NOx Emission Control: Strategies and Technologies for Reduction
NOx (Nitrogen Oxide) emissions have risen considerably over the last century, mainly due to industrial activities and the widespread use of fossil fuels for power generation, transportation, and industrial processes. These emissions have serious negative impacts on public health and the environment, contributing to respiratory illnesses, smog formation, acid rain, and the degradation of ecosystems. Therefore, reducing these emissions from various combustion sources has become a critical focus for industrialized nations.
Various technologies have been developed to control Nitrogen Oxides emissions, focusing on both preventing Nitrogen Oxides formation and removing NOx post-combustion. The development of these technologies relies heavily on an in-depth understanding of the chemical reactions involved in Nitrogen Oxiedes formation. The main parameters influencing Nitrogen Oxides formation include the combustion temperature, oxygen concentration, and residence time (the duration of gases staying in high-temperature combustion zones). By altering these parameters, this pollutant emissions can be significantly reduced.
| Formula | Name | Nitrogen Valence | Properties |
|---|---|---|---|
| N2O5 | dinitrogen pentoxide | 5 | white solid very water soluble decomposes in water |
| N2O4 NO2 | dinitrogen tetroxide nitrogen dioxide | 4 | red-brown gas very water soluble decomposes in water |
| N2O3 | dinitrogen trioxide | 3 | black solid water-soluble decomposes in water |
| N2O2 NO | dinitrogen dioxide nitric oxide | 2 | colorless gas slightly water soluble |
| N2O | nitrous oxide | 1 | colorless gas water-soluble |
Key Factors Influencing Nitrogen Oxides Formation
Nitrogen Oxides emissions occur predominantly through the following mechanisms:
Thermal NOx: Generated when nitrogen in the air reacts with oxygen at high temperatures, usually above 1300°C. This is the most common form of NOx in combustion systems.
Fuel NOx: Forms when nitrogen compounds present in fuels like coal, oil, or biomass are oxidized during combustion.
Prompt NOx: Produced through complex reactions between nitrogen and hydrocarbon radicals in fuel-rich combustion zones. This form is significant in systems where fuel-rich combustion occurs, such as in engines.
The Zeldovich mechanism describes the chemical reactions responsible for thermal NOx formation:
N + O → NO + N
N + O2 → NO + O
N + OH → NO + H
At lower combustion temperatures (below 760°C), NO formation is much less significant. However, reducing the combustion temperature, optimizing burner design, and implementing other mitigation strategies are essential for controlling NOx emissions.
NOx Reduction and Control Strategies
NOx control strategies fall into two broad categories: combustion modification and post-combustion treatments. These approaches are based on one or a combination of the following methods:
- Reducing Combustion Temperature
- Reducing Residence Time
- Removing Nitrogen from Fuels
- Chemical Reduction of NOx
- Oxidation of NOx
- Sorption Technologies
- Combination of the Above Methods
Below, we discuss several key technologies used for reducing NOx emissions, either during combustion or post-combustion.
NOx Reduction Technologies
1. Combustion Product Recirculation
Combustion product recirculation (CPR) from the combustion chamber stack is a process that returns Products of combustion (POCs) to the flame formation zone. At first, it seems that the CPR process will increase the NOx formation because of the direct relation of NOx Emission with temperature. It is noticeable that the Flue gas temperature is much lower than the flame temperature so CPR will reduce NOx production. The NOx formation diagram according to flame temperature has been shown below.
Optimizing the burner’s designed geometry in terms of aerodynamics is necessary to achieve a high fuel and air mixing rate. The goal is to prevent the formation of hot spots and create a homogeneous temperature in the flame so that while increasing the heat released at low flame temperature, the rate of NOx production will decrease. The standard methods of combustion products recirculation to the flame formation zone are Furnace Gas Recirculation (FuGR) and Flue Gas Recirculation (FGR) from the chamber stack. In the FGR method, according to the Figure, the products of combustion products recirculate from the stack to the burner.
In this process, a fan or device that can circulate the POCs inside the furnace or burner is needed. The burner must be designed to control the excess flow due to the return of POCs and the increase in the temperature of the reactants in the combustion process due to the return of hot gases.
In the FGR method, an additional fan is needed to extract the POCs from the stack to the burner. If the exhaust gas temperature is low enough, the burner fan can direct the combustion air and the flow of hot exhaust gases of the stack into the burner. This method is used in steam boiler burners where the exhaust gas temperature is generally much lower. One of the disadvantages of the FGR method is the need to insulate the passageways of the flow of hot gases exiting the stack, which leads to an increase in the dimensions of the burner. The internal components of the burner must also be able to withstand the high temperature of the recirculated flue gas.
In the FuGR method, a process in furnaces, the POCs inside the furnace are returned to the burner. Returned gases balance the flame temperature. This process is shown below.
In another method, the POCs from the furnace will be returned to the path built in the combustion burner head and, combined with the air entering the burner, will decrease the flame’s temperature.
2. Fuel Replacement
Replacing nitrogen-containing fuels with cleaner alternatives is one of the simplest methods to reduce NOx emissions. Fuels like coal and oil contain nitrogen compounds that directly contribute to fuel NOx. Natural gas (NG), on the other hand, contains little to no nitrogen, leading to much lower NOx emissions when burned.
For example, switching from heavy oil to natural gas in industrial burners or power plants can significantly reduce NOx emissions. In systems where complete fuel replacement isn’t feasible, blending fuels (such as natural gas with hydrogen or biogas) can also reduce emissions.
3. Oxidizer Replacement
Air is the most common oxidizer. Significant results in NOx reduction can be achieved using pure oxygen as a substitute for air. For example, in methane (CH4) combustion, if the air with 79% nitrogen on a volumetric scale is replaced with oxygen, NOx emission can be removed entirely from the process because there are no nitrogen molecules to produce NOx.
Usually, NOx is reduced by reducing the amount of Nitrogen in the process. However, using high pure oxygen instead of air has its problems, like high extraction cost, but with the reduction of inexpensive methods in the future to separate oxygen from the air, it’s possible to expand this method in industries.
4. Excess Air Percentage (EA%)
Increasing the amount of excess air before the stochiometric conditions (Fuel Rich Zone) increases the emission rate of NOx. With a further increase in the percentage of excess air, the NOx emission rate will decrease. There are two reasons for the increase of NOx in Fuel rich zone and its decrease at a higher level of EA. The first one, or the reason for the increase of NOx in the low levels of excess air, is that the reaction with oxygen is the priority in chemical reactions. The high flame temperature is the second reason for the increase in NOx near low levels of EA (close to stoichiometric conditions). The combination of available oxygen and high temperature leads to an increase in thermal NOx.
5. Flameless Combustion
Flameless combustion is a novel technique that reduces NOx emissions by achieving more uniform flame temperatures and eliminating the visible flame. In this method, combustion occurs in a distributed manner without localized high-temperature zones, significantly reducing thermal NOx formation.
Flameless combustion also reduces noise and thermal stress on equipment, making it an attractive option for industries where high thermal efficiency and low emissions are required.
6. Staging
Staging combustion is an effective way to reduce NOx. Some of the fuel, oxidizer, or both in the staging are added to the stage before the primary combustion. For example, it is possible to inject the amount of fuel in the primary and secondary stages, as a part of the total input fuel, into the flame formation zone and create a chemical balance in the presence of flame. This method causes the formation of a fuel-lean zone, which has a lower tendency to emission than stoichiometric conditions. The general stoichiometric conditions in this method are the same as a conventional burner. The peak temperature of the flame in fuel staging mode is much lower than the normal mode because the combustion process takes place discretely, while the heat is emitted simultaneously and continuously from the flame. The lower peak temperature in fuel staging helps to reduce NOx emissions. Fuel staging is one of the cost-effective ways to reduce NOx.
7. Water Injection
One of the essential points in emission reduction methods is to prevent combustion efficiency from being reduced. Water injection into the flame is one of the ways of Nitrogen Oxides reduction. In this case, the water absorbs the flame’s heat and directs some of the energy from the combustion along with the combustion products from the stack to the outside of the chamber. This method will reduce combustion efficiency. Another idea is to use steam. Using steam has many advantages compared to liquid water. The steam temperature is much higher than liquid water and includes the latent heat of vaporization, which is required to turn water into steam. When liquid water is injected into the combustion process, it can impose a large thermal load on the process because liquid water can absorb a large amount of energy before vaporization due to its high latent vaporization heat. The thermal efficiency in using water vapor is much more suitable than liquid water because it absorbs less energy than water, and as a result, it does not reduce thermal efficiency as much as liquid water. A nozzle is needed to spread the water evenly in the combustion gases if liquid water is used. No nozzle is needed if water vapor is used, and the steam easily mixes with the combustion gases, so mixing water vapor in combustion products is much easier. Another advantage of water injection is that the water flow rate is easily adjustable.
Effective Strategies for Controlling and Reducing Nitrogen Oxides Emissions
The emissions of Nitrogen Oxides are a significant environmental and health concern, especially as industrial and power generation demands continue to grow globally. Technologies to control and reduce NOx emissions have evolved, with a range of strategies available for both preventing NOx formation and removing it post-combustion.
Key strategies such as Combustion Product Recirculation, fuel replacement, excess air control, and flameless combustion offer effective ways to manage Nitrogen Oxides formation during the combustion process. Post-combustion treatments like Selective Catalytic Reduction (SCR), water injection, and staging further enhance emission control capabilities. By understanding the fundamental mechanisms of NOx formation, industries can implement the most effective combination of these technologies to achieve compliance with environmental regulations and reduce their impact on the environment.
