Continuous Hydrothermal Liquefaction Research Articles

Thermal and Oxidative Stability of Biocrude Oil Derived from the Continuous Hydrothermal Liquefaction of Spirulina

Thermal and Oxidative Stability of Biocrude Oil Derived from the Continuous Hydrothermal Liquefaction of Spirulina

Hydrochar fractionation and composition in batch and continuous hydrothermal liquefaction

Understanding differences in hydrochar characteristics in batch and continuous hydrothermal liquefaction (HTL) is crucial in determining suitable valorisation routes for this byproduct, given its high nutrient load and carbon sequestration potential.This study thoroughly characterised hydrochar from batch and continuous HTL at 300, 325 and 350 °C, elucidating their main differences and shedding light on the operational parameters causing them. For this purpose, a bench-scale continuous HTL unit with in-line solids separation was commissioned. It was possible to differentiate primary and secondary char and infer their formation mechanism.The results showed that batch hydrochar yields were higher (24–26%) than continuous (primary and secondary) char yields (15–19%). In both reactions, higher temperatures led to chars with lower carbon and nitrogen contents. The ash content of batch hydrochars was lower than that of continuous primary chars, revealing that the in-line char separator effectively removed inorganic impurities at reaction conditions and produced a cleaner biocrude. The nutrient distribution in the HTL products showed batch biocrudes were more contaminated by Na, K and Fe, while in the continuous biocrudes, only Fe was detected. Moreover, less carbon migrated to the solids from continuous HTL, indicating that removing inorganics may reduce secondary char formation.Batch hydrochars showed a higher presence of oxygenated and nitrogenated compounds, while the continuous primary chars had a higher share of alkanes and alkenes.These differences may imply that batch reactions may not serve as indicators, in terms of hydrochar characteristics, for HTL upscaling to industrial plants.

Continuous Hydrothermal Liquefaction of Mexican Sargassum Seaweed—An Analysis of Hydrocarbon Fractions and Elemental Composition

Hydrothermal liquefaction (HTL) is often mooted as a promising and sustainable processing methodology for converting biomass into usable products, including bio-oils, which can potentially alleviate humanity’s reliance on fossil fuels. To date, most HTL development work with novel biomasses has been undertaken at the laboratory scale in batch processes, and the results have been extrapolated to the theoretical continuous flow processes required for industrial uptake. Here, we assess the use of a novel continuous flow HTL system, applying it to Sargassum (seaweed) material and generating a bio-oil, which is assessed against typical crude oil fractions.

Process simulation for mass balance of continuous biomass hydrothermal liquefaction with reaction kinetics

Hydrothermal Liquefaction (HTL) process has been investigated for biomass conversion to bio-crude and other valuable materials. However, this process modeling remains a challenge due to the high complexity of its feedstock and product compositions and the numerous chemical reactions occurring simultaneously. Here, we investigate the mass balance of the HTL reaction using a process simulation with a kinetic model previously developed for the HTL of microalgae. This model was implemented in UniSim-Design software, and the simulation's performance was validated against published experimental results for various feedstocks, including microalgae, macroalgae, food waste, and model protein. Bio-crude had the best performance for predicting the products' yields from HTL of microalgae, and aqueous-phase had the poorest. For predicting the bio-crude yield from HTL of different feedstock categories, microalgae had the best performance, and model proteins had the poorest. Insights are suggested for improved modeling towards a more robust system-design-oriented approach for process simulations.

An Ecological Toilet System Incorporated with a Hydrothermal Liquefaction Process

The harmless disposal and resource utilization of human feces is important to the sanitation process. Hydrothermal liquefaction (HTL) can convert toilet feces into bio-crude oil and reduce waste. In this study, an integrated eco-toilet system was developed by combining vacuum micro-flush toilets with a continuous hydrothermal liquefaction reactor. The system operated stably for over 10 h. This system can serve 300 households and save 2759 m3 of water per year compared to traditional flush toilets. The energy recovery from the feces was 2.87 times the energy consumed for the HTL process. The HTL bio-crude oil yield was 28 wt%, and the higher heat value (HHV) of the bio-crude was 36.1 MJ/kg. The biochemical compounds of the bio-crude oil consisted of acid ester, hydrocarbons, phenols, and a nitrogenous heterocyclic compound. The carbon in the human feces was mainly transferred to the bio-crude oil, while nitrogen was mainly transferred to the aqueous phase product. The post-HTL aqueous stream could be treated and used as fertilizer. This system achieves energy self-sufficiency, along with water and energy savings. This integrated eco-toilet effectively converts feces into bio-crude to realize waste reduction and resource utilization of human feces.

Sustainable Valorization of Wood Residue for the Production of Biofuel Materials Via Continuous Flow Hydrothermal Liquefaction

When forests are extensively cleared for infrastructure and agricultural purposes, over half of the wood generated is regarded as waste. To minimize the negative consequences and successfully use renewable energy sources, further applications must be investigated. Biomass has the potential to be converted into biofuels using a process known as hydrothermal liquefaction (HTL). The conversion of wood residue under continuous flow, subcritical HTL, under both untreated and alkaline pretreated conditions was examined in this study. According to the results, pretreatment with 4% NaOH sped up recovery times compared to using raw wood and increased glucose production by 1.8 times (equivalent to 90 g/L) in comparison to using raw wood. Additionally, the fluid's pH was raised from 6.1 to 3.5 due to the alkaline pretreatment, which also switched the hydrolysis of arabinose and cellobiose to rhamnose at 0.2 MPa. The average Net Energy Ratio (NER) for glucose during liquefaction reached values as high as 63%, while the energy output from glucose during the pretreatment reached 246 kJ. The move to green energy transition toward net zero will be aided by these findings, which will usher in new waste conversion techniques to produce sustainable biofuels using continuous flow HTL.Graphical Abstract

Development of a rigorous and generalized model on the hydrothermal liquefaction (HTL) process for bio-oil production

A rigorous process model for hydrothermal liquefaction (HTL) of microalgae is proposed in this work. This research attempts to uncover a complicated simulation scenario for the characterization of microalgae, the development of suitable reaction pathways, kinetics, and thermodynamics. In this study, 55 model compounds and 41 individual reactions were used to describe an HTL system. The kinetic parameters were regressed using literature data, which reported the results under different operating conditions using the three species of microalgae. The proposed kinetic model revealed acceptable predictivity by predicting 117 published biocrude yields (total 160) to within ± 10 wt% and 28 published HHV (total 39) to within ± 5 MJ/kg. Finally, a continuous HTL process was conceptually designed. The trade-off between the biocrude yield and HHV was investigated via multi-objective optimization (MOO), which yielded the best trade-off between the biocrude yields (37.2–60.0 wt%) and HHV (27.5 and 33.5 MJ/kg). This work presents a satisfactory first attempt to rigorously simulate a very complex HTL process. Its application in the preliminary process design, optimization, and economic analysis is recommended.

Convective enhancement of microalgae slurry in continuous tubular reactors for biocrude production during hydrothermal liquefaction

Continuous hydrothermal liquefaction (HTL) reactor is promising in the large-scale production of biocrude from microalgae biomass. To optimize the temperature distribution and biocrude yield of microalgae slurry, the convection of microalgae slurry is enhanced through adjusting flow rate (1 – 24 mL/min) and inserting twist tapes (twist ratio: 3 – 10) in the flow channel, respectively. A computational fluid dynamics (CFD) model integrated HTL kinetics of microalgae biomass is proposed to simulate the contour plots of temperature and biocrude yields at the heating temperature from 523 to 623 K. Comparing to increasing flow rate, the swirl flow induced by the twist tape can significantly increase the convective heat transfer coefficient from 669 to 1707 W·m−2·K−1, while the conversion rate of biocrude from 0.188 to 0.298 g·L-1·s−1 at 8 mL/min and 623 K. As the enhancement factor of convective heat transfer coefficient h /h0 is higher than 2, the biocrude yield YBC of microalgae slurry appears obvious increasing by the enhancing convective heat transfer. This investigation provides the insights to optimize the flow channel of tubular reactors for the optimization of HTL conversion of microalgae biomass.

Techno-economic baseline study for a waste-based biorefinery in South Africa

A baseline techno-economic assessment for converting organic municipal waste via a novel continuous hydrothermal liquefaction (cHTL) pilot plant is presented. Feed streams, product yields, and product quality were based on real-time data. Pilot plant data was used to estimate the capital investment and operating costs, cashflow and economic indicators for a commercial-scale cHTL plant. Results showed that a return on investment of 31 %, an internal rate of return of 31 %, and a profitability index of 5.5–6.0 could be realized with a satisfactory nett cashflow over an estimated plant life of 20 years. Sensitivity analyses showed that both plant capacity and capital investment had a significant effect on product unit cost. The potential of using cHTL technology to sterilize and co-treat primary sewage with municipal waste was demonstrated for underpinning a circular economy where waste would be re-directed toward new energy products, while offsetting localized municipal operating costs.

Comparative investigation on the value-added products obtained from continuous and batch hydrothermal liquefaction of sewage sludge

Hydrothermal liquefaction (HTL) can be considered a promising route for the energy valorisation of waste sewage sludge (SS). However, not much information is available on continuous flow processing. In this study, the mixed SS was subjected to HTL at 350°C for 8 min in a continuous reactor with loadings of 10 wt% in the feed flow. The results show that the mass recovery reached 88%, with a biocrude yield of 30.8 wt% (3.9 wt% N content). The recovered biocrude yields are highly dependent on the selection of the recovery solvent for extraction, and dichloromethane can contribute an additional 3.1 wt% biocrude from aqueous phase, acetone can extract some pyrrole derivatives into the trapped phases. Comparable results were also achieved by performing batch reactions under the same conditions: a slightly higher biocrude yield (33.1 wt%) with an N content of 4.3 wt%. The higher N content observed in the biocrude from the batch process indicates that interactions and chelation between intermediates are enhanced during heating up and cooling period, which lead to more N-containing compounds.

Synergies during hydrothermal liquefaction of cow manure and wheat straw

Agribusiness crop and animal residues constitute some of the major waste streams worldwide. Among them, wheat straw and cow manure are large contributors to the quantities generated. Both materials can be converted using hydrothermal liquefaction (HTL) for recovery of biocrude and here we investigate how combined HTL processing can be of great interest to boost biocrude production and carbon recovery. This study presents batch HTL experiments using individual and blended feedstock mixtures to build predictive models for biocrude, carbon and energy recovery. These models are validated using a continuous HTL pilot plant. The combined approach led to the nitrogen-containing compounds present in cow manure to react with lignocellulosic-derived compounds from wheat straw and divert carbon into the oil phase, the reason for which biocrude, carbon and energy yields were drastically improved. Continuous HTL pilot plant campaigns successfully demonstrated increased carbon yields from 40% to 60% when using optimal feedstock ratios. Continuous data also shows the great benefits of increasing the organic matter concentration input with combined processing, resulting in total energy efficiencies larger than 50% and energy return over investment of 2.6, compared to 1.3–1.9 for individual feedstock processing.

Coupling hydrothermal liquefaction and aqueous phase reforming for integrated production of biocrude and renewable H2

AbstractLignin‐rich stream from lignocellulosic ethanol production was converted into biocrude by continuous hydrothermal liquefaction (HTL) while hydrogen was produced by aqueous phase reforming (APR) of the HTL aqueous by‐product. The effects of Na2CO3 and NaOH were investigated both in terms of processability of the feedstock as well as yield and composition of the obtained products. A maximum biocrude yield of 27 wt% was reached in the NaOH‐catalyzed runs. A relevant amount of dissolved phenolics were detected in the co‐produced aqueous phase (AP), and removed by liquid–liquid extraction using butyl acetate or diethyl ether, preserving the APR catalyst stability and reaching an hydrogen yield up to 146 mmol H2 L−1 AP. Preliminary mass balances integrating HTL and APR showed that the hydrogen provided by APR may account for up to 46% of the hydrogen amount theoretically required for upgrading the HTL biocrude, thus significantly improving the process performance and sustainability.

Blending of Hydrothermal Liquefaction Biocrude with Residual Marine Fuel: An Experimental Assessment

As with all transport modes, the maritime sector is undergoing a drastic transition towards net zero, similar to the path in which Aviation is already engaged through global decarbonization programs such as CORSIA for the International Civil Aviation Organization, or the Emission trading Scheme of the European Union). Maritime indeed shares with Aviation a common element: the difficulty of shifting to electric in the short to medium term. Therefore, the use of sustainable fuels represents the main and only relevant option in this timeframe. As sustainable biofuels will be used as blend components in the case of large-scale deployment, it is necessary to investigate the behavior of bio- and fossil-based fuels when mixed in various percentages, in particular for low quality products such as HTL (HydroThermal Liquefaction) and fast pyrolysis oils from lignocellulosic biomass and waste. Biocrude from subcritical hydrothermal liquefaction of undigested sewage sludge, produced at reaction conditions of 350 °C and 200 bar in a continuous HTL pilot scale unit, was manually mixed at 70 °C with residual marine fuel (low-sulphur type F-RMG-380 per ISO 8217) at two different nominal biocrude shares, respectively 10 wt.% and 20 wt.% in the mixture. While the former blend resulted in the technically complete dissolution of biocrude in the fossil component, the latter sample formed biocrude agglomerates and only partial dissolution of the biocrude aliquot in marine fuel could be achieved (calculated between 14–16 wt.%). The blend with 10 wt.% of SS biocrude in the mixture resulted in compliance with limits of total acid number (TAN), inorganics (in particular vanadium, sodium, silicon and aluminum) and sulphur content, while only the ash content was slightly above the limit.

Microplastics degradation through hydrothermal liquefaction of wastewater treatment sludge

Wastewater treatment plant sludge contains large quantities of microplastics (MPs), which is a problematic substance that impedes sustainability efforts, such as in land management. MPs are resilient to degradation, but extreme conditions, such as high temperature and pressure, can lead to residues that can be used as fertilizers on farmlands. Hydrothermal liquefaction (HTL) creates such conditions, converting sludge into valuable bio-crude. To this end, the current study examined the resilience of MPs in sewage sludge that were treated by continuous HTL operated at supercritical water conditions (400 °C, 30 MPa). MPs were extracted before and after the HTL process and quantified by Fourier Transformation Infrared Spectroscopy (FTIR). Particles of 10–500 μm were quantified using Focal Plane Array (FPA) based micro-FTIR (FPA-μ-FTIR) imaging combined with an automated analysis of the generated spectral image, while Attenuated Total Reflection (ATR)-FTIR was used for MPs >500 μm. The continuous HTL led to an MP reduction of approximately 76% in terms of MP number and 97% in terms of MP mass. The difference in reduction of the number of MPs versus their accumulated mass was the result of MPs being smaller after the HTL process. A total of 18 polymer types were detected in the sludge and slurry entering the continuous HTL while only 11 types were identified in the residual materials. No MPs were detected in the bio-crude, i.e. the most favorable product of the process. The polymer composition changed considerably as a result of the HTL process. Polyurethane, polypropylene, and polyethylene were the dominant polymers in the feedstock, while polypropylene and polyethylene were the most present in the residual products. The findings indicate that HTL can be efficient in reducing MPs in highly polluted sludge from wastewater treatment plants, leaving the byproducts and residuals significantly less polluted, hereby reducing the movement of MPs to the terrestrial environment. Thus the products are better suited for sustainability efforts than the raw material.

The Effect of Dichloromethane on Product Separation during Continuous Hydrothermal Liquefaction of Chlorella vulgaris and Aqueous Product Recycling for Algae Cultivation

Dichloromethane (DCM) is a solvent commonly used in laboratories for microalgae hydrothermal liquefaction (HTL) product separation. The addition of DCM would lead to an “overestimation effect” of biocrude yield and diminish biocrude quality. However, it is currently not clear to what extent this overestimation effect will impact a continuous HTL process. In this study, Chlorella vulgaris microalgae was processed in a continuous stirred tank reactor at different temperatures (300, 325, 350, 375, and 400 °C) at 24 MPa for 15 min holding time. Two separation methods were applied to investigate the effect of using DCM in a cHTL product separation procedure in terms of product yield, biocrude elemental content, and aqueous product (AP) composition. Subsequently, the feasibility of reusing AP for algae cultivation has been evaluated. Results suggest that 350 °C is the optimal temperature for cHTL operation, leading to the highest biocrude yield, and an average increase in biocrude yield of 9 wt % was achieved when using DCM in cHTL product separation. Within the temperature range investigated, an average biocrude yield estimation can be proposed by yieldnon-DCM ≈ 0.818 × yieldDCM. The AP has been characterized by total organic carbon and total nitrogen, high-performance liquid chromatography, and inductively coupled plasma optical emission spectroscopy. Results show that at 350–375 °C more nitrogen and other ions were directed into the AP, which could be advantageous in nutrient recovery. With the help of optical density testing, algae was shown to exhibit a better growth using AP with activated carbon absorption purification treatment as compared to the standard medium. The recovery of water and nutrients from the HTL-AP could improve the economics of a microalgae biorefinery process.

Improving the Yield of Fermentable Sugars Through Alkaline Pretreatment and Continuous Flow Hydrothermal Liquefaction of Lignocellulosic Biomass

Improving the Yield of Fermentable Sugars Through Alkaline Pretreatment and Continuous Flow Hydrothermal Liquefaction of Lignocellulosic Biomass

Biocrude Production from Hydrothermal Liquefaction of Chlorella: Thermodynamic Modelling and Reactor Design

Hydrothermal liquefaction can directly and efficiently convert wet biomass into biocrude with a high heating value. We developed a continuous hydrothermal liquefaction model via Aspen Plus to explore the effects of moisture content of Chlorella, reaction pressure and temperature on thermodynamic equilibrium yields, and energy recoveries of biocrude. We also compared the simulated biocrude yield and energy recoveries with experiment values in literature. Furthermore, vertical and horizontal transportation characteristics of insoluble solids in Chlorella were analyzed to determine the critical diameters that could avoid the plugging of the reactor at different flow rates. The results showed that the optimum moisture content, reaction pressure, and reaction temperature were 70–90 wt%, 20 MPa, and 250–350 °C, respectively. At a thermodynamic equilibrium state, the yield and the energy recovery of biocrude could be higher than 56 wt% and 96%, respectively. When the capacity of the hydrothermal liquefaction system changed from 100 to 1000 kg·h−1, the critical diameter of the reactor increased from 9 to 25 mm.

Biocrude Production via Non-Catalytic Supercritical Hydrothermal Liquefaction of Fucus vesiculosus Seaweed Processing Residues

The potential of using cold water brown macroalgae Fucus vesiculosus for biocrude production via non-catalytic supercritical hydrothermal liquefaction (HTL) was studied. Demineralization, residue neutralization, and high value-added product (alginate and fucoidan) extraction processes were carried out before using the biomass for HTL biocrude production. Acid leaching was carried out using three demineralization agents: distilled water, dilute citric acid solution, and the diluted acidic aqueous by-product from a continuous HTL pilot facility. Alginate was extracted via H2SO4 and NaCO3 bathing, and fucoidan was extracted using CaCl2. Experimental data show that none of the leaching agents was greatly efficient in removing inorganics, with citric acid leaching with extensive neutralization reaching the highest ash removal efficiency of 47%. The produced 6 sets of biocrudes were characterized by elemental and thermogravimetric analyses. Short (10-min retention) HTL and the extent of leaching residue neutralization were also investigated. Highest biocrude yields were recorded when liquefying non-neutralized citric acid leaching, alginate, and fucoidan extraction residues. On the other hand, thermochemical conversions of short retention time HTL, full neutralization extent, and baseline (dried raw macroalgae) biomass performed worse. Specifically, the highest biocrude yield of 28.2 ± 2.5 wt.% on dry ash-free feedstock basis was recorded when liquefying alginate extraction residues. Moreover, the highest energy recovery of 52.8% was recorded when converting fucoidan extraction residues.

Biofuels from spent coffee grounds: comparison of processing routes

Bulk fuel was produced from spent coffee grounds (SCG) using different oil extraction and fuel production methods. The highest oil yield (30.2%) and energy efficiency (50%) from continuous hydrothermal liquefaction (CHTL) of the SCG, in water as a solvent, also had the lowest calorific value (37.4 MJ/kg). Oil from traditional hexane oil extraction had the highest calorific value (HHV) (38.7 MJ/kg). Renewable diesel yields through hydrotreatment of extracted oils were consistently higher than biodiesel yields through enzymatic transesterification of the same oils. The highest renewable diesel yield (15.8%) from batch HTL derived bio-oil had the highest calorific value (46.2 MJ/kg). However, the highest kerosene yield (9%) was obtained from in situ renewable diesel production through direct hydrotreatment of the SCG. The study showed that hydrothermal liquefaction was more efficient in producing oil from SCG than traditional solvent extraction but at the cost of losing valuable phytosterols present in the feedstock. As a bulk fuel, renewable diesel had a higher energy value than biodiesel, making it easier to blend into existing transportation fuels. The study provides useful information for the development of a sustainable waste-based bio-refinery in terms of attainable yields and processing options.

Establishment and performance of a plug-flow continuous hydrothermal reactor for biocrude oil production

It is critical important for continuous production of biocrude oil and scale-up of hydrothermal liquefaction (HTL) technology. Pickering emulsion associated with solid (e.g. ash, char etc.) content is currently identified as a challenge for products recovery. Here, we investigated the continuous biocrude oil production from Spirulina sp., swine manure wastewater-fed Chlorella sp., and biogas wastewater-fed Chlorella sp. with quite different ash content ranging from 5.85% to 26.9%. The continuous reactions were evaluated at 300 °C with a flow rate of 45 mL·min−1 and a solid content of 18%. All the samples were converted into biocrude oil without obvious clogging issues. Subsequent analysis proves the feasibility of steady continuous HTL operation regarding the product yields, nutrients recovery, energy recovery as well as biocrude oil properties. The highest biocrude oil yield (42.3%, dw) was obtained with Spirulina sp. while the highest energy recovery (68.6%) and carbon recovery (59.7%) of biocrude oil were obtained from swine manure wastewater-fed Chlorella sp. Results reveal that higher ash content related to Pickering emulsion led to lower biocrude oil recovery rate. Further studies should focus on addressing the issue, especially for large-scale continuous HTL products separation system.