There are basically three types of material used for SCR catalysts, noble, base metal, and zeolite. The material formulations and manufacturing processes are usu- ally proprietary developments of the manufacturers. The reaction of NOxwith am-
monia takes place at the catalyst macropore surface, which may amount to 60 m2/ per gram of material of noble or base metal catalyst materials. In the case of the ceramic-zeolite-type catalysts, the exothermic reaction takes place inside the vast micropore structure of over 200 m2per gram of zeolite.
Noble catalyst metals are platinum, rhodium, and palladium. They can be used for both NOxreductions and for the oxidation of VOC/HC, CO, PM. Due to the
high cost, primarily oxides of base metal are being used for SCR catalysts. Noble metal oxidation catalysts may be used upstream and/or downstream of the SCR catalyst: for upstream, to enhance the SCR NOxreduction by partially oxidizing
NO to NO2, which in some cases, however, is counterproductive if SO2is converted to SO3as well; for downstream, to reduce possible ammonia slip spikes and CO/ HC not oxidized by the SCR catalyst. In HD diesel engine SCR applications using the Siemens diesel SCR catalyst, oxidation catalysts are generally not recommended because additional PM would be generated when burning sulfur fuel. Also, today’s efficient HD diesel engines emit only minimal amounts of CO and VOC/HC.
Base-metal-based SCR catalysts contain oxides of base metals such as titanium (TiO2), vanadium (V2O5), tungsten (WO3), and additive and ceramic binders. V2O5 is highly reactive and used in small amounts of up to⬃2% only. Catalysts with a high V2O5 content are used in the production of sulfuric acid (H2SO4) as well, which would also form in exhaust gas ducts if SO2 oxidizes to SO3 catalytically (SO3 ⫹ H2O ⬎ H2SO4). Base-metal-type SCR catalysts have nondiscrete mac- ropores and channels, adsorbing ammonia, which is desorbed in a subsequent oper- ation. This allows the adsorption of unreacted ammonia spikes (ammonia slip) rather than passing through the stack as secondary emission. Ammonia slip rates as low as 3–10 ppm have been achieved in continuous operations. The advanced diesel SCR catalyst development allows NOxemission reductions at temperatures
as low as 300 F (150°C). Although originally developed for NOxreduction only,
the advanced SCR catalysts is able to simultaneously reduce VOC/HC by up to 95%, PM by up to 50% and NOxby up to 95% at no extra cost. In Fig. 21, the
(a) The K values, the reactivity value of catalysts, may vary greatly, depending on type, material, and structure of catalyst
Keff.⫽
SV
AV⫻ ln(1 ⫺ n), (1)
where SV is the space velocity, the volume of the exhaust gas flow (N m3) per hour at normal conditions
divided by the volume of the catalyst (m3) resulting in (1/h), AV is the area volume, the catalyst surface
area per catalyst volume (m2/pm3), ln is the natural logarithm, and n is the emission reduction rate (i.e.
95%⫽ 0.95).
(b) Catalyst Volume (Vcat.)
Vcat.⫽ VEGF(N m3/h) SV (1/h) (m 3) (2) SV⫽ Keff.⫽ AV ln(1⫺ n) see Eq. (1)
where VEGFis the total exhaust gas flow at standard or normal condition (N m3/h) and SV is the space
velocity. [For each proprietary catalyst formulation and structure variation, the manufacturer has devel- oped proprietary space velocity table values for reactivity and NOxreduction rate (1/h).]
(c) Exhaust Gas Flow, (VEGF)
VEGF⫽ VEGF min.⫹ NGcons. (N m3/h) (3)
VEGF min.⫽ VAIR min.⫻ NGcons. (N m3/h) (4)
VAIR min.⫽ 2(Cx⫻ Hx) 21 (N m 3/h) (5) l⫽ 21 (21⫺ O2act.) (%) (6)
where VEGF min.is the volume of air consumption times O2content times NG consumption (N m3/h),
VAIR min.is the volume of air with 21% O2required to oxidize total HC (N m3/h), l is the percentage
O2in the air used during combustion, Cx Hy is the various hydrocarbons (HC) of the NG analysis
making up the total HC, O2 act.is the actual oxygen (O2) concentration of the exhaust gas, and NGcons.
is the natural gas consumption (N m3/h).
(d) Reducing Agent Consumption, Example Aqueous NH3Consumption
NH3 cons.⫽ VEGF(N m3)⫻ NOxreduction (ppm)
⫻ 3.3/1,000,000 (kg/h) using aqueous (7) ammonia with a 25% ammonia concentration,
NOx—Reduction⫽ (NOx in⫺ NOx out)(ppm), (8)
ppm NO⫽ ppm NO ⫻ 30 (molecular weight)/22.4 (mg/N m3)
ppm NO2⫽ ppm NO2⫻ 46 (molecular weight)/22.4 (mg/N m3), (9)
ppm NH3⫽ ppm NH3⫻ 17 (molecular weight)/22.4 (mg/N m3),
ppm SO2⫽ ppm SO2⫻ 64 (molecular weight)/22.4 (mg/N m3),
O2act vs. stand⫽ (ppmvd compound)
21⫺ O2 stand.
21⫺ O2 actual.
(ppm vd) (10)
FIG. 18 (a) Reactivity of SCR catalysts; (b) catalyst volume calculation; (c) Exhaust gas flow calcula- tions, natural gas combustion; (d) reducing agent consumption.
FIG. 19 Pellet-type catalyst. (Courtesy of SCAQMD.)
Zeolite, also called molecular sieve, is a ceramic material. Some zeolite struc- tures occur naturally; others like the ZSM5 of Mobile Oil is produce synthetically. Oil refineries heavily depend on them for their gasoline cracker, synthetic lubrica- tion oil, and gasoline from natural gas processes. The extruded honeycomb-type zeolite-based SCR catalyst has a very large micropore structure of over 2000 ft2 or 200 m2per gram of material. NO
xand NH3are attached to the micropore surface upon passing through the discrete pore openings of⬃6–10 A˚ in size. This enor- mous sponge effect compensates for major spikes of NH3and NOxduring rapid
load changes. The exothermic reaction of ammonia and NOxtakes place inside the
micropore structure through electrostatic forces. The reaction products, N2and H2O vapor, are disposed of, back into the exhaust gas. This reaction is relatively slow, requiring a higher volume of catalyst than, for example, base metal catalysts. How- ever, the zeolite catalyst has superior resistance to many compounds such as heavy metals, which are unable to enter the micropore structure through the discrete open- ings, and thereby extending the service life of the SCR catalyst considerably. One example of the over 1000 known different zeolite crystals is shown in Fig. 20.
Some other more novel combinations of zeolite with noble or base metal mate- rials are presently being researched for PM and other emission reductions.
SCR Catalyst Structure
There are three types of SCR catalyst structure. The pellet-type catalyst, the ex- truded monolithic, honeycomb-type catalyst using either oxides of base metals or zeolite, and the coated-substrate-type catalyst, incorporating either corrugated foil
FIG. 20 A zeolite crystal.
or plate-type stainless-steel sheet metal or extruded Corderite ceramic substrates. There is also a novel catalyst development incorporating fiber-based substrates. (See Figs. 20–22).
Pellet-type catalysts are filled in containers through which exhaust gas is passed. The pulsing exhaust gas flow, however, cause the pellets to vibrate, abrade/ erode, and dust. The dust settles, clogging the catalyst bed and prevents an even gas flow. Due to the erosion of the pellets, the catalyst bed shrinks and unreacted exhaust gas will bypass together with the injected ammonia over the top of the catalyst bed into the atmosphere. Thus, pellet catalysts do not work most of the time and were replaced as soon as the honeycomb-type catalysts became available. However, there are still several such reactors operating in southern California today.
FIG. 21 Left: Coated stainless-steel mesh/expanded metal-substrate-based plate-type catalyst for high-dust applications; right: extruded, honeycomb, monolithic, base metal diesel catalyst. (Courtesy of Siemens.)
FIG. 22 Macropore structure of extruded base metal catalyst.
The extruded, monolithic, honeycomb-type SCR catalyst has low back pressure and is widely used for gas turbines and boilers, engines, and other applications. The higher the number of extruded channels per square inch (cps), the higher the reactivity/active surface area and the smaller the catalyst for a specific application. Advanced base-metal-type SCR catalysts are available with 14–300 cps with chan- nel wall thicknesses of 0.3–1.08 mm containing the macropore structure. This advanced catalyst development allows NOxemission reductions at temperatures
as low as 300 F (150°C). The lack of a ‘‘sponge effect’’ may also be the reason for the lower emission reduction rates achieved by the coated-substrate-based cata- lyst at temperatures below 480 F (250°C); see Fig. 23.
The coated-type SCR and oxidation catalyst has usually three layers: the corro- sion-resistant substrate (such as the extruded Corderite monolith, corrugated stain- less-steel foil or mesh plates, the aluminized washcoat to which the third layer, the catalytically active material, is bond. The corrugated foil substrate is primarily used for noble metal catalysts, whereas the Corderite monolith is used for noble and base metal. The plate-type catalyst has been developed for flue gases, con- taining high-dust loads, such as the hard coal utility boiler, industrial and municipal solid-waste incineration, and other industrial applications. Long-term operating experiences in Europe showed that the erosion of the reactive catalyst material at the face of the SCR catalyst bed will terminate upon the exposure of the stainless- steel substrate, extending the service life. Due to the smaller macropore structure and surface area and thus absorption capability, the coated-type catalyst is less reactive.
Conclusion
It would be beyond the scope of this introduction to the SCR technology to go into further details of the process and the application engineering (i.e., review basically 10–20-year-old designs for coal and gas utility boilers). The future of
(a)
(b)
FIG. 23 (a) Extruded monolithic honeycomb catalyst macropore structure versus the reduced coated- metal substrate-based macropore structure. (b) Standardized test: relative reactivity of ex- truded (⫽1) versus coated-catalyst structures depending on thickness of coating (⫽0.6 and 0.2) at space velocity of 60,000/h. (Courtesy of Siemens.)
the SCR technology lies in distributed power generation applications such as gener- ation sets, cogeneration sets, and mobile on-road and non-road applications. There are already close to 1000 IC engine and turbine applications in service worldwide today (Intermacom AG). This number could multiply when HDD trucks and other mobile SCR applications come to market in 2001 through 2010. In the following section, a few examples of SCR projects are summarized. However, because some past design, application engineering, and operation deficiencies gave the SCR tech- nology a bad name in the United States, SCR systems engineers will have to pay more attention to design and application engineering details in future (Table 2).
TABLE 2 Why Certain SCR Systems Have Not Performed in the United States Advanced (SCR)
Failure Cause Technology Solutions
Catalyst
1. Clogging and bypass of Pellet type catalyst Honeycomb or plate-type
NOxand NH3slip catalyst
2. Reactivity loss Masking, poisoning, or de- Special lean burn/diesel cat-
lamination of catalyst alyst, homogeneous ma-
coating with metal or terial, allowing up to 3%
Corderite-based sub- sulfur fuel and an op-
strates erating temperature win-
dow of 300–1020 F
3. Emission spikes at rapid Little to no adsorption/ Diesel catalyst with micro-
load changes desorption capability of pore structure/‘‘sponge
catalyst (nonmonolithic effect’’ with adsorption/
catalysts) at⬍500 F desorption features
(250°C)
System Design
1. Clogging of injection Heavy corrosion/particle All stainless-steel storage,
system valves or noz- volume due to carbon delivery and injection
zles steel aqueous ammonia system
tank and piping material
2. Clogging of catalyst Carbon steel reactor hous- Heat-resistant steel such as
ing, scaling/ particles low Molly steel
due to temperature cycling
3. Low emission reduction Uneven gas flow at front Gas flow modeling, scale
face of catalyst bed or in- model tests, and low
sufficient mixing of ex- back-pressure static mix-
haust gas/NH3 ers such as Parmix TM/
TM Siemens
Controls and Other
1. High emission spikes at Relying only on a down- Feed-forward PEMS-based
load changes stream CEMS with long control with optional
feedback/response time feed back CEMS or
sensor-based control
2. Not cost effective Including a fully certified Electric–chemical sensor-
CEMS, which is often based accurate spot
more costly than the check analyzer with peri-
SCR emission reduction odic emission testing by
system for NOx, VOC, third party
and PM itself
3. Politics: Operator’s Operators avoiding fines, Independent test lab certifi-
good references but bad shutdowns, and lawsuits cation, confirming equip-
performance of air pollu- of poorly maintained sys- ment manufacturer’s
tion control equipment tem or new/unproven long-term performance
in actual operation technology, ‘‘pro- claims during a 3-year