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Module 6: Air Pollutants and Control Techniques - Nitrogen Oxides - Control Techniques
Introduction
Combustion Modifications
Add-On Controls
Objective
- Describe the basic methods used to control nitrogen oxide emissions.
There are two primary control techniques: (1) combustion modifications to suppress the formation of nitrogen oxides, and (2) add-on controls to reduce nitrogen oxides to molecular nitrogen.
The purpose of combustion modifications is altering the conditions that contribute to the formation of both thermal and fuel NOx. Most of these techniques involve a reduction in the peak gas temperatures, a reduction in the oxygen concentrations in the high temperature areas of the burner flames, and/or a reduction in the residence time of combustion products in the high temperature areas of the burner flame. A partial list of the combustion modifications that have been used to reduce NOx formation is provided below:
- Low excess air operation
- Off-stoichiometric combustion
- Flue gas recirculation
- Natural gas reburning
Low excess air operation simply involves a reduction in the total quantity of air used in the combustion process. All combustion systems use slightly more air than theoretically needed to ensure complete combustion of the fuel. By reducing the excess air levels down to the lowest possible level, the oxygen concentrations in the high temperature zone of the combustion process can be minimized, thereby reducing NOx formation.
Off-stoichiometric combustion involves the mixing of the fuel and air in a way that reduces the peak gas temperatures and peak oxygen concentrations. Usually, a portion of the combustion flame is operated with very low oxygen levels (fuel rich) to allow a major portion of the fuel oxidation to occur under conditions where NOx formation is suppressed. Combustion is completed in the remaining portion of the flame and/or combustion chamber by providing the remainder of the oxygen needed for complete fuel oxidation. There are a variety of different approaches for achieving off-stoichiometric firing conditions. These methods include low NOx burners, overfire air (OFA), and burners-out-of service (BOOS). Low NOx burners control the mixing of fuel and air in a pattern that keeps the flame temperature low and dissipates the heat quickly (see Figure 2 below). OFA refers to operating the lower burners as fuel rich and placing air injection nozzles above the burners to complete the combustion process (see Figure 3 below). BOOS is performed by operating alternate burners in the combustion zone as fuel rich, air rich, or air only (see Figure 4 below).
Flue gas recirculation involves the return of combustion gases to the burner area of the boiler. The slightly cooled combustion gas from the boiler exit is mixed back with the burner flame to reduce the peak flame temperatures, thereby suppressing NOx formation. This approach requires a separate recirculation fan and duct system.
Fuel reburning involves the operation of the main burners in a boiler at very low excess air (fuel rich conditions). Between 10 to 20% of the total fuel is injected into the boiler through a series of ports. This creates fuel rich conditions across the entire combustion chamber. The partially oxidized compounds formed in the burner and reburn fuel injection area, which is located in the middle region of the boiler, are then oxidized completely in the upper region of the boiler. A series of overfire air ports are used in this upper region to provide all of the air needed for complete combustion.
These combustion modifications are usually capable of reducing nitrogen oxides levels 30 to 50% from the levels that would exist in less sophisticated combustion system designs. There are several practical limits to the combustion modifications. If the combustion conditions are altered too much, some partially oxidized organic compounds and carbon monoxide can form due to impaired oxidation conditions. Flame instability can occur from minimizing oxygen and temperature levels. Also, by operating at fuel-rich conditions, the combustion processes can become vulnerable to operating problems.
Due to the limitations of combustion modifications, add-on control systems are being developed to decrease nitrogen oxide emissions below the levels possible by means of combustion modifications alone. There are two categories of add-on control systems that are applicable to boilers and other combustion processes.
- Selective Non-Catalytic Reduction (SNCR)
- Selective Catalytic Reduction (SCR)
Both types of systems inject ammonia or urea into the gas stream to reduce nitrogen oxides to molecular nitrogen and water.
Selective Non-Catalytic Reduction
In selective non-catalytic reduction systems, the ammonia (NH3) or urea is injected into a very hot gas zone where thermal reactions leading to the chemical reduction of nitrogen oxides can occur.
These reactions are completed within the boiler, and no waste products are generated. However, if the ammonia or urea is injected into an area that is too cold, the reduction efficiency of nitrogen oxides is low, and some of the reducing gas (NH3) can be emitted to the atmosphere. SCNR systems are capable of reducing nitrogen oxides from 20 to 60%. A conventional SCNR system is shown in Figure 6.
In selective catalytic reduction, beds containing ammonia or urea reduce nitrogen oxides to molecular nitrogen and water. The catalysts are usually composed of tungsten and vanadium deposited through a substrate that is extruded into a honeycomb arrangement. The gas stream passes through the channels in the honeycomb. There are usually two or three separate catalyst beds in series. NOx reduction efficiencies ranging from 75 to 90% are possible when the following conditions are met:
- The amount of catalyst is sufficient
- The catalyst is in good condition
- The ammonia reagent flow is sufficient
- The ammonia is adequately distributed across the gas stream
A conventional SCR system is shown in Figure 7. SCR systems are now being used for numerous gas turbines and a growing number of coal- and oil-fired boilers.
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#1
- Would wet scrubbers be a good control technique for NOx emissions? Please explain.
Practice Problems
Nitrogen Oxides - Control Techniques
- Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.