Biofuel production from pipeline-transported lignocellulosic biomass via hydrothermal liquefaction: Process optimization and product characterization

A number of technological strategies utilizing various types of biomass for the production of hydrocarbons have been put forth but their energy intensive methods are a concern for improved efficiency of biofuel production. Hydrothermal liquefaction (HTL) has emerged as a promising and feasible technology towards utilization of lignocellulosic biomass. The suitability of different biomass feedstock for HTL is intricately tied to their macromolecular composition and process parameters. The comprehensive analysis of feedstock for hydrothermal liquefaction (HTL) signal towards the immense potential of various biomass feedstock, such as corn stover, Miscanthus, pine biomass, Spirulina, sugarcane bagasse, rice bran etc. in contributing significantly to renewable energy production. The study emphasizes that the composition of biomass is critical in influencing bio-oil yield during the HTL process. Biomass components like cellulose, hemicellulose, and lignin, each play distinct roles in determining the efficiency of conversion. Specifically, feedstock with higher cellulose and hemicellulose content, such as Miscanthus and sugarcane bagasse, demonstrate superior bio-oil yields. The analysis of proximate factors affecting HTL efficiency reveals that moisture content, ash content and high heating value (HHV) are pivotal in optimizing the process. In addition to composition and physical characteristics, the article underscores the significance of growth conditions and nutrient utilization in cultivating biomass feedstock. Integrating HTL with biomass cultivation can create a sustainable, closed-loop system where nutrients from the HTL process are recycled back into cultivation. Biomass offers a renewable energy alternative, however it also poses challenges related to land use and potential competition with food production. Sustainable practices, such as utilizing agricultural and forestry residues and optimizing collection as well as storage processes, can alleviate some of these concerns. By optimizing feedstock selection, process parameters, and integrating sustainable practices, HTL can play a decisive role in advancing biofuel production and contributing to a more sustainable energy future. The interplay between biomass composition, processing efficiency, environmental impacts, and economic feasibility is essential for realizing the full potential of HTL technology in the bio-economy. The current analysis sheds light on the relationship of bio-oil yield with macromolecular components including cellulose, hemicellulose, and lignin as well as process parameters like ash content, moisture content, higher heating value, fixed carbon and volatiles. Focusing on process optimization, this study embodies a closer analysis of literature aimed at defining optimum strategies for enhancement of HTL.

Hydro Thermal Liquefaction (HTL) is a transformative process capable of converting algal biomass into four distinct phases: biocrude, gaseous, aqueous products, solid residue. Algal biofuel is regarded as the promising "third" generation biofuel, with three primary routes for liquid biofuel production from algae: biodiesel extraction or transesterification, bio-oil through pyrolysis, and biocrude via hydrothermal liquefaction (HTL). Both pyrolysis and HTL fall under the umbrella of thermochemical liquefaction technologies. HTL, specifically, involves the direct liquefaction of algal biomass into biocrude oil within a closed, oxygen-free reactor. This process utilizes pressurized inert gases like N2 or He, or reducing gases such as H2 or CO, at temperatures ranging from 250 to 380°C and pressures from 5 to 28 MPa. Notably, HTL employs hot compressed water, functioning as both a solvent and a reaction medium, with the advantage of being near-critical water, which is abundant, non-toxic, non-flammable, cost-effective, and naturally present in biomass. The paper further considers key challenge of HTL with organic solvents, which is the relative high cost, the use of hot compressed water which offers significant advantages. HTL with hot compressed water eliminates the need for an expensive solvent, and it has the flexibility to process wet algal feedstock directly, as the total solids (TS) content in the feedstock typically ranges from 10 to 25%. Furthermore, HTL enables the conversion of the entire algal composition, including lipids, proteins, and carbohydrates, resulting in a higher biocrude yield. Unlike other liquid biofuel production technologies such as oil extraction or pyrolysis, HTL offers distinct benefits: elimination of the drying process, broader feedstock applicability, enhanced mass transfer facilitated by sub-/super-critical water acting as both a reaction medium and solvent, and improved energy efficiency due to reduced latent loss during phase change.

Recent advances in hydrothermal liquefaction (HTL) have established this biomass conversion technology as a potent tool for the effective valorization and energy densification of varied feedstocks, ranging from lignocelluloses to microalgae and organic wastes. Emphasizing its application across biomass types, this exploration delves into the evolving landscape of HTL. Microalgae, recognized as a promising feedstock, offer a rich source of biomolecules, including lipids, carbohydrates, and proteins, making them particularly attractive for biofuel production. The comprehensive review explores the biofuel products and platform chemicals obtained through HTL of microalgae, delving into biodiesel production, bio-oil composition, characteristics, and to produce high-valued by-products. Challenges and limitations, such as reactor design, scalability issues, and the impact of microalgal composition on yields, are critically analyzed. The future prospects and research directions section envision advancements in HTL technology, integration with biorefinery processes, and the exploration of hybrid approaches for enhanced biofuel production. Overall, the paper emphasizes the promising potential of HTL for wet microalgal biomass and underscores the need for continued research to overcome existing challenges and unlock further opportunities in sustainable biofuel and platform chemical production.

BackgroundOleaginous microorganisms are attractive feedstock for production of liquid biofuels. Direct hydrothermal liquefaction (HTL) is an efficient route that converts whole, wet biomass into an energy-dense liquid fuel precursor, called ‘biocrude’. HTL represents a promising alternative to conventional lipid extraction methods as it does not require a dry feedstock or additional steps for lipid extraction. However, high operating pressure in HTL can pose challenges in reactor sizing and overall operating costs. Through the use of co-solvents the HTL operating pressure can be reduced. The present study investigates low-temperature co-solvent HTL of oleaginous yeast, Cryptococcus curvatus, using laboratory batch reactors.ResultsIn this study, we report the co-solvent HTL of microbial yeast biomass in an isopropanol–water binary system in the presence or absence of Na2CO3 catalyst. This novel approach proved to be effective and resulted in significantly higher yield of biocrude (56.4 ± 0.1 %) than that of HTL performed without a co-solvent (49.1 ± 0.4 %)(p = 0.001). Addition of Na2CO3 as a catalyst marginally improved the biocrude yield. The energy content of the resulting biocrude (~37 MJ kg−1) was only slightly lower than that of petroleum crude (42 MJ kg−1). The HTL process was successful in removing carboxyl groups from fatty acids and creating their associated straight-chain alkanes (C17–C21). Experimental results were leveraged to inform techno-economic analysis (TEA) of the baseline HTL conversion pathway to evaluate the commercial feasibility of this process. TEA results showed a renewable diesel fuel price of $5.09 per gallon, with the HTL-processing step accounting for approximately 23 % of the total cost for the baseline pathway.ConclusionsThis study shows the feasibility of co-solvent HTL of oleaginous yeast biomass in producing an energy-dense biocrude, and hence provides a platform for adding value to the current dairy industry. Co-solvents can be used to lower the HTL temperature and hence the operating pressure. This process results in a higher biocrude yield at a lower HTL temperature. A conceptual yeast HTL biofuel platform suggests the use of a dairy waste stream for increasing the productivity and sustainability of rural areas while providing a new feedstock (yeast) for generating biofuels.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0345-5) contains supplementary material, which is available to authorized users.

This study introduces and analyzes a novel system for algal biofuel production that synergistically integrates algal wastewater treatment with hydrothermal liquefaction (HTL) of wastewater biosolids and algae into bio-crude oil. This system maximizes the biofuel potential of wastewater inputs by internally capturing and recycling carbon and nutrients – a powerful concept referred to as multi-cycle nutrient reuse, which amplifies waste nutrients into multiple cycles of algal biomass and bioenergy production. We call this system “Environment-Enhancing Energy” (E2-Energy) because it can simultaneously improve conventional wastewater treatment by nutrient removal and production of a large amount of biofuel co-products. Moreover, E2-Energy resolves several key bottlenecks commonly associated with large-scale algal biofuel production including: contamination of target high-oil algal cultures, high nutrient costs/usage, unsustainable fresh water usage, and large energy inputs for dewatering/extraction. A series of algal cultivation and HTL experiments were conducted to confirm the primary steps and performance characteristics of the E2-Energy system. These experiments showed: (1) low-oil, mixed algal–bacterial biomass can be successfully cultured with the recycled HTL aqueous product; (2) both organics and nutrients are removed from wastewater during algal–bacterial biomass production (63–95% reduction); (3) this low-oil, algal–bacterial biomass can be converted into bio-crude oil via HTL with a high yield (∼50%) and a net positive energy balance; and (4) the HTL step re-releases nutrients to an aqueous phase product that can be recycled back to step (1). This repeating loop of steps 1–4 facilitates multi-cycle reuse of nutrients and thus provides biomass amplification. A mathematical model was also developed using STELLA® to simulate mass balances for long-term E2-Energy operations with internal recycling of nutrients and carbon. The model results showed that E2-Energy can amplify the biomass and biofuel production from wastewater by up to 10 times, which gives it the potential to replace all US petroleum imports using only current wastewater feedstocks and carbon dioxide from the atmosphere or point sources. Thus, E2-Energy represents a major paradigm shift—where wastewater treatment systems become optimized biofuel producers with enhanced effluent quality, which provides a viable and advantageous pathway to sustainable, carbon-neutral energy independence.

Hydrothermal liquefaction: Exploring biomass/plastic synergies and pathways for enhanced biofuel production.

Biofuel production by hydro-thermal liquefaction of municipal solid waste: Process characterization and optimization

Economic viability of multiple algal biorefining pathways and the impact of public policies

Hydrothermal liquefaction of biomass for jet fuel precursors: A review

The conversion of wet microalgae biomass into biofuels via hydrothermal liquefaction (HTL) is a promising route for sustainable energy production. This study investigates the use of potassium carbonate (K2CO3) as a catalyst to enhance bio-crude yield through aqueous phase recycling during the HTL process of Arthrospira platensis. The addition of K2CO3 significantly boosts biomass conversion, increasing yield by 20–35%. Optimization using response surface methodology (RSM) identifies peak bio-crude production (37.60 wt%) at 325 °C, 30 min, with 20 g of K2CO3. The catalyst improves bio-oil yield by 5.00 wt% at lower temperatures (275–300 °C). The artificial neural network (ANN) model demonstrates exceptional predictive accuracy, with an R 2 of 0.9991 and a mean squared error of 0.014701. Both ANN and RSM models yield excellent results, with R 2 values exceeding 0.98. In non-catalytic HTL, higher reaction temperatures and longer durations enhance oil production. The presence of K2CO3 increases bio-oil yield, H/C ratio, and calorific value while reducing oxygen content. Gas chromatography–mass spectrometry analysis reveals that temperature, time, and catalysts influence the formation of furan, cyclic, and oxygen-containing compounds in HTL bio-oil. This study highlights the potential of K2CO3-catalyzed HTL for efficient biofuel production from cyanobacterial biomass.

Synthetic polymers constitute one of the largest fractions of solid waste worldwide. From 1950 to 2015, roughly 12 Gton of these materials were deposited either in landfills or in the environment. The absolute majority of these materials are energetically dense, fossil-derived and non-biodegradable, which causes accumulation in the environment, threatening both marine and terrestrial ecosystems. Chemical recycling of these materials can be a management strategy to alleviate pollution and to reuse otherwise wasted energy in the form of solid materials. Agricultural crop residues are composed of both wet and dry streams, summing up to 3600 Mton year-1 (2013 estimate) of wasted resources globally. Besides that, around 3120 MTon year-1 (2017 estimate) of animal manure is generated worldwide. Nowadays, these agribusiness byproducts are underutilized and their conversion to liquid biofuels may present an untapped opportunity to provide the sustainability needed in sectors dependent on liquid hydrocarbons as an energy source. This thesis focuses on understanding how synthetic polymers and agricultural waste interact under hydrothermal liquefaction (HTL) conditions, identifying opportunities and evaluating the engineering challenges to apply the technology in combined processing of waste streams. This work evaluates the possibility of recovering monomer-like structures from synergistic combined HTL (co-HTL) of synthetic materials and lignocellulosic biomasses. It also evaluates how biocrudes derived from highly synergistic co-HTL behave in downstream processing for biofuel production when compared to single-feedstock biocrudes. HTL uses the reactivity of hot-compressed water in near-critical conditions to convert carbon-based materials into useful short chain organic compounds. The interaction of different feedstock materials under this condition allows a beneficial process efficiency and enlarges the opportunities to apply this process in waste handling scenarios. Literature about HTL processing of synthetic polymers present significant achievements within the field, however the non-standardized approach for several studies lead to contradictory results, generating a knowledge gap between laboratory results and practical applications. Here, results of subcritical HTL processing are presented for the 12 most used synthetic polymers worldwide, both individually and combined with lignocellulosic materials. When evaluating synthetic polymers alone, it is found that materials containing heteroatoms in the backbone of the polymer structure are prone to hydrolysis under subcritical water, while carbon-carbon bonds are preserved. In practice, polymers derived from addition polymerization such as polyolefins and polystyrene do not depolymerize under subcritical water, while condensation polymers and others containing heteroatoms in the backbone are decomposed into molecules similar to their original monomers. When these materials are combined with lignocellulosic ones, the synthetic parts containing nitrogen heteroatoms tend to synergistically interact with the organic-derived molecules and act synergistically increasing biocrude production. The reactivity of nitrogen species in synthetic polymers was directly proportional to the intensity of the synergies verified. The largest synergy identified was for polyurethane combined processing due to the presence of highly reactive amines bonded to aromatic groups. This finding led to an improved combinedprocessing of polyurethane foam and lignocellulosic materials, reaching pilot processing carbon and energy efficiencies of 71 and 75%, respectively. The combination of wet and dry agribusiness waste fractions in HTL processing was evaluated using cow manure and wheat straw, respectively, as representatives. Their combination also leads to enhanced biocrude and carbon recovery during subcritical HTL processing through nitrogen species reactions with lignocellulosic-derived compounds. The formation of heteroatom-containing aromatics acts as a carbon carrier to the biocrude products. With this approach, pilot HTL processing carbon yields were enhanced from 40 to 60 wt%, while also providing superior total energy efficiencies (up to 50% based on organic input and output including heating utilities). This increase in carbon efficiency generates further benefits in the production of hydrotreated products, with biomass-to-hydrotreated products carbon balances increasing from 34 wt% for wheat straw in single HTL to 43 wt% in co-HTL of wheat straw and cow manure. The distillation of hydrotreated products depicts that the nitrogen-containing molecules tend to have higher concentration in heavier fractions, which may be an opportunity for more targeted processing of these fractions. Overall, production of biofuels enlarged via co-HTL mainly due to HTL superior carbon and energy yields. Both synthetic-organic and organic-organic waste combined HTL, the reactions involving nitrogen compounds generate high synergistic effects towards biocrude formation. When increasing product stability through nitrogenated species, a consequent increased difficulty for their removal in following hydrotreatment oil upgrading is also verified. Nevertheless, the enhanced carbon and energy recovery and enlarged scope of HTL technologies attainedvia combination of waste materials is an opportunity to take advantage of these sub-utilized streams.

The search for alternatives to hydrocarbon fuels remains an actual task. Microalgae (MA) as raw material for the production of biofuel remain an urgent object of research among other types of biomass, and the cultivation of MA is constantly growing. One of the promising technologies for biofuel production from MA is hydrothermal liquefaction (HTL) that allows processing wet biomass and turning all carbon-containing components (lipids, carbohydrates, proteins) into fuel. However, the hydrothermal liquefaction process leads to the formation of a significant amount of the aqueous phase, which is a by-product with low energy value and also needs to be processed. It is important to consider the possibility of using the aqueous phase after HTL, as well as municipal wastewater, for the cultivation of MA in combination with the production of biofuels. The MA strains capable to grow in a dilute aqueous phase after HTL (Galdieria sulphuraria rsemsu G-1, Chlorella vulgaris rsemsu Chv-20/11-Ps, Arthrospira platensis rsemsu Bios) and in wastewater (Arthrospira platensis rsemsu Bios, Chlorella ellipsoidea rsemsu Chl-el) were experimentally selected from collection of RSE Laboratory at LMSU. The article is devoted to the experimental study of the degree of nutrients utilization by MA from wastewater and an HTL-aqueous phase.

Hydro Thermal Liquefaction (HTL) emerges as a promising method for converting wet biomass into liquid fuels. However, additional processing of the resulting HTL biocrude is imperative. Elevated levels of oxygen and nitrogen in HTL-produced biocrude necessitate deoxygenation and denitrogenation before it can be effectively used as a transport fuel. Managing the by-product aqueous stream is crucial for the success of an algal biorefinery employing HTL. Consequently, maximizing HTL efficiency and optimizing the utilization of biocrude and co-products, especially aqueous by-products, are current research priorities in biorefinery studies. To boost HTL efficiency, the focus is on using only carbon and hydrogen for hydrocarbon liquid fuels, as the presence of oxygen and nitrogen is undesirable for oil applications. Oxygen lacks inherent heating value, and nitrogen, if combusted, contributes to environmental pollution. Hence, HTL involves concurrent deoxygenation and denitrogenation during biocrude formation. The primary role of HTL lies in sustainable energy and chemicals production, aligning with a commitment to environmental preservation. Biocrude, characterized by high oxygen and nitrogen contents, along with elevated molecular weight and viscosity, typically undergoes upgrading processes like solvent extraction/distillation, hydrogenation/hydrodeoxygenation, catalytic cracking, esterification, and hybrid techniques. These processes lead to the partial removal of oxygen as CO2 or H2O and the conversion of nitrogen into ammonium. The study explores potential routes for the thermochemical conversion of microalgae, distinguishing between dry processes (pyrolysis and gasification) and wet processes (near-critical water hydrothermal liquefaction and hydrothermal gasification). The work identifies key engineering advantages and challenges, focusing on biofuel production for transportation. The future perspectives for each route are presented.

Biofuel production from algae feedstock has become a topic of interest in the recent decades since algae biomass cultivation is feasible in aquaculture and does therefore not compete with use of arable land. In the present work, hydrothermal liquefaction of both microalgae and macroalgae is evaluated for biofuel production and compared with transesterifying lipids extracted from microalgae as a benchmark process. The focus of the evaluation is on both the energy and carbon footprint performance of the processes. In addition, integration of the processes with an oil refinery has been assessed with regard to heat and material integration. It is shown that there are several potential benefits of co-locating an algae-based biorefinery at an oil refinery site and that the use of macroalgae as feedstock is more beneficial than the use of microalgae from a system energy performance perspective. Macroalgae-based hydrothermal liquefaction achieves the highest system energy efficiency of 38.6%, but has the lowest yield of liquid fuel (22.5 MJ per 100 MJalgae) with a substantial amount of solid biochar produced (28.0 MJ per 100 MJalgae). Microalgae-based hydrothermal liquefaction achieves the highest liquid biofuel yield (54.1 MJ per 100 MJalgae), achieving a system efficiency of 30.6%. Macro-algae-based hydrothermal liquefaction achieves the highest CO2 reduction potential, leading to savings of 24.5 resp 92 kt CO2eq/year for the two future energy market scenarios considered, assuming a constant feedstock supply rate of 100 MW algae, generating 184.5, 177.1 and 229.6 GWhbiochar/year, respectively. Heat integration with the oil refinery is only possible to a limited extent for the hydrothermal liquefaction process routes, whereas the lipid extraction process can benefit to a larger extent from heat integration due to the lower temperature level of the process heat demand.

Microalgae are an alternative source for the renewable biofuels production. One of the promising technologies of microalgae fuel production is the hydrothermal liquefaction (HTL) with obtaining bio-oil as the target product. We have carried out a number of experiments on the hydrothermal liquefaction the biomass of blue-green microalgae Arthrospira platensis rsemsu 1/02-P (collection of Renewable Source Energy Laboratory at Lomonosov Moscow State University). For the HTL of arthrospira biomass the reactor of the Institute of High Temperatures RAS have been used. The outputs of bio-oil, gaseous products, solid residue and aqueous solution were 34-46%, 12-18%, 26-30%, 10-24% respectively. The aqueous solution hydrothermal liquefaction is a by-product, it has a limited energy value and needs to be recycled. Aqueous solution contains the nutrients necessary for growing algae but in quantities that are orders of magnitude higher than the standard ones. Studies growth of different algae species in aqueous solution after HTL have shown that in order to prevent the growth inhibitors toxic effect intensive its dilution is necessary. Microalgae strains, which can be cultivated in 500-fold diluted aqueous solution (Galdieria sulphuraria rsemsu G-1, Chlorella vulgaris rsemsu Chv-20/11-Ps), have been experimentally selected. It allows partially recycling the by-product of bio-oil from microalgae