Processes for Converting Coal into Gaseous and Liquid Fuels
Authors: Gillmore D.W. 
Published: December 2007

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The future of coal usage to alleviate the energy shortage depends on our ability to burn coal cleanly and to convert it to gaseous and liquid products. This paper discusses the state-of-the-art of coal conversion and reviews briefly by simplified flow sheets the major gasification and liquefaction processes. At present technology has not advanced beyond pilot plant or demonstration stages; even the optimistic experts predict that commercial operations will not be realized before the 1980's. Hence, a large and concerted effort is needed now to implement conversion of coal to gas and oil.

Introduction

Rapid depletion of our oil and gas resources has focused attention on our vast reserves of coal as a source of clean, gaseous and liquid fuels. Several coal gasification and liquefaction processes are currently in various stages of development and it remains to be seen which will reach commercial operation first and which will prove to be most economical. Familiarity with this field is increasing rapidly, but a broadening front of developments makes it difficult to keep up. This paper discusses the fundamentals of coal gasification and liquefaction and reviews the current status of processes now under active development.

Coal is a rock composed mostly of organic matter (carbon, hydrogen, oxygen, sulfur acid nitrogen) and some mineral species. In order to convert coal to gas or liquid fuels, either carbon must be removed or hydrogen must be added to the coal molecule; precursors of pollutants, such as sulfur, must be converted to removable pollutants, such as sulfur, must be converted to removable compounds, and undesirable inorganic matter separated. As the fuel type changes from coal to liquid to gas, the C/H weight ratio decreases. For example, as given in Figure 1, in approximate terms, coal has a carbon-to-hydrogen weight ratio (C/H) of 16; crude oil, 7; fuel oil, 6; gasoline, 5; and natural gas, 4.

The aim of conversion processes usually is to add hydrogen to convert coal to a gaseous or liquid feedstock. Conversion of coal by removing carbon, such as by pyrolysis, will decrease the C/H ratio, but these processes are less efficient and non-selective. processes are less efficient and non-selective. In the United States, there are seven major processes to convert coal to pipeline quality gas and five processes to convert coal to liquid fuels (table 1). Simplified flow-sheets for these processes depict the essential features of each and the significant differences between them. In each case, the status of the project, whether laboratory, pilot plant or semi-commercial (demonstration), is pilot plant or semi-commercial (demonstration), is indicated.

In addition to these seven processes, there are several consortiums and partnerships of power, coal, oil, gas and earlier companies being formed to investigate fixed-bed, fluid-bed and entrained-bed coal conversion processes. processes.