Catalytic hydrothermal gasification of microalgae : start-up of a new process demonstration unit

Abstract

A new PDU (Process Demonstration Unit) for continuous catalytic hydrothermal gasification of wet biomass was developed. The plant, which had a capacity of 1 kg/h, consisted of six main sections: a feeding section, a salt separator for hydrothermal liquefaction and salt separation, a salt removal section, a catalytic reactor for hydrothermal gasification, a pressure control section and a phase separator. This work aimed at assessing and improving the stability of the new PDU for the production of methane. The assessment was conducted by an experimental approach including salt separation tests with model solutions and liquefaction- and gasification tests of microalgae (Phaeodactylum tricornutum). After implementation of salt separation- and liquefaction tests, hydrothermal gasification tests of microalgae were carried out. The lower section of the reactor was filled with ZnO for sulfur adsorption, whereas the higher section was filled with the catalyst (5%-Ru/C) for subsequent gasification. The working pressure was 280 bar and the temperature inside the reactor was in the range of 375-410 °C. No catalyst deactivation could be detected while processing microalgae for 6.4 h. The TOC content of the reactor effluent stayed below 1000mg/L and a gas containing mostly methane (57 vol%) and carbon dioxide (36 vol%) could be produced. The carbon content in the brine effluent was low, too (1300-1500 mg/kg), but tar could be found on the filer. The nitrogen concentration of brine- and reactor effluent reached values close to the nitrogen concentration of the feed. The pH values of brine- and reactor effluent were 5.94 and 8.44, respectively. The presence of ammonia in the reactor effluent could be expected. ICP analysis showed that K, Na, P and S could be separated and concentrated in the brine effluent very efficiently. An elemental separation efficiency in the following order could be identified: K > Na > P > S. Although none of those elements could be found in the reactor effluent, its electrical conductivity reached very high values. In summary, the overall goal of starting up the new PDU and reaching a stable operating point for a continuous production of bio-methane from microalgae could be achieved. Nevertheless, a number of unsolved issues could be identified leaving room for process optimization.