NH3-SCR

The removal of NOx emissions from stationary electricity generating stations is a far more tractable problem than NOx removal from mobile combustion power sources. There are two reasons for this. 

Firstly, fuel combustion under these conditions is a relatively static process. Once operational, the A/F ratio (and therefore the combustion temperature, and the concentration of the emissions components) are constant. This differs from the dynamic situation seen when combustion is carried out in a vehicle s internal combustion engine and different concentrations of various pollutants continuously emerge. 

Secondly, this combustion generally takes place in a large-scale power plant with the capacity and space to safely construct and operate reductant reservoirs which contain NHj (or Nffj precursors) that can be added to the exhaust stream in order to reduce NOx over a suitable heterogeneous catalyst. Since the [NOx] emitted from these power sources is constant the [reductant] that must be added to reduce this is also constant, making process control relatively straightforward. 

The addition of such reservoirs to the aftertreatment system of a vehicle would be expensive both to install and to transport (adding weight to the vehicle). Also, since the [NOx] from a vehicle s engine is variable the [reductant] would also be variable and therefore such a system would require a much more sophisticated process control system than that needed for the stationary power source. 

The use of NH3 as a reductant (rather than the VOC and CO used in the TWC) is interesting [18-22]. This molecule will (with the correct catalyst) selectively react with NOx in an exhaust stream. In the case of the TWC the redox reactions are xmselective, i.e. NO and O would react with VOC and CO and in order for full reaction to take place their relative concentrations had to be equivalent (see above). In the case of a selective reduction the added reductant (NH3) will selectively react with (and reduce) the NOx component of the exhaust gas rather than the O component, NO + NHj + xs02 N + H O + xsO.  

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A wide range of catalytic materials have been investigated for the selective catalytic reduction of NOx. For stationary emissions, NH3-SCR using vanadium-tungsten oxides supported on titania is the most used method however, when there is a simultaneous emission of NO and NOz (in tail gas from nitric acid plants), copper-based zeolites or analogous systems have been proven to be preferable [31b], In fact, there are two main reactions for NH3-SCR ... 

Various other classes of catalysts have been investigated for NH3-SCR, in particular, metal-containing clays and layered materials [43 15] supported on active carbon [46] and micro- and meso-porous materials [31b,47,48], the latter also especially investigated for HC-SCR [25,3lb,48-53], However, while for NH3-SCR, either for stationary or mobile applications, the performances under practical conditions of alternative catalysts to V-W-oxides supported on titania do not justify their commercial use if not for special cases, the identification of a suitable catalyst, or combination of catalysts, for HC-SCR is still a matter of question. In general terms, supported noble metals are preferable for their low-temperature activity, centred typically 200°C. As commented before, low-temperature activity is a critical issue. However, supported noble metals have a quite limited temperature window of operation. 

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. 

Several aspects regarding the reaction mechanism and the new trend in research for NH3-SCR in stationary and mobile sources, as well as for NO,-SR, will be discussed in the following chapters of this book or in recent reviews [1-20,62-68], Even so there are some points which deserve comments, in particular, issues and questions which should be clarified regarding future prospects and new directions of research in this field. 

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. 

Figure 1.3. Possible options for NH3-SCR for mobile sources (A) conventional and (B) coupled to a CRT catalyst (developed by Johnson Matthey) which oxidizes NO to NO, besides to remove HC, CO and PM.

Another possibility for the plasma device is the generation of N02 for the enhancement of NH3-SCR at low temperature, (the so-called fast SCR reaction) which occurs if there is a 1 1 N0 N02 stoichiometry over V-Ti02 type catalysts and with variable stoichiometry over Fe-beta zeolite. Being able to switch on and tune N02 production over a limited temperature range will help to avoid N02 slip issues, that can be an issue for oxidation catalysts. Also, if tuned correctly, plasma can do the NO oxidation without in turn doing S02 oxidation and so generate N02 without making sulphates (and associated particulates). 

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