Bench‐Scale Evaluation of Hydrothermal Processing Technology for Conversion of Wastewater Solids to Fuels

Abstract

Hydrothermal Liquefaction (HTL) and Catalytic Hydrothermal Gasification (CHG) proof-of-concept bench-scale tests were performed to assess the potential of hydrothermal treatment for handling municipal wastewater sludge. HTL tests were conducted at 300 to 350 °C and 20 MPa on three different feeds: primary sludge, secondary sludge, and digested solids. Corresponding CHG tests were conducted at 350 °C and 20 MPa on the HTL aqueous phase output using a ruthenium-based catalyst. Biocrude yields ranged from 25 to 37%. Biocrude composition and quality were comparable to biocrudes generated from algae feeds. Subsequent hydrotreating of biocrude resulted in a product with comparable physical and chemical properties to crude oil. CHG product gas methane yields on a carbon basis ranged from 47 to 64%. Siloxane concentrations in the CHG product gas were below engine limits. The HTL-CHG process resulted in a chemical oxygen demand (COD) reduction of > 99.9% and a reduction in residual solids for disposal of 94 to 99%.

Continuous Hydrothermal Liquefaction of Biomass: A Critical Review

“…On the other hand, nitrogen was not removed to a high extent, which constitutes an issue in view of wastewater disposal. Similar results were also obtained in other studies from the same group from the processing of grape pomace [45] and wastewater solids [46].…”

“…[44] and Chlorella, with both standard and high lipid content [43]. Some experience was also gained in the processing of agricultural waste, such as grape pomace [45], and of wastewater solids [46]. In general, the tests conducted at PNNL are carried out at temperatures around 350 • C and cover a wide range of dry matter concentrations, ranging from 5% up to 34.4% in the case of algae slurry.…”

Continuous Hydrothermal Liquefaction of Biomass in a Novel Pilot Plant with Heat Recovery and Hydraulic Oscillation

“…At these conditions, minerals have a low solubility in water, which leads to precipitation of salts, which can settle out in a precipitation vessel and with the combined filtration avoids any precipitates passing through the system. This can be particularly attractive for the recovery of phosphorous for future nutrient recycling applications [28]. PNNL report an ash content in the bio-crude from primary sludge of 0.4 wt% versus 20 wt% in the current study, also produced from primary sludge [24].…”

“…Jazrawi et al reported up to 50 wt% and 30 wt% of carbon retention in the bio-crude and process water respectively during continuous flow HTL of microalgae [13]. Marrone et al found approximately 60 wt% and 40 wt% of the carbon present in primary and secondary sludge respectively to partition in the bio-crude during continuous flow HTL while 20 wt% and 40 wt% of carbon was retained in the process water [28]. The literature data shows that higher carbon recoveries to the bio-crude are possible in continuous flow HTL reactors and this is an area we aim to improve the operation by, for example, recycling process water, optimising the use of oscillation for increased mixing, increasing the DM content of slurries to the pumpable maximum, improved product separation and potentially longer residence times.…”

“…The Pacific Northwest National Laboratory (PNNL) in the USA established a continuous HTL reactor system incorporating a continuous-flow stirred-tank reactor and a horizontal tubular reactor [16,24,26]. The system has been demonstrated for microalgae [27], macro-algae [16], food waste [26], sewage sludge and digested solids [24,28]. The main part of the system consists of a horizontal preheater with a capacity of 210 mL, a stirred tank reactor with a capacity of 415 mL where the slurry is heated to the reaction temperature (350 • C), a horizontal oil jacketed reactor (capacity 300 mL) for maintaining the slurry's temperature and a high temperature-pressure filter for removing solids at the reaction conditions.…”