An investigation of changes in structural parameters and organic functional groups of inertinite rich lignite during acid treatment processes

• Published in: Energy sources. Part A, Recovery, utilization, and environmental effects Recovery, utilization, and environmental effects, 2021-05, Vol.ahead-of-print (ahead-of-print), p.1-18
• Main Authors: Jaiswal, Yash, Pal, Sunder Lal, Jaiswal, Harsh, Jain, Anusha, Kush, Lav, Rai, Devendra, Tatar, Dhiraj
• Format: Article
• Language: English
• Subjects: acid treatment; C NMR; FTIR; lignite coal; organic functional groups; Structural parameters; XRD
• ISSN: 1556-7036; 1556-7230
• DOI: 10.1080/15567036.2021.1923867

In the recent past, researchers have paid much attention to HF, HNO 3 , and HCl acids for demineralization of various types of coals with the assumption that they bring negligible amount of change in the coal structure. Here we report the changes occurring in the structural parameters and the nature of organic functional groups present in an inertinite-rich lignite coal during acid demineralization processes. The investigation has been carried out by analyzing the coal using multiple analytical tools, namely, Fourier transform infrared spectroscopy (FTIR), solid-state 13 C nuclear magnetic resonance ( 13 C-NMR), and X-ray diffraction techniques (XRD). The maximum demineralization (90.82%) and surface area (252m 2 /g) of coal was achieved using HF/HCl/HF process. A broad peak of =N-OH functional group was observed in FTIR spectrum of HF/HNO 3 process which can be associated with the interaction of coal and nitric acid. This result is also consistent with an increase in nitrogen (from 2.0% to 5.6%) and oxygen (from 12.6% to 19.3%) content of HF/HNO 3 -treated coal. The NMR results indicate that the coal structure contains two aromatic rings per cluster, which remains intact even after demineralization. Some increase in the amount of phenolics or phenolic ester (from 1.4% to 3.6%), oxy-methylene (from 1.96% to 2.4%), and oxymethine (1.0% to 1.5%) in the coal structure was observed by demineralization of coal using HF/HNO 3 process whereas it remains constant when HF/HCl/HF process is used, indicating that the presence of HNO 3 can remove organic sulfur (up to 53.96%) from the coal sample during acid treatment processes. The aromaticity (f a ) of raw coal was found to be in the range of 0.71-0.75, which increases with HF/HNO 3 treatment but remains constant with HF/HCl/HF treatment. The other crystallite structural parameters obtained from FTIR, 13 C-NMR, and XRD were influenced to a relatively greater extent by HF/HNO 3 treatment when compared with HF/HCl/HF treatment of coal. This is the first detailed analysis of the structural change in the macromolecular arrangement of coal due to demineralization, which will aid toward new techniques for utilization of Indian coal in clean coal technology.