Experimental investigation of emissions from a light duty diesel engine utilizing urea spray SCR system

摘要

Stringent pollutant regulations on diesel-powered vehicles have resulted in the development of newtechnologies to reduce emission of nitrogen oxides (NOx). The urea Selective Catalyst Reduction (SCR) systemand Lean NOx Trap (LNT) have become the two promising solutions to this problem. Whilst the LNT results in afuel penalty due to periodic regeneration, the SCR system with aqueous urea solution or ammonia gasreductants could provide a better solution with higher NOx reduction efficiency.This thesis describes an experimental investigation which has been designed for comparing the effect NOxabatement of a SCR system with AdBlue urea spray and ammonia gas at 5% and 4% concentration. For thisstudy, a SCR exhaust system comprising of a diesel particulate filter (DPF), a diesel oxidation catalyst (DOC) andSCR catalysts was tested on a steady state, direct injection 1998 cc diesel engine. It featured an expansion can,nozzle and diffuser arrangement for a controlled flow profile for CFD model validation. Four different lengthsof SCR catalyst were tested for a space velocity study. Chemiluminescence (CLD) based ammonia analysershave been used to provide high resolution NO, NO2 and NH3 measurements across the SCR exhaust system. Bymeasuring at the exit of the SCR bricks, the NO and NO2 profiles within the bricks were found. Comparison ofthe measurements between spray and gas lead to insights of the behaviour of the droplets upstream andwithin the SCR bricks.From the analysis, it was deduced that around half to three quarters of the droplets from the urea sprayremain unconverted at the entry of the first SCR brick. Approximately 200 ppm of potential ammonia wasreleased from the urea spray in the first SCR brick to react with NOx. The analysis also shows between 10 to100 ppm of potential ammonia survived through the first brick in droplet form for cases from NOx-matchedspray input to excess spray. Measurements show NOx reduction was complete after the second SCR bricks.Experimental and CFD prediction showed breakthrough of all species for the short brick with gas injection dueto the high space velocity. The long brick gas cases predictions gave reasonable agreement with experimentalresults. NO2 conversion efficiency was found higher than NO which contradicts with the fast SCR reactionkinetics.Transient response was observed in both cases during the NOx reduction, ammonia absorption and desorptionprocess. From the transient analysis an estimate of the ammonia storage capacity of the bricks was derived.The amount of ammonia slippage was obtained through numerical integration of the ammonia slippage curveusing an excel spreadsheet. Comparing the time constant for the spray and gas cases, showed a slightly fastertime response from the gas for both NOx reduction and ammonia slippage.