Separative Refining of Coals through Solvolytic Extraction under Milder Conditions: A Review

Abstract

Not only cleaning but refining of coal has become an area of utmost importance both industrially and academically because of stringent environmental norms being imposed globally for arresting the climate change. However, coal is a polluting fuel, therefore, there is a grave need of deep refining of coal to obtain ashless coal for increasing its efficiency of utilization in order to minimize its impact on environment. The presence of ash in coal also greatly reduces the coal utility because of its deleterious effect on the boiler parts and gasification equipment. Solvent extraction (organo-refining) techniques produce Super Clean Coal (SCC) or Ultra Clean Coal (UCC) with almost zero ash content. The utilization of the SCC in the Integrated Gasification Combined Cycle (IGCC) Power Generation would increase the availability of this otherwise efficient and cleaner coal technology. Research work on different processes of solvent extraction of coal under varying conditions of temperature, pressure, and the nature of the solvent used and other parameters, their effect on the extraction yield have been discussed presently. Extraction of coal under milder ambient pressure conditions is desirable as this affords the recovery of coal without its fractionation, structural dislocations, or changes. The effect of the use of single solvent, mixture of two solvents (of solvent and cosolvent), i.e., synergism in enhancing the extraction yield under milder atmospheric pressure conditions have been reviewed. Utilization of high boiling solvents and coal derived solvents along with successive sequential solvent extractions for obtaining the enhanced extraction yields mainly under ambient pressure conditions have been discussed in the present paper. Hypercoal production under high pressure conditions, its characteristics, and utilization have also been described. Use of ionic liquids as potential solvents for coal extraction (especially of lignites) has also been discussed. Physical and chemical pretreatments greatly enhance the coal solvent extraction yield. Different potential uses of SCC such as in production of carbon nanotubes as a super clean fuel dust for car engines, synthesis of chemicals, etc. have been discussed. Separate discussion on nanotechnology of coals as related to solvent extraction of coal has also been included presently.

Fractionation of coal through organo-separative refining for enhancing its potential for the CO2-gasification

“…The high swelling of the coals in NMP and creation of channels and fissures in coal when subjected to dispersion in the solvent as reported through SEM images could be reason for such high reactivity. It was also reported for the e, N solvent system where NMP containing a small amount of EDA results in a product (SCC(e, N)) by disruption of non-covalent interactions such as Van der waal forces, London forces, hydrogen bondings, p-p interactions, charge-transfer interactions (Sharma and Dhawan 2018;Pande 2000). The use of DETA instead of EDA as a promoter/co-solvent for the solvent extraction to generate SCC (NMP-DETA) was found to affect the CO 2 gasification reactivities.…”

“…Since a large reserves of coal are available as an organic feedstock world over it is logical to think about the exploitation of coal as a source of chemicals, materials, products etc. besides of course as a cleaner fuel after refining (Sharma and Dhawan 2018). Coal is a contaminated organic feedstock, however, it is possible to produce super clean coal that is a refined coal having almost zero ash contents through separativeorgano refining techniques under milder ambient pressure contents (Dhawan et al 2018;Sharma and Dhawan 2018).…”

“…besides of course as a cleaner fuel after refining (Sharma and Dhawan 2018). Coal is a contaminated organic feedstock, however, it is possible to produce super clean coal that is a refined coal having almost zero ash contents through separativeorgano refining techniques under milder ambient pressure contents (Dhawan et al 2018;Sharma and Dhawan 2018). Though the process is economically not viable but this holds a great potential for future research and later exploitation to obtain value added chemicals, materials and cleaner fuels.…”

“…The presence of ash has been reported to account for the loss in catalyst activity (Yoshida et al 2002;Okuyama et al 2004). Amongst these techniques solvent extraction of coals results in producing almost zero ash super clean coals (Sharma and Dhawan 2018).…”

Metal ion-induced separation of valuable organic acids from a depolymerized mixture of lignite without using organic solvents

“…Therefore, how to fulll the green and valuable utilization of lignite, particularly based on their unique structures and properties, is still a challenge. 3,4 Obtaining valuable chemicals from lignite is attractive because lignite is rich in valuable structural units, such as oxygen-containing groups, aromatic rings, and/or N, Scontaining heterocyclic structures. 5 Compared to the traditional thermal processes, a more rational choice is to maintain these structures as much as possible during the utilization process and convert them into valuable products.…”