POWER July 2017 - 32

ENVIRONMENTAL ISSUES
catalyst material confirmed that the HBGS
catalyst was a titania-type material with an
850F maximum allowable continuous operating
temperature. Prior to July 2014, engine
load had been kept between 14.5 MW
and full load. The result was that the catalyst
temperature remained below 850F. However,
after July 2014 the peak catalyst temperature
increased to 870F when the engines
operated at minimum permit allowable load.
Tests performed by Environex found that
the first catalyst layer had sustained considerable
damage and degradation due to
the higher operating temperatures, which
could account for the increased emissions
performance degradation noticed by station
personnel.
HBGS staff was confident that Wärtsilä
technicians could tune the engines to operate
at lower outlet temperatures, so tuning was
scheduled for completion in late spring 2017.
Tuning engines to lower exhaust temperatures
was expected to result in increased CO
and decreased NOx
.
The testing ultimately concluded that excellent
emissions control equipment health
was critical for dealing with higher CO production
while not exceeding permit limits.
Catalyst Poisons. Another problem discovered
during the Environex study was sodium
and calcium poisons found on the first
layers of the catalyst.
HBGS is situated on the east shore of
Humboldt Bay, directly across from the entrance
channel. Large waves regularly break
upon the rock breakwater just west of the
facility, causing a salty spray to spread over
the plant. Although the source of the poisons
is still up for debate, one theory on how sodium
entered the system is that the engines
routinely ingest the salt spray, which then
contaminates the catalyst. HBGS intends to
continue monitoring SCR catalyst performance,
focusing on the first layer, which is
affected most by the sodium.
The calcium source remains a mystery.
However, one possible source is the water
component of the supplied 19% aqueous
NH3
. For that matter, sodium poison could
be introduced via the same mechanism.
The station's environmental specialist
has been tasked with gathering an aqueous
NH3
sample and analyzing it for calcium
and sodium. In the meantime, quality
control visits to the aqueous NH3
found that the demineralized water used
in the production of the aqueous NH3
supplier
was
not monitored continuously. That could allow
potential sodium and/or calcium breakthroughs
to go unnoticed during the aqueous
NH3
tions, it has been difficult to predict catalyst
life accurately at HBGS-it is one of the first
dual-fuel plants built and operating in a heavily
cycling configuration (multiple starts,
low load). Hug engineers had not seen such
rapid catalyst deterioration at other facilities,
most of which are spark gas (not dual-fuel)
or are baseload stations. Engines used in the
shipping industry typically operate at 75%
capacity while steaming. One positive development,
however, is that throughout these
efforts HBGS staff has gained valuable experience
predicting catalyst degradation and the
order of replacement.
Structural Damage Caused by Thermal
Cycling. The engines have between
1,400 and 1,600 starts since commissioning.
High cycle rates combined with the
high catalyst inlet temperature at low load
has caused brick damage, such as crushing
at the bottom brick rows, and resulted in
gaps opening above the top brick rows. Engines
are taken off-line periodically to clean
the brick and to plug gaps identified during
inspections.
High-temperature non-titania-based SCR
catalyst is available. HBGS staff intends to
consult with Wärtsilä and catalyst suppliers
to identify high-temperature catalyst that is
compatible with its operating conditions, if
such catalyst is available.
NH3 Flow Control
The plant's originally supplied NH3
flow
control hardware did not allow good NH3
flow control at low engine loads in dualfuel
operation. In addition, the injectors,
atomizing components, and exhaust mixing
equipment supplied were sized for full-load
diesel operation and were highly inefficient
at low loads in dual-fuel mode. The result
was high over-injection rates and NOx
production
in the oxidation catalyst. Facility
staff has continued to fine-tune atomizing
air rates and adjust injectors at low loads to
find the optimum setpoint.
HBGS staff was hesitant to add a paralinjection
system sized for dual-fuel
lel NH3
mode operation. The exhaust pipe mixing
equipment was sized to mix NH3
and
exhaust gas in diesel mode and cannot be
changed. In addition, no additional ports
were available to house low-flow injectors.
However, the addition of new dual-range
variable-flow-coefficient valves has greatly
improved NH3
load dual-fuel operation. The NH3
trends have stabilized and NH3
tion has decreased.
manufacturing process.
Due to a lack of industry experience operating
facilities under tight permit condi32
consumpWaste
Gate Tuning
As mentioned previously, the waste gate bypasses
exhaust gas around the turbocharger
www.powermag.com
A Successful Effort
In HBGS's case, there was almost no previous
operating experience to draw upon.
HBGS is one of the first facilities utilizing
the Wärtsilä W18V50DF A engine technology
in a heavily cycling mode with such a low
permit level condition. The expected catalyst
life was based on the only available information,
which came from the shipping industry.
The shipping industry, however, typically is
operated under different conditions, including
less-stringent permit conditions, and the
engines operate near full load while steaming
(>75%).
The HBGS team has been fully engaged
in the effort of identifying corrective actions
to help operate the plant within its permit
parameters by fully understanding the limitations
of its catalyst system. The staff's
philosophy is one of continual improvement,
and it has found studying the issue and identifying
further improvement to be an excitflow
control during lowflow
ing
endeavor. ■
-Silas Biggin is power generation plant
engineer; Chris Brittain is operations and
maintenance supervisor; Chuck Holm
is plant manager; Jill Lorenc is power
plant assistant; and Scott Washington is
environmental field specialist for PG&E
Humboldt Bay Generating Station.
POWER | July 2017
turbines to control charge air (combustion
air) pressure. Operating experience indicated
that a charge air pressure swing of 2 psid or
greater would cause the NH3
injection control
system to over-inject, resulting in higher
stack NOx
values.
At HBGS, all charge air pressure swings
have been the consequence of a malfunctioning
waste gate. Once the waste gate
swing was eliminated, the result has been
more stable emissions and more stable NH3
flow control.
Additionally, engine temperature control
swings also cause emissions swings. In that
case, CO and NOx
of NH3
can swing and result in loss
control. However, temperature control
issues have proven to be rare at HBGS.
Catalyst Brick Availability
Catalyst brick availability is highly volatile.
In HBGS's experience, catalyst suppliers do
not maintain large catalyst brick inventories.
Orders placed by other facilities often deplete
stocks, resulting in lead times of three
months or more.
HBGS has developed relationships with
three different catalyst suppliers familiar
with engine-based generation. In addition,
the plant has started looking into the future
more than usual to maintain in-house
inventory levels higher than originally recommended.
http://www.powermag.com

POWER July 2017

Table of Contents for the Digital Edition of POWER July 2017

Contents
POWER July 2017 - Cover1
POWER July 2017 - Cover2
POWER July 2017 - Contents
POWER July 2017 - 2
POWER July 2017 - 3
POWER July 2017 - 4
POWER July 2017 - 5
POWER July 2017 - 6
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