Reaction Mechanism for the Removal of NOx by Wet Scrubbing Using Urea Solution: Determination of Main and Side Reaction Paths

Abstract

Secondary problems, such as the occurrence of side reactions and the accumulation of by-products, are a major challenge in the application of wet denitrification technology through urea solution. We revealed the formation mechanism of urea nitrate and clarified the main and side reaction paths and key intermediates of denitrification. Urea nitrate would be separated from urea absorption solution only when the concentration product of [urea], [H+] and [NO3−] was greater than 0.87~1.22 mol3/L3. The effects of the urea concentration (5–20%) and reaction temperature (30–70 °C) on the denitrification efficiency could be ignored. Improving the oxidation degree of the flue gas promoted the removal of nitrogen oxides. The alkaline condition was beneficial to the dissolution process, while the acidic condition was beneficial to the reaction process. As a whole, the alkaline condition was the preferred process parameter. The research results could guide the optimization of process conditions in theory, improve the operation efficiency of the denitrification reactor and avoid the occurrence of side reactions.

Kinetic Study on the Reduction of Room-Temperature NOx in Etching Waste Gas by Na2S2O5 and Urea in Alkaline Pretreatment

“…Concurrently, (NH 2 ) 2 CO eliminates NO 2 – and NO 3 – in the solution via eqs and , pushing eq to the right, thus accelerating NO 2 dissolution and reaction rates, enhancing the NO x capture and elimination efficiency of the eluent. However, when urea is added excessively, the diminished H + concentration hampers urea solution hydrolysis, and the content of ammonium carbamate (NH 2 COONH 4 ) produced by urea hydrolysis decreases, which is not easy to react with NO 2 – and NO 3 – , and reduces the reaction rate, thus inhibiting the removal efficiency. − Consequently, when the concentration of (NH 2 ) 2 CO exceeded 0.5 mol L –1 , the removal rate slowed. 4 false( normalN normalH 2 false) 2 normalC normalO + 6 normalN normalO 2 → 7 normalN 2 + 4 normalC normalO 2 + 8 normalH 2 normalO 2 false( normalN normalH 2 false) 2 normalC normalO + 6 normalN normalO → 5 normalN 2 + 2 normalC normalO 2 + 4 normalH 2 normalO false( normalN normalH 2 false) 2 normalC normalO + normalN normalO + normalN normalO 2 → 2 normalN 2 <...…”

“…Concurrently, (NH 2 ) 2 CO eliminates NO 2 – and NO 3 – in the solution via eqs and , pushing eq to the right, thus accelerating NO 2 dissolution and reaction rates, enhancing the NO x capture and elimination efficiency of the eluent. However, when urea is added excessively, the diminished H + concentration hampers urea solution hydrolysis, and the content of ammonium carbamate (NH 2 COONH 4 ) produced by urea hydrolysis decreases, which is not easy to react with NO 2 – and NO 3 – , and reduces the reaction rate, thus inhibiting the removal efficiency. − Consequently, when the concentration of (NH 2 ) 2 CO exceeded 0.5 mol L –1 , the removal rate slowed. …”

Application of TiO2 Supported on Nickel Foam for Limitation of NOx in the Air via Photocatalytic Processes

“…Their presence in the air has harmful effects on human health and ecosystems. Therefore there are some processes applied in the industry for the limitation of nitrogen oxides to the atmosphere, such as SCR (selective catalytic reduction) or wet denitration in ammonia or urea solutions [1]. Some other processes of NO x abatement have been also studied, which are carried out at low temperatures such as catalytic ozonation, electrochemical reduction by single-atom catalysts, or photo-deNOx reactions [2][3][4].…”

“…In addition, the presence of alkaline ions on the TiO 2 surface, such as K + , results in the trapping of NO 3 − ions and their stronger binding to the titania [6]. In the case of other technology used for NO x removal such as wet denitration in the urea solution, the presence of alkali conditions allowed for dissolving urea nitrate, which was formed as a side product and being in excess was disadvantageous [1]. Another way to increase nitrogen oxide removal efficiency while reducing NO 2 release is to modify TiO 2 with noble metals.…”