*2.3. Solid Biomass-Fuelled Boiler*

For providing steam by solid biomass, it is burned in a modern BMB. The heat, generated in the combustion chamber, is transferred to water via a heat exchanger and evaporated accordingly. The amount of steam to be produced is regulated by the amount of fuel supplied. If the amount is reduced or increased, there is a delay in the supply of steam. Although a reduction in the steam demand results in a lower fuel supply, the remaining fuel in the combustion chamber is still used and provides steam to the system. With an increase of the steam, more biomass is fed into the combustion chamber, which ignites with a delay due to the increased quantity [32]. With these plants a wide range of performance can be covered, whereby strict regulations exist in Germany and plants larger than 1000 kW are subject to legal approval [33]. These legal requirements are caused by the emission of nitrogen oxides and ashes, among other things, whereby large plants must be equipped with special exhaust gas purification filters. Besides, the amount of occurring pollutants depends on the choice of biomass and the moisture content of the fuel [34].

It exists a variety of biomass energy sources, from which wood pellets and chips are most widely used. These are characterised by their good storage properties and their relatively high energy content (5.21 kWh/m³). Irrespective of the energy source, a continuously operated system requires both a corresponding storage area and a supply chain for the fuel. Due to the low energy content of the biomass fuels, a large amount of fuel is required, depending on the plant design. The fuel demand of the BMB *EBM* depends on the steam demand *Esteam*,*demand* for the boiler and the part load efficiency *η* of the operating state of the boiler. This is presented by Equation (1).

$$E\_{BM} = E\_{stam,demand} \cdot \eta (\frac{P\_{BM,partload}}{P\_{BM,max}}) . \tag{1}$$
