An improved method for selective catalytic reduction of nitrogen oxides in exhaust gases

The selective catalytic reduction (SCR) of nitrogen dioxide in an air flow modeling the exhaust gas from an internal combustion engine is studied. Granulated V₂O₅ (13.5%)–MnO₂ (0.7–1.0%)/Al₂O₃ powder (AVK-10M catalyst) and ammonia injected into a SCR catalytic cell are used as a heterogeneous catalyst of NO₂ reduction and a reducing agent, respectively. If the efficiency of NO₂ removal is high enough and satisfies the requirements of the State Sanitary Standards for the maximum permissible concentrations of substances emitted into the atmosphere (\(MP{C_{N{O_2}}}\) = 0.085 mg/m³), the reducing agent (ammonia) is not completely consumed during SCR, so a considerable amount of NH₃ can be released into the atmosphere. Therefore, a strict control of both NO₂ and unreacted ammonia emissions is needed. The dependences of the concentrations of [NH₃] and [NO₂] on the [NH₃]/[NO₂] ratio for the model air flow passed through the AVK-10M granular heterogeneous catalyst are measured. It is found that the maximum degree of removal of NO₂ from the air takes place at [NH₃]/[NO₂] = 1.3. In the conventional process, the concentration of [NO₂] drop from 530.00 to 0.07 mg/m³, i.e., below the \(MP{C_{N{O_2}}}\). At the same time, the ammonia concentration increases to [NH₃] = 3.4 mg/m³, which becomes 85 times the \(MP{C_{N{O_2}}}\), 0.04 mg/m³. To remove unreacted ammonia from air flows, we developed [P–(SO₃^-)₂ · Me²⁺] sulfocationites, where Me are the Cu and/or Ca ions, P is a styrene–divinylbenzene copolymer. It is shown that the concentration of ammonia passed through the adsorption cell filled with a freshly sulfocationite drops below \(MP{C_{N{H_3}}}\) = 0.04 mg/m³. The dependences of the dynamic exchange capacity (DEC) before ammonia breakthrough for the [P–(SO₃^-)₂ · Cu²⁺] delta-sulfocationite on the air flow rate, [NH₃] concentration, and humidity are measured. The maximum value of the DEC, δ = 59.5 mg/cm³, is observed at an air flow velocity of 2.171 m/s, [NH₃] = 0.0035 mg/L, and 75% humidity. To illustrate practical applications of the proposed improved SCR method, it is shown that a 3-L replaceable [P–(SO₃^-)₂ · Cu²⁺] sorbent cartridge in a SCR exhaust gas purifier for a car internal combustion engine does not need replacement more frequently than every 50000 km.