Thermal Oxidizer NO_x Emissions

NO_x is a term used to describe gases consisting primarily of NO (mono-nitrogen oxides) and NO₂ (nitric oxide and nitrogen dioxide). They are produced from the reaction among nitrogen, oxygen, and even hydrocarbons (during combustion), especially at high temperatures. NO_x often appears as a brownish gas. It is a strong oxidizing agent and plays a major role in the atmospheric reactions with volatile organic compounds (VOC) that produce ozone (smog) on hot summer days. NO_x emissions also contribute to the formation of acid rain and have been shown to be detrimental to human health.

The NESTEC Regenerative Thermal Oxidation (RTO) not only purifies exhaust gases at significant fuel savings but provides optimum conditions for the control of NO_x.

NESTEC’s Thermal Oxidation Equipment (such as the MCC RTO System) offers several specific features that minimize NO_x production:

1. NESTEC provides an inlet distribution nozzle to ensure uniform air flow to heat exchange media.
2. Modern, high efficiency RTOs require less fuel than older models, which minimizes the associated NO_x produced during the oxidation process.
3. Unlike other systems, Regenerative Thermal Oxidizers do not require an intense flame for the quick heating of the process gases. RTOs are therefore much better suited for a cooler (long) flame which provides better heat distribution within the chamber. A cooler flame results in sharply lower NO_x production than common afterburner systems.
4. Process gases in a RTO are not required to pass through the burner flame, further minimizing the probability of NO_x generation because the gases can bypass the area of highest temperature.
5. Auto-ignition of hydrocarbons and/or Natural Gas Enhancement (NGE) in the heat exchange media occurs at a much lower temperature, substantially reducing NO_x generation.
6. Gradual cooling of RTO exhaust gases permits Oxides of Nitrogen to return to their natural state of nitrogen and oxygen.

In summary, the NESTEC Regenerative Thermal Oxidation (RTO) not only purifies exhaust gases at significant fuel savings but provides optimum conditions for the control of NO_x.

Thermal oxidation combustion of fossil fuels is a source of NO_x emissions. The key factors to minimize NO_x are:

• Airstream temperature and constituents
  • Lower operating temperatures, i.e. catalytic oxidation to reduce NO_x.
  • RTO Uniform air flow and temperature profile to minimize the potential for NO_x.
• NO_x type and concentration
  • Fuel NO_x is formed by oxidation of nitrogenous matter contained in the airstream/process/chemically-bound nitrogen in VOCs. (volatile organic compounds) or other compounds.
  • Thermal NO_x is most often formed with gaseous fuels and results from temperatures that cause the breakdown of O₂ to O₁ which reacts with N₂ to form NO and N₁. In turn the N₁ combines with O₂ to make more NO and 0₁, OH, “prompt NO,” and HCN (formed when operating fuel rich, even with gaseous fuels) aggravate the situation. All three of these are prolific at high temperature. The mechanism for forming thermal NO is strongly temperature-dependent; whereas that for fuel NO_x is not. Determining whether the NO_x emission problems are from thermal or fuel NO is essential, because abatement techniques that utilize flame temperature are very effective for thermal NO, but have little effect on fuel NO. The effects are similar with preheated air and with oxygen enrichment, both of which raise flame temperatures.

Fuel NO_x

Minimization of thermal oxidation fuel NO_x formation can be accomplished by controlling the availability of 0₁ atoms at the critical conditions that would otherwise produce NO_x.

Variables that affect 0₁ availabilities are:

• Excess air (keep it low)
• Rich atmosphere (use staged air addition)
• Staged combustion with first stage scavenging of 0₁ atoms using highly reactive radicals from polymerization (cracking) of fuel molecules
• In incineration of wastes containing N, the “starved air” procedure in the first phase of incineration is therefore a step in the right direction

Points to Remember

• Eliminate high temperature areas
• Minimize process contact with the high temperature zones
• Maximize the process hydrocarbon concentration to provide a self-sustaining operation:
  • Ensure uniform air flow through the RTO heat exchange bed
  • Include a NESTEC NGE design that insures uniform mixture of the NG and process air stream prior to the heat recovery media, maximizing the RTO thermal efficiency
• Utilize catalyst (RCO) where the process conditions are favorable.
• A well-designed RTO allows for a reduction in the purification control temperature, which reduces the energy consumption and reduces NO_x.

For assistance from our extensive experience, call or e-mail NESTEC Inc. for a free evaluation of NESTEC’s air emission control and/or energy conservation equipment needs.

Office: 610.323.7670

Jim Nester, CEO: jnester@nestecinc.com

Rick Reimlinger, Vice President: rick.reimlinger@nestecinc.com

Rodney L. Pennington, PE, Vice President of Key Accounts: rpennington@nestecinc.com

William Holden, Aftermarket & Services Manager: wholden@nestecinc.com