Performance and exhaust emissions of diesel engines using microalgae FAME and the prospects for microalgae HTL biocrude

Microalgae's potential as a third generation biofuel has attracted recent attention. Two key methods for obtaining biofuel from microalgae are compared and contrasted, namely solvent extraction followed by transesterification, producing Fatty Acid Methyl Esters (FAME) biodiesel, and hydrothermal liquefaction (HTL) to producing biocrude. The resulting differences in physicochemical characteristics affect engine performance and emissions as well as engine selection. Most microalgae engine research has been conducted on high speed diesel engines to investigate the performance and exhaust emissions of FAME whereas the HTL literature has mostly presumed biocrude would be upgraded for similar engines. However, growing awareness of the significant contribution of shipping emissions to public health brings into focus alternatives to heavy fuel oil (HFO) for low speed diesel (marine) engines. Such engines are thermodynamically very similar to high speed diesel engines but have the economic advantage that they can use low cost, high density fuels such as HFO/HTL. Microalgae FAME contains 10.5-11% (wt) oxygen and 36.2-39.2 MJ/kg calorific value, (10-15% lower than for petroleum diesel). High speed diesel engines tests using microalgae FAME generally decrease particulate emissions with a small penalty in engine power owing to the high oxygen content and lower Higher Heating Value (HHV). Conversely, biocrude obtained by HTL from green freshwater microalgae contained 10-11% (wt) oxygen, 4-8% (wt) nitrogen and 32-35 MJ/kg calorific value (20-27% lower than for petroleum diesel). HTL biocrude would be expected to reduce emissions for low speed marine diesel engines compared to HFO, especially soot emissions due to their very low sulphur content, although NOx emissions may increase. HTL biocrude may reduce the engine performance (output power) compared to HFO due to the higher oxygen content and lower HHV. This review' aim is to compare microalgae FAME and HTL biocrude and their suitability for diesel engines regarding engine performance and emissions.