Radicals from homogeneous decomposition of H2O2 react with unscrubbable NO to produce scrubbable gases.
Pilot scale experiments have demonstrated a method of reducing the amounts of oxides of nitrogen (NOx) emitted by industrial boilers and powerplant combustors that involves (1) injection of H2O2 into flue gases and (2) treatment of the flue gases by caustic wet scrubbing like that commonly used to remove SO2 from combustion flue gases. Heretofore, the method most commonly used for removing NOx from flue gases has been selective catalytic reduction (SCR), in which the costs of both installation and operation are very high. After further development, the present method may prove to be an economically attractive alternative to SCR.
The primary constituent of NOx is NO. Although the nitrogen acid gases HNO2 and HNO3 (and, to a lesser extent, NO2) can be removed from flue gas by caustic scrubbing, NO is almost completely insoluble and thus not scrubbable. If, however, the NO in NOx could be economically converted to HNO2 and HNO3, then scrubbing could be an effective means for removing NOx
NO can be oxidized to HNO2, HNO3, and NO2 at low to moderate flue-gas temperatures by use of hydroxyl radicals (OH). In the present method, the OH needed for oxidation of NO is generated by thermal decomposition of the H2O2 injected into the flue gas.
For efficiency, it is necessary to maximize the proportion of OH and HO2 radicals, relative to other products of decomposition of H2O2. In particular, it is necessary to suppress a competing reaction in which H2O2 decomposes into water and oxygen. This competing reaction occurs readily at surfaces. In general, peroxides are preserved at acidic surfaces and are decomposed at basic ones. Experiments have shown that the incidence of the competing decomposition into H2O and O can be reduced by treating reactor surfaces with boric acid to render them acidic.
In one of several sets of experiments to demonstrate the feasibility of the method, the optimum temperature for conversion of NO to HNO2, HNO3, and NO2 by use of injected H2O2 was found to be about 500 °C (see Figure 1).
A study performed under the guidelines of the EPA, EPRI comparing the economics of SCR and the experimental H2O2-injection/scrubbing method was conducted for a design base case and a variety of alternative cases. This study illustrated the tradeoff between capital and operating costs for the two methods. The single largest factor in determining the total cost of one method relative to the other method was found to be the H2O2:NOx molar ratio. At the H2O2:NOx molar ratio of 1.92, which was previously demonstrated in the laboratory, the H2O2-injection/scrubbing method was shown to be uneconomical. However, it was also concluded that the molar ratio in a full-size coal-fired power plant could be lower than that found in the laboratory, and that on the basis of all the assumptions of the study, at an H2O2:NOx molar ratio of 1.37, the H2O2-injection/scrubbing method could be an economically feasible alternative to SCR.
Pilot-scale tests run at Kennedy Space Center demonstrated the feasibility and competitiveness of this new technology. The H2O2 to NO molar ratio, at 500 °C shown to achieve a NO conversion efficiency of > 90 percent was 1:1, which is significantly lower than the required 1.37:1 (See Figure 2).
This work was done by Michelle M. Collins of Kennedy Space Center and C. David Cooper and Christian A. Clausen III of the University of Central Florida.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Technology Commercialization Office, Kennedy Space Center, (321) 867-1463. Refer to KSC-12056.