Supercritical water combustion

Tester JW, Webley PA, Holgate HR. Revised global kinetic measurements of methanol oxidation in supercritical water. Ind Eng Ch R 1993 32(l) 236-239 Helling RK, Tester JW. Fundamental kinetics and mechanisms of hydrogen oxidation in supercritical water. Combust Sci Technol 1993 88(5-6) 369-397. 

Concluding these calculations, NIR/IR-spectroscopy can obtain following results for supercritical water combustion ... 

Because oxygen, carbon dioxide, methane, and other alkanes are completely miscible with dense supercritical water, combustion can occur in this fluid phase. Both flameless oxidation and flaming combustion can take place. This leads to an important application in the treatment of organic hazardous wastes. Nonpolar organic wastes such as polychlorinated biphenyls (PCBs) are miscible in all proportions in supercritical water and, in the presence of an oxidizer, react to produce primarily carbon dioxide, water, chloride salts, and other small molecules. The products can be selectively removed from solution by dropping the pressure or by cooling. Oxidation in supercritical water can transform more than 99.9 percent of hazardous organic materials into environmentally acceptable forms in just a few minutes. A supercritical water reactor is a closed system that has no emissions into the atmosphere, which is different from an incinerator. 

It could be expected, that combustion reactions and possibly flames can be produced in such dense supercritical mixtures. Technical aspects of hydrothermal oxydation at moderate pressures have already been tested and discussed [7,8]. The study of combustion and flames in supercritical phases offers several possibilities 1. The variation of pressure over wide ranges should influence reaction mechanisms and flame characteristics because the density can be changed from low, gas-like, to high, liquid-like, values. 2. The variable temperature of the dense, fluid environment can have an influence on reactions and flames. 3. The chemical and physical character of this environment can be varied considerably, for example by using supercritical water as the major component, as in the present experiments. Certainly, the knowledge of transport coefficients of gases involved is desirable. For water the viscosity has been determined to... 

Various kinds of information can be expected from the high pressure combustion and flame experiments Reaction kinetics data for conditions of very high collision rates. Results about combustion products obtained at high density and with the quenching action of supercritical water, without or with flame formation. Flame ignition temperatures in the high pressure aqueous phases and the ranges of stability can be determined as well as flame size, shape and perhaps temperature. Stationary diffusion flames at elevated pressures to 10 bar and to 40 bar are described in the literature [12 — 14]. 

The aim of the present work was to design and operate an apparatus in which stationary combustion and flames can be produced and sustained to pressures of 2000 bar and with environmental temperatures up to 500°C. Visual observation of the interior of the reaction vessel should be possible. Arrangements had to be made by which a gas flow of only a few microlitres per second could be fed steadily into the reaction vessel at pressures to two kilobar. A similar provision was necessary to extract small samples for product analysis at constant conditions. The principle of design and operation will be described. First results will be given for experiments with oxygen introduced into supercritical water-methane mixtures. 

T0733 Sonotech, Inc., Cello Pulse Combustion Burner System T0746 STC Remediation, Inc., Solidification/Stabilization Technology T0756 Supercritical Water Oxidation—General... 

As water is subjected to temperatures and pressures above its critical point (374.2°C, 22.1 MPa), it exhibits properties that differ from both liquid water and steam. At the critical point the liquid and vapor phases of water have the same density. When the critical point is exceeded, hydrogen bonding between water molecules is essentially stopped. Supercritical water sustains combustion and oxidation reactions because it mixes well with oxygen and with nonpolar organic compounds. Some organic compounds that are normally insoluble in liquid water become completely soluble (miscible in all proportions) in supercritical water. Some water-soluble inorganic compounds, such as salts, become insoluble in supercritical water. 

The cost of transporting wood chips by truck and by pipeline as a water slurry was determined. In a practical application of field delivery by truck of biomass to a pipeline inlet, the pipeline will only be economical at large capacity (>0.5 million dry t/yr for a one-way pipeline, and >1.25 million dry t/yr for a two-way pipeline that returns the carrier fluid to the pipeline inlet), and at medium to long distances (>75 km [one-way] and >470 km [two-way] at a capacity of 2 million dry t/yr). Mixed hardwood and softwood chips in western Canada rise in moisture level from about 50% to 67% when transported in water the loss in lower heating value (LHV) would preclude the use of water slurry pipelines for direct combustion applications. The same chips, when transported in a heavy gas oil, take up as much as 50% oil by weight and result in a fuel that is >30% oil on mass basis and is about two-thirds oil on a thermal basis. Uptake of water by straw during slurry transport is so extreme that it has effectively no LHV. Pipeline-delivered biomass could be used in processes that do not produce contained water as a vapor, such as supercritical water gasification. 

In addition to these cost elements, transport of biomass for a direct combustion application by water creates a prohibitive drop in the LHV of the fuel because of absorbed water. These issues limit the application of pipeline transport of biomass to large applications that use oil as a carrier medium, or that supply a process for which the heat content of the fuel is not degraded by the requirement to remove absorbed water as vapor, such as a supercritical water gasification process. 

Furthermore, water transport of mixed hardwood and softwood chips causes an increase in moisture level to 65% or greater, which so degrades the LHV of the biomass that it cannot be economical for any process, such as direct combustion, that produces water vapor from water contained in the biomass. The impact on straw is greater, in that moisture levels are so high that the LHV is negative. Pipeline transport of biomass water slurries can only be utilized when produced water is removed as a liquid, such as from supercritical water gasification. 

Matsumura, Y., Sasaki, M., Okuda, K., Takami, S., Ohara, S., Umetsu, M. and Adschiri, T. 2006. Supercritical Water Treatment of Biomass for Energy and Material Recovery. Combust. Sci. Technol., 178, 509-536. 

The objective is to provide a waste disposal technology with the high pressure combustion in supercritical water which does not burden air and water with harmful effluents. The quality of this technology is that the wastes are completely mineralised to C02, water and salts. The products like CO, S02, NOx, dioxins, arsenic, mercury, etc., typical for incineration, are avoided only harmless end-products remain. 

U. Franck, Th. Hirth, G. Pohsner, J. Steinle "High Pressure Combustion Flames in Supercritical Water" 22nd ICT Conference, Karlsruhe, Germany, July 2-5,1991... 

Th. Hirth, H. Krause, N. Eisenreich, "Degradation and Combustion of Organic Materials in Supercritical Water " Institut Superieur Industriel Liegeois (Veranst.) Industrie et Environnement (Liege 1992). Liege Selbstverl., 1992... 

J. M. Ploeger, P. A. Bielenberg, J. L. DiNaro-Blanchard, R. P. Lachance, J. D. Taylor, W. H. Green and J. W. Tester, Modeling Oxidation and Hydrolysis Reactions in Supercritical Water—Free Radical Elementary Reaction Networks and Their Applications, Combust. Sci. and Tech., 178, 363-398 (2006). 

The aim of supercritical water oxidation is to have complete oxidation, with no products of incomplete combustion remain in solution. 

A combustion process is described using a slurry of pulverized coal in a mixture of water and air. The mixture also contains some alkali which is stated to serve as a combustion catalyst. Combustion of the coal raises the temperature to above the critical temperature of water, and the ash that is present in the coal remains suspended in the supercritical medium. This supercritical fluid combustion stream is used to boil water and superheat steam in a countercurrent superheater-boiler train. The steam that is formed in the boiler is used to generate power. 

The calculations have been done not only for stoechiometric combustion conditions, but also for negative oxygen balances, which are relevant for partial combustion conditions and for thermal decomposition reactions. By taking into account an excess of water, also the reaction products of a supercritical water oxidation can be calculated. 

The steam temperature can be raised to levels as high as 580°C-600°C (930 F-1110°F) and pressure over 4500 psi. Under these conditions, water enters a supercritical phase with properties in between those of liquid and gas. This supercritical water can dissolve a variety of organic compounds and gases, and when hydrogen peroxide and liquid oxygen are added, combustion is triggered. Dirbines based on this principle (supercritical turbines) offer outputs of over 500 MW. 

Studies of the reaction mechanisms for a number of simple compounds in supercritical water have been carried out, but even these seem to be complex. For example the oxidation of carbon monoxide can involve 21 elementary reactions [23]. Computer modelling is required and programs designed for combustion processes are sometimes used. The rates of oxidation of carbon monoxide are substantially lower than those predicted by gas-phase models and the proportion of hydrogen produced by the concurrent water-gas shift reaction is unexpectedly high. The differences are explained in terms of lower diffusivities compared with the gas phase, as a result of solvent cages . Because of difficulties with fundamental studies, much work on reactions is empirical and directed towards particular processes. Some of this work relevant to toxics destruction is given in the next section. 

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