Liquefaction of coal in hydrogen-donor and non-donor vehicles

A Review of Thermal Co-Conversion of Coal and Biomass/Waste

“…They found that the degree of coal conversion was a function of the amount of hydrogen transferred and relatively independent of the solvent composition employed. Neavel [127] noted that the presence or lack thereof of a hydrogen donor vehicle had little effect on the thermal rupture of the coal fragment. This thermal cleavage results in the formation of highly reactive free radicals.…”

“…Products are then relatively low molecular weight substances capable of being dissolved in most solvents. Progressively more donor hydrogen is abstracted for each increment of conversion as the reaction proceeds [127]. Vernon [128] conducted liquefaction of coal model compound dibenzyl and observed that when heated for 30 min at 723 K with an excess of a good donor solvent, tetralin, the conversion was 47% by mass and the only major product was toluene; very little benzene and ethyl benzene were produced.…”

“…So the essential approach for increasing oil yield and conversion of coal is to provide enough H• to stabilize the free radicals from coal pyrolysis. Unlike Neavel [127], McMillen [130] offered that the direct transfer of hydrogen atoms from solvent derived radicals to substituted positions in aromatic rings is substantial as preliminary steps in depolymerization of coal structures. Wei et al [131] believe molecular hydrogen enhances the thermolysis, hydrogenation and hydrocracking of coal model compounds, as oppose to hydrogen donating solvents which inhibit these reactions.…”

Subtask 3.3 - Feasibility of Direct Coal Liquefaction in the Modern Economic Climate

“…There are two mechanisms proposed by which DCL occurs. In the model that was widely accepted during the 1970s, DCL occurs via transient-state thermolysis products Neavel, 1976). This is based both on the observed temperatures at which coal liquefaction first becomes favorable and on the types of products that are formed.…”

“…In the revised model, some solvents-in particular, the poorer donor solvents containing polyaromatic groups-can directly hydrogenate key atoms of the coal molecule, which assists in bond-breaking and enhances thermolysis (Chawla et al, 1994;Malhotra and McMillen, 1990;McMillen, et al, 1987). This model was put forward primarily to explain why a mix of good and poor donor solvents is often a better overall solvent than either solvent alone (Probstein and Hicks, 1982), why some coals hydrogenate most readily in poor donor solvents (Neavel, 1976), and how some of the bonds holding coal molecules together could be broken at liquefaction conditions (Malhotra, 2005). In this model, thermolysis is still a factor for breaking weak bonds, but solvents are critical to breaking strong bonds (Malhotra and McMillen, 1990).…”