Biomass boilers
A
boiler consists of a furnace and a heat exchanger (boiler tubes).
Furnance heat from the flue gas is transferred in the heat exchanger,
producing hot water or steam.
The furnace determines the type and quality of the fuel that can be burnt in the boiler.
The different biomass furnaces available for industrial purposes can be broadly categorised as:
Fixed-bed furnaces include grate furnaces and underfed stokers.
Primary air passes through a fixed bed, where drying, gasification and
charcoal combustion take places in consecutive stages. The combustible
gases are burned in a separate combustion zone using secondary air.
Grate furnaces are appropriate for burning biomass fuels with medium
moisture content, different particle sizes and high ash content. The
capacity of grate furnaces is up to about 20 MWth.
Underfed stokers are an appropriate technology for small- to
medium-scale systems up to about 6MWth. The fuel is fed into the
combustion chamber by screw conveyors from below and is transported
upwards on a grate.
Underfed stokers are suitable for biomass
with high M.C andlow ash content (wood chips, sawdust) and small
particle sizes (up to 50 mm).
In a fluidised bed furnace, biomass fuel is burned in a self-mixing
suspension of gas and solid bed material (usually silica sand and
dolomite) in which air for combustion enters below. Due to the good
mixing achieved, fuel flexibility is high.
Fluid bed combustion plants are of special interest for large-scale applications (normally exceeding 30 MWth).
Dust furnaces are suitable for fuels available as small, dry
particles such as sawdust. A mixture of fuel and primary combustion air
is injected into the combustion chamber. Combustion takes place while
the fuel is in suspension; the transportation air is used as primary
air. An auxiliary burner is used to start the furnace.
Due to the explosion-like gasification process of biomass particles, careful fuel feeding is essential.
For more information on the various combustion technologies, and a review of the state of the art see IEA Bioenergy: Biomass Combustion and Co-firing.
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