POWER July 2016 - 32

ENVIRONMENTAL ISSUES
1. Conventional baseload vs. cycling operation. Courtesy: Environex Inc.
Conventional baseload unit
80
70
60
50
40
30
20
10
Below 60 60 65 70
80
70
60
50
40
30
20
10
Below 60 60 65
70
75
80 85
Load (% of baseload)
2. Typical cycling operation of a modern frame turbine unit. Courtesy:
Environex Inc.
■ Exhaust flow (klb/hr/10) ■ Load (MW/10) ■ Turbine exit NOx (ppmvdc) ■ Turbine exit CO (ppmvdc)
120
100
80
60
40
20
0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Time (days)
perature and low mass flows could be an issue
for current catalyst designs.
Challenges for Emissions Catalyst
Operation
Catalyst-based systems for control of CO,
NOx, and volatile organic compounds (VOC)
are strongly affected by the conditions of the
turbine exhaust gas. Oxidation catalysts are
passive catalysts used to oxidize CO and VOC
to CO2. Selective catalytic reduction (SCR)
systems are active catalyst systems that require
injection of a reagent containing ammonia
(NH3) to reduce NOx to N2 and H2O.
32
Prior to 2001, turbine catalyst systems were
designed primarily for steady-state, baseload
operation. These systems had generous
startup/shutdown windows, normally of 1 to
3 hours, and typically operated above 80% of
maximum load. Between 2001 and 2005, the
startup/shutdown windows were reduced to
30 to 60 minutes. Today, these systems can be
required to start up or shut down in 10 minutes,
while at the same time dealing with more
frequent cycling, faster ramp rates, and wider
load swings than ever before.
As a unit cycles, the temperature, flow rate,
and concentration of emissions in the turbine
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60
50
40
30
20
10
5.0
90 95
100
75 80 85 90 95
Load (% of baseload)
Modern cycling unit
100
exhaust entering the SCR/CO catalyst system
vary. Exhaust temperatures will often depend
on the pressure and steam production in the
HP steam system and can vary by +/-100F entering
the catalyst system. Also, because the
lower-load operation introduces lower flow
into the same fixed cross-section, the flow distribution
differs from baseload, imparting gas
velocity, ammonia concentration, and temperature
variations at the catalyst inlet.
Turbine exhaust NOx and CO also spike
during load changes, which can affect the
stack emissions and require greater emissions
reduction
to
maintain compliance
during short averaging periods. Maintaining
peak SCR system performance and low
levels of both stack NOx and ammonia slip
through these cycles requires precise control
of ammonia injection. In some cases this may
require updates to the ammonia flow control
logic or even upgrades to the equipment and
additional monitoring hardware.
An example of SCR operation with turbine
cycling is shown in Figure 5. The variations
from cycling operation are most apparent in the
ammonia slip (the unreacted ammonia), shown
in orange. Variations of 3 to 4 ppm are common.
These variations in ammonia slip are most
often caused by a lag in response time of the
ammonia vaporization and injection system in
reacting to changing ammonia demands. Some
progress is being made to improve response
times, but more improvement is needed.
In the permitting phase for modern cycling
unit sites, permit applications must take into
account these variations, but they currently do
not. Regulations at the state and federal levels
have reduced best available control technology
(BACT) guidelines for turbines to between 2
and 5 ppm NOx and 2 to 5 ppm ammonia slip,
both with averaging times as little as 30 minutes.
In ozone nonattainment areas, air permit limits
of 2 ppm NOx and 2 ppm ammonia slip are normal.
These BACT limits required by regulations
are not practical in cases with cycling operation,
as illustrated in Figure 5.
Fast-start combined cycle systems are capable
of startup in 10 minutes. Conventional
SCR/CO system designs have demonstrated
startup in approximately 30 minutes. Though
the difference of 20 minutes may seem small,
much happens in those 20 minutes, including
heating of the SCR catalyst and ammonia
vaporization system that are required for
optimum performance. Figures 6 and 7 compare
a conventional SCR system start (with a
1-hour startup window) to a modern fast-start
system (with a 10-minute start requirement).
Even with a warm start, the fast-start system
struggles to achieve compliance within 15 minutes.
In many cases, particularly for cold starts,
SCR systems cannot meet, or have difficulty
meeting, the 10-minute startup requirement.
POWER | July 2016
Exhaust flow (klb/hr/10)
Operating hours (%)
Operating hours (%)
Load (MW/10) and CT exit NOx/Exit
CO (ppmvdc)

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POWER July 2016

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