Contribution to Circular Economy options of mixed agricultural wastes management: Coupling anaerobic digestion with gasification for enhanced energy and material recovery

Anaerobic digestion (AD) is an established process for the treatment of organic wastes and the production of renewable energy. However, high amounts of digestate produced by AD plants require enhancement for further use. This study investigates a conceptual model for the digestate enhancement by using a downstream gasification. It is based on a 'systemic approach’ considering the interactions of every contributing process into the dual system. The digestate was provided by an Italian AD plant, that treats mixed agricultural wastes of pig manure (43%), cow manure (20%), maize and triticale silages (25%), and cereal bran (12%). Digestate air gasification experiments were conducted, in a downdraft fixed-bed reactor, at temperature range from 750 °C to 850 °C, with λ varying from 0.14 to 0.34. Results have shown that gasification of digestate at 850 °C with λ = 0.24, increased producer gas yield (65.5 wt %), and its LHV (2.88 MJ Nm−3). The gas is classified as medium heating value fuel, suitable to generate electricity of 971 kWhel day−1 to enhance the AD plant's economic viability. A carbonaceous material rich in macronutrients (P, K, Ca, Mg) was produced, with R50 = 0.48, suitable for carbon sequestration. The study offers a resource closed loop approach of converting AD digestate into energy and soil fertilizer. Useful suggestions for policy makers and business can be drawn.