For gas turbine operators, NOx emissions remain an environmental concern, even in a relaxed regulatory environment. They pose compliance risks, raise operating costs, and threaten access in markets with strict air quality rules.
Simple or combined-cycle gas turbines with conventional diffusion combustors typically emit NOx in the 25-45 ppm range at 15% O₂ with diluent injection to meet permit levels. Some sites use SCR to meet stricter limits, often as low as 2 ppm.
The solution isn’t replacing turbines—retrofit combustion systems with proven low-emissions technology that delivers single-digit NOx and maintains fuel flexibility for LNG and hydrogen blends.
What Drives NOx Formation in Conventional Combustors
Understanding how to reduce NOx starts with understanding how it forms.
In conventional diffusion combustors, fuel is injected and mixes directly with air, igniting to form a diffusion flame. This creates localized hot spots above 3,400°F (1,871°C), driving chemical reactions between nitrogen and oxygen in the combustion air that generate NOx.
The primary mechanism driving this process is thermal NOx, also called Zeldovich NOx after the Russian physicist who first described it. The reactions occur as follows:
N₂ + O → NO + N
N + O₂ → NO + O
N + OH → NO + H
These reactions are highly temperature-dependent. Below approximately 2,800°F (1,538°C), NOx formation is minimal. Above that threshold, formation rates increase exponentially with temperature. Research shows that reducing peak flame temperature from 2,912°F (1,600°C) to 2,732°F (1,500°C), a drop of just 180°F, can reduce NOx formation by more than 50%.
The challenge with conventional diffusion combustors is that they operate well above this threshold. Primary zone temperatures routinely hit 3,800-4,000°F (2,093-2,204°C) to ensure complete combustion and fuel flexibility. This design prioritizes efficiency and multi-fuel capability but produces NOx as an unavoidable byproduct.
Compounding the problem, NOx formation is also dependent on pressure and residence time. The longer combustion gases remain at high temperature, the more NOx is formed. Conventional combustors with relatively long primary zones give nitrogen and oxygen plenty of time to react.
The result: legacy diffusion combustors produce NOx emissions that exceed modern regulatory limits by an order of magnitude.
Early NOx Control: Water and Steam Injection
The first widespread approach to reducing NOx in gas turbines was wet low-emissions (WLE) technology, which injects water or steam into the combustion chamber to lower flame temperature.
Water injection works through two mechanisms. First, the latent heat of vaporization absorbs thermal energy from the flame, directly reducing peak temperatures. Second, the steam produced dilutes the combustion gases, lowering oxygen concentration and further suppressing NOx formation.
To achieve meaningful NOx reductions, typical water-to-fuel ratios range from 0.5:1 to 1:1 by mass. At a 1:1 ratio, water injection can reduce NOx by 40-60% compared to uncontrolled diffusion combustion.
But WLE systems come with significant operational penalties:
Steam injection offers similar benefits but requires an external steam source, typically available only in combined-cycle or cogeneration plants. And steam is less effective than water because it lacks the latent heat absorption that makes water so effective at cooling flames.
Given the operational limitations of WLE systems, by the late 1990s, stricter emissions targets and ongoing challenges drove the industry toward dry low-NOx (DLN) combustion technology, which eliminates the need for water or steam.
How Low Emissions Combustors (LEC) Work
With LEC (aka DLN) technology, combustion systems achieve low emissions through lean-premixed combustion [TB1]. Instead of injecting fuel directly into the combustion chamber as a diffusion flame, LEC systems mix fuel and air upstream of the flame zone, creating a homogeneous lean mixture before ignition.
Premixing: The Foundation of Low-NOx Combustion
In a LEC combustor, fuel is injected into the airstream at the fuel nozzle or swirler assembly, where it mixes thoroughly with 50-60% of the total combustion air before entering the primary combustion zone. This premixing ensures that the fuel-air mixture is lean (excess air) and uniform, eliminating the fuel-rich hot spots that drive NOx formation in diffusion flames.
Lean combustion operates at an equivalence ratio (Φ) significantly below 1.0 (stoichiometric). Typical LEC systems target Φ = 0.5-0.7, meaning they use 40-100% more air than required for complete combustion. The excess air acts as a thermal diluent, reducing peak flame temperature to 2,600-2,900°F (1,427-1,593°C), well below the threshold for significant NOx formation.
The result: NOx emissions drop from 75-150 ppm in diffusion combustors to 9-25 ppm in first-generation LEC systems, and as low as 2-5 ppm in advanced designs.
Fuel Staging: Managing Combustion Across Load Range
Lean premixed combustion is highly effective at full load, but it creates stability problems at low loads. As turbine output decreases, airflow remains relatively constant (especially in single-shaft machines), driving the fuel-air mixture even leaner. Below a certain threshold, the mixture becomes too lean to sustain combustion, causing flameout.
To maintain stable operation across the load range, LEC combustors use fuel staging. Multiple fuel circuits allow operators to control which nozzles receive fuel at different load points, effectively managing the fuel-air ratio to keep combustion within the stable lean range.
A typical fuel-staged LEC combustor operates in several modes:
Each transition between modes must be carefully managed to avoid combustion instabilities, dynamic issues, or flameout.
Combustion Dynamics: The Trade-Off for Low NOx
Lean premixed combustion is inherently less stable than diffusion combustion. Small variations in fuel flow, air distribution, or flame position can trigger pressure oscillations called combustion dynamics. These oscillations can damage hardware, shorten component life, and force operators to detune the combustion system (thereby increasing NOx) to restore stability.
Managing dynamics is a key challenge in LEC combustor design. Advanced systems use:
Modern LEC combustor retrofits incorporate decades of field experience to minimize dynamics risk while maintaining low emissions.
Low-Emissions Combustor Retrofits for Simple-Cycle Plants
For operators running simple-cycle gas turbines, retrofitting to low-emissions combustion provides clear advantages: it enables immediate compliance with tightening emissions standards and offers operational flexibility, helping plants remain competitive in markets with strict air quality regulations. Take action to assess your retrofit options now and secure your plant’s future.
Hanwha Power’s FlameSheet Technology
Hanwha Power’s FlameSheet™ combustion system is a proven DLN retrofit solution for GE Frame 7EA, 7E, and 6B, as well as Frame 5 turbines. FlameSheet replaces conventional diffusion combustors with advanced lean premixed hardware designed for:
The retrofit involves replacing combustion liners, transition pieces, fuel nozzles, and associated hardware. Depending on the turbine model and site-specific requirements, installation typically occurs during a standard hot-gas-path (HGP) inspection outage, minimizing downtime.
LNG and Hydrogen Compatibility
One of the critical advantages of modern LEC combustor retrofits is the flexibility they offer in fuel selection. As the power generation industry transitions toward cleaner fuels, operators need combustion systems that can handle:
Liquefied Natural Gas (LNG): LNG has a higher heating value and slightly different combustion characteristics than pipeline natural gas. DLE combustors designed for multi-fuel operations accommodate LNG without hardware changes, maintaining low NOx across the fuel spectrum.
Hydrogen Blending: Hydrogen burns hotter and faster than natural gas, creating challenges for combustion stability and NOx control. Advanced DLE systems like FlameSheet are engineered to handle hydrogen blends up to 30% by volume with minimal NOx penalty. This capability positions operators to meet future decarbonization mandates without replacing combustion hardware.
Refinery Off-Gas and Byproduct Fuels: Industrial applications often involve variable-composition fuels with fluctuating heating values. Fuel-flexible DLN combustors maintain stable operation and low emissions across a wide range of fuel compositions, reducing the need for fuel conditioning or blending.
Operational Benefits Beyond Emissions Compliance
Low-emissions combustor retrofits deliver value beyond regulatory compliance:
Avoided SCR Costs: Selective catalytic reduction (SCR) systems cost $2-5 million for a typical simple-cycle turbine and require ongoing catalyst replacement, ammonia reagent, and maintenance. Achieving sub-10 ppm NOx with combustor technology alone eliminates the need for SCR in many jurisdictions.
Improved Dispatch Competitiveness: In markets with emissions-based dispatch penalties or environmental credits, lower NOx improves unit economics. Plants with single-digit emissions can bid more competitively than units burning through emissions allowances.
Extended Operating Permits: Many existing plants operate under grandfathered permits with NOx limits that would not be granted today. Retrofitting to low-emissions combustion can secure permit renewals, extend plant life, and unlock capacity uprates that would otherwise trigger stricter New Source Review (NSR) standards.
Fuel Switching Readiness: As hydrogen infrastructure develops and carbon pricing evolves, fuel-flexible combustors provide optionality. Operators can transition to hydrogen blends without replacing combustion systems.
Implementation Considerations
Hanwha Power’s engineering team evaluates site-specific conditions to determine feasibility, performance impacts, and integration requirements before recommending retrofits.
Outage Planning and Installation
LEC combustor retrofits are typically executed during planned HGP inspections to minimize incremental downtime. Scope includes:
For a typical Frame 7EA simple-cycle unit, retrofit installation adds 3-5 days to a standard HGP outage. The extended downtime is offset by immediate compliance with emissions regulations and avoided future outages resulting from SCR installation.
Performance Validation and Tuning
Achieving guaranteed emissions requires post-installation tuning. Variables include:
Hanwha Power’s field service team conducts full-load and part-load emissions testing, dynamic sweeps, and operational tuning to deliver on the contracted performance.
The Bottom Line
Conventional diffusion combustors were designed for an era when NOx emissions weren’t regulated, and fuel flexibility mattered more than air quality. That era is over.
Today’s operators need combustion systems that deliver single-digit NOx levels, accommodate hydrogen blends, and provide operational flexibility to respond to changing dispatch requirements, fuel availability, and market conditions.
Low-emissions combustor retrofits provide a proven, cost-effective path to compliance without replacing turbines or installing expensive post-combustion treatment systems. For simple-cycle plants facing tightening emissions standards, the question isn’t whether to retrofit, but when. It’s when, and with which technology partner.
Evaluating low-emissions combustor upgrades for your fleet? Hanwha Power’s FlameSheet combustion system delivers field-proven NOx reductions for Frame 7EA, 9E, Frame 6B, and Frame 5 turbines. Our engineering team can assess your units for retrofit feasibility, performance impacts, and regulatory compliance strategy.
Contact us to discuss your emissions challenges and fuel flexibility requirements.