Pulverised Coal Pyrolysis and Char Combustion Characteristics In

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 Moghtaderi Presented by Rohan Stanger Oxy-fuel Working Group, Chemical Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia.

Priority Research  Centre for Energy  2 OUTLINE OF PRESENTATION

. Oxy‐fuel (O2/CO2) Combustion . Pulverised Coal Reactivity in Oxy‐fuel Conditions . Hypothesis & Objectives of the Project . Experimental . Results and discussion . Conclusions

Priority Research  Centre for Energy  3 OXY-FUEL (O2/CO2) COMBUSTION

N2 COAL FURNACE H2O AIR O CO + ASU 2 2 gases WET DRY RECYCLED FLUE GAS

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

Properties of CO2 in comparison to N2 2 . Higher emissivity – affects radiative 1.8 heat transfer 1.6 (no unit) (no 2 1.4 . Higher molar heat capacity – affects for N 1.2 ee 1 gas and partilicle temperatures

/ Valu / 0.8 2 0.6 . Lower O2 diffusivity – affects char 0.4 combustion reactivity 0.2 lue for CO

Va 0 Thermal Molar Heat Density Mass Diffusivity . Ignition and flame stability Conductivity Capacity of O2

Properties of CO 2 are significantly different when compared to N2

Priority Research  Centre for Energy  4 PULVERISED COAL REACTIVITY IN OXY-FUEL CONDITIONS

. Pyrolysis characteristics O2/CO2 pg . Char formation

. Char reactivity Tp . Ash characteristics . Emissions Tg ps

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

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

Char‐CO2 gasification reaction ‐ chemical reactivity of char

Priority Research  Centre for Energy  5 OBJECTIVES

HthiHypothesis “The reactivity of pulverised coal is significantly different during oxy-fuel combustion when compared to air combustion under high temperature and low oxygen partial pressure conditions.”

o To measure and compare the reactivity of a range of coals in air and oxy‐fuel conditions and evaluate the impact of these differences on the design and operation of oxy‐fuel technology o To identify the conditions under which the reactivity is significantly different in air and oxy conditions o To obtain the intrinsic and apparent kinetic parameters (activation energy, pre‐ exponential factor, reaction ord)der) of char combustion

Practical applications o Estimate differences in furnace operation and performance between air and oxy‐fuel for a range of coals

o Propose modifications to obtain better burnout in an oxy‐fuel combustor or a reduction in O2 usage

Priority Research  Centre for Energy  6 EXPERIMENTAL TECHNIQUES

Coals selected for study A 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 measurements Thermogravimetric Analyzer (TGA) Drop Tube Furnace (DTF)

o Coal and Char characterisation Proximate and ultimate analysis, ash analysis, maceral content analysis Scanning Electron Microscope (()SEM) analysis – char structure characterisation Internal Surface Area Measurements – char internal surface area Particle Size Analysis – coal and char particle size distribution

Priority Research  Centre for Energy  7 COAL PROPERTIES & RANKING

100 Carbon 91.40 95 90 83.91 Volatile matter 81.60 81.86 78.40 79.10 90 Coal H 80 75.00

70 67.51 85

57.19 60 80 wt. % daf basis) daf % wt. % daf basis) basis) daf % 51.46 (( 50 75 37.93 40 37.01 35.51 36.51 32.61 70 30

Coal A 20 65 C content in coal coal in content C 9.83 Coal property (wt. 10 60 0 0 0.5 1 1.5 2 2.5 3 Coal A Coal E Coal G Coal D Coal F Coal C Coal B Coal H Vitrinite Reflectance (%)

Wt. % air dried basis 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 3803.80 22. 90 23. 90 49. 40 60. 00 3293.29 1461.46 1941.94 6616.61 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

Priority Research  Centre for Energy  8 PYROLYSIS & CHAR REACTIVITY MEASUREMENTS

Air Oxy-fuel

Volatiles N2 , CO2 CO2 H2O

O2 , CO2 O2 , N2

coal char carbon + unburnt carbon + mineral matter ash drying pyrolysis char oxidation + ash formation gasification

Coal Pyrolysis reactivity Coal Char O2 Reactivity in N2 & CO2 in O2/N2 & O2/CO2 atmospheres atmospheres TGA – coal slow heating TGA- coal slow heating DTF – coal rapid heating DTF – coal rapid heating TGA – char isothermal

Priority Research  Centre for Energy  9 THERMOGRAVIMETRIC ANALYSIS

 Thermo gravimetric analysis is an useful tool to perform reactivity 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

Priority Research  Centre for Energy  10 ISOTHERMAL TGA EXPERIMENTS EXPERIMENTAL PROCEDURE

1200 Step Atmosphere Process Temperature Program 5 6 1000 1 N2 Flow stabilization C)

oo 2 N Heatinggygp to drying temp. 800 2 3 N2 Drying 4 N Heating to T 600 2 f 4 5 No gas Gas change (if required) 6 N , CO , Reactivity measurement 400 2 2

emperature ( O2/N2, TT O /CO 200 2 2 3 1 2 0 0 50 100 150 200 Time (()min)

EXPERIMENTAL CONDITIONS Parameter Condition

Initial sample (char) mass 3 to 5 mg Temperature range (isothermal) 800 to 1000 oC Total gas flow rate (carrier + 50 mL/min auxiliary)

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

Priority Research  Centre for Energy  11 DROP TUBE FURNACE (DTF) EXPERIMENTS A drop tube furnace (()DTF) was used to measure volatile yield and char burnout, and form char for TGA experiments. Coal Feeder • Volatile yields in N2 and CO2 atmospheres at various temperatures

Coal • Coal burnouts at various O2 levels and Secondary gas temperatures

Primary gas o •Char formation in N2 atmosphere at 1400 C for TGA experiments Coal feeder probe

EXPERIMENTAL CONDITIONS

Vertical furnace Parameter Condition Coal feed rate 3to5g/h Total gas flow rate 5.2 LPM (primary + secondary) Collector probe Quench gas flow rate 3LPM Curtain gas flow rate 3LPM To vacuum O2 concentration in N2/CO2 0.5 to 30% v/v basis pump Furnace temperature 900 to 1400 oC

Cyclone

Aerosol filter

Priority Research Drop Tube Furnace (DTF)  Centre for Energy  12 EXPERIMENTAL – DROP TUBE FURNACE

Coal particles + Primary gas

Cooling water in Cooling water out Secondary gas

Feeder probe Rotameter Primary gas - top bed O2 O2 Gas MFC in Mixing Rotameter Coil N2 / CO2 N2 / CO2 Fluidised bed coal feeder MFC in

Fluidised coal particles Hot zone

Primaryyg gas - bottom bed

Rotameter Curtain gas (N2) in

Curtain gas (N2) out

Collection probe

Cooling water out Cooling water in Quench gas (N2) in

Filter paper

Exhaust gas to atmosphere Pump

ClCyclone arrangemen t for collecting char particles

Two-way valve Water bottle

Schematic of the DTF Experimental Facility at the University of Newcastle

Priority Research  Centre for Energy  13 PYROLYSIS OF COAL D IN N2 & CO2 ATMOSPHERES IN TGA EXPERIMENTS 0.0016

0.0014 Coal 100% N2 Coal 100% CO2

) 000120.0012 CO2 reactiv ity -1 with char (s 0.001

m,p Pyrolysis

R reactivities are , 0.0008 yy similar CO2 Char gasification 0.0006 begins

Reactivit 0.0004

0.0002 N2 0 500 600 700 800 900 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.

Priority Research  Centre for Energy  14 COMPARISON OF APPARENT VOLATILE YIELDS OF TEST COALS o IN N2 & CO2 ATMOSPHERES IN DTF EXPERIMENTS AT 1400 C

100 90 N2 80 CO2 daf basis) basis) daf Proximate Analysis %% 70 60 50 yield (wt. .45 66 77

40 8 76.3 74.3 70.2 30 66.6 60.0 59.4 57.19 50.9 51.46 48.7 49.19 46.7 20 48.57 37.93 37.01 36.51 36.18 35.51 35.80 ent volatile 32.61 rr 10 0 Appa 67.51 75.00 78.40 79.10 81.60 81.86 83.91 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 2 atmosphere - attributed to the char-CO2 gasification reaction.

Priority Research  Centre for Energy  15 CHAR SWELLING IN N2 & CO2 ATMOSPHERES

Chars were formed in the DTF at 1400 oC

100 100 90 Coal 90 Coal N2 Char 80 N2 Char 80 CO2 Char CO2 Char ercentage

ercentage 70 70 pp pp 60 60 50 50 40 40 Larger CO2 30 30 chars 20

20 volume ulative ulative volume volume ulative 10 Coal A mm 10 Coal B Cu Cum 0 0 1 10 100 1000 1 10 100 1000 Particle diameter (μm) Particle diameter (μm)

ClACoal A 100 ClBCoal B 90 Coal 80 N2 Char 70 CO2 Char

Similar N e percentage 60 Swollen

2 mm 50 chars and CO2 40 chars 30 20 10 Coal C umulative volu umulative

CC 0 1 10 100 1000 Coal C Particle diameter (μm) Priority Research  Centre for Energy  16 CHAR SWELLING IN N2 & CO2 ATMOSPHERES

Chars were formed in the DTF at 1400 oC

4.5 4.0 N2 Char CO2 Char 3.5 3.0 2.5 g factor (-) g factor

nn 2.0 1.5 Swelli 1.0 0.5 0.0 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 dchar / dcoal

Priority Research  Centre for Energy  17 SEM PICTURES OF CROSS SECTIONS OF COAL A

N2 & CO2 CHARS Chars were formed in the DTF at 1400 oC

Coal A - N2 CHAR Coal A - CO2 CHAR

Priority Research  Centre for Energy  18 SEM PICTURES OF CROSS SECTIONS OF COAL B

N2 & CO2 CHARS Chars were formed in the DTF at 1400 oC

Coal B - N2 CHAR Coal B - CO2 CHAR

Priority Research  Centre for Energy  19 SEM PICTURES OF CROSS SECTIONS OF COAL C

N2 & CO2 CHARS Chars were formed in the DTF at 1400 oC

Coal C - N2 CHAR Coal C - CO2 CHAR

Priority Research  Centre for Energy  20 CHAR MICROPORE SURFACE AREAS OF N2 & CO2 CHARS OF TEST COALS

Chars were formed in the DTF at 1400 oC

500

)) N2 Char /g 450 2 CO2 Char 400

area (m area 350

300

250

ore surface surface ore 200

150 Microp 100 60 65 70 75 80 85 90 C content in coal (wt. % daf basis)

Priority Research  Centre for Energy  21 COAL A REACTIVITY IN O2/N2 & O2/CO2 IN SLOW HEATING TGA EXPERIMENTS REACTIVITY Constant Heating Rate – 25C/min 0.06 0.2 Coal A 0.05 21% O2 - Lower reactivities & heat flows at low ) 0.16 -1 temperatures in O2/CO2 combustion 0.04 CO2 0.12

- Endothermic gasification reduces heat 0.03 10% O2 0.08 flows and increases reactivities at high 0.02 3% O2 temperatures eactivity (min RR 0.01 000.04 N2 0 0 HEAT FLOW 0 200 400 600 800 1000 200 o Coal A Tempp(erature ( C)

150 21% O2 ) W 100 10% O2

ow (m 50 ll 3% O2

N2 0 Endothermic

Heat F 0 200 400 600 800 1000 -50 gasification CO2 -100 Temperature (oC)

Priority Research  Centre for Energy  22 COAL A BURNOUT/REACTIVITY IN O2/N2 & O2/CO2 ATMOSPHERES

DTF EXPERIMENTS ISOTHERMAL TGA EXPERIMENTS

1 100 COAL A 1400N2 CHAR 1000 OC, 50% CONVERSION s)

90 ) 080.8 -1 80 0.6 70 Coal A 60 0.4 O2/N2 Coal burnout Coal

50 (min Reactivity 0.2 (wt. %, daf basi %, (wt. O2/CO2 Air - O2/N2 40 Oxy - O2/CO2 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0 0.05 0.1 0.15 0.2 0.25 Oxygen concentration (atm) Oxygen Concentration (atm)

100 1 Air - O2/N2 COAL A 1400N2 CHAR 10% O , 50% CONVERSION ) 0.8 Oxy - O2/CO2 2 90 -1 0.6 80 Coal A 0.4 oal burnout oal activity (min activity . %, daf basis) %, . 70 O2/N2 ee CC

R 0.2

(wt O2/CO2 60 0 800 900 1000 1100 1200 1300 1400 1500 600 700 800 900 1000 1100 1200 o Furnace temperature (oC) Temperature ( C)

Priority Research  Centre for Energy  23 COAL B BURNOUT/REACTIVITY IN O2/N2 & O2/CO2 ATMOSPHERES

DTF EXPERIMENTS ISOTHERMAL TGA EXPERIMENTS

100 0.25 COAL B 1400N2 CHAR O s) 90 1000 C, 50% CONVERSION ) 0.2 80 -1 0.15 70 Coal B 60 0.1 O2/N2 Coal burnout Coal 50 Reactivity (min Reactivity (wt. %, daf basi %, (wt. O2/CO2 0050.05 Air - O2/N2 40 Oxy - O2/CO2 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0 0.05 0.1 0.15 0.2 0.25 Oxygen concentration (atm) Oxygen Concentration (atm)

90 0.125 COAL B 1400N2 CHAR 10% O , 50% CONVERSION 80 ) 0.12 2 -1 70 0.115

60 0.11 Coal B 50

oal burnout oal 0.105 activity (min

t. %, daf basis) %, t. O2/N2 ee CC

ww 40 R Air - O2/N2 ( O2/CO2 0.1 30 Oxy - O2/CO2 0.095 800 900 1000 1100 1200 1300 1400 1500 600 700 800 900 1000 1100 1200 o Furnace temperature (oC) Temperature ( C)

Priority Research  Centre for Energy  24 COAL C BURNOUT/REACTIVITY IN O2/N2 & O2/CO2 ATMOSPHERES

DTF EXPERIMENTS ISOTHERMAL TGA EXPERIMENTS

0.06 100 COAL C 1400N2 CHAR

) O

ss 0050.05 1000 C, 10% CONVERSIONCONVERSION

90 ) -1 80 0.04 70 60 Coal C 0.03 50 O2/N2 0.02 t. %, daf basi %, t. Coal burnout Coal eactivity (min ww RR

( 40 O2/CO2 0.01 Air - O2/N2 30 Oxy - O2/CO2 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0 0 0.05 0.1 0.15 0.2 0.25 Oxygen concentration (atm) Oxygen Concentration (atm) 80 0.04 COAL C 1400N2 CHAR 70 O2/N2 , 10% CONVERSION

) 10 % O 2 O2/CO2 -1 0.035 60

50 0.03 tivity (min tivity al burnout al %, daf basis) %, 40 cc oo

C 0.025

30 Rea Air - O2/N2

(wt. (wt. Coal C 20 Oxy - O2/CO2 0.02 800 900 1000 1100 1200 1300 1400 1500 600 700 800 900 1000 1100 1200 o Furnace tempp(erature (oC) Temperature ( C)

Priority Research  Centre for Energy  25 SUMMARY OF RESULTS

Variable Technique Difference in O2/CO2 or CO2 atmosphere Apppparent Volatile Yield DTF Higher Pyrolysis Rate TGA Similar

Char Appearance DTF/SEM Similar Char Swelling DTF/PSD Similar for most coals

Micropore Surface DTF/BET Slightly higher for CO2 Area chars of most coals

Coal Burnout DTF Similar or slightly higher in

O2/CO2 conditions

Char Reactivity TGA Higher in O2/CO2 conditions

Priority Research  Centre for Energy  26 CONCLUSIONS . There are significant differences in pulverised coal reactivity in air and oxy‐fuel 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 char‐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.

Priority Research  Centre for Energy  27 ACKNOWLEDGEMENTS

☺ Cooperative Research Centre for Coal in Sustainable Development (CCSD) for financial support ☺ Vatten fall AB, Stockhol m, SSedeweden, for fin anc ial suuotpport ☺ The University of Newcastle for PhD scholarships (Renu Rathnam) ☺ Ms. JJyenny Martin, Ms. Jane Hamson, and Dr. Artur Ziolkowski, for technical support in the DTF and TGA experiments ☺ The University of Newcastle Electron Microscope & X‐ray Unit for support in the SEM analysis of char samples ☺ SRC for Multi‐phase Processes for help with the particle size measurements ☺ Mr. Rohan Stanger for presenting this work

Priority Research  Centre for Energy  28

THANK YOU! Oxy‐fuel Working Group The University of Newcastle [email protected]

Priority Research  Centre for Energy  29 COMPARISON OF BURNOUTS OF TEST COALS IN N2 & CO2 ATMOSPHERES IN DTF EXPERIMENTS AT 1400 oC

98.59 98.44 97.59 100 96.44 96.2 O2/N2 95 92.69 92.16 90.68 2 2 91.06 O2/CO2 is) 90 88.00 % O % O 22 00 ss 86.28 1 1 2 2 2 2 2

85 2 2 2 8% O 12% O 10% O 10% O 12% O

80 12% O 10% O 10% O

t. % daf ba 75 72.92 ww 70 66.16 2

65 2 59.49 l burnout ( burnout l 0% O 10% O 22 11

aa 60 2 Co

55 10% O

50 10% O 67.51 75.00 78.40 79.10 81.60 81.86 83.91 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.

Priority Research  Centre for Energy 