Nitrogen oxides are known as one of the most important and dangerous pollutants resulting from combustion, leading to serious damage to the environment and human health. This article will present the three main mechanisms responsible for the formation of these pollutants. These pollutants include thermal NOx, fuel NOx, and prompt NOx. Also, the method of reducing these pollutants will be analyzed. Regarding the importance of controlling Nitrogen Oxides emission, understanding the mechanisms is crucial for developing effective strategies to reduce air pollution and enhance environmental quality.
What is NOx?
Nitrogen oxides, commonly referred to as NOx, are harmful products generated from combustion processes in industry.
These compounds are generated during combustion processes and are major contributors to urban air pollution. By participating in chemical reactions in the atmosphere, Nitrogen Oxides can cause environmental and health problems. The group of Nitrogen Oxides consists of seven chemical compounds, which are listed in the table below.
Among the seven chemical compounds mentioned, NO is the most common form of Nitrogen Oxides in the atmosphere. For this reason, the U.S. Environmental Protection Agency (EPA) considers this molecule as the key Nitrogen Oxides pollutant. When discussing Nitrogen Oxides emissions in combustion, the focus is primarily on NO and NO2. Although other forms of Nitrogen Oxides can be generated during combustion, NO and NO2 are produced in significant quantities. As a result, regulations and standards have been established for these two compounds. It is worth noting that NO produced during combustion eventually reacts with oxygen and ozone in the air, converting into NO2.
Characteristics of Nitrogen Oxides
In order to prevent from the emissions of this pollutant, it is important to know its characteristics.
Nitric oxide (NO)
It is a colorless and odorless gas produced during combustion processes at high temperatures. NO easily reacts with oxygen and turns into nitrogen dioxide (NO₂).
Nitrogen dioxide (NO2)
This gas is brown and has pungent smell. In the atmosphere, NO2 continuously engages in chemical reactions with other compounds, in the atmosphere continuously reacts with other chemical compounds and converts to acid rain and surface-level ozone.
Impacts of NOx in the Atmosphere
NOx in the atmosphere can result in significant environmental and health consequences.
Formation of Acid rain
NOx reacts with water vapor in the atmosphere to produce nitric acid (HNO₃), resulting in the formation of acid rain. The acid rain can damage soils, freshwater resources, and urban structures natural ecosystems. Furthermore, it can be detrimental for natural ecosystems and decline agricultural products.
Formation of Ground-Level Ozone
In the presence of sunlight, NOx reacts with volatile organic compounds (VOCs) to form ground-level ozone (O₃). Ground-Level Ozone can cause serious respiratory issues, worsen lung diseases, and harm vegetation.
Impact on Human Health
Inhaling NOx can irritate the respiratory system, heighten the risk of respiratory and cardiovascular diseases, and worsen chronic conditions such as asthma and bronchitis.
Considering harmful effects and dangers associated with NOx, growing awareness of environmental health issues, some strict standards have been implemented to significantly limit NOx emissions from industrial facilities, internal combustion vehicles, and other sources. These laws and standards have effectively reduced Nitrogen Oxides concentrations in the atmosphere in many industrialized countries. For instance, the chart below shows the trend in average NOx concentration in the United States from 1980 to 2023.
In Iran, the use of existing technologies to reduce these emissions, such as pre-mixed burners, low-NOx burners, and flue gas recirculation (FGR) systems, can significantly decrease the emission of harmful pollutants. Additionally, improving the performance of combustion engines can also contribute to reducing Nitrogen Oxides emissions to acceptable levels.
Formation of Nitrogen Oxides
The formation of nitrogen oxides during combustion processes generally occurs through three main mechanisms: thermal NOx, fuel-bound NOx, and prompt NOx. In the following, each of these mechanisms is explained in detail.
1- Thermal NOx
In this mechanism, NOx is produced at very high temperatures, particularly in combustion systems like power station boilers and internal combustion engines. This mechanism is based on chemical reactions of nitrogen (N2) and oxygen (O2) at temperatures exceeding 1,300 degrees Celsius. At high temperatures, sufficient energy is provided to break nitrogen double bonds and form nitrogen oxides. While the thermal NOx mechanism involves several steps, it can be simplified into two primary steps for clarity.
Reaction steps:
Step 1: A nitrogen atom (N) collides with an oxygen molecule (O ), resulting in the formation of Nitric oxide molecule (NO) and an oxygen atom (O). This reaction starts a series of subsequent reactions.
Step 2: The oxygen atom generated in the first step collides with a nitrogen molecule (N ), resulting in the formation of another nitric oxide (NO) molecule and a nitrogen atom (N). The newly produced nitrogen atom then re-enters the first step reaction, and this chain of reactions continues.
Final Result
When both steps are considered together, the outcome is that the reaction of one nitrogen molecule (N₂) with one oxygen molecule (O₂) produces two nitric oxide (NO) molecules.
As mentioned earlier, these reactions occur at very high temperatures because a significant amount of energy is required to break the nitrogen and oxygen molecules apart and form individual atoms. The high temperatures present in the flames of industrial burners provide the necessary energy for this process.
Oxygen molecule (O2), with its double bond, is more easily dissociated than the nitrogen molecule (N₂), which has a triple bond. As a result, the second step, where this dissociation occurs, is the stage that limits the overall reaction rate. In simpler terms, since the second step is slower, it will determine the total reaction rate.
Equation of Changes in NO Concentration During the Reaction
Assuming that atomic oxygen is in equilibrium with molecular oxygen, the concentration of NO changes over time according to the following equation.
Important Points
Effect of temperature: Higher temperature significantly increases nitric oxide production. So, the hot spots within the flame, characterized by high temperatures, are the main sources of NO production. Thus, by reducing the flame temperature, NOx formation can be effectively decreased.
Effect of Oxygen Concentration: This factor also plays a crucial role in NO production. The higher the oxygen concentration at the reaction site, the more NO is produced. Therefore, if the concentration of oxygen is reduced, the emissions of NO will also be decreased.
Constancy of nitrogen: During the chemical reaction, the concentration of nitrogen does not change significantly and can be considered almost constant. For this reason, the impact of nitrogen concentration in the equation is taken as a constant coefficient.
Strategies for Reducing Thermal NOx
Based on this equation and the explanations provided, there are three main methods to reduce Nitrogen Oxides emissions during combustion:
Reducing Flame Temperature: Since temperature has a significant impact on Nitrogen Oxides production, lowering the flame temperature is one of the most effective ways to reduce Nitrogen Oxides.
Reducing Oxygen Concentration: Nitrogen Oxides production can be reduced by decreasing the amount of oxygen in the fuel and air mixture.
Minimizing Combustion Duration at High Temperatures: If the duration of combustion occurring at high temperatures in the presence of oxygen is shortened, Nitrogen Oxides production will also be reduced.
These are key methods for controlling and reducing thermal NOx emissions, which can help to improve air quality and reduce pollution.
2- Fuel-bound NOx
This type of Nitrogen Oxides is formed from nitrogen present in fossil or biofuel molecules. The nitrogen contained in these fuel molecules is converted into Nitrogen Oxides during combustion. Fuel-bound is usually found in fuels such as coal, oil, and some biodiesel fuels. The chemical reactions that occur during this process generally lead to the formation of Nitrogen Oxides from the nitrogen present in the fuel.
The Role of Nitrogen Molecule (N2) in Fuel
It should be noted that if the fuel contains only free nitrogen molecules (N₂), this nitrogen cannot contribute to fuel-bound NOx formation. In fact, it may even reduce Nitrogen Oxides production. In fact, nitrogen molecules (N₂) present in the fuel gas do not participate in the combustion process due to their high chemical stability. Instead, they can act as a diluent. This diluent lowers the concentration of reactive substances and the flame temperature, which in turn reduces the production of both thermal and fuel-bound NOx.
Steps of Fuel-bound NOx Formation
Step 1: When a fuel containing nitrogen atoms (such as C H N) burns, it first breaks down into intermediate substances like HCN (hydrogen cyanide) and CN (cyanide). This step is shown in the equation below:
C H N → HCN+CN+…
Step 2: In the next step, the intermediate substances react with oxygen (O2), resulting in the production of nitric oxide (NO). This process is given in the following equation:
HCN+CN+ O₂ →NO+…
How to Minimize Fuel-bound NOx?
Here are two effective methods for reducing fuel-bound NOx emissions:
Reducing oxygen concentration: Lowering the oxygen concentration in the combustion chamber will also decrease the amount of fuel-bound NOx. It is because of reducing the reaction rate that converts intermediate compounds (HCN and CN) into nitric oxide.
Using Nitrogen-free fuels: If a fuel that does not contain nitrogen, such as natural gas is used, the formation of fuel-bound NOx will not occur. For this reason, one effective method in the industry for reducing these emissions is to use fuels that are free of nitrogen.
3- Prompt NOx
The mechanism of prompt NOx is similar to fuel-bound. The main difference is that in prompt NOx, the nitrogen needed to form nitrogen oxides comes from nitrogen molecules (N2) in the air, not from fuel.
Reaction steps of prompt NOx formation:
Step 1: In this step, fuel molecules (such as CH ) react with nitrogen molecules (N2) present in the air. This reaction breaks the strong triple bond of nitrogen (N≡N) and forms intermediate compounds such as HCN and CN. This step is more challenging than the formation of fuel-bound NOx because breaking the strong bond between nitrogen atoms requires a greater amount of energy.
CH + N →HCN+CN+…
Step 2: As in fuel-bound NOx mechanism, here the intermediates (HCN and CN) react with oxygen (O2) and nitric oxide (NO) is formed.
CHN+ O →HCN+CN+…
Effective Factors on Prompt NOx?
The following section briefly and clearly examines two factors that influence prompt NOx formation.
Fuel concentration: The higher the concentration of fuel in combustion chamber, the more likely it is to react with nitrogen and produce prompt NOx.
N2 concentration: Since nitrogen is abundant in the air, reducing its concentration is usually not possible. This means that the focus should be on lowering fuel concentration.
How to reduce prompt NOx?
The best solution to reduce prompt NOx is diluting fuel before combustion. That is, before the fuel reaches the flame zone, it should be mixed with the combustion products already present in the chamber. This can be achieved through burner design in such a way that high-velocity fuel jets draw in and mix with the existing combustion products. In this way, the fuel becomes more diluted, and Nitrogen Oxides production is reduced.
Review of Innovative Technologies for Reducing NOx in Various Industries
With technological advancements and growing environmental concerns, industries are moving towards reducing hazardous pollutants like nitrogen oxides (NOx). NOx is one of the most significant urban air pollutants, causing harm to human health and the environment. In this regard, innovative NOx reduction technologies have been introduced as effective solutions. Below, we review some of these technologies.
1- Flue Gas Recirculation (FGR) System: This system reduces flame temperature and prevents the formation of thermal NOx by recirculating a portion of the combustion exhaust gases back into the combustion chamber. Due to its simplicity and high efficiency, FGR is widely used in boilers and gas turbines.
2- Selective Catalytic Reduction (SCR): SCR technology is one of the most effective methods for reducing NOx emissions, commonly employed in power plants and automotive industries. In this system, ammonia or urea is injected into exhaust gases, and specific catalysts convert NOx into harmless nitrogen and water.
3- Direct Steam or Water Injection: Injecting steam or water into the combustion chamber lowers the flame temperature and limits thermal NOx production. This method is advantageous due to its relatively low cost and compatibility with existing systems.
4- Pre-Mixed and Low-NOx Burners: Pre-mixed burners reduce flame temperature and NOx formation by thoroughly mixing fuel and air before combustion. The uniform flame temperature in these burners results in low thermal NOx production.
5- Low-NOx Burners: Low-NOx burners feature specialized combustion designs that enable uniform flame temperatures, significantly reducing NOx emissions. These burners are commonly used in industrial boilers and furnaces.
6- Staging Combustion: In staging combustion, fuel or air is injected in phases to slow the reaction rate and prevent high-temperature flames, thereby significantly reducing NOx emissions. This method is widely utilized in power plant burners and petrochemical industries.
7- Artificial Intelligence and Digital Simulation: A recent innovation is the use of artificial intelligence and computer simulations to optimize combustion processes and reduce NOx emissions. This approach helps industries identify the most efficient combustion settings by analyzing data and simulating various scenarios.
8- Use of Clean Fuels: Switching from fossil fuels to cleaner alternatives such as natural gas or biofuels can significantly reduce NOx production. When combined with NOx reduction technologies, this transition plays a critical role in mitigating air pollution.
Ways to enhance air quality by Nitrogen Oxides reduction
Nitrogen Oxides formation in combustion processes occurs through various mechanisms, including thermal NOx, fuel-bound NOx, and prompt NOx. Each of these mechanisms has its own influence on production of this emission. Understanding these mechanisms is essential for reducing emissions of this pollutant and improving environmental air quality. Considering the negative effects of Nitrogen Oxides on public health and environment, it is necessary to develop new strategies and technologies for controlling and reducing these emissions. Using cleaner fuels, optimizing combustion processes, and implementing reduction technologies such as raadman low-NOx burners, premixed burners, and flue gas recirculation systems (FGR) can significantly reduce these emissions in various combustion systems.
