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The study reviews and compares the most utilised techniques to obtain high quality biomethane by upgrading biogas from anaerobic digestion of the organic fraction of municipal solid waste. Environmental and economic aspects of membrane separation, water scrubbing, chemical absorption with amine solvent, and pressure swing adsorption have been quantified in a life cycle perspective. An attributional environmental Life Cycle Assessment has been implemented with the support of a Material Flow Analysis and in combination with a complementary environmental Life Cycle Costing. The analyses are based on data largely obtained from Italian existing plants but they can be generalised to the whole European Union, as demonstrated by a companion sensitivity analysis. The comparative assessment of the results indicates all the examined options as fully sustainable, also identifying the “win-win” situations. In particular, the membrane separation technique appears to have the best performances, even though in some cases with limited differences. With reference to base case scenarios, this technique shows better results for the respiratory inorganics potential (up to 34%, i.e. up to 328 kgPM2.5eq/y), global warming potential (up to 7%, i.e. up to 344 tCO2eq/y), and non-renewable energy potential (up to 12%, i.e. up to 6400 GJprimary/y) as well as for life cycle costs (up to 3.4%, i.e. about 60 k€/y). The performances of the examined techniques appear anyway dependent on site-specific conditions (such as the injection pressure in the gas grid or the existence/amount of local economic incentives) and commercial strategies for the market of interest.

Air gasification tests have been carried out in a pilot scale bubbling fluidized bed gasifier, able to treat up to 100 kg/h of Eucalyptus wood chips, and operated, under chemical and thermal steady-state conditions, at different values of equivalence ratio. The results are reported in terms of the main process performance parameters and transfer coefficients, which indicate the fate of carbon in the different output streams (syngas, fines, tars). The results can be transferred to commercial scale reactors and confirm the technical feasibility of the air gasification of the biomass in the range 0.25–0.34 of equivalence ratio, yielding a good quality syngas, with a low heating value between 5000 and 6000 kJ/m3N,syngas. The measured axial syngas composition profiles along the reactor indicate a monotonic reduction of methane and carbon dioxide concentrations and an increase of that of hydrogen. This suggests that water gas shift and steam and dry reforming reactions play a significant role, together with a limited contribution of tar cracking reactions, probably enhanced by alkali and alkaline earth metallic species contained in the inorganic fraction of elutriated fines. Results can support the implementation of reliable numerical models of bubbling fluidized bed gasifiers and the optimisation of design/operating criteria.

The gasification of biomass and waste is a potentially high efficiency process for energy generation. However, the appropriate cleaning of the produced fuel gas is a major issue, in particular for tar removal. This study investigates the performance of three new-generation activated carbons, which are able to adsorb a wide spectrum of tars at high temperature, from 750°C up to 900°C. For this purpose, a laboratory scale hot gas filtration apparatus was designed and set-up. Experimental tests were carried out with naphthalene as the reference tar compound. The selected activated carbons were tested under different conditions of temperature, test duration and naphthalene concentration. The procedure allows evaluating the contributions of adsorption and cracking phenomena on the overall tar removal efficiency. Complete naphthalene removal was obtained with two of the selected activated carbons, at 850 °C and 900 °C, respectively. Simultaneously, a partial reforming of adsorbed naphthalene was observed, and molecular hydrogen was detected in the gas phase. Results were correlated to the differences in porosimetry and chemical surface of the selected activated carbons, as well as to those highlighted by Scanning Electronic Microscopy and X-ray analyses. Finally, a procedure for activated carbon regeneration by carbon dioxide is reported, together with results from preliminary tests. The gasification of biomass and waste is a potentially high efficiency process for energy generation. However, the appropriate cleaning of the produced fuel gas is a major issue, in particular for tar removal. This study investigates the performance of three new-generation activated carbons, which are able to adsorb a wide spectrum of tars at high temperature, from 750 degrees C up to 900 degrees C. For this purpose, a laboratory-scale hot gas filtration apparatus was designed and set-up. Experimental tests were carried out with naphthalene as the reference tar compound. The selected activated carbons were tested under different conditions of temperature, test duration and naphthalene concentration. The procedure allows evaluating the contributions of adsorption and cracking phenomena on the overall tar removal efficiency. Complete naphthalene removal was obtained with two of the selected activated carbons, at 850 degrees C and 900 degrees C respectively. Simultaneously, a partial reforming of adsorbed naphthalene was observed, and molecular hydrogen was detected in the gas phase. Results were correlated to the differences in porosimetry and chemical surface of the selected activated carbons, as well as to those highlighted by Scanning Electronic Microscopy and X-ray analyses. Finally, a procedure for activated carbon regeneration by carbon dioxide is reported, together with results from preliminary tests. (C) 2016 Elsevier B.V. All rights reserved.

The presence of tar compounds in the syngas is the main bottleneck for a wider commercial adoption of the gasification process. Tar is a mixture of heavy hydrocarbons that condense when syngas is cooled below 350°C, leading to clogging of pipes and downstream apparatus. In the last two decades, the scientific community has shown a growing interest on innovative solutions for syngas cleaning. One of the most promising options for tar conversion in medium-scale gasifiers is the use of biomass char as catalyst. However, the mechanism of tar conversion over char surfaces is still not well known. A deeper understanding of this conversion process could be useful to define the best pre-treatments for chars, able to improve their performances. This work aims at acquiring reliable data to study the influence of char internal structure and operating conditions on tar conversion kinetics. Three different chars have been obtained starting from the same parent coal. The activation protocol includes the coal devolatilisation at 900ºC and the char activation by steam at the same temperature, for different times. The specific surface area increases from 1 m2/g for the char without activation (raw char) up to 627 m2/g for the char activated for a time of 150 minutes. The activity of the different chars produced has been tested in a tubular reactor. The tests utilised a nitrogen stream doped with naphthalene. The tested reactor temperatures were 800°C and 900°C, by keeping the gas residence time and naphthalene concentration constant. More tests were carried out at 900°C under different residence times in the reactor. Preliminary results show a high conversion of naphthalene, especially with chars having higher specific surface areas. Reactor temperature and residence time as well as the char internal structure appear to have a significant influence on naphthalene conversion.