| Urea-SCR,the selective catalytic reduction using aqueous urea solution as reducing agent,has been investigated for decades and becomes a well established technique for de NO_x of diesel engines.However,the application of urea-SCR technique has a number of issues at low temperatures,including freezing below-11℃,solid deposit formation in the exhaust pipes,and difficulties in dosing with exhaust temperature below 200℃.The temperature limitations of aqueous urea solution dosing readevs limited de NO_x efficiencies of urea-SCR systems at real-world urban driving conditions when de NO_x is needed at low temperatures.The application of urea-SCR aftertreatment system on heavy-duty diesel engine has some obstacles to be addressed for high NO_x efficiencies at real-world urban traffic conditions.A solid selective catalytic reduction technology,solid-SCR,using a solid ammonia precursor to store ammonia reducing agent,has been developed and introudced as another diesel aftertreatment solution in recent years.It is designed as an alternative de NO_x technique to urea-SCR system.Due to the reducing agent generation of solid-SCR system doesn’t rely on exhaust temperature,the system may solve the major drawbacks of urea-SCR system by injecting gaseous ammonia directly to the exhaust gas stream and promote the de NO_x efficiencies of SCR catalysts in terms of NO_x conversion at low temperature.The present researches are devoted to evaluating the de NO_x performances of solid-SCR technique and urea-SCR technique at low exhaust temperature conditions.The primary aims of this work are to carry out comparative researches on the de NO_x efficiencies of heavy duty diesel engines between solid-SCR and urea-SCR at low temperature conditions.The de NO_x performances of solid-SCR system and urea-SCR system were investigated based on heavy duty diesel engine test bench with the same ANR(ammonia to NO_x ratio)of reducing agent.The experimental results indicated that the solid SCR system didn’t generate urea crystallization.Solid SCR system can improve the de NO_x efficiency of diesel engine at low to medium exhaust temperature,the average efficiency was promoted by 6%than that of urea SCR when exhaust temperature below 300℃.Solid SCR and urea SCR system had a comparable NO_x conversion efficiency when the exhaust temperature was above 400℃.The copper-iron zeolite based SCR catalysts showed more de NO_x performance than the vanadia SCR catalysts at low temperature.The light-off temperature of copper-iron zeolite SCR catalyst applied with solid SCR system can be lower to 170℃.Based on NO_x signal closed-loop of solid-SCR dosing control strategy,the NO_x efficiency of copper-exchanged zeolite was much higher than that of vanadia SCR catalyst,due to the copper-exchanged zeolite SCR catalystSolid SCR system using closed-loop SCR control strategy,copper-exchanged zeolite SCR catalyst than the vanadium based catalyst obtained lower NO_x emissions.Compared with the NO_x closed loop injection strategy,the injection strategy based on transient correction can better control the NO_x emission and the average ammonia leakage value of the catalyst.The application of solid SCR technology in diesel engine can meet the requirements of China 5 emission regulations for the NO_x emission and ammonia slip limits.NO_x emission performances of urea and solid SCR system with same dosing control strategies were tested by using PEMS at real-world driving conditions.The work-based window method was used to calculate bu brake-specific NO_x emissions.The experimental results showed that the average vehicle speed and engine load were low.The average vehicle speed was 18.19km/h on urea SCR system test and 18.61km/h on solid SCR system test,and the engine load levels were 24.60%,25.26,respectively.Low vehicle speed and low engine resulted in low catalyst temperature.The average NO_x conversion efficiencies of solid SCR system test cycle increased by 4% than urea SCR system,and the tailpipe NO_x emissions reduced by 9.1% than urea SCR system. |
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