Flexibility of Fossil Fuel Plant in a Renewable
Scenario
F. Starr, E. Tzimas, & S. Peteves
European Commission - DG JRC
Institute for Energy
Petten, The Netherlands
Coping With Variability: UK Open University Conference 24th Jan 2006
Presentation Aims
Highlight the fact that plant cycling tends to be overlooked
in future plant design concepts
Indicate some of the problems which currently constrain
rapid start ups in CCGT and coal fired steam plant
Highlight the issues which cycling can create in advanced
fossil fuel electricity-only plants in a renewables scenario
Introduce an IGCC based hydrogen/electricity plant design
which will be capable of rapid response to electricity demand
Commonly agreed priorities for future fossil fuel plants:
High efficiency
Ability to capture CO2 is a preferred option
Capital costs similar to today
Priorities that tend to be overlooked:
Need for increased load following, two shifting and stop/start
operation resulting from any or all of the following:
Wind and solar renewable electricity
Nuclear base load plants
Large scale cogeneration
Today There Are Four Ages of Fossil Fuel
Plant Operation
Years 1-2: Post Commissioning with frequent plant trips
Years 3-9: Base Load Operation with biannual shutdowns
By year 9 plants will need to cycle
although many components have aged and corroded
Years 10-14: Load Following down to 80% output (gas) 40% (coal)
Years 14-30: Two Shifting (stop-start operation) with shutdown at
night and weekends
Running for 10-15 hours a day
Shut down at weekends
Frequent load changes
Thermal and pressure cycling
Over temperature running
Bad water conditions in boiler
Poor furnace conditions
Stop-start operation will be worse in renewables scenario
FOR THE FUTURE 2010-2025 PERIOD
Big CHP
PWR Nuclear
Hydro
Will grab the base load
Wind Power
Photovoltaic
Will be the growing intermittent suppliers
Fossil fuel plant will have to cycle much more
Electrical Demand or Output
(GW)
Estimated German Power Output and Demand in October
2020
100
Total Demand
80
60
40
Fossil
20
Renewables
0
0
2
1
Number of Weeks
For original data see refs at end
In the future peak to trough ratio
for fossil plants increases to 3/1
3
Combined Cycle Gas Turbine Plant
•Gas turbines and steam turbines produce the power
•Steam is produced for steam turbines in a HRSG (Heat Recovery
Steam Generator )
The HRSG Section of CCGT
•Most current designs intended for base load operation
• Thermal stress and fatigue is a problem due condensation of steam during
start up and shutdown
• HRSGs are difficult to repair- avoid thermal stress by careful operation
Typical Layout of a Horizontal HRSG
Image source: www.bhpi.com.ph
Man sized object
Image source: www.bhpi.com.ph
Steam condensation in HRSGs tends to limit quick start up after
CCGT plant shutdowns and poor drainage of condensate
can result in uneven temperature distribution and thermal stress
Outlet Header
Steam
Front View
of
HRSG Superheater
during
Shutdown
Combination of cold air from gas turbine
needed to flush
potentially explosive gases
out of duct from gas turbine,
plus poor drainage of condensed steam,
causes tubes to cool down unevenly
Condensate
CCGT Cycling Issues of Today
• Hot section of gas turbines and HRSGs suffer from thermal stress
because of temperature changes and condensation effects
• Boiler water conditions can be poor
• CCGTs can give relatively fast start ups and shutdowns at the cost of
increased maintenance
• Big changes in output, when load following can be difficult
with current units
• Drop in grid frequency will result in reduced power output
will require some over- temperature running by the GT to restore
frequency
• Acting as spinning reserve is probably not practical with most
current CCGTs
Future CCGTs
Gas turbines will be more complex
Blade cooling using steam, reheat or interstage combustion
but
Controls and water treatment should be better than today
HRSGs can be made to drain better, reducing build up of
condensate and reducing thermal stress
Higher inlet temperatures from GT to the HRSG may permit
better load following
CCGTs may still be difficult to run in a spinning reserve mode
Conventional CCGTs have the disadvantage that high amount of
excess air increases difficulty in capturing CO2
Advanced Coal Fired Steam Plant
• Superheater Metal
Temperatures 675-780°C
• Pressures 300-400 bar
• Inlet HP and IP Turbine Rotor
650-720°C
Image source www.ocp.tudelft.ph
Heavier walled pipe work, need
for stainless steels, and
increased
temperatures implies start up
times of several hours
Spinning reserve capability may be compromised in advanced plants
High carbon level in fuel make it more essential to capture CO2
IGCC- Hypogen Carbon Capture Concepts
Coal-Oxygen-Water Into Gasifier
Production of hydrogen for CCGT
•Removes sulphur and chlorine
•Removes CO2
•Similar electrical efficiency to steam
plants with CO2 capture
Raw Gas Out
Slag
and
Water
( for purification and
conversion to
hydrogen for use in CCGT)
•More efficient at producing hydrogen
from fossil fuel than steam plant plus
electrolysis
Note : IGCC based concepts are
not easy to start up or
to use for load following
Main Characteristics of Flexible IGCC-Hypogen Plant
Plant works as a base load energy producer all the time
Gasifier and gas purification systems in constant operation
Able to change energy output from 100% electricity to 100% hydrogen
Does not require external source of electricity for ancillaries
Electricity from CCGT section of plant can be
produced
extremely rapidly- can be used as spinning reserve
because
HRSG can be kept hot and ready using steam supply from gasifier
Flexible IGCC-Hypogen Plant Layout
Conclusions for 2010-2025
CCGT plant can be made more efficient, and at some additional cost, can be
made to be more flexible than today’s units (1-2 hour start ups)
CCGTs may still have problems when used for spinning reserve
With steam plant, at some point, efforts to improve efficiency will greatly
compromise the ability to change to two – shift operation
The preferred option for future fossil plants is to have the facility to capture
carbon
This could be a problem for CCGT and Steam Plant
as CO2 capture systems probably need to be run continuously
Flexible IGCC - Hypogen Plants will be extremely good at responding
to changes in the demand for electricity
and capturing CO2
Thank You
The views in this presentation are those of the
authors and not necessarily those of the
European Commission
References:
Quaschning, V.: Simulationserebnisse fűr die regenerative Erzeugung im Jahr 2020 c/o V. Quaschning (2001)
Starr F, Tzimas E, Steen M , Peteves SD : Flexibility in the production of hydrogen and electricity from fossil fuel plants c/o EU Institute for Energy
Starr F : Background to the Design of HRSGs and Implications for CCGT Plant Cycling in OMMI Power Plant Internet Journal Vol 2/1 2002