Improving power plant efficiency using analytical instrumentation

Generating electricity is a fundamentally inefficient process. The efficiency of existing European generating capacity averages at about 35%. On a global scale, this drops to 30%. In essence, 65-70% of the potential energy in all fossil fuels is wasted. With fuel accounting for around 75% of the operating cost of a coal-fired power station, the need to ensure optimum energy efficiency is critical.

The power generation process is at its most efficient when the plant is in constant operation. If a plant is well maintained and runs smoothly, it will achieve better combustion efficiency, consume less fuel and emit less C02. This article will examine how carrying out monitoring throughout the plant using analytical instrumentation can help ensure optimum efficiency throughout the water and steam loop.

The following are a few tips on maximising power plant efficiency and output using analytical instrumentation.

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 regime, it is not possible to totally eliminate the presence of potential contaminants present in boiler feedwater. For example, in a 500 megawatt boiler, 1,500 tons 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 condenser, steam from the turbines is condensed using cooling water from a local source. Although this water is pretreated to remove mud, silt and any organic matter, problems can still occur if it becomes mixed with the condensate from the turbine steam. Over time, condenser leaks are almost inevitable, enabling contaminated cooling water to enter the condensate compartment.

With the feedwater system, de-ionised water is preheated and chemically treated before it enters the boiler. Although chemical treatment can help to reduce contamination, it can also cause immense damage to the boiler. For example, certain solid chemicals, such as sodium hydroxide or sodium phosphate, can 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 water, which is itself added to the condenser to offset any losses. However, this 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 frequently, this operation can become expensive and inefficient.

Why measure boiler chemistry?
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 cases, boilers with poor water chemistry have been known to last just six weeks before failure. As further proof, the Electric Power Research Institute (EPRI) estimates that 50% 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 chemistry, but also other areas around a power plant, it is possible to obtain a better overview of current conditions. When incorporated into a planned preventative maintenance programme, this information can help to substantially reduce the risk of unplanned outages.

Controlling contamination
To keep the steam raising process running at peak efficiency, the following parameters should be monitored constantly:

Dissolved oxygen: Even small parts per billion concentrations of oxygen dissolved in the feedwater stream can cause pitting in the boiler, drastically its operating life. The concentration of dissolved oxygen therefore needs to be checked throughout the feedwater loop, from 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 liquid, which is highly soluble in water. It is a powerful reducing agent that reduces oxygen to form nitrogen and water. At high temperatures and pressure, it will also form ammonia, which increases the feedwater pH level, reducing the risk of acidic corrosion. As an oxygen scavenger, hydrazine 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 which acts to protect 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 costly, whilst too little will not be able to adequately control dissolved oxygen levels and will prevent the formation of magnetite. Typically, the most effective dosage of hydrazine is 3:1 parts hydrazine to the expected level of dissolved oxygen.

pH & Conductivity: pH is an extremely important parameter to measure, as 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 (that is, its ability of the feedwater to pass an electrical current), it is possible to ascertain the level of contamination present, which can then be used to dictate the level or duration of treatment required. For example, where boiler blowdown is used, conductivity will be one of the main parameters used to control the frequency of the blowdown process.

Silica: Despite having no direct corrosive effect on plant, silica can form extremely hard and dense scales in the boiler and turbines, hampering 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 1,500 tonnes of water per hour will result in 1 tonne of silica being deposited in the boiler in just one month.

As dissolved silica is only weakly ionised, it is difficult to detect by conductivity measurement. For this reason, dedicated silica analysers are necessary if accurate information is to be obtained.

Depending on the type of power plant, typical sampling points for silica monitoring include the water treatment plant, the 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 conductivity, it lacks adequate sensitivity. As sodium is present in the critical dissolved compounds, it can be detected with on-line sodium monitors, which are very sensitive.

Other parameters that operators may also wish to monitor for include phosphate, ammonia and chloride, using sensors that offer quick response times, are 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. Ideally, monitoring equipment should be situated in an environment that has less potential for damage, has easy access for maintenance and allows for enhanced measurement accuracy.

Sampling instruments should also be located together, where possible, in a clean and accessible environment. The conditions for sampling must also be ideal, preferably with samples brought down to 25oC for measurement.

One way to achieve this is to use pre-manufactured packaged monitoring stations. Incorporating a full array of sampling instruments, including coolers and pressure reducers, these stations can be built to an operator’s requirements and can simply be connected up to the power plant’s existing sampling lines, greatly reducing the time, cost and disruption typically associated with installing and commissioning sampling systems.

Summary
The ability to gauge maintenance frequency, coupled with enhanced life cycle costs, offers a golden opportunity to improve reliability of supply and minimise unscheduled disruptions.

For this reason, it is important to ensure that online monitoring systems are themselves well maintained and that, where possible, they utilise the latest developments in technology to ensure they deliver maximum benefits.