the high volatile and free-burning Mid-Western type coals. The feature of the traveling
grate stoker that provides for the utilization of such a wide variety of fuel types is
the undergrate air zoning. These units normally have from five to nine individual air
zones which can control the amount of air admitted to the fuel bed as it travels from
the free end of the stoker to the discharge. This provides the stoker operation with
tremendous flexibility to obtain complete combustion with the various sizes and types of
fuel. Since the fuel bed on the traveling grate stoker is not agitated by vibration as
the bed usually 4 inches (101.6 mm) to 6 inches (152.4 mm) depth is moving from the feed
end toward the discharge end, the amount of particulate fluidization is very low. This
means that the traveling grate stoker has a low particulate pollution characteristic as
compared to other fuel burning stokers. Chain grate stokers are not recommended except
to burn low fusion coals with high clinkering tendencies.
Overfeed Spreader Stoker with Traveling Grate. The spreader stoker is
characterized by a thin bed and partial burning of coal particles in suspension.
Suspension burning gives rapid response to load changes which is an important
characteristic for many industrial process steam plants that need rapid changes in steam
production. This characteristic, together with a nonclinkering thin bed on the grate,
provides a unit capable of firing a wide range of coal grades and types. The spreader
stoker has high availability, ease of operation, and good efficiency. The suspension
burning causes a high particulate loading of the burning gases within the furnace which,
without fly ash reinjection, would result in a high carbon loss in the fly ash. Front
discharge traveling grates are commonly used with spreader stokers. (Dump, vibrating,
reciprocating, and oscillating grates are also available). With a high particulate
loading, the spreader stoker requires the use of electrostatic precipitator or baghouse
Stoker Criteria. See Table 5, Stoker Selection Criteria, for information
necessary for proper selection of a stoker type. Information included in the table are
average criteria gathered from several boiler-stoker manufacturers' recommendations.
Coal Feeders. For use with each pulverizer, the coal feeding function can be
accomplished by the use of a separate rotary feeder or combined with the weighing
function (see para. 4.5.6, Coal Scales), using a volumetric or gravimetric feeder.
Pulverizers, depending on type, may operate with either a negative or positive internal
pressure and will also contain hot circulating air. Coal feeders cannot act as a seal
for the pulverizer air, therefore, a height (head) of coal must be provided and
maintained above the feeder inlet to prevent pulverizer air backflow. See also MIL-
HDBK-1003/6, Section 5.