Review of Gasification of Organic Compounds in Continuous-Flow, Supercritical Water Reactors

Hydrothermal Destruction and Defluorination of Trifluoroacetic Acid (TFA)

Per- and polyfluoroalkyl substances (PFAS) have received increased attention due to their environmental prevalence and threat to public health. Trifluoroacetic acid (TFA) is an ultrashort-chain PFAS and the simplest perfluorocarboxylic acid (PFCA). While the US EPA does not currently regulate TFA, its chemical similarity to other PFCAs and its simple molecular structure make it a suitable model compound for studying the transformation of PFAS. We show that hydrothermal processing in compressed liquid water transforms TFA at relatively mild conditions (T = 150-250 °C, P < 30 MPa), initially yielding gaseous products, such as CHF3 and CO2, that naturally aspirate from the solution. Alkali amendment (e.g., NaOH) promotes the mineralization of CHF3, yielding dissolved fluoride, formate, and carbonate species as final products. Fluorine and carbon balances are closed using Raman spectroscopy and fluoride ion selective electrode measurements for experiments performed at alkaline conditions, where gas yields are negligible. Qualitative FTIR gas analysis allows for establishing the transformation pathways; however, the F-balance could not be quantitatively closed for experiments without NaOH amendment. The kinetics of TFA transformation under hydrothermal conditions are measured, showing little to no dependency on NaOH concentration, indicating that the thermal decarboxylation is a rate-limiting step. A proposed TFA transformation mechanism motivates additional work to generalize the hydrothermal reaction pathways to other PFCAs.

Comparison study of supercritical water gasification for hydrogen production on a continuous flow versus a batch reactor

This study compares batch and continuous supercritical water gasification (SCWG) processes for green hydrogen production from biomass. It offers insights for optimizing processes, enhancing yields, quality, and energy efficiency, assessing scale-up feasibility, and supporting techno-economic analyses. Glucose, glycerol, and black liquor were SCWG-treated at 500 °C with K2CO3 catalyst in a self-built continuous-flow reactor (150 g/h) and a batch reactor (75 mL). Comparisons primarily focused on gas product yields. Batch reactors outperformed continuous-flow reactors in hydrogen (glucose: 1.53 to 0.9 mmol/g, glycerol: 7.22 to 1.14 mmol/g, black liquor: 2.88 to 1.74 mmol/g) and total gas yields due to differences in reaction time and heating behavior. Temperature effects on continuous operation (450-600 °C) were studied, with glycerol showing the highest hydrogen yield increase (from 1.21 to 4.30 mmol/g). The study discusses the applicability of both reactors for biomass SCWG processes and their implications for sustainable green hydrogen production from renewable feedstocks.

Destruction and defluorination of PFAS matrix in continuous-flow supercritical water oxidation reactor: Effect of operating temperature

Cleanup and disposal of stockpiles and waste streams containing per- and polyfluoroalkyl substances (PFAS) require effective end-of-life destruction/mineralization technologies. Two classes of PFAS, perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs), are commonly found in legacy stockpiles, industrial waste streams, and as environmental pollutants. Continuous flow supercritical water oxidation (SCWO) reactors have been shown to destroy several PFAS and aqueous film-forming foams. However, a direct comparison of the SCWO efficacy for PFSAs and PFCAs has not been reported. We show the effectiveness of continuous flow SCWO treatment for a matrix of model PFCAs and PFSAs as a function of operating temperature. PFSAs appear to be significantly more recalcitrant than PFCAs in the SCWO environment. The SCWO treatment results in a destruction and removal efficiency of 99.999% at a T > 610 °C and at a residence time of ∼30 s. Fluoride recovery lags destruction PFAS at 510 °C and reaches >100% above 610 °C, confirming the formation of liquid and gaseous phase intermediate product during lower temperature oxidation. This paper establishes the threshold for destroying PFAS-containing liquids under SCWO conditions.

Recycling of carbon fiber reinforced polymers in a subcritical acetic acid solution

A novel single-stage solvolysis process is demonstrated for recycling carbon fibers from an epoxy-based composite material using 50 wt% acetic acid solution under subcritical conditions. The process yields 100% fiber recovery efficiency in less than 30 min at 300 °C. Qualitative SEM/EDS analysis of the fibers reveals that the recovered fibers are entirely free of resin, and the carbon fiber surfaces were not damaged. SEM images and gravimetric measurements of the composites treated at lower temperatures and short residence times show an initial increase in mass of the CFRP samples, suggesting a two-step process consisting of initial composite swelling due to uptake of solvent, followed by depolymerization and chemical decomposition of the polymer. FTIR and GC-MS analyses confirm resin decomposition and production of aromatic and aliphatic compounds.

Technological innovations on direct carbon mitigation by ordered energy conversion and full resource utilization

Coal consumption leads to over 15 billion tons of global CO₂ emissions annually, which will continue at a considerable intensity in the foreseeable future. To remove the huge amount of CO₂, a practically feasible way of direct carbon mitigation, instead of capturing that from dilute tail gases, should be developed; as intended, we developed two innovative supporting technologies, of which the status, strengths, applications, and perspective are discussed in this paper. One is supercritical water gasification-based coal/biomass utilization technology, which orderly converts chemical energy of coal and low-grade heat into hydrogen energy, and can achieve poly-generation of steam, heat, hydrogen, power, pure CO₂, and minerals. The other one is the renewables-powered CO₂ reduction techniques, which uses CO₂ as the resource for carbon-based fuel production. When combining the above two technical loops, one can achieve a full resource utilization and zero CO₂ emission, making it a practically feasible way for China and global countries to achieve carbon neutrality while creating substantial domestic benefits of economic growth, competitiveness, well-beings, and new industries.

Review: Hydrothermal treatment of per- and polyfluoroalkyl substances (PFAS)

PER: and polyfluoroalkyl substances (PFAS) are a concerning and unique class of environmentally persistent contaminants with biotoxic effects. Decades of PFAS discharge into water and soil resulted in PFAS bioaccumulation in plants, animals, and humans. PFAS are very stable, and their treatment has become a global environmental challenge. Significant efforts have been made to achieve efficient and complete PFAS mineralization using existing and emerging technologies. Hydrothermal treatments in subcritical and supercritical water have emerged as promising end-of-life PFAS destruction technologies, attracting the attention of scholars, industry, and key stakeholders. This paper reviews the state-of-the-art research on the behavior of PFAS, PFAS precursors, PFAS alternatives, and PFAS-containing waste in hydrothermal processes, including the destruction and defluorination efficiency, the proposed reaction mechanisms, and the environmental impact of these treatments. Scientific literature shows that >99% degradation and >60% defluorination of PFAS can be achieved through subcritical and supercritical water processing. The limitations of current research are evaluated, special considerations are given to the challenges of technology maturation and scale-up from laboratory studies to large-scale industrial application, and potential future technological developments are proposed.

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.

Raman spectroscopic data from Formic Acid Decomposition in subcritical and supercritical water

The spectra presented correspond with the research article entitled "Kinetics of Formic Acid Decomposition in Subcritical and Supercritical Water - A Raman Spectroscopic Study" [1]. Data set contains in situ Raman spectra of the quenched effluent stream, which includes varied concentrations of formic acid, water, CO, CO₂, and H₂ as reaction products. Each spectrum is collected downstream of the subcritical or supercritical water gasification of formic acid, which occurs at a specified temperature, residence time, a constant pressure of 25 MPa, and a constant initial feedstock concentration of 3.6 wt% formic acid. Additionally, calibration spectra of formic acid in water, and spectra of pure carbon dioxide and high concentration formic acid are provided for model development. Finally, a MATLAB code used for baseline subtraction of raw data files is included with the dataset. The full dataset is hosted in Mendeley Data, https://doi.org/10.17632/hjn8xwskng.1.

Supercritical water gasification: practical design strategies and operational challenges for lab-scale, continuous flow reactors

Optimizing an industrial-scale supercritical water gasification process requires detailed knowledge of chemical reaction pathways, rates, and product yields. Laboratory-scale reactors are employed to develop this knowledge base. The rationale behind designs and component selection of continuous flow, laboratory-scale supercritical water gasification reactors is analyzed. Some design challenges have standard solutions, such as pressurization and preheating, but issues with solid precipitation and feedstock pretreatment still present open questions. Strategies for reactant mixing must be evaluated on a system-by-system basis, depending on feedstock and experimental goals, as mixing can affect product yields, char formation, and reaction pathways. In-situ Raman spectroscopic monitoring of reaction chemistry promises to further fundamental knowledge of gasification and decrease experimentation time. High-temperature, high-pressure spectroscopy in supercritical water conditions is performed, however, long-term operation flow cell operation is challenging. Comparison of Raman spectra for decomposition of formic acid in the supercritical region and cold section of the reactor demonstrates the difficulty in performing quantitative spectroscopy in the hot zone. Future designs and optimization of continuous supercritical water gasification reactors should consider well-established solutions for pressurization, heating, and process monitoring, and effective strategies for mixing and solids handling for long-term reactor operation and data collection.