You may think I’m talking about putting plastic in a gasifier. That’s not so great either, but in small quantity it can be done.
No, I’m talking about plastic on the hot areas of the gasifier, specifically close to the hearth, flares or engine exhaust. First let’s look at the advantages of plastic vs steel.
It’s cheap. It never rusts or corrodes. Sometimes it’s flexible. Many times it’s easy to seal up, and welding it can be done at low temperatures. It’s many times lighter than steel. And there’s tons of it around. Sounds great!
However, when you get near the heat, that all changes.
Plastic gets hot, and softens. It loses all it’s strength. An internal “bang” can cause plastic pieces to shred or tear. Or they can warp and deform from the heat.
If it gets too hot, it can even melt, holes will appear, and occasionally the molten plastic will drip out. (And if your hand happens to be in the way, it acts like napalm - you can’t get it off. Molten plastic is very sticky until it cools. Ref picture below.)
If it actually ignites, it can be difficult to put out. The smoke is noxious, particularly from PVC.
Summary; plastic is really good for cold places on your gasifier. Just keep it away from the heat.