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Modeling the reaction of oxygen with coal and biomass chars
Proceedings of the Combustion Institute, Volume 29, 2002/pp. 415–422
MODELING THE REACTION OF OXYGEN WITH COAL AND BIOMASS CHARS R. I. BACKREEDY, J. M. JONES, M. POURKASHANIAN and A. WILLIAMS Department of Fuel and Energy School of Process, Environmental and Materials Engineering The University of Leeds Leeds, LS2 9JT, UK
The combustion of coal is responsible for nearly 40% of the world’s electricity production, and char combustion accounts for about half of that amount. Clearly, an understanding of the combustion mechanism of carbon is of great importance not only because of its industrial significance but because it is a model heterogeneous reaction. A number of recent studies have been concerned with ab initio molecular orbital calculations on graphite including model chemistry and the reactions with molecular oxygen. This study is concerned with oxidation steps involving the attachment of oxygen to a graphene layer at high temperature leading to the formation of carbon monoxide, and particular attention is paid to the subsequent oxidation reactions. In addition, the reaction of oxygen with carbon catalyzed by metals inherent within the char matrix and the reaction of molecular oxygen with the analogous biomass char are investigated and their reaction paths are discussed.
Introduction A considerable amount of research [1–10] has been directed toward the mechanism of oxidation of carbon and a number of recent studies [6,11–14] have been concerned with ab initio molecular orbital calculations on the reaction of graphite, as a model of carbon and chars, with molecular oxygen. Indeed, the revolution in carbon oxidation kinetics at a molecular scale through ab initio modeling is now beginning to have an impact on char oxidation model validation through the greater understanding and detail unraveling of such complex processes afforded by this technique. As combustion of coal char proceeds, the largely amorphous carbon undergoes restructuring toward a more aromatic and ordered structure and becomes less reactive, resulting in some unburned carbon remaining in the ash; the losses of unburned carbon in ash are about 1% of all coal burned [1]. To support ongoing experimental research directed toward the char oxidation process, theoretical studies on the molecular chemistry of the representative elements from char have been initiated. The combustion of carbon is complex, and although many experimental and theoretical studies have been made, it has been difficult to unravel the detailed chemistry. The use of computational quantum mechanics offers an intriguing way of studying and determining the elementary heterogeneous reaction kinetics, which, when combined with experimental data and theory, can provide predictions of accurate kinetics that can then be applied to char 415
oxidation models. The availability of computer programs for ab initio molecular orbital calculations facilitates this type of research and we have used the technique here. The combustion of biomass is of some interest because of the development of computer-based biomass combustion models. Few studies have been made of the combustion of biomass at high temperatures or indeed the catalytic influences and effects of metals inherent within coal char matrix on reactivity. This study first sets the scene by recalling recent work [6] on high temperature oxidation steps involving the attachment of molecular oxygen to the graphene layer, the formation of carbon monoxide and, in particular, the subsequent oxidation reactions. Following, this study then offers some novel insights on the reaction of molecular oxygen with biomass char and the nature of the catalytic influence of inherent metals within the char matrix on combustion using computation techniques as described in Ref. [6].
The Combustion Kinetics of Char Oxidation The overall oxidation reaction of carbon has been described in detail in ref 6 (and references therein) and so only pertinent points are recalled in this study. At high temperatures, the major initial combustion product is CO, but at low temperatures both CO and CO2 are formed [5]. In most cases the CO
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ENERGY PRODUCTION—Biomass, Coal, and Char Combustion
is later converted to either CO2 in the boundary layer of the carbon particle or in the ambient gases. In order to explain these experimental observations, models based on the adsorption of oxygen on active sites have been developed over a number of years and only some pertinent references are given here [4,5,10]. Two types of interaction of oxygen with the surface carbon have been identified (see for example Refs. [4,15]). Type A results in the adsorption of oxygen without the gasification of carbon, that is, the reaction can be represented by Cf Ⳮ O2 ⳱ C(O2) and Type B, where there is gasification, namely, 2Cf Ⳮ O2 ⳱ C(O) Ⳮ CO. Here Cf is a surface carbon. The activation energy of the former is 10–80 kJ/mol, and for the latter, 40–140 kJ/mol, and the Type B mechanism dominates at a temperature of about 900 K [16]. The main features of carbon oxidation are usually set out as follows. 2Cf Ⳮ O2 ⳱ 2C(O)
(1)
C(O) ⳱ CO Ⳮ Cf
(2)
C(O) Ⳮ C(O) ⳱ CO2 Ⳮ Cf
(3)
The surface carbons where gasification takes place are identified as active sites, Cf, but they are not specifically identified in the literature as individual structures and are usually considered to involve defects in the structure and edges of the aromatic lamellae. These active sites have a range of reactivities as pointed out above. The mechanism is effectively a pseudocatalytic mechanism where the active sites are regenerated by a turnover mechanism. Molecular Modeling Technique By means of empirical and ab initio modeling [6,11–14], it is possible to calculate the geometric parameters for the structures set out in Figs. 1–2 for both the initial model and the subsequent oxygen intermediates using GAUSSIAN 98 revision A8.21 [17]. The molecular system used here is the graphene model that is based on studies [11,12] that show C25H9 replicates the behavior of a one-dimensional sheet of graphite. The modeling techniques and method has been outlined and detailed in [6]. We employed the unrestricted Hartree-Fock (UHF) method with a basis set of 3-21G(d) for geometry optimization followed by density functional method using the B3LYP with the basis set of 6-31G(d) for single-point energy calculation. The geometry and multiplicity of each structure, including those with oxygen, were determined by optimizing the spin for a given structure. All the structures in Figs. 1–2 were calculated using B3LYP/6-31G(d). The bond energies of specific critical bonds in the structures can in principle be obtained using the single-point total
self-consistent field (SCF) energies of the reacting and product structures. In addition, by performing single-point energy calculations, for each model, at the same level of theory for different electronic states, the ground state could be established, that is, the model with the lowest energy. In the work undertaken here, we have used the technique to calculate bond lengths and energies and from this deduce the most likely subsequent structures and thus make deductions about reaction paths. Modeling Results and Discussions Mechanism of Char Combustion Various attempts have been made to translate turnover models of the type given above into specific chemical mechanisms [4–6]. From a mechanistic point, Chen and Yang [12] proposed a low-temperature model that involves both zigzag and armchair edge attack by molecular oxygen and also the formation of epoxy off-plane oxygen complexes. We have restricted this study to only the high-temperature mechanism where CO is the dominant product. The first stage involves the adsorption and desorption of oxygen, and this is followed by the formation of surface oxides and the release of CO during their thermal desorption; these processes and their interpretation have been well described. In our previous work [6], we have shown that molecular oxygen is adsorbed onto the active site, as defined here, and then forms an oxygenated species, the semiquinone molecule, structure II (refer to Fig. 2 in [6]). Alternatively, it can also form a loosely bonded oxygen compound labeled IIIB or its precursor IIIA. We proposed that structures II and IIB can be identified with the active sites, tight and loose bonds, respectively. Chen and Yang [12] have suggested that the semiquinone (structure II) decomposes to give CO by the fission of two C–C bonds and this energy has been estimated to be about 320 kJ/mol per bond. Our studies, based on the kinetic analysis of Bews et al. [5], suggest that there is an additional reaction route involving the formation and subsequent decomposition of IIIB to give structure IV and CO by one route (via reaction 8 above) or, alternatively, to form molecular oxygen (via reaction 9). See Ref. [6] for structures. Computations of the bond lengths and bond dissociation energy of graphene and its oxidation products using geometry optimization and total energy calculations have been undertaken previously [6]. The Subsequent Reaction Mechanism The next stage involves the formation of the structure IV that can decompose and lose a CO molecule
MODELING REACTION OXYGEN COAL BIOMASS CHARS
417
Fig. 1. Molecular models of carbon structure with Fe metal additive to simulate the effect of catalyses on combustion.
and form structure V (see Fig. 2 in [6]). It can be seen from Table 1 that, in structure IV, the bond C5– C6 is somewhat elongated, which results in easy fission. It should be noted that the carbonyl bond length (C6 ⳱ O, 120.8 pm) is slightly shorter than that in semiquinone. This results in the formation of structure V, which involves a dangling carbon. This dangling carbon can be identified with a superreactive site. Consequently, at any time, there is a distribution of reactive and less reactive sites, but the total active surface area is self-preserving except at the very beginning of the reaction and near the end when the graphite structure disintegrates. This is in accord with experimental observations. It is seen
that the first seven steps result in the oxidation of the first row of the graphene leaving a dangling carbon in a reactive site. The next stage must involve the reaction of an oxygen molecule with a dangling carbon and a carbon with a free electron either side of it, that is, along the first layer of aromatic rings or it will start to form a cavity. The evidence from steric factors and from the electronic charges suggests that it reacts along the first layer of carbons by C–C fission, and this then leaves the second row of four carbons with free electrons. That is, it is exactly the same situation as when the oxidation started but one layer of aromatic rings have been oxidized away. This is shown as structure VIII in Fig. 2 of [6].
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ENERGY PRODUCTION—Biomass, Coal, and Char Combustion
Fig. 2. Molecular model of (a) simplified biomass char matrix, (b) oxidized biomass structure a, and (c) highertemperature structure used in this study. (See text for details.) R represents a repeated structural unit.
There is experimental evidence [9] from studies on the morphology of char oxidation to support the arguments put forward for the non-catalyzed mechanism set out in Fig. 2 of [6]. At oxidation temperatures, for example, of 1000 K, oxygen reacts in such a way that defects are enlarged during reaction and show pronounced anisotropy parallel to the (100) surface. This growth in defects can come about via the mechanism proposed. It has been observed that at 1000 K oxidation does not lead to a significant increase in the disorder of the graphene layer. The number of defective graphene layers increases only slightly, although the surface becomes disordered on a macroscopic scale. In studies [18] that we have made, we have injected 100 lm graphite particles into a flame and examined the surfaces of the product by high-resolution scanning electron microscopy (SEM). We came to the same conclusion, that there is reaction by edge recession. This enables a repeat of the sequence leaving terraces in the graphite crys-
tal. A similar course of events will take place at higher temperatures. Catalysis of Char Combustion by Metallic Impurities The presence of metallic impurities catalyzes the rate of combustion of carbon, and this effect shows up most markedly at low-temperature (723 K) oxidation because the higher activation energy of reaction 1 overwhelms the reaction at temperatures greater than 1273 K. The char morphology during combustion of the catalyzed reaction is different and, during the initial part of the reaction, the carbon surface is etched around the catalyst and later on this is reflected in the increase in porosity; subsequently, the metallic atoms can migrate together and sinter and lose their catalytic effect. We have attempted to simulate the catalysis of metallic atoms in a graphene matrix by a model
MODELING REACTION OXYGEN COAL BIOMASS CHARS
419
TABLE 2 Bond lengths (pm) in active sites and oxygenated products for catalyzed carbon and biomass char
TABLE 1 Bond lengths (pm) in active sites and oxygenated products Structure
Structure
Bond
IV
V
VI
VIII
C1–C2 C2–C3 C3–C9 C5–C6 C6–O
142.0 142.0 142.0 150.9 120.8
140.6 138.3 141.2 N/A N/A
141.6 137.4 142.4 N/A N/A
139.7 125.3 139.5 N/A N/A
See Fig. 2 of [6] for label details. N/A ⳱ not applicable to structure.
structure in which an Fe atom is bonded to the graphene molecule in one of two ways as shown in Fig. 1, a and b. Iron is a well-known catalyst for reducing soot or carbons. The model structure attempts to represent highly dispersed iron such as may arise from coalification of porphyrin-type or other organometallic precursors [19,20]. While it is recognized that iron and/or iron oxide crystallites will be present on the carbon surface, this model structure seeks to explore the influence of the metal-support interaction (i.e., the Fe–C or Fe–O–C bond) and the perturbations of the carbon structure and how this in turn influences the reactivity. The structures in Fig. 1 can also represent as a ‘‘model’’ edge iron atom in a small cluster of iron atoms/iron oxide on the carbon surface. It is known that dispersion of such iron clusters in the surrounding gas catalyzes the reverse effect, the formation of carbon [21], but here we consider the surface reactions. We performed geometry optimization and single-point energy calculations on the structures shown in Fig. 1, a and b, and the results show that the structure in Fig. 1b is sterically more probable. Thus, the proposed catalyzed graphene is shown in Fig. 1b and it is seen that the addition of iron can have a significant catalytic effect. The influence of the iron is twofold. It weakens the C5–C6 and C6–C7 bonds resulting in greater bond lengths, as indicated in Table 2, and thereby permits an easy route for the CO to desorb. Here, the CO can react further to form CO2 either through another surface-catalyzed reaction or in the boundary layer or free stream gases. The second influence of iron is as an oxygen-transfer medium. This is in accordance with earlier work from which the following mechanism was developed: Cf Ⳮ (–FeO) r C(O) Ⳮ (–Fe)
(4)
Cf Ⳮ (–FeO) r CO Ⳮ (–Fe) Ⳮ Cf
(5)
Bond C1–C2 C2–C3 C3–C4 or (O)a (O)a or C4–C5 C5–C6 C6–C7 C7–C8 C6–O C4–Fe
Fig. 1b
Fig. 2b
Fig. 2 of [6]: Structure I (for comparison)
139.0 140.5 143.7 135.7 150.1 150.2 138.2 142.0 193.8
139.5 140.1 136.5 136.6 149.3 145.7 147.0 123.7 N/A
142.9 142.9 142.2 141.6 140.1 139.2 140.2 N/A N/A
See Figs. 1 and 2 for label details. N/A ⳱ not applicable to structure. a Relates to Fig. 2b only.
In this case, a third reaction step in the sequence above completes the redox cycle, namely 2(–Fe) Ⳮ O2 r (–FeO)
(6)
(–Fe) Ⳮ Cf Ⳮ O2 r (–FeO) Ⳮ C(O)
(7)
or
For this mechanism, structure 1b would represent an intermediate in oxygen transfer. However, this is still open to considerable debate at the moment and we have no experimental evidence for one route over the other. Modeling Biomass Char Combustion The combustion of biomass is of some interest because of the development of computer-based biomass combustion models. Few studies have been made of the combustion of biomass at high temperatures and, therefore, we have extrapolated lowtemperature data to observations at high temperatures. There is considerable evidence that the basic biomass char structure is different from graphene and we propose two types of structures based on analytical and structure studies [22–24]. There seems to be evidence that the biomass char consists of small aromatic structural units and that the oxygen is present mostly within heterocyclic and phenolic groups. The structural units are cross-linked by ether and olefinic linkages. Such an arrangement yields a disordered structure observed in biomass chars,
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ENERGY PRODUCTION—Biomass, Coal, and Char Combustion
such as charcoal. Chars produced under severe conditions will have a higher percentage of aromatic carbon and a lower oxygen content than young (lowtemperature char) [22]. An approximate structure is given in Fig. 3a that also enables a comparison to be made with the graphene structure (structure I in Fig. 2 of [6]). The structural unit given in Fig. 2a is simplified in that phenolic –OH groups have been removed. The adsorption of oxygen onto the biomass structure set out in Fig. 2a results in an oxygenated biomass structure, shown in Fig. 2b, that can be compared with the semiquinone, structure II in Fig. 2 of Ref. [6]. We find, from bond energy and bond length calculations shown in Table 2, that the O atom adjacent to the reactive site (C6) weakens the carbon bonds (C5–C6) much more in this biomass structure (Fig. 2b) than in the semiquinone (structure II in Fig. 2 of [6]), and thus facilitates the subsequent reaction; this is consistent with experiments [25]. Similar perturbations in the C5–C11 structure are expected if the dioxin portion is replaced by ether linkages to other structural units. In addition, it is clear that these units can react to form more ordered units as has been observed in flames in an analogous way to coal char. Presumably, this arises from the further loss of ring oxygen and aromatization. Fig. 2c represents an oxidized form of this type of structure. Similar bond lengthening at C7–C8 is observed in the modeling studies indicating higher reactivity compared with the graphene structure. Conclusions 1. It has been shown possible in previous work [6] to model active sites, the formation of carbon monoxide, and the formation of new active sites as the reaction proceeds. This previous study has set the scene and then been furthered to include the reactions of oxygen with metal catalyzed char and biomass char. 2. A model has been proposed for the study of metal-catalyzed carbon reactions, whereby an Fe atom is bonded to the graphene structure, and the results of these model simulations indicate that Fe atom influences an increase in the structure model reactivity. 3. A biomass char oxygen reaction model is proposed and ab initio simulation studies of the structure indicate distortion of the structural units compared with graphene such that the oxidized structure can desorb CO more easily. Acknowledgments We wish to acknowledge financial support for this work under project no. 127 from the UK DTI Cleaner Coal Programme.
REFERENCES 1. Hurt, R. H., Proc. Combust. Inst. 27:2887 (1998). 2. Suuberg, E. M., in Fundamental Issues in Control of Carbon Gasification (J. Lahaye and P. Ehrburger, eds.), Kluwer Academic, Dordrecht, 1991, pp. 269– 280. 3. Hurt, R. H., and Calo, J. M., Combust. Flame 125:1138–1149 (2001). 4. Haynes, B. S., Combust. Flame 126:1421–1432 (2001). 5. Bews, M., Hayhurst, A. N., Richardson, S. M., and Taylor, S. G., Combust. Flame 124:231 (2001). 6. Backreedy, R. I., Pourkashanian, M., Jones, J. M., and Williams, A., R. Soc. Chem. Faraday Discuss. 119:385 (2001). 7. Smith, I. W., Proc. Combust. Inst. 19:1045 (1982). 8. Williams, A., Pourkashanian, M., and Jones, J. M., Proc. Combust. Inst. 28:2141 (2000). 9. Thomas, J. M., in Chemistry and Physics of Carbon, Vol. 1 (P. Walker, ed.), Marcel Dekker, New York, 1965, pp. 121–142. 10. Essenhigh, R. H., ‘‘Fundamentals of Coal Gasification,’’ in Chemistry of Coal Utilisation: Second Supplementary Volume (M.A. Elliott, ed.), Wiley, New York, 1981, Chapter 19. 11. Chen, N., and Yang, R. T., Carbon 36:1061 (1998). 12. Chen, N., and Yang, R. T., J. Phys. Chem. A 102:6348 (1998). 13. Radovic, L. R., and Skakova, K. A., in American Chemical Society, Division of Fuel Preprints, 219th ACS National Meeting, Vol. 45, American Chemical Society, Washington, DC, 2000, pp. 225–226. 14. Kyotani, T., and Tomita, A., in American Chemical Society, Division of Fuel Preprints, 219th ACS National Meeting, Vol. 45, American Chemical Society, Washington, DC, 2000, pp. 221–223. 15. Brown, T. C., Lear, A. E., and Haynes, B. S., Proc. Combust. Inst. 24:1199 (1992). 16. Ma, M. C., and Haynes, B. S., Proc. Combust. Inst. 26:3119 (1996). 17. Frisch, M. J., et al., GAUSSIAN 98, Revision A.8., Gaussian, Pittsburgh, PA, 1998. 18. Chan, M.-L., Moody, K. N., Mullins, J. R., and Williams, A., Fuel 66:1694 (1987). 19. Jones, J. M., Zhu, Q., and Thomas, K. M., Carbon 37:1123–1131 (1999). 20. Jones, J. M., Agnew, J., Kennedy, J., and Watts, B., Fuel 76:1235–1240 (1997). 21. Tanke, D., Wagner, H. G. G., and Zaslonko, I. S., Proc. Combust. Inst. 27:1597 (1998). 22. Wooten, J. B., Crosby, B., and Hajalogol, M. R., Fuel Chem. Div. Preprints 46(1):191 (2001). 23. Horne, P., and Williams, P. T., Fuel 75:1051–1059 (1996). 24. Jones, J. M., Pourkashanian, M., Williams, A., and Hainsworth, D., Renewable Energy 19:229–234 (2000). 25. Wornat, M. J., Hurt, R. H., Davis, K. A., and Yang, N. C., Proc. Combust. Inst. 26:3075 (1996).
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COMMENTS Karina Sendt and Brian Haynes, University of Sydney, Australia. You have not provided any details of the PES so it is difficult to evaluate your proposed mechanism. However, formation of complex II from I Ⳮ O2 is about 800 kJ/mol exothermic, and it seems highly unlikely that this oxygen could ever be recovered through formation of IIIB and elimination of O2. Perhaps IIIB might be seen better as an intermediate between I Ⳮ O2 and II? Montoya et al. [1] have shown that loss of CO from a semiquinone site produces a five-membered ring (C3–C5 bond). Our own calculations have shown that adjacent oxides are stabilized by the presence of this five-membered ring, making it considerably harder to remove the second CO. Even if a dangling carbon could be formed, then step IV–V would be rate controlling rather than the formation of IV. As a general observation, we believe that kinetically accessible routes cannot really be identified simply based on bond length changes.
REFERENCE 1. Montoya, A., Mondragon, F., and Truong, T. N., J. Phys. Chem. 106:4236 (2002). Author’s Reply. Our modeling studies showed that both oxygen complexes II and IIIB were possible together with an oxygen complex IIIA (not shown, and thought to be less stable). We believe there are parallel routes that lead to the formation of the complexes and there can be interchange between them. The reaction scheme that we have set out is the oxidative route with an adequate supply of oxygen. Alternative routes are possible where there is competition between oxidation and routes that involves deactivation of active sites and annealing. The formation of the five-membered ring as suggested in this question and by Montoya et al. [1] can occur,
but we believe that this is in competition with the oxidative route that we have set out. The dominance of each route would be dictated by the reaction conditions, especially the oxygen concentration and temperature. We agree that generally that full potential energy curves should be calculated. However, in our model, we have used a seven-ring structure because of the extent of our oxidative mechanism, whereas Montoya et al. (Ref. [1] in Comment) have used a five-ring structure to simplify the calculation. Clearly the use of methods such as ONIOM can permit advances to more realistic models. ● Piero Salatino, Universita di Napoli ‘‘Federico II’’, Italy. It has been reported in the literature that oxygen chemisorption can occur at surfaces characterized by high density of delocalized p electrons, for example, graphene layers. Could you comment on what the relevance of this chemisorption pathway might be? Author’s Reply. We agree that oxygen chemisorption can occur on the surfaces of graphene characterized by a high density of delocalized p electrons. However, there are significant differences between the behaviors of granular graphite and coal or biomass chars [1]. Thus with chars, this route is not the predominant one since chemisorption on the edges of the layers seems to occur more readily because of the higher concentration of the active sites. Chemisorption on the graphene layer would seem to be more important in the case of annealed graphite where the route due to edge recession is less active, but this will depend on the surface-layer reaction conditions.
REFERENCE 1. Li, C., and Brown, T. C., Carbon 39:725 (2001).
MODELING THE REACTION OF OXYGEN WITH COAL AND BIOMASS CHARS R. I. BACKREEDY, J. M. JONES, M. POURKASHANIAN and A. WILLIAMS Department of Fuel and Energy School of Process, Environmental and Materials Engineering The University of Leeds Leeds, LS2 9JT, UK
The combustion of coal is responsible for nearly 40% of the world’s electricity production, and char combustion accounts for about half of that amount. Clearly, an understanding of the combustion mechanism of carbon is of great importance not only because of its industrial significance but because it is a model heterogeneous reaction. A number of recent studies have been concerned with ab initio molecular orbital calculations on graphite including model chemistry and the reactions with molecular oxygen. This study is concerned with oxidation steps involving the attachment of oxygen to a graphene layer at high temperature leading to the formation of carbon monoxide, and particular attention is paid to the subsequent oxidation reactions. In addition, the reaction of oxygen with carbon catalyzed by metals inherent within the char matrix and the reaction of molecular oxygen with the analogous biomass char are investigated and their reaction paths are discussed.
Introduction A considerable amount of research [1–10] has been directed toward the mechanism of oxidation of carbon and a number of recent studies [6,11–14] have been concerned with ab initio molecular orbital calculations on the reaction of graphite, as a model of carbon and chars, with molecular oxygen. Indeed, the revolution in carbon oxidation kinetics at a molecular scale through ab initio modeling is now beginning to have an impact on char oxidation model validation through the greater understanding and detail unraveling of such complex processes afforded by this technique. As combustion of coal char proceeds, the largely amorphous carbon undergoes restructuring toward a more aromatic and ordered structure and becomes less reactive, resulting in some unburned carbon remaining in the ash; the losses of unburned carbon in ash are about 1% of all coal burned [1]. To support ongoing experimental research directed toward the char oxidation process, theoretical studies on the molecular chemistry of the representative elements from char have been initiated. The combustion of carbon is complex, and although many experimental and theoretical studies have been made, it has been difficult to unravel the detailed chemistry. The use of computational quantum mechanics offers an intriguing way of studying and determining the elementary heterogeneous reaction kinetics, which, when combined with experimental data and theory, can provide predictions of accurate kinetics that can then be applied to char 415
oxidation models. The availability of computer programs for ab initio molecular orbital calculations facilitates this type of research and we have used the technique here. The combustion of biomass is of some interest because of the development of computer-based biomass combustion models. Few studies have been made of the combustion of biomass at high temperatures or indeed the catalytic influences and effects of metals inherent within coal char matrix on reactivity. This study first sets the scene by recalling recent work [6] on high temperature oxidation steps involving the attachment of molecular oxygen to the graphene layer, the formation of carbon monoxide and, in particular, the subsequent oxidation reactions. Following, this study then offers some novel insights on the reaction of molecular oxygen with biomass char and the nature of the catalytic influence of inherent metals within the char matrix on combustion using computation techniques as described in Ref. [6].
The Combustion Kinetics of Char Oxidation The overall oxidation reaction of carbon has been described in detail in ref 6 (and references therein) and so only pertinent points are recalled in this study. At high temperatures, the major initial combustion product is CO, but at low temperatures both CO and CO2 are formed [5]. In most cases the CO
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ENERGY PRODUCTION—Biomass, Coal, and Char Combustion
is later converted to either CO2 in the boundary layer of the carbon particle or in the ambient gases. In order to explain these experimental observations, models based on the adsorption of oxygen on active sites have been developed over a number of years and only some pertinent references are given here [4,5,10]. Two types of interaction of oxygen with the surface carbon have been identified (see for example Refs. [4,15]). Type A results in the adsorption of oxygen without the gasification of carbon, that is, the reaction can be represented by Cf Ⳮ O2 ⳱ C(O2) and Type B, where there is gasification, namely, 2Cf Ⳮ O2 ⳱ C(O) Ⳮ CO. Here Cf is a surface carbon. The activation energy of the former is 10–80 kJ/mol, and for the latter, 40–140 kJ/mol, and the Type B mechanism dominates at a temperature of about 900 K [16]. The main features of carbon oxidation are usually set out as follows. 2Cf Ⳮ O2 ⳱ 2C(O)
(1)
C(O) ⳱ CO Ⳮ Cf
(2)
C(O) Ⳮ C(O) ⳱ CO2 Ⳮ Cf
(3)
The surface carbons where gasification takes place are identified as active sites, Cf, but they are not specifically identified in the literature as individual structures and are usually considered to involve defects in the structure and edges of the aromatic lamellae. These active sites have a range of reactivities as pointed out above. The mechanism is effectively a pseudocatalytic mechanism where the active sites are regenerated by a turnover mechanism. Molecular Modeling Technique By means of empirical and ab initio modeling [6,11–14], it is possible to calculate the geometric parameters for the structures set out in Figs. 1–2 for both the initial model and the subsequent oxygen intermediates using GAUSSIAN 98 revision A8.21 [17]. The molecular system used here is the graphene model that is based on studies [11,12] that show C25H9 replicates the behavior of a one-dimensional sheet of graphite. The modeling techniques and method has been outlined and detailed in [6]. We employed the unrestricted Hartree-Fock (UHF) method with a basis set of 3-21G(d) for geometry optimization followed by density functional method using the B3LYP with the basis set of 6-31G(d) for single-point energy calculation. The geometry and multiplicity of each structure, including those with oxygen, were determined by optimizing the spin for a given structure. All the structures in Figs. 1–2 were calculated using B3LYP/6-31G(d). The bond energies of specific critical bonds in the structures can in principle be obtained using the single-point total
self-consistent field (SCF) energies of the reacting and product structures. In addition, by performing single-point energy calculations, for each model, at the same level of theory for different electronic states, the ground state could be established, that is, the model with the lowest energy. In the work undertaken here, we have used the technique to calculate bond lengths and energies and from this deduce the most likely subsequent structures and thus make deductions about reaction paths. Modeling Results and Discussions Mechanism of Char Combustion Various attempts have been made to translate turnover models of the type given above into specific chemical mechanisms [4–6]. From a mechanistic point, Chen and Yang [12] proposed a low-temperature model that involves both zigzag and armchair edge attack by molecular oxygen and also the formation of epoxy off-plane oxygen complexes. We have restricted this study to only the high-temperature mechanism where CO is the dominant product. The first stage involves the adsorption and desorption of oxygen, and this is followed by the formation of surface oxides and the release of CO during their thermal desorption; these processes and their interpretation have been well described. In our previous work [6], we have shown that molecular oxygen is adsorbed onto the active site, as defined here, and then forms an oxygenated species, the semiquinone molecule, structure II (refer to Fig. 2 in [6]). Alternatively, it can also form a loosely bonded oxygen compound labeled IIIB or its precursor IIIA. We proposed that structures II and IIB can be identified with the active sites, tight and loose bonds, respectively. Chen and Yang [12] have suggested that the semiquinone (structure II) decomposes to give CO by the fission of two C–C bonds and this energy has been estimated to be about 320 kJ/mol per bond. Our studies, based on the kinetic analysis of Bews et al. [5], suggest that there is an additional reaction route involving the formation and subsequent decomposition of IIIB to give structure IV and CO by one route (via reaction 8 above) or, alternatively, to form molecular oxygen (via reaction 9). See Ref. [6] for structures. Computations of the bond lengths and bond dissociation energy of graphene and its oxidation products using geometry optimization and total energy calculations have been undertaken previously [6]. The Subsequent Reaction Mechanism The next stage involves the formation of the structure IV that can decompose and lose a CO molecule
MODELING REACTION OXYGEN COAL BIOMASS CHARS
417
Fig. 1. Molecular models of carbon structure with Fe metal additive to simulate the effect of catalyses on combustion.
and form structure V (see Fig. 2 in [6]). It can be seen from Table 1 that, in structure IV, the bond C5– C6 is somewhat elongated, which results in easy fission. It should be noted that the carbonyl bond length (C6 ⳱ O, 120.8 pm) is slightly shorter than that in semiquinone. This results in the formation of structure V, which involves a dangling carbon. This dangling carbon can be identified with a superreactive site. Consequently, at any time, there is a distribution of reactive and less reactive sites, but the total active surface area is self-preserving except at the very beginning of the reaction and near the end when the graphite structure disintegrates. This is in accord with experimental observations. It is seen
that the first seven steps result in the oxidation of the first row of the graphene leaving a dangling carbon in a reactive site. The next stage must involve the reaction of an oxygen molecule with a dangling carbon and a carbon with a free electron either side of it, that is, along the first layer of aromatic rings or it will start to form a cavity. The evidence from steric factors and from the electronic charges suggests that it reacts along the first layer of carbons by C–C fission, and this then leaves the second row of four carbons with free electrons. That is, it is exactly the same situation as when the oxidation started but one layer of aromatic rings have been oxidized away. This is shown as structure VIII in Fig. 2 of [6].
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Fig. 2. Molecular model of (a) simplified biomass char matrix, (b) oxidized biomass structure a, and (c) highertemperature structure used in this study. (See text for details.) R represents a repeated structural unit.
There is experimental evidence [9] from studies on the morphology of char oxidation to support the arguments put forward for the non-catalyzed mechanism set out in Fig. 2 of [6]. At oxidation temperatures, for example, of 1000 K, oxygen reacts in such a way that defects are enlarged during reaction and show pronounced anisotropy parallel to the (100) surface. This growth in defects can come about via the mechanism proposed. It has been observed that at 1000 K oxidation does not lead to a significant increase in the disorder of the graphene layer. The number of defective graphene layers increases only slightly, although the surface becomes disordered on a macroscopic scale. In studies [18] that we have made, we have injected 100 lm graphite particles into a flame and examined the surfaces of the product by high-resolution scanning electron microscopy (SEM). We came to the same conclusion, that there is reaction by edge recession. This enables a repeat of the sequence leaving terraces in the graphite crys-
tal. A similar course of events will take place at higher temperatures. Catalysis of Char Combustion by Metallic Impurities The presence of metallic impurities catalyzes the rate of combustion of carbon, and this effect shows up most markedly at low-temperature (723 K) oxidation because the higher activation energy of reaction 1 overwhelms the reaction at temperatures greater than 1273 K. The char morphology during combustion of the catalyzed reaction is different and, during the initial part of the reaction, the carbon surface is etched around the catalyst and later on this is reflected in the increase in porosity; subsequently, the metallic atoms can migrate together and sinter and lose their catalytic effect. We have attempted to simulate the catalysis of metallic atoms in a graphene matrix by a model
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419
TABLE 2 Bond lengths (pm) in active sites and oxygenated products for catalyzed carbon and biomass char
TABLE 1 Bond lengths (pm) in active sites and oxygenated products Structure
Structure
Bond
IV
V
VI
VIII
C1–C2 C2–C3 C3–C9 C5–C6 C6–O
142.0 142.0 142.0 150.9 120.8
140.6 138.3 141.2 N/A N/A
141.6 137.4 142.4 N/A N/A
139.7 125.3 139.5 N/A N/A
See Fig. 2 of [6] for label details. N/A ⳱ not applicable to structure.
structure in which an Fe atom is bonded to the graphene molecule in one of two ways as shown in Fig. 1, a and b. Iron is a well-known catalyst for reducing soot or carbons. The model structure attempts to represent highly dispersed iron such as may arise from coalification of porphyrin-type or other organometallic precursors [19,20]. While it is recognized that iron and/or iron oxide crystallites will be present on the carbon surface, this model structure seeks to explore the influence of the metal-support interaction (i.e., the Fe–C or Fe–O–C bond) and the perturbations of the carbon structure and how this in turn influences the reactivity. The structures in Fig. 1 can also represent as a ‘‘model’’ edge iron atom in a small cluster of iron atoms/iron oxide on the carbon surface. It is known that dispersion of such iron clusters in the surrounding gas catalyzes the reverse effect, the formation of carbon [21], but here we consider the surface reactions. We performed geometry optimization and single-point energy calculations on the structures shown in Fig. 1, a and b, and the results show that the structure in Fig. 1b is sterically more probable. Thus, the proposed catalyzed graphene is shown in Fig. 1b and it is seen that the addition of iron can have a significant catalytic effect. The influence of the iron is twofold. It weakens the C5–C6 and C6–C7 bonds resulting in greater bond lengths, as indicated in Table 2, and thereby permits an easy route for the CO to desorb. Here, the CO can react further to form CO2 either through another surface-catalyzed reaction or in the boundary layer or free stream gases. The second influence of iron is as an oxygen-transfer medium. This is in accordance with earlier work from which the following mechanism was developed: Cf Ⳮ (–FeO) r C(O) Ⳮ (–Fe)
(4)
Cf Ⳮ (–FeO) r CO Ⳮ (–Fe) Ⳮ Cf
(5)
Bond C1–C2 C2–C3 C3–C4 or (O)a (O)a or C4–C5 C5–C6 C6–C7 C7–C8 C6–O C4–Fe
Fig. 1b
Fig. 2b
Fig. 2 of [6]: Structure I (for comparison)
139.0 140.5 143.7 135.7 150.1 150.2 138.2 142.0 193.8
139.5 140.1 136.5 136.6 149.3 145.7 147.0 123.7 N/A
142.9 142.9 142.2 141.6 140.1 139.2 140.2 N/A N/A
See Figs. 1 and 2 for label details. N/A ⳱ not applicable to structure. a Relates to Fig. 2b only.
In this case, a third reaction step in the sequence above completes the redox cycle, namely 2(–Fe) Ⳮ O2 r (–FeO)
(6)
(–Fe) Ⳮ Cf Ⳮ O2 r (–FeO) Ⳮ C(O)
(7)
or
For this mechanism, structure 1b would represent an intermediate in oxygen transfer. However, this is still open to considerable debate at the moment and we have no experimental evidence for one route over the other. Modeling Biomass Char Combustion The combustion of biomass is of some interest because of the development of computer-based biomass combustion models. Few studies have been made of the combustion of biomass at high temperatures and, therefore, we have extrapolated lowtemperature data to observations at high temperatures. There is considerable evidence that the basic biomass char structure is different from graphene and we propose two types of structures based on analytical and structure studies [22–24]. There seems to be evidence that the biomass char consists of small aromatic structural units and that the oxygen is present mostly within heterocyclic and phenolic groups. The structural units are cross-linked by ether and olefinic linkages. Such an arrangement yields a disordered structure observed in biomass chars,
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such as charcoal. Chars produced under severe conditions will have a higher percentage of aromatic carbon and a lower oxygen content than young (lowtemperature char) [22]. An approximate structure is given in Fig. 3a that also enables a comparison to be made with the graphene structure (structure I in Fig. 2 of [6]). The structural unit given in Fig. 2a is simplified in that phenolic –OH groups have been removed. The adsorption of oxygen onto the biomass structure set out in Fig. 2a results in an oxygenated biomass structure, shown in Fig. 2b, that can be compared with the semiquinone, structure II in Fig. 2 of Ref. [6]. We find, from bond energy and bond length calculations shown in Table 2, that the O atom adjacent to the reactive site (C6) weakens the carbon bonds (C5–C6) much more in this biomass structure (Fig. 2b) than in the semiquinone (structure II in Fig. 2 of [6]), and thus facilitates the subsequent reaction; this is consistent with experiments [25]. Similar perturbations in the C5–C11 structure are expected if the dioxin portion is replaced by ether linkages to other structural units. In addition, it is clear that these units can react to form more ordered units as has been observed in flames in an analogous way to coal char. Presumably, this arises from the further loss of ring oxygen and aromatization. Fig. 2c represents an oxidized form of this type of structure. Similar bond lengthening at C7–C8 is observed in the modeling studies indicating higher reactivity compared with the graphene structure. Conclusions 1. It has been shown possible in previous work [6] to model active sites, the formation of carbon monoxide, and the formation of new active sites as the reaction proceeds. This previous study has set the scene and then been furthered to include the reactions of oxygen with metal catalyzed char and biomass char. 2. A model has been proposed for the study of metal-catalyzed carbon reactions, whereby an Fe atom is bonded to the graphene structure, and the results of these model simulations indicate that Fe atom influences an increase in the structure model reactivity. 3. A biomass char oxygen reaction model is proposed and ab initio simulation studies of the structure indicate distortion of the structural units compared with graphene such that the oxidized structure can desorb CO more easily. Acknowledgments We wish to acknowledge financial support for this work under project no. 127 from the UK DTI Cleaner Coal Programme.
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COMMENTS Karina Sendt and Brian Haynes, University of Sydney, Australia. You have not provided any details of the PES so it is difficult to evaluate your proposed mechanism. However, formation of complex II from I Ⳮ O2 is about 800 kJ/mol exothermic, and it seems highly unlikely that this oxygen could ever be recovered through formation of IIIB and elimination of O2. Perhaps IIIB might be seen better as an intermediate between I Ⳮ O2 and II? Montoya et al. [1] have shown that loss of CO from a semiquinone site produces a five-membered ring (C3–C5 bond). Our own calculations have shown that adjacent oxides are stabilized by the presence of this five-membered ring, making it considerably harder to remove the second CO. Even if a dangling carbon could be formed, then step IV–V would be rate controlling rather than the formation of IV. As a general observation, we believe that kinetically accessible routes cannot really be identified simply based on bond length changes.
REFERENCE 1. Montoya, A., Mondragon, F., and Truong, T. N., J. Phys. Chem. 106:4236 (2002). Author’s Reply. Our modeling studies showed that both oxygen complexes II and IIIB were possible together with an oxygen complex IIIA (not shown, and thought to be less stable). We believe there are parallel routes that lead to the formation of the complexes and there can be interchange between them. The reaction scheme that we have set out is the oxidative route with an adequate supply of oxygen. Alternative routes are possible where there is competition between oxidation and routes that involves deactivation of active sites and annealing. The formation of the five-membered ring as suggested in this question and by Montoya et al. [1] can occur,
but we believe that this is in competition with the oxidative route that we have set out. The dominance of each route would be dictated by the reaction conditions, especially the oxygen concentration and temperature. We agree that generally that full potential energy curves should be calculated. However, in our model, we have used a seven-ring structure because of the extent of our oxidative mechanism, whereas Montoya et al. (Ref. [1] in Comment) have used a five-ring structure to simplify the calculation. Clearly the use of methods such as ONIOM can permit advances to more realistic models. ● Piero Salatino, Universita di Napoli ‘‘Federico II’’, Italy. It has been reported in the literature that oxygen chemisorption can occur at surfaces characterized by high density of delocalized p electrons, for example, graphene layers. Could you comment on what the relevance of this chemisorption pathway might be? Author’s Reply. We agree that oxygen chemisorption can occur on the surfaces of graphene characterized by a high density of delocalized p electrons. However, there are significant differences between the behaviors of granular graphite and coal or biomass chars [1]. Thus with chars, this route is not the predominant one since chemisorption on the edges of the layers seems to occur more readily because of the higher concentration of the active sites. Chemisorption on the graphene layer would seem to be more important in the case of annealed graphite where the route due to edge recession is less active, but this will depend on the surface-layer reaction conditions.
REFERENCE 1. Li, C., and Brown, T. C., Carbon 39:725 (2001).