ignition and volatile combustion of pulverized coals in … rate: 2.5~5g/h high speed camera fastcam...

煤燃烧国家重点实验室 State Key Lab Coal Combustion

Ignition and volatile combustion of Pulverized Coals in Lower Oxygen Content

O2/CO2 Atmosphere

Xiaohong HUANG, Jing LI, Zhaohui LIU*, Ming YANG, Dingbang WANG, and Chuguang ZHENG

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology,

Wuhan 430074, China

2nd Oxyfuel Combustion Conference(2OCC) Yeppoon, Queensland, Australia, Sept. 13~16, 2011

2nd Oxyfuel Combustion Conference(2OCC) Yeppoon, Queensland, China 12th - 16th September 2011



Background

-Pulverized Coal Combustion ----Conventional condition (High O2 content, N2 background) ----Oxy-fuel condition (High O2 content, CO2 background)

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-The flame propagation speed in O2/CO2 environment is lower than that in O2/N2 (Kiga, 1997)

-The pulverized coals are transported by recycle flue gas(low O2 content).

-An increased oxygen concentration for oxy-fuel recycle combustion can produce ignition times and volatile flames similar to those obtained under coal/air combustion. (Shaddix, 2009)

-Could the pulverized coals be ignition in low oxygen flue gas without addition pure oxygen？



Previous Research

C.R.Shaddix(2009) employed the Sandia’s optical entrained flow reactor facility to capture the transient image of the burning particle, studied the ignition and devolatilization of bituminous and subbituminous coal at a gas temperature of 1700K over a range of 12~36% O2 in both N2 and CO2 diluent gases.

Zhang(2010) employed the high speed camera to capture the particle images in the DTF, investigated the combustion of a brown coal at 1073K and 1273K over a range of 21~36% O2 in both N2 and CO2 diluent gases.

They found that an increased oxygen concentration can reduce the ignition delay in oxy-fuel condition.

The ignition and volatile combustion of coal in low oxygen are studied rarely, can’t be seen in the references.

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Experimental Setup

Flat Flame supported Entrained Flow Reactor (FF-EFR)

Flame temperature: ~1773K Post flame gases: O2: 0%~30% CO2: 3%~87% H2O: 8%~16% N2: 0%~87% Heating rate: 105~106K/s Residence time: 10~1000ms Feeder rate: 2.5~5g/h High Speed Camera Fastcam SA1.1 5400fps

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Flue Gas Compositions and Temperature History

NO. O2 N2 CO2 H2O Flux* Tad(K) 1 2 75.6 7.5 14.9 22.9 1940 2 5 72.6 7.5 14.9 22.9 1940 3 10 67.6 7.5 14.9 22.9 1940 4 20 57.6 7.5 14.9 22.9 1940 5 30 47.6 7.5 14.9 22.9 1940 6 2 78 4 16 22.2 1770 7 5 75 4 16 22.2 1770 8 10 70 4 16 22.2 1770 9 20 60 4 16 22.2 1770

10 30 50 4 16 22.2 1770 11 2 79 4 15 22.9 1670 12 5 76 4 15 22.9 1670 13 10 71 4 15 22.9 1670 14 20 61 4 15 22.9 1670 15 30 51 4 15 22.9 1670 16 2 0 82 16 20.5 1769 17 5 0 79 16 20.5 1769 18 2 0 82 16 14.8 1670 19 5 0 79 16 14.8 1670

Calculated Composition and Adiabatic Flame Temperature of Combustion Products (mol%)

*: 298K, 1atm

The gas residence time were calculated with the Plug flow reactors of CHEMKIN

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0 10 20 30 40 50 60 70 80 90 100

400

600

800

1000

1200

1400

1600

1800

2000

Tem

pera

ture

(K)

Residence time(ms)

O2_N2_1940 O2_N2_1770 O2_CO2_1770 O2_N2_1670 O2_CO2_1670



Coal Properties

Analysis DQ

coal DT

coal JC

coal Rank lignite bituminous anthracite Proximate analysis (wt.%ad)

Moisture 6.55 2.59 0.89 Ash 22.62 28.31 23.52 Volatile 34.11 24.78 7.44 Fixed Carbon 36.73 44.32 68.15

Ultimate analysis (wt.%daf) Carbon 76.72 80.83 88.54 Hydrogen 4.72 4.08 3.48 Nitrogen 1.22 1.34 1.09 Sulphur 1.15 3.28 2.02 Oxygen (by diff.) 16.19 10.47 4.87

Proximate and Ultimate analysis of Coal Samples

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Particle Image Capture

Calibration Particle Image

CMOS sensors 1024×1024 pixels Frame Rate:5400fps Shutter Speed: 1/5400s

Photron FASTCAM SA1.1

51μm/pixel

500 Images Captured

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Single Particle

Image Processing

Raw Image Gray Image Filtered Image

Particle Central Position:

Particle mean brightness:

Particle Size: The number of pixels contained in the particle beam

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Optical Intensity Profiles along the reactor

0 20 40 60 80 100 1200

50

100

150

200

250

Ignition Point

Char Combustion

Opt

ical

Inte

nsity

ResidenceTime (ms)

DT-1773K-2% O2/CO2 DT-1773K-2% O2/N2 DT-1773K-20% O2/N2

Volatile Combustion

X-Po

sitio

n

Gas Flow

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0 20 40 60 80 100 1200

50

100

150

200

250

Opt

ical

Inte

nsity

ResidenceTime (ms)

DQ-1773K-2% O2/CO2 DQ-1773K-2% O2/N2 DQ-1773K-20% O2/N2

0 20 40 60 80 100 1200

50

100

150

200

250

O

ptic

al In

tens

ity

ResidenceTime (ms)

JC-1773K-2% O2/CO2 JC-1773K-2% O2/N2 JC-1773K-20% O2/N2



The Principle of Ignition

300 600 900 1200 1500 1800 21000

5

10

15

20

25

30

35

40

45

50

http://webbook.nist.gov/chemistry/fluid/C

v (J

/mol

*K)

Temperature (K)

H2O O2 Ar CO2 N2

300 600 900 1200 1500 1800 21000.00

0.03

0.06

0.09

0.12

0.15

Ther

mal

Con

duct

ivity

(W/m

*K)

Temperature (K)

H2O O2 Ar CO2 N2

Particle Heat-up:

For different conditions, the gas thermal conductivity(λ) is the only factor affecting the particle temperature.

Auto-ignition time(adiabatic thermal explosion theory)

The ignition delay is affected by the heat capacity (CV) and the reactivity(k) of the mixture gas

There is no identifiable heterogeneous ignition stage. {By Mclean et al. 1981 and Molina and Shaddix 2007}

∝ (Cv，1/k)

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Ignition Characteristic at the Different Conditions: N2 dilution

0 5 10 15 20 25 305

6

7

8

9

10

11

12

13

14

15

Igni

tion

Tim

e(m

s)

Oxygen Concentration (%)

DQ-N2-1940K DQ-N2-1773K DQ-N2-1673K

0 5 10 15 20 25 305

6

7

8

9

10

11

12

13

14

15

Igni

tion

Tim

e(m

s)


DT-N2-1940K DT-N2-1773K DT-N2-1673K

0 5 10 15 20 25 305

10

15

20

25

30

Igni

tion

Tim

e(m

s)


JC-N2-1940K JC-N2-1773K JC-N2-1673K

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C.R. Shaddix, A. Molina.. Proc. Combust. Inst. 2009, 32, 2091-2098.

According to adiabatic thermal explosion theory for a one-step overall reaction, the auto-ignition time is linear correlation with fuel concentration and oxygen concentration. From the results, we think that the adiabatic thermal explosion theory is improper.



Ignition behavior in Lower Oxygen Content O2/CO2 Atmosphere

0

2

4

6

8

10

12

14

16

18

20

17731670

Igni

tion

Tim

e(m

s)

Temperature (K)

DQ-N2-5% DQ-N2-2% DQ-CO2-5% DQ-CO2-2%

0

2

4

6

8

10

12

14

16

18

20

Igni

tion

Tim

e(m

s)

1673 1773

Temperature (K)

DT-N2-5% DT-N2-2% DT-CO2-5% DT-CO2-2%

0

2

4

6

8

10

12

14

16

18

20

22

24

26

Temperature (K)

JC-N2-5% JC-N2-2% JC-CO2-5% JC-CO2-2%

Igni

tion

Tim

e(m

s)

17731670

CO2 instead of N2 : Heat capacity (CV) Ignition delay

For low oxygen content: Gas temperature O2 concentration

Ignition delay

More data(at high oxygen content) are needed in order to illustrate the reaction order in O2/CO2 atmosphere.

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Volatile Combustion

O2

• Volatile

CO H2

CH4 rmax

Flame Front

H2

CO

CH4

O2

Coal

The maximum radius of soot clouds or the farthest position of flame front.

rmax:

-Volatile Combustion Time

(the diffusion rate of volatile and oxygen, temperature, et al.)

----the yield of volatile matter (coal property, Heating rate, et al.)

----Volatile combustion rate

-Volatile combustion Coal →CnHmOJ+char C+CO2 →2CO CnHmOJ+(n+m/4-j/2)O2 →nCO2+(m/2)H2O

----the diffusion rate of volatile and oxygen

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Volatile Combustion Time at the Different Conditions

0 5 10 15 200

20

40

60

80

100

Dev

olat

iliza

tion

Tim

e(m

s)


DQ-N2-1940K DQ-N2-1773K DQ-N2-1673K

0 5 10 15 200

20

40

60

80

100

D

evol

atili

zatio

n Ti

me(

ms)

DT-N2-1940K DT-N2-1773K DT-N2-1673K


N2 diluent 14

0

20

40

60

80

100

120

140

160

180

17731673

Dev

olat

iliza

iton

Tim

e(m

s)

Temperature (K)

DQ-N2-5% DQ-N2-2% DQ-CO2-5% DQ-CO2-2%

0

20

40

60

80

100

120

140

160

180

Dev

olat

iliza

iton

Tim

e(m

s)

Temperature (K)

DT-N2-5% DT-N2-2% DT-CO2-5% DT-CO2-2%

17731673

Low

oxy

gen



15 30 45 60 75 90200

210

220

230

240

250

X-p

ositi

on (p

ixel

)

Residence Time (ms)

10.00

58.75

107.5

156.3

205.0

253.8

302.5

351.3

400.0

1773

K2%

O2/

CO2

Volatile Flame Optical Intensity and Soot Cloud Size

0 20 40 60 80 100 1200

50

100

150

200

250

Ignition Point

Char Combustion

Opt

ical

Inte

nsity

ResidenceTime (ms)

DT-1773K-2% O2/CO2 DT-1773K-2% O2/N2 DT-1773K-20% O2/N2

Volatile Combustion

When replacing the nitrogen diluent with carbon dioxide , there appears to be a slight trend towards larger soot cloud size, and the optical intensity during the volatile combustion stage are higher.

20 30 40 50 60 70 80 90200

210

220

230

240

250

X-p

ositi

on (p

ixel

)Residence Time (ms)

10.00

58.75

107.5

156.3

205.0

253.8

302.5

351.3

400.0

1773

K2%

O2/

N2

At Low Oxygen Concentration

0 50 100 150 200 250 300 350 400100

125

150

175

200

225

250

Par

ticle

_Opt

ical

Inte

nsity

Particle_size (pixel)

Through the correlation analysis among soot cloud size and optical intensity, the optical intensity isn’t related with soot cloud size. So the luminous intensity is a manifestation of temperature.

The phenomenon at low oxygen concentration is significantly different with that obtained by Shaddix at high oxygen concentration.

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For the cases of low oxygen concentration, --Oxygen content relative to volatile isn’t too much. --stoichiometric ratio of volatile combustion increased, which after the use of CO2 instead of N2. --the volatile flame temperatures in CO2 were higher than those in N2. --the decline in the diffusion rate of volatile is small compared to the decline in the diffusion rate of oxygen, --the maximum size of soot clouds increasing



Conclusions

With the gas temperature and oxygen content increased, the ignition delay time and volatile combustion time for the lower oxygen content cases decreased for both N2 and CO2 atmosphere. They are similar with that at high oxygen content(such as 20%). But volatile combustion intensity is different.

From the nonlinear ignition time profile, it can be get that the adiabatic thermal explosion theory is improper.

But the use CO2 in place of N2 leads to the rise in volatile flame temperature, which results from that the CO2 accelerated the char gasification reaction, promoted the formation of CO, while increased the release of the volatile. They lead to that the stoichiometric ratio of volatile combustion increases.

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Thank you for your attention!

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ignition and volatile combustion of pulverized coals in … rate: 2.5~5g/h high speed camera fastcam...

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