Roll and race
The various types are described in detail in the following boiler
manufacturers' literature: (1) Babcock & Wilcox, 1978, (2) Combustion Engineering, Inc,
b) Turndown Ratio. The operating range of all types of pulverizers,
without reducing the number of burners fed from the pulverizer(s) is approximately 35
percent to 100 percent of the maximum pulverizer coal capacity. This is usually stated
as not more than 3 to 1 turndown range or ratio.
c) Pulverizer Sizing. Base pulverizer selection on coal feed is required
at maximum boiler load plus 10 percent for load pickup and continuous boiler output at
maximum steam load. Pulverizer output varies with coal grindability index and fineness
(percent through 200 mesh) of grind. These factors must also be taken into account in
selecting number and size of pulverizers. Emergency loss of one pulverizer must be
considered and the remaining pulverizer capacity must be sufficient to carry maximum
boiler steam load. The minimum boiler load will depend on the number of pulverizers and
burners installed and primary air velocities in the coal-air piping and coal burners.
It is desirable to have at least a 3 to 1 turndown on automatic control with all burners
and pulverizers in service. During boiler startup, the firing rate may be further
reduced by reducing the number of pulverizers and number of burners per pulverizer in
service. Sizing of pulverizers must be coordinated with the boiler manufacturer and
usually requires the development of a set of coordination curves of the various factors
involved such as shown on Figure 10.
d) Coal Feed Size. Crushed coal is used as the feed stock for
pulverizers. The maximum coal feed size is dependent upon pulverizer size. The larger
the pulverizer size, the larger is the coal size which can be accommodated. Coal feed
size ranges from 3/4" (19.05 mm) x 0" to 1-1/2" (38.1 mm) x 0" with 3/4" x 0" being a
size which is commonly used.
Pulverized Coal Firing vs. Stoker Coal Firing. The choice between the use of
pulverizers or stokers can only be determined by making an economical evaluation of life
cycle costs which include cost of equipment and installation, fuel, maintenance labor
and parts, operating labor, electrical energy, electrical demand, and supplies. For
many years, for industrial power applications, the boiler size breakpoint was
approximately 300,000 pph (136 000 kg/hr) with pulverizers predominantly used at this
boiler load and above. Presently there is a downward trend and the breakpoint for
boiler size is approximately 250,000 pph (113 000 kg/hr). Pulverized coal systems are
of high installation costs, high power costs to drive mills, more rigid coal
specifications, and need highly trained personnel.