CCT Experience of Tong-Hae CFB Boiler Using Korean Anthracite

Jong-Min Lee, Jae-Sung Kim and Jong-Jin Kim

Combustion & Thermal Engineering Group, Power Generation Laboratory, Korea Electric Power Research Institute, Korea Electric Power Corporation

103-16 Munji-dong , Yusung-gu, TaeJon 305-380, Korea

Abstract

The 200MWe Tong-Hae thermal power plant CFB boiler (2-units) in Korea is the largest boiler to fire Korean anthracite for electric power generation. The CFB units 1 and 2 have been operated commercially since October 1998, and October 1999, respectively. The initial operation period of the units clearly demonstrated that the full load operation was successful. However, there were apparently some rooms for optimization, particularly regarding a furnace and cyclones/loopseals temperatures and SOx emissions that were somewhat higher than expected. Also, combustion efficiency was somewhat lower than designed. Some modification of the boiler system such as cyclones, fluidizing nozzles and ash reinjection system was carried out to improve the performance of the boiler. It allowed temperatures of the furnace and cyclones/loopseals and the emissions of SOx to be lower. It also achieved the stable operation and increased the combustion efficiency of the CFB boiler. INTRODUCTION

The design engineering and construction project of the Tong-Hae thermal power plant

CFB(Circulating Fluidized Bed) boiler located in Tong-Hae, Korea, was begun by Korea Electric Power Corporation (KEPCO) to utilize Korean anthracite in early 1993. Combustion Engineering Inc. (ABB-CE) designed a 200 MWe CFB boiler and KOPEC (Korea Power Engineering Company) served as the project architectural engineering. Additionally, Hanjung Heavy Industry Corporation was responsible for the boiler equipment fabrication and procurement [1]. The Tong-Hae CFB boiler was selected over the conventional pulverized coal boiler because of its improved efficiency and air emission capability. The unit of Tong-Hae CFB boiler designed in such a manner that it could be operated more efficiently and could be able to meet the Korean environmental standards without installation of external environmental control systems. Also, the CFB boiler doesn’t need to fire supplemental fuel oil during normal operation. The 2 units of CFB boilers utilize the domestic anthracite as its fuel source approximately 1.2 million ton per year. This will encourage the local coal mining industry of Korea.

The CFB unit 1 and unit 2 have been operated commercially since October 1998 and 1999, respectively [2]. The initial operation period clearly demonstrated that the full load operation was successful. However, there were apparently some rooms for optimization, particularly regarding a furnace and cyclones/loopseals temperatures and SOx emissions that were somewhat higher than expected. Also, combustion efficiency was somewhat lower than design. It was difficult to maintain the stable operation at high temperature due to the formation of clinker in the cyclones/loopseals. Some modification of the system was carried out to improve the performance of the CFB boiler, and allowed to get stable and efficient operation of the CFB boiler.

This paper gives a description of the unit identifying the design features and considerations. The history of operation and modification to improve the performance of the CFB boiler from initial start up to the present is also discussed. POWER PLAN AND STATUS OF CFB BOILER IN KOREA Electricity forecast and long-term power plan in Korea Since the available domestic energy source is very limited, Korea mainly depends on imports for the most of the primary energy source. From the viewpoint of energy strategy and global environmental problem, CCT such as IGCC, FBC and FGD will be essential for the 21st century. The stringent environmental regulation has been implemented since January 1999 in Korea. KEPCO is planning to install some CCT units such as CFBC, PFBC or IGCC by the year 2015. KEPCO expects that the annual growth rate of electricity demand will be about 3.3% on average in the long term from 2002 to 2015. To meet future electricity demands, the power development plan requires to build additional 71 power generating units with a capacity of 32,740MW between 2002 and

2015. These facilities will comprise 12 nuclear units (13,600MW), 16 coal-fired plants (8,900MW), 20 LNG combined cycle units (7,570MW), 21 pumped storage power plants and hydro plants (2,520MW), 2 oil-fired plants (150MW) including some units of CCT power plants.

According to the above program, the installed capacity by 2015 will be 77,023MW. The composition of capacity will be 26,637MW(34.6%) nuclear, 22,240MW(28.8%) coal, 21,762MW (28.3%) LNG and oil, 6,385MW(8.3%) hydro and pump storage power. This is summarized in Table 1.

Table 1. Long-Term Electric Power Plan in Korea [3]

Sources / Year 2001 2005 2010 2015

Nuclear MW (%) 13,720 (27.0) 17,720 (28.6) 23,120 (29.2) 26,640 (34.6)

Coal-fired MW (%) 15,530 (30.5) 18,170 (29.3) 24,270 (30.7) 22,240 (28.8)

LNG-fired MW (%) 12,870 (25.3) 16,810 (27.2) 20,440 (25.9) 19,550 (25.4)

Oil-fired MW (%) 4,870 (9.6) 4,670 (7.6) 4,820 (6.1) 2,210 (2.9)

Hydro MW (%) 3,880 (7.6) 4,490 (7.3) 6,390 (8.1) 6,390 (8.3)

Total MW (%) 50 ,860 (100) 61,850 (100) 79,020 (100) 77,020 (100)

Commercial CFB boiler in Korea Table 2 presents the commercial CFB boilers in Korea [4]. Most boilers in operation are utility scale boilers with a capacity of 100-250 steam tons per hour. The common fuel is a bituminous coal imported from abroad. The 200MWe Tong-Hae CFB plant, which is the largest power plant adopted beyond utility scale in Korea, uses Korean anthracite to demonstrate the capability of anthracite burning and thus encourages the domestic coal industry. Table 2. Distribution of Commercial CFB boiler in Korea

Company (Location) Category Capacity(MW)

Steam(T/H) Fuel Start Supplier

Oriental Chemical (Inchon) Chemical 12.5 120 Bitu. Coal,

Pet. Coke 1985 Hyundai /Ahlstrom

Sunkyung (Suwon) Textile - 130 Bitu. Coal 1988 “

Sunkyung (Ulsan) Textile 27 200 Bitu. Coal, Pet. Coke 1989 “

Hyundai Oil (Susan) Refinery - 120 Pet. Coke 1989 “ LG Chemical (Yochon) Chemicals 25 210 Bitu. Coal 1989 “ Petrochemical Service

Co. (Ulsan) Cogen. - 250 Bitu. Coal, Pet. Coke 1990 “

Sunil Glucose (Inchon) Food 9.2 60 Bitu. Coal, Heavy Oil 1991 “

Dyeing Complex (Taegu) Dye 54.1 130 x

3 Bitu. Coal 1986 Samsung/ Babcock

Jeil Sugar (Seoul ) Food 5.4 40 Bitu. Coal 1988 “ Hansol Paper (Jeunju ) Paper 23 130 Bitu. Coal 1990 “

Dyeing Complex (Pusan) Dye - 80 Bitu. Coal,

Heavy Oil 1991 Daewoo/ B&W

Korea Energy (Ulsan) Metal 43.5 175 Bitu. Coal 1991 Hanjung/Lurgi

KEPCO (Tong-Hae) Electricity 200 x 2 693 x 2 Anthracite 1998

1999 Hanjung/ ABB-CE

TONG-HAE CFB BOILER FEATURES

The Tong-Hae thermal power plant CFB boiler was designed to fire Korean anthracite by Combustion Engineering, Inc. [ABB-CE, 1994]. The Tong-Hae CFB combustor is shown in Fig. 1. It consists of feeding parts of coal and limestone, PA, SA and FA supplier, main combustion part (furnace, cyclones, loopseal, FBHEs and FBAC), and convective backpass.

Furnace

Special consideration was paid to the 200MWe Tong-Hae thermal power plant CFB to minimize the erosion potential in the furnace. Design considerations were the use of a low fluidizing velocity, the use of specially designed tubes at the lower furnace refractory/water wall interface with super hardened alloy coated, and the exclusion of the hanging heat exchanger surface in the furnace due to the erosiveness of the fuel. Some CFB designs have employed velocities of 6 m/sec or more, but Tong-Hae CFB is operated at a much lower velocity (at about 4.6m/sec). These velocities resulted in lower solid impact velocities and lower solid fluidization rates; both of which significantly lessened the furnace erosion potential.

The furnace of the Tong-Hae CFB (19m(W) x 7m(L) x 32m(H)) has a rectangular footprint and is significantly wider than it is deep, incorporating as an aspect ratio of more than 2:1. As all fuel feed points are aligned on the furnace front wall,

the rectangular geometry was chosen to allow for good fuel mixing. Limestone is injected with the fuel in the fuel feed chutes and it is also introduced in two injection ports along the rear wall. Flue gas and solid particles exit the furnace through three openings in the upper rear wall. Solids are returned to the furnace from the cyclones and external fluid bed heat exchangers (FBHE) via the lower front wall.

Backpass

Coal Silo

Boiler

Cyclone

Primary air is introduced to the furnace via the fluidization grate. T-style fluidizing nozzle patented by ABB-CE has relatively large openings to reduce the potential of plugging associated with many nozzle designs, while maintaining a pressure drop to preclude backsifting. Secondary air is introduced at the lower furnace along the front and rear walls. All secondary air nozzles are furnished with butterfly style dampers to maintain a steady secondary air supply-duct pressure. Secondary air is also introduced into four start-up burners. Bottom ash is removed from the bottom of the furnace via two ash control valves (ACV). The bottom ash is introduced into a fluidized bed ash cooler (FBAC) which contains economizer and cooling water heat transfer surface. Heated fluidizing air is returned from the FBAC vents into the CFB combustor at four locations along the front wall. Cyclones and Loopseal Most entrained particles from the flue gas are captured by three cyclone separators and are returned

FBAC

FBHE

Primary Air

Air

Air

Solid Flow

Air

Solid Flow

Gas FlowGas Flow

Windbox

Fig. 1. Tonghae CFB Boiler Feature

to the furnace, passed through the loopseals. The loopseals serve to create a pressure seal from positive pressure in the furnace to the negative pressure in the cyclone. This pressure seal prevents the flow of material back up the cyclone from the bottom of the furnace. Fluidized Bed Heat Exchangers (FBHE) The Tong-Hae unit was designed to operate at 35% load without support fuel and at 30% load with minimal co-firing of the start up fuel oil. This stringent boiler turndown requirement, coupled with the difficulty to burn Korean anthracite, resulted in offering external fluid bed heat exchangers. At each of the loopseal, a stream of solid particles is diverged and introduced into FBHE. As the solid particles from the loopseal flow over the FBHE heat transfer surfaces, the ash is cooled. The ash is then returned to the furnace. By placing superheat and reheat heat transfer surface in separate FBHEs into which solids can be introduced in a controlled fashion, optimum turn down control is accomplished. Convective Backpass The total unit heat duty is distributed in a fairly even fashion among the furnace, backpass, and FBHEs. The convective backpass contains the first stages of superheater and reheater heat transfer surfaces. The convective back pass is steam cooled as used on typical utility boiler designs. A typical air-through, gas-over tubular air heater is located below the convective backpass economizer. Properties of Coal and Limestone and Operating Conditions

Korean anthracite contains 39% ash, 4% volatile matters(VM), 53.7% fixed carbon(FC) and 3.3% moisture(MS). The coal has relatively low content of S (0.6%) and N (0.2%), and has low heating value and the combustion reactivity. The fraction of coal in the range of 0.1-6mm is over 95% on the design basis. Limestone used as a sorbent for desulfurization, contains 90% of CaCO3 and 4.2% of MgCO3. The particle size of limestone is smaller than 1.0mm (<0.7mm-95%, <0.5mm-90%). The proximate and ultimate analysis of the anthracite is shown in Table 3. The operation condition with variation of nominal rates is also shown in Table 4. Table 3. Analyses of design coal used in the Tong-Hae CFBC.

Proximate analysis wt.% Ultimate analysis wt.% (dry basis)

Moisture Volatile Matters Fixed Carbon

Ash Heating Value (dry basis)

3.59 4.12 57.80 34.49

4600~4800 (kcal/kg)

C H O N S

Ash

60.41 0.79 2.16 0.42 0.45 35.75

Table 4. Operation conditions in the Tong-Hae CFB boiler.

Operation conditions aBMCR bMGR 100%cNR 75%NR 50%NR 30%NR

Primary air (kg/h) 382,700 382,700 382,700 334,760 287,740 299,830 Secondary air (kg/h) 256,300 245,890 180,080 77,320 77,320 77,320

FBHE & loopseal air (kg/h) 40,100 40,100 40,100 40,100 40,100 32,210 FBAC air (kg/h) 49,300 49,300 49,300 49,300 49,300 49,300

Bed temperature (oC) 830 ~ 880 Pressure drop (mmH2O) 1400 ~ 1600 (including pressure drop of distributor)

Coal (kg/s) 30.1 29.7 27.3 20.7 14.5 7.9 Limestone (kg/s) 0.92 0.91 0.83 0.63 0.44 0.38

a: Boiler Maximum Continuous Rating , b: Maximum Guaranteed Rating, c: Nominal Rating

OPERATION EXPERIENCES Basic guidelines for start up and operating the CFB boiler to complement control loop tuning and automatic controls have been prepared via some operating experience. Some changes and modifications were required to the procedure based upon the operating conditions encountered at the time. To improve the performance of the CFB boiler, some values and systems have been added in the combustor and boiler air/gas systems.

In the steady-state CFB operation at 100% NR, the achieving conditions are as follows: - Furnace low temperature: 830-880oC - loopseal dipleg temperature: 900-970oC - Furnace overall differential pressure (including grid pressure): 1,400-1,600 mmH2O - Furnace upper differential pressure (ΔL=23.3m; from 6.9 to 30.2m): 100-300 mmH2O - loopseal dipleg pressure: 500-1,000 mmH2O

Initial Experience and Technical Problems

During the initial operating period of the CFB, the following problems were found; longer time initial heat-up of bed material, formation of clinkers in loopseals and cyclones, higher emission of unburned carbon in fly ash, higher temperature profiles and higher SOx emission. These problems may be due to lower combustion reactivity and particle size distribution of Korean anthracite.

The ignition temperature of the coal sized of 43μm was about 510oC, and rose with increasing particle size. This was about 100oC higher than that of bituminous coal under same condition. The activation energy, determined in TGA, was about 51 kcal/mol in the chemical reaction control regime. It showed similar trend obtained in previous study [5]. However, combustion rate of anthracite char was much slower than that of bituminous coal char as compared with previous work [5]. Consequently, the temperature of hind part of the CFB, such as freeboard, cyclones and loopseals, was made to increase by low combustion reactivity and post combustion reaction of the anthracite.

The particle size distribution of the coal, as well as low combustion reactivity, may cause to increase the temperature of freeboard, cyclones and loopseals. The coal included finer (<0.075mm) and coarser (>6mm) particles than PSD designed and is shown in Table 5. Especially, the fine particles were considered more important when the full circulation of the bed material was not achieved. At this time, the temperature of the hind part of the CFB also increased, because the heat of coal combustion was not transferred and distributed well to the wall membrane tube due to low fraction of the bed material in the freeboard. Consequently, the high temperature operation caused to form clinkers in the loopseals, and to deactivate the sorbent (CaO) for desulfurization reaction. The unburned carbon fraction in the fly ash was also high because of short residence time of fine particles in the combustor. Table 5. Particle size distribution of design and sampled coal (#1, 2, 3) in field. Size distribution (mm) Design PSD (wt.%) #1-PSD (wt.%) #2-PSD (wt.%) #3-PSD (wt.%)

>9.5 5.6 – 9.5 4.75 - 5.6 2.8 – 4.75

2 – 2.8 1.0– 2.0 0.6 – 1.0 0.25 – 0.6 0.1– 0.25

0.075 – 0.1 <0.075

0 0

1.0 2.0

16.0 31.0 16.0 17.0 10.0 2.0 5.0

1.0 14.4 5.1

12.3 7.2

20.0 14.6 14.6 5.8 3.0 2.0

0 5.0 1.2 2.9 2.4

14.0 20.0 26.0 17.9 3.1 11.0

0 24.4 5.1 11.5 4.0 11.0 6.0

12.0 13.6 2.4

10.0

To solve these problems, it is necessary to reduce the time for initial heat-up of the bed materials, which allows the circulation to be achieved quickly, and to remove the defluidization region in the loopseals, as well as to increase the solid circulation rate allowing the decrease of the temperature of loopseals and cyclones. Therefore, the lance burners were mounted to heat up the bed materials quickly, and the grease air lines in the loopseals were also added to improve the fluidization. The

cyclone was also modified to improve the efficiency by way of extension of the vortex finder and reduction of width of cyclone inlet duct.

On the other hand, the fly ash reinjection system was installed to reduce unburned carbon in fly ash, which was relative high than designed. The reinjection system returned a portion of fly ash from electric precipitator (EP) and air preheater (AH) hoppers into the combustor. Also, the T-type nozzles were modified to prevent the backsift of solid particles through the fluidizing nozzles by introducing longer and steeper orifice-tube lines.

These modifications increased solid circulation rate and consequently, allowed being lower temperatures of the furnace and cyclones/loopseals and the emissions of SOx as well as increasing the combustion efficiency. Table 6 shows modification items and current status of the boiler. Table 6. Modifications of the device for the Tong-Hae CFBC

Item Contents for modifications Marks Grease air line at loopseal(dipleg ) 2.5”pipe – 35 places at each loopseal Addition/use Grease air line around ACVs on FBHE 1” and 1.5” – 10 place at each ACV Addition/use Window for observation at loopseal 1 window at each loopseals New/use Duct water spray nozzles at cyclone inlet 2” pipe – 3 places New/not use Injection port of bed media (sand) to bed 6” pipe – 3 places New/not use Injection port of bed media to loopseals 6” pipe at each loopseals New/not use Solids drain line at fluidizing air line 2” nozzles – 7 places New/use Solids drain line at FBAC Screw type at FBAC New/use Lance burner 5 places in the combustor New/not use Fluidizing nozzles Longer and steeper orifice tube Exchange Cyclone modification Vortex finder and inlet width Modification Fly Ash reinjection system from EP and AH hoppers to combustor Modification Improvement of the CFB performance after modification of cyclone

After the modification of cyclones, the differential pressure of upper part (Δ L=23.3m from combustor top ) in the combustor, regarded as basis of complement of solid circulation, was about 190-210 mmH2O, which was higher than that of unmodified. The differential pressure of middle part also increased. These are resulted from the change of solid hold up in the combustor due to increase of cyclone efficiency. The axial solid hold up and pressure profiles before and after modification of cyclones are shown

Solid Fraction [-]

0.00 0.05 0.10 0.15 0.20 0.25

Com

bust

or H

eigh

t [m

]

0

5

10

15

20

25

30

35

Pressure [mmH2O]

0 200 400 600 80010001200

Com

bust

or H

eigh

t [m

]

0

5

10

15

20

25

30

35

PT

PT

ΔPupper, after : 190-210 mmH2O

PT

ΔPmiddle, after : 450-550 mmH2O

ΔPupper, before : 100-160 mmH2O

ΔPmiddle, before : 400-500 mmH2O

before modificationafter modification

Fig. 2. Axial solid hold up and pressure profiles in the combustor

in Fig. 2, with predicted results by IEA (International Energy Agency) - CFBC model [6-7]. The increases of solid circulation rate and solid hold up in lean phase are estimated from increase of the differential pressure. In the model calculation for the solid hold up and the pressure profiles in the combustor, the predicted results for before and after cyclone modification had good fits [8].

The temperature profiles before and after modification of cyclone in the combustor are shown in Fig. 3 with predictions by IEA-CFBC model. The temperature in the combustor after modification of cyclone was lower than that of before modification of cyclone. The cyclones inlet temperatures as well as loopseals temperatures were also lower than before, and the stable operation and control could be achieved successfully.

Temperature [oC]

700 750 800 850 900 950 1000 1050

Com

bust

or H

eigh

t [m

]

0

5

10

15

20

25

30Cyclone inlet T

annulus core

before modificationafter modification

Fig. 3. Comparison of temperature profiles between before and after modification of cyclones

The emissions of SO2 also became to be lower and its current emission level is about 120~130ppm.

In Fig. 4, the calculated cumulative size fractions of discharged bottom ash, recycle ash and filter ash are depicted with sample taken from after modification of cyclone. The calculated results predict that smaller particles are more circulated, so the particles in all parts appear to be smaller after modification of cyclone. This is because of the increase of the cyclone efficiency.

Therefore, the modification of cyclone to improve separation efficiency allowed temperatures of the furnace and cyclones/loopseals and the emissions of SOx to be lower and more stable.

Particle Size [μm]

10 100 1000 10000

Cum

ulat

ive

Wei

ght F

actio

n [%

]

0

20

40

60

80

100

Feed

Overflow

Recirculation

Filter

Bed-content

before modificationafter modificationsample from field

Fig. 4. Comparison of particle size distribution in the CFB boiler.

Improvement of the CFB performance after installation of ash reinjection system

The ash reinjection system for improvement of the combustion efficiency was installed at EP hopper and AH hoppers and a portion of fly ash was reinjected into the combustor. About 2~3% of unburned carbon in fly ash was reduced and the horizontal temperature gradient in the combustor was decreased after installation of the ash reinjection system. The decrease of temperature gradient of the combustor may be caused from the activation of mixing of the particles through adding of fine particles to the bed as well as introducing some air for ash transport to the combustor. Therefore, the installation of the ash reinjection system allowed the combustion

efficiency to increase and the stable operation to be achieved. CONCLUSIONS

During the initial operation period, the Tong-Hae CFB boiler demonstrated successfully in spite of some problems. The temperatures of the combustor and cyclones/loopseals became to be lower than before cyclone modification, and the SO2 emissions were also reduced. The combustion efficiency was somewhat increased and the temperature gradient in the bed was decreased after installation of the ash reinjection system. Consequently, the stable and improved operation of the Tong-Hae CFB boiler could be achieved. REFERENCES [1] Maitland J, Schaker Y. Design of the 200MWe Tonghae thermal power plant circulating fluidized

bed steam generator. In: Preto FDS, editor. Proceedings of the 14th International Conference on Fluidized Bed Combustion, Vancouver, Canada: ASME, 1997. p.191-196.

[2] Kim JS, Lee JM. Status of the 200MWe Tonghae thermal power plant circulating fluidized bed boiler using Korean anthracite coal. In : Proceedings of the 2nd Korea-China Joint Workshop, Taejon, Korea: KIER, 1998. p. 445-454.

[3] Ministry of Commerce, Industry and Energy in Korea. The first plan for supply and demand of electric power in Korean, MOCIE 2002-158, 2002.

[4] Lee SH, Lee JM, Kim JS, Choi J.H, Kim S.D. Combustion Characteristics of Anthracite Coal in the D CFB boiler. HWAHAK KONGHAK, 2000. p. 516-522.

[5] Lee JM, Kim, YJ, Lee WJ, Kim, SD. Coal Gasification Kinetics Derived from Pyrolysis in a Fluidized Bed Reactor, Energy, 1998, 23, p 475-484.

[6] Hannes JP. Mathematical modeling of circulating fluidized bed combustion. PhD Thesis, Delft University of Technology, The Netherlands, 1996.

[7] Hannes JP, Bleek CM, Renz U. The IEA model for circulating fluidized bed combustion. In: Heinschel KJ, editor. Proceedings of the 13th International Conference on Fluidized Bed Combustion, Orlando, Florida: ASME, 1995. p. 287-296.

[8] Lee JM, Kim, JS, Kim, JJ. Evaluation of the 200MWe Tonghae CFB boiler performance with cyclone modification, Energy, 2003, 28, p 575-589.