“Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts”
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The introduction and development of catalytic control for exhaust gas emissions from vehicles has been one of the major technical achievements over the last four decades. A huge number of cars were manufactured during this time that provided society with a high degree of personal mobility and without the continuous development of emissions control technologies the atmospheric pollution derived from them would have been overwhelming. Three-way catalysts (TWC) were introduced on traditional gasoline powered cars in the early 1980s to control the emissions of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) and have since been developed so that today tailpipe emissions of these pollutants can be reduced by more than 99.5% and tailpipe emission levels can be less than in the surrounding ambient air. During more recent years, and especially in Europe, the proportion of diesel powered cars has increased rapidly so now about half of new European cars have a diesel engine. Control of their tailpipe emissions has been particularly challenging because of their low exhaust gas temperature and the presence of excess oxygen. Under these conditions TWCs cannot be used and alternative technologies were developed for the control of HCs and CO by oxidation catalysts. An undesirable characteristic of older diesel engines was the black soot they produced. This was considerably reduced by fuelling and combustion engineering improvements and was effectively eliminated by the use of diesel particulate filters (DPFs) which were introduced a decade ago. The remaining difficult challenge has been the control of NOx emissions from both light and heavy duty diesel vehicles. Two technologies have been recently introduced to do this, though only one, ammonia selective catalytic reduction (SCR), appears to be able to provide the necessary performance for future demands under a wide range of driving conditions. The present book is about diesel engine NOx emissions control by ammonia (derived from urea) SCR, and before detailing the book’s contents some background information is given which provides a suitable context. Because of higher exhaust gas temperatures control of emissions from heavy duty diesel vehicles is less demanding than with light duty ones, so the emphasis here is on diesel cars.