Catalysis in Coal Liquefaction
AUTHOR: Frank J. Derbyshire
DATE: June 1988
The economic viability and operability of processes to convert coals to useful liquid products could be greatly improved by the successful development and application of suitable catalysts. New and improved catalysts can lead to more favourable process economics by increasing the rates of reaction and product selectivity and by lowering reaction temperatures and pressures.
Coal liquefaction catalysis has been reviewed with the aims of assessing the present state of knowledge and identifying new directions and strategies for research. Recommendations are made which are intended to provide a basis for the formulation of future R&D programmes. The processes of primary coal dissolution and coal liquids upgrading are distinguished. Concerning the former, the discussion has centred on two broad groups of catalysts; hydrogenation catalysts and acid catalysts. In both cases, effective control of the dissolution process requires intimate coal-catalyst contact.
Hydrogenation catalysts, such as certain metal sulphides, may function by providing a source of H atoms through the dissociation of molecular hydrogen. The H atoms can promote bond cleavage reactions and assist in stabilising the products of these reactions. The partial pressure of H2S is important to the catalyst mechanism. Acid catalysts, such as metal halide, promote bond cleavage by an ionic mechanism. In general, they are not particularly active hydrogenation catalysts which creates the potential for the stabilisation of cracked products hrough the formation of high molecular weight adducts. Consideration has been given to the limitations of these catalysts and to approaches which could lead to improvements in their performance and utilisation.
Multicomponent systems appear to offer excellent prospects for the development of more active and selective catalysts. Existing support catalysts are quite adequate to the task of hydroprocessing distillate coal liquids. In the presence of high boiling and nondistillable coal-derived liquids they are rapidly deactivated by the deposition of carbonaceous materials and metals. One prospective solution to this problem is to generate feeds which are more amendable to upgrading by enhanced catalytic control of the dissolution process. There is also a need for the development of supported catalysts which are resistant to deactivation. This will require exploratory research into non-conventional catalyst phases and novel supports.
Other topics which are addressed in this study include the effects of low-temperature and consecutively staged reactions, conversion in CO-H2O and CO-H2O-H2 atmospheres and coal degradation by biological action.