THE ENERGY INDUSTRY TIMES - AUGUST 2018
Special Project Supplement
Samcheok takes
CFBs to the next level
The massive
Samcheok Green
Power Project in
South Korea has
now marked just
over one year of
full commercial
operation. Utilising
the largest and
most advanced
ultra-supercritical
circulating fluidised
bed (CFB) boilers
in the world, it
signifies the next big
step for clean and
flexible power from
economical solid
fuels. Junior Isles
that even at supercritical conditions,
tubes that receive more heat will naturally
flow more steam and water. This
provides additional cooling to limit
temperature differences between different
wall tubes.
SFW says that out of the hundreds
of boilers sold in the market, only a
few use this relatively unique technology.
“This is belt and suspenders for
the CFB,” said Giglio. “Since the
combustion temperature and heat flux
is so uniform in a CFB, overheating of
tubes is a rare occurrence.”
The material used for SFW’s supercritical
boilers is much the same
as for its subcritical boilers, except in
the final superheater section, which
is subjected to very high temperatures.
With PC or PF boilers, final
superheating takes place in coils
hung from the top of the boiler. This
is not a great place to put these coils,
since they operate at the highest
metal temperatures and are fully exposed
to the corrosive flue gases
containing molten ash.
This design weakness is avoided in
SFW’s CFB’s by placing these coils
in high efficiency fluidised bubbling
bed heat exchangers called INTREXs.
These heat exchangers are integrated
into the bottom of the furnace extracting
heat from the hot circulating bed
material at a rate 4-5 times higher
than in PC boilers.
As the hot solids return from the
solid separators, they are collected
and returned to the furnace through
the INTREXs and since the solids are
fluidised by air and not flue gas, they
are protected from the corrosive flue
gases. Further, instead of causing
fouling, the fuel’s ash and added
limestone are used to achieve very
high heat conduction rates allowing
the CFB to achieve very high ultrasupercritical
steam temperatures with
less heat transfer surface area and a
much lower combustion temperature
than a PC boiler.
This means the same supercritical
temperatures can be reached using a
lower grade coil material than would
be needed in a PC boiler or that a
lower quality fuel can be used without
sacrificing steam temperature or
Aunique power generation project
aimed at providing cheap,
green energy to South Korea,
is now demonstrating what is possible
in terms of burning fossil and
carbon-neutral fuels with minimal
emissions.
The final unit of the Samcheok
Green Power Project, located 40 km
outside the city of Samcheok, in
Gangwon-do province on the east
coast of South Korea, was taken into
operation in June last year. Notably,
the plant, owned by state-owned
power company Korea Southern
Power Company (KOSPO), sees the
world’s first use of advanced ultrasupercritical
circulating fluidised bed
(CFB) boilers – a technology that
enables it to burn a combination of
biomass and coal of varying quality,
including low rank coals.
The plant is equipped with four
boilers supplied by Sumitomo SHI
FW (SFW), in tandem with two steam
turbine generators with a total gross
capacity of 2200 MW. It is an impressive
example of a project that uses a
diversified mix of clean burning fossil
fuel technology and renewable energy
sources. In addition to the four CFB
boilers, Samcheok Green Power
Project is also the site of renewable
power generation projects based on
hydro, wind and solar sources that are
in the design phase. There is also a
fuel cell plant at a nearby facility
owned by Korea Gas Corporation.
In many ways Samcheok reflects
South Korea’s energy strategy. Strong
economic growth drives power demand
in the country but like Japan,
South Korea has no significant natural
energy resources and is heavily dependent
on fuel imports. It is also in a
region of relatively high gas prices,
compared to the US and Europe. This,
combined with a reduced focus on
nuclear post-Fukushima, leaves coal
and renewables as the main options
for meeting future electricity demand.
Going forward, however, the situation
with regard to coal is complicated by
rising coal prices and the decreasing
availability of high quality coal.
This led KOSPO to think about
how it could use low quality coal.
After a comprehensive evaluation
of boiler technologies, KOSPO
found that CFB technology could
reliably fire cheaper low-quality
coal together with biomass and resolve
the problem.
“CFB technology has the flexibility
to burn the lower quality coals that
they saw becoming more abundant,
namely from Indonesia,” said Robert
Giglio, Vice President of Strategic
Business Development and Strategy
at SFW, adding: “If you use a pulverised
coal boiler, it has to be designed
for a narrow fuel range. The coal has
to be finely ground before being fed to
the boiler and you could only burn a
limited amount of biomass.”
Another advantage of the CFB
technology relates to the fact that it
does not need traditional flue gas
desulphurisation (FGD) systems
since the CFB boilers capture the
sulphur in the fuels as it burns.
Samcheok Green Power Project
uses four 550 MWe (gross) ultra-supercritical
boilers – making it the
largest CFB plant in the world. The
boilers – which represent the first
phase of a plant that could be expanded
at a later date – are configured
as two blocks (Units 1 and 2) with
each block having two boilers feeding
into an 1100 MW steam turbine.
Coal and biomass are burned in
once-through ultra-supercritical
(OTUSC) CFB boilers. Supercritical
steam conditions represent a physical
point just above the triple point of
water. When the boiler pressure
reaches above the critical pressure of
221.2 bar (the critical point of water)
and a temperature of 374°C, twophase
mixtures of water and steam
cease to exist, and are replaced by a
single supercritical fluid. These steam
conditions allow a once-through
boiler design where the high steam
temperature and pressure results in
much higher efficiency compared to a
conventional drum-type boiler.
The normal operating mode of the
Samcheok unit is coordinated control
with sliding pressure operation. In
normal operation the boilers are operated
at the same load level and any
load change requests are forwarded
to the boilers simultaneously and
with similar control parameters.
Steam temperatures are individually
controlled in order to ensure that the
required temperatures in the main
steam and reheated steam systems
are achieved. Reheated steam shared
between the boilers is continuously
monitored and controlled in accordance
with the applicable firing
rates.
Each boiler at Samcheok produces
main steam at a temperature of 603°C
and reheat steam at 603°C. Superheat
steam pressure is 257 bar g. They
utilise advanced vertical tube, lowmass
flux Benson evaporator technology,
which is more efficient and easier
to build and maintain than
conventional spiral-wound supercritical
boiler technology.
The vertical tube design has several
advantages over a spiral tube design.
It has a lower pressure drop across the
boiler, resulting in higher efficiency.
Samcheok has an estimated net electrical
efficiency of 42.4 per cent
(LHV) compared to the 38-39 per
cent typically achieved with traditional
boilers.
SFW says its vertical tube supercritical
CFB design also makes it
easier to build and maintain because
structurally, it avoids the complicated
support system needed for a spiral
tube boiler.
Commonly used for conventional
pulverised coal (PC) or pulverised
fuel boilers (PF), spiral wrapping the
tubes around the furnace helps evenout
the temperature differences from
boiler tube to boiler tube but the spiral
winding has a natural tendency to
structurally unwind, putting a twisting
force on the boiler since it is hung
from the top. To counter the twisting
forces, an elaborate bracing and lateral
support system is required adding
cost to the boiler and making it more
difficult to build and then maintain.
SFW’s CFBs use the same simple
vertical tube hanging design common
in subcritical drum boilers, but instead
uses much smaller tubes. This reduces
the weight of the steam/water mixture
in each tube allowing buoyancy
forces to become strong enough so