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Identification and monitoring of byproducts generated from a MEA absorbent during post-combustion CO2 capture (PCC) from brown coal flue gases
thesisposted on 22.02.2017, 02:00 by Reynolds, Alicia Jayne
During post-combustion capture (PCC) of CO2, amine based absorbents undergo undesirable reactions which increase human and environmental impacts (due to emission and disposal of amine absorbents and degradation products), as well as operational and maintenance costs (due to amine consumption and corrosion). This thesis describes: (a) the degradation of a 30% (w/w) aqueous MEA absorbent during a six month campaign at CSIRO’s PCC pilot plant located at AGL’s Loy Yang brown coal-fired power station; (b) the chemical and physicochemical factors that are likely to affect the amine degradation; and (c) provides recommendations for technologies and strategies for monitoring and managing amine condition. The aims and objectives of this thesis are defined in chapter 1. The samples of 30% (w/w) aqueous MEA absorbent from a CSIRO PCC pilot plant are crucial for the research presented in this thesis, and for this reason, the background of the sample set is also described in chapter 1. Chapters 2-7 contain publications that have been published in or submitted to respected peer-reviewed journals. Chapter 2 contains a literature review that was published in Environmental Science and Technology during 2012. This literature review considers reasons for the consumption of amine during PCC (including oxidative degradation and carbamate polymerisation), other aspects of amine management (including fly ash, foaming, corrosion and amine reclamation), and environmental management (including nitrosation, atmospheric emissions, waste management). This publication is followed by a brief review of the relevant literature that has been published since 2012. Chapter 3 describes the development, validation and use of a GC-FID method for quantification of MEA concentrations in the samples from a CSIRO PCC pilot plant. The MEA concentrations measured using GC-FID were statistically equivalent to the MEA concentrations measured by acid-base titration, which is the industry standard method. Importantly, this GC-FID method could be used to measure the concentrations of individual species in blended amine absorbents. Chapter 4 uses spectroscopic and elemental methods to: characterise the changes in the organic structure and physiochemical environment of the MEA absorbent; and to evaluate potential methods for on- or at-line monitoring of MEA degradation during PCC. UV-Vis was identified as an effective on- or at-line technique for monitoring the production of N-(2-hydroxyethyl)imidazole (HEI), an oxidative MEA degradation product. Chapter 5 investigates the processes responsible for the accumulation of inorganic anions, cations and metals in the MEA absorbent. The results obtained in this chapter demonstrate that fly ash was the most significant source of cations and indicates that oxidative degradation was a more significant source of heat-stable salts (HSS) than SOx and NOx from the flue gas. A potential change in corrosion mechanisms was identified from the change in ratios of iron, chromium, manganese and nickel concentrations and a rapid increase in the iron concentration was measured after the tenth sampling event. To enable the quantification of organic degradation products other than organic acids, two sample preparation methods are described and validated in chapter 6. The cation exchange, derivatisation and GC-MS method was used to investigate the changes in the concentrations of organic degradation products during a pilot scale PCC campaign. These changes are discussed in chapter 7and confirm that HEI is a suitable marker for oxidative degradation of MEA. The results in chapter 7 also suggest that the difference in ratios of organic acid concentrations at different PCC pilot plants may be due to the influence of operating parameters on the reaction pathways. This chapter also identifies coinciding discontinuities or disturbances in (a) the concentration profiles of all organic degradation products excluding the organic acids; and (b) the ratio of transition metal concentrations and increase in iron concentration. Two possibly related explanations provided for these changes are: (a) a decrease in the alkalinity due to the accumulation of HSS; and (b) the oxidation of residual aldehydes and/or sulphite during extended periods of absorbent storage prior to the last three sampling events. Overall, the information presented in this thesis provides valuable insight into: (a) the rates of different amine degradation pathways; (b) accumulation rates of flue gas and corrosion byproducts during pilot-scale PCC; and (c) enabled the identification of oxidative degradation of MEA and accumulation of iron due to corrosion as the two most significant issues for managing amine condition during PCC.