Hello Stijn,
are you wondering which existing ecoinvent process to use or rather how to model the incineration yourself? In any case, you can check how ecoinvent did it, e.g. with "treatment of waste polypropylene, municipal incineration | waste polypropylene | Consequential, U" in CH, and then maybe adapt. You don't need to model anything with a negative sign, the incineration takes up waste, as input.
Hth, Andreas