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STUDY OF THE KEY TECHNOLOGY OF SNCR DENITRIFICATION AND APPLICATION ON 300 MW CFB BOILERS
Gao Hongpei, Lin Weirong, Wang Haitao, Han Ying, Zhang Jirui, Liu Haifeng, Liu Bin
Huaneng Clean Energy Research Institute, 102209, Beijing, China, [email protected]
ABSTRACT
This paper describes the summarization of the key technology of denitrification and application on
300MW CFB boilers. Based on the literatures about the mechanism of NOx formation and destruction,
the low NOx emission optimized control combustion experiments demonstrated the obvious
denitrification effect, for the purpose to control NOx emission to be lower than 50mg/m3. The domestic
proprietary spray guns used on SNCR system have successfully been developed with the optimized CFD
technology application, the SNCR denitrification system can operate with the authorized denitrification
efficiency higher than 85% and have successfully been applied on more than 50 300 MW degree CFB
boilers.
KEYWORDS: CFB, Denitrification, High Efficiency SNCR Technology, 300MW
CFB Boilers Application
1. Introduction
Circulating Fluidized Bed (CFB) boiler technology has rapidly developed in the past decades, due to
its advantages in high combustion efficiency, wide fuel source, good load adjustability, low pollutant
emission, and so on[1]. The boiler capacity has increased from 100 MW in the 1970s to 600 MW with
Sichuan Baima Demonstration Power Plant as the first 600 MW scale unit in the world. There exist
around 300 CFB units with capacity over 100 MW in China, ~57 units of which are 300 MW scale.
Compared with Pulverised Coal (PC) boiler, pollutant emission is much lower in CFB boiler, but not
low enough to meet the severe environmental policy. For pollutant of NOx, the original emission varies
from 150~400 mg/Nm3, much higher than the upper limit of 50 unit for ultra-low emission. Therefore,
denitrification solution seems necessary [2].
Efficient solution to control NOx emission in CFB boiler operation contains NOx formation
suppression during coal combustion and post-combustion NOx removal with the aid of reductants, such
as ammonia or urea. The former process lies in the optimization of boiler operation parameters while the
latter is based on the denitrification reaction: NH3 + NOx→N2 + H2O. For denitrification technology,
there are Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR). SCR
requires utilization of catalyst and is widely applied in PC boiler flue gas treatment while SNCR
denitrification can directly work in flue gas without catalyst and the reaction condition, e.g., temperature
(850~1150 ℃), is close to CFB flue gas. Compared with SCR denitrification, SNCR technology has
advantages, such as low invest and operation cost, compact structure, simple installation and operation,
rapid response, no SO2 oxidation and so on. Nevertheless, the weak points for SNCR is still obvious, that
is, low denitrification efficiency (40~60%) and high NH3 leak [3].
In this paper, the key technology of SNCR denitrification based on urea is studied, including spray
experiments/simulation, flow field observation in 300 MW scale CFB boiler as well as the SNCR
2
simulation. Then, combustion adjustment/SNCR system optimization operation is carried out for ultra-
low emission of NOx after the optimal SNCR scheme is designed and implemented.
2. Method 2.1 CFB combustion optimization
Air-staging combustion optimization is carried out on 300 MW CFB boiler (type of DG1100/17.4-II3
by Dongfang Boiler Co., Ltd, see Table 1) in Datang Wu’an Power Plant and the goal is to suppress the
formation of NOx during coal combustion and decrease the original NOx emission level. Aar, Nar and
Qnet,ar of the coal utilized is 43.4%, 0.79% and 3630 kCal/kg, respectively. Operation parameters of
primary air amount, secondary air amount, air ratio, bed temperature, etc. under load of 100%, 80% and
70% are optimized under the criteria of boiler heat efficiency and NOx emission.
Table 1 Main thermal parameters of 300 MW CFB boiler
Parameters Unit B-MCR BRL
Steam gas flux t/h 1100 1049.264
Steam gas pressure MPa.g 17.4 17.4
Steam gas temperature ℃ 541 541
Reheat steam flux t/h 908.953 864.618
Reheat steam inlet/outlet pressure MPa.g 4.06/3.87 3.857/3.677
Reheat steam inlet/outlet temperature ℃ 338/541 332/541
feed-water temperature ℃ 282 278
2.2 Spray experiments
Air-blast nozzle is the key component of SNCR denitrification system. For the patented spray gun by
Huaneng Clean Energy Research Institute [4], shown in Fig. 1, air and solution are primarily mixed in the
gun chamber and then the liquid is further atomized after leaving the nozzle. A test bench is designed to
investigate its performance, in which the flow rate and particle diameter could be measured, as shown in
Fig. 2.
Fig. 1 Structure of spray gun by HNCERI
3
Fig. 2 Scheme of nozzle experimental setup
2.3 Spray simulation
For the primary breakup of two-phase flow in nozzle, Volume of Fluid (VOF) model is applied with
gas as the first phase and liquid as the second phase [5]. Surface tension between gas and liquid is
considered and the liquid surface tension coefficient is 0.073 N/m. Mesh shown in Fig. 3(a) created by
ICEM contains ~0.8 million unstructured tetrahedral cells and the maximum size is 0.8 mm. 3 boundary
layers are attached by the ratio of 1.2. Temperature change is ignored in nozzle and the flow is assumed
as fully developed. Gas and liquid inlet is set pressure-inlet while the outlet is 1 atm.
For the secondary breakup simulation outside the nozzle, a cylindrical zone is built with diameter of
400 mm and length of 500 mm (see Fig. 3b). The field contains ~1.5 million unstructured tetrahedral cell
and the maximum size is 15 mm. Calculation zone near the nozzle is densified, with the size of 2, 6 and
10 mm, respectively.
(a) Primary breakup simulation mesh (b) Secondary breakup simulation mesh
Fig. 3 Mesh of nozzle in primary and secondary breakup simulation
Since the spray gun is inserted into flue or cyclone in SNCR system, the influence of high-speed flue
gas on liquid secondary breakup should be taken into consideration. Fig. 4(a) shows the simplified zone
of spray gun installed on the flue. In order to reduce the cell number, hexahedral cell is adopted and
denser mesh is set in the cylindrical zone. The total cell number is up to 1.7 million.
(a) Scheme of nozzle and flue (b) Mesh at z=0 mm
Fig. 4 Simplified scheme and mesh for flue gas influence on spray breakup
Spray secondary breakup process is investigated in transient mode. Turbulence of realizable k-ε is
selected while Discrete Phase Model (DPM) is used for interaction between continuous and discrete
4
phase, in which breakup, collision and coalescence of particles are considered. Injection type of Air-
blast-atomizer is applied, where WAVE model by Reitz is adopted for particle breakup simulation [6].
SIMPLE scheme is applied for pressure-velocity coupling discretization. For spatial discretization,
PRESTO! and second-order upwind method is adopted for pressure and momentum treatment,
respectively.
2.4 Flow and SNCR denitrification simulation
Fig. 5(a) shows the structure of flue and cyclone in the flow and SNCR denitrification simulation.
Layout of multiple nozzles is designed in which the nozzles are installed on the flue wall or cyclone wall,
with an insert depth of ~200 mm. Hexahedral mesh is used for less cell number and the maximum size
is 150 mm. The total cell number is ~0.94 million.
(a) Scheme of flue and cyclone (b) Mesh
Fig. 5 Scheme and mesh of 300MW CFB boiler flue and cyclone
Since dimension of flue and cyclone is 3~4 times more than nozzle in order of magnitude, meshing is
of great difficulty if structure of multiple nozzles is combined. For simplification, injection type of solid-
core in DPM is used instead of air-blast-atomizer and the relevant parameter setup is based on the spray
simulation of section 2.3. The other model set is similar to those above with the exception of Reynolds
Stress Model (RSM) utilization for turbulence calculation in cyclone.
2.5 SNCR system optimization
After the SNCR denitrification system is optimized by simulation and the engineering retrofitting is
carried out on the 300 MW CFB boiler, the system adjustment is necessary to obtain the optimal
operation parameters. Under the rated load, influence of main operation parameters such as urea solution
concentration, ratio of reductant for 3 cyclones, reductant inlet pressure and air inlet pressure, etc. on the
SNCR denitrification efficiency is to investigate.
3. Results 3.1 Air-staging combustion optimization
Under load of 300 MW, unit heat loss by unburned carbon in ash/slag slightly decreases as total air
amount increases, since the coal is easy to burn out, little excess air coefficient for combustion is
5
sufficient. Therefore, the boiler heat efficiency decreases as air amount increases. In the aspect of NOx
original emission as Fig. 6 shows, it increases from ~225 mg/Nm3 to a maximum of 280 mg/Nm3 as the
total air amount increases. In the dense-phase combustion zone, more existence of oxygen promotes more
nitrogen in coal to change into NOx, while the product of NOx is suppressed in the oxygen-deficient
conditions.
Fig. 7 indicates the influence of primary air percentage on NOx emission. The primary air is supported
into the dense-phase zone from the boiler bottom mainly for fluidization. Lower primary air proportion
means that the dense-phase zone is oxygen-poor which helps to make reducing ambient so that the
nitrogen in volatile components is more translated into N2 other than NOx. The coal particle with most
nitrogen volatilized is to burn out in dilute-phase zone with secondary air supported.
Considering unit heat efficiency, the NOx original emission is controlled ~250 mg/Nm3 with the
optimal parameter of 96~97×104 Nm3/h of total air amount and 43% of primary air percentage.
Fig. 6 Influence of total air amount on NOx original emission
Fig. 7 Influence of primary air percentage on NOx original emission
3.2 Spray simulation
Fig. 8(a) shows the velocity distribution of water at the inlet total pressure of 5 atm in nozzle central
plane. Due to high-pressure air expansion and shrink of flow path, air flow is accelerated and before the
nozzle outlet, the rate is up to 246.6m/s. Meanwhile, energy of static pressure forces the water flow till
its pressure equals the air. In Fig. 8(b), obvious boundary exists between air and water phase. Owing to
shear of air flow and multiple backflow zone in the chamber, primary mixing of water is completed with
the average volume fraction of water up to 0.62%.
6
(a) velocity distribution (b) water volume fraction distribution
Fig. 8 Distribution of velocity and volume fraction of water in central plane of Z=0mm
The simulated liquid flux feature is compared with the experimental results, as Fig. 9 shows, where
the gas inlet pressure is set identical to liquid inlet pressure. Although the simulated water flux is around
25 kg/h lower than measured value, experiment and simulative flux have a similar trend as gas/water
inlet pressure increases, since the slope of fitting curve is close. On the whole, the method in primary
breakup simulation is reasonable and feasible.
Fig. 9 Comparison of experimental and simulative liquid flux.
On the secondary breakup simulation, the spray field is found to stay in steady state after 0.1 s of DPM
addition. Fig. 10 indicates the influence of air velocity at the nozzle outlet. The air inlet pressure varies
from 4 to 6 atm with a constant water inlet pressure of 5 atm. The cone angle is 37.1º,32.6ºand 28.5º,
respectively. Higher outlet gas velocity changes the movement of sap flow more greatly and leads to a
smaller cone angle.
(a) Vair=145.9 m/s (b) Vair=246.6 m/s
380
360
340
320
300
280
260
240
220
Flu
x [k
g/h]
65432
Inlet Pressure [atm]
165.6+36.6*P
149.6+37.0*P
Exp. Flux Fitting Exp. Flux Sim. Flux Fitting Sim. Flux
7
(c) Vair=333.3 m/s Fig. 10 Track of spray velocity at air inlet pressure of 3, 4 and 5 atm.
As to the influence of outlet velocity on particle diameter, the concept of Sauter diameter defined
below is adopted. It assumes that the spray particles has one diameter equal to the whole particle volume
divided by the entire particle surface. A better secondary breakup leads to a smaller Sauter diameter. 3 3
332
322 2 2
32
4 4
3 2 3 2
4 42 2
ii
i i
iii
i
d dd
ddd d
(Equation 1)
In Fig. 11, spatially along the spray direction, the spray Sauter diameter basically increases, indicating
a better breakup near the nozzle outlet, since the gas velocity is the largest at the outlet and it gradually
decays downstream. In most calculation field, the particle Sauter diameter is over 100 μm if the outlet
velocity is relatively small at the inlet pressure of 4 atm. Contrarily, the diameter is reduced by ~40 μm
with pressure increased by 1 atm. It continuously decreases if the velocity is up to 333.3m/s, although
the variation amplitude is reduced (The overall Sauter diameter is 107.7, 61.2 and 43.7 μm, respectively).
Fig. 11 Particle Sauter diameter distribution in nozzle downstream direction
Higher gas velocity leads to a better spray breakup, however, other factors should also be considered
for the combination of a simplified DPM set. Since the spray gun works in flue gas, the extra effect of
flue gas velocity on secondary breakup is observed. Fig. 12 shows spatial distribution of particle Sauter
diameter due to the auxiliary influence of flue gas on secondary breakup after.0.1 s of spray. The
gas/water inlet pressure is 5 atm and the outlet gas velocity is 246.6 m/s.
Due to interference of large-amount flue gas, the direction of spray gas and water is both changed and
the primary particle is carried to move along the flue. For its spatial distribution (see Fig. 12), in the most
140
120
100
80
60
40
20
Sau
ter
Dia
met
er [
um]
55050045040035030025020015010050
Posion in x direction [mm]
145.9 m/s 246.6 m/s 333.3 m/s
8
calculation zone, the Sauter diameter distributes uniformly except the zone near nozzle outlet where both
spray gas and flue gas work on spray breakup. Higher flue gas velocity serves as a better breakup source
once it interacts with the gas flow from the nozzle outlet (Sauter diameter of 37.2 μm @40 m/s Vs 46.7
μm @ 8 m/s of flue gas). In addition, in the other zone of flue where spray gas decays, large particles
carried away by flue gas start to breakup by flue gas. Since the velocity scale is lower and spatially
uniform, Sauter diameter keeps stable alone the flue. Note that higher flue gas velocity below 20 m/s has
obvious effect on liquid breakup while the difference is relatively small for velocity of 20 and 40 m/s.
Fig. 12 Influence of flue gas velocity on particle breakup in flue downstream direction
At the condition of flue gas velocity of 12 m/s and liquid inlet pressure of 5 atm, Fig. 13 shows the
particle Sauter diameter distribution by various nozzle outlet gas velocities, with an extra assumed
velocity of 0 m/s for comparison. The flue gas can only break the liquid into particles larger than ~200
μm. Together with high-speed nozzle inject gas, the particles are fully atomized: higher gas rate leads to
smaller particles. The particle breakup results are integrated for next-stage CFD simulation.
Fig. 13 Influence of spray gas velocity on particle breakup in flue downstream direction
3.3 Flow and SNCR denitrification simulation
60
50
40
30
20
10
0
Sau
ter
Dia
met
er [
um]
-1435-1335-1235-1135-1035-935-835-735-635-535-435-335-235-135-35
Position in Z direction [mm]
8 m/s 12 m/s 20 m/s 40 m/s
500
400
300
200
100
Sau
ter
Dia
met
er [
um]
-1435-1335-1235-1135-1035-935-835-735-635-535-435-335-235-135-35
Position in Z direction [mm]
0 m/s 145.9 m/s 246.6 m/s 333.3 m/s
9
Flow simulation in flue and cyclone using RSM results in a typical flow path and tangential velocity
distribution in radial direction, indicating the accuracy of model applied. For 300 MW CFB boiler,
various typical schemes using 6 guns with an insert depth of 200 mm are designed, shown in Table 2,
where the exact positions of guns are hidden.
Table 2 Design of 6 spray guns layout scheme
Scheme Layout of guns Scheme Layout of guns
S1 4 on outer flue sidewall and 2 on the
inner. S5 3 on flue top wall and 3 on bottom wall
S2 3 on each flue sidewall S6 6 on cyclone top wall arranged in a
circle.
S3 6 on the outer flue sidewall S7 6 on cyclone sidewall arranged in a
line.
S4 6 on the inner flue sidewall S8 ideal case of uniform injection at the
flue inlet
Take scheme of S1 for example (see Fig. 14), once the urea solution particles are injected, The particles
move towards cyclone wall by centrifugal force. Water in particles vaporizes first because of radiation
heating from flue gas; Subsequently, the urea starts to sublime and then dissociate or hydrolyse into NH3.
The distribution of NSR (Normalized Stoichiometric Ratio, defined below) could judge the mixing
degree of reductant and flue gas. In Fig. 14(b), mixing is poor in zones where particles have not fully
vaporized, causing ultra-high or low NSR zones. On the contrary, a relatively uniform NSR zone is
attained after particle vaporization and mixing, e.g., in cyclone cone.
X
2 (urea)
(NO )
nNSR
n
(Equation 2)
(a) Residence time of particles (b) NSR distribution in central plane of cyclone
Fig. 14 Particle vaporization and NSR distribution of S1
Table 3 compares the volume fraction of NSR range in cyclone. Utilization of identical guns leads to
great difference in given NSR range (1.5~2). Even for schemes in which guns are arranged on outer or
inner flue sidewall, the volume fraction differs, with S1 ~10% higher than S2 or S3. Since scheme 1 is
the optimal result by adjusting vertical and horizontal positions, arrangement position optimization is of
necessity for certain gun numbers.
Table 3 Cyclone volume fraction of NSR range in various layouts
NSR S1 S2 S3 S4 S5 S6 S7 S8
<1.5 6.6 9.2 13.2 10.4 30.1 78.0 21.8 0.0
1.5~2 80.8 69.2 69.8 60.9 13.9 9.1 12.4 99.4
>2 12.5 21.5 17.0 28.6 56.0 12.9 65.8 0.6
10
Residence time is another factor for scheme evaluation. As Fig. 15 shows, reductant residence time
frequency distribution is similar to that of flue gas, with a highest frequency between 1~1.2 s. For guns
arranged on cyclone body, residence time is obviously shorter. For those arranged on flue sidewall, the
residence time is determined by gun position and relative direction of spray and flue gas movement.
Since gas velocity is changed toward outer sidewall by flue shape, residence time of spray from the inner
sidewall should be shorter than that from the outer. NSR distribution and residence time frequency
distribution should be both considered for an optimal layout design.
Fig. 15 Residence time frequency distribution in various schemes
In SNCR denitrification simulation, a reduced kinetic mechanism is integrated instead of detailed
NOxOUT mechanism for turbulence-chemistry calculation simplification. Mechanism of Urea 2000 by
Rota [7] is presently the most comprehensive and accurate kinetics and the 9-step reduced mechanism [8]
listed in Table 4 including urea dissociation and hydrolysis is widely used (Here, mechanism validation
with SNCR denitrification data is neglected). Simulation by the reduced mechanism has good agreement
with experiments in denitrification efficiency and NH3 leak.
Table 4 9-step reduced kinetic mechanism for SNCR denitrification
NO. Reactions Pre-exponential
factor(A)
Temp.
exponent(b)
Activation
Energy (Ea,
Cal/mol)
1 NH2CONH2 → NH3 + HNCO 1.27E+04 0 65048.1
2 NH2CONH2 + H2O→ 2NH3 + CO2 6.13E+10 0 87819.1
3 NH3 + NO → N2 + H2O + H 4.24E+08 5.3 349937.0
4 NH3 + O2 →NO + H2O + H 3.50E+05 7.7 524487.0
5 HNCO + M → H + NCO + M 2.40E+14 0.8 284637.0
6 NCO + NO →N2O + CO 1.00E+13 0.0 -1632.5
7 NCO + OH → NO + CO + H 1.00E+13 0.0 0.0
8 N2O + OH → N2 + O2 + H 2.00E+12 0.0 41858.5
9 N2O + M → N2 + O + M 6.90E+23 -2.5 271075.6
Influence of CFB boiler operation parameters such as gas temperature, NSR, oxygen/water content,
residence time. etc., is observed by CFD with denitrification mechanism, and a brief results is listed in
Fig. 16. There exists an optimal temperature for SNCR denitrification ~920 ℃, close to the designed
0.20
0.15
0.10
0.05
0.00
Fre
qu
ency
dis
trib
uti
on
4.03.02.01.00.0
Residence Time /s
S1 S2 S3 S4 S5 S6 S7 S8
11
flue gas temperature while the efficiency continuously increases as NSR is enlarged. Note that more
input of reductant helps to remove more NOx, however, the addition emission of NH3 and N2O should
not be neglected.
(a) DeNOx effect Vs gas temperature (b) DeNOx effect Vs NSR
Fig. 16 Influence of gas temperature and NSR on SNCR denitrification efficiency.
In Fig. 17, on assumption of inlet NOx 250 mg/Nm3 and NSR 1.5, denitrification efficiency and NH3
leak has opposite features, regardless the scheme. High NH3 leak indicates its absence in NOx reduction,
inducing low efficiency. For those schemes with guns arranged on the flue sidewall, the efficiency is
much higher and S2 is the highest as 69.9%. It discloses that volume fraction of NSR of 1.5~2 indicates
mixing more than denitrification efficiency and other determined factors such as residence time should
be considered as well.
Fig. 17 Influence of spray guns layout scheme on denitrification efficiency and pollutant emission
3.4 SNCR system optimization
SNCR denitrification efficiency is no larger than 70% on NSR of 1.5, therefore, optimization of spray
guns number is carried out (See Table 5 with installed positions hidden). Fig. 18 re-discloses the
independency of denitrification efficiency and volume fraction of NSR 1.5~2. Instead, the efficiency
trend is similar to the fraction of NSR >1.5. Moreover, as guns number increases from 6 to 12, the
efficiency is increased by as much as 7.5% (from 69.0% to 76.5%).
0.8
0.6
0.4
0.2
Den
itri
fica
tion
Eff
icie
ncy
110010501000950900850800
Gas Temperature /ºC
NSR1 NSR1.5 NSR2 NSR2.5
0.8
0.6
0.4
0.2
Den
itri
fica
tion
eff
icie
ncy
2.52.01.51.0
NSR
800ºC 900ºC 1000ºC 1100ºC
0.72
0.70
0.68
0.66
0.64
0.62
0.60
0.58
0.56
Den
itri
fica
tion
Eff
icie
ncy
Spray guns layout scheme
35
30
25
20
15
Em
ission [m
g/Nm
3]
S1 S2 S3 S4 S5 S6 S7 S8
DeNOx Efficiency NH3 emission N2O emission
12
Table 5 Optimization scheme of spray guns number
NO. Spray num. On flue outer sidewall On flue inner sidewall
S1 6 4 2
OS7 7 4 3
OS8 8 4 4
OS9 9 5 4
OS10 10 5 5
OS11 11 6 5
OS12 12 6 6
Fig. 18 Influence of spray guns number on denitrification efficiency
SNCR denitrification of 10 and 12 guns scheme is additionally compared since the efficiency is so
close in Fig. 18. For conditions of inlet NOx concentration of 250 mg/Nm3, the ultra-low emission of 50
mg/Nm3 requires an efficiency no less than 80%. Therefore, the NSR should be increased. As Fig. 19
shows, on the whole, efficiency of 12 guns scheme is higher than that of 10, nevertheless, the increase
amplitude is less than 0.5% on NSR>1.5, indicating limitation of guns number increase on denitrification.
The efficiency of 10 guns scheme is 80% on NSR ~1.8 and up to 81.9% on NSR of 2, meeting the
requirement of ultra-low emission. Therefore, the 10 guns scheme seems sufficient for relevant
engineering retrofitting.
0.95
0.90
0.85
0.80
0.75
0.70
0.65
Per
cen
tage
1211109876
Number of spray guns
DeNOx efficiency Volume fraction of NSR 1.5~2 Volume fraction of NSR>1.5
13
Fig. 19 Influence of NSR on denitrification efficiency of 10 and 12 guns scheme
After the retrofitting, the SNCR denitrification system operation is tested for optimization. A typical
result of urea solution concentration adjustment with constant NSR is listed below. As concentration
increases, NOx emission decreases first, and then increases. Spray of low concentration solution may
bring in too much water and water content increase as well as temperature decrease may inhibit the
denitrification, contrarily, if concentration is high, less spray of solution may inhibit sufficient mixing of
gas and reductant. In Fig. 20, the NOx emission comes to its minimum at the concentration of ~8%.
Fig. 20 Optimization of urea solution concentration
Operation optimization results are summarized in Table 6. In this method, the ultra-low emission of
NOx is achieved on NSR between 1.5~1.6 [9] with efficiency over 80%. If the NSR is continuously
increased, the efficiency could be higher than 85% [10]. Since 2012, the SNCR denitrification technology
with high efficiency and low cost has been widely applied in CFB boiler retrofitting in China, including
over 50 300 MW scale boiler.
Table 6 Set of optimal SNCR denitrification system operation parameters
NO. Parameters Unit Value
0.82
0.80
0.78
0.76
0.74
Den
itri
fica
tion
eff
icie
ncy
2.12.01.91.81.71.61.5
NSR
10 guns scheme 12 guns scheme
NOx Emission
DeNOx efficiency
Urea solution concentration / %
NO
x em
issi
on m
g/N
m3
DeN
Ox
effi
cien
cy/ %
14
1 Solution concentration % 8~10
2 Ratio of 3 cyclones - 3:4:3
3 Liquid injection pressure MPa 0.5~0.6
4 Gas injection pressure MPa 0.5~0.6
4. Conclusions
In this study, spray nozzle breakup feature is observed mainly by numerical simulation and validated
by experiments. Flow in flue and cyclone as well as the SNCR denitrification is studied for the
optimization of multiple spray guns layout design. Better denitrification performance is available through
coal combustion and SNCR system operation optimization. The major conclusions are:
(1). The total air amount and primary air fraction are mainly factors that affect NOx original emission
in air-staging combustion adjustment. The combustion optimization is able to reduce the NOx emission
to ~250 mg/Nm3 under full load of the test plant.
(2). Higher gas inlet pressure leads to higher gas velocity at the nozzle outlet as well as a better spray
secondary breakup. The spray breakup is also influenced by flue gas velocity, especially near the nozzle
outlet. The cold simulation of spray could supply simplified particle parameters for CFD simulation in
flue and cyclone.
(3). Spray guns layout scheme is investigated by NSR distribution and reductant residence time.
Integration of 9-step SNCR reduced mechanism helps to observe denitrification process and compare
various layout schemes. Finally, guns number optimization is carried out for higher denitrification
performance: 10 guns layout design is expected to decrease NOx emission to 50 mg/Nm3 on NSR ~1.8.
(4). 10 guns layout scheme is implemented in engineering retrofitting in demonstrated 300MW scale
plant. SNCR system operation is optimized for NOx ultra-low parameters on NSR between 1.5~1.6.
Higher denitrification efficiency than 85% is achieved by continuously increasing NSR.
(5). Air-staging combustion optimization integrated with SNCR denitrification turns out to be effective
NOx ultra-low emission control method with high efficiency as well as low cost and it is potential for
wider application.
Acknowledge:
This research is supported by the technical research projection of “Flow and kinetics simulation of
SNCR denitrification in CFB boiler” by Huaneng Group Clean Energy Research Institute, 2014.
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