Environmental impacts and aspects of absorbents used for CO2 capture
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If the CO2 capture processes based on absorption are to be employed on a global level, there will be a large scale use of solvents. The absorbents used should have favourable characteristics for energy requirement, reaction rate and stability at process conditions - such as thermal and chemical stability. However, it is also important that the chemicals used are benign and have low or no detrimental environmental effects, as emissions may occur through the cleaned exhaust gas, as degraded solvent and as accidental spills. Potential solvents should be tested with regard to possible detrimental environmental effects and to determine acceptable emission levels for the process. One of the main objectives of this work was to find absorbent systems used in CO2 capture which were acceptable from an environmental viewpoint, while still being stable with regards to thermal degradation with and without CO2 under process conditions. The main points for attaining this goal was to assess the environmental impact of solvents on the marine environment according to the Norwegian Activities Regulations (PSA, 2010) and the OSPAR convention, and to study the thermal degradation of the solvents with and without CO2 at process conditions as well as attain a detailed understanding of the degradation reactions with CO2. A total of 43 absorbent systems – 42 amines and one organo-sulfur compound – have been evaluated for the environmental impact on a marine environment, testing ecotoxicity and biodegradability with standardised tests. The selection of compounds was based on known compounds used in CO2 capture such as MEA, DEA, MDEA and AMP. However novel solvents were also tested, and some compounds were purely added to test the correlation between chemical structure and the biodegradation, ecotoxicity and thermal degradation. Of the 43 compounds tested for biodegradation and ecotoxicity, 30 were in addition tested for thermal degradation with and without CO2. The compound loss was determined as well as degradation products for most of the compounds tested. The environmental characteristics for MEA were good as the compound was shown to be non-toxic and biodegradable; however for thermal degradation with CO2 MEA had relatively high degradation. In general the toxicity of the compounds tested were not a problem, however for some compounds the toxicity needs to be taken into account. While there does not seem to be a strong correlation between the biodegradability and the thermal degradation, the compounds stable at process conditions – cyclic amine such as PZ, alkanolamines with steric hindrance such as AMP and tertiary alkanolamines such as MDEA – are also the most likely to be resilient with regard to biodegradation. However, natural compounds in these groups may be stable at process conditions while still being biodegradable. Four compounds were found to have favourable characteristics based on the performed tests: AB, MIPA, DMMEA and Alanine. None of these belong to the same group of amines and all of them are naturally occuring compounds. The degradation products identified for the thermal degradation with CO2 were in line with the carbamate polymerisation mechanism for most of the compounds tested. The degradation went through the corresponding carbamates forming cyclic compounds such as oxazolidinone and imidazolidinone and piperazines, as well as polymeric compounds such as dimers and trimers. As a part of attaining a detailed understanding of the degradation reactions with CO2, the first step of the degradation mechanism for MEA was investigated using molecular modeling. Another goal of this work was to check for correlation between chemical structure, toxicity and degradation as well as develop models which could be used as screening tool for potential new solvents. This was accomplished using PLS-R, and QSAR screening models were developed for biodegradation, ecotoxicity and thermal degradation with CO2. The models developed for ecotoxicity and biodegradability can act as a first step screening model with some reservations. However, there is room for improvement and the models should be developed further. For thermal degration with CO2 the models developed can act as a screening model based on the data available now. However the models should be tested on a separate validation set before they are used.