Organic substances are substances containing primarily carbon. Organic materials such as wood, biomass, plants, fruits, etc., and materials such as natural gas, oil, coal, plastics, etc. are all substances containing carbon and therefore react with oxygen (oxidation). These substances are all flammable, since the carbon reacts with oxygen releasing heat.
Inorganic substances are mainly acids and alkaline, which do not contain carbon; therefore they are usually not directly flammable. There is no combustion or gasification system that could work with inorganic materials. Nevertheless, gasification with inorganic materials is used, for example, in the steel making industry. As an organic material, blast furnace coke (carbonaceous) is added, together with inorganic iron oxide. The carbon removes the iron oxide (oxygen) and high quality steel is produced. At the same time the resulting heat is used to melt the steel.