| Selective catalytic reduction vector-1


Velocity Magnitude 29991.69


27108.27 24224.85 21341.43 18458.01 15574.59 12691.17 9807.75 6924.33 4040.91 1157.49


Figure 1. Example of CFD modelling: LM6000 gas turbine exhaust CFD model output showing velocity vectors coloured according to velocity magnitude (ft/min) at turbine outlet location. Source: EnergyLink (2023).


The catalyst is composed of a base metal (usually a compound of titanium dioxide (TiO2


vanadium pentoxide (V2


) and tungsten trioxide (WO3 O5


)), with ) as the active


catalyst material. Catalyst technology and performance are continually improving, with various manufacturers developing and offering different catalyst types. Selecting the right catalyst for the application is crucial. Emissions reduction performance depends on how effectively ammonia is distributed to react with NOx


, as well as on catalyst


volume and type. Exhaust flow, which varies by turbine model, is analysed using CFD (computational fluid dynamics) modelling (Figure 1) and physical flow modelling. Models closely match actual conditions and help guide the design of the ammonia distribution system. Achieving a smooth, evenly distributed ammonia flow is crucial for optimal performance, and analysing swirl velocities and flow patterns across different turbine exhaust streams informs the design. Also important is sealing the SCR catalyst inside the turbine exhaust duct to minimise


Theoretical chemistry Actual NOX


removal


NH3 slip @ 9ppm NOX NH3 slip @ 15ppm NOX NH3 slip @ 25ppm NOX NH3 slip @ 35ppm NOX NH3 slip @ 40ppm NOX


Ammonia to NOX ration (v/v)


Figure 2. Graph of ammonia slip values (ppm) for various turbine exit NOx while achieving 90% NOx


removal with the EnergyLink system.


Source: Jeff Wirt, EnergyLink International, and Dan Ott, Environex, Inc, SCR and CO catalyst systems [conference presentation], ProEnergy Services 2024 customer conference, Houston, Texas, USA, 19 November 2024.


90% NOX removal is achieved across a range of inlet NOX


For example, starting from 25 ppm at the turbine exhaust outlet with a target of 90% NOX approximately 4.0 ppm ammonia slip is achieved.


levels, resulting in varying ammonia slip values. removal,


levels


mandated emission limits but are still partially removed by the SCR/CO catalyst. The ideal temperature range for catalyst bed performance is between 427 and 482°C (800 and 900°F). Smaller industrial turbines typically produce exhaust gases within this range, while larger central station utility turbines and aero-derivative turbines can exceed 538°C (1000°F), reaching up to 675°C (1247°F). When the exhaust gas temperature exceeds 468°C (875°F), it must be cooled with tempering air to maintain a temperature suitable for acceptable catalyst performance. In gas turbine SCR systems to date – see Figure 4 – pressurised ambient air from dilution air blowers, heated with electric heaters or, optionally, recirculated turbine exhaust gas, is used to vaporise aqueous ammonia via an ammonia vaporisation skid (AVS). Gaseous ammonia is then injected through an ammonia injection grid (AIG) after the exhaust gases have been cooled to the required temperature by tempering air fans. Typically, the CO catalyst is placed upstream of the AIG and SCR catalyst.


www.modernpowersystems.com | May/June 2026 | 19


the amount of unreacted ammonia, aka “ammonia slip” as mentioned earlier. Achieving 90% or more NOx


Notes on terminology ● Ammonia slip: An SCR uses ammonia ● NOx


as a reducing agent to control NOx emissions from gas turbines. The part of the unreacted ammonia that passes through the catalyst and exits the exhaust stack is called “ammonia slip.” ,


: Collective term for NO and NO2


which are significant components of harmful air pollution, ground-level ozone formation, and acid rain.


● CO: Colorless, odourless gas that forms from the incomplete burning of carbon-based fuels like natural gas.


removal and


keeping ammonia slip at 9.41 mg/Nm³ (5 ppm or less) throughout the catalyst’s lifespan (typically 5 years or 1000 hours) is possible thanks to the design of EnergyLink’s system. Figure 2 shows a graph of ammonia slip


values (ppm) for various turbine exit NOx levels while achieving 90% NOx


EnergyLink technology.


The same design principles apply to removing CO, a toxic gas produced by turbine combustion. And maintaining the CO catalyst’s cleanliness is just as important. Poor performance or deactivation of the CO catalyst often results from fouling, usually caused by zinc and phosphorus from lube oil or sulphur in the fuel. Figure 3 shows a plot of CO catalyst performance. In addition to NOx


and CO, trace amounts


of volatile organic compounds (VOCs) and particulates (PM2.5, PM10) are found in the exhaust stream. These include hydrocarbons such as ethene, propene, propane, and butane, which are usually well below the


removal with


● VOCs: Volatile organic compounds (VOCs) emissions include total hydrocarbon (THC) emissions or unburned hydrocarbons (UHC), excluding methane and ethane. Organic hazardous air pollutants (HAPs) are mostly classified with VOCs. HAP emissions from gas turbines are very low. Formaldehyde is the main HAP.


● PM: The term particulate matter refers to a mixture of solid particles and liquid droplets in the air. PM10 and PM2.5 (with diameters of 10.0 and 2.5 micrometers or smaller, respectively) have legislated limits. : This indicates parts


● ppmvd @ 15% O2


per million by volume, measured as dry and adjusted to 15% oxygen or 10 g/kWh. Regulations for specific exhaust gas levels are issued as if the gases were diluted with enough air to reach a 15% O2


level.


● % root-mean-square (RMS): This metric quantifies variability in the inlet ammonia-to-NOx


distribution,


with a lower %RMS indicating a more uniform distribution.


NOX


conversion (%)


Ammonia slip (ppm)


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