Contents Issue 3(1999)

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Albert Bursik and Jørgen Peter Jensen:

Comments on Carbon Dioxide Behavior in Power Plant Cycles

Carbon dioxide is one of the most common contaminants in fossil and PWR secondary cycles. Unfortunately, effects of carbon dioxide on plant cycle components are only incompletely understood. Nevertheless, the well-founded assumption is that the presence of carbon dioxide does not have any positive effects on the reliability of major plant cycle components and on the overall plant operation and maintenance costs. In this paper, the behavior of carbon dioxide is considered from some important points of view as removal of carbon dioxide in condensers, removal of carbon dioxide in deaerators, impact of carbon dioxide on the composition of liquid films formed in the low-pressure turbine, and influence of carbon dioxide on plant cycle monitoring. The authors show that the presence of carbon dioxide in plant cycle streams affects the composition of the first condensate and contributes to the formation of high-conductivity liquid films in the low-pressure turbine even if they are not any other contaminants present in the steam.

With respect to the cycle surveillance, the consequence of carbon dioxide presence in the cycle is unfavorable. Particularly during startup, cation conductivity does not provide any reliable information. The operator cannot differentiate between the carbon dioxide content and a serious cycle contamination and must rely only on his operational experience. This situation is not very satisfactorily considering that the surveillance of many units, particularly of new combined cycles, is based on cation and specific conductivity monitoring.

PowerPlant Chemistry 1999, 1(3)

Heinz Gutberlet

Operating Experience and Process Optimization of High-Dust SCR Systems at 2000 MW Staudinger Power Station of PreussenElektra AG

In April 1984, the Environment Ministers of the German states made a decision that all bituminous coal-fired power stations in Germany have to reduce the nitrogen oxides (NOx) emissions. The NOx removal process (DeNOx) of choice was the selective catalytic reduction (SCR). These regulations applied equally to both new and old stations, insofar as the old stations were to continue operating indefinitely. A concentration of 200 mg/m³ of (NOx) corresponding to 100 ppmv or approx. to 0.11 lbs/MBtu must be adhered to as an emission limit for nitrogen oxides for coal-fired power plants. From 1985 onwards, power plants producing approximately 30,000 MW have been equipped with SCR technology. Most of these facilities, in particular the retrofitted ones, went into operation about 10 to 15 years ago. The paper describes the specific problems and problem solutions taken in Staudinger power plant as bonding of arsenic as catalyst poison through the addition of limestone, optimization of the flue gas ducts, combustion optimization, and the occurrence of ammonia in the fly ash.

The example of the DeNOx reactors of the Staudinger power plant has shown that in order to arrive at an optimal design, it is necessary to look at the entire power plant, from the fuel to the boiler and the entire path of the flue gas up to the DeNOx reactors. Under these conditions, the SCR technology can be mastered, both technologically and economically, including its application as a high-dust SCR system downstream of a wet-bottom cyclone-fired boiler. The number of iterative steps to an optimal station design can be noticeably reduced, however, if one takes advantage of the expert advice right from the start.

PowerPlant Chemistry 1999, 1(3)

Matthias Meierer

Operation and Optimization of Flue Gas Desulfurization Systems in Coal-fired Power Plants

Flue gas desulfurization has become a standard technology in coal-fired power generation units. This paper deal with the experience with flue gas desulfurization systems (FGD) of Grosskraftwerk Mannheim AG (GKM). GKM has FGD systems in the coal-fired co-generation units with an electric capacity of 475 MW (Unit #7) and 480 MW (Unit #8). The systems are operated on limestone as absorbent; the final product is dry gypsum powder used in the building industry.

The applied double-loop flue gas desulfurization process is described and the major technical parameters stated; then, the operating experience gained from 1988 through today is discussed. The focus of attention are all operation-related phenomena as corrosion aspects and material issues and problems caused by gypsum deposits. The taken optimization measures are explained in detail. Despite of all discussed problems, the overall results and experience gathered over a decade of base-load and intermediate-load operation with the FGD systems of the coal-fired Units #7 and #8 of Grosskraftwerk Mannheim are very positive. The mandated SO2 concentration of 100 mg/m³ maximum in the scrubbed gas could always be safely maintained. The availability of the two systems is very high. So far, no significant restriction with respect to the plant units' availability caused by FGD failures was experienced. The major components scrubber (including internals), suspension cycles, flue gas ducts, heat exchangers for reheating the flue gas, and FGD fans function reliably. In addition, the reaction product FGD gypsum complies with the strict quality requirements of the building materials industry. This is warranted by maintaining the respective target values for the physical and chemical process parameters (pH, solids level, chloride, sulfite, carbonate concentrations etc.).

PowerPlant Chemistry 1999, 1(3)

Peter Odermatt

Catalysts for the Removal of NOx and Dioxins in Various Applications

This paper gives an overview on the catalytic removal of ni-trogen oxides, NOx, (DeNOx), and its development in Europe. Possible locations of the selective catalytic reduction (SCR) installations are presented. Theoretical aspects and a brief description of BASF's DeNOx experience are given. The author deals with the most important catalyst deactivation mechanisms as deactivation on catalysts activity centres, deactivation due to sintering, deactivation due to surface blockage, and deactivation due to ammonium sulphates deposition. The techniques adequate for a simultaneous removal of nitrogen oxides and dioxins and furans at municipal and industrial waste incinerators as well as the catalyst replacement strategies are briefly touched on.

PowerPlant Chemistry 1999, 1(3)

Wilfried Rühle

Water Chemistry in BWR and PWR Nuclear Power Plants

During the last three decades, water chemistry in nuclear power plants with light water reactors has been developed to a high standard, approaching in case of boiling water reactors (BWR) extremely low conductivity in the coolant close to that of pure water. Chemistry in pressurised-water reactors (PWR), where different chemical additives are used, has reached a high level of operational perfection, leaving the impression that no further development is necessary. Nevertheless, for both reactor types there are some requirements for improvements. Concerning BWR, the main item is the occasional appearance of intergranular stress corrosion cracking (IGSCC) in stainless steel components. In PWR, chemists have to deal among others with activity build-up and, depending on the plant design and materials concept, with IGSCC in nickel based alloys. Referring to PWR, the paper deals with primary coolant chemistry particularly with zinc addition for mitigation of crack propagation and for mitigation of activity build-up and with secondary coolant chemistry. In case of BWR, crack corrosion, irradiation-assisted stress corrosion cracking, and countermeasures against crack corrosion are focussed. The application of hydrogen water chemistry, noble metal coating, noble metal chemical addition, and zinc addition are discussed.

Radiochemistry for light-water reactors is a further aspect covered in this paper. Predominantly responsible for long lasting activity build-up is the nuclide 60Co with a half-life of about 5.3 years. The elements responsible for dose rates during outages are 60Co, 59Fe, 65Zn (in BWR), 122/124Sb and, in some plants, 110mAg. Because of the importance of these nuclides for plant operation, their chemical status and behaviour in the reactor is described in more detail by the example of a PWR.

PowerPlant Chemistry 1999, 1(3)