coal chars as a function of burn-out


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On the other hand, from Figure 11, it is possible to see a remarkable decrease in the amorphous carbon content (or ID3/IG ratio) during the first step of carbonaceous burn-out where devolatilization process occurs at an oxygen content of 8.0 %. However, after the devolatilization process, the amorphous carbon content reaches a constant value and no significant differences were found between the two oxygen concentrations used.

Figure 11. ID3/IG and ID4/IG ratios of coal char as a function of burn-out. Similarly, the decrease in the ID4/IG ratio with coal burn-out at low oxygen concentration suggest that some structural defects of the carbon network were gradually eliminated during the three dimensional arrangement of the carbonaceous network. This fact can be associated with loss of active sites on the material surface that leads to the decrease in the reactivity in the final step of coal burn-out.


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CONCLUSION This study showed the experimental setup used to follow the evolution of carbonaceous structure during pulverized-coal oxidation in a vitiated environment with low oxygen concentration, which emulates MILD conditions. The results indicate that the early stage of burn-out was characterized by some changes in the organic constituents of coal since the low molecular weight compounds reacts quickly and are the first to be removed in a process that depends weakly on oxygen content. However, the positive effect associated with the increase in the specific surface area and porosity observed on samples after the devolatilization stage was overshadowed by the deactivation of the carbonaceous material followed by a reduction in the crystalline size domain. This behavior should be explained not only by the low oxygen content in the combustion environment, but also, by the loss of active sites and carbonaceous network rearrangement which agrees well with the hydrogen content depletion at burn-out level of samples increases making them more resistant to oxidation. So, under the emulated MILD combustion conditions, it is expected that the consumption of coal particles occurs through an


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“oxidative pyrolysis” process which means that the coal oxidation and carbonaceous network arrangement assisted by the high temperature regime may occur through a parallel way.

Acknowledgments The authors thank the “Sostenibilidad 2016–2017” of University of Antioquia for the financial support. Pedro N. Alvarado thanks the Colombian Administrative Department of Science, Technology and Innovation (COLCIENCIAS) for his PhD scholarship. REFERENCES (1)

Energy Information Administration. http://www.eia.gov/forecasts/ieo.

International

Energy

Outlook

2013

(2)

Wünning, J. Prog. Energy Combust. Sci. 1997, 23 (1), 81–94.

(3)

Katsuki, M.; Hasegawa, T. Symp. Combust. 1998, 27 (2), 3135–3146.

(4)

Weber, R.; Orsino, S.; Lallemant, N.; Verlaan, A. Proc. Combust. Inst. 2000, 28, 1315– 1321.

(5)

Cavaliere, A.; De Joannon, M. Prog. Energy Combust. Sci. 2004, 30 (4), 329–366.

(6)

Saha, M.; Dally, B. B.; Medwell, P. R.; Chinnici, A. Fuel Process. Technol. 2016.

(7)

Mardani, A.; Fazlollahi Ghomshi, A. Energy 2016, 99, 136–151.

(8)

Weber, R.; Smart, J. P.; Kamp, W. vd. Proc. Combust. Inst. 2005, 30 (2), 2623–2629.

(9)

Ye, J.; Medwell, P. R.; Varea, E.; Kruse, S.; Dally, B. B.; Pitsch, H. G. Appl. Energy 2015, 151, 93–101.

(10)

Li, P.; Dally, B. B.; Mi, J.; Wang, F. Combust. Flame 2013, 160 (5), 933–946.

(11)

Li, P.; Dally, B. B.; Mi, J.; Wang, F. Combust. Flame 2013, 160 (5), 933–946.

(12)

Dally, B. B.; Shim, S. H.; Craig, R. a.; Ashman, P. J.; Szegö, G. G. Energy and Fuels 2010, 24 (6), 3462–3470.

(13)

Saha, M.; Dally, B. B.; Medwell, P. R.; Cleary, E. M. Energy and Fuels 2014, 28 (1), 6046–6057.

(14)

Stadler, H.; Ristic, D.; Förster, M.; Schuster, A.; Kneer, R.; Scheffknecht, G. Proc. Combust. Inst. 2009, 32 (2), 3131–3138.

(15)

Manabendra Saha; Alfonso Chinnici; Bassam B. Dally and Paul R. Medwell. Energy &


21

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Page 22 of 37

Fuels 2015, 29 (11), 7650–7669. (16)

Saha, M.; Dally, B. B.; Medwell, P. R.; Chinnici, A. Combust. Flame 2016, 172, 252– 270.

(17)

Smart J P., R. G. S. J. Energy Inst. 2012, 85 (3), 131–134.

(18)

Suda, T.; Takafuji, M.; Hirata, T.; Yoshino, M.; Sato, J. Proc. Combust. Inst. 2002, 29 (1), 503–509.

(19)

Wornat, M. J.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100 (1–2), 131–143.

(20)

Hurt, R. H. Energy & Fuels 1993, 7 (6), 721–733.

(21)

Russell, N. V.; Gibbins, J. R.; Man, C. K.; Williamson, J. Energy and Fuels 2000, 14 (4), 883–888.

(22)

Davis, K. a.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100 (1– 2), 31–40.

(23)

Weidmann, M.; Verbaere, V.; Boutin, G.; Honoré, D.; Grathwohl, S.; Goddard, G.; Gobin, C.; Dieter, H.; Kneer, R.; Scheffknecht, G. Appl. Therm. Eng. 2015, 74, 96–101.

(24)

Weidmann, M.; Honoré, D.; Verbaere, V.; Boutin, G.; Grathwohl, S.; Godard, G.; Gobin, C.; Kneer, R.; Scheffknecht, G. Combust. Flame 2016, 168, 365–377.

(25)

Schaffel, N.; Mancini, M.; Szle¸k, A.; Weber, R. Combust. Flame 2009, 156 (9), 1771– 1784.

(26)

Tamura, M.; Watanabe, S.; Komaba, K.; Okazaki, K. Appl. Therm. Eng. 2015, 75, 445– 450.

(27)

Vascellari, M.; Cau, G. Fuel 2012, 101, 90–101.

(28)

Donnelly, L. J.; De La Cruz, H.; Asmar, I.; Zapata, O.; Perez, J. D. Eng. Geol. 2001, 59 (1–2), 103–114.

(29)

Tay, H.-L.; Li, C.-Z. Fuel Process. Technol. 2010, 91 (8), 800–804.

(30)

Quyn, D. M.; Wu, H.; Hayashi, J.; Li, C.-Z. Fuel 2003, 82 (5), 587–593.

(31)

David Tillman. The Combustion of Solid Fuels and Wastes, 1st ed.; Academic Press, INC: New York, 1991.

(32)

Ponzio, A.; Senthoorselvan, S.; Yang, W.; Blasiak, W.; Eriksson, O. Fuel 2008, 87 (6), 974–987.

(33)

Ponzio, A.; Senthoorselvan, S.; Yang, W.; Blasiak, W.; Eriksson, O. Fuel 2009, 88 (6), 1127–1134.

(34)

Jiménez, F.; Mondragón, F.; López, D. J. Anal. Appl. Pyrolysis 2012, 95, 164–170.

(35)

Chen, Y.; He, R. J. Anal. Appl. Pyrolysis 2011, 90 (1), 72–79.

(36)

Alvarez, D.; Borrego, A. G. Energy and Fuels 2007, 21 (2), 1085–1091.

(37)

Tuinstra, F. J. Chem. Phys. 1970, 53 (3), 1126.

(38)

Makovsky, Leo E., Peter Waldstein, W. H. E. Nat. Phys. Sci. 1971, 231, 154–155.

(39)

Friedel, R. a.; Carlson, G. L. Fuel 1972, 51 (3), 194–198.


22

Page 23 of 37

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Energy & Fuels

(40)

Tsu, R.; H, J. G.; Hern, I.; Luengo, C. A. 1977, 24, 6–9.

(41)

Vigil de la Villa, R.; Frías, M.; García-Giménez, R.; Martínez-Ramirez, S.; FernándezCarrasco, L. Int. J. Coal Geol. 2014, 132, 123–130.

(42)

Matthews, M.; Pimenta, M.; Dresselhaus, G.; Dresselhaus, M.; Endo, M. Phys. Rev. B 1999, 59 (10), R6585–R6588.

(43)

Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Pöschl, U. Carbon N. Y. 2005, 43 (8), 1731–1742.

(44)

Beyssac, O.; Goffé, B.; Petitet, J.-P.; Froigneux, E.; Moreau, M.; Rouzaud, J.-N. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2003, 59 (10), 2267–2276.

(45)

Chabalala, V. P.; Wagner, N.; Potgieter-Vermaak, S. Fuel Process. Technol. 2011, 92 (4), 750–756.

(46)

Dippel, B.; Jander, H.; Heintzenberg, J. Phys. Chem. Chem. Phys. 1999, 1 (20), 4707– 4712.

(47)

Bar-Ziv, E.; Zaida, A.; Salatino, P.; Senneca, O. Proc. Combust. Inst. 2000, 28 (2), 2369–2374.

(48)

Schwan, J.; Ulrich, S.; Batori, V.; Ehrhardt, H.; Silva, S. R. P. J. Appl. Phys. 1996, 80 (1), 440.

(49)

Li, X.; Hayashi, J.; Li, C. Fuel 2006, 85 (10–11), 1509–1517.


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Table 1. Proximate and ultimate analysis of pulverized coal sample Proximate analysisa (wt.%) Amagá Coal

Ultimate analysisb (wt.%)

Ash

Volatiles

Fixed carbon

C

H

O*

N

S

5.1

46.8

48.1

65.4

5.2

22.5

1.6

0.2

Gross calorific value MJ/kgc 26.1

a)

Determined by TGA using the standard method D7582-15

b)

Determined by elemental analyzer using the standard method D3176-15. * The oxygen content was calculated by difference

c)

Determined by oxygen bomb calorimeter using the standard method ASTM D5865


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Table 2. Flame environment conditions for pulverized coal combustion Flame sampling position (mm) 25

Equivalent ratio (φ) 0.65

O2 (% v/v) 8.0 ± 0.3

CO2 (% v/v) 6.9 ± 0.2

CO (ppm) 45± 6

Temperature (K) 1147±21

30

0.85

4.0 ± 0.2

9.7 ± 0.3

83± 4

1144±15


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Table 3. Raman parameters of unburned coal particles coming from two different oxygen environments O2

4.0 %

8.0 %

Burn-out level (%) 0 22 44 62 84 0 39 43 62 84

Band position (cm-1) G D 1576 ± 2 1357 ± 3 1582 ± 1 1351 ± 2 1593 ± 1 1337 ± 2 1591 ± 1 1332 ± 1 1585 ± 3 1355 ± 1 1588 ± 1 1355 ± 2 1592 ± 1 1334 ± 1 1591 ± 2 1336 ± 2

Bandwidth G D 99 ± 3 267 ± 4 79 ± 2 255 ± 2 77 ± 2 226 ± 3 75 ± 1 215 ± 3 88 ± 2 258 ± 3 85 ± 1 257 ± 1 80 ± 3 218 ± 4 79 ± 2 228 ± 1


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FIGURES (1-11)

Figure 1. Experimental setup for coal combustion under simulated MILD conditions


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Energy & Fuels

Ignition behaviour

Flame propagation

Volatiles flame

Char oxidation

0-1

1-2

2-150

150-900

8.0 %

O2

4.0 %

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t(s)

0-46%

α (%)

> 46 %

α = Coal burn-out level

Figure 2. Image sequences of coal combustion under two oxygen diluted conditions


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Figure 3. Coal burn-out at two diluted oxygen content as a function of time


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Figure 4. Reactivity of carbonaceous material as a function of conversion from TGA


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Burn-out level

4.0 % O2

8.0 % O2

0%

44%

52%

62%

Figure 5. SEM images of the coal particles obtained at different burn-out levels


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Figure 6. Surface area development as a function of coal burn-out at two different oxygen contents.


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Figure 7. Surface area and specific reactivity of coal char samples as a function coal burn-out.


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Figure 8. Raman spectra of the partially reacted coal obtained during combustion at two different oxygen contents. (x= burn-out, t = time)


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Figure 9. Deconvoluted Raman spectra of partially reacted coal particle obtained from coal combustion at 4.0% v/v O2 concentration.


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Figure 10. ID/IG ratio of the coal chars as a function of burn-out


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Figure 11. ID3/IG and ID4/IG ratios of coal char as a function of burn-out.


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Reactivity and Structural Changes of Coal during ... - ACS Publications

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