The future of fuel supply will undoubtedly involve the utilization of heavy crude oils, including those from nonconventional sources, such as bitumen and oil shale. Because of their dense nature and poor compositional characteristics, heavy oils cannot be admitted straightly as refinery feeds, since the direct processing of such oils hardly produces engine fuels of commercial standard. The currently available refinery setups also require substantial retrofitting in order to process such heavy feeds. Thus, heavy oils must undergo an initial upgrading called hydrotreatment (HDT) by which the feeds are converted to qualified fuel oils or synthetic crude (syncrude) for easy handling. Removing the considerable amount of sulfur (S) and nitrogen (N) compounds present in the heavy crude oils selectively by hydrodesulfurization (HDS) and hydrodenitrogenation (HDN), respectively, is among the most critical and challenging aspects of the upgrading. However, the mechanism of these two reactions, in relation to different catalytic sites, temperature, pressure, and other operation variables, is not fully understood or well-documented. By analyzing the possible reaction routes involved in S and N removal by HDT, this review sets to bridge the gap that has been left void for a long period of time, to serve as a guide for innovative heavy crude oil upgrading technologies. It finally reports the current challenges impeding the speedy inclusion of heavy crude oils into the global oil supply stream, and proffer perspective solutions together with future research trends.

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The detailed chemical reaction mechanism of high temperature pyrolysis is constructed through the comparison of various current reaction mechanisms and sub-models. This mechanism can better predict the formation of polycyclic aromatic hydrocarbons. Through path analysis, the interrelationship of the growth mechanisms of polycyclic aromatic hydrocarbons is expounded. And the important contribution pathways of fivemembered/six-membered rings at high temperatures are investigated. Through component flux analysis and sensitivity analysis, the results show that the main formation path of the external five-membered ring is the acetylene addition of naphthalene. Its stability decreases with increasing temperature and is the main introduction route of molecular curvature during low temperature pyrolysis. However, the embedded five-membered ring obtained by benzene cycloaddition is less affected by temperature and is the most important chemical reaction process introduced by high temperature molecular curvature. The hydrogen abstraction acetylene addition mechanism still plays a dominant role at different temperatures and is a classical pathway for the formation of six-membered rings. By cooperating with multiple reaction mechanisms, the formation mechanism of polycyclic aromatic hydrocarbons at different temperatures can be well predicted.