• Energy Research

To obtain drop-in fuel properties from 3rd generation biomass, we herein report the catalytic hydrotreatment of microalgae biocrude, produced from hydrothermal liquefaction (HTL) of Spirulina. Our contribution focuses on the effect of temperature, initial H2 pressure, and residence time on the removal of heteroatoms (O and N) in a batch hydrotreating setup. In contrast to common experimental protocols for hydrotreating at batch scale, we devised a set of two-level factorial experiments and studied the most influential parameters affecting the removal of heteroatoms. It was found that up to 350 °C, the degree of deoxygenation (de-O) is mainly driven by temperature, whereas the degree of denitrogenation (de-N) also relies on initial H2 pressure and temperature-pressure interaction. Based on this, complete deoxygenation was obtained at mild operating conditions (350 °C), reaching a concurrent 47% denitrogenation. Moreover, three optimized experiments are reported with 100% removal of oxygen. In addition, the analysis by GC-MS and Sim-Dis gives insight to the fuel quality. The distribution of heteroatom N in lower (<340 °C) and higher (>340 °C) fractional cuts is studied by a fractional distillation unit following ASTM D-1160. Final results show that 63–68% of nitrogen is concentrated in higher fractional cuts.

Hydrothermal liquefaction (HTL) of biomass is emerging as an effective technology to efficiently valorize different types of (wet) biomass feedstocks, ranging from lignocellulosics to algae and organic wastes. Significant research into HTL has been conducted in batch systems, which has provided a fundamental understanding of the different process conditions and the behavior of different biomass. The next step towards continuous plants, which are prerequisites for an industrial implementation of the process, has been significantly less explored. In order to facilitate a more focused future development, this review—based on the sources available in the open literature—intends to present the state of the art in the field of continuous HTL as well as to suggest means of interpretation of data from such plants. This contributes to a more holistic understanding of causes and effects, aiding next generation designs as well as pinpointing research focus. Additionally, the documented experiences in upgrading by catalytic hydrotreating are reported. The study reveals some interesting features in terms of energy densification versus the yield of different classes of feedstocks, indicating that some global limitations exist irrespective of processing implementations. Finally, techno-economic considerations, observations and remarks for future studies are presented.

Intense farming activities and the growth of the population produce increasing amounts of wastes, which represent an environmental concern and require an adequate disposal. Animal manure, fish sludge, and sewage sludge are all wet wastes consisting of organic, but also inorganic material. Hydrothermal liquefaction is proposed to treat these wastes as wet feedstocks can be processed without any drying. The organic fraction is valorized, being converted into biocrude oil, while the inorganics are recovered primarily in the solid products. The decomposition of these wastes is investigated under sub- (350 °C) and supercritical (400 °C) conditions, and with and without the addition of K2CO3 catalyst with focus on the biocrude yield and quality. High yields of biocrude are obtained from the liquefaction of all the feedstocks, especially from fish sludge (ca. 50% d.a.f.) and sewage sludge (ca. 45% d.a.f.). A reduction in biocrude production is observed at supercritical conditions for animal wastes, however, the quality of manure-derived biocrudes is improved when using supercritical conditions and by the addition of the catalyst. Carbon is primarily recovered in the biocrude: 50–60% for swine and cow manure, 55–80% for fish and sewage sludge. Considerable quantities of nitrogen and sulfur are transferred to the biocrude, respectively 26–60% and 33–66%. Most of the inorganics (e.g. Ca, Mg, P) are recovered in the solids (above 70%), except for potassium and sodium, which show a higher degree of solubility in the aqueous phase.

The management and optimization of the aqueous phase are the major challenges that hinder the promotion of hydrothermal liquefaction (HTL) technology on a commercial scale. Recently, many studies reported about the accumulation of the N-content in the bio-crude with continuous recycling of the aqueous phase from high protein-containing biomass. In the present study, sewage sludge was processed at 350 °C in an autoclave. The produced aqueous phase was treated with activated carbon, and its subsequent recycling effect on the properties of the bio-crude and aqueous phase was investigated. By contacting the aqueous phase with activated carbon, 38–43% of the total nitrogen was removed from the aqueous phase. After applying the treated aqueous phase recycling, the energy recovery of the bio-crude increased from 50 to 61% after three rounds of recycling. From overall carbon/nitrogen recoveries, 50 to 56% of the carbon was transferred to the bio-crude phase and more than 50% of the nitrogen remained in the aqueous phase. The aqueous phase contained mostly of N&O-heterocyclic compounds, small chain organic acids, and amides. ICP-AES analysis showed that more than 80% of the inorganic elements were concentrated into the solid phase.

Aqueous phase recirculation increased the bio-crude yield and energy recovery along with promoting the production of N-heterocyclic compounds that lead to harsher required hydrotreating conditions.

Willows are increasingly used as natural filters to treat nutrient-rich wastewater. Their natural tendency to absorb minerals is exploited both for the nutrients and the metals, which are contained in the wastewater. This application allows addressing environmental concerns related to wastewater management and, at the same time, achieving higher biomass yields. However, the end-use of this biomass is often a simple incineration for production of heat and power. The present study proposes, alternatively, to use willow biomass, grown on wastewater irrigated fields, as feedstock for the hydrothermal liquefaction process. The thermochemical conversion route allows the valorization of the organic fraction of the biomass into a biocrude oil, and simultaneously collecting and preserving the inorganic elements in the effluent products. The willow was converted at supercritical water conditions (400 °C) for 15 min in a micro-batch reactor (10 cm 3), and high mass yields (39.7%) of energy dense (38.6 MJ kg −1) biocrude oil were obtained. It was found that most inorganics, including phosphorus (76% of total P on a mass basis), are mainly transferred to the solid products. The concentration of the elements in the solids eases their recovery and re-use for soil amendment. A different tendency was observed for potassium and sodium, which were almost exclusively collected in the aqueous phase (above 88% for both K and Na on a mass basis). Significant quantities of nitrogen and sulfur, and some metals, were transferred to the biocrude oil, however its quality resulted overall unaffected.