Catalytic steam gasification of large coal particles

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Catalytic steam gasification of large coal

particles

School of Chemical and Minerals Engineering

Coal Research Group

Sansha Nel

B. Eng (Chem. Eng.)(North-West University)

Dissertation submitted in fulfilment of the requirements for the degree

Masters in Chemical Engineering

at the Potchefstroom campus of the

North-West University, South Africa

Supervisor: Prof. H.W.J.P. Neomagus

Co-supervisor: Prof. J.R. Bunt

Co-supervisor: Prof. R.C. Everson

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Declaration

Catalytic steam gasification of large coal particles ii

Declaration

I, Sansha Nel, hereby declare that the dissertation entitled: “Catalytic steam gasification

of large coal particles”, submitted in fulfilment of the requirements for a Masters degree in

Chemical Engineering (M. Eng.), is my own work, unless otherwise specified in text, and that this dissertation has not been submitted to any other tertiary institution either in part or as a whole.

Signed at Potchefstroom, on the ... day of ..., 2011.

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Acknowledgement

Catalytic steam gasification of large coal particles iii

Acknowledgement

I would like to acknowledge and thank the following people who have motivated and guided me through the course of this study, and without whom I could not have succeeded:

• Firstly and most importantly to my Heavenly Father, who has blessed me with the knowledge and insight to further my studies, and who always gives me strength and guidance in everything I do;

• Professors Hein Neomagus, John Bunt and Ray Everson for their much appreciated guidance, support and motivation throughout this investigation;

• Sasol for the financial support to conduct this study;

• Jan Kroeze and Adrian Brock for their technical assistance;

• Dr. Lourens Tiedt for conducting all the SEM scans and EDS analyses;

• Paul Keanly from X-Sight X-ray Services for conducting all the tomography scans and for assisting me with the data processing;

• Liezl Schoeman at UIS Analaytical Services for all her help and suggestions regarding the XRF analysis;

• My parents and brother for their unconditional love, support and motivation;

• And lastly, my significant other, Hennie, for all his love and support and the much needed motivational speeches.

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Abstract

Catalytic steam gasification of large coal particles iv

Abstract

Catalytic gasification has been studied extensively in order to develop more efficient and economic coal conversion processes. Fundamental studies regarding catalytic gasification have thus far focused on experimentation with small coal particles and powders. The lack of knowledge regarding the application of large coal particles in steam gasification studies, in particular catalytic steam gasification, is the motivation behind this investigation.

A washed bituminous, medium rank-C Highveld coal (seam 4) was selected for this study, and a general characterisation of the coal was conducted. It was found that the ash content of the washed coal is 12.6 wt.% (air-dried basis). Based on the gross calorific value of 26.6 MJ/kg (air-dried basis), the coal sample was graded as a grade B coal. XRF analysis of the ash indicated that the coal is rich in SiO2 and Al2O3, with a low potassium oxide content

(0.53 wt.%) which is typical for South African coal.

Potassium carbonate (K2CO3) was selected as catalyst, and the excess solution

impregnation method was used to impregnate large coal particles (5 mm, 10 mm, 20 mm and 30 mm). The pH of the impregnation solution stabilised after three weeks, which led to the assumption that impregnation is complete. Two methods were used to determine the catalyst loading obtained after impregnation: XRF was used to determine the wt.% K in the ash, while ion specific electrode (ISE) was used to measure the [K+] decrease in the impregnation solution. XRF results indicated the maximum catalyst loading obtainable for large coal particles, with the specific impregnation method, to be between 0.68 – 0.83 wt.% K (coal basis). XRF can be used to determine the catalyst loading by measuring the K content in the ash, while ISE can be used to semi-quantitatively predict the catalyst loadings of large coal particles. The catalyst distribution was studied using SEM and tomography analyses. SEM scans showed that the formation of cracks occurred as a result of impregnation, and EDS analysis indicated that the majority of the catalyst is concentrated around the outer surface of the particles. Tomography scans, and mineral volume analysis, indicated that the mineral matter of the coal particles increased after impregnation.

The effect of catalyst addition on reactivity was investigated by conducting steam gasification experiments with 5 mm and 10 mm particles, in a large particle TGA. The 20 mm and 30 mm particles did not remain intact after impregnation and were therefore not used for the reactivity experiments. Reactivity experiments were performed at temperatures ranging from 800 °C to 875 °C, with a steam concentration of 80 mol%. Graphs illustrating conversion as

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Abstract

Catalytic steam gasification of large coal particles v

a function of time indicated that the addition of K2CO3 to the coal samples increased the

reaction rate. This was quantified by determining the reactivities of the raw and catalysed samples using linearised homogeneous model plots. The reaction rate was found to be temperature sensitive, and independent of particle size, which indicated that experiments were conducted in the chemical reaction control regime. A slight decrease in activation energy was observed with the addition of K2CO3, from 191 kJ/mol (raw coal) to 179 kJ/mol

(catalysed coal). Microscope images of raw and catalysed chars indicated that the addition of a catalyst may reduce agglomeration.

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Opsomming

Catalytic steam gasification of large coal particles vi

Opsomming

Katalitiese vergassing is voorheen al breedvoerig ondersoek met die doel om meer doeltreffende en ekonomiese steenkool vergassingsprosesse te ontwikkel. Fundamentele studies aangaande katalitiese vergassing het tot dusver gefokus op eksperimentering met klein steenkool partikels en poeiers. Die gebrek aan kennis rakende die aanwending van groot steenkool partikels vir steenkool vergassing studies, in besonder katalitiese vergassing, is die motivering agter dié ondersoek.

‘n Bitumineuse, medium rang-C Hoëveld steenkool (laag 4) is gekies vir die studie, en ‘n algemene karakterisering van die steenkool was gedoen. Dit was gevind dat die as-inhoud van die steenkool 12.6 % (massa basis) is, wat relatief laag is vir Suid-Afrikaanse steenkool. Die steenkool was gegradeer as ‘n graad B steenkool, volgens ‘n verbrandingswarmte waarde van 26.6 MJ/kg. XRF analise het getoon dat die steenkool ryk is in SiO2 and Al2O3,

met ‘n lae kalium oksied inhoud (0.53 %, massa basis), wat kenmerkend is van Suid-Afrikaanse steenkool.

Kalium karbonaat (K2CO3) was die gekose katalis, en die groot steenkool partikels (5 mm,

10 mm, 20 mm and 30 mm) was geïmpregneer deur middle van die oormatige oplossing metode. Die pH van die impregneringsoplossing het gestabiliseer na drie weke, wat gelui het tot die aanname dat impregnering klaar is. Twee metodes was gebruik om die katalisinhoud in die steenkool te bepaal na impregnering: XRF was gebruik om die massa % K in die as te bepaal, terwyl ISE gebruik was om die afname in [K+] in die oplossing te bepaal. XRF resultate het aangedui dat ‘n maksimum katalisinhoud van tussen 0.68 – 0.83 % K (massa basis) vergrygbaar is vir die impregnering van groot steenkool partikels. XRF kan gebruik word om die katalisinhoud te bepaal deur die K inhoud in die as te analiseer, terwyl die katalisinhoud op ‘n semi-kwantitatiewe manier voorspel kan word d.m.v. ISE. Die katalis verspreiding was bestudeer met SEM en tomografiese analises. SEM foto’s het gewys dat die impregneringsmetode die vorming van krake veroorsaak in die partikels, en EDS analise het aangedui dat die meerderheid van die katalis rondom die buite oppervlakte van die steenkool partikels gekonsentreerd is. Tomografiese foto’s, en mineral volume analises, het gewys dat die mineraalinhoud van die partikels vermeerder na impregnering.

Die effek van katalis byvoeging op die reaktiwiteit van die steenkool was bestudeer deur stoom vergassing eksperimente te doen met die 5 mm en 10 mm partikels, in ‘n groot partikel TGA. Die 20 mm en 30 mm partikels was nie gebruik vir stoom

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Opsomming

Catalytic steam gasification of large coal particles vii

vergassingseksperimente nie, aangesien hulle verbrokkel het na impregnering. Eskperimente was uitgevoer by temperature in die omgewing van 800 °C tot 875 °C, met ‘n stoomkonsentrasie van 80 mol.%. Grafieke wat die omsetting as ‘n funksie van tyd vooorstel het aangedui dat die byvoeging van K2CO3 tot die steenkool partikels, die reaksie

tempo verhoog. Dit was gekwantifiseer deur die reaktiwiteite van die rou en gekataliseerde partikels te bepaal deur gebruik te maak van gelineardiseerde homogene model grafieke. Dit was ook gevind dat die reaksie tempo temperatuur-sensitief is en onafhanklik van partikelgrootte, wat aandui dat die eksperimente uitgevoer is in die chemiese-reaksie beherende regime. ‘n Afname in aktiveringsenergie is ook waargeneem met die toevoeging van K2CO3, van 191 kJ/mol (rou) na 179 kJ/mol (gekataliseerd). Mikroskoop foto’s van rou

en gekataliseerde steenkool het ook gewys dat die byvoeging van katalis agglomerasie verminder.

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Table of Contents

Catalytic steam gasification of large coal particles viii

List of figures

Figure 2.1: Comparison of gasification rates for coal char catalysed with various ... 22

Figure 2.2: Competition of alkali cation... 23

Figure 2.3: Oxygen transfer and intermediate hybrid mechanism ... 31

Figure 4.1: Bulk coal sample in a cone-shaped pile... 42

Figure 4.2: Four cone-shaped piles of equal volume ... 43

Figure 4.3: Rotary Splitter ... 43

Figure 5.1: 30 mm hand selected particle ... 52

Figure 5.2: FEI QUANTA 200 ESEM integrated with Oxford INCA X-SIGHT EDS ... 56

Figure 5.3: Sample preparation for SEM and EDS analyses ... 56

Figure 5.4: HMXST CT system ... 58

Figure 5.5: pH measurements during impregnation ... 59

Figure 5.6: Influence of impregnation on 20 mm and 30 mm particles ... 60

Figure 5.7: Comparison of XRF and ISE results ... 64

Figure 5.8: SEM scans of raw coal particle (outside and inside) ... 65

Figure 5.9: SEM scans of impregnated coal particle (outside and inside) ... 66

Figure 5.10: SEM images of the outer surface of a raw and impregnated particle ... 67

Figure 5.11: CT scan of impregnated 30 mm particle ... 69

Figure 5.12: Clipping plane of 30 mm particle ... 69

Figure 5.13: Slice view of raw 30 mm particle ... 70

Figure 5.14: Slice view of impregnated 30 mm particle ... 70

Figure 6.1: Experimental set-up of the large particle TGA ... 74

Figure 6.2: Mass loss curve for raw 5 mm particles, at 825 °C ... 78

Figure 6.3: Normalised mass loss curve for raw 5 mm particles, at 825 °C ... 79

Figure 6.4: Conversion-time graph for raw 5 mm particles, at 825 °C ... 80

Figure 6.5: Effect of impregnation on reactivity... 81

Figure 6.6: Conversion as a function of time, for 5 mm particles ... 82

Figure 6.7: Conversion as a function of time, for 10 mm particles ... 83

Figure 6.8: Influence of temperature on reactivity ... 84

Figure 6.9: Influence of particle size on reactivity at 800 °C ... 85

Figure 6.10: Linearised HM plot (-ln(1-X) vs. t) ... 87

Figure 6.11: Linearised HM plot up to 70 wt.% conversion (5 mm) ... 88

Figure 6.12: Linearised HM plot up to 70 wt.% conversion (10 mm) ... 88

Figure 6.13: Combined Arrhenius plot for 5 mm and 10 mm particles ... 91

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List of figures

Catalytic steam gasification of large coal particles xiii

Figure 6.15: Char samples of raw and impregnated 10 mm particles ... 93

Figure 6.16: SEM scans of 5 mm char samples (100 µm) ... 94

Figure 6.17: SEM scans of 5 mm char samples (1 mm) ... 94

Figure A.1: ISE calibration curve ... 107

Figure A.2: Positioning of 5 mm particles for CT scans ... 111

Figure A.3: Positioning of 20 mm particle for CT scans ... 111

Figure A.4: Volume rendering tool interface ... 112

Figure A.5: CT scan obtained from volume rendering analysis ... 114

Figure A.6: CT scan illustrating minerals in coal particles ... 115

Figure A.7: Volume analyser data histogram ... 116

Figure A.8: CT scan of 5 mm raw coal particles ... 117

Figure A.9: CT scan of 5 mm impregnated coal particles ... 117

Figure A.10: CT scan of 10 mm raw coal particles ... 118

Figure A.11: CT scan of 10 mm impregnated coal particles ... 118

Figure A.12: Clipping plane of 10 mm particles ... 119

Figure A.13: Slice view of raw 10 mm particles ... 119

Figure A.14: Slice view of impregnated 10 mm particles ... 120

Figure A.15: CT scan of raw 20 mm coal particle ... 120

Figure A.16: CT scan of impregnated 20 mm coal particle ... 121

Figure A.17: Clipping plane of 20 mm particles ... 121

Figure A.18: Slice view of raw 20 mm coal particle ... 122

Figure A.19: Slice view of impregnated 20 mm coal particle ... 122

Figure B.1: Conversion-time graphs for 5 mm particles ... 125

Figure B.2: Conversion-time graphs for 10 mm particles ... 126

Figure B.3: Influence of particle size on reactivity at 825 °C ... 128

Figure B.6: Determination of reactivity, k, from slope (5 mm) ... 130

Figure B.7: Determination of reactivity, k, from slope (10 mm) ... 131

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List of tables

Catalytic steam gasification of large coal particles xiv

List of tables

Table 2.1: Summary of large particle gasification research ... 15

Table 2.2: Summary of catalytic gasification research ... 35

Table 4.1: Size fractions of export coal sample. ... 42

Table 4.2: Coal characterisation procedures ... 44

Table 4.3: Chemical analysis of coal sample ... 46

Table 4.4: XRF analysis of ash sample ... 48

Table 5.1: Sieve size ranges for particle selection ... 51

Table 5.2: 4-Star Plus Benchtop Multiparameter Meter Specifications ... 53

Table 5.3: Potassium ISE Specifications ... 53

Table 5.4: XRF results for catalyst loading ... 61

Table 5.5: K content (in wt.%) of K2CO3 impregnated char samples ... 62

Table 5.6: ISE results for catalyst loading ... 63

Table 5.7: EDS results ... 67

Table 5.8: Results for mineral volume (Vol.%)... 71

Table 6.1: Equipment specifications for large particle TGA ... 75

Table 6.2: Reaction specifications for gasification experiments ... 76

Table 6.3: Reactivity values, k (h-1) ... 89

Table 6.4: Catalytic effectiveness ... 90

Table 6.5: Activation energies, Ea (kJ/mol) ... 92

Table A.1: XRF results in wt.% K2O ... 109

Table A.2: Validation of volume analyser measurements ... 123

Table A.3: Initial and final impregnation solution concentrations ... 124

Table B.1: Experimental values for inherent moisture and volatile matter and ash contents ... 127

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List of symbols

Catalytic steam gasification of large coal particles xii

List of symbols

Roman Symbols

Symbol Description Units

A Pre-exponential factor min-1.bar-m

Ea Activation energy kJ/mol

∆H0 Enthalpy of reaction kJ/mol

k Reaction rate constant/ reactivity h-1

kcat Reactivity of catalysed coal samples h-1

kraw Reactivity of raw coal samples h-1

K2Oinitial Mass K2O present in coal before impregnation wt.%/g ash

K2Oloaded Mass K2O impregnated wt.%/g ash

K2Ototal Total mass K2O present in coal after impregnation wt.%/g ash

Kmass Mass K impregnated g/g coal

Kloaded Mass K impregnated wt.%/g coal

[K2CO3] K2CO3 concentration decrease mol/L

[K2CO3]i Initial K2CO3 of impregnation solution mol/L

[K2CO3]f Final K2CO3 of impregnation solution mol/L

[K2CO3]adsorbed K2CO3 concentration impregnated/adsorbed mol/L

[K+]adsorbed K+ concentration impregnated/adsorbed mol/L

M0 Initial mass of coal sample g

Mt Mass of coal sample at time t g

Mash Mass of residual ash after gasification g

r Reaction rate molgas/mol.s

R Molar gas constant J/mol.K

t time hr or s

T Temperature K or °C

X Conversion -

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List of symbols

Catalytic steam gasification of large coal particles xiii

Nomenclature

Abbreviations Description

AFT Ash fusion temperature

ASTM American Society for Testing Materials CCG Catalytic coal gasification

CT Computer tomography

DAEM Distributed activation energy model d.a.f. Dry ash free basis

ECI Excess co-impregnation

EDS Energy dispersive spectrometry

ESI Excess solution impregnation

FSI Free swelling index

FTIR Fourier transform infrared spectroscopy GCV Gross calorific value (MJ/kg)

HM Homogeneous model

IM Inherent moisture

ISE Ion-specific electrode

ISO International Standards Organization

LH Langmuir-Hinshelwood rate equation

PVI Pore volume impregnation

RCM Random capillary model

ROM Run-of-mine

RPM Random pore model

SABS South African Bureau of Standards SANS South African National Standards

SCM Shrinking core model

SEM Scanning Electron Microscope

SNG Substitute natural gas

SS-NMR Solid-state nuclear magnetic resonance

TGA Thermogravimetric analyser

TG-DTA Thermogravimetric/Differential Thermal Analyser

VM Volatile matter

WCI World Coal Institute

WCIM Wetness co-impregnation method

WSI Wetness sequential impregnation

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List of symbols

Catalytic steam gasification of large coal particles xiv XRF X-ray fluorescence analysis