ReviewMolten salt oxidation: A versatile and promising technology for the destruction of organic-containing wastes
Highlights
► There is no summary and review reported on the MSO process. ► It describes the history of MSO, as well as the design and engineering details. ► It focuses on the reaction mechanism and its applications in various wastes. ► The current status for the MSO and directions for future research are considered.
Introduction
Rapid urbanization and industrial diversification have led to the generation of considerable quantities of wastes; a considerable fraction of which is made up of organic matter. These wastes have been determined to be detrimental to either public health or the environment, or both, if not properly managed. The economic impacts of these wastes cannot be ignored and their management has become a major environmental concern. Incineration is an effective technique for organic combustible waste treatment and disposal, mainly because of the substantial volume reduction, energy recovery, and the destruction of pathogens and other toxic organic compounds (Quina et al., 2010). There are, however, some disadvantages with incineration, related to the production of toxic gases (especially PCDD/Fs) and hazardous solid wastes. At high operating temperatures (around 2300 K), carbon and hydrogen in the wastes are converted into CO/CO2, and H2O, respectively, while S, N, P, and halogens transform to SO2/SO3, N2O/NO/NO2, P2O3/P2O5, and X2/HX (X being halogen), respectively. These gases are subsequently released into the exhaust stream. The carbon oxide contributes to the greenhouse effect, while sulfur, nitrogen, phosphorus, and halogen-bearing gases contribute to acid rain and global warming. In addition, NOx emissions can react with unburned hydrocarbons in the presence of sunlight, thereby playing a major role in ground-level ozone formation (Mauzerall et al., 2005). Therefore, the exhaust stream from incineration needs a secondary processing step to eliminate these undesirable gases. However, their complete destruction would require extraordinary precautions, which are costly and often not very effective (Rubel, 1974). It is imperative to develop an alternative process that can destroy these wastes economically and in an environmentally sound manner.
Molten salt oxidation (MSO) is a robust thermal treatment process for destroying organic-containing wastes. In this process, various kinds of wastes are infused with oxidant air and fed into a molten salt bath. Flameless oxidation occurs within the melt, thus converting organic constituents into CO2 and steam. Halogens and heteroatoms (such as sulfur) are converted into acidic gases, which are then “scrubbed” and captured in the alkali melt, as are other inorganic compounds in their highest oxidation state. The off-gas product is treated to remove entrained particulates and sometimes steam before being discharged to the off-gas treatment system. A high destructive efficiency (>99.9999%) was reported for PCBs, trichloro and hexachloro-benzene, ion exchange resins, silicon carbide, energetic materials, and chemical warfare agent sarin (Navratil and Steward, 1996). The MSO process thus offers several advantages over incineration: (1) The operating temperature is hundreds of degrees lower than that of normal incineration, thereby minimizing the emissions of radioactive or hazardous materials. (2) It generates less off-gas, since the process is an exothermic reaction and does not require supplemental fuel to sustain a flame. (3) Flameouts could be avoided, as it is a non-flame process proceeding catalytic liquid-phase organic oxidation reactions. (4) Acid gases are “scrubbed” by the alkali salts, avoiding the requirement of a wet off-gas scrubbing system. (5) The large thermal mass of the salt provides a stable heat transfer medium that resists thermal surges and ensures temperature uniformity, thus mitigating the effects of rapid process fluctuations (Yang et al., 2008). And (6) permitting the MSO process should be easier, while approval for the construction and operation of incinerators is difficult to obtain, and public opposition to incineration is growing (Bache et al., 1992, Hsu et al., 2000, Rae and Brown, 2009). These advantages provide the main reasons for developing MSO as an alternative.
Section snippets
History
MSO was pioneered by the Rocketdyne Division of Rockwell International (now part of the Boeing) for the US Atomic Energy Commission in the 1950s for nuclear fuel processing. Initial research with molten sodium carbonate began with experiments to scrub sulfur dioxide from flue gas and as a catalyst for coal gasification. It has been under sporadic development since the early 1970s. Rockwell pursued the development and testing of MSO as a treatment for hazardous organic wastes in the early 1980s;
Applications
The MSO process provides a method for removing more than 99.999% of the organic matrix from the combustible waste. It has been regarded as one of the emerging technologies that could treat PCBs, various dioxins, pesticides and herbicides, chemical warfare agents, explosives and propellants, etc. (Dustin et al., 1977, Yosim et al., 1980, Edwards et al., 1985, Hsu et al., 1997a, Alam and Kamath, 1998, Heslop, 1998, Yang et al., 2004).
Conclusions and future research
The MSO process offers many advantages over incineration and has potential applications in various wastes. However, there exist some limitations and drawbacks as well: (1) It is not readily adaptable to aqueous wastes. Water-based wastes require additional fuels for vaporization before being processed, increasing the treatment cost. (2) This technology is not effective for wastes with greater than 20% ash and organic constituents that are not easily thermally desorbed. (3) The spent salt must
Acknowledgments
We gratefully acknowledge financial support from the Chinese National 863 High Technology (Grant No. 2009AA064001). We also would like to thank the anonymous referees for their helpful comments on this Paper.
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