Maturation grade of coals as revealed by Raman spectroscopy: progress and problems.

The present study questions the sensitivity and the accuracy of Raman spectroscopy as a tool for determining the maturity of natural organic matter (NOM). It focuses on the definition of optimized experimental parameters in order to maximize the quality of the Raman signal and control the accuracy and reproducibility of measurements. A series of 11 coals has been investigated, sampling a wide maturity range (2-7% vitrinite reflectance VR). The role of experimental parameters is first investigated. An excitation wavelength of 514.5 nm gives better results than 457.9 and 632.8 nm, minimizing the fluorescence background observed in the spectra of low-rank coals. Both Raman and fluorescence spectra were investigated with time-resolved experiments in air and argon. These data show that fluorescence and Raman spectra are sensitive to acquisition time and laser power parameters, and reveal a physicochemical instability of the samples under laser irradiation, mostly due to photo-oxidation processes. These data clearly show that the experiments, especially in air, should be performed with strictly constant acquisition parameters. In addition, the results of a whole series of coal measurements performed in air under constant experimental conditions show that Raman spectroscopy is definitely sensitive to the maturity of coal samples with VR> approximately 1%. The most sensitive spectral maturity tracers are the width of the D-band (FWHM-D), the ratio of the peak intensities of the D- and G-bands (I(D)/I(G)), the normalized ratio of the band integrated intensities A(D)/[A(D)+A(G)] for the maturity range VR=3-7% and the width of the G-band (FWHM-G) for VR=1-5%. However, the accuracy and reproducibility are definitely weaker in such measurements compared to the standard VR. Future work must solve the problem of sample stability under laser irradiation, and greatly increase the number of samples to improve the statistical significance of the results.

Performance of PAHs emission from bituminous coal combustion.

Carcinogenic and mutagenic polycyclic aromatic hydrocarbons (PAHs) generated in coal combustion have caused great environmental health concern. Seventeen PAHs (16 high priority PAHs recommended by USEPA plus Benzo[e]pyrene) present in five raw bituminous coals and released during bituminous coal combustion were studied. The effects of combustion temperature, gas atmosphere, and chlorine content of raw coal on PAHs formation were investigated. Two additives (copper and cupric oxide) were added when the coal was burned. The results indicated that significant quantities of PAHs were produced from incomplete combustion of coal pyrolysis products at high temperature, and that temperature is an important causative factor of PAHs formation. PAHs concentrations decrease with the increase of chlorine content in oxygen or in nitrogen atmosphere. Copper and cupric oxide additives can promote PAHs formation (especially the multi-ring PAHs) during coal combustion.

Investigations on desulphurisation of some high-sulphur indian coals by gamma-ray-induced chlorinolysis.

Results of investigations on desulphurisation of high-sulphur (2-8 wt%) coals of Meghalaya by radiation chlorinolysis, effected in different media of chlorination (CCl4, CCl4/H2O, and CCl4/CH3OH) via radiolysis of CCl4, for the removal of both the inorganic (pyritic and sulphate) and the organic forms of sulphur as well as mineral matter are reported. In general, maximum removal of total sulphur (up to 37%) and mineral matter (up to 10%) takes place in CCl4/CH3OH medium, which is attributed to the CH3OH acting as a better wetting agent for coal. It is found that while pyritic and sulphate sulphur are removed almost quantitatively, there is variation in the removal of organic sulphur from the coals. Furthermore, the coal matrix is not affected/degraded to any appreciable extent and the caking property of the coals is either improved or slightly degraded after desulphurisation. Mechanism of desulphurisation of coal via radiation chlorination is proposed.