Synthetic fuels and combustion

Abstract As the supply of hydrocarbons for transportation fuels includes an increasing proportion of low hydrogen-to-carbon ratio sources, such as coal, the cost and waste of energy of converting these materials to the high hydrogen-to-carbon ratio fuels now required by land and air propulsion systems will increase. In the extreme, where coal is the major source of liquid fuel, elimination of restrictions on aromatics content (H/C ratio) could reduce refining energy cost by as much as 20% of the heat of combustion of the syncrude being processed. Refining costs are approximately proportional to refining energy consumption, and an energy saving of this magnitude would reduce the total cost of refined products by one-third. For a syncrude product cost of 30 $/bbl, this would be a cost saving of 25 ¢/gal. of product. Such a large conservation and economic driving force provides a powerful incentive for choice of power plants capable of burning fuels of low hydrogen-to-carbon ratio in a clean and environmentally acceptable manner. The main combustion problem is the increasing difficulty of avoiding the emission of soot, and the relative ability of power plants to completely burn out the soot formed in the early stages of combustion will be an important selection criterion. In automotive systems, the combustion problems appear much more easily solved for the Stirling cycle and the gas turbine because of the steady flow conditions and the potentially longer time that can be provided for soot burnout. The liquid injection Diesel and stratified charge engines are at a disadvantage in this regard and may not be able to compete successfully with the Otto cycle engine, for which aromatics offer an improvement in efficiency because of their high octane number. Improvement in the ability of aircraft engines to burn highly aromatic, wide boiling range fuels offers the possibility of advances in economics and fuel conservation in air transportation. Fortunately, there is every indication that combustion research and development can be expected to eliminate the need for high levels of hydrogenation and boiling range conversion in fuels manufacture. Much more research is needed in the chemistry of soot formation and burnout, and the mechanics of reactive flows involving high molecular weight liquids and vapors and soot, for the complex systems of practical interest. In development programs, highly aromatic fuels should be used even though the economic and conservation driving force for fuel specification changes might appear well into the future. While the examples and numbers used in this discussion are based on an extreme that is indeed well into the next century, the trend toward lower hydrogen-to-carbon ratio feed stocks is already underway, and it is timely to begin moving toward less energy intensive fuels manufacture in addition to working on more thermodynamically efficient propulsion machinery.