Selective catalytic reduction with ammonia

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 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. 

In earlier work, Thomas and coworkers (1-3) found that NOx in a simulated NWP offgas (containing 2% CO, 3% CO2, 14% O2, 20% H2O and the remainder of N2) could be removed from levels of 10,000 - 30,000 ppm to 300-1,000 ppm by selective catalytic reduction with ammonia over a commercial H-mordenite catalyst (Norton NC-300) at 300-500 C. Based on these results and pilot plant tests, a conceptual process for NOx and CO removal has been designed by Westinghouse Idaho Nuclear Company, Inc. (WINCO). This process consists of three fixed bed... 

The reaction pathway of the NOx selective catalytic reduction with ammonia (NH3-SCR) is described by the following reactions (reactions 19.21-19.24) ... 

Activated Coke or Char. Processes based on active forms of carbon resemble the copper oxide processes in that they remove NO, by selective catalytic reduction with ammonia using the sorbent as a catalyst. However, the mechanism for SO2 removal is entirely different. Sulfur dioxide is adsorbed on the active carbon, which also acts as a catalyst for the oxidation of adsorbed SO2 to SO3. In the presence of moisture, the sulfur trioxide forms sulfuric acid on the char. Regeneration of the sulfuric acid-laden char is accomplished in a separate vessel where the sorbent is heated to about 400°C. At this temperature, the sulfuric acid reacts with a portion of the carbon, forming a gas phase containing sulfur dioxide, carbon dioxide, moisture, and various impurities. This gas stream is further processed to produce sulfuric acid or elemental sulfur, and the remaining char is recycled, with makeup, to the contactor. Several variations of the basic process are discussed in Chapter 7 under the broad heading Adsorption Processes. ... 

In the case of the selective catalytic reduction using ammonia as reductant but in excess 02, Ce-exchanged sodium-type mordenite (CeNa-MOR) has been reported as an active catalyst in the 250-560°C temperature range with respect to non-redox La,Na-and H-mordenite catalysts (Ito et al. 1994). In this case, the reduction of nitric oxide is thought to proceed with crucial involvement of a Ce3+/Ce4+ redox couple, although the intermediate reaction pathway depends on the reaction temperature. 

The most efficient method for NO removal from stationary and mobile sources is catalytic reduction with ammonia, hydrocarbons, CO or H2. Modified zeolites are active catalysts in these processes. For Cu-ZSM-5 especially high activity and stability have been reported. In this work the properties of copper-containing ZSM-5 zeolites prepared by wet or solid state ion-exchange have been investigated. The Bronsted acidity of the Cu -exchanged samples was much lower than that of the parent zeolites, and they had high activity in selective reduction with ammonia, propene or propane. A comparison of Cu-ZSM-5 activity in the decomposition of NO and in the reaction of NO with propene or propane revealed that the hydrocarbons as well as the nitrogen oxides play important role in the performance of NO reduction catalysis. 

Xu L, McCabe RW, Hammerle RH (2002) NOx self-inhibition in selective catalytic reduction with urea (ammonia) over a Cu-zeolite catalyst in diesel exhaust. Applied Catalysis B Environmental 39 51-63... 

Scrubbers are also used to prevent the release of nitrogen oxides, NO , into the air. In this process called selective catalytic reduction, gaseous ammonia is sprayed into flue gases and the mixture passed over or through a variety of metal oxide catalysts. The product is nothing more than nitrogen gas. The catalytic reduction of NO2 with ammonia is shown in Equation (17.38). 

Selective catalytic reduction (SCR) is cmrently the most developed and widely applied FGT technology. In the SCR process, ammonia is used as a reducing agent to convert NO, to nitrogen in the presence of a catalyst in a converter upstream of the air heater. The catalyst is usually a mixture of titanium dioxide, vanadium pentoxide, and hmgsten trioxide. SCR can remove 60-90% of NO, from flue gases. Unfortunately, the process is very expensive (US 40- 80/kilowatt), and the associated ammonia injection results in an ammonia slip stream in the exhaust. In addition, there are safety and environmental concerns associated with anhydrous ammonia storage. 

Selective Catalytic Reduction (SCR) SCE is a process to reduce NO, to nitrogen and water with ammonia in the presence of a catalyst between 540-840 F (282-449 C). Ammonia is usually injected at a 1 1 molar ratio with the NOx contaminants. Ammonia is used due to its tendency to react only with the contaminants and not with the oxygen in the gas stream. Ammonia is injected by means of compressed gas or steam carriers. Efficiencies near 90% have been reported with SCR. See Exxon Thermal DeNO. ... 

Selective catalytic reduction is based on selective reactions of a continuous gaseous flow of ammonia or similar reducing agents with the exhaust stream in the presence of a catalyst. The reaction that occurs is as follows ... 

Postcombustion processes are designed to capture NO, after it has been produced. In a selective catalytic reduction (SCR) system, ammonia is mixed with flue gas in the presence of a catalyst to transform the NO, into molecular nitrogen and water. In a selective noncatalytic reduction (SNCR) system, a reducing agent, such as ammonia or urea, is injected into the furnace above the combustion zone where it reacts with the NO, to form nitrogen gas and water vapor. Existing postcombustion processes are costly and each has drawbacks. SCR relies on expensive catalysts and experiences problems with ammonia adsorption on the fly ash. SNCR systems have not been proven for boilers larger than 300 MW. 

Emission control from heavy duty diesel engines in vehicles and stationary sources involves the use of ammonium to selectively reduce N O, from the exhaust gas. This NO removal system is called selective catalytic reduction by ammonium (NH3-SGR) and it is additionally used for the catalytic oxidation of GO and HGs.The ammonia primarily reacts in the SGR catalytic converter with NO2 to form nitrogen and water. Excess ammonia is converted to nitrogen and water on reaction with residual oxygen. As ammonia is a toxic substance, the actual reducing agent used in motor vehicle applications is urea. Urea is manufactured commercially and is both ground water compatible and chemically stable under ambient conditions [46]. 

Rather than selective non-catalytic reduction, the reduction can be carried out over a catalyst (e.g. zeolite) at 150 to 450 C. This is known as selective catalytic reduction. Figure 25.31 shows a typical selective catalytic reduction arrangement10. Either anhydrous or aqueous ammonia can be used. This is mixed with air and injected into the flue gas stream upstream of the catalyst. Removal efficiency of up to 95% is possible. Again, slippage of excess ammonia needs to be controlled. 

Example 25.5 A gas turbine exhaust is currently operating with a flowrate of 41.6 kg s-1 and a temperature of 180°C after a heat recovery steam generator. The exhaust contains 200 ppmv NOx to be reduced to 60 rng rn 3 (expressed as N02) at 0°C and 1 atm. The NOx is to be treated in the exhaust using low temperature selective catalytic reduction. Ammonia slippage must be restricted to be less than 10 mgm 3, but a design basis of 5 mg-rn 3 will be taken. Aqueous ammonia is to be used at a cost of 300 -1 1 (dry NH3 basis). Estimate the cost of ammonia if the plant operates... 

Devadas, M., Krocher, O., Elsener, M. et al. (2006) Influence of N02 on the Selective Catalytic Reduction of NO with Ammonia over FE-ZSM5, Appl. Catal. B, 67, 187. 

Devadas, M. (2006) Selective catalytic reduction (SCR) of nitrogen oxides with ammonia over Fe-ZSM5, PhD. Thesis No. 16524, ETH Zurich. 

Presently the catalytic selective NOx reduction by ammonia is efficient and widespread through the world for stationary sources. The remarkable beneficial effect of 02 for the complete reduction of NO into nitrogen is usually observed between 200 and 400°C. However, such a technology is not applicable for mobile sources due to the toxicity of ammonia and vanadium, which composes the active phase in vanadia-titania-based catalysts. Main drawbacks related to storing and handling of ammonia as well as changes in the load composition with subsequent ammonia slip considerably affect the reliability of such a process. On the other hand, the use of urea for heavy-duty vehicles is of interest with the in situ formation of ammonia. 

SCR [Selective Catalytic Reduction] A general term for processes which destroy nitrogen oxides in gaseous effluents by reacting them with ammonia in the presence of a catalyst ... 

Selective Catalytic Reduction (SCR) process is very similar to SNCR with the exception that a catalyst is used to accelerate the reactions at lower temperatures allowing it to be applied to both full and partial-burn regenerators. An SCR system consists of a catalyst bed installed in the flue gas line of a combustion system. Ammonia is injected into the flue gas with air in the presence of a catalyst. The catalyst is typically oxide forms of vanadium and tungsten. The ammonia selectively reacts with NOx to form molecular nitrogen and water via an exothermic reaction that has achieved > 90% reduction in NO when applied to an FCCU. 

Reduction of Nitric Oxide with Ammonia. - Control of the emission of NO from stationary sources is possible by selective catalytic reduction, for which up to now NH3 is the only effective reductant in the presence of excess 02. Beside noble metal catalysts Bauerle etal.101 109 and Wu and Nobe108 studied Al2 03-supported vanadium oxide and found this to be highly effective in NO removal which is considerably enhanced by the presence of 02. Alkali metal compounds which are usually added as promoters for S02 oxidation completely inactivate the catalysts for NO reduction. Adsorption kinetic studies indicated first-order dependence on NH3 adsorption. Similar results were obtained for NO on reduced vanadium oxide, but its adsorption on... 

The operability and reliability of processes using ammonia must also be studied. With the potential for increased ammonia use in these systems (in the selective catalytic reduction of nitrogen oxides and as an absorbent), research documenting ammonia emissions and the effects of ammonia on process equipment should be conducted. Furthermore, additional investigations should be performed to determine whether ammonium salts are formed and to document their effects on both the environment and the flue gas treatment system. 

Selective Catalytic Reduction (SCR) is normally used in new nitric acid plants. In this process ammonia reacts with nitric oxide and nitrogen dioxide but only to a lesser extent with oxygen to selectively reduce the NOx compounds to N2. The reactions are shown below97,104 ... 

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