Measurement of Low Concentration NH
in Diesel Exhaust using Tunable Diode Laser Adsorption Spectroscopy (TDLAS)
It is expected that aftertreatment for oxides of nitrogen (NOx) will be the main strategy to meeting 2010 and beyond regulations for heavy-duty diesel engines. The introduction of liquid or gaseous ammonia (NH3) in conjunction with selective catalytic reduction (SCR) is one of the primary strategies being considered to meet these regulations. The development and verification of SCR systems will require NH3 measurement systems with sufficiently low detection limits to minimize NH3 slip and ensure slip levels below the regulatory limits. The application of Tunable Diode Laser Adsorption Spectroscopy (TDLAS) to this application offers many distinct advantages for the measurement of NH3 from exhaust including specificity, sensitivity, and the response time necessary to investigate low-level concentrations of exhaust gases.
Testing was conducted on a VTA/Extengine anhydrous ammonia NOx reduction system at the CATTS heavy duty chassis dynamometer facility. The results indicated that the average raw exhaust integrated concentrations for the New York Bus Cycle (NYBC), the Urban Dynamometer Driving Schedule (UDDS) and CLE567 tests were below the detection limit of 300 ppbV when using the TDLAS. The aggregate NH3 slip for all tests is well below the standard of 25 ppmV set forth in California Code of Regulations (CCR), Title 13, Section 2706 (b) (3).
Injection recovery tests were used to characterize losses in the transfer tube and dilution tunnel. Recovery rates for the system without the vehicle attached ranged from 70% to 74%, with slightly lower recoveries found for the heated dilution air turned on. Injection recoveries with the vehicle attached to the dilution tunnel and the engine operating at an idle were reduced to 46-47%. Finally, cold start injection recoveries with the engine idling were less than 5%, which could be due to the effects of a cold surface area on the transfer tube and condensation from water from the combustion process.
Additional measurements of NH3 emissions from light-duty gasoline vehicles also showed good comparisons between the raw exhaust TDL measurements and an older FTIR system sampled from the dilution tunnel.