Combustion and NOx Emission Characteristics of a Down-Fired

Dec 18, 2013 - ABSTRACT: With the focus on improving coal burnout and controlling NOx ... emissions of 683 mg/Nm3 at O2 = 6% and coal burnout, 3.07% o...
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Combustion and NOx Emission Characteristics of a Down-Fired Furnace with the Hot Air Packing Combustion Technology Weijuan Yang,* Zhijun Zhou, Wenchuang Yang, Junhu Zhou, Zhihua Wang, Jianzhong Liu, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: With the focus on improving coal burnout and controlling NOx emissions in down-fired boilers, a new deep airstaging combustion technology, the hot air packing technology (HAPT), was investigated by experiments and numerical simulations. The effects of the special secondary air ports added in the furnace ash hopper (SA-H) and the furnace bottom (SAB) were analyzed by comparing the three cases: the HAPT, no-SA-H, and no-SA-B cases. The experiments with Guizhou anthracite coal in a down-fired 3.5 MW pilot facility showed that the HAPT case presented a good performance of both NO emissions of 683 mg/Nm3 at O2 = 6% and coal burnout, 3.07% of unburned coal in fly ash. Simulation results using Fluent software satisfactorily coincided with the experiment results of the HAPT case. It was found by simulation that the HAPT case formed a rational aerodynamic field in the furnace, refrained dead recirculation zones from emerging in the ash hopper, and implemented an air-packed and deep air-staging coal combustion inside the furnace. SA-H flows took the responsibility of destroying dead recirculating zones in the ash hopper, and SB-H flows affected the penetration depth of primary air flow and the utilization rate of the ash hopper.

1. INTRODUCTION Down-fired boilers are widely applied for burning anthracite and lean coal, owing to a long distance of flame penetration and a long residence time of pulverized coal. Down-fired combustion technology (DFCT) has developed quickly in the past 20 years. However, high nitrogen oxides (NOx) emissions often occur in the running of down-fired boilers, and the uncontrolled NOx emissions can reach up to 1700−2100 mg/ Nm3 (at 6% O2 dry).1−3 Among all kinds of NOx control technologies, including low-NOx combustion, selective catalytic reduction, and selective non-catalytic reduction, low-NOx combustion technologies are more cost-effective and can generally obtain a NOx reduction of 30−50% with a tiny operation cost.4−10 Low-NOx combustion technologies are mature to a higher degree and more widely used in several types of boilers burning different kinds of fuel.11−14 Down-fired combustion technologies absorb some concepts of low-NOx combustion technologies for reference, and some new DFCTs have been researched, demonstrated, and applied.15−21 Li et al. investigated the opening of the vent air valve in a down-fired 300 MWe utility, and the NOx content was up to 2000 mg/m3 with a opening of 40%.1,2 Ren et al. researched the influence of the adjustable vane position of cyclone burners in a down-fired 300 MWe utility and found raising the vanes led to a rise of unburned carbon content (UBC) in fly ash and NOx emissions.3 Overfire air (OFA) is one of effective ways to remove NOx for a variety of coals boilers, and the impacts of the angle, location, and flow rate ratio of OFA have been investigated in 300 and 350 MWe down-fired utility boilers by experiments and numerical simulation.21−24 According to their results, the OFA angle had a great effect on unburned carbon in fly ash but not NO.22 The flow rate ratio of OFA to the total fed air, the OFA ratio, was optimal in the range of 20−25% considering both UBC and NOx emissions.21 NOx emissions could drop down to 800−1300 mg/Nm3, and reduction efficiency could be 30−50%, by OFA optimization. Besides © 2013 American Chemical Society

OFA, secondary air (SA) arrangement is another key factor of NOx control. The influence of damper opening of the E-tier secondary box on a 1025 ton/h down-fired boiler was established, and 30% of the opening was better for both NOx and UBC.2 A new technology based on multiple injections and multiple staging was demonstrated in a down-fired 350 MW boiler and obtained a NOx reduction of 50% without an UBC increase.23,24 A down-fired 660 MW boiler retrofitted with an air-surrounding-fuel concept achieved a NOx emission reduction from 2594 to 1895 mg/m3.25 The influences of the F-tier secondary air angle in down-fired 660 and 300 MW boilers were investigated, and inclining downward, the F-tier SA increased the flame penetration depth and improved the boiler efficiency.19,26 Fang et al. found that the bigger inclining angle showed the better reduction effect on NOx emissions.19 However, Li’s group obtained a contrary effect.26 The hot air packing technology (HAPT) is an innovative deep air-staging technology for down-fired pulverized-coal utility boilers. The HAPT implements the addition of special secondary air ports in the furnace ash hopper and in the furnace bottom.27 Besides improving the aerodynamic characteristics in the downward part of the lower furnace and ash hopper, the advancement of the HAPT also includes promoting flow filling in the ash hopper by deep air staging. In the prior work, we found that the aerodynamic characteristics of HAPT in the furnace is rational and conducive to NOx control and coal burnout.27,28 In this paper, hot experiments were conducted in a down-fired 3.5 MW pilot facility. Combustion and NOx emission characteristics were investigated and analyzed in detail by numerical simulation. The results would be good references for the design of a down-fired boiler and its combustion system. Received: September 17, 2013 Revised: December 18, 2013 Published: December 18, 2013 439

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2. METHODOLOGY

Table 2. Air and Coal Distribution in the HAPT Case

2.1. Experimental Facility and Measurement. Figure 1 presents a schematic diagram of the 3.5 MW furnace with the

item

Za (m)

air ratiob (%)

temperature (K)

velocity (m/s)

coal ratio (%)

PA-rich PA-lean SA-W SA-H SA-B OFA

3.05 3.15 2.1 1.0 0 3.6, 4.0

21.5 10.8 9.6 23.6 17.2 17.3

378 378 655 655 655 655

12.59 6.30 7.00 6.13 6.57 19.79

85 15

a Z position of the port central line. bThe ratio of the flow rate injected from the certain ports to the total flow rate.

in the furnaces. A water-cooling suction thermocouple measured the gas temperature along both the furnace height and the PA injection direction. A water-cooled probe was used to sample the gas in the furnace, and in situ O2, CO, and NO concentrations were analyzed by a Testo 350M system. Fly ash was collected to analyze UBC by a particle sampling device with a constant suction speed. 2.2. Numerical Simulation Method. A computational fluid dynamics (CFD) program, Fluent, version 6.3.26, was used to conduct numerical simulations here. The time-averaged conservation equations were adopted for the gas and particle phases: Eulerian description for the former and Lagrangian description for the latter. The standard k−ε model was adopted for the turbulence flow simulation. The P1 method was used to simulate radiation heat transfer in the furnace. The gasphase combustion model used here was the transported probability density function (PDF) model of non-premixed combustion. The equilibrium assumption model described the system chemistry. The SIMPLE algorithm was selected to couple the velocity and pressure fields. A partition meshing method was adopted to achieve high-quality grids. Unstructured grids were generated in the near-burner region and ash hopper region, which were the irregular shapes. All other grids were structured. Grids were refined near the PA ports with a higher concentration of grid lines because the flow was expected to change rapidly there. There were 526 153 nodes and 503 360 cells in total in our simulation case, and the smallest grid was 6.83 cm3 in volume and small enough to guarantee the grid independency. In our case, most grids were structured hexahedral grids and no strong whirling flow was presented in the furnace without turbulent burners. The numerical diffusion should be naturally low; therefore, we used the first-order upwind discretization scheme instead of the second-order scheme without any significant loss of accuracy. Convergence criteria were absolute. Absolute convergence residues of NO, HCN, and NH3 were 10−7. The residues of continuity, velocity, k, and ε were 10−3, and the residues of energy and P1 were 10−6. Coal particle type was set as combustible, and the kinetic diffusion reaction model was the char oxidation model. A single-rate kinetic model was selected as the devolatilization model, and the kinetic rate was k = A exp(−E/RT), where A was the pre-exponential factor of 49 200 s−1 and E was the activation energy of 7.4 × 107 J/mol. No hightemperature volatile yield was considered specially. The particle− radiation interaction was activated to calculate radiative heat transfer of particles, and particle emissivity was set as a constant of 0.9. Coal dimensional distribution was Rosin−Rammler. The turbulent dispersion model was suited, and the discrete random walk model was selected. There were 10 particle diameters in the range of 1−100 μm in each injection surface. In total, 6400 coal particles were injected from PA-lean and -rich ports. The interaction of the discrete phase model (DPM) with a continuous phase was carried out every 300 times of continuous phase iterations. With regard to a steady-state

Figure 1. Schematic diagram of the 3.5 MW furnace with the HAPT. HAPT. The zone above the arches was the upper furnace with a net cross-section of 1500 × 1200 mm, and the lower furnace was 3000 × 1200 mm in net size. The furnace wall was 169 mm in thickness and composed of three layers of different materials from inside out: refractory material of 70 mm, water-cold tube, and insulation material of 50 mm. The height of the entire facility was 13.7 m, and the net height of the furnace is 9.6 m. The outlet temperature of cooling water should not be beyond 353 K. The primary air (PA) flows were inclined at an angle of 5° vertically downward through two double-cyclone burners located above the arches. The cyclone burners conducted a pulverized coal bias combustion and divided the pulverized coal flow into lean PA flows and rich PA flows. The lean PA flows approximately injected 15% of pulverized coal and 30% of air of PA flow rate, and the rich PA flows fed 85% of pulverized coal. Guizhou anthracite coal was used, and its components were given in Table 1. Pulverized coal powder was filtered by a filter with 90 μm diameter holes, and the weight residue on the filter was 5.5%. The air ports included two SA ports on the vertical wall (SA-W), two SA ports in the furnace ash hopper (SA-H), a SA port on the bottom (SA-B), two layers of OFA, two rich PA nozzles, and two lean PA nozzles. Because it was found that serious slagging might happen on the side walls of the lower furnace,20 45°-inclined SA-W and horizontal SA-H were designed to weaken slagging to some degree. Table 2 lists the pulverized coal and air distribution in the HAPT case. The openings of air valves had been tuned to guarantee the designed air ratios of all air ports. It took about 2 h for the furnace to reach steady state on the condition of no operation tuning. The tests would begin after the combustion had been stable for 1 h. Besides the air velocities measured by a Pitot-tube anemometer, the temperature and gas were sampled and measured through several observation ports

Table 1. Proximate and Ultimate Analyses of Guizhou Anthracite Coal (as Air Dried) proximate analysis (wt %)

ultimate analysis (wt %)

Mad

Aad

Vad

FCad

Cad

Had

Nad

St,ad

Oad

Qb,ad (J g−1)

0.87

24.1

6.47

68.56

65.1

2.54

0.94

2.6

3.85

26382

440

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combustion simulation, the interaction time step between the DPM and continuous phase did not change the final result but just the converging performance. Thermal NO and fuel NO were taken into consideration when simulating NOx. NOx simulation was conducted as a post-process of the flow-combustion-coupling simulation because the reactions of NOx generation did not impact the temperature or the flow at all.

3. RESULTS AND DISCUSSION 3.1. Experimental Results. The SA-H and SH-B ports in HAPT are special compared to the other DFCTs, and their Table 3. Experimental Results in the W Facility item coal feeding rate (kg/h) hot air temperature (K) NO emissions (mg/m3, at 6% O2) O2 at the furnace exit (%) average temperature at the furnace exit (K) maximum temperature in the furnace (K) UBC in fly ash (%) incomplete combustion loss (%)

without HAPT

HAPT

480 681 1289

480 655 683

480 663 626

480 685 737

3.8 1543

3.3 1218

2.9 1173

3.8 1246

no-SA-H no-SA-B

Figure 4. O2 and NO concentrations along the furnace central line and the PA flow direction.

Table 4. Simulated Results under Different Air Distributions

1731

1643

1656

1631

8.91 2.93

3.07 0.95

2.2 0.67

1.28 0.39

items

HAPT

no-SA-H

no-SA-B

air ratio of SA-W, SA-H, and SA-B (%) NO emissions (mg/m3 at 6% O2) O2 at the furnace exit (%) UBC (%) average temperature at the furnace exit (K) maximum temperature in the hopper (K) average temperature in the hopper (K) average NO in the hopper (mg/m3 at 6% O2) average velocity in the hopper (s/m) ignition distance (m) flame penetration depth (m)

9.6/23.6/17.2

28.2/0/22.8

17.5/33.7/0

671

663

755

3.01 3.2 1249

3.03 2.2 1240

3.05 0.9 1238

1579

1614

1603

1206

1298

1159

542

697

307

2.92

3.81

2.79

0.8 2.25

0.8 2.4

0.8 1.85

effects should be focused on and discovered. By closing down the SA-H or SH-B ports, the effects of them were researched. Table 3 gives the hot experimental results in the 3.5 MW facility, including four cases: the without HAPT, HAPT, closing down SA-H (no-SA-H), and closing down SA-B (no-SA-B). The without HAPT case was a normal down-fired technology without SA-H and SB-H ports, and it injected the secondary air by a layer of horizonal ports located on the same position of SA-W ports. NO emissions at the furnace exit were 683 mg/ Nm3 at 6% of O2, and UBC in fly ash was 3.07% in the HAPT case, which indicated that the HAPT could obtain a good performance of both NO emissions and coal burnout. In comparison to the without HAPT case, the HAPT presented a much lower furnace temperature, a significant decrease of UBC from 8.9%, and a NO reduction of 47%. The no-SA-H and noSA-B cases performed less UBC than the HAPT. Closing down SA-B induced NO emissions to increase, while closing down SA-H led to NO reduction. The effect of NO emissions associated with SA-B or SA-H was approximately 50 mg/m3. Figure 2 shows the gas temperature along the furnace central line by the experiment and simulation. In all figures, the Z-axis direction presented the height of the furnace and the origin of the Z axis was the furnace bottom, as shown by Figure 1. The simulation temperatures coincided generally with the experi-

Figure 2. Gas temperature along the furnace central line.

Figure 3. Central temperature of PA flow along the PA injection direction.

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Figure 5. Temperature and flow fields in the central cross-section in the three cases.

3.2. Effects of the SA-H and SH-B Ports. Table 4 lists the simulated results under different air distributions, and there are three cases: the HAPT, no-SA-H, and no-SA-B. The flow rates of SA-W and SA-B were enhanced in the no-SA-H case, and the rates of SA-W and SA-H were enhanced in the no-SA-B case. The simulation of the three cases generally agreed with the experimental data in Table 3. In comparison to the HAPT case, according to both the simulation and experimental results, the no-SA-H case reduced approximately 5% NO emissions and the no-SA-B case increased 10% NO emissions. Both cases dropped down UBC 1−2% from 3% of the HAPT case. The no-SA-B case showed larger NO emissions, less NO concentration in the ash hopper, shorter flame penetration depth, and lower temperature and velocity in the ash hopper. The latter three provided the ash hopper with harmful conditions to support coal particles burning and heat transfer to water and resulted in a wicked utilization of the ash hopper, especially the lower part of the ash hopper, which no hot flow filled with enough velocity. Thus, the utilization rate of the ash hopper should be smaller in the no-SA-B case. The flame penetration depth was defined as the vertical distance from the PA nozzle to the position where the temperature began being below 1450 K along PA flow streamlines. The gas temperature in the hopper was higher and flame penetration depth was larger obviously at the no-SA-H case. The temperature at the furnace exit was 1240 ± 9 °C, and the maximum temperature in the hopper appeared to have little difference in the three cases. The temperature and velocity distributions in the central cross-section in the three cases are graphed in Figure 5. The temperature had a similar distribution at the three cases, and the highest temperature zone was located in the height close to the SA-W ports. The no-SA-H case performed a higher temperature in the ash hopper as well as a longer SA-W penetration distance without disturbing SA-H. It was accompanied by a lower temperature distribution in the ash hopper center. The larger SA-H flows at the no-SA-B case blocked the flame penetrating down, which resulted in a lower temperature in ash hopper. There were good recirculating

ment. The experimental results were slightly higher in the lower furnace but lower in the upper furnace. This might be caused by the difference of the furnace wall. The furnace wall temperature was higher in the combustion zone and lower near the furnace exit during the test. However, the temperature of the entire furnace wall was fixed at the same value in simulation. This resulted in less difference between the peak temperature and the valley temperature of the furnace gas compared to the experiments. In Figure 3, the PA flow central temperatures along the PA injection direction are given and the ignition point was defined as the first position at 1000 K on the PA injection direction that can be achieved. The ignition point was located at the position that was 0.8 m away from the PA ports, and it showed that the pulverized coal was ignited quickly by recirculating hot gas flow. The ignition distance was much shorter than that of a 300 MW down-fired furnace, which was about 2−3 m.19 Figure 4 graphs the O2 and NO concentration curves along the furnace central line and the PA flow direction. Along the PA injection, oxygen gas was consumed by volatile oxidization first, which led to an increase of NO. After coal ignition beginning, the char burnt and NO was formed continuously but the NO concentration increased weakly because new air was fed and the volume of flue gas was increasing continuously. NO reached the minimum and O2 reached the maximum at the furnace bottom. In the central furnace at Z = 1.3−3.5, the NO concentration was higher and the O2 concentration was lower because of a strong combustion. When OFA was injected, the NO concentration dropped down as O2 increased and then NO went up with O2 consumption. When Z was above 5.5 m, NO and O2 changed little. Finally, NO reached 671 mg/m3 with 3.01% O2 by the simulation at the furnace exit and 683 mg/m3 with 3.3% O2 by the experiment. The experimental and simulation data coincided well with each other according to Figures 1−4, and the simulations were trustworthy to show detailed data. The detailed comparison of the experimental and simulation results in the no-SA-H and no-SA-B cases were not conducted because they are very similar to the HAPT case. 442

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Figure 6. Flow fields in the ash hopper in the three cases.

case. It could reach a balance between the utilization rate of the lower furnace and the safety, such as avoiding serious slagging and partial wall tube overheating. It was worth mentioning that dead recirculating zones appeared in the ash hopper in both noSA-H and no-SA-B cases. The flow fields in the ash hopper are presented in Figure 6 to discuss the dead recirculating zones in detail. At the HAPT case, small eddies appeared in the A region in Figure 6a, instead of the dead recirculating zone, because of the flow supplements through SA-H and SA-B ports. A long and narrow dead recirculating zone (the B region in Figure 6b) emerged close to the ash hopper tube wall. It was caused by the strong SA-W flow and no flow supplement from SA-H ports. The coal particle pathlines in Figure 7b also showed the dead recirculating zone at the no-SA-H case. This phenomenon would be dangerous and bring serious slagging in the ash hopper during the boiler operation. The no-SA-B case presented a dead recirculating flow as shown by Figure 6c. The dead recirculating zone was inside the hopper (the C region in Figure 6c) and not dangerous, as Figure 7c implies, because of no particles moving toward the tube wall. However, it was disadvantageous to the utilization rate of the ash hopper and boiler performance. Particle pathlines at the HAPT case

Figure 7. Particle pathlines from the left PA ports in the three cases.

zones, which took primary responsibility to heat and ignite the PA flow at all cases. Because of different rates of SA-W and SAH, the shapes of the recirculating zones were different. The noSA-H case had a longer recirculating zone, and the no-SA-B case the smaller recirculating zone. With the help of both SA-H and SA-B, the flame penetration of the HAPT case was rational, not short as the no-SA-B case and not long as the no-SA-H 443

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Figure 8. Species O2 and NO fields at the HAPT case.

Figure 9. Species O2 and NO fields at the no-SA-H case.

SA-H flows had early secondary air injection and bigger oxygen concentration for char burning, which resulted in bigger NO generation along coal particle streamlines and greater NO emissions as well as less UBC at the furnace exit. At the no-SAH case, closing down SA-H presented a larger combustion region with a low oxygen concentration, which led to a deeper air staging char combustion in the ash hopper to some extent and obtained less NO emissions finally. The longer flame penetration depth guaranteed the carbon burnout. The average NO concentration in the lower furnace was less, and the region with a greater NO concentration was obviously smaller at the HAPT case than the others. Furthermore, at the HAPT case, there was a lower NO distribution in the junction region between the lower and upper furnaces. It could be concluded that the HAPT case could achieve a better air-staging performance according to multi-factors, including NO emissions, UBC, and flow field. As one of the important factors of slagging as well as combustion, CO concentrations at the three cases are shown in Figure 11. There was a large region of high CO concentration

were similar to those at the no-SA-B case, except a deeper penetration. The shorter penetration depth at the no-SA-B case would give a passive impact on the furnace utilization rate. Figures 8−10 present the O2 and NO fields at the HAPT, no-SA-H, and no-SA-B cases, respectively. At the HAPT case, a long slim region with greater O2 concentration lied above the tube wall surface and formed a protecting layer from coal particle knocking and corrosion. When the no-SA-H case was compared to the HAPT case, there was a respectively less O2 zone in the under part of the ash hopper at the no-SA-H case, which meant that the SA-W flow probably was not able to provide a protection as strong as at the HAPT case for the tube wall of the under part of the ash hopper. During the anthracite combustion process, char N conversion to NO dominated the fuel N conversion and the oxygen concentration had an obvious impact on NO generation during char carbon combustion. The no-SA-B case performed less NO with more O2 in the ash hopper; however, there were some dead recirculating flows, and little coal particles were burning there. Thus, it did not mean something to NO reduction. The no-SA-B case with greater 444

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Figure 10. Species O2 and NO fields at the no-SA-B case.

Figure 11. CO concentrations in the three cases.

4. CONCLUSION On the basis of our prior research of a new deep air-staging technology, the HAPT, hot experiments were conducted in a down-fired 3.5 MW pilot facility. Moreover, combustion and NOx emission characteristics were investigated. The effects of SA-H and SH-B ports were focused on and discovered by simulations. The following can be concluded: (1) The HAPT case presented good performance of both NO emissions and coal burnout in our experiments. NO emissions at the furnace exit were 683 mg/Nm3 at 6% of O2, and UBC in fly ash was 3.07%. (2) The results of numerical simulation and experiments had a satisfactory consistency at the HAPT case. The HAPT case formed good aerodynamic fields, refrained dead recirculation zones from emerging in the ash hopper, and implemented the air-packed and deep air-staging particle combustion inside the furnace. (3) In comparison to the HAPT case, the no-SA-H case reduced approximately 5% NO

in the central furnace and a lower CO concentration near the tube wall at the HAPT case, as shown by Figure 11a, which meant that coal particles were packed by SA flow and burning inside the furnace. The SA-B port was located far away from the SA-W ports, and no air flow was injected between them in the no-SA-H case. As a consequent of delayed air feeding through the SA-B, a region of lower CO concentration emerged among the high CO ranges and the lower CO region was located above the tail of SA-W flow, as presented by Figure 11b. This should bring a passive effect on the combustion process and UBC. On the contrary, the no-SA-B case had greater SA-W and SA-H flows. The SA-H flow brought abundant cool air, and the air mixed with the burning main flow in advance. Thus, the SA-H flows cooled the combustion first, and then they assisted with coal particle combustion. As a result, the high CO region was broken and divided into two regions, as displayed by Figure 11c and implied by Figure 5c. 445

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(25) Ren, F.; Li, Z. Q.; Liu, G. K.; Chen, Z. C.; Zhu, Q. Y. Energy 2011, 36, 70−77. (26) Li, Z. Q.; Ren, F.; Liu, G. K.; Shen, S. P.; Chen, Z. C. Combust. Sci. Technol. 2011, 183, 238−251. (27) Yang, W. C.; Yang, W. J.; Zhou, Z. J.; Yuan, W. D.; Chen, Y. J.; Zhou, J. H.; Cen, K. F. J. Zhejiang Univ., Eng. Sci. 2012, 47, 139−145. (28) Yang, W. J.; Yang, W. C.; Zhou, Z. J.; Zhou, J. H.; Huang, Z. Y.; Liu, J. Z.; Cen, K. F. Fuel Process. Technol. 2014, 118, 90−97.

emissions and the no-SA-B increased 10% NO emissions. Both cases dropped down UBC 1−2% from 3% of the HAPT case. (4) It could be obtained by comparing the HAPT, no-SA-H, and no-SA-B cases that the SA-H flows took the responsibility of destroying the dead recirculating zones in the ash hopper and the SB-H flows affected the penetration depth of the main flow and the utilization rate of the ash hopper.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-571-8795-2885. Fax: +86-571-8795-1616. Email: [email protected].. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the China National High Technology Research and Development Program (2009AA05Z301) and the National Basic Research Program of China (2012CB214906).

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REFERENCES

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dx.doi.org/10.1021/ef4018652 | Energy Fuels 2014, 28, 439−446