Sources of NOx Emissions

This article discusses the sources of NO_x emissions, including what is NO_x, the opportunities for its formation, and why it is a concern.

What is NO_x?

First and foremost, you might ask, “What is NO_x?” Well, here’s the chemical explanation: The term NO_x refers to nitrogen oxides—Nitric oxide (NO) and Nitrogen Dioxide (NO₂)—which are a set of chemically reactive gases comprised of varying amounts of nitrogen and oxygen molecules.

NO_x gas formation (primarily NO and NO₂) originates from an endothermic reaction during the combustion process between Oxygen, O₂ (making up 20.9% of air) and Nitrogen, N₂ (making up 78% of air).

Nitrogen oxides are highly toxic and acidic, and they are considered air pollutants by the US EPA and other environmental agencies. Now that we have explained what NO_x is, let’s go into why NOx is a concern.

Why is NOx a Concern?

The highly reactive nature of NO and NO₂ increases the likelihood and frequency of the production of hazardous airborne compounds. When NOx gases react with oxygen in the atmosphere, ground-level ozone (O₃), corrosive nitric acid, and toxic organic nitrates are formed.

The presence of these substances in the air causes significant human health issues and contributes to the formation of acid rain and to the deterioration of water and air quality. Additionally, these weakened environmental conditions negatively affect agriculture by killing plant tissue and decreasing plant growth rate.

Opportunities for NO_x Formation

In fact, there are three types of NO_x that can be produced during the combustion process:

• Thermal NO_x
• Fuel NO_x
• Prompt NO_x

Thermal NO_x

This is the most produced form of NO_x during the combustion process, which typically occurs at very high temperatures (i.e., above 2200° F). At these high temperatures, nitrogen molecules present in the combustion air react with oxygen molecules to form NO_x. The higher the temperature and the longer the nitrogen molecules are exposed to this temperature, the greater the formation of thermal NO_x.

Fuel NO_x

This type of NO_x forms when nitrogen molecules in fuels, such as coal and oil, are released and react with excess oxygen present in the combustion air. Fuel NO_x emissions are a significant environmental concern with the potential to account for up to 50% of total emissions when burning oil and up to 80% of total emissions when burning coal.

Prompt NO_x

This is the least produced form of NO_x that occurs when atmospheric nitrogen reacts with radicals in the air at the early stages of combustion. Prompt NO_x is not considered a main contributor to the NO_x emissions targeted by environmental regulators.

As you can see, NO_x from combustion comes from three different sources: thermal NO_x, fuel NO_x and prompt NO_x. Let’s now talk specifically about the kinds of combustion processes that produce NO_x emissions.

Sources of NO_x

Most industries depend on combustion systems, such as boilers and engines, to provide heat or other forms of power to run their processes. Importantly, each man-made fuel combustion system produces NO_x emissions at varying levels.

The sectors that account for the largest share of NO_x emissions are transportation, power generation, industrial processing, and commercial and industrial heating. The sources of NO_x emissions are separated into two main categories: Mobile Sources and Stationary Sources.

Mobile Sources of NO_x

Mobile sources are the leading contributors to global NO_x emissions and include “vehicles, engines, and equipment, that can be categorized as either on-road mobile sources (e.g. trucks, buses, passenger cars, and motorcycles) or non-road mobile sources (e.g. locomotives, marine vessels, construction equipment, lawn, garden and snow equipment, personal recreation equipment, etc.).”

Stationary Sources of NO_x

Stationary sources are non-moving and can be categorized as either Point Sources or Area Sources.

• A Point Source is a fixed or specific location from which significant NO_x emissions are generated. An example of a point source is a single smoke stack.
• An Area Source is a group of multiple sources in a given area that each emit NO_x pollution. Together, these individual sources contribute significant NO_x emissions. An example of an area source is a group of natural gas compression engines along a pipeline.

Examples of specific stationary sources include power plants, chemical plants, refineries, cement plants, foundries, manufacturing plants, etc. At these facilities, the specific fuel-burning combustion systems that generate the emissions include boilers, burners, furnaces, gas or diesel generators, kilns, ovens, incinerators, and smelters.

To give you a complete picture, here is a chart of NO_x emissions by source:

Manage Your NO_x Emissions Effectively

Ultimately, NO_x emissions contribute to smog, acid rain and eutrophication. For this reason, various federal and state regulatory bodies require the measurement of NO and NO₂.

With the increasing demand for air pollution control and the increased possibility of fines or legal actions, you need to be able to demonstrate, in a tangible way, that you have performed your job as a professional and have done everything possible to ensure compliance.

Therein lies the benefit of a portable combustion gas and emissions analyzer, which allows you to test, tune and certify commercial and industrial boilers, engines, compressors, and generators.

At the end of the day, businesses have a responsibility to manage their NOx emissions effectively, and Bacharach is here to help you ensure that monitoring your emissions is fast, easy and accurate. For more information on how to achieve high accuracy gas measurements, check out our article on the Fundamentals of NOx Testing. ∎

How are you currently managing your NO_x emissions? Let us know by leaving a comment below.