Abstract
In response to increasing energy demand, various types of organic wastes, including industrial and municipal wastewaters, or biomass wastes, are considered reliable energy sources. Wastes are now treated in supercritical water (SCW) for non-fossil fuel production and energy recovery. Considering that SCW technologies are green and energetically effective, to implement them on a large scale is a worldwide interest. However, issues related to the stability and functionality of materials used in the harsh conditions of SCW reactors still need to be addressed. Here we present an overview on materials used in the SCW technologies for energy harvesting from wastes. There are catalysts based on metals or metal oxides, and we discuss on these materials’ efficiency and selectivity in SCW conditions. We focus on processes relevant to the waste-to-energy field, such as supercritical water gasification (SCWG) and supercritical water oxidation (SCWO). We discuss the results reported, mainly in the last decades in connection to the current concept of supercritical pseudo-boiling (PB), a phenomenon occurring at the phase change from liquid-like (LL) to gas-like (GL) state of a fluid. This review aims to be a useful database that provides guidelines for the selection of the abovementioned functional materials (catalysts, catalyst supports, and sorbents) for the SCW process, starting from wastes and ending with energy-relevant products.
Highlights
Waste is a material or a good that has reached the end of its life cycle, lost its economic value, and the function it had for the owner, and has become a source of pollution
We focus on processes relevant to the waste-to-energy field, such as supercritical water gasification (SCWG) and supercritical water oxidation (SCWO)
From industrial pharmaceutical wastewater treated by SCWG at 673 K, the maximum H2 yield of 126.5 mmol/L was obtained after 45 min., while after only 5 min, near-complete conversion of 99.5% was reached by SCWO using H2O2 and an oxidation coefficient of 4, at the same operating temperature [110]
Summary
Waste is a material or a good that has reached the end of its life cycle, lost its economic value, and the function it had for the owner, and has become a source of pollution. The treatment in supercritical water of wastes is nowadays one of the most efficient options to obtain energy-relevant products and valuable compounds through recycling or recovery. Proposed initially at the Massachusetts Institute of Technology (MIT) in the 80s, for use on NASA’s space facilities, the MODAR process was developed for the treatment of human wastes by oxidation in supercritical water [13], showing the capability of achieving high destruction efficiencies of hazardous organic constituents in a single step [14]. Materials employed in the SCW processes as catalysts allow cost-efficient operation of SCW reactors at lower temperatures, shorten the reaction time by increasing the process rates, and suppress the formation of by-products while enhancing the selectivity to the specific reactions and products with desirable properties, as for example the elemental composition and the heating value of the combustible gases/fuels.
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