Minimizing the consumption of reducing agents for NOx removal in a wet scrubber without H2S formation

Abstract To control NOx (nitrogen oxide) emitted from point-of-use (POU) scrubbers during semiconductor production, O3 was injected into the inlet of a wet scrubber to oxidize NO to NO2, and Na2S, Na2SO3, and Na2S2O3 were added to remove NO2 in the scrubber. Although Na2S is superior to the other reducing agents, it generated H2S below pH 11. Therefore, NaOH was added to adjust the pH, rapidly increasing Na2S consumption up to 7.13 times the theoretical consumption. In order to reduce chemical consumption, we manufactured a novel wet scrubber in which the liquid reservoir and scrubbing zone were separated according to the chemical. The novel scrubber dramatically reduced the consumption of Na2S and NaOH to levels below 1.42 and 1.19 times the theoretical values. Although Na2SO3 and Na2S2O3 do not generate H2S, their NOx removal performances were inferior to Na2S. Hence, when Na2SO3 and Na2S2O3 were used alone as reducing agents, the chemical consumption rates were 38 times and 95 times their theoretical values, respectively. In order to reduce the chemical consumption rate, we mixed Na2SO3 and Na2S2O3 to prevent fast depletion of Na2SO3 by the free-radical scavenger reactions of thiosulfate (S2O32–). The total chemical consumption rates depending on the mixing ratio of Na2SO3 and Na2S2O3 were experimentally evaluated. Finally, an optimal mixing ratio of 1:1 was determined, which corresponded to a consumption rate of only double the theoretical value.