Nowadays, the decarbonization of the sector of energy and the transformation of the whole economy chain in climate-neutral economy are targets in the political agenda of more than 50 Countries. The replacing of carbon-based fuels with hydrogen is one of the main ways to reach the decarbonization of the energy sector and reducing of impact on climate. Anyway, the production of hydrogen by recurring to electrolysis of the water molecules requires a large energy amount so that its sustainability must be increased by realizing more efficient system or synergy with other industrial sectors such as clean energy recovery. Possible well recognized sources of sustainable energy are biomasses, solar, and wind; in this paper an additional resource, a biowaste produced by anthropic activity, the sewage sludge, is considered as a possible source of energy for green hydrogen on the basis of a life-cycle perspective: the avoided burdens of landfill and incineration of sludges balance the low efficiency of the overall system for hydrogen production.
The paper illustrates a model that combines biowaste treatment with hydrogen production. The mathematical model of the system allowed to calculate the mass balance and the feedstock energies distribution. The output variables of the model are hydrogen yield, electricity to the grid (surplus), and CO2 sequestrated; the input data to the model are the biowaste mass rate (and the composition), the power demand for hydrogen production by electrolysis and the electricity and heat consumption of the plant. The model results demonstrated that the hydrothermal carbonization of the given biowaste, that is sewage sludge from a waste-water treatment plant and the enriched air gasification of the resulted hydrochar converted into biofuel allowed to produce about 20kg/h of green hydrogen by covering all the energy demand of the plant, both heat, and electricity.