Industrial processes need simple, robust and effective equipment to meet environmental regulations.

Since the passing of the Clean Air Act in 1970, downward pressure on the allowable concentrations of NOx emissions from stationary stacks has continued. The last 50 years have produced new research on the health impacts of air pollution, particularly ground-level ozone. The result has been a continuing focus on reducing point source emissions of NOx.

Combustion equipment is a particularly tricky obstacle, especially as the combustion of hydrocarbons guarantees the production of carbon dioxide (CO₂) proportional to the amount of fuel burned. The use of air as an oxidizer leads to nitrogen oxides (NO and NO₂). Unlike carbon dioxide, the production of nitrogen oxides can be reduced through clever engineering.

Nitrogen oxides (NOx) are a family of highly reactive and poisonous gases typically formed when fuel is burned at high temperatures. Sources of NOx include automobiles, trucks and other vehicles, as well as industrial processes. NO₂, in the presence of air in the atmosphere, can react with ultraviolet (UV) light to form tropospheric ozone (one of the primary constituents of smog), acid rain and nitric oxide (NO).

NOx formation can occur through several mechanisms, depending on the source of nitrogen and the system conditions, all requiring different means of mitigation. Thermal NOx is controlled by the nitrogen and oxygen molar concentrations and the temperature of combustion. Fuel NOx is the result of the oxidation of fuels already containing nitrogen atoms. Prompt NOx is formed from molecular nitrogen in the air (diatomic N₂) combining with fuel in fuel-rich conditions, which exist, to some extent, in all nozzle-mix combustion.

Manufacturers of combustion equipment recognize many methods to reduce NOx production, ranging from changing process inputs to adjusting burner operation to employing creative techniques in design. Several pathways will often be utilized at once to optimize low-NOx performance.

Nutrien’s furnace underwent comprehensive shop testing in August 2021 before being fully installed in the field in February 2022.

The Solution

In general terms, there are three main pathways to NOx reduction: lower peak combustion and furnace temperatures, reducing local oxygen levels and minimizing residence time at the highest temperatures. The opposite is often true for low-CO systems, and a trade-off is expected when juggling both concentrations at the same time.

A direct way to reduce the NOx in a system is to manage the inputs of the burner. Using low-nitrogen fuels, such as hydrocarbons and hydrogen, eliminates the prevalence of fuel-bound nitrogen that occurs most commonly in liquid and solid fuels. Fuel-bound nitrogen can also be managed through air staging. A fuel-rich primary flame zone maximizes the amount of fuel-bound nitrogen that is converted into diatomic nitrogen (N₂). Lastly, replacing air with high-purity oxygen can potentially reduce the formation of NOx. It is critical that sources of nitrogen into the furnace are controlled. Nitrogen can be introduced through ambient air infiltrating into the furnace or with the fuel.

Preheating air is a commonly used technique among a plethora of applications. However, higher preheated air temperatures can lead to higher flame temperatures, corresponding with increased NOx generation. Typically, as preheated air is used for energy savings, adjusting the temperature to reduce NOx may lead to decreases in overall process efficiency. Burners can be specially designed to allow for use of preheated air without significant increases in NOx.

Outside of stoichiometric operation, the percent of excess air in the system may be manipulated to minimize NOx emissions. In typical premix systems running lean combustion, additional air is the solution, which reduces peak flame temperatures. In nozzle-mix systems, the operating point must be considered before determining whether a leaner or richer reaction chamber will result in less NOx. The answer will depend on the design of the burner and which NOx reduction strategies have been implemented.

Without compromising efficiency or air-fuel ratios, a series of burner design techniques are available to drive NOx emissions down. Air and fuel staging allow for the above methods to occur in sections of the burner. These methods react only portions of the reactants at any given time before mixing them together in the outlet stream. Alternatively, lean premix technologies mix the air and fuel before sending the flow through a combustion zone, allowing for total control over the air-to-fuel ratio throughout the combustion process. Burners utilizing lean premix methods typically operate at higher excess air percentages, leading to the potential for single-digit NOx ppm levels. Using both fuel staging and lean premix technology in tandem can reduce NOx concentrations under a widespread series of conditions.

Other methods include flameless combustion such as low-NOx injectors (LNI) and diffuse mode combustion (DMC), both of which can achieve extremely low NOx concentrations. These methods require that the furnace be over a threshold “auto-ignition” temperature to be used safely. Applications with surfaces significantly below the auto-ignition temperature are typically excluded from using these approaches due to safety concerns. The use of flue gas recirculation (FGR) can also improve NOx concentrations in the outlet stream, which is utilized by the automotive industry and referred to as exhaust gas recirculation (EGR).

The EcoFornax SLEx can be utilized in equipment from air heaters to furnaces to dryers and in any application that could benefit from a less complicated ultra-low-NOx burner capable of meeting sub-30 ppm NOx emissions requirements (when operated at full capacity).

The Problem with the Solutions and How to Solve that Problem

A burner employing several NOx-reduction techniques at the same time, resulting in ultra-low-NOx emissions, has been developed. Known as the EcoFornax LEx, it requires a complicated control system, which, while efficient, increases the cost to the customer. In many cases, a slight sacrifice in NOx emission performance can eliminate the need for such complexity and result in a simple but effective burner with a high price-to-performance ratio. Enter the EcoFornax SLEx.

The “S” in SLEx stands for “simple,” “LE” for “low emissions” and “x” for “excess air.” The EcoFornax SLEx was designed to maintain the performance of its more complicated counterpart, delivering low-NOx emissions and high efficiencies while eliminating the necessity of the complex control system in favor of a simpler, easy-to-operate low-NOx burner. Both the LEx and the SLEx burners operate using lean premix technology and incorporate fuel staging. However, the SLEx adds a stabilization method.

Originally designed to focus on the regenerative thermal-oxidizer market, the simplified burner has found a home in a series of applications outside of the thermal-oxidizing world. It can be utilized in equipment from air heaters to furnaces to dryers and in any application that could benefit from a less complicated ultra-low-NOx burner capable of meeting sub-30 ppm NOx emissions requirements (when operated at full capacity), with the potential to drop even lower with the addition of excess air.

With two EcoFornax SLEx burners in place, Nutrien was successfully able to run fully metered combustion into their 1472°F (800°C) furnace while maintaining NOx emissions levels below 30 ppmdv (corrected to 3% O₂) processing 1.5 tons of ammonia per minute.

A Case Study: Vaporizing Ammonia

Nutrien, a Canadian fertilizer producer, had been vaporizing ammonia on a 30-year-old system before they decided to make a change. Their load-out heater was necessary to bring liquid ammonia temperature up to meet the necessary regulation for transport (from -40 to 32°F/-40 to 0°C) to avoid damaging the metal in the transport vehicles.

Low-NOx emissions were one of the primary requirements for the burners on the new vaporizer. While other combustion solution manufacturers were unable to meet this specification, the EcoFornax SLEx burner with innovative premix technology alone fit the bill.

With the help of Spartan Controls as system integrator and two EcoFornax SLEx burners in place, Nutrien was successfully able to run fully metered combustion into their 1472°F (800°C) furnace while maintaining NOx emissions levels below 30 ppmdv (corrected to 3% O₂) processing 1.5 tons of ammonia per minute.

A Case Study: Fresh Process Air Heating

Process Combustion has provided ambient air heating to a series of mine shafts utilizing the EcoFornax LEx burner to stay below the required threshold of 0.02 ppm NO2 in the fresh air entering the mine. In these environments, the raw concentration of NO2 is regulated rather than the oxygen-corrected values. This represents the concentration for human exposure in the air of the mine. It is the absolute value of NO2 that is critical rather than the potential for concerning ground-level ozone. In conjunction with their new heating concept, Process Combustion pursued burner technologies capable of keeping NOx emissions low in response to updated standards.

When a smaller fresh process air heating system opportunity for a mine shaft in Manitoba came along, Process Combustion recognized an opening to use an EcoFornax SLEx burner. The burner would provide similar emissions to the EcoFornax LEx in a fresh process air heating environment in which the additional airflow used to meet lower temperature requirements would also dilute the NOx emissions released from the burner. Using the simplified burner allowed Process Combustion to relay savings to their customer because it allowed for a less-complicated control system and maintained a high efficiency compared to an indirect-fired heating system.

Conclusion

Industrial processes need simple, robust and effective equipment to meet environmental regulations and reduce the impact of technological progress on the world. Lowering NOx emissions from combustion reactions can result in cleaner air with lower concentrations of ground-level ozone, making it easier to breathe and reducing the effects of industry on vegetation.

The EcoFornax SLEx is one such way to mitigate the negative effects of NOx emissions while saving money and maintaining process efficiency. It is versatile enough to meet the requirements of a variety of applications and simple enough to avoid the need for complex control systems.

A mine shaft in Manitoba uses an EcoFornax LEx burner for fresh process air heating.

For more information: Ashley Graybill is a Strategic Innovation Engineer at Fives North American Combustion in Cleveland, Ohio. She can be reached at 216-373-8497 or ashley.graybill@fivesgroup.com. Visit Fives North American Combustion at www.fivesgroup.com.