Optimising the ‘energy performance’ of key power plant components, such as the gas turbine is vital in making the power sector more energy efficient, explains independent consultant Dr Thomas Peters.

Amid soaring energy demand and rising concern over limited natural resources, interest in energy efficiency is also growing across a whole host of industry sectors – from manufacturing to construction – with public awareness combining with economic and environmental considerations to drive the trend.

The turbulent market for oil and gas back in 2008, when prices hit unexpected highs, has also shifted attention onto energy efficiency, shortening the timescale for achieving improvements. In the European Union (EU), this is apparent in a clear statement from the European Commission, released in March last year, relating to its Energy Efficiency Plan 2011.

“Energy efficiency is at the heart of the EU’s Europe 2020 Strategy for smart, sustainable and inclusive growth and of the transition to a resource efficient economy,” read the announcement. Energy efficiency is one of the most cost-effective ways to enhance security of energy supply and to reduce emissions of greenhouse gases and other pollutants.

“In many ways, energy efficiency can be seen as Europe’s biggest energy resource. This is why the Union has set itself a target for 2020 of saving 20 per cent of its primary energy consumption compared to projections, and why this objective was identified in the Commission’s Communication on Energy 2020 as a key step.”

Even if substantial steps have been taken towards this objective, recent Commission estimates indicate that the EU is on course to achieve only half of the 20 per cent objective, the statement continued.

Thus, “responding to the call of the European Council of 4 February 2011 to take ‘determined action to tap the considerable potential for higher energy savings of buildings, transport and products and processes’, the Commission has therefore developed a comprehensive new Energy Efficiency Plan.”

The Energy Efficiency Plan gives a very broad definition of ‘energy efficiency’ as using less energy while maintaining an equivalent level of economic activity. But what is the relevance to the electric power generation sector? For plant operators, an important aspect of the energy efficiency debate relates to safeguarding the ‘energy performance’ of their equipment, so that they work as efficiently as possible.

Gas based power fleet

We can now generate electricity from many fuels – ranging from burning fossil fuels and biomass through to harnessing the power of wind, water and the sun. But gas turbine-based combined-cycle power plants are expected to remain one of the world’s most important options to generate electricity over the coming decades.

A German market research company, trend:research, has predicted that most of the new power plants to be built in Europe until 2030, equivalent to about 98 GW, will be fuelled by natural gas. The need to develop new gas-based plant concepts that optimise both performance and operational flexibility will therefore continue.

To facilitate and support these efforts, VTU Energy developed a Gas Turbine Library based on performance test procedures according to ISO or ASME. All relevant data on design and operational conditions is specified and summarised in a professional engineering tool that facilitates analysis for improving the design of combined-cycle plants to enhance turbine performance.

This library offers a wide variety of input data and evaluation methods for further processing predictions of gas turbine performance. Ambient conditions, which can significantly affect gas turbine performance, also feature in the data resource. As combustion is affected by changes to the quality of the air as well as the natural gas, ambient conditions play a key role in a gas turbine’s long-term operation – and their influence can be unpredictable because it is governed by local environmental conditions.

Safeguarding turbine performance

In some circumstances, high efficiency but costly filtration systems can reduce the negative impact of impurities in the air. But although they can help cut the passage of contamination into the compressor, they can never fully prevent the ingestion of oily vapours and unburned hydrocarbons.

Once an oily film is laid down on the stationary and rotating blades, fine particulates of suspended solids – and potentially corrosive salt crystals – that pass through the filtration system can easily be trapped in the oily surface film and build up quite rapidly to negatively affect the adiabatic process, reducing mass flow and consequently the turbine’s power output.

Over the long term, this fouling inevitably raises flow resistance in a gas turbine’s compressor stages, through growing surface roughness or shape changes due to blade deposits. The discharge pressure can then fall – and the heat rate can rise – increasing fuel consumption. The overall effect can reduce the gas turbine’s performance by up to a few percentage points.

Although designs can be optimised to minimise the impact of fouling, a gas turbine’s high energy performance can only be guaranteed through a regular cleaning programme, which, if conducted according to the right specifications, can control and reduce the negative impact of fouling.

For this purpose, operators can routinely apply wet cleaning – either ‘online-washing’ with the turbine in full operation or ‘offline-washing’ when the turbine is at a low rotation velocity or during cranking. At specified intervals, demineralised water, alternated with cleaning agents, is injected under high pressure into the air intake of the gas turbine via specially-designed nozzles.

The cleaning agent usually consists of a chemical concentrate diluted with demineralised water. Ideally, this cleaning chemical has been specified on a case-by-case basis by specialists using a scientifically proven selection methodology, or produced to meet special requirements based on experience and on demand.

Alternative approaches for cleaning turbine blades include spot blasting with, for example, dry ice, aluminum oxide or pieces of walnut shells. But a strategic evaluation of downtime, impact on the different mechanical parts, health and safety issues, potential post-operation repairs, will usually conclude that wet cleaning is the best option. This procedure is also endorsed by many of the world’s leading gas turbine manufacturers.

Figure 1 shows the effect of wet cleaning on a GE Frame 7 gas turbine. The flow deviation trend illustrates that cyclic cleaning using offline washing enables the unit to recover the air flow necessary for normal operation. By way of comparison, Figure 2 shows the impact of online washing with water only, followed by a successful offline wash and then cyclic online washing with water that every third day incorporates a cleaning agent. In this case, the cleaning agent is Fyrewash, developed by Rochem Technical Services (R.T.S).

R.T.S
Figure 1: Flow deviation versus effect of offline washing
Credit: R.T.S.
R.T.S
Figure 2: Flow deviation versus effect of online cleaning
Credit: R.T.S.

To identify the most efficient cleaner for each situation, test procedures are conducted under scientific conditions. The purpose of the tests is to determine the cleaning performance of various compressor cleaning products. The result of one such cleaning test, which is based on a simple spray bottle test, is shown in Figure 3.

Figure 3
Figure 3
Figure 3: Cleaning performance tests on fouled small compressor blades using the spray bottle test, with black deposits visible in the before image (top) and the blades’ original colour reappearing after (bottom)
Credit: R.T.S.

Water saving options

As demineralised water is an important element in wet cleaning, R.T.S. developed a compact reverse osmosis (RO) unit, which produces the demineralised water from potable quality water. The unit has been designed on the basis of an easy-to-clean, open-channel membrane element, and is suitable for operation even under the most challenging conditions (see Figure 4).

Figure 4
Figure 4: Compact RO unit for production of demineralised water with an open-channel membrane element (stack with membrane cushions and hydraulic disks)
Credit: R.T.S

As water is now seen as a dwindling resource, R.T.S. has developed a new separation unit, using the same open-channel design in its membrane elements, for purifying wastewater from offline gas turbine washing, significantly reducing the volume of the residual liquid. It delivers significant cost savings or – depending on the situation – serves as basis for partial reuse.

Depending on the situation and the reason for treating this type of wastewater, membranes for microfiltration, ultrafiltration, nanofiltration or RO could be used in the system-specific membrane stack.

Keys to successful cleaning

The success of wet gas turbine cleaning – whether online or offline – depends on three key preconditions:

  • The availability of equipment that has been adapted to the gas turbine (see specially designed spray nozzles in Figure 5),
    Figure 5
    Figure 5: Nozzle system installed in turbine compressor unit
    Credit: R.T.S
  • The use of demineralised water and of chemicals selected to meet the local specific requirements, and;
  • A specific cleaning programme elaborated under consideration of the frame conditions of each case.

Wet cleaning equipment should include: a system with chemical storing capacity or a unit for delivering the necessary amount of cleaning solution each cleaning cycle, a high-pressure pumping unit, control instruments and related tubing; custom-made nozzles – the size of droplets and their uniform distribution in the inlet air stream is of extreme importance – designed and manufactured in accordance with HAZOP criteria and adapted to the construction of the specific gas turbine; performance monitoring to be adapted on a case-by-case basis for producing demineralised water to guarantee the high quality of the water used for injection or the dilution of the cleaning chemical concentrate; and a unit for the treatment of the wastewater produced during offline washing to manage the water balance and reduce wastewater discharge costs

Products from specialised companies such as R.T.S. with long proven track records enable successful implementation not only with new-build equipment but also with retrofitted and modernised installations and where related parts and devices have been repaired.

Why offline cleaning is critical

Many of the directives and regulations in the EU are aimed at addressing the need for improvements in the area of ‘energy consumption’, and it is only more recently that we have seen political consideration being given to issues related to energy efficiency, and especially those that apply to electric power generation.

As highlighted above, a key way to improve energy performance in power generation is through optimising the operation of gas turbines. High energy performance can be achieved through implementing optimisation strategies during design, after commissioning and during operation.

Wet cleaning – either online or offline – is one of the optimisation options for gas turbines, and has been shown to safeguard the turbines’ reliable long-term operational performance.

But the success of this type of preventive solution depends on picking the right tools, including the cleaning chemicals for each application, custom-designed nozzles, and peripheral devices and units, such as a self-contained high-pressure delivery system for the cleaning agents and units for producing the demineralised water and for treating the wastewater from offline washing.