Methane Production from Microalgae via Continuous Catalytic Supercritical Water Gasification: Development of Catalysts and Sulfur Removal Techniques

This doctoral thesis aims at demonstrating the technical feasibility of methane production from microalgae via continuous catalytic supercritical water gasification (CSCWG). This work focuses on three different research approaches: (i) assessment and improvement of the catalytic performance of Ru/C catalysts, (ii) optimization of the salt separation, and (iii) continuous removal of sulfur. With all the acquired knowledge, a new process demonstration unit so called KONTI-C with a pumping capacity of 1-2 kg h-1 was built in the frame of the SunCHem project. The catalytic performance of Ru/C catalysts was assessed with isopropanol (IPA) at 450 °C and 30 MPa in a fixed-bed plug flow reactor. Ru/C was able to convert efficiently IPA to a CH4-rich gas (65 vol %) over a period of 96 h at a relatively low weight hourly space velocity (WHSVgRu). By working at a higher WHSVgRu, a deactivation of the catalyst was observed. The decomposition of IPA over the carbon surface to coke that has progressively covered the Ru nanoparticles, was responsible for the loss of the catalytic activity. The Ru loading was found to be a crucial parameter for the improvement of the coking resistance. As a result, the stability of our standard catalyst was enhanced by increasing the Ru loading from 2 wt % to 5 wt %. When assessing the effect of some synthesis factors it was found that a higher Ru dispersion, the use of acetone during the catalyst impregnation, and the choice of a chloride free salt precursor (Ru(NO)(NO3)3) were needed for the improvement of the catalytic activity. Whereas the presence of carboxylic groups on the carbon support during the catalyst preparation did not improve the performance. The catalyst prepared with Ru(NO)(NO3)3 exhibited a better catalytic activity and stability than our standard commercial catalyst. By comparing the performance of a Ru/C catalyst with other Ru catalysts supported on metal oxides (Ru/TiO2, Ru/ZrO2, and Ru/Al2O3), Ru/C was found to be the most stable and active catalyst. Hence, Ru/C is the most suitable catalyst for CSCWG. The design of the salt separator was improved by modifying the configuration of the feed entrance (feeding from the bottom of the salt separator). As a result, the salts and the inorganic sulfur (SO42-) were efficiently removed from the reactor effluent when processed with a model salt solution (Na2SO4/K2SO4 in 10 wt % IPA). A commercial ZnO adsorbent showed a high mechanical stability in supercritical water (SCW) (400 °C, 30 MPa). The adsorbent was able to adsorb sulfur (S2-) when processed with a model sulfur solution (Na2S¿9H2O) at 400 °C and 30 MPa. Characterization of the spent ZnO adsorbent confirmed that sulfur was adsorbed on ZnO to form ZnS. The improved commercial catalyst (5% Ru/CBASF), the new design of the salt separator, and the commercial ZnO adsorbent were implemented in the development of KONTI-C. As a result, microalgae (Chlorella vulgaris) were successfully gasified (400 °C, 28 MPa) to a CH4-rich gas (55-60 vol %) over a period of 55 h during the gasification campaign performed in Wädenswil (CH). The low total organic carbon in the reactor effluent recorded over that period showed the good catalytic performance of the 5% Ru/CBASF. A brine effluent rich in nutrients (N, K, S, P, and Na) was obtained with the salt separator. The promising results obtained during the gasification campaign allowed to demonstrate the technical feasibility of continuous CSCWG of microalgae.

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