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

Abstract

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.

Gasification Kinetics in Continuous Supercritical Water Reactors

“…Achieving high temperatures and pressures, mitigating corrosion of reactor components, rapid heating and quenching of the reagent, acquiring accurate experimental data, and strategies for achieving a well-mixed, uniform flow must all be considered in the design of a supercritical water reactor for studies of reaction chemistry. Solutions for mitigating some of these challenges have been reported in the literature, but open questions remain regarding the best methods to mitigate char formation and salt precipitation in reactors designed to process complex feedstocks [3].…”

“…The most reliable way to achieve independent pressure and mass flow control in a continuous SCWR is to operate a constant flow rate pump(s) in series with a back pressure regulator (BPR). Spring-loaded or dome-loaded BPRs are simple to use and reliable [3,5]. High-performance liquid chromatography (HPLC) pumps are often used for pumping liquid reagents to high pressures with precise flow rate control in the range of 0.01-30 mL/min [6][7][8][9].…”

“…However, some researchers prefer to report the total organic carbon (TOC) concentration in the liquid phase by using a TOC analyzer, which is sufficient for calculating carbon CE. Occasionally solid products are analyzed ex situ, using scanning electron microscopy (SEM), proton-induced X-ray emission (PIXE), Raman spectroscopy, or FTIR spectroscopy [3].…”

“…It is beyond the scope of this chapter to review the body of work investigating various catalysts for SCWG. We direct the reader to the comprehensive reviews on the subject [2,3]. However, it is worth mentioning some broad findings from this extensive area of research.…”

Design of a Small-Scale Supercritical Water Oxidation Reactor. Part I: Experimental Performance and Characterization

“…Heating of the reactor section is typically accomplished by a combination of (i) external heating (e.g., with a radiant furnace), (ii) internal heating from the fuel value of the feedstock, and/or (iii) internal heating with a pilot fuel (e.g., diesel, ethanol, isopropanol, etc.). 21 Note that "pilot" can also be considered as co-fuel; the need for its use depends on the heating value of the waste stream.…”