**3. Results**

#### *3.1. Reaction and Heat Transfer in the Reference Case*

The key results for the reference case (C05-S20) are explained in this section to understand the flow, reaction, and heat transfer characteristics of the boiler before comparisons between the simulation cases varying the CCOFA and SOFA ratios. As shown in Figure 2, the flow and combustion patterns were characterized by the formation of a large swirling fireball across the burner zone which is typical for a TF boiler. The fireball was pushed to the center by the large momentum delivered by the jets from the burners on the corners, whereas it expanded towards the wall by the centrifugal force in the spacings between the burners B and C, and D and E. This created the crescent shapes of a high velocity region above 20 m/s in the vertical cross-section of Figure 2a. With the increase of flow rate from the burners a to E, the fireball size became larger. Above the SOFA, the fireball finally expanded, before the flow entered the tube bundles. The path-lines from the burner a in Figure 2b show that the flow from this burner filled the bottom section, and then moved upward through the center of the fireball. The jet flows from the other burners above joined the swirling flow of the fireball mostly at its outer part.

**Figure 2.** Velocity contours and path-line from burner a for the reference case (C05-S20) (**a**) Velocity and (**b**) Path-line.

Figure 3a shows the temperature on the selected cross-sections and heat flux on the wall. The bottom cone region was filled with the flow from burner a and was cooled down to a low temperature by heat transfer to the wall. It then entered the center of the fireball, which created the bell-shaped low-temperature (<1000 ◦C) region in the lower furnace. The temperature gradually increased by introduction of more coal along the height to form the high-temperature zone above 1500 ◦C, which stretched from the near-wall region of the burner D level to the center at the SOFA level. The cold SOFA jets at 326 ◦C caused a temperature drop by dilution although it delivered additional air for burnout of the remaining char and combustible gases. The temperature decreased rapidly above the primary SH which virtually stopped further gaseous or heterogeneous reactions. The wall heat flux shown in Figure 3b was large in the burner zone with a peak of 220.6 <sup>W</sup>/m<sup>2</sup> appearing on the burner D level which coincided with the temperature contours.

**Figure 3.** Temperature and wall heat flux for the reference case (C05-S20). (**a**) Temperature and (**b**) Wall heat flux.

Figure 4 shows the solid carbon concentration and the rates of char conversion reactions. The solid carbon in char particles had high concentrations (>0.01 kg/m3) close to the wall caused by the inertia of the swirling flow. In addition, some char particles originating from the lowermost burners filled the bottom cone and then entered the center of the fireball from below with insufficient char conversion. Because the burner zone was fuel-rich (SR of 0.847), the solid carbon remained at the top of the burner zone. The CCOFA and SOFA delivered the rest of the air for char oxidation. Although the overall SR became fuel-lean, solid carbon was present in the heat exchanger zone because the mixing by the OFA jets was not perfect. In particular, unburned char particles were present mostly on the corners of cross-section (1) with a concentration over 5 × 10−<sup>5</sup> kg/m<sup>3</sup> in Figure 4a. On cross-section (2), the carbon concentration decreased along the SOFA paths to below 2 × 10−<sup>6</sup> kg/m<sup>3</sup> by slow oxidation but the corners still had concentrated char particles. Above this section, the temperature was not high enough for further char reactions, which led to the UBC in fly ash. The contour on the vertical cross-section in Figure 4a appears to have an increased carbon concentration in the top furnace because the mixing of the particles from the corners with the gas flow slowly progressed. Therefore, the trajectories of OFA jets and its mixing with the char particles are important in reducing the UBC before they enter the heat exchangers.

**Figure 4.** Solid carbon concentration and char conversion rates for the reference (C05-S20). (**a**) Solid carbon concentration; (**b**) Char-O2 rate and (**c**) Char-CO2 rate.

As shown in Figure 4b, the char oxidation reactions were active in the outer part of the fireball where most char particles were present, and the combustion air was delivered. However, no oxidation occurred in the bottom cone, at the center of the fireball, and above the primary SH, because the temperature was not high enough and/or O2 was depleted. Because the temperature was high enough in the burner zone, the gasification by CO2 and H2O also made a significant contribution to the char conversion (Figure 4c). Integrating the reaction rates over the entire volume, the two gasification reactions accounted for 36.7% of char conversion and the remainder by oxidation.

Figure 5a shows that O2 delivered by the burner SA was consumed rapidly and was depleted in most of the burner zone. By contrast, O2 from the SOFA penetrated deeper, because in this case, the SOFA ratio was relatively large (20%). The excess O2 then approached the wall as the fireball expanded. O2 was fully consumed at the central region above the SOFA to have a very low solid carbon concentration. After the flow entered the heat exchanger zones, the fireball quickly disappeared, and the mixing slowly progressed. The CO mole fraction above 5% appeared along the path of coal particles in the burner zone, as shown in Figure 5b. In particular, its concentration was very high on the sidewalls of burners C and D. The excessively reducing atmosphere can increase the possibility of high-temperature corrosion of the wall. Without further fuel supply, the CO mole fraction decreased rapidly by the CCOFA injection and became below 0.25% on the SOFA level. On the platen SH, the CO concentration was 46 ppm.

**Figure 5.** Mole fractions of O2 and CO for the reference case (C05-S20). (**a**) O2 mole fraction and (**b**) CO mole fraction.

Figure 6 shows the reaction rate and concentration of NO. Along the trajectories of coal particles, the fast devolatilization with very high heating rate released the N-intermediates which were partially oxidized to produce NO under the oxygen-rich atmosphere with the PA and burner SA around. This was followed by char conversion releasing NO directly from char-N. This led to the regions of fast NO formation rates of over 1 × 10−<sup>6</sup> kmol/m3.s, which coincided with those of high concentrations of solid carbon (Figure 4a) and O2 (Figure 5a). Outside these regions, however, O2 was depleted and the reducing atmosphere caused the removal of NO (negative reaction rates). In particular, rapid reduction reactions (<sup>&</sup>lt;−1 × 10−<sup>6</sup> kmol/m3.s) took place by the remaining N-intermediates and by char with the already produced NO in the region around the coal jets where O2 was depleted. Also, the volume between the CCOFA and SOFA provided the spaces for further reduction reactions of NO, mainly by residual char. Integrating the reaction rates, the NO emission was dominated by the fuel NO mechanism, and the contribution of the thermal NO mechanism was only approximately 10%.

**Figure 6.** Reaction rate and concentration of NO for the reference case (C05-S20). (**a**) NO rate and (**b**) NO concentration.

Table 4 compares the design data and CFD results for the reference case. The difference of O2 concentration between by design and the CFD result was 0.02%, owing to the release of UBC in fly ash and bottom ash. The predicted NO concentration was below the guaranteed value. The predicted heat absorption on the furnace wall using the measured thermal resistance was very close to the design value with a deviation of 0.04 MWth. The heat absorption on the tube bundles calibrated using fconv and frad was reasonably close to the design values. These results also imply that the predicted temperature distribution along the furnace height would be reasonable. However, this study was for the proposed boiler retrofit and the modeling approach was not validated by experiments. Therefore, the focus was on the comparative evaluation of the key performance parameters between simulation cases.


**Table 4.** Comparison of design data and CFD results for the reference case.

#### *3.2. Influence of SOFA Ratios*

Figure 7 compares the mass-weighted average profiles of temperature, O2, solid carbon, and NO concentrations for various SOFA ratios at a fixed CCOFA ratio of 5% and total OFA ratio of 25% (case set #1 in Table 1). In the burner zone, the temperature and O2 profiles were spread with a deviation of approximately 150 ◦C and 1%, respectively, with an increase in the SOFA ratio and corresponding decrease in the burner zone SR. The trends were inverted at the SOFA level by fast oxidation reactions of combustible gas species. The temperature profile influenced the distribution of heat absorption between the wall and tube bundles, which will be presented later. The carbon concentration exhibited acute changes responding to the fuel air supply from each coal burner in the burner zone. On the

burner E, it varied from 0.00095 kg/m<sup>3</sup> (C05-S15) to 0.00175 kg/m<sup>3</sup> (C05-S35). With no further fuel supply above this level, it showed a continuous decrease to a value below 0.0001 kg/m3, with a final carbon conversion of 99.92% (C05-S15) and 99.80% (C05-S35). However, the differences between the cases in terms of UBC in fly ash were significant, which will be presented later.

**Figure 7.** Mass-weighted average profiles of gas temperature and concentrations of O2, solid carbon, and NO in the burner zone for different SOFA ratios (CCOFA ratio fixed at 5%). (**a**) Temperature; (**b**) O2; (**c**) Solid carbon and (**d**) NO.

In Figure 7d, the NO concentration sharply increased between the burners A–E, owing to the dominant contribution of fuel NO. Comparing the value between cases, both the thermal and fuel NO formations were suppressed at higher SOFA ratios having lower O2 concentration and temperature. In particular, NO reduction reactions were active in the region where O2 was locally depleted, as indicated by the decreases in the concentration between the burner levels of B and C, and D and E. On the burner F (standby) level where all the fuel has already been introduced, NO concentration was 128 ppm for C05-S35 and 251 ppm for C05-S15. However, between the volume between the F burner and the first SOFA ports, low SOFA ratios exhibited active NO reduction reactions which lowered the NO concentration to 181 ppm (−70 ppm from the value on the F level) for C05-S15. By contrast, NO concentration of C05-S35 on the first SOFA level was 110 ppm (−18 ppm). Then, the SOFA injection through the two port levels had an immediate dilution effect that further lowered the NO concentration to 151 ppm (−30 ppm) for C05-S15 and 93 ppm (−17 ppm) for C05-S35. Above the SOFA level, NO concentration was not significant. These results sugges<sup>t</sup> that a high level of air staging is effective for low NO concentration within the burner zone and, therefore, the OFA distribution between the CCOFA and SOFA would not be crucial. However, for a moderate level of air staging, securing the volume (i.e., time) for NO reduction reactions is essential by the installation of SOFA and increasing its ratio larger than the CCOFA ratio.

Although large SOFA ratios effectively reduced NOx emission, negative impacts were accompanied on the boiler performance. Figure 8 shows the NOx emission and UBC in fly ash at the boiler exit and FEGT. FEGT was determined from the average gas temperature entering the first tube bundle (primary SH). The UBC represents the boiler efficiency, while the FEGT is associated with the propensity of ash slagging on the tube bundles. NOx emission was reduced from 109.9 ppm to 73.2 ppm (a 33.4% reduction) on a 6% O2 basis by the increase in the SOFA ratio, but the FEGT was increased by 62.6 ◦C, while the UBC was more than tripled. Therefore, the use of SOFA ratio as high as 35% was not favorable.

**Figure 8.** Comparison of NOx emission, unburned carbon (UBC) in fly ash, and furnace exit gas temperature (FEGT) for different SOFA ratios (CCOFA ratio fixed at 5%).

Figure 9 compares the heat absorption by heat exchangers for different SOFA ratios, which was closely associated with the temperature profile shown in Figure 7. On increasing the SOFA ratio, the heat absorption by the furnace wall was decreased from 599 MWth for C05-S15 to 524 MWth for C05-S35 by the lower gas temperatures of the burner zone. Because the trend in the temperature was inverted above the SOFA level, the heat absorption in the tube bundles of SHs, RHs, and ECO increased from a total of 543 MWth for C05-S15 to 614 MWth for C05-S35. The resultant boiler efficiency was 87.61%–87.30% on an HHV basis, which is also plotted in Figure 9. Together with the lower boiler efficiency, the increase in the SHs and RHs at the larger SOFA ratios has an unfavorable impact on the boiler operation. This is because it increases the possibility of high steam temperature at the final SH and RH exits that requires more water spray by the attemperator to maintain the value below the limit.

**Figure 9.** Comparison of heat absorption by heat exchangers and boiler efficiency for different SOFA ratios (CCOFA ratio fixed at 5%).

One additional issue to assess is the influence on corrosion of the water wall. With the increase in the OFA ratio, the burner zone SR decreased from 0.90 (C05-S15) to 0.68 (C05-S35). The more reducing environment in the furnace increases the possibility of high-temperature corrosion by the presence of H2S, COS, and CO [33], which shortens the lifetime of the boiler. As summarized in Table 5, the wall of the burner zone was exposed to more CO and less O2 with a decrease in the burner zone SR. Here, the CO concentration can directly represent the degree of reducing atmosphere. Using a CO mole

fraction larger than 0.5% as the criterion of the strongly reducing atmosphere, the difference in each case was within 3% for the OFA ratio up to 30% (C05-S25). Above this value, the area increased rapidly. Therefore, an OFA ratio of 30% can be considered as the limit to avoid excessive corrosion, and 25% would be acceptable, considering the trade-off between NO emission and overall boiler performance.

**Table 5.** Average mole fractions of CO and O2 and the area with CO > 0.5% on the wall of the burner zone.


#### *3.3. Influence of Di*ff*erent Air Distributions between SOFA and CCOFA*

Figure 10 compares the profiles of gas temperature and concentrations narrowed to the region between burner E level and primary SH for different CCOFA/SOFA distributions with a total OFA ratio fixed at 25% (case set #2 in Table 1). Because the operation conditions of the burner zone were identical, the profiles below E level were the same as those of C05-S20 shown in Figure 7. High CCOFA ratios caused immediate temperature drops from approximately 1510 ◦C along the four ports, but the fresh air supply recovered the temperature more, by increased oxidation reactions above the CCOFA. It was followed by another temperature drop by the SOFA injection, but the temperature increase above the SOFA was noticeable only at higher SOFA ratios (C10-S15, C05-S20, and C00-S25). The solid carbon concentration above the CCOFA ports was higher for C00-S25 and C05-S20 due to the shortage of O2, but the rest of the cases had similar values. After the SOFA injection, C00-S25 exhibited more active decrease in solid carbon concentration but its rate was not as fast as those in the burner zone. This led to the highest UBC in the fly ash for this case.

**Figure 10.** Mass-weighted average profiles of gas temperature and concentrations of O2, solid carbon, and NO between E burner level and primary superheater for different CCOFA/SOFA distribution (total OFA ratio fixed at 25%). (**a**) Temperature; (**b**) O2; (**c**) Solid carbon and (**d**) NO.

In the NO profile, the CCOFA injection caused immediate decrease by dilution, splitting the values from 218 ppm on the lowermost CCOFA port level to a range between 174−210 ppm at the top CCOFA port level. The degree of NO reduction reaction in the volume between the CCOFA and SOFA depended on the O2 concentration, as indicated by the fact that the slope in the NO concentration was proportional to the SOFA ratio. This significantly reduced the di fference in the NO concentration before the SOFA level. Then, the trend was almost inverted after the SOFA injection by dilution, but the values of C05-S20 and C00-S25 were similar to that for C10-S15. The NO reduction reactions slowly continued until the gas entered the primary SH and the temperature decreased rapidly.

Figure 11 compares the key performance parameters for di fferent CCOFA/SOFA distributions in case set #2. The UBC content in fly ash, NO emission, and FEGT had favorable results by increasing the proportion of SOFA up to 15% for a total OFA ratio of 25%. Therefore, C10-S15 was the ideal case in this case set. Above this SOFA ratio, both NO emission and FEGT did not change noticeably. However, the UBC content increased rapidly. This was mainly because too strong jets of CCOFA or SOFA led to an ine fficient mixing between the char particles and fresh oxygen. This can be confirmed from the path-lines shown in Figure 12 for CCOFA for C25-S00 and SOFA for C10-S15 and C00-S25 drawn on the contours of solid carbon concentration. In the lowermost cross-section, the char particles were present mainly near the wall by the centrifugal force of the fireball. The strong jets by the largest CCOFA or SOFA ratios (C25-S00 and C00-S25, respectively) penetrated deeper into the furnace, and the solid carbon concentration decreased rapidly along their trajectories. However, a significant fraction of char particles escaped the cross-sections through the spaces not covered by the OFA trajectories. In contrast, the jets of CCOFA and SOFA in C10-S15 had a moderate momentum that supplied fresh oxygen to the region near the wall where char particles were more concentrated. This explains the low UBC in this case. The quenching e ffect by the OFA jets appeared not to be significant, because in all cases, the entrained char was e ffectively converted. The heat absorption pattern and boiler e fficiency were little influenced by the CCOFA/SOFA distribution, as shown in Figure 13.

**Figure 11.** Comparison of NOx emission, UBC in fly ash, and FEGT for di fferent CCOFA/SOFA distributions (total OFA ratio fixed at 25%).

**Figure 12.** Solid carbon concentration and path-lines of CCOFA (foe C25-S00) and SOFA (for C10-S15 and C00-S25).

**Figure 13.** Comparison of heat absorption by heat exchangers and boiler efficiency for different CCOFA/SOFA distributions (total OFA ratio fixed at 25%).

Figure 14 compares the key performance parameters for different CCOFA/SOFA distributions in case set #3 with a total OFA ratio fixed to 30%. The trends in the parameters were similar with those in case set #2 shown in Figure 11, but the NO emissions were lower whereas the FEGT was higher. From the results, C10-S20 could be considered ideal at 30% OFA ratio. Compared to C10-S15 (ideal in case set #2 at 25% OFA ratio), the NO emission (90 ppm) was lowered by 10 ppm whereas the FEGT (1320 ◦C) and the UBC content (0.39%) were 14 ◦C and 0.14% higher, respectively.

**Figure 14.** Comparison of NOx emission, UBC in fly ash, and FEGT for different CCOFA/SOFA distributions (total OFA ratio fixed at 30%).

Overall, the results indicate that the CCOFA/SOFA distribution for a fixed total OFA ratio can be optimized for improved boiler performance. Compared to the case with CCOFA only (C25-S00) before the retrofit of this boiler, the overall performance can be significantly enhanced in terms of the NOx emission, the FEGT, and the UBC content in fly ash. With the SOFA installation, cases C10-S15 and C10-S20 were ideal for the OFA ratios of 25% and 30%, respectively. The actual air distribution can be adjusted around these cases, depending on the fuel properties such as ash slagging propensity, fuel N and S contents, heating value, etc.
