of low-cost subbituminous coal or lignite, removal of oxygen in coal as carbon dioxide, and elimination of product hydrocarbon gases (increased selectivity).
Integration of direct coal liquefaction with an existing petroleum refinery could take advantage of existing facilities and ease the transition between petroleum and coal feedstock. DOE sponsored work on simultaneous processing ("coprocessing") of coal with heavy petroleum fractions in an ebullated-bed hydroprocessing reactor. One CCT program submission utilizing this technique was selected for funding but was unable to find the private sector funding needed to proceed.
Pyrolysis of coal dates back to the eighteenth century, using temperatures below 700 °C (1290 °F) in fixed- or moving fixed-bed reactors. The primary product was a low-volatile smokeless domestic solid fuel, although the value of the liquid products was also soon recognized. During the 1920s and 1930s there was a great deal of R&D in low-temperature processes, but interest dwindled in the mid-1940s when gas and oil became readily available at low prices. With the oil embargo and increased oil prices of the early 1970s, interest renewed in coal pyrolysis, but in more recent times interest has again declined along with petroleum prices (Khan and Kurata, 1985).
Pyrolysis kinetics are reasonably well understood and have been modeled extensively (Solomon et al., 1993). Both yield and liquid fuel properties depend on pyrolysis conditions. Pyrolysis under mild temperatures (500 °C to 700 °C [930 °F to 1290 °F]) and pressures (up to 50 psig) with rapid heat-up can produce high liquid yields without adding hydrogen. However, a significant part of the feed coal remains as char with market value comparable to or somewhat less than that of the feed coal. Coal pyrolysis offers some promise of lower liquid costs if the char can be upgraded to higher-value specialty products, such as form coke, smokeless fuel, activated carbon, or electrode carbon, or if the liquid yield can be significantly increased by using low-cost reactants (steam and carbon dioxide) or catalysts. Pyrolysis liquids have a low hydrogen-to-carbon ratio, generally less than one, in contrast to petroleum tars and bitumens (around 1.4) and high-quality petroleum products (approximately 2.0). They also contain substantial amounts of oxygen, compared to tars, and thus require more extensive hydrogen addition to produce specification fuels. Their tendency to polymerize on standing can cause operational problems, which also must be addressed.
Little heat is required to produce pyrolysis liquids from coal, however, and production as a side stream to coal gasification or fluidized-bed combustion is efficient. Pyrolysis reactors generally operate at modest pressures and temperatures compared to other coal conversion systems and offer high throughput. Both of these features lead to low capital cost. The cost of pyrolysis liquids could thus be low and might be competitive with bitumen or for integration with oil refinery