Ammonia Storage and Release in SCR Systems for Mobile Applications

摘要

The SCR reaction requires ammonia for the reduction of nitrogen oxides. While compressed or liquefied ammonia is used for the supply of ammonia in stationary applications, the odorous ammonia gas had to be replaced in mobile SCR applications by an ammonia precursor compound for the safe storage and reliable release of ammonia in the right quantities and right dynamics. Thus, significant innovation was needed before mobile SCR systems could be realized, even though commercial operation of ammonia SCR systems in coal power plants (>250 MW) started already around 1980 [1]. After the invention of HC-SCR for mobile NO_x sources, to avoid handling NH3 [2], and the discovery of cyanuric acid as a safe NH3 storage compound, [3] urea was proposed as a storage material for NH3 in 1988 [4]. By then, urea was already known to work as an NH3 precursor for SCR in stationary applications for 3 years [5]. First results on mobile urea-based SCR were publically presented in 1990, already showing NO_x conversions above 90 % from 250 °C at gas hourly space velocities (GHSV) of 12,900 h~(-1) [6]. The first patented mobile applications of urea solutions for SCR were registered in 1990 [7].