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Energy & Fuels 2009, 23, 3078–3082
Oxidative Decomposition of Formaldehyde Catalyzed by a Bituminous Coal Haim Cohen*,†,‡ and Uri Green† Biological Chemistry Department, Ariel UniVersity Center in Samaria, Israel and Chemistry Department, Ben-Gurion UniVersity of the NegeV P.O.Box 653, Beer SheVa 84105, Israel ReceiVed February 23, 2009. ReVised Manuscript ReceiVed April 5, 2009
It has been observed that molecular hydrogen is formed during long-term storage of bitumimeous coals via oxidative decomposition of formaldehyde by coal surface peroxides. This study has investigated the effects of coal quantity, temperature, and water content on the molecular hydrogen formation with a typical American coal (Pittsburgh No. 6). The results indicate that the coal’s surface serves as a catalyst in the formation processes of molecular hydrogen. Furthermore, the results also indicate that low temperature emission of molecular hydrogen may possibly be the cause of unexplained explosions in confined spaces containing bituminous coals, for example, underground mines or ship holds.
1. Introduction
Surface oxides f CO2, H2O (main reaction)
Hydrogen is a minor product released during the low temperature oxidation of bituminous coals under long-term atmospheric storage conditions.1 The emission is temperature dependent, occurring at evolving hot spots in coal piles. These hot spots are formed usually during long-term storage of bituminous coals at the yards of coal-fired power stations2 and also in deep underground mines where the temperature can reach 65 °C at the coal seam.3 The emission of hydrogen is linearly dependent on the amount of oxygen consumed by the coal4 and increases with temperature above 32 °C. The H2 emission process is a surprising fact, due to the observations that: (i) a reduction product, H2, is produced in conjunction with, or as a result of, an oxidation process; and (ii) emission of H2 is, usually, a well-known process in coals (pyrolysis processes), however it is observed only at much higher temperatures (>250 °C). The ambient temperature oxidation of bituminous coals can be summed up by the following reaction scheme:5 Coal(s)+ O2(g) f O2(ads)
(1)
O2(ads) f O2(chemisorbed)
(2)
O2(chemisorbed) f Surface oxides
(3)
* To whom correspondence should be addressed. Phone: 00972-39066632; fax: 00972-3-9066634 e-mail:
[email protected] or urigr@ ariel.ac.il. † Ariel University Center in Samaria. ‡ Ben-Gurion University of the Negev. (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. (2) Nelson C. R., Coal Weathering: Chemica Processes and Pathways. In Chemistry of Coal Weathering; Nelson C. R. Ed.; Elsevier: Amsterdam, 1989; pp1-32. (3) Wanzl, W. DMT, Essen, Personal Communication, 1993. (4) Davidi, S.; Grossman, S. L.; Cohen, H. Fuel 1995, 74, 1357. (5) Schmal, D. Chemistry of Coal Weathering, Nelson C. R. Ed.; Elsevier: 1989; Ch 6.
(4a)
Surface oxides# f CO, CnH(m-2u)Ou, H2 (side reactions) (4b) Reactions1-3 are the main processes occurring during the atmospheric oxidation of the coal, but only reactions2,3, and 4a are relatively exothermic and are the source of the selfheating process and formation of hot spots inside the coal pile, if the heat dissipation by the pile is not efficient. The term “surface oxides” refers to several types and composition of oxygenated functional groups. Thus, only a small percentage of the surface oxides (1-15%, depending on the temperature) decompose via eq 4a to yield carbon dioxide and water and a much smaller percentage of certain other surface oxides (denoted as surface oxides# in eq 4b) decompose via eq 4b to yield the residual gases.6 The emission of dihydrogen via the atmospheric oxidation might be an important process, as accumulation of hydrogen in confined spaces should be considered as another possible cause of explosions in underground coal mines.7 Usually, such explosions are assigned to methane or fine coal dust accumulation in the mine, and this is why methane detectors are installed and also good air ventilation/filtration processes are required in the mines. Despite the above precautions, there have been some unexplained explosions that took the lives of many miners.8 The precursor to H2 formation in these processes is formaldehyde.9 It was proposed that formaldehyde is formed by cleavage of certain coal surface oxides# that are then oxidized (6) (a) Smith, A. C.; Miron, Y.; Lazzara, C. P. Large Scale Studies of Spontaneous Combusion of Coal, Report of InVestigations; United States Department of the Interior: 1991. (b) Bouwman, R.; Freriks, L. C. Fuel 1980, 59, 315. (c) Seki, H.; Ito, O.; Lino, M. Fuel 1990, 69, 317. (d) Liotta, R.; Brons, G.; Isaacs, J. Fuel 1983, 62, 781. (7) Grossman, S. L.; Davidi, S.; Cohen, H. Fuel 1995, 74, 1772. (8) (a) Davies, A. W.; Isaac, A. K. Inst. Min. Mettall. Trans. Sect. A 1999, 108, 85. (b) Smith, G. L.; Plessis, J. J. J. S. Afr. Inst. Min. Metall. 1999, 99, 117. (c) McGregor, R. Miners pay high price for China’s coal. In Financial Times, July 17th, 2002. (d) Coal mine explosion kills 48 in China [Online]; Available at: http://www.ananova.com/news/story/sm268418; April 22nd, 2001.
10.1021/ef9001583 CCC: $40.75 2009 American Chemical Society Published on Web 04/27/2009
OxidatiVe Decomposition of Formaldehyde by Coal
Energy & Fuels, Vol. 23, 2009 3079
by coal-derived hydroperoxides to form the triatom heterocycle dioxirane, CH2O210
which subsequently decomposes to yield hydrogen and carbon dioxide. Thus, as the concentration of the released formaldehyde and the surface hydroperoxides is determined by the formation of certain surface oxides formed via atmospheric oxidation of the coal, the hydrogen emission is correlated to the amounts of oxygen consumed by the coal. The oxidative decomposition of the formaldehyde is catalyzed by the coal’s surface.11 This reaction is not common. In most reactions of formaldehyde, when it serves as a reducing or hydrogenating reagent it decomposes to yield carbon monoxide and not carbon dioxide.12 Furthermore, oxidative decomposition of formaldehyde has also been noted in homogeneous phases with hydrogen peroxide and t-butyl-hydroperoxide as the oxidizing reagents.13 In all three systems studied, the yields of the carbon dioxide equals that of the oxidized formaldehyde, but the yield of hydrogen is only 10-30% of the oxidized formaldehyde. The following scheme of reactions has been proposed9,11 in order to describe the oxidation processes in the coal system, which produces CO2 and H2: (-C-H)coal + O2 f (-C-O-OH)coal + other surface oxides(5) decomposition
surface oxides 98 CH2O
(6)
All these reactions (5-11) are gas/solid processes that occur at the surface of the coal macromolecule. In the presence of dioxygen, the carboxyl radical, •COOH might be oxidized to yield hydroperoxyl radical and carbon dioxide via: COOH + O2 f HO2+CO2
(11)
It was decided to study the effects of different parameters on the yield of carbon dioxide and hydrogen in order to better understand the mechanism of the oxidative decomposition of formaldehyde that is catalyzed by bituminous coals. A typical American coal from Pittsburgh area (Consol-Bailey coal) has (9) Nehemia, V. Emission of Pollutant Gases from Stored Coal: OxidatiVe Decomposition of Formaldehyde Catalyzed by Bituminous Coal; Ph.D. Thesis, Chemistry Department, Ben-Gurion University of the Negev: Beer-Sheva, Israel, 2001. (10) (a) Busca, G.; Lamotte, J.; Lavalley, J.; Lorenzelli, V. J. Am. Chem. Soc. 1987, 109, 5197. (b) M ao, C. F.; Vannice, M. A. J. Catal. 1995, 154, 230. (c) Gomes, J. R. B.; Gomes, J. A. N. F.; Illas, F. J. Mol. Catal A 2001, 170, 187. (11) Nehemia, V.; Davidi, S.; Cohen, H. Fuel 1999, 78, 775. (12) Barannik, G. B.; Babkin, V. S. Combustion and Shock WaVes, 1973, 9 (3), 363. (13) Nehemia, V.; Richter, U. B.; Haenel, M. W. Cohen, H., Proc. 9th ICCS ’97, Essen, Germany 1997, 1, 345. (14) Taieb, H.; Davidi, S.; Cohen, H.; Meyerstein, D. Proc. 11th ICCS ’97, San Francisco 2001, 1, 221. (15) Bailey, A. J.; Griffith, W. P.; Parkin, B. C. J. Chem. Soc. Dalton. Trans. 1995, 1833.
Table 1. Properties of Consol-Baily Coala proximate analysis (wt %)
ultimate analysis (wt %, daf)
moisture
ashdb
VMdaf
C
H
S
CV (J · g-1)
5.87
7.78
37.20
78.07
5.18
1.50
3.047× 104
a
VM ) volatile matter; CV ) calorific value; daf ) dry ash free.
been selected for this purpose. The effects of methanol, water, sample size, and temperature on the yield of H2 have been studied and are described below. 2. Experimental Section All chemicals and gases used throughout the study were of AR grade and were supplied by Aldrich, Fluka, Merck, or Maxima. The water used was purified water (via ion exchange columns). The formaldehyde used (supplied by Aldrich) was 37 wt % in water and also contained 12-15% of methanol (the addition of the methanol is in order to suppress formldehyde polymerization to paraformaldehyde {CH2O}n).18 Coal. All of the experiments in this work were carried out with Consol-Baily coal from the United States of America (USA). Baily coal is a bituminous coal used by coal-fired power plants in Israel. The properties of Baily coal are presented in Table 1. The experiments were performed in sealed glass vials (30 mL) used as batch reactors. The reactors were charged with Baily coal (particle size e74 µ) and formaldehyde (the amount of formaldehyde is equivalent to 2.0 vol % CH2O as gas in the reactors atmosphere), in air atmosphere and heated at 55-95 °C for various periods in a nu¨Veoven model FT 30. All the coals in the present work were prepared by grinding and separating to several grain sizes by sieves. The coal samples were then dried in a Heraeus vacuum oven model VT6060 for 24 h at 60 °C. Gas Chromatography. The amount of the gases (CO2, H2, O2) 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”, 9’ ss column using He or N2 as the carrier gas. The experimental error in the GC determination is (5%. The gaseous atmosphere was sampled (0.5 mL samples) after the reaction, with gastight syringes (Precision Syringes, model A2), and its composition was measured in the gas chromatograph. The gases that could be determined are hydrogen, nitrogen, oxygen, carbon dioxide, carbon monoxide, and methane. The argon gas present is not separated from oxygen, thus the value determined for oxygen contains also 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 duplicate reactors to reduce the total experimental error. However, still the error is (15%, mainly due to the nature of the experiments.
3. Results and Discussion The effect of several parameters have been studied in detail, in order to obtain a better understanding of the oxidative decomposition process of the formaldehyde to molecular hydrogen and carbon dioxide. Effect of Methanol. The formaldehyde used during the previous studies was taken from concentrated aqueous solutions (16) (a) Dengel, a. A. C.; Griffith, W. P.; Parkin, B. C. J. Chem. Soc. Dalton. Trans. 1993, 2683. (b) Venturello, C.; D’Aloisio, R.; Bart, J. C. J.; Ricci, M. J. Mol. Catal. 1985, 32, 107. (c) Venturello, C.; D’Aloisio, R. J. Org. Chem. 1988, 53, 1553. (17) Sauer, A.; Cohen, H.; Meyerstein, D. Inorg. Chem. 1988, 27, 4578. (18) Grossblatt, N. Formaldehyde and Other Aldehydes; National Academy Press: Washington, D.C., 1981; Ch 5.
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Cohen and Green
Table 2. Methanol Effect on Yields of CO2 and H2 by Low Temperature Oxidation of Coala sample name blank methanol methanol
samplea
gas analysis
mass (gr) H2 (ppmv) CO2 (wt %) O2 (wt %) N2 (wt %) 5 5 0
456 648 0
5.34 6.50 0
0 0 21.1
Table 3. Water Effect on Yields of CO2 and H2 by Low-temperature Oxidation of Coal
94.6 93.4 78.9
a 0.8% Methanol was added to 30 mL reactors and thereafter heated at 95 °C for 5 h.
of formaldehyde (37 wt %) that also contains methanol (12-15 wt %). Methanol is added to the formaldehyde solution as an oxidation and polymerization inhibitor of the formaldehyde. In the absence of methanol, formaldehyde might be oxidized to formic acid or may polymerize to paraformaldehyde (PFA) (CH2O)n18 via radical reaction initiated by dissolved dioxygen, O2 (which is a precursor to formation of radicals, •R).
Since one of the possible pathways reaction (12a) for methanol to react might be oxidation to give formaldehyde and thereby be involved in carbon dioxide or hydrogen emission during the low temperature coal oxidation, it is of interest to study if methanol has an effect on the amount of carbon dioxide and/or hydrogen released during the oxidation process. Thus, the effect of methanol on the oxidation process was investigated. The samples were prepared by charging 30 mL batch reactors with 5 g of Baily coal samples (particle size e74 µ) and injecting 0.1 mL of an aqueous solution containing a concentration that releases 0.8 v % of methanol (this was the concentration in the gas phase in the reactor in all the experiments described below). After sample preparation, the batch reactors were sealed with rubber septa and aluminum caps in an air atmosphere and heated at 95 °C for 5 h until all the oxygen in the gas phase has been consumed by the coal oxidation process reactions 1-4. The concentration of methanol, 0.8 v %, was chosen in order to simulate the methanol concentration present as gas in the reactors atmosphere when using the formaldehyde solution with which previous experiments were performed. Blank experiments, without the addition of methanol to the coal, or 0.8 v % without coal have also been performed. The composition of the gas phase in the reactor was determined via gas chromatography. The results of experiments to determine the effect of methanol are summed up in Table 2: It is important to note that the slight difference in the yields of the gases observed between the methanol + coal sample and the blank is within the reproducibility of the experiments performed with coal samples (see above). Hypothetically, if the methanol did indeed follow, even partially, pathway 12a, then we would have expected to observe a massive increase (2000-8000 ppm, 4-16 times more then observed) in H2 production. However, only a 40% increase in H2 yield is observed, which is close to the reproducibility of the experiment. Thus, the results in Table 2 indicate that addition of methanol did not affect the yield of hydrogen or carbon dioxide that is produced during the low-temperature oxidation of the coal. These observations clearly show that methanol is not involved nor affects the chemical reactions that are present in the lowtemperature oxidation of the coals, reactions 1-4. Therefore it (19) Mori, M.; Weil, J. A.; Ishiguro, M. J. Am. Chem. Soc. 1968, 90, 615. (20) Lever, A. B. P.; Gray, H. B. Acc. Chem. Res. 1978, 11, 348.
gas analysis
water content in coal (wt %)
H2 (ppmv)
CO2 (wt %)
O2 (wt %)
N2 (wt %)
1.96 3.84 7.41 16.6
552 495 360 359
6.25 6.40 6.16 6.21
0 0 0 17.8
93.7 93.6 93.8 75.9
a Distilled water was added to 30 mL reactors and subsequently heated at 95 °C for 5.0 h.
is safe to conclude that the methanol presence in the formaldehyde solution used throughout the studies did not change the amounts of H2 or CO2 produced. Effect of Water. The coal used was vacuum-dried (see above Experimental section) resulting in an evaporation of most of the water content (coal moisture). However, the treated coal still contains 1-2% of strongly bound water (usually defined as intrinsic water).1 The low-temperature coal oxidation is a surface process occurring only inside the macropores (diameter g30 Å, surface area ∼2-3 m2/g coal) and not inside the micropores (diameter e10Å, surface area ∼120-250 m2/g coal). Thus, if the water content is high enough, it can block the entrance of oxygen into the macropores and reduce appreciably the active surface area available to the low-temperature oxidation of the coal. The effect of water on the oxidation processes in the coal was investigated. The 30 mL glass reactors were charged with 5 g of Baily coal samples containing 1.9-16.7 wt % water (wt % of the coal) in an air atmosphere and sealed with a rubber septa and aluminum cap. The reactors were isothermally heated in the simulation oven at 95 °C for 5 h. The composition of the gas phase of the reactor was determined via gas chromatography. The results of experiments to determine the effect of water are summed up in Table 3. The results in Table 3 demonstrate that when water content reaches 7-17 wt % it reduces appreciably the oxidation processes that produce hydrogen, reactions 7-10. However, carbon dioxide yields remained unchanged within experimental error. The fact that the carbon dioxide yields have not been altered show that most of the carbon dioxide is formed not via the formaldehyde route, reactions 7-10, but rather via reaction 4a. In this case, even if carbon dioxide production via the oxidative decomposition of formaldehyde is reduced appreciably (∼50% inhibition) the net reduction in the total yield of carbon dioxide is negligible. It is interesting to comment that a general procedure commonly used in order to reduce the explosion risks due to accumulation of dust in coal mines is a process known as coal wetting. Namely, while excavating the coal the tool-heads of the mining units are equipped with water nozzles that sprinkle small water droplets in order to control the plume of dust. This procedure will simultaneously quench or slow coal bed production of hydrogen, reducing the risk of hydrogen explosions. Effect of Sample Size. As it has been suggested, the coal surface, probably that of the macropores, serves as the catalyst. As the reactions occur at the surface, it is important to examine the effect of the size of the coal sample on the products’ yields. Thus, the effect of coal sample size (as the weight of the coal is correlated to its surface area if the grain size is more or less constant) has been studied, whereas the amount of formaldehyde is kept constant. The effect of the sample size on the oxidation processes in the coal was investigated. 30 mL glass reactors were charged
OxidatiVe Decomposition of Formaldehyde by Coal
Energy & Fuels, Vol. 23, 2009 3081
Table 4. Dependence of the Yields of CO2 and H2 Produced by Low-temperature Oxidation of Coal on Sample Sizea sample
gas analysis
name mass (gr) H2 (ppmv × blank 1 2 3
2 0.5 1 2
0 0.668 0.861 1.227
103)
Table 5. Initial Rates of Oxygen Consumption, Carbon Dioxide Release, and Hydrogen Formed As Function of the Temperature, 55-95 °Ca sample
CO2 (wt %) O2 (wt %) N2 (wt %) 0 0.780 1.09 1.92
21.1 19.2 17.7 16.1
78.9 79.9 81.1 81.8
a The 30 mL reactors contained also 0.80% Methanol and 2.0% of Formaldehyde and were heated at 95 °C for 2.0 h.
gas analysis
time (h)
temperature (°C)
H2 (ppmv × 103)
CO2 (wt %)
O2 (wt %)
N2 (wt %)
2 8 12 20 30
95 85 75 65 55
0.938 1.477 1.284 1.091 0.783
1.12 2.18 1.43 1.54 0.917
19.0 15.2 17.0 17.4 18.7
79.8 82.5 81.4 80.9 80.3
a The 30 mL reactors were charged with 1.0 g of Baily coal and also contained 0.80% methanol and 2.0% of formaldehyde in air and were heated at 55-95 °C for 2.0-30 h.
take into account that these activation energies are apparent activation energies, as every product is formed via several consecutive reactions and not as a result of a single step. The rates of the reactions were measured using the initial rate method. The method of initial rates involves measuring the rate of reaction at low O2 conversion (
