NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Whole Algae Hydrothermal 

Liquefaction Technology 

Pathway

Mary Biddy and Ryan Davis

National Renewable Energy Laboratory

Susanne Jones and Yunhua Zhu

Pacific Northwest National Laboratory

NREL is a national laboratory of the U.S. Department of Energy, 

Office of Energy Efficiency & Renewable Energy, operated by the 

Alliance for Sustainable Energy, LLC, under contract 

DE-AC36-08GO28308.

Pacific Northwest National Laboratory is operated by Battelle for the 

United States Department of Energy under contract 

DE-AC05-76RL01830.

Technical Report

NREL/TP-5100-58051

PNNL-22314

March 2013

Prepared for the U.S. Department of Energy Bioenergy Technologies Office
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Pacific Northwest National Laboratory

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Whole Algae Hydrothermal 

Liquefaction Technology 

Pathway 

Mary Biddy and Ryan Davis

National Renewable Energy Laboratory

Susanne Jones and Yunhua Zhu

Pacific Northwest National Laboratory

NREL is a national laboratory of the U.S. Department of Energy, 

Office of Energy Efficiency & Renewable Energy, operated by the 

Alliance for Sustainable Energy, LLC, under contract 

DE-AC36-08GO28308.

Pacific Northwest National Laboratory is operated by Battelle for the 

United States Department of Energy under contract 

DE-AC05-76RL01830.

Technical Report 

NREL/TP-5100-58051

PNNL-22314

March 2013

Prepared for the U.S. Department of Energy Bioenergy Technologies Office
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1 

Whole Algae Hydrothermal Liquefaction

Developing Technology Pathway Cases to Understand the Cost of 

Converting Biomass to Hydrocarbon Fuels

In support of the Bioenergy Technologies Office, the National Renewable Energy Laboratory 

(NREL) and the Pacific Northwest National Laboratory (PNNL) are undertaking studies of 

biomass conversion technologies to hydrocarbon fuels to identify barriers and target research 

toward reducing conversion costs. 

Process designs and preliminary economic estimates for each of these pathway cases were 

developed using rigorous modeling tools (Aspen Plus and Chemcad). These analyses 

incorporated the best information available at the time of development, including data from 

recent pilot- and bench-scale demonstrations, collaborative industrial and academic partners, and 

published literature and patents. The economic results of these analyses are in the process of 

further refinement and will be published in FY13 and FY14 design reports. This report 

summarizes the preliminary technical data used for the models and identified data gaps.

This technology pathway case investigates the feasibility of using whole wet microalgae as a 

feedstock for conversion via hydrothermal liquefaction. Technical barriers and key research 

needs have been assessed in order for the hydrothermal liquefaction of microalgae to be 

competitive with petroleum-derived gasoline-, diesel-, and jet-range hydrocarbon blendstocks. 

Process Block Diagram (Co-Located Conversion and Hydrotreating 

Scenario)

-

Heat Integration and Power Generation

[Catalytic Hydrogasification (CHG) + Biogas Combustion] 

Hydrogen 

Plant 

Catalytic 

Hydrotreating 

and Product 

Fractionation

Flue 

Gas

Process 

Heat and 

Power 

Integration

Off-gas H2

Hydrocarbon 

Biofuels (i.e., 

Gasoline, Diesel, 

Jet) 

Cell 

Concentration/ 

Dewatering

(Settling, dissolved 

air flotation, 

centrifuge) 

Algae 

Growth

Water & 

Nutrients Evaporation

Aqueous 

Phase

Off-gas

Process 

Heat and 

Power 

Integration 

(if located 

at HTL 

plant)

CO2 +

Nutrient

Recycle

Hydrothermal 

Liquefaction 

(HTL)
2 

Key Highlights

• Microalgae grown via autotrophic pathways have the potential to make sizeable 

contributions to renewable fuel mandates, particularly due to their rapid growth rates and 

other favorable cultivation characteristics relative to terrestrial biomass feedstocks. 

• An advantage of hydrothermal liquefaction (HTL) is that it uses the whole algae. Thus it 

is not necessary to promote lipid accumulation, nor is it necessary to extract the lipids.

• HTL is especially suited for conversion of wet feedstocks such as algae since the feed to 

the conversion unit is water based slurry. Thus no energy is expended for evaporative 

algae drying.

• For the algal HTL pathway, the overall economics for hydrocarbon biofuels are more 

strongly influenced by improvements in productivity rather than extractable lipid content. 

• Hydrocarbon fuel can be produced by hydrotreating HTL oil under relatively mild 

conditions. 

• Important research needs for this pathway include enhancing algal oil productivity on a 

sustained basis, optimizing the nutrient recycle, demonstrating wastewater treatment, and 

characterizing both HTL oil and hydrocarbon products. 

Process Design Details

Wet whole algae source: The HTL plant is assumed to be co-located with algae production. The 

algal growth and cell concentration and dewatering steps are based on the Algal Lipid Upgrading 

(ALU) design (Davis et al. 2012), namely cultivation in open raceway ponds, steady-state 

harvesting of algal biomass at a rate equal to the biomass growth rate, and dewatering through a 

series of operations to concentrate the material to 20 wt% solids. Recycle of CO2-containing 

HTL off-gas and HTL wastewater with dissolved ammonia to the algae ponds is also assumed. 

Hydrothermal liquefaction (HTL): Whole algal biomass slurry of ~20 wt% solids in water is 

pumped to the HTL reactor pressure of ~3,000 psia. Operating conditions and economics for the 

pumping system are based on experimental work from the National Alliance for Algal Biofuels 

and Bio-Products (NAABB) operating on wet whole algae and lipid extracted algae. Lab-scale 

experiments indicate that algal slurries are not difficult to pump (Zhu et al. 2013). 

Whole wet algae at a 20 wt% solids content is hydrothermally treated in subcritical water, 

nominally 2,000–3,000 psia and 300°–350°C. The reaction temperature dictates the reactor 

pressure (e.g., the reactor pressure is sufficiently high to maintain water in the liquid state at the 

reaction conditions). The slurry entering the reactor is preheated by exchange with reactor 

effluent. During HTL of woody feedstocks, a buffering agent is employed to prevent formation 

of high molecular weight compounds (Zhu et al. 2012). Unlike HTL with wood, no buffering 

agent is needed when processing algae. This suggests that algae feedstocks produce a more 

stable HTL oil product than oil derived from processing wood. Algal nitrogen and sulfur 

partition between each phase to varying degrees.

The reactor effluent is a mixture of solids, liquids, and gas. Effective filtering is an important 

means of protecting downstream conversion catalysts. Further research is needed to identify the 

most effective means for doing so. The two liquid phases and the non-condensable gases are 
3 

cooled and then separated. The non-condensable gases are sent to a steam boiler for power 

generation. The predominately organic liquid phase is sent to catalytic upgrading, and the 

predominately aqueous phase is sent to wastewater cleanup for carbon recovery. Solids removed 

by filtration might be recycled back to the algae ponds as nutrients. 

There are a number of HTL studies in the literature, for example, Minowa et al. (1995), Brown et 

al. (2010), Duan and Savage (2011), Biller and Ross (2011), Jena et al. (2011), Valdez et al.

(2011), Yu et al. (2011), Vardon et al. (2011), and Alba et al. (2012). However, these are all 

batch experiments with long residence times and variable operating conditions. Product recovery 

is typically by extraction. The scales, except for Jena et al. at 1,800 mL, are in the 35–75 mL

range. Literature derived yields are in the 15–52 wt% range. 

HTL experimental work at PNNL employs a continuous system with simple phase separation (no 

extraction systems). The scale is approximately 1 L, on the order of that used by Jena et al.

(2011). HTL oil quality is very good, with expected ranges of 5%–10 wt% oxygen, 3–5 wt% 

nitrogen and yields of 45–55 wt% (all on a dry basis). The details of these runs, including 

element analysis will be reported in the Zhu paper (Zhu et al. 2013). 

Wastewater treatment: The nitrogen content in the wastewater (assumed to be dissolved 

ammonia) likely precludes the use of anaerobic digestion. Therefore, wastewater from the HTL 

process (and upgrading if it is co-located) is sent to a catalytic hydrothermal gasification (CHG) 

process to convert all organics to CO2 and CH4 similar to that described in Elliott et al. (2009). It 

is assumed that ammonia remains dissolved in the CHG wastewater. For CHG, the wastewater 

stream is pumped to ~3,000 psia, preheated to 370°C, then fed to a fixed bed catalytic reactor. 

Assumptions used in the model will be verified in FY13 by continuous lab-scale (1-liter fixed 

bed reactor) experiments using the aqueous phase derived from the aforementioned HTL 

reactions. Treated water should be recycled to the algae farm to reduce fresh nutrient demands 

during cultivation. 

The produced CHG gas can be used to generate hydrogen if the HTL plant is co-located with the 

upgrading plant. If the HTL plant is not co-located with the upgrader, off-gas from the CHG 

system can be sent to a boiler to generate steam for process use. 

Steam and power: Superheated steam from the steam cycle boiler is used to power the process 

compressors. Depending upon the overall plant configuration, power demand in the conversion 

plant was balanced by either purchasing from or selling electricity to the grid. Future work will 

consider integration of algae production and harvest with the conversion plant. 

Product upgrading— catalytic hydrotreating: Algal HTL oil requires further catalytic 

processing to remove oxygen and nitrogen. Sulfur removal also occurs, but sulfur is present in 

the HTL oil to a much lower extent. The organic phase from HTL processing is catalytically 

hydrotreated to remove oxygen and nitrogen; the former is converted to CO2 and water, while the 

latter is converted to ammonia. Sulfur is converted to hydrogen sulfide. The ammonia remains 

dissolved in the water phase, which in turn should be recycled to the algae farm assuming colocation. 
4 

Algal HTL oil requires milder upgrading conditions than fast pyrolysis oil, which requires 2-3 

reactors in series, each operating at different temperatures and space velocities. Typical 

hydrotreating conditions are 250°–420°C, 1,000–2,000 psig, and 0.2–1.1 liquid hourly space 

velocity. Preliminary experimental work found that a single hydrotreater (continuous operation) 

was able to reduce the oxygen content to 1 wt% and the nitrogen content to <0.3 wt%. Short term 

testing showed no appearance of catalyst deactivation. Further optimization work will continue 

in FY13 to narrow the range of operating conditions. 

The hydrotreater effluent is cooled to condense the produced water and hydrocarbons. The 

organic phase is fractionated into four boiling point cuts: C4 minus, gasoline range, diesel range, 

and heavy oil range material. The heavy oil type material is assumed to be cracked in a 

conventional hydrocracker to produce additional gasoline- and diesel-range products. It is 

assumed that the product quality is sufficiently suitable for gasoline- and diesel-range 

blendstocks. Detailed product analysis has not yet been undertaken. 

Hydrogen plant: Hydrogen is assumed to be produced onsite. If the HTL plant is co-located with 

the upgrading facilities, off-gases from HTL and the hydrodeoxygenation section are sent to a 

conventional hydrogen plant consisting of a steam reformer, water gas shift reactor, pressure 

swing adsorption unit, and heat recovery. If the HTL plant is not co-located, then only the offgases from hydrodeoxygenation are available for use in the hydrogen plant. The off-gases are 

mixed with enough supplemental natural gas to satisfy the hydrogen demand. No processing 

penalty is assumed for mixing off-gas with natural gas. The plant size is within typical 

commercial scale.

Data Gaps, Uncertainties, and Research Needs

While the HTL oil production and hydrodeoxygenation (and de-nitrification) results are based on 

experimental data, the CHG wastewater processing model is literature based. Experimental 

results from FY13 tasks will be used to update the HTL, hydrotreating, and CHG portions of the 

model. The goal of this pathway is to reach a minimum fuel selling price of $3/gallon of gasoline 

equivalent (in 2011 U.S. dollars). To reach this targeted product price, the key bottlenecks, 

uncertainties, and areas for further development are summarized as follows: 

• Validate algae growth and oil productivity rates based on data from large scale 

demonstrations. Previous work has demonstrated a strong economic sensitivity to lipid 

content and algal growth rate. Much of the existing data is based on literature values in 

controlled environments. It will be important to incorporate first-hand operational data on 

large-scale, outdoor, year-round operation to validate the assumptions and projections for 

these metrics.

• Confirm design requirement of pond liners. The costs for liners in algae cultivation 

exhibit the largest single-unit impact on overall economics and there is a great deal of 

uncertainty on their requirement. In reality, their use will depend on local soil 

characteristics, algal strain employed, and governing regulatory policies. 

• Reduce cost and increase efficiency of algae dewatering and concentration steps. 

While the performance for dewatering operations is supported to a degree in literature 

(for operations utilizing wastewater dewatering technologies), such operations leave 
5 

room for further cost reduction if more cost-effective alternatives can be developed and 

proven at large scale.

• Verify feed handling. Feedstock handling is not anticipated to present issues because 

lab-scale experiments indicate that algal slurries are easier to pump than wood slurries 

and do not appear to have plugging issues. However, this will need to be definitively 

verified.

• Understand effect of feedstock variability. There are known compositional differences 

between strains of algae, and typically seasonal variations as well. The assumption that 

HTL yield and that of subsequent upgrading only affects product quality but not yield 

should be demonstrated. Preliminary tests in FY13 will address this, but additional 

research may be needed.

• Assess nutrient recycle. Future work must consider the impact and practical application 

of nutrient recycle on both the sustainability and economics of the process. Nutrient 

recycle has been identified as a key driver in meeting GHG reduction goals in the life 

cycle assessment of ALU. However, nothing is known about the actual practice, either as 

benefits (such as uptake of recycled nutrients) or as issues (such as accumulation of 

undesired compounds). This will involve a better understanding, for example, of the 

partitioning of nutrients, such as nitrogen and phosphorus, and non-nutrients among the 

various phases. Cultivation with real recycle also needs exploring. 

• Develop detailed reactor design and cost estimates. A large high pressure reactor is 

required. The economic analysis for this system was based on hydrothermal liquefaction 

reactor experiments conducted within a continuously stirred tank reactor (CSTR). It is 

likely that a hybrid CSTR-plug flow reactor (PFR) would be much less expensive to scale 

up than a CSTR alone, and this will be investigated in the current fiscal year. Work is still 

needed to determine the most appropriate reactor(s) configuration that promotes full 

conversion and reduced capital costs. 

• Characterize feed and products. Detailed characterizations of all the products and 

intermediates are needed. For example, better understanding of the quality of the HTL oil 

will help reveal the underlying HTL reactions and subsequent upgrading requirements. In 

addition to ultimate analysis, phase densities, gas analysis and whole oil distillation 

curves, analysis by GC-MS, HPLC, and 13C NMR would be useful as the next step. Also 

needed is similar analysis on the major distillation fractions: gasoline range, diesel range, 

and gas oil range for the HTL oil and the hydrotreated oil. Understanding the speciation 

of alkenes, aromatics, and oxygenates, particularly as a function of processing conditions, 

will help manage hydrogen usage. Off-gas composition by GC is available from the HTL 

and hydrotreating experiments, but this may not be sufficient to assess the need for gas

conditioning prior to use in a hydrogen plant. Detailed algal feed characterization is 

needed to assist determination of the trade-offs (if any) between lipid content and final 

product yield and quality, and the availability of recoverable nutrients. Lastly, testing for 

final fuel qualities, such as octane, cetane, and freeze, is desirable.

• Optimize separations and carbon efficiency. HTL oil and aqueous phase separation 

needs further work to recover more of the organic material into the HTL oil phase. 

Additionally, research is needed to understand the conversion of organics in the aqueous 
6 

phase to hydrogen, bio-products, and species that can rejoin the predominantly organic 

phase. Hydrocracking yields of the gas oil fraction should be demonstrated. 

• Demonstrate wastewater treatment. Wastewater treatment is largely unexplored and 

anaerobic digestion is not considered appropriate for the types of species present. 

Catalytic hydrothermal gasification needs to be further explored to understand the impact 

of organic compounds on wastewater treatment, the toxicity of trace compounds, and the 

impact of ammonia in the conversion, and to minimize catalyst costs and carbon loss to 

wastewater treatment. 

• Improve catalyst performance. Hydrotreating catalyst maintenance and stability are 

unknown, as are regeneration protocols and lifetimes. Longer-term testing with HTL oil 

and detailed characterization of catalyst performance and deactivation modes are needed. 

Parameter testing and development of compounds’ structure-reactivity relationships is 

needed. There is a need to understand mass transfer issues of hydrogen in bio-oil and on 

catalyst.

• Consider co-products. Co-product opportunities have the potential to further lower the 

cost of hydrocarbon production from algal biomass and should be explored in future 

economic evaluations. The impact of any process modifications should be evaluated via a 

life cycle assessment as well, since the current algal models have been developed and 

integrated to ultimately achieve a sustainable process design. Key also for the production 

of co-products is to ensure that the volumes of co-products produced via these routes will 

not overwhelm the market demand. 

Summary and Next Steps

This study assessed the processing of whole algal biomass via conversion by hydrothermal 

liquefaction followed by upgrading and finishing to gasoline-, diesel-, and jet-range hydrocarbon 

blendstocks. A combination of preliminary experimental data and literature-based technoeconomic analysis was performed to identify technology gaps, uncertainties, and research needed 

to achieve a minimum fuel selling price of $3/gallon of gasoline equivalent. A design case 

detailing this pathway will be developed by the end of FY13.

References

Alba, L.; Torri, C.; Samor, C.; van der Spek, J.; Fabbri, D.; Kerstn, S.; Brilman, D. (2012). 

“Hydrothermal Treatment (HTT) of Microalgae: Evaluation of the Process as Conversion 

Method in an Algae Biorefinery Concept.” Energy & Fuels (26); pp. 642-657. 

Biller, P.; Ross, A. (2011). “Potential Yields and Properties of Oil from the Hydrothermal 

Liquefaction of Microalgae with Different Biochemical Content.” Bioresource Technology 

(102); pp. 215-225. 

Brown, T.; Duan, P.; Savage, P. (2010). “Hydrothermal Liquefaction and Gasification of 

Nannochloropsis sp.” Energy Fuels (24); pp. 3639-3646. 

Davis, R.; Fishman, D.; Frank, E.D.; Wigmosta, M.S.; Aden, A.; Coleman, A.A.; Pienkos, P.T.; 

Skaggs, R.J.; Venteris, E.R.; Wang, M.Q. (June 2012). Renewable Diesel from Algal Lipids: An 

Integrated Baseline for Cost, Emissions, and Resource Potential from a Harmonized Model. 
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ANL/ESD/12-4; NREL/TP-5100-55431; PNNL-21437. Argonne, IL: Argonne National 

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Northwest National Laboratory. http://www.nrel.gov/docs/fy12osti/55431.pdf. 

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Elliott, D.; Neuenschwander, G.; Hart, T.; Rotness, L.; Zacher, A.; Santosa, D.; Valkenburg, C.; 

Jones, S.; Tjokro Rahardjo, S. (October 2009). Catalytic Hydrothermal Gasification of LigninRich Biorefinery Residues and Algae. PNNL-18944. Richland, WA: Pacific Northwest National 

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Marker, T.; Petri, J.; Kalnes, T.; McCall, M.; Mackowiak, D.; Jerosky, B.; Reagan, B.; Nemeth, 

L.; Krawczyk, M.; Czernik, S.; Elliott, D.; Shonnard, D. (2005). Opportunities for 

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1738. 

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Vardon, D.; Sharma, B.; Scott, J.; Yu, G.; Wang, Z.; Schideman, L. (2011). “Chemical 

Properties of Biocrude Oil form the Hydrothermal Liquefaction of Spirulina Algae, Swine 

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Yu, G.; Zhang, Y.; Schideman, L.; Funk, T.; Wang, Z. (2011). “Distributions of Carbon and 

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and Environmental Science (4); pp. 4587-4595. 

Zhu, Y.; Biddy, M.; Jones, S.; Elliott, D.; Schmidt, A. (2012). “Gasoline and Diesel Production 

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Two Different Microalgae Conversion Methods to Liquid Hydrocarbon Fuels.” National 

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