Deciphering the Differential Influence of Organic Additives on Coal Fluidity Development: A First-Principles Investigation

Abstract

Formation of high-quality coke is essentially controlled by the development of the extended stability of the coal fluid phase during industrial carbonization processes. Stabilization of coal free radicals through hydrogenation leads to this development of coal fluidity. The donatable hydrogens are usually supplied by the donor hydrogen species formed through the thermal decomposition of a coal macromolecular network. In view of the limited reserve of the donor hydrogen species within a coal system, organic additives with characteristic hydrogen-donating ability are conventionally added to the system to maintain the desirable coal fluidity. However, the choice of compounds requires a clear understanding of their chemistry and associated reaction features. In this context, factors controlling hydrogen-donating properties of some organic donor hydrogen compounds are investigated herein within the framework of density functional theory. A mechanistic interpretation of the reaction of the compounds with the coal radical is offered. In addition to structural modification, the influence of the functional group, namely, the phenolic hydroxyl group, on the hydrogen-donating efficiency of the systems is validated through our calculations. The key role of molecular orbitals in facilitating the release of hydrogens from them is also elucidated. Thus, the work features an advanced understanding of a fundamental industrial process combining a detailed electronic description of the systems. The efficiency of a new compound to act as a superior hydrogen donor candidate is identified. The insights obtained are believed to help derive a rational understanding of the systems and structure−property relationships, which, in turn, can inspire future studies to engineer desired compounds with improved properties. The relevance of our findings to practical carbonization processes is also illustrated in the present work.

A Molecular Level Interpretation of the Underlying Chemical Phenomenon of Semi‐coke and Coke Formation: A Treatise from Ab Initio Calculations

“…As simple model systems are often useful in elucidating the fundamental features associated with complex chemical processes, we primarily attempt to obtain the mechanistic insights considering benzyl radical as the representative coal free‐radical system [12,17] and without the presence of any additional donor‐hydrogen species that may influence the structural development of semi‐coke and coke. The choice of benzyl radical is also guided by its high abundance in non‐coking coal liquefaction products [18] .…”

“…Monomer unit of para phenol formaldehyde resin has been chosen to be the hydrogen donor additive in this context. The choice of the compound is guided by its superior performance as donor hydrogen species contributing to the development of enhanced coal fluidity and coke strength [17,41,43] …”

“…The M06 family of functionals, developed by Zhao and Truhlar, are known to exhibit excellent performance for neutral and radical reactions without the need to refine the energies by post Hartree‐Fock methods. The adoption of 6‐31+G(d) basis set in the present work is guided by some recent investigations that certify M06/6‐31+G(d) as the preferred level of theory to produce an accurate description for the currently investigated systems [12,17] . Additionally, it should be noted that the current work aspires to provide primarily a clarity of understanding of the underlying molecular mechanism for which the current level of theory can conveniently be applied.…”

“…Coal chemical processes have been the subject of active research since many years [1–31] given the utility of coal as a chief component in many practical industrial conversions. This has contributed to the advancement of our understanding of a variety of coal chemical reactions along with their utilities.…”

“…Stabilization of coal free radicals, produced during the pyrolysis of coal, by H-transfer is known to be critical to the development of coal fluidity [34,37] and recent density functional theory investigations have been able to offer some molecular and electronic level understanding of the key factors controlling the phenomenon. [17,36] On the other hand, crosslinking and condensation of species is believed to dominate at a higher temperature regime, and this leads to the formation of a solid porous residue termed as semi-coke. [7,33,38] Semi-coke is characterized by a more ordered structure than the liquid phase.…”