There is no sustainable plastic until we are able to seed the earth with a genetically modified bacteria that can break down this plastic in the environment. Combustion is also not perfectly clean.
It can be if done in a sealed device which then processes the remainder.
Which is important because aerobic life is basically a combustion process.
Isn't there a huge risk that this bacteria would also break down the plastics that are in use, so cars would just melt away, all plastic parts that are currently needed would break. So general industrial collapse.
This sounds good when talking about land fills.
It doesn't sound so good when all technology stops working.
https://www.goodreads.com/book/show/86452.Ill_Wind but it appears there are a few books that explore this idea.