Energy valorisation of the residual biomass from greenhouses in the framework of a circular economy

Thrace Greenhouses S.A. cultivates 5000 t y−1 cucumbers and 3500 t y−1 tomatoes hydroponically on 170,000 m2 of land. During cultivation (from February to November), suckers and leaves are removed regularly, producing 7,000 – 8,000 t y−1 residual green biomass. The anaerobic digestion technology was applied at laboratory scale, to explore the potential of valorising the generated residues for biogas production. The latter ranged between 18 and 30 NL methane per kg of fresh biomass (suckers or leaves, respectively). Considering a typical mixture of suckers and leaves produced during the cultivating season, the methane produced annually was estimated at 140,000 m3. The biogas can be used as a fuel in a combined heat and power (CHP) system (cogeneration). CHP was considered the central unit in a hybrid energy system (HES) integrated with batteries, aiming to provide energy autonomy to the greenhouses. Moreover, the flue gas from the CHP containing 10% CO2 and 9% O2 could be used as a source of CO2 in the greenhouses. In this case, trace gases (NOx: 401±37 ppm, NO:168±15 ppm, NO2: 150±18ppm, CO: 401±60, SO2: 1.4±2.2) present in the flue gas should be removed and/or diluted with air before entering the greenhouses.