Аннотация
Podroiko I. V., Butuzova L. F., Marinov S., Romanko V. V. Research of structure of coals of the Donetsk basin and their semi-coke by DRIFT-spectroscopy In the report, a study was made of the change in the structural-group composition of coals of different types of reductivity in the processes of semi-coking. The influence of sulfur on the pyrolysis process is established.
Formulation of the problem
Infrared spectroscopy with Fourier transform and diffuse reflection technique (DRIFT) has recently been used in coal chemistry and has significant advantages over other methods. An important advantage of this method is the possibility of using a computer for digital accumulation and processing of data, which allows programmatic control of the experiment, correction of the baseline. The baseline method allows to partially eliminate the consequences of deviations from the Lambert-Beer law, due to the heterogeneity of the samples, the presence of particles larger than the wavelength, etc.
DRIFT is a surface localized FTIR spectroscopy, since it can provide both chemical and structural information for all types of solid surfaces. When infrared radiation reaches a sample surface, one or several processes can occur: light can be adsorbed, reflected from the surface, or it can penetrate the sample before being scattered. If scattering centers, which are fibers in the case of ACM, are randomly oriented, the phenomenon is isotropic and generates a diffuse reflectance. The scattered light is then collected and relayed to the IR detector, where the absorption by chemical groups is revealed. DRIFT spectrometry has many advantages with respect to the conventional transmission (or reflection) FTIR method: DRIFT is a fast and non-destructive technique, since the sample can be directly analyzed, both as it is or in its powdered form; moreover, DRIFT is better suited to the analysis of strongly absorbing materials, whose main characteristic are very low signal and sloping baselines when analyzed in transmission. Since the optical phenomena that generate DRIFT signals are different from those involved in transmittance spectrometry, the spectra obtained by these methods cannot be considered equivalent. It is therefore mandatory to verify that DRIFT, which is a powerful experimental technique, can be usefully used as an analytical quantitative method in measuring the amount of asbestos contained in bulk materials.
Diffuse-reflectance IR Fourier (DRIFT) spectroscopy is one of the few direct and informative methods of qualitative and quantitative molecular analysis of coal. It is possible to obtain the spectra of powder samples with particles no larger than 200 μm (standard analytical grinding). There is no need to prepare the coal sample in special ways for spectral analysis, thereby eliminating a source of mechanical or chemical modification. Diffuse-reflectance spectra are characterized by high contrast and low noise and match the quality of those obtained using dispersion spectrometers in tablets with KBr. However, a problem with DRIFT spectroscopy is the significant difference in the levels of reflection of parallel spectra of the same sample, with reproducibility of their overall shape and the position of the bands. The reasons for this difference are unclear, and correspondingly we cannot determine which of the parallel spectra is best for quantitative measurements. We may assume that such distortion is due to the following uncontrollable factors: the particle size of the coal powder at the light spot; the orientation of the coal grains relative to the incident radiation flux; and the formation of empty space between the grains. In the first and second cases, some of the scattered radiation is lost on account of deflection beyond the instrument’s optical system. In this case, there is a total loss of scattered radiation.
Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) is an infrared spectroscopy technique used on powder samples with no preparation. The sample is added to a sample cup and the data is collected on the bulk sample. The infrared light on a sample is reflected and transmitted at different amounts depending on the bulk properties of the material. The diffuse reflection is produced by the sampleʼs rough surfacesʼ reflection of the light in all directions and is collected by use of an ellipsoid or paraboloid mirror. Shape, compactness, refractive index, reflectivity and absorption of the particles are all characteristic of the material being analyzed. If the sample is too absorbent, then it can be diluted with a nonabsorbent material such as potassium bromide, potassium chloride, etc. The particle size should be smaller than the wavelength of the incident light, so this would infer that it should be less than 5 μm for mid-range infrared spectroscopy. The spectra are plotted in units of log inverse reflectance versus wavenumber. Alternative plots of Kubelka-Munk units can be used, which relate reflectance to concentration using a scaling factor.
The aim of the work is to assess the influence of the degree of metamorphism and the type of reduction on the structure of coal by the method of DRIFT-spectroscopy and determine the parameters that best reflect these characteristics.
Methodology
The object of the research is the coal of the Donetsk basin of the grades Д, Г, Ж, whose characteristics are given in Table 1.
Table 1 – Elemental and technical composition of coal
In analyzing the spectra, the ratio of different bands to the most stable band in the experimental conditions (1600 cm-1) was considered, which allows comparing different spectra [1,2]. The results are presented in table 2.
Table 2 – Relative intensityIx/I1600 Ix/I2920 for coals Д, Г, Ж and their product of pirolysis (SC* – semi-coke, obtain at 520°)
The main differences in the character of the spectra of long-flame coals of different types in terms of reduction are observed for the bands of 3050 cm-1 and 1640 cm-1. These differences show that when semi-coking of long-flame coals of type a decreases the relative content of aromatic hydrogen, aliphatic paraffin chains and carbonyl groups not associated with aromatics, and for semi-cokes of long-flame coals of type B, the opposite changes in the intensities of these bands occur.
Estimating the relative intensity Ix/I2920, we can conclude that thermal transformations at the semi-coking stage generally lead to an increase in the strength of intermolecular interactions, an increase in the relative amount of short paraffin chains (except for grade Zh coal), as well as an increase in the amount of aromatic hydrogen relative to aliphatic , which is consistent with the data [2]. The relative intensity of the 1250 cm-1 band, which is responsible for the absorption of ethers, decreases for semi-cokes of grades G and W, which indicates the destruction of oxygen-containing and sulfur-containing bridges, which contributes to the transition of coal to a plastic state.
Conclusions
Thus, the difference in the behavior of low-sulfur and sulfur coals during semi-coking is that the semi-coke obtained from sulfur coals is characterized by a large relative amount of aromatic hydrogen and C = O groups compared to low-sulfur coals. In addition, during the pyrolysis of reduced-type coal, paraffin chains are more intensively destroyed and the proportion of short substitutes is much higher than during the pyrolysis of type a coal.
References
1. Machnikowska, H. The characterization of coal macerals by diffuse reflectance infrared spectroscopy [Text] // H. Machnikowska, A. Krzton, J. Machnikowski // Fuel. –2002. –V.81 –P. 245 – 252.
2. Bechtel, A., Butuzova, L., Turchanina. O. Thermochemical and geochemical characteristics of sulphur coals [Text] // Fuel Processing Technology, 2002. –V. 77 – 78, p. 45 – 52