Heterogeneous catalytic conversion of solid anaerobic digestate waste to biofuels and value-added chemicals

Abstract

Increasing use of anaerobic digestion (AD) in agri-food waste management for the production of renewable methane has presented issues surrounding AD. One of the challenges of AD is handling the solid fraction of the digestate. Whilst the digestate, especially the liquid fraction, can currently be used as bio-fertiliser, there are still limitations to its use because of regulations on the nutrient loading in soil and possible run off resulting in eutrophication of water. Hence, there is a need for alternative forms of valorisation of both the liquid and solid digestate (SD). This work will focus on the potential of SD being a feedstock for biofuel and fine chemicals. The valorisation of the lignin constituent of SD would be a significant contribution to the success of the lignocellulosic biorefinery economy. This paper investigates the hydrogenolysis (HDO) of SD using Ni/ZnO₂, Cu/Al₂O₃ and platinum group metal (PGM) catalysts such as Rh/C, Pd/C and Ru/C to extract the biofuel. The major products from the depolymerisation of the phenolic lignin monomer constituent of the SD are the known octane booster ethylguaiacol and the structurally related antioxidant butylated hydroxytoluene. Furthermore, using 2-methyltetrahydrofuran as a green solvent increased substrate conversion (by ∼13%) and bio-oil yield (by ∼17 wt%) compared to dioxane solvent used in previous studies. The life cycle assessment of the catalytic extraction of anaerobic digestate showed a total energy consumption of 849 MJ per tonne of digestate utilised and a net energy ratio of 1.7, which is considerably better than fossil petrol and diesel and comparable to conventional temperate biofuels.