Hydrogen applications in selective catalytic reduction of NOx emissions from diesel engines
Introduction
NOx emissions (NO and NO2) make major contribution to air pollution. They cause many adverse effects on environment and people health [1], [2]. Acidification of rain, photochemical smog, greenhouse effect and ozone depletion are the main damages of NOx on environment while respiratory tract diseases are the most harmful effect of NOx on people health [3], [4], [5]. Although many sources such as agriculture, thermal plants, industry etc. have been considered as contributor to NOx emissions, transport sector especially diesel powered vehicles is the main contributor of NOx emissions [6], [7].
Road transport is responsible for around 40% of total NOx emissions and 85% of NOx emissions from transport sector are emitted by diesel-powered vehicles [8], [9]. To eliminate NOx emissions from diesel engines without causing to any decrease in engine performance have been a major barrier for vehicle manufacturers and researchers. To provide this aim, researches have been adopted to SCR technology for diesel vehicles since 2000s and SCR of NOx emissions from diesel vehicles by urea solution have become one of the most effective methods for vehicles [1], [10].
SCR system is currently best choice to eliminate NOx emissions from diesel engines [7]. NOx emissions in exhaust gas can be eliminated with a reductant in SCR system [4]. Although different reductants such as alcohols, hydrocarbons, hydrogen, etc. have been sampled, Ammonia (NH3) is the most commonly used reductant and has high conversion efficiency in SCR of NOx [6], [11]. To prevent the burning of NH3 in high exhaust temperatures before the reaction, NH3 is obtained from urea solution [12]. Urea-SCR is the current emission control technology to reduce NOx emissions from vehicles and other power generating systems. However, many problems such as low activity at low temperatures, ammonia slip, high running cost, NH3 storage, ash odour, deterioration of catalysts and complex of systems have been direct the researches to alternative SCR technologies [13], [14], [15], [16].
Low exhaust temperature is a major problem for urea-SCR applications [17]. At low exhaust temperatures below 200 °C, the reactions of urea solution (thermolysis and hydrolysis) cannot occur properly. Biurea, cyanide acid, ameline, melamine and amelide are constituted due to faulty of reactions [18]. Light off temperature of urea-SCR catalyst is generally around or above 200 °C and NOx conversion efficiency of urea-SCR is limited at low temperatures [12].
Another SCR technology, the use of HC as reductant in SCR of NOx, has been worked by many researches currently [19]. Ag/Al2O3 is the most promising catalyst type in HC-SCR system [20], [21] while V2O5-WO3/TiO2 catalyst is conventional for urea-SCR system [12]. Compared to urea-SCR, HC-SCR systems provide advantages such as existence of HC in exhaust gas and eliminated the urea tank [19]. However, H2O vapour leads to significantly decrease in activity of HC-SCR [17].
In urea-SCR and HC-SCR systems, to reduce NOx emissions at low temperature with high O2 presentation in exhaust gas is a major problem [22]. The low activities of catalysts at low temperatures have been focused on intensively by many researches for decades. To increase activity at low temperature can be possible to use different reductants or additives.
H2 has been used in many studies as an alternative fuel for internal combustion engines to eliminate pollutant emissions and improve engine performance [23], [24]. The combustion of hydrogen in combustion chamber has improved activity of SCR systems enhancing NO2/NOx ratios [25].
Hydrogen also has been considered as a reductant or promoter in SCR systems for mobile applications [26]. Use of hydrogen in SCR of NOx emissions has been considered as a green environmental technique because of only H2O formation following the H2 combustion. Also high NOx conversion efficiencies of H2-SCR at low temperatures have increased the attention of H2-SCR systems [27]. In this study, the H2-SCR was explained and the studies on the effects of H2-SCR on NOx emissions were discussed.
Section snippets
H2-SCR system
Generally, H2 is not used as a reductant in SCR of NOx. Urea and hydrocarbons are the mainly reductants for SCR reactions. However, hydrogen offers high NOx conversion efficiency for SCR applications at low temperatures. This advantage makes it to use a promoter [22], [28], [29] or reductant [13], [30], [31] in SCR applications. Compared to conventional urea-SCR, H2-SCR reduces NOx emissions without forming another pollutants such as biurea, cyanide acid, etc. [1], [32]. Fig. 1 presents a
Conclusions
This review focused on H2 applications in SCR technology. The H2-SCR technology was expressed and the researches on H2-SCR was scrutinised in this study. The researches carried out on H2-SCR showed that the addition of hydrogen resulted in conversion of the oxidized nitrogen to N2 on the catalyst surface. H2 has an improving effect on SCR activity especially at low temperature. Pt and Pd based catalysts are the most feasible and current for H2-SCR applications while Ag based catalysts have high
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