NO_X is a term that describes nitric oxide (NO) and nitrogen dioxide (N₂O). Nitric oxide is not hazardous to our health at normal atmospheric levels. Nitrogen dioxide, however, is a known poisonous gas that can cause inflammation of the airways, headaches and reduced lung function, depending on concentration and length of exposure.

Not only does NO_X pose health threats, NO_X — along with volatile organic compounds (VOCs) — react with sunlight to form smog.

The three largest contributors to the formation of NO_X are natural sources, agricultural sources and industrial sources. The largest contributors — and focus of this article — are industrial sources. The industrial segment is comprised of four main groups:

• Thermal NO_X formation.
• Fuel NO_X formation.
• Process-derived NO_X formation.
• Prompt NO_X formation.

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Thermal NO_X formation is the most relevant source when combusting organic fuels such as natural gas or oil. NO_X is formed in the combustion chamber by the combining of atmospheric nitrogen with atmospheric oxygen. At elevated temperatures, they disassociate and recombine to produce NO_X.

Fuel NO_X is formed from the combustion of nitrogen rich fuels. During combustion, nitrogen bonds are broken, allowing the formation of NO, N₂ and NO₂.

 Process-derived NO_X is produced primarily during manufacturing, especially when producing nitric acid or incorporating the use of nitric acid. Here the emission concentrations are frequently in the thousands of PPMv — and can be much more difficult to control.

Prompt NO_X is yet another source of NO_X. It is produced during the early stages of combustion; however, it is a much less significant contributor than other forms.

A catalytic candle filtration unit is used to control particulate matter, volatile organic compounds, SOx and NOx emissions from a glass manufacturing process.

How to Control NO_X Emissions

Abatement and control technology for NO_X is complex, with a variety of solutions. Following are proven methods for reducing — and, in some cases, eliminating — the formation of NO_X.

Temperature Reduction

Temperature reduction prevents nitrogen from becoming ionized, thus preventing the formation of NO_X. Combustion temperatures can be reduced by adjusting the stoichiometric ratio of the fuel being used. This is accomplished by adding additional fuel, injecting excess air, injecting steam, or recirculation of flue gas.

Residence Time

Reducing the residence time at peak temperature can prevent nitrogen from ionizing. Strategic burner placement, combustion chamber design and flue-gas injection location are examples of design changes that can be made to adjust residence time.

Burner Tuning

Poor burner tuning can play a part in NO_X production. After many hours of operation, it is a best practice to evaluate the burner flame because air-to-fuel ratios may have changed. Minimizing excess combustion air can aid in the reduction of NO_X production.

A selective catalytic reduction unit is used for controlling NOx emissions in a catalyst manufacturing process.

Low NO_X Burners

Low NO_X burners provide a high reduction of NO_X — as much as 80 percent —at a relatively low cost. By limiting the amount of combustion air, flame temperatures are reduced, which prevents the formation of NO_X. With a variety of low NO_X burner designs available, solutions can be retrofitted to an existing unit with minimal cost and downtime.

Selective Noncatalytic Reduction

Selective noncatalytic reduction (SNCR) is a chemical process used to reduce NO_X emissions. In the SNCR process, the combustion chamber functions as a reactor. A reagent, typically ammonia or urea, is injected where the temperature is high enough (1600 to 2100°F [871 to 1149°C]), to facilitate the reduction of NO_X molecules to nitrogen (N₂) and water vapor (H₂O).

In order for a selective noncatalytic reduction unit to successfully operate, there are several requirements that must be met. The first is temperature, which needs to be between 1600 and 2100°F. The second is a residence time of approximately 1 second. The final requirement is a high concentration of NO_X. A destruction efficiency of approximately 60 to 70 percent is achievable.

Selective Catalytic Reduction

Selective catalytic reduction is a chemical process in which NO_X is converted into water vapor (H₂O) and nitrogen (N₂). A reagent such as ammonia or urea is injected into the process exhaust prior to the combustion chamber. The process gas and reagent, after mixing, pass through a catalyst. The reagent selectively reacts with NO_X in the process gas at a specific temperature in the presence of catalyst and oxygen.

A direct-fired thermal oxidizer is used in tandem with a selective catalytic reduction unit to control volatile organic compounds and NOx emissions from a chemical manufacturing facility.

The basic building blocks of an SCR system include:

• SCR catalyst.
• Ammonia of urea injection grid.
• Ammonia control systems.
• NO_X analyzers.
• Heat recovery equipment.
• Auxiliary boilers.

The reduction of NO_X is dependent on the amount, density, temperature and type of catalyst. Catalyst selection is based on the process gas temperature and presence of potential contaminants such as sulfur and particulate.

The reagent injection design also heavily influences the effectiveness of a SCR system. The design of the injection system needs to ensure that the reagent completely mixes with the flue gas prior to contact with the catalyst.

Catalytic Candle Filtration

Catalytic candle filtration (CCF) is a three-in-one process that can effectively control particulate matter, SO_X and NO_X. In this process, ridged filter media captures acid gas and particulate matter. The media is coated with SCR-based catalyst. A reagent is injected and causes the reduction of NO_X.

These systems are capable of reducing the amount of SO_X, NO_X and particulate matter by more than 90 percent. In addition, the filter design is effective in protecting the catalyst from becoming plugged.

In conclusion, whether it is existing equipment that has to be upgraded for more stringent emission requirements, a remediation project such as a greenfield or a new system being installed at a newly constructed plant, effective pollution control solutions exist to seamlessly integrate into any process. Carefully selecting pollution abatement equipment can allow industrial processors to successfully abate NO_X emissions from a host of applications.