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Chapter 1 Biomass and technologies for its exploitation the grate, in order to well adjust primary air injection: if this is done, they can work efficiently also at part load (down to 25% of the design point), right thanks to primary air injection control. Fixed grate is mostly used when the desired thermal power is lower than 500 kWth, while moving grate, with its different versions, is more suitable above this value (up to some thermal megawatts). For larger scales, i.e. tens of megawatts, fluidised bed furnaces are instead more efficient. In these plants grate is no more necessary because the bed (formed by biomass and a refractory, typically sand, the latter having the aim of homogenising heat distribution) is kept in suspension by primary air that is insufflated from the bottom. This involves a better mixing of fuel and air, and thus a better combustion quality and a lower production of unburnt. Moreover, this determines the possibility to provide a lower excess air (the air quantity percentage exceeding the stoichiometric need), that can be limited to 10 ÷ 20% for circulating beds (for fixed bed at least 30 ÷ 40% is required, but values up to 100% can also occur). Fluidised bed furnaces are flexible in terms of different fuel types input, but are quite rigid concerning biomass size and part load operation. They can operate at ambient pressure or be pressurised (up to 25 bar). Temperature is limited to 800°C (in fixed bed plant it can be 100 ÷ 200°C higher) to avoid bed sintering: this can be achieved with heat exchangers, gas recirculation or water injection. Moreover, this fact has positive effects on NOx production. In bubbling type devices, the bed is kept in suspension but not in a turbulent condition (primary air velocity is the minimum required to keep the bed fluidised, i.e. 1 ÷ 3 m/s), while in circulating type ones, thanks to higher air velocity (5 ÷ 10 m/s) and lower bed particle size, sand and biomass are dragged out with hot gases, separated in cyclones and the re- injected in the bed: the higher turbulence in the bed involves a better heat exchange and a more uniform temperature, and thus a higher efficiency, but costs are higher as well. In pulverised biomass furnaces (sawdust, wood shavings, etc.), where peak temperature are much higher (up to 1500°C), the solid fuel is pneumatically injected into the device through the primary combustion air. Obviously biomass particles must be very small (smaller than 10 mm, and preferably than 2 mm), thus gasification and char combustion occur very quickly, while gases are afterwards burnt by the secondary air. A low moisture content (lower than 20%) is also required. Typically the air/fuel mix is injected tangentially in the cylindrical furnace in order to obtain a vortex flow that 33PDF Image | SMALL-SCALE BIOMASS POWER GENERATION
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