Transesterification of high-acidity spent coffee ground oil and subsequent combustion and emissions characteristics in a compression-ignition engine

Abstract Lipids extracted from spent coffee grounds (SCG) are a potentially promising feedstock for biodiesel production if the relatively high free fatty acid (FFA) portion of the oil can be successfully converted into methyl esters, and the resulting biodiesel found to have acceptable combustion and emissions performance. This study presents experimental results obtained from transesterification of SCG-extracted oil with a FFA content of ∼30% w/w through a two-step process, followed by fuel characterization and combustion experiments with SCG-derived biodiesel, pure and blended with fossil diesel, and untreated SCG oil in a single cylinder research compression-ignition engine. The acid-catalyzed pretreatment reduced the FFA content of the oil below 1.5% w/w, with minor losses, and showed the methanol-to-FFA molar ratio to be more significant relative to the quantity of sulphuric acid used as a catalyst within the range of investigated conditions, while the subsequent base-catalyzed step converted ∼87% of the pretreated oil into biodiesel with a higher heating value (HHV) of 39.7 MJ/kg. The combustion and emission characteristics of pure and blended SCG biodiesel revealed similarities with those of commercial rapeseed and soya biodiesel samples tested. While ignition delay decreased with increasing SCG biodiesel content, the comparatively higher CO, total hydrocarbon (THC) and particulate emissions of the SCG biodiesel were attributed to higher fuel kinematic viscosity (KV). Combustion of the untreated SCG oil produced low in-cylinder peak pressure and peak heat release relative to other samples tested despite a longer ignition delay, suggesting that the oil physical properties were more important in determining combustion performance.