Direct liquefaction of coal has been carried out in a hydrogen donor solvent at a temperature between 350-450°C, pressure between 2,000-5,000 psig, and in the presence of a hydrogenation catalyst along with hydrogen or another suitable reducing agent (such as carbon monoxide). During the 1970s and 1980s, significant research on direct coal liquefaction was carried out and this was summarized by Shah [115]. Although liquefaction is technically feasible, high hydrogen consumption, high pressure, short catalyst life, and poor liquid composition (without upgrading) made the process economically unattractive. The oil produced was stable, insoluble with water but not of the same quality as that derived from crude oil. Although direct liquefaction can be applied to bituminous, subbituminous, and lignite coals, the liquid production per ton of coal was higher for bituminous and subbituminous coals compared to lignite coals. Lignite coals are more active due to the high amount of hetero atoms and oxygen, but they also contain a large amount of water which reduces the oil production per ton of coal.

To some extent biomass is similar to lignite (see Figure 7.1) in that it con­tains a high amount of oxygen, a large amount of volatile matter, and its H/C and O/C ratios are not too far from the ones for biomass (see Figure 7.1). This is to be expected considering the fact that lignite is the lowest rank and youngest coal. The process of biomass liquefaction in the presence of a hydrogen donor solvent, catalyst, and hydrogen is likely to be similar to that for lignite coal liquefaction. Biomass liquefaction generally requires a tem­perature between 250-450°C and pressure between 700 to 3,000 psig [116]. A number of different hydrogen donor solvents have been used including creosote oil, ethylene glycol, tetralin, methanol, phenol, and recycled oil. The catalysts used for biomass liquefaction are alkaline oxides, carbonates and bicarbonates, metals such as zinc, copper, and nickel, formate, iodine, cobalt sulfide, zinc chloride, ferric hydroxide, and so on. Some of these are very similar to those used in coal liquefaction, for example, Fe, Co, Mo, Zn, Cu, and their derivatives. Both biomass and coal can be dissolved by solvolysis using a reactive liquid solvent. A review of biomass liquefaction research from 1920-1980 is presented by Moffatt and Overend [117].

In recent years, studies [118-123] have been reported to co-liquefy a mix­ture of coal and biomass. These studies have used lignite coal and various types of cellulosic waste materials. These studies indicated that the conver­sion is not significantly affected by the particle size of coal and biomass. Although all biomass gave good liquefaction results, waste paper gave the most desirable product distribution under both catalytic and noncatalytic conditions. The optimum reaction conditions were a solvent/solid ratio of 3, temperature of 400°C, and a reaction time of 90 min. The most suitable cata­lyst for the mixture was Fe2O3, although Cr(CO)6, Mo(CO)6, and MoO3 were also effective catalysts.

The studies on direct liquefaction of mixed feedstock carried out thus far have come to the same conclusion that was arrived at for direct coal liquefac­tion. High pressure, hydrogen, and low catalyst life make the co-liquefaction economically unattractive at the present time. A cheap catalyst or a catalyst with long life, low pressure, use of recycled solvent, and low hydrogen par­tial pressure will make this process more attractive. The quality of bio-oil is better than that produced from fast pyrolysis, however, it can only be used as boiler fuel and the like without further upgrading.