Emmissions – Nitrogen Oxides Control

Posted on April 1, 2013 by
n1What Is NOx?
Nitrogen oxide (NOx) is a generic term that describes a group of gases that contain varying amounts of nitrogen and oxygen. NOx is formed at high temperatures during fossil fuel combustion. NOx emissions contribute to a variety of environmental problems, including smog, reduced visibility, acid rain and acidification of aquatic systems.
NOx is formed in furnaces in two ways:
  1. Excess air, above stoichiometric requirements, combines with nitrogen in the combustion air through high temperature thermal fixation of the nitrogen. This produces thermal NOx.
  2. Nitrogen that is chemically bound in coal combines with oxygen in the furnace to produce fuel derived NOx. Because there can be significant nitrogen in coal, there is a greater potential for NOx generation in coal-fired boilers.
The amount of NOx formed in a furnace depends on several factors:
  • Flame temperature
  • Nitrogen content of the fuel
  • Amount of excess air
  • Amount of turbulence
  • The residence time at a high temperature
An increase in any of these factors results in increased formation of NOx. One way to reduce boiler NOx emissions is to reduce any of these factors.

NOx Control Technologies

NOx control technologies can be grouped into two broad categories: combustion modifications and post-combustion processes.
Combustion modifications control the mixing of fuel and air to reduce one of more of the five factors listed above. For example, by reducing furnace temperature and initial turbulence, thermal NOx formation in the furnace is reduced.
Post-combustion control uses chemical processes in back-end equipment to convert the NOx formed in the furnace to inert nitrogen and water.
In addition to NOx control processes, sophisticated combustion control systems are being developed to further reduce NOx emissions while optimizing overall boiler performance.

Post-Combustion NOx Reduction – Selective Catalytic Reduction (SCR) Systems

Selective Catalytic Reduction (SCR) is the proven NOx control technology used for back-end reduction of NOx. It utilizes a catalyst to reduce NOx to nitrogen and water. An NOx reduction rate in excess of 90% can be achieved with a Selective Catalytic System
The SCR process operates at approximately 570-840 degrees F. In an SCR ammonia (NH3) is sprayed into the flue gas. The catalyst facilitates a chemical reaction between the NOx and ammonia to produce nitrogen and water.
A typical SCR system includes:
  • SCR reactor with catalyst
  • Ammonia delivery, storage, piping, vaporization and injection system
  • Soot blowers for cleaning the SCR reactor
  • Instrumentation and control system

NOx Reduction – SCR Design Considerations

The design of the catalyst determines the economics and effectiveness of the SCR. The SCR catalyst needs to be designed for the specific fuel being burned and the furnace conditions. A catalyst will be a combination of oxides of vanadium as the active catalyst, titanium as a catalyst dispersing and supporting agent, and tungsten to improve mechanical stability and reduce sulfur oxidation. The concentrations of these three catalyst materials is customized to meet the specific boiler requirements.
Some coals contain materials that can deactivate (poison) the catalyst. For example, arsenic is found in some coals and will cause localized deactivation of the SCR catalyst. Poison-resistant catalyst formulations use molybdenum oxide to capture and localize the poisons and thus prevent deactivation.
Structurally catalysts are manufactured as supported honeycomb extrudates (homogenous catalysts) or catalyst coatings on plates (non-homogenous catalysts).
For coal fired boilers one of three back-end configurations is used.
  1. High dust
  2. Low dust
  3. Tail end
The high dust configuration is the most common because it places the SCR in a location where the gas temperature needed for reduction of NOx is present. “High dust” name refers to the SCR reactor’s location between the economizer and the air preheater. In this configuration the reactor and catalyst is exposed to flyash and the chemical compounds present in the flue gas. These have the potential to degrade the catalyst mechanically and chemically. However, the SCR system can be designed to mitigate the mechanical and chemical impacts on the catalyst.
In the low dust configuration the SCR reactor is placed in a location after the ash has been removed from the flue gas. This is usually downstream from the electrostatic precipitator (ESP) or baghouse. This location is used when the amount of ash needs to be reduced before the flue gas enters the SCR to reduce the degrading effect of flyash on the catalyst. The disadvantage of this location is the lower temperature of the flue gas.
In the tail end configuration the SCR reactor is moved further downstream to the outlet of the flue gas desulfurization (FGD) unit. The tail end configuration results in a very low level of dust entering the SCR. However, because of the low flue gas temperature at this point, the flue gas must be reheated, using duct burners, to bring it up to the temperature at which the reduction of NOx will occur. The tail-end SCR configuration is only used when the amount and type of flue gas ash prevents either of the other two configurations from being used.

NOx Reduction – SCR Operation

The critical parameter that needs to be considered when operating a SCR system is the injection, distribution, and mixing of the ammonia in the flue gas. When these are optimized the level of ammonia slip will be reduced and the reduction of NOx maximized.
What is ammonia slip?
Ammonia skip is the amount of unreacted ammonia that passes through the SCR reactor. Not only is this ammonia wasted, ammonia can contribute to air heater pluggage due to the formation of ammonium bisulfates. In addition, flyash contaminated with ammonia has a lower market value. Ammonia slip of less than 5 ppm minimizes air preheater plugging and generally assures the marketability of the flyash.

Using Combustion Controls For NOx Reduction

NOx combustion controls involve controlling how the fuel and air enters the furnace, and may include recirculating some of the flue gas back to the furnace.
Without regard to how it is accomplished, all NOx combustion control techniques are aimed at reducing the flame temperature below the level at which NOx will form, and limiting the amount of excess air so that it is as close to stoichiometric as possible. If it were possible, having exactly the amount of air needed to burn the fuel with no excess air (stoichiometric conditions), would be ideal. However, because mixing is not perfect and each carbon atom can not be brought into contact with an oxygen atom under those conditions, more air than is required for combustion must be supplied to the furnace. This is called excess air. In addition to providing the air needed for complete combustion under imperfect mixing conditions, the excess air provides the oxygen that results in NOx.
Combustion control technologies used to reduce NOx formation include:
  • Wall Fired – Low-NOx Burners without Over Fire Air (OFA)
  • Wall Fired – Low-NOx Burners with OFA
  • Tangetially Fired – Low Nox Coal-And-Air Nozzles with Close Coupled OFA
  • Tangetially Fired – Low Nox Coal-And-Air Nozzles with Separated Coupled OFA
  • Tangetially Fired – Low Nox Coal-And-Air Nozzles with Close Coupled and Separated OFA

NOx Reduction – Combustion Control

Without regard to the technology, the objectives are the same: to control the flame shape and temperature to produce a flame larger in area that does not have high temperature zones that will produce NOx. When burners are used, each burner creates a flame that is controlled by the design of the burner. When tangential firing is used there is a single flame, and the the design and location of the fuel and air nozzles controls the flame.
Overall the objective is to have the initial combustion take place under sub-stoichiometric conditions. Having less air than is needed for combustion will result in unburned carbon leaving this area to be burned at a later time. This reduces the amount of heat generated in the initial combustion zone, reducing flame temperature.
Thorough mixing in the sub-stoichiometric area is also important. The second objective is to use all available oxygen for combustion. To accomplish this the secondary air flow, direction and spin is controlled to optimize mixing. In addition, the primary air/pulverized coal stream may also have controlled turbulence introduced as it enters the furnace, to help improve the flame characteristics.
Overfire air is introduced into the furnace to provide the oxygen needed for complete combustion. This results in the unburned carbon leaving the initial combustion zone being burned in a lower temperature zone in which NOx does not form. Idealy the sub-stoichiometric conditions for initial combustion keep the initial flame below NOx formation temperatures, and the stretched out flame in the over-fire “air-rich” zone keeps that area below the NOx formation temperature.
If the design of the firing system and overfire air is not sufficient to reduce NOx to desired levels, then gas recirculation may be used. This involves introducing a portion of the flue gas back into the lower furnace. This reduces the amount of available oxygen and reduces flame temperature, resulting in a reduction in NOx.

Labeling NOx Reduction Systems

Labels, signs and tags are used throughout NOx reduction systems. The SCR piping, valves, dampers, actuators, wiring, control system, instruments and transmitters should all be identified with labels and tags. In addition, color coding of labels and tags may be appropriate to readily identify SCR system components.
Components of NOx combustion control systems should also be clearly marked and identified. This includes dampers, vanes, actuators, instrumentation, and components related to gas recirculation.
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