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The boiler shall be the dry bottom type, discharging ash to the hopper
compartments. Pulverized coal boiler ash shall be handled hydraulically
rather than pneumatically. Facilities for pulverized coal boiler ash
handling shall be as indicated. Fly ash shall not be mixed with furnace
ash but economizer ash can be combined with furnace ash as indicated. Dual
ash dewatering bins to allow recirculation of ash conveying water shall be
as indicated. Each bin shall be provided with automatic controls and
hydraulically operated gate for ash unloading to trucks or railroad cars.
Hopper shall be maintained in flooded condition and hydraulically operated
sluice gates shall allow for periodic removal of ash. Each ash hopper
compartment shall be fitted with double-roll, electric motor driven clinker
from the grinders shall be to an adaptor or to a sump for feed to the
centrifugal pump. Centrifugal pump for each hopper compartment shall be
provided and shall incorporate special abrasive resistant metals and
special seals for abrasive slurry handling. Drives shall be totally
enclosed electric motors as indicated. Ash slurry conveying pipelines
shall be made of abrasive resistant alloy metal with a Brinell hardness
number of approximately 280. Wall thickness shall be not less than 13 mm
1/2 inch and pipe lengths shall not exceed 5.5 m 18 feet. Fittings shall
be of the tangent end type. Fittings shall have a Brinell hardness number
of approximately 400 and shall have removable wearbacks, where applicable,
or shall be of the integral wearback type. Loading facilities for ash
removal of dewatering bins shall be as indicated.
2.4.4
Pulverizer Rejects
Pulverizer rejects such as tramp iron or pyrites shall be collected in
hoppers adjacent to each mill discharge spout. High-level hopper
indicators shall be provided to initiate reject removal, in sequence, from
each hopper to the central holding bin by properly sized jet pumps. Mill
discharge control gates shall be provided and the control system shall be
interlocked with the furnace ash removal system to provide a completely
automatic disposal system. Discharge of mill rejects into the ash hopper
is not acceptable. The central holding bin and loading facilities shall be
as indicated.
2.4.5
Control Systems
Pulverized coal plant control systems shall be in accordance with NFPA 8501
and shall be interlocked to provide for a completely automatic boiler
operation. Automatic controls for coal feed, pulverizer operation,
combustion, and ash disposal systems shall be coordinated.
2.4.5.1
Coal Master Control System
Coal master control system shall regulate the total coal feed from the coal
[bunkers] [silos] to the pulverizers in response to changing demand. A
parallel metering combustion control system that limits the firing rate to
the actual total air flow available shall be provided. The cross limiting
of air and fuel shall be arranged so the air leads the fuel on load
increase and trails the fuel on a load decrease. A gain changer shall be
provided to compensate the fuel system for the number of pulverizers in
service. Depending on the requirements of the pulverizer, the coal master
control system shall be actuated by the coal feeder speed or by the
coal/air mixture. The total fuel controller shall be a standard,
proportional plus integral controller with a derivative or rate term on the
SECTION 15559A
Page 38
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