CO2-Free Energy and Hydrogen Production from ... - ACS Publications

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Energy & Fuels 2006, 20, 1242-1249

CO2-Free Energy and Hydrogen Production from Hydrocarbons Alexander Fridman and Alexander Gutsol* Department of Mechanical Engineering and Mechanics, Drexel UniVersity, 3141 Chestnut Street, Philadelphia, PennsylVania 19104

Alexander Dolgopolsky Department of Mathematics, Drexel UniVersity, 3141 Chestnut Street, Philadelphia, PennsylVania 19104

Emil Shtessel Exotherm Corporation, 1035 Line Street, Camden, New Jersey 08103 ReceiVed August 3, 2005. ReVised Manuscript ReceiVed January 24, 2006

Partial oxidation or conversion of hydrocarbons to polymerized carbon suboxides can result in energy and/ or hydrogen production without CO2 emission. Despite the lower energy efficiency of this process in comparison with complete oxidation, this approach is promising because it avoids many problems related to fossil fuel combustion. Carbon suboxides, which are similar to the humic components of soils, may be used at a large scale as a soil conditioner. Addition of organic carbon to soil may counteract widespread carbon losses from soils due to climate change and land use patterns. This article discusses the conversion process thermodynamics for coal, biomass, and natural gas as the most abundant hydrocarbon feedstocks, as well as possible process organization and limitations. Conversion of methane into energy or hydrogen without CO2 emission has a thermodynamic efficiency as high as 65%.

1. Introduction Reduction of CO2 emission is an important task of the modern energy generation industry. The goal of this article is to discuss energy generation from hydrocarbons without any CO2 production. While it may seem strange to the reader, this approach to energy generation may use even such feedstocks as coal. To explain this idea, we would like to bring to the reader’s attention the existence of a carbon oxide other than CO and CO2, namely C3O2, that can be polymerized and, as a polymer, forms a chemically and thermodynamically stable substance.1 Vast amounts of carbon on earth exist as a similar polymer substance, the so-called humic acids,2 which are major organic component of soils. The conversion of hydrocarbons into humic acids with an effectively organized energy release process appears to be an advantageous approach for the energy industry. It is necessary to emphasize that conversion of solid hydrocarbons, such as coal and biomass, into carbon suboxides by a relatively lowtemperature process that preserves the major structure of the initial fuel would solve many important problems simultaneously (we will show later that the structure of suboxides is very close to that of cellulose). In addition to energy generation without CO2 emission, such a process would eliminate the problems of high-temperature emission of heavy metals, sulfur oxides, and nitrogen oxides. These coal “impurities” remain in the solid residue, which has approximately the same volume as the coal. * Corresponding author. Phone: 1-215-895-1485. Fax: 1-215-895-1633. E-mail: [email protected]. (1) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press: New York, 1984. (2) Bio Ag Technologies International. Humic Acid Structure and Properties. http://www.phelpstek.com/graphics/bioag/humic_acid.pdf, Sept 1999.

The problems of ash utilization and inorganic component addition into organic fertilizers (conditioners) are also addressed. Polymerized suboxide, containing a primarily humic component with nitrogen and sulfur trapped in the chemical structure, as well as micronutrients associated with the fuel ash (K, Mg, P, etc.), may be used as a soil conditioner. Land application can recycle nutrients to the agricultural ecosystem; improve soil ion transport, water penetration and water retention; and disaggregate clay structures. Current price for humic-based soil conditioners is on the level of $2/lb, and some of them are produced from low-quality coal, for example, lignite.3 Addition of organic carbon to soil may also counteract widespread carbon losses from soils due to climate change and land use patterns.4 Recent estimates of the magnitude of this loss are on the order of 13 Tg/yr (1.3 × 1010 kg/yr) for the United Kingdom only.5 Practical realization of such hydrogen energy and humic acid production (HEHAP) process looks like a promising solution to the CO2 emission problem, at least temporarily. Of course, this solution is not “for free”: incomplete oxidation of hydrocarbons means less energy release. Therefore, this approach raises a couple of key questions: (1) How much energy may be obtained from such a process, and what will be the quality of this energy? The quality of the extracted energy is of extreme importance because later it should be effectively converted into other forms (i.e., mechanical, (3) United Nations Earth Summit+5 Success Stories. Lignite and Biotechnology. http://www.un.org/esa/earthsummit/unido4.htm, Nov 1, 1997. (4) Schulze, E. D.; Freibauer, A. Carbon Unlocked from Soils. Nature 2005, 437, 205-206. (5) Bellamy, P. H.; Loveland, P. J.; Bradley, R. I.; Lark, R. M.; Kirk, G. J. D. Carbon Losses from All Soils across England and Wales 19782003. Nature 2005, 437, 245-248.

10.1021/ef050247n CCC: $33.50 © 2006 American Chemical Society Published on Web 03/04/2006

CO2-Free Energy and Hydrogen Production

Figure 1. Basic structure of (C3O2)n.6

electrical, etc.). Low-temperature heat, for example, is lowquality energy, and it can be converted into the mechanical energy only with significant losses, in accordance with the second law of thermodynamics. (2) What would the energy generating technology process look like if carbon suboxides will be produced simultaneously? This question is relevant with respect to coal, biomass, and natural gas as the most abundant hydrocarbon feedstocks. In this article, we attempt to answer some of these key questions, at least in principle, to show the alternative to the existing paradigm of energy generation. 2. Thermodynamics of the Process Carbon suboxide is a foul-smelling lachrymatory gas produced by the dehydration of malonic acid, CH2(COOH)2, with P4O10. The carbon suboxide molecule is a linear and symmetric structure that can be represented as OdCdCdCdO. It is stable at -78 °C, but at 25 °C the compound is unstable; it polymerizes to form highly colored solid products. The basic structure of all its polymers appears to be a polycyclic six-membered lactone (Figure 1).1,6 Since carbon suboxide is the acid anhydride of malonic acid, it slowly reacts with water to produce that acid. In the laboratory, carbon suboxide is widely used as a source of atomic carbon. As a gas, it can be stored in a bulb at a pressure of a few Torrs, but under conditions of standard temperature and pressure (300 K, 1 atm), C3O2 forms a yellow, red, or brown polymer (C3O2)n (ruby red above 100 °C, violet at 400 °C, and it decomposes into carbon at 500 °C).1 Formation energy for gaseous C3O2 is -97.6 kJ/mol7 or -95.4 kJ/mol.6 The formation energy for liquid polymerized carbon suboxide (C3O2)n is -58.8 kJ/mol of carbon.8,9 Estimates based on a thermodynamic database10 show that formation energy for solid polymerized carbon suboxide (C3O2)n is about -112 kJ/mol of carbon. As mentioned earlier, similar polymer substances are major organic components of soils called humic acids or humic substances. In their “pure” state, they are black solids with approximately 55-60% carbon and 2-3% hydrogen, the balance being predominantly oxygen.11 The chemical composition of humic substances can vary, but its basic component is (C3O2)n. Because of composition variations in natural humic (6) Ballauff, M.; Li, L.; Rosenfeldt, S.; Dingenouts, N.; Beck, J.; KriegerBeck, P. Analysis of Poly(carbon suboxide) by Small-Angle X-ray Scattering. Angew. Chem., Int. Ed. 2004, 43, 5843-5846. (7) Chase, M. W. NIST-JANAF Thermochemical Tables, Fourth Edition. J. Phys. Chem. Ref. Data 1998, 1-1951; Monograph 9. (8) Kybett, B. D.; Johnson, G. K.; Barker, C. K.; Margrave, J. L. The Heats of Formation and Polymerization of Carbon Suboxide. J. Phys. Chem. 1965, 69, 3603-3606. (9) McDougall, L. A.; Kilpatrick, J. E. Entropy and Related Thermodynamic Properties of Carbon Suboxide. J. Chem. Phys. 1965, 42, 23112321. (10) Kondratiev, V. N. Chemical Bonding Energies, Enthalpy of Chemical Processes, Ionization Potentials and Electron Affinities; Nauka: Moscow, 1974. (11) Davidson, R. M. Natural Oxidation of Coal; IEACR/29; IEA Coal Research, University of Kentucky: Lexington, KY, 1990.

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substances (as well as in coal and biomass), it is difficult to find their formation energy. Therefore, the data for chemically pure substances will be used for our estimates: carbon in the form of graphite instead of coal, methane instead of natural gas, polymerized suboxide (C3O2)n instead of humic substances, and cellulose, (C6H10O5)n (with the formation energy of -962 kJ/ mol of monomer),12 instead of biomass. Coal is one of the most commonly used sources of energy in modern industry. It easily ignites at temperatures above 300 °C, sometimes spontaneously under natural conditions. At higher temperatures, it burns and generates energy that is converted into other forms thus participating in the industrial cycle. Heat release accompanying coal burning obviously depends on its composition. However, to specify the numeric values, one can illustrate the process by oxidation of carbon (in the form of graphite):

C (s) + O2 f CO2 (g), enthalpy: ∆H ) -393.5 kJ/mol (1) It is well-known that under natural conditions of weathering slow oxidation of coal occurs. As a result of this process, condensed phase material that can be identified as carbon suboxide is left as a residual with no, or very little, carbon dioxide generated. This process is described in numerous scientific publications and has its own peculiar features and subtleties. In all recent sources, the reference would be made to many not fully investigated aspects of coal oxidation and weathering.11 Heat release accompanying coal oxidation to the suboxide state, just as in the case of burning, obviously depends on its composition. Similarly to eq 1, one can illustrate the incomplete oxidation process by carbon oxidation to its polymerized suboxide:

C (s) + 1/3 O2 f 1/(3n) (C3O2)n (s), ∆H ) -112 kJ/mol carbon (2) Apparently, carbon oxidation to a suboxide phase still provides considerable heat release, but in contrast to burning, it occurs without any CO2 emission. Taking into account essential hydrogen content in coal, water will also be produced during the process, and heat release of the oxidation can, therefore, be higher than the enthalpy shown above. It should also be emphasized that suboxides can be utilized further, as fertilizers, and/or in biochemical technologies that do not generate large-scale CO2 emission. Natural conversion of coal and other organic fuels (peat, wood) into the polymerized carbon suboxides of soil goes slowly and without formation of a monomer that can interact with water. We are going to consider different approaches to accelerate and utilize these natural processes, and we do not consider the analytical method of suboxide production from malonic acid (see above) as promising from the industrial point of view. Kinetics of “slow” oxidation of coal, illustrated by eq 2, proceeds, at low temperatures, at rates prohibiting the effective use of the process in energy generation; at low temperatures it takes many hours or days.11 The question is whether it is possible to speed up this process. If possible, the exothermic oxidation reaction can possibly be organized in such a way that oxidizing coal could be used as a clean energy source. Unfortunately, acceleration of the process by means of quasi(12) Perks, H. M.; Liebman, J. F. Estimation of the Enthalpies of Formation of Some Common, Solid-Phase Compounds of Considerable Theoretical Importance. J. Struct. Chem. 2000, 11, 325-329.

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equilibrium temperature increase is impossible. Essential temperature increase (i.e., above 300 °C) immediately results in a formation of gas-phase carbon oxides (CO and CO2).12 This means that the quality of energy that can be obtained in the form of heat, as a result of oxidation process (eq 2), is low. However, the situation will become radically different if this energy can be extracted in the chemical form. For example, water vapor can be added to the oxygen, or air, to make the oxidation process thermoneutral with ∆H ≈ 0. Once more, neglecting the hydrogen content of coal, the thermoneutral process of steam-air conversion of carbon into suboxides without CO2 emission can be illustrated as follows:

2.16 C (s) + 0.22 O2 + H2O f 0.72/n (C3O2)n (s) + H2, ∆H ≈ 0 (3) Hydrogen obtained in this process, is a source of very highquality chemical energy. For hydrogen the low caloric value (LCV) is 241.8 kJ/mol H2, or 112 kJ/mol carbon used in eq 3, the same value as that in eq 2. Chemical energy of hydrogen can be easily converted into mechanical energy (i.e., as in turbines) or directly into electrical energy (i.e., as in fuel cells). An assembly of a reactor for the thermoneutral process (eq 3), together with a fuel cell, will result in production of energy of the highest qualityselectrical energysand the water will be used as catalytic and chemical transport medium only. It is necessary to emphasize here that coal contains a considerable amount of hydrogen (typically 0.5-1 hydrogen atom per carbon atom), so that in technological production of hydrogen the yield should be much higher. Additional high-quality energy can be obtained if a part of oxygen in eq 3 can be substituted by water vapor and a low-temperature waste heat. A discussion of the organization of this process in technological terms will be presented in the next section. Complete oxidation of methane, which actually illustrates the combustion process, results in significant energy release:

CH4 + 2 O2 f CO2 + 2 H2O, ∆H ) -802.5 kJ/mol

(4)

Replacing the complete oxidation process by partial oxidation with the formation of polymerized solid suboxides without CO2 emission leads to the following reaction:

CH4 + 1/3 O2 f 1/(3n) (C3O2)n (s) + 2 H2, ∆H ) -37.4 kJ/mol (5) Addition of a small amount of water can lead to the thermoneutral reaction:

1.3 CH4 + 1/3 O2 + 0.2 H2O f 1.3/(3n) (C3O2)n (s) + 2.8 H2, ∆H ≈ 0 (6) The LCV of the produced hydrogen is about 520.8 kJ/mol of methane. Taking into account that the LCV of methane is 802.5 kJ/mol (that can be released in combustion eq 4), we find that the conversion of methane into hydrogen (eq 5) without CO2 emission has the efficiency of 65%. This efficiency is obviously better than that of coal conversion discussed earlier, which is the result of a high hydrogen content of methane. The rest of the energy (in this case, about 35%) stays in the form of suboxides, which can be further used as organic fertilizer. Again, additional high-quality energy in the form of hydrogen can be obtained if part of the oxygen in eq 6 can be substituted by a low-temperature waste heat.

Figure 2. Polymerized carbon suboxide film deposited on the inner surface of the plasma reactor quartz tube.

It is interesting to compare the process of methane conversion into polymerized carbon suboxides (eq 5) proposed in this article with direct methane conversion into carbon and hydrogen:

CH4 f C (s) + 2 H2, ∆H ) +74.6 kJ/mol

(7)

This process can be effectively organized in thermal plasma, as demonstrated by Steinberg.13 Though this plasma process also permits conversion of methane into hydrogen and avoids CO2 emission, the thermodynamic energy efficiency in this case (about 51%) is not as good as that in the case of suboxide production. It should also be mentioned that this process requires the use of high-quality energy (i.e., electrical or concentrated solar energy).14 It is also important that the solid product of Steinberg’s conversion is elemental carbon, which may be a less valuable coproduct than carbon suboxides in a large-scale energy production process. 3. Physicochemical Aspects of Tentative Technological Processes for Energy Production from Hydrocarbons without CO2 Emission a. Methane and Coal Conversion. We already have some experience in obtaining polymerized carbon suboxide from methane in plasma-assisted “super-rich” (equivalence ratio > 4) oxidation of methane. These experiments were made within the framework of plasma-catalytic conversion of methane into syn-gas and hydrogen.15 Plasma reactor quartz tubes with deposited film of polymerized carbon suboxide are shown in Figure 2. Earlier, a similar product was obtained during a study of microwave discharge in CO. Formation of polymerized C3O2 in nonthermal plasma of low-pressure glow discharge was also reported earlier.16,17 The results obtained show that formation of polymerized carbon suboxides is possible even at relatively high temperatures (at about 500 °C) under oxygen-deficient conditions. Nonequilibrium plasma can effectively stimulate this process. It is necessary to mention that, under nonequilibrium plasma stimulation or plasma catalysis, we employ relatively cold plasma as (13) Steinberg, M. Fossil Fuel Decarbonization Technology for Mitigating Global Warming. Int. J. Hydrogen Energy 1999, 24, 771-777. (14) Hirsch, D.; Steinfeld, A. Solar Hydrogen Production by Thermal Decomposition of Natural Gas Using a Vortex-Flow Reactor. Int. J. Hydrogen Energy 2004, 29, 47-55. (15) Kalra, C. S.; Gutsol, A.; Fridman, A. Gliding arc discharge as a source of intermediate plasma for methane partial oxidation. IEEE Trans. Plasma Sci. 2005, 33, 32-41. (16) D’Amico, K. M.; Smith, A. L. S. Mechanism and Rate of Loss of CO in Glow Discharges in CO, CO-He and CO-N2-He. J. Phys. D: Appl. Phys. 1977, 10, 261-267. (17) Mori, S.; Akatsuka, H.; Suzuki, M. Carbon and Oxygen Isotope Separation by Plasma Chemical Reactions in Carbon Monoxide Glow Discharge. J. Nucl. Sci. Technol. 2001, 38, 850-858.


a source of radicals and other active species (ions, excited molecules and atoms, etc.) but not as a source of energy. In such plasma-catalytic systems with high selectivity,15 plasma energy constitutes a very small fraction (i.e., on the order of 1%) of the total energy balance. Separation of polymerized carbon suboxides from gaseous reagents and products is a separate technological task that can probably be accomplished by (C3O2)n condensation on an additional solid-phase material passing through the reactor, for example, on the surface of sand particles, or even on residues of coal particles. In the latter case, it is possible to organize a thermoneutral partial methane oxidation-steam conversion reaction with hydrogen production by an arbitrary combination of eqs 3 and 6. For example:

4.9 CH4 + 3.3 C (s) + 6.3/3 O2 + 2.6/3 H2O f 8.2/(3n) (C3O2)n (s) + 33.2/3 H2, ∆H ≈ 0 (8) In addition to using nonequilibrium plasma as a volume catalyst for such a process, other methods of equilibrium shift can also be applied. For example, the increase of pressure in a process together with simultaneous hydrogen separation using a hydrogen-permeable or electrochemical membrane should shift the reaction in the direction of polymerized carbon suboxide formation. It is relatively easy to separate hydrogen from other gases due to its unique properties. For example, it is known that palladium- and nickel-based membranes can separate pure hydrogen at elevated temperatures. Polymer membranes also have very high selectivity for hydrogen separation. Another option of hydrogen “separation” is electricity production using a fuel cell inside the reactor. This would also result in effective hydrogen depletion in the reactor atmosphere. Continuous operation of a solid-fed process (with coal or biomass; see below) can be organized, for example, in a countercurrent flow reactor: solid fuel comes from one side and a gaseous reagent from the opposite side, while gaseous products flow out from the first side and solid products exit from the second side. An example of such an industrial process is use of rotational furnaces for metal sulfide oxidation. During conventional high-temperature coal combustion, new portions of solid reagent (e.g., coal) enter chemical reaction (eq 1) after the removal of gaseous product (CO2) and a subproduct (CO) from the reagent surface. The key question regarding conversion of coal into solid polymerized carbon suboxides is the following: What would be the mechanism of chemical reaction (eq 2) propagation? To answer this question, one needs to consider mechanochemical processes of coal conversion. Before doing this, let us briefly review the extensively described but still not fully understood process of coal oxidation. Oxidation of coal at low temperatures starts with adsorption of oxygen at the surface with its further propagation inside the volume by means of diffusion. The process is exothermic, and the rate of oxidation markedly increases with temperature. The release of energy at temperatures above 70 °C leads to the formation of alkali-soluble products (i.e., humic acids). Under natural conditions, the oxidation process lasts from 80 to 100 hours, leading to degradation of the coal substance. At temperatures in the range of 150-250 °C, acidic products are reputed to be stable, but at higher temperatures the reaction becomes rapid and strongly exothermic: the coal reaches the “ignition” point and starts to burn. Thus, it is reasonable to expect that the excess of oxygen in exothermic reaction leads to combustion, while at low oxygen levels and moderate temperatures slow oxidation occurs with condensed phase products as a residual. The acceleration of a slow oxidation process by a few orders


Figure 3. Basic structure of cellulose.

of magnitude may be achieved in special cases (e.g., with the use of nonequilibrium low-temperature plasma as volume catalyst, and/or by changing pressure and composition of reacting gas). Coal is a heterogeneous material. Factors such as mineral structure, moisture content, thermal chemistry, particle size, and surface area affect the oxidation process over a range of temperatures. The role of thermodynamic parameters affecting the slow oxidation process is not always quite clear. Structural changes in organic matter intricately interact with the oxidation chemistry. During low-temperature coal oxidation, the temperature of coal as a result of the coal-oxygen reaction starts to rise. If it reaches the “threshold” value somewhere between 80-120 °C, a steady reaction resulting in the production of gaseous products such as carbon dioxide ensues. In this temperature range, the oxidation rate significantly increases and becomes dependent on the structural properties of coal, such as porosity, presence of microdefects, microcracks, and so forth. At this point, the combination of mechanical and chemical properties of coal substance becomes important because further continuation of the process involves both. The adsorption of oxygen by coal gives further rise to microdefects in the coal structure. Microdefects interact, creating pores, microcracks, and so forth, that, in their own turn, enhance oxygen diffusion, thus creating a positive feedback loop for propagation of the degradation zone. The range of thermodynamic parameters that characterize the degradation zone is of critical importance, since in this range and region the intense conversion of coal into gaseous and acid products occurs. An analysis and a mathematical model of the mechanochemical process of coal oxidation are described in the Appendix. b. Biomass Conversion. As mentioned earlier, cellulose (C6H10O5)n (Figure 3) can be considered as representative material for modeling of biomass conversion. Under natural anaerobic conditions, the conversion of biomass into energy carriers proceeds over millennia. Thermal processes, pressure, and presence of bacteria all work to transform biomass into natural gas, oil, coal, and humic acids. To use biomass as a renewable source of energy, the conversion process should be arranged at a much faster pace. Conversion of dry biomass into energy in a conventional combustion process occurs as follows:

1/n (C6H10O5)n + 6 O2 f 6 CO2 + 5 H2O,

∆H ) -2608 kJ

From the standpoint of greenhouse effect prevention, biomass releases more energy than coal, but less than natural gas: production of energy per mole CO2 is 434.7 kJ for cellulose, 393.5 kJ for carbon, and 802.5 kJ for methane. Similarly to coal, the use of biomass causes a lot of technical problems because of residual ash formation. Therefore, most research efforts concerning biomass use are directed toward production of high-quality biofuel (i.e., ethanol, biodiesel, etc.). Neverthe-


less, in all methods of processing, the final result of biomass usage as a source of energy is its full conversion into water and CO2, with some solid residual left. This case raises a general question: what is the advantage of using biomass in comparison with methane (which is a product of natural conversion of biomass) from the standpoint of greenhouse effect prevention? Indeed, we produce more CO2 per unit energy from biomass because energy quality is lower for biomass than for methane. The positive result can be achieved only if, in the process of biomass production and conversion, part of biomass carbon can be turned into solid products. It is supposed that biomass production using modern agriculture results in carbon fixation in soils because a significant part of formed biomass (roots) remains in soil. However, a recent study shows that this process cannot compensate for widespread carbon losses from soils due to climate change and land use patterns.4 Data obtained from the National Soil Inventory of England and Wales between 1978 and 2003 show that carbon was lost from soils across England and Wales over the survey period at a mean rate of 0.6% per year.5 Moreover, recently researchers from the Max Planck Institute for Nuclear Physics made the surprising discovery that plants release methane, a greenhouse gas, and this goes against all previous assumptions.18 All this means that use of biomass as a renewable source of energy is rather questionable from the standpoint of greenhouse effect prevention, and if biomass is used, it is necessary to ensure that a significant part of carbon content remains in solid residue. In this case, the conversion of biomass into carbon suboxides as organic conditioner is obviously the best choice. Technically, this process should be even simpler than conversion of coal because dry biomass has a porous structure and a significant amount of oxygen content. Therefore, no limitation on the diffusion process can be expected in this case. Moreover, cellulose structure (see Figure 3) is quite similar to the structure of (C3O2)n (see Figure 1). The excess of oxygen demands conversion of part of hydrogen into water:

Fridman et al.

posed processes. This concept, which can be called HEHAP, involves the oxidation process of energy generation (that uses the energy carriers such as hydrogen) from hydrocarbons without CO2 production. Conversion of solid hydrocarbons such as coal and biomass into carbon suboxides by a relatively lowtemperature process would solve many existing important problems simultaneously. In addition to energy generation without CO2 emission, benefits may include the elimination of ash accumulation, elimination of heavy metal, nitrogen, and sulfur component emission, and addition of microelements (e.g., N, S, P, K, Na, etc.) into organic fertilizers. Production of polymerized carbon suboxides with soil conditioner quality at large scale may counteract widespread carbon losses from soils due to climate change and land use patterns. Appendix

The approach presented in this article suggests a significant shift of the paradigm of hydrocarbon feedstock use as an energy source. The presented concept shows that it is possible to release a substantial portion of chemical energy of hydrocarbons in the form of hydrogen or electricity, together with simultaneous bonding of carbon into polymerized carbon suboxides that can be used as organic fertilizers. Of course, further experimental validation of the presented approach is necessary, but there are no fundamental restrictions precluding realization of the pro-

Mechanochemical Process of Coal Oxidation and Mathematical Model. Adequate supply of air and/or water vapor into the volume of coal is necessary for the conversion process to proceed. Therefore, the presence of microcracks, fissures, and porosity is of extreme importance. The process of motion of the degradation zone is mechanochemical by nature. The initial oxygen flux into the surface of coal substance by the diffusion mechanism penetrates some distance inside the volume, creating simultaneously microcracks, pores, and other inhomogeneities in the coal particle structure. Loosening of the coal structure, in turn, accelerates oxygen diffusion, thus enhancing the process. A similar mechanism has been described earlier19 for the so-called, steam-zirconium reaction (i.e., SZR) in nuclear reactors. The characteristic feature of the mechanism is the breakaway phenomenon characterized by transition from parabolic to linear dependence of the oxidation zone moving inside the volume over time. In the case of oxidation in nuclear reactors, the breakaway phenomenon obviously plays a negative role, accelerating the reactor’s loss of structural capacity, while in the present situation rapid oxidation in the oxygen/steam atmosphere can make the process conducive to hydrogen production. A significant amount of data on physical and chemical properties of coal is given in ref 11. Coal’s pore structure is strongly influenced by weathering under natural conditions; the presence of water weakens its structure. It is worth mentioning that the affinity of oxygen to coal sometimes leads, as mentioned before, to a spontaneous combustion under natural conditions. This combination of physical and chemical properties with, for example, catalyzing assistance of low-temperature plasma or high pressure will allow for the process of oxidation to proceed at an accelerated pace, so that hydrogen would be produced with solid humic acids as a residual. In the described process, generation of CO2 and other unwanted gaseous components can be avoided. The key feature of the mathematical model is the use of a combination of diffusion equations for oxygen penetration into coal and of a mechanical deterioration model in terms of modern fracture and damage mechanics. It was noted19 that physical mechanisms of deterioration can be considered as brittle or ductile (or a combination of both), and in each case an appropriate mathematical model can be developed. Obviously, each model (i.e., brittle or ductile) is only a rough approximation to the real situation and, therefore, has

(18) The Forgotten Methane Source. Physorg.com: Science, Technology, Physics, Space News. http://www.physorg.com/news9792.html, Jan 11, 2006.

(19) Baranov, I. E.; Fridman, A. A.; Kirillov, I. A.; Rusanov, V. D. Breakaway Oxidation Effect and Its Influence on SeVere Accident; I. V. Kurchatov Institute of Atomic Energy: Moscow, 1991.

1/n (C6H10O5)n f 2/n (C3O2)n (s) + 4 H2 + H2O, ∆H ) 496.2 kJ To compensate for energy demand for this reaction, it is necessary to oxidize about half of the hydrogen produced by this process:

1/n (C6H10O5)n + O2 f 2/n (C3O2)n (s) + 2 H2 + 3 H2O, ∆H ) 12.6 kJ or to use additional low-temperature heat. Once more, in our opinion, this process can be stimulated by low-temperature plasma, selective hydrogen removal, and increased pressure. 4. Conclusion


an illustrative character. Any such model reflects only certain features of one physical process. There is no doubt, however, that chemical reaction of oxidation triggers some microkinetic process leading to the formation of microdefects, in particular pores and cracks, that appear in any process of destruction of solids. In the present article, the degradation process of coal substance starting at its surface as a result of oxidation reaction is considered in terms of linear fracture mechanics which describes the key features of surface oxide film macroscopic disintegration. This process can be described as switching from the initial diffusion process accompanied by a fracture zone formation to a breakaway oxidation phenomenon.19 The latter occurs because of interaction between the diffusion, chemical reaction, structural and morphological transitions, and degradation. The breakaway mechanism can be thought of as a positive feedback loop with the following pivots: Oxygen diffusion via vacancies, oxidation reaction and oxygen dissolution, geometrical and morphological transformations (i.e., molar volume increase and onset of new grains and formation of microdefects), mechanical stress evolution, oxide layer degradation (i.e., pore interconnection, microcrack system formation), plastic flow, elastic deformation, and again, oxygen diffusion through microdefects. Oxygen diffusion deep into coal triggers oxide film dissolution and propagation of the chemical oxidation reaction front. This chemical reaction, in turn, brings about the oxygen transition from a labile to a bound state, which results in the appearance of mechanical stresses. Growth of elastic stresses, appearing as a result of molar volumetric increase and the nonconformity of the reacting components and product structures, is usually accompanied by phase transformations in the oxide layer that finally result in a formation of a scale-defective structure, which is a system of interconnected cracks and pores. The scale degradation ensures fast migration of gas through macroscopic defects and enhanced oxygen anion diffusion through the oxide. The formation of a permeable system of cracks and/or pores makes the oxidation process autocatalytic. The key feature of this autocatalysis is that it occurs due to morphological and structural changes in the oxidized coal while these changes are caused by the elastic stresses affected by the topochemical reaction. The conceptual basis of the breakaway oxidation process, viewed as a degradation of the oxide film resulting from a combined action of diffusion, chemical reaction, structural rearrangement, and its consequent destruction, allows construction of a mathematical model of the described phenomenon that is briefly described below. The diffusion-driven propagation of the oxidation reaction front can be analyzed by using self-similar approximation to the solution of the diffusion equation for the concentration of oxygen in coal. After that, invoking the concepts of fracture mechanics, the dependence of the breakaway oxidation rate on the main thermodynamic parameters (e.g., such as temperature), and elastic and morphological oxide characteristics can be explicitly calculated. The rate of each process component can be evaluated so that its contribution to the overall degradation zone propagation process becomes apparent. Let us consider first the structure of the chemical oxidation reaction zone as a whole. Its schematic representation is given in Figure A1. Part c of Figure A1 shows the graph of qualitative distribution of oxygen concentration. The meaning of LC will be seen from equations in the Appendix. Part b of Figure A1 describes different stages of the process in terms of their morphological


Figure A1. Breakaway oxidation wave front structure: (a) morphological changes in oxide, (b) zones of substantial influence of different conjugated processes, and (c) oxygen distribution in coal and oxide.

structure. The whole Figure A1 describes steady motion of the breakaway oxidation front at temperatures 150-250 °C, the range of temperatures characteristic for the process of natural slow oxidation of coal. For the description of the oxidation zone, only diffusion and chemical processes need to be considered. Let us introduce the surface where the destruction begins in order to describe the process. This is the surface separating compact oxide from the scale permeable for gaseous oxygen and steam. In the predestruction zone (part b of Figure A1), microscopic pores and cracks nucleate and grow. In the destruction (fracture) zone, macroscopic interaction and growth of pores and cracks take place. The macroscopic consequence of the oxide degradation (i.e., enhanced oxygen penetration through permeable oxide scale) may be phenomenologically delineated by the following equation:

∂c ∂ 2c ) D 2 + S(c(x),ξr - ξf) ∂t ∂x

(A1)

where c(x) is the oxygen concentration in the oxide film, D ) D0 exp(-Ea/RT) is the diffusion coefficient, and S(c(x),ξr - ξf) is oxygen source describing the rate of oxygen infiltration due to the film permeability, and ξr and ξf are the coordinates of the reaction and degradation fronts, respectively. This equation can be rewritten in terms of self-similar variable ξ ) x + Vt. Then it takes the form:

V

∂2c ∂c ) D 2 + S(c,ξ) ∂ξ ∂ξ

(A2)

To determine oxygen source dependence on the characteristics of the degraded scale, it would be necessary to solve the problem of nucleation, growth, and interaction of microdefects with subsequent transport of oxygen via permeable system of defects. However, this is not the purpose of the present article, and instead, for illustrative purposes an assumption on the mathematical form of the source is made:

S(c,ξ) )

c0Dψ LB

2

[

exp -

c2 c(ξ - LB)

-

]

c(ξ) c1

(A3)

where J0 ) c0Dψ/LB2 is a diffusion flux through the oxide barrier of width LB, and ψ is the volume fraction of the interconnected permeable cracks or pores. The exponential form of the factor provides for a rapid fall off the source power outside the compact region of the


Fridman et al.

deteriorated material. This property of nonlinear source admits the use of asymptotic methods of analysis. The solution of the homogeneous equation with the source absent (i.e., S ) 0) takes the form:

V c(ξ) ) c1 exp ξ D

( )

Then the nonhomogeneous eq A2 with the described source to the first approximation takes the form:

[

]

{

[

]}

c2 (LB - ξ) ∂c ∂ Vc(ξ) - D ) J0 exp - exp V ∂ξ ∂ξ c1 D

(A4) kL ) Jdiff ) D

Matching the solutions for the regions ξ < 0 and ξ > 0, one can obtain the following result for parameter V in eq A4:

V)

[ ( )]

c1 c0D2ψ D ln ln 2 2 LB c2 L V c B 2

(A5)

This means that the velocity of the reaction front is determined uniquely by the above equation. Disregarding the weak logarithmic dependence on ψ (the volume fraction of the interconnected permeable cracks) allows the approximation:

D c1 ln V≈ LB c 2

(A6)

The obtained result represents velocity V of the steady state, self-similar, diffusion-driven front of the oxidation reaction, supported by the degradation front staying at a distance LB behind the oxidation reaction front. In summary, one can state that theoretical description of the breakaway oxidation wave requires simultaneous consideration of two fronts: chemical oxidation reaction front and degradation front. Their mutual position and common velocity of self-similar motion must be determined by taking into account evolution of mechanical stresses and destruction of an oxide layer. Mechanical Process: Elastic Stresses and Fracture. Under isothermal conditions, the sources of mechanical stresses appearing because of growth of an oxide film can be listed as follows: (a) differences in molar volumes between the oxide and coal, (b) epitaxial differences in lattice parameters, (c) changes in chemical composition, for instance, due to the oxygen solution, (d) recrystallization process, and (e) specific features of sample geometry. Although not all the details of physical mechanism of stress generation in coal are clear at the present time, it is known that the degradation front advances from the surface into the volume of coal.11 This happens within the temperature interval of 150-250 °C. A reasonable hypothesis is that this range of temperatures allows other thermodynamic parameters, such as internal stresses in oxide film, to rise to a critical level σc characteristic for the formation of microcracks. As in ref 19, spherical stresses satisfy the equations of linear elastostatics (i.e., equilibrium and compatibility equations, plus Hooke’s law). The stresses are caused by oxygen diffusion and are proportional to the concentration of oxygen in the oxide layer:

σy ) σx ) f(x) ) -

ωE c(x) 1-ν

This is an important factor since compression enhances shearing deformation which is conducive to the creation of pores, cracks, and the overall softening of the material. Following the model considered in ref 19, only brittle fracture is taken into consideration. Suppose that the only defects in the oxidation layer are macrocracks, parallel and perpendicular to the reaction front, as in part a of Figure A1. The system of macrocracks is assumed to migrate into the interior of coal with certain velocity V that on average is constant. A breakaway oxidation rate coefficient kL can be introduced that is equal to a diffusion flux Jdiff, which is defined as follows:

(A7)

where ω is volumetric dilatation, and E and ν are the modulus of elasticity and Poisson’s ratio for coal, respectively. The minus sign in the formula characterizes the stresses as compressive.

(∂x∂c)(x ) ξ ) f

(A8)

For calculation of kL, the stability of a system of macrocracks has to be considered. In doing so, new parameters such as stress intensity factor k, Griffith’s surface energy γ, and critical stress σc, characteristic for coal, must be considered.20 Stress intensity factor k is given by the usual expression:

kc )

∫

σ(x) dx

xLC - x

(A9)

where LC is critical crack length, and σ(x) is the elastic stresses on the crack surfaces in the crack tip region. Stress intensity factor kc can be expressed in terms of γ, as follows:20

kc ) xπγE

(A10)

When the integral in eq A9 exceeds some critical value (specific for the material), the crack becomes unstable. For the situation under consideration, the growing crack length is limited by the size (the width) of the oxidized zone because at the oxide-healthy material interface the conditions change; there are no elastic stresses due to the oxidation and, consequently, no more cracks. Neglecting crack interactions, which is a complex problem itself, and assuming the distribution of the elastic stresses in the oxide layer to be

( )

σ(x) ) σ/ exp -

x LB

(A11)

one can obtain the following balance condition:

kc

xLB

) 2σ/F(u)

(A12)

where LB is the width of the oxidized layer barrier illustrated in Figure A1, and F(u) is an anti-derivative of the integral in eq A9. The width of the oxidized layer barrier LB in its relation to the whole oxidized zone and the function F(u) are illustrated in Figures A2 and A3. Balance eq A12 can be interpreted as an equilibrium condition between the tensile stresses in the right-hand side of eq A12 and the aggregative cohesive forces keeping cracking material in the oxidized barrier zone together. Not going into the details of the analysis and referring the reader to ref 19, we will only state here that the condition of crack stability is fulfilled when the horizontal straight line in Figure A3 becomes tangent to the curve for the function σ/F(u). Since the maximum value for the integral is 1.02, achieved at u ) LC/LB ) 0.82, the (20) Broek, D. Elementary Engineering Fracture Mechanics; Kluwer: Boston, 1982.



Figure A2. Nonhomogeneous stress distribution in oxide layer.

Figure A3. Graphic representation of the balance between aggregative and tensile forces at the crack tip.

relations between critical crack length LC and LB and the mechanical constants kc and σc are given by the following equations:

LC ) 0.82LB LB )

(

kc

2σ/Fmax(u)

) ( ) 2

)

kc 2.04σ/

2

(A13)

The breakaway coefficient kL becomes:

kL ) -Ddiff

c2 - c 1 LB

(∂x∂c)(x ) ξ ) ) -D f

A8), which can also be expressed in the form shown in eq A15, where the characteristic length LB of the barrier is given in terms mechanical parameters σ, γ, kc. This means that elastic stresses are counterbalanced by the resultant of the cohesion forces, characterized by Griffith’s surface energy γ on the length scale LB. Summarizing the results of this section, the expression for the breakaway coefficient kL has been obtained. It characterizes the rate of propagation of the oxidized, deteriorated zone. This is the region of intense mechanochemical transformation of coal, which can be, in our opinion, significantly enhanced by the assistance of special conditions (high pressure or special gas composition, including low-temperature plasma). In this region, intense hydrogen and humic acid generation occurs. Rough estimates lead us to believe that the increase in the diffusion coefficient of about 5 orders of magnitude (i.e., from hours to seconds) is possible. It should be noted that the functional form of dependence of kc on the mechanical parameters such as σc and γ can be somewhat different for the different models of fracture (or general “softening” due to oxygen diffusion) of coal. Perhaps σc, γ, and kc can be combined into one parameter and interpreted in terms of the J-integral of fracture mechanics,20 which is a more sophisticated analytical tool used in the more advanced practice of the discipline. Introduction of such a technique will better account for the details of the mechanical process but will not change the essence of the model. The important thing is that the expression (eq A8) for kL is linear in the diffusion coefficient D and the coefficient representing the mechanical constants of coal. The distance traveled by the deterioration zone X is proportional to the time t, so that

(A14)

Relating the breakaway coefficient kL to the diffusion and fracture parameters of coal, on the basis of eqs A6 and A7, one can write the following equation:

( )

2.04σ/ 2 4.0016 D(c1 - c2)σ/2 ) (A15) kL ) D(c1 - c2) kc π γE Equation A15 states that breakaway oxidation kinetics through the oxidation barrier is determined by the diffusion flux (eq

X ) kLt

(A16)

The larger the value of kL, the faster the deterioration zone propagates. The discussion above leads to the conclusion that by providing for high enough value of kL one can achieve a relatively fast propagation rate for the degradation zone and, therefore, can organize a “fast” process of the coal conversion without CO2 emission. Such a fast process of coal conversion into solid polymerized oxides would be extremely beneficial for industrial energy production. To achieve practically good results, the process should be stimulated by moderate heating, by reaction equilibrium shift (i.e., using oxygen deficit, hydrogen removal, increased pressure), and by special nonequilibrium thermodynamic influence, for example, by nonthermal plasma. EF050247N

CO2-Free Energy and Hydrogen Production from ... - ACS Publications

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