I received an unwanted letter and, my county not allowing open-burning, I used my closed-top propane grill to burn it. After sticking it into the slots, I realized that this is imprudent, because of the plastic in the envelope (and possibly in the letter: I didn't open in, only rip it in half). The letter is now thoroughly burned, though the charred burnt paper is still in the bottom of the grill. The only question now is how safe or unsafe is the grill? What should I do to clean the grill? Other than charred paper, there is no other visible residue. I have read this question, but the linked question is about rust and concerns bacterial poisoning, not chemical.
Heat the grill, like good and hot (I have used the hottest setting). Once it is hot, just scrape it down with a brush or wooden scraper. After that, let it "cook" for a few minutes. Once all that is done, I would say you are good to go.