I.- Ping Chung, PhD, is a senior development engineer in the Technology and Commercial Development Group at
3.3 Techniques for Reducing NOx Formation Two molecules, nitrogen oxide (NO) and nitrogen
dioxide (NO2) together constitute the definition of NOx. During the combustion process, most of the NOx formed will be NO. In the atmosphere, the NO is then oxidized to NO2 in the presence of sunlight and certain hydrocarbons. When computing NOx emissions on a weight basis, all the NOx is considered as NO2. When measured out of a combustion chamber, the instru- ment should measure both NO2 and NO as parts per million dry volume (ppmdv).
As described in Volume 1, Chapter 15, there are three generally recognized significant sources of NOx emis- sions formed during the combustion process:
• Thermal NOx, formation which was first described by Zeldovich,7 comes from oxida- tion of atmospheric nitrogen (N2) to NO inside the areas of the flame where temperatures are developed in excess of about 2600°F (1400°C).
• Prompt NOx experimentally discovered first by Fenimore8 is also the result of atmospheric nitrogen oxidation formed predominantly in the initial parts of the flame via reaction with fuel radicals where local stoichiometric condi- tions are fuel rich.
• NOx originated from oxidation of nitrogen present in the fuel in several forms other than N2 is also called fuel bound nitrogen (FBN).
An approximate theoretical relation for thermal-formed NO as a function of temperature is shown in Figure 3.1.
The chart takes into account only the forward-going reactions of nitrogen oxidation that in boilers are almost always the case. It shows about 10 times increase in the NO formation rate with about 220°F (∼120°C) increase in temperature. However, this steep increase in the NO with temperature never directly translates to the rela- tion between the boiler effluent NOx and the peak theo- retical flame temperature, also called the adiabatic flame temperature (AFT). The explanation of this is that the temperatures close to the AFT occur in the flame in only small areas at low residence times due to heat transfer from the flame to the surrounding lower-temperature gas and combustion chamber walls and the rate of heat dissipation from these areas is greatly accelerated with the temperature. Furthermore, some experimental NOx data presented in Section 3.3.4 indicate a much lower dependence of NO formation of with temperature that is difficult to explain on the basis of known kinetic reactions.
If uncontrolled, the concentration of thermally pro- duced NOx in medium-size boilers can reach the level of a few hundred parts per million (ppm) and higher if preheated air or oxygen enriched air is used. In large boilers, the uncontrolled NOx could be much higher reaching 500–1200 ppm level and more.9,10
When burning conventional fossil fuels, the produc- tion of prompt NOx is typically considered in the range of 10–15 ppm. This number, however, is very approxi- mate as the formation of prompt NOx also depends on the peak flame temperature and there is no easy experimental way to separate it from the thermal NOx and produce data for validation of theoretical models.
The control of prompt NOx is only considered after the
thermal NOx is already reduced to a very low level.
Utilizing lean premixed burners accomplishes control of both thermal and prompt NOx.
FBN is mostly a concern in fuel oils and biofuels.
Gaseous fuels on occasion may contain ammonia or other elemental nitrogen compounds that at medium temperatures readily oxidize to NOx. The percent con- version of FBN to NOx depends on many factors: spe- cific molecules with nitrogen-carrying radicals, rate of release of species with nitrogen into the gas phase, and amount of FBN in the fuel. A conversion rate of 100% of FBN present in fuel oil in the amount of 0.1% by weight would result in about 130 ppm of NOx in a low-oxygen exhaust. Number 2 Oil typically contains from 0.01%
to 0.05% of FBN11 with the rate of conversion to NOx of 40%–75%. Heavy oil usually contains between 0.3% and 0.6% of FBN with a conversion to NOx in the range of 30%–50%. The operation of the burners with minimum overall excess air and/or using air staging explained in Section 3.3.2 reduces the conversion of FBN to NOx.
Extensive literature exists on the subject of NOx for- mation and control for those who want to study deeper into the subject.
All practical ways of reducing NOx formation dur- ing combustion that are described as follows complicate the combustion process and make it more susceptible to increases in other emissions (CO, UHC, PM). So the art of delivering lowest NOx combustion often becomes a bal- ancing act of designing the process to lower NOx with minimum trade-offs to the increase of other pollutants.
3.3.1 Flue-gas recirculation and injection of Steam into the Flame
The peak flame temperature and thus thermal NOx can be effectively reduced when the initial reactants—fuel or combustion air—are mixed with non-reacting gases like atmospheric nitrogen, carbon dioxide, or water vapor
introduced into the combustion zone at temperatures sub- stantially below the temperature of the flame. As all the aforementioned inerts for the combustion gases are pres- ent in the combustion products, the recirculation of cooled flue gas back to the burner (flue-gas recirculation [FGR]) becomes a powerful way of controlling thermal NOx.
The quantity of FGR is usually defined as the ratio of the mass of recirculating FGR—Mflue gas—coming from the same combustion process to the mass of combustion products exiting the stack Mfgr:
FGR fgr
flue gas
= M
M (3.1)
A system designer needs to be careful in sizing the FGR delivery system if the outgoing flue gas is diluted with combustion air (e.g., leakages in the air heaters) or taken from another process that may produce flue gas with a different composition.
As a generic heat balance equations will show, the recirculation of a typical flue gas in the amount of 10%
results in 6%–8% reduction in the theoretical peak flame temperature (AFT). Relations between NOx and FGR for several specific cases when firing fuel without FBN are shown on Figure 3.2.12
The same effect can also be achieved when steam or finely dispersed water is injected directly into the flame, or mixed with combustion air or fuel. As FGR is almost always available, the monetary penalty of using FGR for NOx control is mostly associated with the cost of mechanically moving FGR back to the burner and is usually insignificant compared to a change in any heat losses with the exhaust due to a slight increase in out- going flue-gas temperature. For retrofit applications, the increase in stack temperature and thermal losses is on the order of 10°F (6°C) per 10% of FGR depend- ing on the design of the boiler convection section. This rule of thumb was determined by observations and
1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05
2500 2600 2700 2800 2900 3000 3100 3200 3300 3400 3500
Formation rate of NO at 2% O2 (vol)
NO, ppm /s
Temperature, °F Figure 3.1
Approximate rate of thermal (Zeldovich) NO formation.
calculations of the heat transfer inside the furnace and the convection section. For comparison, a 10°F (6°C) increase in stack temperature reduces boiler thermal efficiency by about 0.25%.
The extent that thermal NOx can be controlled with FGR strongly depends on the ability of the burner to remain stable and provide sufficient completeness of combustion with the rate of combustion reactions reduced due to reduced temperatures. This issue will be discussed further in this chapter for specific types of burners.
The addition of FGR to the combustion air prior to the forced draft fan demands a substantial increase in the fan capacity. The increase depends on whether the boiler and the burner were originally designed for oper- ation with FGR, Case 1, or the FGR is a retrofit addition to the system, Case 2.
Using fan laws (see Volume 2, Chapter 3), it is easy to estimate factors for the fan power increase prior to making the selection of the specific equipment or determining if the existing fans will work. In the first case, the power of the fan for the same boiler capacity needs to be increased at least about linearly with the amount of FGR as defined by the approximate ratio shown in Equation 3.2:
P P
T T
fan fan
fgr amb
FGR 1 1 1 8 FGR 1
0 = +
. . * %
% *
0 00 (3.2)
In Case 2, the power increase factor with FGR is much more substantial:
P P
T T
n fan
fan
fgr amb
FGR 1 1 1 8 FGR 1
0 = + +
0 0
. . * %00
% * (3.3)
where the power factor n varies between 2.3 and 2.8 depending on the relative hydraulic resistance of the burner to the boiler convection section with n being lower for boilers with convection section pressure drops much lower than the burner resistance. In the afore- mentioned equations, the temperatures of FGR and air passing through the fan—Tfgr and Tamb—are absolute temperatures.
The equations given above are rough references and approximately account for the increased pressure drop across the burner and the boiler convection section as well as additional pressure losses for inducing the flue gas. For any particular application, the requirements of the fan characteristics can be accurately determined based on fundamental considerations of the pressure drop characteristics of the boiler and selected burner.
FGR can also be added to the combustion air using a separate FGR fan. This “forced FGR” can be mixed with air with the use of a custom-designed mixing device or delivered to the burner exiting cross section through a plenum with ports. In the latter case, the FGR can be strategically directed to the parts of the flame where it is most effective. The method is also called selective FGR—the term that is contrary to the forced or induced FGR uniformly (or in bulk) mixed with combustion air.
There are pro and cons with either way of introducing the FGR. In retrofits, the forced FGR may allow a better chance of reusing the existing combustion air fan, but would require a more complicated FGR ducting arrange- ment and more complicated combustion controls (see also Volume 2, Chapter 2). Some burners, described later in this chapter, are also better suited for operation with forced FGR if delivered directly to the burner. Utilizing induced FGR, consideration must be given to additional water and sulfuric acid condensation and corrosion problems that are better contained when delivering forced FGR to the burner. The benefit of induced FGR is its simplicity.
In some cases, the flue gas can be mixed with a gas- eous fuel. For a given percentage of FGR, this method potentially achieves somewhat better NOx reduction than the FGR mixed with combustion air. This can be explained by concentrating the FGR in the high- temperature combustion zone rather than spending it partially on the dilution of the excess combustion air initially bypassing the combustion zone. Except for a few specific cases, the technique is difficult to use.
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%0% 10% 20% 30%
FGR %
NOx reduction
Power boilers Package boilers Boilers with high air preheat Figure 3.2
Approximate relation of NOx reduction with FGR for fuels without FBN. (Data for power boilers are per Rawdon, A.H. and Johnson, S.A., Application of NOx control technology to power boilers, Presented at the American Power Conference, May 10, 1973; other data are from several COEN applications.)
It requires specially designed burners and also complicates the controls and safety system.
A small amount of FGR can be mixed inside the burner with a gaseous fuel delivered at high pressure using the fuel gas mechanical energy to aspirate the flue gases from the boiler exhaust and pass the fuel/
FGR blend through the fuel injectors, also called fuel- induced recirculation (FIR). FIR works if the burner is designed for the high fuel supply pressure usually in excess of 20 psig (1.3 barg) and with burners designed for low-pressure injectors of the fuel–flue gas blend into the combustion zone. In other cases, an FGR fan with substantial static pressure is needed. The levels of NOx reduction with this technique may be quite sub- stantial—up to about 80%–85%.
Contrary to the low operating cost of using flue gas to control NOx, the use of steam or water injection for NOx control carries substantial penalties for the efficiency of the boiler or combustion system. The amount of steam that is equivalent to 10% FGR would be equal to about 4.5% of the boiler steam output—an unacceptable loss of the system efficiency as well as capacity if the produced steam is used. In some cases, a low-pressure waste steam is available and can be used for NOx control with mini- mal capital costs and operating cost as waste steam has minimum impact on the efficiency. It may also be justifi- able to use steam when it is reserved for some infrequent occasions of peak power production in a very tight reg- ulatory environment. Some modern burners can utilize steam much more effectively by using selective injection points rather than just injecting it into the air with an increase of its effectiveness by a factor of two or more.
However, this method usually involves modifications to the burner negating to some degree the advantage of its low capital costs.
Water injection into the flame is even more problem- atic than the injection of steam. First of all, water needs to be very finely atomized to below 100 μm diameter particles to be effective, and that requires using com- pressed air and special atomizing nozzles. Secondly, its injection has to be properly distributed throughout the flame body, but in a way not to impact flame stability.
Lastly, all this water in the flame will be converted to water vapor again substantially degrading the overall efficiency of the process on the order of ∼50% to 75% of the efficiency loss with the injection of produced steam.
The ultimate ability of mixing air with inerts for com- bustion gases depends on the burner. Conventional burners designed for the use of FGR can utilize up to 20% FGR when firing natural gas and a somewhat lesser amount up to ∼15% when firing oil and achieve anywhere from 40% to 75% reduction in NOx when compared with the baseline (no FGR) level. As will be discussed further in this chapter, advanced burner designs can utilize much more FGR.
3.3.2 air Staging
Thermal NOx may also be reduced by burning fuel under local conditions substantially different from stoi- chiometric such as lean and rich when combustion does not generate high NOx-forming temperatures.
Using conventional combustion equipment, the fuel burning typically starts after engaging only a partial amount of air well below the stoichiometric amount.
The balance of the air is added to the combustion after the products of combustion lose a substantial amount of heat. Thus, the initial rich section of the flame and the lean final part of the flame are at lower temperatures due to prior heat transfer from the rich section. The result is a reduction in the peak flame temperature over the entire flame zone and a reduction in thermal NOx.
When the burner is designed to enhance this described effect, the technique is often called air staging. For the concept to work, it is essential for the products of the ini- tial rich flame to lose a substantial amount of heat before the rest of the air being introduced.
When firing fuels containing FBN, the use of air stag- ing also helps to reduce conversion of FBN to NOx as the release of fuel nitrogen is taking place in a low-oxygen environment. The use of air-staged combustion usually generates flames with increased volume. Air-staging implementation and potential effectiveness depends on the space available for the fuel oxidation to be com- pleted. Possible problems include increased CO and UHC emissions and potential flame stability problems as the burner becomes more susceptible to the devia- tions from optimum operating conditions.
Practically, air staging can be achieved with redis- tribution of the fuel within a burner by creating a very coarse mixing pattern using fewer fuel injectors, injecting fuel into the flame zone with low momen- tum or directing fuel to mix differently in specific flow streams. Multiple air passages and some means of con- trolling air distribution to those passages can also be used to achieve the desired effects of air staging. These techniques were the first embraced by early combus- tion technologies for controlling the NOx as it required relatively minor changes to the combustion systems.
On the air side, the staging can be achieved by impos- ing minimum intensity swirling motion (generating flows with low swirl numbers as defined by Ref. [5]) or by introducing a portion of the air through ports spaced away from the burner.
Air staging alone can typically reduce the NOx by 20%–60%. Any deeper reduction may become prob- lematic and not reliable. Air-staged burners are usu- ally poor candidates for operation with FGR as the combination of fuel-rich conditions with reduced oxy- gen results in marginal flammability for most hydro- carbon fuels.
3.3.3 Fuel Staging
Fuel staging requires injecting a portion of the fuel away from or completely out of the flow of the combus- tion air. This portion of the fuel is termed staged fuel or a secondary fuel, while the other portion is referred to as primary fuel. For the fuel-staging technique to be effective, the secondary fuel needs to enter the com- bustion zone at some distance from the burner exit cross section after the flame created by the primary fuel loses a substantial amount of heat by radiation and convection to the boiler water walls. The staged fuel may also be injected in a way to bring into the com- bustion zone products of combustion from the furnace volume surrounding the flame. When this volume con- tains low-oxygen gas at a temperature much lower than the flame, the effect of the furnace gas re-entrainment into the flame on the NOx will be somewhat similar to the use of selective FGR.
Burners with fuel staging are quite different from conventional burners and burners with air staging as fuel-staged burners are equipped with fuel injectors external to the combustion air passage. Fuel-staged burners, if improperly designed, may be susceptible to partial loss of flame and combustion instabilities as the location of the flame front and ignition areas for the staged fuel may be shifting by the highly turbulent combustion process. Fuel-staged burners can reduce NOx emissions by up to 50% and can be designed to accept moderate amounts of FGR for a deeper NOx reduction up to ∼80% overall.
3.3.4 lean Premixed Combustion
Lean premixed combustion applied to natural gas can reduce prompt NOx to below 2 ppm. With high excess air or FGR, the technique is capable of reducing thermal NOx almost to the level of prompt NOx formation. Lean pre- mixing technology was developed to reduce prompt NOx and operate with higher levels of FGR at the same time.
The simple delivery of combustion air to the burner in excess of the stoichiometric amount alone does not make the combustion process fuel lean. As the fuel is injected into the air flow, the mixing process always generates fuel-rich mixtures first, and that is where combustion easily starts with any source of ignition.
In order to achieve fuel oxidation under lean condi- tions throughout the flame, the fuel and the air need to be premixed to a fuel lean blend prior to ignition.
Premixing of the fuel and air upstream of the burner for industrial size burners would require large volumes of flammable mixture that would be very difficult to han- dle with adequate safety and would present significant issues with respect to controlling any leakages. So, in large burners, the premixing needs to be accomplished by the burner. To differentiate large premix burners
from small burners where premixing may take place in the duct or a pipe upstream of the burner, large premix burners are designed differently and are sometimes called simulated premixed combustion burners.
In order to achieve the benefits of thermal NOx reduc- tion by premixed combustion over the alternative ways of controlling NOx, the mixture of fuel and air requires substantial excess air and/or inert gas like FGR. When firing natural gas, for example, the substantial benefits of premixed combustion on the NOx are realized only if the excess air levels are over ∼40%. In combination with bulk mixed air and FGR, the excess air may be reduced to “normal” levels of 10%–15% or even less with full benefits of the lean premixed combustion.
Turbulent premixed flames are prone to generat- ing intense combustion instabilities with frequencies defined by the burner and the overall system. Only spe- cially designed burners with some of them described further in this chapter, along with accurate controls, allow satisfactory operation on a large industrial scale with significant turndown. The key for reducing premix flames instabilities is diffusing—stretching the flame front either with the increased velocity and turbulence of the flow or with the reduction of the oxidation kinet- ics with high amounts of excess air or flue gas. Some other techniques incorporated into the burner design may help to improve operation with desynchronizing the combustion instabilities developed in different parts of the flame.13
Premixed burners are typically designed for operation with NOx emissions below 15 ppm (corr. 3% stack O2, dry) with the most advanced boiler burners currently operating in selected equipment with sub 5 ppm NOx and low excess air. A typical relation between the rates of FGR, excess air, and NOx for a medium-size package boiler equipped with a premixed burner is shown on Figure 3.313 with a series of iso-NOx curves.
When lean premixed combustion is substantially diluted with FGR or excess air to the level below 10 ppm, the effect of the NOx formation beyond the flame zone becomes significant. Figure 3.4 shows NOx emis- sions measured when firing lean premixed burners with a substantial amount of FGR and about 20% excess air recorded during the commissioning of QLA burn- ers in 1999.14 The fuel in this case was natural gas and the combustion air was preheated to 425°F (218°C). The 60,000 lb/h (27,000 kg/h) CE boiler furnace was sized for about 1 s residence time and had ∼50% refractory coverage of the flame surrounding walls. The two burners were generating relatively short flames not reaching the furnace back wall. NOx, CO, O2, and com- bustible emissions were measured in proximity to the boiler back wall at locations about 11 ft (3 m) opposite to the burners and at the stack using well-calibrated and accurate instruments. The NOx was measured using a