Alternative NH3 delivery materials for enabling low-temperature SCR
Zihao Zhang, Washington State University and Pacific Northwest National Laboratory
The increasingly stringent emission standard for gaseous pollutants has called for the development of the next-generation exhaust abatement technology. Nitrogen oxides (NOx), derived from both mobile and stationary sources, are one of the major air pollutants that cause the severe environmental issues. It is generally known that the selective catalytic reduction (SCR) with ammonia as reductant is the efficient protocol to reduce the NOx emissions. Considering the nontoxicity, safety and easy-to-store requirements, the aqueous solution of urea (32.5 wt %), also known as Diesel Exhaust Fluid (DEF) or AdBlue, was usually utilized in practical application as an alternative for liquid ammonia on-board in a pressure vessel. Although the current urea-SCR technology has witnessed a huge progress, it however still has several limitations: (1) high freezing point (-12 ºC) limits its application in some cold regions; (2) solid deposit formation usually derived from isocyanic acid-derived polymeric products in the exhaust line; (3) low efficiency in releasing ammonia at exhaust temperatures below 200 ºC. Additionally, another drawback of DEF urea solution is its limited ammonia capacity, due to which there has to be an additional automotive fluid distribution system at every gas station.
Since NH3 is the actual reductant during SCR process, we here mainly focus on NH3 generation from alternative ammonia precursors and processes. We will present recent update on TGA-MS and fixed-bed experiments for decomposition of ammonium salts and urea in both dry and wet conditions. Introduction of H2O accelerates the decompostion of ammonium carbamate while it had limited impact on decomposiation of urea and ammonium carbonate. Furthermore, we have synthesized and evaluated a wide range of acidic catalysts, which can significantly reduce the formation of solid deposits and attain a higher NH3 evolution content compared to other catalysts. Studying these different materials and catalysts will help us in develop an advanced process that enables NH3 generation from solid ammonia precursors for NOx abatement with higher low-temperature efficiency and less solid deposits.