The particularity of this process is that oil shale in the retort moves from the bottom upward to the top, countercurrent to the descending hot gases, by a mechanism known as a rock pump.
The process technology was invented by the American oil company Unocal Corporation in late 1940s and was developed through several decades.
In 1921, it acquired an oil shale tract in the Parachute Creek area of Colorado, southern Piceance Basin.
[7] The Union A (direct) process is similar to the gas combustion retort technology, classified as an internal combustion method, while the Union B (indirect) process is classified as an externally generated hot gas method.
Hot gases, generated by internal combustion or circulated through the top of the retort, decompose the oil shale while descending.
The reducing atmosphere in the retort allows the removal of sulfur and nitrogen compounds through the formation of hydrogen sulfide and ammonia.