State of the art CFB technology for flexible large scale utility …€¦ ·  · 2018-04-19The fluid temperatures were carefully ... During boiler start-up and shut down a circulation

Kalle Nuortimo

Amec Foster Wheeler

Varkaus, Finland

Presented at

PowerGen Russia

Moscow

Russia

3-5 March 2015

© Amec Foster Wheeler 2015.

State of the art CFB technology for flexible large scale utility power production

ABSTRACT

Circulating fluidized bed (CFB) has emerged among the leading combustion technologies for utility-scale, solid-fuel-fired power plants, especially when multi-fuel capabilities are required. Established benefits of high efficiency and reliability, supreme fuel flexibility, with low emissions and installation costs, have been demonstrated on nearly 440 Amec Foster Wheeler’s CFB boilers including both, subcritical and once-through supercritical boilers. CFB technology is a efficient method with which to generate large scale power with a broad variety of solid fuels. This makes CFB technology ideal to meet the current challenges in the power production markets. This paper will discuss the background behind, and reasons for, the success of CFB technology. This will be achieved by highlighting the key market drivers behind CFB technology’s development, and the technical challenges that needed to be overcome during its development in the past 30 years. CFB technology is currently available in large scale sizes (up to 800 MWe) for a wide array of solid fuels. CFB technology has proven to be ideal for firing a variety of solid fuels, such as bituminous and anthracite coals, lignite, petroleum coke, peat and biomass. This fuel flexibility, the possibility of fuel switching, and the possibility of co-firing are some of the significant economic advantages behind the technology. Users of CFB technology are able to choose/use the most cost effective fuel at any given time. This opens up the opportunity to use lower cost local fuels for power generation instead of high quality fuels with higher transportation costs. Furthermore, CFB technology is well positioned to meet the current pressure for increased fuel- and operational flexibility in the power production sector that has been caused by the rise of renewable power, such as solar and wind. Finally, this paper will presents latest utility CFB technology references built or are under construction to meet the requirements of the current power production market, such as the Samcheok Green Power plant (the world’s largest CFB technology based power plant (4x550 MWe) that both fires coal and co-fires biomass), and the lignite fired Kladno (135 MWe) power plant in Czech and lignite fired Kolin Soma plant (2x225MWe) in Turkey. Keywords: Utility CFB technology, Coal combustion, Renewable Energy

1 INTRODUCTION

New boundaries for power plant operation are being set in the global energy production markets.

Several international factors have led to increased pressure to develop power plant technology that is

capable of both fuel flexible and highly efficient operation.

The shale gas boom in the US has lowered the global coal price, making coal plants more economical

than gas plants and favouring solid fuel firing in large scale in rest of the world. At the same time,

initiatives for the reduction of greenhouse gas emissions have encouraged the use of mixtures of

biomass with other solid fuels. Also air pollution regulation has been tightening on a global scale,

resulting stricter emission limits for large combustion plants. This means that boiler technology needs to

be developed in order to be able to increase fuel flexibility and meet the stringent emission limits.

To achieve these goals, supercritical steam parameters and specialised boiler designs have been

developed. In particular, CFB technology has been a focus of development due to its capacity for

economical large utility power production and its enhanced environmental and operational performance

in terms of reduced fuel needs, reduced pollutants emitted and the capability to adapt to sudden

changes in the power demand.

Circulating Fluidized Bed (CFB) boiler technology has been growing in size and number (Figure 1) over

the past three decades and it has now established its position as a viable and ideal utility scale boiler

technology.

Figure 1. Amec Foster Wheeler CFB reference (as by Jun, 2014).

2 DEVELOPMENT STEPS TO ACHIEVE A UTILITY SIZE CFB

2.1 Technology development background

CFB technology development started with small demonstration units constructed in the 1970s. Following

this, the size of the technology increased, with industrial size units constructed in the 1980s and utility size

units of over 200 MWe constructed in the 1990s. Since the 1990s, CFB technology has increasingly

challenged the popularity of Pulverised Coal (PC) technology in large scale energy generation. Today,

there are over 30 Amec Foster Wheeler CFB units firing a wide range of fuels of a scale of over 200 MWe

in operation, or under construction, worldwide. Furthermore, CFB technology has now advanced to the

point where it is possible to construct once through units (OTU) utilising supercritical steam parameters.

Amec Foster Wheeler has built up extensive experience of CFB power plants which utilise supercritical

steam parameters with once-though steam cycle technology. Examples of this are the Łagisza power plant

(460 MWe) in Poland, the Novocherkasskaya power plant (330 MWe) in Russia, and the latest Samcheok

power plant (4 X 550 MWe) in South Korea. These references are shown in Table 1 below.

Table 1. Amec Foster Wheeler OTU CFB references

Country In

production

MWe Main fuel

Łagisza, Poland 2009 460 Polish Bituminous coal

Novocherkasskaya,

Russia

2015 330 Russian Anthracite, Bituminous

coal

Samcheok, South Korea 2015 4 x 550 Indonesian Sub-Bituminous coal,

biomass

These examples demonstrate that Amec Foster Wheeler is on the cutting edge of CFB technology

development and has the capability to design and construct utility size CFB technology boilers with

supercritical steam parameters and once-through technology. Knowledge and experience has been

achieved by committing to CFB technology and its continuous and determined development work, which

has included an experience database of over 400 reference boilers in operation. Emphasis has been on

the mechanical design issues of CFB technology and on understanding the process conditions affecting

heat transfer, flow dynamics, combustion characteristics, gaseous emission control, and thermo

hydraulics. By carrying out work in bench-scale test rigs and pilot plants, field testing of operating units,

model development, and simulations carried out using developed semi-empirical models or more

theoretical models, a detailed understanding of these processes has successfully been built up. This has

led to development of detailed design criteria for larger units, which has been successfully implemented in

boiler projects. Design criteria has also been supported by data collected, model development work, and

correlations with conventional boiler design. Figure 2 below shows the development of the size of CFB

boilers.

Figure 2. Increase of the size of CFB boilers

The viability of CFB technology and its main design components has been proven in utility scale power

production. The scale-up of CFB technology with super-critical steam parameters of up to 800 MWe is now

technically ready and commercially available. The special features of CFB technology with supercritical

steam parameters are presented in the next chapter.

3 AMEC FOSTER WHEELER LARGE-SCALE OTU CFB TECHNOLOGY

The Basic OTU CFB concept is based on a CFB process that provides high plant efficiency. The concept

incorporates supercritical steam parameters accompanied by Benson vertical tube technology and is based

on in-line boiler arrangement (presented in Figure 3). The furnace and the separators in the design form a

compact hot loop package and the convection pass consists of a steam-cooled enclosure containing the

convection superheaters and reheaters. This is then followed by the economizer and the rotary regenerative

air heaters. The design of the convection pass follows the same principles used in large two-pass PC

boilers. The hot loop and convection pass are connected with steam cooled cross over ducts (CODs).

Figure 3. CFB boiler in-line concept

The water and steam design of the OTU CFB boiler is based on the low mass flux BENSON once-through

technology licensed by Siemens AG. This technology is ideal for the CFB design because it utilises vertical

furnace tubes instead of the spiral wound tubing that is used in many other once-through designs. In proven

CFB designs, natural circulation is achieved by using vertical tubing as the normal arrangement and it is

beneficial to use a similar design for supercritical OTU boilers. The heat transfer rate in CFB boilers is very low

and is uniform in comparison to Pulverised Coal (PC) boilers and the required water mass fluxes are relatively

low. The low heat fluxes also allow the use of normal smooth tubes in the furnace walls with a mass flux of 550 -

650 kg/m2s at full load. The fluid temperatures were carefully analysed after each evaporator tube system in

different load conditions when creating the OTU CFB design and it was found that the low and uniform heat flux

of the CFB furnace and the BENSON low mass flux technology makes the fluid temperatures very uniform.

The OTU CFB design requires the plant to be operated with sliding steam pressure so that the boiler pressure

follows the turbine load. At lower loads (below ca. 70%), the main steam pressure is typically below the critical

pressure (221 bar) and at higher loads the boiler operates at supercritical pressures. During boiler start-up and

shut down a circulation pump is used to ensure that water flow through the evaporator is maintained to ensure

proper cooling. The two-phase flow from the outlet headers of the evaporator walls is collected in the vertical

water/steam separators where the water is then separated from the steam and led to a single water-collecting

vessel (see Figure 4).

When the boiler load exceeds the BENSON point at approximately 30% load, the steam exiting the evaporator

walls is slightly superheated. At this point, the circulation system can be closed and the boiler will have achieved

the once-through operation mode.

Figure 4. Steam circuitry

3.1 Latest milestone in the OTU CFB technology: Samcheok Green Power 4 x 550 MWe

In July 2011, full notice to proceed was given by Hyundai Engineering and Construction for the design and

supply of supercritical Circulating Fluidized Bed (CFB) steam generators for the Samcheok Green Power Project

in South Korea. The contract included the design and supply of four 550 MWe advanced vertical tube, once-

through supercritical CFB steam generators (Figure 5) feeding two steam turbines. The CFB steam generators

have been designed to burn imported coal mixed with biomass whilst at the same time meeting all

environmental requirements. Once the Samcheok CFB units enter commercial operation in 2015, they will be

the world's most advanced CFBs and will provide a new level of fuel flexibility, reliability and environmental

performance.

Figure 5. Samcheok Green Power 4 x 550 MWe

3.1.1 Design details

The Samcheok boiler design is based on OTU CFB concept and follows the same basic design features used in

the Łagisza plant. The boiler material requirements for most sections of the Samcheok boilers are very

conventional and normal boiler materials have been used. Furthermore, the design for the Samcheok boilers is

free of T24-steel.

3.1.2 Design fuel

The CFB steam generators at the Samcheok plant are designed to burn imported coals mixed with biomass

whilst at the same time meeting strict environmental requirements. The fuel that will be primarily used for the

Samcheok plant is sub-bituminous coal sourced from several international coal mines (mainly in Indonesia). The

boilers have also been designed to be able to co-fire wood pellets (Table 2).

Table 2. Fuel specifications

Bituminous Coal Biomass

LHV (a.r.) MJ/kg 14,2 – 24,9 15,8-18

Moisture % 20 – 43 5 – 15

Ash (a.r.) % 1,2 – 15,3 0,7 – 5

Sulphur (a.r.) % 0.1-1 0-0,16

3.1.3 Steam parameters

The steam pressure and temperature that has been selected for the Samcheok plant has been shown to be

viable in other supercritical units and conventional boiler steel materials can be used for the boiler’s

construction. Table 3 below presents the main design steam parameters of the 4 x 550 MWe

(gross) CFB

boilers that will be built at Samcheok.

Table 3. Design Steam parameters at 100 % load

SH flow kg/s 437,7

SH pressure bar(g) 257

SH temperature °C 603

RH flow kg/s 356,4

RH pressure bar(g) 53

RH temperature °C 603

Feed water temperature °C

297

3.1.4 Emission limits

The CFBs will meet the stringent emission values given below in Table 4 without needing any additional back-

end flue gas desulphurisation equipment for SOx control.

Table 4. Emission values

Item Unit Limit value Method to meet

SOx ppm (as SO2) Max. 50 (6% O2) Limestone injection to furnace; no back-

end desulphurization equipment needed

NOx ppm (as NO2) Max. 50 (6% O2) SCR between economizer and air heaters

Particulate matter mg/m3n Max. 20 (6% O2) ESP

3.1.5 Unit Operation

The normal operating mode of the Samcheok unit is co-ordinated control with sliding pressure operation. The

boilers will normally be operated at the same load level and any load change requests will be forwarded to the

boilers simultaneously and with similar control parameters. The steam temperatures will be individually

controlled in order to ensure that the required temperatures in the main steam and reheated steam systems

are achieved. Reheated steam share between the boilers will be continuously monitored and controlled in

accordance with the applicable firing rates.

4 NATURAL CIRCULATION CFB TECHNOLOGY FOR UTILITY POWER PRODUCTION

4.1 CFB boiler technology in Kladno power plant

In 2010, the Swiss utility company Alpiq awarded a 135 MWe lignite fired EPC power plant contract to

Kraftanlagen München (KAM) for the Kladno power plant in the Czech Republic. The project was designed to

replace old technologies with a more efficient and environmentally cleaner plant, producing energy for the local

community whilst meeting the requirements of flexibility from the electrical grid system. For the boiler at the

Kladno power plant, Alpiq required CFB boiler technology that was specifically designed to co-fire lignite and

biomass.

The Kladno boiler represents the new generation of utility power boilers. The Kladno CFB boiler has been

designed to co-fire lignite and biomass with the capability of adapting to sudden load change requirements in

the electricity grid.

The design of the new Kladno CFB boiler (Figure 6) incorporates solids separators built from steam cooled

panels integrated with the combustion chamber. The steam cooled separator design avoids the occurrence of

heavy refractory linings in the separator. The final superheating stage and the final reheating stage are

INTREXTM

heat exchangers located in special enclosures at the bottom of the furnace (adjacent to the main

combustion chamber). The INTREXTM

heat exchangers are located outside the main combustion area, which

enables them to be used as the last superheating and reheating stages. This results in higher steam

temperatures because the INTREXTM

heat exchangers are protected from the fouling and corrosive

environment of the boiler’s hot flue gas. The INTREXTM

heat exchangers also provide high load-following

capabilities and turndown ratios.

Figure 6. Kladno K7 CFB boiler design

The design basis of the Kladno boiler is presented in tables 5-7.

Table 5. Kladno fuel data

Lignite Biomass

Sulphur 1,35% d.s. 0.13% d.s.

Nitrogen 0,67% d.s. 1% d.s.

Moisture 26,6% a.r 40% a.r

Ash 19,78 % d.s. 3,33% d.s.

LHV 15,61 MJ/kg 9,7 MJ/kg

Table 6. Kladno boiler steam data

STEAM DATA

Total Heat Output 303 MWth

Steam Flow 105/102 kg/s

Steam Pressure 133/33 bar(a)

Steam Temperature 541/541 °C

Feedwater Temperature 251 °C

Table 7. Kladno boiler design performance

Flue Gas Exit Temperature 130 °C

Boiler Efficiency 93,2%

Emission* Guarantees 40 % to 100 % BMCR ½ hour average

- NOx

- SO2

- CO

- NH3

- NH3

Particulate Matter

*) Emissions expressed in dry fluegases @ 6%O2

<190 mg/Nm3

<190 mg/Nm3

<95 mg/Nm3

<10 mg/Nm3 (slip cat. installed)

<20 mg/Nm3 (w/o slip cat.)

<20 mg/Nm³

The new Kladno K7 lignite firing CFB boiler unit replaced an old coal-fired unit, which was commissioned at

the Kladno power plant in the late 1970s. The new unit will be operated alongside two 135 MWe CFB units,

which were previously commissioned in the 1990s, and is located adjacent to the old boilers. This allows the

new boiler to utilise many of the existing plant systems, such as the coal handling and water treatment plant.

Scope of work for the CFB boiler included the design of the boiler; the delivery of the boiler house enclosure

with its steel structures, the boiler pressure parts, the auxiliary equipment, the lignite crushers, the fuel silos for

solid biomass fuel and lignite, the fuel feeding equipment for biomass fuel and lignite, and the bag filter; the

erection and construction of the boiler; and the start-up, performance testing and commissioning of the boiler.

The time schedule that was adopted for the project is presented in Table 8.

Table 8. Project execution schedule

Contract Award December 2010

Start of Erection November 2011

Commercial Operation December 2013

The main fuel utilised for the new CFB boiler at the Kladno power plant is lignite obtained from a local mine

(Bilina). The CFB Boiler is designed for biomass co-firing of a maximum of 10 % heat input. Despite the fact that

the lignite is obtained from only one source, there is substantial variation in the fuel quality, especially in terms of

in-organic matter, which results in a dry solids range of 13 to 30 %. For biomass, the variation is even wider,

giving a moisture range of 25 to 55 %. The CFB boiler’s ability to use fuel with such a wide variation in quality

clearly demonstrates its excellent fuel flexibility.

The Kladno CFB boiler incorporates the latest design of solids separators build from water or steam cooled

panels integrated with the combustion chamber. The design also features INTREXTM

superheaters. The high

performance of the solids separator leads to a high solids circulation rate and a uniform combustion temperature

profile across the whole operation range. After commencing commercial operation in December 2013, the

operation of the Kladno power plant has been excellent. According to the performance tests, all performance

guarantees have been fulfilled within a large margin, demonstrating that CFB boiler technology is able to meet

and surpass all the requirements set for a modern utility power unit.

4.2 Harbin Electric International Co. Ltd. (HEI), Soma, Turkey

Amec Foster Wheeler has been awarded a contract by Harbin Electric International Co. Ltd. (HEI) for the design

and supply of two circulating fluidized-bed (CFB) boiler islands and flue gas scrubbers for HIDRO-GEN Energy

Import, Export, Distribution and Trading Inc., a subsidiary of Kolin Group of Companies. HEI is acting as EPC

contractor for HIDRO-GEN’s power project to be built close to lignite mines, near the town of Soma, west part of

Turkey, 135 km north from Izmir (Figure 7).

Figure 7:Location of the Turkish lignite fields and Soma CFB boilers (picture:IEA)

Amec Foster Wheeler has received a full notice to proceed on this project in January 2014. Commercial

operation of the new steam generators is scheduled for the beginning of 2017 (Table 9).

Table 9. Project execution schedule

Contract Award January 2014

Start of Erection April 2015

Commercial Operation January 2017

Amec Foster Wheeler’s scope includes design and supply two 255 MWe (gross megawatt electric) steam

generators and auxiliary equipment for the boiler islands, flue gas cleaning systems with Amec Foster Wheeler

CFB scrubbers, and technical advisory services during erection and commissioning. The CFB boilers will be

designed to burn local lignite, due to the significant economic benefit of using this fuel for power generation. This

is also the largest CFB project awarded in Turkey until today.

4.2.1 Soma boiler design

Soma CFB boilers are designed to utilize local lignite reserves located near Soma town, with design heating

value of 6.77 MJ/kg (see Table 11). Boilers are natural circulation type of drum boilers with reheat (see steam

parameters in Table 10). The CFB boilers design incorporates solids separators built from steam cooled panels

integrated with the combustion chamber. The final superheating stage are INTREXTM

heat exchangers located

in special enclosures at the bottom of the furnace (adjacent to the main combustion chamber)(Figure 8).

Figure 8: Soma CFB boilers

Table 11. Soma boiler steam data

SH flow kg/s 198,5

SH pressure bar(a) 173

SH temperature °C 565

RH flow kg/s 173

RH pressure bar(a) 53

RH temperature °C 565

Feed water temperature °C

262

Table 10. Soma fuel data

DESIGN FUEL DATA Lignite

Sulphur 0,95 % d.s.

Nitrogen 0,53% d.s.

Moisture 23,3% a.r

Ash 42,9 % d.s.

LHV 6,77 MJ/kg

4.2.2 Soma CFB flue gas cleaning

To reach required emission values (Table 12) in economic way, the circulating fluidized bed scrubbing (CFBS)

technology has been applied in the Soma power plant. This is a viable pathway for addressing multi-pollutant

control in a cost effective manner. There are two identical flue gas cleaning lines, which consist of CFB scrubber

including bag-house with solids re-circulation (Figure 9).

Figure 9. CFB scrubber

Table 12. Design performance

Boiler Efficiency 89,98%

Emissions

- NOx

- SO2

- CO

Particulate Matter

*) Emissions expressed in dry fluegases @ 6%O2

<200 mg/Nm3

<200 mg/Nm

<200 mg/Nm

<30 mg/Nm³

Construction costs can be reduced as the major system components can be pre-assembled on the ground and

lifted into place during system erection. The technology provides high pollutant removal efficiencies for SO2,

SO3, HCl and HF. Further the absorber/fabric filter arrangement is highly adaptable for sorbent injection for

removal of heavy metals including mercury. The main advantages of this technology combined with CFB boiler

is reduced operating cost due to lower limestone consumption.

5 DYNAMIC BOILER PERFORMANCE

When designing a CFB boiler, it is important to ensure safe and efficient steady-state operation, high flexibility in

fuel or fuel mixtures to be combusted, and high flexibility in the boiler’s operational range. The increased

requirements for solid fuel fired power plant operation and control dynamics in some of the power markets have

resulted in a particular focus on the development of flexible operation with fast load following in order to fulfil the

load change requirements. There has also been an increased emphasis on boiler designs needing to meet

stringent emission limits.

The trend of needing to increase flexibility requirements in power markets will set new targets for the

development of conventional solid fuel based power plants. New functions and capabilities, such as low load

operation and fast load following, are expected to result from this rapid development, particularly within the field

of CFB technology.

In comparison with conventional PC technology, CFB technology provides the possibility to utilise various fuel

blends. This includes raw lignite together with dried lignite, coal, and mixtures of biomass. Moreover, when firing

these fuels CFB technology is able to meet all of the relevant emission requirements as well as the boiler

minimum load/load change requirements. Consequently, the need for dynamic capability is constantly

increasing. Furthermore, there is also an increasing need to be able to shut down the plant for the night and

achieve a fast start up in the morning.

6 SUMMARY

CFB technology development has undergone extensive development since the 1970s and Circulating Fluidized

Bed (CFB) technology has now established its position as a viable and efficient utility-scale boiler technology.

When considering whether to build new plants, or to repower old plants, efficiency, environmental performance

and operational flexibility are the key issues that developers need to consider. High efficiency means lower fuel

consumption and lower levels of ash and air emissions, including lower emissions of carbon dioxide (CO2). CFB

technology has been proven to be capable of achieving these goals by means of both supercritical steam

parameters and specialised boiler designs for biomass firing in the utility scale. CFB boiler designs have also

achieved a high level of operational flexibility, which represents the next generation in utility power generation.

New more tighten capabilities e.g. for low load operation and fast load following, are expected from current and

new power generation capacity. These aims have been directly taken into account when undertaking CFB

development programs in order to ensure compliance with future load control requirements. All of these

achievements, as well as the further development efforts currently being undertaken, make CFB technology the

optimum choice to meet the market’s demands both now and in the future to utilise a broad range of fuels in

large scale power generation.