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A comprehensive comparison is conducted to explore the conversion performance of torrefaction and hydrothermal carbonization of high-moisture microalga for producing solid biofuel with low expense and environmental impact. The results suggest that when their solid yields are close, the hydrochar has higher HHV (higher heating value) (21.10–25.39 MJ kg−1) and energy yield (72.48–92.29%) than the biochar. The graphitization degrees of the biochar and hydrochar are 1.298–3.107 and 2.193–3.049, respectively. Considering fuel grade, the biochar is in biomass and peat zones, but the hydrochar is only in the biomass zone. The biochar's elemental valence transformation and graphitization degree are better than hydrochar's. The pyrolysis and combustion characteristics of the hydrochar are better than those of the biochar when considering the thermodegradation data, leading to a lower expense and environmental impact. The results are conducive to efficiently figuring out a solution with a comparative advantage for microalgal thermochemical conversion. The hydrothermal carbonization conditions at 160 °C for 2 h are the best operation for microalgal solid biofuel production. [Display omitted] [ABSTRACT FROM AUTHOR]

Microalgae are recognized as promising feedstocks for biofuel production and have been intensively studied. However, a systematic investigation of the relationship between cellular components and fuel properties in microalgae is currently absent. This review introduces recent advances in microalgal torrefaction and critically evaluates the properties of torrefied products from the viewpoint of molecular conversion. We also critically review how to regulate biosynthesis to better modulate microalgal metabolism and how cellular components impact fuel properties. Process optimization to improve solid fuel properties using different torrefaction technologies are furthermore comprehensively compared and summarized. This review provides new insights into developing innovative microalgal solid biofuels and increasing their commercial viability. The proportions of microalgal components affect solid biofuel properties. Synthetic biology is a promising approach to regulate microalgal components. Torrefied microalgae have excellent fuel properties with superior storage and transportation characteristics. Several torrefaction technologies for microalgal solid biofuel upgrading are currently being evaluated. [ABSTRACT FROM AUTHOR]

To evaluate the potential of microwave heating for biomass torrefaction, the torrefaction performances and energy utilization of coffee grounds and microalga residue, under conventional and microwave heating were investigated and compared with each other. For the two biomass samples, the dehydrogenation of the coffee grounds was more sensitive to torrefaction severity, whereas the microalga residue consumed more energy under the same torrefaction conditions. Microwave heating under lower torrefaction severity had a higher energy efficiency. As regard to the lower solid yields or higher torrefaction severity, the energy efficiency of microwave heating was close to that of conventional heating, irrespective of the feedstocks. This revealed the comparable energy consumption state between the two heating modes. Accordingly, it is concluded that microwave torrefaction is more efficient for biomass upgrading and densification than conventional torrefaction. [ABSTRACT FROM AUTHOR]

Biomass torrefaction is a promising method for carbon-neutral solid biofuel production. The economic feasibility and environmental pollution potential of the torrefaction operation are of great concern to developing a sustainable energy system. In this study, spent coffee grounds are selected as the feedstocks for comparing the economic feasibility of different torrefaction methods and evaluating the environmental pollution potential of conventional torrefaction and microwave torrefaction. The result shows that microwave torrefaction possesses a more economically efficient approach for biomass upgrading, as a result of lower energy input and carrier gas consumption. For environmental impact assessment, several different indicators are adopted for evaluating the torrefaction process, which is based on openLCA software of CML 2001 method. The life cycle assessment results indicate that the environmental impact of microwave torrefaction is lower than conventional torrefaction, especially when torrefied at light conditions. Thus, microwave light torrefaction is the optimal condition for spent coffee grounds upgrading. However, the growth rate of the environmental impact of microwave torrefaction is greater than conventional torrefaction, including resource depletion and terrestrial ecotoxicity. Moreover, the environmental impact of the torrefaction process is strongly correlated to the torrefaction severity. [Display omitted] • Microwave light torrefaction is energy-saving for fuel upgrading. • Microwave torrefaction saves expenses of biochar production. • The eco-invent 3.4 databases of openLCA with the CML 2001 method are adopted. • Microwave torrefaction is more feasible than conventional torrefaction. • Microwave light torrefaction possesses a lower environmental impact. [ABSTRACT FROM AUTHOR]

Sludge dewatering is a matter of great concern to reduce the volume of sludge, stabilize its organic components, and achieve resource utilization. This study investigates sludge dewatering by microwave torrefaction along with the production of sludge solid biofuel at 480–800 W combined with durations of 5–25 min. Proximate analysis, calorific value analysis, thermogravimetric analysis, and scanning electron microscopy observations are employed to evaluate the dewatering degree, fuel properties, and energy efficiency of the torrefaction process. The independent parallel reaction (IPR) model and particle swarm optimization (PSO) analysis are also adopted for sludge pyrolysis kinetics calculation. The results show that microwave torrefaction is efficient for sludge dewatering with a short duration. The produced sludge solid biofuel is similar to stone-like coal, and can be used for civil or industrial boilers after flotation or just co-firing with briquette. The ash content of sludge solid biofuel shows a declining trend and the surface characteristics change from smooth to rough and fluffy with increasing the torrefaction severity. The bio-oil is mainly composed of phenols, siloxanes, and cholesterol. In addition, hydrogen is detected in the torgas. Furthermore, it is found that lower torrefaction power with a shorter duration yields a higher energy efficiency of the torrefaction process. [Display omitted] • Sludge dewatering and solid biofuel production are implemented simultaneously. • Sludge biofuel obtained by microwave torrefaction is similar to stone-like coal. • Hydrogen is produced from the sludge dewatering process. • Phenols, siloxanes, and cholesterol are the main components of bio-oil. • Short torrefaction duration possesses better energy efficiency. [ABSTRACT FROM AUTHOR]