Abstract
Active phase loss mechanisms from Ru/AC catalysts were studied in continuous supercritical water gasification (SCWG) for the first time by analysing the Ru content in process water with low limit-of-detection time-resolved ICP-MS. Ru loss was investigated alongside the activity of commercial and in-house Ru-based catalysts, showing very low Ru loss rates compared to Ru/metal-oxides (0.2–1.2 vs. 10–24 μg gRu−1 h−1, respectively). Furthermore, AC-supported Ru catalysts showed superior long-term SCWG activity to their oxide-based analogues. The impact on Ru loss of several parameters relevant for catalytic SCWG (temperature, feed concentration or feed rate) was also studied and was shown to have no effect on the Ru concentration in the process water, as it systematically stabilised to 0.01–0.2 μgRu L−1 for Ru/AC. Looking into the type of Ru loss in steady-state operation, time-resolved ICP-MS confirmed a high probability of finding Ru in the ionic form, suggesting that leaching is the main steady-state Ru loss mechanism. In non-steady-state operation, abrupt changes in the pressure and flow rate induced important Ru losses, which were assigned to catalyst fragments. This is directly linked to irreversible mechanical damage to the catalyst. Taking the different observations into consideration, the following Ru loss mechanisms are suggested: 1) constant Ru dissolution (leaching) until solubility equilibrium is reached; 2) minor nanoparticle uncoupling from the support (both at steady state); 3) support disintegration leading to the loss of larger amounts of Ru in the form of catalyst fragments (abrupt feed rate or pressure variations). The very low Ru concentrations detected in process water at steady state (0.01–0.2 μgRu L−1) are close to the thermodynamic equilibrium and indicated that leaching did not contribute to Ru/AC deactivation in SCWG.
Highlights
In this work, the focus is set on low-temperature supercritical water (SCW) (374–500 °C) using an active catalyst in order to selectively form CH4, thermodynamically favoured at these temperaturesPaper.[12,14,15,16,17,18] In this temperature range, biomass readily hydrolyses in the absence of a catalyst
We conclude that the active phase loss from Ru/activated carbon (AC) catalysts during supercritical water gasification (SCWG) is governed by several mechanisms in parallel: 1. Constant Ru dissolution until solubility equilibrium is reached; 2
Support disintegration leading to the loss of larger amounts of Ru caused by abrupt feed rate or pressure variations
Summary
The focus is set on low-temperature SCW (374–500 °C) using an active catalyst in order to selectively form CH4, thermodynamically favoured at these temperaturesPaper (higher temperatures will favour H2 formation).[12,14,15,16,17,18] In this temperature range, biomass readily hydrolyses in the absence of a catalyst. To ensure high carbon gasification efficiency and methane selectivity, a stable and active methanation catalyst is required.[12,19] Many support materials have been investigated for their stability in SCW, but only a very narrow selection remains stable (physically and structurally). The conclusions from those different studies are that only α-Al2O3, rutile-TiO2, monoclinic-ZrO2, CeO2, Ce– ZrO2 and activated carbon (AC) can be considered as stable materials for supercritical water gasification (SCWG).[12,18,20,21,22,23,24]. The exact cause for this still remains unclear.[31]
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