The UTSR Biomass Boiler is designed for wood fuels including chips, pellets, residuals bark and some alternative fuels, up to 60% moisture. The UTSR boiler unit is equipped with a flat feed grate and also suitable for fuel with a high ash content. The fuel is passed into the furnace from the rear of the furnace on the flat feed grate with high-alloy casting grate bars. The grate bars are designed that they overlap on both sides and the minimum fuel slips through the grate. Furthermore, the grate is designed to enable a clear, uniform distribution of the primary air supplied to the various zones, thereby creating a steady, undisturbed gasification zone. The grate carriage has a hydraulic drive, so that the movement intervals can be automatically adjusted to the capacity. The complete combustion chamber is lined with fireproof material, thereby producing a high furnace temperature even in case of wet fuel. An additional, full-combustion roof with furnace lining is provided above the radiation cover in the combustion room, which varies according to the fuel. The secondary air supplied to the combustion chamber and the full-combustion stage makes possible a stepped combustion that results in optimum combustion and a reduction in the NOX value in the exhaust gases. This results in low emission values. The ash is transported slowly to the end of the grate and then to the automatic ash removal system. If so desired, an ash removal system can also be installed under the grate. With this system, any fuel that falls through the grate is transported with the grate ash directly to an ash bucket or container, by an ash screw conveyor.