Experimental Study Of Urea Solution Spray And Decomposition And Ammonia Storage In Selective Catalytic Reduction System For Diesel Engines

This thesis studied the characteristics of urea solution spray, mixing of the spraywith exhaust gas, urea decomposition, deposit formation and ammonia storage, whichwere the key issues in the application of urea Selective Catalytic Reduction （SCR） as aneffective technology to reduce the NOxemission of diesel engines.The urea solution spray cone angle and penetration of the air assisted urea injectionsystem and the airless urea injection system were investigated through high speedphotography. The spray droplet size distribution of the two systems was measured usingPhase Doppler Particle Analyzer （PDPA）. The impingement of the spray on the exhaustpipe inner wall under real exhaust gas condition was studied through visualization.Experimental results showed that the spray atomization of the air assisted urea injectionsystem was better than that of the airless urea injection system. The spray impingementof the two systems was easy to occur, and the impingement of the airless injectionsystem was more serious than that of the air assisted system under the same runningcondition. The experimental data can be used for the simulation of urea solution spray.In order to study the effect of the mixing uniformity of the injected urea solutionwith exhaust gas on the performance of the SCR system, two types of compact mixerswere designed. The experimental results indicated that both the NOxconversionefficiency and the distribution uniformity of the reductant in the SCR catalyst wereenhanced when the mixers were added before the catalytic convertor. The ureadecomposition process under different temperatures was analysed through thermalgravimetric analysis. The experimental results showed that the urea decompositionprocess was different when the temperature programming rate was changed. However,the temperature range of each stage in the decomposition process was almost notchanged. The urea was completely decomposed when the temperature reached550℃inthe temperature programming process. But the urea could completely decompose if thetemperature was kept constant at360℃in the experiment. The deposit formation in theSCR system was investigated on the test bench. The experimental results showed thatthe inner wall temperature of the exhaust pipe downstream the urea injector was muchlower than the exhaust gas temperature and the low temperature increased the risk of deposit formation. The exhaust gas temperature, gas velocity, urea solution injectionrate and the position of urea injector all had significant effect on the deposit formation.The characteristics of chemical reaction and ammonia storage were investigatedquantitatively in a full-size vanadium base SCR convertor on a diesel engine test bench.A model was built to estimate the ammonia storage amount based on the experimentdata and then the urea solution injection control strategy was proposed considering theammonia storage. Experimental results showed that the exhaust temperature hadsignificant effect on the SCR reaction rate while the space velocity affected the reactionrate slightly. Both the exhaust temperature and the space velocity obviously affected theadsorption and desorption of the ammonia on the catalyst surface. The effect of theexhaust temperature on the ammonia storage amount was greater than that of the gasvelocity. The NOxconversion efficiency strongly depends on the amount of ammoniastorage in the temperate range of200℃～240℃and with the increase of thetemperature above240℃the dependence get weak. The ammonia storage amount hadsignificant effect on ammonia slip in transient processes. There was an appropriateamount of ammonia storage at each running condition that could maximize the NOxconversion efficiency in rapid heating process while keeping ammonia slip below10×10^-6.