Hydrothermal Liquefaction of Biomass
Hydrothermal liquefaction technology is describes in its relationship to fast pyrolysis of biomass. The scope of work at PNNL is discussed and some intial results are presented. HydroThermal Liquefaction (HTL), called high-pressure liquefaction in earlier years, is an alternative process for conversion of biomass into liquid products. Some experts consider it to be pyrolysis in solvent phase. It is typically performed at about 350 C and 200 atm pressure such that the water carrier for biomass slurry is maintained in a liquid phase, i.e. below super-critical conditions. In some applications catalysts and/or reducing gases have been added to the system with the expectation of producing higher yields of higher quality products. Slurry agents ('carriers') evaluated have included water, various hydrocarbon oils and recycled bio-oil. High-pressure pumping of biomass slurry has been a major limitation in the process development. Process research in this field faded away in the 1990s except for the HydroThermal Upgrading (HTU) effort in the Netherlands, but has new resurgence with other renewable fuels in light of the increased oil prices and climate change concerns. Research restarted at Pacific Northwest National Laboratory (PNNL) in 2007 with a project, 'HydroThermal Liquefaction of Agricultural and Biorefinery Residues' with partners Archer-Daniels-Midland Company and ConocoPhillips. Through bench-scale experimentation in a continuous-flow system this project investigated the bio-oil yield and quality that could be achieved from a range of biomass feedstocks and derivatives. The project was completed earlier this year with the issuance of the final report. HydroThermal Liquefaction research continues within the National Advanced Biofuels Consortium with the effort focused at PNNL. The bench-scale reactor is being used for conversion of lignocellulosic biomass including pine forest residue and corn stover. A complementary project is an international collaboration with Canada to investigate kelp (seaweed) as a biomass feedstock. The collaborative project includes process testing of the kelp in HydroThermal Liquefaction in the bench-scale unit at PNNL. HydroThermal Liquefaction at PNNL is performed in the hydrothermal processing bench-scale reactor system. Slurries of biomass are prepared in the laboratory from whole ground biomass materials. Both wet processing and dry processing mills can be used, but the wet milling to final slurry is accomplished in a stirred ball mill filled with angle-cut stainless steel shot. The PNNL HTL system, as shown in the figure, is a continuous-flow system including a 1-litre stirred tank preheater/reactor, which can be connected to a 1-litre tubular reactor. The product is filtered at high-pressure to remove mineral precipitate before it is collected in the two high-pressure collectors, which allow the liquid products to be collected batchwise and recovered alternately from the process flow. The filter can be intermittently back-flushed as needed during the run to maintain operation. By-product gas is vented out the wet test meter for volume measurement and samples are collected for gas chromatography compositional analysis. The bio-oil product is analyzed for elemental content in order to calculate mass and elemental balances around the experiments. Detailed chemical analysis is performed by gas chromatography-mass spectrometry and 13-C nuclear magnetic resonance is used to evaluate functional group types in the bio-oil. Sufficient product is produced to allow subsequent catalytic hydroprocessing to produce liquid hydrocarbon fuels. The product bio-oil from hydrothermal liquefaction is typically a more viscous product compared to fast pyrolysis bio-oil. There are several reasons for this difference. The HTL bio-oil contains a lower level of oxygen because of more extensive secondary reaction of the pyrolysis products. There are less amounts of the many light oxygenates derived from the carbohydrate structures as they have been further reacted to phenolic Aldol condensation products. The bio-oil is more hydrophobic because of the lower oxygen content and resulting lower polarity and therefore has a lower amount of dissolved water. Without the light oxygenates acting as solvents along with the water, the bio-oil product is much more viscous. Related results are that the bio-oil is less dense and has a higher energy content. These differences in properties led to the earlier held belief that the HTL bio-oils could be upgraded by catalytic hydroprocessing in a manner more similar to simple petroleum hydrotreating. Some initial results from the HydroThermal Liquefaction of kelp are shown in Table 1. The experiments were performed with kelp slurries of 5-13 wt.% dry solids in water. Oil yields from kelp are low at 24% on ash-free basis but up to 41% calculated on a carbon basis. The bulk of the rest of the carbon ends up dissolved in the water stream.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1032423
- Report Number(s):
- PNNL-SA-76509; BM0101010; TRN: US201202%%39
- Journal Information:
- IEA Bioenergy Task 34 Pyrolysis, Vol. 28; ISSN 2040-2759
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
09 BIOMASS FUELS
36 MATERIALS SCIENCE
AGRICULTURAL WASTES
BIOFUELS
BIOMASS
BY-PRODUCTS
CHEMICAL ANALYSIS
GAS CHROMATOGRAPHY
HYDROCARBONS
MAIZE
NUCLEAR MAGNETIC RESONANCE
OIL YIELDS
OXYGEN
PETROLEUM
PROCESSING
PYROLYSIS
PYROLYSIS PRODUCTS
SECONDARY REACTIONS
SPECTROSCOPY
STAINLESS STEELS
hydrothermal
liquefaction
biomass
bio-oil