Assessing Hydrothermal Liquefaction of Lignocellulosic Biomass, Microalgae and Sewage Sludge at Pilot Scale

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1 Assessing Hydrothermal Liquefaction of Lignocellulosic Biomass, Microalgae and Sewage Sludge at Pilot Scale Biological and Chemical Engineering, Department of Engineering, Aarhus University, Denmark K. Anastasakis, I. Johannsen, R. B. Madsen, P. Biller

2 HTL OVERVIEW Terrestrial biomass Aquatic biomass Heat HTL Bio-crude Solids HTL water Wastes/sludges Water Gases (CO 2 ) Processing of aqueous slurries No need for drying Any biomass able to be suspended in water can be processed High pressure helps maintain the water in the liquid phase eliminating the energy intensive step of water vaporization HTL has been demonstrated for a variety of different feedstocks at small laboratory batch scale

3 HTL PROCESS DEVELOPMENTS Scaling up HTL reactors from batch-lab scale to continuous-pilot scale: PNNL (Pacific Northwest National Laboratory, US): continuous HTL-system with oil jacketed preheater, electrically heated stir tank reactor, oil jacketed reactor, high T & P filter. Total volume of the system ~1.6 l Sydney University (Australia): continuous HTL-system with two heat exchangers and four SS coils immersed into a fluidized bed as a reactor. Total volume of the reactor ~2l Aalborg University & Steeper Energy (Denmark): Supercritical HTL (400 C and 300 bar), feeds slurry is pressurized prior to feeding into two heaters. Capacity of 30-kg/h slurry throughput

4 AU PILOT-SCALE HTL SYSTEM PFD 7 sections: Feed introduction system Heat exchanger Trim heater Reactor Oscillation system Take-off system Product collection zone Typical flow rate 60 l/h, P=220 bar, T=350ºC Total Volume of the system ~20 l

5 AU PILOT-SCALE HTL SYSTEM HEX: 8 pipes divided in 4 convolutions of 2x6 meter each, total length of both sides is 49.2 m Trim heater: 4 pipes divided in 2 convolutions of 2x3 meter each, making a total length of 12.6 m equipped with 32 independent controlled electrical heaters Reactor: 10 pipes divided in 5 convolutions of 2x6 meter each, making a total pipe length of 65.1 m - equipped with 5 independent controlled electrical heating tapes Main parts of reactor system, different sections from bottom to top: heat exchanger, trim heater, reactor Pilot-scale HTL reactor at AU

6 SLURRY PREPARATION (PRE-TREATMENT) Freshly harvested Miscanthus (early-january 2018) from AU fields Spirulina powder was bought from Inner Mongolia Rejuve Biotech Co. Ltd (China) Sewage sludge collected locally from Viborg WWT Key issue in continuous HTL systems Produce a homogenous, stable and easily pumpable slurry Depending on the nature of the feedstock different pretreatment is needed Presence of lignin leads to formation of hard spherical particles upon milling or grinding blocking the piston of the pump Tackle this issue by extrusion and addition of carboxymethyl cellulose (CMC) as a thickener Pre-treatment flow diagram (solid lines=main flow, dotted lines=recirculation flow) Feedstock Extrusion Catalyst (KOH) CMC Slurry mass (kg) Slurry DM (wt%) Miscanthus Yes Yes Yes Spirulina No No No Sewage sludge No No No

7 TEMPERATURE AND RESIDENCE TIME DISTRIBUTION Incoming feed is heated from ~20 C to ~ C in the heat exchanger in ~2.5 min Heated to the reaction temperature (350 C) in the trim heater in ~1 min Temperature is maintained, with some minor fluctuations ( C), in the reactor for ~5.6 min Product stream is then directed to the return-flow side of the heat exchanger, where it transfers heat to the incoming feed and is being cooled from the reaction temperature to ~80-85 C in ~3.2 min Finally, the product stream passes through the cooling section where is being cooled to approximately 60 C in 1.5 min before entering the product collection zone Temperature profile and residence time in the different process units of the HTL reactor (heat exchanger-feed flow, trim heater, reactor, heat exchanger-return flow and cooler) at 220bar for HTL runs of miscanthus Heating profiles and residence time distribution are similar for the three different biomass slurries

8 HEAT RECOVERY BY THE HEAT EXCHANGER Temperature profile at the exit of the heat exchanger (feed flow) and inside the reactor and energy consumption by the trim heater at 220bar for HTL runs of miscanthus Sudden drop in temperatures once the feed slurry is introduced After re-equilibration temperature in the reactor is maintained constant indicating steady-state operation Temperature at the HX outlet monotonously increasing till the end of the feed Direct effect on the heat recovered (HR%) by the HX Energy requirements by the trim heater showed reverse trend Longer run times should further increase the heat recovered by the heat exchanger, to higher than 80%, leading to significantly less energy input for heating of the system Heat recovery (HR %) by the heat exchanger for HTL runs of miscanthus Time HR (%) 12:00-13: :00-14: :00-15: :00-16: :00-17: :00-18: Average 75.2

9 EFFECT OF OSCILLATION Temperature profile at the exit of the heat exchanger (feed flow) and inside the reactor and energy consumption by the trim heater at 220bar for HTL runs of sewage sludge Oscillation on for about an hour Effect of oscillation is apparent in the HX graph and is characterized by a peak in T Significant increase in HR during the time intervals oscillation was on Oscillation system is believed to increase turbulence in the whole reactor system leading to improved mixing and enhanced heat transfer Heat recovery (HR %) by the heat exchanger for HTL runs of sewage sludge Time HR (%) 11:00-12: :00-13: :00-14: * 14:00-15: * 15:00-16: Average 75.3 * Oscillation ON for certain time intervals

10 BIO-CRUDE YIELDS Samples were collected in duplicate every half-hour to 45 min Bio-crude yields for miscanthus were found to vary between 18.5wt.% and 37.5 wt.% Average bio-crude yield ~26 wt.% Big variation in yields due to: Changes in T in the pipes after the exit of the HX and the take-off system Pipe network needs to get saturated with oil or be hot enough to avoid bio-crude condensation Each piston in the take-off system can give different volumes of liquid product during its discharge due to the produced gases Appears to be an increase in bio-crude yields with time HHV between 29 MJ/kg and 36.5 MJ/kg Average HHV ~30.5 MJ/kg Bio-crude yields and HHVs over time for HTL of miscanthus Similar behavior for the rest of feedstock Average values: Spirulina: Bio-crude yield = 32.9 wt.% - HHV = 35.6 MJ/kg Sewage sludge: Bio-crude yield = 24.5 wt.% - HHV = 26.8 MJ/kg

11 PROCESS EFFICIENCY Miscanthus Spirulina Sewage sludge Flow rate (l/h) DM content Time (h) Feedstock consumed (kg, dry) Energy in feedstock (kw, dry) 42.7 (HHV=17.1MJ/kg) 63.1 (HHV=23.1MJ/kg) 13.2 (HHV=19.8MJ/kg) Bio-crude yield (wt.%) Energy in bio-crude (kw, dry) 19.9 (HHV=30.6 MJ/kg) 32 (HHV=35.6 MJ/kg) 4.4 (HHV=26.8 MJ/kg) n th (%) Trim heater energy requirement (kw) Reactor energy requirement (kw) Main pump energy requirement (kw) n tot (%) EROI* * Calculated by considering energy requirements of the HTL process

12 CONCLUSIONS HTL of three biomass feedstocks (terrestrial, aquatic, waste) with different biochemical composition has been successfully processed by pilot-scale HTL Biomass slurries up to 16 wt.% dry matter content were able to be processed HR by the heat exchanger was up to 80% Longer run times should further increase the heat recovered by the heat exchanger, to higher than 80%, leading to significantly less energy input for heating of the system Hydraulic oscillation system appears to improve mixing and enhance heat transfer Samples collected during the runs were found to have big variation with each other indicating the difficulty in accurately sampling in pilot-scale continuous HTL systems Average bio-crude yields for miscanthus, spirulina and sewage sludge were 26.2, 32.9 and 24.5 wt.%, respectively EROI for miscanthus, spirulina and sewage sludge was 2.8, 3.5 and 0.5, respectively Further improvement on operation conditions (e.g. longer run times) and on sampling procedure is needed

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