M. Jesus Sanchez-Montero

Conversion of a resistant pollutant, phenol, into green fuels by gasification using supercritical water compressed up to 1000 bar

For green sustainable chemistry, it is crucial to investigate the destruction of such a common pollutant as phenol. This study reports the gasification of phenol with steam and supercritical water (SCW) and shows that gasification under high-pressure SCW is a method that destroys and efficiently converts phenol into valuable products. To the best of our knowledge, the widest pressure range ever investigated in this field is utilized, i.e., from atmospheric pressure steam to SCW at 1000 bar. The high temperature used (700 °C) leads to fast degradation of phenol under all the conditions studied but the amount of phenol gasified does strongly depend on pressure. During gasification, polymeric compounds such as naphthalene and phenanthrene are generated. They play a key role in the proposed degradation–gasification mechanism since they are difficult to degrade and can lead to the formation of char. High-pressure SCW can more efficiently degrade and gasify such polymeric compounds compared to steam. Then, the supercritical fluid leads to the conversion of a greater amount of phenol into gas than steam: 68% of the pollutant is gasified at 750 bar and 700 °C after 16.5 min with no generation of pollutant by-products, except CO2. Furthermore, the gaseous stream contains the valuable green gases H2 and CH4. The use of highly compressed SCW implies not only the production of more gases but also the enrichment of the gas mixture in H2 and CH4. H2 and CH4 concentrations up to 35 and 30%, respectively, are obtained at 1000 bar.

Regeneration of Adsorbent Carbonaceous Materials with Supercritical Water

The aim of this work was to study a new procedure for the regeneration of activated carbon saturated with phenol. The study was accomplished in two steps: extraction of the pollutant with supercritical water at 410 °C and 275 bar, and gasification of phenol with supercritical water at temperatures ranging between 600 - 650 °C. It was observed that the regeneration process was very rapid and effective. The regenerated activated carbon always recovered its original adsorption capacity, even after several regeneration cycles. The gasification of phenol afforded CO2 and H2O, with a very fast first-order kinetic process. The activation energy was very low (0.192 kJ mol-1).

Importance of Textural Characteristics in the Adsorption of Hydrogen onto Activated Carbon and Activated Carbon Fibers

This paper highlights the importance of micropore size of carbonous materials in the storage capacity of hydrogen. A study is made of a series of carbon fibers with different burn off, activated by a new procedure in which supercritical CO2 is used, as well as of several activated carbons. The best storage results at 77 K and 1 bar corresponded to the activated carbon fibers. The maximum value obtained for these materials was 2.86wt%, significantly higher than that obtained previously by other authors. The study shows that storage increases rapidly with the degree of activation of the fiber and is closely linked to the micropores. Micropores of around 0.6 nm are those responsible for the greater increase in storage. This study confirms that activation with supercritical CO2 may lead to microporous solids with enormous capacity for adsorbing H2.

Effect of pressure on the gasification of dodecane with steam and supercritical water and consequences for H2 production

Supercritical water (SCW) is widely known to be a powerful gasifying agent, but the supercritical gasification of linear paraffins is a method whose ability to produce H2 has not been studied significantly. Herein, an analysis of the gasification of dodecane, a representative diesel compound, with steam and SCW and the ability of the method to produce H2 under different pressures is reported. In this study, the broadest pressure (1–500 bar) and temperature (550–800 °C) ranges ever studied in this field are covered. We found that a fraction of the short-chain hydrocarbons generated in the thermal cracking of dodecane are turned into polycyclic aromatic compounds and phenol, compounds that hinder gasification. These reactions become more significant as steam at atmospheric pressure is progressively compressed up to SCW at 500 bar; consequently, steam gasification is faster than supercritical gasification. A gasification mechanism that gathers all of the possible pathways is proposed. Despite the slow gasification kinetics in SCW, a pressure slightly above the critical point (250 bar) is the most efficient to produce H2. At this pressure, the long reaction times related to the high SCW density allow a significant amount of CH4 and CO to be reformed into H2; however, further compression is not recommended because gasification is significantly slowed down and H2 production decreases.