Nannochloropsis Hydrothermal Carbonization for Hydrochar Production

Abstract

Hydrothermal carbonization (HTC) is a hydrothermal treatment that converts various types of high moisture biomass into hydrochar. Several operational parameters must be considered, such as temperature, reaction time, biomass/water ratio and initial pressure, which ultimately affect the performance of the process. One of the main advantages of HTC is the possibility to use wet feedstock, which promotes the decrease of the operational energy cost. In this work, Nannochloropsis microalgae was tested for the production of hydrochar. A Batch reactor was used, initially pressurized with 20 bars of nitrogen. A 2k factorial design was used to plan the experiments, with three different factors tested at two different levels: temperature (160-190 ºC), reaction time (30-45 min) and biomass/water ratio (1/10-1/15 w/w). Four different products were obtained: solid fraction (hydrochar), aqueous and gas phases, and an insignificant amount of bio-oil. The gas collected was analyzed by the gas chromatograph to determine the compounds and the mass produced. All the other products were separated by vacuum filtration and decantation, and then weighed to calculate the corresponding product yield. The results indicate that the HTC process is suitable for processing biomass with high moisture content. The reaction temperature was a key parameter that significantly decreased the hydrochar yield, resulting in a higher production of the other products. The effects of reaction time on the hydrochar yield depend on the biomass/water ratio. Overall, HTC process produced a carbon-rich solid and a small amount of gaseous products mainly composed of carbon dioxide.