To be exact, it is a wt% of the feedstock they are changing. Not the fuel itself. (I am not commenting on the fact that they likely would be exposed to combustion products/uncombusted products).

From a chemical perspective, it also chemically needs to be similiar to the original feedstock, or else you would not be producing the fuel you want in the correct ratios. These systems are a very complex system of multiple primary reactions + side chain reactions. They are not trying to reinvent the wheel here, they would want the system to behave as close to the same as possible so that they can just apply this in their existing plants.

IF (BIG IF) they can prove that the waste plastic feedstock does not change the output products of the combusted fuel (included non combusted products, or products of incomplete combustion), this could be a big win for making use of waste plastics, otherwise I have a very sick feeling that this is trending towards the next tetra-ethyl lead event