Underfed stoker boilers can burn wood pellets and
wood chips up to 30% MC. Some may be designed
specifically for use only with pellets, in which
case very little refractory lining may be installed.
For wood chips up to 30% MC a moderate level of
refractory lining will be installed, and the boiler is
usually limited in the size of wood chip it can accept.
Again, they are designed for a reasonably fast
response to heating load demands.
Underfed stoker boilers are always auger fed and
do not have a HWC capability. It is not possible to
install a flap valve beneath the grate.
Most underfed stoker boilers have separate primary
and secondary air fans to provide an independent
combustion control on the grate and in the final gas
combustion zone.