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Role of an additive in retarding coal oxidation at moderate temperatures
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Proceedings of the
Proceedings of the Combustion Institute 33 (2011) 2515–2522
Combustion Institute www.elsevier.com/locate/proci
Role of an additive in retarding coal oxidation at moderate temperatures Jing Zhan, Hai-Hui Wang *, Sheng-Nan Song, Yuan Hu, Jiao Li State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China Available online 7 August 2010
Abstract The impact of an additive on coal oxidation process is studied from the mechanistic perspective, aiming at development of a guideline in search of effective inhibitors for controlling the coal self-heating phenomena. The salt Na3PO4 was chosen as an additive. Behaviors of samples with/without the additive were examined at temperatures up to 400 °C both in oxidative and inert atmosphere using a TGA instrument, and the compounds on coal surface during oxidation and pyrolysis were monitored by FT-IR technique. The TGA data show that the impact of the additive on coal oxidation process can be directly evaluated using a parameter defined as the percentage of mass increase at 265 °C, and the addition of Na3PO4 slows down the rates of oxygen uptake and decomposition reactions. FT-IR results also indicate that the additive suppresses both the coal oxidation and pyrolysis processes essentially by accelerating the formation of saturated ether linkages. Further analysis suggests that Na3PO4 plays a role in modifying the routes for decomposition of hydroxyl, and subsequently improving the coal thermal stability. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Coal; Oxidation at moderate temperatures; Retarding additives; Control of self-heating phenomena
1. Introduction Self-heating and spontaneous combustion of coal are quite common to occur during its exploitation, storage and transportation. This phenomenon is directly due to the nature of the coal as an active organic material. Once a coal is exposed to air, it absorbs oxygen both physically and chemically, which triggers a series of exothermic reactions taking place at the internal surface of coal pores [1]. The release of heat increases temperature of the coal itself and induces the reactions owning higher activation energies; the
*
Corresponding author. Fax: +86 (0) 551 360 1669. E-mail address: [email protected] (H.-H. Wang).
acceleration of oxidation reactions may eventually lead to spontaneous combustion of the coal. An inhibitor can suppress the coal oxidation processes, and hence has the application in preventing the self-heating and spontaneous combustion of coal. The role of an inhibitor in slowing down the coal oxidation process can be classified into two folds, with one of them being direct isolation of the active centers on coal surfaces from oxygen. This is usually achieved by jetting mud, water glass and polymer emulsion to cover the coal surface, for instance. Their role is physically-based, which implies low efficiency in controlling the coal oxidation process. Chemicals have also been used in retarding coal oxidation process by either decreasing the formation of active groups or restraining the free radical reactions. Such chemicals include inorganic salts,
1540-7489/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2010.06.046
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antiaging agents, and antioxidant [2–5]. Due to the potential application of such chemically-based inhibitors, development of chemical suppressants has been a focus for scientists working in the fire safety in coal mining and utilization sectors. Alkali compounds were recognized as inhibitors by early researchers. Smith et al. [2] reported that NaNO3, NaCl and CaCO3 could suppress the coal oxidation process, while NaCOOH and some other substances had the reversed effect. By testing the ‘critical ambient temperatures’ of a coal mixed with various inorganic salts, Zhang’s research group [4,5] examined the impact of additives on the coal oxidation process. Their results suggested that CaCl2, Ca(Ac)2, Mg(Ac)2, MgCO3, NaCl and NaOH might inhibit the oxidation process. So far, criteria have been developed for evaluating the effect of inorganic compounds on coal oxidation and the associated self-heating phenomena, which are based upon the observation of heat evolution mass change during coal oxidation and the determination of apparent activation energies for oxygen uptake by coals [2–6]. However, no rigorous studies have been done in regard to how an additive plays a role in affecting the series of reactions occurred during coal oxidation and consequently slowing down the overall rate of coal oxidation. This paper is aiming at developing a routine technique for examining the impact of additives on coal oxidation at moderate temperatures from the chemistry perspective. By designing experimental procedures and conducting experiments using modern analytical instruments, the behaviors of oxidation of a coal are studied in the presence of various amount of additives. Mechanism of the inhibitors (additives) in slowing down the oxidation reactions is then explored. 2. Experimental A bituminous coal was collected from the Gunzhou Colliery, Shandong Province. The coal was crushed and ground into particles. Size-distribution of the particles was measured by Laser particle-size-distribution tester JL-1177, and the volumetric mean diameter of the particles was found to be about 1.61 lm. Proximate and ultimate analyses of the coal are given in Table 1. Na3PO412H2O at a purity of 99% was selected as an additive for the experiments, which was purchased from a local medical station. Sodium chloride at a purity of 99.5%, supplied by Sinopharm Chemical Reagent Co., was also used as an additive in the current study. Thermal stability of these compounds was examined at temperatures up to 400 °C using TGA technique both in air and nitrogen atmosphere. Prior to use, an additive was first dried at 80 °C for 4 h in order to eliminate its crystalline water. The product was then
ground and blended with the raw coal particles in a mortar to make samples containing 0, 3, 6 and 9 wt.% Na3PO4, accordingly. Samples containing 6 wt.% NaCl were also prepared for measurements. All the samples were kept in tightlysealed plastic bags and stored in a deep freezer prior to measurements. Thermo-gravimetric analysis (TGA) was used to evaluate the oxidation and pyrolysis behaviors of raw coals and those mixed with the additives. Measurements were performed using a TA Q5000IR thermal analysis system. A sample of 10 mg in mass was quickly and loosely placed on the aluminum crucible. The sample temperature was raised from room temperature to 400 °C at a heating rate of 4 °C/min. The reason to choose such a low heating rate was to reduce the thermal lag of a sample during its heating. Industrial grade air and nitrogen gas bottles were used for conducting oxidation or pyrolysis experiments. The flow rate of atmosphere purging into the furnace was set at 50 ml/min. The experimental uncertainties for mass change are below 0.01 mg. Samples for FT-IR analysis were prepared by heating coal particles to a temperature of 265 or 400 °C in a set atmosphere using the TGA instrument. A sample collected was then mixed with KBr at a mass ratio of about 5:100, and finely ground in an agate mortar for 5 min or so. The KBr pellet containing coal particles was then made by pressing the mixture at a pressure of 20 MPa for 1 min. Absorbance spectra were picked up by the Nicolet 6700 FT-IR spectrometer with a nominal spectral resolution of 4 cm 1 and a signal-to-noise ratio of 50,000:1. Spectra ranged from 4000 to 400 cm 1, and the scanning time for the background and sample were both 32 times. All the spectra were baseline corrected. The SiAO bond is relatively stable within the experimental temperature range [7–9], and its bending vibration peak appears at 472 cm 1; therefore, this peak for the raw coal was chosen as a benchmark for calibrating all the spectra. Differential spectra were generated using the Data General processing system. 3. Results and discussion 3.1. TGA measurements Typical TGA data obtained from the aerial oxidation measurements for coal samples containing various weight percent of Na3PO4 are shown in Fig. 1. As observed from the curves, with the temperature increasing, a sample firstly undergoes a slight decline in its mass, which is followed by a progressive increase. The sample mass reaches the maximum at 265 °C with a magnitude exceeding 100% of its initial mass. As the temperature increases further, the curve begins to decline
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Table 1 Proximate and ultimate analyses for the coal used in the current study.
Table 2 Percentage of mass increase at 265 °C for various samples.
Proximate analysis (air dried basis) (%)
Content of Na3PO4 (wt.%)
Percentage of mass increase (%)
Relative reduction (%)
0 3 6 9
3.62 3.30 2.80 2.83
0 8.8 22.7 21.8
Moisture Ash Volatile matter Fixed carbon Total sulfur
2.8 13.1 32.5 51.1 0.5
Ultimate analysis (dry ash free basis) (%) C H N S O
83.5 5.0 1.2 0.6 9.7
Calorific value of the coal (air dried basis): 4533 kcal/kg.
Fig. 1. Results of the TGA measurements with coal samples containing various amount of Na3PO4 and oxidized in air.
rapidly, and the weight percent of all the samples maintains about 80% of their initial mass at 400 °C. Difference among the TG curves for the samples containing various percentages of additives in weight is mainly reflected by the mass increase occurred at the temperatures between 150 and 300 °C. It is seen that the peak for mass increase gets flat due to an introduction of the additive. We define a parameter to evaluate the extent of mass increase, i.e. the percentage of mass increase, which is determined by a takeaway of the maximum mass between 150 and 300 °C to the minimum mass from room temperature to 150 °C with the outcome divided by the initial sample mass. The results determined for various types of samples are reported in Table 2. It is evident that, the percentage of mass increase reduces due to the incorporation of Na3PO4 in samples, while an amount of 6 wt.% Na3PO4 makes the mass increase percentage reduce from 3.62% to 2.80%, with a decrease extent by 22.7%, compared to that for the raw coal. However, while the amount of Na3PO4 increases to 9 wt.%, the percentage of mass increase tends to stop reducing further. Small magnitudes determined for the percentage
of mass increase are also true reflection on the slow process of coal oxidation at temperatures below 300 °C. It is shown that for the coal sample undergoing oxidation and pyrolysis in a dynamic oxidative atmosphere, the mass loss at the initial stage is mainly ascribed to the removal of moisture and light volatiles. At 150 °C or so, the sample starts to exhibit an increase in its mass as a result of oxygen uptake and formation of solid oxygenated complexes. With the temperature increasing further, pyrolysis of unstable oxygenated complexes may become eminent, and a direct interaction of coal with oxygen, i.e. the so-called direct ‘burnoff’ reaction, may also come into action [1,10,11]. However, prior to 265 °C the rates of oxygen adsorption by coal and the formation of unstable solid products are larger than the rates of thermal decomposition and the direct ‘burnoff’ reaction. As a result, the overall sample mass remains increasing. After the temperature exceeds a threshold of 265 °C, the decomposition reactions and the direct ‘burn-off’ reaction become dominated, which lead to a drop in the sample mass. An addition of Na3PO4 results in a reduction in the percentage of mass increase, indicating that the rates of oxygen uptake by coal and formation of unstable solid oxygenated complexes have slowed down. As the temperature increases further, due to significant decomposition of oxygenated complexes and the emergence of direct ‘burn-off’ reaction, the mass changes for various samples are getting close. In addition, with a further increase in the amount of Na3PO4 in a coal sample, the percentage of mass increase does not continue to reduce. This is an evidence to suggest that the role of Na3PO4 in inhibiting the oxidation process is not physically-based, and the salt Na3PO4 may act as a negative catalyst by either increasing the activation energies for oxidation reactions or modifying the reaction steps. This should also explain the fact that an addition of 6 wt.% of this compound lead to a significant reduction in the percentage of mass increase, as observed by TGA measurements for coal oxidation (refer to Table 2). Figure 2 illustrates the TG curves for coal samples with addition to 6 wt.% Na3PO4 and NaCl. It can be seen that unlike what happens to the
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Fig. 2. TG curves for the samples containing different types of additives and heated in air.
sample containing Na3PO4, the addition of NaCl to a coal sample does not have an obvious impact on the oxidation process, which is in disagreement with the results reported by other workers. In literature [2,5], NaCl has often been considered as an inhibitor for coal oxidation. For example, Watanabe and Zhang [5] found that the addition of NaCl in a coal sample results in a raised critical self-heating temperature evaluated by a self-developed facility. The disparity observed in the role of NaCl may be attributed to the different sample preparation procedures and test methods. It is also possible that different types of coals may not be sensitive to the same inhibitors. Using an adiabatic heating oven, Smith and co-workers [2] found that the addition of Na3PO4 promoted the self-heating of the No. 80 seam coal collected from Wyoming. By heating a raw coal sample and a sample containing 6 wt.% Na3PO4 under nitrogen, the TGA results are collected and plotted in Fig. 3. Similar to the results obtained under oxidative atmosphere, the TG curves both experience a slight decline at the initial stage; however, the
Fig. 3. Comparison of TG data collected for coal samples heated either in nitrogen or in air.
sample mass remains decrease slowly after 150 °C, which is followed by a rapid drop after 320 °C. It can be readily seen that the effect of Na3PO4 on the coal pyrolysis process becomes apparent after 200 °C, especially after 265 °C, with the rate of mass loss being reduced significantly. In fact, at 400 °C the mass of the sample containing 6 wt.% Na3PO4 was 5.14% more than that of the raw coal, which implies a mass loss rate of 50% lower than that of the raw coal. As discussed above, with the temperature rise, the samples undergo dehydration and the release of certain volatile molecules. After 150 °C, the sample mass declines because of progressive decomposition reactions in the inert environment. Different to what happened in the oxidative atmosphere, these decomposition reactions are not accompanied by the pyrolysis of unstable oxygenated complexes formed during oxygen adsorption, but the relatively unstable substances retained by the raw coal itself [1,11]. At 265 °C, the pyrolysis rate for the sample containing Na3PO4 becomes slower than that for the raw coal, and this phenomenon becomes more obvious with the temperature increasing further. This clearly indicates that the addition of Na3PO4 either inhibits the occurrence of certain thermal decomposition reactions, or promotes the generation of relatively stable compounds. The TG results both for coal oxidation and pyrolysis have demonstrated that at around 265 °C, unstable compounds begin to decompose rapidly, while the presence of Na3PO4 relatively improves the stability of the coal structure. 3.2. Results from FT-IR analyses The samples after being ramped to 265 °C using the TGA instrument, are collected for FTIR characterization. A comparison of FT-IR spectra for the raw coal sample, and those oxidized at 265 °C are shown in Fig. 4. In general,
Fig. 4. FT-IR spectra for a raw coal (a), a sample ramped to 265 °C in air (b); and a sample containing 6 wt.% Na3PO4 ramped to 265 °C (c).
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the most striking modifications are those appeared in the 3000–1300 cm 1 region, with the main absorption peaks as well as the corresponding functional groups being identified as follows. The 3600–3100 cm 1 region corresponds to –OH stretching vibration (m) peak for water, alcohols, phenols, carboxylic acids, etc., among which the lower wave number corresponds to the major association state –OH [12]. The bending vibration (d) peaks of C–H are located at 1450 and 1380 cm 1, it can be seen that the 1450 cm 1 peak area is much larger than that of 1380 cm 1, indicating that the coal contains abundant CH2. The weak absorption peak at 1700 cm 1 or so corresponds to C@O stretching vibration, mainly from COOH, COOR and COR. The interpretation of the 1300–900 cm 1 region, where all single C–O bonds absorb, is complicated due to the superimposition of numerous similar functionalities and mineral matters, e.g. Kaolinite bands emerged between 1040 and 1010 cm 1 [8,13]. As a result of the addition of Na3PO4 to a coal sample, the absorption of P–O bonds is detectable in the region of 1100–1000 cm 1; however, its peak intensity is too weak to make any impact on the major species identified in coal structure. Compared to curve a in Fig. 4, most of the absorption bands change more or less for the oxidized coal. The bands in the region of 3300– 3100 cm 1 attributed to O–H species weaken since water is removed and hydroxyl is decomposed by heating the sample; meanwhile, the intensity of C– O bands corresponding to C–OH also decreases. However, the higher peak at about 3400 cm 1 is still obvious due to the relatively stable phenolic hydroxyl. Since active hydrocarbons are oxidized into aldehydes, ketones, carboxyl, and esters [14–16], the intensity of the peaks for mC–H and dC–H decreases significantly; meanwhile, the intensity of C@O peaks increases and new peaks between 1300 and 1000 cm 1 emerge for C(O)– O–R(H). The peaks in 900–700 cm 1 are related to the substitution of aromatic structures, and the wave number shifts to a higher region when more aromatic hydrogen is replaced. As for the oxidized coal, the intensity of the bands at a higher region decreases, indicating that the number of the substituent groups reduces, which may be a result of the pyrolysis of carboxyl (or the oxygenated products of alkyl) with an evolution of CO2 [1,11,17]. Figure 5 reports the differential FT-IR spectra for a sample containing 6 wt.% Na3PO4 and a raw coal sample after the oxidation at 265 °C. The appearance of all the peaks at about 2900 and 1450 cm 1 corresponding to C–H, 1700 cm 1 to C@O, and 1600 cm 1 to benzene skeleton vibration, indicating that the presence of Na3PO4 improves thermal stability of the coal. The outstanding peak in the region of 1200–1000 cm 1 should be contributed by the species containing
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Fig. 5. FT-IR difference spectra for coal samples with and without addition of Na3PO4 oxidized at 265 °C.
O@C–O, C–OH, and C–O–C bonds. The peak areas for C@O at 1700 cm 1 are too small to contribute to such large peak, while the peak between 3400 and 3100 cm 1 for –OH is also very trivial. Therefore, this outstanding peak is mainly for C–O–C species, and should be assigned to the statured ether bonds because the absorption peak is located in a region below 1200 cm 1. The FT-IR spectra directly point to a slowing down of the oxidation process due to the presence of the inhibitor, which is consistent with the trend demonstrated by the TGA data. Such phenomena are essentially related to the formation of more statured ether bonds in coal during its oxidation towards 265 °C. Differential FT-IR spectra are obtained for the coal samples with and without addition of Na3PO4 heated towards 265 or 400 °C in nitrogen atmosphere. As shown in Fig. 6a, the appearance of the negative absorption peak of –OH and the positive peak of C–O indicates that more –OH
Fig. 6. FT-IR difference spectra for coal samples with and without addition of Na3PO4 heated under nitrogen at temperatures of 265 (a), and 400 °C (b), respectively.
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species are decomposed or participate in the reactions for converting into ethers. These ether bonds correspond to the saturated ones, as observed in the oxidative atomsphere. Positive absorption peaks, including significant ether bands, are presented in the differential spectra for the samples collected at 400 °C (Fig. 6b). This is indicative to the role of Na3PO4 in inhibiting the decomposition of major bonds in raw coal (i.e. C–H, C@C and –OH) and in promoting the formation of ether bonds. It is convincing that, it was the formation of ether bonds in the coal sample containing Na3PO4 that improves its thermal stability during heating in nitrogen. 3.3. Mechanism of Na3PO4 in slowing down coal oxidation As acknowledged from the FT-IR analysis, a large number of saturated ether bonds are generated for the samples containing the inhibitor during the heating process both in air and nitrogen. Formation of the ether bonds is definitely a key for the inhibitor to retard the oxidation process. The oxygen-containing compounds in coal mainly include alcohols, phenols, aldehydes, carboxyl and esters. The last three ones may release CO or CO2 by decomposition, while the phenols are responsible for the formation of unsaturated ether bonds. This suggests that the saturated ether bonds should primarily come from alcoholic species. Under heating environment, alcoholic species can be converted into ether by dehydration, or react with the carboxylic acids to form esters; however, both of them are accomplished via acid-catalysed nucleophilic substitution reactions. Since Na3PO4 is an alkaline reagent, the formation of ether bonds would mainly rely on the radical-driven reactions. As shown in Fig. 7, the breaking of chemical bonds in alcoholic species might occur at either location a or b, corresponding to two distinct reactions. The presence of Na3PO4 should promote the formation of ether bonds, i.e. the reactions related to the breaking of O–H bonds. With reference to the bond dissociation energies for ethylene glycol [18,19], it is expected that the dissociation energy for bond a (BDE2) is slightly lower than that for bond b (BDE1), implying that bond a is more likely to break under usual situations. However, the comprehensive dissociation energy for bond b could be reduced by the commencement of a transition state in the presence of Na3PO4. Existing studies [20–22] have indicated that the formation of radical ions may significantly decrease the dissociation energies for specific bonds during pyrolysis and oxidation of the coal. It is postulated that Na3PO4 also promotes a radical ion transition state for O–H in the coal structure, and consequently lessens the dissociation energy for this bond. Thus, we have
BDE1 > BDE2 > BDE3 + BDE4, as shown in Fig. 8. The formation of saturated ether bonds not only implies a significant reduction in the number of free radicals, but also specifies an improvement of the thermal stability of coal structure. In coal structure, the ether bond is a relatively stable one. Petersen and co-workers [23] used FT-IR to detect the changes in oxygen-containing functional groups of the coal during heating process, and found that elimination of oxygen-containing groups is generally in the order of quinone, carboxyl, ketones/esters, hydroxyl, and ether. Using a different approach, Kidena et al. [24] confirmed that ether bond is the most stable one among the oxygen-containing groups during pyrolysis. A number of studies have shown that the compounds containing ether linkages are the major end products during low-temperature oxidation of coal [15–17,25,26]. The reaction steps for coal oxidation and pyrolysis are given in Fig. 9. During coal oxidation, free radicals adsorb oxygen molecules to form unstable solid oxygenated complexes; meanwhile, unstable oxygenated complexes begin to decompose, which leads to the release of small gaseous molecules (reactions 2, 7, 11, 12) [1,11,15,17]. After 150 °C, the former reactions dominate and the overall sample mass increases. As the temperature increases further, the rate of oxygen uptake speeds up, which leads to significant increase in the sample mass. With the addition of Na3PO4, reaction 5 is suppressed, which prevents the formation of free radicals for further oxygen adsorption; on the other hand, reaction 6 is promoted, which implies additional consumption of free radicals remained. A combination of these two effects results in a significant decrease in the percentage of mass increase during the TGA measurement. Under nitrogen atmosphere, the release of water and light volatiles also result in a progressive decrease in the sample mass at the initial stage. At a temperature higher than 265 °C, the unstable compounds are decomposed, and gaseous products are released by the free radical chain reactions, which leads to obvious mass loss (reaction 2, 7, 11 and 12) [15,17]. Due to the presence of Na3PO4 reactions 6 and 8 are enhanced. This prevents further mass loss, since the breaking of –OH bonds from hydroxyl species results in a release of H2O (refer to reactions 5 and 7). In addition, the formation of ether bonds also reduces the number of free radicals, and consequently suppresses reaction 2 during coal pyrolysis. As relatively stable species, the ethers formed may improve thermal stability of the coal. Therefore, the mass loss at 400 °C has been significantly prohibited due to the addition of Na3PO4, compared to that for the raw coal sample, as shown in Fig. 3.
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a b
Fig. 7. Possible reactions related to the decomposition of hydroxyl species.
Fig. 8. Role of Na3PO4 in affecting the breakdown of bonds in hydroxyl.
Fig. 9. An illustration of reaction steps proceeded during coal oxidation and pyrolysis.
4. Concluding remarks The effect of Na3PO4 on coal oxidation can be assessed using a TGA instrument in conjunction with a parameter defined as the percentage of mass
increase at 265 °C. The TGA measurements for various coal samples have shown that the addition of Na3PO4 leads to significant reduction in the percentage of mass increase, with the optimal amount found to be 6 wt.%. The inhibiting effect of NaCl
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is not observable with the current sample preparation procedure and test method. Differential FT-IR spectra for samples oxidized at specific temperatures show that the oxidation of aromatics and alkanes slows down due to the addition of Na3PO4, with more saturated ether bonds being formed. Observations on the solid species on coal surfaces during the pyrolysis measurements also indicate that extra ether bonds are formed in the presence of Na3PO4. The addition of Na3PO4 to the coal mainly influences the routes for the decomposition of hydroxyl by promoting its conversion into ether linkages. The formation of ether bonds not only improves the thermal stability of the coal, but also reduces the number of free radicals in the coal structure, which allow an inhibition of further oxidation and decomposition of coal. References [1] H.-H. Wang, B.Z. Dlugogorski, E.M. Kennedy, Prog. Energy Combust. Sci. 29 (6) (2003) 487–513. [2] A.C. Smith, Y. Miron, C.P. Lazzara, Inhibition of Spontaneous Combustion of Coal, RI 9196, US Bureau of Mines, 1988. [3] X.-L. Dong, D.D. Drysdale, in: Proc. 5th Inter. Symp. on Fire Safety Sci., IAFSS, 1997, pp. 571–580. [4] W. Sujanti, D.-K. Zhang, Combust. Sci. Technol. 152 (1) (2000) 99–114. [5] W.S. Watanabe, D.-K. Zhang, Fuel Process. Technol. 74 (3) (2001) 145–160. [6] S.-Z. Hou, Y. Wang, Coal Technol. 27 (10) (2008) 142–143 (in Chinese). [7] M.J. Iglesias, G. de la Puente, E. Fuente, J.J. Pis, Vib. Spectrosc. 17 (1) (1998) 41–52.
[8] V. Go´mez-Serrano, M.C. Ferna´ndez-Gonza´lez, M.L. Rojas-Cervantes, M.F. Alexandre-Franco, A. Macı´as-Garcı´a, Bull. Mater. Sci. 26 (7) (2003) 721–732. [9] L. Giroux, J.P. Charland, J.A. MacPhee, Energy Fuels 20 (5) (2006) 1988–1996. [10] A.Y. Kam, A.N. Hixson, D.D. Perlmutter, Chem. Eng. Sci. 31 (9) (1976) 821–834. [11] H.-H. Wang, B.Z. Dlugogorski, E.M. Kennedy, Combust. Flame 134 (1–2) (2003) 107–117. [12] C. Chen, J.-S. Gao, Y.-J. Yan, Energy Fuels 12 (3) (1998) 446–449. [13] A.L. Goodman, Energy Fuels 23 (2) (2009) 1101– 1106. [14] J.S. Gethner, Fuel 64 (10) (1985) 1443–1446. [15] A.H. Clemens, T.W. Matheson, D.E. Rogers, Fuel 70 (2) (1991) 215–221. [16] B. Gong, P.J. Pigram, R.N. Lamb, Fuel 77 (9/10) (1998) 1081–1087. [17] V. Calemma, R. Rausa, R. Margarit, E. Girardi, Fuel 67 (6) (1988) 764–770. [18] Y.-R. Luo, Handbook of Bond Dissociation Energies in Organic Compounds, CRC Press, Washington, DC, 2003, pp. 171 and 213. [19] J. Wang, Y.-Y. Li, T.-C. Zhang, et al., Astrophys. J. 676 (1) (2008) 416–419. [20] D.M. Camaioni, J. Am. Chem. Soc. 112 (26) (1990) 9475–9483. [21] M. Farcasiu, C. Smith, Energy Fuels 5 (1) (1991) 83–87. [22] J.W. Larsen, T.P. Eskay, Energy Fuels 10 (4) (1996) 964–969. [23] H.I. Petersen, P. Rosenberg, H.P. Nytoft, Int. J. Coal Geol. 74 (2) (2008) 93–113. [24] K. Kidena, R. Adachi, S. Murata, M. Nomura, Fuel 87 (3) (2008) 388–394. [25] R. Liotta, G. Brons, J. Isaacs, Fuel 62 (1983) 781– 791. [26] D.L. Perry, A. Grint, Fuel 62 (1983) 1024–1033.
Proceedings of the
Proceedings of the Combustion Institute 33 (2011) 2515–2522
Combustion Institute www.elsevier.com/locate/proci
Role of an additive in retarding coal oxidation at moderate temperatures Jing Zhan, Hai-Hui Wang *, Sheng-Nan Song, Yuan Hu, Jiao Li State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China Available online 7 August 2010
Abstract The impact of an additive on coal oxidation process is studied from the mechanistic perspective, aiming at development of a guideline in search of effective inhibitors for controlling the coal self-heating phenomena. The salt Na3PO4 was chosen as an additive. Behaviors of samples with/without the additive were examined at temperatures up to 400 °C both in oxidative and inert atmosphere using a TGA instrument, and the compounds on coal surface during oxidation and pyrolysis were monitored by FT-IR technique. The TGA data show that the impact of the additive on coal oxidation process can be directly evaluated using a parameter defined as the percentage of mass increase at 265 °C, and the addition of Na3PO4 slows down the rates of oxygen uptake and decomposition reactions. FT-IR results also indicate that the additive suppresses both the coal oxidation and pyrolysis processes essentially by accelerating the formation of saturated ether linkages. Further analysis suggests that Na3PO4 plays a role in modifying the routes for decomposition of hydroxyl, and subsequently improving the coal thermal stability. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Coal; Oxidation at moderate temperatures; Retarding additives; Control of self-heating phenomena
1. Introduction Self-heating and spontaneous combustion of coal are quite common to occur during its exploitation, storage and transportation. This phenomenon is directly due to the nature of the coal as an active organic material. Once a coal is exposed to air, it absorbs oxygen both physically and chemically, which triggers a series of exothermic reactions taking place at the internal surface of coal pores [1]. The release of heat increases temperature of the coal itself and induces the reactions owning higher activation energies; the
*
Corresponding author. Fax: +86 (0) 551 360 1669. E-mail address: [email protected] (H.-H. Wang).
acceleration of oxidation reactions may eventually lead to spontaneous combustion of the coal. An inhibitor can suppress the coal oxidation processes, and hence has the application in preventing the self-heating and spontaneous combustion of coal. The role of an inhibitor in slowing down the coal oxidation process can be classified into two folds, with one of them being direct isolation of the active centers on coal surfaces from oxygen. This is usually achieved by jetting mud, water glass and polymer emulsion to cover the coal surface, for instance. Their role is physically-based, which implies low efficiency in controlling the coal oxidation process. Chemicals have also been used in retarding coal oxidation process by either decreasing the formation of active groups or restraining the free radical reactions. Such chemicals include inorganic salts,
1540-7489/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2010.06.046
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antiaging agents, and antioxidant [2–5]. Due to the potential application of such chemically-based inhibitors, development of chemical suppressants has been a focus for scientists working in the fire safety in coal mining and utilization sectors. Alkali compounds were recognized as inhibitors by early researchers. Smith et al. [2] reported that NaNO3, NaCl and CaCO3 could suppress the coal oxidation process, while NaCOOH and some other substances had the reversed effect. By testing the ‘critical ambient temperatures’ of a coal mixed with various inorganic salts, Zhang’s research group [4,5] examined the impact of additives on the coal oxidation process. Their results suggested that CaCl2, Ca(Ac)2, Mg(Ac)2, MgCO3, NaCl and NaOH might inhibit the oxidation process. So far, criteria have been developed for evaluating the effect of inorganic compounds on coal oxidation and the associated self-heating phenomena, which are based upon the observation of heat evolution mass change during coal oxidation and the determination of apparent activation energies for oxygen uptake by coals [2–6]. However, no rigorous studies have been done in regard to how an additive plays a role in affecting the series of reactions occurred during coal oxidation and consequently slowing down the overall rate of coal oxidation. This paper is aiming at developing a routine technique for examining the impact of additives on coal oxidation at moderate temperatures from the chemistry perspective. By designing experimental procedures and conducting experiments using modern analytical instruments, the behaviors of oxidation of a coal are studied in the presence of various amount of additives. Mechanism of the inhibitors (additives) in slowing down the oxidation reactions is then explored. 2. Experimental A bituminous coal was collected from the Gunzhou Colliery, Shandong Province. The coal was crushed and ground into particles. Size-distribution of the particles was measured by Laser particle-size-distribution tester JL-1177, and the volumetric mean diameter of the particles was found to be about 1.61 lm. Proximate and ultimate analyses of the coal are given in Table 1. Na3PO412H2O at a purity of 99% was selected as an additive for the experiments, which was purchased from a local medical station. Sodium chloride at a purity of 99.5%, supplied by Sinopharm Chemical Reagent Co., was also used as an additive in the current study. Thermal stability of these compounds was examined at temperatures up to 400 °C using TGA technique both in air and nitrogen atmosphere. Prior to use, an additive was first dried at 80 °C for 4 h in order to eliminate its crystalline water. The product was then
ground and blended with the raw coal particles in a mortar to make samples containing 0, 3, 6 and 9 wt.% Na3PO4, accordingly. Samples containing 6 wt.% NaCl were also prepared for measurements. All the samples were kept in tightlysealed plastic bags and stored in a deep freezer prior to measurements. Thermo-gravimetric analysis (TGA) was used to evaluate the oxidation and pyrolysis behaviors of raw coals and those mixed with the additives. Measurements were performed using a TA Q5000IR thermal analysis system. A sample of 10 mg in mass was quickly and loosely placed on the aluminum crucible. The sample temperature was raised from room temperature to 400 °C at a heating rate of 4 °C/min. The reason to choose such a low heating rate was to reduce the thermal lag of a sample during its heating. Industrial grade air and nitrogen gas bottles were used for conducting oxidation or pyrolysis experiments. The flow rate of atmosphere purging into the furnace was set at 50 ml/min. The experimental uncertainties for mass change are below 0.01 mg. Samples for FT-IR analysis were prepared by heating coal particles to a temperature of 265 or 400 °C in a set atmosphere using the TGA instrument. A sample collected was then mixed with KBr at a mass ratio of about 5:100, and finely ground in an agate mortar for 5 min or so. The KBr pellet containing coal particles was then made by pressing the mixture at a pressure of 20 MPa for 1 min. Absorbance spectra were picked up by the Nicolet 6700 FT-IR spectrometer with a nominal spectral resolution of 4 cm 1 and a signal-to-noise ratio of 50,000:1. Spectra ranged from 4000 to 400 cm 1, and the scanning time for the background and sample were both 32 times. All the spectra were baseline corrected. The SiAO bond is relatively stable within the experimental temperature range [7–9], and its bending vibration peak appears at 472 cm 1; therefore, this peak for the raw coal was chosen as a benchmark for calibrating all the spectra. Differential spectra were generated using the Data General processing system. 3. Results and discussion 3.1. TGA measurements Typical TGA data obtained from the aerial oxidation measurements for coal samples containing various weight percent of Na3PO4 are shown in Fig. 1. As observed from the curves, with the temperature increasing, a sample firstly undergoes a slight decline in its mass, which is followed by a progressive increase. The sample mass reaches the maximum at 265 °C with a magnitude exceeding 100% of its initial mass. As the temperature increases further, the curve begins to decline
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Table 1 Proximate and ultimate analyses for the coal used in the current study.
Table 2 Percentage of mass increase at 265 °C for various samples.
Proximate analysis (air dried basis) (%)
Content of Na3PO4 (wt.%)
Percentage of mass increase (%)
Relative reduction (%)
0 3 6 9
3.62 3.30 2.80 2.83
0 8.8 22.7 21.8
Moisture Ash Volatile matter Fixed carbon Total sulfur
2.8 13.1 32.5 51.1 0.5
Ultimate analysis (dry ash free basis) (%) C H N S O
83.5 5.0 1.2 0.6 9.7
Calorific value of the coal (air dried basis): 4533 kcal/kg.
Fig. 1. Results of the TGA measurements with coal samples containing various amount of Na3PO4 and oxidized in air.
rapidly, and the weight percent of all the samples maintains about 80% of their initial mass at 400 °C. Difference among the TG curves for the samples containing various percentages of additives in weight is mainly reflected by the mass increase occurred at the temperatures between 150 and 300 °C. It is seen that the peak for mass increase gets flat due to an introduction of the additive. We define a parameter to evaluate the extent of mass increase, i.e. the percentage of mass increase, which is determined by a takeaway of the maximum mass between 150 and 300 °C to the minimum mass from room temperature to 150 °C with the outcome divided by the initial sample mass. The results determined for various types of samples are reported in Table 2. It is evident that, the percentage of mass increase reduces due to the incorporation of Na3PO4 in samples, while an amount of 6 wt.% Na3PO4 makes the mass increase percentage reduce from 3.62% to 2.80%, with a decrease extent by 22.7%, compared to that for the raw coal. However, while the amount of Na3PO4 increases to 9 wt.%, the percentage of mass increase tends to stop reducing further. Small magnitudes determined for the percentage
of mass increase are also true reflection on the slow process of coal oxidation at temperatures below 300 °C. It is shown that for the coal sample undergoing oxidation and pyrolysis in a dynamic oxidative atmosphere, the mass loss at the initial stage is mainly ascribed to the removal of moisture and light volatiles. At 150 °C or so, the sample starts to exhibit an increase in its mass as a result of oxygen uptake and formation of solid oxygenated complexes. With the temperature increasing further, pyrolysis of unstable oxygenated complexes may become eminent, and a direct interaction of coal with oxygen, i.e. the so-called direct ‘burnoff’ reaction, may also come into action [1,10,11]. However, prior to 265 °C the rates of oxygen adsorption by coal and the formation of unstable solid products are larger than the rates of thermal decomposition and the direct ‘burnoff’ reaction. As a result, the overall sample mass remains increasing. After the temperature exceeds a threshold of 265 °C, the decomposition reactions and the direct ‘burn-off’ reaction become dominated, which lead to a drop in the sample mass. An addition of Na3PO4 results in a reduction in the percentage of mass increase, indicating that the rates of oxygen uptake by coal and formation of unstable solid oxygenated complexes have slowed down. As the temperature increases further, due to significant decomposition of oxygenated complexes and the emergence of direct ‘burn-off’ reaction, the mass changes for various samples are getting close. In addition, with a further increase in the amount of Na3PO4 in a coal sample, the percentage of mass increase does not continue to reduce. This is an evidence to suggest that the role of Na3PO4 in inhibiting the oxidation process is not physically-based, and the salt Na3PO4 may act as a negative catalyst by either increasing the activation energies for oxidation reactions or modifying the reaction steps. This should also explain the fact that an addition of 6 wt.% of this compound lead to a significant reduction in the percentage of mass increase, as observed by TGA measurements for coal oxidation (refer to Table 2). Figure 2 illustrates the TG curves for coal samples with addition to 6 wt.% Na3PO4 and NaCl. It can be seen that unlike what happens to the
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Fig. 2. TG curves for the samples containing different types of additives and heated in air.
sample containing Na3PO4, the addition of NaCl to a coal sample does not have an obvious impact on the oxidation process, which is in disagreement with the results reported by other workers. In literature [2,5], NaCl has often been considered as an inhibitor for coal oxidation. For example, Watanabe and Zhang [5] found that the addition of NaCl in a coal sample results in a raised critical self-heating temperature evaluated by a self-developed facility. The disparity observed in the role of NaCl may be attributed to the different sample preparation procedures and test methods. It is also possible that different types of coals may not be sensitive to the same inhibitors. Using an adiabatic heating oven, Smith and co-workers [2] found that the addition of Na3PO4 promoted the self-heating of the No. 80 seam coal collected from Wyoming. By heating a raw coal sample and a sample containing 6 wt.% Na3PO4 under nitrogen, the TGA results are collected and plotted in Fig. 3. Similar to the results obtained under oxidative atmosphere, the TG curves both experience a slight decline at the initial stage; however, the
Fig. 3. Comparison of TG data collected for coal samples heated either in nitrogen or in air.
sample mass remains decrease slowly after 150 °C, which is followed by a rapid drop after 320 °C. It can be readily seen that the effect of Na3PO4 on the coal pyrolysis process becomes apparent after 200 °C, especially after 265 °C, with the rate of mass loss being reduced significantly. In fact, at 400 °C the mass of the sample containing 6 wt.% Na3PO4 was 5.14% more than that of the raw coal, which implies a mass loss rate of 50% lower than that of the raw coal. As discussed above, with the temperature rise, the samples undergo dehydration and the release of certain volatile molecules. After 150 °C, the sample mass declines because of progressive decomposition reactions in the inert environment. Different to what happened in the oxidative atmosphere, these decomposition reactions are not accompanied by the pyrolysis of unstable oxygenated complexes formed during oxygen adsorption, but the relatively unstable substances retained by the raw coal itself [1,11]. At 265 °C, the pyrolysis rate for the sample containing Na3PO4 becomes slower than that for the raw coal, and this phenomenon becomes more obvious with the temperature increasing further. This clearly indicates that the addition of Na3PO4 either inhibits the occurrence of certain thermal decomposition reactions, or promotes the generation of relatively stable compounds. The TG results both for coal oxidation and pyrolysis have demonstrated that at around 265 °C, unstable compounds begin to decompose rapidly, while the presence of Na3PO4 relatively improves the stability of the coal structure. 3.2. Results from FT-IR analyses The samples after being ramped to 265 °C using the TGA instrument, are collected for FTIR characterization. A comparison of FT-IR spectra for the raw coal sample, and those oxidized at 265 °C are shown in Fig. 4. In general,
Fig. 4. FT-IR spectra for a raw coal (a), a sample ramped to 265 °C in air (b); and a sample containing 6 wt.% Na3PO4 ramped to 265 °C (c).
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the most striking modifications are those appeared in the 3000–1300 cm 1 region, with the main absorption peaks as well as the corresponding functional groups being identified as follows. The 3600–3100 cm 1 region corresponds to –OH stretching vibration (m) peak for water, alcohols, phenols, carboxylic acids, etc., among which the lower wave number corresponds to the major association state –OH [12]. The bending vibration (d) peaks of C–H are located at 1450 and 1380 cm 1, it can be seen that the 1450 cm 1 peak area is much larger than that of 1380 cm 1, indicating that the coal contains abundant CH2. The weak absorption peak at 1700 cm 1 or so corresponds to C@O stretching vibration, mainly from COOH, COOR and COR. The interpretation of the 1300–900 cm 1 region, where all single C–O bonds absorb, is complicated due to the superimposition of numerous similar functionalities and mineral matters, e.g. Kaolinite bands emerged between 1040 and 1010 cm 1 [8,13]. As a result of the addition of Na3PO4 to a coal sample, the absorption of P–O bonds is detectable in the region of 1100–1000 cm 1; however, its peak intensity is too weak to make any impact on the major species identified in coal structure. Compared to curve a in Fig. 4, most of the absorption bands change more or less for the oxidized coal. The bands in the region of 3300– 3100 cm 1 attributed to O–H species weaken since water is removed and hydroxyl is decomposed by heating the sample; meanwhile, the intensity of C– O bands corresponding to C–OH also decreases. However, the higher peak at about 3400 cm 1 is still obvious due to the relatively stable phenolic hydroxyl. Since active hydrocarbons are oxidized into aldehydes, ketones, carboxyl, and esters [14–16], the intensity of the peaks for mC–H and dC–H decreases significantly; meanwhile, the intensity of C@O peaks increases and new peaks between 1300 and 1000 cm 1 emerge for C(O)– O–R(H). The peaks in 900–700 cm 1 are related to the substitution of aromatic structures, and the wave number shifts to a higher region when more aromatic hydrogen is replaced. As for the oxidized coal, the intensity of the bands at a higher region decreases, indicating that the number of the substituent groups reduces, which may be a result of the pyrolysis of carboxyl (or the oxygenated products of alkyl) with an evolution of CO2 [1,11,17]. Figure 5 reports the differential FT-IR spectra for a sample containing 6 wt.% Na3PO4 and a raw coal sample after the oxidation at 265 °C. The appearance of all the peaks at about 2900 and 1450 cm 1 corresponding to C–H, 1700 cm 1 to C@O, and 1600 cm 1 to benzene skeleton vibration, indicating that the presence of Na3PO4 improves thermal stability of the coal. The outstanding peak in the region of 1200–1000 cm 1 should be contributed by the species containing
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Fig. 5. FT-IR difference spectra for coal samples with and without addition of Na3PO4 oxidized at 265 °C.
O@C–O, C–OH, and C–O–C bonds. The peak areas for C@O at 1700 cm 1 are too small to contribute to such large peak, while the peak between 3400 and 3100 cm 1 for –OH is also very trivial. Therefore, this outstanding peak is mainly for C–O–C species, and should be assigned to the statured ether bonds because the absorption peak is located in a region below 1200 cm 1. The FT-IR spectra directly point to a slowing down of the oxidation process due to the presence of the inhibitor, which is consistent with the trend demonstrated by the TGA data. Such phenomena are essentially related to the formation of more statured ether bonds in coal during its oxidation towards 265 °C. Differential FT-IR spectra are obtained for the coal samples with and without addition of Na3PO4 heated towards 265 or 400 °C in nitrogen atmosphere. As shown in Fig. 6a, the appearance of the negative absorption peak of –OH and the positive peak of C–O indicates that more –OH
Fig. 6. FT-IR difference spectra for coal samples with and without addition of Na3PO4 heated under nitrogen at temperatures of 265 (a), and 400 °C (b), respectively.
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species are decomposed or participate in the reactions for converting into ethers. These ether bonds correspond to the saturated ones, as observed in the oxidative atomsphere. Positive absorption peaks, including significant ether bands, are presented in the differential spectra for the samples collected at 400 °C (Fig. 6b). This is indicative to the role of Na3PO4 in inhibiting the decomposition of major bonds in raw coal (i.e. C–H, C@C and –OH) and in promoting the formation of ether bonds. It is convincing that, it was the formation of ether bonds in the coal sample containing Na3PO4 that improves its thermal stability during heating in nitrogen. 3.3. Mechanism of Na3PO4 in slowing down coal oxidation As acknowledged from the FT-IR analysis, a large number of saturated ether bonds are generated for the samples containing the inhibitor during the heating process both in air and nitrogen. Formation of the ether bonds is definitely a key for the inhibitor to retard the oxidation process. The oxygen-containing compounds in coal mainly include alcohols, phenols, aldehydes, carboxyl and esters. The last three ones may release CO or CO2 by decomposition, while the phenols are responsible for the formation of unsaturated ether bonds. This suggests that the saturated ether bonds should primarily come from alcoholic species. Under heating environment, alcoholic species can be converted into ether by dehydration, or react with the carboxylic acids to form esters; however, both of them are accomplished via acid-catalysed nucleophilic substitution reactions. Since Na3PO4 is an alkaline reagent, the formation of ether bonds would mainly rely on the radical-driven reactions. As shown in Fig. 7, the breaking of chemical bonds in alcoholic species might occur at either location a or b, corresponding to two distinct reactions. The presence of Na3PO4 should promote the formation of ether bonds, i.e. the reactions related to the breaking of O–H bonds. With reference to the bond dissociation energies for ethylene glycol [18,19], it is expected that the dissociation energy for bond a (BDE2) is slightly lower than that for bond b (BDE1), implying that bond a is more likely to break under usual situations. However, the comprehensive dissociation energy for bond b could be reduced by the commencement of a transition state in the presence of Na3PO4. Existing studies [20–22] have indicated that the formation of radical ions may significantly decrease the dissociation energies for specific bonds during pyrolysis and oxidation of the coal. It is postulated that Na3PO4 also promotes a radical ion transition state for O–H in the coal structure, and consequently lessens the dissociation energy for this bond. Thus, we have
BDE1 > BDE2 > BDE3 + BDE4, as shown in Fig. 8. The formation of saturated ether bonds not only implies a significant reduction in the number of free radicals, but also specifies an improvement of the thermal stability of coal structure. In coal structure, the ether bond is a relatively stable one. Petersen and co-workers [23] used FT-IR to detect the changes in oxygen-containing functional groups of the coal during heating process, and found that elimination of oxygen-containing groups is generally in the order of quinone, carboxyl, ketones/esters, hydroxyl, and ether. Using a different approach, Kidena et al. [24] confirmed that ether bond is the most stable one among the oxygen-containing groups during pyrolysis. A number of studies have shown that the compounds containing ether linkages are the major end products during low-temperature oxidation of coal [15–17,25,26]. The reaction steps for coal oxidation and pyrolysis are given in Fig. 9. During coal oxidation, free radicals adsorb oxygen molecules to form unstable solid oxygenated complexes; meanwhile, unstable oxygenated complexes begin to decompose, which leads to the release of small gaseous molecules (reactions 2, 7, 11, 12) [1,11,15,17]. After 150 °C, the former reactions dominate and the overall sample mass increases. As the temperature increases further, the rate of oxygen uptake speeds up, which leads to significant increase in the sample mass. With the addition of Na3PO4, reaction 5 is suppressed, which prevents the formation of free radicals for further oxygen adsorption; on the other hand, reaction 6 is promoted, which implies additional consumption of free radicals remained. A combination of these two effects results in a significant decrease in the percentage of mass increase during the TGA measurement. Under nitrogen atmosphere, the release of water and light volatiles also result in a progressive decrease in the sample mass at the initial stage. At a temperature higher than 265 °C, the unstable compounds are decomposed, and gaseous products are released by the free radical chain reactions, which leads to obvious mass loss (reaction 2, 7, 11 and 12) [15,17]. Due to the presence of Na3PO4 reactions 6 and 8 are enhanced. This prevents further mass loss, since the breaking of –OH bonds from hydroxyl species results in a release of H2O (refer to reactions 5 and 7). In addition, the formation of ether bonds also reduces the number of free radicals, and consequently suppresses reaction 2 during coal pyrolysis. As relatively stable species, the ethers formed may improve thermal stability of the coal. Therefore, the mass loss at 400 °C has been significantly prohibited due to the addition of Na3PO4, compared to that for the raw coal sample, as shown in Fig. 3.
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a b
Fig. 7. Possible reactions related to the decomposition of hydroxyl species.
Fig. 8. Role of Na3PO4 in affecting the breakdown of bonds in hydroxyl.
Fig. 9. An illustration of reaction steps proceeded during coal oxidation and pyrolysis.
4. Concluding remarks The effect of Na3PO4 on coal oxidation can be assessed using a TGA instrument in conjunction with a parameter defined as the percentage of mass
increase at 265 °C. The TGA measurements for various coal samples have shown that the addition of Na3PO4 leads to significant reduction in the percentage of mass increase, with the optimal amount found to be 6 wt.%. The inhibiting effect of NaCl
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is not observable with the current sample preparation procedure and test method. Differential FT-IR spectra for samples oxidized at specific temperatures show that the oxidation of aromatics and alkanes slows down due to the addition of Na3PO4, with more saturated ether bonds being formed. Observations on the solid species on coal surfaces during the pyrolysis measurements also indicate that extra ether bonds are formed in the presence of Na3PO4. The addition of Na3PO4 to the coal mainly influences the routes for the decomposition of hydroxyl by promoting its conversion into ether linkages. The formation of ether bonds not only improves the thermal stability of the coal, but also reduces the number of free radicals in the coal structure, which allow an inhibition of further oxidation and decomposition of coal. References [1] H.-H. Wang, B.Z. Dlugogorski, E.M. Kennedy, Prog. Energy Combust. Sci. 29 (6) (2003) 487–513. [2] A.C. Smith, Y. Miron, C.P. Lazzara, Inhibition of Spontaneous Combustion of Coal, RI 9196, US Bureau of Mines, 1988. [3] X.-L. Dong, D.D. Drysdale, in: Proc. 5th Inter. Symp. on Fire Safety Sci., IAFSS, 1997, pp. 571–580. [4] W. Sujanti, D.-K. Zhang, Combust. Sci. Technol. 152 (1) (2000) 99–114. [5] W.S. Watanabe, D.-K. Zhang, Fuel Process. Technol. 74 (3) (2001) 145–160. [6] S.-Z. Hou, Y. Wang, Coal Technol. 27 (10) (2008) 142–143 (in Chinese). [7] M.J. Iglesias, G. de la Puente, E. Fuente, J.J. Pis, Vib. Spectrosc. 17 (1) (1998) 41–52.
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