Carbon dioxide capture from humid exhaust air and flue gases

Abstract

Carbon capture and storage (CCS), as well as utilisation of carbon dioxide (CO2), has become an increasingly important research area in order to achieve the goals set by the European Commission for the EU to become climate neutral by 2050. The purpose of this work was to evaluate the most feasible carbon capture technologies for both humid exhaust air and flue gas. In the literature study, state-of-the-art carbon capture technologies were compared in terms of energy requirements, capture costs, capture rates as well as the technology readiness level (TRL) of each technology. In addition, the focus was set on comparing the equipment size and the effect of humidity. CO2 capture processes by zeolites, calcium looping and membrane gas separation were identified as the most profitable technologies for flue gases, whereas conventional absorption using amine mixtures and absorption utilising membrane contactors were also identified as feasible technologies. For humid exhaust air, amine functionalised adsorbents and metal-organic-frameworks (MOFs) were revealed to be the least energy demanding technologies. Furthermore, absorption using hydroxides, such as sodium hydroxide (NaOH) and potassium hydroxide (KOH), was identified as possible, however, this technology was shown to be very energy consuming. In the case of using hydroxides for capturing carbon dioxide from air, KOH was shown to be less expensive than NaOH. In terms of equipment size, absorption was revealed to require less area occupied per unit compared to adsorption. Moreover, transport, storage and utilisation of CO2 was studied, as well as methods for measuring CO2 in the gas and aqueous phase.

As relevant literature lacks easy-to-implement equations for predicting CO2 absorption efficiency as a function of CO2 concentration in the gas phase and hydroxide (OH-) concentration of alkaline solutions at low OH- concentrations, the experimental part of this thesis focused on studying the effect of respective parameter on the CO2 absorption efficiency. The experiments were done in a small-scale bubble column reactor, where the CO2 outlet gas concentration was measured with a gas analyser. Moreover, a semi-empirical mathematical model representing the experimental data was developed. The CO2 removal efficiency was shown to increase with lower CO2 concentration in the gas phase and higher pH of the alkaline solution. The developed semi-empirical mathematical model was proved to fit the experimental data reasonably well and can be used to predict CO2 absorption as a function of CO2 concentration in the gas phase and OH- concentration in the alkaline solution at low OH- concentrations. However, when implementing the model, the difference in mass transfer coefficient and interfacial area of both the small-scale bubble column reactor and the contactor for which the model will be used should be considered.