NOx 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. NOx 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. NOx 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 NOx.
NESTEC’s Thermal Oxidation Equipment (such as the MCC RTO System) offers several specific features that minimize NOx production:
- NESTEC provides an inlet distribution nozzle to ensure uniform air flow to heat exchange media.
- Modern, high efficiency RTOs require less fuel than older models, which minimizes the associated NOx produced during the oxidation process.
- 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 NOx production than common afterburner systems.
- Process gases in a RTO are not required to pass through the burner flame, further minimizing the probability of NOx generation because the gases can bypass the area of highest temperature.
- Auto-ignition of hydrocarbons and/or Natural Gas Enhancement (NGE) in the heat exchange media occurs at a much lower temperature, substantially reducing NOx generation.
- 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 NOx.
Thermal oxidation combustion of fossil fuels is a source of NOx emissions. The key factors to minimize NOx are:
- Airstream temperature and constituents
- Lower operating temperatures, i.e. catalytic oxidation to reduce NOx.
- RTO Uniform air flow and temperature profile to minimize the potential for NOx.
- NOx type and concentration
- Fuel NOx is formed by oxidation of nitrogenous matter contained in the airstream/process/chemically-bound nitrogen in VOCs. (volatile organic compounds) or other compounds.
- Thermal NOx 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 NOx is not. Determining whether the NOx 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.
Minimization of thermal oxidation fuel NOx formation can be accomplished by controlling the availability of 01 atoms at the critical conditions that would otherwise produce NOx.
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 NOx.
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.
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