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Innovative small-scale biogas plants, including upgrading solutions to affordable biomethane, are necessary to tap into the spatially distributed potentials of organic waste. This research identified and assessed novel small-scale technologies before market-entry maturity in the key process steps of the biomethane chain. We assessed technical, economic, and ecological indicators, and compared them to larger-scale references. The assessment included 7 pre-treatment, 13 digester, and 11 upgrading systems all at the small scale. We collected recently available data for Europe (2016–2018) for small-scale technologies (<200 m3; raw biogas per hour). In the literature we did not find such a comprehensive assessment of actual European small-scale innovative non-market-ready technologies for the production of biomethane. Several conclusions were drawn for each of the individual process steps in the biomethane chain, e.g., the economic indicator calculated for the upgrading technologies shows that the upgrading costs, for some of them, are already close to the larger-scale reference (about 1.5 €ct/kWh raw biogas). Furthermore, biomethane production is absolutely context-specific, which dramatically limits the traditional way to evaluate technologies. Hence, new ways of integration of the technologies plays a major role on their future R&D.

Disintegration of lignocellulosic biomass for energy purposes has been extensively studied. The study aimed to investigate the influence of crushed and uncrushed lignocellulosic biomass on biogas production in innovative reactor. The substrate fed to the reactor was Sida hermaphrodita silage mixed with cow manure. The bioreactor had innovative design mixing cage system. Mixing system of the bioreactor consisted of two cylindrical stirrers in the form of cage. The cages by rotation around the axis of the bioreactor at the same time turn against its own axis. The bioreactor is currently presented under the program Record Biomap (Horizon 2020). The bioreactor was operated at organic compounds loading 2 kg/(m3∙d) and 3 kg/(m3∙d) and hydraulic retention time was 50 d and 33 d, respectively. The biogas production under organic compounds loading 2 kg VS/(m3∙d) was 680 L/kg VS from crushed lignocellulosic biomass and 570 L/kg VS from uncrushed lignocellulosic biomass. The biogas production under organic compounds loading 3 kg VS/(m3∙d) was 730 L/kg VS from crushed lignocellulosic biomass and 630 L/kg VS from uncrushed lignocellulosic biomass. The crushing of substrate did not influence on methane content in the biogas. In all experiments about 54% of methane was in the biogas. The net energy efficiency was also calculated.

Physical pretreatment methods using ultrasound and hydrothermal cavitation were compared in the process of mesophilic anaerobic co-digestion of cattle manure mixed with straw wheat. To evaluate the anaerobic biodegradability of pretreated lignocellulosic biomass, 20-days batch anaerobic digestion experiments were used. The results showed that both methods achieved similar biomass solubilization (ca. 30% as CODsol), but in hydrothermal cavitation the pretreatment time was shorter and energy required was lower than in ultrasonic pretreatment. Solubilization of nitrogen compounds was higher in hydrothermal cavitation. Application of sonication with energy input of 4839 kJ/kg TS increased biogas production rate to 177 mL/mg VS·d, compared to 194 mL/mg VS·d with hydrothermal cavitation. However, biogas productivity was higher with ultrasound, where biogas enhancement ranged 59.6–64.2% in contrast to 35.6–39.4% hydrothermal cavitation. Both methods did not contribute to increase methane concentration in biogas. The optimization of pretreatments for the production of biofuels from lignocellulosic residues is needed to be feasible at industrial scale. Physical pretreatment based on cavitation enhanced lignocellulosic biomass solubilization as well as biogas productivity, but the process yield was related to the cavitation equipment and mechanism.