It's oil before it is distilled in the distill tanks. If this is redirected before it is distilled into fuel then its not only htf but its preheated!
The cost of htf is directly proportional to the output value. The higher cost the better the output and less need to worry about potentially harmful contaminants that cost a lot to deal with.
When its all said and done its a wash at best on cost+process cost to output value. But it sure boosts output numbers when you need to sell a story.
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