NH3-SCR technology

The previous section has evidenced that NH3-SCR technology has been used successfully for more than two decades, to reduce NOx emissions from power stations fired by coal, oil and gas, from marine vessels and stationary diesel engines. NH3-SCR technology for high-duty diesel (HDD) vehicles has also been developed to the commercialization stage and is already available as an option in the series production of several European truck-manufacturing companies starting from 2001. For mobile source applications, the preferred reductant source is aqueous urea, which rapidly hydrolyses to produce ammonia in the exhaust stream. 

Beside the NHs/urea SCR process, the NOx reduction from lean bum exhaust gas can be achieved using the cycled NOx-Storage Reduction or NSR system (also called Lean NOx-trap (LNT) system). In Europe, the NH3-SCR technology could be quickly implemented on heavier cars, as it is already the case for tmcks, while the NSR system is rather envisaged to be implemented in light passenger car. 

Finally, the literature essentially focuses on the association of NSR catalyst with usual NH3-SCR samples. However, in this coupled NSR + SCR system, ammonia is not directly injected in the feed gas, but produced during the regeneration step of the NSR process. The additional NOx reduction occurs during the lean phases (in O2 excess) between NOx from gas phase and stored ammonia. It makes a prominent difference with the usual NH3-SCR technology. The development of specific SCR catalyst is desirable in a near future to achieved DeNOx efficiency and nitrogen yield even higher. 

For the NH3-SCR technology, as a well-established DeNOx method for purifying lean exhaust gas, different catalyst technologies exist, e.g., Fe- or Cu-exchanged zeolite systems, Vanadia SCR, and nonzeolite catalysts. Each technology shows advantages and disadvantages so that their selection depends on the specifications of application conditions. For the application considered here, Fe- and/or Cu-SCR are the choice. 

We may thus conclude after this short overview on DeNO technologies that NH3-SCR using catalysts based on V-W-oxides supported on titania is a well-established technique for stationary sources of power plants and incinerators, while for other relevant sources of NO, such as nitric acid tail gases, where emissions are characterized from a lower temperature and the presence of large amounts of NOz, alternative catalysts based on transition metal containing microporous materials are possible. Also, for the combined DeNO -deSO, alternative catalysts would be necessary, because they should operate in the presence of large amounts of SO,.. Similarly, there is a need to develop new/improved catalysts for the elimination of NO in FCC emissions, again due to the different characteristics of the feed with respect to emissions from power plants. 

The development of new combustors will reduce the formation of NOx and further development of selective catalytic oxidation (SCO) of NH3 and HCN will most likely fiirther reduce the emissions. The conventional SCR technology, the cost of which has lately been reduced, can of course also be used. 

Can be used in dirty/fouling services (particulates and/or high sulfur). However, any SO3 can react with the NH3 and form ammonium bisulfate precipitates (see the section on SCR technology). 

The actual issues of EuroV standards aim at optimizing engine s design to decrease the engine-out N(), emissions in order to avoid the need for expensive after-treatments in the exhaust line. Only some heavily loaded applications would need such NOx after-treatment. Today, two major technological ways of NOx treatment are identified the NOxTrap and the selective catalytic reduction with ammonia (SCR-NH3). 

The SCR-NH3 is a continuous process for NO treatment and shows very efficient treatment efficiency. But this technology needs to put in the vehicle an additional tank for the urea storage. Moreover, this technology is constrained from an architectural point of view because two DOCs are necessary before and after the DeNO catalyst to hydrolyze urea and form ammonia and to prevent the NH3 release in the exhaust line. 

The second DeNOx technology, the selective catalytic reduction with ammonia (SCR-NH3) commercially available in heavy-duty vehicles since 2006, seems to present an interesting potential in terms of efficiency, reliability, HC penalties, etc. 

By 2010, in EuroVI context with more stringent NO limits and a possible outbreak of normative restrictions such as the off-cycle risk, it will be necessary to make a technico-economic comparison between the SCR-NH3 and the NO Trap technologies. 

The SCR with NH3/urea is emerging as the most promising technology for the abatement of NOx emissions from diesel vehicles (ACEA, 2003 Heck et al., 2002). This has stimulated a renewed interest in the investigation of fundamental aspects of the SCR catalytic chemistry, also in view of the need of the transportation industry to develop design and simulation tools incorporating SCR kinetic schemes. 

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