Re:<"They use nitrogen gas to exclude oxygen from the system and for safety.">
Oh, that way. I thought you were talking about getting rid of the oxygen contained in the chemical structure of plastics like PET.
Yes, pyrolysis needs to happen in the absense of oxygen otherwise it will combust. Nitrogen can be effective to displace regular air.
For the stack test, the gas product is obviously burned to provide heat and that requires oxygen from the air. More air than is needed to completely combust the carbon and hydrogen and the output from the stack will contain oxygen.
However that doesn't help for oxygen-containing plastics such as PET [chemical formula (C10H8O4)n] that are about 1/3 oxygen. That won't make a nice clean hydrocarbon-only oil. The oil will be filled with hard-to-removed acids and other components. Unfortunately starting with sorted plastics would make it too expensive because the sorted waste plastic is too expensive.
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