POWER August 2020 - 31

TOP PLANT
more formidable challenge, Li said. " The
drum is the boundary of water and steam;
therefore, the main steam temperature
can't be as flexibly adjusted as a oncethrough
boiler, " he explained. Engineers
came up with an innovative and economical
solution that involved retrofitting the
boilers heating surfaces (Figure 1), and
installing a new " radiation superheater. "
Replacing Piping. To accommodate
efficiency improvements in the turbine
and the boiler, this part involved replacements
with P92 piping, Super 304H boiler
tubing, and other high-temperature alloys
that are commonly used in USC plants.
Applying Energy Saving Technologies.
1. Engineers came up with an innovative
boiler retrofit solution as part of a project to
dramatically increase the efficiency at the Xuzhou
3 subcritical unit. The retrofit achieves
high boiler temperatures of 600C but it requires
only minimal modification costs. Courtesy:
Shenergy Power Technology
generator systems, and though the efficiency
gains would have been valuable, it
was too costly to consider, Li said.
Engineering a Solution
The retrofit solution the engineers ultimately
drew up for Xuzhou 3 was based
on an invention developed over 11 years
by Prof. Feng Weizhong, who is also
general manager of Shenergy Power
Technology. Weizhong calls the solution,
" High-Temperature Comprehensive Upgrading
Retrofit of Subcritical Unit. " It
included the the steps that follow.
Improving Turbine Efficiency. Determining
that ramping up efficiencies would
require increasing steam temperatures
from 538C (1,000F) to 600C (1,112F),
while keeping the main steam pressure
unchanged, Shenergy recognized the
unit's turbine efficiencies would also need
to be ramped up dramatically, so it collaborated
with turbine heavyweights Siemens
and GE Power to develop a retrofit solution.
The customized turbine reaps benefits
from an efficiency improvement of
about 7%, which was achieved via higher
feedwater temperature, a better flow path,
and higher exhaust steam quality, Li said.
The 600C subcritical turbine design also
showed that the heat rate could be as low
as 7,500 kilojoules/kWh (7,109 Btu/kWh)-
much lower than that of a supercritical one.
Ramping Up Boiler Temperatures.
Achieving a 600C temperature for the
2004-built subcritical drum boiler was a
August 2020 | POWER
To further improve efficiencies, engineers
also added a set of Shenergy-developed
and patented energy-saving technologies,
including efforts to recover heat from flue
gas, " generalized regeneration " technologies
that reduce the turbine exhaust loss,
and improvements to the air preheater.
These technologies also essentially reduce
power consumption of existing auxiliary
equipment such as boiler fans and turbine
pumps. " The evidence is performance related, "
Li said. " They actually reduced power
consumption by 10%, " which pays off in
long-term efficiency gains, he said.
Environmental Controls. While Xuzhou
3 already had environmental controls, including
selective catalytic reduction (SCR)
technology, electrostatic precipitators, and
wet flue gas desulfurization, the retrofit
improved the equipment's performance
and efficiency, mainly because the reduced
coal consumption decreased the flue gas
mass flow.
An Enduring Model
Despite the wide scope of these retrofits,
many of them requiring first-of-their-kind
applications and rigorous testing, Shenergy
and China Resources completed
physical construction of the project at the
end of July 2019-within a remarkable
six months. So far, the combined effect
of these improvements on the total thermal
cycle for Xuzhou 3 are paying off in a
big way, Li said, and Shenergy has shown
that if applied to a bigger plant, such as
a 600-MW unit, efficiency levels could be
improved further by about 0.7%.
" Compared with USC units, where the
typical parameters are about 26 Megapascal
(MPa, 3,771 psia) and 600C, our subcritical
unit has 6.7 MPa (972 psia) and 600C.
So, we just have a lower pressure-which
we could not change because it would
ramp up the cost-but the influence of
the pressure sensitivity is not that large,
and we can compensate this gap with our
www.powermag.com
POWER POINTS
Winning Attributes
✔ Xuzhou 3, a 2004-built subcritical
coal unit, ramped up its net efficiency
to 43.56%, which surpassed all
existing supercritical and even many
ultrasupercritical units.
✔ The high-temperature retrofit
technology increased steam
temperatures from 538C to
600C while controlling costs and
maintaining environmental profiles.
✔ Since it was commissioned, the
novel 320-MW unit has garnered
certification to run stably and safely
at ultra-low loads of less than 20%.
energy-saving technologies, " he said.
The total retrofit project, meanwhile,
cost about 350 million RMB, or $50 million,
which is less than a third of the cost
to build a new unit in China, Li said. That's
also important because getting the government's
approval to build larger new
supercritical or USC coal units in China
is currently difficult, owing to climate
and overcapacity concerns. " The government
actually encourages subcritical
retrofits for efficiency gains, " but before
the Xuzhou 3 demonstration, previous
efforts had proven costly and were limited
in performance, Li noted.
Since the project was commissioned,
Xuzhou 3 has also found a new purpose-as
a flexible, load-following generator.
" Flexibility wasn't a requirement
in the project contract, but Shenergy still
took it into its design, " Li said. " It turns
out that load can be as low as 19%, and
the boiler combustion will still be stable
and the SCR will still operate normally. "
A three-stage test witnessed by a local
power grid authority institute in December
confirmed the unit can operate safely
at a load of 19% for more than four hours.
That achievement garnered the unique
subcritical unit one of China's first certifications
under " deep load regulation. "
That's crucial because it opens up a
new avenue for subcritical coal utilization
as China tries to balance its growing share
of intermittent renewable power, Li said.
It's also a big bonus that could encourage
market uptake of the " very promising and
economic solution for all the subcritical
units in the world to dramatically increase
the unit efficiency, " he said. ■
-Sonal Patel is a POWER senior
associate editor.
31
http://www.powermag.com
POWER August 2020

Table of Contents for the Digital Edition of POWER August 2020
