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• Nitric acid / Caprolactum plants: nitric acid may be manufactured by the
catalytic oxidation of ammonia at around 950°C to give a gas rich in
nitrogen oxides. The next step of the process requires a gas temperature of
around 250°C creating a requirement for a HRSG. Either water tube or fire
tube designs may be used – in either case the design needs to take into
account a typical gas side pressure of 5 – 7 barg.
• Ammonia plants
• Hydrogen gas plants
• Fluidised catalytic converter units
3.10 Conclusions
• Current state of the art utility scale HRSGs operate at HP steam conditions
of up to 124 bar/565°C and allow the associated CCGT to deliver
electrical power at a claimed net efficiency of up to 60%. They are
generally two pressure or three pressure with reheat, and may be of either
vertical or horizontal gas flow.
• The capital cost of new-build CCGT plant is around £425/kW, with the
HRSG accounting for 10-15% of this total. The estimated delivered energy
cost for a CCGT in the UK is around 2.2p/kWh.
• Current state of the art industrial HRSGs generally operate at lower steam
conditions than utility scale plant, and are usually of single pressure
design. They are integrated into a wide range of industrial plant and often
include provision for supplementary or auxiliary (stand-alone) firing.
Highly fired units may incorporate a water-cooled furnace. Lower pressure
industrial boilers are usually of shell rather than water tube design.
Designs tend to be bespoke for particular process applications.
• The recent trend has been for CCGT plant to be built under turnkey
contract. Whilst this does have advantages to the user in terms of
accountability, it does tend to mean that the user has less influence on the
detailed HRSG design.
• Operational experience with HRSGs indicates that inclusion of specific
design features and attention to detail during fabrication are just as
important as the overall HRSG design, and that non pressure parts can be
as problematic as pressure parts.
• Key areas for improvement include build quality, access for in service
inspection & maintenance, control & instrumentation and capability for
flexible operation. Overall cycle chemistry philosophy also needs to be
more thoroughly considered at the design stage.
The current challenge for operational HRSGs, particularly in the UK, is the
need to cycle plant which has been designed for and/or previously operating at
base load. Many users are currently carrying out investigations/trials and plant
modifications.
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