NOX scrubbing

Anon., Degussa Pushing Hydrogen Peroxide for NOx Scrubbing , Chemical Engineering, 20 September, p.18 (1982). 

Figure 5.6 Reactions taking place during NOx scrubbing.

The reactions taking place during NOx scrubbing are as follows [325] ... 

The most popiilar dry scrubbing systems for incinerators have involved the spray drying of hme slurries, followed by dry coUection in electrostatic precipitators or fabric filters. Moller and Christiansen [Air Poll. Cout. Assoc. 84-9.5 (1984)] published data on early European technology. Moller et al. [U.S. Patent no. 4,889,698 (1989)] describe the newer extension of that technology to include both spray-dryer absorption and dry scrubbing with powdered, activated carbon injection. They claim greatly improved removal of mercury, dioxins, and NOx. 

SOLINOX SO,. Linde NO,] A process for removing both NOx and SOx from fluegases. The SOx is removed by scrubbing with tetra-ethylene glycol dimethyl ether, circulated in a packed tower (the Selexol process). The NOx is destroyed by Selective Catalytic Reduction ( SCR). The sorbent is regenerated with steam the SOx is recovered for conversion to sulfuric acid. Developed by Linde in 1985 and used in a lead smelter in Austria and several power stations in Germany. In 1990 it was announced that it would be used at the titanium pigment plant in The Netherlands operated by Sachtleben. 

The electron-rich oxygen anions exhibit basic electron donor capacity. Basic metal oxides are commonly used for neutralizing or scrubbing acidic gases. Alkaline earth metal oxides have been used for the removal of NOx. The surfaces of cubic alkaline metal oxide such as MgO, CaO, and BaO are dominated by the Lewis basicity of surface oxide anions. The basicity increases down the alkaline earth family as the metal ion radii become larger and the chaige on the metal ion becomes more positive. 

The LoTOx process represents a high-efficiency alternative to SCR. When combined with the EDV Wet Scrubbing System, LoTOx can provide similar or higher NOx reductions as an SCR, but without any of the associated concerns. 

The demonstration achieved all objectives and proved that the EDV Wet Scrubbing System with LoTO, technology can easily be applied to FCCU applications for reducing both the NOx and SO, together. The demonstration proved that even when used on an FCCU flue gas, the combination system can reduce SO2 greater than 99% and NO, greater than 90%. In addition, the EDV system s ability to reduce particulate emissions makes this a real all-pollutants reduction system. Figure 17.27 documents the results from the demonstration. 

Marathon proceeded to purchase and install LoTOx technology for application to their existing EDV wet scrubbing system at Texas City based upon the results of the technology demonstration. This system started commercial operation in February, 2007 [7]. The initial NOx reduction results from the commercial application have confirmed the results of the demonstration. Results from the first week of operation on the first LoTOx unit installed on an FCCU is shown in Figure 17.28. Subsequent testing has confirmed operation below 10 ppm. This outlet NOx is adjusted by varying the set point on the system controller. 

Carbon impregnated with potassium iodide was used as an ozone-scrubbing filter in a chemiluminescence NOx analyser. When the level of iodide was inadvertently increased to the high level of 40%, the fdter exploded violently during replacement after use. This was attributed to oxidation of iodide to iodate by ozone, and frictional initiation of the iodate—carbon mixture when the filter was dismantled. 

Air Pollution Control. A number of processes use ammonia for scrubbing sulfur oxides (SOx) and/or nitrogen oxides (NOx) from industrial and electrical power plant stack gases.57... 

LoTOx A process for removing NOx from FCC off-gases. Injection of ozone oxidizes the NOx to N205, which is easily removed by water scrubbing. Developed by BOC. 

TRI-NOx A process for removing NOx and nitric acid mists from the waste gases from the manufacture of electronic devices. It uses multistage scrubbing with a proprietary solvent. Developed by Wacker Siltronic Corporation, based on a scrubber engineered and manufactured by Tri-Mer Corporation. 

NOx Absorption by Salt Solutions. A number of aqueous salt solutions have been screened for NO r scrubbing potential and some of these... 

Postformation nitrogen oxide emission control measures include selective catalytic and noncatalytic reduction with ammonia, which between them are used by some 900 power station installations worldwide [51]. The catalytic removal methods are 70-90% efficient at NOx removal, but are more expensive to operate than the noncatalytic methods which are 30-80% efficient. Ammonia or methane noncatalytic reduction of NOx to elemental nitrogen is also an effective method which is cost-effective for high concentration sources such as nitric acid plants (Chap. 11). NOx capture in packed beds is less expensive, but this method is not particularly effective [23]. It is also not a very practical method either for utilities or for transportation sources. Two-stage scrubbing has also been proposed as an effective end-of-pipe NOx control measure. The first stage uses water alone and the second uses aqueous urea. 

Dust (especially from industrial activities) and salt spray will also exacerbate atmospheric corrosion (Section 16.4). In enclosed industrial premises, atmospheric corrosion could be minimized by preventing noxious emissions, filtering the air to remove particulate matter, and scrubbing the air with water to remove SO2 and other objectionable gases, although the humidity should itself be kept as low as possible (e.g., steam leaks should not be tolerated). On the global scale, however, the cost to the public of atmospheric corrosion could be substantially reduced by sharply limiting SO2 and, to a lesser extent, NOx emissions from power plants, smelters, automobiles, and other industrial functions. This is an aspect of the acid rain threat (Chapter 8) that is usually overlooked. 

The solubilities of the various gases in [BMIM][PF6] suggests that this IL should be an excellent candidate for a wide variety of industrially important gas separations. There is also the possibility of performing higher-temperature gas separations, thanks to the high thermal stability of the ILs. For supported liquid membranes this would require the use of ceramic or metallic membranes rather than polymeric ones. Both water vapor and C02 should be removed easily from natural gas since the ratios of Henry s law constants at 25 °C are -9950 and 32, respectively. It should be possible to scrub C02 from stack gases composed of N2 and Oz. Since we know of no measurements of H2S, SOx, or NOx solubility in [BMIM][PF6], we do not know if it would be possible to remove these contaminants as well. Nonetheless, there appears to be ample opportunity for use of ILs for gas separations on the basis of the widely varying gas solubilities measured thus far. 

Rare earths in SOx and NOx abatement additives for FCC. Sulfur oxides can be formed in the regenerator due to the oxidation of the sulfur compounds contained in the coke. There are four options to abate SOx emissions in FCC units Processing low-sulfur feeds (S < 0.3%), hydrodesulfurization of the feed, flue gas scrubbing, and the use of SOx additives. 

Sulfur oxides and nitrogen oxides may be removed from combustion gases by various processes. One of the cost-effective and efficient methods involve dry scrubbing of SOx and NOx over lanthanide-oxygen-sulfur compounds (Jalan and Desai 1992). Cerium sulfate is found to be an effective catalyst toward the reduction of NOx by ammonia. A combined removal of NOx and SOx has been achieved using cerium oxide doped with strontium oxide, lanthanum oxide, calcium oxide, or cerium sulfate. 

Jalan, V., and M. Desai. 1992. Dry scrubbing of SOx and NOx over lanthanide-oxygen-sulfur compounds. Abstracts of Small Business Innovative Research Program. Washington, DC U.S. EPA. 

The majority of the stack gas scrubbing processes are designed to remove sulfur dioxide from the gas streams some processes show the potential for the removal of NOx. However, there is the current line of thinking that pursues the options that enable SOx and NOx to be controlled, at least as far as possible, by modification of the combustion process. Sulfur (as already noted) can be removed by injecting limestone, with the coal into a boiler while modifications of the combustion chamber, as well as methods of flame temperature regulation and techniques that lower combustion temperatures, such as injection of steam into the combustion region are claimed to reduce emissions of NOx. 

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