POWER December 2015 - 64

FUNDAMENTALS
6. A negative correlation. The three
main factors that constitute pulverized capacity
are Hardgrove Grindability Index (HGI),
fineness, and coal throughput. Increasing
throughput will adversely affect fineness.
Source: Storm Technologies
210
200
190
180
170
160
150
140
130
120
110
100
90
80
70
60
For variations in HGI and fineness
Example:
Given: Coal grindability - 55
Coal fineness leaving
Pulverizer 80% through
200 mesh
Max capacity - 111,000
Compares to 83,000 lb/hr at 40 HGI fuel
and 80% passing 200 mesh
60%
65%
70%
75%
80%
7. Missing the mark. The air-to-fuel (A/F)
flows shown in this graph are much higher
than optimum. Installing properly sized rotating
throats is often required to achieve targeted
A/F ratios. Source: Storm Technologies
- Recommended primary air-fuel ramp ■ " As found "
Unit 2 mill fuel line measurements ▲ Unit 2 installed airfuel
curve ◆ " As found " Unit 1 fuel line measurements
20 30 40 50 60 70 80
Grindability % (HGI)
fuel is best. For some pulverizer types, such as
ball tube mills and high-speed attrition mills,
often a 1.6 A/F ratio is optimum. Never have
we observed good combustion conditions or
good mill performance with A/F ratios of 2.5
or greater. However, it is common to find A/F
ratios of 2.2 to 2.5 during baseline testing.
Results of as-found airflow to fuel flow
testing from a sample plant are shown in Figure
7. In this particular case, the A/F ratios
tested were well above the desired A/F ramp.
When operators bias the primary airflow up,
above the optimum, it may improve wet-coal
drying, load response, and reduce coal spillage
from the grinding zone, but it is not good
for the furnace burner belt performance.
All combustion airflow inputs should be
measured and controlled, if possible. We prefer
to use the tried and proven venturi or flow
nozzles for this purpose because they are rugged,
reliable, offer repeatable results, and are
less prone to impulse line plugging.
MPS Mill Recommendations
Several components can be retrofitted to
improve the performance of MPS mills.
The changes may cost a significant amount
of money, but the work will usually pay for
itself through improved heat rate. One 450MW
coal-fired unit in the Midwest spent
$750,000 on testing, changes, and tuning, but
calculated that it saved millions by improving
heat rate and by allowing higher-slagging
fuel to be used at a reduced cost, which greatly
increased its market competitivness.
The locations corresponding to the following
improvements are shown in Figure 8:
64
300
280
260
240
220
200
180
160
140
120
100
8. Extending component lives. Getting
8,000 hours per year performance requires
condition-based maintenance utilizing periodic
isokinetic coal sampling and venturi hot " K " testing
and calibration. Source: Storm Technologies
0% PA bias, Mill 5
-6% PA bias, Mill 5
-10% PA bias, Mill 5
Recommended mill
inet airflow ramp
40,000 55,000 70,000 85,000 100,000 115,000 130,000
Coal flow (lb/hr)
■ Install an optimized outlet skirt component
(A).
■ Install new optimized extended course
particle guide classifier blades (B) for better
classification and homogenization.
■ Set conical baffle (C) clearances and tolerances
in accordance with optimized specifications.
■
Install optimized flapper/reject door (D)
design.
■ Install clean air system resistance balance
(E) with optimized orifice housings.
■ Retrofit primary airflow measurement device
(F).
■ Install optimized deflector assembly (G).
■ Install optimized rotating throat assembly
(H).
■ Add extension ring (I).
Cold air has a different density than hot air,
which can result in a variance in measured velocity
at similar mass flow rates. Because the Kfactor
will vary, we prefer to conduct " Hot-K "
airflow calibrations that use typical operational
air or gas density when developing an average
K-factor. That information is important when
developing a pulverizer primary airflow curve
and when measuring all combustion airflows.
Most instrumentation technicians can calibrate
and check using the " Hot-K " method
to verify calibration accuracy. As previously
mentioned, high primary airflows are one of
the most common root causes of poor pulverizer
performance, in our experience, so
obtaining accurate and representative measurements
is very important.
www.powermag.com
Getting the Best Burner Belt
Combustion
The goal is to obtain the best possible burner
belt combustion because it improves heat rate,
reduces slagging/fouling, lowers emissions,
and reduces cost. All of the following actions
can help improve burner belt combustion:
■ Balancing airflow using fuel line orifice
housings.
■ Retrofitting gravimetric coal feeders with
new microprocessors and properly calibrated
load cells.
■ Tuning pulverizers to tight mechanical
specifications.
■ Maintaining a desirable A/F ratio of about
1.8.
■ Keeping pulverizer throat velocity within
design parameters.
■ Calibrating flowmeters using the " Hot-K "
method.
■ Checking fuel lines for fineness and distribution
using isokinetic coal sampler testing.
Excellent pulverizer performance and fuel
line balancing results in uniform flames in the
boiler, which offers the following advantages:
■ Reduced CO and NOx
emissions
■ Less slagging
■ Less fouling
■ Better fly ash loss on ignition
■ Reduced dry gas loss
■ Improved fuel flexibility
■ Reduced popcorn ash to the selective catalytic
reduction system
■ Decreased furnace exit gas temperature
■ Less desuperheating water flow
■ Reduced sootblowing
With all of these benefits, it makes good
sense to take action and implement changes
that will improve plant performance. ■
-Richard F. (Dick) Storm, PE is senior
consultant for Storm Technologies Inc.
and a long-time POWER contributor. The
staff of Storm Technologies contributed to
this article.
POWER | December 2015
Capacity (1000 lb/hr)
Primary airflow (1,000 lb/hr)
Fineness through 200 mesh
http://www.powermag.com

POWER December 2015

Table of Contents for the Digital Edition of POWER December 2015

Contents
POWER December 2015 - Cover1
POWER December 2015 - Cover2
POWER December 2015 - Contents
POWER December 2015 - 2
POWER December 2015 - 3
POWER December 2015 - 4
POWER December 2015 - 5
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