Two-cycle power plants

Diagram of two-circuit water-steam reactor


The two-circuit system shown here is made up of the water-steam circuit and the cooling water circuit.

Water, the operating medium, is pumped into the steam generator (on the very left), where it is heated by fission or combustion. The steam then migrates through the pipe system to the turbine and drives it, producing electricity. In the condenser, the steam condenses by emitting heat energy to the cooling water. Following this, the water is fed back into the steam generator and the process begins again.


Adjusting the water chemistry for optimum reactor efficiency

Power plant cooling towers

The conditions within the steam generator are conducive to corrosion and the formation of deposits, both of which can greatly reduce the efficiency of the power plant. To counteract this, the water chemistry has to be optimized.

On the one hand, the water must be ultrapure, and on the other, the concentration of conditioning agents that are added, e.g., phosphates or oxygen scavengers, must be continuously monitored.

The parameters and limit values for the water circulating in the water-steam circuit are precisely defined, for example in the standards EN 12952 (water-tube boilers and auxiliary installations) and EN 12953 (shell boilers).

> Overview of the requirements for the feed water of steam boilers and hot-water boilers (EN 12953)

Water-chemistry monitoring with online analyzers from Metrohm Process Analytics

Water drop

If continuous monitoring of substance concentrations and parameter values is required, then Metrohm Process Analytics instruments are the best choice. These monitor the required parameters fully automatically and can be configured to trigger a specific action if one of the values is not within the expected range. As a result you can reduce your interventions to a minimum.

Metrohm Process Analytics analyzers are capable of monitoring a wide range of analytes and are available as easy-to-use single-parameter, single-method, or multiparameter instruments.

> Find here an overview of analytes that can be monitored colorimetrically with Metrohm Process Analytics instruments

Hydrazine: An effective oxygen scavenger

Alert Analyzer with nuclear power plant in background

Dissolved oxygen is one of the main corrosive agents in water circuits. Thermal degassing of the boiler feed water removes a large part of this oxygen, but residual oxygen is still present.

Reducing agents such as hydrazine or sulfite are usually used to remove the residual oxygen chemically. While sulfite suffers from the drawback that it introduces corrosive sulfate in the water-steam circuit, hydrazine decomposes only to volatile nitrogen and ammonia. As an added benefit, the latter increases the pH value and thus lowers the corrosion potential. The only drawback of hydrazine is its toxicity.

The hydrazine concentration has to be monitored closely in boiler feed water. Alert analyzers are ideal for this purpose. They can determine the hydrazine content colorimetrically in just ten minutes. The detection limit is in the lower µg/L range.

> Read more about the Plug and Analyze series from Metrohm Process Analytics

 

Silica: Not so harmless in water-steam circuits

Detail of turbine blade

The presence of excessive levels of silica in boiler feed or makeup water is critical. Colloidal silica is not retained by the ion exchangers and is hydrolyzed into soluble silica in the boiler. Owing to its volatility, it can enter the steam circuit at elevated pressures and then deposit on turbine blades, particularly in the presence of alkaline earth metals.

Silica can be determined colorimetrically to levels in the µg/L range, for instance with an Alert analyzer.

> Read more about the Plug and Analyze series from Metrohm Process Analytics

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Determining copper with voltammetry

797 VA Computrace, 2x 800 Dosino

Copper alloys are now virtually used in all condensers of the water-steam circuit. The drawback is the susceptibility of copper and its alloys to corrosion by ammonia. The resulting corrosion products initiate further corrosive attack. Copper compounds already precipitate from steam in the high-pressure regions of steam turbines and deposit on the blades.

Copper compounds are determined voltammetrically according to DIN 38406-16. Sample preparation is not necessary.

> Read more about the voltammetric determination of copper compounds

Determining iron with voltammetry

Voltammogram Iron

At high temperatures, steam reacts with the iron in the carbon steel of steam boilers. This leads to the formation of a thin layer of magnetite, an iron(II,III) oxide, which passivates the steel surface protecting it against further corrosion (Schikorr reaction).

Under unfavorable conditions, the inhibiting magnetite layer can flake off, which leads to elevated iron concentrations in the water-steam circuit. A regular iron determination enables monitoring of not only corrosion processes but also the formation and destruction of the protective magnetite layer.

Adsorptive stripping voltammetry (AdSV) provides fast and sensitive detection of iron in process waters of the water-steam circuit (boiler feed water, makeup water, condensate) in power plants. This is achieved by adding suitable complexing agents to convert the iron into adsorbable complexes that are reduced on the electrode surface after a defined preconcentration time. Detection limits in the lower μg/L range can be achieved using 2,3-dihydroxynaphthalene (DHN) as the complexing agent.

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Corrosion inhibitors

Chromatogram of corrosion inhibitors
Zinc ions, phosphates, and phosphonates are commonly used as corrosion inhibitors in steel piping. If copper and copper alloys are involved, then triazoles, e.g., tolytriazole, benzotriazole, and 2-mercaptobenzothiazole can be used to inhibit corrosion. The copper compounds of the triazoles are prone to oxidation and also react with microbiocides that are added. As a result, the triazoles have to be replenished, making regular determinations of the triazole concentration necessary.

This can be done by ion chromatography with spectrophotometric detection.

> Learn more about Metrohm ion chromatography

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Monitoring of corrosive anions: Chloride and sulfate

Chromatogram for chloride and sulfate
Chloride causes pitting corrosion on turbine blades and rotors. In combination with sulfate, it also leads to corrosion fatigue and stress corrosion cracking (SCC). To prevent these detrimental effects, power plants have to monitor these anions in the water-steam circuit to trace levels.

You can do this using ion chromatography with inline preconcentration and matrix elimination.

> Learn more about Metrohm ion chromatography

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Further applications and products

Combustion IC complete system

Combustion IC for fuels

Fossil fuels may contain large amounts of sulfur. When these fuels are burned, sulfur dioxide (SO2) is produced and released into the environment, making monitoring of the sulfur content of fossil fuels indispensable.

Halides promote corrosion in the cooling circuits of power plants and thus require monitoring.

Both sulfur and halogens can be determined in solid or liquid samples by way of Combustion Ion Chromatography (CIC). In this technique, the sample undergoes pyrohydrolysis, and the resulting gas is absorbed in a solution and then analyzed by ion chromatography.

Read more about Combustion IC
Power plant with flue gas

Flue gas cleaning

To remove the CO2 from flue gas, the gas is treated with an amine-containing scrubbing solution. The acidic CO2 is reversibly chemically bound by the amines and afterwards released again by heating, compressed, dried, and liquefied.

You can determine the CO2 binding capacity of the scrubbing solution with an ADI 2045TI Process Analyzer. This analyzer is capable of monitoring several sample streams and determining the CO2 binding capacity of several amine scrubbers in succession.

ADI 2045TI Process Analyzer Read the application
Detail of turbine

Turbine and lubricating oils

Turbine and lubricating oils are exposed to extreme conditions in power plants. Numerous international standards define the requirements and test procedures for in-service maintenance of the turbines.

The ASTM D 4378 standard specifies that the acid and base numbers are to be determined by potentiometric titration and that the water content is to be determined by Karl Fischer titration.

Metrohm titration Karl Fischer titration

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