1. CONCEPTUAL DESIGN OF THE PLANT

1.1 Basis
1.2 Combined cycle plant arrangements
1.3 Plant operation
1.4 Utility load growth
1.5 Thermal energy and power system
1.6 Buildings and constructions
1.7 Conclusion

Typical efficiency and costs

The introduction, over the past four decades, of larger capacity gas turbine designs (30 MW to 420 MW) with increased specific power has lead to the paralleled development of highly efficient and economical combined cycle systems consisting of gas turbines, steam turbines, generators, heat recovery steam generators and controls.

The combined cycle system is an optimized and matched system of high technology power generation equipment, software and services configured for convenient integration with the owner’s auxiliaries and balance of the plant to form an economical power plant.

Economical performance of function, either power generation or power and thermal energy production with an efficiency of up to 60 %, is the outstanding characteristic of combined-cycle systems.

The combined-cycle system includes single-shaft and multi-shaft configurations. The single-shaft system consists of one gas turbine, one steam turbine, one generator and one Heat Recovery Steam Generator (HRSG), with the gas turbine and steam turbine coupled to the single generator in a tandem arrangement on a single shaft. The key advantage of this single-shaft arrangement is its operating simplicity with higher reliability than multi-shaft blocks. Further operational flexibility is provided with a steam turbine which can be disconnected, using a hydraulic clutch, for start up or for simple cycle operation of the gas turbine.

Multi-shaft systems have one or more gas turbine-generators and HRSGs that supply steam through a common header to a separate single steam turbine-generator. In terms of overall investment a multi-shaft system is about 5 % higher in costs.

Single- and multiple-pressure non-reheat steam cycles are applied to combined-cycle systems equipped with gas turbines having rating point exhaust gas temperatures of approximately 540 °C or less. Selection of a single- or multiple-pressure steam cycle for a specific application is determined by economic evaluation which considers plant installed cost, fuel cost and quality, plant duty cycle, and operating and maintenance cost.

Multiple-pressure reheat steam cycles are applied to combined-cycle systems with gas turbines having rating point exhaust gas temperatures of approximately 600 °C.

The most efficient power generation cycles are those with unfired HRSGs with modular pre-engineered components. These unfired steam cycles are also the lowest in cost. Supplementary-fired combined-cycle systems are provided for specific application.

The primary regions of interest for cogeneration combined-cycle systems are those with unfired and supplementary fired steam cycles. Theses systems provide a wide range of thermal energy to electric power ratio and represent the range of thermal energy capability and power generation covered by the product line for thermal energy and power systems.

Steam production and steam turbine output vary with the gas flow and temperature supply to the HRSG. Gas turbine exhaust flow and temperature vary with ambient temperature and barometric pressure while steam turbines can be selected to suit specific application requirements. The steam turbines are sized so that their rated flow matches the steam production when the gas turbines are operating at base rating. The turbines use the steam produced by the HRSGs, with the gas turbines at base capability over the ambient air temperature range expected for most applications.

Excellent part-load heat rate is achieved on multi-shaft systems or multiple single-shaft units by sequentially loading gas turbines to meet system requirements.

Power generation economics can be enhanced by the installation of generation capacity in small increments as utility load grows.

Flexibility is also available with the progeneration approach to capacity addition. Initial natural gas/distillate oil-fired, simple-cycle gas turbine installations can be converted to combined-cycle later, when power demands require capacity increases. Plot area for the steam cycle equipment and transmission line capability are the main considerations during the initial commitment for simple cycle gas turbines. Future conversion to coal derived fuels is also an option for dealing with the long range uncertainties of conventional fuel availability and price.

Thermal energy and power combined cycle systems (cogeneration and district heating systems) are designed with structured flexibly to provide a wide range of power and thermal energy capacities to suit varied application requirements. The most commonly supplied systems are:

• Steam generation at Process Conditions with HRSG (no Steam Turbine)

Unfired HRSG

Supplemental-Fired HRSG

• HRSG and Non-condensing Steam Turbine Exhausting to Process

Unfired, One-Pressure HRSG

Unfired, Two-Pressure HRSG

Supplemental-Fired, One-Pressure HRSG

• HRSG with extraction/condensing steam turbine

Unfired, One-Pressure HRSG

Unfired, Two-Pressure HRSG

Supplemental-Fired, One-Pressure HRSG

The capabilities of the thermal energy and power systems are unique for each gas turbine frame size as well as each set of process steam conditions for those systems with both unfired HRSGs and unfired HRSGs with non-condensing steam turbines. The systems with fired HRSGs and condensing steam turbines provide extraordinary flexibility in both thermal energy and power generation capacity for each gas turbine frame size.

Buildings are depending on the local conditions. The equipment is suitable for outdoor installations, semi-outdoor installations or for fully housed installations.

The combined-cycle power plant, including the power generation systems and thermal energy and power systems are efficient, low cost systems that meet the environmental requirements of all countries. The attractive economics, reliability and operating flexibility of these systems suggest their consideration for all power generation applications.

Typical efficiency and costs

With a gas turbine only an efficiency of about 35 % can be achieved, whereas with a modern gas and steam plant peak efficiencies of up to 60 % are possible. It is however interesting to see, that with a gas and steam combi plant not only the highest efficiency can be achieved but also the plant specific investment costs are low.

Another advantage is the relatively short construction time, which can be organised on a step by step basis as needed or required for a plant refitting and/or extension.

To summarise a gas and steam combi plant is today the most economical solution as it is from an ecological point of view. This, since it provides also various possibilities to reduce the harmful emissions and at the same time an increase in plant capacity and efficiency.

Process flow sheet