The aviation industry is a significant contributor to global carbon dioxide emissions, with over 920 million tonnes per year, and there is a growing need to reduce its environmental impact. The production of biojet fuel from renewable biomass feedstocks presents a promising solution to address this challenge, with the potential to reduce greenhouse gas emissions and dependence on fossil fuels in the aviation sector. This review provides an in-depth discussion of current and emerging biojet fuel conversion technologies, their feasibility, and their sustainability, focusing on the promising conversion pathways: lipids-to-jet, sugar-to-jet, gas-to-jet, alcohol-to-jet, and whole biomass-to-jet. Each technology is discussed in terms of its associated feedstocks, important chemistries, and processing steps, with focus on recent innovations to improve yields of biojet product at the required specifications. In addition, the emerging power-to-liquid technology is briefly introduced. With the integrated biorefinery approach, consideration is given to biomass pretreatment to obtain specific feedstocks for the specific technology to obtain the final product, with the embedded environmental sustainability requirements. In addition, the review highlights the challenges associated with the biojet production technologies, with embedded suggestions of future research directions to advance the development of this important and fast-growing sustainable fuel industry.

In this present work, the hydrolysis of a sample of rapeseed oil (RSO) and two waste cooking oil (WCO) samples in subcritical water has been carried out in a stirred batch stainless-steel reactor to produce fatty acids. Using RSO as a model triglyceride, the effects of reaction parameters on the yields of fatty acids were investigated to determine the optimum set of hydrolysis conditions to be a temperature of 300 °C, a reaction time of 60 min, and a vegetable oil-water mass ratio of 1:2. The set of optimum conditions was applied to the hydrolysis of the WCOs. Oleic acid was the dominant fatty acid with yields of 74.4 wt % from RSO and 57.5 and 72.4 wt % from the two WCOs, respectively, while palmitic acid was the second most abundant fatty acid with yields of up to 31 wt %. The feedstocks and fatty acid products were characterized by acid-base titration and thermogravimetric analysis (TGA). Thereafter, the hydrolysis products from the optimum conditions were esterified to their fatty acid methyl esters (FAMEs), which were further characterized by gas chromatography-mass spectrometry (GC/MS) and TGA. With RSO at the optimum hydrolysis conditions, acid-base titration gave a fatty acid yield of 97.2 wt %, while TGA gave 86 wt %. Under the same conditions, the yield of FAMEs from GC/MS analysis was 88.6 wt %, while TGA gave a FAMEs' yield of 91 wt %. This study showed that the simple TGA method provided detailed and complete characterization of lipid feedstocks and their conversion products. In addition, subcritical water hydrolysis can be used to valorize WCOs to fatty acids, with little or no extensive feedstock purification, for various applications including biodiesel production.

Conventional biomass gasification involves a complex set of chemical reactions leading to the production of a gas mainly composed on carbon monoxide, hydrogen, carbon dioxide and some methane. Some C₂–C₅₊ hydrocarbon gases are also formed in the gasifier. This review has uniquely focused on the thermochemistries of conventional biomass gasification with emphasis on gasification temperature, gasifying agents (pure oxygen, air, carbon dioxide, steam or combinations of these) and the types of gasifiers as the key parameters that determine the yields and compositions of gas products. With air as the gasifying agent, the product gas is highly diluted with nitrogen (>45 vol%) and is known as producer gas, which is often more suitable for direct energy application via combustion. With nitrogen-free gasifying agents, syngas with ≤5 vol% nitrogen content is produced and therefore suitable for various downstream uses including enhanced hydrogen production via the water–gas shift reaction and, especially, the synthesis of organic compounds such as methanol and dimethyl ether as well as hydrocarbons (liquids and waxes via Fischer–Tropsch synthesis). The contributions of kinetic and thermodynamic studies to the understanding and progress of biomass gasification have been explored. In addition, the review covers the challenges of tar formation during biomass gasification and various strategies to reduce/eliminate this major bottleneck via catalysis and reactor design or configuration. The historical perspective of biomass gasification and current trends are presented, highlighting the exponential growth in high-quality research publications around biomass gasification over the last decade, possibly driven by current Net Zero initiatives.

In this paper, biodiesel production from waste frying oils was investigated with zinc aluminate (ZnAl2O4) as a heterogeneous catalyst, synthesised by the combustion of urea with aluminium nitrate and zinc nitrate. The transesterification was studied at temperatures between 60 and 200°C, in an alcohol:oil molar ratio of 40:1, 2h reaction time, stirring at 700rpm and different catalyst amounts (from 1% to 10% based on oil weight). The biodiesel yield was analyzed in a gas chromatographer. Synthesised and recovered catalyst were characterized for their physical and textural properties. Thermogravimetric analysis (TGA) and Differential scanning calorimetry (DSC) have been used to study the thermal behavior of the zinc aluminate spinel. Powder X-ray diffraction (XRD) was used to determine the spinel-type structure, which was confirmed by Raman and Fourier Transform Infrared (FTIR) Spectroscopy. The particle and pore sizes were measured by transmission electron microscopy (TEM) and He pycnometry respectively. The specific surface area was determined by BET surface area analysis to be 17.3084 ± 0.0804 m2.g-1, and the density 4,61g.cm-3. Residual glycerin and biodiesel were also observed in the recovered catalysts. Optimal yields above 95% were measured for both methylic and ethylic routes. Moreover, the same catalyst was utilized for three reaction cycles without significant loss of biodiesel yield at the best reaction conditions, which indicates the feasibility of the proposed method