Energy efficient disposal of hazardous waste material by supercritical water oxidation
1. Introduction
The results of developments in structural materials have also made high corrosion, heat, and pressure resistant materials available for general mechanical engineering applications (e.g., INCONEL alloy 740H [1]). The use of these materials has also made it possible to utilize ultra-supercritical water (USCW) industrially. Using the USCW, effective technologies have been born for the unconventional oil industry; for example, for the conversion of heavy oils into lighter oils before transportation, as well as for environmental protection, in particular for the energy-efficient disposal of hazardous waste material. Most hazardous wastes have a significant organic matter content e.g. halogenated solvents, unsorted mixed plastic wastes, waste oils, various sewage sludges, etc. Below we describe a process and equipment for converting the organic content of hazardous wastes (HW) into useful and easy-to-use energy. The technology shown in Figure 1 covers, among other things, the energy needs of the waste processing itself.
Waste disposal technology is based on supercritical-water oxidation. Supercritical water (SCW) is an excellent organic solvent and also an effective oxidizer in case of appropriate oxygen content.
There are already available continuously operating technologies in the industry - e.g. Chematur SCWO [2], Aqua Critox [3], Duke University SCWO [4], General Atomics SCWO [5], Athos Veolia SCWO [6]. The technologies of these companies and research groups are operational technical solutions, but their main shortcomings are the following:
There are two known methods for an efficient processing of organic wastes: USCW gasification of the materials and their USCW oxidation. The purpose of the presented technology and equipment is to address most of the listed deficiencies in the technologies developed and described for USCWO and to improve the deficiencies. This was achieved mainly by changing the design of the tubular reactor.
2. TECHNOLOGY AND ITS DESCRIPTION
The details of the new type of supercritical-water oxidation waste disposal technology we have developed are described below.
2.1. Technological description
Figure 1 shows the theoretical structure of SCW oxidation technology for higher-capacity hazardous waste disposal.
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Figure 1.. Energy efficient disposal of hazardous waste material by supercritical water oxidation
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During the oxidation of the waste, the heat energy released is converted into superheated steam by a steam-generating tube coil that is hydraulically independent of the reactor tube but in close thermal contact. The steam, leaving the tubular reactor, is utilized in a small steam turbine power plant that is part of the technology. In the case of higher-capacity oxidation plants, a significant waste-SCW mixture is to be expected. This version of the pressure (240-390 bar) required to produce the supercritical operation of the tubular reactor in the system is preferably utilized by expansion to atmospheric pressure with a single-disc turbine. The “carrier material” of the HW to be utilized - the ultra-supercritical water - is produced by a special container boiler unit. The USCW boiler has variable control for parameters in the temperature range of 600-700 ° C and pressure in the range of 240-500 bar. The boiler pipe bundle consists of three separate pipe coils, each connected to collectors, placed one above the other, ensuring excellent heat transfer and good thermal efficiency. The defining part of the SCW oxidation technology is the supercritical tubular reactor, the structure of which is shown in Figure 2.
Figure 2 - Structure of the supercritical tubular reactor (left: in main view, right: in cross section)
The tube reactor equipment comprises two thermally coupled but hydraulically independent tube coils. One possibility is to build into each other, in a pipe-in-pipe heat exchanger design method. Reactor tubes are made of NiCr alloy (INCONEL alloy 740H), which is highly resistant to corrosion, pressure and heat. Thermally coupled pipes for heat recovery are made of P91 boiler steel.
The tube reactor is a complex equipment. On the one hand, it ensures that the HW-SCW-oxidizing agent mixture remains at the pressure and temperature required by the reaction parameters for the required period of time, and that the HW to be destroyed is completely oxidized. On the other hand, it ensures the continuous dissipation of the heat energy released during the exothermic reaction, thus ensuring the thermal stability of the system. It further comprises a power ultrasonic generator together with its vibrating scrapers for keeping the internal surfaces of the reaction tubes of the tubular reactor clean.
2.2. Functional description
The USCW boiler receives feed water from the feed water pump, at the pressure and capacity which are determined by the tube reactor technological parameters required by HW as well as the HW / SCW ratio. The feeding unit is a water jet pump device that effectively solves the uniform mixing of the SCW and the prepared, homogenized WH, the oxidizing agent, and the control fuel, and then feeds the resulting mixture into the reactor tube of the tube reactor unit. The mixture fed to the reactor tube passes through the reactor tube coil for the required reaction time (typically 0.5-3.5 min) determined by the HW material, for the oxidation of the HW. Meanwhile, the heat energy of the heat generation accompanying the oxidation is continuously transferred to the steam generating tube coil in direct thermal contact with the reactor tube coil. In parallel, the ribbed surface of the outer tube coil heats and preheats the combustion air of the USCW boiler passing through the tube reactor. This technical solution reduces the gas consumption of the boiler by about 70%.
The superheated steam generated by the steam generator coil enters the steam condenser of the steam turbine and then condenses the condensate water back to the steam generator coil through the feed water pump heat exchanger. There, the feed water preheated by the heat exchanger again generates superheated steam. The process is thus repeated, cyclically, producing electricity. Through a heat exchanger, the feed water utilizes a significant portion of the thermal energy content of the mixture (supercritical fluid) exiting the reactor tube coil of the tube reactor. The thermal energy of the fluid leaving the reactor tube coil is utilized by a group of elegy turbina and electric power generator sets by expanding the pressure in the reactor tube coil to atmospheric pressure. The pressure reduction is necessary for the boiler water treatment plant.
The prepared homogenized HW is preheated by heat exchangers. HW containing organic matter can be preheated to about 180-300 ° C, depending on the type of organic matter. Separators clean the fluid leaving the tubular reactor of its inert material content. The water treatment plant produces boiler water from the fluid cooled by the heat exchangers and purified by the separators. The feed pump delivers the feed water thus obtained, supplemented with treated fresh water, to the high-pressure feed pump of the USCW boiler, and thus the HW utilization is cyclically repeated.
The thermal efficiency of the described technology and process is between 60 and 70%, depending on the type and material of HW to be utilized and the capacity of the equipment.
3. Summary and Evaluation
Proper use of the high-performance ultrasonic generator and its auxiliaries eliminates the settling of insoluble salts in the mixture of supercritical water and the HW in the tube reactor and the forming of deposition in the reactor tubes. Ultrasound generates gas bubbles in the mixture, which keep the reactor tubes clean and ensure continuous, fine mixing of the mixture and thus perfect distribution of the oxidizing agent. Thus shortening the reaction time required for oxidation and consequently also minimizing the size of the tube reactor.
Preheating the combustion air of the USCW boiler to 400-500 ° C by the outer finned tube of the tube reactor enables energy-efficient HW disposal operation. The water jet pump is a feeding unit that reduces the pump pressure requirements and improves the mixing of the materials fed to the unit. A properly selected elegy-turbina operating in a supercritical state can result in significant electricity savings, improving energy efficiency. The automatically controlled feeding of the control fuel allows the destruction of various organic and inorganic HW material, ensuring that the thermal parameters of the tube reactor are within narrow limits. Thus, it facilitates the utilization of the generated oxidizing heat energy, at the same time improving the efficiency of the small steam turbine power plant and the entire technology. The construction of the tube reactor enables the establishment of high-capacity HW processing plants, which are also power plants producing renewable energy.
LITERATURE REFERENCES
[1] J. J. deBarbadillo, "14 - INCONEL alloy 740H", in Materials for Ultra-Supercritical and Advanced Ultra-Supercritical Power Plants, A. Di Gianfrancesco, Ed. Woodhead Publishing, 2017, p. 469-510.
[2] A. Gidner and L. Stenmark, “Supercritical water oxidation of sewage sludge: state of the art,” vol. 430, 0 2001.
[3] D. Patterson, L. Stenmark, F. Hogan, and A. Water, “Pilot-scale supercritical water oxidation of sewage sludge,” 0 2021.
[4] “Neighborhood-Scale Sewage Treatment Using Supercritical Water Oxidation | Sanitation Solutions ”. https://sanitation.pratt.duke.edu/community-treatment/about-community-treatment-project (accessed Feb. 19, 2021).
[5] “Industrial Supercritical Water Oxidation (iSCWO) | General Atomics ”. https://www.ga.com/hazardous-waste-destruction (accessed Feb. 19, 2021).
[6] K. Hii, S. Baroutian, R. Parthasarathy, D. Gapes, and N. Eshtiaghi, “A review of wet air oxidation and Thermal Hydrolysis technologies in sludge treatment,” Bioresour. Technol., Bind. 155C, p. 289–299, 0 2013, doi: 10.1016 / j.biortech.2013.12.066.