PRELIMINARY INVESTIGATION OF THE GLOBAL

KINETIC PARAMETERS OF LOW-RANK COALS

UNDER OXY-FUEL CONDITION

Adriano Carotenuto1, Rodrigo Corrêa da Silva2,

René Lucio Rech1, Paulo Smith Schneider1,

Nilson Romeu Marcílio1, Hans Joachim Krautz2

1-UFRGS 2-BTU Cottbus

III Congresso Brasileiro de Carvão Mineral

21 - 24 August 2011 – Gramado, RS, Brazil

• Is the combustion of fossil or renewable fuels under oxygen stream

diluted by recycled exhaust gas (CO2 and H2O);

• Has the aim of making easier the post-combustion capture and

storage of CO2, considered one of the main green house gases;

• Has a 7-10% efficiency penalty on the thermal power generation due

to oxygen production and CO2 compression, about the same level of

CO2 post-combustion capture with air.

Oxy-Fuel Combustion

Oxy-Fuel Combustion Process

From Wall et al. An overview on oxyfuel coal combustion – State of the art research and technology development,

Chemical Engineering Research and Design 87 (2009) p. 1003-1016.

• Objective: Study the combustion kinetics of low-rank coals

under air and oxy-fuel conditions in an atmospheric

laboratory test facility (ALVA 20).

• Three low-rank coals: pre-dried Lusatian lignite -

Germany, Leão and Bonito coals - Brazil.

• Three combustion temperatures: 1073, 1173 and 1273 K.

• Two oxidant compositions: air and 21% O2 / 79% CO2.

• Particle size: 1.25 - 2.00 mm.

Objective and Experimental Parameters

Tested Coals Characteristics

Test Facility ALVA 20

Cyclone Combustion Reactor

ALVA 20

Oxygen and Temperature Curves

Time (s)

0 50 100 150 200

Tem

per

ature

(K

)

950

1000

1050

1100

1150

1200

O2

(vo

l. %

)

12

14

16

18

20

22

Temperature at Bottom

Temperature at Middle

Temperature at Top

O2 (vol. %)

Combustion of test coal sample

Stop of base lignite feed

Restart of base lignite feed

Air - 1073 KK4 (1.250-2.000 mm)LTBK - 3 g

Combustion Behavior with Time

Time (s)

0 3 6 9 12 15 18 21 24 27 30

O2 (

vo

l. %

)

15

16

17

18

19

20

21

22

Air - 1073 KK4: 1.250-2.000 mm Leao - 3 g

Pre-Dried Lignite Results

time (s)

0 2 4 6 8 10 12 14 16 18 20

O2

(vo

l. %

)

12

13

14

15

16

17

18

19

20

21

1073 K

1173 K

1273 K

21/79

K4: 1.250-2.000 mm

LTBK - 1g

time (s)

0 2 4 6 8 10 12 14 16 18 20

O2

(vo

l. %

)

12

13

14

15

16

17

18

19

20

21

1073 K

1173 K

1273 K

Air

K4: 1.250-2.000 mm

LTBK - 1g

Air Oxy-fuel

Leão Coal Results

time (s)

0 2 4 6 8 10 12 14 16 18 20

O2

(vol.

%)

12

13

14

15

16

17

18

19

20

21

1073 K

1173 K

1273 K

Air

K4: 1.250-2.000 mm

Leão - 1g

time (s)

0 2 4 6 8 10 12 14 16 18 20

O2

(vol.

%)

12

13

14

15

16

17

18

19

20

21

1073 K

1173 K

1273 K

21/79

K4: 1.250-2.000 mm

Leão - 1g

Air Oxy-fuel

Bonito Coal Results

time (s)

0 2 4 6 8 10 12 14 16 18 20 22 24

O2

(vo

l. %

)

12

13

14

15

16

17

18

19

20

21

1073 K

1173 K

1273 K

21/79

K4: 1.250-2.000 mm

Bonito - 1g

time (s)

0 2 4 6 8 10 12 14 16 18 20 22 24

O2

(vo

l. %

)

12

13

14

15

16

17

18

19

20

21

1073 K

1173 K

1273 K

Air

K4: 1.250-2.000 mm

Bonito - 1g

Air Oxy-fuel

• Coal combustion occurs in two steps

1) Coal pyrolysis and fast volatiles burning

2) Slow char burning

• The complex multiple reactions mechanism can be

simplified by a global reaction C + O2 CO2

COAL

BURNING

RATE =

OXYGEN

CONSUMPTION

RATE = n

Oeff Ck2

keff : global reaction rate constant : oxygen concentration

2OC

n : global reaction order

Coal Combustion Kinetics

• Reaction order n = 0.5, which gave the best fit of experimental data for the global reaction rate equation, was used for all tested coals.

• Global reaction rate constant keff varied with coal, temperature and oxidant composition.

Experimental Kinetics Constants

• The lower the rank of the coal, the higher the

combustion rate constant:

keff Lignite > keff Leão > keff Bonito

• For the same coal, keff increases with the combustion

temperature.

• Under oxyfuel conditions, keff is higher than under air

probably due to the contribution of the CO2 gasification

reaction C + CO2 2 CO, followed by fast CO burning.

Conclusions

THAT’S ALL, FOLKS !

THANK YOU

FOR YOUR ATTENTION