Title: Hydrothermal Liquefaction of Biopulp: Waste Management and Sustainable Bioblendstock Production
Abstract: In the past decades, increasing attention has been drawn to reducing the transportation sector's carbon footprint by substituting conventional fossil fuels with renewable liquid fuels.Moreover, the challenges associated with the environmental-friendly disposal of municipal solid waste have urged the scientific communities and policymakers to explore alternative technologies.As a prospective technology, hydrothermal liquefaction (HTL) stands out as a competent technology for urban waste disposal along with a promising method of producing liquid fuel precursors.The flexibility of HTL in treating highly moist feedstocks enables it to be a great candidate in valorizing the embedded calorific value of the organic fraction of municipal solid waste (MSW) into an energy-dense intermediate product so-called biocrude.Despite the significant benefits, HTL and the resultant product-of-interest still encounter serious challenges that if remain untacked, can potentially dominate the technology readiness level, and risk its commercialization.The present work is built upon addressing various challenges in the development of continuous HTL, valorization of the aqueous phase affluent, and utilization of the upgraded biocrude as an alternative diesel blendstock.In response to the heavily contaminated MSW (by non-biodegradable substances), the development of a seamless waste-to-biocrude scheme through the integration of a biomass pretreatment technique into the HTL process is closely studied in terms of biomass properties.The Ecogi® process yielding a moist feedstock (biopulp) was targeted as the primary mechanical pretreatment.The various aspects of the biopulp were thoroughly inspected.The biomass containing 15 to 25 wt.% of moisture, less than 1 wt.% of plastic contamination (db), and over 90 wt.% of convertible organics (db) was verified as an ideal slurry to be used in HTL as such.The chemical conversion of the biomass macromolecules to the intermediate product was investigated in different HTL operating conditions seeking the optimal temperature and homogeneous catalyst in lab-scale setups.Furthermore, the obtained results were utilized to establish a continuous bench-scale campaign.The results have shown that K2CO3-assisted subcritical (350 °C) yielded the highest biocrude yield (over 35 wt.%) with a superior saturation (H/C: 1.64).It has also been found that the biocrude was majorly composed of fatty acids and fatty amides originating from the high crude lipid content of the biopulp.The results of the continuous campaign showed that the presence of K2CO3 as a homogeneous catalyst can ensure a steadier operation by preventing intermediates repolymerization in the unit's preheaters.Moreover, the involvement of the alkali catalysts secured transferring the inorganic elements to the solid residue phase, facilitating feasible NPK recoveries.To cope with the energy-intensive HTL wastewater treatment requirement and simultaneously aim to increase the energy distribution to biocrude, recirculation of the concentrated aqueous phase was performed in this study.Removal of the surplus water allows the value-chain to maintain uninterrupted operation without the need for biomass pre-drying.The primary results have shown that the evaporation of the aqueous phase could eliminate up to 30 wt. % of total nitrogen and 60 wt.% of ammonia while maintaining over 80 wt.% of the carbon into the cycle.The mass Abstract iv distribution results revealed a significant increase in biocrude mass (over 8 wt.%), carbon (over 4 %), and energy recovery after consecutive recirculation.The chemical composition results on the other hand indicated a greater presence of N-containing compounds mainly in the form of fatty amides and N-heterocyclics in the subsequent cycles that if end up in the final fuel, might inversely affect the performance of the engine and the exhaust gases.Therefore, a hydrotreating step operating at two operating conditions was carried out.The results projected a higher N distribution in the consecutive hydrotreated biocrude possibly due to the refractory N-heterocyclics originating from the cyclization and Maillard reactions.Achieving a high nitrogen removal requires a severe hydrotreating condition (high temperature, H2 partial pressure, and WHSV) that can in return negatively influence the process economy and trigger catalyst deactivation.The resultant biocrude from the continuous campaign was subjected to another study to assess the capability of HTL fuel in being viewed as an on-specification diesel blendstock.The impact of a mild pre-stabilization step along with vacuum distillation was investigated in terms of distillate recovery and the chemical and physicochemical properties.Later, a multi-objective optimization method was developed to optimize the selection of the biocrude distillates (biodistillates mixture) and blend the mixture to the reference diesel.The produced blend fuels were then combusted in an internal combustion engine to monitor the fuel emission profiles.The results have shown that the raw and hydrotreated biocrudes substantially deviated from the specifications announced by EN590 for road diesel.Moreover, the challenges with blending and fuel storage stability were also detected resulting in disregarding them as the potential bioblendstocks.On the other hand, vacuum distillation considerably augmented the physicochemical properties of the fuel, allowing it to produce on-spec blendstocks.The mild hydrotreating (pre-stabilization) step significantly contributed to a higher distillate recovery from 22.6 to 55.6 wt.% through the removal of oxygenated compounds and prevention of repolymerization reactions during distillation operation.The results also indicate that the HTL value-chain could be optimized based on the most limiting blend walls to maximize the Bioblend share in the final fuel.The emission results heralded promising results for the hydrotreated biodistillate mixtures with comparable SO2, THC, CO, and CO2 to the reference diesel.The outcome of the present Ph.D. thesis highlights the flexibility of the HTL process in valorizing highly moist urban waste.It also contributes to providing a beneficial pathway for the recirculation of the aqueous phase residual carbon to the process while treating wet biomass.The thesis addresses the challenges that correspond to the continuous HTL operation of the biopulp and provides a mathematical method to obtain on-spec blend road diesel.All in all, the results of the activities carried out in the framework of this thesis support the upscaling and development of future HTL value-chain schemes.