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Investigation of the flow, combustion, heat-transfer and emissions from a 609 MW utility tangentially fired pulverized-coal boiler
Fuel 81 (2002) 997±1006
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Investigation of the ¯ow, combustion, heat-transfer and emissions from a 609 MW utility tangentially ®red pulverized-coal boiler q Chungen Yin a,*, SeÂbastien Caillat b, Jean-Luc Harion b, Bernard Baudoin b, Everest Perez c a
Institute of Energy Technology, Aalborg University, Pon. 101, DK-9220 Aalborg Oe., Denmark b DeÂpartement EnergeÂtique Industrielle, EÂcole des Mines de Douai, 59508 Douai, France c ALSTOM Power Boilers, 19-21 av. Morane Saulnier-BP74, 78141 VeÂlizy Cedex, France
Received 7 September 2001; revised 7 December 2001; accepted 8 December 2001; available online 4 February 2002
Abstract A numerical approach is given to investigate the performance of a 609 MW tangentially ®red pulverized-coal boiler, with emphasis on formation mechanism of gas ¯ow deviation and uneven wall temperature in crossover pass and on NOx emission. To achieve this purpose and obtain a reliable solution, some different strategies with the existing researches are used. Good agreement of simulation results with design parameters and site operation records indicates this simulation is pretty reasonable and thus the conclusions of the gas ¯ow deviation, emissions, combustion and heat transfer are reliable. These conclusions can be used to guide the design and operation of boilers of similar types. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Coal combustion simulation; Tangentially ®red boiler; Gas ¯ow deviation; NOx emission
1. Introduction
2. Method
Tangentially ®red pulverized-coal (PC) boilers are widely used in power generation. However, there still remain some problems, such as heater pipes over-temperature and pollutant emission. Numerical simulations have been successfully used to study coal combustion and investigate performances of large-scale boilers, for example, in Refs. [1±5]. Most of the existing simulations are mainly aimed at the ¯ow and combustion properties inside furnace and therefore, usually terminate at furnace exit. This research extends the calculation domain from the furnace exit, where the ¯ow is still quite unmixed, to the crossover pass and part of the rear pass, based on a new structured mesh whose grid in the furnace cross-section is approximately along the ¯ow direction to resolve the major property of the ¯ow [6]. These measures are proved to be able to get a better understanding of performances of a large-scale boiler and make it possible to investigate gas ¯ow deviation and uneven wall temperature in crossover pass.
2.1. Simulation object and calculation domain
* Corresponding author. E-mail address: [email protected] (C. Yin). q Published ®rst on the web via Fuel®rst.comÐhttp://www.fuel®rst.com
The 609 MW boiler can be shown schematically in Fig. 1, where P1, P2 and P3 are the three sections used frequently in the following analysis. This simulation is carried out on the basis of full-load operation. The analysis data of the coal ®red boiler are shown in Table 1. Different burners, listed in Fig. 2, are located at each of the four corners (A, B, C and D corners in Fig. 3), and make up a concentric ®ring system in the furnace, as shown in Fig. 3. In Fig. 2, the mark SA represents different secondary air inlets, PA denotes primary air inlets, `Leaking Air' is a controlled leakage air for some closed nozzles around the exit of operating burners, and OFA indicates over-®re air inlets. These marks together with the number followed are used for data presentation at a certain cross-section in the furnace in the following ®gures. The air velocity is about 25 m/s at PA inlets, about 50 m/s at SA (including OFA) inlets at full-load operation. The leaking air velocity is set to about 5 m/s. Coal particles are injected into the furnace from each PA inlet. To model the discrete phase coupling to the continuous phase ¯ow, 24 coal injections are de®ned at the 24 PA inlets. Each of these injections is divided into 90 particle
0016-2361/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0016-236 1(02)00004-2
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C. Yin et al. / Fuel 81 (2002) 997±1006
Fig. 1. Simpli®ed volume of the 609 MW boiler.
streams according to nine different locations and 10 groups of initial particle size. The 10 groups of sizes have particle diameters of 6, 20, 34, 48, 62, 76, 90, 104, 118 and 132 mm, whose mass fractions obey the Rosin±Rammler distribution, with a mean diameter of 57 mm. Since the ¯ow is still quite unmixed at the furnace exit (i.e. at the plane P1) where most of the existing simulations terminate, the upper furnace, the crossover pass and part of the rear pass are included in this investigation. In the upper furnace, there are three groups of panel super-heaters along the main ¯ow direction, i.e. in the y-direction. The ®rst two groups of panel super-heaters (®ve plates in each group in the x-direction) are modelled as constant-temperature (783 K) double-sided walls, and the third group of super-heaters (29 plates in the x-direction) is considered as a porous medium whose inertial resistance coef®cients in both the y- and z-directions are set to 0.35 according to the actually measured pressure dynamics across it, and that in x-direction is set to 50 to limit the anisotropy of these coef®cients to two orders of magnitudes. In the crossover pass, there are also three groups of panel super- and/or re-heaters arranged in the main ¯ow direction. All are modelled as porous media, whose inertial resistance coef®cients in both y-direction and z-direction are set to 0.72, 0.63 and 0.45, respectively, based on the actual
Fig. 2. Burners at each corner.
measured pressure losses. Their resistance coef®cients in x-direction are set to a same value of 50. In the rear pass section, two groups of heaters are included, and they are also modelled as porous zones, with the same inertial resistance set to 2 in all the three directions. 2.2. Mesh of the calculation domain Mesh is one of the most important factors in the simulation of large-scale boilers. To resolve as much of the major ¯ow property in the furnace as possible and thus reduce the pseudo-diffusion, a new structured mesh with grid approximately along the swirl ¯ow direction in furnace is created, as shown in Fig. 4. Since the ¯ow in the cross-section of the crossover pass and the rear pass is relatively simple, the commonly used quadrilateral grid is kept. There are, in total, 454 776 hexahedral cells, with 42 £ 37 £ 234 363 636 cells in the furnace.
Table 1 Analysis data of the coal ®red boiler Proximate analysis, % (dry)
Ultimate analysis, % (DAF)
Volatile
Fixed carbon
Ash
C
H
O
N
S
35.1
47.41
17.49
85.01
4.79
7.18
1.70
1.32
Lower heating value, MJ/kg (DAF) 33.066
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from combustion simulation. All these are general models, widely used and well documented. Here, the detailed formulations are not given. Some references can be found for details [5,7]. This calculation is conducted within the framework of FLUENT. 3. Results and discussions 3.1. Validation of combustion simulation with design parameters
Fig. 3. Concentric ®ring system in the furnace.
2.3. Overall models This simulation is based on a structured mesh with boundary ®tted coordinates. SIMPLE method is used in ¯ow ®eld prediction; standard k± e model is used as closure of turbulent Reynolds equations. Lagrangian particle trajectory is traced. To take the effects of turbulence on the particle trajectories into account, stochastic tracking model are used in which three trajectory calculations are performed to include turbulent velocity ¯uctuations into the particle force balance. During travelling through gas and interacting with gas, coal particles devolatilize and undergo char combustion, creating a source of fuel for reaction in gas phase, where two-competing-rates devolatilization model and kinetics/diffusion-limited char combustion model are used. Species and chemical reactions are modelled using the mixture-fraction/PDF approach and the full equilibrium chemistry, where the turbulence± chemistry interaction is modelled using a double-delta probability density function (PDF). The discrete ordinates (DO) model is employed to simulate radiation heat transfer. NOx, including thermal NOx and fuel NOx, are post-processed
The combustion calculation begins by solving the gas ¯ow ®eld equations assuming that the particles are absent. This simulation is well converged after more than 3000 iterations, as shown in Fig. 5. The convergence rate is relatively slow. The de®nition of some porous media is one of the important factors, through which the pressure losses are not very small. The porous media pressure drop appears as a momentum source termÐyielding a loss of diagonal dominanceÐin the matrix of equations solved. Another possible reason is the time of transfer of a perturbation from one point to another point; in this case the rear pass is not closely connected to the furnace. Based on this cold calculation, the coal combustion simulation is then carried out. The combustion simulation is validated with some key global design parameters, including the total heat transfer rate in the furnace, the average temperature and the average O2 mass fraction at the furnace exit (i.e. P1 in Fig. 1) and so on. According to the design of the boiler, the total heat transfer in the furnace should be around 609 MW; the average gas temperature and O2 mass fraction at the furnace outlet should be about 1600 K and 3.5%, respectively. The reported data from the simulation results agree very well with the design parameters, as shown in Table 2. Here, it should be stated that the emissivity of the furnace wall is set to 0.7 in this study for the radiation heat transfer simulation, since the average temperature in section P1 is quite dependent on this parameter. 3.2. Flow ®eld and gas temperature ®eld in the furnace
Fig. 4. Grid scheme at the furnace cross-section.
The gas ¯ow ®eld and the corresponding temperature distribution at some cross sections in the furnace are shown in Figs. 6 and 7, respectively, where the PA5 and SA3 can be found in Fig. 2. To present the simulated data, especially the range of the data, as precisely as possible, we neither round the data in most of the ®gures, nor employ a same data scale for a same group of ®gures. From Fig. 6(b) and (c), one can see the air injection forms a strong tangential circle ¯ow in the furnace, just as expected as design. From Fig. 6(a), one can see the platen super-heaters in the upper furnace cut the supposed circle and thus reduce the residual air¯ow swirling. Fig. 7 re¯ects the change trend of the ¯ame fullness degree along the furnace height, as well as the change trend of the gas temperature. In low plane SA3, because
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Fig. 5. Plot of scaled residuals in cold simulation (Unit: -).
the swirling and the mixing are not strong enough, the temperature appears non-uniform, high near the walls and low near the furnace centre. With the height increased, for example, at plane PA5, because of relatively high combustion intensity, strong swirl ¯ow and increased mixing, the mean temperature increases and appears quite uniform at this plane, except the zones near the four corners where cold air (358 K) of high velocity is injected. With further increase of the height and beyond the combustion section, for example at plane P1, because most of combustion is ®nished, the overall temperature level decreases under the cooling of the furnace walls. Meanwhile, because of the weakened gas ¯ow swirling and mixing, the high-temperature zones lie in the furnace centre and low near the furnace walls, which is opposite to that in plane SA3. The ¯ow at cross-sections in the furnace is shown earlier. To have a better understanding of ¯ow in the boiler, the z-directional velocity ®eld is shown in Fig. 8, from which one can see clearly the ¯ow mode in the furnace, i.e. `downTable 2 Validation of combustion simulation with key global design parameters Item
In boiler design
From simulation results
Average temperature at section P1 (K) Average O2 mass fraction at section P1 (%) Total heat transfer in the furnace (MW)
, 1600
1602
, 3.5
, 609
3.4
614
ward in near-wall zone and upward in the core'. In the bottom furnace, the ¯ow enters through the burners and part of the ¯ow goes downward along the furnace wall into the ash hopper, and then ¯ows upward near the furnace centre. The gas ¯ow is relatively quiescent in most part of the ash hopper. The ¯ow has relatively uniform upward velocity over the furnace cross section above the burnerszone, and bends around the nose of the furnace and exits the furnace through the platen heaters in the crossover pass. The overall temperature ®eld is shown in Fig. 9. From it, one can see clearly the approximate shape of the ¯ame in the furnace. This overall temperature ®eld also indicates that combustion causes gas temperature to increase from a relatively low inlet value at the burner throat to a maximum temperature about 2000 K at the furnace centre. Radiant heat loss from the ¯ue gas to the furnace walls causes temperatures to decreases as gas ¯ows upward in the furnace. The mixture exits the furnace at plane P1 with a mean value of about 1602 K. Temperature level near the furnace walls is also an important parameter, which can affect the slagging potential in the furnace. The average gas temperature near the furnace walls is about 1438 K in this calculation. The averages of the corresponding heat ¯ux distribution and the heat transfer coef®cient at the furnace walls are about 2230 kW/m 2 and 2597 W/(m 2 K), respectively. 3.3. Investigation of gas ¯ow deviation in the crossover pass Uneven wall temperature, i.e. gas temperature deviation between the two sidewalls, in the crossover pass is one of the most important problems for tangentially ®red pulverized coal boilers. It may lead to pipes over-temperature and
C. Yin et al. / Fuel 81 (2002) 997±1006
Fig. 6. Gas ¯ow ®eld in the furnace. (a) Temperature at the P1 section (average 1602 K). (b) Temperature at the PA5 section (average 1778 K). (c) Temperature at the SA3 section (average 1705 K).
vapour-leaking accidents of super-heaters or re-heaters nearby, which takes a large proportion in boiler accidents. As for its cause, different researchers hold different opinions. One typical opinion is that the platen super-heater is one of the important factors which cause the gas ¯ow and temperature deviation. However, from Fig. 6(a), one can see that the platen super-heaters in the upper furnace cut the circle, reduce the residual air¯ow swirling and thus
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Fig. 7. Temperature distribution in the furnace.
make the ¯ow uniform between the two sidewalls. The platen super-heaters in the crossover pass can also help distribute the gas ¯ow uniformly between the two sidewalls. So it might be concluded that the platen super-heaters can somehow correct the gas ¯ow and temperature deviation. However, the function is limited, and they cannot eliminate it completely. Another typical opinion is that one has to examine the coal particle trajectories and their combustion histories and how they affect the overall thermal characteristics inside and beyond the combustion section of the boiler. To see
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Fig. 8. Z-directional gas velocity distribution in the boiler.
the possible in¯uence, a quantitative summary of particle track is given (as shown in Table 3) instead of a presentation of some qualitative coal particle trajectories. In tangentially ®red boilers, the particles sweep around the furnace volume, and provide suf®cient residence time, so the char conversion is pretty high, more than 99%. During coal combustion, all of the fuel species except char are consumed within 1 or 2 m of the burner throats, while char burns at a slower rate, and normally is not consumed until it reaches the furnace centre. It can also be re¯ected from the temperature distribution in the furnace in the above section. However, the char concentration is very low above the furnace exit (P1 in Fig. 1), with mass fraction less than 2 £ 10 210 according to the simulation. So, the particle combustion in the upper furnace and its effect on the uneven wall temperature in the crossover pass may be negligible. The effect of the particle combustion
Fig. 9. Temperature distribution in the boiler.
histories inside the combustion section on the uneven wall temperature in the crossover pass might be also neglected because of the reasonable design distance between the upmost burner and the crossover pass and the long period of intensive mixing in the furnace. It is well known that the particles are not perfectly coincident with the gas ¯ow because of the turbulent ¯uctuation, here about 90% following gas ¯ow out of the boiler exit. However, generally speaking, it is the gas ¯ow that determines the trajectories of particles, especially in the upper furnace and in the crossover pass since the particles are very tiny and the particle concentration is pretty low. So essentially, one perhaps should have to check the gas ¯ow to investigate the uneven wall temperature, or more clearly, by checking the ¯ow deviation induced by the residual swirling in upper furnace and in the crossover pass. Gas ¯ow deviation formation in crossover pass might be explained in Fig. 10, a top view of the boiler. M1 denotes the momentum of the residual air¯ow swirling in upper furnace; M3 is the momentum introduced by induction fan, which can be supposed as constant here along the width of crossover pass; and M2 represents the composite momentum of gas in crossover pass. This kind of gas ¯ow deviation, together with the similar particles concentration deviation induced by the ¯ow deviation, will probably result in an uneven wall temperature distribution in upper furnace and in crossover pass. Figs. 11 and 12 show the gas ¯ow and the corresponding gas temperature distribution at the exit of the crossover pass where the heater surface over-temperature and vapourleaking accidents are mostly taking place, respectively. Here, we have to state the temperature peak in Fig. 9 may be a little higher than the actual value because the heat transfer between gas ¯ow and the platen heaters in the crossover pass, modelled as porous media in this study, is not taken into account. However, it has little in¯uence on the results of the temperature deviation between the two sidewalls in the crossover pass. From the ®gures, one can see that there exist some deviations between the two sidewalls of the crossover pass, just as analysed earlier; however, the deviations are pretty small. The small
Fig. 10. Simpli®ed mechanism of gas ¯ow deviation.
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Table 3 Summary of particle track (number tracked 6480, among which, escaped (i.e. the trajectories that reach the boiler exit) 5714 incomplete (i.e. the trajectories that were terminated after a large enough time-steps) 766) Elapsed time (s) Fate Min Incomplete 3.174 Escaped 3.930
Max 21.47 23.95
Average 14.12 8.012
Mass ¯ow (kg/s) Fate Initial Incomplete 6.105 Escaped 48.36
Final 1.162 8.461
Change 24.943 239.899
Heat content (W) Fate Initial Incomplete 25:262 £ 106 Escaped 24:168 £ 107
Final 1:167 £ 106 7:431 £ 106
Change 6:429 £ 106 4:911 £ 107
Combusting particles Volatile content (kg/s) Fate Incomplete Escaped
Initial 2.143 16.98
Final 0 0
%Conv 100 100
Final 0.09482 0.002716
%Conv 96.74 99.99
Combusting particles Char content (kg/s) Fate Incomplete Escaped
Initial 2.894 22.93
deviations are probably due to two factors. One is that the residual air¯ow swirling at the upper furnace is not strong because of the reasonable design of the concentric ®ring system in the furnace and the distance between the upmost burner and the upper furnace. The other is that the residual swirlingat the furnace exit may be somehow corrected by the presence of super-heater platens in the upper furnace and in the crossover pass. The platen super-heaters in the upper furnace cut the supposed circle and reduce the residual air¯ow swirling to some extent, and the platen heaters in the crossover pass can help distribute ¯ue gas between the two sidewalls and thus reduce the ¯ow deviation. So, the gas temperature deviation in the crossover pass is not serious, less than 60 K higher on the left side. This is in accordance with the safe operation of the site platen heaters in the crossover pass. In fact, not all the tangentially ®red coal boilers will necessarily suffer from a serious gas temperature deviation in crossover pass although this problem has been widely reported. It depends on the structural design and the organization of the ¯ow and combustion in furnace. For example, Yuan et al. (1995) measured the temperature deviations at 14 pairs of points with distance to wall from 1 to 4 m between the two sidewalls in upper furnace of a 600 MW utility tangentially ®red boiler. The measured mean deviation is about 85 K, less than their modelled deviation (about 145 K in average).
Fig. 11. Gas ¯ow ®eld at cross sections in the crossover pass.
Fig. 12. Gas temperature distribution at cross-sections in the crossover pass. (a) O2 mass fraction. (b) CO2 mass fraction.
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3.4. Species distribution All the coal volatiles are formed in the near-burner region, chemical species closely related to this process are distributed over a fusiform convected zone coaxial with the burners. The combustion pattern is illustrated by the distribution of oxygen concentrations shown in Fig. 13. It is observed that O2 concentration is relatively high near the burners, and rapidly decreases to around 3% after complete combustion of the fuel. The steepest gradients in O2 concentration are near the burners where the fuel volatile species are more rapidly consumed. Fig. 13 also indicates the strong relation between the oxygen concentration and the temperature ®eld in the furnace: the high-temperature zones
Fig. 13. Distributions of some main components in the boiler.
correspond to low-oxygen concentrations. The distribution of CO2 is approximately the reverse of the oxygen concentration: zero at the burner throats increasing to around 20% after complete combustion of the fuel. CO reaches a maximum near the burner throat and rapidly decreases to a few parts per million outside the ¯ame. The char burns at a slower rate, and is not consumed until it reaches the centre of the furnace. The predicted species at the outlet agree well with the site everyday operation records. 3.5. NOx emission simulation NOx emission is also an important topic of coal combustion, and some works have been successfully done, such as in Refs. [8,9]. In this paper, it is believed a more reliable ¯ow and combustion solution was achieved, which in fact have great in¯uence on the NOx predictions, so an investigation of NOx emission is conducted. On the grounds that the NOx concentration is very low and has negligible impact on coal combustion prediction, a `post-processing' mode is used in this calculation, with the ¯ow ®eld, temperature, and all the species concentrations frozen. Both thermal and fuel mechanisms are taken into account for the NO formation, while prompt NO is neglected because of its insigni®cant amounts. The nitrogen in the coal is considered to partition between the volatiles and char such that its concentration in the volatiles is identical to that in the dry, ash-free parent coal. Fig. 14 shows the total NO emission in the boiler. From the results, the NO distributions in the furnace are found to have close relationship with the corresponding ¯ame shape and species distributions. Since the fuel-NO formation is strongly in¯uenced by oxygen concentration and temperature, this species is mostly formed within the envelope of the ¯ames. In the near-burner region, rapid release of volatiles along with the incoming oxygen injected from burners leads to rapid formation of HCN, sequentially NO, which results in a high NO level in those regions. In the centre of the
Fig. 14. Total NO distribution at the x-directional middle plane.
C. Yin et al. / Fuel 81 (2002) 997±1006
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Table 4 NO emissions Section
Evaluation (m)
Temperature (K)
Mass fraction of O2 (%)
Thermal NO (ppm)
Fuel NO (ppm)
Total NO (ppm)
PA1 SA3 PA4 PA6
,18 ,21 ,23 ,29
1620 1705 1791 1829
2.1 3.1 2.9 2.8
29.7 50.1 64.9 125.6
180.4 164.0 182.1 204.2
180.5 176.3 205.6 271.7
furnace, the temperature is not the highest, and the oxygen concentration is very low. They both show a lower NO concentration than the near-burner regions. The results show that the NO emission is fairly low. In tangentially coal-®red boilers, the particles sweep around the furnace volume and have long residence times. By staging combustion, the maximum temperatures are lower than those generated in the intense, concentrated combustion of a swirl burner. This kind of boiler has lower temperature peaks and relatively lower mixing rates, which favour low NO generation. Moreover, the staged air injection can also help reduce NO emission. The predicted total NO emission is 247 ppm at 13.6% CO2 (mole fraction) at the furnace exit, and 226 ppm at 14.5% CO2 (mole fraction) at the boiler exit, which agree well with the site operation data. Table 4 shows the individual NO emission at several PA or SA sections in the furnace. It can be found that the individual thermal and fuel NO values at one section do not add up to the levels predicted with the two models combined at the same section. It is because reversible reactions are involved. NO produced in one reaction mechanism can be destroyed in another reaction. However, the difference between them is not very large, about 20%. The thermal NO at section PA1 is fairly low, about 29.7 ppm, because of the low temperature level at the lowest PA section in the furnace. Thermal NO is mainly sensitive to the temperature level. When the maximum temperature is less than 1800 K, the thermal NO will be very low. Table 4 also shows that the fuel NO is the dominant mechanism of NO formation in pulverized coal ®red boilers. Based on this calculation, the fuel NO takes about 70% of the total NO on average. 4. Conclusions A calculation, based on a new structured mesh and extended to almost the full boiler, was successfully carried out to investigate the performances of a large-scale tangentially ®red coal boiler. Good agreement of the simulation results with global design parameters and available site operation records indicate that this calculation is reliable and can be used to optimise the design and operation of similar boilers. From this investigation, some main conclusions can be summarized as follows. 1. It is residual air¯ow swirling at the furnace exit that
essentially causes the gas ¯ow deviation and therefore, the uneven wall temperature in the crossover pass in tangentially ®red boilers. The residual swirling is somehow corrected by the presence of platen super-heaters in upper furnace and in crossover pass, however, it cannot be eliminated completely. So, gas ¯ow deviation and uneven wall temperature are inherent phenomena in tangentially ®red boilers. Reasonable design of boiler's structures, such as concentric ®ring system, and the distance between the top burner and the crossover pass inlet and so on, may help reduce the gas ¯ow deviation and thus reduce the uneven wall temperature in crossover pass to an acceptable range. The gas ¯ow deviation and the uneven wall temperature in the crossover pass of this 609 MW boiler are small and will not endanger its operation. 2. In this boiler, the average gas temperature near the furnace walls is about 1438 K at full load operation. The averages of heat ¯ux and the heat transfer coef®cient at the furnace walls are about 2230 kW/m 2 and 2597 W/(m 2 K), respectively. These parameters may be helpful for boiler design, as well as boiler operation. 3. Most of the NO is formed from fuel nitrogen. The NO emission is fairly low in this boiler because of the relatively low temperature peaks, use of low NOx burners and staging combustion technology. Staging the combustion air produces fuel-rich/fuel-lean sequencing favourable for the conversion of fuel bound N to N2; staging the coal controls the NOx emission in the way that the NOx formed earlier in the ¯ame can be reduced by its reactions with hydrocarbon radicals.
Acknowledgements The authors gratefully acknowledge ADEME and ALSTOM Company (France) for their ®nancial support on this work, and Associate Prof. Dr Thomas J. Condra and Associate Prof. Dr Lasse A. Rosendahl in Department of Energy Technology, Aalborg University (Denmark) for their discussions and advice. References [1] Fiveland WA, Latham CE. Combust Sci Technol 1993;93:53. [2] Yuan J, Xu M, Han C, Ding S, Cao H. In: Xu X, Zhou L, editors.
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Proceeding of the Third International Symposium on Coal Combustion Science and Technology, 1995. p. P234. Lockwood FC, Mahmud T, Yehia MA. Fuel 1998;77:1329. Fan J, Sun P, Zha X, Cen K. Energy Fuels 1999;13:1051. Fan J, Zha X, Cen K. Fuel 2001;80:373. Yin C. Numerical simulation of the ¯ow, combustion, heat-transfer and
emissions from a 609 MW tangentially ®red pulverized-coal boiler. Post-Doctoral Research Report, DeÂpartement EnergeÂtique Industrielle, EÂcole des Mines de Douai, France, 2000. [7] FLUENT5-user's guide. Fluent Inc., 1998. [8] Coimbra CFM, Azevedo JLT, Carvalho MG. Fuel 1994;73:1128. [9] Stanmore BR, Visona SP. Fuel Process Technol 2000;64:25.
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Investigation of the ¯ow, combustion, heat-transfer and emissions from a 609 MW utility tangentially ®red pulverized-coal boiler q Chungen Yin a,*, SeÂbastien Caillat b, Jean-Luc Harion b, Bernard Baudoin b, Everest Perez c a
Institute of Energy Technology, Aalborg University, Pon. 101, DK-9220 Aalborg Oe., Denmark b DeÂpartement EnergeÂtique Industrielle, EÂcole des Mines de Douai, 59508 Douai, France c ALSTOM Power Boilers, 19-21 av. Morane Saulnier-BP74, 78141 VeÂlizy Cedex, France
Received 7 September 2001; revised 7 December 2001; accepted 8 December 2001; available online 4 February 2002
Abstract A numerical approach is given to investigate the performance of a 609 MW tangentially ®red pulverized-coal boiler, with emphasis on formation mechanism of gas ¯ow deviation and uneven wall temperature in crossover pass and on NOx emission. To achieve this purpose and obtain a reliable solution, some different strategies with the existing researches are used. Good agreement of simulation results with design parameters and site operation records indicates this simulation is pretty reasonable and thus the conclusions of the gas ¯ow deviation, emissions, combustion and heat transfer are reliable. These conclusions can be used to guide the design and operation of boilers of similar types. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Coal combustion simulation; Tangentially ®red boiler; Gas ¯ow deviation; NOx emission
1. Introduction
2. Method
Tangentially ®red pulverized-coal (PC) boilers are widely used in power generation. However, there still remain some problems, such as heater pipes over-temperature and pollutant emission. Numerical simulations have been successfully used to study coal combustion and investigate performances of large-scale boilers, for example, in Refs. [1±5]. Most of the existing simulations are mainly aimed at the ¯ow and combustion properties inside furnace and therefore, usually terminate at furnace exit. This research extends the calculation domain from the furnace exit, where the ¯ow is still quite unmixed, to the crossover pass and part of the rear pass, based on a new structured mesh whose grid in the furnace cross-section is approximately along the ¯ow direction to resolve the major property of the ¯ow [6]. These measures are proved to be able to get a better understanding of performances of a large-scale boiler and make it possible to investigate gas ¯ow deviation and uneven wall temperature in crossover pass.
2.1. Simulation object and calculation domain
* Corresponding author. E-mail address: [email protected] (C. Yin). q Published ®rst on the web via Fuel®rst.comÐhttp://www.fuel®rst.com
The 609 MW boiler can be shown schematically in Fig. 1, where P1, P2 and P3 are the three sections used frequently in the following analysis. This simulation is carried out on the basis of full-load operation. The analysis data of the coal ®red boiler are shown in Table 1. Different burners, listed in Fig. 2, are located at each of the four corners (A, B, C and D corners in Fig. 3), and make up a concentric ®ring system in the furnace, as shown in Fig. 3. In Fig. 2, the mark SA represents different secondary air inlets, PA denotes primary air inlets, `Leaking Air' is a controlled leakage air for some closed nozzles around the exit of operating burners, and OFA indicates over-®re air inlets. These marks together with the number followed are used for data presentation at a certain cross-section in the furnace in the following ®gures. The air velocity is about 25 m/s at PA inlets, about 50 m/s at SA (including OFA) inlets at full-load operation. The leaking air velocity is set to about 5 m/s. Coal particles are injected into the furnace from each PA inlet. To model the discrete phase coupling to the continuous phase ¯ow, 24 coal injections are de®ned at the 24 PA inlets. Each of these injections is divided into 90 particle
0016-2361/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0016-236 1(02)00004-2
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C. Yin et al. / Fuel 81 (2002) 997±1006
Fig. 1. Simpli®ed volume of the 609 MW boiler.
streams according to nine different locations and 10 groups of initial particle size. The 10 groups of sizes have particle diameters of 6, 20, 34, 48, 62, 76, 90, 104, 118 and 132 mm, whose mass fractions obey the Rosin±Rammler distribution, with a mean diameter of 57 mm. Since the ¯ow is still quite unmixed at the furnace exit (i.e. at the plane P1) where most of the existing simulations terminate, the upper furnace, the crossover pass and part of the rear pass are included in this investigation. In the upper furnace, there are three groups of panel super-heaters along the main ¯ow direction, i.e. in the y-direction. The ®rst two groups of panel super-heaters (®ve plates in each group in the x-direction) are modelled as constant-temperature (783 K) double-sided walls, and the third group of super-heaters (29 plates in the x-direction) is considered as a porous medium whose inertial resistance coef®cients in both the y- and z-directions are set to 0.35 according to the actually measured pressure dynamics across it, and that in x-direction is set to 50 to limit the anisotropy of these coef®cients to two orders of magnitudes. In the crossover pass, there are also three groups of panel super- and/or re-heaters arranged in the main ¯ow direction. All are modelled as porous media, whose inertial resistance coef®cients in both y-direction and z-direction are set to 0.72, 0.63 and 0.45, respectively, based on the actual
Fig. 2. Burners at each corner.
measured pressure losses. Their resistance coef®cients in x-direction are set to a same value of 50. In the rear pass section, two groups of heaters are included, and they are also modelled as porous zones, with the same inertial resistance set to 2 in all the three directions. 2.2. Mesh of the calculation domain Mesh is one of the most important factors in the simulation of large-scale boilers. To resolve as much of the major ¯ow property in the furnace as possible and thus reduce the pseudo-diffusion, a new structured mesh with grid approximately along the swirl ¯ow direction in furnace is created, as shown in Fig. 4. Since the ¯ow in the cross-section of the crossover pass and the rear pass is relatively simple, the commonly used quadrilateral grid is kept. There are, in total, 454 776 hexahedral cells, with 42 £ 37 £ 234 363 636 cells in the furnace.
Table 1 Analysis data of the coal ®red boiler Proximate analysis, % (dry)
Ultimate analysis, % (DAF)
Volatile
Fixed carbon
Ash
C
H
O
N
S
35.1
47.41
17.49
85.01
4.79
7.18
1.70
1.32
Lower heating value, MJ/kg (DAF) 33.066
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999
from combustion simulation. All these are general models, widely used and well documented. Here, the detailed formulations are not given. Some references can be found for details [5,7]. This calculation is conducted within the framework of FLUENT. 3. Results and discussions 3.1. Validation of combustion simulation with design parameters
Fig. 3. Concentric ®ring system in the furnace.
2.3. Overall models This simulation is based on a structured mesh with boundary ®tted coordinates. SIMPLE method is used in ¯ow ®eld prediction; standard k± e model is used as closure of turbulent Reynolds equations. Lagrangian particle trajectory is traced. To take the effects of turbulence on the particle trajectories into account, stochastic tracking model are used in which three trajectory calculations are performed to include turbulent velocity ¯uctuations into the particle force balance. During travelling through gas and interacting with gas, coal particles devolatilize and undergo char combustion, creating a source of fuel for reaction in gas phase, where two-competing-rates devolatilization model and kinetics/diffusion-limited char combustion model are used. Species and chemical reactions are modelled using the mixture-fraction/PDF approach and the full equilibrium chemistry, where the turbulence± chemistry interaction is modelled using a double-delta probability density function (PDF). The discrete ordinates (DO) model is employed to simulate radiation heat transfer. NOx, including thermal NOx and fuel NOx, are post-processed
The combustion calculation begins by solving the gas ¯ow ®eld equations assuming that the particles are absent. This simulation is well converged after more than 3000 iterations, as shown in Fig. 5. The convergence rate is relatively slow. The de®nition of some porous media is one of the important factors, through which the pressure losses are not very small. The porous media pressure drop appears as a momentum source termÐyielding a loss of diagonal dominanceÐin the matrix of equations solved. Another possible reason is the time of transfer of a perturbation from one point to another point; in this case the rear pass is not closely connected to the furnace. Based on this cold calculation, the coal combustion simulation is then carried out. The combustion simulation is validated with some key global design parameters, including the total heat transfer rate in the furnace, the average temperature and the average O2 mass fraction at the furnace exit (i.e. P1 in Fig. 1) and so on. According to the design of the boiler, the total heat transfer in the furnace should be around 609 MW; the average gas temperature and O2 mass fraction at the furnace outlet should be about 1600 K and 3.5%, respectively. The reported data from the simulation results agree very well with the design parameters, as shown in Table 2. Here, it should be stated that the emissivity of the furnace wall is set to 0.7 in this study for the radiation heat transfer simulation, since the average temperature in section P1 is quite dependent on this parameter. 3.2. Flow ®eld and gas temperature ®eld in the furnace
Fig. 4. Grid scheme at the furnace cross-section.
The gas ¯ow ®eld and the corresponding temperature distribution at some cross sections in the furnace are shown in Figs. 6 and 7, respectively, where the PA5 and SA3 can be found in Fig. 2. To present the simulated data, especially the range of the data, as precisely as possible, we neither round the data in most of the ®gures, nor employ a same data scale for a same group of ®gures. From Fig. 6(b) and (c), one can see the air injection forms a strong tangential circle ¯ow in the furnace, just as expected as design. From Fig. 6(a), one can see the platen super-heaters in the upper furnace cut the supposed circle and thus reduce the residual air¯ow swirling. Fig. 7 re¯ects the change trend of the ¯ame fullness degree along the furnace height, as well as the change trend of the gas temperature. In low plane SA3, because
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Fig. 5. Plot of scaled residuals in cold simulation (Unit: -).
the swirling and the mixing are not strong enough, the temperature appears non-uniform, high near the walls and low near the furnace centre. With the height increased, for example, at plane PA5, because of relatively high combustion intensity, strong swirl ¯ow and increased mixing, the mean temperature increases and appears quite uniform at this plane, except the zones near the four corners where cold air (358 K) of high velocity is injected. With further increase of the height and beyond the combustion section, for example at plane P1, because most of combustion is ®nished, the overall temperature level decreases under the cooling of the furnace walls. Meanwhile, because of the weakened gas ¯ow swirling and mixing, the high-temperature zones lie in the furnace centre and low near the furnace walls, which is opposite to that in plane SA3. The ¯ow at cross-sections in the furnace is shown earlier. To have a better understanding of ¯ow in the boiler, the z-directional velocity ®eld is shown in Fig. 8, from which one can see clearly the ¯ow mode in the furnace, i.e. `downTable 2 Validation of combustion simulation with key global design parameters Item
In boiler design
From simulation results
Average temperature at section P1 (K) Average O2 mass fraction at section P1 (%) Total heat transfer in the furnace (MW)
, 1600
1602
, 3.5
, 609
3.4
614
ward in near-wall zone and upward in the core'. In the bottom furnace, the ¯ow enters through the burners and part of the ¯ow goes downward along the furnace wall into the ash hopper, and then ¯ows upward near the furnace centre. The gas ¯ow is relatively quiescent in most part of the ash hopper. The ¯ow has relatively uniform upward velocity over the furnace cross section above the burnerszone, and bends around the nose of the furnace and exits the furnace through the platen heaters in the crossover pass. The overall temperature ®eld is shown in Fig. 9. From it, one can see clearly the approximate shape of the ¯ame in the furnace. This overall temperature ®eld also indicates that combustion causes gas temperature to increase from a relatively low inlet value at the burner throat to a maximum temperature about 2000 K at the furnace centre. Radiant heat loss from the ¯ue gas to the furnace walls causes temperatures to decreases as gas ¯ows upward in the furnace. The mixture exits the furnace at plane P1 with a mean value of about 1602 K. Temperature level near the furnace walls is also an important parameter, which can affect the slagging potential in the furnace. The average gas temperature near the furnace walls is about 1438 K in this calculation. The averages of the corresponding heat ¯ux distribution and the heat transfer coef®cient at the furnace walls are about 2230 kW/m 2 and 2597 W/(m 2 K), respectively. 3.3. Investigation of gas ¯ow deviation in the crossover pass Uneven wall temperature, i.e. gas temperature deviation between the two sidewalls, in the crossover pass is one of the most important problems for tangentially ®red pulverized coal boilers. It may lead to pipes over-temperature and
C. Yin et al. / Fuel 81 (2002) 997±1006
Fig. 6. Gas ¯ow ®eld in the furnace. (a) Temperature at the P1 section (average 1602 K). (b) Temperature at the PA5 section (average 1778 K). (c) Temperature at the SA3 section (average 1705 K).
vapour-leaking accidents of super-heaters or re-heaters nearby, which takes a large proportion in boiler accidents. As for its cause, different researchers hold different opinions. One typical opinion is that the platen super-heater is one of the important factors which cause the gas ¯ow and temperature deviation. However, from Fig. 6(a), one can see that the platen super-heaters in the upper furnace cut the circle, reduce the residual air¯ow swirling and thus
1001
Fig. 7. Temperature distribution in the furnace.
make the ¯ow uniform between the two sidewalls. The platen super-heaters in the crossover pass can also help distribute the gas ¯ow uniformly between the two sidewalls. So it might be concluded that the platen super-heaters can somehow correct the gas ¯ow and temperature deviation. However, the function is limited, and they cannot eliminate it completely. Another typical opinion is that one has to examine the coal particle trajectories and their combustion histories and how they affect the overall thermal characteristics inside and beyond the combustion section of the boiler. To see
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Fig. 8. Z-directional gas velocity distribution in the boiler.
the possible in¯uence, a quantitative summary of particle track is given (as shown in Table 3) instead of a presentation of some qualitative coal particle trajectories. In tangentially ®red boilers, the particles sweep around the furnace volume, and provide suf®cient residence time, so the char conversion is pretty high, more than 99%. During coal combustion, all of the fuel species except char are consumed within 1 or 2 m of the burner throats, while char burns at a slower rate, and normally is not consumed until it reaches the furnace centre. It can also be re¯ected from the temperature distribution in the furnace in the above section. However, the char concentration is very low above the furnace exit (P1 in Fig. 1), with mass fraction less than 2 £ 10 210 according to the simulation. So, the particle combustion in the upper furnace and its effect on the uneven wall temperature in the crossover pass may be negligible. The effect of the particle combustion
Fig. 9. Temperature distribution in the boiler.
histories inside the combustion section on the uneven wall temperature in the crossover pass might be also neglected because of the reasonable design distance between the upmost burner and the crossover pass and the long period of intensive mixing in the furnace. It is well known that the particles are not perfectly coincident with the gas ¯ow because of the turbulent ¯uctuation, here about 90% following gas ¯ow out of the boiler exit. However, generally speaking, it is the gas ¯ow that determines the trajectories of particles, especially in the upper furnace and in the crossover pass since the particles are very tiny and the particle concentration is pretty low. So essentially, one perhaps should have to check the gas ¯ow to investigate the uneven wall temperature, or more clearly, by checking the ¯ow deviation induced by the residual swirling in upper furnace and in the crossover pass. Gas ¯ow deviation formation in crossover pass might be explained in Fig. 10, a top view of the boiler. M1 denotes the momentum of the residual air¯ow swirling in upper furnace; M3 is the momentum introduced by induction fan, which can be supposed as constant here along the width of crossover pass; and M2 represents the composite momentum of gas in crossover pass. This kind of gas ¯ow deviation, together with the similar particles concentration deviation induced by the ¯ow deviation, will probably result in an uneven wall temperature distribution in upper furnace and in crossover pass. Figs. 11 and 12 show the gas ¯ow and the corresponding gas temperature distribution at the exit of the crossover pass where the heater surface over-temperature and vapourleaking accidents are mostly taking place, respectively. Here, we have to state the temperature peak in Fig. 9 may be a little higher than the actual value because the heat transfer between gas ¯ow and the platen heaters in the crossover pass, modelled as porous media in this study, is not taken into account. However, it has little in¯uence on the results of the temperature deviation between the two sidewalls in the crossover pass. From the ®gures, one can see that there exist some deviations between the two sidewalls of the crossover pass, just as analysed earlier; however, the deviations are pretty small. The small
Fig. 10. Simpli®ed mechanism of gas ¯ow deviation.
C. Yin et al. / Fuel 81 (2002) 997±1006
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Table 3 Summary of particle track (number tracked 6480, among which, escaped (i.e. the trajectories that reach the boiler exit) 5714 incomplete (i.e. the trajectories that were terminated after a large enough time-steps) 766) Elapsed time (s) Fate Min Incomplete 3.174 Escaped 3.930
Max 21.47 23.95
Average 14.12 8.012
Mass ¯ow (kg/s) Fate Initial Incomplete 6.105 Escaped 48.36
Final 1.162 8.461
Change 24.943 239.899
Heat content (W) Fate Initial Incomplete 25:262 £ 106 Escaped 24:168 £ 107
Final 1:167 £ 106 7:431 £ 106
Change 6:429 £ 106 4:911 £ 107
Combusting particles Volatile content (kg/s) Fate Incomplete Escaped
Initial 2.143 16.98
Final 0 0
%Conv 100 100
Final 0.09482 0.002716
%Conv 96.74 99.99
Combusting particles Char content (kg/s) Fate Incomplete Escaped
Initial 2.894 22.93
deviations are probably due to two factors. One is that the residual air¯ow swirling at the upper furnace is not strong because of the reasonable design of the concentric ®ring system in the furnace and the distance between the upmost burner and the upper furnace. The other is that the residual swirlingat the furnace exit may be somehow corrected by the presence of super-heater platens in the upper furnace and in the crossover pass. The platen super-heaters in the upper furnace cut the supposed circle and reduce the residual air¯ow swirling to some extent, and the platen heaters in the crossover pass can help distribute ¯ue gas between the two sidewalls and thus reduce the ¯ow deviation. So, the gas temperature deviation in the crossover pass is not serious, less than 60 K higher on the left side. This is in accordance with the safe operation of the site platen heaters in the crossover pass. In fact, not all the tangentially ®red coal boilers will necessarily suffer from a serious gas temperature deviation in crossover pass although this problem has been widely reported. It depends on the structural design and the organization of the ¯ow and combustion in furnace. For example, Yuan et al. (1995) measured the temperature deviations at 14 pairs of points with distance to wall from 1 to 4 m between the two sidewalls in upper furnace of a 600 MW utility tangentially ®red boiler. The measured mean deviation is about 85 K, less than their modelled deviation (about 145 K in average).
Fig. 11. Gas ¯ow ®eld at cross sections in the crossover pass.
Fig. 12. Gas temperature distribution at cross-sections in the crossover pass. (a) O2 mass fraction. (b) CO2 mass fraction.
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3.4. Species distribution All the coal volatiles are formed in the near-burner region, chemical species closely related to this process are distributed over a fusiform convected zone coaxial with the burners. The combustion pattern is illustrated by the distribution of oxygen concentrations shown in Fig. 13. It is observed that O2 concentration is relatively high near the burners, and rapidly decreases to around 3% after complete combustion of the fuel. The steepest gradients in O2 concentration are near the burners where the fuel volatile species are more rapidly consumed. Fig. 13 also indicates the strong relation between the oxygen concentration and the temperature ®eld in the furnace: the high-temperature zones
Fig. 13. Distributions of some main components in the boiler.
correspond to low-oxygen concentrations. The distribution of CO2 is approximately the reverse of the oxygen concentration: zero at the burner throats increasing to around 20% after complete combustion of the fuel. CO reaches a maximum near the burner throat and rapidly decreases to a few parts per million outside the ¯ame. The char burns at a slower rate, and is not consumed until it reaches the centre of the furnace. The predicted species at the outlet agree well with the site everyday operation records. 3.5. NOx emission simulation NOx emission is also an important topic of coal combustion, and some works have been successfully done, such as in Refs. [8,9]. In this paper, it is believed a more reliable ¯ow and combustion solution was achieved, which in fact have great in¯uence on the NOx predictions, so an investigation of NOx emission is conducted. On the grounds that the NOx concentration is very low and has negligible impact on coal combustion prediction, a `post-processing' mode is used in this calculation, with the ¯ow ®eld, temperature, and all the species concentrations frozen. Both thermal and fuel mechanisms are taken into account for the NO formation, while prompt NO is neglected because of its insigni®cant amounts. The nitrogen in the coal is considered to partition between the volatiles and char such that its concentration in the volatiles is identical to that in the dry, ash-free parent coal. Fig. 14 shows the total NO emission in the boiler. From the results, the NO distributions in the furnace are found to have close relationship with the corresponding ¯ame shape and species distributions. Since the fuel-NO formation is strongly in¯uenced by oxygen concentration and temperature, this species is mostly formed within the envelope of the ¯ames. In the near-burner region, rapid release of volatiles along with the incoming oxygen injected from burners leads to rapid formation of HCN, sequentially NO, which results in a high NO level in those regions. In the centre of the
Fig. 14. Total NO distribution at the x-directional middle plane.
C. Yin et al. / Fuel 81 (2002) 997±1006
1005
Table 4 NO emissions Section
Evaluation (m)
Temperature (K)
Mass fraction of O2 (%)
Thermal NO (ppm)
Fuel NO (ppm)
Total NO (ppm)
PA1 SA3 PA4 PA6
,18 ,21 ,23 ,29
1620 1705 1791 1829
2.1 3.1 2.9 2.8
29.7 50.1 64.9 125.6
180.4 164.0 182.1 204.2
180.5 176.3 205.6 271.7
furnace, the temperature is not the highest, and the oxygen concentration is very low. They both show a lower NO concentration than the near-burner regions. The results show that the NO emission is fairly low. In tangentially coal-®red boilers, the particles sweep around the furnace volume and have long residence times. By staging combustion, the maximum temperatures are lower than those generated in the intense, concentrated combustion of a swirl burner. This kind of boiler has lower temperature peaks and relatively lower mixing rates, which favour low NO generation. Moreover, the staged air injection can also help reduce NO emission. The predicted total NO emission is 247 ppm at 13.6% CO2 (mole fraction) at the furnace exit, and 226 ppm at 14.5% CO2 (mole fraction) at the boiler exit, which agree well with the site operation data. Table 4 shows the individual NO emission at several PA or SA sections in the furnace. It can be found that the individual thermal and fuel NO values at one section do not add up to the levels predicted with the two models combined at the same section. It is because reversible reactions are involved. NO produced in one reaction mechanism can be destroyed in another reaction. However, the difference between them is not very large, about 20%. The thermal NO at section PA1 is fairly low, about 29.7 ppm, because of the low temperature level at the lowest PA section in the furnace. Thermal NO is mainly sensitive to the temperature level. When the maximum temperature is less than 1800 K, the thermal NO will be very low. Table 4 also shows that the fuel NO is the dominant mechanism of NO formation in pulverized coal ®red boilers. Based on this calculation, the fuel NO takes about 70% of the total NO on average. 4. Conclusions A calculation, based on a new structured mesh and extended to almost the full boiler, was successfully carried out to investigate the performances of a large-scale tangentially ®red coal boiler. Good agreement of the simulation results with global design parameters and available site operation records indicate that this calculation is reliable and can be used to optimise the design and operation of similar boilers. From this investigation, some main conclusions can be summarized as follows. 1. It is residual air¯ow swirling at the furnace exit that
essentially causes the gas ¯ow deviation and therefore, the uneven wall temperature in the crossover pass in tangentially ®red boilers. The residual swirling is somehow corrected by the presence of platen super-heaters in upper furnace and in crossover pass, however, it cannot be eliminated completely. So, gas ¯ow deviation and uneven wall temperature are inherent phenomena in tangentially ®red boilers. Reasonable design of boiler's structures, such as concentric ®ring system, and the distance between the top burner and the crossover pass inlet and so on, may help reduce the gas ¯ow deviation and thus reduce the uneven wall temperature in crossover pass to an acceptable range. The gas ¯ow deviation and the uneven wall temperature in the crossover pass of this 609 MW boiler are small and will not endanger its operation. 2. In this boiler, the average gas temperature near the furnace walls is about 1438 K at full load operation. The averages of heat ¯ux and the heat transfer coef®cient at the furnace walls are about 2230 kW/m 2 and 2597 W/(m 2 K), respectively. These parameters may be helpful for boiler design, as well as boiler operation. 3. Most of the NO is formed from fuel nitrogen. The NO emission is fairly low in this boiler because of the relatively low temperature peaks, use of low NOx burners and staging combustion technology. Staging the combustion air produces fuel-rich/fuel-lean sequencing favourable for the conversion of fuel bound N to N2; staging the coal controls the NOx emission in the way that the NOx formed earlier in the ¯ame can be reduced by its reactions with hydrocarbon radicals.
Acknowledgements The authors gratefully acknowledge ADEME and ALSTOM Company (France) for their ®nancial support on this work, and Associate Prof. Dr Thomas J. Condra and Associate Prof. Dr Lasse A. Rosendahl in Department of Energy Technology, Aalborg University (Denmark) for their discussions and advice. References [1] Fiveland WA, Latham CE. Combust Sci Technol 1993;93:53. [2] Yuan J, Xu M, Han C, Ding S, Cao H. In: Xu X, Zhou L, editors.
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