Energy Fuels 2010, 24, 6366–6374 Published on Web 11/08/2010
: DOI:10.1021/ef100897m
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Modes of Formation of Carbon Oxides (COx (x = 1,2)) From Coals During Atmospheric Storage: Part I Effect of Coal Rank Zeev Aizenstat,† Uri Green,*,†,‡ Sven Stark,‡, Christoph Weidner,‡, and Haim Cohen*,‡,§
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† Chemistry Institute, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel, ‡Department of Biological Chemistry, Ariel University Center at Samaria, Ariel, Israel, §Chemistry Department, Ben-Gurion University of the Negev, Beer Sheva, ur Energieverfahrenstechnik und Chemieingenieurwesen, Freiberg, Germany Israel, and TU Bergakademie Freiberg, Institut f€
Received July 14, 2010. Revised Manuscript Received September 7, 2010
Emissions of carbon oxides, particularly carbon dioxide, from fossil fuels have an enormous environmental and economical impact on coal (under atmospheric storage) utilization as a fuel for utility plants. Therefore, a deeper understanding of the processes controlling their formation is crucial. This work investigates the effect of coal rank on the carbon oxides emissions. It offers a new perspective on the subject as it encompasses three classes of coals (bituminous, sub-bituminous, and lignite) and can shed light on the specific coal parameters that effect the formation of carbon oxides at low temperatures. It is suggested that the main product carbon dioxide is not only a direct product of the consumed oxygen but that its formation is also dependent on the pre-existing O content in the coal macromolecule. The effect of coal rank on the formation of carbon oxides during low temperatures at the coals surface is discussed in detail.
the pile might occur which in extreme cases results in fire eruptions.3 The major product that is released from coals undergoing low temperature aerial oxidation processes is carbon dioxide. The emission of carbon dioxide is an exothermic process which is dependent on the ambient temperature and the oxygen concentration in the vicinity of the coal macromolecule. Carbon monoxide is also known to evolve at these low temperatures (room temperature (RT) - 150 °C). However, as it is not the main oxidation product, the amounts are smaller (typically, 1 order of magnitude smaller than that of carbon dioxide). The formation of these carbon oxides is suggested to be the result of low-temperature aerial oxidation (LTO). As mentioned above, the aerial oxidation of coal is composed of several series of complex gas-solid reactions.4 Some studies have suggested that the oxidation process involves radicals as intermediates, although no direct proof or determination of these intermediates has been reported (only electron spin resonance (ESR) spectroscopy indicates the occurrence of carbon based radicals during the oxidation processes). The group of Liotta has suggested that the attack of atmospheric oxygen on the coal and the formation of oxycoal occurs via the following scheme of radical reactions (Scheme 1).5 This chain radical mechanism for the coal oxidation is widely accepted even at temperatures as low as 30 °C. Additionally, Beier6
1. Introduction Large coal piles undergo the weathering process during storage prior to combustion in utility plants.1 The process is relatively fast and is dependent on the rank of the coal. The geographical location of the mine also proves an important factor as the composition of the coal can differ from site to site.2 As a result, lignite coals undergo deterioration due to weathering processes in a matter of days while it can take bituminous coals several weeks to reach the same state.1 This multistage mechanism is quite complicated and even today it is not fully understood. The initial stages involve physical adsorption and chemisorption of atmospheric oxygen. The second stage is the formation of surface oxides and hydroperoxides which can partially decompose to yield low molecular weight inorganic gases like carbon oxides (CO, CO2), water, hydrogen (H2), and some organic gases (C1-5).1,11,12 If the heat formation (exothermic processes) is greater than the heat dissipation, self-heating of *To whom correspondence should be addressed. E-mail: urigr@ ariel.ac.il (U.G.) or
[email protected] (H.C.). (1) (a) Grossman, S. L. Low Temperature Atmospheric Oxidation of Coal. Ph.D. Thesis, Chemistry Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel, 1994. (b) Grossman, S. L.; Davidi, S.; Cohen, H. Fuel 1991, 70, 897. (c) Grossman, S. L.; Wegener, I.; Wanzl, W.; Davidi, S.; Cohen, H. Fuel 1994, 73, 762. (d) Kornetzky, S. Investigation of Enviromentally and Safely Releveant Emissions of Gases during Storage and Handling of Solid Fuels. M.Sc. Thesis, TU Bergakademie Freiberg, Freiberg, Germany, 1997. (2) (a) Stach, E.; Mackowsky, M. Th.; Teichmuller, M.; Taylor, G. H.; Chandra, D.;Teichmuller, R. Textbook of Coal Petrology, 3rd ed.; Borntraeger: Berlin, Germany, 1982.(b) Wu, M. M.; Robbins, G. A.; Winschel, R. A.; Burke, F. P. Low Temperature Coal Weathering: its chemical nature and effects on coal properties. Energy Fuels 1988, 2, 150–157. (3) (a) Kuenzer, C.; Zhang, J.; Tetzlaff, A.; van Dijk, P.; Voight, S.; Mehl, H.; Wagner, W. Uncontrolled coal fires and their enviromental impacts: investigating two arid mining regions in north-central China. Appl. Geogr. 2007, 27, 42–62. (b) Stracher, G. B.; Taylor, T. P. Coal fires burning out of control around the world: thermodynamic recipe for enviromental catastrophe. Int. J. Coal Geol. 2004, 59, 7–17. r 2010 American Chemical Society
(4) (a) Nelson, C. R. Chemistry of Coal Weathering; Nelson, C. R., Ed.; Elsevier: Amsterdam, The Netherlands, 1989; Chapter 1, pp 1-32. (b) Van Krevelen, D. W. Coal: Typology, Chemistry, Physics, Constitution, 3rd ed.; Elsevier: Amsterdam, The Netherlands, 1993; pp 627-658. (c) Berkowitz, N. The Chemistry of Coal; Elsevier: Amsterdam, The Netherlands, 1985; pp 143-151. (5) Liotta, R.; Brons, G.; Isaacs, J. Oxidative weathering of Illinois Nu.6 Coal. Fuel 1983, 62, 781. (6) Beier, E. Gasaustausch von Stienkohlen und anderen Stoffen bei jahrzentelanger lagerung an Luft. Erdol und Kohle 1985, 38, 127. Presented in the Proceedings of 1991 International Conference on Coal Science, Newcastle, 1991; p 223.
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Energy Fuels 2010, 24, 6366–6374
: DOI:10.1021/ef100897m
Aizenstat et al. Table 1. Properties of Coalsa
Scheme 1. Mechanism of the Coal Oxidation at Low Temperatures As Proposed by Liotta et al.5
proximate analysis (wt %) type
b
SA IN HA LUS
ultimate analysis (wt %, db)
moisture
ashwf
VMdb
C
H
O
S
CV(J g-1)
1.20 2.61 34.53 10.57
13.77 10.65 5.09 4.16
28.27 36.38 52.39 52.78
74.19 73.25 66.12 63.24
4.10 4.63 4.32 4.50
5.59 9.05 23.65 27.24
0.46 0.67 0.16 0.20
28 416 28 564 25 323 24 516
a VM = volatile matter; CV = calorific value; db = dry basis; wf = water free. b SA = South Africa; IN = Indonesia; HA = Hambach, Germany; LUS = Lusatia, Germany.
suggested that the decomposition of the hydroperoxides can yield carbonyls and water.
effects of carbon oxide readsorption on the coal surface at low temperatures were determined in order to obtain a more accurate estimation of the amounts of carbon oxides formed at low-temperature oxidation. 2. Experimental Section All chemicals and gases used throughout the study were of AR grade and supplied by Aldrich, Fluka, Merck, or Maxima. The water used throughout this study was purified water (via ion exchange columns). 2.1. Coal. Experiments in this work were carried out with three classes of coals: bituminous, sub-bituminous, and lignite. The bituminous coal was from South Africa and the subbituminous coal from Indonesia. The lignite coals were of German origin. The South African bituminous coal used in this work serves as the major fossil fuel in coal fired power plants in Israel (more than 60% of the coal consumption). However the sub-bituminous Indonesian coal is also fired in the Israeli utilities. The properties of all the coals are presented in Table 1. The experiments were performed in sealed glass vials (40 mL) used as batch reactors. The reactors were charged with coal (particle size 74 μm e X e 250 μm) in an air atmosphere and heated at 55-95 °C for various periods in a n€ uve oven model FT 300. The effect the oxygen concentration has on the processes was also studied under an atmosphere of pure argon. Atmosphere substitution was accomplished via a glass vacuum system monitored with a Thyracont pressure gauge model VD83M. As all the coals in the present study were supplied by the power companies in Israel and Germany (with the intention that the results can have practical ramifications for Israeli coal usage), the sample age is up to 6 months (from the moment they were mined). After the samples reached the lab, they were dried under N2 in a Heraeus vacuum oven model VT6060 for 24 h at 60 °C and then crushed in a standard ball bearing grinder. Sieving to the different grain sizes was carried out with standard mesh sifters in an electric column shaker. This procedure was conducted in a chilled room in stages to avoid weathering of the coal as much as possible during preparation. Thereafter, samples were sealed in tinted glass canisters and stored at room temperature under N2. In order to verify that this storage method is sufficient, reference samples were stored at -5 °C and then subjugated to the atmospheric simulation experiments where results were identical to the room temperature samples within the experimental error . 2.2. Gas Chromatography (GC). The amount of the gases (CO2, CO, N2, O2, hydrocarbons) in the reactors was determined using a gas chromatograph (Varian model 3800) equipped with a thermal conductivity detector and a flame ionization detector connected in series. The gases were separated on a carbosieve B 1/8 in., 9 ft stainless steel column using a temperature programmed mode. The experimental error in the GC determination is (5%. The gaseous atmosphere was sampled (1 mL samples) after the reaction, with gastight syringes (Precision Syringes, model A2) and the composition was determined in the gas chromatograph. The gases that could be determined are hydrogen, nitrogen, oxygen, carbon dioxide, carbon monoxide, methane, and ethylene. The argon gas present is not separated from oxygen in the GC
It is also accepted,7 that the radical processes mentioned above result in the formation of unstable surface oxides. These unstable surface oxides subsequently decompose according to the following reactions: surface oxides f CO2 , H2 O surface oxides f CO, Cx Hy , H2
ðmain reactionÞ ðside reactionÞ
ð11Þ ð12Þ
However, the mechanisms detailed above tie the formation of carbon oxides only to a gas/surface reaction of the impinging oxygen molecule with the coal macromolecule surface. As the coal macromolecule contains also oxygen atoms, either as a part of the backbone of the structure (e.g., ether groups, carbonyls, or carboxylic groups) or as chemisorbed water trapped in the pore structure, decomposition of these groups might also contribute to carbon oxides formation. Indeed, our studies1d have corroborated this option. For example: the amount of CO2 emission from lignite coal as a result of low temperature oxidation exceeds the amount of O2 which is consumed by the coal. Thus, it is suggested that the carbon oxides which are emitted from the coal are not only due to the oxidation reaction by atmospheric oxygen but also stem from the thermal decomposition of the inherent oxygen content within the coal macromolecular structure. Presumably, moisture in the coal could be also the precursor to oxygen in the carbon oxides formed. However, this is unlikely as at this relatively low temperature region (55-95 °C) it is not reasonable that oxygen would transfer from water to yield CO or CdO bonds in the coal. It is of interest to investigate and to try to determine the different modes by which carbon oxides are released during the low temperature oxidation/weathering processes. Furthermore, as LTO is dependent on coal rank, it is also very interesting to study the effect of coal rank on these reactions. We have decided to study three types of coals: bituminous, sub-bituminous and lignite; each containing an increasing amount of inherent O. For this purpose, several coals which are fired either in the Israeli utility plants (bituminous and sub-bituminous coals) or in German power plants (lignite coals) have been chosen. Researching a range of coal classes allows a broader comparison of the mechanisms behind low temperature oxidation processes. In addition, the (7) Wang, H.; Dulgogorski, B. Z.; Kennedy, E. M. Thermal decomposition of Solid oxygenated complexes formed by coal oxidation at low temperatures. Fuel 2002, 81, 1913–1923.
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Table 2. Carbon Dioxide Adsorption in Coalsa coal sample
gas analysis
goal type
coal (g)
CO2 (vol %)
% CO2 correctionb
SA
0 0.5 1 2 5 0 0.5 1 2 5
2 1.8 1.8 1.4 1.3 10 8.2 7.1 4.9 3.2
0 0 16 39 142 0 27 46 82 199
IN
It is important to note that the adsorption experiments for all coals were carried out with several carbon dioxide concentration levels, and it was found that the amount of carbon dioxide that is adsorbed by the coal is dependent on the partial pressure of the gas within the measured medium. The effect of CO2 adsorption by the coal is demonstrated in Figure 1. As can be clearly seen from Figure 1, there is an appreciable deviation from the real value due to adsorption of carbon dioxide by the coal itself and the deviation from the real value increases upon increasing the amount of the coal in the reactor. However, upon compensation of the amounts of adsorbed CO2, there is a linear correlation between the CO2 concentration and the GC reading. This means that when studying LTO of bituminous or sub-bituminous coals one should correct the values of CO2 emissions to the amounts absorbed by the coal, otherwise a large deviation from the real value will be recorded. Similar experiments conducted with the low rank coals (lignite, HA and LUS) have shown that there is no appreciable absorption of CO2 by the lignite coals. In these experiments, the vials were charged with 5% (vol %) carbon dioxide thereafter analyzed. The conclusion is, of course, that adsorption of CO2 does not play a role during low-temperature oxidation of low rank coals. Absorption of gases by lignites has been the subject of several studies;18 however, the experiments were carried out at a high pressure of CO2, within the framework of CO2 sequestration. 3.2. Possible Carbon Monoxide Adsorption. Carbon monoxide is not expected to be adsorbed by the coals. This is in line with work reported by Brown et al.8 where no noticeable absorption of carbon monoxide was observed. This stems from the fact that the boiling temperature of CO2 is much higher than that of CO (216 K vs 68 K, respectively9); thus, the CO2 interaction with the coal macromolecule is much stronger. We have checked the adsorptivity of CO at 25 °C (same as in the CO2 experiments) within the four coals and have found that indeed as reported8 there is no adsorption of CO. In line with our results above, in all experiments (see below) of LTO of the bituminous and sub-bituminous coals, the results of the CO2 emissions have been corrected for its adsorption by the coal. 3.3. Temperature Dependence. It is well-known4b,17 that ambient temperature has a profound influence on the atmospheric oxidation processes affecting coal. As such, this effect expresses itself directly as a function of the rate of oxidation. As the ambient temperature increases, so too do
a Carried out at 25 °C. b The % CO2 correction is the % increase of the measured amount of CO2 in order to obtain the actual amount produced
column, thus the value determined for oxygen includes ∼0.93% argon gas. As the reactions studied are gas/surface reactions, the reproducibility of the results is not good. Therefore, each experiment has been carried out with duplicates in order to reduce the total experimental error. However, the error is (15%, mainly due to the nature of the heterogeneous reactions studied in the experiments.
3. Results and Discussion 3.1. Carbon Dioxide Adsorption. Because of the chemical nature of carbon dioxide and the natural adsorption properties of coal, appreciable amounts of the CO2 that is formed at the surface is adsorbed within the coal pore structure. Naturally, the adsorption capacity of each coal is different and depends on its unique chemical properties. The adsorption characteristics of the coals are dependent on several parameters such as temperature, the partial pressure of CO2, C/H ratio of the coal (rank), or porosity (surface area and distribution of macropores vs micropores). This effect is important and in light of it, the following experiments were carried out in order to create a calibration guideline for correcting the results of the measured amounts of carbon dioxide. The experiments were performed at 25 °C in the standard batch reactors as detailed above. The reactors were charged with samples of the bituminous (SA) and the sub-bituminous coal (IN), 0.5, 1, 2, and 5 g, (each coal was pretreated at 95 °C for 24 h in order to work with weathered coal) and sealed. After the vials were sealed, carbon dioxide was injected into the vial until a relative concentration in the vial was reached. Because of the fact that each coal is inherently different, the concentration of carbon dioxide injected into the reactors was tailored specifically to the amounts of carbon dioxide produced, via LTO, by the individual coal. As the South African (SA) coal produces carbon dioxide (in the reactor’s atmosphere) in the range of 2-3% (vol %), the set of reactors charged with SA coal were filled with 2% (vol %) carbon dioxide. The Indonesian (IN) coal produces carbon dioxide in the range of 7-9% (vol %), thus the set of reactors charged with IN coal were filled with 10% (vol %) carbon dioxide. For each set, a reference vial was filled with carbon dioxide in the same manner but without addition of coal. All samples were duplicated to reduce the experimental error. The samples were then set aside for an hour (assuming that equilibrium between CO2 ads in the coal and CO2 in the gas phase has been reached), sampled, and measured via gas chromatography. The results of the experiments to determine the coals ability to adsorb carbon dioxide are presented in Table 2.
(8) Brow, T. C.; Haynes, B. S. Energy Fuels 1992, 6, 154–159. (10) Jakab, E. Effects of Weathering on the Molecular Structure of Coal. In Chemistry of Coal Weathering; Nelson, C. R., Ed.; Elsevier: Amsterdam, The Netherlands, 1989. (11) Schmal, D. Chemistry of Coal Weathering; Nelson, C. R, Ed.; Elsevier: Amsterdam, The Netherlands, 1989; Chapter 6. (12) Kok A. Spontaneous heating and calorific losses in stored coal. KEMA Report WSK/20649, 1987 (13) Belkin, H. E.; Tewalt, S. J.; Hower, J. C.; Stucker, J. D.; O’Keefe, J. M. K. Geochemistry and petrology of selected coal samples from Sumatra, Kalimantan, Sulawesi, and Papua, Indonesia. Int. J. Coal Geol. 2009, 77, 260–268. (14) Gupta, R. Advanced Coal Characterization: A Review. Energy Fuels 2007, 21, 451–460. (15) Solomon, P. R. In New Approaches in Coal Chemistry; ACS Symposium Series, Vol. 169, American Chemical Society: Washington, DC, 1981. (16) Wang, H.; Dulgogorski, B. Z.; Kennedy, E. M. Pathways for Production of CO2 and CO in Low-Temperature Oxidation of Coal. Energy Fuels 2003, 17, 150–158. (17) Schmidt, L. D. In Chemistry of Coal Utilization Lowery, H., Ed.; Wiley: New York, 1945. (18) Stanton, R.; Flores,R.; Warwick, P. D.; Gluskoter, H.; Stricker, G. D. Coal Bed Sequestration of Carbon Dioxide; U.S. Geological Survey Report, www.netl.doe.gov/publications/proceedings/01/carbon_seq/3a3.pdf
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Figure 1. Measured CO2 emissions for (a) IN sub-bituminous coal and (b) SA bituminous coal with corrected graphs (measured at 25 °C in 40 mL vials 1 h after CO2 addition). Table 3. Yields of CO2 and CO as a Function of Temperaturea gas analysis [mL] temperature [°C] type
a
SA IN HA LUS
CO2
CO
65
75
85
95
65
75
85
95
0.200 0.495 2.65 3.67
0.375 0.912 4.44 5.60
0.731 1.58 7.58 9.04
0.974 2.82 10.1 10.6
0.032 0.040 0.052 0.071
0.044 0.063 0.075 0.095
0.069 0.114 0.112 0.126
0.111 0.160 0.136 0.144
0.5 g of coal oxidized in a sealed reactor for 24 h.
the reaction rates. As a result, increasing the temperature by 10 °C can double the reaction rate. Emissions from the coal piles at low-temperature oxidation are naturally effected by the rate of formation. The effect temperature has on LTO emissions from the different coals is demonstrated in Table 3. As can be seen in Table 3, as we increase the temperature by 10 °C increments from 65 to 95 °C, all the coals exhibit the described temperature effect. In light of this effect, we have selected the temperature of 95 °C for the work presented in this paper as the amount of emissions produced at this temperature over 24 h as ideal for analytical purposes. It is important to mention that the ability of the coal to adsorb the carbon dioxide (which was detailed above) has a distinct temperature dependence. At the high temperature, 95 °C, there is no appreciable adsorption of CO2, thus sampling at 95 °C does not require any compensation for CO2 adsorption. 3.4. Mass Dependence. 3.4.1. Oxidizing Environment. It has been reported that the mass of the sample during LTO has a profound influence on the emission of gases under air atmosphere. In previous reports,1 it has been suggested that the coal’s macropore surface serves as the catalyst and carbon supplier for the low-temperature oxidation processes. Thus, an increase in the amount of coal will lead to a proportional increase in the net active surface sites prone to adsorb and react with oxygen to yield carbon oxides. The amount of the carbon oxides produced should also be dependent on the amount of the reacting atmospheric oxygen. In order to check the validity of this assumption, it is important to examine if there is an effect of the size of the coal sample on product yields (as the weight of the coal is correlated to its surface area for the same grain size fraction)
when the amount of atmospheric oxygen is not changed (namely, the volume of the reactor is constant). The effect of the sample size on the oxidation processes has been studied for the different coals. The volume of the reactors was kept constant (40 mL), and they were charged with a range of coal masses (0.5-5 g) in an air atmosphere and sealed with a rubber septa and aluminum cap. The reactors were isothermally heated for a period of 24 h in the simulation oven at 95 °C. This period is sufficient for total consumption of the oxygen in the gas phase (7.0-8.0 mL O2 at STP conditions) by the coal sample. The composition of the different gases in the reactors atmosphere after heating was determined via the gas chromatograph. The carbon dioxide emissions that are presented have already been corrected for adsorption effects with the coal. The results of experiments to determine the effect of sample size are summed up in Table 4. The effect of coal mass for the four types of coals studied on the emission of CO2 is given in Figure 2 and for CO is given in Figure 3. 3.4.2. CO2 Emission. The important role that the mass of the sample plays in the carbon oxides yields is evident in Table 4. One can see that for the low rank coals (lignite, HA and LUS) and the sub-bituminous coal (IN), the amount of CO2 produced per O2 consumed is dependent on the size of the oxidized coal sample even though the amount of the reacting atmospheric O2 is constant (∼7.0-8.0 mL). However, only in the bituminous coal (SA) there is no dependence on the mass (within experimental error). This observation might indicate that only in the case of the South African bituminous coal the source of the CO2 formed is only from 6369
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: DOI:10.1021/ef100897m
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Table 4. Dependence of the Yields of CO2 and CO Produced by Low-Temperature Oxidation of Coal on the Sample Sizea sample type SA
IN
HA
LUS
gas analysis coal [g]
CO2 [mL]
CO [mL] 10
CO2/O2 cons
CO/O2 cons
O2 cons [mL]b
0.5 1 2 5 0.5 1 2 5 0.5 1 2 5 0.5 1 2 5
0.974 1.46 2.29 2.00 2.82 4.21 7.25 12.7 10.1 11.4 13.6 21.3 10.6 12.6 16.3 24.0
1.10 1.79 2.41 3.08 1.60 1.88 2.33 3.28 1.38 1.32 1.25 1.40 1.44 1.44 1.49 1.58
0.252 0.187 0.301 0.286 0.413 0.540 0.954 1.81
0.028 0.023 0.032 0.045 0.023 0.024 0.031 0.047 NAc 0.017 0.016 0.020 NAc 0.018 0.020 0.023
3.86 7.80 7.60 7.00 6.82 7.80 7.60 7.00
1.46 1.79 3.04 1.62 2.14 3.43
7.80 7.60 7.00 7.80 7.60 7.00
a 40 mL glass reactor in air containing 0.5-5 g of 74-250 μm coal particles heated for 24 h at 95 °C. b Volume of O2 consumed by the coal. c NA = not available.
produced for the lignite coals compared to ∼8 mL of O2 consumed). Thus, appreciable amounts of CO2 are produced via direct thermal decomposition of oxygenated groups at the coal macromolecule backbone itself. In other words, aside from low-temperature oxidation by atmospheric oxygen, thermal decomposition of the coal plays an important role in the overall process. Moreover, it is well-known that most of the reacting atmospheric oxygen forms certain surface oxides which are stable and do not decompose at 95 °C (during the time span at the temperature range studied in the experiments) to yield CO2 in the gas phase. When comparing the amounts of CO2 produced by the various coals, one can see that, although not completely linear, the amounts emitted from the coals increase appreciably with the sample size. This assumption is reasonable as it can be explained as resulting from the fact that a younger coal (low rank) has much more inherent O (see Table 4) in the coal which undergoes thermal decomposition to produce CO2. It has been noticed that the amounts of CO2 that are produced during LTO are significantly higher for coals with higher inherent O content. It should be pointed out that in these observations, an appreciable part of the CO2 stems from thermal decomposition of the coals and is in contrast to previous reports in Jakab et al.10 The difference between our observations and the previous reports could be due to the fact that the coals studied were from bituminous rank, and the large amounts of CO2 that are adsorbed inside the coal pores were not taken into consideration in these studies. 3.4.3. CO Emission. Another interesting observation is that the emission of CO from the different coals is dependent appreciably on the coal mass only in the higher rank coal. The bituminous SA has a clear linear dependence, and although not completely linear, the same is true for the sub-bituminous IN coal (Table 3 and Figure 3). Regarding the lignite coals, the amount of CO produced per O2 consumed is 0.017 for the HA coal and 0.020 mL in the LUS coal, independent of the mass (0.5-5 g). As can be clearly seen from Figures 2 and 3, the dependence of carbon oxide emission on sample mass is reversed when introducing coal rank as a factor. It is fascinating to note the similarity of the plotted curves of both CO and CO2 in the case of the subbituminous (IN) coal. This behavior can possibly correlate to the petrography of the IN coal.13 In this work we have
Figure 2. CO2 emissions after 24 h; controlled aerial oxidation at 95 °C as a function of coal mass.
Figure 3. CO emissions after 24 h; controlled aerial oxidation at 95 °C as a function of coal mass.
atmospheric oxygen and not from inherent oxygen of the coal macromolecule. Whereas in the case of the lower rank coals, most of the CO2 formed stems from the inherent oxygen of the coal macromolecule. This can also be clearly seen in Figure 2, which shows the difference between the low rank lignite and sub-bituminous coals and the bituminous coal. Another observation which corroborates this suggestion is the fact that the amount of emitted CO2 for sub-bituminous and lignite coals is appreciably greater than the amount of the consumed atmospheric oxygen (e.g., up to 24 mL of CO2 6370
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Table 5. Dependence of the Yields of CO2 and CO Produced by Thermal Decomposition of Coals at Low Temperature in Argon Atmosphere on Mass and Inherent O Content of the Coala sample type SA
IN
HA
LUS
a
gas analysis
coal [g]
O [wt %]
CO2 [mL]
CO [mL]
CO2/coal [mL/g]
CO/coal [mL/g]
0.5 1 2 5 0.5 1 2 5 0.5 1 2 5 0.5 1 2 5
5.59 5.59 5.59 5.59 9.05 9.05 9.05 9.05 23.67 23.67 23.67 23.67 27.24 27.24 27.24 27.24
0.235 0.407 0.669 NAb 0.727 1.64 4.01 9.70 2.09 3.91 7.98 14.7 1.81 3.12 6.10 12.8
0.038 0.047 0.062 0.132 0.043 0.058 0.086 0.167 0.007 0.014 0.030 0.058 0.004 0.011 0.023 0.055
0.47 0.407 0.335 0.24 1.45 1.64 2.01 1.94 4.18 3.91 3.99 2.94 3.62 3.12 3.05 2.56
0.076 0.047 0.031 0.026 0.086 0.058 0.043 0.033 0.014 0.014 0.015 0.012 0.008 0.011 0.012 0.011
40 mL glass reactor in Ar containing 0.5-5 g of 74-250 μm coal particles heated for 24 h at 95 °C. b NA = Not available.
addressed the rank factor only regarding to inherent O content. However, it appears that the coal petrography of each coal is important in order to further understand each coal’s reactivity.14 As the lignite coals have not proved to be dependent on mass, this leads to the conclusion that the CO produced via the LTO process stems solely from atmospheric oxygen. This is a very surprising conclusion because the amount of inherent oxygen in the lignite coals is relatively high (in the range 15-24 wt %). Furthermore, it leads to the conclusion that during the LTO process there is no decomposition of oxygenated groups in the coal macromolecule to produce CO. Rather, it seems that certain carboxylic groups are oxidized and then decompose yielding CO2. We can however see that the amounts of CO emitted from the bituminous coal during LTO display a linear dependence on the coal mass. The CO emission from the sub-bituminous coal while not linear is also dependent on the coal mass. This observation leads to the conclusion that in the sub-bituminous and bituminous coals there are inherent oxygenated groups in the coal macromolecule which serve as specific precursors that decompose during LTO to produce CO. This conclusion is illustrated very clearly in Figure 3. These results are surprising as they indicate that in the case of low rank coals the main source of oxygen for CO emissions might stem from direct atmospheric oxidation of the coal macromolecule, while in the case of the higher rank coals inherent O plays a major role. In order to gain better insight into the two potential precursors of the carbon oxides emission, atmospheric oxygen vs inherent oxygenated groups and to determine the relationship between the inherent O content in the coal and the amounts of carbon oxides produced via thermal decomposition of the coals during LTO, we have decided to study the emission of the carbon oxides under inert atmosphere conditions when no atmospheric oxygen is present. 3.5. Inert Environment. The experiments were carried out in 40 mL glass reactors charged with the 4 different coals with different sample mass (0.5-5 g) under an atmosphere of argon and sealed with a rubber septa and aluminum cap. The reactors were isothermally heated for 24 h in the simulation oven at 95 °C. The composition of the different gases in the reactors atmosphere was determined via the gas chromatograph. These experiments will exclude of course the amounts
Figure 4. CO2 emissions after 24 h heating at 95 °C under inert atmosphere (AT.) (1 At. of argon gas) as a function of coal mass.
of carbon oxides which are produced via the atmospheric oxygen reaction with the different coals. The CO2 emissions that are presented in Table 5 have already been corrected for it is adsorption in the coal. 3.5.1. CO2 Emission. It is clearly seen (Table 5 and Figure 4) that under inert atmosphere (argon gas) the amount of CO2 emissions is dependent on coal mass. For the SA bituminous coal, the effect observed is not significant. Namely, the decomposition of inherent oxygen within the coal macromolecule does not contribute appreciably to the CO2 formed. This observation corroborates our earlier suggestion that in the case of the bituminous coal most of the CO2 formed is due to decomposition of surface oxides originating from the reaction of atmospheric oxygen with the coal macromolecule. Furthermore, for the young coals, the % of CO2 formed under inert atmosphere compared to aerial conditions is dependent on coal mass (yielding 50-70% from the 5 g coal samples, Table 6). 3.5.2. CO Emission. Under inert atmosphere, the emission of CO from the different coals studied is dependent on the coal mass, Table 5 and Figure 5. The higher rank coals IN and SA emit larger amounts of CO compared to the lignite coals. This might indicate that the activation energy for decomposition of surface oxides to CO is much higher in the lignite coals than in the bituminous and sub-bituminous coals. Another possibility is that due to the nature of coal petrography each coal has a unique array of functional groups. The fact that the younger coals are less prone to produce CO could indicate that we are less likely to discover CO forming functional groups. This observed trend is 6371
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Table 6. Calculation of the % of CO2 Formed under Inert Atmosphere Compared to Emissions in an Air Environment sample type SA
IN
HA
LUS
a
Scheme 2. Surface Oxides Produced by Weathering of Coal by Atmospheric Oxygen
gas analysis
coal [g]
CO2(Argon) [mL]
CO2(air) [mL]
CO2 (Ar/air) [%]
0.5 1 2 5 0.5 1 2 5 0.5 1 2 5 0.5 1 2 5
0.235 0.407 0.669 NAa 0.727 1.64 4.01 9.7 2.09 3.91 7.98 14.7 1.81 3.12 6.1 12.8
0.974 1.46 2.29 2 2.82 4.21 7.25 12.7 10.1 11.4 13.6 21.3 10.6 12.6 16.3 24
24 28 29
eqs 11 and 12 of surface oxides. They are formed either from inherent oxygen or surface oxides decomposition: Route 1 ð13Þ inherent oxygen 1a f CO
26 39 55 76 21 34 59 69 17 25 37 53
inherent oxygen 1b f CO2
ð14Þ
surface oxides 2a f CO
ð15Þ
surface oxides 2b f CO2
ð16Þ
Route 2
NA = not available.
If this reaction scheme is indeed correct, then the amounts of carbon oxides produced from inherent oxygen (Route 1) should be dependent only on the mass of the coal sample. Whereas, the amounts of carbon oxides produced from surface oxides (Route 2) should be dependent only on the amount of the atmospheric oxygen consumed producing surface oxides, which subsequently decompose to yield carbon oxides. The type of coal will also have an effect on the amounts of carbon oxides produced. Measurements of CO and CO2 carried out under aerial oxidation include their production via both routes. Whereas, experiments under an argon atmosphere account mainly for CO and CO2 yields which were produced via route 1. (Although, one cannot exclude some formation of CO and CO2 via decomposition of surface oxides, route 2, generated during storage at room temperature prior to the simulation experiments). However, the difference between the yields under air compared to those under argon should account only for route 2. Thus, CO and CO2 yields which originate from atmospheric oxygen weathering of the coals, route 2, normalized to the amount of consumed oxygen by the coal is given in Table 7. Table 7 clearly proves that the CO and CO2 produced are not dependent on the coal mass but only on the amount of consumed atmospheric oxygen. Furthermore, there is almost no change in the amounts of CO produced from the low temperature atmospheric oxidation in all the coals. This indicates that rank is not a factor in the process of CO emission via the reaction of atmospheric oxygen with the coal at low temperatures (30-150 °C). Namely, for the South African bituminous, Indonesian sub-bituminous, German HA lignite, and the German LUS lignite, the average normalized rates at 95 °C are 0.022, 0.019, 0.014, and 0.016 [mL/ mL O2(cons)], respectively. On the other hand, the amounts of CO2 produced from the low-temperature atmospheric oxidation of the coals are very much dependent on the coal rank and the emission rate decreases upon an increase of coal rank. Namely, for the South African bituminous, Indonesian sub-bituminous, the German HA lignite, and the German LUS lignite, the average normalized rates at 95 °C are 0.17, 0.36, 0.92, and 1.32 [mL/mL O2(cons)], respectively. As the normalized number should not be more than 1 mL of CO2 per 1 mL of O2 consumed, the value of 1.32 for the LUS
Figure 5. CO emissions after 24 h while heating at 95 °C under inert atmosphere (1 At. of argon gas) as a function of coal mass.
reversed when compared to the coal’s behavior regarding CO2 emission in inert atmosphere. The % of CO produced upon heating the coals under argon atmosphere is dependent on the size of the sample (Table 5) and increases appreciably by a factor of 4-10 (depending on coal type) when the weight of the sample is increased by a factor of 10 (from 0.5 to 5 g). 3.5.3. Modes of Carbon Oxides Emission. The precursors to CO and CO2 emissions from the coal under aerial oxidation are oxygenated sites at the coal macromolecule. These sites can be either oxygen, which is part of the backbone structure of the coal macromolecule (denoted as inherent oxygen), namely, functional groups such as ethers C;O;C, carbonyls CdO, esters ;C;OR(dO), carboxylates ;Cd O(OH);, see Figure 6, or surface oxides produced via atmospheric oxygen absorption/chemisorption (decomposition of the chemisorbed oxygen is denoted surface oxides). Several type of surface oxides have been suggested,1,7 such as hydroperoxides ;C;O;OH, hydroxyls ;C;OH, etc., Scheme 2. A third source of oxygen could possibly originate from the moisture (H2O) in the coal. However, water molecules are not expected to decompose at such a low temperature range (30-150 °C) to supply oxygen atoms required for carbon oxide production. Thus, we suggest that the reactions of the coal during weathering processes to yield carbon oxides (CO and CO2) can occur via two types of precursors and not as the simple definition in 6372
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: DOI:10.1021/ef100897m
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Figure 6. Typical model of the bituminous coal macromolecule (by Solomon et al.15). Table 7. Yields of CO2 and CO Produced by Atmospheric Oxygen Weathering of Different Coals at 95 °Ca sample type SA
IN
HA
LUS
gas analysis coal [g]
O2 cons [mL]
ΔCO2 [mL]
ΔCO [mL]
ΔCO2/O2 cons
ΔCO/O2 cons
0.5 1 2 5 0.5 1 2 5 0.5 1 2 5 0.5 1 2 5
3.86 7.80 7.60 7.00 6.82 7.80 7.60 7.00 NAb 7.80 7.60 7.00 NAb 7.80 7.60 7.00
0.70 1.10 1.61 0.82 2.10 2.60 3.20 3.01 8.01 7.51 5.60 6.62 8.79 9.50 10.2 11.2
0.070 0.13 0.18 0.18 0.12 0.13 0.15 0.16 0.13 0.12 0.10 0.080 0.14 0.13 0.13 0.10
0.18 0.14 0.23 0.11 0.31 0.34 0.42 0.43 1.02 0.96 0.74 0.94 1.13 1.22 1.34 1.60
0.018 0.016 0.026 0.026 0.017 0.017 0.020 0.023 0.017 0.013 0.013 0.011 0.018 0.017 0.017 0.014
a
The yields of the carbon oxides were calculated as the difference between the amounts of gas produced under air atmosphere and argon atmosphere in 40 mL glass reactors containing 0.5-5 g of 74-250 μm coal particles heated for 24 h at 95 °C. b NA = not available.
lignite indicates that the low-temperature oxidation process results not only in oxidation of active carbon atoms at the Lignite coal macromolecule but also causes chemical transformations of the inherent oxygen of the coal which end up as CO2. The normalized emissions of carbon oxides via the LTO by atmospheric oxygen can be written as % O2 yields. Thus, for the CO2 emissions, the % of atmospheric oxygen transformed for SA, IN, HA, and LUS are 17, 36, 92 and 132, respectively. Since all the atmospheric oxygen has reacted with the coal, this means that in the bituminous/sub-bituminous coal only 17%/36% of the surface oxides 2b formed during aerial oxidation decomposed to yield CO2. This portion of the emissions has been reported17 to be temperature dependent, increasing with temperature. The most reasonable assumption is that reaction 16b involves decomposition of carboxylic groups to yield the
carbon dioxide and C;H bonds: surface oxides 2b f CO2
ð16aÞ
The detailed mechanism is quite complex and will need further extensive research to fully understand it. For the production of CO, the most reasonable assumption is that reaction 15b involves decomposition of aldehyde groups ;(H)CdO at the coal macromolecule to yield CO 6373
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and C-H bonds: a
surface oxides 2 f CO
be the result of the formation of stable oxygen complexes in the coal macrostructure.16 Thus, assuming that the detection of carbon monoxide in the inert atmosphere thermal decomposition is indeed the result of LTO, it follows that since the concentration of the stable oxygen complexes are relatively small, the unstable oxygen complexes, carbon dioxide precursors, are insignificant by comparison. This leads to the assumption that the CO2, which is detected at low-temperature thermal decomposition, is not an oxidation product. It is important to emphasize that the amount of carbon dioxide released from lignite coals is 1 order of magnitude higher than bituminous coals while carbon monoxide emissions are comparable. The probable source for this result is the nature of the functional groups, which are the precursors to the carbon oxides produced.
ð15aÞ
It is interesting to note that the amount of CO produced by the LTO via surface oxide decomposition is at least 1 order of magnitude lower than the yield of CO2 in all coals studied. This observation indicates that the functional groups at the coal macromolecule surface oxides 2a, which are precursors for CO emission, are rare compared to the surface oxides 2b, which decompose to produce CO2. Measurements of CO and CO2 produced in experiments conducted under argon atmosphere are assumed to occur via route 1. The oxygen content of the coal is very much dependent on the coal rank; 5.52, 8.81, 15.5, and 24.4 for South African (bituminous), Indonesian (sub-bituminous), and the German HA and LUS (lignite coals), respectively. As suggested (see above), the average results of carbon oxide emissions which were normalized to the sample mass should be dependent linearly on the coal mass if indeed they are produced from inherent oxygen present in the coal. The coal mass normalized yields of CO and CO2 under argon atmosphere due to the thermal decomposition of the coal for several coal sample masses are presented in Table 4. The results clearly indicate that there is almost no effect of mass on CO and CO2 emissions, namely, that the source of these emissions (in Ar atmospheres) are oxygenated groups, inherent oxygen, from the coal macromolecule backbone (route 1). An additional indirect proof can be offered toward the suggestion that the CO2 that is emitted during inert thermal decomposition stems from the coal oxy-structure is the following reasoning. The formation of carbon monoxide is thought to
4. Conclusions (i) CO and CO2 produced during weathering of stored coal piles results from two parallel processes: thermal decomposition of oxygenated groups occurring at the coal macromolecule backbone and chemisorption of atmospheric oxygen, which subsequently decompose to yield the carbon oxides. (ii) The thermal decomposition of oxygenated groups to yield CO is not dependent on coal rank while thermal decomposition of oxygenated groups to yield CO2 is dependent very much on coal rank. (iii) On the other hand, decomposition of chemisorbed atmospheric oxygen to yield CO is very much dependent on coal rank while decomposition of chemisorbed atmospheric oxygen to yield CO2 is not dependent on coal rank. (iv) Atmospheric oxygen activates the pre-existing oxygen inherent to the low rank coals. (v) This work has only compared the different coal ranks based on the O/C ratio. However, it is apparent that a deeper understanding of the coal petrography and further experimental work is required in order to understand the weathering processes of coals.
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