Pulverised Coal Pyrolysis and Char Combustion ... 4_C/1_Renu... · Coal Proximate analysis Ultimate...

1st Oxy-fuel Combustion Conference, Cottbus, Germany - 09 September 2009

Pulverised Coal Pyrolysis and Char Combustion Characteristics in Simulated Air (O2/N2) and Oxy-fuel (O2/CO2) Conditions Renu K Rathnam | Terry F Wall | Liza K Elliott | Yinghui Liu | Behdad MoghtaderiRenu K Rathnam | Terry F Wall | Liza K Elliott | Yinghui Liu | Behdad MoghtaderiPresented by Rohan StangerOxy-fuel Working Group, Chemical Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia.

Priority ResearchCentre for Energy

2

OUTLINE OF PRESENTATION

Oxy‐fuel (O2/CO2) Combustion

Pulverised Coal Reactivity in Oxy‐fuel Conditions Pulverised Coal Reactivity in Oxy‐fuel Conditions

Hypothesis & Objectives of the Project

Experimental

Results and discussion

Conclusions


3

OXY-FUEL (O2/CO2) COMBUSTION

FURNACEASUAIR

N2

O2 CO2 + gases

H2O

RECYCLED FLUE GAS

COAL

WET DRY

Properties of CO2 in comparison to N2

“High CO2 concentrations are encountered during oxy-fuel combustion”

Higher emissivity – affects radiative heat transfer

Higher molar heat capacity – affects d i l

1.2

1.41.6

1.8

2

e fo

r N2 (

no u

nit)

Properties of CO2 in comparison to N2

gas and particle temperatures

Lower O2 diffusivity – affects char combustion reactivity

0.2

0.40.6

0.8

1

lue

for C

O2 /

Val

ue

Ignition and flame stability0Thermal

ConductivityMolar HeatCapacity

Density Mass Diffusivityof O2

Va

Properties of CO are significantly different when compared to N


Properties of CO2 are significantly different when compared to N2

4PULVERISED COAL REACTIVITY IN OXY-FUEL CONDITIONS

Pyrolysis characteristics pgO2/CO2

Char formation

Char reactivity

Ash characteristics

Tp

Ash characteristics

Emissions psTg

Lower O2 diffusivity ‐ O2 availability on the surface of the particle (ps)

Higher specific heat ‐ gas & particle temperatures (Tg & Tp)g p

Char‐CO2 gasification reaction ‐ chemical reactivity of char


5

OBJECTIVES

H th iHypothesis“The reactivity of pulverised coal is significantly different during oxy-fuel combustionwhen compared to air combustion under high temperature and low oxygen partial pressureconditions.”

o To measure and compare the reactivity of a range of coals in air and oxy‐fuelconditions and evaluate the impact of these differences on the design andoperation of oxy‐fuel technology

o To identify the conditions under which the reactivity is significantly differentin air and oxy conditions

o To obtain the intrinsic and apparent kinetic parameters (activation energy, pre‐l f d ) f h bexponential factor, reaction order) of char combustion

Practical applications

o Estimate differences in furnace operation and performance between air and oxy‐fuel for ao Estimate differences in furnace operation and performance between air and oxy‐fuel for arange of coals

o Propose modifications to obtain better burnout in an oxy‐fuel combustor or a reduction in O2usage


6

EXPERIMENTAL TECHNIQUESCoals selected for studyCoals selected for studyA suite of coals of varying ranks

o Three international coals (Coals A, B, C)o Five Australian coals (Coals D, E, F, G, H)

Techniques and Methodology

o Pyrolysis and char reactivity measurementsThermogravimetric Analyzer (TGA)Drop Tube Furnace (DTF)

o Coal and Char characterisationProximate and ultimate analysis, ash analysis, maceral content analysisScanning Electron Microscope (SEM) analysis – char structure characterisationg p ( ) yInternal Surface Area Measurements – char internal surface areaParticle Size Analysis – coal and char particle size distribution


7

COAL PROPERTIES & RANKING100

67.51

75.0078.40 79.10 81.60 81.86 83.91

91.40

57.1951.46

60

70

80

90

100

% d

af b

asis

)

CarbonVolatile matter

80

85

90

95

(wt.

% d

af b

asis

)

Coal H

37.01 37.93 35.5132.61

36.51

9.8310

20

30

40

50

Coa

l pro

perty

(wt.

60

65

70

75

C c

onte

nt in

coa

l (

Coal A

Wt. % air dried basis

0

10

Coal A Coal E Coal G Coal D Coal F Coal C Coal B Coal H

600 0.5 1 1.5 2 2.5 3

Vitrinite Reflectance (%)

Coal Proximate analysis Ultimate analysis

Moisture Ash V M F C C H N S O

Coal A 11.20 5.40 47.70 35.70 56.30 4.16 0.55 0.96 21.43

Coal B 2.00 17.20 29.50 51.30 67.80 4.08 1.19 0.68 7.05

Coal C 3 80 22 90 23 90 49 40 60 00 3 29 1 46 1 94 6 61Coal C 3.80 22.90 23.90 49.40 60.00 3.29 1.46 1.94 6.61

Coal D 8.00 19.90 25.60 46.50 57.03 3.25 0.84 0.17 10.81

Coal E 5.20 28.60 24.50 41.70 49.65 3.08 0.80 0.22 12.45

Coal F 1.70 19.60 40.50 38.20 64.22 5.38 0.99 0.50 7.60

Coal G 5.90 5.00 33.80 55.30 69.85 4.57 1.91 0.46 12.30


Coal H 1.7 9.8 8.7 79.8 80.89 3.34 1.66 0.67 1.95

8PYROLYSIS & CHAR REACTIVITY MEASUREMENTS

Air Oxy-fuel

H2O Volatiles N2 , CO2

O CO

CO2

O2 , N2O2 , CO2

drying pyrolysis char oxidation + gasification

ash formation

coal char carbon + mineral matter

unburnt carbon + ash

gasification

Coal Pyrolysis reactivity in N2 & CO2atmospheres

Coal Char O2 Reactivity in O2/N2 & O2/CO2atmospheres p

TGA – coal slow heating

DTF – coal rapid heating

p

TGA- coal slow heating

DTF – coal rapid heating

TGA char isothermal


TGA – char isothermal

9

Thermo gravimetric analysis is an useful tool to perform reactivity

THERMOGRAVIMETRIC ANALYSISg y p y

measurements

It involves the measurement of the mass loss/gain of a substance subjected to a controlled temperature program under controlled conditions

ISOTHERMAL

NON-ISOTHERMAL (HEATING)

SETARAM Setsys Evolution TGA


10

ISOTHERMAL TGA EXPERIMENTSEXPERIMENTAL PROCEDURE

Step Atmosphere Process

12

N2N2

Flow stabilizationHeating to drying temp.800

1000

1200

o C)

Temperature Program65

3456

2N2N2

No gasN2, CO2, O2/N2,

g y g pDryingHeating to TfGas change (if required)Reactivity measurement400

600

800

Tem

pera

ture

(o

4

O2/CO2

0

200

0 50 100 150 200

Time (min)

T

321

Parameter Condition

EXPERIMENTAL CONDITIONS

( )

Initial sample (char) mass Temperature range (isothermal)Total gas flow rate (carrier + auxiliary)

3 to 5 mg800 to 1000 oC50 mL/min


Oxygen concentration in N2/CO2 0 to 21 % v/v basis

11

A drop tube furnace (DTF) was used to measure volatile yield and char

DROP TUBE FURNACE (DTF) EXPERIMENTSp ( ) y

burnout, and form char for TGA experiments. Coal Feeder

• Volatile yields in N2 and CO2 atmospheres at various temperatures

Primary gas

Secondary gas

Coal

p

• Coal burnouts at various O2 levels and temperatures

• Char formation in N2 atmosphere at 1400 oC for TGA experiments

Vertical furnace

Coal feeder probe TGA experiments

Parameter Condition

EXPERIMENTAL CONDITIONS

Collector probe

To

Coal feed rateTotal gas flow rate(primary + secondary)Quench gas flow rateCurtain gas flow rate

3 to 5 g/h5.2 LPM

3 LPM3 LPM

Cyclone

Aerosol filter

To vacuum pump

O2 concentration in N2/CO2Furnace temperature

0.5 to 30% v/v basis900 to 1400 oC


Drop Tube Furnace (DTF)

12

Coal particles + Primary gas

EXPERIMENTAL – DROP TUBE FURNACE

Primary gas - top bed

Feeder probe

Cooling water inCooling water out Secondary gas

GasO2MFC

O2in

Rotameter

Primary gas - bottom bed

Fluidised coal particlesHot zone

Fluidised bed coal feeder

MixingCoil N2 / CO2

MFCN2 / CO2in

Rotameter

y g

Curtain gas (N2) in

Collection probe

Curtain gas (N2) out

Rotameter

Pump

Filter paper

C l t

Quench gas (N2) in

Cooling water in

Cooling water out

Exhaust gas to atmosphere

Two-way valveWater bottle

Cyclone arrangementfor collecting char particles

Schematic of the DTF Experimental Facility at the


University of Newcastle

13PYROLYSIS OF COAL D IN N2 & CO2 ATMOSPHERES IN TGA EXPERIMENTS

0 0012

0.0014

0.0016

Coal 100% N2Coal 100% CO2

CO reacti it

0.0008

0.001

0.0012

y, R

m,p

(s-1

)

CO

CO2 reactivity with char

Pyrolysis reactivities are similar

0.0004

0.0006

0.0008

Rea

ctiv

ity

Char gasification begins

CO2similar

0

0.0002

500 600 700 800 900 1000 1100 1200 1300

N2

500 600 700 800 900 1000 1100 1200 1300

Temperature, T p (K)

Pyrolysis reactivities are similar in both N2 and CO2 atmospheres.


Char-CO2 gasification reaction is clearly evident in a CO2 atmosphere.

14COMPARISON OF APPARENT VOLATILE YIELDS OF TEST COALS IN N2 & CO2 ATMOSPHERES IN DTF EXPERIMENTS AT 1400 oC

80

90

100

% d

af b

asis

)

N2CO2Proximate Analysis

6 7.45

50

60

70

yiel

d (w

t. % Proximate Analysis

76.3

6

46.7 60

.0

50.9

66.6

36.1

8 48.5

7

8 7

48.7

74.3

59.4 70

.2

35.8

0 49.1

9

57.1

9

37.0

1

37.9

3

35.5

1 51.4

6

32.6

1

36.5

1

10

20

30

40

rent

vol

atile

0

10

67.51 75.00 78.40 79.10 81.60 81.86 83.91

App

ar

C in Coal (wt. %, daf basis)

Higher apparent volatile yield at higher temperatures and heating rates in DTF at 1400 oC compared to the proximate analysis volatile matter.

Higher apparent volatile yield in CO atmosphere - attributed to the char-CO


Higher apparent volatile yield in CO2 atmosphere - attributed to the char-CO2gasification reaction.

15CHAR SWELLING IN N2 & CO2 ATMOSPHERES

Chars were formed in the DTF at 1400 oC

708090

100

perc

enta

ge CoalN2 CharCO2 Char 70

8090

100

perc

enta

ge CoalN2 CharCO2 Char

2030405060

ulat

ive

volu

me

p

2030405060

mul

ativ

e vo

lum

e p

Larger CO2chars

010

1 10 100 1000Particle diameter (μm)

Cum Coal A

010

1 10 100 1000Particle diameter (μm)

Cum Coal B

C l A C l B

60708090

100

me

perc

enta

ge CoalN2 CharCO2 Char

Coal A Coal B

Swollen h

Similar N2

01020304050

Cum

ulat

ive

volu

m

Coal C

chars2and CO2chars


01 10 100 1000

Particle diameter (μm)

C

Coal C

16CHAR SWELLING IN N2 & CO2 ATMOSPHERES


3.5

4.0

4.5N2 CharCO2 Char

2.0

2.5

3.0

ng fa

ctor

(-)

0.5

1.0

1.5

2.0

Swel

lin

0.0

0.5

60 65 70 75 80 85 90

C content in coal (wt. % daf basis)

Swelling factor is the ratio of the median diameter of the char particle to the corresponding coal particle diameter

coalchar dd /


17SEM PICTURES OF CROSS SECTIONS OF COAL AN2 & CO2 CHARS


Coal A - N2 CHAR Coal A - CO2 CHAR


18SEM PICTURES OF CROSS SECTIONS OF COAL BN2 & CO2 CHARS


Coal B - N2 CHAR Coal B - CO2 CHAR


19SEM PICTURES OF CROSS SECTIONS OF COAL CN2 & CO2 CHARS


Coal C - N2 CHAR Coal C - CO2 CHAR


20CHAR MICROPORE SURFACE AREAS OF N2 & CO2CHARS OF TEST COALS

500) N2 Ch


350

400

450

area

(m2 /g

) N2 CharCO2 Char

200

250

300

ore

surfa

ce

100

150

200

Mic

rop

60 65 70 75 80 85 90

C content in coal (wt. % daf basis)


21COAL A REACTIVITY IN O2/N2 & O2/CO2 IN SLOW HEATING TGA EXPERIMENTS

REACTIVITYConstant Heating Rate – 25C/min

- Lower reactivities & heat flows at low 0.05

0.06

-1) 0.16

0.2Coal A

21% O2

temperatures in O2/CO2 combustion

- Endothermic gasification reduces heat flows and increases reactivities at high temperatures

0.02

0.03

0.04

Rea

ctiv

ity (m

in

0 0

0.08

0.12CO2

3% O2

10% O2

temperatures

0

0.01

0 200 400 600 800 1000

Temperature (oC)

R

0

0.04N2

200Coal A

HEAT FLOWp ( )

50

100

150

low

(mW

)

Coal A

3% O2

10% O2

21% O2

Endothermic gasification

100

-50

00 200 400 600 800 1000H

eat F

l 3% O2

N2

CO2


-100Temperature (oC)

22COAL A BURNOUT/REACTIVITY IN O2/N2 & O2/CO2 ATMOSPHERES

90100

s)

DTF EXPERIMENTS ISOTHERMAL TGA EXPERIMENTS

0 8

1COAL A 1400N2 CHAR1000 OC, 50% CONVERSION

5060708090

Coa

l bur

nout

(wt.

%, d

af b

asi

O2/N2

O2/CO2

Coal A

0.2

0.4

0.6

0.8

Rea

ctiv

ity (m

in-1

)

Air - O2/N2

400 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Oxygen concentration (atm)

O2/CO2

100

00 0.05 0.1 0.15 0.2 0.25

Oxygen Concentration (atm)

Oxy - O2/CO2

1COAL A 1400N2 CHAR

70

80

90

100

Coa

l bur

nout

. %, d

af b

asis

)

O2/N2

Coal A 0.4

0.6

0.8

eact

ivity

(min

-1)

Air - O2/N2Oxy - O2/CO2

COAL A 1400N2 CHAR10% O2, 50% CONVERSION

60

70

800 900 1000 1100 1200 1300 1400 1500

Furnace temperature (oC)

C(w

t O2/N2

O2/CO2

0

0.2

600 700 800 900 1000 1100 1200

Temperature (oC)

Re


23COAL B BURNOUT/REACTIVITY IN O2/N2 & O2/CO2 ATMOSPHERES

90100

s)

0.25COAL B 1400N2 CHAR1000 OC 50% CONVERSION


5060708090

Coa

l bur

nout

(wt.

%, d

af b

asi

O2/N2

O2/CO2

Coal B

0 05

0.1

0.15

0.2

Rea

ctiv

ity (m

in-1

)

Air O2/N2

1000 C, 50% CONVERSION

400 0.05 0.1 0.15 0.2 0.25 0.3 0.35


O2/CO2

90 0.125COAL B 1400N2 CHAR

0

0.05

0 0.05 0.1 0.15 0.2 0.25Oxygen Concentration (atm)


4050607080

Coa

l bur

nout

wt.

%, d

af b

asis

)

O2/N2

Coal B0.105

0.11

0.115

0.12

eact

ivity

(min

-1)

COAL B 1400N2 CHAR10% O2, 50% CONVERSION

3040

800 900 1000 1100 1200 1300 1400 1500


C(w O2/CO2

0.095

0.1

600 700 800 900 1000 1100 1200

Temperature (oC)

Re



24COAL C BURNOUT/REACTIVITY IN O2/N2 & O2/CO2 ATMOSPHERES

90100

s) 0 05

0.06COAL C 1400N2 CHAR1000 OC 10% CONVERSION


405060708090

Coa

l bur

nout

wt.

%, d

af b

asis

O2/N2

O2/CO2

Coal C

0.02

0.03

0.04

0.05

Rea

ctiv

ity (m

in-1

) 1000 OC, 10% CONVERSION

3040

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35


(w O2/CO2

80

0

0.01

0 0.05 0.1 0.15 0.2 0.25Oxygen Concentration (atm)

R Air - O2/N2Oxy - O2/CO2

0.03

0.035

0.04

ctiv

ity (m

in-1)

COAL C 1400N2 CHAR10 % O2, 10% CONVERSION

4050607080

oal b

urno

ut%

, daf

bas

is) O2/N2

O2/CO2

0.02

0.025

600 700 800 900 1000 1100 1200

Temperature (oC)

Rea

c


30

800 900 1000 1100 1200 1300 1400 1500


Co

(wt.

Coal C


p ( )

25

SUMMARY OF RESULTS

Variable Technique Difference in O2/CO2 or CO2 atmosphere

Apparent Volatile Yield DTF Higherpp gPyrolysis Rate TGA Similar

Char Appearance DTF/SEM SimilarChar Appearance DTF/SEM SimilarChar Swelling DTF/PSD Similar for most coalsMicropore Surface Area

DTF/BET Slightly higher for CO2chars of most coals

Coal Burnout DTF Similar or slightly higher in O2/CO2 conditions

Char Reactivity TGA Higher in O2/CO2 conditions


26

There are significant differences in pulverised coal reactivity in air and oxy‐fuel

CONCLUSIONSg p y y

conditions. The extent of the differences depend on the coal type (rank).

The pyrolysis reactivity of coal in N2 and CO2 is similar as shown by TGA results. The apparent volatile yield appears to be higher in CO2 due to mass loss during the pp y pp g gchar‐CO2 gasification reaction.

Depending on the coal type, there are also significant differences in the characteristics of char formed in N2 and CO2 atmospheres. Some coals exhibit greater swelling in a CO2 atmosphere.

Higher char reactivity in O2/CO2 conditions, especially at high temperatures and low O2 levels, is attributed to the char‐CO2 gasification reaction.

TGA and DTF results complement one another and are consistent with literature data.

Reactivity parameters for char combustion need to be estimated separately for oxy‐fuel conditions including the char‐CO2 gasification reaction and other relevant differences need to be accounted for when modelling pulverised coal combustion in O2/CO2 conditions.


27

ACKNOWLEDGEMENTS

☺ Cooperative Research Centre for Coal in Sustainable Development(CCSD) for financial support

☺ Vatte fall AB Sto khol S ede fo fi a ial u o t☺ Vattenfall AB, Stockholm, Sweden, for financial support

☺ The University of Newcastle for PhD scholarships (Renu Rathnam)

☺ Ms. Jenny Martin, Ms. Jane Hamson, and Dr. Artur Ziolkowski, for J y Jtechnical support in the DTF and TGA experiments

☺ The University of Newcastle Electron Microscope & X‐ray Unit for support in the SEM analysis of char samplespp y p

☺ SRC for Multi‐phase Processes for help with the particle size measurements

☺ Mr Rohan Stanger for presenting this work☺ Mr. Rohan Stanger for presenting this work


28

THANK YOU!THANK YOU!Oxy‐fuel Working Group

The University of NewcastleThe University of [email protected]


29COMPARISON OF BURNOUTS OF TEST COALS IN N2 & CO2ATMOSPHERES IN DTF EXPERIMENTS AT 1400 oC

88.00

96.2

92.1690.68

96.44

92.6991.06

86 28

98.59 97.5998.44

90

95

100

sis)

O2/N2O2/CO2

0% O

2

2% O

2

72.92

86.28

75

80

85

wt.

% d

af b

as

10%

O2

10

10%

O2

10%

O2

8% O

2

10%

O2

12%

O2

12%

O2

12

12%

O2

59.49

66.16

60

65

70

al b

urno

ut (w

10%

O2

10%

O2

2

50

55

60

67.51 75.00 78.40 79.10 81.60 81.86 83.91

Coa

1

10%

O2 10%

O2

C in Coal (wt. %, daf basis)

Similar or even higher burnout in O2/CO2 conditions at similar or lower O2 levels for all the test coals at 1400 oC in the DTF.


Higher char reactivity in O2/CO2 conditions for high rank coals.

Pulverised Coal Pyrolysis and Char Combustion ... 4_C/1_Renu... · Coal Proximate analysis Ultimate...

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