As industries advance at a rapid pace, the demand for energy continues to rise, making the use of fossil fuels in industrial burners a significant source of environmental pollution, particularly nitrogen oxides (NOx). The formation of NOx during high-temperature fuel combustion can have serious negative impacts on both the environment and public health.
To address this issue, NOx reduction technologies in industrial combustion systems have been introduced as a key approach to controlling pollution and improving environmental standards. These technologies primarily consist of low-NOx combustion methods, flue gas recirculation, and catalytic systems.
This article comprehensively examines various NOx reduction methods in industrial burners and analyzes the challenges and advantages of each of them. To gain a foundational understanding of NOx, you may first refer to the article “NOx Formation in Combustion; 3 Key Mechanisms, and the Most Damaging Environmental Effects”.
NOx and Its Impacts on the Environment and Health
NOx (Nitrogen Oxides) is a term used for a group of nitrogen oxide compounds, mainly consisting of nitric oxide (NO) and nitrogen dioxide (NO₂), which are predominantly released from combustion processes in industries, power plants, transportation, and heating systems. These pollutants are among the major air pollutants and have widespread impacts on the environment and human health.
Environmental Impacts of NOx
NOx is a significant factor in air pollution and the formation of acid rain. These compounds react with water vapor and other pollutants in the atmosphere to form nitric acid (HNO₃), which then falls to the earth as acid rain. Acid rain can cause soil fertility decline, damage to forests, destruction of aquatic life, and erosion of historical and urban structures. Moreover, NOx, in the presence of sunlight and combined with volatile organic compounds (VOCs), contributes to the formation of ground-level ozone (O₃), a major pollutant in photochemical smog. This type of ozone has harmful effects on the environment by damaging vegetation, decreasing agricultural product quality, and inducing stress in natural ecosystems.
Effects of NOx on Human Health
NOx, notably nitrogen dioxide (NO₂), is a toxic gas and a powerful irritant to the respiratory system, causing lung inflammation, more frequent asthma attacks, bronchitis, and other respiratory diseases. Prolonged exposure to this pollutant can result in reduced lung capacity, an increased risk of cardiovascular illnesses, and damage to the immune system. Additionally, NOx contributes to the formation of secondary particulate matter (PM2.5), heightening the risk of lung and heart diseases, especially in children, the elderly, and people with underlying health conditions.
Necessity of Reducing NOx Emissions
Given the harmful effects of NOx on the environment and health, controlling and reducing NOx emissions is critically important. Technologies for NOx reduction, including low-NOx burners, staged combustion, flue gas recirculation (FGR), and emissions control systems such as SCR and SNCR, can significantly aid in reducing this pollutant.
NOx Formation Mechanisms in Combustion Processes
The type of NOx formation mechanism determines the appropriate method and technology for NOx reduction. Taking this into account helps in selecting more cost-effective and efficient solutions for specific industrial conditions. The following provides a brief overview of these mechanisms.
Thermal NOx
The first type of NOx is thermal NOx, which forms at very high temperatures (typically above 1300°C) in combustion processes such as boilers and internal combustion engines. In these high-temperature environments, nitrogen (N₂) and oxygen (O₂) in the air react, causing the bonds between these elements to break and producing nitrogen monoxide (NO). The amount of thermal NOx produced depends on factors such as flame temperature, the duration of gas contact at high temperatures, and oxygen concentration. Key strategies for reducing thermal NOx include lowering the flame temperature and controlling the oxygen level in the combustion process.
Fuel NOx
Fuel NOx is generated when the nitrogen in the chemical composition of fossil or biofuels, like coal, heavy oil, and petroleum, reacts with oxygen during the combustion process, resulting in the formation of nitrogen oxides (NOx). The amount of this type of NOx produced depends on the nitrogen content in the fuel and the combustion conditions, including temperature and reaction time. A primary strategy for reducing fuel NOx is to use cleaner fuels, like natural gas, which contain lower nitrogen levels.
Prompt NOx
It results from the reaction of nitrogen and oxygen in the air with hydrocarbon fuel molecules. In this mechanism, the nitrogen in the air directly reacts with the fuel molecules, and the intermediate products of these reactions are converted into nitrogen monoxide (NO). This process can take place at lower temperatures and is typically seen when the fuel concentration in the combustion chamber is high. The amount of prompt NOx produced is usually lower than thermal and fuel NOx. To reduce prompt NOx, strategies such as fuel dilution and reducing fuel concentration in the combustion chamber can be used.
Innovative NOx Reduction Technologies in Industrial Burners
NOx reduction technologies focus on modifying combustion conditions, such as flame temperature and oxygen levels at the flame site. By reducing flame temperature, decreasing oxygen concentration, minimizing the contact time of fuel gases with oxygen, and using nitrogen-free fuels, NOx formation can be significantly reduced. Some of the most common technologies include staged combustion, pre-mixed combustion, and flue gas recirculation.
Raadman burners, with a specially designed configuration for NOx reduction, pre-mixed combustion, and flue gas recirculation systems, achieve NOx classes 3, 4, and 5 in accordance with Iranian national standard 7595. Moreover, Raadman’s water-tube series burners, employing the staged air technique, can reduce NOx emissions.
In cases where NOx formation in the burner cannot be avoided, technologies such as SCR and SNCR can be employed to eliminate NOx from the combustion products. Therefore, SCR and SNCR are not directly related to industrial burners and necessitate the installation of separate systems and equipment; nevertheless, their importance and efficiency warrant their inclusion in this discussion.
In industries like power plants, petrochemical plants, and industrial furnaces, NOx reduction technologies play an essential role in reducing air pollution and improving environmental quality. This section provides an explanation of the various NOx reduction technologies.
Design of Low-NOx Burners
Low-NOx burners, with their special design, prevent the formation of extremely high-temperature spots, which are the primary source of thermal NOx production. These burners achieve a uniform and controlled combustion process by reducing flame temperature, staged combustion, optimizing fuel and air mixing, and creating swirling flows. Additionally, their compatibility with technologies such as Flue Gas Recirculation (FGR) and pre-mixed combustion further reduces NOx emissions. Low-NOx burners not only reduce pollutant emissions by up to 90%, but also enhance combustion efficiency, optimizes fuel consumption, and reduce the formation of other pollutants like carbon monoxide.
Staged Combustion
Staged combustion is an effective method for NOx reduction in industrial burners. It can be carried out in two forms: air staging and fuel staging.
In the air staging method, air is supplied to the combustion chamber in two stages. In the fuel staging method, fuel is injected in two stages. By staging air or fuel, the combustion process occurs in two stages, forming primary and secondary combustion zones. In one zone, combustion occurs in a lean mixture (low fuel-to-air ratio), while in the other, it occurs in a rich mixture (high fuel-to-air ratio). In the lean zone, lower temperatures, and in the rich zone, low oxygen concentration contribute to NOx reduction. The fuel staging method also enhances the circulation of combustion products within the chamber, allowing them to mix with the fuel and air flow, which further reduces NOx emissions.
Pre-mixed Combustion
In pre-mixed combustion, fuel and air are thoroughly mixed before entering the combustion chamber, which leads to uniform flame temperature and prevents the creation of high-temperature zones. This method helps to avoid thermal NOx formation by minimizing the flame temperature and contributes to improved combustion efficiency. Moreover, the accurate control of the fuel-to-air ratio ensures optimal combustion and reduces pollutants such as prompt NOx and carbon monoxide (CO).
Pre-mixed combustion is often used in advanced burners, gas turbines, and industrial boilers. When integrated with technologies such as air staging and flue gas recirculation (FGR), it can contribute to the reduction of NOx and other pollutants.
Pre-mixed burners are one of the effective technologies for reducing NOx pollutants, which, provide more optimal performance in NOx reduction due to their pre-mixed combustion. For further details on these burners, we suggest reading the article “Comprehensive Guide to Premixed Burners: Advantages and Applications“.
Flue Gas Recirculation (FGR)
Flue Gas Recirculation (FGR) is an effective technology for reducing NOx in combustion systems. In this method, a portion of the flue gases is reintroduced into the combustion chamber after the combustion process. These flue gases, which consist of carbon dioxide (CO2), water vapor (H2O), and nitrogen (N2), act as diluting agents and lower the flame temperature without entering the chemical combustion process. As a result, it prevents the formation of NOx, particularly thermal NOx, which is generated at high temperatures.
Selective Catalytic Reduction (SCR)
This method is used to remove nitrogen oxides from the exhaust gases. As stated at the beginning of this section, this technology is independent of industrial burners and requires additional equipment that is installed in the flue gas path. In the SCR process, a reducing agent such as ammonia (NH₃) or urea is injected into the flue gas stream.
In the presence of a catalyst, the reducing agent undergoes a chemical reaction, transforming NO into nitrogen (N₂) and water vapor (H₂O). This process generally takes place in the temperature range of 300 to 400°C and can eliminate 90-95% of the NO in the combustion products. The catalysts used in this method are typically made from materials such as zeolites or metal oxides, including vanadium and tungsten, with their optimal performance reliant on maintaining the correct temperature and accurate ammonia injection.
Selective Non-Catalytic Reduction (SNCR)
In this technology, reducing agents such as ammonia (NH₃) or urea are employed to convert NO into nitrogen (N₂) and water vapor (H₂O). Similar to SCR, SNCR is a method for removing NOx from the combustion products and is independent of the burner. However, unlike SCR, SNCR does not require a catalyst, and the reducing agent is directly injected into the flue gas. this process typically occurs at higher temperatures (between 900 and 1100°C).
Compared to the SCR method, SNCR is simpler and more cost-effective in terms of complexity and expense. However, it has a lower efficiency, typically reducing NOx emissions by 40-70%. The selective non-catalytic reduction method works most effectively when the pollutant concentration is high, and the environmental temperature is favorable for the reduction reactions.
Raadman Burners: Innovative Solutions for Reducing Industrial Pollutants
Advanced technologies for reducing pollutant emissions have become a crucial necessity for industries today. Raadman burners, designed with the specialized Low-NOx series, contribute to lowering NOx emissions in the air. These burners utilize techniques such as staged combustion, pre-mixed combustion (including post-mixed burners and pre-mixed burners), and flue gas recirculation (FGR). In addition to reducing NOx and other pollutants, they ensure high efficiency in industrial thermal processes. By utilizing raadman burners, industries can not only reduce environmental impacts and meet global standards but also take steps toward energy efficiency and sustainable development. These burners provide an effective and economical response for a greener, more responsible industrial future.
