Quantum chemical perspective of coal molecular modeling: A review

Considering the utility of coal in many practical industrial processes, this review aims to discuss the key developments and thereby significant contributions from quantum chemistry investigations to the demystification of complex coal systems and chemistry thereof. The scope does not include a comprehensive review of all the work done till date. While it addresses our current state of understanding, utilization of quantum chemical techniques in unravelling several fundamental features is also discussed. The possibility of occurrence of a variety of decarboxylation reactions in low-rank coals is explained in terms of varying activation energies of the processes. It can be seen that areas of coal liquefaction and carbonization experience significant shortage of novel applications of quantum chemical methods. Thus, these areas have longstanding fundamental questions which remain unanswered. Considering gasification of coal with carbon dioxide, the efficiency of the process is found to be strongly dependent on the chemical nature of the active sites for CO2 adsorption as well as surface active site concentration. The calculations also shed light into the reason behind enhanced rate of catalytic desorption of CO while, interestingly, the activation energy for both the uncatalyzed and catalyzed reaction routes remain unaltered. Low-temperature chemisorption of O2 on coal surface is also successfully explained from the calculations. Despite all these efforts, varying conditions for coal gasification and combustion still require advanced analysis. Coal rank and coal structural features call for increasing and judicious modelling that can ultimately contribute to the advent of advanced coal technology.