Generating electricity is a fundamentally inefficient process. The efficiency of existing European generating capacity averages about 35percent. On a global scalethis drops to 30percent.
For fossil fuelsstill by far the most popular fuel for power generationit is estimated that around 65–0percent of potential energy is wasted. With fuel typically accounting for around 75percent of the operating cost of a fossil fuel-fired power stationthe need to ensure optimum energy efficiency is critical.
The process of generating electricity is at its most efficient when the plant is in constant operation. If a plant is well maintained and runs smoothlyit will achieve better combustion efficiencyconsume less fuel and emit less carbon dioxide (C02). Using analytical instrumentation for monitoring throughout the plant can help operators ensure optimum efficiency throughout the water and steam loop.
Better boiler chemistry
A key culprit behind many boiler failures is the accumulation of scale and corrosion brought about by contaminated water entering the boiler.
Even in a well-controlled regimeit is not possible to totally eliminate the presence of potential contaminants in boiler feedwater. For examplein a 500MW boiler1500tonnes of water is boiled off per hour – that equates to one million tons per month. Consider that most of the resulting contaminants that are present in the water will remain in the boiler and the need for close monitoring and control becomes apparent.
Two areas most likely to introduce contamination into the system are the condenser and the feedwater system. In the condensersteam from the turbines is condensed using cooling water from a local source. Although this water is pretreated to remove mudsilt and any organic matterproblems can still occur if it becomes mixed with the condensate from the turbine steam. Over timecondenser leaks are almost inevitableenabling contaminated cooling water to enter the condensate compartment.
With the feedwater systemde-ionised water is preheated and chemically treated before it enters the boiler. Although chemical treatment can help to reduce contaminationit can also cause immense damage to the boiler. For examplecertain solid chemicalssuch as sodium hydroxide or sodium phosphatecan actually speed up boiler corrosion if applied in overly high concentrations.
Applications using a boiler drum may also use boiler blowdown to slowly ‘flush’ out feedwater contaminants such as scale and high chemical concentrations. The level in the drum is maintained by adding make-up waterwhich is itself added to the condenser to offset any losses. Howeverthis process can be very wasteful in terms of loss of heat and high purity water and so should be utilised only when it is essential. If it occurs too frequentlythis operation can become expensive and inefficient.
The elevated temperatures and pressures inherent in power generation applications greatly increase the speed of the chemical reactions taking place in the boiler. The result is an aggressive environment that can dramatically reduce the life of the boiler if not properly controlled. In some casesboilers with poor water chemistry have been known to last just six weeks before failure. As further proofthe Electric Power Research Institute (EPRI) estimates that 50percent of forced outages in power plants in the US are due to boiler failures directly attributable to corrosion damage.
By measuring and monitoring not just the boiler chemistrybut also other areas around a power plantit is possible to obtain a better overview of current conditions. When incorporated into a planned preventative maintenance programmethis information can help to substantially reduce the risk of unplanned outages.
Controlling contamination
To keep the steam raising process running at peak efficiencythe following parameters should be monitored constantly:
- Dissolved oxygen. Even parts per billion concentrations of oxygen dissolved in the feedwater stream can cause pitting in the boilerdrastically reducing its operating life. The concentration of dissolved oxygen therefore needs to be checked throughout the feedwater loopfrom the extraction pump through to the deaerator and the boiler inlet.
One way to control dissolved oxygen levels is by dosing boiler feedwater with hydrazine. Hydrazine is a colourless liquidwhich is highly soluble in water. It is a powerful reducing agent that reduces oxygen to form nitrogen and water. At high temperatures and pressureit will also form ammoniawhich increases the feedwater pH levelreducing the risk of acidic corrosion. As an oxygen scavengerhydrazine is widely used to remove trace levels of dissolved oxygen in the boiler feedwater.
Hydrazine is also ideal as it reacts with soft haematite layers on the boiler tubes to create a hard protective magnetite layer that protects the tubes from further corrosion.
Placing a hydrazine monitor at the feedwater inlet will help check that feedwater is being dosed with the correct amount of hydrazine. Too much hydrazine is both wasteful and costlywhilst too little will not be able to adequately control dissolved oxygen levels and will prevent the formation of magnetite. Typicallythe most effective dosage of hydrazine is 3:1 parts hydrazine to the expected level of dissolved oxygen.
- pH and conductivity. pH is an extremely important parameter to measureas it gives an indication of the degree of acidity or alkalinity of the feedwater.
Measurement of conductivity in the feedwater and steam loops provides an indication of water and steam purity. By measuring the electrolytic conductivity of the feedwater – its ability to pass an electrical current – it is possible to ascertain the level of contamination presentwhich can then be used to dictate the level or duration of treatment required. For examplewhere boiler blowdown is usedconductivity will be one of the main parameters used to control the frequency of the blowdown process.
- Silica. Despite having no direct corrosive effect on plantsilica can form extremely hard and dense scales in the boiler and turbineshampering heat transfer efficiency and increasing the risk of mechanical failure such as turbine blade malfunction. Silica entering a high-pressure boiler can concentrate very quickly.
Just 1ppm of silica in the feedwater for a 500W boiler evaporating 1500tonnes of water per hour will result in one tonne of silica being deposited in the boiler in just one month. As dissolved silica is only weakly ionisedit is difficult to detect by conductivity measurement. For this reasondedicated silica analysers are necessary if accurate information is to be obtained.
Depending on the type of power planttypical sampling points for silica monitoring include the water treatment plantthe boiler drum and the saturated steam.
- Sodium. Sodium is one of the most important parameters to measure on a boiler plant. Although conductivity measurement is typically used to indicate total dissolved solids or chemical conductivityit lacks adequate sensitivity. As sodium is present in the critical dissolved compoundsit can be detected with on-line sodium monitorswhich are very sensitive.
Other parameters that operators may also wish to monitor for include phosphateammonia and chlorideusing sensors that offer quick response timesare temperature tolerant and require minimal maintenance.
Tips for online monitoring
Tips for maximising the efficiency of online monitoring systems include using instruments that can respond quickly to changes in boiler chemistry and have self-diagnostic capabilities where possible.
The location of monitoring equipment is a vital component in ensuring the best return on investment in a power plant. Ideallymonitoring equipment should be situated in an environment that has less potential for damagehas easy access for maintenance and allows for enhanced measurement accuracy.
Sampling instruments should also be located togetherwhere possiblein a clean and accessible environment. The conditions for sampling must also be idealpreferably with samples brought down to 25°C for measurement.
One way to achieve this is to use pre-manufactured packaged monitoring stations. Incorporating a full array of sampling instrumentsincluding coolers and pressure reducersthese stations can be built to an operator’s requirements and can simply be connected up to the power plant’s existing sampling linesgreatly reducing the timecost and disruption typically associated with installing and commissioning sampling systems.
Summary
The ability to gauge maintenance frequencycoupled with enhanced life cycle costsoffers a golden opportunity to improve reliability of supply and minimise unscheduled disruptions.
For this reasonit is important to ensure that online monitoring systems are themselves well maintained and thatwhere possiblethey utilise the latest developments in technology to ensure they deliver maximum benefits.
Jim Plumley is Analytical Product Manager for ABB Instrumentation. www.abb.com
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