Rheological properties of microalgae slurry under subcritical conditions for hydrothermal hydrolysis systems

Hydrothermal hydrolysis of algal biomass for biofuels production: A review

Hydrothermal hydrolysis is an energy-efficient and economical pretreatment technology to disrupt the algal cells and hydrolyze the intracellular compounds, thereby promoting the biofuels production of fermentation. However, complex reaction mechanisms, unpredictable rheological properties of algal slurry, and immature continuous reactors still constrain the commercialization of such a process. To systematically understand the existing status and lay a foundation for promoting the technology, the chemical mechanism of hydrothermal hydrolysis of algal biomass is elaborated in this paper, and the influences of temperature, residence time, total solid content, and pH, on the biomethane production of hydrolyzed algal biomass are summarized. Besides, a comprehensive overview of the rheological behavior of algal slurries is discussed at various operational factors. The recent advances in flow, heat and mass transfer model coupling with the generic kinetics model in continuous reactors and the application of energy-saving strategies for efficient algal biomass pretreatment are detailed reviewed.

Hydrothermal treatment of Chlorella sp.: Influence on biochemical methane potential, microbial function and biochemical metabolism

This study focused on the effect of hydrothermal treatment (HTT) on biochemical methane potential (BMP) of Chlorella sp. The BMP was in the range of 119.16-485.90 mLCH₄/gVS, and increased by 80.31%-210.16% after HTT, while reduced 23.94% at hydrothermal treatment severity (HTS) 5.21. The cell wall was more greatly disrupted with increasing HTS, accompanied with the increase of volatile fatty acids (VFAs) and fermentation inhibitors (5-HMF and more complex chemical compositions) recoveries. The reducing sugar yields were 0.94-3.65% and obtained its maximum at a retention time of 30 min. Illumina MiSeq sequencing clarified that, the phylum Chloroflexi with functions of hydrolysis and acidogenesis, decreased with increasing HTS. The family Methanosaetaceae belonging to acetoclastic methanogens, had an unexpected decrease at HTS 5.21. As the response, VFAs concentration was less than 1 g/L after biochemical metabolism, while high concentrations of VFAs and inhibitors at HTS 5.21 led to the poor performance.

Rheokinetics of microalgae slurry during hydrothermal pretreatment processes

Hydrothermal pretreatment is an efficient process for improving the productivity of biofuels from wet microalgae biomass. The rheological behavior of microalgae slurry is a significant parameter affecting the performance of hydrothermal pretreatment reactors. Herein, the dynamic rheological behavior of microalgae slurry during hydrothermal pretreatment was investigated for the first time. The results revealed that the insoluble organics released from microalgae cells was the main factor affecting the rheological behavior of microalgae slurry. The denaturation and hydrolysis of starch and protein in liquid phase at different temperature regions caused the increasing and decreasing of viscosity of the microalgae slurry, respectively. The rheokinetics equations were established based on four-parameter cross-linking rheokinetics equation to describe the variation of viscosity with reaction time in different temperature. The variation of the rheokinetics model parameters with temperature revealed that the temperature has an obviously positive influence on the hydrothermal pretreatment process of the microalgae slurry.

Enhancing Hydrogen Production from Chlorella sp. Biomass by Pre-Hydrolysis with Simultaneous Saccharification and Fermentation (PSSF)

Simultaneous saccharification and fermentation (SSF) and pre-hydrolysis with SSF (PSSF) were used to produce hydrogen from the biomass of Chlorella sp. SSF was conducted using an enzyme mixture consisting of 80 filter paper unit (FPU) g-biomass−1 of cellulase, 92 U g-biomass−1 of amylase, and 120 U g-biomass−1 of glucoamylase at 35 °C for 108 h. This yielded 170 mL-H2 g-volatile-solids−1 (VS), with a productivity of 1.6 mL-H2 g-VS−1 h−1. Pre-hydrolyzing the biomass at 50 °C for 12 h resulted in the production of 1.8 g/L of reducing sugars, leading to a hydrogen yield (HY) of 172 mL-H2 g-VS−1. Using PSSF, the fermentation time was shortened by 36 h in which a productivity of 2.4 mL-H2 g-VS−1 h−1 was attained. To the best of our knowledge, the present study is the first report on the use of SSF and PSSF for hydrogen production from microalgal biomass, and the HY obtained in the study is by far the highest yield reported. Our results indicate that PSSF is a promising process for hydrogen production from microalgal biomass.

Drag reduction and shear-induced cells migration behavior of microalgae slurry in tube flow

To optimize the designing of microalgae slurry pumping system and enhance the efficiency of microalgae products production, the flow characteristics of microalgae slurries (Chlorella pyrenoidosa) in tube flow were for the first time investigated combining experiments and numerical simulation. The drag reduction behavior of microalgae slurry in the fully developed laminar flow regime was studied. In addition, the transition Reynolds number of microalgae slurries from laminar flow to turbulent flow was about 1000-1300, which was similar to the expression of two-phase flow. To provide a further understanding of flow feature of microalgae slurries in tube, a two-phase mixture model was proposed by considering the heterogeneity of concentration due to the shear-induced microalgae cells migration behavior. Simulation results revealed that the heterogeneous distribution of concentration was affected by average velocity and volume fraction of microalgae slurries, significantly affecting the flow resistance and flow stability of microalgae slurry in the tube flow.