Integrated conversion technologies for sustainable agri-food waste valorization: A critical review

The global increase in agri-food production results in a consequential societal requirement to better manage the unavoidable and copious agri-food waste (AFW) generated throughout the agri-food chain. Currently, attaining a zero AFW status is not feasible due to the inefficiencies in the agri-food chain. However, technologies to recover energy (electricity, heat, biofuel) and value-added biomaterials from AFW exist, and their effective adoption offers significant environmental and economic benefits. However, implementing efficient utilisation and valorisation of AFW at the scale needed is a non-trivial engineering challenge that must accomplish optimised energy yield (and conversion efficiencies) without environmental harm from fugitive emissions and discharge of secondary pollutants. Despite advances in standalone conversion technologies (SCTs), including thermal (incineration), biochemical (fermentation/anaerobic digestion), and thermochemical (pyrolysis, gasification and hydrothermal processes), there are still unresolved challenges (e.g., selectivity, low conversion efficiency, downstream upgrading of energy carriers), exacerbated by the accompanying production of polluting intermediates and by-products that scale with the process; potentially significant sources of GHG emissions and environmental risk if discharged untreated. Integrated conversion technologies (ICTs), that combine SCTs have been proposed as an approach that processes intermediates and by-products for further energy recovery at source. Effective implementation has the potential to enhance conversion efficiencies, energy carriers' diversification and yield. This critical appraisal of SCTs, advances in SCTs exploitation as ICTs, and developmental challenges describes the opportunities for efficient utilisation of AFW. The resultant characterisation of barriers to adopting efficient conversion technologies for AFW exploitation also aligns with growing country-level assessments of AFW bioenergy potentials and the next generation of clean energy policy priorities.