Waste-derived catalysts for tar cracking in hot syngas cleaning.

Catalytic tar cracking is a promising technique for hot syngas cleaning unit in gasification plants because it can preserve tars chemical energy, so increasing the syngas heating value. The cost associated with catalyst preparation is a key issue, together with its deactivation induced by coke deposition. Iron is a cheap and frequently used catalyst, which can also be found in some industrial wastes. The study aims to assess the catalytic efficiency for tar cracking of two waste-derived materials (red mud and sewage sludge) having high content of iron. The catalysts were supported on spheres of γ-Al2O3, and their efficiency was compared to that of a pure iron catalyst. The role of support was investigated by testing pure red mud, with and without the support. A series of long-term tests using naphthalene as tar model compound were carried out under different values of process temperatures (750 °C-800 °C) and steam concentrations (0 %-7.5 %). The waste derived catalysts showed lower hydrogen yields compared to pure iron catalyst, due to their lower content of iron. On the other hand, the conversion efficiencies of all the tested catalysts resulted rather similar, since the Alkali and Alkaline-Earth Metallic species present on the surface of waste-derived catalyst help in preventing coke deposition. The iron oxidation state appears to play an important role, with reduced iron more active than its oxidised form in the tar cracking reactions. This indicates the importance of tuning steam concentration to keep constant the reduced state of iron while limiting coke deposition.

Environmental performances of a modern waste-to-energy unit in the light of the 2019 BREF document.

The study compares the environmental performances of a new-generation, large scale, combustion-based waste-to-energy unit, active since 2010, with those of different "virtual" units, defined in the light of the Best Available Techniques REFerence document (BREF) for Waste Incineration published by the European Community on December 2019. The average performances of these units have been evaluated in terms of air emissions, material consumptions and energy recovery, based on data related to 355 "existing" European waste incineration lines and those established for the future "new" plants. An attributional Life Cycle Assessment has been used to compare and quantify the environmental performances of the selected units, all equipped with a moving grate furnace and similar air pollution control systems. A sensitivity analysis quantifies how even more severe requests for emission and energy performances as well as the evolution of the European electricity mix until the year 2030 can affect the comparative assessment. The results indicate that the considered large scale waste-to-energy plant has good environmental performances, even in an electricity mix characterised by 45% of renewable sources. This allows an easy compliance with the Best Available Techniques Associated Emission Levels of the new waste incineration BREF document. Possible further improvements of its performances should be focused mainly on a further increase of the energy efficiency, provided that it is economically viable.