# US 4,400,362 A · Removal of nitrogen oxides from gas > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 4,400,362 A - Document ID: US4400362A - Status: Granted - Kind: A - Application number: 06/317,948 - Filed: 1981-11-04 - Granted: 1983-08-23 - Earliest claimed: 1981-11-04 - Inventors: Bernard J. Lerner - Web page: https://patentgrep.com/patent/US4400362A ## CPC - C01B 21/20: Nitrogen oxides; Oxyacids of nitrogen; Salts thereof - B01D 53/56: Nitrogen oxides (B01D53/60 takes precedence) - C01B 21/48: Methods for the preparation of nitrates in general (particular individual nitrates, see the relevant groups of subclasses C01B - C01G, according to the cation) - C01B: NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; {METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C} - B01D: SEPARATION (separating solids from solids by wet methods B03B, B03D; by pneumatic jigs or tables B03B; by other dry methods B07; magnetic or electrostatic separation of solid materials from solid materials or fluids, separation by high-voltage electric fields B03C; centrifuges, vortex apparatus B04; presses per se for squeezing-out liquid from liquid-containing material B30B9/02; treatment of water C02F, e.g. softening by ion-exchange C02F1/42; {arrangements of air intake cleaners in gas turbine plants F02C7/05} ; arrangements or mounting of filters in air-conditioning, air-humidification or ventilation F24F13/28) ## Abstract Gas containing nitrogen oxides is treated in two separate stages. In the first stage, the nitrogen oxides in the gas are reacted with sulfurous acid. The gaseous product of this reaction is reacted with an alkaline solution, typically caustic alkali. As a result of these treatments, the nitrogen oxides are converted into neutral salts. ## Claims ### Claim 1 The method of removing nitrogen oxide and nitrogen dioxide from a gas comprising reacting the nitrogen oxides with sulfurous acid, and oxygen in a wet scrubbing operation, thereafter reacting the gaseous product of the reaction of the sulfurous acid and oxygen and the nitrogen oxides with an alkaline solution in a wet scrubbing operation to convert the said product into a salt, the reaction of the nitrogen oxides with the sulfurous acid and oxygen converting the nitrogen oxides into a compound which is readily converted into a salt. ### Claim 2 (depends on 1) The method of claim 1 wherein the sulfurous acid is continuously formed by flowing sulfur dioxide into water. ### Claim 3 (depends on 2) The method of claim 2 wherein, prior to the reaction of the nitrogen oxides in the gas with sulfurous acid, the gas is contacted by water to remove any components of the gas which inhibit the formation of sulfurous acid from the sulfur dioxide and water. ### Claim 4 (depends on 1) The method of claim 1 wherein liquid effluent from the reaction of the gas with sulfurous acid which liquid effluent includes excess sulfur dioxide and nitrogen oxides is added back to the reaction of the sulfurous acid and the gas. ### Claim 5 (depends on 1) The method of claim 1 wherein the product of the reaction of the nitrogen oxides in the gas and sulfurous acid, which is converted into a salt by reaction with alkaline solution, is predominantly compounds which are more soluble in and more reactive with said alkaline solution than nitric oxide and nitrogen dioxide. ### Claim 6 (depends on 1) The method of claim 1 wherein the reaction of the nitrogen oxides with the sulfurous acid is carried out by countercurrent contacting the sulfurous acid and the gas containing the nitrogen oxides. ### Claim 7 (depends on 1) The method of claim 1, wherein the sulfurous acid is produced by reacting water with sulfur dioxide and wherein sulfuric acid is produced in the reaction of the nitrogen oxides and the sulfurous acid and the concentration of the said sulfuric acid is monitored and controlled to prevent the sulfuric acid from reaching a high level at which it inhibits the reaction of the sulfur dioxide and the water. ### Claim 8 (depends on 7) The method of claim 7, wherein the liquid containing the sulfuric acid is drained off and replaced by water to prevent the sulfuric acid concentration from reaching the high level. ### Claim 9 (depends on 7) The method of claim 7, wherein the solution capacity for sulfur dioxide of the liquid in which the nitrogen oxides are reacted with the sulfurous acid in the wet scrubbing operation is monitored by measuring the oxidation-reduction potential of the liquid and the concentration of sulfuric acid is monitored by measuring the pH of the liquid. ### Claim 10 The method of removing nitrogen oxide and nitrogen dioxide, from gas comprising reacting the nitrogen oxides in the gas with sulfurous acid and oxygen in a wet scrubbing operation to convert the nitrogen oxides into a nitrogen-oxygen gaseous compound which reacts readily with alkaline solution, then reacting the nitrogen-oxygen compound with an alkaline solution in a wet scrubbing operation to produce a nitrite salt, and then oxidizing the nitrite salt to convert said nitrite salt into a nitrate salt. ### Claim 11 The method of removing nitrogen dioxide from gas comprising reacting the nitrogen dioxide in the gas with sulfurous acid in a wet scrubbing operation to convert the nitrogen dioxide into a compound which reacts readily with alkaline solution, then reacting the said compound with an alkaline solution in a wet scrubbing operation to produce a nitrite salt, and then oxidizing the nitrite salt to convert said nitrite salt into a nitrate salt. ### Claim 12 (depends on 11) The method of claim 11, wherein the reaction of the nitrogen dioxide with the sulfurous acid is carried out by countercurrent contacting the sulfurous acid and the gas containing the nitrogen dioxide. ### Claim 13 (depends on 11) The method of claim 11, wherein the sulfurous acid is produced by reacting water with sulfur dioxide and wherein sulfuric acid is produced in the reaction of the nitrogen dioxide and sulfurous acid and the concentration of the said sulfuric acid is monitored and controlled to prevent the sulfuric acid from reaching a high level at which it inhibits the reaction of the sulfur dioxide and the water. ### Claim 14 (depends on 13) The method of claim 13, wherein the liquid containing the sulfuric acid is drained off and replaced by water to prevent the sulfuric acid concentration from reaching the high level. ### Claim 15 (depends on 13) The method of claim 13, wherein the solution capacity for sulfur dioxide of the liquid in which the nitrogen dioxides are reacted with the sulfurous acid in the wet scrubbing operation is monitored by measuring the oxidation-reduction potential of the liquid and the concentration of sulfuric acid is monitored by measuring the pH of the liquid.