Fundamental aspects of solvent management for carbon capture and storage

The increased awareness of climate change has amplified interest in the development of
approaches to mitigate greenhouse gas emissions, such as carbon capture and storage (CCS).
While it goes without saying that energy switching to renewable sources is the key to achieving
a sustainable future, it is not a viable option in a short timeframe. Therefore, the challenge for
the foreseeable future is the development of solutions such as CCS to allow the continued use of
current technology in sectors such as energy and manufacturing whilst limiting greenhouse gas
emissions.

Post-combustion carbon dioxide capture via amine absorption-stripping is an evolving strategy
towards the mitigation of CO2 emissions, and aqueous amine solutions are considered the most
mature and industrially developed technology for this purpose. However a significant issue
hindering large deployment is the economic and operational challenges originating from the
chemical instability of amine-based solvents. Although in recent years several solvents have
received more interest, monoethanolamine (MEA) is the benchmark solvent used and tested at a
commercial scale for CO2 capture due to its high absorption rate and capacity to capture CO2.
However, degradation mechanisms are not yet fully understood despite studies being conducted
for more than two decades.

Initial work focussed on the development of a protocol for studying the species of degradation
products formed from MEA. Previously, the quantification of degradation products has been
characterised using chromatography-MS methods or 1D NMR spectroscopy, however severe
signal overlap in 1D 1H NMR spectra of degraded solvent samples hinders accurate
quantification. This work employs the application of 2D 1H-13C HSQC quantitative NMR
spectroscopy, which leads to the dispersion of peaks along the 13C dimension to considerably
reduce signal overlap. This improved methodology for monitoring could help to better predict
and monitor degradation and thus help design better mitigation technologies.

Chapter 3 describes the impact of N-(2-hydroxyethyl)imidazole-N-oxide (HEINO), a newly
discovered degradation product from MEA, on the laboratory scale degradation using 30% w.t.
MEA in the presence of varying metallic catalysts under thermal conditions, where HEINO
could act as a possible oxidising agent. These experiments confirmed that the introduction of
HEINO to thermal degradation experiments caused oxidation of MEA, and suggested that
HEINO could act as an oxygen shuttle between the absorber and stripper, and oxidise MEA
under harsher conditions than previously investigated. These findings strengthen the idea that
oxidation of MEA is not necessarily limited to the absorber section of a capture plant. While
degradation mechanisms are complex and not yet fully understood, this work adds data to the
current knowledge base and points in the direction of which compounds seem to have
significant impact and ‘high risk’ for the degradation of MEA.

Chapter 4 describes possible routes for the formation of N-(2-hydroxyethyl)imidazole (HEI) and
HEINO under CCS conditions with rationalised MEA degradation products. Previous literature
has proposed a route for the formation of HEI from glyoxal, formaldehyde and MEA. This work
has shown that the oxidation of HEI to HEINO is unlikely to occur under oxidative conditions
that are typically seen within a CCS facility, and therefore it is likely that HEINO is formed
from the cyclisation of suitable fragments. The results of the study indicate that the formation of
N-oxidation products of MEA are possible under conditions relative to CCS, which is a
relatively untouched area of research within the CCS community. The study found that the
formation of HEINO was possible via the degradation of N-hydroxymonoethanolamine under
thermal conditions relatively quickly, suggesting that this could be a key intermediate in the
route to HEINO. The results from this study support the idea that the formation of an imine
intermediate is a key reaction in the formation of HEINO, however a further open chain
intermediate was not detected. Further research is needed to better understand the complexity of
the formation of HEI and HEINO under CCS conditions.

Chapter 5 describes the use and fate of phenolic antioxidants within physical solvents such as
dimethylated polyethylene glycols at a pilot-scale. It was found that BHT oxidised to a
cyclohexadienone derivative via a base-catalysed oxidation, limiting its use as a radical-trapping
antioxidant. Analysis of a selection of other phenolic antioxidants showed limited oxidation,
and could provide viable alternatives for protection against autoxidation of capture solvents.
Finally, an environmental impact assessment of various solvents was completed using lifecycle
assessment. The evaluation of the environmental performance from a holistic perspective is
important to assess the extent of the implementation of CCS will change environmental impacts
before large-scale implementation of a carbon capture solvent. This chapter discusses the routes
for the formation of solvents and impact of degradation. The comparison of non-amine based
solvents such as potassium acetate showed a materialistic increase in environmental
performance across impact categories when compared to solvents such as ethylenediamine
within both manufacturing and solvent lifetime.