So far in my experience with the tar extractors, after a certain point when you return they are still running but "off" and out of fuel. Then you have the option to refuel and return it to the "on" state. The production of tar remains constant, and I'm uncertain but I think it is stacking as well.
Steps to Reproduce
Refuel tar extractors and turn on again after depletion. Repeat.
This issue has been closed either because it is not a bug or does not belong in the bug tracker.