Unfortunately plastic is pretty stable stuff, some of it wont biodegrade for thousands of years.

As a rule of thumb anything that stable will consume lots of energy to break the bonds that will have to be broken to turn really long chain cross linked hydrocarbons into the relatively short chain hydrocarbons that can be burned as fuel. I haven't done the math on it, I never was that good at PChem, but I would be willing to bet it will turn out to be an energy consuming process. By that I mean it will consume more energy than will be made available.

We will just have to find somthing else to do with all those old computer cases.