"The gas combustion process was described in a previous paper, for the purposes of this discussion it is desirable to do so again but in slightly different terms Figure 1 shows the retort proper as two gas - to- solids heat exchangers joined by a combustion zone. In the upper heat exchanger the shale is gradually heated to retorting temperature, while at the same time the gas stream from the combustion zone is cooled and the oil vapors released in the retorting zone are condensed as a mist prior to leaving the shale bed at the top. In the lower heat exchanger the flow of heat is from the retorted shale to the gas stream . Thus most of the heat content of the retorted shale is recovered . Enough heat is released in the combustion zone to supply the net heat requirements of the process . The unique feature of the gas - combustion process is the manner in which the oil vapors are cooled and condensed in the product cooling zone of the upper heat exchanger . For such a process to operate properly, the oil vapors must be condensed preferentially in the gas stream as mist droplets ,which are small enough to travel through the shale bed without being separated from the gas stream by impingement on the shale particles. The requirements for the formation of a suitable mist have been studied extensively and represent one of two topics which are discussed in detail in this paper . The net heat requirements for a gas- combustion retorting plant producing 10,000 barrels of shale oil per day would be 250 million B. t . U . Per hour . This is about equivalent to the heat released in the boilers of a 20,000 kw steam power plant. By releasing this heat within the retort proper, a substantial saving in investment cost would be achieved . The ether topic discussed concerns the method of releasing the heat or maintaining a combustion zone in the retort."