Combustion and Emissions Study of New Renewable Hydrothermal Liquefaction Biofuel: An Experimental and Numerical Investigation

This doctoral research investigates the potential of biofuel derived from municipal solid waste (MSW) via hydrothermal liquefaction (HTL) as a diesel blendstock for compression ignition (CI) engines. Two variants of HTL biofuel were evaluated: one processed solely by distillation (nonupgraded HTL biofuel) and another subjected to both hydrotreatment and distillation (upgraded HTL biofuel), each derived from raw bio-crude. The study assessed the influence of blending these HTL biofuels with reference diesel (RD) on combustion and emission characteristics under engine-like conditions in an optically accessible compression ignition chamber (OACIC). Key combustion parameters measured included heat release rate (HRR), in-cylinder pressure, ignition delay (ID), flame lift-off length (FLOL), and in-flame soot, while emissions such as particulate matter (PM), carbon monoxide (CO), nitrogen oxides (NO_x), and carbon dioxide (CO₂) were recorded. In parallel, an HTL surrogate model was investigated to examine the effect of fuel-bound nitrogen on NO_x emissions. Pyridine was employed as a representative surrogate for nitrogen in HTL biofuels, and its impact was analyzed using a Stochastic Reactor Model (SRM). The simulated combustion and emission data were validated against experimental results from the OACIC, ensuring model reliability across various HTL-diesel blend ratios. The findings indicate that HTL biofuel blends exhibit combustion and emission behaviors closely resembling those of reference diesel. Notably, upgraded HTL blends demonstrated improved combustion performance—evidenced by a shorter ignition delay and flame lift-off length—compared to both RD and nonupgraded blends, although they produced moderately higher NO_x emissions. Analysis revealed that the increase in NO_x emissions with higher HTL content is primarily due to fuel-derived NO_x, as thermal NO_x production remained relatively constant across all blends. The proportion of fuel-based NO_x in overall NO_x formation was quantitatively determined. Overall, this study substantiates the viability of HTL biofuels as renewable diesel blendstocks and highlights the importance of further reducing fuel-bound nitrogen, particularly in upgraded HTL biofuels, to mitigate NO_x emissions.