Hydrogen applications in selective catalytic reduction of NOx emissions from diesel engines

https://doi.org/10.1016/j.ijhydene.2017.02.011Get rights and content

Highlights

  • Use of hydrogen as a reductant or promoter in SCR of NOx.

  • Catalysis for H2-SCR applications.

  • Higher NOx conversions with H2-SCR at low temperatures.

  • NOx elimination with SCR applications in vehicles.

Abstract

Diesel engines have been considered as a major source in nitrogen oxide (NOx) formation worldwide. The widespread use of diesel engines in consequence of their low fuel consumption, high durability and efficiency increases NOx emissions day by day. NOx emissions from diesel engines cause unavoidable damage on environment and people health. Although so many technologies such as exhaust gas recirculation (EGR), lean burn combustion, electronic controlling fuel injection systems, etc. have been developed to control NOx emissions from diesel engines, they couldn't meet the desired reduction in NOx emissions. In any case, Selective Catalytic Reduction (SCR) as one of the most promising aftertreatment-emission control technologies is an effective solution in restriction of NOx emissions. The use of SCR systems especially in heavy-duty diesel powered vehicles has been increasing nowadays. In these systems, to use of hydrogen (H2) as a reductant or promoter have been improved the conversion efficiency especially at low exhaust temperatures. Many researchers have been focused on the use of H2 in SCR systems for controlling NOx emissions.

In this study, the applications of H2 in SCR of NOx have been discussed. The studies on use of H2 in SCR of NOx emissions were examined and the effects on NOx conversions were determined. Consequently, it is confirmed that H2 is a promising and alternative reductant in SCR of NOx and it has been kept as an attracting subject for many researchers.

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

References (60)

  • X. Li et al.

    Influence of support properties on H2 selective catalytic reduction activities and N2 selectivities of Pt catalysts

    Chin J Catal

    (2015)
  • P. Schaber et al.

    Thermal decomposition (pyrolysis) of urea in an open reaction vessel

    Thermochim Acta

    (2004)
  • M.M. Azis et al.

    Kinetic modeling of H2-assisted C3H6 selective catalytic reduction of NO over silver alumina catalyst

    Chem Eng J

    (2015)
  • J.H. Zhou et al.

    Combustion, performance and emissions of a diesel engine with H2, CH4 and H2–CH4 addition

    Int J Hydrogen Energy

    (2014)
  • N. Saravanan et al.

    An insight on hydrogen fuel injection techniques with SCR system for NOx reduction in a hydrogen–diesel dual fuel engine

    Int J Hydrogen Energy

    (2009)
  • J.J. Chong et al.

    Enhancing the NO2/NOx ratio in compression ignition engines by hydrogen and reformate combustion, for improved aftertreatment performance

    Int J Hydrogen Energy

    (2010)
  • A. Abu-Jrai et al.

    The influence of H2 and CO on diesel engine combustion characteristics, exhaust gas emissions, and after treatment selective catalytic NOx reduction

    Int J Hydrogen Energy

    (2007)
  • C. Xu et al.

    Highly efficient Pd-doped aluminate spinel catalysts with different divalent cations for the selective catalytic reduction of NO with H2 at low temperature

    Chem Eng J

    (2017)
  • K. Arve et al.

    Kinetic considerations of H2 assisted hydrocarbon selective catalytic reduction of NO over Ag/Al2O3 I. Kinetic behaviour

    Appl Catal A General

    (2006)
  • S. Furfori et al.

    NO SCR reduction by hydrogen generated in line on perovskite-type catalysts for automotive diesel exhaust gas treatment

    Chem Eng Sci

    (2010)
  • C. Xu et al.

    Highly efficient Pd-doped ferrite spinel catalysts for the selective catalytic reduction of NO with H2 at low temperature

    Chem Eng J

    (2016)
  • J. Li et al.

    NO selective reduction by hydrogen over bimetallic Pd–Ir/TiO2 catalyst

    Catal Commun

    (2012)
  • S.M. Park et al.

    H2-SCR of NO on Pt–MnOx catalysts: reaction path via NH3 formation

    Appl Catal A General

    (2011)
  • L. Wang et al.

    Selective catalytic reduction of nitric oxide with hydrogen on supported Pd: enhancement by hydrogen spillover

    Appl Catal A General

    (2016)
  • J.P. Breen et al.

    NOx storage during H2 assisted selective catalytic reduction of NOx reaction over a Ag/Al2O3 catalyst

    Catal Today

    (2009)
  • P.M. More

    Effect of active component addition and support modification on catalytic activity of Ag/Al2O3 for the selective catalytic reduction of NOx by hydrocarbon–a review

    J Environ Manag

    (2017)
  • D.H. Lee et al.

    Hydrogen in plasma-assisted hydrocarbon selective catalytic reduction

    Int J Hydrogen Energy

    (2012)
  • Q. Yu et al.

    Selective catalytic reduction of NO by hydrogen over Pt/ZSM-35

    Catal Today

    (2010)
  • F.J.P. Schott et al.

    Reduction of NOx by H2 on Pt/WO3/ZrO2 catalysts in oxygen-rich exhaust

    Appl Catal B Environ

    (2009)
  • A. Abu-Jrai et al.

    The effect of H2 and CO on the selective catalytic reduction of NOx under real diesel engine exhaust conditions over Pt/Al2O3

    Int J Hydrogen Energy

    (2007)
  • Cited by (0)

    View full text