Effects of Air Staging Conditions on the Combustion and NOx

Article pubs.acs.org/EF

Effects of Air Staging Conditions on the Combustion and NOx Emission Characteristics in a 600 MW Wall Fired Utility Boiler Using Lean Coal Hu Liu,† Yinhe Liu,† Guangzhou Yi,‡ Li Nie,‡ and Defu Che*,† †

School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China Dongfang Boiler Group Co., Ltd., Dongfang Electric Corporation, Chengdu, Sichuan 611731, China

‡

ABSTRACT: A three-dimensional numerical investigation is presented on the coal combustion and NOx emission in a supercritical 600 MW wall fired utility boiler fed with lean coal. The distributions of velocity, temperature, and species were obtained using a finite volume method and some validated submodels. The influence of air staging condition, i.e., overfire air (OFA) ratio and OFA port position, on combustion and especially on NOx emission was studied. The temperature of the burner zone and the carbon content in fly ash increased with increasing OFA ratio, while the NOx emission decreased with increasing OFA ratio. An increase of the OFA port position resulted in a decrease of NOx emission and an increase of carbon content in fly ash. In addition, high burnout rate of pulverized coal and low NOx emission can be achieved in the present boiler, in which OFA was injected from Port 3 with a ratio of 34.2%. This study provides new insights into the physical and chemical processes in a wall fired utility boiler fed with lean coal and also illustrates a method to reduce NOx emission.

1. INTRODUCTION Coal accounts for about 30% and 70% of world and China energy consumption,1 respectively. Obviously, it is one of the most crucial energy resources that must be effectively used to meet future energy needs, especially for China. In China, power generation takes about 50% of total coal consumption. As the most abundant fossil fuel, coal will continue to serve as a main fuel for power generation in the future. However, compared to other fuels, such as oil and natural gas, coal induces more serious pollution problems. For example, one-third of NOx emission in China results from the pulverized coal fired power generation.2 NOx is mainly responsible for the formation of acid rain and photochemical smog, which has caused continuous deterioration to the ecological environment and human health. As a response, China established strict environmental legislation3 to control NOx emission from pulverized-coal fired power generation. According to the legislation, NOx emission of new thermal power generating units, which were approved for construction after 2012, should be less than 100 mg/Nm3 (dry basis, 6% O2). Currently, the capacity of most of the new coal fired units with supercritical or ultrasupercritical parameters is equal to or larger than 600 MW in China. In these power units, a wall fired pulverized-coal boiler with swirl burners is widely used due to the uniform distribution of gas temperature and the good capability of minimum stable load without auxiliary fuel support. Lean coal is very abundant in China, thus there are many utility boilers working with it. However, compared to bituminous coal, lean coal was found to form much more NOx during combustion because of its low volatile content. Therefore, it is of great importance for these utility companies to reduce NOx emission while remaining profitable. Both the primary techniques such as fuel/air staging, low NOx burners, OFA, reburning, flue gas recirculation, and the © XXXX American Chemical Society

secondary techniques using catalytic denitrification facilities were applied in pulverized coal fired power generation units in order to meet the above NOx emission restriction. Minimum NOx emission may be achieved by combining the primary and secondary methods. Compared to the secondary techniques, most of the primary techniques are much more cost-effective and are preferred to reduce NOx emission. Generally, the simplest technique, air staging along the furnace height which is commonly known as OFA, has been widely adopted. NOx emission is closely related with the complex physical and chemical processes such as turbulent flow, combustion, and NOx formation reactions. Great efforts have been made both experimentally4−6 and numerically7−9 to understand these complex processes for the sake of reducing NOx emissions. Previous studies focused on the effects of OFA ratio and OFA port position. Fan et al.10,11 and Zhang et al.12 experimentally investigated the influence of air stoichiometric ratio and OFA port position on NOx emission for bituminous coal in an electrically heated one-dimensional furnace and a 3 MW pilot scale facility, respectively. Huang et al.,13 Liu et al.,14 and Zhou et al.15 numerically studied the effect of various OFA parameters on the NOx emission. Wu et al.16 found that NOx emission could be reduced by about 50% when the OFA ratio raised from 25% to 30% for a 600 MW wall fired furnace fed with bituminous coal. These studies indicate that NOx formation is sensitive to both OFA ratio and OFA port position. Combustion test in the pulverized coal fired utility boiler is a costly and arduous task, since a large amount of sophisticated instruments are needed for measurements. Only a small quantity of deficient data sets with a large measurement Received: July 15, 2013 Revised: September 9, 2013

A

dx.doi.org/10.1021/ef401354g | Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Schematic configurations and mesh system of the wall fired pulverized-coal boiler.

burner is reduced to three annular air nozzles with the size equal to the actual structure of the burner in the mesh system. The pulverized coal is fed into the furnace by the swirl-free primary air. The secondary air was divided into an inner and an outer part, without and with swirl, respectively. In this manner, each burner will carry out a two-stage-combustion by means of air staging. Combination of this technology with 12 OFA nozzles above the burners on the front and rear walls is very helpful for NOx reduction.

uncertainty was gathered from limited combustion tests. Compared to experiments, numerical simulations are more flexible and cost-effective to generate detailed data sets in wide ranges of conditions. Therefore, the numerical method is more convenient and suitable to analyze the combustion and pollutant formation process in a utility boiler under various operating conditions. In the past few decades, the numerical method was widely applied to investigate the complex phenomena in pulverized coal fired utility boiler, such as turbulent flow, combustion, heat transfer, and NOx emission. Some studies focused on the combustion characteristics in a wall fired utility boiler under different operating conditions.17−19 Several researchers studied the temperature deviation and the methods for decreasing gas flow and temperature deviation for a tangentially fired boiler.20−23 The characteristics of oxy-fuel combustion24−27 and cofiring28−31 in a utility boiler were also reported. Recently, more and more scholars have paid attention to pollutant emission from boilers under different operation conditions.7,12,15,32−35 All these studies have proven the numerical method to be a feasible and powerful tool to understand pulverized-coal combustion in a utility boiler. However, limited studies have focused on the effect of OFA operating conditions on NOx emission of a wall fired utility boiler fed with lean coal. In the present study, a numerical simulation of a wall fired boiler fed with lean coal was carried out to predict the flow pattern, combustion process, temperature distribution, and NOx emission under different OFA ratios and OFA port positions.

3. COMPUTATIONAL MODELING APPROACH AND OPERATING CONDITIONS 3.1. Mesh of the Calculation Domain. Mesh system is important for reliable and accurate calculations. A partition meshing method was applied to generate high quality mesh with hexahedral cells. The mesh was then refined around the burner regions where combustion vigorously takes place. A grid independency test was conducted to eliminate the numerical error due to dependence of prediction results on the grid sizes. The primary calculations were carried out for three grid numbers, i.e., 983 821, 1 467 228, and 1 906 982. As can be seen in Figure 2, predicted gas phase temperature distributions along the centerline of the furnace at different grid numbers are compared to determine the appropriate grid number. In the present work, the medium mesh system with 1 467 228 cells was used. 3.2. Numerical Models. The simulations were carried out using a commercial computational fluid dynamics code Fluent. Proper submodels were chosen to simulate devolatilization, volatile combustion, char oxidation, thermal radiation, and NOx emission processes in the boiler. The time averaged conservation equations for mass, momentum, and energy were solved using the SIMPLE algorithm. Instead of RNG and the realizable k-ε model, the standard k-ε model was chosen in the present study because it had proven to be the most suitable model in terms of computational economy, stability, and reliability of results in combustion applications.18,19,36,37 Coal particle trajectories were calculated using the Lagrangian approach. The P-1 radiation model is used to simulate radiation heat transfer between gas and particle in the furnace since it is applicable to any complex geometry.36 The domain based weighted-sum-of-gray-gases model8 is used to calculate the absorption coefficients of the gas phase. The two-competing-reactions model7−9,38 proposed by Kobayashi is used to simulate coal devolatilization. The process of char oxidation is

2. THE WALL FIRED PULVERIZED-COAL BOILER The simulations were performed for an inverted-U type wall fired 600 MW unit with supercritical parameters utilizing lean coal. The utility boiler is shown schematically in Figure 1. The height of the furnace is 67 m, and the horizontal cross section of the furnace is rectangular with a width of 19.4 m and a depth of 15.4 m. Totally 24 swirl burners equally divided into 6 rows are installed on the front and rear walls of the furnace. Each B


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volatile nitrogen and char nitrogen during coal devolatilization. It is often assumed that HCN and NH3 are the dominant intermediate species formed from the volatile nitrogen, which are further oxidized to NO while being competitively reduced to N2. Generation of N2 and NO is mainly related to the local environment, higher N2 production in fuel-rich regions while higher NO production in fuel-lean regions k4

HCN + O2 → NO + ···

(5)

k5

HCN + NO → N2 + ···

(6)

k6

NH3 + O2 → NO + ···

(7)

k7

NH3 + NO → N2 + ···

(8) 39

where the reaction rate constants are proposed by DeSoete. Additionally, with the interaction between char and gas phase intermediates, NO can be reduced to N2 through both heterogeneous and homogeneous reactions. However, in low NOx burners which are designed to create a fuel-rich region, the homogeneous reactions seem to be the dominate reduction reaction.39 3.3. Numerical Analysis. In the present study, the basic simulation case considered is based on the measurement case of 100% BMCR operation with all 6 mills working. The feed rate of coal to the furnace is 62.3 kg/s, and the mass flow rate and temperature of primary air and secondary air is 116.6 kg/s at 363 K and 487.5 kg/s at 608 K, respectively, including 14% excess air, with equal flow rates for all the burners. The designed swirl intensity of outer secondary air is set as the swirl boundary condition in the model. The corresponding properties of the lean coal used in experiments and simulations are presented in Table 1. The size distribution of

Figure 2. Mesh independence test based on temperature distribution along the centerline of the furnace. described by the kinetics/diffusion-limited model7,8,38 of Field, and the interaction between turbulence and chemical reaction can be simulated by the single mixture probability density function. All the models above were widely used to simulate the complex physic and chemical phenomena in large capacity boilers, which ensures the reliability of the present calculated results. NOx concentration in the combustion systems is assumed to have no significant influence on the flow field and the major combustion process.33,39 Thus, NOx calculation is carried out as a postprocessing procedure based on the solution of the main combustion calculation. Three mechanisms are responsible for the NOx formation in combustion systems: thermal NOx, prompt NOx, and fuel NOx. In large pulverized-coal flames, more than 90% of the total NOx production is NO, thus only NO is considered in our model.33 Thermal NOx is generated by the oxidation of nitrogen in the combustion air at a temperature higher than 1800 K.39 This process can be modeled by the extended Zeldovich mechanism as follows.39 The last reaction is usually negligible except in fuel-rich flames

Table 1. Proximate and Ultimate Analysis of the Coal and Coal Particle Diameter Distribution parameter proximate analysis [%](dry basis)

k1

N2 + O XoooY N + NO k −1

ultimate analysis [%](as-received basis)

(1)

k2

N + O2 XoooY O + NO k −2

(2)

k3

N + OH XoooY H + NO k −3

higher heating value [MJ/kg](Daf) particle diameter distribution

(3)

where ki represents the forward and reverse reaction rates, determined according to the evaluation of Hanson. Then, the net rate of thermal NO species via eqs 1−3 is given by

d[NO] = k1[O][N2] + k 2[N][O2 ] + k 3[N][OH] dt − k −1[NO][N] − k −2[NO][O] − k −3[NO][H]

volatile matters fixed carbon ash carbon(C) hydrogen(H) oxygen(O) nitrogen(N) sulfur(S) under #200 [Wt%] under #400 [Wt%] min./max. diameters [Μm] mean diameter [Μm] Rosin-Rammler spread parameter [-]

value 11.03 65.36 23.61 64.25 3.55 2.62 1.15 0.33 35.3 73.88 47.19 10/200 57.5 1.11

pulverized coal is assumed to obey the Rosin-Rammler algorithm which is widely used to calculate the particle size distribution of different types and sizes of powders. In this study, the diameter of coal powders ranges from 10 to 200 μm, and the mean diameter is 57.5 μm with a spread parameter of 1.11. Normally, 720 K and 0.8 are imposed as the wall temperature and wall emissivity boundary conditions. Five different OFA ratios (i.e., 21.1%, 25.4%, 29.8%, 34.2%, and 38.6%) and four different OFA port positions (i.e., Port 1, Port 2, Port 3, and Port 4) were used to study the effect of OFA ratio and port position on the characteristics of NOx emission. The OFA ratio varies from 21.1% to 38.6% with the total excess air ratio of 1.14. Different OFA port positions correspond to different residence times for the pulverized coal in the reducing zone which influences NOx emission significantly.12,15 Three imaginary layers of OFA nozzles are located above the burner zone as shown in Figure 1 (b). For each case, only

(4) The concentrations of O, H, and OH are computed by the partial equilibrium approach. Prompt NOx is produced by the reaction of atmospheric nitrogen with hydrocarbon radicals in fuel-rich regions of flames, which is subsequently oxidized to NO. In this study, prompt NOx is neglected because of its small amount in most combustion systems.39 Fuel NOx is the dominant mechanism of NOx formation in large pulverized-coal boiler. It is produced by oxidation of the nitrogen bound in the coal and typically accounts for more than 80% of the NOx formed in these systems.39 Formation of fuel NO is easier than that of thermal NO since the most common N−H and N−C bonds in fuel-bound nitrogen are much weaker than the triple bond in molecular nitrogen in the air. Fuel-bound nitrogen is usually split into C


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one layer OFA nozzles are in operation, while the other OFA nozzles are not in operation. The operating conditions of the boiler are listed in Table 2. Case 3 is the basic simulation case in this study.

4.2. Flow Fields and Coal Particle Trajectories. Figure 4 shows the gas flow velocity distribution in the vertical cross section and velocity vectors in six specific horizontal cross sections along the furnace height. It is evident that each burner generates a basically separate flame. Gas flow is pushed to the side wall in the center of the furnace and then forms a vortex at each corner of the furnace. The velocity distribution is more uniform in the higher height (sections E, F) than that in the lower height (sections A, B, C, D). Between the opposed OFA nozzles, insufficient mixing between the OFA and the flue gas is found. Part of the upward flue gas, mainly at the center of the furnace, is observed to pass between the OFA jets without being impeded, which may cause the increase of the carbon content in fly ash. In addition, it can be seen that the furnace is filled with the flue gas. Figure 5 depicts the trajectories of coal particles injected from different rows of burners and the streamlines of the flue gas. As can be seen, particle trajectories show very complicated flow behavior. Moreover, the particle trajectories and the flue gas streamlines are very similar but not exactly the same due to different densities and turbulent fluctuations. Most of the gas moves upward to the furnace exit after colliding in the furnace center, while a small part of the gas injected from the lowest burner rushes into the hopper section and forms a stagnant vortex. Part of the particles injected from the lowest burners initially circulate in the ash hopper section and then travel upward through the high temperature region, while particles from the upper burners directly travel upward to the high temperature region because of the significant affection from the gas flow from the lower zone. Therefore, the particles from the lower burners reside in the furnace for a longer time than those from the upper burners. Four well-defined steps, i.e., heating-up, devolatilization, volatile combustion, and char combustion, will take place after the coal particles enter the furnace. According to Figure 5 (b), the average residence time of coal particles is about 8 s, which is sufficient for most coal particles to complete the combustion process since volatile combustion times are of the order of 10 ms and char takes about 300 ms to burn out.40 4.3. Temperature Distributions. Figure 6 shows the temperature distributions in the vertical cross section and six horizontal cross sections along the furnace height. From both Figure 6 (a) and Figure 3, it can be seen that the temperature increases along the furnace height in the burner zone. At the OFA nozzles layer, the temperature decreases obviously since a large amount of OFA with relative low temperature is injected into the furnace. Above the OFA nozzles, the temperature begins to rise due to the combustion of the residual CO and char. When the height further increases, the temperature gradually decreases because of the heat transfer from the flue gas to the water cooled walls. In the horizontal cross sections, symmetrical temperature distributions are identified, and the maximum temperature and temperature gradient can be seen in the vicinity of the burner exit. The temperature is relatively low and uniformly distributed in the central region of the cross section. It can be seen from Figure 6 (b) that the temperature distribution is more uniform along the furnace width, which is the outstanding feature of the wall fired boiler. However, the temperature is too high to prevent coking and high temperature corrosion on the central region of the side wall. Therefore, more attention should be paid to prevent these negative impacts on the furnace.

Table 2. Test Case Information of the 600 MW Wall Fired Boiler OFA ratio Case Case Case Case Case Case Case Case

1 2 3 (basic case) 4 5 6 7 8

21.1% 25.4% 29.8% 34.2% 38.6% 34.2% 34.2% 34.2%

OFA position OFA OFA OFA OFA OFA OFA OFA OFA

Port Port Port Port Port Port Port Port

1 1 1 1 1 2 3 4

29465.7 mm 29465.7 mm 29465.7 mm 29465.7 mm 29465.7 mm 32504 mm 37504 mm 42504 mm

4. RESULTS AND DISCUSSION 4.1. Model Validation. In order to obtain creditable and reasonable simulation results, the numerical calculation results were validated with the measured values provided by Dongfang Boiler Group Co., Ltd. Both the area-weighted average temperature along the furnace height and the NOx concentration values at the furnace exit were compared with the measured values, as shown in Figure 3 and Table 3.

Figure 3. Comparison of the calculated and measured area-average temperature profiles along the furnace height.

Table 3. Comparison between the Measured Data and the Predicted Results parameter

carbon content in fly ash (%)

NOx concn (mg/m3 6% O2)

measured calculated

5.04 4.36

508 512

Figure 3 illustrates the numerical results and the corresponding experimental data of area-weighted average temperature distributions. Table 3 presents predicted and measured NOx concentration values at the furnace exit. On the whole, the maximum deviation of the present simulation is 12.1% for the average temperature over section A. Such an agreement is quite satisfactory from an engineering perspective. Consequently, the above-described numerical method is valid for the current problem and is used in this paper. D


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Figure 4. Velocity distribution and velocity vectors in different cross sections.

4.4. Species Distributions. The mass fraction distributions of O2, CO2, and CO are shown in Figure 7. Obviously, O2 concentration rapidly decreases when flows downstream from the burners since O2 is consumed during the coal particle combustion processes. As a result, CO2 concentration significantly increases in contrast. As shown in Figure 7 (c) and (d), high CO concentration is identified at the central region of flame zone since the specially designed low NOx burners can establish both fuel-rich and fuel-lean regions in the flame zone of each burner through dividing the secondary air into two streams, and CO generates in the fuel-rich region. A large amount of CO accumulates at the center of the furnace, and part of it appears near the side wall, as shown in Figure 7 (c), because part of the CO generated at the center of the flame zone was pushed to the side wall after colliding at the center of the burner zone. 4.5. NOx Distributions. NOx emission is a serious problem in coal fired utility boilers. Much effort has been made to reduce NOx emission.12,15,16,32,35 In this study, a postprocessing procedure is adopted to calculate the NOx formation based on reliable flow and combustion results, which is critical for the NOx predictions. Only NO is considered in the present study with both thermal NO and fuel NO taken into account, while prompt NO formation is neglected.39 Fuel nitrogen is

Figure 5. Streamlines of flue gas and coal particle trajectories.

Figure 6. Temperature distributions. E


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Figure 7. Species distributions.

Figure 8. NOx distributions.

concentration is observed since the oxygen concentration is very low and the CO concentration is relatively high. 4.6. Effects of Staging Conditions on Combustion and NOx Emission. Figure 9 shows the predicted area-weighted average temperature and NOx distributions along the furnace height under different OFA ratios. From Case 1 to Case 5, the OFA ratio increases from 21.1% to 38.6%. Similar average temperature and NOx distributions are identified for the five cases. In addition, temperature distributions are similar for all five cases around the burner zone, while the combustion details are different. The average temperature increases with an increased OFA ratio, which agrees with the results of Ren.7 Ren found that combustion air should not be supplied too much so that the temperature could rise suddenly for low volatile content coals. It differs from the results of Wu16 for bituminous coal, which indicated a decrease of the average temperature in the burner zone when OFA ratio is increased from 25% to 35%. For bituminous coals, volatiles release easily, and combustion

considered to evolve during devolatilization and char combustion processes, and HCN and NH3 are assumed as the intermediates. The swirl burner can generate an intense and concentrated combustion zone which affects coal burnout and pollutant emission directly. Figure 8 shows the NO distribution over the boiler cross sections. From Figure 6 and Figure 7, it is clearly shown that the NO distribution is closely related to the flame shape and species concentration field since high NO concentration regions are found in the vicinity of the burner nozzles where the temperature and oxygen concentration are high. Near the burner nozzle, the volatiles are rapidly released from the coal and then mix with the O2 injected from the burners, which lead to the rapid generation of HCN and NH3. Then a fraction of N contained in HCN and NH3 is oxidized to NO in fuel-lean regions and reduced to N2 in fuel-rich regions. In the central region of the horizontal cross sections, a low NO F


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in the burner zone descends as CCO(38.6%) > CCO(34.2%) > CCO(29.8%) > CCO(25.4%) > CCO(21.1%); however, CCO above the OFA nozzles reduces as C CO (38.6%) > CCO(25.4%) > CCO(21.1%) > CCO(29.8%) > CCO(34.2%), which means the highest CO consuming rate appears at the ratio of 34.2%, leading to the highest average temperature under this condition, but not at the ratio of 38.6%. When the furnace is operated under deep air staging conditions at the ratio of 38.6%, CO generates in the burner zone more than that at the ratio of 34.2%, which flows to the side wall with flue gas and moves upward near the side wall and will not mix with the OFA. Therefore, as shown in Table 4, more CO remains under Table 4. Simulation Results of Various Concentrations at the Furnace Exit under Different OFA Ratios OFA ratios (%)

Figure 9. Area-weighted average temperature and NO distribution under different OFA ratios.

position furnace exit

occurs intensely due to its high volatile content. Therefore, the intense combustion process of pulverized bituminous coal is inhibited, and the heat released in the burner zone is reduced under air staging conditions, resulting in the decrease of the average temperature with increased OFA ratio. Compared with bituminous coal, the present lean coal is more difficult to ignite and burn because of its low volatile content and reactivity. The average temperature is further reduced, and the initial ignition of the fuel is delayed when a large amount of cold secondary air is injected into the burner zone. Therefore, the present average temperature in the burner zone rises slightly when the OFA ratio is increased from 21.1% to 38.6%. It is evident that the area-weighted average temperature Tav above the OFA nozzles decreases as Tav(34.2%) > Tav(38.6%) > Tav(29.8%) > Tav(25.4%) > Tav(21.1%) with the highest temperature appearing at the ratio of 34.2%. The reason may be explained as follows. The OFA injection velocity and consequently the penetration length of the OFA jet increase with increased OFA ratio, which leads to better mixing between OFA and the upward flue gas.41 As shown in Figure 10, the area-weighted average O2 concentration CO2 descends as CO2(21.1%) > CO2(25.4%) > CO2(29.8%) > CO2(34.2%) > CO2(38.6%). The area-weighted average CO concentration CCO

items

21.1

25.4

29.8

34.2

38.6

O2 concn (%) CO concn (%) area average temp (K) carbon content in fly ash (%) NO concn (mg/ m3 6% O2) average residue time (s)

2.96 0.08 1304

3.08 0.11 1313

3.15 0.08 1320

2.99 0.04 1347

2.83 0.71 1361

3.54

3.60

4.43

1.08

0.65

552

528

512

491

461

7.91

8.06

8.37

8.42

8.59

this condition, even though the penetration length of OFA injection is greater. Additionally, carbon content in fly ash under different OFA ratios are plotted in Figure 11, and the

Figure 11. Carbon content in fly ash under different OFA ratios.

peak value is observed at the ratio of 29.8%. Carbon content in fly ash increases with an increased OFA ratio when the ratio is lower than 29.8% but decreases when the ratio is higher than 29.8%. This result indicates that the velocity of OFA injection has a critical value in a wall fired boiler, and the OFA injection can sufficiently mix with the upward flue gas only when the velocity of OFA injection is higher than the critical value. Figure 9 plots the average temperature and NO distribution under different OFA ratios. It is seen that NO concentration distributions of the five cases present similar characteristics and NO reduction efficiency increases with increased OFA ratio. Combining with Figure 10, it can be found that both the temperature and the oxygen concentration are high in the

Figure 10. Area-weighted average species distribution under different OFA ratios. G


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vicinity of the burner, which finally leads to the rapid formation of NO near the burner zone. Then the NO concentration is significantly diluted by the OFA near the OFA nozzles, while the NO concentration slightly increases above the OFA nozzles due to the residual char combustion. The NO emission in deep air staged condition at the ratio of 38.6% are about 17% lower than that at the ratio of 21.1%, which agrees with the results of Fan.11 This means that the ratio of OFA can slightly affect the NO emission during lean coal combustion, which may be explained with the study conducted by Yan42 and Pershing.40 Yan reported that the char derived from the coal of high rank shows great nitrogen retention, and Pershing stated that the dependence of char NO formation on the stoichiometric ratio is relatively small. For lean coal, due to its low volatile content, more nitrogen remained in the char after devolatilization, thus the NO formation has an insignificant dependency on the stoichiometric ratio. However, serious slagging and high temperature corrosion may occur when the flame impinges the side wall under deep air staging. Therefore, it is important to choose the proper OFA ratio for the boiler. In this study, 34.2% is chosen as the optimal OFA ratio to obtain a high burnout rate of pulverized coal and low NO emission, which can be seen in Table 4. 4.7. Effects of OFA Position on NOx Emission. Figure 12 and Figure 13 respectively show the area-weighted average

Figure 13. Area-weighted average species distribution under different OFA positions.

while CO2 concentration increases gradually after OFA is injected into the furnace. The area-weighted average gas temperature at the furnace exit, species concentrations, and carbon content in fly ash for each case are presented in Table 5. The area-weighted average Table 5. Simulation Results of Various Concentrations at the Furnace Exit under Different OFA Positions OFA ports position furnace exit

items

Port 1

Port 2

Port 3

Port 4

O2 concn (%) CO concn (%) area average temp (K) carbon content in fly ash (%) NO concentration (mg/m3 at 6% O2)

2.99 0.04 1347 1.00

2.90 0.05 1349 1.04

2.83 0.07 1353 1.96

2.71 0.15 1373 3.68

488

482

435

403

temperature, CO concentration, and carbon content in fly ash all increase with enlarging the reducing zone, which agrees with the results in the literature.10,12 The highest carbon content in fly ash is observed when OFA is injected from Port 4. The reason for this is that enlarging the reducing zone leads to less burnout time in the burnout zone, which consequently determines the carbon content in fly ash. Obviously, the residence time in the burnout zone is an important factor affecting the carbon content in fly ash. The average temperature at the furnace exit increases from 1347 to 1373 K as the burnout zone moves upward since more CO and char burn near the furnace exit. Figure 12 shows the impacts of OFA injection port position on NOx emission. NO is greatly reduced when the injection port position of OFA moves from the bottom nozzles (OFA Port 1) to the top nozzles (OFA Port 4) under the same OFA ratio, and similar results were reported by Fan.10,11 The NO concentration decreases by 17.4% as the OFA switches from Port 1 to Port 4. The reducing zone expands, and the residence time of the pulverized coal in the reducing zone increases when enlarging the distance between section C and the OFA port. Consequently, more NO is reduced to N2 by gaseous N species and char in the reducing zone since a strong reducing atmosphere is formed there. Although air staging is helpful for NO reduction, it may give rise to the change of the carbon content in fly ash, which is a significant index for evaluating the

Figure 12. Area-weighted average temperature and NOx distribution under different OFA positions.

temperature and species distributions at different OFA port positions. From Case 4 to Case 8, the OFA switches from Port 1 to Port 4. Obviously, almost the same temperature and species distributions are observed for the four cases sharing the same combustion condition in the burner zone, but each case shows its own characteristics above the burner zone with different OFA port positions. As can be seen in Figure 12, the temperature gradually decreases in the reducing zone, which is defined as the zone between section C and the OFA port of each case, and is higher than the temperature in the burnout zone, which is defined as the zone above the OFA port of each case. As shown in Figure 13, O2 concentration decreases continuously, while CO concentrations increase gradually with the increase of furnace height in the reducing zone for each case. It is obvious that a strong reducing atmosphere with high temperature and low O2 concentration is formed in the reducing zone, and more NO will be reduced to N2 when enlarging the zone. The concentrations of CO and O2 decrease, H


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combustion efficiency of the coal fired utility boiler. Port 3 is the optimal OFA port position in the present study, producing low NOx emission and having a weak effect on the combustion characteristics as well.

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5. CONCLUSION A three-dimensional numerical study was performed on the flow, combustion, temperature, and species distributions in a supercritical 600 MW wall fired pulverized coal boiler. Models used in this paper were well validated with the experimental data. Combustion and NOx formation characteristics were studied with the validated models under various combinations of OFA ratio and OFA port position. The following main conclusions can be drawn: (1) Each burner is found to generate an intense and concentrated combustion zone, which affects the burnout feature of coal particles and pollutant emission directly in the wall fired furnace. A fraction of N is oxidized to NO in the fuellean regions near the burner, where the temperature and O2 concentrations are high. (2) Gas flow is found to impinge the side wall in the center of the furnace. Thus, effective measures must be taken to prevent high temperature corrosion and coking on the side wall when deep air staging is used. (3) The air staging combustion technology, which is helpful for reducing NO emission for boiler using bituminous coal, is not effective enough for the present wall fired boiler with lean coal. NO emission only decreases by 101 mg/Nm3 when the OFA ratio is increased from 21.1% to 38.6%. (4) The velocity of the OFA jet significantly influences the carbon content in fly ash in a wall fired boiler under the same burnout time, and it should be larger than a critical value. With increasing velocity of the OFA jet, the penetration depth of the OFA jet is increased, and the mixing between the OFA and upward flue gas is enhanced, which finally leads to the decrease of the carbon content in the fly ash. (5) The position of OFA port has a significant effect on NOx emission. Simulation results show that less NOx is produced when OFA is injected through Port 4, as the residence time of pulverized coal is extended in the reducing zone and more NOx is reduced to N2.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-29- 82665185. Fax: +86-29-82668703. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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NOMENCLATURE OFA = over fire air SIMPLE = Semi-Implicit Method for Pressure Linked Equations RNG = renor-malization group BMCR = Boiler Maximum Continuous Rating Tav = area-weight average temperature (K) CO2 = area-weight average mass fraction of O2 (%) CCO = area-weight average mass fraction of CO (%)

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REFERENCES

(1) Petroleum, B. BP Statistical Review of World Energy 2012; 2012. (2) Chen, W.; Xu, R. Energy Policy 2010, 38 (5), 2123−2130. I


Energy & Fuels

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