NOx, or Oxides of Nitrogen is formed in all combustion processed. It is a general term that includes two forms; Nitric Oxide (NO) and Nitrogen Dioxide (NO2). NO2 is formed when NO is subjected atmospheric Oxygen.
What are the Environmental and Health Concerns?
When NOx reacts with hydrocarbons and oxygen in the presence of sunlight, Smog is formed. Smog is a kind of air pollution, originally named for the mixture of smoke and fog in the air. Classic smog results from large amounts of coal burning in an area and is caused by a mixture of smoke and sulfur dioxide. In the 1950s a new type of smog, known as Photochemical Smog, was first described.
Photochemical Smog is what you are most likely thinking of when you think of the hazy cloud that forms in modern, large cities.
According to Encyclopedia Britannica: “The highly toxic gas ozone arises from the reaction of nitrogen oxides with hydrocarbon vapours in the presence of sunlight, and some nitrogen dioxide is produced from the reaction of nitrogen oxide with sunlight. The resulting smog causes a light brownish coloration of the atmosphere, reduced visibility, plant damage, irritation of the eyes, and respiratory distress. Surface-level ozone concentrations are considered unhealthy if they exceed 70 parts per billion for eight hours or longer; such conditions are fairly common in urban areas prone to photochemical smog.”
In the stratosphere, ozone filters out harmful ultraviolet rays and is helpful to living things on earth. At ground level, ozone when inhaled causes health problems.
NOx contributes to the formation of PM-10, suspended particulate matter smaller than 10 microns in size – known to effect the human respiratory system. It also contributes to Acidic Deposition including Acid Rain and Fog.
How is NOx Formed in Combustion?
NOx is produced from the nitrogen present in the combustion air and in the fuel being burned. The generation of NOx results from high flame temperatures in the presence of oxygen, over a period of time.
There are three main areas of NOx formation:
- Prompt NOx – The start of combustion. That moment at the base of the flame when the process of combustion starts is the area of prompt NOx. This is one of the most difficult areas of NOx formation to control because there are minimum temperatures required to maintain stable combustion.
- Thermal NOx – Results when nitrogen and oxygen in the air combine at the elevated temperatures of the combustion process.
- Fuel Bound NOx – Is the result of nitrogen organically bound in the fuel hydrocarbon chain combining with air at the elevated temperature present in the combustion process. Fuel bound nitrogen has a high propensity to form NOx and is difficult to eliminate from the fuel.
Contributing factor in boiler design:
- Combustion Chamber Design – The generation of NOx relates directly to peak flame temperature, heat release volume per cubic foot of the combustion chamber as well as the heat absorbing (and reflecting) characteristics of the chamber surfaces. The larger the combustion chamber volume (for a given BTU input), the lower the NOx emission rate. By derating a boiler firing rate, the NOx formed will be lower than at full design input capacity. A chamber with a floor and sidewalls of refractory materials will, due to re-radiation of heat back into the flame, generate a higher level of NOx than a chamber formed by heat absorbing water legs, a wet base or water tube design.
NOx Compliance Requirements
Under the Federal Clean Air Act Amendments of 1990 (CAAA), states have been required to submit to the EPA for approval, then enforce, State Implementation Plans (SIP’s) to attain and maintain National Ambient Air Quality Standards (NAAQS). NOx emissions regulation can be required in geographical areas that are in non compliance with applicable NAAQS standards.
In addition, CAAA Title V requires a Federal EPA permit to be administered by the individual state air pollution control agencies, for major facilities having the potential to emit NOx and other air pollutants. The NOx threshold, per facility, varies from 10 tons per year in an extreme ozone nonattainment area, like Los Angeles, to 25 tons per year in a severe nonattainment area like Philadelphia, to 100 tons per year in areas meeting NAAQS standards for ozone. In a future article we will discuss
Your local or state Air Quality Regulatory Agency can provide information on any specific NOx regulations impacting your area. NOx regulations in some jurisdictions have been changing rapidly. If you are in doubt as to which Air Quality Agency has jurisdiction you can contact the Federal Environmental Protection Agency (EPA).
In a future article, we will discuss current methods for controlling NOx formation in boilers. If you have questions regarding NOx, boiler room safety, efficiency, reliability, or if you have any other boiler equipment or boiler service questions, please give us a call, we are here to help. We have the expertise to support you during the purchase, installation and startup, and experienced factory trained technicians to support you and keep your equipment in tip-top shape, long-term. Our technicians are available 24/7/365, for emergency services.
 Much of the material referenced above are from Power Flame’s white paper titled “Combustion”. For the latest in combustion technology, please also visit their website powerflame.com