Theoretical investigation of sodium capture mechanism on kaolinite surfaces

The influence of surrounding gases, such as He, N2, atmospheric air, and Ar, and gas flow rate on the laser-induced breakdown spectroscopy (LIBS) characterization of coals in free space is studied. The atomic and molecular carbon (C2 and CN) emission intensities are observed to be higher in Ar and N2 ambiance. Quantitative analysis of carbon and ash content in different coal samples is carried out using the carbon bound atomic and molecular emission signals and the ash forming elements (Si, Fe, Mg, Al, Ca, Na, and K) signals. The sum of the LIBS emission of the all and major ash forming elements increased linearly with an increase in the ash content. Similarly, the ratio between the carbon signals (C I, CN, and C2) and the sum of major ash forming elements (Si, Al, Fe, and Ca) also showed a linear increase with the increase in carbon content in coal samples. The linear coefficient of regression, R2, was estimated to be 0.67, 0.58, and 0.85, and the root mean square of calibration samples was estimated to be 5.71, 5.82, and 5.57 wt% using the partial least square regression (PLSR) method for air (no flow), N2, and Ar atmosphere, respectively. The precision and accuracy of the carbon measurement in coal samples by the LIBS technique using the PLSR method were higher in the presence of Ar than air or N2 atmosphere due to the plasma shielding effect.

Effect of ambiance on the coal characterization using laser-induced breakdown spectroscopy (LIBS)

A simple metamaterial absorber with ultra-thin structure has been proposed for X-band applications with enhanced absorption bandwidth. The proposed structure comprises of circular rings embedded in L-shaped resonators. This ultra-thin structure (0.0420λ0 thick with respect to the center frequency of the operating bandwidth) exhibits wide absorption of 2.3 GHz above 90% absorptivity from 9.4 to 11.7 GHz. The designed structure was tested for different polarization for transverse electric mode under normal and oblique angles of electromagnetic wave incidence. It is polarization sensitive because of its asymmetrical design, and has diverse impacts on absorption at various incidence angles. The electromagnetic fields and surface current distributions were analysed to understand the high absorption of the presented metamaterial absorber. The proposed structure has been fabricated and the experimental responses were matched closely with the simulated responses. This metamaterial absorber will be suitable for applications like stealth technology in X-band frequencies.

An Ultra-Thin, Bandwidth Enhanced Metamaterial Absorber for X-Band Applications

A benchtop laser-induced breakdown spectroscopy is demonstrated to determine the elemental carbon content present in raw coal used for combustion in power plants. The spectral intensities of molecular CN and C2 emission are measured together with the atomic carbon (C) and other inorganic elements (Si, Fe, Mg, Al, Ca, Na, and K) in the laser-induced breakdown spectroscopy spectrum of coal. The emission persistence time of C2 molecule emission is measured from the coal plasma generated by a nanosecond laser ablation with a wavelength of 266 nm in the Ar atmosphere. The emission persistence time of molecular C2 emission along with the spectral intensities of major ash elements (Fe, Si, Al, and Ca) and carbon emissions (atomic C, molecular CN, and C2) shows a better relationship with the carbon wt% of different coal samples. The calibration model to measure elemental carbon (wt%) is developed by combining the spectral characteristics (spectral intensity) and the temporal characteristics (emission persistence time of C2 molecule emission). The temporal characteristic studies combined with the spectroscopic data in the partial least square regression model have resulted in an improvement in the root mean square error of validation, and the relative standard deviation is reduced from 10.8% to 4.1% and from 11.3% to 6.0%, respectively.

Laser-Induced Breakdown Spectroscopy Combined with Temporal Plasma Analysis of C2 Molecular Emission for Carbon Analysis in Coal.

Laser-Induced Breakdown Spectroscopy (LIBS) technique is used to determine the emission intensity of inorganic elements, such as Si, Fe, Mg, Al, Ca, Na, and K in coal samples. The average emission intensity of the inorganic elements increases with the ash content of the coal sample with R2 = 0.82. Using the LIBS data, partial least square regression (PLSR)-based multivariate analysis was performed to determine the coal ash content. The PLS approach combined with the LIBS technique resulted in the coefficient of correlation R2 = 0.93, root-mean-square error RMSE = 3.5%, standard deviation SD = 11.7% and resolution = 3.7% corresponding to the reference ash data.

Determination of Ash Content in Coal Using Laser-Induced Breakdown Spectroscopy with Multivariate Analysis

Alkali vapors present in the flue gas generated during coal-based combustion form fouling deposits as they condense. An additive added to coal can trap alkali elements in ash, therefore suppress the growth rate of fouling deposits, and increase thermal efficiency of a coal-fired thermal power plant. Laser-induced breakdown spectroscopy (LIBS) technique is proposed and demonstrated to screen potential additives to trap alkali elements in ash. Five additives—namely, kaolinite, alumina, silica, magnesia, and pumice—were analyzed for their effectiveness on four Indian coals for retaining/confining alkali elements in ash during coal combustion. Ratio analysis based on LIBS emission intensity values clearly shows that kaolinite and pumice are promising additives to trap sodium. Similarly, kaolinite, pumice, and silica exhibited good potassium retention.

A. Mehta

Papers

Suitability of laser-induced breakdown spectroscopy in screening potential additives to mitigate fouling deposits