study of the key technology of sncr · pdf filestudy of the key technology of sncr...

15
1 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/m 3 . 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/Nm 3 , 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: NH 3 + NOxN 2 + H 2 O. 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 SO 2 oxidation and so on. Nevertheless, the weak points for SNCR is still obvious, that is, low denitrification efficiency (40~60%) and high NH 3 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

Upload: truongthuy

Post on 27-Mar-2018

227 views

Category:

Documents


3 download

TRANSCRIPT

1

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.

References

[1] Xianbin S. Development and application of large scale CFB boiler. Plant system engineering. 2009;

(04): 1-4+8.

[2] Xianbin S, Zhenghai S, Senwang J. Study of Ultra-low emission technology in CFB boiler [J]. China

Power. 2014; (01): 142-5

[3] Muzio L J, Quartucy G C. Implementing NOx control: Research to application [J]. Progress in Energy

& Combustion Science, 1997, 23(3):233-266.

[4] Hongpei G, Haitao W, ect., Water-cooled air spray nozzle for SNCR denitrification[P],China Patent:

ZL 2012 1 0267061.8,2014-06-11.

[5] Laryea G N, No S Y. Spray angle and breakup length of charge-injected electrostatic pressure-swirl

nozzle [J]. Journal of Electrostatics, 2004, 60(1):37-47.

[6] Reitz R D. Modeling of atomization processes in high-pressure vaporizing sprays[J].Atomization and

Spray Technology,1987,3(4):309~337

15

[7] Rota R, Antos D, Éverton F Zanoelo, et al. Experimental and modeling analysis of the NO x OUT

process [J]. Chemical Engineering Science, 2002, 57(1):27-38.

[8] Brouwer J, Heap M P, Pershing D W, et al. A model for prediction of selective noncatalytic reduction

of nitrogen oxides by ammonia, urea, and cyanuric acid with mixing limitations in the presence of co [J].

1996, 26(2):2117-2124.

[9] Haifeng L, Hongpei G, etc., Technical report of 300 MW CFB boiler air-staging combustion

adjustment and SNCR denitrification system operation optimization[R]. TB-14-BJKW01, 2016

[10] Environmental acceptance of Flue gas denitrification operation of 5# boiler in Qinhuangdao

Thermal Power Plant[R], Hebei Province Environment Protection Department, 2016.