POWER November 2014 - 38

HEAT RATE IMPROVEMENT
1. Two-stage coal dryer. This schematic
of a moving bed fluidized bed dryer
shows how the raw coal is cleaned of sulfur
and mercury in the first stage and coal is
dried in the second stage. Hot fluidizing air
is provided from plant low-grade waste heat.
Source: Great River Energy
Feed stream
Fluidization air and
evaporated coal moisture
1st stage
In-bed
HXE
In-bed
HXE
2nd stage
In-bed
HXE
Bed surface
Fuel
In-bed
HXE
In-bed
HXE
Test
1st stage fluidizing air
Segregated
stream
Note: HXE = heat exchanger
guishes this coal-drying technology and provides
a very important cobenefit of emissions
reduction.
A moving bed FBD was selected for the
DryFining process due to its high heat and
mass transfer coefficients, which produce a
compact dryer design. Air is employed as a
fluidization medium instead of the commonly
used steam. Potential devolatization of coal
during the drying process is avoided by drying
with plant low-grade waste heat (Figure 1).
2nd stage fluidizing air
Product
stream
Wet coal
Dried coal
Difference
(%)
moisture
(%)
37.1
32.1
-13.5
HHV
(Btu/lb)
6,251
6,914
10.6
Fuel flow
(klb/hr)
946
856
-9.5
Stack
flow
(kscfm)
1,557
1,467
-5.8
Notes: HHV = fuel higher heating value; ID = induced draft.
Crushed coal is fed to the first stage of the
fluidized bed dryer, where non-fluidizable
material such as rocks and other higher-density
fractions are segregated at the bottom of
the dryer, while less-dense and smaller particles
tend to float. Therefore, the segregated
stream discharged from the dryer has higher
mineral matter content (including pyrite) in
comparison to the dried coal (product stream).
Because most of the inorganically associated
sulfur is contained in pyrite forms, in the case
of North Dakota (ND) lignite, about 30% of
the sulfur and mercury (Hg) from coal are
segregated out in the first stage of the FBD.
The fluidizable material next enters the
www.kral.at
Pumps
Flow Measurement
dryer second stage, where the surface and
a portion of the inherent coal moisture are
evaporated by the heat supplied by the fluidizing
air and the in-bed heat exchanger.
The in-bed heat exchanger increases the
temperature of the fluidizing (drying) air
and fluidized coal bed, improving drying
kinetics. The drying process affects the microstructure
of coal particles that disintegrate
during drying. The drier and finer coal
is discharged from the FBD as the product
stream. The bed residence time and temperature
are the main parameters affecting the
residual moisture content.
Fluid Handling Solutions
Services
KRAL AG, Austria, e-mail: kral@kral.at
CIRCLE 13 ON READER SERVICE CARD
38
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POWER | November 2014
Four Years of Operating Experience
Three series of controlled tests were conducted
on Coal Creek Unit 1 at full load (600
MW, gross), steady state conditions before
and after the implementation of DryFining
to quantify the improvement in unit performance
and emissions. The coal-drying and
-cleaning system is sized for 1,100 tons/hr of
raw ND lignite with moisture content in the
range of 38% to 40%. DryFining has been
in continuous commercial operation at Coal
Creek Station since December 2009.
The tests with the wet (raw) coal were performed
in September 2009 to establish baseline
unit performance and emissions. Tests
with DryFining in service were performed in
March-April 2010 and October 2011. Those
test results are summarized below. The complete
test report may be found at " Lignite Fuel
Enhancement Final Technical Report, " DOE
Award Number: DE-FC26-04NT41763, released
June 2010.
Operating Conditions. Air preheater
(APH) air leakage decreased due to the lower
drafts with dried coal. Also, with dried coal
the temperature of flue gas at the APH exit
decreased, resulting in lower volumetric flow
of flue gas entering the flue gas desulfurization
(FGD) system, thus allowing a larger
proportion of the flue gas to be scrubbed
(Coal Creek employs a FGD gas bypass to
maintain dry stack conditions).
As a consequence of the lower FGD bypass
flow, the stack temperature decreased
but remained well above saturation temperature.
Also, with lower flue gas temperature,
flue gas velocity through the electrostatic
precipitator (ESP) decreased, resulting in improved
particulate collection efficiency and
lower opacity. The test operating conditions
are summarized in Table 3.
Unit Performance. The effect of DryFining
on net unit heat rate and boiler efficiency,
fuel and stack flow, and mill and induced
draft (ID) fan power is summarized in Table
4. During the October 2011 tests, the coal
moisture content was reduced by 5 percentage
points or 13%, resulting in ~11% increase
in coal heating value. Further reduction of
fuel moisture at Coal Creek was limited by
steam temperatures, which began to decrease
due to the lower flow rate of flue gas through
the convective pass of the boiler. Additional
boiler heat transfer surface is planned to alMill
power
(kW)
3,989
3,596
-9.9
ID
fan
power
(kW)
8,767
7,251
-17.3
Heat rate,
net (Btu/
kWh)
10,246
9,890
-3.5
Boiler
efficiency
(%)
80.34
83.06
3.4
Table 3. Unit 1 operating conditions during September 2009 and October
2011 tests. Source: Great River Energy
Test
Wet coal
Dried coal
Power
(MWgross)
600
606
O2 entering
APH (%) APH leakage (%) FGD inlet (F) Stack (F) Opacity (%)
2.54
2.76
8.2
4.1
345
309
188
156
6.6
5.8
Table 4. The effect of DryFining on the performance of Coal Creek Unit
1. Source: Great River Energy
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POWER November 2014

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