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The environment
Published in Tom Denton, Alternative Fuel Vehicles, 2018
Euro 6 imposed a significant reduction in NOx emissions from diesel engines (67% reduction compared with Euro 5) and established similar standards for petrol and diesel. Exhaust gas recirculation (EGR) reduces the amount of nitrogen available to be oxidised to NOx during combustion but further exhaust after treatment may be required in addition to the diesel particulate filters (DPFs) required to meet Euro 5. To comply with Euro 6, diesel cars may also be fitted with: ► A NOx absorber (also known as a lean NOx trap), which stores NOx and reduces it to nitrogen by catalytic action.► Selective catalytic reduction (SCR) using an additive (diesel exhaust fluid (DEF) or AdBlue) containing urea, which is injected into the exhaust to convert NOx into nitrogen and water. ► The use of Cerium,8 a fluid injected into the fuel tank each time the vehicle is refuelled which assists the DPF regeneration by nanoparticle catalytic action lowering the regeneration temperature.
x Emissions
Published in Ozcan Konur, Petrodiesel Fuels, 2021
R. J. G. Nuguid, F. Buttignol, A. Marberger, O. Kröcher
As the term implies, SCR requires a reductant that will preferentially react with NOx to form environmentally benign products. During the early stages of SCR development, several chemical substances were extensively investigated as potential reducing agents. The most studied ones are hydrocarbons, H2, and NH3. Hydrocarbon-SCR is particularly convenient because the fuel itself and/or the unburned hydrocarbons in the exhaust can serve as the reducing agent directly (Mrad et al., 2015). However, the overall efficiency of the process was found to be low because most of the hydrocarbons are actually oxidized to CO2 instead of selectively reacting with NOx. Side reactions can also form HCN, an extremely poisonous gas, at concentrations above the threshold limit value (Radtke et al., 1995). H2-SCR was proposed as a much greener alternative, but it requires a large excess of H2 to achieve sufficient conversions and expensive Pt-based catalysts (Costa et al., 2007). Due to these serious limitations, hydrocarbon-SCR and H2-SCR did not achieve widespread use. Indeed, most modern SCR installations use ammonia (NH3) as a reducing agent for the conversion of NOx. While this technology is more expensive to implement, its efficiency and selectivity make up for the added cost. To solve the problem of toxicity and storage of NH3, urea was proposed as a safe NH3 precursor compound. Under SCR-relevant conditions, urea easily decomposes in situ yielding two equivalents of NH3. With the introduction of the SCR system in diesel vehicles, 32.5 wt% urea solution (trade name AdBlue®) became the most used NH3 precursor worldwide (Bowers, 1988; Gabrielsson, 2004).
The effect of fusel oil as a reductant over the multi-metallic catalyst for selective catalytic reduction of NOx in diesel exhaust at low-temperature conditions
Published in Petroleum Science and Technology, 2023
Şilen U. H. Sümer, Sinan Keiyinci, Ali Keskin, Himmet Özarslan, Zeycan Keskin
Reductant and catalyst structures are generally used together in the SCR system. (Liu et al. 2018). Adblue which consists of 33% of urea, and 67% pure water by mass is commercially used as a reductant. Besides, different types of catalysts are tested for SCR systems in the literature. Particularly, the vanadium-based catalysts are used commercially in NH3 systems. However, it has negative aspects such as the toxic effect of vanadium, being active in the narrow temperature range (300-400 °C), and catalytic activity with low efficiency at low temperatures (Busca et al. 1998; Brandenberger et al. 2008). Besides, NH3 exerts negative effects on the environment and humans such as being toxic, corrosive, unpleasant odor, eye irritation, and respiratory tract effect (Horne et al. 2018). In order to cope with these negative aspects, the researchers have intended to efforts not only to develop catalysts but also to test new reductants. Examining the literature about the reductants used in the SCR system, several studies were reported including hydrogen, hydrocarbon (HC), carbon monoxide, etc. (Souza, Araújo, and Oliveira 2020; Keskin et al., 2021; Oton et al. 2020).