Suspension firing

Solid bio-fuels that are naturally available in the form of particles (husk, shells, nuts, grains, etc.) or that are reduced to particles by some sort of pretreatment process (crushing, chipping, pelletising, etc.) are often suitable for suspension firing.

This means that the fuel is either mechanically or pneumatically thrown into the furnace and spread over the whole area of the grate in such a way that the smaller particles burn in suspension and the larger particles are spread onto the grate and burn on the grate surface.

Application:

Features:
  • Outstanding load changing capacity – up to 10% per minute.
  • All heat values above 6000 kJ/kg
  • Low NOx emission due to low excess air at the bottom of the furnace.
  • All capacities up to the maximum of the BWV boiler and higher for retrofits to existing boilers.
  • A high heat release rate from the grate ensures a small boiler plant footprint.
   

Design

The fuel, which may be a specific fuel such as bark or woodchips or a mixture of different fuels (e.g. wood chips, sawdust and pellets), is carried to a day silo placed in a highly elevated position in front of the boiler front wall.

If a high load changing capacity is desired, which is a typical requirement e.g. at a paper factory, this day silo is provided with a hydraulic push floor (see description of the HF silo system), which carries the fuel down to a number of smaller dosing silos. These dosing silos are provided with speed controlled screw feeders.

Each of these screw feeders supplies a controlled amount of fuel to a chute, from which it slides into a pneumatic biomass thrower (ASA). The thrower has a bottom plate, the angle of which is adjustable. By trimming this bottom plate and adjusting the air flow, the throw can be adjusted to the nature of the fuel, thereby achieving the best possible distribution of fuel across the grate and the entire cross-section of the furnace.

If the boiler is operating in a system that requires less control, e.g. in a CHP plant, the day silo is typically provided with speed-controlled screw conveyors at the bottom. The screw conveyors operate in pairs and carry the fuel down to the chutes to the throwers.

Secondary air nozzles which draw hot flue gas from the middle of the furnace are placed around the pneumatic throwers to ensure the heating and ignition of the fresh suspended fuel.

Secondary and tertiary air nozzles are also placed strategically in stages up through the furnace to ensure sufficient turbulence to achieve a good temperature distribution, a low CO level and a good combustion of the particles.

For a plant fired with biomass or RDF alone, a water-cooled vibrating grate (HVB) is preferred. If biomass and coal are fired together, a travelling grate is the correct choice.