Parametric Study of NOx Reduction by Reburning in a Carbon Monoxide Boiler

4th International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'214) Jan. 28-29, 214 Bangkok (Thailand) Parametric Study of NOx Reduction by Reburning in a Carbon Monoxide Boiler Chun-Lang Yeh Abstract In this study, NOx reduction by reburning in a carbon monoxide (CO) boiler is investigated. This study adopts a CO boiler from the Formosa Petrochemical Corporation (FPC) in Taiwan as the model for numerical investigation. The combustion and fluid flow in the FPC CO boiler is examined with emphasis on the effect of reburning upon NOx reduction. The influences of some relevant parameters to reburning are inspected, including the size, position and number of reburn fuel or air holes, the primary/reburn fuel ratio, as well as the inlet/reburn air ratio. It is found that NOx reduction occurs mainly behind the DeNOx section where recirculation is prominent. For a smaller reburn hole, the injection velocity and the penetration depth are larger and this yields a better NOx reduction. On the other hand, the injection velocity and the penetration depth are smaller for a larger reburn hole and this leads to a local high temperature region near the reburn hole and a worse NOx reduction. NOx reduction is better for a lower primary/reburn fuel ratio and a higher inlet/reburn air ratio. In addition, NOx reduction is better when the reburn air hole is closer to the reburn fuel hole and when the reburn fuel is injected from where recirculation is prominent. The cross-sectional average temperature arises near the reburn fuel hole and the temperature rise is higher when the reburn fuel is injected more upstream where recirculation is stronger. In addition, the cross-sectional average temperature decreases first and arises then near the reburn air hole. Finally, based on a specified reburn fuel and air flow rates, a single-reburn-hole arrangement yields lower NOx concentration and temperature than a three-reburn-holes arrangement does. Keywords CO boiler, reburning, NOx reduction. I. INTRODUCTION EBURNIBG technology is one of the most promising and Rcost-effective NOx reduction strategies for combustion systems. Wang et al. [1] carried out the reburning experiments with six kinds of biomass and one biochar in an entrained flow reactor. Their results indicated that NO reduction efficiency behaves a trend of first increase and then decrease with decreasing of stoichiometric ratio in the reburn-zone or increasing of reburn fuel fraction. Kim et al. [2] carried out experimental and numerical studies to investigate the effect of the fuel lean reburning process on the NOx reduction and CO emission. The detailed numerical results showed that the recirculation flow developed inside the boiler was found to play an important role in improving the effectiveness of fuel lean reburning process. Giménez-López et al. [3] performed an experimental parametric study on the NO reduction efficiency by reburning in the 8 to 18 K temperature range. A Chun-Lang Yeh, Professor Department of Aeronautical Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan, R.O.C. Phone: 886--627; fax: 886--63124; e-mail: clyeh@nfu.edu.tw significant NO reduction can be obtained at moderately high temperatures, fuel-rich conditions, high values of the reburn fuel/no ratio, sufficiently high residence times and low water vapor contents. Oh et al. [4] investigated reburning of cattle manure-based biomass (CB) with coals to develop environmentally friendly thermochemical energy conversion technologies for NOx reductions and Hg captures and removals from existing pulverized coalfired power plants. Their results indicated that the CB can serve as a very effective fuel supplementing coals on NOx reductions and Hg captures and removals in pulverized coal-fired boilers. Kim and Baek [] presented an experimental study focused on a fuel-lean reburn system to control NOx emission from combustion process. The fuel-lean reburn system, even with only an amount of reburn fuel of 13% of total heat input, was observed to achieve a maximum of 48% in NOx reduction. Nguyen et al. [6] evaluated the technical and economical feasibility of utilizing a low-calorific fuel gas as a reburn fuel. Their study indicated that the proposed reburn fuel and OFA injection systems will provide adequate mixing and reasonable residence time for the reburn application. Dimitriou et al. [7] presented a reburn chemistry modelling method for predicting the reduction of NOx emissions in coal combustors. The model is applied to predict NOx reductions in a large-scale laboratory pulverised fuel combustor with gas as well as coal reburn strategies. Ahn et al. [8] performed an experimental study on the combustion characteristics of a low NOx burner using reburning technology. When the amount of reburn fuel reaches to 2-3% of the total fuel used, the overall NO reduction of % is achieved. Carbon Monoxide (CO) boiler plays an important role in the petroleum-refining process. It can retrieve the thermal energy of CO from the regenerator. CO boiler utilizes the burning gas (CO) from the regenerator as fuel. Then CO reacts to form CO 2 and release large amount of heat. The high-temperature flue gas then flows through the superheating section and exchanges heat with the water in the pipelines to produce superheated steam which can be used by other equipment. A CO boiler is composed of an oxidizer section, a DeNOx section, a flue gas cooler section, a DeSOx section, and a stack. The operating temperatures in oxidizer and DeNOx sections can be as high as 12 o C. This can result in interior or exterior problems of a CO boiler. The loss caused by shutdown/inspection/maintenance of a CO boiler can be ten million $NT per day. Consequently, performance of a CO boiler has a detrimental influence on the operation and production of petrochemical industry, steel & iron industry, and relevant industries. Configuration of a CO boiler for a typical petroleum refinery is shown in Fig.1. 6

4th International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'214) Jan. 28-29, 214 Bangkok (Thailand) Fig. 1 Configuration of a CO boiler in a typical petroleum refinery This study adopts a CO boiler from the Formosa Petrochemical Corporation (FPC) in Taiwan as the model for numerical investigation. Configuration and dimensions of the CO boiler investigated are shown in Fig.2. The combustion and fluid flow in the FPC CO boiler is examined with emphasis on the effect of reburning upon NOx reduction. The influences of some relevant parameters to reburning are inspected, including the size, position and number of reburn fuel or air holes, the primary/reburn fuel ratio, as well as the inlet/reburn air ratio. (a) dimensions of the CO boiler investigated (b) numerical model of the CO boiler investigated (c) enlarged view of the CO boiler inlet (d) dimensions of the CO boiler inlet Fig. 2 Configuration and dimension of the CO boiler investigated II. NUMERICAL METHODS AND PHYSICAL MODELS In this study, the FLUENT commercial code is employed to simulate the reacting and fluid flow in the FPC CO boiler. The SIMPLE algorithm by Patankar is used to solve the governing equations. The discretizations of convection terms and diffusion terms are carried out by the power-law scheme and the central difference scheme, respectively. In respect of physical models, by considering the accuracy and stability of the models and by referring to the evaluation of other researchers, the standard k-ε Model, P-1 radiation model and non-premixed combustion model with β type probability density function, and partial equilibrium model [9] are adopted for turbulence, radiation, combustion and reburning simulations, respectively. The standard wall functions are used to resolve the flow quantities ( velocity, temperature, and turbulence quantities ) at the near-wall regions. III. RESULTS AND DISCUSSION In this study, the numerical model of the CO boiler is constructed by unstructured grid. In the author s previous study [], five cell densities for the CO boiler (i.e. without reburn holes) were tested to ensure a grid independence solution. They include 49,68 cells, 73,87 cells, 198,46 cells, 349,411 cells, and,73 cells. Computational results showed that the recirculation zone sizes of the DeNOx section and the cross-sectional average temperature profiles obtained by the last two meshes nearly coincide with deviation within.%. Therefore, the mesh of 349,411 cells is adopted for the CO boiler in subsequent discussion. For the CO boiler with reburn holes, the numerical model is constructed by adding reburn holes to the above CO boiler. In this study, we examined four reburn hole diameters,.1m,.2m,.3m and.4m. We tested three mesh sizes,.1m,.m and.m, in constructing reburn holes. It is found that the above three mesh sizes have minor influences on the computational results for the CO boilers with reburn hole sizes of.2m,.3m and.4m. The recirculation zone sizes of the DeNOx section and the cross-sectional average temperature profiles obtained by the three mesh sizes nearly coincide with deviation within.%. However, for the CO boiler with reburn hole sizes of.1m, the computational results with mesh size of.m is a little 7

4th International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'214) Jan. 28-29, 214 Bangkok (Thailand) different from the results with mesh sizes of.m or.1m, especially, in the neighborhood of reburn holes. This may be due to the insufficient mesh density near the reburn holes with mesh size of.m. Therefore, the reburn hole with mesh size of.m is adopted in subsequent discussion. The total number of meshes is around 4, and may differ with the reburn hole sizes and positions. In this study, we examined four reburn fuel injection positions, as shown in Fig.2(a), and three reburn/total fuel mass fractions (7%, 8% and 9%), as well as three reburn/total air mass fractions (7%, 8% and 9%). Species compositions at the flue gas inlet and the fuel inlet (including primary and reburn fuels ) are listed in Tables 1 and 2, respectively. Inlet pressure is 1 atm. The primary air inlets are facilitated by swirlers at a 6 o swirl angle. Axial and tangential velocity components are 6.31 and.93 m/sec, respectively, for the boiler (i.e. without reburning). Temperature is 3K. Turbulence kinetic energy is % of the inlet mean flow kinetic energy and turbulence dissipation rate is computed from Eq.(1) with hydraulic diameter L=.944m. 3/ 2 4 k ε = C 3/ (1) µ l where l=.7l and L is the hydraulic diameter. At the secondary air inlet, velocity is 4.4m/sec for the boiler, temperature is 3K, turbulence kinetic energy is % of the inlet mean flow kinetic energy, and the turbulence dissipation rate is computed from Eq.(1) with hydraulic diameter L of either.2m or.m (Fig.2(d)). At the fuel inlet, velocity is.33m/sec for the boiler, temperature is 323K, turbulence kinetic energy is % of the inlet mean flow kinetic energy, and the turbulence dissipation rate is computed from Eq.(1) with a hydraulic diameter L=.1m. At the flue gas inlet, velocity is 13.m/sec, temperature is 877K, turbulence kinetic energy is % of the inlet mean flow kinetic energy, and the turbulence dissipation rate is computed from Eq.(1) with a hydraulic diameter L=.912m. The heat absorption rate of the flue gas cooling tubes is 26,72 w/m 2 and the other walls are adiabatic. No slip condition is applied on any of the solid walls. The atmosphere at the exit is taken as a cube with a side length of twenty times the exit diameter of the DeSOx section. The atmosphere is at 3K and 1 atm. TABLE I SPECIES COMPOSITION AT THE FLUE GAS INLET HOLES (MOLE FRACTION) Carbon dioxide(co 2) 13.1% Carbon monoxide(co) 2.71% Nitrogen(N 2) 7.92% Water(H 2O) 13.27% TABLE II Species Composition At The Fuel Inlet Holes (Mole Fraction) Methane(CH 4).482 Ethane(C 2H 6) 8.41-2 Propane(C 3H 8) 4.647-2 Propylene(C 3H 6) 2.276-2 Carbon monoxide(co) 1.699-2 Oxygen(O 2) 4.249-3 Nitrogen(N 2).472-2 Hydrogen(H 2).292 Sulphur(S) 3.98-4 A. Effect of Reburn Hole Size Fig.3 shows the comparison of cross-sectional average NO concentration for different size of reburn hole. The reburn fuel is injected from the first reburn hole and the reburn/total fuel mass fraction is 3%. The reburn air is injected from the second reburn hole and the reburn/total air mass fraction is %. We adopt these mass fractions for discussing the effect of reburn hole size because, in next section, we will demonstrate that these mass fractions is a better operation condition for NOx reduction by reburn. The result shows that NOx reduction occurs mainly behind the DeNOx section (x 11m) where recirculation is prominent. The recirculation flow is conducive to the NOx reduction reaction by reburn. It is also observed that NOx reduction is better for a smaller reburn hole because the injection velocity and penetration depth of the reburn fuel and air are larger for a smaller reburn hole and this is conducive to the NOx reduction reaction of the reburn fuel and air. This can also be seen from Fig.4 which shows the temperature profiles at the symmetric plane (z=) for different reburn hole sizes. On the contrary, NOx reduction is worse for a larger reburn hole because the injection velocity and penetration depth of the reburn fuel and air are smaller and this results in a local high temperature region near the reburn fuel hole as shown in Fig.4(b). D=.1m D=.2m D=.3m D=.4m 4 3 2 Fig. 3 Comparison of cross-sectional average NO concentration for different size of reburn hole 8

4th International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'214) Jan. 28-29, 214 Bangkok (Thailand) (a) D=.1m B. Effect Of Reburn Fuel Fraction And Reburn Air Fraction Fig.6 shows the comparison of cross-sectional average NO concentration for different reburn/total fuel mass fractions. The reburn fuel is injected from the first reburn hole and three reburn/total fuel mass fractions, %, 2 and 3%, are examined. The reburn air is injected from the second reburn hole and the reburn/total air mass fraction is kept at %. From Fig.6(a), it is seen that, for reburn hole size of.4m, NOx reduction is best for reburn/total fuel mass fraction of 3%. The result implies that NOx reduction is better when reburn/total fuel mass fraction is increased, i.e. fuel lean inlet condition. This is consistent with the results of Ahn et al. [8]. In a practical combustion system, a fuel lean inlet operation may result in a low heat release in the primary combustion zone which may influence the combustion efficiency. Therefore, the highest reburn/total fuel mass fraction is restricted to 3% in this study. For reburn hole size of.1m, it can be observed from Fig.6(b) that NOx reduction is better than for reburn hole size of.4m. As mentioned in last section, NOx reduction is better for a smaller reburn hole because the injection velocity and penetration depth of the reburn fuel and air are larger and this is conducive to the NOx reduction reaction of the reburn fuel and air. In addition, because the NOx reduction reaction is more complete for a smaller reburn hole, NOx concentration nearly coincides for different reburn/total fuel mass fractions. (b) D=.4m Fig. 4 Temperature profiles at the symmetric plane (z=) for different reburn hole sizes 4 reburn / total fuel = 3% reburn / total fuel = 2% reburn / total fuel = % Fig. shows the comparison of cross-sectional average temperature for different reburn hole size. It is seen that temperature arises near the reburn fuel injection location (x=21.6m). This is consistent with the results of Fig.4(b) which show that a larger reburn hole results in a local high temperature region near the reburn fuel hole because the injection velocity and penetration depth of the reburn fuel and air are smaller and therefore the reburn fuel concentrates near the reburn fuel hole. 3 2 (a) D=.4m 9 8 4 reburn / tatal fuel = 3% reburn / tatal fuel = 2% reburn / tatal fuel = % 7 6 4 D=.1m D=.2m D=.3m D=.4m 3 2 3 Fig. Comparison of cross-sectional average temperature for different reburn hole size (b) D=.1m 9

4th International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'214) Jan. 28-29, 214 Bangkok (Thailand) Fig. 6 Comparison of cross-sectional average NO concentration for different reburn fuel fractions Fig.7 shows the comparison of cross-sectional average temperature for different reburn/total fuel mass fractions. For reburn hole size of.4m, because the injection velocity and penetration depth of the reburn fuel and air are smaller and therefore the fuel concentrates near the reburn fuel hole, temperature arises near the reburn fuel hole (x=21.6m), as shown in Fig.7(a). On the other hand, for reburn hole size of.1m, the reburn fuel and air do not concentrate near the reburn holes because their injection velocity and penetration depth are larger, temperature is lower than for reburn hole size of.4m, as shown in Fig.7(b). In addition, because the reaction of reburn fuel and air is more complete for a smaller reburn hole, the temperatures for different reburn/total fuel mass fractions do not differ significantly. In a practical combustion system, heat release occurs mainly in the primary combustion zone. The major function of the reburn zone is to convert NOx to N 2. Therefore, a lower reburn/total fuel mass fraction yields a higher temperature due to its higher heat release. reburn / total fuel = 3% reburn / total fuel = 2% 9 reburn / total fuel = % 8 Fig.8 shows the comparison of cross-sectional average NO concentration for different reburn air fractions. The reburn fuel is injected from the first reburn hole and the reburn/total fuel mass fraction is kept at 3%. The reburn air is injected from the second reburn hole and three reburn/total air mass fractions, %, 2 and 3%, are examined. It is seen that NOx reduction is best for reburn/total air mass fraction of %. The result implies that NOx reduction is better when reburn/total air mass fraction is decreased, i.e. fuel lean inlet condition. This is consistent with the results of different reburn/total fuel mass fractions discussed in this section and the results of Ahn et al. [8]. In addition, it can be observed by comparing Figs.8(a) and (b) that NOx reduction is better for a smaller reburn hole size and this is consistent with the results in previous section. 4 3 2 reburn / total air = 3% reburn / total air = 2% reburn / total air = % 7 6 (a) D=.4m 4 3 9 8 (a) D=.4m reburn / total fuel = 3% reburn / total fuel = 2% reburn / total fuel = % 4 3 2 reburn / total air = 3% reburn / total air = 2% reburn / total air = % 7 6 4 (b) D=.1m Fig. 8 Comparison of cross-sectional average NO concentration for different reburn/total air mass fractions 3 (b) D=.1m Fig. 7 Comparison of cross-sectional average temperature for different reburn/tatal fuel mass fractions Fig.9 shows the comparison of cross-sectional average temperature for different reburn/total air mass fractions. For reburn hole size of.4m, because the injection velocity and penetration depth of the reburn fuel and air are smaller and therefore the fuel concentrates near the reburn fuel hole, temperature arises near the reburn fuel hole (x=21.6m), as shown in Fig.9(a). On the other hand, for reburn hole size of.1m, the reburn fuel and air do not concentrate near the reburn

4th International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'214) Jan. 28-29, 214 Bangkok (Thailand) holes because their injection velocity and penetration depth are larger, temperature is lower than for reburn hole size of.4m, as shown in Fig.9(b). C. Effect of Reburn Hole Positions And Numbers Fig. shows the comparison of cross-sectional average NO concentration for different reburn hole positions. In previous sections we have demonstrated that NOx reduction is better when reburn fuel is increased or reburn air is decreased. Therefore, we adopt a reburn/total fuel mass fraction of 3% and a reburn/total air mass fraction of % for discussing the effect of reburn hole positions and numbers. For reburn hole size of.4m, it can be observed from Fig.(a) that NOx reduction is better when the reburn fuel is injected from an upstream region of strong recirculation and when the reburn air hole is closer to the reburn fuel hole. This is because a reburn hole in an upstream position is in a strong recirculation flow region and this is conducive to the NOx reduction reaction by reburn. On the other hand, a reburn hole in a downstream position is in a redevelopping flow region and this is not conducive to the NOx reduction reaction by reburn. For reburn hole size of.1m, because the NOx reduction reaction is more complete, it can be observed from Fig.(b) that the influence of reburn hole position on NOx concentration is not obvious and the NOx concentrations for different reburn hole positions nearly coincide. reburn / total air = 3% reburn / total air = 2% 9 reburn / total air = % 8 7 6 4 3 8 7 6 4 (a) D=.4m reburn / total air = 3% reburn / total air = 2% 9 reburn / total air = % 3 (b) D=.1m Fig. 9 Comparison of cross-sectional average temperature for different reburn/total air mass fractions 4 3 2 reburn fuel hole : No.1 ; reburn air hole : No.2 reburn fuel hole : No.1 ; reburn air hole : No.3 reburn fuel hole : No.1 ; reburn air hole : No.4 reburn fuel hole : No.2 ; reburn air hole : No.3 reburn fuel hole : No.2 ; reburn air hole : No.4 reburn fuel hole : No.3 ; reburn air hole : No.4 3 2 (a) D=.4m reburn fuel hole : No.1 ; reburn air hole : No.2 reburn fuel hole : No.1 ; reburn air hole : No.3 reburn fuel hole : No.1 ; reburn air hole : No.4 reburn fuel hole : No.2 ; reburn air hole : No.3 4 reburn fuel hole : No.2 ; reburn air hole : No.4 reburn fuel hole : No.3 ; reburn air hole : No.4 (b) D=.1m Fig. Comparison of cross-sectional average NO concentration for different reburn hole positions Fig.11 shows the comparison of cross-sectional average temperature. It is observed that, for reburn hole size of.1m, the temperature is lower than that for reburn hole size of.4m. Further, as discussed in previous sections, because the injection velocity and penetration depth of the reburn fuel and air are larger and therefore the fuel and air do not concentrate near the reburn fuel hole, there is no local high temperature region for reburn hole size of.1m. In addition, because the reaction of reburn fuel and air is more complete for a smaller reburn hole, the temperatures for different reburn hole positions do not differ significantly. On the other hand, for reburn hole size of.4m, because the injection velocity and penetration depth of the reburn fuel and air are smaller, the reburn fuel concentrates and therefore temperature arises near the reburn fuel hole (x=21.6m,.43mand 31.99m), as shown in Fig.11(a). From Fig.11(a), it is also observed that the temperature rise near the first reburn fuel hole is the highest. This is because the first reburn fuel hole is located in a region of stronger recirculation which is conducive to the reburn reaction. In addition, it is also observed that the temperature decreases first and arises then near the reburn air hole (x=.43m, 31.99m and 38.448m). 11

4th International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'214) Jan. 28-29, 214 Bangkok (Thailand) This is because the reburn air is injected at atmospheric temperature and therefore the temperature decreases first, but when the reburn air reacts with the reburn fuel, the temperature arises then due to the reaction heat release. 9 8 7 6 4 reburn fuel hole : No.1 ; reburn air hole : No.2 reburn fuel hole : No.1 ; reburn air hole : No.3 reburn fuel hole : No.1 ; reburn air hole : No.4 reburn fuel hole : No.2 ; reburn air hole : No.3 reburn fuel hole : No.2 ; reburn air hole : No.4 reburn fuel hole : No.3 ; reburn air hole : No.4 3 9 8 7 6 4 (a) D=.4m reburn fuel hole : No.1 ; reburn air hole : No.2 reburn fuel hole : No.1 ; reburn air hole : No.3 reburn fuel hole : No.1 ; reburn air hole : No.4 reburn fuel hole : No.2 ; reburn air hole : No.3 reburn fuel hole : No.2 ; reburn air hole : No.4 reburn fuel hole : No.3 ; reburn air hole : No.4 3 (b) D=.1m Fig. 11 Comparison of cross-sectional average temperature for different reburn hole positions Fig.12 shows the comparison of cross-sectional average NO concentration for different number of reburn holes. From the above discussion, we have demonstrated that NOx reduction is better when reburn fuel is injected from the first reburn hole and reburn air is injected from the second reburn hole and when the reburn/total fuel mass fraction is 3% and the reburn/total air mass fraction is %. Therefore, we adopt this reburn hole arrangement and mass fractions for discussing the effect of number of reburn holes. We compare a three-reburn-hole arrangement, as shown in Fig.13, with the conventional single-reburn-hole arrangement, as shown in Fig.2(a). For the purpose of the same basis of comparison, the reburn fuel and air mass flow rates for the two arrangements are identical. Therefore, the reburn fuel and air mass flowrates in each hole of the three-reburn-hole arrangement is only 1/3 of that in each hole of the single-reburn-hole arrangement. From Fig.12, it is seen that a single-reburn-hole arrangement is better for NOx reduction. This is because, as mentioned above, the reburn fuel and air mass flowrates in each hole of the three-reburn-hole arrangement is only 1/3 of that in each hole of the single-reburn-hole arrangement, the penetration depth of the reburn fuel and air is smaller for the three-reburn-hole arrangement and therefore the NOx reduction by reburn is less complete. The temperature rise near the reburn fuel hole (x=21.6m) can also be seen from Fig.14 which shows the comparison of cross-sectional average temperature for different number of reburn holes. It can be seen from Fig.14 that the temperature rise for the three-reburn-hole arrangement is higher than that for the single-reburn-hole arrangement. IV. CONCLUSION In this study, NOx reduction by reburning in a CO boiler is investigated numerically. The combustion and fluid flow in the FPC CO boiler is examined with emphasis on the effect of reburning upon NOx reduction. From the simulation results, it is found that NOx reduction occurs mainly behind the DeNOx section where recirculation is prominent. NOx reduction is closely connected with the reburn fuel penetration depth. For a smaller reburn hole, the reburn fuel penetration depth is larger and this yields a better NOx reduction. On the other hand, the penetration depth is smaller for a larger reburn hole and this leads to a local high temperature region near the reburn hole and a worse NOx reduction. NOx reduction is better for a lower primary/reburn fuel ratio and a higher inlet/reburn air ratio. In addition, NOx reduction is better when the reburn air hole is closer to the reburn fuel hole and when the reburn fuel is injected from an upstream region of strong recirculation. The cross-sectional average temperature arises near the reburn fuel hole and the temperature rise is higher when the reburn fuel is injected from an upstream region of strong recirculation. In addition, the cross-sectional average temperature decreases first and arises then near the reburn air hole. Finally, based on a specified reburn fuel and air flow rates, a single-reburn-hole arrangement yields lower NOx concentration and temperature than a three-reburn-holes arrangement does. 4 3 2 single renburn hole ; D=.1m three renburn holes ; D=.1m 2 4 6 8 (a) D=.1m 12

4th International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'214) Jan. 28-29, 214 Bangkok (Thailand) 4 single renburn hole ; D=.4m three renburn holes ; D=.4m 9 8 3 2 7 6 single reburn hole ; D=.1m three reburn holes ; D=.1m 4 (b) D=.4m Fig. 12 Comparison of cross-sectional average NO concentration for different number of reburn holes 3 (b) D=.1m Fig. 14 Comparison of cross-sectional average temperature for different number of reburn holes ACKNOWLEDGMENT The author gratefully acknowledges the grant support from the National Science Council, Taiwan, R.O.C., under the contract NSC2-2221-E--31. Fig. 13 Illustration of the three-reburn-hole arrangement 9 8 7 6 4 single reburn hole ; D=.4m three reburn holes ; D=.4m 3 (a) D=.4m REFERENCES [1] P. Lu, Y. Wang, F. Lu and Y. Liu, Cleaner Combustion and Sustainable World, Qi H and Zhao B (eds.), Springer-Verlag Berlin Heidelberg and Tsinghua University Press, 213, pp.28-289. [2] H. Y. Kim, S. W. Baek and S. W. Kim, Investigation of fuel lean reburning process in a 1. MW boiler, Applied Energy, vol.89, pp.183-192, 212. [3] J. Giménez-López, V. Aranda, A. Millera, R. Bilbao and M. U. Alzueta, An experimental parametric study of gas reburning under conditions of interest for oxy-fuel combustion, Fuel Processing Technology, vol.92, pp.82-89, 211. [4] H. Oh, K. Annamalai and J. Sweeten, Reburning of cattle manure-based biomass with coals in a small scale boiler burner facility for NOx and Hg reduction, in Proceedings of the ASME/JSME 211 8th Thermal Engineering Joint Conference, AJTEC211, Honolulu, Hawaii, USA, March 13-17, 211. [] H. Y. Kim and S. W. Baek, Investigation of fuel lean reburning, in Proceedings of the ASME 29 International Mechanical Engineering Congress & Exposition, IMECE29, Lake Buena Vista, Florida, USA, November 13-19. [6] Q. H. Nguyen, W. Zhou, G. Xu, L. W. Swanson and D. K. Moyeda, Comprehensive modeling study on fuel gas reburn in an opposed wall fired boiler, in Proceedings of IMECE27, 27 ASME International Mechanical Engineering Congress and Exposition, November 11-, 27, Seattle, Washington, USA. [7] D. J. Dimitriou, N. Kandamby and F. C. Lockwood, A mathematical modelling technique for gaseous and solid fuel reburning in pulverised coal combustors, Fuel, vol.82, pp.27-2114, 23. [8] K. Y. Ahn, H. S. Kim, M. G. Son, H. K. Kim and Y. M. Kim, An experimental study on the combustion characteristics low NOx burner using reburning technology, KSME International Journal, vol.16, no.7, pp.9-98, 22. [9] N. Kandamby, G. Lazopoulos, F. C. Lockwood, A. Perera, and L. Vigevano, Mathematical modeling of NOx emission reduction by the use of reburn technology in utility boilers, presented at the ASME Int. Joint Power Generation Conference and Exhibition, Houston, Texas, 1996. [] C. L. Yeh, Numerical analysis of the combustion and fluid flow in a carbon monoxide boiler, International Journal of Heat and Mass Transfer, vol.9, pp.172-19, April, 213. 13