The 37 th International Technical Conference on Clean Coal & Fuel Systems, Clearwater, Florida, USA June 3-7, 2012 USE OF CFD MODELING IN DESIGNING ADVANCED NO X CONTROL TECHNOLOGY FOR UTILITY BOILERS Guisu Liu 1, Baiyun Gong 1, Brian Higgins 1, Muhammad Sami 2 1 Nalco Mobotec Inc., 1601 W. Diehl Road, Naperville, IL 60563 2 ANSYS Inc., 1007 Church St., Suite 250, Evanston, IL 60201 ABSTRACT In this paper, a 350 MW tangential coal-fired utility boiler in China was studied using ANSYS FLUENT Computational Fluid Dynamics (CFD) code to evaluate the application of Nalco Mobotec advanced ROFA system for NOx reduction. The simulations showed that ROFA, which delivers a large amount of air up to the upper furnace, significantly reduces the NOx through deep staging. Due to enhanced turbulent mixing, CO and LOI are rapidly burned out in the upper furnace. Heat transfer becomes stronger than the baseline case due to mixing. An overall 50%-60% NOx reduction is achieved during performance test. The boiler efficiency has been slightly increased by 0.38%. All these results are the consequence of strong mixing from ROFA system. These results have been validated against the performance testing results in great detail. INTRODUCTION China s rapid economic growth has led to concerns about environmental protection. The state utilities are facing more aggressive and stringent regulations for SO 2, NOx and particulate matter. In accordance with the new regulations of China State Environmental Protection Administration come into effect on January 1, 2012, the coal-fired boilers have to meet the stringent NOx emissions requirements by January 1, 2014. Therefore, there is an urgent task for the power plants to employ advanced NOx reduction technologies. Existing NOx reduction technologies are typically classified into three categories: low NOx combustion technologies (e.g. Low-NOx Burner, Over-Fired Air, etc), selective non-catalytic reduction (SNCR), and selective catalytic reduction (SCR) technology. In all these technologies, low NOx combustion technologies are low capital cost, and are preferred to be installed in all units. The Nalco Mobotec advanced ROFA system is a deep-stage combustion technology to reduce NOx emissions. Meanwhile, the high-speed ROFA air can increase the upper furnace mixing to burn out CO and LOI to improve combustion performance. The ROFA system air flow, air pressure and nozzle position are designed and optimized through CFD simulation to achieve the best NOx reduction performance and combustion performance 1,2,3,4. ROFA was selected to be implemented on a 350 MW tangential coal-fired utility boiler in China. 1 Liu, G., et al., CFD Evaluation of ROFA on Limestone Utilization in a Circulating Fluidized Bed, CIBO Conference, April 2005 2 Higgins, B., et al., Evaluation of ROFA and Sorbent Injection on NOx and SOx Reduction at Hoosier Energy s Frank E. Ratts Unit 1, Coal- Gen, August 20, 2009 3 Higgins, B., Gong, B., Pozzobon, E., Liu, G., ROFA and Rotamix Systems Reduced NOx below 200 mg/nm 3 at Elektrownia Opole, the 35 th International Clean Coal Conference, Clearwater, Florida, June 6-10, 2010 4 Gong, B., Liu, G., Higgins, B., Williamson, T., et al., CFD-based Design and Installation of Cost-effective ROFA/Rotamix System for NOx Reduction at RPU Silver Lake Unit 4,, the 35 th International Clean Coal Conference, Clearwater, Clearwater, Florida, June 6-10, 2010
In this paper, the details of the project are presented. First, the technology is evaluated through a Computational Fluid Dynamic (CFD) modeling. The actual performance of ROFA system is also presented to validate the model predictions. BOILER INFORMATION The boiler is a Japanese Mitsubishi company's 1160 t/h subcritical reheat forced circulation furnace built in 1998. The unit main parameters are as follows: unit capacity: 350MW, main steam flow: 1210 t/h, main steam temperature: 541, main steam pressure: 17.5Mpa, flue gas volume 1,200,000-1,600,00 Nm 3 /h, exhaust gas temperature: 130-200. The boiler is equipped with three induced fans, flue gas exhausts into the atmosphere through the 200-meter-hige chimney. The design coal is Shenfu Dongsheng coal, the check coal is Datong coal; The unit usually fires a blended coal. Blast furnace gas (BFG) and coke oven gas (COG) are often fired as auxiliary fuel. Under normal circumstances, the amount of blast furnace gas is about 100-200 knm 3 /h, and in extreme case the BFG co-firing capacity is of 350kNm 3 /h. The baseline NOx is in a wide range of 400-900 mg/nm 3 with an average of 650 mg/nm 3. NOx level is lower when co-firing blast furnace gas, and it becomes high when 100% coal firing. ROFA SYSTEM DESIGN The ROFA system was installed to reduce NOx through furnace staging. In the meantime, the ROFA system was designed to increase mixing in the upper furnace for CO and LOI burnout. As shows in Fig. 1, a ROFA system includes a boosted-pressure ROFA fan, interconnecting air ducting, and air injection nozzles. The ROFA air is taken from ducts at the outlet of the air preheaters. It is boosted in pressure by the ROFA fan and delivered through specially located nozzles into the furnace. The air pressure at the nozzles is optimized as required to achieve mixing as determined during the CFD modeling. All air flow to the ROFA nozzles is controlled based on a relationship to boiler steam flow to maintain tuned box pressures as load changes. Additionally, feed-forward and feed-back control strategies can be implemented to reduce system upsets during load fluctuations. The ROFA system consists of a variable frequency drive (VFD) controlled centrifugal ROFA fan. The ROFA fan takes air through an individual duct from the outlet of the air preheater. Each duct has a venturi flow meter to allow the boiler control system to account for air flow through the ROFA system. Figure 1: ROFA system design
Extensive on-site investigations were carried out to determine the most efficient and cost effective way to redirect secondary air to the upper furnace ROFA ports. The ROFA air suction and discharge duct routing was meticulously engineered as the existing boiler and building structure contained significant duct path interferences particularly adjacent to the furnace walls. CFD MODELING Combustion Model Overview ANSYS FLUENT has been selected as the CFD code for this project because it is likely the best commercially available CFD package. Grids (or meshes) used in the CFD simulations are first constructed using FLUENT s companion software, GAMBIT. FLUENT then solves for the density, velocity, temperature, and species (including coal volatiles) concentrations fields of the gas phase and coal particle properties and combustion within the furnace to steady state. The gas phase conservation equations are solved using a variable density, quasi-incompressible formulation embedded in an Eulerian reference frame. These governing equations are the gas phase continuity, momentum, turbulent kinetic energy, turbulent dissipation, enthalpy, and the species conservation equations for each gas species in the turbulent combustion model. These conservation laws have been described and formulated extensively in standard CFD textbooks. A k-ε turbulence model was implemented in our simulations. Standard Eddy- Breakup (EBU) turbulence combustion model is used. The following two step mechanism was utilized for coal combustion: Coal + a O 2 b CO + c CO 2 + d H 2 O + e SO 2 CO + 0.5 O 2 CO 2 where the stoichiometric coefficients (a, b, c, d, and e) were determined from the fuel proximate and ultimate analyses. For lower temperatures found in the back-pass, a modification to the carbon monoxide reaction is also included to more accurately predict CO concentration. Coal is injected through the burners by specifying a Rosin-Rammler particle size distribution and a particle velocity slightly less than the gas phase velocity within the primary injectors. Parameters for this distribution are derived from sieve data collected onsite. Gas phase air flow rates are specified at the primary, fuel air, auxiliary air and CCOFA, and ROFA ports using appropriate inlet velocities, temperatures, turbulence intensities. The CFD model solves the particle/liquid phase (coal, limestone, water/urea, etc.) in a Langrangian reference frame. The gas phase and particle phase conservation equations are solved separately by FLUENT in order to make the computation more tractable; however, these two phases are strongly coupled through iterative updates of the source terms that occur less often than the iterative updates of the gas phase variables. Particle motion is obtained through solutions of the bulk gas velocity. Turbulent dispersion of particles was modeled using the stochastic discrete-particle approach. The CFD model uses different expressions for particle heating and reaction at each stage of the process. An inert heating law applies when particle temperature is less than the onset temperature for devolatilization. Particle heating is caused by convective heat transfer from the gas phase and the radiant flux from the furnace. During devolatilization and char oxidation, the particle energy balance also includes a heat of devolatilization and heat of combustion, in addition to the convective and radiative heat transfer rates. Both diffusion and intrinsic kinetics were included in the char oxidation sub-model. FLUENT NOx submodel involves sophisticated fuel-n conversion pathways. After fuel devolatilization, fuel-n is partitioned into volatiles-n and char-n. HCN is the dominant nitrogen species in volatile-n released from coal. Char-N is released into the gas phase at a rate that is proportional to the carbon
burnout rate. Because char-n conversion chemistry is complex, we used a fixed fraction of char-n directly converted to NO with the rest of N converted to N 2. This assumption is often used in literature 5. The gas phase NO can be reduced by CO, on the char surface, or through ammonia/urea injection. Geometry The CFD computational domain is shown in the left panel of Fig. 2, respectively. Coal is fed into the furnace through rows of burners in each of four corners. Secondary air comes from the windbox, and the flow is generally above, below, and between the coal pipes. The superheater and reheater pendants are suspended above the nose, leaving the open radiant furnace free of any obstructions. Figure 2: The CFD domain (left) of the furnace and surface mesh (right). The furnace enclosure or CFD model domain for baseline and ROFA cases is defined as beginning at coal burners and ending at the vertical plane right after the tertiary reheater (as shown in Fig. 2). The same geometry was used for both baseline and ROFA cases; however, the baseline and ROFA cases were distinguished by switching off and on the boundary conditions for ROFA ports. The waterwall, and all superheat and reheater pendants are included in the model to account for heat absorption and flow stratification, and are accurately depicted with equivalent surface areas of each of the sections. The burner geometry including primary air flow and secondary air flow was well represented in the model. An isometric view of the furnace surface mesh is shown in the right panel of Fig. 2. This furnace is represented by about 1,440,000 computational cells. Most of the cells are unstructured, hybrid, hexahedral cells. This large number of computational cells is adequate to resolve the most relevant features of the three-dimensional combustion process. The grid size is relatively uniform over the entire domain, except the burner region. Because the near-burner zone combustion is complex and important, an extra care was taken when burner zone was meshed, and the burner zone is subject to much finer mesh. 5 Niksa, S., and Liu, G.-S., Incorporating detailed reaction mechanisms into simulations of coal-nitrogen conversion in p.f. flames, Fuel 81(18), pp. 2371-2385 (2002)
Model Inputs Key inputs for the furnace CFD baseline simulations at full load (350 MW) are listed in Table 1. The coal proximate and ultimate analysis, fineness are listed in Table 2. Coal flow for full load were provided in the performance data during testing, and the firing rate (i.e. total heat input) for both cases are calculated based on the fuel flow and the fuel high heating values. The exit O 2 for two cases were also taken from the performance data during testing and the total air flow (TAF) was calculated based on the stoichiometric (S.R.) analysis with given fuel flow and exit O 2. Table 1: Baseline System Operating Conditions Thermal Firing Rate [MWt] 917.0 Load [MWe gross] 350 Excess Air [%] 22 Excess O 2 [% dry] 3.84 Excess O 2 [% wet] 3.50 Coal Flow [t/h] 138.1 Total Air Flow [t/h] 1322.9 Table 2: Coal Analysis and Fineness Proximate Analysis Volatiles Matter [wt % ar] 27.33 Fixed Carbon [wt % ar] 47.67 Moisture [wt % ar] 14.0 Ash [wt % ar] 11.0 HHV [kj/kg] 23919 Ultimate analysis C [wt % ar] 60.33 H [wt % ar] 3.62 O [wt % ar] 9.94 N [wt % ar] 0.70 S [wt % ar] 0.41 Fineness < 297 µm (50 mesh) [wt %] 100 < 149 µm (100 mesh) [wt %] 99.5 < 74 µm (200 mesh) [wt %] 85 Modeling Results Baseline Validation The baseline model results are compared with the testing data in Fig. 3. The testing was conducted at nose elevation for full load 350 MW and mid load 260 MW. Only the full load data is compared here. The x- axis of the plots in Fig. 3 represents the testing port locations from A at left wall, A1 through A6 on front wall and B on right side wall. All testing ports are located around nose elevation. There were two temperature measurements obtained during testing: one by thermocouple at the tip of the HVT probe and the other by a portable infra-view pyrometer, which measures the maximum and minimum temperatures around the view port region. Modeled gas temperature, O 2, CO and NOx at furnace exit (F.E.) are also
included in the plot. In this case, the furnace exit is between tertiary superheater pendants and second reheater pendants. No HVT measurements were taken at furnace exit location due to lack of accessible viewports. Overall, modeling results are in good agreement with the testing data. In Table 3, the CFD modeled FEGT is almost exactly the same as the OEM design value of 1317 K, indicating that the heat distribution feature in the furnace and backpass in the model is well captured. CFD modeled O 2 and NOx data at the furnace outlet is also in good agreement with the tested emission data at the stack for furnace outlet comparison. Figure 3. Comparison of temperature, O 2, CO and NOx between testing and CFD modeling Comparison at Furnace Exit and Model Outlet The baseline model results are compared with other data in this section. Two locations are compared. One is the furnace exit which is typically defined as the vertical plane in between radiant and convective section of steam tubes. In this case, the furnace exit is between tertiary superheater pendants and second reheater pendants. The other location is the furnace outlet in CFD which is immediately after the tertiary reheater pendants. No HVT measurements were taken in these locations due to lack of accessible viewports. Instead, we used the furnace exit temperature in a design document for furnace exit comparison and the tested emission data at the stack for furnace outlet comparison. In Table 3, the OEM design document indicated that at full load the furnace exit gas temperature (FEGT) is 1317 K. The CFD modeled FEGT are almost exactly the same, indicating that the heat distribution in the furnace and backpass in the model. CFD modeled O 2 and NOx data at the furnace outlet is also in good agreement with the reported data. The following conclusions can be drawn from the comparisons.
CO decreased from baseline to ROFA. As discussed later, even though in the lower furnace a large amount of CO is formed, it is burnt away rapidly and results in even lower CO at the exit in ROFA cases. Upper furnace combustion is greatly improved by high turbulent ROFA jets. The furnace exit gas temperature decreased about 25 C from baseline to ROFA case. The heat transfer rate however increased slightly from baseline to ROFA case. This indicates that the heat transfer in ROFA case is stronger than that in baseline case due to the high turbulence induced by high velocity ROFA jets and better use of the furnace volume. The outlet NOx concentration reduced 56% from baseline, due to air staging. LOI also went down in the ROFA case. Table 3: Comparisons of results between baseline and ROFA case Baseline Testing CFD Baseline CFD ROFA Fur. Exit O 2 [%] 3.8 3.8 H 2 O [%] 7.3 7.3 CO [ppm] 122 129 CO 2 [%] 14.1 14.1 Temp [K] 1317 1320 1296 NO [ppm] 323 141 Outlet O 2 [%] 3.5 3.7 3.7 H 2 O [%] 7.3 7.3 CO ppm [ppm] 58 39 CO 2 [%] 14.1 14.1 Temp [K] 1109 1091 NOx [ppm] 307 324 141 NOx Red. [%] - 56 LOI [%] 2.7 1.1 Temperature Distribution The temperature distributions for seven horizontal planes are shown in Fig. 4. These figures show that the majority of the combustion occurs in the region well below the nose. In fact, by the ROFA level the majority of the coal is combusted, though high CO levels still need to be burned out. The maximum flame temperature in the baseline furnace is about 2100 K. This is also true for the ROFA case but the temperature is more evenly distributed through the lower furnace as the mass flow in the lower furnace is approximately 30% less. The temperature distribution also shows that coal ignites soon after being injected into the furnace. As can been seen from looking into the furnace (and from the control room furnace camera), the flames are not attached to the coal nozzles (as is usual for T-fired boiler). Fig. 2 and later illustrations also show how the ROFA jets penetrate deep into the flue gas cross flow. O 2 Distribution The O 2 distribution in the furnace in the left panel of Fig. 5 shows high O 2 in the near-wall region of the furnace that does not quickly mix with the combustion products until well after the nose. These O 2 inhomogeneities persist well into the upper furnace. For the ROFA case, the lower furnace is clearly staged sub-stoichiometrically. This is, of course, the mechanism for NOx reduction. The key to efficient NOx reduction is the ability to stage sub-stoichiometrically and still burn out the CO before exiting the furnace. CO burnout requires an evenly mixed O 2 concentration in the ROFA case. The O 2 at furnace outlet is more evenly distributed in the ROFA case than in the baseline case, as evidenced in Fig. 6.
Figure 4. Gas temperature of baseline and ROFA. Figure 5. O 2 distribution of baseline and ROFA. CO Distribution CO is the primary intermediate species during the oxidation of coal. In general, all of the coal carbon that eventually burns must first be partially oxidized to CO before further oxidized to CO 2. In fact, half of the heat release from combustion occurs during this CO to CO 2 oxidation. Therefore CO is as important as O 2 to characterize the combustion in furnace. In Fig. 7, the CO is formed but burnt progressively as the flow moves upwards to the upper furnace in the baseline. Clearly there is much more CO in the ROFA case below the nose. This is again a desirable effect of staging. The key is to burn it all out before leaving the furnace. The deep penetration by the ROFA jets is seen to quickly react with CO. Not all the CO is burned in the model before it exits. Stack CO with ROFA is typically below 20 ppm, and we expect this to be the case after ROFA is installed in this unit. NOx Distribution Figure 6. Furnace outlet O 2 distribution of (left) baseline and (right) ROFA. The CFD analysis can calculate NOx reduction using the chemistry described above. While each coal and boiler geometry leads to different NOx generation, the fundamentals remain the same. The baseline NOx model parameters were adjusted within normal ranges to model the measured base case NOx rates and then these model parameters were fixed when the ROFA case was modeled.
The NOx results of baseline and ROFA are compared in Fig. 8. Clearly the ROFA case shows reduced NOx. Importantly, the NOx reduction comes directly from affecting the conversion of the fuel-bound nitrogen. In a staged environment, HCN comes off the coal and preferentially reduces to N 2, where in the baseline case much of the HCN forms NOx. This is particularly evident in the horizontal plane between the ROFA nozzles and the burners, where the NOx concentrations for the ROFA case are clearly lower across the entire horizontal plane. Due to air staging of ROFA, the lower furnace is overall reducing. As a result, the NO concentration is dramatically lower in the entire furnace as shown in the right panel of Fig. 8. Even with the secondary combustion after injection of ROFA air, NO is not increased significantly. Figure 7. CO distribution of baseline and ROFA. Figure 8. NO distribution of baseline and ROFA. Turbulent Kinetic Energy One of the two variables in the turbulence model - kinetic energy is plotted in Fig. 9. Coal combustion is mixing limited. That is, in the open furnace, it is the mixing of the fuel and air that limits the progress of combustion. Once mixed, combustion occurs quickly at high temperatures. Turbulence is a very instructive method of gauging mixing. As a fluid body moves, turbulence dissipates through mixing. Large eddies break into smaller eddies and the smaller eddies break into many more smaller eddies. Each eddy functions to mix unburned carbon or CO with O 2. In Fig. 9, it is obvious that there is significant kinetic energy in the burner zone in both baseline and ROFA case, but once the coal leaves the burner zone in the baseline case there is nothing left to continue the mixing. The one exception to this is the nose, which is why the nose is so important in boiler designs. In the ROFA case, there are not only high levels of kinetic energy in the upper furnace, but it is clear that the turbulence is dissipated throughout the upper furnace. Upper Furnace Temperature Distribution For tangentially fired boiler, furnace outlet steam temperature is usually biased, this problem is universal. This is due to the reason of the fluid in one direction of rotation. Overheating discipline and reheater tubes serious temperature deviation will increase the thermal stress of the pipe and gradually make the tubes leak, eventually had to replace the pipe. Frequent replacement of the steam pipe cost is expensive.
Figure 9. Kinetic energy of baseline and ROFA. In ROFA the design, the introduction of air to the upper part of the furnace again can effectively solve this problem. Figure 10 shows the upper furnace temperature distribution of before and after ROFA. In the boiler right side of the wall and two and three overheating near region, compared with the early-mode model, the entire furnace within the flue gas temperature distribution is more uniform. The flue gas temperature cannot be a good adjustment, but such a comparison also shows that, of ROFA the flexible design for this problem. A large number of on-site commissioning according to customer needs, so as to solve the corresponding problem Figure 10. Upper furnace temperature distribution of baseline and ROFA. FIELD PERFORMANCE The entire ROFA installation includes ROFA fan, the air duct, boiler water wall openings, ROFA air dampers, and control system. The installation took approximately 3-4 months. ROFA air duct installation is difficult, mainly due to the transformation of the old units, insufficient space on-site construction, and the larger ROFA duct size. During the unit outage, the boiler water wall openings, ROFA dampers, and
the connection between air duct and dampers were installed. Special attention was paid to the boiler thermal expansion installation. Turbulent mixing of the high-speed ROFA airflow enhanced the upper furnace combustion; as a result the boiler system can be operated in a relatively low oxygen conditions. The original O 2 curve was then corrected. Figure 11 shows the O 2 curves before and after ROFA in service. Over the entire range of steam load, the O 2 is reduced by 0.5%-1.5%, due to improved combustion by ROFA. This reduction of O 2 has resulted in the increase of thermal efficiency by 0.38%. Figure 11. The furnace O 2 operating curves of baseline and ROFA. Furnace flue gas temperatures were measured through the front wall viewport (right above nose elevation) using infra-view pyrometer. Table 4 shows the temperature measurements between ROFA on and off conditions. The average furnace temperature reduced about 12 C when ROFA is in service. CFD model predicted the flue gas temperature reduced by 25 C. While there is some discrepancy between prediction and actual performance, the qualitative trend is consistent. The reduction on flue gas temperature is caused by enhanced heat transfer due to strong mixing by ROFA air jets. The ROFA fan is turned on, the NOx concentration decreased significantly, as can be seen from Figs. 12 and 13. When the ROFA air increased from 70 t/h to 380 t/h, the NOx concentration reduced from 680 mg/nm 3 to 300 mg/nm3. When the ROFA air flow reduced from 380 t/h to 60 t/h, NOx increased from about 300 mg/nm 3 to 600 mg/nm 3. In general, ROFA system reduced NOx by 50% to 60% based on various operating conditions. Similar level of NOx reduction was achieved when co-burning BFG and COG gas. This is in good agreement with CFD predictions. The boiler fly ash LOI increased slightly, but remained to below 4.0% limit. The increase of LOI is thought to be due to lowered O 2 from baseline to ROFA case. The CO content measured at the inlet of the air preheater reduced from about 22 ppm to 2ppm when ROFA is in service. This is consistent with CFD prediction in Table 3. Superheater steam temperature deviation from left to right is a common operational problem. Due to enhanced mixing through ROFA box setting, ROFA improved the steam temperature deviation by reducing from 20 C to 10 C.
Table 4: Comparisons of Measured Upper Furnace Temperatures Condition ROFA Out of Service ROFA In service Infra-view Temperatures Measured Through Front Wall Viewports F1 F2 F3 F4 F5 F6 Run1 1176 1204 1315 1319 1287 1154 Run2 1209 1256 1338 1333 1226 1129 Run3 1237 1247 1365 1348 1348 1166 Mean 1259 Standard Deviation 209 Run1 1182 1196 1261 1322 1328 1211 Run2 1206 1241 1242 1260 1298 1179 Run3 1206 1223 1292 1307 1312 1186 Mean 1247 Standard Deviation 146 Figure 12. NOx change when ROFA is tuned on. Figure 13. NOx change when ROFA is tuned off. CONCLUSIONS A ROFA system has been designed and installed on a 350-MWe tangential coal-fired boiler to reduce NOx emission. Over 50% NOx reduction was achieved and NOx level remained at or below 300 mg/nm 3 when ROFA is in service. The combustion was improved by reduced CO and reduced furnace O 2, and the boiler efficiency increased by 0.38%. Due to enhanced mixing in upper furnace, the flue gas temperature became much more uniform with less potential to form slagging at the bottom of superheater platens. 6 Nalco and the logo are Registered Trademarks of Nalco Company. Nalco Mobotec and the logo are Registered Trademarks of Nalco Mobotec, Inc. Ecolab is a trademark of Ecolab USA Inc. ROFA and Rotamix are Trademarks of Mobotec AB, used with permission. 2012 Ecolab USA Inc. All Rights Reserved.