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 NH₄HSO₄ is a sticky, corrosive material that can cause corrosion problems unless more costly, corrosion resistant materials of construction are used.

        Factors that contribute to NH₄HSO₄ formation are the temperature, catalyst composition 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 NH₄HSO₄ formation problems.

        The SCR Project (see next section) conducted by Southern Company Services, Inc. (SCS) in conjunction with the U.S. Department of Energy as part of the Innovative Clean Coal Technology Program (ICCT) was aimed at determining the applicability of SCR technology to U.S. coal-fired boilers. The project demonstrated the use of several SCR catalysts on a slip stream from a commercial boiler burning medium- to high-sulfur U.S. coal.

 

SCR Demonstration Project Summary

Introduction

        This project, titled "Demonstration of Selective Catalytic Reduction (SCR) technology for the control of Nitrogen Oxides (NOx) Emissions from high sulfur Coal-Fired Boilers," is part of the Department of Energy's Innovation Clean Coal Technology Program. The project had a total budget of approximately $23 million.

        The Innovative Clean Coal Technology Demonstration Program (Also referred to as the CCT Program) is a $7.14 billion cost-shared industry/government technology development effort. The program is to demonstrate a new generation of advanced coal-based technologies, with the most promising technologies being moved into the domestic and international marketplace.

        The CCT Program has a key role in advancing three goals of the DOE Strategic Plan under the Energy Resource business line:

• Reduces adverse environmental impacts associated with energy production, delivery, and use.
• Ensure reliable energy services with the reduced vulnerability to energy price and supply volatility.
• Enhance energy productivity to strengthen the U.S. economy and improve living standards.

        The technologies being demonstrated under the CCT Program reduce the emissions of the sulfur oxides, nitrogen oxides, greenhouse gasses, hazardous air pollutants, solid and liquid wastes, and other emissions resulting from coal use or conversion to other fuel forms. These emission reductions are achieved with efficiencies equal to or greater than currently available technologies.

        Clean coal technologies being demonstrated under the CCT Program are creating the technology base that allows the nation to meet it's energy and environmental goals efficiently and reliably. The fact that most of the demonstrations are being conducted at commercial scale, in actual user environments, and under conditions typical of commercial operations allows the potential of the technologies to be evaluated in their intended commercial applications. The technologies are categorized into four market sectors:

• Advanced electric power generation systems
• Environmental Control devices
• Coal processing equipment for clean fuels.
• Industrial technologies.

        The project objective was to demonstrate the applicability of SCR technology to provide a cost-effective means of reducing NOx emissions from power plants burning U.S. high-sulfur coal. The process was demonstrated on both high and low-dust loading of flue gas. The performance of eight commercially available SCR catalysts was evaluated under various operating conditions while achieving NOx reduction as high as 98%. Specifically, the project addressed several technical uncertainties which are associated with applying SCR technology to U.S. coals. These include.

1. Potential catalyst deactivation due to poisoning by trace metal species in U.S. coals that are not present in other fuels.
2. Performance of the technology and effects on the balance-of-plant equipment including air preheaters of high amounts of SO₂ and SO₃
3. Performance of a wide variety of SCR catalyst compositions, geometries, and methods of manufacturer under these new operating conditions.
4. Performance of and requirement for auxiliary equipment associated with SCR technology.

        The test facility was operated for a period of two years to provide adequate exposure time to determine important long-term catalyst operating parameters. Actual exposure time on most catalysts was 10-12 thousand hours.

        The results of laboratory tests have shown that for all catalysts, deactivation rates are similar to those noted in European and Japanese installations. No unusual deactivation trends were noted and it appears that at least for the coals tested at this facility, catalyst poisoning and deactivation is similar in significance to other world-wide installations.

        Unfortunately, the catalytic reactions that result in deNOx activity also contribute to SO₂ oxidation activity. Since increased SO₂ oxidation is detrimental to equipment downstream of the SCR, the competing reactions tend to bound the catalyst design. In general, as requirements to minimize SO₂ oxidation relax, deNOx activity per volume of catalyst can be increased. The upper bound for SO₂ oxidation for the test facility catalysts was set at 0.75%. While some catalysts designs essentially met this maximum oxidation requirement, others exceeded (improved upon) it greatly. In practice, all suppliers would likely be able to meet a customer's specific SO₂ oxidation requirements.

        As measured by the drop in number of transfer units (Ntu) from the initial values to the final values, the thermal performance of the test facility's air preheaters declined an average of 14%. In general, all three air preheaters showed generally steady increases in gas-side pressure drop during the test period. In general, the high DP's could be reduced by aggressive cleaning methods, including soot blowing at 4-hour intervals, thorough water washing, and occasional increases in the gas outlet temperature. It was not possible, however, to maintain the original, clean DP of any of the air preheaters. Corrosion tests on various heat transfer surfaces showed that enameled heat transfer surfaces should be used when sulfur and ammonia compounds are both present in the gas stream.

 

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