Daniil Salionov
The dependency on fossil fuels and their impact on the environment is a matter of great concern for the future sustainability of modern society. The development of the "green" technologies which utilize renewable energy sources is now under investigation. One of the promising solutions is the conversion of different biomass types to bio-crude, which could serve as a potential substitution for transportation fuel, as this sector contributes to 16% of total carbon dioxide emission. The "HyFlexFuel" project, supported by the Horizon 2020 program, aims to develop the next generation of transportation biofuels using the emerging technology of hydrothermal liquefaction (HTL) of organic wastes, phototrophic algae, and other advanced biomass feedstocks. The primary process during HTL is the depolymerization of the feed using near- or supercritical water (Tcr = 374 oC, pcr = 22.1 MPa). As a result, four main fractions are generated: a desirable bio-crude, an aqueous phase (HTL-AP), and a minor fraction of solid residue and gas. The efficient valorization of the by-product - HTL-AP, is needed to improve the sustainability of the HTL technology on the industrial level. Catalytic hydrothermal gasification (cHTG) is a promising technology for converting wastes or wet biomass feedstocks into renewable natural gas. Its performance strongly depends on the chemical composition of the feedstock and produced effluents. Those streams require new method development for the analysis of such complex mixtures. The cHTG process includes two main parts: a) salt separation, where inorganic components of the feed are precipitated and concentrated into the brine phase under supercritical water (SCW) conditions, and b) supercritical water gasification (SCWG), where remaining organics (desalinated stream) is converted to methane with the help of the catalyst. Former works were conducted on the behavior of water-salts mixtures under SCW conditions to optimize the salt separation step. However, during the conversion of a model or real biomass, a considerable amount of organic carbon was detected in the brine phase, resulting in the overall low gasification efficiency of the cHTG process. Until now, no information has been provided regarding the qualitative and quantitative chemical composition of the salt separation process streams. Also, the SCWG catalyst can rapidly undergo deactivation through different mechanisms, namely poisoning, coking, sintering, or leaching. A particular focus is set on coking as it is inevitable during the cHTG process. This doctoral thesis is motivated to better understand the chemical composition of the salt separator effluents and to provide a basis for the rational optimization of the process. Another aspect of the work is related to the investigation of the chemical composition of the SCWG process operating under high weight hourly space velocity to gain insights into the coke formation and to tailor an active and robust SCWG catalyst.
EPFL2023
