Challenges and opportunities in continuous hydrothermal liquefaction of high nitrogen containing biomass with respect to its aqueous phase treatment

Climate change concerns and shrinking fossil fuel resources have led to extensive research towards liquid and gaseous fuel production from renewable sources such as biomass and municipal waste (MW). Energy generation from urban and industrial municipal waste such as sewage sludge is an environmental friendly way of dealing with huge volume of waste. In addition it eliminates a part of indirect greenhouse gas emission from energy crop derived biofuels. Various factors such as population density, level of urbanization, number of operational industrial factories can overwhelm the municipal waste management system. Conventional treatments for MW include composting, incineration, landfilling etc. Landfilling and composting cannot destroy micro-plastics and heavy metals from the sludge, which are harmful for land. Incineration adds very little value and also cannot recover useful nutrients from sludge. This demands more efficient treatment methods for MW treatment and water purification. Hydrothermal liquefaction (HTL) is one such widely researched method. HTL is a thermochemical process in which raw sludge is heat treated at sub (T < 374⁰C and P < 22.1 MPa) or supercritical (T ≥ 374⁰C and P ≥ 22.1 MPa) conditions. This
eliminates the need to dewater/dry biomass which can be a major energy input for biofuel production via other processes such as pyrolysis or gasification. HTL can be used for treating waste water with or without biogas generation hence HTL technology can be retrofitted to existing wastewater treatment plants or incorporated in Greenfield projects. Since hydrothermal liquefaction is a promising technology for conversion of high-water-content biomass without the need of costly sludge dewatering, it could replace the conventional sludge treatment by making valuable energy products out of a waste material as well.

During this PhD; a lab scale continuous HTL plant will be designed, build and commissioned for experimentation with various feedstock. This will help gathering data necessary for process optimization and scale up. Batch experiments with HTL will also be carried out for initial screening of parameters such as temperature, pressure and catalyst. State of the art continuous HTL faces challenges with respect to product separation which consists of oil, aqueous and solid phases and pressure let down systems. New innovative ideas will be investigated with this lab scale unit. Both yield and quality of bio oil are important for its further processing hence its complete characterization will be done to compare composition with various feedstock and opportunities to synthesize commercially valuable chemical along with oil.

Collaborator: Aalborg Forsyning and Steeper Energy.
Funding: Innovationsfonden