Hydrothermal liquefaction (HTL) is a thermal depolymerization process used to convert wet

biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bio ...

, and other macromolecules, into crude-like oil under moderate temperature and high pressure. The crude-like oil has high

energy density In physics, energy density is the amount of energy stored in a given system or region of space per unit volume. It is sometimes confused with energy per unit mass which is properly called specific energy or . Often only the ''useful'' or extrac ...

with a

lower heating value The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it. The ''calorific value'' is the total energy rele ...

of 33.8-36.9 MJ/kg and 5-20 wt% oxygen and renewable chemicals. The process has also been called hydrous pyrolysis. The reaction usually involves homogeneous and/or heterogeneous

catalysts Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

to improve the quality of products and yields. Carbon and hydrogen of an organic material, such as biomass,

peat Peat (), also known as turf (), is an accumulation of partially Decomposition, decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, Moorland, moors, or muskegs. The peatland ecosystem covers and ...

or low-ranked coals (

lignite Lignite, often referred to as brown coal, is a soft, brown, combustible, sedimentary rock formed from naturally compressed peat. It has a carbon content around 25–35%, and is considered the lowest rank of coal due to its relatively low heat ...

) are thermo-chemically converted into hydrophobic compounds with low viscosity and high solubility. Depending on the processing conditions, the fuel can be used as produced for heavy engines, including marine and rail or upgraded to transportation fuels, such as diesel, gasoline or jet-fuels. The process may be a significant in the creation of

fossil fuel A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels ma ...

s. Simple heating without water, anhydrous

pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements '' ...

has long been considered to take place naturally during the catagenesis of

kerogen Kerogen is solid, insoluble organic matter in sedimentary rocks. Comprising an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold. It ...

s to fossil fuels. In recent decades it has been found that water under pressure causes more efficient breakdown of kerogens at lower temperatures than without it. The carbon

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...

ratio of

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...

also suggests that

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

from water has been added during creation of the gas.

History

As early as the 1920s, the concept of using hot water and alkali catalysts to produce oil out of biomass was proposed. In 1939, U.S. patent 2,177,557, described a two-stage process in which a mixture of water,

wood chips Woodchips are small- to medium-sized pieces of wood formed by cutting or chipping larger pieces of wood such as trees, branches, logging residues, stumps, roots, and wood waste. Woodchips may be used as a biomass solid fuel and are raw materia ...

, and

calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has ma ...

is heated in the first stage at temperatures in a range of , with the pressure "higher than that of saturated steam at the temperature used." This produces "oils and alcohols" which are collected. The materials are then subjected in a second stage to what is called "dry distillation", which produces "oils and ketones". Temperatures and pressures for this second stage are not disclosed. These processes were the foundation of later HTL technologies that attracted research interest especially during the 1970s oil embargo. It was around that time that a high-pressure (hydrothermal) liquefaction process was developed at the Pittsburgh Energy Research Center (PERC) and later demonstrated (at the 100 kg/h scale) at the Albany Biomass Liquefaction Experimental Facility at Albany, Oregon, US. In 1982, Shell Oil developed the HTU™ process in the Netherlands. Other organizations that have previously demonstrated HTL of biomass include Hochschule für Angewandte Wissenschaften Hamburg, Germany, SCF Technologies in Copenhagen, Denmark, EPA’s Water Engineering Research Laboratory, Cincinnati, Ohio, USA, and Changing World Technology Inc. (CWT), Philadelphia, Pennsylvania, USA. Today, technology companies such a
Licella/Ignite Energy Resources
(Australia)
Arbios Biotech
a Licella/Canfor joint venture
Altaca Energy
(Turkey)
Circlia Nordic
(Denmark)
Steeper Energy
(Denmark, Canada) continue to explore the commercialization of HTL. Construction has begun in Teesside, UK, for a catalytic hydrothermal liquefaction plant that aims to process 80,000 tonnes per year of mixed plastic waste by 2022.

Chemical reactions

In hydrothermal liquefaction processes, long carbon chain molecules in biomass are thermally cracked and oxygen is removed in the form of H₂O (dehydration) and CO₂ (decarboxylation). These reactions result in the production of high H/C ratio bio-oil. Simplified descriptions of dehydration and decarboxylation reactions can be found in the literature (e.g. Asghari and Yoshida (2006) and Snåre et al. (2007).

Process

Most applications of hydrothermal liquefaction operate at temperatures between 250-550 °C and high pressures of 5-25 MPa as well as catalysts for 20–60 minutes, although higher or lower temperatures can be used to optimize gas or liquid yields, respectively. At these temperatures and pressures, the water present in the biomass becomes either subcritical or supercritical, depending on the conditions, and acts as a solvent, reactant, and catalyst to facilitate the reaction of biomass to bio-oil. The exact conversion of biomass to bio-oil is dependent on several variables: * Feedstock composition * Temperature and heating rate * Pressure * Solvent * Residence time * Catalysts

Feedstock

Theoretically, any biomass can be converted into bio-oil using hydrothermal liquefaction regardless of water content, and various different biomasses have been tested, from forestry and agriculture residues, sewage sludges, food process wastes, to emerging non-food biomass such as algae. The composition of

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...

hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all terrestrial plant cell walls.Scheller HV, Ulvskov Hemicelluloses.// Annu Rev ...

, protein, and

lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity a ...

in the feedstock influence the yield and quality of the oil from the process. Zhang et al., at the University of Illinois, report on a hydrous pyrolysis process in which swine manure is converted to oil by heating the swine manure and water in the presence of carbon monoxide in a closed container. For that process they report that a temperatures of at least is required to convert the swine manure to oil, and temperatures above about reduces the amount of oil produced. The Zhang ''et al.'' process produces pressures of about 7 to 18 Mpa (1000 to 2600 psi - 69 to 178 atm), with higher temperatures producing higher pressures. Zhang et al. used a retention time of 120 minutes for the reported study, but report at higher temperatures a time of less than 30 minutes results in significant production of oil. A commercialized process using hydrous pyrolysis (see the article Thermal depolymerization) used by Changing World Technologies, Inc. (CWT) and its subsidiary Renewable Environmental Solutions, LLC (RES) to convert turkey offal. As a two-stage process, the first stage to convert the turkey offal to hydrocarbons at a temperature of and a second stage to crack the oil into light hydrocarbons at a temperature of near . Adams et al. report only that the first stage heating is "under pressure"; Lemley, in a non-technical article on the CWT process, reports that for the first stage (for conversion) a temperature of about and a pressure of about 600 psi, with a time for the conversion of "usually about 15 minutes". For the second stage (cracking), Lemley reports a temperature of about .

Temperature and heating rate

Temperature plays a major role in the conversion of biomass to bio-oil. The temperature of the reaction determines the depolymerization of the biomass to bio-oil, as well as the repolymerization into

char Char may refer to: People * Char Fontane, American actress * Char Margolis, American spiritualist * René Char (1907–1988), French poet *The Char family of Colombia: ** Fuad Char, Colombian senator ** Alejandro Char Chaljub, mayor of Barranquill ...

. While the ideal reaction temperature is dependent on the feedstock used, temperatures above ideal lead to an increase in char formation and eventually increased gas formation, while lower than ideal temperatures reduce depolymerization and overall product yields. Similarly to temperature, the rate of heating plays a critical role in the production of the different phase streams, due to the prevalence of secondary reactions at non-optimum heating rates. Secondary reactions become dominant in heating rates that are too low, leading to the formation of char. While high heating rates are required to form liquid bio-oil, there is a threshold heating rate and temperature where liquid production is inhibited and gas production is favored in secondary reactions.

Pressure

Pressure (along with temperature) determines the super- or subcritical state of solvents as well as overall reaction kinetics and the energy inputs required to yield the desirable HTL products (oil, gas, chemicals, char etc.).

Residence Time

Hydrothermal liquefaction is a fast process, resulting in low residence times for depolymerization to occur. Typical residence times are measured in minutes (15 to 60 minutes); however, the residence time is highly dependent on the reaction conditions, including feedstock, solvent ratio and temperature. As such, optimization of the residence time is necessary to ensure a complete depolymerization without allowing further reactions to occur.

Catalysts

While water acts as a catalyst in the reaction, other catalysts can be added to the reaction vessel to optimize the conversion. Previously used catalysts include water-soluble inorganic compounds and salts, including KOH and Na₂CO₃, as well as transition metal catalysts using

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...

palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself ...

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...

and

ruthenium Ruthenium is a chemical element with the symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most other chemical ...

supported on either

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

or alumina. The addition of these catalysts can lead to an oil yield increase of 20% or greater, due to the catalysts converting the protein, cellulose, and hemicellulose into oil. This ability for catalysts to convert biomaterials other than fats and oils to bio-oil allows for a wider range of feedstock to be used.

Environmental Impact

Biofuels that are produced through hydrothermal liquefaction are

carbon neutral Carbon neutrality is a state of net-zero carbon dioxide emissions. This can be achieved by balancing emissions of carbon dioxide with its removal (often through carbon offsetting) or by eliminating emissions from society (the transition to the " ...

, meaning that there are no net carbon emissions produced when burning the biofuel. The plant materials used to produce bio-oils use photosynthesis to grow, and as such consume carbon dioxide from the atmosphere. The burning of the biofuels produced releases carbon dioxide into the atmosphere, but is nearly completely offset by the carbon dioxide consumed from growing the plants, resulting in a release of only 15-18 g of CO₂ per kWh of energy produced. This is substantially lower than the releases rate of fossil fuel technologies, which can range from releases of 955 g/kWh (coal), 813 g/kWh (oil), and 446 g/kWh (natural gas). Recently, Steeper Energy announced that the

carbon intensity An emission intensity (also carbon intensity or C.I.) is the emission rate of a given pollutant relative to the intensity of a specific activity, or an industrial production process; for example grams of carbon dioxide released per megajoule o ...

(CI) of its Hydrofaction™ oil is 15 CO₂eq/MJ according t
GHGenius model
(version 4.03a), while diesel fuel is 93.55 CO₂eq/MJ. Hydrothermal liquefaction is a clean process that doesn't produce harmful compounds, such as

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...

, NO_x, or SO_x. Instead the

heteroatom In chemistry, a heteroatom () is, strictly, any atom that is not carbon or hydrogen. Organic chemistry In practice, the term is usually used more specifically to indicate that non-carbon atoms have replaced carbon in the backbone of the molecula ...

s, including nitrogen, sulfur, and chlorine, are converted into harmless byproducts such as N₂ and inorganic acids that can be neutralized with bases.

Comparison with pyrolysis and other biomass to liquid technologies

The HTL process differs from

as it can process wet biomass and produce a bio-oil that contains approximately twice the energy density of pyrolysis oil. Pyrolysis is a related process to HTL, but biomass must be processed and dried in order to increase the yield. The presence of water in pyrolysis drastically increases the heat of vaporization of the organic material, increasing the energy required to decompose the biomass. Typical pyrolysis processes require a water content of less than 40% to suitably convert the biomass to bio-oil. This requires considerable pretreatment of wet biomass such as tropical grasses, which contain a water content as high as 80-85%, and even further treatment for aquatic species, which can contain higher than 90% water content. The HTL oil can contain up to 80% of the feedstock carbon content (single pass). HTL oil has good potential to yield bio-oil with "drop-in" properties that can be directly distributed in existing petroleum infrastructure. The energy returned on energy invested (EROEI) of these processes is uncertain and/or has not been measured. Furthermore, products of hydrous pyrolysis might not meet current fuel standards. Further processing may be required to produce fuels.

References

External links

A Possible Deep-Basin High-Rank Gas Machine Via Water Organic-Matter Redox Reactions
Leigh C. Price
Surreptitiously converting dead matter into oil and coal - Water, Water Everywhere
Science News, February 20, 1993, Elizabeth Pennisi
Hydrogen isotope systematics of thermally generated natural gases
{dead link, date=January 2018 , bot=InternetArchiveBot , fix-attempted=yes , Chris Clayton Organic reactions Chemical processes Industrial processes Biodegradable waste management Waste treatment technology