Hydrothermal liquefaction of municipal sludge and biosolids

The conversion of municipal sludge and biosolids into bio-oil via hydrothermal liquefaction (HTL) can simultaneously provide a replacement to non-renewable crude oil while dealing with waste disposal issues. Hydrothermal liquefaction takes advantage of liquid water’s interesting properties at high temperatures near the critical point, which facilitate the break down and reformation of biomass into a more energy dense bio-oil. Several laboratory-scale batch HTL experiments have been conducted with algae, woody biomass and livestock manure, but few have considered municipal sludge or biosolids. Suzuki et al. (1988) and Vardon et al. (2011) conducted studies on the HTL of sludge and biosolids, however neither study explored the effect of processing conditions (1,2). This thesis presents a study that explored how bio-oil composition and yield were affected by residence time, heating rate, initial biomass solids percentage and initial biomass composition. The highest quality bio-oil had a higher heating value (HHV) of 31.46±0.37 MJ/kg with a conversion yield of 39.42±1.4%. The HHV of the bio-oil was increased when the initial biomass had a higher solids concentration or higher HHV. The conversion yield was larger at lower solids percentages and at a heating rate of 270˚C/min. Furthermore, very few continuous HTL systems have been developed even though they may be the most viable option for scaling up. This thesis also presents the design and construction of a continuous HTL system for the processing of municipal sludge and biosolids.