with the combustion zone at the bottom of fuel-filled conduits ending at the surface of a fuel bed

Full Title

Combustion apparatus in which the fuel is fed into or through the combustion zone by gravity, e.g. from a fuel storage situated above the combustion zone > the fuel forming a column, stack or thick layer with the combustion zone at its bottom > with the combustion zone at the bottom of fuel-filled conduits ending at the surface of a fuel bed