Technical Paper
BR-1849
Successful Operation of Jinzhushan 3,
the World’s First PC-Fired Low Mass
Flux Vertical Tube Supercritical Boiler
Authors:
A.J. Bennett
M.J. Albrecht
C.S. Jones
Babcock & Wilcox Power
Generation Group, Inc.
Barberton, Ohio, U.S.A.
Q. Zhang
Babcock & Wilcox
Beijing Company, Ltd.
Beijing, People’s Republic of
China
Y. Chen
Datang Huayin Jinzhushan
Thermoelectricity Power
Generation Co., Ltd.
Hunan Province, People’s
Republic of China
Presented to:
Power-Gen Asia
Date:
November 2-4, 2010
Location:
Singapore, Republic of Singapore
Successful Operation of Jinzhushan 3, the World’s First
PC-Fired Low Mass Flux Vertical Tube Supercritical Boiler
Q. Zhang
Babcock & Wilcox Beijing Company, Ltd.
Beijing, People’s Republic of China
A.J. Bennett
M.J. Albrecht
C.S. Jones
Babcock & Wilcox Power
Generation Group, Inc.
Barberton, Ohio, U.S.A.
Y. Chen
Datang Huayin Jinzhushan
Thermoelectricity Power
Generation Co., Ltd.
Hunan Province, People’s
Republic of China
BR-1849
Presented to:
Power-Gen Asia
November 2-4, 2010
Singapore, Republic of Singapore
Abstract
Introduction
June 2009 marked the initial coal-fired operation of the
world’s first supercritical pulverized coal-fired low mass
flux vertical tube Benson® boiler. Jinzhushan 3, located
in Hunan Province in the People’s Republic of China, is a
600 MW Babcock & Wilcox Power Generation Group, Inc.
(B&W PGG) VTUP™ once-through boiler with vertical tube
furnace construction designed to burn Chinese anthracite
using downshot pulverized coal (PC) technology. The boiler
was supplied to the Datang Jinzhushan Fossil Power Generating Company by Babcock & Wilcox Beijing Company,
Ltd. (BWBC) under license from B&W PGG.
Supercritical steam cycles are an integral part of low carbon electric generating strategies being deployed world-wide
because of their inherently higher efficiency than traditional
subcritical cycles. This paper reviews the development
history of supercritical, once-through steam generator technology and product advancements. Spiral and vertical tube
furnace circuitry designs for once-through variable pressure
boilers are compared and contrasted.
Downshot pulverized coal combustion technology is
used to efficiently utilize low volatile anthracite coal that is
difficult to burn conventionally. The unique design features
of the downshot combustion system are reviewed.
B&W PGG’s VTUP steam generator product and the
adaptation of this technology for the Jinzhushan 3 downshot
application are described. Results from the successful 168
hour reliability run of the Jinzhushan 3 boiler completed
on July 4, 2009, and subsequent performance testing are
presented.
The next development steps to use B&W’s VTUP technology, for downshot and wall-fired applications are also
presented.
In March 2007, Datang Huayin Jinzhushan Thermoelectric Power Generation Co., Ltd. placed an order with BWBC
for the supply of a 600 MW supercritical steam generator
as part of a planned expansion for their power plant located
near the city of Leng Shuijiang in Hunan province of the
People’s Republic of China. To efficiently utilize the plant’s
low volatile anthracite coal, a downshot pulverized coalfired combustion system was required for the project. This
was the first downshot boiler designed to take advantage of
the higher plant efficiency of a supercritical pressure steam
cycle. Modern supercritical boilers have typically used high
mass flux spiral tube furnace designs that are inherently
resilient to furnace gas-side heat flux upsets. However, the
complex geometry of the lower furnace burner zone makes
it very difficult to adapt a spiral tube design to a downshot
boiler arrangement.
BWBC is a joint venture company owned with equal
shares by Beijing Jingcheng Machinery Electric Holding
Co., Ltd. and Babcock & Wilcox Power Generation Group,
Inc. (B&W PGG). BWBC licenses a full range of B&W PGG
utility boiler products and technologies focusing primarily
on the China market and is a key partner supporting B&W
PGG as a primary detail engineering and fabrication shop
for utility and industrial products sold world-wide. When
Jinzhushan 3 required supercritical boiler technology for a
downshot arrangement, B&W PGG worked closely with
BWBC to develop a solution that utilized low mass flux,
vertical tube Benson boiler technology originally pioneered
by Siemens AG. B&W PGG licenses this technology from
Siemens and has further developed it into a commercial
product, the B&W PGG VTUP boiler. The low mass flux
technology offers advantages of natural circulation flow
Babcock & Wilcox Power Generation Group
1
characteristics in the furnace tubes which can help selfcompensate for inherent upsets in heat absorption rates in
the furnace. The vertical orientation of the tubes in the VTUP
boiler furnace panels was also much easier to adapt to the
lower furnace arrangement of the downshot boiler. Other
advantages include a lower furnace water/steam pressure
drop resulting in significant savings in the power consumption of the feedwater pump as well as a simpler mechanical
support system for the furnace and less complexity for shop
and field construction and fit-up.
In July 2009, Jinzhushan unit 3 successfully completed
its 168 hour test run and in September 2009, passed its
performance test, proving the successful application of the
world’s first supercritical, pulverized coal-fired low mass
flux vertical tube boiler. This successful demonstration
establishes B&W PGG’s VTUP low mass flux supercritical
boiler technology as a viable alternative supercritical steam
generation technology for the electric utility generation
industry, whether for a downshot or wall-fired pulverized
coal application.
Jinzhushan 3 boiler description
The Jinzhushan 3 boiler is designed to supply main steam
to a 660 MW supercritical steam turbine generator set at a
rate of 1900 ton/hr (4,189.5 klb/h) at 25.4 MPa (3683 psi)
and 571C (1060F) with reheated steam at 569C (1056F). A
“Heater Above Reheat Point” (HARP) steam cycle is used,
providing feedwater to the boiler at 289C (552F). The B&W
VTUP downshot boiler arrangement is illustrated in Fig.
1. The unit is a two pass once-through supercritical boiler
with a vertical tube membrane water wall furnace with a
parallel down pass. Reheat steam temperature is controlled
by biasing flue gas between the reheater and primary super-
heater paths in the downpass. Major dimensions are shown
in Table 1.
Fuel analysis
The coal is delivered from a mine near the plant. The
design coal analysis is as follows:
The unique coal characteristics significantly influence
Ultimate (% wt)
C
49.60
H
O
Proximate (% wt)
VM
3.82
1.71
FC
50.80
1.53
H2O
9.39
S
1.20
Ash
35.99
N
0.58
HHV
19.4 MJ/kg
H2O
9.39
Ash
35.99
Ash Analysis (% wt)
SiO2
53.97
Al2O3
32.00
Fe2O3
4.18
CaO
2.72
MgO
1.35
Na2O
1.00
K2O
1.86
TiO2
1.06
SO3
1.86
the boiler design. The ash is primarily silica and alumina,
which exhibits a very high ash fusion temperature minimizing slagging concerns. However, because silica and alumina
is very erosive in nature and the total content of the ash is
very high, the gas velocities through the convection heating
surface were designed to be very low, approximately 7.5
meters per second.
Low volatile anthracite requires a hotter furnace to sustain
combustion compared to more common, higher volatile,
lower rank coals. The downshot furnace with refractory
lined burner zone provides an effective combination of
long residence time and high temperature. The burners fire
downward into the refractory lined zone of the furnace. The
combustion products then turn upwards to leave the burner
Table 1
Jinzhushan 3 Boiler Dimensions
Furnace
Width – 31.81 m
Depth – 16.6/9.4 m (lower/upper)
Height – 62.1 m
Horizontal Convection Pass
Fig. 1 Jinzhushan 3 VTUP downshot boiler.
2
Reheater pass depth – 4.4 m
Primary SH pass depth – 6.6 m
Babcock & Wilcox Power Generation Group
zone as shown in Fig. 2. As the flame turns upward to exit the
lower furnace, the char reactions return heat to the ignition
area, increasing the effectiveness of the combustion process.
The enlarged lower furnace also provides extended residence
time of the fuel particles allowing the slower burning char
to burn out more completely.
B&W PGG and BWBC have designed and supplied many
downshot furnaces for subcritical boilers. However, there
is a special challenge when designing the downshot furnace
for a supercritical application. The spiral furnace geometry
traditionally used on variable pressure supercritical boilers
is very difficult to adapt to the complex arrangement of the
downshot furnace geometry. A vertical tube configuration
is needed to adapt to the lower arch or burner shelf of the
downshot furnace. To ensure that all the furnace tubes are exposed to nearly even heat distribution through the refractory
covered combustion zone, the corners of the lower furnace
are mitered as shown in Fig. 3. Subcritical downshot furnace
designs have typically used square corners.
The furnace walls are constructed of vertically oriented
membrane tube panels. The lower section of the furnace uses
optimized multi-lead ribbed (OMLR) tubes and extends up
to a set of transition headers at an elevation approximately
midway up the furnace shaft. The transition headers interconnect through mixing bottles which equalize the enthalpy
entering the upper furnace panels. The upper furnace panels
use smooth tubing. Riser tubes extend from the upper furnace
enclosure headers to a manifold header where the fluid is
mixed and routed to two vertical steam separators.
The steam passes from the vertical steam separators
through the furnace roof and then to the horizontal convection pass (HCP) enclosure walls and the rear pendant convection pass (PCP) enclosure panels. The steam from the rear
roof header flows down the rear HCP wall and up the HCP
side and front walls. The flow is then routed to the upper
baffle wall header and down the baffle wall to the primary
superheater inlet. The steam then flows through the primary
superheater, the platen superheater in the furnace and finally
the secondary superheater. Spraywater attemperators are
located at the platen and secondary superheater inlets.
Fuel/Air
Vent Air
Staging Air
NOx Controlled
by Multi-Step
Air Supply
Fig. 2 Downshot furnace.
Babcock & Wilcox Power Generation Group
Particle
Heating,
Drying and
Devolatilization
Main Flame
Development
Sub-Critical
Square
Corners
Supercritical
Mitered
Corners
Fig. 3 Mitered furnace corners.
Reheat steam is first heated in the front section of the
horizontal convection pass then passes through the final reheater sections in the pendant convection pass. The reheater
is arranged with double end inlet and outlet steam connections at the headers. Reheat steam temperature is controlled
during steady-state conditions by biasing gas between the
front and rear parallel gas passes in the horizontal convection
pass using dampers located at the boiler outlet. Attemperation at the reheater inlet provides reheat steam temperature
control during load transients.
Combustion system (fuel and air systems)
To supply pulverized coal to the burners, the boiler uses
a direct firing system that includes twelve gravimetric coal
feeders, six ball mill-type pulverizers and 24 B&W PGG half
primary air exchange (H-PAX™) burners. Twelve burners
are arranged on each front and rear wall burner shelf. Combustion air is supplied by two axial flow primary air fans
and two axial flow forced draft fans. Two trisector-type air
heaters are used to preheat the combustion air. A selective
catalytic reduction (SCR) system with two initially installed
layers and space for one future layer of catalyst is located
above the air heater. The balance of the flue gas and emissions control system includes an electrostatic precipitator
(ESP) and two axial flow ID fans per unit.
Boiler startup system
A schematic of the boiler water-steam and startup system arrangement is shown in Fig. 4. The startup system
equipment consists of two steam water separators, a water
collection tank, a boiler circulating pump and the associated
3
piping and control valves to return the fluid from the water
collection tank to the economizer inlet.
During startup, the unit is operated much like a drum
boiler where water is recirculated to maintain a minimum
flow through the furnace equivalent to 30% of full load flow.
The system is similar to a pumped circulation drum boiler
with the steam water separators and the water collection
tank functioning like the steam drum. The water flowing
through the furnace is a combination of water from the water
collection tank and boiler feedwater. The boiler feed pump
controls the total flow through the furnace so the minimum
required mass flow is maintained. Steam generated through
the furnace circuits is separated from the water in the vertical separator, routed to the superheater and then to either
the steam turbine or the turbine bypass system. The water
from the vertical separator is returned to the water collection tank and then to the circulating pump. The 381 valve,
located at the discharge of the circulating pump, controls
the flow proportionally to tank level to maintain the water
inventory in the collection tank. Water is also recirculated
from the pump discharge to the collecting tank to assure that
the minimum flow required through the pump is maintained.
Fig. 4 Steam/water circuitry and startup system.
4
Babcock & Wilcox Power Generation Group
Above the minimum boiler load (or Benson load) the unit
switches to once-through operation. The circulating pump
is taken out of service but is kept pressurized. A small flow
of feedwater from the economizer outlet is routed to the
circulating pump inlet and back to the separator to maintain
the components in the ready state for use during shutdown.
50% of Primary Air
85%-90% of Coal
Coal
and
Primary Air
H-PAX Vent
50% of Primary Air
10%-15% of Coal
Boiler technology background
Downshot combustion system for low volatile
coal
China has one of the largest reserves of anthracite coal
in the world. Anthracite is a dense, hard, brittle and homogenous type of coal which has relatively higher heating value
(HHV) than lower rank coals. It has the highest fixed carbon
and the lowest volatile matter of any coal type. The low
volatile matter content makes anthracite difficult to ignite
and slow burning. The ratio of fixed carbon (FC) to volatile
matter (VM) is known as the fuel ratio, and is often used to
compare the relative combustion characteristics of different
coals. A coal with a high fuel ratio is typically more difficult
to burn. The fuel ratio for anthracite coal is typically 10:1
whereas this ratio for typical bituminous or lignite coals is
in the range of 1:1 to 2:1.
B&W PGG determined that FC and VM alone are not
good indicators of whether a fuel is difficult to burn. Even
reactive (low fuel ratio) coals have proven to be difficult to
burn if there is excessive moisture and/or ash. This is because high inorganic content can inhibit carbon utilization
by providing a heat sink and interfering with the opportunity
for combustible portions of the fuel to come in contact with
the oxygen. B&W PGG has developed an empirical ignition factor derived from full-scale experience with difficult
coals. This factor relates variations in fuel ratio, moisture
content, ash content, and coal heating value to ignition
behavior. This factor allows the accurate matching of the
proper burner to the fuel.
H-PAX burner
B&W PGG has pioneered the use of the H-PAX burner
for use with burning low volatile coals (Fig. 5). The H-PAX
burner includes several features to improve ignition of the
low volatile anthracite coal. First, the fuel-air temperature
in the ignition zone is increased by directing a portion of
the low temperature primary air away from the coal at
the burner nozzle and injecting it later in the combustion
process. This diverted portion of primary air is referred to
as vent air. Extracting the vent air reduces the amount of
heat needed to support the ignition of the fuel-air mixture,
improving the effectiveness of the ignition process. The
velocity of the pulverized coal stream through the burner
nozzle is determined empirically to optimize the ignition of
the fuel while providing long residence time of the fuel in
the ignition zone. The pulverizers supply the coal with high
Babcock & Wilcox Power Generation Group
Sliding
Air Damper
Secondary
Air
Inner Zone
Spin Vane
Outer Zone
Spin Vane
Fig. 5 H-PAX burner.
particle fineness which increases the heating rate of the coal
particles. The H-PAX burner is equipped with B&W PGG’s
enhanced ignition dual register system which recirculates
gases from the char reaction zone back to the ignition zone
of the flame. Finally, the H-PAX burner develops a fuel-rich
zone near the high temperature secondary air layer which
provides more efficient heating of the coal particles. Staging
air is injected below (downstream of) the vent air, as shown
in Fig. 2, to minimize nitrogen oxides (NOx) formation.
Supercritical boiler
Babcock & Wilcox's (B&W) research in once-through
boilers began at the company’s Bayonne, New Jersey,
laboratory in 1916. B&W increased its research work and
in 1951, established another heat transfer test facility in Alliance, Ohio, capable of operating at 34.5 MPa (5000 psi).
The vision of the high efficiency, supercritical power
plant was also shared by American Electric Power (AEP) and
General Electric (GE). AEP entered into a contract with both
B&W and GE to build the world’s first ultra-supercritical
power plant. With the commencement of commercial operation of the B&W® 125 MW Philo steam generator in 1957,
the United States (U.S.) electric utility industry moved into
the supercritical era.
During the 1960s there was rapid growth in power plant
size with most of the large units using the supercritical
steam cycle. During this period, B&W’s once-through
boiler design capability grew from 125 MW to more than
1100 MW. The year 1972 marked the startup of the world’s
largest supercritical boiler, the B&W 1300 MW unit at TVA
Cumberland. During the next 18 years, B&W supplied and
started up eight (8) additional 1300 MW units, including the
5
AEP Mountaineer unit which holds the world record of 607
days of continuous operation without an outage.
The U.S. utility market originally used supercritical
units for base load duty only, so constant furnace pressure
operation was not a concern. However, today’s market in
the U.S. and world-wide favors units capable of full variable pressure furnace operation, which allows the flexibility
to operate the units either as base-loaded or with on/off or
load cycling. B&W brought the first fully variable pressure
operation-capable unit to the U.S. by converting the Jacksonville Electric Authority Northside Unit 1 from a constant
pressure to a variable pressure operating unit with a spiral
wound furnace in 1981. B&W grew its experience base for
the variable pressure supercritical spiral wound furnace
design with its supply of 2 x 425 MW supercritical units in
Australia, several units in North America ranging from 530
to 800 MW, and BWBC’s supply of 350, 600 and 1000 MW
units in China. Currently B&W PGG and BWBC have more
than 26,500 MW (41 units) of variable pressure supercritical
units in operation or under contract.
B&W PGG now extends its supercritical experience to
the vertical tube universal pressure (VTUP) supercritical
boiler. The concept of a low mass flux once-through variable
pressure furnace design was first introduced by Siemens in
the 1990s. B&W PGG has performed extensive research
over many years and developed a commercial design for
pulverized coal applications based on the Siemens concept.
This research included laboratory testing, in-furnace heat
flux testing, as well as detailed thermo hydraulic performance modeling. The concept of a low mass flux vertical
tube furnace has been applied in the industry for subcritical
once-through PC boiler applications. A supercritical low
mass flux design adds a special challenge as fluid side heat
transfer rates reduce when the furnace is operated near critical pressure (22.1 MPa/3208 psig). B&W PGG’s design has
been proven by more than a year of successful operation of
the world’s first supercritical PC-fired low mass flux VTUP
boiler, Jinzhushan unit 3.
In addition to the Jinzhushan unit 3 project, BWBC has
four other downshot-type VTUP boilers under contract.
China Guodian Corporation Xingyang units 1 and 2 are
scheduled to achieve first fire in late 2010 and early 2011.
Guizhou Xingyi Power Development Xingyi units 1 and 2
are scheduled to start up in 2012. All are 600 MW downshot
units with steam conditions of 571C /569C (1060F/1056F).
In addition to the downshot contracts, the technology is ready
to be applied to a wall-fired design project.
Guohua Diandong Power Co for their Diandong power plant
located in Yunnan Province. B&W PGG and BWBC have
sold a total of 37 anthracite-fired downshot units (16,800
MW) to customers in Canada, China and Vietnam.
With the central utility planning for China requiring the
use of the more efficient supercritical steam cycle technology for new coal-fired power plants, B&W PGG and BWBC
developed an enhanced version of its 600 MW downshot
boiler by adapting B&W PGG’s VTUP boiler technology.
The differences between a natural circulation and a oncethrough boiler are shown in the simplified configurations
illustrated in Fig. 6. In a natural circulation drum boiler, the
steam flow from the boiler is controlled by the fuel firing
rate. The steam temperature from the boiler depends on the
sizing of the superheating surfaces in the boiler. The flow
of the fluid within the furnace tubes is obtained through the
density difference between the steam-water mixture in the
furnace tubes and the water in the downcomers of the circulation system. Adequate circulation flow within the furnace
tubes must be obtained to properly cool the furnace enclosure
tubes. With a once-through boiler furnace, the steaming rate
of the boiler is established by the flow from the boiler feed
pump, and the superheat temperature is controlled by the
feedwater flow rate in conjunction with the fuel firing rate.
B&W PGG has developed several types of furnace
designs for once-through fossil fuel boiler applications.
Fig. 7 provides a comparison of the three (3) basic types
of B&W PGG once-through boiler furnaces. B&W PGG
has experience with both vertical tube and spiral wound
furnace designs. The multi-pass vertical tube design was
developed in the late 1950s and has been proven by more
than 600 designed and operating worldwide. B&W PGG’s
multi-pass design, known as the UP boiler, utilizes multiple
flow passes in the furnace with a constant pressure high
mass flux furnace design. The multi-pass UP uses variable
pressure operation in the superheater to match the variable
pressure requirements of the steam turbine over the operating load range.
The proven spiral wound boiler design has been used
worldwide on over 425 designed and operating units. The
Natural Circulation
(Drum)
Once-Through
(Benson)
Advanced VTUP furnace circuitry
Evolution of the Downshot Circulation Design for
Jinzhushan 3 B&W PGG’s development of the downshot
boiler in China started in 1986 with the sale of 2 x 350 MW
subcritical natural circulation-type boilers to Huenang International Power Development Corporation for the Shang-an
power plant. In 2003, the product was scaled up to 600
MW when BWBC sold four subcritical downshot boilers to
6
Fig. 6 Boiler circulation types.
Babcock & Wilcox Power Generation Group
Fig. 7 B&W supercritical, once-through boiler designs.
B&W PGG spiral furnace design, known as the SWUP™
boiler, utilizes a spiral wound furnace where the tubes are
wrapped around the furnace periphery at a specific angle
so that each tube in the spiral portion of the furnace makes
approximately one turn around the boiler walls. The wrapping of the tubes around the furnace provides nearly equal
heat absorption for each tube in the spiral portion. This
averaging effect minimizes the heat absorption difference
encountered by tubes around the furnace perimeter in a
vertical tube furnace resulting from the inherent imbalances
in the combustion system. The spiral wound furnace tubes
typically transition to vertical tubes in a region of the furnace
just below the furnace arch. The vertical tubes in the upper
furnace run from this transition point up to the furnace roof.
The B&W PGG SWUP boiler has the advantage of operating with full boiler variable pressure and is used for base
load, load cycling and on/off cycling operation. The newest
once-through design offered by B&W is the low mass flux
vertical tube VTUP boiler, which also operates with variable
furnace pressure and can be used for base load, load cycling
and on/off cycling operation.
Another type of furnace design using vertical tubes that
has been recently developed and supplied in the industry is
the medium mass flux furnace design. The promising and
evolving development of the VTUP was compared to a
medium mass flux furnace design. As shown in Fig. 8, the
medium mass flux furnace design exhibits negative or forced
circulation flow characteristics. With forced circulation flow
characteristics, the mass flux of fluid within a tube decreases
when the heat applied to that tube increases. This reduced
mass flow in tubes exposed to higher heat flux upsets in a
medium mass flux furnace results in less cooling and a hotter
tube, potentially risking overheat failure of that tube. This
can result in a less conservative design that will require
a specific and detailed circuit orifice strategy to ensure
adequate distribution of flow to all circuits of the furnace
needed to achieve sufficient cooling of those circuits over the
load range and operating conditions for the unit. As shown
in Fig. 9, the medium mass flux circulation design does not
provide much design margin over the lower design limit for
a given furnace tube. In comparison with the high mass flux
furnace design such as the B&W PGG’s multi-pass UP and
Babcock & Wilcox Power Generation Group
Fig. 8 Mass flux effects.
SWUP boilers, the medium mass flux furnace design has a
lower design margin.
By contrast, the B&W PGG VTUP design has natural
circulation characteristics, where the mass flux of the fluid
in the tube increases with increasing heat flux to provide an
increased cooling effect for tubes that are exposed to heat
flux surges. This effect provides a natural self-adjustment
characteristic that is more resistant to the inherent variations in combustion within the furnace from unexpected
turbulence as well as different burner and pulverizer firing
combinations and when changing from one fuel to another.
The low mass flux design will also result in more even
enthalpy leaving the tubes around the furnace perimeter,
and tube metal temperatures will be more balanced than
with other vertical tube type variable pressure furnaces.
Therefore, the B&W PGG VTUP furnace design was chosen
for Jinzhushan 3. The low mass flux and the vertical tube
geometry provided a combination of features that best fit the
needs of the supercritical downshot application.
The low mass flux feature provided lower overall furnace
pressure drop and natural circulation characteristics across
all boiler operating loads. The natural circulation characteristics allows for the fluid flow to distribute itself to the
furnace circuits based upon each circuit’s heat absorption.
Circuits with more heat absorption draw more flow. One of
the disadvantages of the multi-pass, spiral and medium mass
flux designs is a higher furnace pressure drop, which requires
Fig. 9 Circulation characteristics of design mass flux.
7
greater feed pump power, reducing the net generating efficiency of the supercritical steam cycle. Other drawbacks of
the multi-pass vertical tube furnace design are that it is not
capable of full variable furnace pressure operation, reducing
the cycle efficiency at partial loads. The multiple pass and
spiral furnace configurations are also not easily adaptable to
the unusual furnace geometry of the downshot boiler. The
VTUP furnace design proved to be the most advantageous
fit for the Jinzhushan downshot boiler.
Advantages of the B&W PGG VTUP furnace design
include:
• Full variable pressure operation - best net heat rate
at all loads
• Same gas-side arrangement as subcritical and other
supercritical designs
• Natural circulation flow characteristics (flow increases
with heat absorption)
• Low tube-to-tube temperature differences at the furnace circuit exits
• Low water-steam pressure drop - feed pump power
savings (more than 1MPa lower pressure drop than
high mass flux designs)
• Simple construction and support system - easier to
manufacture, erect and maintain
• Good fuel flexibility – can be designed to operate with
a wide range of coals
• Forgiving operation – can accommodate a wide range
of upset operating conditions
Application of B&W PGG VTUP to the Jinzhushan downshot furnace design
The Jinzhushan 3 downshot furnace design has an arrangement with 35mm OD OMLR SA213T12 tubes in the
lower furnace and 28mm OD smooth 15CrMo tubes in the
upper furnace. The fluid leaving the lower furnace tubes is
fully mixed before entering the upper furnace tube circuits.
The tubes in the burner zone region of the lower furnace
are covered by refractory. Since the furnace cross-section
is rectangular and the corners are mitered, the heat flux
around the furnace perimeter was carefully assessed and
applicable adjustments were made in the evaluation of the
circulation design. Heat flux variations due to potential upset
conditions were also evaluated to achieve acceptable metal
temperatures at all loads. Computational combustion models
of the furnace design were developed to obtain vertical and
horizontal heat flux distributions. Fig. 10 shows the heat flux
distribution in the Jinzhushan 3 furnace for full load firing
conditions with all burners in service.
The circulation design was evaluated for loads from 30
to 100% of the maximum rated load for the boiler (BMCR).
Balanced and unbalanced firing schemes were evaluated
and used for checking the maximum expected tube metal
temperatures for the operating range of loads. Stability
evaluations were performed on the unit for the most severe
circuits in the lower furnace. The tube stability within the
panels as well as the panel to panel stability were met. Fig.
8
Fig. 10 Jinzhushan 3 heat flux profile.
11 shows typical stability plots for two different front wall
circuits in the Jinzhushan furnace at the 40% load case. The
stability graphs show that flow variations caused by a heat
disturbance to the circuit diminish relatively quickly without
the need for additional equalization/stabilization measures
such as the use of tube inlet orifices or an equalization/
stabilization header.
Jinzhushan 3 operating results
The Jinzhushan unit 3 boiler completed its 168 hour
reliability run on July 4, 2009. The boiler performance test
was completed on September 12, 2009. The performance
test results show all design values were achieved. Emissions
values of 700 to 900 mg/Nm3 for NOx were obtained. Boiler
efficiency was 91.7%, which exceeded the guarantee of
91.1%. The carbon content of the fly ash was 3 to 4 % which
makes the fly ash marketable for the concrete industry. The
carbon content of the bottom ash was 2 to 3%.
The two keys to once-through boiler furnace water-side
design are maintaining uniform tube-to-tube temperatures
and maintaining adequate heat transfer on the inside of the
tubes in high heat flux areas. The Jinzhushan boiler operation
shows that both of these design goals are met.
Fig. 12 shows the measured fluid temperatures at the
outlet of the lower furnace at 450 MW and 600 MW load
conditions. The largest temperature differential between
tubes is 18C. Historically, many vertical tube once-through
boilers operating at subcritical pressures would have tubeto-tube temperature differentials in excess of 80C. High
tube-to-tube differentials produce severe stresses which
can, in turn, lead to possible tube failures. The low tube
temperature differentials indicate that the low mass flux
natural circulation design concept used for the Jinzhushan
3 boiler is operating as expected.
B&W PGG installed heat flux instrumentation and additional thermocouples on the lower furnace to help evaluate
the functional operation of the VTUP furnace. The heat flux
instrumentation was installed in critical areas above the
refractory covered burner zone (Fig. 10). Measurements
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Fig. 12 Lower furnace outlet fluid temperatures.
Fig. 11 Jinzhushan 3 circuit stability plots.
indicate that heat flux values are in line with values used to
design the furnace metals.
Ribbed tubing has been used for many years in high
heat flux zones of boiler circuits to create turbulence along
the tube wall, cooling the tube to prevent overheating that
can be caused by departure from nucleate boiling (DNB).
Traditionally, the extruded multi-lead ribbed (MLR) type
tubing has been used for once-through supercritical furnace panels. Siemens has developed a specification for an
optimized multi-lead ribbed (OMLR) tube that can also
be extruded which provides more effective cooling, but is
more expensive to produce and has higher pressure drop
characteristics than MLR tubes.
B&W PGG uses an MLR tube specification that is similar
to the OMLR tube in rib height, differing from the OMLR
tube primarily by the larger rib lead angle. Fig. 13 shows
the differences between two similar sized MLR and OMLR
tubes. As a result of the reduced rib lead angle, the OMLR
tube will have a higher pressure drop for an equivalent
tube size and length when compared to the MLR tube. The
heat transfer performance of the OMLR has been shown
to be somewhat better than the MLR tube. However, when
evaluating alternative furnace designs, the mass flux required
to meet the design metal temperature of the tube using an
Future development of the downshot VTUP
furnace design
Since the successful operation of Jinzhushan 3, B&W
PGG has been advancing the circulation design of the
furnace. Future projects will utilize B&W PGG’s patented
concept of incorporating multiple lead ribbed (MLR) tubes
within the furnace design where different combinations of
ribbed (MLR and/or OMLR) and smooth tubes can be used
in sliding pressure once-through boiler furnace circuits to
optimize the circulation performance. A combination of
tube types can be selected based on the best evaluation of
economics while providing a design that meets required
tube metal temperatures across the load range of the boiler.
The optimized furnace design typically has lower furnace
pressure drop and better flow stability at lower loads while
still maintaining natural circulation flow characteristics at
all loads. In most cases, the B&W PGG MLR tube can be
used without the OMLR tubing.
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Equivalent Rib
Height
Reduced Rib
Lead Angle
B&W Multi Lead Rib Tube
Optimized Multi Lead Rib Tube
Fig. 13 Multi-lead rib tubes.
9
MLR tube is almost equivalent to that required using an
OMLR tube.
In Fig. 14, the performance of the MLR to OMLR tube
is shown for a typical furnace design. These curves show
the minimum required mass flux for a typical furnace tube
throughout the load operating load range. The performance
of the OMLR and the MLR tubes can be seen to be similar
except between approximately 50 to 75% load. The furnace
is typically operating at critical pressure near 70% load. The
upset heat flux value used in this figure is determined at
selected load points and represents the maximum heat flux
expected in the critical regions of the furnace wall. The mass
flux data for the curves in Fig. 15 are based on using the upset
heat flux and the furnace operating pressure for each load.
The mass flux data for the curves in the figure is generated
for an OMLR and MLR tube of the same size. Since the
mass flux required for the furnace would be set by meeting the minimum mass flux at all loads, and the difference
in minimum mass flux between MLR and OMLR tubes is
relatively small, a tube size can be selected that would allow
the use of either type of tube. Therefore, a furnace design
can be optimized to use MLR, OMLR or a combination of
OMLR and MLR. Because the MLR tube has lower pressure drop characteristics than the OMLR tube, a design that
can use MLR tubing will have a performance advantage.
The next generation of downshot furnaces will incorporate the use of MLR tubing in combination with OMLR in
the furnace circuits. The circulation design results in a boiler
that has a slightly higher overall average mass flux through
the furnace circuits, but will still have natural circulation
characteristics, a lower overall pressure drop and a boiler
design that has better stability characteristics at low loads.
Fig. 15 shows that an unstable circuit that uses all OMLR
tubes can be made stable by using a combination of OMLR
and MLR in the lower furnace. For this application, the
results shown in the upper stability plot in Fig. 15 show
the unstable flow characteristics for a furnace circuit that is
designed with only OMLR tubing. Correcting the instability
would normally require either using an equalization/stabilization header or circuit tube inlet orifices. However, by using
the OMLR and MLR furnace design, the flow characteristics
given in the lower stability plot in Fig. 15, show that the
Fig. 14 Comparison of MLR to OMLR tubes.
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Fig. 15 Circuit stability improvement with combination of MLR and
OMLR tubes.
furnace circuit would be stable. Improved flow stability
with the OMLR and MLR furnace design is achieved by
incorporating the lower pressure drop characteristics of the
MLR tube in the appropriate locations of the furnace while
still maintaining good heat transfer characteristics within the
furnace. Use of all MLR for the entire lower furnace may
also be developed in the future.
VTUP PC wall-fired design
B&W PGG has used the experience gained from the successful operation of the furnace design of the Jinzhushan unit
3 downshot supercritical boiler, to adapt the VTUP design
for wall-fired pulverized coal applications. A traditional PC
wall-fired boiler has some additional challenges, but the
principles of design and operation proven on the downshot
design remain the same.
A wall-fired PC boiler does not have refractory covering the furnace walls in the burner zone like the downshot
design. Therefore, the furnace tubes are exposed to high
rates of heat flux over a larger portion of the tube wall. As
a result, the enthalpy rise of the fluid passing through the
furnace tubes is greater and the upset steam temperatures
leaving the lower furnace tend to be higher. However, the
thermohydraulic principles of the VTUP design remain the
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same and are predictable and repeatable. In a wall-fired application, the tube materials may need to be of a higher alloy
grade, and/or the tube diameter and the mix of materials may
be somewhat different than on the downshot.
Additionally, the path and geometry differences between
straight tubes and tubes bent around burner openings, is
more significant on a wall-fired boiler than a downshot. In
a wall-fired boiler burners are arranged in columns such that
the tubes that run through those burner columns must bend
around multiple burner openings, while tubes between burners remain straight. The relative resistance to flow through
the tubes located in the burner column is significantly greater
than in a downshot unit where there is typically only one
burner opening to pass around. This means that there is
inherent potential for a wider range of flow between tubes
in a given circuit. However, this resistance can be modeled
and predicted accurately such that tube size and materials
can be selected to accommodate this difference.
B&W PGG has developed a 600-700 MW VTUP design
for a PC wall-fired application which will utilize only MLR
tubes and does not require any OMLR tubes. The design
will still have natural circulation flow characteristics to
accommodate heat flux upsets in the furnace and operates
with stable flow at low loads. The VTUP furnace can also
be applied to 350 to 1000 MW PC-fired boilers and can be
used for a variety of world-wide coal types. The operational
characteristics of the furnace for these boilers are expected
to be similar to the Jinzhushan 3 downshot boiler and will
offer the same advantages as those of the downshot VTUP
furnace design.
Advanced steam cycles
Today’s new coal fired power plants are also facing
greater pressures to minimize carbon dioxide (CO2) emissions leading to more focus on carbon capture and sequestration (CCS) technologies which reduce the net generation
efficiency of the plant. To minimize the efficiency impacts
of CCS there is an international drive for advanced ultra
supercritical (A-USC) plants with ever higher efficiency.
The situation in the U.S. is used as an example. The net efficiency of the current fleet of coal-fired plants in the U.S.
is around 36 to 39% (HHV). The goal of the U.S. advanced
plant development project is to achieve net efficiencies in
the range of 46 to 47% (HHV). Increasing cycle efficiencies
to these levels along with advancing the CCS technology
should nearly offset the increased parasitic energy of the
CCS process and bring net plant efficiencies for new A-USC
plants equipped with CCS in line with the current aggregate
fleet efficiency.
The current state-of-the-art for supercritical units
have main steam temperatures ranging up to about 605C
(1121F), and hot reheat temperatures ranging up to about
620C (1148F). The highest main steam pressures of about
30.5 MPa (4423 psi) have been designed in Europe while
the highest main steam and reheat temperatures have been
commissioned in Japan. The most advanced cycle conditions
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for the current market in the U.S. have been developed for a
project in which B&W PGG is supplying a boiler designed
with outlet steam conditions of 26.1 MPa (3785 psi) and
602C/608C (1116F/1126F). These USC steam conditions are
approaching a practical upper limit for steam temperatures
while still allowing use of ferritic outlet header materials.
For significantly higher service temperatures, advanced
austenitic steels or nickel based super alloys will be needed.
Nickel based alloys offer an alternative to austenitic steels,
but are higher in cost and the expense needs to be evaluated
and justified. The key technology improvement needed to
realize the A-USC plant is the development of stronger,
high-temperature materials, capable of operating under high
stresses at very high temperatures.
Either the spiral wound SWUP or vertical tube VTUP
furnace design can be used for the A-USC boiler design.
Selection of the type of furnace design will be based on an
economic evaluation of required materials of construction
and functional performance for each alternative.
Europe, Japan, and the U.S. all have programs to develop
materials for use in advanced ultra-supercritical power
plants. The goal of the European Thermie AD 700 program
on advanced steam power plants is to identify materials for
use in steam at 700C/720C (1292F/1328F), 37.5 MPa (5438
psi). The Japanese Cool Earth initiative is attempting to develop coal-fired power plants capable of operating at a range
of temperatures from 700C (1292F) to 750C (1382F). The
U.S. DOE/OCDO project has a goal of developing materials
for a 732C/760C (1350F/1400F), 350 bar (5075 psi) plant.
All of the programs have multiple facets for development and
testing which lead up to the goal of a commercial advanced
ultra supercritical plant.
The European program has completed first round testing
in a component test facility. The COMTES700 project was
completed at the Scholven F Plant of E.ONKraftwerke, in
Gelsenkirchen, Germany. The test loop consists of an HP
bypass valve, startup valve, stop valve, safety valve, and
evaporator and superheater panels. It produces up to 36
tons/h (79.4 klb/h) of steam at temperatures greater than
700C (1292F) and pressures up to 220 bar (3190 psi). The
COMTES700 test loop had been in operation since July
2005. Subsequent inspections found cracking in alloy 617
material. These results emphasize the need for further development. Eon, Gelsenkirchen, Germany, had previously
announced the planned development of a 500 MW A-USC
demonstration plant at Wilhelmshaven. This 700C plant
which would have started in 2014 is now delayed due to
market conditions and technical reasons.
B&W PGG is a consortium member of the U.S. Department of Energy (DOE) / Ohio Coal Development Office
(OCDO) “Boiler Materials for Ultra Supercritical Coal
Power Plants” program. The specific objectives of the Ultra
Supercritical Materials Project are to:
• Identify materials performance issues that limit operating temperatures and thermal efficiency of coal-fired
electricity generating plants.
• Identify improved alloys, fabrication processes, and
11
coating methods that will permit boiler operation of
steam temperatures up to 760C (1400F) and steam
pressures up to 37.9 MPa (5500 psi).
• Work with alloy developers, fabricators, equipment
vendors and power generation plants to develop cost
targets for the commercial deployment of the alloys
and processes developed.
• Define issues impacting designs that can permit power
generation at temperatures greater than or equal to
870C (1600F).
• Lay the groundwork for ASME Code approval.
The U.S. DOE/OCDO project is proposing COMTEST1400, a component test facility similar to the European
COMTES700, for demonstrating proof of concept for very
high temperature components. The test facility, which is
proposed to be in a slip stream off of an operating unit,
would operate at up to 350 bar (5075 psi) and 760C (1400F).
The proposed timeline for the COMTEST1400 project is
between 2010 and 2015. The experience gained from the
component test facility would lead into future projects for
an air-fired A-USC demonstration plant, and subsequently
a demonstration of an oxy-combustion A-USC plant. The
air fired A-USC demonstration plant is proposed to operate
from 2015 to 2020 while the oxy-combustion A-USC demonstration should take place in the 2020 to 2025 time period.
Summary
Both the SWUP and VTUP boiler designs can be supplied
to meet the market requirements of today’s supercritical
pulverized coal applications. The SWUP boiler represents
a well-proven solution that provides consistent heat distribution to all tubes flowing through the furnace circuits
making it insensitive to the natural variations that occur in
furnace heat flux patterns that result from variations in fuel
characteristics and types burned, boiler cleanliness conditions, air staging for combustion NOx control, and changes
in pulverizer operating conditions. The VTUP boiler offers
a solution that takes advantage of the benefits of natural
circulation where tubes exposed to surges in heat flux will
draw more flow to cool the tube instead of reducing flow due
to higher resistance. The successful operation of Jinzhushan
3 demonstrates that this process works as expected and offers
an alternative that has advantages of simpler construction,
a less complicated wall support system, and lower furnace
pressure drop contributing to more efficient net plant electric generation. Both the SWUP and VTUP furnace designs
represent good alternative technologies for supercritical
boiler applications that can be used to meet the needs of the
modern power plant.
References
1. J.W. Smith, “Babcock & Wilcox Company Supercritical (Once-Through) Boiler Technology,” published
1998.
2. DK McDonald/Jean-Pierre Tranier – Oxy-Coal is
Ready for Demonstration – Babcock & Wilcox/Air
Liquide – 35th International Technical Conference on
Clean Coal & Fuel Systems.
3. J. Wheeldon - Engineering and Economic Evaluation of 1300°F Series Ultra-Supercritical Pulverized
Coal Power Plants: Phase 1 EPRI - Technical Update,
March 2008.
4. R. Viswanathan, J.F. Henry, J. Tanzosh, G. Stanko,
J.Shingledecker, and B. Vitalis, “U.S. Program on
Materials Technology for USC Power Plants,” Fourth
International Conference on Advances in Materials
Technology for Fossil Power Plants, October 2004,
Hilton Head, South Carolina, U.S.A.
5. A.J. Bennett, P.S. Weitzel, “Boiler Materials for Ultra
Supercritical Coal Power Plants Task 1B, Conceptual
Design, Babcock & Wilcox Approach” – Topical Report, February 2003.
H-Pax, B&W, UP, VTUP and SWUP are trademarks of
Babcock & Wilcox Power Generation Group, Inc.
Benson is a trademark of Siemens AG.
Copyright © 2010 by Babcock & Wilcox Power Generation Group, Inc.
a Babcock & Wilcox company
All rights reserved.
No part of this work may be published, translated or reproduced in any form or by any means, or incorporated
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Box 351, Barberton, Ohio, U.S.A. 44203-0351. Or, contact us from our Web site at www.babcock.com.
Disclaimer
Although the information presented in this work is believed to be reliable, this work is published with the
understanding that Babcock & Wilcox Power Generation Group, Inc. and the authors are supplying general
information and are not attempting to render or provide engineering or professional services. Neither Babcock
& Wilcox Power Generation Group, Inc. nor any of its employees make any warranty, guarantee, or representation, whether expressed or implied, with respect to the accuracy, completeness or usefulness of any information, product, process or apparatus discussed in this work; and neither Babcock & Wilcox Power Generation
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