There are two prominent types of fixed-bed combustion: underfeed stokers and grate firings. With these methods of combustion, air is primarily supplied through the grate from below, and initial combustion of solid fuel takes place on the grate and some gasification occurs. This allows for secondary combustion in another chamber above the first where secondary air is added. Generally, fixed-bed combustion is used in small-scale batch furnace for biomass containing little ash. Typical examples of fixed-bed systems are manual-fed systems, spreader-stoker systems, underscrew systems, throughscrew systems, static grates, and inclined grates.