Optimizing operation

New developments in gas turbine technology and their effect on combined cycle power plants have brought advantages as well as challenges for power plantoperators. While higher pressure ratios and firing temperatures have increased efficiency, they have also had an impact on life cycle costs and controllable losses.

A new generation of combined cycle power plants has replaced the large steam turbine plants that were the main fossil power plants of the 1980s. Combined cycle power plants are not a new concept, having been in operation since the mid-1950s, but their high capacity and high efficiency have made them the plant of choice for the new millennium.

Advanced combined cycle power plants operate at thermal efficiencies of 53-58 per cent and in some cases as high as 60-64 per cent. They use innovations such as solar heat for preheat, water flashing for intercooling, reheating, and steam for gas turbine hot component cooling schemes. But the high efficiencies attained by these power plants is mainly due to the introduction of a new generation of gas turbines with thermal efficiencies of 45-48 per cent.

The performance of this new generation of gas turbines in combined cycle operation is complex and presents new problems that have to be addressed. Advanced gas turbines operate at very high pressure ratios and turbine firing temperatures. This high firing temperature affects the performance and life of the components in the hot section of the turbine, while the high compressor pressure ratio leads to a very narrow operation margin. This makes the turbine susceptible to compressor fouling. The turbines are also very sensitive to backpressure exerted on them by the heat recovery steam generators (HRSGs). The pressure drop through the air filter also results in major deterioration of the performance of the turbine.

The performance of the combined cycle is also dependent on the steam turbine performance. The steam turbine is dependent on the pressure, temperature, and flow generated in the heat recovery steam generator (HRSGs), which in turn is dependent on the turbine firing temperature and the air mass flow through the gas turbine. The entire system is very intertwined and deterioration of one component will lead to off-design operation of other components, leading to an overall drop in cycle efficiency. Thus, determining component performance and efficiency is the key to determining overall cycle efficiency.

New problems

Gas turbine: The new gas turbines are the cornerstone of the rise in importance of the combined cycle. They have a very high pressure ratio, a high firing temperature, and in some cases a reheat burner in the gas turbine. The gas turbines have also dry low NOx combustors. The combination of all these components has dramatically increased the thermal efficiency of the gas turbine.

The efficiency of the gas turbine since the early 1960s has increased from 15-17 per cent to 45 per cent. This has been due to a pressure ratio increase and an increase in the firing temperature. With these changes, we have also seen the efficiency of the major components in the gas turbine increase dramatically.

The increase in compressor pressure ratio has decreased the operating range of the compressor. Figure 1 indicates the operating range as being from the surge line at the low flow end of the compressor speed line to the choke point at the high flow end. The low pressure speed line has a larger operational range than the high pressure speed line. The higher pressure ratio compressors are therefore subject to fouling, resulting in surge problems, blade excitation problems and blade failure.

A drop in pressure ratio at the turbine inlet due to filter fouling will result in a loss in the overall turbine efficiency and the power produced. An increase in the pressure drop of about 25 mm amounts to a drop of about 0.3 per cent in power.

The gas turbine has to be operated at a constant speed, as any slight variation in speed could result in major problems for the grid. Load control is done by controlling the fuel input and therefore the turbine firing temperature, the inlet guide vane position and the airflow. This maintains the exhaust temperature of the gas turbine.

Compressor fouling also has serious consequences for the overall performance of the gas turbine as it uses nearly 60 per cent of the work generated by the gas turbine. Therefore a one per cent drop in compressor efficiency equates to nearly a 0.5 per cent drop in the gas turbine efficiency and a 0.3 per cent drop in the overall cycle efficiency. The cleaning of these blades by on-line water washing is a very important operational requirement.

In many cases, on-line washing has contributed literally hundreds of thousands of dollars to the bottom line of a plant. In addition, washing using demineralized water is as effective as using a detergent in an on-line water wash. On-line water washing is not the answer to all the problems, however, since after each wash the full power is not regained, and the unit will need to be cleaned off-line.

The time for off-line cleaning must be determined by calculating the loss of income in power as well as the cost of labour and equate it against the extra energy costs. An off-line soak and water wash without opening the casing usually restores about two percentage points to the compressor efficiency above that which could be recovered from an on-line water wash, while the opening of the casing and scrubbing the blades restores about four to six percentage points.

The lifetime of the various hot section components of the gas turbine depends on the following operational parameters:

  • Type of fuel: natural gas is the base fuel against which all other fuels are measured. Diesel fuel reduces the average life by about 25 per cent, and residual fuel reduces life by as much as 65 per cent.
  • Type of service: peaking service tends to reduce life by as much as 20 per cent as compared to base load operation
  • Number of starts: each start is equivalent to about 50 h of operation.
  • Number of full load trips: this is very hard on the turbine and is nearly equivalent to about 400-500 h of operation
  • Type of material: the properties of the blade and nozzle vanes are a very important factor. The new blade materials, which are single crystal structures, have done much to help the life of these blades in the higher temperatures. If more than about eight per cent of the air is used in cooling then the advantage of going to higher temperatures is lost. The use of the Larson-Miller parameters, which describes an alloy’s stress rupture characteristics over a wide temperature, life, and stress range, is very useful in comparing the elevated temperature capabilities of many alloys.
  • Types of coatings: the use of coatings in both compressors and turbines has extended the life of most of the components. Coatings are also being used on combustor liners. The new overlay coatings are more corrosion-resistant than diffusion coatings, and allow compressors to operate at very high pressure ratios. Compressor coatings also tend to reduce the frictional losses and can have a very rapid payback.

HRSG: The heat recovery steam generators are the key to high efficiency in combined cycles. The HRSG transfers the energy from the gas turbine to the steam turbine by converting the water entering the HRSG into superheated steam. The advanced cycles require the steam to have higher pressures and temperatures compared to conventional combined cycles. HRSGs are very effective at this over a wide range of operation.

Steam turbine: The steam turbines in most combined cycle applications are condensing steam turbines that can take an advantage of the low temperatures. The steam in the low pressure (LP) steam turbine is usually in a two-phase (wet) stage. Care must be taken to ensure that the steam has a steam quality of about 85-90 per cent to prevent blade erosion.

The computation of the power of the LP condensing turbine is very difficult since the quality of the steam at the turbine exit cannot be measured. The only way to ascertain the quality of the steam is to model a heat balance between the condenser and the exit conditions at the steam turbine. To conduct the heat balance, the cooling water flow and the inlet and outlet temperatures of the cooling water must be measured. However, measurement of the cooling water flow is often inaccurate, leading to large discrepancies in the computation of steam quality at the LP turbine.

Condenser: The steam condensers used in most large power plants are surface condensers whose function is to reduce the exhaust pressure of a prime mover to below atmospheric pressure and to condense the steam to allow the condensate to be reused as boiler feedwater. Most large plants use fresh or salt water for condensing the steam. Some plants also use air as the cooling medium, while advanced condenser design uses titanium, which has good heat transfer qualities, for the tubes carrying the water.

The fouling of condenser tubes in service must be carefully monitored otherwise heat transfer efficiency can be affected, increasing the exit pressure of steam leaving the LP steam turbine. This in turn will reduce turbine output.

The cleanliness factor is a term used to express the degree of tube fouling. It is a ratio of the thermal transmittance of tubes in service to the thermal transmittance of new clean tubes all operating under identical operating conditions. The cleanliness factor is a unique value since it applies to only one specific operating condition, and so to use it as a constant factor of new clean tube heat transfer for other operating conditions introduces error.

Controlling losses

The losses that are encountered in a power plant can be divided into two groups, uncontrollable losses, and controllable losses. The uncontrollable losses are usually environmental conditions, such as temperature, pressure, humidity, and turbine aging. Table 1 shows the approximate impact of such changes.

The controllable losses are those that the operator can have some degree of control over and can take corrective actions:

  • Pressure drop across the inlet filter: this can be remedied by cleaning or replacing the filter.
  • Compressor fouling: on-line water cleaning can restore part of the drop encountered.
  • Fuel lower heating value: in many plants on-line fuel analysers have been introduced to not only monitor the turbine performance but to also calculate the fuel payments, which are usually based on the energy content of the fuel.
  • Turbine back pressure: in this case the operator is relatively limited since nothing can be done about the downstream design. If there is some obstruction in the ducting to the HRSG, or if the duct has collapsed, the duct could be replaced.
  • HRSG effectiveness: a properly instrumented HRSG has thermocouples placed so that the effectiveness of each section of the HRSG can be monitored.
  • Steam turbine fouling: most of the fouling will occur in the LP turbine along with some blade erosion problems. By trying to calculate the quality content of the steam, as well as the power output of that section of the turbine, problems can be noted and corrective action taken.
  • Condenser pressure: the fouling of tubes and insufficient water flow through the condenser lead to increased pressure at the LP turbine exit and an increase in the exit pressure and temperature.
  • Condenser condensate sub cooling: this can lead to losses as the condensate has to be heated up and the overall energy available in the HRSG is reduced.

Life cycles

The division in power between the gas turbine and the steam turbine in a combined cycle varies considerably with load as seen in Figure 2. At lower loads, the steam turbine produces more power than the gas turbine because in a utility application, the mechanical speed must remain constant to prevent frequency fluctuations. The inlet guide vane is adjusted to reduce the flow at off-design loads, and to maintain the high exhaust gas temperature.

Figure 3 shows the variation in firing temperature and exhaust gas temperature as a function of load. It is interesting to note that the firing temperature of the turbine is greatly reduced while the exhaust temperature remains nearly constant, accounting for the steam turbine producing more work at low part loads. Figure 4 shows the energy available in the exhaust gas from the gas turbine, and the energy converted into power in a typical large combined cycle power plant, which is about 36 per cent.

Figure 5 shows the typical efficiencies that one could expect from the different major sections of a typical combined cycle power plant. The gas turbine efficiency drops quickly at part load as would be expected, as the gas turbine is very dependent on turbine firing temperature and mass flow of the incoming air.

Performance evaluation of power plants is important in determining at the beginning whether the plant meets all its guarantee points and downstream to ensure that the plant is being operated at or near its design operating point. Maintenance practices are increasingly being combined with operational practices to ensure that plants have the highest reliability with maximum efficiency.

The life cycle costs of any machinery are dependent on the life expectancy of the various components, the efficiency of its operation through out its life. New costs are about 7-10 per cent of the life cycle costs. Maintenance costs are approximately 15-20 per cent of the life cycle costs.

Operating costs, which essentially consist of energy costs, make up the remainder – 70-80 per cent of the life cycle costs – of any major utility plant. It is therefore clear why the new purchasing mantra for a utility plant, or in fact any major plant operating large machinery, has to be “life cycle cost”.

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