Coronado Emissions Control Project SRP Doc #:1700-02-M-SZZ-6435 

SRP PO #: SRP - 0000322042 Dist: G91, SRP Rev A

Project No. Plant and Unit No. Document No. 

100551 Salt River Project 

Coronado Generating 

Station Unit 2 

100551-090220000-04 

Document 

FUNCTIONAL DESCRIPTION 

The SCR Reactor 

Note: 

 

 

1st Issue Phone No. Date Name Signature 

Written by X1178 4/26/11 Kellie Bresnahan Kellie Bresnahan 

Reviewed by X3877 4/27/11 Kenneth Hurlburt Kenneth Hurlburt 

Approved by X1192 4/29/11 Michael Jasinski Michael Jasinski 

Approved by X1174 5/3/11 William Braband William Braband 

Rev. Date Item Changed 

by 

Approved 

by 

01 10/24/11 Updates per SRP / S&L Comments KSB MLJ 

02 3/26/12 Update section 2.2, 3.1 KSB MLJ 

03 2/4/13 Remove reference to Alarm & Setpoint List KSB MLJ 

04 9/4/13 Update Figures 1 and 2 KSB MLJ 

Distribution list no. pages 

 10 

This document and the information, design and material contained and/or illustrated therein (hereinafter “proprietary material”), is the property of BABCOCK POWER Inc., 

P.O. Box 15040, Worcester, MA 01615-0040, and is submitted, lent and furnished to recipient in strict confidence with the express understanding that the recipient shall not 

reproduce, copy, loan, dispose of, or disclose to anyone outside recipient’s organization, directly or indirectly, or use said proprietary material for any purpose other than that 

for which it is furnished and submitted. Recipient by receiving said proprietary material agrees not to use the same in any way injurious to the interest or BABCOCK POWER 

Inc., and agrees to return same upon request. Copyright  BABCOCK POWER Inc., 2011. All Rights Reserved.
Salt River Project Coronado Generating Station Unit 2 

Selective Catalytic Reduction System

SCR Operating and Maintenance Manual September 04, 2013 

SCR Reactor Functional Description Page 2 of 10 Revision 04 

The Selective Catalytic Reduction Reactor 

Table of Contents 

1.0 GENERAL INFORMATION AND REFERENCES 3

2.0 SCR REACTOR PROCESS DESCRIPTION 4

2.1 The NOx Reduction Chemical Reaction 4

2.2 SCR Reactors 4

3.0 SCR REACTOR OPERATION AND CONTROL 7

3.1 SCR Reactor Startup 7

3.2 Normal Operation 8

3.3 Normal Shutdown 9

3.4 Emergency Shutdown (SCR Reactor Trip) 10

3.5 SCR Reactor Purge 10
Salt River Project Coronado Generating Station Unit 2 

Selective Catalytic Reduction System

SCR Operating and Maintenance Manual September 04, 2013 

SCR Reactor Functional Description Page 3 of 10 Revision 04 

1.0 General Information and References 

The design of the SCR Reactor system complies with the following codes: 

• American National Standard Institute (ANSI) 

• American Society of Mechanical Engineers (ASME) 

• National Fire Protection Association (NFPA) 

• American Institute of Steel Construction (AISC) 

• American Welding Society (AWS) 

• Occupational Safety and Health Administration Standards (OSHA) 

• American Society of Civil Engineers (ASCE) 

• American Society for Testing and Materials (ASTM) 

Refer to the following documents for detailed information such as the process flow rates, instrument tag 

numbers, DCS I/O points and alarm setpoints. 

Reference 

No. RPI Doc. Number SRP Doc. Number Title 

1 100551-093000100 1700-02-M-SZF-6435 Process Flow Diagram 

2 100551-093001040 1700-02-M-SZM-6435 SCR Material Balance – 450 MW 

3 100551-093001041 1700-02-M-SZM-6436 SCR Material Balance – 200 MW 

4 100551-093001042 1700-02-M-SZM-6437 SCR Material Balance – 50 MW 

5 100551-094900050 1700-02-M-BAD-6435 P&ID – SCR Reactor 2-N 

6 100551-094900051 1700-02-M-BAD-6436 P&ID – SCR Reactor 2-S 

7 100551-094751001 1700-02-M-SZL-6435 Manual Valve List 

8 100551-094752001 1700-02-M-SZL-6436 Actuated Valve List 

9 100551-094904200 1700-02-J-SZZ-6436 Instrument List 

10 100551-094915000 – 1700-02-J-XXF-6435 – Analog Logic Diagrams 

 100551-094917088 1700-02-J-SHF-6438 

11 100551-094920000 – 1700-02-J-XXL-6435 – Binary Logic Diagrams 

 100551-094928022 1700-02-J-SHL-6440 

12 100551-094904600 1700-02-J-SZZ-6437 Input/Output List 
Salt River Project Coronado Generating Station Unit 2 

Selective Catalytic Reduction System

SCR Operating and Maintenance Manual September 04, 2013 

SCR Reactor Functional Description Page 4 of 10 Revision 04 

2.0 SCR Reactor Process Description 

2.1 The NOx Reduction Chemical Reaction 

The SCR System is designed to reduce nitrogen oxides (NOx) in the boiler flue gas to nitrogen gas (N2) 

and water vapor (H2O). The reduction is accomplished using ammonia as the reducing agent in the 

presence of a catalyst. The catalyst allows the exothermic reactions to take place in the SCR reactor. 

As long as a mixture of NO and NO2 is present, the following faster reaction happens in parallel, 

2.2 SCR Reactors 

Installed on the boiler of the Salt River Project (SRP) Coronado Unit 2 are two SCR reactors operating in 

parallel at the hot gas side ESP outlet. Anhydrous ammonia is the reducing reagent for the SCR reactors. 

The initial charge of reactor catalyst is a honeycomb design. 

Figure 1 is a schematic of one of the SCR reactors. Each SCR reactor is comprised of the following 

equipment: 

• Four layer reactor with three layers of catalyst installed and one empty layer for future catalyst 

• Redundant inlet (2-BA-AE-6451N / S, 2-BA-AE-6452N / S) and outlet (2-BA-AE-6491N / S, 2-BAAE-6492N / S) NOx / CO2 Analyzer (See Figure 1) 

• Outlet ammonia slip analyzer (2-BA-AE-6490N / S) 

• Expansion joints (2-M-BA-M-6435N / S, 2-M-BA-M-6436N / S, 2-M-BA-M-6437N / S, 2-M-BA-M6438N / S) 

• Pressure instrumentation located throughout the system (See Figure 1): 

2-BA-PIT-6440NA / NB / NC, SA / SB / SC 

2-BA-PIT-6464NA / NB / NC, SA / SB / SC 

2-BA-PIT-6470N / S 

2-BA-PIT-6481NA / NB / NC, SA / SB / SC 

2-BA-PIT-6510NA / NB / NC, SA / SB / SC 

• Temperature instrumentation located throughout the system (See Figure 1): 

2-BA-TE-6443NA / NB / NC, SA / SB / SC 

2-BA-TE-6461NA / NB / NC, SA / SB / SC 

2-BA-TE-6472NA / NB, SA / SB 

NO NH N H O

NO NH O N H O

2 3 2 2

3 2 2 2

+ → +

+ + → +

NO + NO2 + NH3 →N2 +H2O
Salt River Project Coronado Generating Station Unit 2 

Selective Catalytic Reduction System

SCR Operating and Maintenance Manual September 04, 2013 

SCR Reactor Functional Description Page 5 of 10 Revision 04 

2-BA-TE-6473NA / NB, SA / SB 

2-BA-TE-6474NA / NB, SA / SB 

2-BA-TE-6494NA / NB / NC, SA / SB / SC 

• Permanently installed temperature measurement grid at inlet 

• Permanently installed temperature measurement grid at outlet for initial testing 

• Delta Wing mixers with locations for injecting ammonia vapor 

• Four sonic horns on each layers two (2-M-SH-B-6435N / S – 6438N / S), three (2-M-SH-B-6439N 

/ S – 6442N / S), and four (2-M-SH-B-6443N / S – 6446N / S) of catalyst (See Sonic Horn and 

Sootblower Functional Description) 

• Three sootblowers on catalyst layer one (2-M-SS-B-6435N / S, 6436N / S, 6437N / S) 

• Manual test ports 

In the case of three redundant instruments in a row, RPI uses median select which uses only good signals 

as determined by the DCS. 

During boiler operation, flue gas flows from the outlet of the economizer through a hot gas side ESP and 

on to the inlet of the SCR reactor. Ammonia vapor diluted with warm air is injected at the Delta Wing

mixers at the SCR reactor inlet to thoroughly mix the ammonia with the flue gas. Turning vanes and 

crossmixers are located throughout the SCR system ductwork to aid with flow and ash distribution and 

NH3/ NOx mixing. After mixing with ammonia, the flue gas enters the reactor and first layer of catalyst. 

The SCR initially contains three layers of honeycomb type catalyst and one empty layer to be filled in the 

future. Each layer of catalyst contains 66 modules in a 6 x 11 arrangement. The catalytic reaction 

between the NOx and NH3 occurs in the reactor. Once the flue gas passes through the active reactor 

layers, it flows to the inlet of the Air Heater. 

In cases where there are multiple instruments installed in one location, the median select using only good 

signals is used. 
Salt River Project Coronado Generating Station Unit 2 

Selective Catalytic Reduction System

SCR Operating and Maintenance Manual September 04, 2013 

SCR Reactor Functional Description Page 6 of 10 Revision 04 

FIGURE 1: SCR Reactor System Schematic

P1 – HESP Outlet Pressure x 3 

P2 – SCR Reactor Inlet Pressure x 3 

P3 – SCR Reactor Inlet Pressure x 1 

P4 – SCR Reactor Outlet Pressure x 3 

P5 – Air Heater Inlet Pressure x 3 

A1 – SCR Reactor Inlet NOx and CO2 x 2

A2 – SCR Reactor Outlet NOx and CO2 x 2 

A3 – NH3 Slip 

T1 – HESP Outlet Temperature x 3 

T2 – SCR Reactor Inlet Temperature x 3 

T3 – Permanently Installed Temperature Grid 

T4 – Catalyst Layer N-1 Temperature x 2 

T5 – Catalyst Layer N-2 Temperature x 2 

T6 – Catalyst Layer N-3 Temperature x 2 

T7 – Catalyst Layer N-4 Temperature x 2 

T8 – Permanently Installed Temperature Grid for Testing 

T9 – SCR Reactor Outlet Temperature x 3 
Salt River Project Coronado Generating Station Unit 2 

Selective Catalytic Reduction System

SCR Operating and Maintenance Manual September 04, 2013 

SCR Reactor Functional Description Page 7 of 10 Revision 04 

3.0 SCR Reactor Operation and Control 

3.1 SCR Reactor Startup 

The objective of the SCR start-up is to warm the reactor in a controlled procedure to the minimum catalyst 

operating temperature before injecting ammonia. SCR warm up is controlled manually by the DCS 

operator by limiting the boiler ramp up rate. Ammonia cannot be injected below the minimum temperature 

because ammonia sulfates would condense in the catalyst pores and prematurely reduce the activity of 

the catalyst. The catalyst minimum continuous operating temperature (Tmincot) is calculated based on an 

analysis of the fuels. The catalyst maximum continuous operating temperature (Tmaxcot) is set to avoid 

thermal sintering of the catalyst. 

Based on the specified fuel and the catalyst design the catalyst minimum and maximum continuous 

operating temperatures at Coronado Unit 2 are: 

T F

T F

= °

= °

800

599

max cot

min cot

The SCR reactor starts along with the boiler. It is important to monitor and control the SCR reactor 

temperature ramp-up rate during start-up. It is the responsibility of the DCS operator to limit boiler ramp 

up rate to avoid exceeding the limitations. There are two limitations that must be considered: 

a) The maximum allowed temperature ramp up rate for the catalyst: The catalyst manufacturer 

defines this temperature. The supplier supplied start up temperature ramping rate up to the 

desired catalyst temperature is 18°F per minute. O nce the catalyst reaches the desired 

temperature, the ramp rate is increased to 180°F pe r minute. 

b) The maximum allowed temperature ramp up rate of the SCR reactor steel structure: Riley Power, 

Inc. defines the maximum temperature ramp up rate as 3°F per minute (180°F/hr) when 

measured with gas thermocouples in the inlet, reactor, and outlet ductwork. 

If the maximum allowed temperature rise for the catalyst is exceeded, the catalyst pores may be damaged 

due to the quick evaporation of moisture. If the maximum allowed temperature rise for the steel structure 

is exceeded, serious thermal stress damage of the reactor steel structure may occur. The maximum 

allowed steel structure temperature rise of 3°F per minute and is the limiting temperature ramp-up rate. 

Figure 2 illustrates a typical temperature trend for the SCR reactor during start-up. At a reactor inlet 

temperature (T2) above ambient, a temperature rise of 2.5°F per minute (150°F/hr) produces an alarm to

the operator. The operator receives a second alarm at 3°F per minute (180°F/hr). At the second alarm , 

the operator must take steps to lower the temperature rise rate in the SCR such as limiting the firing rate 

in the boiler. Alarms also occur for high catalyst temperatures or high SCR reactor differential pressure. 

The logic diagrams provide the values of the alarm setpoints. At a reactor outlet temperature (T9) above 

500°F, the reactor steel structure is close to the normal operating temperature and with reduced thermal 

stresses there are no temperature rise alarms, but other alarms remain active. 

When the temperature of the SCR reactor outlet and the last catalyst layer reaches the minimum 

continuous operating temperature for ammonia injection, the ammonia injection trip valve is permitted to 

open. 
Salt River Project Coronado Generating Station Unit 2 

Selective Catalytic Reduction System

SCR Operating and Maintenance Manual September 04, 2013 

SCR Reactor Functional Description Page 8 of 10 Revision 04 

3.2 Normal Operation 

Figure 3 shows the SCR temperature as a function of load. The reactor operates in a temperature range

between the catalyst minimum continuous operating temperature and the maximum continuous operating 

temperature. A low temperature alarm is generated for the operator at an SCR inlet temperature of Tmincot

+ 15°F. A low-low temperature alarm occurs at Tmincot. If the SCR inlet temperature falls below Tmincot - 

15°F, the ammonia injection trip valves close. The ammonia injection trip valve will not close if the SCR

temperature exceeds Tmaxcot, however catalyst deactivation can occur. If Tmaxcot is exceeded, it is 

necessary for the operator to limit firing rate on the boiler to bring the temperature back into the operating 

range. The ammonia injection trip valve also closes when the SCR reactor outlet temperature or the 

catalyst temperature falls below Tmincot - 15°F. A high alarm occurs at Tmaxcot - 25°F and at Tmaxcot for any 

SCR reactor temperature element. Alarms are also generated for high-pressure drop across the SCR 

reactor and high outlet NOx concentration. The logic diagrams provide the temperature setpoints and 

alarms to aid the operator. 

FIGURE 2: SCR Reactor Start-up Temperature Trend 
Salt River Project Coronado Generating Station Unit 2 

Selective Catalytic Reduction System

SCR Operating and Maintenance Manual September 04, 2013 

SCR Reactor Functional Description Page 9 of 10 Revision 04 

3.3 Normal Shutdown 

A normal shutdown occurs when the temperature of the SCR reactor inlet, SCR reactor outlet or lowest 

filled catalyst layer drops below Tmincot - 15°F. The SCR reactor cool down rate is not monitored or 

controlled, as the insulation and heat capacity of the reactor and structure will not allow the SCR reactor to 

exceed a maximum cool down rate. A normal shutdown results in the following action: 

• The ammonia injection trip valves (2-SN-V6496NA / SA, NB / SB) for both reactors close 

• The ammonia injection flow control valves (2-SN-V6506NA / NB, SA / SB) for both reactors close 

and transfer to manual mode 

The operating ammonia pump isolation valve remains open, the ammonia pump remains on, and the 

ammonia tank supply remains selected. 

The operator shuts down the dilution air system approximately 30 minutes after ammonia injection has 

ceased in order to purge the ammonia injection piping. The presence of residual ammonia can be 

determined based on inlet and outlet NOx meter values. Once the boiler is shut down standard NPFA 

purge requirements clear the SCR reactor and ductwork of any potential explosive gases. (See the SCR 

System Operating Procedure and the Catalyst Manual). 

FIGURE 3: SCR Reactor Temperature versus Load 
Salt River Project Coronado Generating Station Unit 2 

Selective Catalytic Reduction System

SCR Operating and Maintenance Manual September 04, 2013 

SCR Reactor Functional Description Page 10 of 10 Revision 04 

3.4 Emergency Shutdown (SCR Reactor Trip) 

An emergency shutdown occurs in response to a Master Fuel Trip (MFT). The MFT signal from the BMS 

will result in the following actions in the SCR control system logic: 

• The ammonia double block valves (2-SN-V6501NA / NB, SA / SB, 2-SN-V6504NA / NB, SA / SB) 

close and vent valves (2-SN-V6503NA / NB, SA / SB) open 

• The ammonia injection trip valves (2-SN-V6496NA / SA, NB / SB) for both reactors close 

• The ammonia injection flow control valves (2-SN-V6506NA / NB, SA / SB) for both reactors close 

after delay (Delay is required to bleed ammonia into reactor) 

• Sonic horns and soot blowers shutdown. Catalyst cleaning not required when there is no flue gas 

flowing 

The dilution air system remains in service in order to purge the ammonia injection piping of residual 

ammonia. A time period of 30 minutes is recommended to ensure the residual ammonia has been 

vaporized. 

In addition hard-wired MFT relay contacts will close the double block valves and open the vent valve on all 

of the ammonia injection headers preventing injection of ammonia while the unit is shutdown. 

3.5 SCR Reactor Purge 

After a Master Fuel Trip or normal shutdown, a unit purge should be completed according to NFPA 85 and

plant procedures.