POWER August 2020 - 30

TOP PLANT
Xuzhou 3 Shows the
Future of Subcritical
Coal Power Is Sublime
Courtesy: Shenergy Power Technology
A remarkable retrofit at Xuzhou Unit 3 boosted the 320-MW subcritical coal
unit's efficiency to beyond 43.56%-higher than all existing Chinese supercritical
units, and even many ultrasupercritical units. The groundbreaking demonstration,
a POWER 2020 Top Plant award winner, offers crucial hope and new purpose
for subcritical units around the world.
Sonal Patel
I
n April 2017, engineers from Shanghai
Shenergy Power Technology sat together
contemplating how they would
execute a formidable task. Owing partly
to their success at boosting the annual
average net efficiency at Shanghai Waigaoqiao
3, an ultrasupercritical (USC) unit, to
a world-class 44.5%, Hong Kong-based
power generation giant China Resources
Power had invited the technology arm of
Shanghai government-owned Shenergy
Group to conduct a high-temperature
retrofit at one of four units at the 1.2-GW
Xuzhou China Resources Power Plant in
Jiangsu province. Though it featured some
of China Resources's very first coal units,
some units at the Xuzhou plant, like the
June 2004-commissioned Unit 3, were
relatively young. But China Resources was
concerned the efficiency of these and its
other subcritical units wouldn't meet the
39.6% average unit net efficiency (UNE)
level (net lower heating value basis) mandated
by the Chinese government in
2014, which would force a vast slice of the
company's thermal generation portfolio to
close as required by 2020.
As Li Li, a Shanghai Shenergy Power
Technology engineer, explained to POWER
in June, China had already made
sizable leaps to modernize its coal fleet
at the time. Yet, while the nation had installed
more than 100 1-GW USC units, it
30
still had a substantial 350-GW subcritical
fleet, and most-more than 880 units-
were small 300-MW designs.
The outlook for the subcritical fleet was
dismal, Li said. Considering the fact that
the typical annual average net efficiency
level of subcritical units was about 37%-
some were even lower than 35%-
achieving the mandated 39.6% UNE
target seemed overly ambitious. Though
the large 3% UNE gap was challenging,
shuttering those small, still relatively young
units would have been " a great waste of
investment and resources, " he said.
Exploring the Options
Ultimately, the team chose to conduct a
high-temperature retrofit customized to Xuzhou
3, a 320-MW subcritical unit that best
resembled many Chinese-built units in age
and equipment. Before the retrofit, Xuzhou
3 contained a two-path drum boiler made
by Dongfang Boiler and a turbine made by
Shanghai Turbine. But although the unit's
high-pressure-intermediate-pressure (HPIP)
turbine flow paths had been modified
once before in 2012, the efficiency benefits
had degraded over time, and the unit's
UNE efficiency under turbine heat rate acceptance
conditions was about 38.6%,
according to a 2016 performance test. The
retrofit project's requirements clearly called
for reducing the coal consumption rate
of the unit from 318 grams/kWh to 287
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grams/kWh when it was running loads at
100%. That meant raising its efficiency to
42.8%-a level that surpassed all existing
supercritical units and several USC units.
However, Shenergy's engineers recognized
that their task also required a
long-term vision, Li said. For one, the
demonstration needed to show the technology
could be widely applicable, so the
retrofit would need to show good return
on investment (ROI). Essentially, " The
solution needed to substantially increase
the efficiency in an economical way so
that it would be accepted by the market, "
he explained. And as significantly, it would
need to show exemplary performance
that could be sustained " so unit owners
wouldn't need to be concerned about future
operation efficiency. " Finally, the solution
also needed a " low technical risk "
profile, which means it needed to be based
on " mature materials and equipment to reduce
the risk and ensure safety, " he said.
These parameters helped the team rule
out several existing solutions. The typical
modification of turbine flow paths, a
common retrofit solution usually recommended
by turbine manufacturers, would
have yielded an efficiency improvement of
only 1.2%, engineers determined. Upgrading
to a " double-reheat " unit, meanwhile,
would have required a complete dismantling
and rebuilding of the boiler, as well
as replacing the feedwater systems and
POWER | August 2020
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POWER August 2020

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