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W. S. Hinton & Associates
Research and Consulting Engineers

Selective Catalytic Reduction (SCR)

Whether you are buying catalyst, assessing SCR, or are installing or operating SCRs, we can help.

We have significant experience within the U.S. in the design, operation, testing, and maintenance of SCR reactors. Our reactor design experience can aid you in the purchase of an SCR reactor and catalyst which optimizes NOx reduction with a minimum of downstream problems developed by ammonia slip. We have close working relationships with various catalyst manufacturers including, Cormetech, Haldor Topsoe, Hitachi America, and Argillon.

Our testing partners have have capabilities which include the full range common to SCR testing as well as other specialized tests. Analyses such as ammonia distribution, ammonia slip, NOx reduction, etc. can be very difficult if the intricacies of the testing are not recognized. Our experience prevents these mistakes allowing for accurate results the first time. We can design and perform acceptance and performance tests to ensure maximum NOx reduction at a minimum cost.

We also have experience in catalyst development, and have worked with various firms to develop proprietary catalytic products for use in the pollution control industry.

SCR Support Areas:

Feasibility Studies
Economic Evaluations
Preparation of RFPs
Bid Evaluation
Reactor Design
Ammonia System Design
Monitoring System Design
Catalyst Selection
Catalyst Development
Catalyst Management Plans

Catalyst Testing
Acceptance Testing
Optimization Testing
Troubleshooting
Operator/Engineer Training
Air Heater Design
Air Heater Testing
Mitigation of APH Effects
Specialized SCR Short Courses

SCR Process Description

SCR technology involves the catalytic reaction of ammonia (NH₃) which is injected into the flue gas containing NO_x to produce molecular nitrogen (N₂) and water vapor. These reactions take place in the SCR reactor.
Specifically, hot flue gas leaving the economizer section of the boiler is ducted to the SCR reactor. Prior to entering the reactor, NH₃ is injected into the flue gas at a sufficient distance upstream of the reactor to provide for complete mixing of the NH₃ and flue gas. The quantity of NH₃ can be adjusted as it reacts with the NO_x in the presence of the catalyst to remove NO_x from the flue gas. The flue gas leaving the catalytic reactor enters the air preheater where it transfers heat to the incoming combustion air. Provisions are made for ash removal from the bottom of the reactor since some fallout of fly ash is expected. Ductwork is also provided to bypass some flue gas around the economizer during periods when the boiler is operating at a reduced load. This is required to maintain the temperature of the flue gas entering the catalytic reactor at the optimum reactor temperature of about 700 ºF. The flue gas leaving the air preheater goes to the electrostatic precipitator (ESP) where the fly ash is removed. The ESP is part of the existing plant and is generally unaffected by the SCR system except as higher SO₃ content affects the electrical resistivity of the fly ash or if ammonia bisulfate (NH₄HSO₄) co-precipitates with the fly ash.

Current formulations of SCR catalyst are based upon patented discoveries by the Japanese and are typically comprised of vanadium pentoxide (V₂O₅) as the active material deposited on or incorporated with a substrate. The V₂O₅ composition typically ranges between one and five weight percent depending upon the flue gas SO₂ content. Tungsten trioxide (WO₃) is often added as a co-catalyst/promoter in the cases where additional catalyst activity is needed. But, the V₂O₅ concentration does not typically exceed 2% when using high sulfur fuel due to concerns about the oxidation of SO₃. The catalyst substrate is typically composed of pure titanium dioxide (TiO₂), although some manufacturers use modifications to this standard material. The catalyst is offered commercially in Europe and Japan in two basic geometric shapes, honeycomb and plate. When using standard SCR design and operating conditions, deNO_x efficiency is directly proportional to the ratio of NH₃ to NO_x up to NO_x removal (deNO_x) levels of approximately 80%. Above this value, some unreacted NH₃ can pass through the SCR reactor (referred to as a NH₃ slip) due to the low concentration of the reactants and to the inhibiting effects of the water vapor. Minimization of NH₃ slip is a major operational and design concern as discussed below.

Slip NH₃ is a concern in the application of SCR to coal-fired boilers to the formation of ammonium bisulfate (NH₄HSO₄), and its subsequent condensation on downstream equipment. The condensation of ammonium bisulfate results in a sticky corrosive material that can cause corrosion problems unless more costly corrosion resistant materials of construction are used.

Factors that contribute to ammonium bisulfate formation are temperature, SO₃ level, catalyst behavior, and the concentration of NH₃ and NO_x in the flue gas. The influence of temperature and catalyst composition are interdependent. The quantity of NH₃ available can be controlled by the plant operator. The amount of SO₃ present is due to two factors: the amount formed in the boiler itself and the amount that formed by the catalytic oxidation of SO₂ to SO₃ in the SCR unit. The combustion of low-sulfur coal typically results in very little SO₂ formation in the boiler. In addition, the SO₂ concentration in the flue gas is also very low which results in less SO₂ to SO₃ conversion. Thus, NH₃ slip is of less concern when burning low sulfur coals. However, U.S. high-sulfur coal may form much more SO₃ in the boiler. Moreover, the higher flue gas SO₂ content will likely cause more SO₂ to be converted to SO₃ in the SCR reactor, thereby aggravating the ammonium bisulfate formation problems.

Catalysts

In the simplest definition, a catalyst is a substance which speeds up or causes a chemical reaction, but remains unchanged. There are two general types of catalytic systems; homogeneous and heterogeneous. Homogeneous catalysis refers to systems in which the catalyst and the reactants are in the same phase, such as a liquid catalyst acting upon liquid reactants. Heterogeneous systems are more common to air pollution control. These systems operate with the catalyst and the reactants in different phases. A good example of this is the catalytic converter in automobiles. In this case the catalyst is a solid material, while the reactants are contained as gases in the automobile exhaust. Some basic characteristics of catalysts are as follows

In the catalytic reaction, the catalyst is unchanged at the final step of the reaction, although at different phases of the reaction, the catalyst may be present as a different chemical compound.

When more than one possible reaction can occur between reactants, the catalyst may help to promote a specific desired reaction. This phenomenon is called selectivity, and lends its name to "selective catalytic reduction" referring to the ability of SCR catalyst to catalyze primarily the nitrogen oxide reduction reaction.

The rate of reduction is generally proportional to the amount of catalyst present. For solid catalyst, parameters such as surface area, number of active sites, and porosity are all important to reaction rate.

As a result of the many technical issues related to SCR technology, catalyst design is extremely complicated and vendor offerings for any particular installation will represent a compromise between catalyst volume and cost, deNOx activity, SO₂ conversion, and pressure drop. Catalyst manufacturers typically address the many catalyst design issues by utilizing their technical expertise developed over many years of experience.

For SCR catalysts, an active catalytic species (principally vanadium) is dispersed on a substrate or carrier (titanium dioxide), which has a high surface area. This high surface area allows for an adequate number of active sites to accomplish the degree of reaction needed. When active sites are reduced in number, or are otherwise rendered incapable of performing their function, the term “deactivation” is used. The figure below shows an elementary depiction of an SCR catalyst surface, with internal pores and active sites. The internal pore structure is what actually gives the catalyst its high surface area. This surface area can be thousands or millions of times greater than the simple geometric surface area would indicate.

The specific design of any particular catalyst addresses many different factors. These include the total porosity, pore size distribution, internal surface area, geometrical surface area, vanadium content, promoter content (promoters are chemicals that may help increase activity or inhibit SO₂oxidation, for instance), dispersion of the vanadium and promoters, physical strength, substrate design, etc. Thus, many technical factors must be addressed when designing a catalyst and the number of firms capable of providing SCR catalyst on a commercial basis is relatively small. When a particular catalyst design is offered for an application, all of these design factors must be considered to insure that an offering is being made which will meet the required guarantees.

Microscopic Diagram of Catalyst, with Internal Pores and Active Sites

Catalysts for use in SCR technology are generally divided into three types, honeycomb, plate, and hybrid (sometimes called corrugated), referring to the particular geometry and manufacturing process used. “Pitch” is used as a standard method of denoting the opening size of the flue gas channels that are formed via the catalyst geometry. Large-pitch catalysts are typically applied to coal-fired boilers, while small-pitch catalysts are applied to installations where no particulate (dust) is expected, such as when natural gas is the only fuel. SCR reactors are typically constructed of several layers (two to four) of catalyst.

Plate catalysts typically use a metallic screen to help support the ceramic-like catalyst material, which includes the catalyst substrate and the active catalytic components. Honeycomb catalysts are typically extruded from a clay-like ceramic, forming square channels - the results in a monolith of ceramic material which does not require a screen support. Hybrid catalysts have characteristics of both plate and honeycomb catalysts, and have a corrugated appearance. In all cases, the catalyst substrate consists primarily of titanium dioxide, which offers the high surface area required to increase the number of active sites available to the flue gas. Active components are principally comprised of vanadium, tungsten, molybdenum, and other proprietary constituents and promoters. Vanadium is considered the principle active catalytic component for NOx reduction, although in some cases, very little vanadium may actually be required.