Many hydrous pyrolysis processes use superheated water in the liquid phase, whereas steam cracking uses water in the gas phase. Steam cracking is used in the petroleum industry to produce the lighter alkenes.
Hydrous pyrolysis may be a significant process in the creation of fossil fuels. Simple heating without water, anhydrous pyrolysis has long been considered to take place naturally during the catagenesis of kerogens 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 ratio of natural gas also suggests that hydrogen from water has been added during creation of the gas.
U.S. patent 2,177,557, describes 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 220 to 360 °C (428 to 680 °F), 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.
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 275 °C (527 °F) is required to convert the swine manure to oil, and temperatures above about 335 °C (635 °F) 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 200 to 300 °C (392 to 572 °F) and a second stage to crack the oil into light hydrocarbons at a temperature of near 500 °C (932 °F). 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 260 °C (500 °F) 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 480 °C (896 °F).
The EROEI [Energy Returned On Energy Invested] 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.
- Pennisi, Elizabeth, Surreptitiously converting dead matter into oil and coal - Water, Water Everywhere, Science News, February 20, 1993.
- Bergstrom et al., Method of treating wood or lignine or cellulose containing materials for obtaining valuable products, U.S. Patent 2,177,557, issued 24 October 1939
- Zhang et al.,. (1999). Thermochemical Conversion of Swine Manure to Produce Fuel and Reduce Waste. University of Illinois.
- See Appel et al., published U. S. patent application US 2004/0192980, publication date September 30, 2004, now patent 8,003,833, issued August 23, 2011.
- Adams, Terry N.; Appel, Brian S.; P. J., Samson; Michael J., Roberts (2004-03-01). Converting Turkey Offal into Bio-derived Hydrocarbons Oil (PDF). Power-Gen Renewable Energy Conference. Las Vegas, Nevada.
- Lemley, Brad (May 2003). Anything Into Oil. Discover.
- Ramirez, Jerome; Brown, Richard; Rainey, Thomas (1 July 2015). "A Review of Hydrothermal Liquefaction Bio-Crude Properties and Prospects for Upgrading to Transportation Fuels". Energies. 8: 6765–6794. doi:10.3390/en8076765.
- 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, Chris Clayton