Chapter 2 Selective Catalytic Reduction

1 Chapter 2 Selective Catalytic Reduction John L. Sorrels Air Economics Group Health and Environmental Impacts Division Office of Air Quality Planning and Standards Environmental Protection Agency Research Triangle Park, NC 27711 David D. Randall, Karen S. Schaffner, Carrie Richardson Fry RTI International Research Triangle Park, NC 27709 June 2019 DISCLAIMER This document includes references to specific companies, trade names and commercial products. Mention of these companies and their products in this document is not intended to constitute an endorsement or recommendation by the Environmental Protection Agency.

2 CONTENTS 2. Selective Catalytic Reduction .. 2-1 Introduction .. 2-1 Process Description .. 2-9 Reduction Chemistry, Reagents, and Catalyst .. 2-10 SCR Performance Parameters .. 2-15 SCR System Configurations .. 2-28 SCR System Primary Equipment .. 2-34 SCR System Auxiliary Equipment .. 2-41 Other Considerations .. 2-44 Design Parameters .. 2-49 Boiler Heat 2-50 Heat Rate Factor .. 2-51 System Capacity Factor .. 2-51 Inlet NOx and Stack NOx .. 2-52 NOx Removal Efficiency.

3 2-53 NOx Removal Rates .. 2-53 Stoichiometric Ratio Factor .. 2-54 Flue Gas Flow Rate .. 2-54 Space Velocity and Area Velocity .. 2-55 Theoretical NOx Removal Efficiency .. 2-56 Catalyst 2-56 SCR Reactor Dimensions .. 2-57 Estimating Reagent Consumption and Tank Size .. 2-60 Cost Analysis .. 2-61 Total Capital Investment .. 2-62 Total Annual Costs .. 2-70 Example Problem #1 Utility Boiler .. 2-77 Design Parameter Example #1 .. 2-79 Cost Estimation Example .. 2-82 Example Problem #2 Industrial Boiler.

4 2-85 Design Parameter Example #2 .. 2-86 Cost Estimation Example #2 .. 2-89 References .. 2-93 List of Figures Figure : SCR Process Flow Diagram .. 2-10 Figure : NOx Removal versus Temperature .. 2-16 Figure : Change in Catalyst Volume vs. Temperature .. 2-17 Figure : Typical Catalyst Deactivation per Equation with KO = ; = 55,000 .. 2-21 Figure : Pitch for a Honeycomb Catalyst Configuration .. 2-23 Figure : Typical Catalyst Management Plan .. 2-27 Figure : High-Dust SCR Arrangement.

5 2-29 Figure : Low-Dust SCR Arrangement .. 2-30 Figure : Tail-end SCR Arrangement .. 2-32 Figure : SCR Arrangement for a Combined- cycle Gas Turbine .. 2-33 Figure 2-11: Urea-Derived Ammonia Production System Using U2A system .. 2-38 Figure : Urea-Derived Ammonia Production System Using NOx ULTRA System .. 2-39 List of Tables Table : Summary of SCR Cost Data for Utility 2-4 Table : Summary of SCR Cost Data for Miscellaneous Industrial Sources .. 2-5 Table : Ammonia Reagent Properties.

6 2-12 Table : Major Equipment List for an SCR Application .. 2-35 Table : Comparison of Ammonia Delivery Systems .. 2-36 Table : Higher Heating Values for Various Coals .. 2-50 Table : Estimated Flue Gas Volumetric Flow Rate Factors for Various Coals .. 2-55 Table Atmospheric Pressure at Different Elevations.. 2-63 Table : Comparison of Power Consumption for High-Dust and Low-Dust SCR .. 2-73 2. Selective Catalytic Reduction Introduction Selective Catalytic Reduction (SCR) has been applied to stationary source fossil fuel-fired combustion units for emission control since the early 1970s and is currently being used in Japan, Europe, the United States, and other countries.

7 In the alone, more than 1,000 SCR systems have been installed on a wide variety of sources in many different industries, including utility and industrial boilers, process heaters, gas turbines, internal combustion engines, chemical plants, and steel mills [1]. Other sources include fluid Catalytic cracking units (FCCUs), ethylene cracker furnaces, nitric acid plants, catalyst manufacturing processes, nitrogen fixation processes, and solid/liquid or gas waste incinerators [2, 3]. In the , SCR has been installed on more than 300 coal-fired power plants ranging in size from less than 100 megawatt equivalent (MWe) to 1,400 MWe [1, 4].

8 Other combustion sources with large numbers of SCR retrofits include more than 50 gas-fired utility boilers ranging in size from 147 MWe to 750 MWe, more than 50 industrial boilers and process heaters (both field-erected and packaged units), and more than 650 combined cycle gas turbines [1]. SCR can be applied as a stand-alone nitrogen oxides (NOx) control or with other technologies, including Selective non- Catalytic Reduction (SNCR)1 and combustion controls such as low NOx burner (LNB) and flue gas recirculation (FGR) [2].

9 SCR is typically implemented on stationary source combustion units requiring a higher level of NOx Reduction than achievable by Selective non- Catalytic Reduction (SNCR) or combustion controls. Theoretically, SCR systems can be designed for NOx removal efficiencies up close to 100 percent. In practice, commercial coal-, oil-, and natural gas fired SCR systems are often designed to meet control targets of over 90 percent. However, the Reduction may be less than 90 percent when SCR follows other NOx controls such as LNB or FGR that achieve relatively low emissions on their own.

10 The outlet concentration from SCR on a utility boiler is rarely less than lb/million British thermal units (MMBtu) [1].2 In comparison, SNCR units typically achieve approximately 25 to 75 percent Reduction efficiencies [5]. Either ammonia or urea may be used as the NOx Reduction reagent in SCR systems. Urea is generally converted to ammonia before injection. Results of a survey of electric utilities that operate SCR systems indicated that about 80 percent use ammonia (anhydrous and aqueous), and the remainder use urea [4].