Selective catalytic reduction filters

Flue-gas from boilers fired with liquid or solid fuels contains fly-ash and gaseous contaminants such as CO, NOx, SO2, or volatile organic compounds (VOCs). Emission regulations require their removal, which is achieved by a sequence of after-treatment processes. The after-treatment usually comprises a filter to remove solid particulates operated at approximately 150 °C, a wet scrubber for the removal of SO2 with an alkaline solution operated at approximately 50 °C, and finally a selective catalytic reduction (SCR) unit, which converts NOx to N2 with the help of NH3 at approximately 370 °C (Fig. 15.1) [4]. During this process, the flue gas is cooled down and then heated up again, which requires additional heat transfer equipment, with its inherent energy losses. 

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]).

Figure 9 Mass transfer and reaction in (a) honeycomb catalysts and in (b) catalytic filters for the selective catalytic reduction of NO, with NH3.

DPF= Diesel Particulate Filter EG R = Exhaust Gas Recirculation SCR = Selective Catalytic Reduction LNT = Lean NOx Trap... 

Diesel emission control has been a major feature in catalyst development in Europe. Growth in sales of diesel cars, which in 1996 reached 22% of car sales across the whole of Europe and around 50% in France, has been a factor in raising concern on the possible health effects of diesel emissions particularly ultra fine partieles. The key areas for research in diesel exhaust control are Lean NOx control, which is covered later, and oxidation catalysts, particulate filters and selective catalytic reduction. 

Offgas treatment is extensive and involves use of sintered metal filters, quench systems, venturi scrubbers, a condenser, a mist eliminator, an offgas heater, parallel HEPA filters, a carbon filter for radioactive iodine removal, a baghouse, and a selective catalytic reduction unit a packed tower scrubber system is used as a backup. 

Some methods used to clean off gases from incinerators employ catalytic bag filters with the injection of PAG or selective catalytic reduction (for NO removal) in a combination with catalyzed oxidation of the dioxins [ISO]. The use of activated carbon packed in adsorption units or sprayed into flue gas streams together with lime, has become a standard in gas cleaning liom dioxins on all sizes of plants. Typically, the treatment with activated carbon maintains the dioxin emission range below 0.1 ng m [181]. 

The washed and filtered flue gas passes through another Selective Catalytic Reduction (SCR) catalyst, which, with ammonia, reduces nitrogen oxides to nitrogen and steam and removes the remaining traces of organic substances. 

Bayer operates a number of. similarly engineered waste combustion units. At the Dormagen plant, a further combustor for solid and liquid wastes has been placed in service. It consists of a rotary furnace, afterburner, waste-heat boiler, and gas scrubber. The plant incorporates a condensation-type electrostatic filter specially developed by Bayer [259] and an SCR unit for selective catalytic reduction of nitrogen oxides (with ammonia) and for degradation of dioxins in the tail gas (see Fig. 115). 

A key research area is the study of the effect of selective catalytic reduction (SCR), selective noncatalytic reduction (SNCR), dry particulate adsorbent injection, and SOx removal systems on the removal efficiency, pressure drop, and lifetime of the baghouses. The use of fabric filters in PC boilers is increasing every year due to the stringent particulate emission standards. Pennsylvania Power and Light was the first utility to install a bag-house in 1973, and up until 2005, more than 129 RGFFs were installed on 28 GW of power plant capacities [64] (Figure 18.10). 

Hsieh M-F, Wang J (2011) Design and experimental validation of an extended Kalman filter-based NO,t concentration estimator in selective catalytic reduction system applications. Control Engineering Practice, 19(4) 346-353. 

Hsieh M-F, Wang J (2010) An extended Kalman filter for NO sensor ammonia crosssensitivity elimination in selective catalytic reduction applications. Proceedings of the 2010 American Control Conference. 

Two chemical processes have been selected by automotive makers lean NO e trap (LNT) [11] or NO c storage reduction (NSR) and selective catalytic reduction by ammonia (SCR-NH3) [12]. For LNT process, the cost is very expensive, because a large amount of noble metal is required. It is why the second possibility (SCR-NH3) has paid attention and has been developed first on trucks and bus. The result is positive in terms of efficiency and decrease of pollutants, but the required location remains a major problem especially when the technology must be applied to a particular vehicle. In order to improve the process, a new concept has been proposed and involves bringing together two technologies into one. In this chapter, we will describe the integration of the SCR-NH3 catalyst in the structure of the particles filters for application in diesel posttreatment. 

In order to decrease the number of bricks in the posttreatment exhaust line, some combinations can be found such as integrating catalytic treatment and filtration step. The aim of this single brick is to reduce the overall size of the posttreatment system and to reduce the cost of the final engine. One approach to achieve this goal is to coat the soot filter with a catalyst composition effective for the conversion of NO in innocuous components. With this selective catalytic reduction filtration (SCRF) concept, the catalyzed soot filter assumes two functions removal of the particulate and conversion of the NO species to N2 of the exhaust stream (Scheme 35.6). 

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