Spectral characterization of the key parameters and elements in coal using terahertz spectroscopy
Introduction
Coal, always known as the ‘black gold’ on earth, has been one of the most important energy resources since eighteenth century. However, the combustion of coal also leads to a series of environmental problems. Most countries are turning to the optimization of the industrial structure. Technological innovation is the most significant way to improve the benefit of coal processing [1], [2], [3], [4]. The monitoring of key parameters and elements is an important part in processing and utilization of coal [5]. One of the key parameters is ash, which is the solid residue after combustion. The larger ash content is, the lower combustion efficiency reflects. Volatile matter is another key parameter to characterize the classification and quality. Coal quality and rank often augment with the dwindling of volatile. Coal with high volatile matter produces unburned carbon particles under combustion and leads to more carbon monoxide, polycyclic aromatic hydrocarbons, aldehydes pollutants et al., especially when the combustion conditions are not appropriate [6], [7]. In the process of combustion, carbon and hydrogen are the principal elements to generate heat. Nitrogen hardly generates energy, but transmits to nitrogen oxides and ammonia [8]. Moreover, sulfur leads to the largest harm to the health of animals, plants and human, and corrodes the equipment [9].
A series of researches focused on the dynamics behavior and interaction between the elemental components in gasification, pyrolysis and combustion process [10], [11], [12], [13]. Conventional methods such as derivative thermo-gravimetric and designed reactor represented clear features of special composites in pyrolysis. Moreover, some new technique such as computer controlled scanning electron microscopy and numerical simulation also presented advantages on the potential of various inorganic and organic components. Based on the data of dynamics behavior and interaction in pyrolysis or combustion, the key parameters such as volatile and ash can be revealed and the elemental components including carbon, hydrogen, oxygen et al. can be determined for further processing. In terms of direct characterization of the components in coal without pyrolysis, a series of techniques, such as proximate analysis, ultimate analysis, gross calorific value determination, chemical analysis, X-ray diffraction et al., are often required to determine multiple parameters [14].
In the last decades, optical technologies, especially spectral methods, were used to realize the on-line characterization of coal because of actual technical requirements in present coal industry [15], [16], [17]. THz (Terahertz) spectroscopy, a developing spectral technique bridging the gap between microwave and infrared spectroscopy, is becoming a hot research because of the unique advantages [18], [19], [20]. A series of studies reported the practicability of THz technique to apply in energy field such as petroleum industry [21], [22], [23], [24], [25]. Due to the strong absorption effect of water in THz range, water content in coal can be qualitatively and quantitatively determined [26]. In this research, nine kinds of coal materials were studied by THz-TDS (THz time domain spectroscopy). PCA (principal component analysis) was then employed and PC1 scores were extracted to analyze in detail the relationship between volatile matter as well as ash and THz radiation. In terms of anthracite versus bituminous and clean versus meagre coal, the different trends were reflected in volatile matter and ash dependent PC1 scores, respectively. In addition, based on the PC1 scores, the contents of carbon, hydrogen, nitrogen and sulfur can be determined according to linear and non-linear models.
Section snippets
THz-TDS setup
The experimental setup used in this research is comprised of a transmission THz-TDS system and a mode-locked femtosecond Ti-sapphire laser. As shown in Fig. 1, the femtosecond laser with the central wavelength of 800 nm is initially attenuated by an attenuator to obtain an input beam with an appropriate power ∼100 mW. The input laser is then split into two beams. The probe beam has the lower power. After transmitting through the delay stage, the probe beam is reflected by a mirror (M3) to reach
Statistical analysis of THz spectra
Initially, we perform a basic characterization of the THz dielectric effect of nine coal materials. Fig. 3(a) shows the THz field signal amplitude as a function of time after the transmission of the THz pulses through the nitrogen (reference) and nine coal tablets. The THz signal peaks and relative delay time remain different from each other, showing that the physical properties can be differentiated using THz spectra. Based on the THz-TDS, the THz-FDS (THz frequency domain spectroscopy) can be
Conclusions
In summary, THz technique was employed in this research to determine nine coal materials. PCA was used with the input of absorbance spectra over the whole range and PC1 scores were extracted to represent the general absorption effect in the selected range. The significant parameters to evaluate coal quality, including volatile matter and ash, indicated different trend with the increase of PC1. The PC1 dependent volatile matter revealed that anthracite and bituminous coals have clear demarcation
Acknowledgments
We acknowledge the National Key Basic Research Program of China (Grant 2014CB744302), the Specially Founded Program on National Key Scientific Instruments and Equipment Development (Grant 2012YQ140005) and the National Natural Science Foundation of China (Grant No. 11574401) for the financial support of this work.
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