By-products are defined as “any saleable or usable value incidentally produced in addition to the main product”. By-products means secondary or subsidiary products arising in the course of manufacturing the main product(s). In the process of producing the main product it frequently occurs that materials or other products emerge which are of smaller value. These are the by-products and even if subsequent processing enhances their value, the resulting profit will be less than that from the main product; otherwise, of course the by-product would become the main product and vice-versa. For example, in oil refinery crude oil is processed but by-products, i.e. bitumen, chemical fertilizer are obtained with the main product-refined oil. Similarly in coke ovens, gas and tar are incidentally produced in addition to the main product coke. Gas and tar are therefore treated as by-products.