Combustion technology for biomass necessary, and very complex, NOx reduction and the lack of uniform combustion conditions. A higher oxygen surplus reduced the efficiency of the plant. Low emission light-load operation generally proves to be difficult. Very good load control and quick load change are possible with direct firing. A low surplus of oxygen increases the efficiency and reduces the flue gas volume flow. Air grading and swirling generally make a good level of NOx reduction possible. The limitations in the use of this technology are the limited fuel particle size (< 10-20 mm) and the high erosion and thermal loading of the firing chamber walls. The use of an ignition burner is also necessary. Fluidised bed firing is divided into stationary and circulating fluidised bed firing. The firing equipment essentially comprises a cylindrical combustion chamber with a perforated base plate. The fuel is fed into the preheated bed material suspended in the firing equipment (glass sand, ash or limestone/dolomite). The fuel fraction is 2-10% by weight in this mixture. Primary combustion air is injected over the base plate in an amount that results in fluidising the bed. The intensive mixture of bed material, fuel and combustion air can result in a very low surplus of air. The incineration temperature must be kept low (800-900°C) to prevent the melting of ash and bed agglomeration. Firing method Cyclone Superheater Superheater 790 890°C Superheater Air Vaporiser Economiser Fuel Fig. 2: The circulating fluidised bed in detail The benefits of the stationary fluidised bed are that there are no moving parts in the hot combustion chamber and a good level of NOx reduction is possible using air grading. Furthermore, the technology is very flexible in regard to particle size, water content and type of biomass and even the use of fuel mixtures is possible. As with direct firing, a lower oxygen surplus increases efficiency and reduces the flue gas volume flow. Fluidised bed firing is generally associated with very high investment and operating costs. Even the fuel particle size is limited (<80mm). Also, the flue gas is loaded with more dust and the susceptibility in regard to melting ash is also higher. The circulating fluidised bed (see Fig. 2) has all the benefits of the stationary Performance range Particle size Direct firing 870°C 910°C < 5- 20 mm Water content variant plus others due to the high thermal transfer capacity as a result of higher turbulence. The fact that the addition of additives is easier is another benefit. The level of dust loading in the flue gas in circulating fluidised beds, investment and operating costs, and susceptibility to melting ash are all high. Other limiting factors for use are the limited fuel particle size (<40mm) and the susceptibility of the thermal exchange pipe to erosion. Table 2 provides an overview of the separate firing technologies, their performance ranges and correlation of fuel properties. Generally speaking, fluidised bed and grate firing are suitable for use in the “top-end“ performance range whilst direct and underfeed firing are possible in the low-end performance brackets. Type of fuel < 30 % by weight Underfeed firing < 6 MWth < 50 mm flexible Low ash content; possible homogeneous fuel properties; use of bark not possible; Grate firing < 50 MWth < 500 mm flexible High fuel flexibility (type, particle size, water content); no wood and stem material mixture possible; Stationary fluidised bed firing > 20 MWth < 80 mm flexible High fuel flexibility (type, particle size, water content); also suitable for fuel mixtures; Circulating fluidised bed firing > 30 MWth < 40 mm flexible High fuel flexibility (type, particle size, water content); also suitable for fuel mixtures; Table 2: Overview of fuel properties correlation to firing types 7