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Energy Conversion Conversion and Management Management 49 (2008) 1530–1537 www.elsevier.com/locate/enconman
Numerical investigation of spray combustion in jet mixing type combustor for low NOx emission Hirotatsu Watanabe a,*, Yoshikazu Suwa b, Yohsuke Matsushita a, Yoshio Morozumi a, Hideyuki Aoki a, Shoji Tanno a, Takatoshi Miura a a
b
Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, Sendai, Miyagi 980-8579, Japan SHI Mechanical and Equipment Inc., Development and Process Group, Engineering Department, 1501 Imazaike, Saijyo, Ehime 799-1393, Japan
Received 27 April 2007; accepted 6 December 2007 Available Available online 21 February February 2008
Abstract
The present paper describes a numerical investigation of spray combustion in a jet mixing type combustor. In this combustor, kerosene spray was injected with a pressure atomizer, and high speed combustion air was introduced towards the spray flow through some inlet air nozzles to improve mixing of the spray and the air. In the numerical simulation, the conservative equations of mass, momentum and energy in the turbulent flow field were solved in conjunction with the k – e two equation turbulence model. The effects of the diameter and the number of air inlet nozzles on the combustion behavior and NO emission were numerically investigated. When the diameter of the inlet air nozzle decreased from 8 to 4 mm, the calculated NO mole fraction in the exhaust gas was drastically decreased by about 80%. An increase in the inlet velocity resulted in improvement of the mixing of the spray and the air, and hence, the high temperature region where thermal NO was formed became narrow. As a result, the exhaust NO mole fraction decreased. Furthermore, a decrease in exhaust NO mole fraction was explained by a decrease in the residence time in the high temperature region above 1800 K. 2007
Elsevier Ltd. All rights reserved.
Keywords: Spray combustion; Numerical simulation; Low-NOx emission
1. Introduction
Spray combustion systems are widely utilized in utility boilers, gas turbine combustors and internal combustion engines. engines. Similar to other combustion combustion systems, systems, reduction reduction of NOx emiss emissio ion n from from spra sprayy combu combust stion ion syste systems ms is required. To reduce NOx emission, study of the spray combustion process involving NO x formation plays an important tant role role in the the desi design gn of comb combus usto tors rs with with low low NOx emission. Two stage combustion based on controlling the equivalence ratio has been used in practical spray combustors for reduction of NOx emission. A new low NOx combust bustion ion syste system m calle called d jet jet mixin mixingg type type comb combus ustor tor was was developed by Arai et al. [1] al. [1].. The principle of the jet mixing
*
Corresponding author. Tel.: +81 22 795 7251; fax: +81 22 795 6165. E-mail address:
[email protected] [email protected] (H. (H. Watanabe).
0196-8904/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2007.12.002
type combustor was revealed as a rapid mixing of the air and the spray, which was achieved by a high speed air jet. This type of combustor can reduce NOx formation as well as THC (total hydrocarbon) emission, however, it is difficult to reduce both of them further by controlling the equivalence ratio. Numerical simulation is a useful and powerful tool for investigating the effect of uncertain factors on the combustio bustion n charac character teristi istics cs and the design design of a combus combustor tor.. Recently, several numerical investigations were conducted for spray spray combus combustion tion.. Hampar Hampartso tsoumia umian n et al. [2] have deve develo lope ped d a post post proce process ssin ingg mode modell for for pred predic ictio tion n of NOx formation in the spray combustion process and performed a spray combustion simulation in a staged combustor. Aoki et al. [3] have investigated the combustion characteristics in a practical boiler by using three dimensional sional spray spray combus combustion tion simulat simulation ion and predic predicted ted the efficiency of combustion, the concentration of unburned
H. Watanabe et al. / Energy Conversion and Management 49 (2008) 1530–1537
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Nomenclature
pre-exponential factor of rate constant () A C l, C k, C e1,C e2 constants in k – e two equation model () diameter (m) d activation energy (J mol1) E turbulent kinetic energy (m2 s1) k mass fraction for chemical species i () mi number of air inlet nozzles n number of total computational grids () q universal gas constant (m) R Reynolds number () Re chemical reaction rate (mol m3 s1) Ri stoichiometric oxygen weight required to burn r 1 kg of fuel () source term for / S u source term evaluated using a PSI-CELL model S du temperature (K) T time (s) t U , V , W time mean velocity component (m s1) x, r , h cylindrical coordinate
char and soot in the gas phase and NO concentration in the exhaust gas. Furuhata et al. [4] have modeled heavy fuel oil spray flames stabilized by a baffle plate and compared their predictions with experimental data. Sharma and Som [5] have investigated numerically the effect of spray parameters on the combustion characteristics and NOx emission numerically. However, there were few numerical investigations for a jet mixing type spray combustor. Therefore, it is needed to understand the combustion characteristics in the jet mixing type system in detail using numerical simulation. Previously, we have performed spray combustion simulation for a jet burner with the jet mixing type spray combustion system [6]. The jet burner, in which a fuel and an oxidizing agent burn under high pressure conditions, generates a high speed and high temperature jet of exhaust gas. It is mainly used for grinding and drying various processed products. The validity of the spray combustion simulation has been investigated, and the calculated results were in good agreement with experimental results. Moreover, to improve the combustion behavior in the jet burner, a numerical simulation considering a baffle plate was also performed. As a result, it was shown that the NO mole fraction in the exhaust gas decreased by about 40% when a suitable baffle plate was applied to the combustor. However, the effects of the mixing of the air and the spray on combustion behavior have not been discussed, although that is a very important part of the spray combustion system. In this study, the spray combustion simulation model developed in the previous study [6] is applied to the investigation of the effect of the mixing of the air and the spray
Greek symbols C/ diffusion coefficient for / e eddy dissipation rate (m2 s3) / dependent variable k air ratio () viscosity (Pa s) l q density (kg m3) Subscripts
Arr eddy eff fu g h ox t
Arrhenius eddy diffusion effective fuel gas high temperature region oxidant turbulence
on the combustion behavior and exhaust NO mole fraction. 2. Jet burner
Fig. 1 shows a schematic diagram of a jet burner applying the jet mixing type spray combustion system. This jet burner has a preheating and a combustion chamber. The dimensions of the combustion chamber are /38.4 mm in inner diameter and 420 mm in length. The diameter ( d ) of the inlet air nozzle in the combustion chamber is /8 mm. A pressure atomizer whose spray pattern is a full cone is used in the combustor. Kerosene is sprayed from the atomizer. Preheated air in the outer annular preheating chamber is introduced into the combustion chamber through 6 air inlet nozzles towards the spray flow. 3. Numerical simulation
3.1. Governing equations
The k – e two equation turbulence model [7] is used to describe the turbulent flow. The transport equations used in this study can be expressed in a three dimensional cylindrical coordinate system as: o o x
ðqU /Þ þ
1
o
1
o
ðqW /Þ r or r oh o o/ o/ o/ 1 o 1 o ¼ þ þ C/ C/ r C/ o x o x or r or r oh r oh
ðr qV /Þ þ
þ S / þ S d / ;
ð1Þ
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H. Watanabe et al. / Energy Conversion and Management 49 (2008) 1530–1537
Fig. 1. Schematic diagram of a jet burner.
where / represents the dependent variables which denote mass (1), momentum (U , V , W ), turbulence energy (k ), dissipation rate of turbulence energy (e), enthalpy (h) and mass fraction (mi ; i = C12H24, O2, CO2, H2O, N2, CO, H2, C(soot), NO). C/ is the diffusion coefficient, S / is the source term in gas phase, S d/ is the source term arising from the droplet phase. x , r , h are the axial, radial and tangential coordinates and q is the gas density, respectively. The gas density is obtained from the equation of state. The governing equations are discretized using a control volume method. C12H24 is used to represent kerosene. A multi-step global reaction mechanism is used to express the kerosene–air combustion reaction. The reaction rate is expressed by Eq. (2), using both the rate of dissipation of the eddies (Eq. (3)) derived from the EDC (eddy dissipation concept) [8] and the chemical reaction rate (Eq. (4)). Ri ¼ minð Reddy ; RArr Þ
ð2Þ
e mox Reddy ¼ 4:0 min ; mfu k r fu a
b
RArr ¼ A ½fuel ½oxygen expð E = RT Þ
ð3Þ ð4Þ
where r fu is the stoichiometric oxidant requirement to burn 1 kg fuel, A is the pre-exponential rate factor, E is the activation energy and T is the gas temperature. The soot distribution in the combustor was calculated by solving a transport equation for the soot mass fraction using simple expressions for the soot formation and oxidation rates. A post processing NO formation model is used to predict the concentration of thermal and prompt NO. Radiative heat transfer is calculated using a 6 flux method [9] coupled with a WSGGM (weighted sum of gray gases model). In
the WSGGM, Beer’s model [10], including the radiative properties of CO2, H2O and soot is used. The motions of fuel droplets in the turbulent combustion flow field are calculated using a stochastic separated flow (SSF) model [11]. The Lagrangian equations for the rate of change of droplet velocity, mass and temperature are solved simultaneously, and the source term S d / in Eq. (1) is calculated by the PSI-CELL model [12]. The influence of the turbulent fluctuation velocity of the gas flow is considered in the droplet velocity calculation. To obtain average injection velocities, the angle of the droplets and the standard deviation of the Gaussian distribution, we perform the spray calculations so as to agree with the measured data for the size distribution and spray flux. The details of the spray combustion simulation model are described in Ref. [6]. 3.2. Case study
The configuration of the combustion chamber is shown in Fig. 1. Our previous research [6] showed the validity of the spray combustion simulation above mentioned in the jet burner with d = /8 mm and n = 6. In this study, the spray combustion simulations are performed to investigate the effect of mixing of the air and the spray by changing the diameter and the number of the air inlet nozzles. When the effect of the diameter of the air inlet nozzle on the combustion behavior is investigated, the diameter of the air inlet nozzle is set to 4, 6, 8 and 10 mm. In this case, the number of air inlet nozzles is set to 6. When the effect of the number of air inlet nozzles is investigated, their number is set to 4, 6 or 8 without changing the total area of the inlets as shown in Fig. 2. Table 1 shows the number of computational grids
Fig. 2. Schematic diagram of a jet burner (r – h plane).
H. Watanabe et al. / Energy Conversion and Management 49 (2008) 1530–1537
Table 1 The number of computational grids and calculation domain for tangential direction for each air inlet nozzle The number of air inlet nozzles ()
Nozzle diameter (mm)
Calculation domain for tangential direction
The number of computational grids
4 6 8
7.3 4.0, 6.0, 8.0, 10 5.2
1/4 1/6 1/8
122 21 48 122 21 30 122 21 25
Table 2 Calculation conditions Inlet air flow rate (N m3 h1) Fuel flow rate (kg h1) Air ratio () Inlet air temperature (K)
122 3.60 2.0 593
for the axial, radial and tangential directions, respectively, and the calculation domain for the tangential direction. When the calculation domain for the tangential direction is changed with the number of air inlet nozzles, the grid width for the tangential direction is set to almost the same. The calculation conditions are shown in Table 2.
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4. Results and discussion
4.1. The effect of the diameter of the air inlet nozzle
In this section, the effect of the diameter of the air inlet nozzle related to the mixing of the spray and the air is investigated numerically. Fig. 3 shows the effect of the diameter of the air inlet nozzle on the temperature distribution and the peak temperature. When the diameter of the air inlet nozzle decreases, the high temperature region shifts towards the upstream because an increase in the inlet velocity causes improvement of the mixing of the spray and the air. Fig. 4 shows the effect of the diameter of the air inlet nozzle on the distribution of NO mole fraction and exhaust NO mole fraction at 0% O2 concentration. When the diameter of the air inlet nozzle decreases, the exhaust NO mole fraction decreases because the residence time in the high temperature region is drastically decreased. 4.2. The effect of the number of the air inlet nozzles
Fig. 5 shows the effect of the number of air inlet nozzles on the temperature distribution and the peak temperature. No significant change is observed when the number of air inlet nozzles is changed.
Fig. 3. Predicted temperature distribution (The effect of the diameter of the air inlet nozzle, n = 6).
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H. Watanabe et al. / Energy Conversion and Management 49 (2008) 1530–1537
Fig. 4. Predicted NO mole fraction distribution (The effect of the diameter of the air inlet nozzle, n = 6).
Fig. 5. The effect of the number of the inlet nozzles on predicted temperature distribution.
Fig. 6 shows the effect of the number of air inlet nozzles on the predicted temperature distribution (r – h plane at x = 100 mm). When the number of air inlet nozzles increases, the temperature gradient for the tangential
direction decreases because the inlet air is uniformly supplied. Fig. 7 shows the distribution of the NO mole fraction and the exhaust NO mole fraction at 0% O 2 concentration.
H. Watanabe et al. / Energy Conversion and Management 49 (2008) 1530–1537
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The exhaust NO mole fraction increases a little with n = 4 compared to that at n = 6 and n = 8. This result is further discussed in the next section. 4.3. The effect of inlet Reynolds number and the residence time on exhaust NO mole fraction
In this section, the effect of the mixing of the air and the spray and the residence time on the exhaust NO mole fraction is investigated. The inlet Reynolds number Reinlet is used as a parameter that expresses the mixing of the air and the spray because an increase in inlet Reynolds number causes an increase in the mixing of the air and the spray. The inlet Reynolds number is determined as follows:
(a) n = 4
Reinlet ¼
1 t h (c) n = 8
2170 K
Fig. 6. The effect of the number of the air inlet nozzles on predicted temperature distribution (r-h plane at x = 100 mm). r
0
duq l
:
ð5Þ
inlet
In this study, the air ratio (k) is 2.0. Prompt NO formation is inhibited in such a lean combustion condition because prompt NO is hardly generated when k > 1.0 [13]. Therefore, Thermal NO dominates the exhaust NO. Thermal NO increases exponentially above about 1800 K. The residence time of the gas in each computational cell is calculated from the cell volume divided by the volume flow rate of combustion gas in the cell. Therefore, the residence time in the high temperature region, th, is expressed as the sum of the residence times in the cells above 1800 K. th is evaluated as follows:
(b) n = 6
630 K
X ¼ q
1 1 1 þ þ D x=U Dr =V r Dh=W
:
Fig. 8 shows the effect of inlet Reynolds number on the exhaust NO mole fraction. Increasing the inlet Reynolds number steadily decreases the exhaust NO mole fraction. This result shows that the mixing process of the fuel and the air has an important effect on low NOx combustion.
Inlet air
(a) n = 4 (Exhasut NO at 0%O2 = 200 ppm)
x
(b) n = 6 (Exhaust NO at 0%O2 = 174 ppm)
x
r
0
r
0
(c) n = 8 (Exhaust NO at 0%O2 = 176 ppm)
0 ppm
ð6Þ
t >1800K
x
450 ppm
Fig. 7. The effect of the number of the air inlet nozzles on predicted NO mole fraction distribution and exhaust NO mole fraction.
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H. Watanabe et al. / Energy Conversion and Management 49 (2008) 1530–1537
Fig. 8. The effect of inlet Reynolds number on exhaust NO mole fraction.
450
(d =8mm, n =6) ] 400 m p p [ 350 ) 2 O % 300 0 t a ( n 250 o i t c a r f e 200 l o m O 150 N t s u 100 a h x E
(d =10mm, n =6)
(d =5.2mm, n =8) (d =7.3mm, n =4) (d =6mm, n =6)
50 0
(d =4mm, n =6) 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Residence time in high temperature region [sec]
Fig. 9. The effect of residence time in high temperature region on exhaust NO mole fraction.
Fig. 9 shows the effect of residence time in the high temperature region on the exhaust NO mole fraction. The exhaust NO mole fraction increases with an increase of the residence time in the high temperature region. When the number of air inlet nozzles decreases, the residence time in the high temperature region increases because the region where the mixing of the spray and the air deteriorates due to the inlet air being not as uniformly supplied. Therefore, the exhaust NO mole fraction increases with a decrease of the number of air inlet nozzles. When the residence time in the high temperature region is higher than 0.66 (at d = 8 mm,
n = 6), the exhaust NO mole fractions not greatly
affected. This is because the NO formation reaction is almost terminated in such long residence times. 5. Conclusion
In this study, the effects of the diameter and the number of air inlet nozzles on the combustion behavior in a jet mixing type combustor were numerically investigated. In addition, the effect of the mixing of the spray and the air on the exhaust NO mole fraction was also investigated numerically.
H. Watanabe et al. / Energy Conversion and Management 49 (2008) 1530–1537
When the diameter of the air inlet nozzle decreased from 8 to 4 mm, the calculated NO mole fraction in the exhaust gas was drastically decreased by about 80%, although no significant change is observed when the number of air inlet nozzles is changed. An increase in the inlet velocity causes the improvement of the mixing of spray and air, and hence, the high temperature region where thermal NO is formed becomes narrow. Consequently, the numerical results indicate that the mixing process of the fuel and the air had an important effect on low NOx combustion. It is noted that the exhaust NO mole fraction correlates with the residence time in the high temperature region. References
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[4] Furuhata T, Tanno S, Miura T, Ikeda Y, Nakajima T. Performance of numerical spray combustion simulation. Energy Convers Manage 1997;38:1111–22. [5] Sharma NY, Som SK. Influence of fuel volatility and spray parameters on combustion characteristics and NOx emission in a gas turbine combustor. Appl Therm Eng 2004;24: 885–903. [6] Watanabe H, Suwa Y, Matsushita Y, Morozumi Y, Aoki H, Tanno S, Miura T. Spray combustion simulation including soot and NO formation. Energy Convers Manage 2007;48:2077–89. [7] Launder BF. The prediction of laminarization with a two equation model of turbulence. Int J Heat Mass Transf 1972;15:301–14. [8] Magnussen BF, Hjertager BH. On mathematical modeling of turbulent combustion with special emphasis on soot formation and combustion. Proceed Combust Inst 1976;16:719–29. [9] Gosman AD, Lockwood FC. Incorporation of a flux model for radiation into a finite-difference procedure for furnace calculation. Proceed Combust Inst 1972;14:661–70. [10] Beer JM. Heat transfer in flames. NY: John Wiley & Sons; 1974. [11] Faeth GM. Evaporation and combustion of sprays. Prog Energy Combust Sci 1983;9:1–76. [12] Crowe CT, Sharma MP, Stock DE. The particle-source in cell (PSICELL) model for gas-droplet flows. Trans ASME J Fluids Eng 1977;99:325–32. [13] Otake K, Fujiwara T. Combustion engineering (in Japanese), Tokyo, Japan: CORONA publishing; 1985.
