Development of Molten Salt Promoted Metal Oxide based Absorbents for CO2 Separation

142 Pages·2017·4.49 MB·English

by  Keling Zhang

University of Connecticut
OpenCommons@UConn
Doctoral Dissertations University of Connecticut Graduate School
3-5-2014
Development of Molten Salt Promoted Metal
Oxide based Absorbents for CO2 Separation
Keling Zhang
University of Connecticut, [email protected]
Follow this and additional works at:https://opencommons.uconn.edu/dissertations
Recommended Citation
Zhang, Keling, "Development of Molten Salt Promoted Metal Oxide based Absorbents for CO2 Separation" (2014).Doctoral
Dissertations. 327.
https://opencommons.uconn.edu/dissertations/327
Development of Molten Salt Promoted Metal Oxide based
Absorbents for CO Separation
2
Keling Zhang, Ph.D.
University of Connecticut, 2014
CO capture and storage from both power generation and industrial activity is a central
2
strategy for stabilization of atmospheric greenhouse gas concentrations to avoid drastic climate
change. The deployment of fully integrated commercial CO capture and storage schemes is
2
hindered by the considerable cost of current CO capture technologies. In pre-combustion
2
(gasification or natural gas reforming) systems, capture of CO at warm temperatures (250-400
2
°C) with solid absorbents can provide a lower energy penalty than the use of low-temperature
liquid absorbers, by avoiding the need to cool and reheat the gas stream. To date, efficient
regenerable CO solid absorbents applicable at warm temperatures are still greatly desired.
2
In this thesis, a molten salt promoting effect was discovered that can significantly
facilitate the CO reaction with bulk metal oxides. This leads to the invention of a series of
2
molten salt promoted metal oxide or metal oxide contained double salt absorbents with superior
performance, applicable to different warm temperature windows. A facile preparation procedure
utilizing ball milling was developed to prepare these absorbent materials. The roles of each
individual component in the absorbent mixture were discussed in the exemplary system of
NaNO promoted MgO-Na CO . For this same system, the chemistry was tuned for optimal
3 2 3
performance, which was demonstrated in a fixed bed reactor.
i
NaNO promoted MgO was chosen as the basic material system to study the origin of the
3
significant promotion effects of molten salts on metal oxides. Comprehensive experimental and
computational calculation results reveal that this facilitation originates from the capability of
molten nitrate to dissolve bulk MgO. Dynamic MgO dissolution/precipitation equilibrium in
molten nitrate provides activated MgO species accessible to CO at gas-solid-liquid triple phase
2
boundaries. This proposed reaction mechanism is also applicable to other systems composed of
different molten salts with other basic metal oxides or double salts, inspiring the design of
absorbents that require activation of the bulk material. It is also proposed here that molten
NaNO acts as a phase transfer catalyst in the gas-solid reaction between CO and MgO, by
3 2
converting the solid reaction environment into liquid and providing an alternate reaction pathway
to traditional gas-solid reactions.

Keling Zhang – University of Connecticut, 2014

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Development of Molten Salt Promoted Metal Oxide based
Absorbents for CO Separation
2
Keling Zhang
B.A., Huazhong University of Science and Technology, 2009

A Dissertation
Submitted in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
at the
University of Connecticut

2014

iii
Copyright by
Keling Zhang

2014
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APPROVAL PAGE
Doctor of Philosophy Dissertation
Development of Molten Salt Promoted Metal Oxide based
Absorbents for CO Separation
2

Presented by
Keling Zhang, B.A.
Major Advisor_________________________________________________
Prabhakar Singh
Associate Advisor______________________________________________
David L. King
Associate Advisor_____________________________________________
Harold D. Brody
Associate Advisor_____________________________________________
Steven L. Suib

University of Connecticut
2014
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Acknowledgements
I am greatly indebted to everyone who has contributed in part to this dissertation. I would
like to express my deepest appreciation to my advisor Professor Prabhakar Singh and mentor Dr.
David King. Both of you have been tremendous mentors and role models for me. Your guidance
on research as well as on my career has been priceless. Most importantly, the understanding,
compassion, and kindness I received from you nurtured me to be a more confident researcher. I
would also like to thank my associate advisors and committee members, Professor Brody,
Professor Suib and Professor Maric for their helpful discussions and comments.
I would like to express appreciation to everyone at Center for Clean Energy Engineering
at University of Connecticut, for the help and encouragement I received throughout my PhD
study. I would like to specially thank Pacific Northwest National Laboratory (PNNL) for
providing me an opportunity to work there as a Ph.D. student. This experience not only allowed
me to work on very interesting research projects, but also exposed me to many outstanding
scholars and a great research atmosphere. Special thanks to Ms. Xiaohong Shari Li for her help
with material synthesis and the numerous inspiring discussions. I would like to thank Dr. Liyu Li
for introducing me to this exciting area. I would like thank Dr. King’s research team at PNNL for
their help and encouragement along the way and many other scientists at the lab for discussions
and collaboration. Furthermore I would like to thank Dr. Yuhua Duan at National Energy
Technology Laboratory for his helpful contributions.
I can’t say thank you enough for the tremendous support I received from my family
through the entire process. I will be grateful forever for your love.

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Contents
Chapter 1: Sorbent materials for warm temperature CO capture .................................................. 1
2
1.1 Introduction to carbon capture and storage (CCS) ................................................................ 1
1.2 What makes a good CO sorbent? ......................................................................................... 2
2
1.3 Typical CO sorbent materials .............................................................................................. 3
2
1.3.1 Low temperature sorbents .............................................................................................. 3
1.3.2 Warm temperature sorbents ............................................................................................ 6
1.3.3 High temperature sorbents .............................................................................................. 9
1.4 Challenges and controversies over warm temperature sorbents ......................................... 11
Chapter 2: Resolving controversies over double salt absorbents .................................................. 20
2.1 Reproduce the double salt absorbent preparation ............................................................... 20
2.2 Sorption test and characterization of the reproduced sample .............................................. 20
2.3 Results and discussion ......................................................................................................... 22
2.3.1 Absorbent chemistry during synthesis .......................................................................... 22
2.3.2 Evaluation of CO absorption ....................................................................................... 24
2
2.3.3 Thermodynamic analysis .............................................................................................. 26
2.4 Discovery of the significance of NaNO in the CO uptake ............................................... 28
3 2
2.5 Effect of operating window on absorbent performance and the discovery of non-
stoichiometric double salt operation ......................................................................................... 30
2.5.1 High initial absorption peak with different operations ................................................. 30
2.5.2 Re-evaluation of MgCO thermodynamic equilibrium ................................................ 31
3
2.5.3 Pre-melting phenomena ................................................................................................ 32
2.6 Summary ............................................................................................................................. 32
Chapter 3: Development of reproducible preparation method and demonstration of broad molten
salt promoting effect ..................................................................................................................... 46
3.1 Development of alternate synthesis method........................................................................ 46
3.1.1 Influence of different process parameters in the precipitation method ........................ 46
3.1.2 Development of NaNO promoted CaCO -MgO based absorbent .............................. 49
3 3
3.1.3 Development of ball milling preparation procedures ................................................... 50
3.2 Composition optimization for NaNO promoted Na CO -MgO absorbent for
3 2 3
stoichiometric double salt operation ......................................................................................... 52
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3.3 Development of a broad range of molten salt promoted absorbents using the ball milling
method ....................................................................................................................................... 53
3.4 Summary ............................................................................................................................. 54
Chapter 4: Demonstration of selected absorbent in fixed bed reactor .......................................... 67
4.1 Effect of operating window on NaNO promoted Na CO -MgO based sorbent for non-
3 2 3
stochiometric double salt operation........................................................................................... 67
4.2 Effect of operating window on NaNO promoted MgO ..................................................... 70
3
4.3 Equilibrium pressure study of NaNO promoted Na CO -MgO for stoichiometric double
3 2 3
salt operation ............................................................................................................................. 70
4.4 Demonstration of NaNO promoted Na CO -MgO based sorbent in fixed bed reactor for
3 2 3
warm temperature CO capture ................................................................................................. 71
2
4.5 Integration of CO capture with methanation catalytic reaction for fuel production .......... 72
2
4.6 Summary ............................................................................................................................. 74
Chapter 5: Mechanistic understanding of the promoting effect of molten salts ........................... 88
5.1 Correlation between promoted CO absorption and melting of salt additive ..................... 88
2
5.2 Isotherm and cyclic absorptions of NaNO -MgO ............................................................... 89
3
5.3 Effect of NaNO concentration and role of triple phase boundary ..................................... 91
3
5.4 Molten salt effect on desorption .......................................................................................... 92
5.5 Phase conversions of NaNO -MgO during absorption cycles ............................................ 92
3
5.6 MgO dissolution in molten NaNO and the formation of MgCO through precipitation ... 93
3 3
5.7 Density functional theory calculation (courtesy of Dr. Yuhua Duan) ................................ 94
5.8 Proposed reaction mechanism ............................................................................................. 97
5.9 Applying the reaction mechanism to general molten salt promoted absorbents ................. 97
5.10 Pre-melting phenomena..................................................................................................... 99
5.11 Summary ......................................................................................................................... 101
Chapter 6: Conclusions and recommendation for future work ................................................... 118
6.1 Conclusions on the practical applicability of the developed absorbent materials............. 118
6.2 Conclusions on the discovery of molten salt promoting effect ......................................... 118
6.3 Recommended future work for fundamental scientific understanding ............................. 119
References ................................................................................................................................... 121

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List of Figures
Figure 1.1. IGCC with SEWGS vs. Conventional IGCC ............................................................. 12
Figure 1.2. CO chemisorption isotherms on K CO -promoted hydrotalcite ............................... 13
2 2 3
Figure 1.3. Pressure swing test results for various MgO contained double salts.......................... 14
Figure 1.4. Effect of temperature on the reaction of 137 μm (-100+120 mesh) CaO particles .... 15
Figure 1.5. Evolution with the number of carbonation/calcination cycles of the maximum
carbonation capacity of CaO from different authors .................................................................... 16
Figure 1.6. TG curves for Li SiO and Li ZrO obtained at 500 ˚C in 20% CO ........................ 17
4 4 2 3 2
Figure 1.7. Schematic illustration of carbonation mechanism on pure and modified Li ZrO .... 18
2 3
Figure 2.1. X-ray diffraction patterns of Na CO -promoted MgO absorbent (a) after filtration; (b)
2 3
after drying; (c) after activation .................................................................................................... 35
Figure 2.2. SEM image of Na CO -promoted MgO absorbent .................................................... 36
2 3
Figure 2.3. First 3 cycles of (a) TSA (b) PSA CO absorption tests of Na CO -promoted MgO
2 2 3
absorbent, ramping up in N ......................................................................................................... 37
2
Figure 2.4. 10-cycle TSA and PSA CO absorption capacity comparison for Na CO -promoted
2 2 3
MgO absorbent.............................................................................................................................. 38
Figure 2.5. In situ X-ray diffraction patterns of Na CO -promoted MgO absorbent during TSA
2 3
(380/470 °C) absorption cycles, ramping up in N ....................................................................... 39
2
Figure 2.6. The illustration of the conversion of compounds involved in absorption and
desorption processes ..................................................................................................................... 40
Figure 2.7. The thermodynamic properties of the reactions studied in this paper calculated from
HSC Chemistry database and ab-initio thermodynamic calculation: (a) enthalpy change versus
temperatures; (b) Gibbs free energy change versus temperatures ................................................ 41
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