A Review of Solid Materials as Alternative Ammonia Sources for Lean NOx Reduction with SCR 2009-01-0907
The need for improved emissions control in lean exhaust to meet tightening, world-wide NOx emissions standards has led to the development of selective catalytic reduction of NOx with ammonia as a major technology for emissions control. Current systems are being designed to use a solution of urea (32.5 wt %) dissolved in water or Diesel Exhaust Fluid (DEF) as the ammonia source. While DEF or AdBlue® is widely used as a source of ammonia, it has a number of issues at low temperatures, including freezing below −12 °C, solid deposit formation in the exhaust, and difficulties in dosing at exhaust temperatures below 200 °C. Additionally creating a uniform ammonia concentration can be problematic, complicating exhaust packaging and usually requiring a discrete mixer. A number of other materials have been proposed as alternative sources of ammonia that would improve many of these issues; materials such as solid urea, ammonium carbamate, and metal ammine chloride salts allow for the direct injection of ammonia gas into the exhaust.
We have examined the relevant properties of a range of ammonia source materials for their ability to provide ammonia for use in SCR. Important properties, including volumetric efficiency (moles of ammonia/liter), mass efficiency (moles of ammonia/gram), and ammonia release temperatures were evaluated. A number of materials provide an equivalent number of moles of ammonia to AdBlue® in roughly one third the volume or one third the mass. After a careful review of these materials, we have found that ammonium salts (e.g., ammonium carbamate) and metal ammine chlorides (e.g., strontium ammine chloride) offer the most desirable range of properties for use as ammonia source materials. Further issues relating to the use of these materials will be discussed.