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The valorization of residual biomass plays today a decisive role in the concept of “circular economy”, according to which each waste material must be reused to its maximum extent. The collection and energy valorization at the local level of biomass from forest management practices and wildfire prevention cutting can be settled in protected areas to contribute to local decarbonization, by removing power generation from fossil fuels. Despite the evident advantages of bioenergy systems, several problems still hinder their diffusion, such as the need to assure their reliability by extending the operating range with materials of different origin. The Italian project “INNOVARE—Innovative plants for distributed poly-generation by residual biomass”, funded by the Italian Ministry of Economic Development (MISE), has the main scope of improving micro-cogeneration technologies fueled by biomass. A micro-combined heat and power (mCHP) unit was chosen as a case study to discuss pros and cons of biomass-powered cogeneration within a national park, especially due to its flexibility of use. The availability of local biomasses (woodchips, olive milling residuals) was established by studying the agro-industrial production and by identifying forest areas to be properly managed through an approach using a satellite location system based on the microwave technology. A detailed synergic numerical and experimental characterization of the selected cogeneration system was performed in order to identify its main inefficiencies. Improvements of its operation were optimized by acting on the engine control strategy and by also adding a post-treatment system on the engine exhaust gas line. Overall, the electrical output was increased by up to 6% using the correct spark timing, and pollutant emissions were reduced well below the limits allowed by legislation by working with a lean mixture and by adopting an oxidizing catalyst. Finally, the global efficiency of the system increased from 45.8% to 63.2%. The right blending of different biomasses led to an important improvement of the reliability of the entire plant despite using an agrifood residual, such as olive pomace. It was demonstrated that the use of this biomass is feasible if its maximum mass percentage in a wood matrix mixture does not exceed 25%. The project was concluded with a real operation demonstration within a national park in Southern Italy by replacing a diesel genset with the analyzed and improved biomass-powered plant and by proving a decisive improvement of air quality in the real environment during exercise.

The linear stability of an inviscid two-dimensional liquid sheet falling under gravity in a still gas is studied by analysing the asymptotic behaviour of a localized perturbation (wave-packet solution to the initial value problem). Unlike previous papers the effect of gravity is fully taken into account by introducing a slow length scale which allows the flow to be considered slightly non-parallel. A multiple-scale approach is developed and the dispersion relations for both the sinuous and varicose disturbances are obtained to the zeroth-order approximation. These exhibit a local character as they involve a local Weber number Weη. For sinuous disturbances a critical Weη equal to unity is found below which the sheet is locally absolutely unstable (with algebraic growth of disturbances) and above which it is locally convectively unstable. The transition from absolute to convective instability occurs at a critical location along the vertical direction where the flow Weber number equals the dimensionless sheet thickness. This critical distance, as measured from the nozzle exit section, increases with decreasing the flow Weber number, and hence, for instance, the liquid flow rate per unit length. If the region of absolute instability is relatively small it may be argued that the system behaves as a globally stable one. Beyond a critical size the flow receptivity is enhanced and self-sustained unstable global modes should arise. This agrees with the experimental evidence that the sheet breaks up as the flow rate is reduced. It is conjectured that liquid viscosity may act to remove the algebraic growth, but the time after which this occurs could be not sufficient to avoid possible nonlinear phenomena appearing and breaking up the sheet.

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