Supercritical Water Gasification of Biomass in a Ceramic Reactor: Long-Time Batch Experiments

A critical review on the influence of operating parameters and feedstock characteristics on microwave pyrolysis of biomass

Biomass pyrolysis is the most effective process to convert abundant organic matter into value-added products that could be an alternative to depleting fossil fuels. A comprehensive understanding of the biomass pyrolysis is essential in designing the experiments. However, pyrolysis is a complex process dependent on multiple feedstock characteristics, such as biomass consisting of volatile matter, moisture content, fixed carbon, and ash content, all of which can influence yield formation. On top of that, product composition can also be affected by the particle size, shape, susceptors used, and pre-treatment conditions of the feedstock. Compared to conventional pyrolysis, microwave-assisted pyrolysis (MAP) is a novel thermochemical process that improves internal heat transfer. MAP experiments complicate the operation due to additional governing factors (i.e. operating parameters) such as heating rate, temperature, and microwave power. In most instances, a single parameter or the interaction of parameters, i.e. the influence of other parameter integration, plays a crucial role in pyrolysis. Although various studies on a few operating parameters or feedstock characteristics have been discussed in the literature, a comprehensive review still needs to be provided. Consequently, this review paper deconstructed biomass and its sources, including microwave-assisted pyrolysis, and discussed the impact of operating parameters and biomass properties on pyrolysis products. This paper addresses the challenge of handling multivariate problems in MAP and delivers solutions by application of the machine learning technique to minimise experimental effort. Techno-economic analysis of the biomass pyrolysis process and suggestions for future research are also discussed.

Subcritical Water Hydrolysis of Fresh and Waste Cooking Oils to Fatty Acids Followed by Esterification to Fatty Acid Methyl Esters: Detailed Characterization of Feedstocks and Products

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.

Heat Transfer Limitations in Supercritical Water Gasification

Supercritical water gasification (SCWG) is a promising technology for the valorization of wet biomass with a high-water content, which has attracted increasing interest. Many experimental studies have been carried out using conventional heating equipment at lab scale, where researchers try to obtain insight into the process. However, heat transfer from the energy source to the fluid stream entering the reactor may be ineffective, so slow heating occurs that produces a series of undesirable reactions, especially char formation and tar formation. This paper reviews the limitations due to different factors affecting heat transfer, such as low Reynolds numbers or laminar flow regimes, unknown real fluid temperature as this is usually measured on the tubing surface, the strong change in physical properties of water from subcritical to supercritical that boosts a deterioration in heat transfer, and the insufficient mixing, among others. In addition, some troubleshooting and new perspectives in the design of efficient and effective devices are described and proposed to enhance heat transfer, which is an essential aspect in the experimental studies of SCWG to move it forward to a larger scale.