Ammonia slip

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. 

The vanadium content of some fuels presents an interesting problem. When the vanadium leaves the burner it may condense on the surface of the heat exchanger in the power plant. As vanadia is a good catalyst for oxidizing SO2 this reaction may occur prior to the SCR reactor. This is clearly seen in Fig. 10.13, which shows SO2 conversion by wall deposits in a power plant that has used vanadium-containing Orimulsion as a fuel. The presence of potassium actually increases this premature oxidation of SO2. The problem arises when ammonia is added, since SO3 and NH3 react to form ammonium sulfate, which condenses and gives rise to deposits that block the monoliths. Note that ammonium sulfate formation also becomes a problem when ammonia slips through the SCR reactor and reacts downstream with SO3. 

Figure 10.14. NO reduction and ammonia slip during SCR as a function of the NH3/NOX ratio for two different space velocities. The maximum admissible level of ammonia is 10 ppm, which dictates the space velocity, or...

Figure 10.16. NO conversion and ammonia slip as a function of the NH3/NO ratio in the presence of O2 and H2O over a V203/Ti02 catalyst at 623 K. The lines represent the model based on reactions (9)-(14) and the parameters in Tab. 10.7. [Adapted from).A. Dumesic, N.-Y. Topsoe, H. Topsoe, Y. Chen, and T. Slabiak, J. Catal. 163 (1996) 409.]...

Unreacted NH3 in the flue gas downstream the SCR reactor is referred to as NH3 slip. It is essential to hold the NH3 slip below 5ppm, preferably 2-3 ppm, to minimize the formation of (NH4)2S04 and NH4HS04, which can cause plugging and corrosion of downstream equipment. In order to avoid the ammonia slip, and to limit the direct oxidation of NH3 to N2, the NH3/NO ratio in the feed is typically maintained below the stoichiometric values, e.g. between 0.90 and 0.95. 

SCR systems at stationary diesel engines profit from the high exhaust gas temperatures of about 350-400 C, caused by the usually constant high load operation conditions of the diesel engine. In this temperature window nearly all known SCR catalysts are very active. Moreover, weight and size of the exhaust gas catalyst are usually not strictly limited, which results in a good NO, reduction efficiency (DeNOJ. However, DeNO, is not the only criterion for an SCR catalyst. Further requirements are excellent selectivities regarding NO and urea/ammonia as well as low ammonia slip, which is an undesired secondary emission of the SCR process. Therefore, all SCR catalysts exhibit surface acidity, which is necessary to store ammonia on the catalyst surface and, thus, to prevent ammonia slip. 

Figure 9.7. N20 formation at 10 ppm ammonia slip in dependency of the vanadia concentration. Extruded V205AV03—Ti02 catalysts with ( ) 3%, (A) 1.9% and ( ) 1.7% V205. (a) Thermal treatment at 550°C for 50 h. (b) Thermal treatment at 550°C for 50 h and at 600°C for 30 h.

By plotting ammonia slip versus DeNOx, the two most important properties of an SCR catalyst are combined in one graph. Starting with a constant amount of NOx at... 

Even in the case of an ideal dosing strategy, ammonia slip cannot be excluded if the SCR catalyst is cold, undersized, deactivated or if load jumps are large and very fast. As a consequence, the addition of an oxidation catalyst downstream of the SCR catalyst has... 

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. 

Figure 13.1 Influence ofthe maldistribution ofthe NHatNO ratio (a) over the cross-section at the inlet of the catalytic converter on the percentage of extra catalyst volume requires to keep the ammonia slip below 5 ppm for a = 0.8. Adapted from ref. [27]. 

Finally, the advances in the identification of the pathway of the reduction of stored NO, where ammonia is suggested as the intermediate product in the formation of nitrogen, may favor the improvement of the combined NSR + SCR technology that has been proposed by several car manufacturers to make NO removal by NSR more effective and at the same time to limit the ammonia slip. 

A basic process flow diagram for the SNCR TDN process is shown in Figure 17.3. Anhydrous or aqueous ammonia is vaporized and mixed with a carrier gas of air or steam for transport to injection/distribution modules. The injection distribution modules distribute ammonia and/or hydrogen reagent and carrier gas to proprietary spray nozzles or injection lances. Reagent flow control can be controlled and trimmed by outlet NOx signals or ammonia slip measurements. 

The SNCR process is capable of high levels of NO, reduction under ideal conditions. The reaction between the reagent and NO, occurs within a specific range of temperature. At about 1800°F (982°C) the reactions essentially go to completion in a short residence time. At 1600°F (871°C), a much longer residence time is required to achieve similar NO reduction. Because the reaction occurs slower at low temperatures, the potential for unreacted ammonia (ammonia slip) increases as temperature is reduced. At lower temperatures, use of an additional reagent (eSNCR) in the flue... 

Ammonia slip on large units like an FCCU translates to wasted chemical expense with no benefit to the refiner. Excess ammonia in a high temperature SCR will oxidize to create NO, which is counter to the intended purpose of the unit. Excess ammonia in a low temperature SCR will tend to form ammonium bisulfate (ABS) per Figure 17.9. 

Ammonia can be injected into the flue gas prior to the ESP to modify the catalyst resistivity and agglomerate the line particles and improve the units collecting efficiency. Ammonia is most effective at operating temperatures lower than 550°F. Injection rates are typically 10-50 ppm of which > 50% of the ammonia is adsorbed on the catalyst lines and the remainder of the injected portion shows up as ammonia slip in the flue gas. Ammonia is good for ESP performance when used in moderation. Excessive injection can cause a degradation in performance due to collecting plate build-up. 

Minkara, R. 2003. Technologies to Mitigate Ammonia Slip. Proceedings 13th International Symposium on Use and Management of Coal Combustion Products, American Coal Ash Association, Paper No. 73, January 2003, St. Petersburg, FL. 

The AIG is used to uniformly inject diluted ammonia into the reactor. Uniform mixing of the ammonia into the flue gas is necessary to maintain catalyst performance at its highest level and to minimize ammonia leakage (ammonia slip) past the catalyst. 

In the SCR process, ammonia, usually diluted with air or steam, is injected through a grid system into the flue/exhaust stream upstream of a catalyst bed (37). The effectiveness of the SCR process is also dependent on the NH3 to NO ratio. The ammonia injection rate and distribution must be controlled to yield an approximately 1 1 molar ratio. At a given temperature and space velocity, as the molar ratio increases to approximately 1 1, the NO reduction increases. At operations above 1 1, however, the amount of ammonia passing through the system increases (38). This ammonia slip can be caused by catalyst deterioration, by poor velocity distribution, or inhomogeneous ammonia distribution in the bed. 

Initially, NOx emissions were problematic, but now seem to be under control. First, the amount of ammonia needed was discovered to be less them originally thought.1 Although tests performed in early 1988 showed a 3-day NOx average that was below the permitted level of 500 lb/day, Modesto was forced to use previously purchased offsets.3 Initially, much breakthrough of unreacted ammonia (ammonia "slip") from the boilers into the wet scrubber occurred, causing emissions to exceed the ammonia limit on some runs.3 Reduced ammonia levels stopped the breakthrough, and NOx emission levels were still within required limits. 

No boiling occurs during the chemical reaction because a constant pressure is maintained in the reactor. This prevents ammonia slip and avoids any hazard related to bubbles in the AN solutions. 

The HD configuration is used mostly in coal-fired applications because the temperature of the flue gas between the economizer and the air preheater is optimal for the catalyst activity ( 3(K)-400X) and because dust removal is usually accomplished by means of cold electrostatic precipitators. Ammonia slip must be kept at low levels (<5 ppm or even below 2-3 ppm), and SO2 oxidation must be as low as 0.5-1.0% in order to minimize the formation and deposition of ammonium sulfates in the heat exchanger and in the fly ash. 

There is a greater operating experience in HD plants [3,21-27] than in LD and TE plants, which are dedicated to specific applications. Typically, design specifications for HD systems include 80-85% NO, reduction efficiency (with a = NH3/NO feed ratios of 0 8-0.85) and ammonia slip lower than 3-5 ppm. 

A second advantage of catalytic fillers over honeycomb converters for the DeNOx reaction lies in the comparatively small degree of SO2 oxidation they should allow. This last reaction has to be kept to a minimum, since the formed SO3 would react with the ammonia slip to form ammonium sulphate deposits in the pipeline and apparatuses downstream of the NO converter, causing their obstruction in a relatively short time. Oxidation of SO2 on V-Ti catalysts is a rather slow reaction compared with NO reduction, so the efficiency factor of honeycomb catalysts for this reaction is practically 1, despite the relatively thick catalyst walls. Figure 10 shows, for kinetics and operating parameters given in Ref. 38, the conversion attained for the NO reduction and the SO2 reaction as a function of the wall s thickness of a typical DeNOx honeycomb catalyst. As expected,... 

High ammonia consumption/ammonia slip. An excess of 20-50% is required to achieve a 40 /o NO c reduction. By comparison, SCRs require < 5%i excess ammonia. Because of the higher ammonia slip, operating costs could be higher than SCRs. 

Less ammonia slip and a greater NO reduction than SNCRs. 

Control of the ammonia injection rate. As a minimum, the ammonia feed rate should be controlled by the exit concentration, possibly adjusted for the firing rate. For larger installations, those with varying NO concentrations or for those with a tight ammonia slip specification, an inlet NO , analyzer or an ammonia analyzer could be used to reset the ammonia/feed ratio. 

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