Views: 0 Author: Cécile Favre, John May & Dirk Bosteels Publish Time: 2025-08-18 Origin: International Paper
With the development of lean burn direct injection gasoline engines and the increased use of diesel engines in passenger cars, there is an increasing need for the control of NOx in lean combustion systems. Lean burn systems limit CO2 emissions and reduce fuel consumption and so are key technologies for the future.
SCR was originally developed and used to reduce nitrogen oxide emissions from coal, oil and gas fired power stations, marine vessels and stationary diesel engines. SCR technology permits the NOx reduction reaction to take place in an oxidizing atmosphere. It is called “selective” because the catalytic reduction of NOx with ammonia (NH3) as a reductant occurs preferentially to the oxidation of NH3 with oxygen.
Figure 7: Selective Catalytic Reduction.
SCR technology is now fitted to most new heavy-duty (i.e. truck and bus) diesel engines in Europe. Systems are also being introduced on light-duty diesel vehicles and on non-road mobile machinery such as construction equipment. The efficiency of SCR for NOx reduction also offers a benefit for fuel consumption. It allows diesel engine developers to take advantage of the trade-off between NOx, PM and fuel consumption and calibrate the engine in a lower area of fuel consumption than if they had to reduce NOx by engine measures alone. Particulate emissions are also lowered and SCR catalytic converters can be used alone or in combination with a particulate filter.
For mobile source applications ammonia is used as a selective reductant, in the presence of excess oxygen, to convert over 70% (up to 95%) of NO and NO2 to nitrogen over a special catalyst system. Different precursors of ammonia can be used; but for vehicles the most common option is a solution of urea in water (e.g. AdBlue®) carefully metered from a separate tank and sprayed into the exhaust system where it hydrolyses into ammonia ahead of the SCR catalyst. AdBlue® is a stable, non-flammable, colorless fluid containing 32.5% urea which is not classified as hazardous to health and does not require any special handling precautions. It is made to internationally-recognized standards. Urea is used as an artificial fertilizer and is found in products such as cosmetics. The consumption of AdBlue® is typically 3-4% of fuel consumption for a Euro IV engine, and 5-7% for a Euro V engine, depending on driving, load and road conditions. A truck can have an AdBlue® tank which will hold enough urea solution to last for up to 10 000 km. On-board systems alert the driver when it is time to fill up the urea tank. An AdBlue® infrastructure was put in place over Europe and a dedicated website www.findadblue.com is available to show facilities where AdBlue® is available.
Figure 8: Urea dosing system.
SCR technology is relatively dynamic and improvements are being made in low temperature performance, urea delivery systems, system design, and alternatives to liquid urea. Indeed, SCR is emerging in the light-duty sector and further NOx reductions are also desired in the heavy-duty sector in urban driving or other low load conditions.
Urea injection quality and mixing are complex and critically important. A study shows (40) that the urea droplet quality from various nozzle designs can impact the deNOx system efficiency by up to 10% while the urea distribution across the catalyst can result in efficiency variations from 60 to 95%.
Modeling studies to improve urea injection and mixing using a variety of devices are numerous (41), (42) and (43). About 10-20% deNOx efficiency improvements can come from good injection practice, with nominally 5-10% coming from using a variety of mixers. Ammonia storage models also help with cold start deNOx (44).
Airless injectors (45), without a urea return line, simplify the urea delivery system and allow accurate delivery of lower volumes. The injector cooling is performed by raising off the exhaust pipe and using convection air and fins cooling rather than by using an excess of urea. Also, upon shut-off, the urea line drains to eliminate freezing issues and the need for line heaters.
Figure 9: Urea injector, mixing device and SCR catalyst
Several types of catalysts are used, the choice of which is determined by the temperature of the exhaust environment. Originally in Europe, China and India, SCR catalysts were based on vanadia. However, if DPFs are used in combination with SCR systems, zeolites are preferred due to the better high temperature durability needed when exotherms associated with DPF regeneration can expose SCR catalysts to temperatures up to 800°C. Currently copper-zeolites have the best low temperature performance and iron-zeolites have the best high temperature performance.
Optimized operation of SCR catalysts depends on control of adsorbed urea and use of oxidation catalysts to deliver the appropriate NO2/NOx ratio. In fact, the ‘fast SCR reaction’ uses both NO and NO2 at an optimum ratio of 1:1 and this is critical for good performance below 200°C. However, excess NO may be needed to oxidize ammonium nitrate (NH4NO3) which can condense and block catalytic sites. The reduction mechanism for the SCR reactions over zeolite catalysts are described in (46). It shows that with NOx present only as NO, the oxidation to NO2 to promote the ‘fast SCR reaction’ is the limiting step.
Copper and iron can be used together for a balanced performance over a broad range of temperatures (47), (48). Vanadia is cheaper and more tolerant to sulfur, but deteriorates at temperatures greater than 600°C whereas zeolites are very little affected with long exposure at 800°C (49). Like vanadia, Fe-zeolites are quite tolerant to sulfur but Cu-zeolite performance deteriorates and can be restored with a desulfation cycle (50). New zeolite are being developed for low temperature conversion without copper (48) and new catalyst families based on acidic zirconia are also emerging (51).
Emerging systems now incorporate the catalyst onto the Diesel Particulate Filter (50), (52) and (53) with a slightly lower deNOx performance (5-10% lower NOx conversion) than using separate substrates. Results are mixed on the impact of soot blocking SCR performance; and backpressure is higher for the combined system due to high catalyst loading on the DPF.
On-Board Diagnostic (OBD) and closed loop SCR control are using either the reputable NOx sensors (54) or a new ammonia sensor (55) which has a ± 5 ppm ammonia detection accuracy up to about 30 ppm ammonia, and has negligible interference from NOx, HC and CO.
Finally, alternative SCR reductants are being developed as solid urea (56) and magnesium dichloride ammonia storage media (57). Both have three times more ammonia per liter than liquid urea.
Lean NOx traps adsorb and store NOx under lean conditions. A typical approach is to speed up the conversion of nitric oxide (NO) to nitrogen dioxide (NO2) using an oxidation or three-way catalyst mounted close to the engine so that NO2 can rapidly be stored as nitrate. The function of the NOx storage element can be fulfilled by materials that are able to form sufficiently stable nitrates within the temperature range determined by lean operating engine points. Thus especially alkaline, alkaline earth and to a certain extent also rare-earth compounds can be used.
When this storage media reaches its capacity, it must be regenerated. This is accomplished in a NOx regeneration step. Unfortunately, alkaline and alkaline earth compounds have a strong affinity for sulfation. As a consequence, alkaline and alkaline earth compounds are almost irreversibly poisoned by the sulfur contained in the fuel during the NOx storage operation mode, leading to a decrease in NOx adsorption efficiency.
The stored NOx is released by creating a rich atmosphere with injection of small amounts of fuel. The rich running portion is of short duration and can be accomplished in a number of ways, but usually includes some combination of intake air throttling, EGR, late ignition timing and post-combustion fuel injection.
The release NOx is quickly reduced to N2 by reaction with CO (the same reaction that occurs in three-way catalysts for spark-ignited engines) on a rhodium catalyst site or another precious metal that is incorporated into this unique single catalyst layer.
Figure 10: NOx adsorber system.
Under oxygen rich conditions, the thermal dissociation of the alkaline and alkaline earth sulfates would require temperatures above 1000°C. Such temperatures cannot be achieved under realistic driving conditions. However, it has been demonstrated in various publications (58), (59) and (60) that it is in principle possible to decompose the corresponding alkaline earth sulfate under reducing exhaust gas conditions at elevated temperatures. In this way, the NOx storage capacity can be restored.
For engines less than 2.0 to 2.5 liters, NOx adsorbers may be more cost effective than SCR (61). Also, mixed-mode engines with reduced low-load NOx allow NOx adsorbers to focus on NOx emitted at higher temperatures (> 350°C), more than half the precious metals might be removed (62) and (63), which may make them economically attractive for light-duty applications with mixed-mode engines of 5-6 liters. Also improved NOx adsorber formulations with greatly improved precious metal dispersion, result in less PGM usage for better performance (64).
Lean De-NOx catalysts, also known as hydrocarbon-SCR systems use advanced structural properties in the catalytic coating to create a rich 'microclimate' where hydrocarbons from the exhaust can reduce the nitrogen oxides to nitrogen, while the overall exhaust remains lean. The hydrocarbon may be that occurring in the exhaust gas (‘native’) or may be added to the exhaust gas through injection of a small amount of additional fuel. This has the advantage that no additional reductant source (i.e. urea) needs to be carried but these systems do not, at present, offer the same performance as ammonia-SCR systems.
A study (65) evaluated the influence of diesel fuel sulfur content on the performance of a passive deNOx catalyst. The program used two specially prepared fuels with different sulfur contents, but with other fuel parameters unchanged. The NOx conversion efficiency of the deNOx catalyst increased from 14 to 26% over the European test cycle when the sulfur content was reduced from 49 to 6 ppm.
Recent developments on HC-SCR using hydrocarbons from the fuel are reported in the literature (66), (67) and a patent (68) specifies very low precious metal loadings (0.7 g/l) but the system needs temperature greater than 300°C to perform well.
A concept is reported (69) to combine an HC-trap and LNT, wherein the zeolite HC-adsorber is applied first on to the honeycomb substrate and the LNT material is placed on top. The HC-adsorber helps reducing cold start HC emissions and adsorbs HC during the lean periods. Upon release during the hotter rich periods, hydrogen and CO are formed to help LNT regeneration.
Diesel Particulate Filters combined with Selective Catalytic Reduction show significant reduction in both PM mass and number and NOx (see § 6) Such systems are already in use for some vehicles but are expected to be widely used to meet light-duty Euro 6 and heavy-duty Euro VI emissions requirements. An example of results from such a system is shown in the following section.
