Converting Waste Agricultural Biomass into a Resource - UNEP

Figure 5: KSM Drop Cell Unit
Some of the smaller, simple woodchip systems units may dispense with the
drop cell and use a single short auger to feed fuel directly to the burner.
Grate (Hearth)
Comprises a short auger that feeds the fuel directly into the combustion zone.
The type and design of the grate varies between individual manufactures.
‘Underfed grates’, as used in KSM boilers, are typically used with wood-fuels
with moisture contents of 30% or less, ‘Overfed grates’ are used for small
wood pellet burners and ‘Moving step grates’ are used for fuels with moisture
contents of 30% to 60%.
Wood requires relatively little primary air (air supplied at the point of solids
combustion, usually under the grate) but requires a good supply of secondary
air (over-grate) to ensure that complete combustion takes place. All modern
woodchip and wood pellet burners have provision for separate
control of the primary and secondary air supply, in order that combustion can
be properly balanced. The combustion zone is usually insulated and
comprises a primary and secondary chamber to ensure thorough mixing
(turbulence) and time for the combustion reactions to occur.
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