Changes in chemical and physical properties of South African caking coals

during pyrolysis.Student: R. White

Supervisors: Prof. C.A. Strydom

Prof. J.R. Bunt

Outline

1. Background1. Caking coals2. Pyrolysis

2. Problem statement3. Hypothesis 4. Aim and objectives5. Methods of investigation6. Experimental work7. Analysis8. Results9. Conclusion

1.

Background

• Coal is currently South Africa’s primary energy source and provides approximately 79% of the total energy needs[1].

• Coal will remain the primary energy source despite alternative energy generation methods such as nuclear power, wind and solar energy[2].

• Higher grade coals will last up to 2050[2].

• Utilization of lower grade coals and caking coals[2].– Industrial coal combustion and gasification processes can be adjusted to fit the

properties of caking coals.

[1] Falcon, R., & Van der Riet, M. (2007). Effect of milling and coal quality on combustion. International Pittsburgh Coal Conference. Sandton: South Africa.[2] Jeffrey, L. (2005). Characterization of the coal resources of South Africa. The Journal of The South African Institute of Mining and Metallurgy, 95-102.

2.

Background: pyrolysis

• Pyrolysis is the thermal decomposition of coal when heated in an oxygen deprived atmosphere[6].

• Pyrolysis is an important industrial process as it is the initial step of thermal decomposition processes.– Volatiles are released and changes occur in the char[7].– Functional groups on the remaining coal-char samples indicate the reaction

mechanism.

• The pyrolysis process affects: – the swelling and agglomeration of the coal.– the structure and reactivity of the char[8].

• Pyrolysis products includes: coke/char, tar and gaseous compounds.

[6] Hambly, E. (1998). The Chemical Structure of Coal Tar and Char During Devolatilization. A Thesis Presented to the Department of Chemical Engineering: Brigham Young University.

[7] Alonso, M., Borrego, A., Alvarez, D., & Mene´ndez, R. (1999). Pyrolysis behaviour of pulverised coals at different temperatures. Fuel 78 , 1501–1513.[8] Gavalas, G. (1982). Coal Pyrolysis. California: Elsevier Scientific Publishing Company.

4.

Background: caking coals

• Caking coal refers to the softening, melting, swelling and re-solidifying of bituminous coals when heated in the absence of air[3].

• The plasticity of coal during pyrolysis is of significant importance[4].– It affects the structure of the char , thus the behaviour during further utilization

processes– Metaplast theory: coal à metaplast à coke/char

• The swelling of a coal particle can be described using the multi-bubble mechanism[5].

[3] Maloney, D., Jenkins, R., & Walker, P. (1982). Low-temperature air oxidation of caking coals. 2. Effect on swelling and softening properties. Fuel, 175-181.[4] Sheng, C., & Azevedo, J. (2000). Modeling the evolution of particle morphology during coal devolatilization. Proceedings of the Combustion Institute, (pp.

2225-2232).[5] Yu, J., J, L., Wall, T., Liu, G., & Sheng, C. (2004). Modeling the development of char structure during the rapid heating of pulverized coal.

Combustion and Flame, 519-532. 3.

Pyrolysis of caking coal

[9] Yu, J., Lucas, J., & Wall, T. (2007). Formation of the structure of chars during devolatilization of pulverized coal and its thermoproperties: a review. Progress in Energy and Combustion Science, 135-170. 5.

Stage I

metaplast

Stage Il

Primary pyrolysis

Stage Ill

Secondary pyrolysis

Raw coal

Tar and aliphatic gases

Decrease in H(al)

(a loss in plasticity and swelling behaviour)

Molten stage where multiple reactions may

occur

Cross-link reaction

Ring condensation

CO and H2

Decrease in H(ar)

Problem statement

• Higher grade coal reserves are limited and new technologies must be investigated to maximize the products derived from coal processes.

• Caking properties will influence process efficiency and may cause operational problems.

• The pyrolysis process of Southern African caking coals need to be characterized to be better understood.

6.

Hypothesis

• By characterizing the changes in chemical and physical properties of caking coals during pyrolysis, the behaviour in terms of structural changes of Southern African caking coals can be described.

• Different types of caking coals may behave differently during pyrolysis and their chemical compositions and physical structural changes may be used as an indication of the differences.

7.

Aim and objective

• The aim was to investigate the behaviour of Southern African caking coals during pyrolysis.

• Compare caking coal with non-caking coal.

• The following objectives were stipulated:

– Characterize three South African coal samples with different swelling indices.– Determine temperatures where the coals have undergone different percentages

of pyrolysis mass loss.– Analyze the chemical and physical changes during pyrolysis.– Explain the pyrolysis behaviour of each coal individually to determine where the

significant changes occur.– Compare the pyrolysis behaviour of the three types of South African coals in

order to distinguish between caking and non-caking coals.

8.

Methods of investigation

• Three different types of Southern African caking coals;

1. Highveld coal Medium rank C vitrinite-rich FSI 02. Grootegeluk Medium rank C vitrinite-rich FSI 6.53. Tshikondeni Medium rank B vitrinite-rich FSI 9

Experimental conditions preparing char samples

Nitrogen atmosphere; flow rate 100 mL/min

Operating temperature up to 900 °C

Mass loss % = 20, 40, 60, 80 and 100

Coal particle size; < 250 microns

9.

Analysis

Chemical analysis

TG/MS

DRIFT

Proximate and ultimate

XRD and XRF

Physical analysis

CO2 surface area

SEM

11.

Table 1: Conventional analysis results for the three raw coals.

TWD raw GG raw TSH raw

Proximate analysis m.f.b m.f.b m.f.b

Inherent moisture 0.0 0.0 0.0

Ash 14.2 8.0 15.0

Volatile matter 28.3 36.9 21.5

Fixed carbon 57.4 55.1 63.5

100 100 100

Ultimate analysis m.f.b m.f.b m.f.b

Sulfur 0.8 1.1 0.8

Carbon 79.4 82.0 89.7

Hydrogen 4.2 5.3 5.1

Nitrogen 2.2 1.8 2.1

Oxygen 13.2 9.7 2.3

100 100 100

Results

Table 2: Temperatures at which specific mass loss observed from TGA.

Highveld TWD Grootegeluk GG Tshikondeni TSH

Sample Temperature

(°C)

Sample Temperatur

e

(°C)

Sample Temperature

(°C)

TWD 1 at 20% 433 GG 1 at 20% 424 TSH 1 at 20% 457

TWD 2 at 40% 468 GG 2 at 40% 450 TSH 2 at 40% 490

TWD 3 at 60% 535 GG 3 at 60% 482 TSH 3 at 60% 520

TWD 4 at 80% 657 GG 4 at 80% 571 TSH 4 at 80% 612

TWD 5 at 100% 900 GG 5 at 100% 900 TSH 5 at 100% 900

Results

TGA Results

50

60

70

80

90

100

110

0 200 400 600 800 1000

Wei

ght (

%)

Temperature (°C)

TWD GG TSH

Figure 2: TG curves for the three coal samples. 12.

DTG curves

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 200 400 600 800 1000

Deriv

. wt.%

(°C/

min

)

Temperature (°C)TWD GG TSH

Figure 3: DTG curves for the three coal samples. 13.

DRIFT spectra

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

40080012001600200024002800320036004000

Kub

elka

-Mun

k

Wavenumber (cm-1)

TWD GG TSH

OH groups Aromatic CH stretch

Aliphatic CH stretch

C=O stretch Aromatic C=C stretch

CH2, CH3

Figure 4: DRIFT spectra of the three raw coal samples. 14.

Highveld coal

Figure 5: DRIFT spectra for the Highveld coal and char samples. 15.

Grootegeluk coal

0

0.05

0.1

0.15

0.2

0.25

0.3

40080012001600200024002800320036004000

Kub

elka

-Mun

k

Wavenumber (cm-1)

GG 1 GG 2 GG 3 GG 4 GG 5 GG 6

Figure 6: DRIFT spectra for the Grootegeluk coal and char samples. 16.

Tshikondeni coal

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

40080012001600200024002800320036004000

Kub

elka

-Mun

k

Wavenumber (cm-1)

TSH 1 TSH 2 TSH 3 TSH 4 TSH 5 TSH 6

Figure 7: DRIFT spectra for the Tshikondeni coal and char samples. 17.

Surface area

0.0

50.0

100.0

150.0

200.0

0 1 2 3 4 5 6 7

BET

sur

face

are

a (m

2 .g -1

)

Coal-char samples at mass loss percentages

TWD GG TSH

Figure 8: CO2 BET surface area for the coal and char samples.

Raw coal 20% 40% 60% 80% 100%

18.

Highveld coal

Figure 9: SEM images for the Highveld coal and char samples. 19.

Grootegeluk

Figure 10: SEM images for the Grootegeluk coal and char samples. 20.

Tshikondeni

Figure 11: SEM images for the Tshikondeni coal and char samples. 21.

Conclusion

• TG and DTG curves:– TSH (highly caking coal) reacts at higher temperatures.– The shift in devolatilization temperature, in the DTG curve, for TSH can be

attributed to the thermoplastic behaviour.

• DRIFT results:– Hydrogen bonding is more evident for TWD and GG– TSH coal exhibited higher CHar/C=C ratio.

• CO2 surface area results:– Coal samples are microporous material.– Decrease in surface area for GG and TSH an indication of thermoplastic

behaviour.

23.

Caking coal vs. non-caking coal

• The most significant differences between non-caking and caking coals occurred mainly in two stages:

1. Between 40 and 60% mass loss

• Re-solidification temperature for GG and TSH is within this stage.• Decrease in BET surface area for the TSH coal.• Significant decrease in the intensity of the DRIFT spectra for GG and TSH.

• Metaplast occurs in the region of maximum fluidity and the most significant changes were observed.

24.

Caking coal vs. non-caking coal

2. Between 80 and 100% mass loss

• Secondary pyrolysis process.

• Decrease in BET surface area for the GG coal sample – indication of swelling behaviour.

• Aromaticity increase for TSH sample thus the end of plastic range and the start of caking phase.

• No signs of thermoplastic behaviour for the TWD coal sample.

25.

Acknowledgement

• This work is based on the research supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation of South Africa.

Any opinion, finding or conclusion or recommendation expressed in this material is that of the author(s) and the NRF does not accept any liability in this regard.