The extensive use of fossil fuels and the emission of greenhouse gases has resulted in severe issues such as global warming and climate change. The energy and transport sector generate a major share of greenhouse gases globally. Though renewable energy can supply some of the energy demands, fossil fuels are still considered as a primary source to balance the world energy requirements. CO2 is the major constituent of the greenhouse gases, carbon capture process (from the source of emission and from the air) has been proposed as a promising technology to alleviate the effects of global warming in the near future. Alongside, technologies have been developed to produce fuels or other useful chemicals from the captured CO2. This process is called carbon re-utilization. Zero Emission Fuels (ZEF B.V.) is a company in the Netherlands that is working towards developing a system that produces Methanol from CO2 and water captured from ambient air viz. Direct air capture (DAC).
In the ZEF DAC process, polyethylenimine (PEI) and tetraethylenepentamine (TEPA) are used as CO2 absorbents. These amines capture CO2 and water at ambient conditions (absorption phase). The CO2 and water is then stripped from the amine at elevated temperature (desorption phase) and the gases are used for methanol production. During the process of absorption and desorption, the amines undergo losses such as evaporation and degradation. This leads to a decrease in the efficiency of the DAC system over time. This thesis is focused
on determining the evaporation and CO2 induced degradation losses that might occur of PEI and TEPA during the direct air capture process. To determine the losses in PEI and TEPA, it is necessary to understand the behavior of amines in a cyclic absorption-desorption. PEI and TEPA were subjected to 30 cycles of alternative absorption and desorption processes. Regarding evaporation losses, both PEI and TEPA showed significant loss of mass during the desorption process (80 ∘C, 120 ∘C were the selected desorption temperatures). Theoretically, the partial pressure of CO2 and water absorbed by the amine reduces the evaporation rate of the mixture (amine+water+CO2). But, during the desorption process, as the CO2 and water are stripped out of the amine, the evaporation rate of the mixture increases. To, determine the maximum evaporation that could take place under a negligible partial pressure of CO2 and water, pure samples of PEI and TEPA were heated at 80 ∘C, 100 ∘C, 120 ∘C. Results showed that, in a long term operation, both PEI and TEPA might undergo evaporation losses (relatively, TEPA evaporates faster than PEI) and replacing the amine frequently would increase the cost of DAC process. Another important observation was that a significant mass of CO2 remained in the sample after every desorption cycle. PEI accumulated more CO2 than TEPA. This suggested that the process of desorption carried out during the experiment was inefficient. A hypothesis was proposed to explain the accumulation taking place in both the amines. Furthermore, Fourier Transform Infrared Spectroscopy (FTIR) was carried out on the samples subjected to cyclic experiments to identify the formation of degradation products. Results showed that during the cyclic absoprtion-desorption process, TEPA might have degraded when desorbed at 120∘C. PEI samples did not show any indications of degradation. Finally, future experiments are proposed to understand the degradation losses which might help in deciding the suitable amine for the DAC process.