**1. Introduction**

NOx emission from coal-fired power plants has become a crucial issue in the power industry in Korea because of its contribution to the formation of secondary particulate matter of less than 2.5 μm (PM2.5) by photochemical reactions. Due to climate change in the Korean Peninsula and the increase of inbound pollutants, severe haze events have become frequent in recent years [1,2]. The contribution of secondary PM originating from coal-fired power plants is ye<sup>t</sup> to be clarified among the inbound and domestic sources. However, air quality concern has a negative impact on the public perception of coal power and has changed energy policy including the temporary shutdown of old plants, as well as more stringent emission regulations. Although advanced combustion technology and efficient gas cleaning equipment are already in use, further lowering the pollutant emission from existing power plants to the minimum has become a most urgen<sup>t</sup> issue. This applies to existing power plants, which include 20 units of 500 MWe tangential-firing (TF) boilers in Korea that have been built since 1993 with an identical design and are referred to as the 'standard' 500 MWe unit.

NOx emission reduction has been a key topic of combustion and gas cleaning technology. As the primary measure, the formation of NOx needs to be minimized by a combination of air staging, fuel staging, and low-NOx burners [3]. NOx can then be removed by secondary measures such as

selective catalytic or non-catalytic reduction. In particular, NOx emission from coal is dominated by the fuel NOx mechanism, and therefore optimizing the reaction stoichiometry by air staging is very effective for its reduction. One advanced technique of air staging is the use of separated overfire air (SOFA) injected distantly above the burner zone. It is di fferentiated from close-coupled OFA (CCOFA) which is injected immediately above the top burners in a TF boiler. Adjusting the CCOFA ratio can be helpful in reducing NOx [4], but multi-level air staging by the use of SOFA can provide increased retention time for the reduction reactions to be more e ffective. The SOFA technique can also be applied to the 20 standard TF boilers in Korea by retrofit.

Together with NO emission, furnace exit gas temperature (FEGT) and unburned carbon (UBC) in ash are the key performance parameters of boiler operation associated with the operability and efficiency [5]. Various studies of the optimization of SOFA to improve boiler performance have been reported in the literature based on experiments and/or computational fluid dynamics (CFD). Increasing the SOFA ratio and optimizing its detailed distribution were found to be e ffective in reducing NOx emission for various boiler types [6–13]. However, the change in reaction stoichiometry can also significantly alter the combustion and heat transfer characteristics. In the study of Zha et al. [9] in a 600 MWe TF boiler, increasing the SOFA ratio from 10% to 40% achieved 50% reduction in NOx emission with more uniform heat flux distribution. However, this accompanied an unfavorable increase in the UBC in the fly ash, in the CO concentration, and in the flue gas temperature at the platen superheater, together with a significant change in the heat absorption pattern between the wall and convective heat exchangers. Li et al. [14] performed experiments on various damper openings of secondary air (SA) in the burner zone, CCOFA, and SOFA for a retrofitted 300 MWe TF boiler, and found that an ideal setup reduced the NOx emission by 44% with the UBC in ash not influenced at a sacrifice in boiler e fficiency of 0.21%. The aerodynamics of SOFA also has a large influence on the UBC and heat absorption pattern in the heat exchangers downstream and adjusting the yaw and tilt angles of SOFA can alleviate such problems [15–17]. The reducing atmosphere in the burner zone by increasing the SOFA ratio may increase the possibility of fireside corrosion by H2S. In this respect, the experimental study of Xu et al. [18] reported that the air distribution can be adjusted to reach a balance between low corrosion, low NOx emission, and high boiler e fficiency.

This study was to optimize the flow rate distribution of CCOFA and SOFA for NOx reduction in the standard 500 MWe TF boiler to be retrofitted including the installation of SOFA. Di fferent ratios of flow rates between the burner secondary air, CCOFA, and SOFA were simulated. The CFD method was validated using the design data for the reference case of the retrofit boiler. From the results, the boiler performance was evaluated to determine the ideal flow ratios, and to understand the reasons for the di fferences in terms of NOx emission, UBC in ash, furnace exit gas temperature (FEGT), heat transfer rates, boiler e fficiency, and the possibility of high-temperature corrosion.

#### **2. Target Boilers and Numerical Methods**

#### *2.1. Target Boiler and Operation Conditions*

Figure 1 shows a schematic of the 500 MWe coal-fired TF boiler modified from its original design. The modification was intended to adjust the operation range to low-rank coals and to improve its efficiency with an increased steam temperature from 538 ◦C to 596 ◦C. Because the original boiler only had CCOFA for air staging, SOFA was to be installed for the e fficient reduction of NOx emission. This retrofit was also expected to extend its lifetime by ten years. The burner zone had six levels of coal burners (A to F levels) installed on the corners to create a swirling flow (fireball) at the center. It was divided into three sections with two burners each having an identical arrangemen<sup>t</sup> of coal and air supply ports as illustrated in the figure. Each coal burner aerodynamically split the coal and primary air flow into concentrated and weak ports depending on the coal concentration. The CCOFA was injected through four ports immediately above the top burner F on each corner. In the retrofit design, the SOFA was located 6.144 m above the CCOFA with two ports on each corner and one each on the adjacent side walls. The convective heat exchangers in the upper part had a new arrangemen<sup>t</sup> consisting of superheaters (SH) and reheaters (RH) with different tube bank geometry and steam conditions. An economizer (ECO) was located in the backpass. The furnace was built with membrane tube walls that act as an evaporator of the preheated water from the ECO. The new steam pressure and temperature leaving the final SH to the high-pressure turbine were 255.4 kg/cm2g and 596 ◦C, respectively.

**Figure 1.** Schematic of the 500 MWe coal-fired tangential-firing (TF) boiler.

Table 1 presents the characteristics of the performance coal and summary of the operating conditions. The coal for the retrofit boiler was sub-bituminous with a higher heating value (HHV) of 5600 kcal/kg which was significantly lower than that for the old design coal (6300 kcal/kg). At the nominal rate load, the coal throughput was 55.583 kg/s which delivered 1303 MWth of thermal input on an HHV basis. The coal was pulverized to an average particle size of 50 μm, and partially dried by hot primary air to have a moisture content of 8.6% and a mass flow rate of 50.494 kg/s. It was transported by the primary air (including the evaporated moisture) from the pulverizers to the burners a to E. The secondary air (SA) with a total flow rate of 355.1 kg/s at 326 ◦C was distributed into different ports of the burner SA, CCOFA, and SOFA. The overall excess air ratio was 12.94%.


**Table 1.** Operating conditions of the 500 MWe coal-fired boiler.

Table 2 summarizes the simulation cases for various distributions of CCOFA and SOFA. In the first set of cases, the SOFA ratio was increased from 15% to 35%, while fixing the CCOFA ratio at 5%. These values corresponded to the burner zone stoichiometric ratio (SR) of 0.904 to 0.678. In the second set, the distribution between CCOFA and SOFA was varied from 0% to 25% by 5% increments, while fixing the total OFA ratio at 25%. Each case was named after the ratio in the two OFA ports. For example, C05-S20 refers to the case with 5% CCOFA ratio and 20% SOFA ratio. This is the reference case to be used for validation of the CFD results by comparison with the design data.


**Table 2.** Cases of various close-coupled overfire air (CCOFA) and separated overfire air (SOFA) ratios for computational fluid dynamics (CFD) simulations.
