Hydrothermal liquefaction

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, and calcium hydroxide 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. 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, 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: sewage sludges, food process wastes, to emerging non-food biomass such as algae. The composition of cellulose, hemicellulose, protein, and lignin 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. Barbero-López et al., tested in the University of Eastern Finland the use of spent mushroom substrate and tomato plant residues as feedstock for hydrothermal liquefaction. They focused in the hydrothermal liquids produced, rich in many different constituents, and found that they are potential antifungals against several fungi causing decay on wood, but their ecotoxicity was lower than that of the commercial Cu-based wood preservative. The effectiveness of the antifungal activity of the hydrothermal liquids varied mostly due to liquid concentration and strain sensitivity, while the different feedstocks did not have such a significant effect. 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. Previously used catalysts include water-soluble inorganic compounds and salts, including KOH and Na2CO3, as well as transition metal catalysts using nickel, palladium, platinum and ruthenium supported on either carbon, silica 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, 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). Hydrothermal liquefaction is a clean process that doesn't produce harmful compounds, such as ammonia, NOx, or SOx. Instead the heteroatoms, 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 pyrolysis 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 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. ==See also==