Coal combustion is used for a wide range of applications, especially industrial plant such as boilers or furnaces and for heating certain materials directly. Coal combustion can be used where the gaseous combustion products, that is, the hot gas, are used to power a gas turbine, but this type of gasification is used together with hot gas cleanup to give clean gaseous products. Coal size is largely determined by the method of mining and/or preparation. Prior to cleaning, coal may be sized by the use of screens. The cleaning process currently involves, depending on the circumstances, some aspect of mechanical cleaning, and washing or wet cleaning. Coal cleaning and upgrading are used to reduce ash content, and this process is used in conjunction with the blending of coals to meet desired commercial specification. The Industrial Revolution in the mid-nineteenth century was based on coal combustion, and the use of fixed-grate combustion was central to the technology.