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Is Your SCR/CO System Ready for Turndown? How Increased NO2/NOx Ratios Require Additional SCR Performance

Tuesday, December 1, 2020 @ 3:00pm EST/12:00pm PST

Presentation Synopsis

When most of us hear the term NOx, we think of it as a single compound. However, NOx actually comprises a group of chemicals called nitrogen oxides, which mainly consists of three different compounds: NO, NO2, and N2O. In gas turbine exhaust, NO is typically the primary component of NOx, with the balance being NO2. Why is this distinction important? Because NO2 requires significantly more SCR catalyst activity than NO to achieve the same level of NOx reduction and recent changes in operating requirements for gas turbine power plants can result in higher NO2/NOx levels in the exhaust.

With combined cycle plants being required to operate at both maximum load and their lowest achievable turndown (and everywhere in between), the performance requirements for SCR and CO catalyst systems are becoming increasingly challenging and varied. In certain cases, the performance requirements at low load are more demanding than the maximum load cases that have historically been used as the basis for catalyst system design.

Lower catalyst operating temperatures and higher NO2/NOx ratios in the GT exhaust will limit performance of the SCR/CO system. Catalyst activity is inherently lower at the lower operating temperatures associated with low load operation. For systems with oxidation catalysts for CO and VOC destruction, the NO2/NOx ratio issue is exacerbated by NO to NO2 oxidation that occurs over the oxidation catalyst (upstream of the SCR). Historically, SCR catalysts for CCGT applications have been designed based on the assumption that less than 50% of NOx enters the SCR catalyst as NO2. However, recent field data shows that the NO2 ratio is higher than 50% in some cases. For systems considering operating at lower loads, these factors must be considered when designing and maintaining SCR and CO catalysts to ensure that the systems will be operable across the full range of operating loads.

This presentation will include field and lab data for NO to NO2 oxidation across the oxidation catalyst and show how increased NO2/NOx ratios at the SCR inlet impact catalyst performance and required activity levels. Plant personnel will learn the importance of knowing the maximum NO2/NOx ratio at the SCR inlet when specifying designs for new and replacement SCR catalysts.

Key Takeaways for Users

  • Why the ratio of NO2/NOx is important
  • How turndown operation impacts the operating conditions at the SCR and CO catalysts
  • How turndown operation affects catalyst performance
  • How high NO2/NOx limits SCR performance and expected catalyst life
  • How to identify the limiting operating case for your catalyst system
  • What actions can be taken to mitigate the impact of high NO2/NOx on stack emissions performance


Andy Toback, Project Manager

007Andrew “Andy” Toback, Project Manager at Environex, Inc. has over 15 years of experience in the field of air pollution control from combustion sources. Andy specializes in maintenance and testing for combustion turbines, boilers, and IC engines.

Andy is an experienced field engineer who routinely provides field service for sampling, inspection, tuning, and troubleshooting of CO and SCR systems in the power industry. Andy is also one of the instructors for the annual NOx and CO short course Environex provides. Andy holds a Bachelor’s and Master’s Degree in Chemical Engineering from Rowan University.


Joe Otto
Marketing Director
570-470-5776  office 484-320-8608

Dan Ott
570-470-5775  office 484-320-8608

Andy Toback
Project Manager
610-209-6136 office 484-320-8608

About Environex, Inc.

Established in 1991, Environex Inc. offers engineering and consulting expertise in air emissions control systems for the power generation industry. We are experts in Post Combustion NOx and CO control technologies. Our primary focus is Selective Catalytic Reduction (SCR) and oxidation catalyst testing and system maintenance, catalyst management and selection, troubleshooting, and design. We operate an independent US-based test laboratory specializing in SCR and CO catalyst testing. Environex provides objective third party testing, engineering and consulting related to emissions control products and systems.

With over 25 years of experience in the power industry, we have an extensive database of catalyst test results and consulting projects to draw from. We have worked with catalyst systems on the full range of prime movers, from small engines to aero-derivative and large frame gas turbines, as well as industrial boilers of all sizes. Our experience includes systems used on all fuel types, including natural gas, fuel oil, biomass, coal, and landfill gas. The Environex database includes catalyst test results and projects on systems from all catalyst manufacturers, system/HRSG suppliers, and gas turbine and boiler makers.
If you are having a catalyst or emissions control problem, chances are we have seen it before.

Products and Services

Full NOx and CO System Evaluation – includes a combination of catalyst core sample testing, operating data analysis, and system inspection. This scope enables us to distinguish system problems from catalyst problems, account for stratification in catalyst performance across the duct, and evaluate catalyst and system performance under all plant operating conditions. This scope allows us to definitively identify the limiting operating case for your system.

SCR Catalyst Testing and Life Projections – includes performance testing, surface area analysis, and chemical analysis. Multiple test points included to span the range of plant operating conditions.
Replacement SCR elements are available. Life projections are based on plant operating/design conditions.

CO Catalyst Testing and Life Projections – includes performance testing, surface area analysis, and chemical analysis. Multiple test points included to span the range of plant operating conditions. Replacement CO buttons are available. Life projections are based on plant operating/design conditions.

Dual Function Catalyst Testing and Life Projections – includes NOx and CO conversion performance testing, surface area analysis, and chemical analysis. Multiple test points included to span the range of plant operation. Life projections are based on plant operating/design conditions.

Ammonia Injection Grid (AIG) Tuning – optimizes NH3-to-NOx distribution at the SCR catalyst inlet. This extends SCR catalyst life and reduces ammonia consumption, stack ammonia slip, and ammonia salt formation on downstream boiler tubes.

CO Catalyst Rejuvenation Studies – determine the feasibility of using different wash solutions to improve performance and extend CO catalyst life.

Catalyst Replacement Services – includes developing the replacement catalyst bid specification, identifying qualified suppliers, and evaluating vendor bids. This service identifies opportunities to improve catalyst design and leverages the best designs, guarantee terms, and lead times for replacement catalysts. Turnkey replacement services are available.

Operator SCR System Training – Environex offers an annual training course on NOx and CO/VOC control systems and technologies.

SCR/NOx and CO System Consulting and Troubleshooting – low turndown operation, GT upgrade impact, start-up emissions, failure analysis, etc.

SCR/CO System Design Reviews

Fleet Catalyst Management Plans

White papers, spec sheets, brochures


Environex Inc. – Emissions Control Catalyst and Lab Services


1 Great Valley Parkway
Suite 4
Malvern, Pennsylvania 19355
United States

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