NOx catalytic treatment

Figure 1.7. Catalytic treatment of NOx in a) mobile b) stationary sources

Granger P, Parvulescu VI. Catalytic NOx abatement systems for mobile sources From three-way to lean burn after-treatment technologies. Chemical Reviews. 2011 111(5) 3155—3207. 

Fig. 15.1. Flue-gas treatment by a conventional process comprising a fabric filter for fly-ash removal, a wet scrubber for SO2 reduction and a selective catalytic reduction unit for NOx treatment (from Ref. [4]).

In future the use of EGR is expected to increase for certain applications, particularly those which do not use Selective Catalytic Reduction NOx after-treatment. When this happens, the levels of acids entering the lubricant via the blow-by gases may increase. Because of this, there is an expectation that drain intervals may need to be lowered if the base number of the lubricant drops too quickly. To counter this, reductions in fuel sulphur will help offset the increased levels of EGR and may allow lubricant drain intervals to be maintained. A good example is in North America where higher cooled EGR rates for US 2007 engines are used in combination with less than 15 ppm sulphur diesel. Maximum lubricant drain intervals have been maintained at around 25,000 miles (40,000 km). 

The second process is known as selective catalytic reduction (SCR). SCR may also be used in the treatment of nitric acid process tail gas and similar processes, but has achieved prominence through its application to NO removal from electricity-generating power stations, especially those that are coal fired. In SCR, a range of reductants for the NOx can be used the most common is ammonia. The primary reactions involved arc shown in Eqs. (13) and (14). Oxygen is required for this form of NO control, and levels of 2-3% are typically needed for optimum catalyst performance. 

A benefit of stoichiometric engine operation, besides the maximized power density, is in fact the possibility to reduce NOx-emissions to negligible levels by using exhaust gas after-treatment systems [64]. With conventional automotive catalytic converters conversion ratios of more than 99.9 % percent can be reached. 

The first case covers for example flue-gas treatment, which requires the filtration of fly-ash and the reduction of NOx, or gasification processes, where particulates and high-boiling tars have to be removed. An example of the second case is that of combustion processes, where incomplete combustion leads to the emission of carbonaceous particulates. The most relevant topic in this category is the reduction of diesel particulate emissions ( diesel soot ) by catalytic filtration. A more exotic example is the reaction cyclone for the thermal conversion of biomass, which also combines chemical reactions and separation in one apparatus, though its separation mechanism is not filtration. 

In order to avoid the unfavorable process conditions, different flue-gas treatment processes for combustion plants based on catalytic filters were developed, which combine fly-ash removal with SCR of ISKh with NH3 [4—8], The advantages of these processes are space and treatment-cost savings, reduced internal and external mass transfer resistances compared to honeycomb SCR catalysts, heat recovery from offgases with good efficiency, and low corrosion problems due to the removal of both dust and NOx at high temperatures. 

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). 

In respect of the severe regulations imposed, the motor car manufacturers have decreased considerably the NOx emissions from their vehicles. Initially this was by a modification of the engine s combustion chamber. More recently these measures have proved less than sufficient and so to prevent pollution an exhaust after-treatment has become necessary. With this in mind, several technologies have been developed in order to decrease the harmful NOx emissions. One of which is selective catalytic reduction by hydrocarbons (HC-SCR). An enormous amount of catalytic materi tls have now been developed [1] and an appropriate choice would seem to be supported noble metal catalysts [2]. A high activity is generally observed at low temperature while the efficiency remains little affected by water. The effects  

Nitrous oxide has received increasing attention the last decade, due to the growing awareness of its impact on the environment, as it has been identified as an ozone depletion agent and as a Greenhouse gas [1]. Identified major sources include adipic acid production, nitric acid and fertilizer plants, fossil fuel and biomass combustion and de-NOx treatment techniques, like three-way catalysis and selective catalytic reduction [2,3]. 

Metals other than A1 have been successfully introduced in numerous zeolite frameworks. Aluminum substitution by other metals, such as Fe, Ga, Zn, Co or Cu in the zeolite framework results in modified acidity, and subsequently modified catalytic activity, for certain reactions such as selective catalytic reduction of NOx by hydrocarbons. For example, a calorimetric and IR spectroscopic study of the adsorption of N2O and CO at 303 K on Cu(II)-exchanged ZSM-5 zeolites with different copper loadings has been performed by Rakic et al. [92]. The active sites for both N2O and CO are Cu (I) ions, which are present on the surface as a result of the pre-treatment in vacuum at 673 K. The amounts of chemisorbed species adsorbed by the investigated systems and the values of the differential heats of adsorption of both nitrous oxide (between 80 and 30 kJ mof ) and carbon monoxide (between 140 and 40 kJ mol ) demonstrate the dependence of the adsorption properties on the copper content. 

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