NOX catalysts

The placement of the NOx bed ahead of the oxidation bed causes a delay of the warm-up of the oxidation bed from a cold start. Since many of the materials considered for the reduction of NO are also excellent oxidation catalysts, the NOx bed is often used as the oxidation bed by the injection of secondary air during the first two minutes from a cold start. After the oxidation bed is warmed up, the secondary air is diverted from upstream of the first bed to upstream of the second bed. This procedure helps the emission reduction when the catalysts are fresh, but hastens the aging of the NOx catalyst as it is being exposed repeatedly to oxidation and reduction conditions. 

Most of the NO reducing catalysts in pellet or monolithic form begin to lose their activity at 2000 miles and fail to be effective at 4000 miles. This lack of durability may well be connected to the usage of the NO bed for oxidation purposes during the cold start, which exposes the NOx catalysts to repeated oxidation-reduction cycles. Better catalyst durability can be anticipated in the single bed redox catalyst with a tightly controlled air-to-fuel ratio, since this oxidation-reduction cycle would not take place. Recent data indicates that the all metal catalysts of Questor and Gould may be able to last 25,000 miles. 

Yaccato K, Hagemeyer A, Lesik A, Volpe A, Weinberg WH. 2004. High throughput screening of low temperature SCR and SCD de-NOx catalysts in scanning mass spectrometer. Top Catal 30/31 127-132. 

Ciambelli, P Corbo, P Migliardini, F. Potentialities and limitations of lean de-NOx catalysts in reducing automotive exhaust emissions, Catal. Today, 2000, Volume 59, Issues 3-4. 279-286... 

In addition, DOCs are also capable of reducing NOx, under certain conditions. Early work on these lean NOx catalysts concentrated on Cu/ZSM-5 catalysts (Amiridis et al., 1996 Walker, 1995), but platinum (Amiridis et al., 1996 Burch and Millington, 1995) and silver (Breen and Burch, 2006) -based catalysts, with better hydrothermal resistance than the zeolite systems, are also available. Unfortunately, NOx reduction under lean conditions only occurs over a narrow temperature range and therefore modelling can aid in optimisation of the catalyst and emissions system. 

There has been a growing demand for a lean NO catalyst in order to decrease the relatively low NO emission of the lean bum engine sufficiendy to meet the future standards. Lean NO catalysts have been developed based on zeolites (see Molecular sieves). Cu-promoted ZSM-5 zeolite has shown ability to reduce NO in an exhaust having excess oxygen at an efficiency of 30 to 50% (153). Durability is not proven. Research has revealed that certain hydrocarbons are preferred for the reduction of NOx, and that CO and H2 apparendy do not reduce NO, over such lean NOx catalysts (154). 

M. J. Heim rich and M. L. DeViney, Lean NOx Catalyst Evaluation and Characterisation, SAE 930736, Society of Automotive Engineers, Warrendale, Pa., 1993. 

In contrast to lead, the possible poisoning by metallic elements, derived from the vehicle system, is not easily documented. Many formulations of automotive catalysts contain both base and noble metals, but the detailed effect of such combinations on the particular reactions is rarely known with precision. One study was concerned with the effect of Cu on noble metal oxidation catalysts, since these, placed downstream from Monel NOx catalysts, could accumulate up to 0.15% Cu (100). The introduction of this amount of Cu into a practical catalyst containing 0.35% Pt and Pd in an equiatomic ratio has, after calcination in air, depressed the CO oxidation activity, but enhanced the ethylene oxidation. Formation of a mixed Pt-Cu-oxide phase is thought to underlie this behavior. This particular instance shows an example, when an element introduced into a given catalyst serves as a poison for one reaction, and as a promoter for... 

In order to decrease the nitrogen oxide (NO ) content in the flue gas, two methods can be applied. The first method is the injection of water into the gas turbine combustor. The second method is to selectively reduce the nitrogen oxide content by injecting ammonia gas in the presence of de-NOx catalyst that is packed in a proper position of the heat recovery steam generator. The latter is more effective than the former to lower nitrogen oxide emissions into the air. 

Diesel engines, which are used in the larger vehicles, are important sources of particles and NOx, but emit relatively low amounts of CO and HCs. Diesel particulate emissions can, over time, be controlled. The control of NOx is problematic, and an appropriate technology is not available. Lean NOx catalysts are being pursued but conversion efficiencies remain low. 

A great effort is underway to develop reliable aftertreatment systems for lowering NOx emissions from diesel and LB engines. A variety of approaches have been proposed for NOx aftertreatment of advanced vehicles including lean NOx catalysts (LNC), NOx storage and reduction (NSR) catalysts, selective catalytic reduction with urea (urea-SCR), and plasma-assisted catalysis (PAC). Lean NOx catalysts are mainly designed to reduce NOx with unburned hydrocarbons already included in the exhaust stream in the presence of O2 but result in... 

L. Lietti, P. Forzatti. G. Ramis. G. Busca. F. Bregani. Potassium doping of vanadia/titania de-NOxing catalysts Surface characterization and reactivity study, Appl. Catal. B- -Environm. J 13 (1993). 

Up to now a lot of attention has been paid to improving the performance of SCR catalysts with respect to making them more resistant to the poisonous compounds present in the flue gases, mainly SOx and As, which are results of burning with coal, oil and gas (ref. 5 and 6). At the same time relatively little information has been presented on the poisoning effect of flue gases from municipal waste incinerators, on the SCR-type de-NOx catalysts. 

The aim of this paper is to study the effect of the emissions from burning wastes, I.e, from municipal waste incinerators, on the SCR-type catalyst, specially when the catalyst is placed at the tail-end of the process, i.e. after the de-SOx unit. By choosing the tail-end position, most of the emissions harmful to the de-NOx catalyst, are absorbed either by the dust separator or during the de-SOx stage. Nevertheless about SO ppm SOx still present in flue gases. It is well known that at temperatures of below 250 C the ammonia sulphates are formed where SOx is present in the flue gases. 

Flue gases from a waste incineration plant in Sweden have been used for the testing and ageing of the de-NOx catalyst. Their temperatures after the de-SOx unit are between 110 and 130 C and they are comprised of about 200 ppm NOx PP traces of different elements most likely In... 

Artificial ageing of the de-N0 catalyst under laboratory conditions Most of the elements listed in Table 1 were further tested for any poisoning effects on the de-NOx catalyst. Individual samples of a fresh catalyst were impregnated with each of these elements which were in the form of either N03 or CH3COO" salts - the quantity used was 3 wt.X and in some cases even 1 wt.% of the catalyst. Impregnation was done using the Incipient... 

Ageing of the de-NQv-catalyst in flue gases from a municipal waste incinerator After ageing the samples of the de-NOx catalyst for about 2,000 hours in flue gases the following elements were detected on the catalyst s surface... 

Characteristics of samples prepared by addition of separate poisonous elements to the fresh de-NOx catalyst. ... 

Catalytic activity and BET surface area of samples [1-9] of the de-NOx catalyst after being impregnated with varying quantities of a mixture composed of Fe, Al, Ca, Mg, Na, K, Ba, Cr, Hi and Zn metals. ... 

The use of a mixture of 10 elements when simulating the ageing effect for the long periods of tine (up to 50,000 hours) at ISO C, confirm that the de-NOx catalyst should remain active (NOx conversion >80 t) for a period of at least 2 years. 

Diesel/Lean NOx Catalyst Technologies , SAE Fuels Lubricants, San Antonio, October 1996, SP-1211. 

The current paper offers an alternative systems approach that broadens the temperature window for managing NOx in a full lean environment. The system has a trap component which adsorbs NOx over a temperature range where current lean NOx catalysts are not active. The trapped NOx is periodically desorbed and presented to a downstream lean NOx catalyst when conditions are optimal for its reduction. The predominant species present in the exhaust is NO. The principle is to oxidize NO to NOx above 150 C to enhance its adsorption. The trapped or stored NOx is desorbed by an exotherm generated within the washcoat by oxidation of a small amount of injected hydrocarbon, i.e. diesel fuel while maintaining the environment lean and not significantly modifying the bulk gas temperature. The injection temperature is controlled to allow for efficient downstream reduction of the NOx over a lean NOx catalyst i.e. 200-250 C for Pt or above 400°C for Cu/ZSM-5. 

The trapping component was formulated into a washcoat and supported on a ceramic monolith with 400 cells per square inch (cpsi). The trap material was chosen for NOx adsorption, regenerability, thermal stability and rate of adsorption/desorption. Platinum is incorporated within the trap to oxidize the NO and the injected hydrocarbon. The lean NOx catalyst was Pt (60 gft- ) deposited on y-Al203 on a 400 cpsi cordierite monolith. 

Experiments were also carried out on the trap + lean NOx catalyst system to demonstrate proof of concept. The trap was maintained at 300 C (maximum adsorption) and the downstream lean NOx catalyst maintained at 210 C where its activity is a maximum. The space velocity was 25,000 h and the inlet gas was 250 vppm NO in background gas [10% O2,10% H2O, 50 vppm SO2, with the balance N2] and 7000 vppm Cj (as propylene) as the injected hydrocarbon. The injected hydrocarbon was cycled in an on/off manner to allow the trap to experience NO adsorption and desorption. The feed to the downstream lean NOx catalyst was a steady addition of propylene at a 4 1 Ci/NO ratio based on the feed NO. Thus the actual Ci/NO ratio is less because of the contribution from the desorbed NOx. 

A system designed to manage NOx between 150 and 500 C in a lean bum environment has been explored as an alternative to current lean NOx catalyst limitations. 

The trap inlet injection temperature must be controlled to be compatible with the maximum activity of the downstream lean-NOx catalyst. 

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