Cleaver-Brooks - Complete Boiler Room Solutions

Reducing NOx Emissions in Large Industrial Boilers

Tip Sheet: November 2015

Key Facts

  • There are three types of NOx emissions: Fuel Bound, Prompt and Thermal
  • Flue gas recirculation is the most efficient method for reducing NOx from 120 ppm to 20 – 30 ppm
  • Selective catalytic reduction or selective non-catalytic reduction can be used to reduce NOx below 9 ppm

There are many types of boilers and several ways to reduce NOx both during and post combustion. For big industrial boilers >100 MMBTU/hr heat input, which include large firetubes and nearly all watertube units, flue gas recirculation (FGR) and selective catalytic reduction (SCR) are the strategies that are primarily used to reduce NOx emissions.

There are three types of NOx emissions: Fuel Bound, Prompt and Thermal. The formation of both Prompt and Thermal NOx is temperature- and time (duration)-dependent. 

Prompt NOx forms at lower temperatures in the early stages of oxidation or combustion, reacting with radicals in the fuel such as carbon and/or methylene. Prompt NOx is a relatively small contributor to overall NOx production in total, but when looking closely at the problem, it still needs to be considered.

Thermal NOx is the type that requires the most attention. It forms at elevated temperatures of approximately 2900° F. At this temperature and normal residence time, natural gas generates about 120 ppm of NOx without control.

This is much higher than the required 9 ppm NOx level in California for units larger than 20MM. In Canada, lawmakers have proposed 13 g/GJ NOx.      

The way the packaged boiler industry has reduced NOx while still maintaining energy efficiency is through combustion and/or engineering accomplishments. For example, low NOx burners can limit NOx production in natural gas from approximately 120 ppm to 80 ppm.

It is important to understand that a low NOx burner is only part of the solution. The furnace is critical too because it allows enough time for the combustion to complete, and it also forms the environment for controlling air delivery and flame shape for proper mixing and NOx reduction. Starting with the diffuser for turbulent mixing of fuel and air, the intent within the confines of the furnace is to cool the flame without quenching it, thereby reducing NOx without forming excessive CO. 

During the design process, the aerodynamics of fuel and air injection are important for proper oxidation, but also for flame shaping, matching the flame to the furnace to mitigate impingement while lowering the flame temperature to minimize NOx formation.

FGR is the most popular and most efficient method for reducing NOx from 120 ppm to 20 – 30 ppm. It works by recirculating a portion of the boiler flue gas into the main combustion chamber. This process reduces the peak combustion temperature and lowers the percentage of oxygen in the combustion air/flue gas mixture, thus retarding the formation of Thermal NOx. 

Achieving sub-20 ppm NOx is possible using induced FGR, but to reach this level, the formation of Prompt NOx must also be addressed. Increasing the burner’s excess air minimizes fuel rich zones that are susceptible to forming Prompt NOx. By using high FGR and excess air rates, NOx reductions down to 9 ppm NOx can be achieved. 

To reduce NOx limits below 9 ppm requires additional assistance. There are a couple post-combustion options, including Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR). In both cases, ammonia is used as the reagent device. It acts on NO and NO2, and through a chemical reaction forms harmless nitrogen gas and water vapor.  

SCR is done using lower-temperature combustion gas and the use of a catalyst for the propagation of the chemical reaction. This technique is common for packaged boilers.  

With SNCR, a catalyst is not involved. Rather the ammonia is injected into the furnace directly in the zone of combustion with a temperature of approximately 1800° F. These applications are highly selective, requiring large furnace volumes and adequate residence time for the chemical reaction to occur. This relegates the technology to be applied to field-erect or large utility boilers.  

To learn more, watch the webinar titled Reducing NOx: Flue Gas Recirculation vs. Selective Catalytic Reduction or visit