"When development of the TOSCO II process began, the gas combustion and other retorting processes which were variations on the vertical kiln gas combustion process had already been subjected to extensive field tests. The U.S. Bureau of Mines discontinued development of the gas combustion process in the 1950's. Unresolved problems at the conclusion of the program included: (1) inability of the process to recover 100'of Fischer assay oil, (2) disposition or use of low heating value (65 to 125 Btu/SCF) by-product gas, (3) inability of the process to handle shale particles below a size of about 1/4 to 1/2 inch, and (4) uncertainties surrounding scale-up to the large size retorts required for economic commercial operation. The TOSCO II process was selected for development in the mid-1950's because of its potential for overcoming the disadvantages of gas combustion-type processes."