Water-lean solvents for biogas upgrading
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Biogas upgrading consists in the removal of CO2 from biogas, a process which produces a highly purified biomethane stream. As such, biogas upgrading has a double positive impact when considering climate change mitigation technologies: at once, it enables the utilization of a sustainable source of energy while it also possibilitates the sequestration and permanent storage of a parcel of CO2. The most mature technique for biogas upgrading is amine scrubbing. Although quite well researched, amine scrubbing faces a big challenge for its widespread implementation – namely, the energy consumption of this process is quite high, bringing inevitable efficiency losses for the biomethane production sector. Hence investigations on alternative solvents for utilization in amine scrubbing plants are still ongoing. Water-lean solvents are a class of amine absorbents which rely on the substitution of water for an organic compound as choice diluent for the amine solution. This substitution brings forth many promises. Some researchers claim that water-lean solvents might have an enhanced capacity for CO2 absorption, while others claim that such absorbents can be more easily regenerated. And yet, any comprehensive discussion on water-lean solvents as a unified category will be complicated by the fact that there are uncountable organic compounds to be used in lieu of water for solvent formulation. A quick glance at the available scientific literature reveals a panoply of alcohols, ethers, esters etc. which could be suitable for generating promising water-lean solvents. Conversely, the amount of published data regarding amine scrubbing with water-lean solvents in commercial (or even pilot-plant) applications is very meager. This thesis has carried out a comprehensive study on water-lean solvents by combining (1) an in-depth literature review, (2) a series of empirical studies to generate and evaluate new data on significative parameters regarding solvent performance, and (3) modelling and simulation assessments of water-lean solvent applications. The main point of this exercise has been to make sense of chaos: to understand a bit more what goes on inside a solvent when water is substituted by an organic diluent, and how this affects the CO2 capture process with regards to kinetics, solubility, heat of absorption, and reaction mechanisms.