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Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging
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Journal of the Energy Institute xxx (2018) 1e13
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Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging Q2
Chaoyang Zhou a, Yongqiang Wang b, Qiye Jin b, Yuegui Zhou b, * a b
School of Power and Mechanical Engineering, Wuhan University, 16 Luojia Hill Road, Wuhan 430072, People's Republic of China School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 October 2017 Received in revised form 4 January 2018 Accepted 8 January 2018 Available online xxx
Low NOx burner and air staged combustion are widely applied to control NOx emission in coal-fired power plants. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single low NOx swirl burner in an existing coal-fired boiler was numerically simulated to analyze the mechanisms of flame stability and in-flame NOx reduction. And the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize NOx emissions for the low NOx burner with air staged combustion of different burner stoichiometric ratios. The results show that the specially-designed swirl burner configures including the pulverized coal concentrator, flame stabilizing ring and baffle plate creates an ignition region of high gas temperature, proper oxygen concentration and high pulverized coal concentration near the annular recirculation zone at the burner outlet for flame stability. At the same time, the annular recirculation zone is generated between the primary and secondary air jets to promote the rapid ignition and combustion of pulverized coal particles to consume oxygen, and then a reducing region is formed as fuel-rich environment to contribute to inflame NOX reduction. Moreover, the NOx concentration at the outlet of the combustion chamber is greatly reduced when the deep air staged combustion with the burner stoichiometric ratio of 0.75 is adopted, and the CO concentration at the outlet of the combustion chamber can be maintained simultaneously at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for lower NOx emission in the existing coal-fired boilers. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Pulverized coal combustion Low NOx swirl burner Combustion flame stability Air staged combustion NOx emission
1. Introduction NOx emission in large scale coal-fired power plants is a major burden to the environment, which can result in serious problems such as acid deposition, ozone depletion and photochemical smog. The stringent standards of NOx emission in different countries in the world have been promulgated to reduce NOx emission in coal-fired power plants. Since 2014, Chinese government has implemented the NOx emission limit of 50 mg/m3 at 6% O2 concentration, dry basis for coal-fired boilers named as ultra-low pollutant emissions. The combined combustion technologies of low NOx burner and air staged combustion are widely applied to control lower NOx emission in the furnace of large scale coal-fired power plants and to decrease the operation cost of Selective Catalytic Reduction (SCR). In general, the low NOx burner is adopted with air staged combustion in the main combustion zone to inhibit fuel-NOx conversion and to promote the reduction of NOx in flame. Many researchers have conducted experimental and numerical investigations on the low NOx burners in coal-fired boilers in the past decades. Zhou et al. [1] investigated the gas-solid two-phase flow characteristics of the single HT-NR3 burner with a particle dynamics anemometer (PDA) system and found an annular recirculation zone between the primary and secondary air jets. The variance of annular recirculation zone and particle concentration distribution near the burner outlet were also analyzed by PDA with different vane angles of the outer secondary air [2]. Sun et al. [3] experimentally studied the combustion characteristics of a novel burner with a non-swirl outer secondary air and concluded that the air staged combustion with the non-swirl outer secondary air could suppress NOx emissions to around
* Corresponding author. E-mail address: [email protected] (Y. Zhou). https://doi.org/10.1016/j.joei.2018.01.006 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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600 mg/m3. Tsumura et al. [4] conducted a single burner combustion test with a NOx reduction load extension (NR-LE) type burner with the flame stabilizing ring and a special air nozzle to achieve a stable flame with minimum load of 50% and low NOx emission of 200 mg/m3. Xue et al. [5] performed the pilot tests of two swirl burners in a 1 MW coal-fired chamber to analyze the influences of the burner parameters on NOx emission and unburned carbon in fly ash, and the results provided important guidance for engineering designs and practical applications of low NOX swirl burners. Computational Fluid Dynamics (CFD) method has become an important tool to predict combustion characteristics of coal-fired boilers and to develop different low NOx burners. Zhou et al. [6] carried out numerical simulation on the coal particle distribution characteristics in the primary air pipe of a swirl burner, and concluded that the spindle body could create a fuel-rich area surrounded by the recirculation zone to stabilize combustion flame and reduce NOx emission. Apte et al. [7] performed the gas-solid two-phase flow in a coaxial jet burner with Large-eddy-simulation (LES) and found that bigger particles could rapidly penetrate the recirculation zone while smaller particles respond quickly to the gas phase. Kurose et al. [8] simulated coal combustion performance of an IHI low NOx burner and concluded that the recirculation flow lengthened coal particle residence time in high gas temperature region and enhanced coal devolatilization and combustion, which promoted a low oxygen region for effective NOx reduction. Zhao et al. [9] modeled coal combustion process in the petal swirl burner (PSB) and indicated that the special structure of the PSB burner enhanced the mixing of primary air and recirculated flue gas for reliable ignition and NOx reduction. Modlinski et al. [10] simulated a single rapid ignition JET-burner in a virtual cylinder chamber and found that rapid ignition phenomenon and stable flame operation were realized even with non-swirl secondary and tertiary air. Li et al. [11] made an optimization retrofit to a swirl burner and achieved a larger recirculation zone, lower peak temperature and less NOx production. The effect of the cone lengths of different primary and secondary air jets were numerically simulated by Ti et al. [12], and an optimized burner outlet structure was presented to achieve excellent performances on both combustion and NOx reduction capacity. Zhou et al. [13] found that the optimized structure of the primary air pipe in a swirl burner could promote the generation of a large reducing region to benefit NOx reduction. On the other hand, air staged combustion technology has been regarded as the main measurement to further reduce NOx emission when combined with the low NOx burners in coal-fired power plants. Choi et al. [14,15] experimentally investigated the NOx emissions and burnout characteristics with air-staged combustion in a 15 kW pulverized coal-fired furnace, and found that the staged-air has a positive impact on NOx emissions but a negative impact on burnout performances. Fan et al. [16,17] studied the NOx and CO formation processes for deep air-staged combustion in a 20 kW down flame furnace, and found that the NOx concentration in the reduction zone significantly decreased with the increasing staging degree and a NO reducing saturation phenomenon existed in the reduction zone once the staging level reached a certain extent. Kuang et al. [18] evaluated the effects of staged air and over-fire air in regulating air-staging conditions within a large-scale down-fired furnace and found that regulating deep-air-staging conditions to sharply reduce NOx emissions relied on OFA openings rather than staged air in the down-fired furnace. Recently, Zha et al. [19] evaluated the NOx emissions under deep-air-staging conditions in a 600 MW tangentially fired pulverized-coal boiler, and distinct NOx reduction was achieved under appropriate vertical air-staging condition. Although HT-NR series burners have been widely applied in coal-fired boilers for many years, it is still unclear about their unique combustion concepts to maintain stable flame stability and low NOx emission in the public documents. The deep understanding of the combustion mechanism is important for the improvement of the burner performance and the development of the next generation ultra-low NOx swirl burner. At the same time, the deep air staging combustion is one of the most important technologies for further reducing NOx emission to satisfy more stringent environmental standards. The objective of this work is to simulate gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single HT-NR3 type low NOx swirl burner operated in an existing 600 MW coal-fired boiler in China, and the detailed NOx formation and reduction model under fuel rich conditions is employed to optimize the NOx emissions for the low NOx swirl burner with deep air staged combustion. The flame stabilization mechanism of the HT-NR3 burner is characterized as the combustion theory of high gas temperature, proper oxygen concentration and high pulverized coal concentration behind the flame stabilizing ring near the burner outlet, and the low NOx mechanism of the burner is in-flame NOx reduction in the annular recirculation zone. And then the air staged combustion with different burner stoichiometric ratios (SRm) were optimized to further achieve lower NOx emission with the positive impact on coal burnout performance. It will provide the optimal solution to further reduce NOx emission in the existing coal-fired boilers and the design of new ultra-low NOx swirl burners. 2. Modeling method 2.1. HT-NR3 burner structure and operation parameters The Hitachi NOx reduction (HT-NR)burner series have an unique combustion concept of in-flame NOx reduction where NOx is subject to a reduction reaction in the flame [20]. The HT-NR3 type low NOx swirl burner has been widely applied in coal-fired boilers to realize low NOx performance in recent years. However, the frequent variation of coal types in the actual coal-fired power plants leads to higher NOx emission and unburnt carbon in fly ash. Therefore, the single low NOx swirl burner was simulated to evaluate the coal combustion and NOx emission performance and to explore the technical solution to further reduce NOx emission in an existing 600 MW class coal-fired power plant in China. The schematic of the burner is shown in Fig. 1. The burner connects with a cylindrical combustion chamber with the diameter of 8 m and the length of 15 m, and it consists of a central primary air stream, an inner secondary air stream and a swirling outer secondary air stream with the vane angle of 25 . In the burner, a recirculation zone formed after the fuel nozzle is expanded by changing the direction of secondary air jets outside along the baffle plate, which is equipped on the flame stabilizing ring [21]. The pulverized coal concentrator has the shape of an artillery shell and is installed in the fuel nozzle of the burner. A simple guide sleeve can effectively separate the NOx reduction reaction from the outer secondary air. The proximate and ultimate analyses of Shaanxi anthracite coal used for the simulation are listed in Table 1. The anthracite coal has low volatile matter content and high ash content. Table 2 shows the operation parameters of the single low NOx swirl burner with the stoichiometric ratio SRm ¼ 0.95 in the main combustion zone. The low NOx swirl burner and the connected combustion chamber are meshed with different grid numbers to evaluate the grid independent solution, and the simulated flue gas temperature profiles on the centerline of Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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Fig. 1. Schematic of the HT-NR3 burner and the combustion chamber: (a) the combustion chamber; (b) the HT-NR3 burner.
Table 1 Proximate and ultimate analyses of Shaanxi anthracite coal. Proximate analysis (wt.%, ar)
Volatile matters Fixed carbon Ash Moisture Carbon Hydrogen Oxygen Nitrogen Sulfur
Ultimate analysis (%, daf)
Net heating value (kJ/kg, as received)
e
8.8 52.4 31.5 7.3 83.8 3.1 8.4 1.4 3.3 18840
Table 2 Operation parameters of a single HT-NR3 burner. Air flow rates (kg/s)
Air inlet temperatures (K)
Coal mass flow rate (kg/s) Burner stoichiometric ratio
Primary air Inner secondary air Outer secondary air Primary air Inner secondary air Outer secondary air e e
6.125 7.038 7.038 353 618 618 3.281 0.95
the combustion chamber with four kinds of the total hexahedral cell numbers (i.e. 342539, 640183, 918193 and 1394597) are compared as shown in Fig. 2. The results show that the mesh number of 918193 cells is appropriate to reach the grid independent solution with the balance between numerical accuracy and computation time. Fig. 3 shows the meshes of the combustion chamber used in the present simulation, and the local grid refinement is conducted to reveal the complex flow structure near the burner outlet. Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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2000 1800
342539cells 640183cells 918193cells 1394597cells
1600
Temperature (K)
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0
1
2
3
4
5
6
7
8
Axis (m)
9
10
11
12
13
14
15
Fig. 2. Comparison on the flue gas temperature profiles at the axial center line of the combustion chamber for different cells.
Fig. 3. Meshes of the combustion chamber.
2.2. Coal combustion and NOx models The commercial software Fluent 15.0 is used to predict pulverized coal combustion process and NOx emission in the low NOx swirl burner and the combustion chamber. The turbulent flow, particle motion, coal combustion, homogeneous chemical reaction, gas radiation models and NOx formation and reduction model are briefly described as follows. The Realizable keε turbulence model [22] is widely used to predict the turbulent flow of both planar and rotation flow and to provide more accurate performance for fluid flows involving strong recirculation flow and swirling flow than standard keε model. Therefore, it is adopted to calculate the turbulent flow in the low NOx burner with strong swirling flow. The trajectories of pulverized coal particles are simulated to track the evolution of coal particles flow and combustion in the chamber with discrete phase model (DPM). The coal particle size is set to obey Rosin-Rammler distribution with the mean diameter of 50 mm and the spread parameter of 1.2 and it is classified into 10 groups with the minimum diameter of 5 mm and the maximum diameter of 200 mm. The radiative heat transfer in the combustion chamber is simulated by discrete ordinates (DO) model and the gas absorption coefficients are calculated with the weighted sum of gray gases model (WSGGM) [23]. The coal combustion process involves coal devolatilization, volatile matter homogeneous combustion, and char heterogeneous oxidation. The coal devolatilization process is simulated with two competing rate models [24] to define the production rate of volatile species. The homogeneous combustion of volatile species released from coal particles is predicted with mixed-is-burnt model [25] which assumes that gas phase combustion is infinitely fast. The mass fraction of individual species is determined by the mean mixture fraction and the fraction variables. The interactions between turbulent flow and chemical reactions has been considered with the beta probability density function (PDF). The coal char heterogeneous oxidation is simulated with a diffusion-kinetics model which assumes the reaction rate is determined by the diffusion rate of oxygen to the char surface and the chemical kinetic rate [26].
Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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The NOx formation and reduction process in the combustion chamber is modeled with a post-processing procedure based on the predicted flue gas velocity, temperature and species concentration fields when the coal combustion calculation is converged. The NOx model includes thermal, prompt and fuel-NOx formation as well as NOx destruction by reburning and char-NO reduction reaction in the coal combustion flame. The formation of thermal NOx is determined by the extended Zeldovich mechanism and the prompt NOx can be neglected because it is insignificant during coal combustion process. The detailed NOx formation and reduction model is illustrated in Fig. 4 which is employed to simulate the in-flame NOx reduction and deep air staged combustion in the low NOx swirl burner, and the reaction rate expressions and the corresponding kinetic parameters for R1~R7 are given in Table 3. The fuel-NOx formation is calculated by the global rates with the conversion of volatile nitrogen and char nitrogen during coal combustion process. In this work, the fuel-bound nitrogen in the volatiles is released to form two nitrogen intermediates XN (HCN and NH3) as R1 reaction during coal devolatilization process [23]. Then HCN and NH3 are partially oxidized to NO or reduced to N2 with the formed NO dependent on the temperature and local oxygen concentration, and the global reaction rate expressions R2 and R3 proposed by De Soete are used to describe homogeneous NOx formation and destruction in the pulverized coal flame [27,28]. The remained nitrogen in the char is directly oxidized to NO during the char conversion through R7 [29], and the heterogeneous reaction of NO reduction on the char surface was modeled as R4 according to Levy et al. [30]. The additional reduction path for the NOx destruction is the formed NO reduction reaction by CHi radicals to form XN (R5) in the high temperature fuel-rich zone [23,31]. 3. Results and discussion The gas-solid two-phase flow and combustion characteristics of the low NOx swirl burner was firstly analyzed to elucidate the flame stabilization and low NOx emission mechanisms, and then the air staged combustion in the combustion chamber with different burner stoichiometric ratios were optimized to evaluate NOx and CO emission levels and to provide the optimal solution to further reduce NOx emission in the existing coal-fired boiler. 3.1. Flue gas velocity field Fig. 5 shows the contour and radial distribution of flue gas axial velocities in the combustion chamber. The swirling outer secondary air flows from the burner into the combustion chamber and a low-pressure zone is formed near the burner outlet. Thus, an annular recirculation zone is generated in the area between the primary and secondary air jets. It is located at the area where x/d ¼ 0.25e3.25 and
Fig. 4. Detailed NOx formation and reduction model employed in the numerical simulation.
Table 3 Reaction expressions for NO formation and reduction model. No. Reaction
Formula
Parameters
R1 Volatile-N/XN(HCN,NH3)
Svol;HCN ¼
R2 XN þ O2/NOþ …
d½HCN dt
¼
R3 XN þ NO/N2þ …
d½HCN dt
¼ k3 XHCN XNO ;
R4 Char þ NO/N2þ …
d½NO dt
¼ k5 AE PNO
k5 ¼ 4.18 104 exp(-145046/RT)
R5 CHi þ NO/XNþ …
d½NO dt
¼ k6 ½CHi½NO
k6 ¼ 2.12 106 T1.54 exp(-27977/RT)þ1.324 1010 T3.33 exp(-15090/RT)
R6 Extended Zeldovich mechanism
d½NO dt
¼ k7 ½N2 ½O2 1=2
k7 ¼ 1.32 1010 T0.5 exp(-65495/T)
R7 Char-N/NO
Schar;NO ¼
Svol YN1 ;vol Mw;HCN ; Mw;N Vcell
Svol;NH3 ¼
3 k1 XHCN XaO2 ; d½NH dt
Schar YN;char Mw;NO MW;N Vcell
d½NH3 dt
Svol YN2 ;vol Mw;NH3 ; Mw;N Vcell
Ref. [23]
¼ k2 XNH3 XaO2
k1 ¼ 1.0 1010 exp(-280452/RT); k2 ¼ 4.0 106 exp(-133900/RT)
¼ k4 XNH3 XNO
k3 ¼ 3.0 1012 exp(-251151/RT); k4 ¼ 1.8 108 exp(-113018/RT)
[27] [27] [30] [23] [23] [29]
Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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Fig. 5. Flue gas axial velocity distributions in the combustion chamber: (a) velocity field contour; (b) radial distribution of axial velocity at typical cross-sections.
r/d ¼ 0.4e1.7, where x is the axial distance from the burner outlet, r is the radial distance from the centerline of the combustion chamber, and d is the diameter of primary air pipe at the burner outlet. The recirculation zone reaches maximum at the cross-section of x/d ¼ 0.85, and then it gradually decreases with the expansion of air jets and the entrainment of surrounding gas. Finally, the recirculation zone disappears at the cross-section x/d ¼ 3.25. For the low NOx burner, the annular recirculation zone delays the mixing of the primary and secondary air jets at the early stage of coal combustion and the recirculated hot flue gas quickly heats coal particles. This contributes to form a region of high gas temperature and low oxygen concentration, which is helpful to rapid ignition of pulverized coal particles and inflame NOx reduction. 3.2. Coal particle concentration Fig. 6 illustrates the distribution of pulverized coal particle concentration when the mixture of primary air and coal particles passes through the venturi and the pulverized coal concentrator in the primary air pipe. The coal particles tend to move towards the lateral side of the fuel nozzle at the parallel part of the pulverized coal concentrator. As the result, the pulverized coal particles are concentrated around the flame stabilizing ring at the outlet of the fuel nozzle. High concentration of pulverized coal particles promotes the rapid ignition and flame stability. The rapid ignition of pulverized coal just after the fuel nozzle exit can promote a large amount of oxygen consumption and the NOx reduction zone with deficient oxygen was formed for in-flame NOx reduction. 3.3. Flue gas temperature Fig. 7 depicts the flue gas temperature field in the combustion chamber. The flue gas temperature is much higher in the annular recirculation zone than in the central area because the entrainment of high temperature combustion products can quickly preheat coal particles and enhance particles rapid ignition. Comparing the flue gas temperature field with the gas-solid two-phase flow fields as stated above, pulverized coal particles are concentrated around the flame stabilizing ring by the pulverized coal concentrator and the coal particles in the recirculation zone are rapidly ignited to maintain high flame temperature. On the other hand, the baffle plate attached to the flame Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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Fig. 6. Coal particle concentration.
Fig. 7. Flue gas temperature in the combustion chamber.
stabilizing ring magnifies the recirculation region and promotes the mixing of coal particles and hot flue gas. The rapid heating of coal particles in high temperature recirculation zone enhances coal rapid ignition. The flue gas temperature in the annular recirculation zone is high at the early stage of coal combustion, and then it is leveled off due to the mixing of the secondary air jets and the entrainment of the surrounding combustion products. 3.4. Flue gas species concentrations Fig. 8 illustrates flue gas species concentrations of oxygen and carbon monoxide in the combustion chamber during coal combustion process. Oxygen in primary air is quickly consumed when coal particles of high concentration is ignited and burned in the annular recirculation zone. In the meantime, the oxygen concentration in the core of the primary air jet is high near the burner outlet and it gradually decreases with coal combustion and flue gas entrainment. The CO concentration in the annular recirculation region is high due to low oxygen concentration in this area. The large fuel-rich region in the combustion chamber is beneficial to NOx reduction by the evolved nitrogen intermediates HCN and NH3 and hydrocarbon radicals during the coal devolatilization. 3.5. NOx concentration Fig. 9 shows the calculated NOx concentration in the combustion chamber. At the early stage of coal combustion, pulverized coal particles injected into the combustion chamber are quickly heated by the recirculated hot flue gas and rapidly devolatilized to release light hydrocarbon mixture and nitrogen intermediates HCN and NH3. The devolatilized species rapidly ignited to consume oxygen to create a reducing atmosphere for effective NOx reduction in the region. It demonstrates that the NOx concentration is high near the burner outlet and decreases dramatically with the reduction of the formed NOx by nitrogen intermediates and hydrocarbon in the fuel-rich region. Then the secondary air stream gradually mixed into the primary air stream, the NOx concentration at the late stage of coal combustion increases because of the conversion of partial fuel-N to NOx at the fuel-lean atmosphere with the increasing oxygen concentration. 3.6. Mechanism analysis of flame stability and in-flame NOx reduction in the low NOx burner It is unclear to some extent why the HT-NR series burners have a unique combustion concept to maintain stable flame stability and low NOx emission in the public documents although they have been widely applied in coal-firing boilers in the past decade. And the deep understanding of the combustion design concept and process mechanisms is beneficial for the improvement of the burner performance. The Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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Fig. 8. Flue gas species concentrations in the combustion chamber: (a) oxygen concentration; (b) CO concentration.
Fig. 9. NOx concentration in the combustion chamber.
profiles of high flue gas temperature, proper oxygen concentration and high pulverized coal concentration after the flame stabilizing ring at the outlet of the primary air are firstly depicted to elucidate the mechanisms of coal ignition and flame stability and in-flame NOx reduction in Fig. 10. As mentioned above, pulverized coal particles are concentrated in the annular region after passing through the venturi and the pulverized coal concentrator in the primary air pipe. The high temperature atmosphere of above 1400 K is also found in the same region of the hot recirculating flue gas induced by the swirl burner with the flame stabilizing ring and the baffle plate. In this region, high flue gas temperature, proper oxygen concentration and high pulverized coal concentration promote the rapid pyrolysis and ignition of coal particles, and make an important contribution to maintain stable flame. The consumption of oxygen is accelerated due to the rapid ignition of coal particles and a NOx reduction zone with extremely low oxygen concentration is expanded to enhance the in-flame NOx reduction at the downstream of the burner exit. Therefore, the formed NOx at the early stage of coal combustion process is greatly reduced into nitrogen by the N-contained intermediates HCN and NH3 and CHi radicals to decrease the NOx emission concentration in the main combustion zone. 3.7. Influences of different stoichiometric ratios in the burner region on NOx emissions In order to further decrease the NOx emission concentration in the existing coal-fired boiler, numerical optimization on coal combustion and NOx emission characteristics was conducted with different stoichiometric ratios in the burner region to simulate the effects of air staged Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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Fig. 10. Compared analysis of flue gas temperature, oxygen concentration and coal particle concentration distributions at the burner outlet: (a) schematic of the HT-NR3 burner; (b) parameters at the cross-section of the burner outlet just after flame stabilizing ring.
combustion. Two over-fired air (OFA) nozzles were oppositely arranged at the cross-section of 8.5 m downstream in the combustion chamber as shown in Fig. 11. Table 4 illustrates the operation parameters with different burner stoichiometric ratios (SRm) of 0.65, 0.75, 0.95 and 1.18 in the burner region and the overall excessive air coefficient in the chamber is fixed as 1.18 for all simulated cases. The combustion conditions with SRm ¼1.18, 0.95, 0.75 and 0.65 in the simulated combustion chamber are corresponding to coal combustion process without air staging, with shallow air staging and with deep air staging in real boiler furnace, respectively. Fig. 12(a) shows the flue gas temperature distributions along the axial direction of the top primary air outlet at different burner stoichiometric ratios. It can be seen that the flue gas temperature increases dramatically near the burner outlet when coal particles are injected into the furnace and they burned intensively to release a large amount of heat. Then the flue gas temperature decreases a little when the coal particle combustion intensity decreases due to lower oxygen concentration in the recirculation zone. The flue gas temperature increases again with the mixing of the secondary air stream to increase the local oxygen concentration and to promote char particle combustion. Finally, the flue gas temperature becomes steady with char burnout at the late stage of coal combustion. When the burner stoichiometric ratio is decreased from 1.18 without air staging to 0.95, 0.75 and 0.65 with different air staging degree, the flue gas temperature gradually decreases at the main burner zone. And then the flue gas temperatures and the oxygen concentrations increase with the mixing of the secondary air stream. The flue gas temperature at the location of overfire air (OFA) injection drastically decreases due to the low temperature of OFA stream. Then the oxygen concentration increases with the OFA injection to promote char particle burnout. The flue gas temperature after the location of OFA injection increases with char particle combustion, and it gradually becomes steady till the char burnout.
Fig. 11. Schematic of the combustion chamber with OFA injections.
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Table 4 Operation parameters with different burner stoichiometric ratios in the main combustion region. Cases
1
2
3
4
Air flow rates (kg/s)
Primary air Inner secondary air Outer secondary air OFA stream
4.191 4.815 4.815 11.270
4.836 5.556 5.556 9.144
6.125 7.038 7.038 4.891
7.608 8.742 8.742
Air inlet temperatures (K)
Primary air Inner secondary air Outer secondary air OFA stream
353 618 618 618
353 618 618 618
353 618 618 618
353 618 618
3.281
3.281
3.281
3.281
0.65
0.75
0.95
1.18
1.18
1.18
1.18
1.18
Coal mass flow rate (kg/s)
e
Burner stoichiometric ratio (SRm)
e
Total excess air coefficients
e
e
e
Fig. 12 (b) and (c) show the oxygen and CO concentration distributions along the axial direction at different burner stoichiometric ratios. For SRm ¼ 1.18 without air staging, the oxygen concentration quickly decreases and the CO concentration sharply increases when coal particles are preheated to release the volatile matter and the volatile matter burns to consume a large amount of oxygen. The oxygen concentration increases with the mixing of secondary air stream and the CO concentration gradually decreases because CO burns to
OFA-Injection
2000
OFA-Injection
SRm=0.65 SRm=0.75 SRm=0.95 SRm=1.18
20
1800
15
SRm=0.65 SRm=0.75 SRm=0.95 SRm=1.18
1400 1200
O2(%)
T(K)
1600
10
1000
5
800 600
0
0
2
4
6
8
X(m)
10
12
14
16
0
2
4
6
10
12
14
16
(b) 4000
OFA-Injection
30
8
X(m)
(a) SRm=0.65 SRm=0.75 SRm=0.95 SRm=1.18
25
OFA-Injection
SRm=0.65 SRm=0.75 SRm=0.95 SRm=1.18
3500 3000 2500
NOx(ppm)
20
CO(%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
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15
2000 1500
10 1000 5
500
0
0 0
2
4
6
8
X(m)
(c)
10
12
14
16
0
2
4
6
8
10
12
14
16
X(m)
(d)
Fig. 12. Flue gas temperature and species concentration distributions along the axial direction of the top primary air outlet at different burner stoichiometric ratios: (a) flue gas temperature; (b) oxygen concentration; (c) CO concentration; (d) NOx concentration.
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consume oxygen and to produce CO2 in this area. Then the oxidation of char particle occurs under the combustion condition of high temperature and low oxygen concentration and the CO concentration increases again. The CO concentration decreases with the residual combustible gas and char burnout at the late stage of coal combustion. However, the residual CO concentration is high to some extent because of the weak mixing of secondary air with flue gas. For SRm ¼ 0.95 with shallow air staging, the corresponding oxygen concentration is lower and the CO concentration is higher than those for SRm ¼ 1.18. The oxygen concentration increases and the CO concentration drastically deceases to low level with the mixing of OFA injection with flue gas. For SRm ¼ 0.75 and 0.65 with deep air staging, higher velocity OFA injection enhances the intensive mixing of fresh air with flue gas, which is beneficial for the burnout of unburned combustible gas and carbon content in fly ash. Fig. 12 (d) show the evolution of the NOx concentration along the axial direction at different burner stoichiometric ratios. The NOx concentration in the main burner zone for shallow air staging with SRm ¼ 0.95 is much lower than that without air staging with SRm ¼ 1.18 because there are lower oxygen concentration and higher CO concentration in the large annular recirculation zone to enhance the in-flame NOx reduction by the nitrogen intermediates HCN and NH3 and hydrocarbon radicals. It is also found that the evolution of NOx concentration with shallow and deep air staging (SRm ¼ 0.65e0.95) is very different from that without air staging (SRm ¼ 1.18). The NOx concentration with air staging first increases with the conversion of volatile nitrogen to NO at the early stage of coal combustion, and then it decreases due to the in-flame NOx reduction by the nitrogen intermediates and hydrocarbon radicals in the recirculation zone of high temperature and low oxygen concentration. Then it increases with the char nitrogen conversion to NOx due to the mixing of secondary air stream to increase the local oxygen concentration. The NOx concentration after the injection of OFA stream increases again because higher oxygen concentration promotes char particle combustion to convert the residual char nitrogen to NOx. The NOx concentration in the main burner zone for deep air staging with SRm ¼ 0.65 and 0.75 is much lower than that for shallow air staging with SRm ¼ 0.95. The numerical results quantitatively indicate that the NOx concentration in the combustion chamber can be decreased to lower level with deep air staging. Fig. 13 shows the NOx and CO concentrations at the outlet of the combustion chamber for the low NOx burner with different burner stoichiometric ratios SRm for case 1 to case 4. The averaged NOx concentration at the outlet of the combustion chamber under the combustion condition without air staging SRm ¼ 1.18 is 1982 ppm. However, the averaged NOx concentration at the outlet of the combustion chamber is reduced to 989 ppm at SRm ¼ 0.95 corresponding to shadow air staging in the burner zone. It can be further reduced to 426 ppm at SRm ¼ 0.75 corresponding to deep air staging in the burner zone. It has no obvious effect on NOx emission when the burner stoichiometric ratio is further decreased to 0.65. It is because the strong fuel-rich reduction atmosphere in the annular recirculation zone is created when the deep air staging combustion is adopted and it contributes to the reduction of the formed NOx into nitrogen by N-contained intermediates HCN and NH3 and hydrocarbon radicals. The NOx reduction with the air staged combustion is more effective when the burner stoichiometric ratio is decreased to 0.75. It is also found that the CO concentration at the outlet of the combustion chamber is significantly deceased when the burner stoichiometric ratio decreases from 1.18 to 0.75 because the over-fired air injection of high velocity enhances the mixing of the fresh air with residual combustible gases and unburn carbon in fly ash and promotes the burnout. This finding shows that it is possible to greatly reduce NOx concentration with deep air staging and to simultaneously maintain the lower CO concentration by enhancing the intensive mixing of OFA stream with the flue gas to increase the local oxygen concentration at the late stage of coal combustion. This result is different from the previous results that air staged combustion has a positive impact on NOx emissions but a negative impact on burnout performance [14e17]. It is noted that the level of CO concentration at the outlet of the combustion chamber is higher than that in the actual boiler furnace because the length of the simulated combustion chamber is much shorter than the height of actual boiler furnace. Therefore, it can be concluded that the combined solution of low NOx burner and deep air staged combustion of SRm ¼ 0.75 in the main combustion zone will significantly reduce the NOx emission concentration to much lower level without decreasing the boiler efficiency when it is adopted in the retrofitting of the low NOx combustion system in the existing coal-fired boilers.
Fig. 13. Comparison on NOx and CO emission concentrations for different burner stoichiometric ratios: (a) NOx emission concentration; (b) CO emission concentration.
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4. Conclusion The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single HT-NR3 type low NOx swirl burner was simulated with CFD method and the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize the NOx emissions for air staged combustion with different burner stoichiometric ratios. The flue gas velocity, temperature, species concentration fields and coal particle concentration distributions were analyzed to elucidate the mechanisms of flame stability and in-flame NOx reduction. And the air staged combustion with different stoichiometric ratios in the burner region were numerically simulated to evaluate their effects on NOx and CO emissions in order to further reduce the NOx concentration in the combustion chamber. The main conclusions in this work are summarized as follows: (1) The specially-designed swirl burner configures including the pulverized coal concentrator, flame stabilizing ring and baffle plate created an ignition region of high gas temperature, proper oxygen concentration and high concentration of pulverized coal particles near the burner outlet for flame stability. (2) The large annular recirculation zone was generated between the primary and secondary air jets, and the recirculated hot flue gas in this region promoted volatile matter release and rapid ignition to form a fuel-rich NOx reduction atmosphere, which can be used to elucidate the mechanism of in-flame NOx reduction in the swirl burner. (3) The NOx concentration at the outlet of the combustion chamber was greatly reduced when the deep air staging combustion with the burner stoichiometric ratio of 0.75 was adopted and the CO concentration can be maintained at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for further lower NOx emission in the existing coal-fired boiler. Acknowledgement This work was supported by the National Key Research and Development Program of China (2016YFB0600701) and Huaneng Group Science and Technology Project (HNKJ17-G07). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.joei.2018.01.006. References [1] H. Zhou, Y. Yang, K. Dong, H.Z. Liu, Y.L. Shen, K.F. Cen, Influence of the gas particle flow characteristics of a low-NOx swirl burner on the formation of high temperature corrosion, Fuel 134 (2014) 595e602. [2] J.P. Jing, Z.Q. Li, Q.Y. Zhu, Z.C. Chen, L. Wang, L.Z. Chen, Influence of the outer secondary air vane angle on the gas/particle flow characteristics near the double swirl flow burner region, Energy 36 (2011) 258e267. [3] G.G. Sun, D.F. Che, Z.H. Chi, Effects of secondary air on flow, combustion, and NOx emission from a novel pulverized coal burner for industrial boilers, Energy Fuels 26 (2012) 6640e6650. [4] T. Tsumuraa, H. Okazaki, P. Dernjatin, K. Savolainen, Reducing the minimum load and NOx emissions for lignite-fired boiler by applying a stable-flame concept, Appl. Energy 74 (2003) 415e424. [5] S. Xue, S.E. Hui, Q.L. Zhou, T.M. Xu, Experimental Study on NOx emission and unburnt carbon of a radial biased swirl burner for coal combustion, Energy Fuels 23 (2009) 3558e3564. [6] H. Zhou, Y. Yang, L.L. Wang, Numerical investigation of gas-particle flow in the primary air pipe of a low NOx swirl burner-The DEM-CFD method, Particuology 19 (2015) 133e140. [7] S.V. Apte, K. Mahesh, P. Moin, J.C. Oefelein, Large-eddy simulation of swirling particle-laden flows in a coaxial-jet combustor, Int. J. Multiphas. Flow 29 (2013) 1311e1331. [8] R. Kourose, H. Makino, A. Suzuki, Numerical analysis of pulverized coal combustion characteristics using advanced low-NOx burner, Fuel 83 (2004) 693e703. [9] L.L. Zhao, Q.T. Zhou, C.S. Zhao, Flame characteristics in a novel petal swirl burner, Combust. Flame 155 (2008) 277e288. [10] N. Modlinski, Computational modeling of a utility boiler tangentially-fired furnace retrofitted with swirl burners, Fuel Process. Technol. 11 (2010) 1601e1608. [11] Y.H. Li, B.X. Zhou, P.A. Zheng, Numerical simulation on an optimized swirl burner with low-NOx emission, J. Chin. Soc. Power Eng. (in Chinese) 34 (2014) 261e266. [12] S.G. Ti, Z.C. Chen, M. Kuang, Z.Q. Li, Q.Y. Zhu, H. Zhang, Z.F. Wang, G.Y. Xu, Numerical simulation of the combustion characteristics and NOx emission of a swirl burner: influence of the structure of the burner outlet, Appl. Therm. Eng. 104 (2016) 565e576. [13] H. Zhou, Y. Yang, H.Z. Liu, Q.J. Hang, Numerical simulation of the combustion characteristics of a low NOx swirl burner: influence of the primary air pipe, Fuel 130 (2014) 168e176. [14] C. Moon, Y. Sung, S. Eom, G. Choi, NOx emissions and burnout characteristics of bituminous coal, lignite, and their blends in a pulverized coal-fired furnace, Exp. Therm. Fluid Sci. 62 (2015) 99e108. [15] Y. Sung, C. Moon, S. Eom, G. Choi, D. Kim, Coal-particle size effects on NO reduction and burnout characteristics with air-staged combustion in a pulverized coal-fired furnace, Fuel 182 (2016) 558e567. [16] Y. Li, W.D. Fan, Effect of char gasification on NOx formation process in the deep air-staged combustion in a 20 kW down flame furnace, Appl. Energy 164 (2016) 258e267. [17] W.D. Fan, Y. Li, Q.H. Guo, C. Chen, Y. Wang, Coal-nitrogen release and NOx evolution in the oxidant-staged combustion of coal, Energy 125 (2017) 417e426. [18] M. Kuang, Z.Q. Li, Z.Q. Ling, X.Y. Zeng, Evaluation of staged air and overfire air in regulating air-staging conditions within a large-scale down-fired furnace, Appl. Therm. Eng. 67 (2014) 97e105. [19] Q.L. Zha, D.B. Li, C.A. Wang, D.F. Che, Numerical evaluation of heat transfer and NOx emissions under deep-air-staging conditions within a 600 MWe tangentially fired pulverized-coal boiler, Appl. Therm. Eng. 116 (2017) 170e181. [20] T. Tsumura, K. Kiyama, S. Nomura, H. Kobayashi, Development and actual verification of the latest extremely low-NOx pulverized coal burner, Hitachi Rev. 47 (1998) 188e191. [21] K. Ochi, K. Kiyama, H. Yoshizako, H. Okazaki, M. Taniguchi, Latest low-NOx combustion technology for pulverized-coal-fired boilers, Hitachi Rev. 58 (2009) 187e193. [22] T.H. Shih, W.W. Liou, A. Shabbir, Z. Yang, J. Zhu, A new k-ε eddy viscosity model for high Reynolds number turbulent flows, Comput. Fluids 24 (1995) 227e238. [23] ANSYS Fluent User's Guide. [24] H. Kobayashi, J.B. Howard, A.F. Sarofim, Coal devolatilization at high temperatures, Symposium (International) on Combustion 16 (1977) 411e425. [25] L. Ma, J.M. Jones, M. Pourkashanian, A. Williams, Modelling the combustion of pulverized biomass in an industrial combustion test furnace, Fuel 86 (2007) 1959e1965. [26] X.L. Zhou, Theory and Numerical Modeling of Turbulent Gas Particle Flows and Combustion, Science Press Ltd, Beijing, 1994. [27] G.G. De Soete, Overall reaction rates of NO and N2 formation from fuel nitrogen, Symp. (Int.) Combust 15 (1975) 1093e1102. [28] P. Glarborg, A.D. Jensen, J.E. Johnsson, Fuel nitrogen conversion in solid fuel fired systems, Prog. Energy Combust. Sci. 29 (2003) 89e113.
Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
1 2 3 4
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[29] F.C. Lockwood, C.A. Romo-Millares, Mathematical modeling of fuel-NO emissions from PF burners, J. Energy Inst. 65 (1992) 144e152. r, NO/char reactions at pulverized coal flame conditions, Symp. Int. Combust 18 (1981) 111e120. [30] J.M. Levy, L.K. Chan, A.F. Sarofim, J.M. Bee [31] M. Taniguchi, Y. Kamikawa, T. Okazaki, K. Yamamoto, H. Orita, A role of hydrocarbon reaction for NOx formation and reduction in fuel-rich pulverized coal combustion, Combust. Flame 157 (2010) 1456e1466.
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Contents lists available at ScienceDirect
Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging Q2
Chaoyang Zhou a, Yongqiang Wang b, Qiye Jin b, Yuegui Zhou b, * a b
School of Power and Mechanical Engineering, Wuhan University, 16 Luojia Hill Road, Wuhan 430072, People's Republic of China School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 October 2017 Received in revised form 4 January 2018 Accepted 8 January 2018 Available online xxx
Low NOx burner and air staged combustion are widely applied to control NOx emission in coal-fired power plants. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single low NOx swirl burner in an existing coal-fired boiler was numerically simulated to analyze the mechanisms of flame stability and in-flame NOx reduction. And the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize NOx emissions for the low NOx burner with air staged combustion of different burner stoichiometric ratios. The results show that the specially-designed swirl burner configures including the pulverized coal concentrator, flame stabilizing ring and baffle plate creates an ignition region of high gas temperature, proper oxygen concentration and high pulverized coal concentration near the annular recirculation zone at the burner outlet for flame stability. At the same time, the annular recirculation zone is generated between the primary and secondary air jets to promote the rapid ignition and combustion of pulverized coal particles to consume oxygen, and then a reducing region is formed as fuel-rich environment to contribute to inflame NOX reduction. Moreover, the NOx concentration at the outlet of the combustion chamber is greatly reduced when the deep air staged combustion with the burner stoichiometric ratio of 0.75 is adopted, and the CO concentration at the outlet of the combustion chamber can be maintained simultaneously at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for lower NOx emission in the existing coal-fired boilers. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Pulverized coal combustion Low NOx swirl burner Combustion flame stability Air staged combustion NOx emission
1. Introduction NOx emission in large scale coal-fired power plants is a major burden to the environment, which can result in serious problems such as acid deposition, ozone depletion and photochemical smog. The stringent standards of NOx emission in different countries in the world have been promulgated to reduce NOx emission in coal-fired power plants. Since 2014, Chinese government has implemented the NOx emission limit of 50 mg/m3 at 6% O2 concentration, dry basis for coal-fired boilers named as ultra-low pollutant emissions. The combined combustion technologies of low NOx burner and air staged combustion are widely applied to control lower NOx emission in the furnace of large scale coal-fired power plants and to decrease the operation cost of Selective Catalytic Reduction (SCR). In general, the low NOx burner is adopted with air staged combustion in the main combustion zone to inhibit fuel-NOx conversion and to promote the reduction of NOx in flame. Many researchers have conducted experimental and numerical investigations on the low NOx burners in coal-fired boilers in the past decades. Zhou et al. [1] investigated the gas-solid two-phase flow characteristics of the single HT-NR3 burner with a particle dynamics anemometer (PDA) system and found an annular recirculation zone between the primary and secondary air jets. The variance of annular recirculation zone and particle concentration distribution near the burner outlet were also analyzed by PDA with different vane angles of the outer secondary air [2]. Sun et al. [3] experimentally studied the combustion characteristics of a novel burner with a non-swirl outer secondary air and concluded that the air staged combustion with the non-swirl outer secondary air could suppress NOx emissions to around
* Corresponding author. E-mail address: [email protected] (Y. Zhou). https://doi.org/10.1016/j.joei.2018.01.006 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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Q1
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600 mg/m3. Tsumura et al. [4] conducted a single burner combustion test with a NOx reduction load extension (NR-LE) type burner with the flame stabilizing ring and a special air nozzle to achieve a stable flame with minimum load of 50% and low NOx emission of 200 mg/m3. Xue et al. [5] performed the pilot tests of two swirl burners in a 1 MW coal-fired chamber to analyze the influences of the burner parameters on NOx emission and unburned carbon in fly ash, and the results provided important guidance for engineering designs and practical applications of low NOX swirl burners. Computational Fluid Dynamics (CFD) method has become an important tool to predict combustion characteristics of coal-fired boilers and to develop different low NOx burners. Zhou et al. [6] carried out numerical simulation on the coal particle distribution characteristics in the primary air pipe of a swirl burner, and concluded that the spindle body could create a fuel-rich area surrounded by the recirculation zone to stabilize combustion flame and reduce NOx emission. Apte et al. [7] performed the gas-solid two-phase flow in a coaxial jet burner with Large-eddy-simulation (LES) and found that bigger particles could rapidly penetrate the recirculation zone while smaller particles respond quickly to the gas phase. Kurose et al. [8] simulated coal combustion performance of an IHI low NOx burner and concluded that the recirculation flow lengthened coal particle residence time in high gas temperature region and enhanced coal devolatilization and combustion, which promoted a low oxygen region for effective NOx reduction. Zhao et al. [9] modeled coal combustion process in the petal swirl burner (PSB) and indicated that the special structure of the PSB burner enhanced the mixing of primary air and recirculated flue gas for reliable ignition and NOx reduction. Modlinski et al. [10] simulated a single rapid ignition JET-burner in a virtual cylinder chamber and found that rapid ignition phenomenon and stable flame operation were realized even with non-swirl secondary and tertiary air. Li et al. [11] made an optimization retrofit to a swirl burner and achieved a larger recirculation zone, lower peak temperature and less NOx production. The effect of the cone lengths of different primary and secondary air jets were numerically simulated by Ti et al. [12], and an optimized burner outlet structure was presented to achieve excellent performances on both combustion and NOx reduction capacity. Zhou et al. [13] found that the optimized structure of the primary air pipe in a swirl burner could promote the generation of a large reducing region to benefit NOx reduction. On the other hand, air staged combustion technology has been regarded as the main measurement to further reduce NOx emission when combined with the low NOx burners in coal-fired power plants. Choi et al. [14,15] experimentally investigated the NOx emissions and burnout characteristics with air-staged combustion in a 15 kW pulverized coal-fired furnace, and found that the staged-air has a positive impact on NOx emissions but a negative impact on burnout performances. Fan et al. [16,17] studied the NOx and CO formation processes for deep air-staged combustion in a 20 kW down flame furnace, and found that the NOx concentration in the reduction zone significantly decreased with the increasing staging degree and a NO reducing saturation phenomenon existed in the reduction zone once the staging level reached a certain extent. Kuang et al. [18] evaluated the effects of staged air and over-fire air in regulating air-staging conditions within a large-scale down-fired furnace and found that regulating deep-air-staging conditions to sharply reduce NOx emissions relied on OFA openings rather than staged air in the down-fired furnace. Recently, Zha et al. [19] evaluated the NOx emissions under deep-air-staging conditions in a 600 MW tangentially fired pulverized-coal boiler, and distinct NOx reduction was achieved under appropriate vertical air-staging condition. Although HT-NR series burners have been widely applied in coal-fired boilers for many years, it is still unclear about their unique combustion concepts to maintain stable flame stability and low NOx emission in the public documents. The deep understanding of the combustion mechanism is important for the improvement of the burner performance and the development of the next generation ultra-low NOx swirl burner. At the same time, the deep air staging combustion is one of the most important technologies for further reducing NOx emission to satisfy more stringent environmental standards. The objective of this work is to simulate gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single HT-NR3 type low NOx swirl burner operated in an existing 600 MW coal-fired boiler in China, and the detailed NOx formation and reduction model under fuel rich conditions is employed to optimize the NOx emissions for the low NOx swirl burner with deep air staged combustion. The flame stabilization mechanism of the HT-NR3 burner is characterized as the combustion theory of high gas temperature, proper oxygen concentration and high pulverized coal concentration behind the flame stabilizing ring near the burner outlet, and the low NOx mechanism of the burner is in-flame NOx reduction in the annular recirculation zone. And then the air staged combustion with different burner stoichiometric ratios (SRm) were optimized to further achieve lower NOx emission with the positive impact on coal burnout performance. It will provide the optimal solution to further reduce NOx emission in the existing coal-fired boilers and the design of new ultra-low NOx swirl burners. 2. Modeling method 2.1. HT-NR3 burner structure and operation parameters The Hitachi NOx reduction (HT-NR)burner series have an unique combustion concept of in-flame NOx reduction where NOx is subject to a reduction reaction in the flame [20]. The HT-NR3 type low NOx swirl burner has been widely applied in coal-fired boilers to realize low NOx performance in recent years. However, the frequent variation of coal types in the actual coal-fired power plants leads to higher NOx emission and unburnt carbon in fly ash. Therefore, the single low NOx swirl burner was simulated to evaluate the coal combustion and NOx emission performance and to explore the technical solution to further reduce NOx emission in an existing 600 MW class coal-fired power plant in China. The schematic of the burner is shown in Fig. 1. The burner connects with a cylindrical combustion chamber with the diameter of 8 m and the length of 15 m, and it consists of a central primary air stream, an inner secondary air stream and a swirling outer secondary air stream with the vane angle of 25 . In the burner, a recirculation zone formed after the fuel nozzle is expanded by changing the direction of secondary air jets outside along the baffle plate, which is equipped on the flame stabilizing ring [21]. The pulverized coal concentrator has the shape of an artillery shell and is installed in the fuel nozzle of the burner. A simple guide sleeve can effectively separate the NOx reduction reaction from the outer secondary air. The proximate and ultimate analyses of Shaanxi anthracite coal used for the simulation are listed in Table 1. The anthracite coal has low volatile matter content and high ash content. Table 2 shows the operation parameters of the single low NOx swirl burner with the stoichiometric ratio SRm ¼ 0.95 in the main combustion zone. The low NOx swirl burner and the connected combustion chamber are meshed with different grid numbers to evaluate the grid independent solution, and the simulated flue gas temperature profiles on the centerline of Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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Fig. 1. Schematic of the HT-NR3 burner and the combustion chamber: (a) the combustion chamber; (b) the HT-NR3 burner.
Table 1 Proximate and ultimate analyses of Shaanxi anthracite coal. Proximate analysis (wt.%, ar)
Volatile matters Fixed carbon Ash Moisture Carbon Hydrogen Oxygen Nitrogen Sulfur
Ultimate analysis (%, daf)
Net heating value (kJ/kg, as received)
e
8.8 52.4 31.5 7.3 83.8 3.1 8.4 1.4 3.3 18840
Table 2 Operation parameters of a single HT-NR3 burner. Air flow rates (kg/s)
Air inlet temperatures (K)
Coal mass flow rate (kg/s) Burner stoichiometric ratio
Primary air Inner secondary air Outer secondary air Primary air Inner secondary air Outer secondary air e e
6.125 7.038 7.038 353 618 618 3.281 0.95
the combustion chamber with four kinds of the total hexahedral cell numbers (i.e. 342539, 640183, 918193 and 1394597) are compared as shown in Fig. 2. The results show that the mesh number of 918193 cells is appropriate to reach the grid independent solution with the balance between numerical accuracy and computation time. Fig. 3 shows the meshes of the combustion chamber used in the present simulation, and the local grid refinement is conducted to reveal the complex flow structure near the burner outlet. Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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2000 1800
342539cells 640183cells 918193cells 1394597cells
1600
Temperature (K)
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1400 1200 1000 800 600 400 200
0
1
2
3
4
5
6
7
8
Axis (m)
9
10
11
12
13
14
15
Fig. 2. Comparison on the flue gas temperature profiles at the axial center line of the combustion chamber for different cells.
Fig. 3. Meshes of the combustion chamber.
2.2. Coal combustion and NOx models The commercial software Fluent 15.0 is used to predict pulverized coal combustion process and NOx emission in the low NOx swirl burner and the combustion chamber. The turbulent flow, particle motion, coal combustion, homogeneous chemical reaction, gas radiation models and NOx formation and reduction model are briefly described as follows. The Realizable keε turbulence model [22] is widely used to predict the turbulent flow of both planar and rotation flow and to provide more accurate performance for fluid flows involving strong recirculation flow and swirling flow than standard keε model. Therefore, it is adopted to calculate the turbulent flow in the low NOx burner with strong swirling flow. The trajectories of pulverized coal particles are simulated to track the evolution of coal particles flow and combustion in the chamber with discrete phase model (DPM). The coal particle size is set to obey Rosin-Rammler distribution with the mean diameter of 50 mm and the spread parameter of 1.2 and it is classified into 10 groups with the minimum diameter of 5 mm and the maximum diameter of 200 mm. The radiative heat transfer in the combustion chamber is simulated by discrete ordinates (DO) model and the gas absorption coefficients are calculated with the weighted sum of gray gases model (WSGGM) [23]. The coal combustion process involves coal devolatilization, volatile matter homogeneous combustion, and char heterogeneous oxidation. The coal devolatilization process is simulated with two competing rate models [24] to define the production rate of volatile species. The homogeneous combustion of volatile species released from coal particles is predicted with mixed-is-burnt model [25] which assumes that gas phase combustion is infinitely fast. The mass fraction of individual species is determined by the mean mixture fraction and the fraction variables. The interactions between turbulent flow and chemical reactions has been considered with the beta probability density function (PDF). The coal char heterogeneous oxidation is simulated with a diffusion-kinetics model which assumes the reaction rate is determined by the diffusion rate of oxygen to the char surface and the chemical kinetic rate [26].
Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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The NOx formation and reduction process in the combustion chamber is modeled with a post-processing procedure based on the predicted flue gas velocity, temperature and species concentration fields when the coal combustion calculation is converged. The NOx model includes thermal, prompt and fuel-NOx formation as well as NOx destruction by reburning and char-NO reduction reaction in the coal combustion flame. The formation of thermal NOx is determined by the extended Zeldovich mechanism and the prompt NOx can be neglected because it is insignificant during coal combustion process. The detailed NOx formation and reduction model is illustrated in Fig. 4 which is employed to simulate the in-flame NOx reduction and deep air staged combustion in the low NOx swirl burner, and the reaction rate expressions and the corresponding kinetic parameters for R1~R7 are given in Table 3. The fuel-NOx formation is calculated by the global rates with the conversion of volatile nitrogen and char nitrogen during coal combustion process. In this work, the fuel-bound nitrogen in the volatiles is released to form two nitrogen intermediates XN (HCN and NH3) as R1 reaction during coal devolatilization process [23]. Then HCN and NH3 are partially oxidized to NO or reduced to N2 with the formed NO dependent on the temperature and local oxygen concentration, and the global reaction rate expressions R2 and R3 proposed by De Soete are used to describe homogeneous NOx formation and destruction in the pulverized coal flame [27,28]. The remained nitrogen in the char is directly oxidized to NO during the char conversion through R7 [29], and the heterogeneous reaction of NO reduction on the char surface was modeled as R4 according to Levy et al. [30]. The additional reduction path for the NOx destruction is the formed NO reduction reaction by CHi radicals to form XN (R5) in the high temperature fuel-rich zone [23,31]. 3. Results and discussion The gas-solid two-phase flow and combustion characteristics of the low NOx swirl burner was firstly analyzed to elucidate the flame stabilization and low NOx emission mechanisms, and then the air staged combustion in the combustion chamber with different burner stoichiometric ratios were optimized to evaluate NOx and CO emission levels and to provide the optimal solution to further reduce NOx emission in the existing coal-fired boiler. 3.1. Flue gas velocity field Fig. 5 shows the contour and radial distribution of flue gas axial velocities in the combustion chamber. The swirling outer secondary air flows from the burner into the combustion chamber and a low-pressure zone is formed near the burner outlet. Thus, an annular recirculation zone is generated in the area between the primary and secondary air jets. It is located at the area where x/d ¼ 0.25e3.25 and
Fig. 4. Detailed NOx formation and reduction model employed in the numerical simulation.
Table 3 Reaction expressions for NO formation and reduction model. No. Reaction
Formula
Parameters
R1 Volatile-N/XN(HCN,NH3)
Svol;HCN ¼
R2 XN þ O2/NOþ …
d½HCN dt
¼
R3 XN þ NO/N2þ …
d½HCN dt
¼ k3 XHCN XNO ;
R4 Char þ NO/N2þ …
d½NO dt
¼ k5 AE PNO
k5 ¼ 4.18 104 exp(-145046/RT)
R5 CHi þ NO/XNþ …
d½NO dt
¼ k6 ½CHi½NO
k6 ¼ 2.12 106 T1.54 exp(-27977/RT)þ1.324 1010 T3.33 exp(-15090/RT)
R6 Extended Zeldovich mechanism
d½NO dt
¼ k7 ½N2 ½O2 1=2
k7 ¼ 1.32 1010 T0.5 exp(-65495/T)
R7 Char-N/NO
Schar;NO ¼
Svol YN1 ;vol Mw;HCN ; Mw;N Vcell
Svol;NH3 ¼
3 k1 XHCN XaO2 ; d½NH dt
Schar YN;char Mw;NO MW;N Vcell
d½NH3 dt
Svol YN2 ;vol Mw;NH3 ; Mw;N Vcell
Ref. [23]
¼ k2 XNH3 XaO2
k1 ¼ 1.0 1010 exp(-280452/RT); k2 ¼ 4.0 106 exp(-133900/RT)
¼ k4 XNH3 XNO
k3 ¼ 3.0 1012 exp(-251151/RT); k4 ¼ 1.8 108 exp(-113018/RT)
[27] [27] [30] [23] [23] [29]
Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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Fig. 5. Flue gas axial velocity distributions in the combustion chamber: (a) velocity field contour; (b) radial distribution of axial velocity at typical cross-sections.
r/d ¼ 0.4e1.7, where x is the axial distance from the burner outlet, r is the radial distance from the centerline of the combustion chamber, and d is the diameter of primary air pipe at the burner outlet. The recirculation zone reaches maximum at the cross-section of x/d ¼ 0.85, and then it gradually decreases with the expansion of air jets and the entrainment of surrounding gas. Finally, the recirculation zone disappears at the cross-section x/d ¼ 3.25. For the low NOx burner, the annular recirculation zone delays the mixing of the primary and secondary air jets at the early stage of coal combustion and the recirculated hot flue gas quickly heats coal particles. This contributes to form a region of high gas temperature and low oxygen concentration, which is helpful to rapid ignition of pulverized coal particles and inflame NOx reduction. 3.2. Coal particle concentration Fig. 6 illustrates the distribution of pulverized coal particle concentration when the mixture of primary air and coal particles passes through the venturi and the pulverized coal concentrator in the primary air pipe. The coal particles tend to move towards the lateral side of the fuel nozzle at the parallel part of the pulverized coal concentrator. As the result, the pulverized coal particles are concentrated around the flame stabilizing ring at the outlet of the fuel nozzle. High concentration of pulverized coal particles promotes the rapid ignition and flame stability. The rapid ignition of pulverized coal just after the fuel nozzle exit can promote a large amount of oxygen consumption and the NOx reduction zone with deficient oxygen was formed for in-flame NOx reduction. 3.3. Flue gas temperature Fig. 7 depicts the flue gas temperature field in the combustion chamber. The flue gas temperature is much higher in the annular recirculation zone than in the central area because the entrainment of high temperature combustion products can quickly preheat coal particles and enhance particles rapid ignition. Comparing the flue gas temperature field with the gas-solid two-phase flow fields as stated above, pulverized coal particles are concentrated around the flame stabilizing ring by the pulverized coal concentrator and the coal particles in the recirculation zone are rapidly ignited to maintain high flame temperature. On the other hand, the baffle plate attached to the flame Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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Fig. 6. Coal particle concentration.
Fig. 7. Flue gas temperature in the combustion chamber.
stabilizing ring magnifies the recirculation region and promotes the mixing of coal particles and hot flue gas. The rapid heating of coal particles in high temperature recirculation zone enhances coal rapid ignition. The flue gas temperature in the annular recirculation zone is high at the early stage of coal combustion, and then it is leveled off due to the mixing of the secondary air jets and the entrainment of the surrounding combustion products. 3.4. Flue gas species concentrations Fig. 8 illustrates flue gas species concentrations of oxygen and carbon monoxide in the combustion chamber during coal combustion process. Oxygen in primary air is quickly consumed when coal particles of high concentration is ignited and burned in the annular recirculation zone. In the meantime, the oxygen concentration in the core of the primary air jet is high near the burner outlet and it gradually decreases with coal combustion and flue gas entrainment. The CO concentration in the annular recirculation region is high due to low oxygen concentration in this area. The large fuel-rich region in the combustion chamber is beneficial to NOx reduction by the evolved nitrogen intermediates HCN and NH3 and hydrocarbon radicals during the coal devolatilization. 3.5. NOx concentration Fig. 9 shows the calculated NOx concentration in the combustion chamber. At the early stage of coal combustion, pulverized coal particles injected into the combustion chamber are quickly heated by the recirculated hot flue gas and rapidly devolatilized to release light hydrocarbon mixture and nitrogen intermediates HCN and NH3. The devolatilized species rapidly ignited to consume oxygen to create a reducing atmosphere for effective NOx reduction in the region. It demonstrates that the NOx concentration is high near the burner outlet and decreases dramatically with the reduction of the formed NOx by nitrogen intermediates and hydrocarbon in the fuel-rich region. Then the secondary air stream gradually mixed into the primary air stream, the NOx concentration at the late stage of coal combustion increases because of the conversion of partial fuel-N to NOx at the fuel-lean atmosphere with the increasing oxygen concentration. 3.6. Mechanism analysis of flame stability and in-flame NOx reduction in the low NOx burner It is unclear to some extent why the HT-NR series burners have a unique combustion concept to maintain stable flame stability and low NOx emission in the public documents although they have been widely applied in coal-firing boilers in the past decade. And the deep understanding of the combustion design concept and process mechanisms is beneficial for the improvement of the burner performance. The Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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Fig. 8. Flue gas species concentrations in the combustion chamber: (a) oxygen concentration; (b) CO concentration.
Fig. 9. NOx concentration in the combustion chamber.
profiles of high flue gas temperature, proper oxygen concentration and high pulverized coal concentration after the flame stabilizing ring at the outlet of the primary air are firstly depicted to elucidate the mechanisms of coal ignition and flame stability and in-flame NOx reduction in Fig. 10. As mentioned above, pulverized coal particles are concentrated in the annular region after passing through the venturi and the pulverized coal concentrator in the primary air pipe. The high temperature atmosphere of above 1400 K is also found in the same region of the hot recirculating flue gas induced by the swirl burner with the flame stabilizing ring and the baffle plate. In this region, high flue gas temperature, proper oxygen concentration and high pulverized coal concentration promote the rapid pyrolysis and ignition of coal particles, and make an important contribution to maintain stable flame. The consumption of oxygen is accelerated due to the rapid ignition of coal particles and a NOx reduction zone with extremely low oxygen concentration is expanded to enhance the in-flame NOx reduction at the downstream of the burner exit. Therefore, the formed NOx at the early stage of coal combustion process is greatly reduced into nitrogen by the N-contained intermediates HCN and NH3 and CHi radicals to decrease the NOx emission concentration in the main combustion zone. 3.7. Influences of different stoichiometric ratios in the burner region on NOx emissions In order to further decrease the NOx emission concentration in the existing coal-fired boiler, numerical optimization on coal combustion and NOx emission characteristics was conducted with different stoichiometric ratios in the burner region to simulate the effects of air staged Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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Fig. 10. Compared analysis of flue gas temperature, oxygen concentration and coal particle concentration distributions at the burner outlet: (a) schematic of the HT-NR3 burner; (b) parameters at the cross-section of the burner outlet just after flame stabilizing ring.
combustion. Two over-fired air (OFA) nozzles were oppositely arranged at the cross-section of 8.5 m downstream in the combustion chamber as shown in Fig. 11. Table 4 illustrates the operation parameters with different burner stoichiometric ratios (SRm) of 0.65, 0.75, 0.95 and 1.18 in the burner region and the overall excessive air coefficient in the chamber is fixed as 1.18 for all simulated cases. The combustion conditions with SRm ¼1.18, 0.95, 0.75 and 0.65 in the simulated combustion chamber are corresponding to coal combustion process without air staging, with shallow air staging and with deep air staging in real boiler furnace, respectively. Fig. 12(a) shows the flue gas temperature distributions along the axial direction of the top primary air outlet at different burner stoichiometric ratios. It can be seen that the flue gas temperature increases dramatically near the burner outlet when coal particles are injected into the furnace and they burned intensively to release a large amount of heat. Then the flue gas temperature decreases a little when the coal particle combustion intensity decreases due to lower oxygen concentration in the recirculation zone. The flue gas temperature increases again with the mixing of the secondary air stream to increase the local oxygen concentration and to promote char particle combustion. Finally, the flue gas temperature becomes steady with char burnout at the late stage of coal combustion. When the burner stoichiometric ratio is decreased from 1.18 without air staging to 0.95, 0.75 and 0.65 with different air staging degree, the flue gas temperature gradually decreases at the main burner zone. And then the flue gas temperatures and the oxygen concentrations increase with the mixing of the secondary air stream. The flue gas temperature at the location of overfire air (OFA) injection drastically decreases due to the low temperature of OFA stream. Then the oxygen concentration increases with the OFA injection to promote char particle burnout. The flue gas temperature after the location of OFA injection increases with char particle combustion, and it gradually becomes steady till the char burnout.
Fig. 11. Schematic of the combustion chamber with OFA injections.
Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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Table 4 Operation parameters with different burner stoichiometric ratios in the main combustion region. Cases
1
2
3
4
Air flow rates (kg/s)
Primary air Inner secondary air Outer secondary air OFA stream
4.191 4.815 4.815 11.270
4.836 5.556 5.556 9.144
6.125 7.038 7.038 4.891
7.608 8.742 8.742
Air inlet temperatures (K)
Primary air Inner secondary air Outer secondary air OFA stream
353 618 618 618
353 618 618 618
353 618 618 618
353 618 618
3.281
3.281
3.281
3.281
0.65
0.75
0.95
1.18
1.18
1.18
1.18
1.18
Coal mass flow rate (kg/s)
e
Burner stoichiometric ratio (SRm)
e
Total excess air coefficients
e
e
e
Fig. 12 (b) and (c) show the oxygen and CO concentration distributions along the axial direction at different burner stoichiometric ratios. For SRm ¼ 1.18 without air staging, the oxygen concentration quickly decreases and the CO concentration sharply increases when coal particles are preheated to release the volatile matter and the volatile matter burns to consume a large amount of oxygen. The oxygen concentration increases with the mixing of secondary air stream and the CO concentration gradually decreases because CO burns to
OFA-Injection
2000
OFA-Injection
SRm=0.65 SRm=0.75 SRm=0.95 SRm=1.18
20
1800
15
SRm=0.65 SRm=0.75 SRm=0.95 SRm=1.18
1400 1200
O2(%)
T(K)
1600
10
1000
5
800 600
0
0
2
4
6
8
X(m)
10
12
14
16
0
2
4
6
10
12
14
16
(b) 4000
OFA-Injection
30
8
X(m)
(a) SRm=0.65 SRm=0.75 SRm=0.95 SRm=1.18
25
OFA-Injection
SRm=0.65 SRm=0.75 SRm=0.95 SRm=1.18
3500 3000 2500
NOx(ppm)
20
CO(%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
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2000 1500
10 1000 5
500
0
0 0
2
4
6
8
X(m)
(c)
10
12
14
16
0
2
4
6
8
10
12
14
16
X(m)
(d)
Fig. 12. Flue gas temperature and species concentration distributions along the axial direction of the top primary air outlet at different burner stoichiometric ratios: (a) flue gas temperature; (b) oxygen concentration; (c) CO concentration; (d) NOx concentration.
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consume oxygen and to produce CO2 in this area. Then the oxidation of char particle occurs under the combustion condition of high temperature and low oxygen concentration and the CO concentration increases again. The CO concentration decreases with the residual combustible gas and char burnout at the late stage of coal combustion. However, the residual CO concentration is high to some extent because of the weak mixing of secondary air with flue gas. For SRm ¼ 0.95 with shallow air staging, the corresponding oxygen concentration is lower and the CO concentration is higher than those for SRm ¼ 1.18. The oxygen concentration increases and the CO concentration drastically deceases to low level with the mixing of OFA injection with flue gas. For SRm ¼ 0.75 and 0.65 with deep air staging, higher velocity OFA injection enhances the intensive mixing of fresh air with flue gas, which is beneficial for the burnout of unburned combustible gas and carbon content in fly ash. Fig. 12 (d) show the evolution of the NOx concentration along the axial direction at different burner stoichiometric ratios. The NOx concentration in the main burner zone for shallow air staging with SRm ¼ 0.95 is much lower than that without air staging with SRm ¼ 1.18 because there are lower oxygen concentration and higher CO concentration in the large annular recirculation zone to enhance the in-flame NOx reduction by the nitrogen intermediates HCN and NH3 and hydrocarbon radicals. It is also found that the evolution of NOx concentration with shallow and deep air staging (SRm ¼ 0.65e0.95) is very different from that without air staging (SRm ¼ 1.18). The NOx concentration with air staging first increases with the conversion of volatile nitrogen to NO at the early stage of coal combustion, and then it decreases due to the in-flame NOx reduction by the nitrogen intermediates and hydrocarbon radicals in the recirculation zone of high temperature and low oxygen concentration. Then it increases with the char nitrogen conversion to NOx due to the mixing of secondary air stream to increase the local oxygen concentration. The NOx concentration after the injection of OFA stream increases again because higher oxygen concentration promotes char particle combustion to convert the residual char nitrogen to NOx. The NOx concentration in the main burner zone for deep air staging with SRm ¼ 0.65 and 0.75 is much lower than that for shallow air staging with SRm ¼ 0.95. The numerical results quantitatively indicate that the NOx concentration in the combustion chamber can be decreased to lower level with deep air staging. Fig. 13 shows the NOx and CO concentrations at the outlet of the combustion chamber for the low NOx burner with different burner stoichiometric ratios SRm for case 1 to case 4. The averaged NOx concentration at the outlet of the combustion chamber under the combustion condition without air staging SRm ¼ 1.18 is 1982 ppm. However, the averaged NOx concentration at the outlet of the combustion chamber is reduced to 989 ppm at SRm ¼ 0.95 corresponding to shadow air staging in the burner zone. It can be further reduced to 426 ppm at SRm ¼ 0.75 corresponding to deep air staging in the burner zone. It has no obvious effect on NOx emission when the burner stoichiometric ratio is further decreased to 0.65. It is because the strong fuel-rich reduction atmosphere in the annular recirculation zone is created when the deep air staging combustion is adopted and it contributes to the reduction of the formed NOx into nitrogen by N-contained intermediates HCN and NH3 and hydrocarbon radicals. The NOx reduction with the air staged combustion is more effective when the burner stoichiometric ratio is decreased to 0.75. It is also found that the CO concentration at the outlet of the combustion chamber is significantly deceased when the burner stoichiometric ratio decreases from 1.18 to 0.75 because the over-fired air injection of high velocity enhances the mixing of the fresh air with residual combustible gases and unburn carbon in fly ash and promotes the burnout. This finding shows that it is possible to greatly reduce NOx concentration with deep air staging and to simultaneously maintain the lower CO concentration by enhancing the intensive mixing of OFA stream with the flue gas to increase the local oxygen concentration at the late stage of coal combustion. This result is different from the previous results that air staged combustion has a positive impact on NOx emissions but a negative impact on burnout performance [14e17]. It is noted that the level of CO concentration at the outlet of the combustion chamber is higher than that in the actual boiler furnace because the length of the simulated combustion chamber is much shorter than the height of actual boiler furnace. Therefore, it can be concluded that the combined solution of low NOx burner and deep air staged combustion of SRm ¼ 0.75 in the main combustion zone will significantly reduce the NOx emission concentration to much lower level without decreasing the boiler efficiency when it is adopted in the retrofitting of the low NOx combustion system in the existing coal-fired boilers.
Fig. 13. Comparison on NOx and CO emission concentrations for different burner stoichiometric ratios: (a) NOx emission concentration; (b) CO emission concentration.
Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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4. Conclusion The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single HT-NR3 type low NOx swirl burner was simulated with CFD method and the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize the NOx emissions for air staged combustion with different burner stoichiometric ratios. The flue gas velocity, temperature, species concentration fields and coal particle concentration distributions were analyzed to elucidate the mechanisms of flame stability and in-flame NOx reduction. And the air staged combustion with different stoichiometric ratios in the burner region were numerically simulated to evaluate their effects on NOx and CO emissions in order to further reduce the NOx concentration in the combustion chamber. The main conclusions in this work are summarized as follows: (1) The specially-designed swirl burner configures including the pulverized coal concentrator, flame stabilizing ring and baffle plate created an ignition region of high gas temperature, proper oxygen concentration and high concentration of pulverized coal particles near the burner outlet for flame stability. (2) The large annular recirculation zone was generated between the primary and secondary air jets, and the recirculated hot flue gas in this region promoted volatile matter release and rapid ignition to form a fuel-rich NOx reduction atmosphere, which can be used to elucidate the mechanism of in-flame NOx reduction in the swirl burner. (3) The NOx concentration at the outlet of the combustion chamber was greatly reduced when the deep air staging combustion with the burner stoichiometric ratio of 0.75 was adopted and the CO concentration can be maintained at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for further lower NOx emission in the existing coal-fired boiler. Acknowledgement This work was supported by the National Key Research and Development Program of China (2016YFB0600701) and Huaneng Group Science and Technology Project (HNKJ17-G07). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.joei.2018.01.006. References [1] H. Zhou, Y. Yang, K. Dong, H.Z. Liu, Y.L. Shen, K.F. Cen, Influence of the gas particle flow characteristics of a low-NOx swirl burner on the formation of high temperature corrosion, Fuel 134 (2014) 595e602. [2] J.P. Jing, Z.Q. Li, Q.Y. Zhu, Z.C. Chen, L. Wang, L.Z. Chen, Influence of the outer secondary air vane angle on the gas/particle flow characteristics near the double swirl flow burner region, Energy 36 (2011) 258e267. [3] G.G. Sun, D.F. Che, Z.H. Chi, Effects of secondary air on flow, combustion, and NOx emission from a novel pulverized coal burner for industrial boilers, Energy Fuels 26 (2012) 6640e6650. [4] T. Tsumuraa, H. Okazaki, P. Dernjatin, K. Savolainen, Reducing the minimum load and NOx emissions for lignite-fired boiler by applying a stable-flame concept, Appl. Energy 74 (2003) 415e424. [5] S. Xue, S.E. Hui, Q.L. Zhou, T.M. Xu, Experimental Study on NOx emission and unburnt carbon of a radial biased swirl burner for coal combustion, Energy Fuels 23 (2009) 3558e3564. [6] H. Zhou, Y. Yang, L.L. Wang, Numerical investigation of gas-particle flow in the primary air pipe of a low NOx swirl burner-The DEM-CFD method, Particuology 19 (2015) 133e140. [7] S.V. Apte, K. Mahesh, P. Moin, J.C. Oefelein, Large-eddy simulation of swirling particle-laden flows in a coaxial-jet combustor, Int. J. Multiphas. Flow 29 (2013) 1311e1331. [8] R. Kourose, H. Makino, A. Suzuki, Numerical analysis of pulverized coal combustion characteristics using advanced low-NOx burner, Fuel 83 (2004) 693e703. [9] L.L. Zhao, Q.T. Zhou, C.S. Zhao, Flame characteristics in a novel petal swirl burner, Combust. Flame 155 (2008) 277e288. [10] N. Modlinski, Computational modeling of a utility boiler tangentially-fired furnace retrofitted with swirl burners, Fuel Process. Technol. 11 (2010) 1601e1608. [11] Y.H. Li, B.X. Zhou, P.A. Zheng, Numerical simulation on an optimized swirl burner with low-NOx emission, J. Chin. Soc. Power Eng. (in Chinese) 34 (2014) 261e266. [12] S.G. Ti, Z.C. Chen, M. Kuang, Z.Q. Li, Q.Y. Zhu, H. Zhang, Z.F. Wang, G.Y. Xu, Numerical simulation of the combustion characteristics and NOx emission of a swirl burner: influence of the structure of the burner outlet, Appl. Therm. Eng. 104 (2016) 565e576. [13] H. Zhou, Y. Yang, H.Z. Liu, Q.J. Hang, Numerical simulation of the combustion characteristics of a low NOx swirl burner: influence of the primary air pipe, Fuel 130 (2014) 168e176. [14] C. Moon, Y. Sung, S. Eom, G. Choi, NOx emissions and burnout characteristics of bituminous coal, lignite, and their blends in a pulverized coal-fired furnace, Exp. Therm. Fluid Sci. 62 (2015) 99e108. [15] Y. Sung, C. Moon, S. Eom, G. Choi, D. Kim, Coal-particle size effects on NO reduction and burnout characteristics with air-staged combustion in a pulverized coal-fired furnace, Fuel 182 (2016) 558e567. [16] Y. Li, W.D. Fan, Effect of char gasification on NOx formation process in the deep air-staged combustion in a 20 kW down flame furnace, Appl. Energy 164 (2016) 258e267. [17] W.D. Fan, Y. Li, Q.H. Guo, C. Chen, Y. Wang, Coal-nitrogen release and NOx evolution in the oxidant-staged combustion of coal, Energy 125 (2017) 417e426. [18] M. Kuang, Z.Q. Li, Z.Q. Ling, X.Y. Zeng, Evaluation of staged air and overfire air in regulating air-staging conditions within a large-scale down-fired furnace, Appl. Therm. Eng. 67 (2014) 97e105. [19] Q.L. Zha, D.B. Li, C.A. Wang, D.F. Che, Numerical evaluation of heat transfer and NOx emissions under deep-air-staging conditions within a 600 MWe tangentially fired pulverized-coal boiler, Appl. Therm. Eng. 116 (2017) 170e181. [20] T. Tsumura, K. Kiyama, S. Nomura, H. Kobayashi, Development and actual verification of the latest extremely low-NOx pulverized coal burner, Hitachi Rev. 47 (1998) 188e191. [21] K. Ochi, K. Kiyama, H. Yoshizako, H. Okazaki, M. Taniguchi, Latest low-NOx combustion technology for pulverized-coal-fired boilers, Hitachi Rev. 58 (2009) 187e193. [22] T.H. Shih, W.W. Liou, A. Shabbir, Z. Yang, J. Zhu, A new k-ε eddy viscosity model for high Reynolds number turbulent flows, Comput. Fluids 24 (1995) 227e238. [23] ANSYS Fluent User's Guide. [24] H. Kobayashi, J.B. Howard, A.F. Sarofim, Coal devolatilization at high temperatures, Symposium (International) on Combustion 16 (1977) 411e425. [25] L. Ma, J.M. Jones, M. Pourkashanian, A. Williams, Modelling the combustion of pulverized biomass in an industrial combustion test furnace, Fuel 86 (2007) 1959e1965. [26] X.L. Zhou, Theory and Numerical Modeling of Turbulent Gas Particle Flows and Combustion, Science Press Ltd, Beijing, 1994. [27] G.G. De Soete, Overall reaction rates of NO and N2 formation from fuel nitrogen, Symp. (Int.) Combust 15 (1975) 1093e1102. [28] P. Glarborg, A.D. Jensen, J.E. Johnsson, Fuel nitrogen conversion in solid fuel fired systems, Prog. Energy Combust. Sci. 29 (2003) 89e113.
Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006
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[29] F.C. Lockwood, C.A. Romo-Millares, Mathematical modeling of fuel-NO emissions from PF burners, J. Energy Inst. 65 (1992) 144e152. r, NO/char reactions at pulverized coal flame conditions, Symp. Int. Combust 18 (1981) 111e120. [30] J.M. Levy, L.K. Chan, A.F. Sarofim, J.M. Bee [31] M. Taniguchi, Y. Kamikawa, T. Okazaki, K. Yamamoto, H. Orita, A role of hydrocarbon reaction for NOx formation and reduction in fuel-rich pulverized coal combustion, Combust. Flame 157 (2010) 1456e1466.
Please cite this article in press as: C. Zhou, et al., Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.01.006