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Exploring biochar from gas-pressurized torrefaction for solid biofuel production is of great concern for the process performance assessment. This study conducts several relative indexes of biochars produced from gas-pressurized and conventional torrefaction, identifying the merits of the former. The results suggest that a higher pressure is more feasible for reducing biochar volume and enhancing energy efficiency. The results of the relative higher heating value (1.01–1.07) and relative enhancement factor (1.00–1.06) indicate that gas-pressurized torrefaction is more efficient for energy density improvement. Furthermore, gas-pressurized torrefaction dramatically facilitates biochar grindability, with the relative Hardgrove grindability index value ranging from 1.20 to 2.40. The proximate and elemental analysis results suggest that a higher pressure facilitates biochar carbonization and deoxygenation with a higher carbonized component. The energy consumption, efficiency, and expense calculation results imply that gas-pressurized torrefaction is more energy-efficient and cost-saving, with the relative upgrading energy index range of 1.00–1.12. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Gas-pressurized torrefaction is more efficient for HHV improvement. • Gas-pressurized torrefaction causes a higher carbonization degree. • The gas-pressurized torrefied biochar yield is 48.59–80.13%. • The carbonization index of gas-pressurized conditions is 1.06–1.48. • Gas-pressurized torrefaction is more economic and environmentally friendly. Torrefaction is a feasible approach to produce biochar. Several new torrefaction methods have been conducted for biochar production. Among them, gas-pressurized torrefaction is considered a new technology to produce solid biofuel with better fuel performance, without carrier gas, and higher deoxygenation. This study conducts a comparative analysis between conventional torrefaction and gas-pressurized torrefaction to identify the potential applications of the latter. The results indicate that increasing reaction pressure can achieve biochar volume reduction and HHV improvement, thus leading to a higher energy density. The conventional torrefied biochar yield is in the range of 54.13–90.11%, and it is 48.59–80.13% for gas-pressurized one. Moreover, a more extensive surface area, higher carbonization degree, and more stable pyrolysis characteristics are obtained compared to conventional torrefaction. The carbonization index values of conventional and gas-pressurized conditions are 1.04–1.42 and 1.06–1.48. The correlation analysis results suggest that biochar grindability is highly correlated to carbonaceous properties, regardless of torrefaction methods. The expense and environmental impact analysis indicates that gas-pressurized torrefaction possesses lower cost and environmental pollution potential for biochar production. [ABSTRACT FROM AUTHOR]

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]

• Oxidative torrefied microalgae can be upgraded as lignite. • The enhanced hydrophobicity is in-depth investigated. • The transformation of carbide carbon to elemental carbon is critically determined. • Decomposition and aromatization mechanisms of microalgae oxidative torrefaction are analyzed. Microalgae are a promising feedstock for carbon-neutral biofuel production due to their superior cellular composition. Alternatively, oxidative torrefaction has been recognized as a potential thermochemical technique for microalgal solid biofuel upgrading. Herein, by using microalga N. oceanica as a feedstock, several characterizations are adopted for evaluating the potential of oxidative torrefaction towards microalgal solid biofuel production. The oxidatively torrefied microalgae can be upgraded as lignite. After in-depth analysis, significant change in the surface microstructure of oxidatively torrefied microalgae is largely changed (via wrinkle and fragmentation) The hydrophobicity, thermal decomposition, thermal stability, and aromatization of oxidatively torrefied microalgae can be largely enhanced as the oxidative torrefaction severity increase. With the increasing torrefaction temperature, the hydrophobicity of oxidative torrefied microalgae gradually improved. The decomposition of C-2/3/5, and −OCH 3 , the CO bonds of CH 3 CO-, and the aromatization occurs via oxidative torrefaction according to the NMR analysis. For XPS analysis, torrefaction operation significantly decreases the carbide carbon and enhances the graphitization. As a result, the thermal stability of oxidatively torrefied microalgae is improved. Conclusively, the information obtained in this study can provide insights into the evaluation of oxidative torrefaction performance and fuel properties of microalgal solid biofuel, which may help accelerate the advancement of oxidative torrefaction industrialization. [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]

Biochar is widely used to remove organic pollutants from the environment. Several studies have focused on pollutant removal via biochar adsorption. However, research on the subsequent processing of pollutant-adsorbed biochar is lacking. This study explored the potential of biochar for the adsorption of an aquatic organic pollutant (tetracycline) and its subsequent use as a solid biofuel. These results suggest that corn straw-derived biochar (torrefaction and pyrolysis) is suitable for two-stage utilization to achieve bioresource valorization for environmental sustainability. Tetracycline-adsorbed biochar, particularly biochar pyrolyzed at 600 °C, is suitable for use as a biofuel. The biochar produced via torrefaction (300 °C) and pyrolysis (600 °C) is the optimal choice, with surface area, contact angle, graphitization degree, calorific value, enhancement factor, and upgrading energy index values of 172.48 m2/g, 120.4°, 3.87, 26.983 MJ/kg, 1.58, and 33.72, respectively. This is supported by the results of expense calculation, comprehensive performance analysis, and life-cycle assessment. Overall, the biochar produced in this study is suitable for organic pollutant removal and as solid biofuel; thus, it can be used to realize waste utilization for environmental sustainability. [Display omitted] • Two-stage treatment enhances the adsorption capacity and HHV by 20% and 11.42%. • Pyrolysis process is conducive for biochar HHV and adsorption capacity enhancement. • The HHV of tetracycline-adsorbed biochar ranges at 21.55–26.98 MJ/kg. • Expense calculation and LCA analysis are adopted for biofuel property evaluation. • The optimal solution for tetracycline removal and solid biofuel usage is provided. [ABSTRACT FROM AUTHOR]

[Display omitted] • The multistage utilization of P-doped biochar from soybean straw is developed. • The maximum sulfadiazine adsorption amount of P-doped biochar is 252.24 mg·g−1. • The HHV of P-doped biochar is 24 MJ.kg -1, which can be a coal alternative. • P-doped biochar ash is rich in K and P for fertilization application. • The optimal choice of P-doped biochar acquires comprehensive analysis. Biochar is of great importance to realizing solid biowastes reduction and environmental remediation. Modifying biochar for better performance is also of great concern to achieve property improvement. P-doped biochar from soybean straw is prepared for multistage utilization to realize water pollutant removal and biofuel usage. The results suggest that the prepared biochar is adequate for sulfadiazine adsorption and has stable performance under coexisting ions and aquatic pH. Furthermore, the higher heating value of the biochar is close to coal and thus can be an alternative to fossil fuel. The maximum sulfadiazine adsorption amount of P-doped biochar is 252.24 mg·g-1, and the P-doped biochar HHV is 24 MJ·kg−1 which can be an alternative to coal. The greenhouse gas and pollutant emission potential are also considered to explore the environmental impact of P-doped biochar production and usage. Overall, the optimal ratio of soybean straw: K 3 PO 4 is 3:1. [ABSTRACT FROM AUTHOR]

[Display omitted] • Oxidative torrefaction is more efficient for upgrading biochar. • The maximum relative mass loss of oxidatively torrefied biochar is much higher. • The relative elemental C content of oxidatively torrefied biochar is higher. • Increasing temperature gradually decreases biochar's carbide carbon proportion. • Strong linear distributions between TSI versus elemental C and O exhibit. This study performs the comparative advantage analysis of oxidative torrefaction of corn stalks to investigate the advantages of oxidative torrefaction for biochar fuel property upgrading. The obtained results indicate that oxidative torrefaction is more efficient in realizing mass loss and energy density improvement, as well as elemental carbon accumulation and surface functional groups removal, and thus leads to a better fuel property. The maximum values of relative mass loss, higher heating value, enhancement factor, and energy yield are 3.00, 1.10, 1.03, and 0.87, respectively. The relative elemental carbon, hydrogen, and oxygen content ranges are 1.30–3.10, 1.50–3.30, and 2.00–6.80, respectively. In addition, an excellent linear distribution is obtained between the comprehensive pyrolysis index and torrefaction severity index, with elemental carbon and oxygen component variation stemming from pyrolysis performance correlating to the elemental component and valance. [ABSTRACT FROM AUTHOR]

Oxidative torrefaction is a promising way for biomass upgrading and solid biofuel production. Alkali metals are considered to be efficient activators for enhancing biofuel upgrading during the thermal reaction process. Herein, the microalga Nannochloropsis Oceanica is selected as the feedstock for assessing potassium carbonate activated effect on solid biofuel production through oxidative torrefaction. The potential of potassium carbonate on microalgal biofuel properties upgrading is deeply explored. SEM observation and BET analysis show that torrefied microalgae can be transformed from a spherical structure with wrinkles to smaller particles with larger surface areas and higher total pore volumes, implying that potassium carbonate is a promising porogen. Moreover, potassium carbonate can significantly change the DTG curve at the temperatures of 250 °C and 300 °C from one peak to two peaks, inferring that the activated effect of potassium carbonate occurs on the torrefied microalgae. 13C NMR analysis reveals that the microalgal components significantly change as the torrefaction severity increases, with the decomposition of carbohydrate and protein components. When the potassium carbonate ratio increases from 0:1 to 1:1, the graphitization degree increase from 3.065 to 1.262, along with the increase in the HHV of solid biofuel from 25.024 MJ kg−1 to 31.890 MJ kg−1. In total, this study has comprehensively revealed the activated effect of potassium carbonate on improving the properties of microalgal solid biofuel. [Display omitted] • The activated effect of potassium carbonate on torrefied microalgae is investigated. • K 2 CO 3 intensifies the surface area and total pore volume by 300% and 50%, respectively. • Pyrolysis characteristics of torrefied microalgae are markedly affected by K 2 CO 3. • K 2 CO 3 ameliorates the microalga's hydrophilicity with the contact angle from 112.2° to 136.7°. • The HHV of torrefied microalgae is enhanced by 27.44% after the activation of K 2 CO 3. [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]