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4 NEW AND DEVELOPING TECHNOLOGIES
4.1 Introduction
This chapter describes and reviews HRSG technologies which are envisaged
as becoming available in the near and longer term. A number of HRSG
suppliers and users were consulted during the preparation of this report about
technological developments and the need for further research. The consensus
of opinion amongst those consulted was that the technology is mature and that
large or revolutionary advances in technology are not expected. However,
small incremental improvements are expected to continue. None of the
industrial scale companies consulted stated that they have research projects of
their own going on currently. At the industrial scale, most businesses are not
large enough to take on large R&D commitments. Developments at the utility
scale, such as the use of higher temperature materials, will cascade down to
the smaller industrial scale market eventually and give gradual advances. A
number of new applications or small areas of technological advance were
identified and the following specific categories have emerged:
• Developments in the design of HRSGs themselves.
• Developments in other parts of combined cycle plant or the overall cycle.
• New applications.
4.2 Developments in HRSG Design
4.2.1 Utility Scale Once Through HRSG Designs
In terms of components, the once-through steam generator is the simplest
HRSG design for recovering heat from the exhaust of a gas turbine. Water
entering at the cold end of the gas-pass, moves through a serpentine tube
bundle where heat absorption occurs and a phase change takes place, and exits
as superheated steam. The circulation ratio is one and there is no requirement
for circulation pumps.
Conventional (i.e. not once through), sub-critical HRSGs utilise drums in
which steam and water from the evaporative part of the cycle are separated.
The water is then recirculated within the evaporator with additional feedwater
while the steam passes to the superheater for further heating. Supercritical
pressure boilers cannot utilise this type of design as there is no distinct
water/steam phase transition above the critical pressure. A once-through
design is therefore required. The OTSG design also has advantages for flexible
operation. The steam drum is the component in a conventional HRSG design
with the thickest wall section and is therefore the most prone to the occurrence
of stresses associated with differential thermal expansion. It is the limiting
component in setting maximum heat-up and cool-down rates and a design that
eliminates the drum is therefore better for flexible operation.
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