Earlier quoted context omitted.
You could probably use it in a diesel engine. People filter used frying oil from restaurants and run their diesels on that.
Yes, you absolutely can, especially in an old diesel engine, they can run on all kinds of oil. But the engine might not fare well, depending on the properties of the oil, and the exhaust might contain "interesting" stuff like dioxins, depending on what was in the mystery plastic to begin with.
I assume it still could be profitable because people who want to get rid of plastic waste would still have to pay. Getting excess power from grid is also something one would get paid for. Then turning on generators when grid needs power to get paid when there is a power shortage.
But as I write this out I imagine that it would require massive amounts of investments for setting up such operation and getting all kind of permits or environmental studies and god knows what. Let alone getting deals with electric companies - so I think that would be something that some electric company would have to set up. They would most likely have most know-how for such setup.