Hydrothermal hydrolysis of algal biomass for biofuels production: A review

Improvement of Asterarcys quadricellulare biomass solubilization and subsequent biogas production via pretreatment approaches: structural changes and kinetic modeling evaluation

This study investigated the effect of enzymatic and hydrothermal pretreatment approaches on the solubilization of organic matter, structure, and biogas yield from microalgal biomass. The soluble chemical oxygen demand (sCOD) concentration increased by 1.21-3.30- and 5.54-6.60-fold compared to control by enzymatic and hydrothermal pretreatments respectively. The hydrothermal pretreatment affected the structural changes in the microalgal biomass markedly; nonetheless, increased enzymatic concentration also had a definite effect on it as determined by qualitative approaches like scanning electron microscopy and Fourier transform infrared spectroscopy. Also, the hydrothermal pretreatment (100 °C for 30 min) resulted in the highest biogas production potential (P) of 765.37 mLg^-1 VS at a maximum biogas production rate (R_m) of 22.66 mLg^-1 day^-1 with a very short lag phase (λ) of 0.07 days. The biogas production of pretreated microalgal biomass particularly at higher enzyme dose (20%, 24 h) and higher hydrothermal pretreatment temperature (120 °C, 30 min) showed a significant but weak correlation (R = 0.53) with sCOD, thus demonstrating that the less organic matter was used up for the biogas production. The modified Gompertz model explained the anaerobic digestion of microalgal biomass more accurately and had a better fit to the experimental data comparatively because of the low root mean square error (3.259-16.728), residual sum of squares (78.887-177.025), and Akaike's Information Criterion (38.605-62.853).

Critical review of biochemical pathways to transformation of waste and biomass into bioenergy

In recent years, biofuel or biogas have become the primary source of bio-energy, providing an alternative to conventionally used energy that can meet the growing energy demand for people all over the world while reducing greenhouse gas emissions. Enzyme hydrolysis in bioethanol production is a critical step in obtaining sugars fermented during the final fermentation process. More efficient enzymes are being researched to provide a more cost-effective technique during enzymatic hydrolysis. The exploitation of microbial catabolic biochemical reactions to produce electric energy can be used for complex renewable biomasses and organic wastes in microbial fuel cells. In hydrolysis methods, a variety of diverse enzyme strategies are used to promote efficient bioethanol production from various lignocellulosic biomasses like agricultural wastes, wood feedstocks, and sea algae. This paper investigates the most recent enzyme hydrolysis pathways, microbial fermentation, microbial fuel cells, and anaerobic digestion in the manufacture of bioethanol/bioenergy from lignocellulose biomass.

Co-pyrolysis technology for enhancing the functionality of sewage sludge biochar and immobilizing heavy metals

Sewage sludge (SS) is a frequent and challenging issue for countries with big populations, due to its massive output, significant hazard potential, and challenging resource utilization. Pyrolysis can simultaneously realize the reduction, harmlessness and recycling of SS. Co-pyrolysis offers a wide range of potential in terms of increasing product quality and immobilizing heavy metals (HMs), thanks to its capacity to use additives to address the mismatch between SS characteristics and pyrolysis. High-value utilization potential of SS biochar is the key to evaluating the advancement of treatment technology. A further requirement for using biochar resources is the immobilization and bioavailability reduction of HMs. Due to the catalytic and synergistic effects in the co-pyrolysis process, co-pyrolysis SS biochar exhibits enhanced functionality and has been applied in soil improvement, pollutant adsorption and catalytic reactions. This review focuses on the research progress of different additives in improving the functionality of biochar and influencing the behavior of HMs. The key limitation and challenges in SS co-pyrolysis are then discussed. Future research prospects are detailed from seven perspectives, including pyrolysis process optimization, co-pyrolysis additive selection, catalytic mechanism research of process and product, biochar performance improvement and application field expansion, cooperative immobilization of HMs, and life cycle assessment. This review will offer recommendations and direction for future research paths, while also assist pertinent researchers in swiftly understanding the current state of SS pyrolysis research field.

Value addition through biohydrogen production and integrated processes from hydrothermal pretreatment of lignocellulosic biomass

Bioenergy production is the most sought-after topics at the crunch of energy demand, climate change and waste generation. In view of this, lignocellulosic biomass (LCB) rich in complex organic content has the potential to produce bioenergy in several forms following the pretreatment. Hydrothermal pretreatment that employs high temperatures and pressures is gaining momentum for organics recovery from LCB which can attain value-addition. Diverse bioprocesses such as dark fermentation, anaerobic digestion etc. can be utilized following the pretreatment of LCB which can result in biohydrogen and biomethane production. Besides, integration approaches for LCB utilization that enhance process efficiency and additional products such as biohythane production as well as application of solid residue obtained after LCB pretreatment were discussed. Importance of hydrothermal pretreatment as one of the suitable strategies for LCB utilization is emphasized suggesting its future potential in large scale energy recovery.

Recent advances in lignocellulosic and algal biomass pretreatment and its biorefinery approaches for biochemicals and bioenergy conversion

As the global demand for sustainable energy increases, lignocellulosic (such as agricultural residues, forest biomass, municipal waste, and dedicated energy crops) and algal (including macroalgae and microalgae) biomass have attracted considerable attention, because of their high availability of carbohydrates. This is a potential feedstock to produce biochemical and bioenergy. Pretreatment of biomass can disrupt their complex structure, increasing conversion efficiency and product yield. Therefore, this review comprehensively discusses recent advances in different pretreatments (physical, chemical, physicochemical, and biological pretreatments) for lignocellulosic and algal biomass and their biorefining methods. Life cycle assessment (LCA) which enables the quantification of the environmental impact assessment of a biorefinery also be introduced. Biorefinery processes such as raw material acquisition, extraction, production, waste accumulation, and waste conversion are all monitored under this concept. Nevertheless, there still exist some techno-economic barriers during biorefinery and extensive research is still needed to develop cost-effective processes.

Hydrothermal processing of microalgal biomass: Circular bio-economy perspectives for addressing food-water-energy nexus

Hydrothermal processing of microalgae is regarded as a promising technology to generate multitude of energy based and value-added products. The niche of hydrothermal technologies is still under infancy in terms of the technical discrepancies related to research and development. Thus, the present review critically surveyed the recent advancements linked to the influencing factors governing the algal hydrothermal processing in terms of the product yield and quality. The sustainability of hydrothermal technologies as a standalone method and in broader aspects of circular bio-based economy for energy and value-added platform chemicals are comprehensively discussed. Process optimization and strategic integration of technologies has been suggested to improve efficiency, with reduced energy usage and environmental impacts for addressing the energy-food-water supply chains. Within the wider economic transition and sustainability debate, the knowledge gaps identified and the research hotspots fostering future perspective solutions proposed herewith would facilitate its real-time implementation.

The correlation between the physicochemical properties and enzymatic hydrolyzability of hydrothermal pretreated wheat straw: A quantitative analysis

Hydrothermal pretreatment with diluted acid or alkali can disrupt the compact structure of wheat straw at a moderate temperature for efficient enzymatic saccharification. However, the quantitative analysis between the physicochemical properties and enzymatic hydrolyzability of hydrothermal pretreated lignocellulose was rarely investigated, which hindered the development of model-based applications for process design and control. Herein, correlation analysis (CA) and principal component analysis (PCA) were conducted to elucidate the dominant factors affecting the enzymatic hydrolyzability and quantitative relationship between them. CA results suggested the major positive factor affecting carbohydrate conversion was cellulose content (r = 0.86). Through logarithmic processing and linear combination, these intercorrelated factors were successfully converted into two newly uncorrelated variables named the first principal component (PC1) and the second principal component (PC2). The initial hydrolysis rate and carbohydrate conversion can be well predicted by PC1 and PC2 scores through multiple linear regression with a high R-squared (0.91 and 0.80).

Brown seaweed hydrolysate as a promising growth substrate for biomass and lipid synthesis of the yeast yarrowia lipolytica

Biomass of the brown algae Fucus vesiculosus and Saccharina latissima is a promising, renewable feedstock because of the high growth rate, accessibility and content of glucose and mannitol. Saccharification of seaweeds is a simple process due to the lack of lignocellulose in the cell wall. The high content of glucose and mannitol makes these seaweeds an attractive feedstock for lipid production in the yeast Yarrowia lipolytica. This study demonstrated that hydrolysates of brown algae biomass can be applied as a substrate for synthesis of yeast biomass and lipids without any supplementation. To increase the lipid titer in yeast biomass, we employed an engineered strain of Y. lipolytica overexpressing DGA1/DGA2. In consequence, the C/N ratio has a lower impact on lipid synthesis. Moreover, the applied substrates allowed for high synthesis of unsaturated fatty acids (UFA); the level exceeded 90% in the fatty acid pool. Oleic (C18:1) and linoleic acids (C18:2) achieved the highest content. The study showed that Y. lipolytica is able to grow on the seaweed hydrolysate and produces a high content of UFA in the biomass.

A review on global perspectives of sustainable development in bioenergy generation

Over the last few decades, the globe has much relied on fossil fuels; however, environmental concerns forced the World to look at biofuel as an alternative for stable economic development. Biofuel also facilitates national energy security maintenance and reduces environmental complications. The present study is focused on an in-depth analysis of bioenergy policy measures undertaken by various federal agencies of different countries in order to shed light on the bottlenecks that impede biofuel's growth as a sustainable and alternative fuel. An in-depth assessment of feedstock utilization, blending targets, and policy assistance schemes have been thoroughly reviewed. In addition, the potential of commercial firms for the production of bioenergy is highlighted in order to grasp the current bioenergy market scenario better. Finally, the article is concluded with the viewpoints of the authors to address the standing issues of global bioenergy generation.