ARTICLE pubs.acs.org/EF
Modes of Formation of Carbon Oxides [COx (x = 1 or 2)] from Coals during Atmospheric Storage. Part 2: Effect of Coal Rank on the Kinetics Uri Green,*,†,‡ Zeev Aizenshtat,† James C. Hower,§ Rachel Hatch,§ and Haim Cohen*,‡,|| †
Chemistry Institute, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Departmet of Biological Chemistry, Ariel University Center at Samaria, Ariel 40700, Israel § Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, Kentucky 40511, United States Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel
)
‡
ABSTRACT: The processes resulting in the formation of carbon oxides from fossil fuels during storage prior to combustion hold significant interest in the power production industry. Coal, which remains the single most viable energy source is mined, transported, and used in plants worldwide in vast quantities. During that time, it undergoes a series of atmospheric oxidation reactions that result in the emission of carbon oxides. In the first paper dealing with these processes (10.1021/ef100897m), it was shown that the pathways resulting in the emission of carbon dioxide and CO are inherently different. In this work, the kinetics of these oxidation reactions under atmospheric storage conditions were studied. Furthermore, an analysis of the coal petrography sheds further light on the organic sources, which indicate specific functional groups that are responsible for variations in carbon oxide emissions.
1. INTRODUCTION Coal is a heterogeneous complex organic substrate that is formed uniquely at different geographical locations.1 The rank of the coal affects its reactivity with atmospheric oxygen upon exposure after being mined; thus, lignites are much more reactive than bituminous or anthracite ranks. As a result, coals of different rank react differently to oxidative weathering processes upon storage under atmospheric conditions.2 These low-temperature oxidation processes compose a multi-stage mechanism that is quite complicated, and even today, it is not fully understood.3 The first stages involve physical adsorption and chemisorption of atmospheric oxygen. The second stage is the formation of surface oxides and hydroperoxides, which can thereafter partially decompose to yield low-molecular-weight inorganic gases, such as carbon oxides (CO and CO2), water, hydrogen (H2), and some low-molecular-weight organic gases (C15).36 If the heat formation (exothermic processes) is greater than the heat dissipation, self-heating of the pile can occur, which, in extreme cases, results in fire eruptions. The major product released from coals undergoing low-temperature aerial oxidation processes is carbon dioxide. The emission of CO2 is an exothermic process, which is dependent upon the ambient temperature and the oxygen concentration in the vicinity of the coal macromolecule. Carbon monoxide is also known7 to evolve at these low temperatures [room temperature (RT)95 °C]. However, because it is not the main oxidation product, the measured amounts are smaller (typically, 1 order of magnitude smaller than carbon dioxide). The formation of these carbon oxides is defined as low-temperature aerial oxidation (LTO). The precursors to CO and CO2 emissions from the coal under aerial oxidation are oxygenated sites at the coal macromolecule, which are either part of the coal macromolecule backbone (denoted as inherent oxygen) or oxides formed via the atmospheric oxygen reactions with the coal macromolecule (denoted as surface oxides). r 2011 American Chemical Society
In part 1 (10.1021/ef100897m),8 we suggested that the reactions (reaction numbers are as presented in part 1) of the coal during weathering processes to yield carbon oxides (CO and CO2) can occur via two different types of precursors, formed from either inherent oxygen or surface oxide decomposition. Route 1: inherent oxygen 1a f CO
ð13Þ
inherent oxygen 1b f CO2
ð14Þ
Route 2: surface oxides 2a f CO
ð15Þ
surface oxides 2b f CO2
ð16Þ
a
a
where superscript a in 1 /2 represents functional groups in the coals that are precursors to CO emission and superscript b in 1b/ 2b represents functional groups in the coals that are precursors to CO2 emission. Thus, the carbon oxides that are emitted from the coal are formed because of not only the oxidation reaction by atmospheric oxygen but also thermal decomposition of the inherent oxygen content within the coal macromolecular structure. Presumably, moisture in the coal could also be the precursor to oxygen in the carbon oxides formed. However, this is unlikely at this relatively low-temperature region (5595 °C) because it is not reasonable that the strong OH bond would break to transfer the oxygen atom to the coal macromolecule to yield CO or CdO bonds in the coal. It is of interest to investigate and try to determine the different modes by which carbon oxides Received: August 16, 2011 Revised: October 23, 2011 Published: November 01, 2011 5626
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Table 1. Properties of Coalsa
Table 2. Rates of CO2 Emission at Different Temperaturesa rate constants (mL g1 h1)
proximate analysis (wt %) ultimate analysis (wt %, daf) typeb moisture
ashc
VMdaf
C
H
O
S
CV (J g1)
T (°C)
SA
IN
HA
LUS
SA
1.20
13.77
28.27
74.19
4.10
5.59
0.46
28,416
55
0.0049
0.0159
0.0514
0.100
IN
2.61
10.65
36.38
73.25
4.63
9.05
0.67
28,564
65
0.0123
0.0476
0.162
0.212
HA
34.53
5.09
52.39
66.12
4.32
23.65
0.16
25,323
75
0.0244
0.0709
0.259
0.328
LUS
10.57
4.16
52.78
63.24
4.50
27.24
0.20
24,516
85
0.0435
0.127
0.493
0.543
95
0.0981
0.237
0.859
0.836
a VM, volatile matter; CV, calorific value; daf, dry and ash-free basis. b SA, South Africa; IN, Indonesia; HA, Hambach, Germany; LUS, Lusatia, Germany. c Ash is water-free.
are released during the low-temperature oxidation/weathering processes. LTO is dependent upon coal rank.6 However, while focusing on the aspect of the O content in the coal (by researching a range of coals with different O/C ratios), it was realized that it would also be very interesting to study how the petrography of each coal will affect the reactivity. Assuming that different maceral assemblages could be isolated and defined, they could prove suggestive of the overall structure required for carbon oxide formation. We have investigated three ranks of coals: bituminous, subbituminous, and lignite, each containing an increasing amount of inherent oxygen. For this purpose, several coals that are fired in either the Israeli utility plants (bituminous and sub-bituminous) or German power plants (lignite) have been chosen. Studying a range of coal ranks allows for a broader comparison of the mechanisms behind low-temperature oxidation processes. In this part, we investigate the kinetics of the reactions responsible for LTO emissions.
2. EXPERIMENTAL SECTION All chemicals and gases used throughout the study were of analytical reagent (AR) grade and supplied by Aldrich, Fluka, Merck, or Maxima. The water used throughout this study was purified water (via ionexchange columns). 2.1. Coal. Experiments in this work were carried out with three coal ranks: bituminous, sub-bituminous, and lignite. The bituminous coal was from South Africa; the sub-bituminous coal was from Indonesia; and the lignite was from Germany. 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 subbituminous Indonesian coal is also fired in the Israeli utilities. The properties of all of 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 of 74 e X e 250 μm) in an air atmosphere and heated at 5595 °C for various periods in a N€uve oven model FT 300. Reactors for experiments testing oxygen concentration effects were also conducted (prepared in the same manner), however, in an argon or oxygen atmosphere. The atmosphere of the reactors was changed using a vacuum system. All of the coals in the present study were prepared by grinding and sieving to a particle size of 74 e X e 250 μm. The coal samples were then dried in a Heraeus vacuum oven model VT6060 for 24 h at 60 °C and kept in sealed containers under a dry nitrogen atmosphere. Petrographic studies were conducted on polished resin-bound particulate pellets using Leitz microscopes, with reflected-light, oil-immersion techniques, and a final magnification of 500 at the Center for Applied Energy and Research, University of Kentucky, Lexington, KY. The internal surface area was measured by a nitrogen adsorption isotherm at 77 K using a Micromeritics ASAP 2020 volumetric
a
Rates measurements were determined in plots from 0.5 to 48 h, with average R2 = 0.95 ( 5%.
Table 3. Rate of CO Emission at Different Temperaturesa rate constants (mL g1 h1) T (°C)
SA
IN
HA
LUS
0.0007
0.0010
0.0015
75
0.0005
0.0014
0.0027
0.0041
85
0.0010
0.0032
0.0042
0.0060
95
0.0041
0.0082
0.0074
0.0087
55 65
a
Rates measurements were determined in plots from 0.5 to 48 h, with average R2 = 0.95 ( 5%.
adsorption analyzer at the Technische Universit€at Bergakademie Freiberg, Institute f€ur Energieverfahrenstechnik and Chemieingenieurwesen, Freiberg, Germany. The surface area of the sample was determined using the BrunauerEmmettTeller (BET) equation. 2.2. Gas Chromatography (GC) . The amount of the gases (CO2, CO, N2, O2, and 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 1/800 , 90 ss 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 measured in the gas chromatograph. The gases that could be determined are hydrogen, nitrogen, oxygen, CO2, CO, methane, and ethylene. The argon gas present is not separated from the oxygen; thus, the value determined for oxygen also contains ∼0.93% argon gas. Because the reactions studied are gas/surface reactions, the reproducibility of the results is not good. Therefore, each experiment was carried out with duplicate reactors to reduce the total experimental error. The error is (15%, mainly because of the nature of the heterogeneous reactions studied in the experiments.
3. RESULTS AND DISSCUSSION 3.1. Kinetics and Rates. By measuring the amounts of carbon oxides produced over a period of time, one can determine the rate of a reaction. The concentrations of the emitted gases were plotted versus time. Data regression for the initial values using the least-squares method results in a straight line whose slope (dX/dt, where X is the concentration of the gas) is equivalent to the appropriate rate, which is constant for the temperature at which it was determined. The kinetic investigations were carried out on all of the coals mentioned above. The experiments were performed in sealed 5627
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Figure 1. CO2/CO ratio depicting the emissions of carbon oxides at 95 °C for various periods of time in simulation reactors containing 0.5 g of coal.
glass vials (40 mL) used as batch reactors. The reactors were charged with 0.5 g of coal (particle size of 74 e X e 250 μm) in an air atmosphere. The samples were heated at different temperatures for various periods of time. To enable an accurate measurement (neglecting the CO2 absorbance effect) of the evolving gases, we sampled the vials at the oven temperature. The results of the experiments to determine the rates of reaction for the formation of CO2 are given in Table 2, and the results for CO are presented in Table 3. CO2 emissions were detected from initiation of the simulation; thus, rates were determined up to 48 h (Table 2). However, with regard to the CO emissions, a noticeable induction period with no emission of CO was observed. This induction period is dependent upon the coal rank as well as temperature. For example, at 95 °C, for IN coal, CO emissions were detected after 1 h of LTO, while for SA coal, emissions were detected only after 3 h of LTO. As noted by previous researchers,9 oxygen uptake by the coal is the fastest process, followed by the carbon oxide emission. The formation of carbon monoxide is a slower process. As seen from Tables 2 and 3, the rate of emission of carbon monoxide is lower by 1 order of magnitude than that of CO2. This observation supports the view that the formation of CO is the result of decomposition of relatively stable oxy-coal complexes.10 It is as yet still unclear as to whether the CO formed results solely from oxy-coal group decomposition (the result of oxidation of the coal matrix by atmospheric oxygen, surface oxides) or also via decomposition of oxygenated groups that are part of the coal macromolecule backbone (the inherent oxygen). Wang et al.10,11 suggested that the ratio of CO2/CO production (R) is directly related to the dissociation of unstable intermediates formed by the reaction of atmospheric oxygen, namely, CO and CO2 precursors. However, inherent oxygen has a significant contribution to the carbon oxide release, at least in low-rank coals. Initially, the contribution from inherent oxygen precursors is much higher than those stemming from surface oxides, which will result in a low value of R. Over time (as the coal ages), the contribution of atmospheric oxygen (surface oxides) is more dominant as the concentration of inherent oxygen decreases appreciably. In other words, coal oxidation via the atmospheric oxygen adsorption/ chemisorption dominates at this stage. A lower value implies that the inherent oxygen content of the coal is high, and thus, there is appreciable emission of CO. In Figure 1, the time dependence of
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Figure 2. CO emission from IN coal.
Figure 3. CO emission from SA coal.
R for the different coals is given. The value of the rate was calculated as the derivative of the rate at 0.5, 1, 2, 5, 10, 20, and 48 h. From Figure 1, it is clear that the behavior of lower rank coals (HA and LUS) is significantly different from that of the higher rank sub-bituminous (IN) and bituminous (SA) coals. Namely, the value of R increases very fast to an asymptotic high value of 9096 for the low-rank coals, whereas the change in R for the high-rank coals is mild, reaching an asymptotic value of 1525. This indicates that the low-rank coals, which have a high content of inherent oxygen, contain relatively much larger concentrations of oxy-coal precursors compared to the higher rank SA bituminous and IN sub-bituminous coals. As the rate of CO2 release diminishes over time, the increase in R until the asymptotic is the result of a more significant decrease in the rate of CO release. In the case of the higher rank coals, there is a mild increase in R until it reaches a much lower asymptotic value of 25 for the sub-bituminous IN coal and 15 for the bituminous SA coal. For the high-rank coals, there is an induction period, even at 95 °C, in which no CO release is observed. However, at lower temperatures, no emission of CO is observed (up to 75 °C for the SA coal). To evaluate this effect properly, experiments were performed in sealed glass vials (40 mL) used as batch reactors. The reactors were charged with 0.5 g of coal (particle size of 74 e X e 250 μm) in an air atmosphere. They were then heated at different temperatures for various periods of time. The emission rates of CO (mL/g of coal) from the IN, SA, LUS, and HA coals for the different temperatures are plotted in Figures 25. The effects of the temperature and rank on the overall emission are apparent from Figures 4 and 5. Because of the propensity of IN 5628
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Table 4. Surface Area of Coalsa type
a
Figure 4. CO emission from LUS coal.
Figure 5. CO emission from HA coal.
coal toward oxidation, the time lag stage (induction period) for this coal is only a third of the time that is noticed in the SA coal (at 95 °C, the emission of CO for the IN coal is observed after 1 h, while for SA coal, the first detection of CO is only after 3 h). On the other hand, with regard to the lignites, it is clear that the induction period for CO emission is shorter and, at T > 75 °C, the thermal effect is less significant in the release of CO from the coal matrix. It is clear that the induction period is temperatureand rank-dependent. This indicates that the CO precursors in the high-rank coals, even at T g 75 °C, must undergo chemical changes prior to decomposition to yield CO. In the low-rank coals, this induction process is observed significantly only at the low-temperature region. The oxygenated groups, which initially decompose to produce CO2, are primarily carboxylic groups, coalCOOH,10 whereas the precursors to CO are much more complicated and cannot be defined clearly. As the LTO process occurs at the surface of the coal (mainly in the macropore structure), it is interesting to evaluate the effect that the macropore surface area of the coals studied has on carbon oxide emissions. 3.2. Pore System and Reactivity. Coal porosity consists of macropores and a meso-/micro- or ultrafine pore system. The fine pore system deviates from a normal pore system and resembles the structure of zeolites.12 In the present research, we focus on atmospheric adsorption/chemisorption processes. Because of these conditions, it is assumed that the interaction occurs primarily with the surface area of the macropores. The macropore system is the result of cracks in the coal, and because
class
C (wt %)
BET surface area (m2/g) 3.92
SA
bituminous
74.19
IN
sub-bituminous
73.25
5.45
HA
lignite
66.12
2.35
LUS
lignite
63.24
2.59
Measured with N2 at 77 K for 1 g of coal particles < 74 μm.
they are the widest pores, the diffusion mechanism is assumed to be the same as in the bulk gas phase. As such, the measured macropore surface area is the phase at which the carbon oxides are formed and released. The macropore surface areas for the coals are presented in Table 4. The mechanism responsible for the carbon oxide emission from low-rank coals (HA and LUS) is different from that for the high-rank coals. Furthermore, as shown above in Figure 1, we can see that these low-rank coals show some similarity in their reactivity. Also, from that same dependence (Figure 1), it is clear that the IN coal is slightly different from the SA coal regarding the concentration of carbon oxides emitted. With that in mind, the results in Table 4 can be a bit perplexing. While it appears that the two low-rank coals, LUS and HA, have a similar lower macroporosity (∼2.47 m2/g) versus a higher porosity for the higher rank coals (∼4.69 m2/g), the concentration of CO2 emitted from the low coals is significantly higher than the high-rank coals SA and IN (at 95 °C, low-rank emission is greater by a factor of 8.6 and 3.6, respectively). Thus, it is suggested that the coal reactivity is not limited to the size of the reactive surface area. This indicates, of course, that these low-temperature oxidation reactions are probably related to the inherent concentration of specific functional groups (i.e., carboxylic groups) available at the coal macropore surface. The different functional groups (i.e., macerals) are indeed dependent upon the coal rank, as observed in the data presented in section 3.4 (see below). It is interesting to note another indication that the surface area measurements are indeed not a hindering aspect to carbon oxide formation. The relative concentration of carbon monoxide in all of the coals is relatively similar. This, as noted in previous work,8 demonstrates that the formation of CO is dependent upon the concentration of atmospheric oxygen. Thus, because the initial concentration in all of the vials was the same and the measured concentration of CO is also similar, it is reasonable to conclude that the density of CO precursors in the surface area of the coals is also similar and not dependent upon the rank. 3.3. Activation Energies. The activation energies for the formation of carbon oxides were studied to shed more light on the processes behind these products. The activation energies reported are the apparent activation energies, because every product is formed via several consecutive steps and not as the result of a single step. Furthermore, the reactions occur at the gassolid interphase and not in a homogeneous phase. It is also recognized that the activation energies for emission of CO or CO2 from inherent oxygen oxygenated groups (reactions 13 and 14) will be different from that of surface oxide oxygenated groups (reactions 15 and 16). Thus, the values cited in Table 6 are a combination of both values. During these experiments, the upper limit for O2 consumption in the atmosphere of the reactor was limited to a maximum of 20% of the initial concentration in air (namely, ∼17% after oxidation compared to the initial concentration in air of ∼21%). The samples were heated isothermally in 5629
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Table 5. Apparent Activation Energiesa
Table 6. Maceral Dispersion in the Various Coals
Ea (kJ mol1)
a
coal type (%)
type
CO
CO2
O2
SA
106
73
54
fusinite
19.4 13.4
3
IN
110
75
61
semifusinite
41.6 11
1
0
HA
76
67
63
micrinite
0
0
LUS
83
77
69
class
inertinite
Coal samples (0.5 g) were heated in 40 mL reactors under air (LTO).
maceral
vitrinite/huminite
Figure 6. Arrhenius plot of CO2 emission for LUS coal.
the temperature range of 5595 °C for different durations in the simulation oven. The activation energies (Table 5) were calculated from the Arrhenius plots [ln(rate) versus 1/T]. A typical Arrhenius plot (CO2 emission for LUS coal) is shown in Figure 6. In all coals, the activation energies for O2 consumption are appreciably lower than those for carbon oxide (CO and CO2) release. Also, Ea for CO emission is higher than that for CO2 emission. This indicates that the oxygenated functional groups, which are precursors to the CO emission, are more stable than those that are precursors to CO2 emission. Furthermore, the stability of the oxygenated functional groups, which are precursors to the CO emission in the higher rank coals SA and IN, is much higher than the stability of those that are in the lignites (Ea of ∼108 and ∼80 kJ/mol1, respectively). Merely from analyzing the time dependence of the formation of the carbon oxides as shown above (see Figures 25), it is apparent that the formation of CO is a different process than the reactions producing CO2. 3.4. Coal Petrography. Coal is a heterogeneous substrate; its formation and properties depend upon a variety of factors. Ultimately, vegetation type, deposition and burial of sediments, tectonic plate movement, and erosion forces determine the nature of the coal. The mode of decaying vegetation can play a significant role in coal formation. The nature of these coal precursors (macerals) serves as markers that are detectable in the coal substrate. As such, it is interesting to check if there is a correlation between the different maceral content of the coals studied and the emission of carbon oxides. An analysis of selected maceral groups in the various coals is presented in Table 6. It has been demonstrated13 that the individual macerals are the product of multiple and varied pathways. There are several apparent differences in the maceral distribution among the four coals in the inertinite and liptinite classes in the data presented in Table 6. In the inertinite class, there is a linear increase in the measured concentration
IN
0.8
0
HA LUS 0.5
macrinite
0.8
2.4
0.5
0
secretinite
0.8
NA
0.5
0
funginite
0
1.2
1.5
5.5
inertodetrinite
0.2
0.4
0
0.5
6.5 1
6.5 1 1.5
total (inertinite) sporinite
liptinite
SA
63.6 28.4 1.4 0.4
cutinite
0.4
0.2
0
resinite
0
0.2
2.5
4
alginite
0
0
0
0
liptodetrinite
0
0
0
0
suberinite
0.2
1.2
3
11.5 0
exsudatinite
0
0
0
total (liptinite) gelinite
2 0
2 0.8
6.5 18 1.5 0.5
corpogelinite/corpohuminite
0.4
1.6
7
25.5
vitrodetrinite/humodetrinite
2.6 17.6 56
26.5
telinite/texinite
6.8
4.6
6.5 13
collotelinite/ulminite
22.8 43.8 16
10
total (vitrinite/huminite)
32.6 68.4 87
75.5
Figure 7. van Krevelen diagram of the different coals.
of fusinite macerals with increasing rank. Accordingly, a linear decrease in the measured concentration of funginite macerals with increasing rank is noted. With regard to the liptinite class, a linear decrease in the concentration of suberinite and resinite macerals with increasing rank was observed. The formation of fusinite occurs when coalification proceeds at a very fast rate (dehydration is the dominant process) and leads to a very low hydrogen content structure.14 As such, as coal rank increases, the measured concentration of fusinite macerals should also increase. The low hydrogen (and oxygen) content of fusinite macerals lead to a fused carbon structure that is appropriately classified as inertinite. Thus, as the concentration of the fusinite maceral group in coal increases, the ability of the coal to react with atmospheric oxygen to produce carbon oxides decreases. The relationship between the fusinite maceral concentration and the degree of coalification is demonstrated with the van Krevelen diagram (Figure 7). 5630
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Energy & Fuels As shown in Table 2, at 95 °C, the amount of CO2 formed decreases as coal rank increases (i.e., SA < IN < LUS and HA). It is worth pointing out that, according to Figure 7, while the HA and IN have relatively the same ratio of H/C, the lower rank HA still produces ∼3 times the amount of CO2 compared to the IN coal. This indicates that possibly the O/C ratio is the more dominant factor in carbon dioxide emission. The differences in reactivity between HA and LUS, basically at the same rank, may be a function of the nature of the maceral assemblages, with the higher amount of funginite in LUS indicating a degree and nature of degradation not present at the time of the development of HA.1519 These initial maceral distribution results indicate that there are inherent functional group differences between the coals. However, more work is required to correlate specific maceral functionality to carbon oxide emission.
4. CONCLUSION The mechanism by which coal under open air storage undergoes aging/degradation processes, which results in the emission of toxic gases, self-heating of the piles, and decrease in calorific values, is complex. (i) The emission of the carbon oxides stems from two major precursors: (1) Decomposition of oxygenated groups from the inherent oxygen is part of the coal macromolecule structure. The rate of this process is reduced appreciably with time. (2) Decomposition of oxygenated groups forms by the reaction of atmospheric oxygen with the coal (defined as surface oxides). The rate of this process is not appreciably affected with time. (ii) The functional groups that are precursors to CO emission are much more stable than those that are precursors to CO2 release and do not appreciably depend upon coal rank. (iii) Coal rank has a greater affect on CO2 emission than CO emission. (iv) The differences in reactivity between the lignites may be a function of the nature of the different maceral groups present in each coal. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (U.G.);
[email protected] (H.C.).
’ ACKNOWLEDGMENT Financial support by the Israeli Electric Corporation is gratefully acknowledged. The authors thank Diplom-Mineraloge Mathias Klinger from Technische Universit€at Bergakademie Freiberg, Germany, for the porosity measurements.
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