There are several different types of grate firing, with both fixed and moving grates commonplace. They have the distinct advantage over underfeed stokers in that they can accommodate fuels with high moisture and ash content as well as with varying fuel sizes. It is very important that fuel is spread evenly over the grate surface in order to ensure that air is distributed uniformly throughout the fuel and thus combustion is kept homogeneous and stable. There are a number of different types of grate firing including fixed grates, moving grates, rotating grates, horizontal/inclined grate, water cooling grate, dumping grate, and travelling grates.

The simplest fixed-bed system is composed of one combustion room with a grate. Generally, as soon as the new biomass feed is added into the furnace, it is pyrolyzed into volatile gases and chars. Primary and secondary air supplies are provided under and above the grate for the combustion of chars and volatile gases, respectively. The heat generated through the combustion of chars is responsible for providing enough heat for the pyrolysis of newly added biomass. Because of the high content of volatile matter in biomass fuels, a greater secondary air supply is required than the primary air supply. This is one of the major differences from the process of coal combustion. Recent developments have been made to enhance the combustion efficiency. One example is the cyclonic combustion system, which may be viewed as a modified fixed-bed system, suitable for the combustion of agricultural residues and particulate wood wastes at a high efficiency (Quaak et al., 1999).