Supercritical water gasification (SCWG) is a thermochemical conversion technology developed to transform various feedstocks, such as raw forest biomass materials, crude bio-oils and bio-wastes, into syngas (a combination of CO and H2) for clean energy production. Despite the intensive research efforts that have been applied on the development of SCWG technology, the optimal SCWG operating parameters (temperature, pressure, and biomass/water ratio, etc.) are not well defined because of the complexity of feedstock types and conversion reactor configurations (batch or continuous mode). Moreover, little information is available to determine which alloys are suitable for the reactor construction in a long-term safe and cost-effective manner. This study investigated the corrosion of UNS N06625 under the catalytic SCWG conversion of lignin using standard high temperature autoclave testing methodology and XRD characterization of the formed corrosion products. It was found that the addition of NaOH catalyst resulted in a remarkable increase in H2 production. After 12 cycle exposures to the catalytic SCWG environment, the corrosion layer formed on the alloy was composed of Cr2O3 and Ni3S2. Surprisingly, the addition of NaOH led to a positive weight change instead of weight loss as that occurred in non-catalytic SCWG processes. Further works, such as accurate weight loss measurements and SEM/FIB/TEM characterizations of corrosion layer, are needed to advance the understanding of how the alloy corroded under the catalytic SCWG of lignin.
Supercritical water gasification (SCWG) is a promising thermochemical conversion technology in which supercritical water is used as the medium to convert different types of wet biomass (such as wastewater sludge, food waste or microalgae) and even crude bio-oils into hydrogen-rich syngas without the need of costive pre-drying process.1 During typical SCWG conversion at temperature and pressure above the critical point of water (i.e., 374°C and 22.1 MPa), alkali metal/metal oxide catalysts, carbon-based catalysts and Ni- or Fe-based catalysts are introduced to significantly improve the conversion efficiency on H2 production.2 Compared to conventional air/steam gasification, SCWG exhibits attractive advantages for industrial application because of the following two major functions of supercritical water (SCW) under the conversions: (1) serving as both a reaction medium and a catalyst, and (2) supplying an extra source of H2 and free radicals.3 As a result, considerable efforts have been employed to develop the optimum SCWG conversion of a wide range of biomass and/or biowaste feedstocks to achieve the desired H2 yield efficiency.1,4-8