Because the flame length is shortened, the time for NOx formation is greatly reduced.
by Roberto Ruiz
September 20, 2016

Faced with strict environmental regulations, industries reliant on combustion systems—including enhanced oil recovery (EOR)—are turning to emissions control technologies for support. A number of technologies have been developed over the years to reduce mono-nitrogen oxide (NOx) emissions from industrial sources, with varying degrees of success. Here, we will examine the abilities and limitations of the industry’s most commonly deployed options, as well as emerging technologies showing considerable promise at the industrial scale.


First developed in the 1960s, selective catalytic reduction (SCR) systems involve the injection of a liquid reagent—usually ammonia or urea—through a catalyst reactor upstream the smokestack. This activates a chemical reaction that breaks down NOx compounds in the flue gas into simple chemical elements:

  • nitrogen
  • water
  • carbon dioxide

Flue gas recirculation (FGR) units redirect some of the flue gases produced during combustion to the burner, mixing these gases with the fresh air used in the combustion process. The addition of inert gases to the air lowers peak flame temperatures, reducing the formation of NOx. However, the recirculation of flue gas affects thermal efficiency, increasing fuel consumption and requiring additional electrical energy for increased fan power.

The lowest NOx burners currently recirculate up to 40 percent of the flue gas. That is a lot of flue gas to recirculate, and it requires significant fan power to do so. The extra fan power results in the consumption of additional generated energy with attendant and additional environmental burdens. For this reason, some air quality districts have contemplated enforcing an upper limit on the amount of flue gases that may be recirculated.

While SCR and FGR have succeeded in lowering air pollution emissions in the past, traditional NOx reduction strategies such as these will fail to meet increasingly stringent air quality standards as they plummet to 5 parts per million (ppm) NOx. In addition, these technologies involve the use of toxic chemicals, which can be harmful to humans and pose additional threats to the environment.

Furthermore, SCR and FGR require the installation of more complex burners and systems, creating higher capital expenditure (CAPEX) and frequent operational issues. These issues include the potential release of toxic chemicals into the environment and the generation of excess particulate matter.

Correcting these operational complications may lead to substantial plant downtime, resulting in significant loss in operator revenue. As global oil prices remain at low levels, operators are focused on one thing: cutting production costs.

Now more than ever, the industry sees the need for emissions control technologies that can effectively mitigate pollutants without sacrificing operators’ bottom line.

The inability of SCR and FGR systems to help operators of combustion equipment economically meet modern environmental mandates has led technology providers to approach the issue of NOx gas formation from a different angle.

Instead of addressing the problem with the use of post-combustion treatment, engineers are tackling the issue from its source: the flame. Recently, a new generation of industrial burners has entered the market, operating at the flame level to eliminate pollutants directly.


Low- and ultra-low NOx burners (LNBs and ULNBs) typically use some variant of staged combustion with the purpose of lowering average temperatures to inhibit NOx formation.

Although cheaper and easier to operate than traditional SGR and FGR systems, LNBs and ULNBs can still add to equipment operating costs. Additionally, these systems often create unstable and elongated flames, which can lead to severe flame impingement in process tubes.

Unstable flames are also prone to blowout conditions at low firing rates, so turndown ranges can be compromised.

Advanced burner architectureFigure 1. Advanced burner architecture (Graphics courtesy of ClearSign)

Advanced Burner Architectures

One advanced burner architecture provides a radically different approach to combustion, resulting in improved NOx emissions reduction. By installing a ducted ceramic tile at a set distance from the fuel nozzles, this technology transforms a single large flame into thousands of smaller flames to optimize heat transfer and thermal efficiency (see Figure 1).

Because the flame length is shortened, the time for NOx formation is greatly reduced.

The ceramic surface has a much higher emissivity than the flame itself, resulting in better thermal efficiency in the radiant section and lower combustion temperatures—another contributing factor to reducing NOx output.

The result is a record-breaking reduction to NOx emissions—less than 5 ppm corrected to 3 percent oxygen without the use of FGR or post-combustion flue gas treatment strategies. (see Figure 2).

Advanced burner architecture NOx emissions performanceFigure 2. Advanced burner architecture NOx emissions performance

Unlike LNB and ULNB options, advanced burner architectures do not suffer from flame impingement issues. Reduction in flame impingement issues significantly lowers the potential for tube failure and enhances tube lifetimes. Therefore, advanced burner architecture has the potential to reduce the need for maintenance outages and the frequency of decoking.

The reduction in unscheduled maintenance also allows greater operational savings. Advanced flame architecture improves process throughput and system efficiency while increasing operators’ profits.

Case Study

In 2014, one of the largest independent oil producers in the U.S. sought to cost-effectively mitigate emissions from a 62.5 mega British thermal unit per hour (MMBtu/hr) rated once-through stream generator (OTSG).

Since the installation of the advanced burner architecture, the company has effectively lowered its NOx output to below 5 ppm, surpassing some of the most stringent regulations in California.

The producer also documented improvements in thermal efficiency. In tests conducted at the normal firing rate, the use of distal flame architecture increased thermal efficiency by 1 percent when compared to a baseline case with a low-NOx burner operating with much higher NOx emissions without FGR.

Today, combustion equipment operators have the opportunity to choose from a variety of technologies to abate NOx emissions. As time has shown, the limited capabilities and high costs of traditional approaches will not adequately serve the market moving forward.

By reducing pollutants directly at the source, advanced burner architecture technologies can greatly reduce NOx emissions at a low cost of system ownership.

The industry now has an economically beneficial solution to meet the environmental requirements of today and tomorrow.