What am I I am a solid substance left after heating coal or petrol?

You are likely referring to "coke," which is a solid carbonaceous material produced by heating coal or petroleum in the absence of air. This process, known as pyrolysis, removes volatile substances and leaves behind a dense, porous material that is primarily composed of carbon. Coke is commonly used in steelmaking and other industrial processes due to its high carbon content and ability to generate heat.