Investigation of Crystalline Powder Precipitates Observed in Engine Exhaust Emission Measurement Systems and NOx Exhaust Gas Analyzers

The application of Selective Catalytic Reduction (SCR) to control nitric oxides (NOx) in diesel engines (2010, Tier 2, Bin5) introduced significant amounts of Ammonia (NH₃) and Urea to the NOx exhaust gas analyzers and sampling systems. Under some test conditions, reactions in the sampling system precipitate a white powder, which can accumulate to block sample lines, rendering the exhaust emission sampling inoperable. NOx gas analyzers used for exhaust measurement are also susceptible to precipitation within the sample path and detector components. The contamination requires immediate maintenance for powder removal to restore baseline performance.

The results of experiments to eliminate the powder are presented. Analysis of the powder identifies it as ammonium nitrate (NH₄NO₃) and ammonium sulfate ((NH₄)₂SO₄), which is consistent with the white crystalline precipitate. Techniques to eliminate or minimize the condensation of the ammonia salts are presented, including removing gaseous ammonia on a phosphoric acid (scrubbing) filter, and maintaining the NOx analyzer and sample lines at a temperature greater than the melting point of the ammonia salts. A review of the temperature dependent crystallization of ammonium nitrate provides an estimate of the condensation rate based on gas concentrations and temperatures.