Urea-Based Selective Catalytic Reduction Systems
Summary
Selective catalytic reduction (SCR) systems employing a urea-water solution inject a measured dose of aqueous urea into exhaust gases, where thermal decomposition and hydrolysis generate ammonia. This ammonia reacts with nitrogen oxides (NOx) over a catalyst, converting harmful emissions into benign nitrogen and water. Critical processes include spray atomisation, droplet evaporation, gas-phase decomposition of urea to isocyanic acid (HNCO) and subsequent hydrolysis to ammonia, and uniform mixing of ammonia with exhaust flow. Challenges encompass wall-film formation and solid deposit build-up, which can impair ammonia distribution and elevate backpressure, and the need to minimise unreacted ammonia slip. Advances in injection strategies, mixer design, catalyst formulation and computational modelling have driven significant improvements in NOx conversion efficiency across on-road, marine and stationary power applications. The global imperative to meet stringent emission standards has spurred innovations in close-coupled SCR architectures and real-time control of urea dosing, ensuring system robustness under transient engine conditions.
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Urea-Based Selective Catalytic Reduction Systems publication trend
The graph below shows the total number of articles in urea-based selective catalytic reduction systems across all publications each year (not limited to Nature Index journals).
Technical terms
Selective catalytic reduction (SCR): a post-combustion process in which a reductant, typically ammonia from a urea-water solution, reacts with nitrogen oxides over a catalyst to form nitrogen and water.
Urea-water solution (UWS): an aqueous mixture of urea used as an ammonia precursor in SCR systems.
Sauter mean diameter (SMD): a measure of the average droplet size in a spray, defined as the diameter of a droplet that has the same volume-to-surface-area ratio as the entire spray.
Wall film: a liquid layer formed by impinging droplets on the exhaust pipe or mixer surfaces, which can decompose and lead to solid deposit build-up.
Ammonia slip: the unreacted ammonia that passes through the SCR catalyst and is emitted to the atmosphere, often controlled to minimise secondary pollution.
References
- The importance of individual spray properties in performance improvement of a urea-SCR system employing flash-boiling injection. Applied Energy (2023).
- Advances and challenges of ammonia delivery by urea-water sprays in SCR systems. Progress in Energy and Combustion Science (2021).
- Kinetic modeling and simulation of high-temperature by-product formation from urea decomposition. Chemical Engineering Science (2021).
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