Implementation of DryFining at
Coal Creek had a significant positive
effect on NO
X
, SO
2
and total mercury
(Hg
T
) emissions (Table 1). A reduction
in NO
X
emissions is attributed to the
lower coal input and lower primary
air (PA) to secondary air (SA) flow
ratio, compared to the wet coal
operation. The lower PA flow results
in lower NO
X
formation at the
burners, while the higher SA flow
allows for deeper furnace staging,
with more overfire air available. The
resulting 30% NO
X
reduction allowed
Coal Creek to meet its new NO
X
emission limits by boiler tuning,
avoiding a costly installation of a
SNSR or SCR reactor.
4
SO
2
emissions reduction may be
attributed to three factors. First, the
lower flow rate of dry coal to the
boiler results in a reduction in the
amount of sulfur entering the boiler.
Second, a significant portion of the
inorganically bound sulfur is
segregated out from the FBD. Sulfur
segregation, measured during the
dryer acceptance tests, was 33%.
Finally, the lower volumetric flow of
flue gas allows a larger proportion of
flue gas to be scrubbed, further
reducing SO
2
emissions.
The 35 – 40% reduction in Hg
T
emissions produced by DryFining is
due to the lower flow rate of dried
coal into the plant, as well as the
removal of approximately 30% of the
pyrite-bound mercury from the coal in
the FBD by gravitational segregation.
It is also due to the change in mercury
speciation and the increased flow rate
of flue gas through the FGD, where
oxidised mercury (Hg
2+
) is removed.
The reduction in Hg
T
emissions has
allowed the Coal Creek power plant to
meet new emission limits with FGD
additives to reduce Hg
2+
re-emission,
thereby avoiding injection of
powdered activated carbon.
Overall, by implementing
DryFining at Coal Creek, GRE
avoided US$366 million in CAPEX,
which would otherwise be needed to
comply with emission regulations.
Plant operation
DryFining has been in continuous
commercial operation at Coal Creek
power plant for over five years,
achieving availability higher than 95%
and not causing a single unit outage.
Its implementation has affected the
performance of components in the gas
path, from the boiler to the stack. Key
impacts include:
Boiler
To maintain the reheat steam
temperature set-points, the
combustion control system has
increased main burner tilts and
closed attemperator valves. Total
sootblowing steam flow has remained
constant, although the usage split
changed. The frequency of cleaning
furnace waterwalls decreased, while
cleaning frequency for the convective
path increased to maintain steam
temperature set-points.
Air preheater
Before DryFining, due to high flows
and fouling of the heat transfer
passages in the cold-end, the air
preheaters experienced high
differential pressure across the PA and
flue gas sectors. The high moisture
content of the flue gas, along with
seasonal variations in the air inlet
temperatures were major culprits of
fouling and corrosion of the air
preheater cold end heat transfer
surfaces, which were replaced every
three years. The high pressure drop
also produced excessive air to gas-side
leakage. DryFining virtually
eliminated those problems, reducing
the ID fan power and the PA airflow.
Mills and coal pipes
Before DryFining, seven to eight mills
were normally run. Feeder trips,
caused by large pieces of coal, rocks
and tramp iron stalling the feeder
were frequent occurrences, resulting
in load derates and numerous feeder
belt replacements. High PA flows,
required to maintain mill exit
temperatures, resulted in high
velocities in the coal pipes and
increased erosion. DryFining has
allowed each unit at Coal Creek to
operate at full load with six mills and
reduced PA flow. With lower PA flow,
there was an increase in the mill
capacity and a reduction in mill power
and mill maintenance. Mill feeder
trips have been eliminated, while
plant availability has improved. Also,
the six mill operation allowed mill
maintenance to be performed outside
the plant outage, which further
reduces the cost.
ID fans
Following the implementation of
DryFining, ID fan power decreased
2 MW to 4 MW per unit, due to lower
flue gas flow rate, higher flue gas
density and reduced air preheater
fouling. The total reduction in station
service power per unit is 5 MW.
Flue gas desulferisation
Each unit at Coal Creek is equipped
with a four-module wet scrubber
capable of scrubbing 75% of the flue
gas. Lower flue gas flow rate and
temperature from DryFining has
increased scrubbed flow to 85 – 100%
of the total flue gas flow, eliminating
the need to install a fifth module.
Electrostatic precipitator
The reduction in flue gas temperature
has decreased the resistivity of the
flyash, thereby improving electrostatic
precipitator performance. The reduced
volume of the flue gas decreased its
velocity and increased the specific
collection area. These effects
combined with reduced particulate
loading helped to improve the ESP
collection efficiency over the past
five years.
Notes
1. Inert fluids, other than steam, may be
used for fluidisation to achieve deep
reductions in coal moisture content.
2. For German and Australian brown coals
containing 55 – 60% moisture, the heat
rate improvement is in the 5 – 7% range.
3. Oxyfuel and oxygen-blown gasification
plants are not subject to the equilibrium
moisture content limit. The studies
conducted with DryFining integrated
with the lignite-fired oxyfuel and CTL
plants employing dry-feed gasifier, have
demonstrated coal moisture reduction
from the 40 – 55% range to the target
moisture level of 8 – 12%.
4. The NO
X
reduction of 25% recorded
in regular plant operation was lower
compared to the test results presented
in Table 1 because changes in unit load
and combustion settings experienced in
regular operation, increase NO
X
.
70
|
World Coal
|
July 2015
