Investigation of anaerobic digested pyrolysis oil and waste derived biodiesel blends as sustainable fuel for marine engine application

In anaerobic digestion (AD) plants around 40% of the feed waste is utilised for biogas generation while the remainder is used as a fertiliser that is rich in oil that could be further utilised through pyrolysis. Dry AD waste, subjected to intermediate pyrolysis, generated a digested pyrolysis oil (DPO) of high viscosity (473 mm2/s) and low energy content (26 MJ/kg). To improve its fuel characteristics, we blended DPO with either used cooking oils (high in unsaturated fatty acids) or lamb fat (high in saturated fatty acids) using solvents of differing polarity including alcohols varying in chain length and hence aqueous miscibility from C1(methanol) to C5 (pentanol) and diethylether (DEE, a non-polar solvent). Five blends were prepared, typically consisting of 10–20% DPO, 20% solvent (alcohol or DEE) and 60–70% cooking oil or lamb fat. Physical properties of DPO blends e.g., viscosity, density, heating value, total acid number and flash point were found to be lower than neat DPO but increased about 25–40% over a storage period of 8 months. The blends were more stable than neat biodiesel as assessed by FT-IR spectroscopy. The elemental analysis (C, H, N, S and O) showed that atomic ratio of DPO blends; H:C, H:O, H:N and C:N were found to be lower than biodiesel and DPO fuels, respectively. Overall, the properties of DPO blends met the Marine Fuel Standard (Marine ISO 8217) and hence could be considered as a suitable replacement for bunker oil to protect marine environments following further testing and scale up studies.