POWER March 2015 - 40

AUXILIARY SYSTEM EFFICIENCY & RELIABILITY
4. Adding burners. NIPSCO's Bailly Generating Station added three natural gas-fired 40
MMBtu/hr register burners to Unit 8 (whose data are shown in the figure) and one to Unit 7 in
2011. Each burner has a maximum firing temperature of 1,000F. Burners are engaged when the
flue gas temperature entering the SCR drops below 633F (marked by the red line) and the gas
is reheated to approximately 633F. The dotted line illustrates the maximum capability of the Unit
8 SCR reheat system. Data are for Unit 8 operations during October 2010. Source: NIPSCO
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650
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550
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450
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The plant has been operating, since
January 2011, under the terms of a legal
settlement between NIPSCO and the U.S.
Environmental Protection Agency regarding
compliance with the Clean Air Act. The consent
decree specifies the maximum 30-day
rolling average emission rates for NOx, SO2
,
and PM emissions for both units combined.
Emissions data are collected by the plant's
continuous emissions monitoring system
mounted on the combined stack, and a new
30-day rolling average is calculated each
calendar day. The average emission rates
that occur during the daily averaging period
specifically include startup, shutdown,
and malfunctions that may occur each day.
The consent decree does allow for specific
malfunctions that qualify as a force majeure
event that aren't factored into the rolling average
emission rate.
Both units are operated based on system
demand and are regularly cycled. During
reduced load conditions, the flue gas temperature
often drops below the 645F minimum
temperature setpoint for the operation
of the Unit 8 ammonia injection system for
the SCR, as the June through October 2010
operating data illustrates (Figure 4). This
system limitation means that when the plant
is dispatched to lower loads that reduce the
flue gas temperature below 645F, the ammonia
injection system does not start, SCR
efficiency remains low, and the plant could
fail to meet the 30-day rolling average NOx
limit (0.18 lb/MMBtu) specified in the consent
decree.
BGS determined that adding four Forney
40
5. Assembling burners. The three
burner systems are preassembled prior to installation
on Unit 8. Courtesy: NIPSCO
6. Burner lift. A register burner is lifted
into place on Unit 8. Courtesy: NIPSCO
SCR reheat system design point with three SCR
reheat burners (1,000F injection temperature)
200
Unit load (MW)
natural gas-fired register burners (three on
Unit 8 and one on Unit 7) minimizes the risk
of the plant failing to meet NOx emissions
limits during low-load operation and thereby
avoids expensive penalties associated with
noncompliance. The four register burners
were installed in 2011.
The register burners were delivered to BGS
three months after drawing approval. The
three Unit 8 burners were lifted into place in
June 2011 and placed onto a structural foundation
adjacent to the SCR inlet ductwork.
Each SCR burner assembly weighs approximately
17,600 pounds (Figures 5 and 6).
The SCR register reheat burners are
monitored and modulated by two separate
control systems: the burner management
system (BMS) and the combustion control
system (CCS). An Invensys I/A Series
distributed control system (DCS) using
a dedicated pair of fault-tolerant CP270
controller modules manages each of the
three control systems. BGS also uses the
Invensys DCS platform in other areas of
plant operation.
During operation, the heat load at the
outlet of the economizer is calculated by
the CCS by multiplying the difference
between the economizer average outlet
temperature and SCR inlet temperature
with the specific heat of the flue gas and
the flue gas mass flow rate. This value is
" trimmed " by the SCR inlet temperature
trim controller, and the final calculated
value is used by the CCS as the heat that
must be added to the economizer outlet
gas (heat) to maintain the necessary SCR
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inlet temperature. The number of burners
required (Unit 8 only) is determined by dividing
the heat by the maximum thermal
energy available from a single burner. The
BMS determines when burners are started
and stopped once the system is in service.
Multiple burners are operated in parallel in
order to maintain a uniform exit gas temperature
across the duct at the ammonia
injection grid.
Compliance Risks Avoided
The 320-MW BGS Unit 8 has experienced
no loss of ammonia injection at reduced load
since the SCR reheat burners were installed
in June 2011. The burners have worked flawlessly
in maintaining the minimum ammonia
injection temperature above 635F under all
operating conditions. BGS also reports that
the 635F permissive temperature for ammonia
injection can be maintained down to 180
MW, which gives the utility additional flexibility
in plant dispatch.
An added benefit is the ability to use the
SCR reheat burners during unit startup time
from cold start to full load. Depending on
the specific boiler conditions during rampup,
one, two, or three reheat burners are engaged.
This can reduce the startup time by 2
to 6 hours as the ammonia injection permissive
temperature is achieved faster. This has
a positive influence in the compliance emission
rate calculations. ■
-Robert Parent (robert.parent
@forneycorp.com) is sales manager for
Forney Corp. Bruce Rivera (brivera
@nisource.com ) was NIPSCO's project
manager for the installations.
POWER | March 2015
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SCR inlet temp. (F)

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POWER March 2015

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