Info

Source: EPA, Technical Bulletin, Nitrogen Oxides (NOx): Why and How They Are Controlled, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, U.S. Government Printing Office, Washington, D.C., November 1999.

flue gas, and particulate control device. Many of these methods can be combined to achieve a lower NOz concentration than can be achieved alone by any one method. In some cases, technologies that are used to control other pollutants, such as SO2, can also reduce NOz.

Combustion Modifications Primary NOz control technologies involve modifying the combustion process. Several technologies have been developed and applied commercially and include:

• Furnace air staging;

• Flue gas recirculation;

• Process optimization.

Options to control NOz during combustion and their effects are different for new and existing boilers. For new boilers, combustion modifications are easily made during construction, whereas for existing boilers viable alternatives are more limited. Modifications can be complicated, and unforeseen problems may arise. When combustion modifications are made, it is important to avoid adverse impacts on boiler operation and the formation of other pollutants such as N2O or CO. Issues pertaining to low-NOz operation include:

• Safe operation (e.g., stable ignition over the desired load range);

• Reliable operation to prevent corrosion, erosion, deposition, and uniform heating of the tubes;

• Complete combustion to limit formation of other pollutants such as CO, polyorganic matter, or N2O;

• Minimal adverse impact on the flue gas cleaning equipment;

• Low maintenance costs.

Combustion modification technologies redistribute the fuel and air to slow mixing, reduce the availability of oxygen in the critical NOZ formation zones, and decrease the amount of fuel burned at peak flame temperatures. In addition, reburning chemically destroys the NOZ formed by hydrocarbon radicals during the combustion process. The commercially applied technologies are discussed in detail in the following sections. One technology listed in Table 6-8, low excess air (LEA), is the simplest of the combustion control strategies but is not discussed in detail here because it has achieved only limited success in coal-fired applications (i.e., 1-15%). In this technique, excess air levels are reduced until there are adverse impacts on CO formation and flame length and stability. Similarly, a technique known as burners out of service (BOOS) has met with limited success with coal and is not discussed in detail. In this technique, the fuel flow to the selected burner is stopped but airflow is maintained to create staged combustion in the furnace. The remaining burners operate fuel rich, which limits oxygen availability, lowers peak flame temperatures, and reduces NOx formation. The unreacted products combine with air from the burners out of service to complete burnout before exiting the furnace.

Low-NOx Burners Prior to concerns being raised regarding NO x emissions in the early 1970s, coal burners were designed to provide highly turbulent mixing and combustion at peak flame temperatures to ensure high combustion efficiency, a condition that is ideal for NOx formation [24]. In 1971, industry began developing low-NOx burners for coal-fired boilers with the promulgation of New Source Performance Standards (NSPSs). By the mid-1970s, low-NOx burners were being demonstrated, and commercial operation started in the late 1970s [25]. They have undergone considerable improvements in design spurred by the 1990 Clean Air Act Amendments Title IV, Phase II acid rain regulations and Title I ozone regulations [26,27]. The technology is well proven for NOx control in both wall- and tangentially-fired boilers and is commercially available; a significant number of them are installed worldwide.

Low-NOx burners work under the principle of staging the combustion air within the burner to reduce NOx formation. Rapid devolatilization of the coal particles occurs near the burner in a fuel-rich, oxygen-starved environment to produce NO. NOx formation is suppressed because oxygen molecules are not available to react with the nitrogen released from the coal and present in the air, and the flame temperature is reduced. Hydrocarbon radicals that are generated under the sub-stoichiometric conditions then reduce the NO that is formed to N2. The air required to complete the burnout of the coal is added after the primary combustion zone where the temperature is sufficiently low so that additional NOx formation is minimized.

Larger and more branched flames are produced by staging the air [21]. This flame structure limits coal and air mixing during the initial devolatiliza-tion stage while maximizing the release of volatiles from the coal. The more volatile nitrogen that is released with the volatiles and the longer the residence time in the fuel-rich zone, the lower the amount of fuel NO that is produced. An oxygen-rich layer is produced around the flame that aids in carbon burnout. An example of this concept is shown in Figure 6-11, which is a schematic of a low-NOx burner (i.e., Ahlstom Power's Radially Stratified Fuel Core burner) that illustrates a typical flowfield emanating from it [28]. A photograph of the burner, which is a 20 million Btu/hr prototype used for developmental work at Penn State prior to its commercialization and worldwide deployment, is shown in Figure 6-12, which depicts the various dampers and air scoops for channeling and controlling the quantity and degree of swirl of the various air streams [29].

Tangential Air Inlets . Primary • Secondary

Burnout Zone

FIGURE 6-11. Schematic diagram of Alstom's RSFC burner depicting flow fields. (From Patel, R. L. et al., Firing Micronized Coal with a Low NOj RSFC Burner in an Industrial Boiler Designed for Oil and Gas, in Proc. of the Thirteenth Annual International Pittsburgh Coal Conference, 1996.)

Burnout Zone ta o b n o b to o

FIGURE 6-11. Schematic diagram of Alstom's RSFC burner depicting flow fields. (From Patel, R. L. et al., Firing Micronized Coal with a Low NOj RSFC Burner in an Industrial Boiler Designed for Oil and Gas, in Proc. of the Thirteenth Annual International Pittsburgh Coal Conference, 1996.)

Gj 3

Secondary Air Inlet

Dampers for Controlling Amount of Air into Each Zone

Primary Air Inlet

Tertiary Air Inlet

Secondary Air Inlet

Tertiary Air Inlet

Dampers for Controlling Amount of Air into Each Zone

Primary Air Inlet

Dampers to

Control Tertiary Air Swirl Number

Dampers to

Control Tertiary Air Swirl Number

FIGURE 6-12. Photograph of the RSFC burner showing internal components.

Low NOZ burners are designed to accomplish the following [30]:

• Maximize the rate of volatiles evolution and total volatile yield from the fuel with the fuel nitrogen evolving in the reducing part of the flame;

• Provide an oxygen-deficient zone where the fuel nitrogen is evolved to minimize its conversion to NOZ but sufficient oxygen is available to maintain a stable flame;

• Optimize the residence time and temperature in the reducing zone to minimize conversion of the fuel nitrogen to NOZ;

• Maximize the char residence time under fuel-rich conditions to reduce the potential for NOZ formation from the nitrogen remaining in the char after devolatilization;

• Add sufficient air to complete combustion.

All low-NOz burners employ the air-staging principle, but the designs vary widely between manufacturers. All of the major boiler manufacturers have one or more versions of low-NOz burners employed in boilers throughout the world. Mitchell [21] reported that over 370 units worldwide were fitted with low-NOz burners at a total generating capacity of more than 125 GW prior to 1998. The number of units installing low-NOz burners has increased significantly, as the DOE reports that low-NOz burners are currently found on more than 75% of U.S. coal-fired power capacity [1]. This is significant, as the DOE reported that ~1030 coal-fired, steam-electric generators today have a nameplate capacity of 328 GW and produce more than 1515 billion kWh of electricity [3,31].

Low-NOZ burners, based on air-staging alone, are capable of achieving 30 to 60% NOZ reduction. In addition, they should perform in such a way that [30]:

• The overall combustion efficiency is not significantly reduced;

• Flame stability and turndown limits are not impaired;

• The flame has an oxidizing envelope to minimize the potential for high temperature corrosion at the furnace walls;

• Flame length is compatible with furnace dimensions;

• The performance should be acceptable for a wide range of coals.

The major concern with low-NOz burners is the potential for reducing combustion efficiency and thereby increasing the unburned carbon level in the fly ash. An increase in the unburned carbon level will lower the fly ash resistivity, which can reduce the efficiency of an ESP. In addition, it may also affect the sale of the ash. Some operating parameters that can be adjusted to mitigate the impact of the unburned carbon include [30]:

• Fire coal with high reactivity and high volatile matter content;

• Reduce the size of the coal particles;

• Balance coal distribution to the burners;

• Use advanced combustion control systems.

Furnace Air Staging One technique to stage combustion is to install secondary and even tertiary overfire air (OFA) ports above the main combustion zone. This is a well-proven, commercially-available technology for NOZ reduction at coal-fired power plants and is applicable to both walland tangentially-fired boilers [30]. When OFA is employed, 70 to 90% of the combustion air is supplied to the burners with the coal (i.e., primary air), and the balance is introduced to the furnace above the burners (i.e., overfire air). The primary air and coal produce a relatively low-temperature, oxygen-deficient, fuel-rich environment near the burner which reduces the formation of fuel-NOz. The overfire air is injected above the primary combustion zone to produce a relatively low-temperature secondary combustion zone that limits the formation of thermal NOZ.

Overfire air in combination with low NOz burners can reduce NOz emissions by 30 to 70%. Advanced OFA systems, such as separated over-fire air (SOFA), where the overfire air is introduced some distance above the burners, and close-coupled overfire air (CCOFA), where the overfire air nozzles are immediately above the burners, can achieve higher NOZ reduction efficiency [21]. Mitchell [21] reports that furnace air staging is used in ~300 pulverized coal-fired units with a total generating capacity of over 100 GW. A number of advanced overfire air systems are commercially available and designs vary among suppliers [30]. Furnace air staging can increase unburned carbon levels in the ash by 35 to 50%, with the degree of increase being dependent on the reactivity of the coal used [30]. In addition, operational problems can be experienced, including waterwall corrosion, changes in slagging and fouling patterns, and a loss in steam temperature.

Flue Gas Recirculation Flue gas recirculation (FGR) involves recirculating part of the flue gas back into the furnace or the burners to modify conditions in the combustion zone by lowering the peak flame temperature and reducing the oxygen concentration, thereby reducing thermal NOZ formation. FGR has been used commercially for many years at coal-fired units; however, unlike gas- and oil-fired boilers, which can achieve high NOZ reduction, coal-fired boilers typically realize less than 20% NOZ reduction due to a relatively low contribution of thermal NOZ to total NOZ. In conventional FGR applications, 20 to 30% of the flue gas is extracted from the boiler outlet duct upstream of the air heater (at ~570 to 750°F) and is mixed with the combustion air. This process reduces thermal NOZ formation without any significant effect on fuel NOZ. A major consideration of FGR is the impact on boiler thermal performance [30]. The reduced flame temperature lowers heat transfer, potentially limiting the maximum heating capacity of the unit, which results in a reduction in steam-generating capacity.

Fuel Staging (Reburn) Reburn is a comparatively new technology that combines the principles of air and fuel staging. In this technology, a reburn fuel (e.g., coal, oil, gas, orimulsion, biomass, coal-water mixtures) is used as a reducing agent to convert NOZ to N2. The process does not require modifications to the existing main combustion system and can be used on wall-, tangential-, and cyclone-fired boilers. Reburn is a combustion hardware modification in which the NOZ produced in the main combustion zone is reduced downstream in a second combustion zone (i.e., the reburn zone). This, in turn, is followed by a zone where overfire air is introduced to complete burnout. This is illustrated in Figure 6-13 [32].

In the primary combustion zone, the burners are operated at a reduced firing rate with low excess air (stoichiometry of 0.9 to 1.1) to produce lower fuel and thermal NOZ levels. The reburn fuel, which can be 10 to 30% of the total fuel input on a heat input basis, is injected above the main combustion zone to create a fuel-rich zone (stoichiometry of 0.85-0.95) [32]. In this zone, most of the NOZ reduction occurs, with hydrocarbon radicals formed

Reheater/ Superheater

Overfire Air

Reburn Fuel-Gas 10-30%

Primary Fuel-Coal 70-90%

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