Modelling and efficiency improvement of a plasma-arc gasification reactor quench probe

193 Pages·2017·5.59 MB·English

Modelling and efficiency improvement of a plasma-arc
gasification reactor quench probe

J. van Rooyen
23425652
ORCid ID: https://orcid.org/0000-0002-3931-8548

Dissertation submitted in fulfilment of the requirements of the degree Master of Engineering in
Chemical Engineering at the Potchefstroom campus of North West University

School of Chemical and Minerals Engineering
North-West University Potchefstroom Campus

Supervisors: Mr. A.F. van der Merwe
Prof. K.R. Uren
Graduation May 2018

_____________________________________________________________________________ ABSTRACT
ABSTRACT
A steady state computational fluid dynamics (CFD) model was developed to describe the
temperature and flow characteristics inside a laboratory scale plasma-arc gasification reactor
quench probe with the aim of efficiency improvement. “Thermal efficiency” was mainly described
according to the thermal and fluid profiles and characteristics inside the quench probe, while the
chemical efficiency, which contributed to a minor part of the study, was based on an experimental
approach.
The CFD model was constructed using the STAR-CCM+ CFD simulation software in the 3-
dimensional Eulerian-Lagrangian framework. The realisable k-ε turbulence two-layer model was
applied to describe the physics properties of the simulation and water droplets were considered
spherical and of constant density and size. An unreactive model was developed as temperatures
were too low to deem the dominant reaction, i.e. the water gas shift reaction, active.
Model data was validated through comparison thereof with experimental thermocouple
measurements positioned inside the laboratory scale quench probe. U-tube measurements
described the chemical gas composition downstream to the quench probe.
Experimental data was categorised into phase 1 (non-reactive approach) and phase 2 (reactive
approach), of which the feed of the latter included organic material as opposed to phase 1, which
consisted of only a carrier gas feed. A variety of experimental cases were investigated by inducing
changes to the experimental setup with regards to the water spray rate, gas feed and type, and
supplied electrical current. Phase 1 was used to construct the base model, of which parameter
analyses were conducted to refine the model for phase 2. The final model could describe quench
probe conditions fairly accurate, with an average error of 9.87 % and a root mean square error of
6.96 °C.
It was found that the temperature distribution inside the quench probe was strongly dependant on
the velocity profile. The development of a recirculation zone inside the quench resulted in a longer
residence time, increasing the cooling effect of the spray water. Furthermore, temperatures within
range of full quenching were achieved relatively early after the first spray injection, indicating
redundant water spray.
Through use of the dimensionless temperature gradient and H /CO ratio, the thermal and chemical
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efficiencies could respectively be investigated. Generally, the dimensionless temperature gradient
averaged 0.8 towards the exit of the quench probe, indicating adequate quenching. Contrarywise,
the H /CO ratio ranged between 0.5 and 0.7 when ideally ratios of 1.0 to 2.5 are preferred for
2
industrial application. It is therefore cardinal to improve chemical efficiency whilst not sacrificing the
integrity of the thermal efficiency.
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Lastly, the model was used to investigate improvements to the current quench probe design with
regards to the water flow rate, nozzle placement, number of nozzles and geometry. It was concluded
that the water flow rate could be reduced in addition to lessening the number of nozzles to effectively
achieve the same quenching results. Additionally, a larger diameter quench probe would achieve
faster quenching rates, but due to the redundant water spray nozzles in the current application,
similar results were achieved for smaller diameter cases.
Key words: computational fluid dynamics, plasma gasification, synthesis gas quenching

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___________________________________________________________________ ACKNOWLEDGEMENTS
ACKNOWLEDGEMENTS
“Knowledge is knowing a tomato is a fruit; wisdom is not putting it in a fruit salad”
– Miles Kington
Writing a Master’s dissertation is no small feat and I would therefore like to extend my gratitude
towards those that did not only help me improve my knowledge, but added spoons full of wisdom
into the mix:
 Mr. Frikkie van der Merwe and Prof. Kenny Uren who guided me with patience, diplomacy
and persistence;
 My family, who always reminded me of the importance to take breaks and treat myself;
 Nico Bijzet for his unwavering patience, motivation and unlimited supply of chocolate.
 Mr. Tobie Loftus for being a good minion and helping immensely with my experimental work.
Furthermore, without the aid of Dr. IJ van der Walt and Mr. P Scheepers at the Nuclear Energy
Corporation of South Africa (NECSA) in the use of their facilities and continuous guidance in the
laboratory, this would not be possible.
Lastly, I would like to thank my Heavenly Father for giving me the opportunity, talent and endurance
to complete this research dissertation and for paving the way forward.

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TABLE OF CONTENTS
ABSTRACT i
ACKNOWLEDGEMENTS .............................................................................................................................. iii
TABLE OF FIGURES ................................................................................................................................... viii
LIST OF TABLES .......................................................................................................................................... xiii
LIST OF ABBREVIATIONS .......................................................................................................................... xv
NOMENCLATURE ........................................................................................................................................ xvi
CHAPTER 1: INTRODUCTION ................................................................................................................... 1
1.1 Background and motivation .............................................................................................................. 2
1.1.1 Waste management prospect ................................................................................................... 2
1.1.2 Plasma gasification vs. traditional gasification .......................................................................... 2
1.1.3 Quenching of syngas .................................................................................................................. 4
1.2 Research focus ................................................................................................................................... 5
1.3 Aim ..................................................................................................................................................... 6
1.4 Objectives .......................................................................................................................................... 6
1.5 Investigation outline .......................................................................................................................... 6
1.5.1 Method of investigation ............................................................................................................ 6
1.5.2 Scope of the study ..................................................................................................................... 7
Bibliography 9
CHAPTER 2: LITERATURE STUDY ........................................................................................................ 12
2.1 Synthesis gas quenching ............................................................................................................. 13
2.1.1 Quenching methods ............................................................................................................. 14
2.1.2 The direct quenching method ............................................................................................. 18
2.2 CFD modelling of synthesis gas cooling systems.................................................................... 25
2.2.1 Approaches in CFD modelling for synthesis gas cooling systems ................................ 25
2.2.2 CFD modelling applicable to direct water quenching – reactivity .................................. 31
2.2.3 CFD modelling applicable to direct water quenching – temperature and flow profiles
40
2.3 Concluding review ........................................................................................................................ 48
Bibliography 50
CHAPTER 3: EXPERIMENTAL METHOD .............................................................................................. 54
3.1 Laboratory-scale experimental setup .............................................................................................. 55
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_____________________________________________________________ TABLE OF CONTENTS
3.2 Material ........................................................................................................................................... 57
3.3 Measuring equipment ..................................................................................................................... 57
3.3.1 Type-R thermocouple .............................................................................................................. 57
3.3.2 Type-K thermocouple .............................................................................................................. 57
3.3.3 Picolog® TC-08 USB thermocouple data logger ....................................................................... 57
3.3.4 Gilian® Gilibrator-2 calibrator .................................................................................................. 57
3.4 Experimental procedure .................................................................................................................. 58
3.4.1 Phase 1: non-reactive experimental procedure ...................................................................... 58
3.4.2 Phase 2: reactive experimental procedure ............................................................................. 59
Bibliography 61
CHAPTER 4: MODEL DEVELOPMENT .................................................................................................. 62
4.1 CFD modelling assumptions ............................................................................................................ 63
4.2 Generation of the quench probe geometry for the CFD model ...................................................... 64
4.3 Mesh continua ................................................................................................................................. 66
4.4 Physics continua .............................................................................................................................. 69
4.5 Boundary conditions ........................................................................................................................ 80
4.6 Summary of final conditions used in model .................................................................................... 83
Bibliography 85
RESULTS AND DISCUSSION .............................................................................................................................. 88
4.7 CFD Modelling results ...................................................................................................................... 89
4.7.1 Velocity profile ....................................................................................................................... 89
4.7.2 General temperature profile ................................................................................................ 91
4.8 Validation of modelled results ..................................................................................................... 95
4.8.1 Phase 1 results ...................................................................................................................... 96
4.8.2 Phase 2 results .................................................................................................................... 102
4.9 Quenching efficiency .................................................................................................................. 104
4.10 Chemical efficiency ..................................................................................................................... 107
4.11 Quench probe improvement study ........................................................................................... 108
4.11.1 Nozzle flow rate ................................................................................................................... 109
4.11.2 Nozzle positioning ............................................................................................................... 109
4.11.3 Efficiency of nozzle clusters .............................................................................................. 110
4.11.4 Geometric variations........................................................................................................... 112
Bibliography ............................................................................................................................................. 117
CHAPTER 5: CONCLUSION AND RECOMMENDATIONS ............................................................... 119
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5.1 Conclusion ................................................................................................................................... 120
5.1.1 Flow and temperature profile ............................................................................................ 120
5.1.2 Experimental data ............................................................................................................... 120
5.1.3 Chemical efficiency ............................................................................................................. 121
5.1.4 Improvement considerations ............................................................................................. 121
5.1.5 Geometric considerations .................................................................................................. 121
5.2 Recommendations ...................................................................................................................... 122
Appendix A: Previous conducted research .......................................................................................... 123
Bibliography 125
Appendix B: Description of physics models ......................................................................................... 127
B.1. List of definitions ......................................................................................................................... 127
Bibliography 131
Appendix C: Calibration of testing equipment ..................................................................................... 132
C.1. Calibration of feed ......................................................................................................................... 132
C.2. Nitrogen flow meter ...................................................................................................................... 132
Appendix D: Nozzle specifications ........................................................................................................ 134
D.1. Determining parameters ............................................................................................................... 134
D.2. Summary of water nozzle (injector) parameters........................................................................... 137
Bibliography 138
Appendix E: Ultimate analysis for pine wood chips ............................................................................ 139
E.1. Ultimate analysis for pine wood chips .......................................................................................... 139
Appendix F: Quench probe technical drawing .................................................................................... 140
F.1. Quench probe technical drawing .................................................................................................. 140
Appendix G: SIMPLE algorithm ............................................................................................................. 144
G.1. SIMPLE algorithm .......................................................................................................................... 144
Appendix H: Temperature profile influence of individual nozzles ..................................................... 145
H.1. Nozzle temperature profile ........................................................................................................... 145
Appendix I: Experimental data ............................................................................................................. 148
I.1. Phase 1: 32.0 ℓ/min N flow rate ................................................................................................... 148
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I.2. Phase 1: 32.4 ℓ/min N and 65 ℓ/min air flow rate ....................................................................... 149
2
I.3. Phase 2: 35.6 ℓ/min air flow rate, 150 A power supply and low (4.8 kg/h) wood chip feed
150
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_____________________________________________________________ TABLE OF CONTENTS
I.4. Phase 2: 35.6 ℓ/min air flow rate, 150 A power supply and medium (6 kg/h) wood chip
feed 151
I.5. Phase 2: 35.6 ℓ/min air flow rate, 150 A power supply and high (7.63 kg/h) wood chip feed
151
Appendix J: Phase 1 model results ...................................................................................................... 153
J.1. 10 parcel streams model results ............................................................................................... 153
J.2. 30 parcel streams model results ............................................................................................... 155
J.3. Statistical significance ................................................................................................................ 156
Appendix K: Phase 2 modelling results ................................................................................................ 158
Appendix L: Dimensionless temperature gradient ............................................................................. 162
Appendix M: Flow rate influence on temperature profile .................................................................... 166
Appendix N: Configurations of moved nozzles ................................................................................... 167
Appendix O: Temperature profile images of moved nozzles ............................................................ 169
Appendix P: Influence of nozzle position on temperature distribution ............................................. 171
Appendix Q: Temperature profile for clusters of nozzles ................................................................... 172
Appendix R: Temperature profile graph for geometry alterations .................................................... 173

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TABLE OF FIGURES

CHAPTER 1
Figure 1-1: A direct water quench used in syngas cooling applications ........................................... 4
Figure 1-2: Simplified scheme of the laboratory scale plasma gasification setup ............................ 5
Figure 1-3: Workflow and chapter contents of the research study ................................................... 8

CHAPTER 2
Figure 2-1: The dimensionless temperature gradient as a function of residence time at T of 850
inlet
°C (Adapted from Hawboldt et al., 1999) .................................................................................. 14
Figure 2-2: Comparison between quenching rates of different quenching methods at typical
quenching conditions (Adapted from Sundstrom & DeMichiell, 1971) ....................................... 16
Figure 2-3: Syngas heat recovery systems as investigated by Uebel et al. (2014) (Taken from Uebel
et al., 2014) .............................................................................................................................. 17
Figure 2-4: Syngas cooling configurations as tested by Ni et al. (2011) where (a) direct water
quenching (b) radiant syngas cooler (RSC) (c) RSC and (d) gas quenching (Taken from Ni et al.,
2011) ........................................................................................................................................ 18
Figure 2-5: Exiting CO conversion for different reactor temperatures and H O/CO ratios at 1 atm and
2
present catalyst (Taken from Choi & Stenger, 2003) ................................................................ 20
Figure 2-6: Influence of steam flow of (a) 20 kg/h, (b) 50 kg/h, (c) 100 kg/h and (d) 300 kg/h in a
quench reactor on the formation of different chemical species (Taken from Yan et al., 2012)... 22
Figure 2-7: The effect of water injection with (i) one nozzle, (ii) four nozzles and (iii) five nozzles on
(a) the CO conversion and (b) the reaction equilibrium (Taken from Kiso & Matsuo, 2011) ...... 23
Figure 2-8: Optimisation study on the influence of (a) outlet temperature, (b) h/d ratio, (c) volume,
(d) wall cooling capability and (e) steam flow as led by Uebel et al. (2016b) ............................. 32
Figure 2-9: Primary and secondary injection points in the study conducted by Wang et al. (2011) 36
Figure 2-10: CO conversion in relation to steam mass flow and H O/CO ratio (Adapted from Uebel
2
et al. 2016a) ............................................................................................................................. 39
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