Techno-economic perspective on the use of pyrolysis oil from digestate in spark-ignition engines

Purpose Fermentation residues from biogas plants (digestate) represent an abundant source of lignocellulose-based biowaste with consistent quality and quantity throughout the year. Given that the popularity of digestate pyrolysis for the production of biochar is growing worldwide, an increasing number of biogas plants obtain pyrolysis oil as a by-product for which economically lucrative uses are urgently sought. The purpose of this paper is to investigate the techno-economic aspects of such efforts. Design/methodology/approach In the current study, the pyrolysis oil was mixed with conventional fossil fuel in various proportions and tested at wide range of engine speed varying from 1,400 rpm to 2,800 rpm to estimate the effects of blends on the engine performance and emissions. The key parameters such as engine torque, engine power, specific fuel consumption, exhaust gas temperature and emissions (CO, CO2, HC and NOx) were determined for various engine operating conditions, and the results were trained using the Gradient Boosting Regressor (GBR) model. This research uses both experimental analysis and a GBR model to evaluate key parameters to find the effect of the bio-oil. Findings Experimental results reveal that increased content of pyrolysis oil reduces engine torque and engine power by 10% and 9%. Meanwhile, the specific fuel consumption increased by 4%. Nevertheless, emissions analysis shows significant reductions in the emissions of CO, CO2 and HC for pyrolysis oil-based blends. However, NOx emissions increased by 3% because of higher combustion temperatures. Originality/value Based on the comparison of actual and predicted data, it is clear that the GBR model was highly efficient in the spark ignition engines. Pyrolysis oil from digestate could mean savings in fuel costs as well as reduction in carbon footprint and, thus, contribute to the concept of circular economy.