Nox Reduction Methods

During the last century, Nitrogen oxide emissions, called NOx, have constantly been increasing. Due to the destructive effects of nitrogen oxides on the health of society and the environment, the emissions from different combustion resources in industrial countries have been evaluated. Various technologies were developed to control NOx and reduce emissions from combustion sources. The development of these technologies is dependent on the understanding of chemical reactions to Nitrogen oxides. The main parameters related to NOX formation reactions include the combustion temperature, oxidizer concentration, and the duration of its presence in the combustion zone with high temperature. Any change in these parameters leads to a decrease or increase in NOx formation. However, reducing the combustion temperature through water or steam injection, optimizing the geometry, and recirculating exit gases can be obtained.

Nitrogen is a relatively inert diatomic molecular gas that makes up about 79% of the air around us. However, Nitrogen as a single atom can be very reactive and have ionization levels from one to five. Therefore, this atom can form several different oxides. NOx family compounds and some prominent properties are gathered in the table below.

NOx emission from combustion is primarily in the form of NO. Based on the Zeldovich equations, NO is generated to the limit of oxygen available in the air at temperatures above 1300 °C. Below 760 °C, NO is produced in much lower concentrations or never. NO in the combustion process is generated as a function of the air-to-fuel ratio. The Zeldovich equations are as follows:

N + O → NO + N

N + O₂ → NO + O

N + OH → NO + H

As mentioned above, combustion processes with temperatures well below 1300 °C emit much smaller amounts of “thermal NOx.” Thermal NOx is controlled by nitrogen and oxygen concentration and combustion temperature. Another shape of NOx is “fuel NOx,” which emits from fuels that contain Nitrogen, like coal. “Prompt NOx” is formed from molecular Nitrogen in the air combined with fuel in fuel-rich conditions that exist, to some extent, in all combustion. This Nitrogen then oxidizes along with the fuel and becomes NOx during combustion, just like fuel NOx.

Technologies to control and abate NOx are relatively complex. This discussion tries to make a structure of NOx reduction and control methods by introducing the methods used. Then the more efficient pollution reduction and emission control strategies are described.

Almost all Combustion sources have a large amount of pollution, like NOx, in large amounts in the flue gas. All reduction strategies to remove or reduce NOx emission rate in the combustion process are based on one of the seven:

1. Reducing combustion temperature
2. Reducing residence time
3. Nitrogen removing
4. Chemical reduction of NOx
5. Oxidation of NOx
6. Make sorption
7. Combination of the above methods

Some of the well-known and efficient abatement and control technologies to destroy or remove pollution, especially NOx, are as follows:

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

Fuel change is one of the easiest ways to reduce pollution. For example, the combustion of light or heavy oil, which contain nitrogen compounds, causes to increase in fuel NOx. Natural gas (NG) usually has no or less amount of nitrogen molecules. Partial or complete replacement of light/heavy oil with NG (if there is no force needed to use liquid oil fuel) can significantly reduce NOx emission.

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 has been developed to reduce greenhouse gas emissions while maintaining high thermal efficiency in combustion systems. Among the prominent features of this type of combustion are the reduction of pollutants, homogeneous distribution of flame temperature, reduction of noise pollution, and reduction of thermal stresses. Flameless combustion cannot be seen without an armed eye. The high temperature in the combustion chamber wall and high radiation from it causes eye fatigue and the inability to see the flame.

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 NOx 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 NOx reduction methods is to prevent combustion efficiency from being reduced. Water injection into the flame is one of the ways of NOx 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.