Biomass Handling - Biomass Gasification, Pyrolysis and Torrefaction - page 432

bed of a bubbling fluidized bed. A highly erosive zone may be formed, near
each outlet nozzle of the feeders, which might corrode the tubes nearby.
Another innovative, but one that is less common, feed system uses pulsed
air. Controlled-air pulses push the biomass into the gasifier, avoiding pyrolysis
of feed in the gasifier feed line. A very small amount of air minimizes dilution
of the product gas with nitrogen.
12.4.3.5 Moving-Hole Feeder
A moving-hole feeder is particularly useful for fluffy biomass or solids, with
flakes, which are not free-flowing. Such solids can cause excessive packing
in the hopper and screw feeder. Unlike other types, moving-hole feeders do
not draw solids from one particular point in the silo.
A moving-hole feeder essentially consists of slots that traverse back and
forth with no friction between the stored material and the feeder deck. At a
desired rate, a moving hole or aperture slides under the hopper. The solids
drop by gravity into the trough or belt that carry the feed at that rate.
With a moving-hole feeder there is no compaction of solids that are typi-
cally seen in screw feeders. Rat holes are also avoided by using vertical
instead of sloped walls in the hopper the only stipulation is that the size of
the hole must be sufficiently large to avoid arching of a given biomass.
12.4.3.6 Fuel Auger
A metering device such as a screw is used to meter biomass like hog fuel
and feed it onto the main fuel belt. The belt carries the fuel to the gasifier
front, where the fuel stream is divided into several 50%-capacity fuel trains.
Each train consists of a surge bin with a metering bottom and a fuel auger to
deliver the fuel into the furnace. The auger is cantilevered and driven at a
constant speed through a gear reducer. The bearing of the auger shaft is
located away from the heat of the gasifier. Cooling air is provided to cool
the auger's inner trough as well as to propel the fuel toward the bed.
12.4.3.7 Ram Feeder for Refuse-Derived Fuel
A ram feeder is essentially a hydraulic pusher. Refuse-derived fuel (RDF) is
at times too fibrous or sticky to be handled by any of the aforementioned
feeders. In this case, a ram-type feeder can be effective in forcing them into
the gasifier. A fuel auger can convey the solids into a hopper at the bottom
of which is the ram feeder. The ram pushes the RDF onto a sloped apron-
type feeder that feeds the fuel chute ( Figure 12.15 ). From the fuel chute, the
RDF drops into the fuel spout, where sweep air transports it into the furnace.
The air also prevents any backflow of hot gases. The RDF stored in the inlet
hopper provides a seal against positive furnace pressure. The apron feeders
are driven by a variable-speed drive for controlling the amount of fuel going
into the system.