Review on solid selective catalytic reduction (SSCR) technology: excellent optimization of selective catalytic reduction technology

Abstract

Selective catalytic reduction (SCR) technology is one of the main measures to achieve the pollutant emission reduction target of diesel engines, and the NH₃-SCR exhaust gas aftertreatment technology systems used at present are all based on the design scheme using liquid urea aqueous solution as a reducing agent. However, the SCR system using urea aqueous solution as the reducing agent has some problems in the process of use, such as freezing of the reducing agent (−11 °C) resulting in blocking of the reducing agent transport pipeline; incomplete hydrolysis of urea aqueous solution will not only form crystallization in the pipeline, but also lead to a certain error between the amount of urea injection and the amount of ammonia (urea pyrolysis and hydrolysis to produce ammonia), which affects the conversion efficiency of SCR system. The effective ammonia content per unit volume of urea aqueous solution is low (17.3%), and urea aqueous solution needs to be supplemented at 1500–2500 km, which is not conducive to the quality control of urea aqueous solution, and it is difficult to ensure the consistency of nitrogen oxide (NOx) conversion effect in the SCR system. Because there are many insurmountable defects in urea SCR technology, researchers at home and abroad have turned their research focus to SSCR (solid SCR) technology. Solid selective catalytic reduction (SSCR) technology utilizes solid ammonia storage, releasing ammonia gas directly upon heating. This approach offers significant advantages over urea-based SCR, addressing its inherent limitations. This paper provides an overview of SCR and SSCR technologies, discusses the formation of nitrogen oxides, the NH₃-SCR mechanism, and the principles of operation for both urea SCR and solid ammonia storage materials. It also explores the development of SSCR systems, highlighting their potential to overcome the challenges faced by conventional SCR methods.