Effect of particle size on coal char----NO reaction

Abstract

Surface nitrogen complex formation upon reaction of coal char with NO at 600°C was studied by X-ray photoelectron spectroscopy. Particle size had a noticeable effect on the magnitude of changes, which was observed on the surface of the coal char in the nitrogen functional group. The surface increased its -NO, pyridine-N-oxide, and -NO₂ functional group contents with a decrease in particle size. The chemisorption processes of NO molecules on the char were simulated using the ab initio Hartree–Fock method and density functional theory. Molecular modeling was applied to determine the thermodynamics of the reactions. Mechanisms were proposed to explain the formation of the -NO, pyridine-N-oxide, and -NO₂ functional groups at 600°C.

Keywords

NO reduction / chemisorption / particle size / X-ray photoelectron spectroscopy / density functional theory (DFT)

Introduction

The use of coal as a source of energy and organic chemical feedstock may become more important in the future. Current coal-fired power generation systems in China and elsewhere in the world are largely based on the combustion of pulverized coal. As is well known, the formation of nitrogen oxides (NO_x), one of the major environmental problems in coal combustion, contributes to acidification, global warming, and depletion of the stratospheric ozone layer. The use of advanced combustion technology is widely accepted as an important method to control the emission of NO_x [1-4]. Since the 1980s, the control of NO_x emission and the development of several successful, low-NO_x combustion technologies such as reburning technology have been extensively studied. Super fine pulverized coal is an excellent reburning fuel to reduce NO_x emission [5]. Therefore, a clear understanding of C–NO reaction is important in the control of NO_x emission. However, relatively little research has been conducted on the effect of coal particle size on the mechanism of C–NO reaction during reburning.

The formation of surface nitrogen species on carbon during C–NO reaction and the increase in carbon reactivity toward NO reduction with particle size decrease were observed [6,7]. The present study aims to clarify the NO reduction mechanism. In particular, the roles of these surface species are investigated in detail using quantum calculations.

Experiment

Yuanbaoshan Brown Coal was chosen for the experiments. All coal samples were washed, air-dried at room temperature, and milled to the air-dry basis samples with three different particle sizes. The experiments were conducted in a horizontal furnace system, as illustrated in Fig. 1. The coal obtained was pyrolyzed at 700°C for 1 h under N₂ atmosphere. NO reduction reaction was carried out in a tubular quartz reactor using 0.5 g of char. The temperature was monitored using a thermocouple attached to the reactor external wall, completely imbibed in the sample bed. The temperature of the sample followed furnace temperature due to the inert atmosphere within the reactor and the small sample amount. The gas concentrations were as follows: NO, 8.00 × 10^-4; and balance, N₂, with flow maintained at 700 ml/min. After exiting the reactor, the product gases passed through the gas analyzer SHIMADZU for NO. After 1.5 h of reaction, the reactor was cooled under N₂ flow to room temperature. The sample was then subjected to X-ray photoelectron spectroscopy (XPS). The apparatus used was a VG-XR5 electron spectrometer with Mg Ka X-ray source. The char that does not react with NO was also characterized by XPS to obtain a reference for comparison.

Results and discussion

N-containing functional group formation

Peak assignment

XPS provided direct evidence on the covalent linkage of nitrogen in the carbon structure during C–NO reaction. Information about the newly formed structures was provided in aid of XPS analysis. Figure 2 presents the most important nitrogen functional groups observed on the char surfaces after reaction with NO at temperatures higher than 400°C. They are [4,5,8-11] the pyridinic functional group (N-6) where the N1s electron binding energy used for its characterization by XPS is 398.9 eV, the pyrrolic functional group (N-5) with binding energy at 400.4 eV, the nirtoso functional group (-NO) with 400.1 eV, the quaternary–N (N-Q) functional group with 401.7 eV, the pyridinic-N-oxide functional group with 403.2 eV (N-X), and the nitro-type functional group (-NO₂) with 406.1.

Curve-fitting analysis

Selected zones of the XPS spectra were studied by curve-fitting analysis using a data-processing program. Previous studies [12,13] conducted on coal sample indicated that the best results were obtained using Lortentz–Gauss combination. As the shapes of the bands were not known, a function with equal Lorentzian-type and Gaussian-type contribution was assayed. The parameters were fitted to the experimental envelope by a least squares iterative procedure. All peaks, heights, band shapes, and widths were allowed to vary from the initial speculations in the iterative procedure. To determine the merits of the fit criteria, standard errors of parameters, differences between the original and fitted spectra, and tendencies of particular bands to approach zero with increasing iterations in the curve-fitting procedure were considered.

N-containing functional group formation

XPS technique was first applied to investigate the surface of the char that reacted with NO. Six nitrogen-containing functional groups were found: pyridinic, pyrrolic, quaternary, N-X, nirtoso, and -NO₂, indicating the formation of covalent nitrogen species on the char surface. To examine the effect of particle size on the nitrogen functional groups, three chars with different particle sizes were prepared, which were allowed to react with NO at 600°C and then examined with XPS. The partition of the nitrogen functional groups is presented in Tables 1 and 2. A remarkable difference was observed in the results, as demonstrated in Figs. 3 and 4, indicating that there were significant increments in the –NO, N-X, and -NO₂ functional groups after reaction. Particle size has a noticeable effect on the magnitude of the changes, which is observed on the surface of the coal char in the nitrogen functional group. A good relationship between the integrated intensity area calculated from the XPS spectra and the particle size was found for the studied coal char. After reaction, the surface of the coal char after reaction increases its -NO, pyridine-N-oxide, and -NO₂ functional group contents with a decrease in particle size.

Molecular simulation

Calculation method

Gaussian 03 package was used to calculate the molecular orbitals of the model compounds for the char–NO system. For simplicity, a graphite model containing five six-carbon rings was selected as the char model. Figure 5 illustrates the structure of the carbon model, whereas Table 3 lists its bond parameters. Some carbon atoms were numbered for convenience. The edge atoms on the upper side were unsaturated, whereas the rest are terminated with hydrogen atoms. A single NO molecule was put at a distance of 0.13-0.15 nm from the unsaturated edge in three different ways, as shown in Fig. 6. For the N-down and O-down modes, the NO bond axis was perpendicular to the edge line with the N and O atoms down, respectively. In the case of the side-on mode, the bond axis is parallel to the edge line. The three approaches (N-down, O-down, and side-on) were also adopted for the chemisorption of the NO molecule. Only the approaches in the plane of the carbon model layer were considered. The whole system, including the model carbon and the gas molecule, was subjected to Hartree–Fock (HF) and density functional theory (DFT) calculations. The unrestricted HF (UHF) method with basis set of 3-21G- (d) for geometric optimization was used, followed by the density functional method using B3LYP functional with basis set of 6-31G(d) for single-point energy calculation (the approach using these two methods sequentially is generally symbolized by B3LYP/6-31G(d)//UHF/3-21G(d)). The applicability of this approach to a polyaromatic molecule was rationalized by the pioneer work of Chen et al. [14]. The spin multiplicity of each compound including the chemisorbed species was determined as follows: B3LYP/6-31G(d)//UHF/3-21G(d) calculations under different values of spin multiplicity were made for a given structure, and the multiplicity that led to a chemically reasonable molecular structure and spin densities of edge carbon atoms was chosen. The selected multiplicity was confirmed to give the smallest difference between the expected value of the total spin operator S² and the expected value S(S + 1)/2 (where S stands for the total spin). When these three conditions were satisfied among different values of multiplicity, the one that gave the most energetically stable state was selected. The heat of adsorption, ∆H, was determined as the difference between the total energy of the optimized system and the sum of energies of the corresponding carbon model and gas molecule. The thermal stability of the resultant chemisorbed species was evaluated from the calculated heat of adsorption.

Chemisorption of NO on the carbon edge site

Figure 7 shows the structure of the surface nitrogen complexes formed on zigzag sites, and Table 4 presents the bond lengths and the atomic bond population for each chemisorbed species. All resultant structures were found to be in one plane. When NO was in the N-down mode above the edge carbon atom (C (5)) of Model A1, a linear C (NO) species was formed (Model A1). In the case of the O-down mode, a linear C (ON) species was formed (Model A2). The atomic bond population of the N–O bond was much smaller than that in Model A1, suggesting that the weak bond strength of N–O appeared in Model A2. For all cases, the production of linear species reduced the bond strength of the most neighboring C–C bonds (C (1)–C (5) and C (5)–C (6)). The side-on approach to C (2) and C (5) of the char model gave A3, respectively, where both N and O atoms were chemically bound to the edge carbon atoms to form a five-member ring. As a result of this chemisorption of NO, the framework of the substrate (char models) was distorted to a greater extent; the bond length of C (5)–C (8) decreased from 144 to 138 pm. Such forcible bond length reduction resulted in a decrease in bond strength (from 0.69 to 0.38 in atomic bond population).

The calculated value of ∆H for each species is tabulated in Table 5. All six chemisorption processes toward the zigzag site were found to be exothermic, i.e., a negative ∆H value. Particularly, structures e and f were the most stable species. The values of ∆H for the N-down approach (A1) were -195 kJ/mol, and the O-down approach (A2) gave a small absolute value of ∆H (-15 kJ/mol). Therefore, the N-down approach is a more thermally favorable process than the O-down one.

Figures 3 and 4 show that there is a significant increment in the -NO functional group after reaction. Particle size had a noticeable effect on the magnitude of the changes, which was observed on the surface of the coal char in the nitrogen functional group. A good relationship between the integrated intensity area calculated from the XPS spectra and the particle size was found for the studied coal char. The surface of the coal char after reaction increased the –NO functional group content with a decrease in particle size. The formation of –NO can be explained by the theoretical calculation above which showed that the chemisorption of NO on an active site was an exothermic process (-195 or -470 kJ/mol). This indicated that this reaction was thermodynamically feasible.

Coal pyrolysis was strongly affected by particle size. A finer coal particle had a greater specific surface area and smaller internal diffusion distance due to the heat and mass transport that affected the release rates of volatile matter. The initial rapid devolatilization out of the coal particle might have led to a drastic structural change on the surface of the particle, which resulted in specific surface area increase and the presence of more active sites. This explains why the finer particle was especially prone to NO adsorption on the char.

-NO₂ functional group formation

The N 1s bonding energy at 406.1 eV can be assigned to the -NO₂ functional group. The presence of the -NO₂ functional group on the char surface indicated that this functional group can be formed by NO chemisorption on sites adjacent to the oxygen functional group. Theoretical calculations showed (Fig. 8) that the chemisorption of NO on an active site located between two oxygen functional groups is an exothermic process [15]. The energy released was sufficient for the endothermic rearrangement to produce the –NO₂ functional group. Experimentally, the content of the -NO₂ functional group increased with a decrease in char particle size. In particular, a significant increase in the -NO₂ functional group on the 11.34 µm char surface after reaction can be observed. These results can be explained by an increase in the oxygen functional group on the surface char of the finer coal char. Figure 9 shows that the content of the oxygen functional group increases as the particle size decreases. The presence of more oxygen functional groups is favorable for -NO₂ formation.

N-oxide formation

The results in Figs. 3 and 4 show that there is a significant increment in the formation of the N-X functional group in the case of the reaction with NO. The content of the N-oxide functional group increased with a decrease in char particle size. Nitrogen incorporation on the char can take place on the active sites generated during the thermal treatment of the coal. Montoya et al. [16] showed that CO desorption from char produces a five-member ring structure. Therefore, this structure generated was a susceptible site where the NO molecule can be chemisorbed to produce the N-oxide functional group. The present study proposed that N-X formation occurred in the sites left by CO desorption during coal pyrolysis at 700°C. The theoretical calculation results (Fig. 10) showed that this process involved an exothermic heat of -124 kJ/mol, indicating that the reaction was thermodynamically feasible.

The content of the N-oxide functional group increased with a decrease in char particle size. The specific surface area, porosity ratio, and pore volume were greatly increased by the pulverization of the coal sample [5,17,18]. Consequently, during the thermal treatment of the coal, more effective cracking was favorable for devolatilization as a result of the more effective heat transfer. The finer coal produced more active sites where the NO molecule can be chemisorbed to produce the N-oxide functional group.

Conclusion

The reactions of coal char with NO at 600°C formed stable nitrogen and oxygen complexes on the char surface. Particle size had a noticeable effect on the magnitude of the changes, which was observed on the surface of the coal char in the nitrogen functional group. A good relationship between the integrated intensity area calculated from the XPS spectra and the particle size was found for the studied coal char. After reaction, the surface of the coal char increased its –NO, pyridine-N-oxide, and -NO₂ functional group contents with a decrease in particle size. The chemisorption process of the NO molecules on char edge sites was calculated using the HF and DFT methods. The adsorption of NO on carbon edge sites resulted in the formation of several types of nitrogen-containing complexes, C (N). From the thermal stability of these complexes, for NO adsorption, the N-down approach toward the edge site was concluded to be more thermally favorable than the O-down one. The chemisorption of the NO molecule with its bond axis parallel to the edge plane gave the most stable chemisorbed species. Theoretical calculations showed that the five-member ring structures were susceptible to the chemisorption of NO, giving pyridine-N-oxide at temperatures where gasification was not relevant. - NO₂ complexes can be formed by NO chemisorption on sites adjacent to oxygen complexes or by the chemisorption of NO₂ present in the gas mixtures containing oxygen. These complexes were responsible for NO reversible desorption.

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