Advanced coal-fired technology for Meri-Pori To meet increasing electricity demand Finland is building a 560 MWe power station known as Meri-Pori. Once complete, it will be one of the world's cleanest and most efficient coal-fired power plants. The project which is being funded by Imatran Voima Oy (IVO) and Teollisuuden Voima Oy (TVO), is scheduled for completion in late 1993.

K e v i n D o d m a n

Finland needs more generating capacity, to meet an expected 3000 MW g r o w t h in e n e r g y demand during the period up to the year 2000. Of this 3000 MW, it is anticipated that 1500 MW will need to be base load capacity, while the rest will only be needed during the winter months.

To help meet th is demand a new 560 MWe c o a l - f i r e d p lant called Meri-Pori is being built, near the town of Pori, 250 km north-west of Helsinki in Fin land. The new plant will use an advanced design of s team tu rb ine to ach ieve an overall efficiency of 43.5 per cent.

It has been designed for base load operat ion and will have an annual output of 3.6 TWh. A combi-nation of low-NO x burners and a flue gas cleaning plant will result in emissions that will be within the recently determined regulatory lim-its of 70 m g / M J for N O x , 140 mg/MJ for S 0 2 and 20 mg/MJ for particulates.

Joint funding The overal l cost estmate for

Meri-Pori is FIM 2.5 billion ($650 mil l ion), including f inancing and interest charges. Of this, 45 per cent is being financed by TVO, the rest by IVO. TVO will have a 250 MW share of the power generated by the plant in exchange for its investment, whi le IVO has been responsible for construction of the plant and will own and oprate it when completed.

IVO owns and operates a range of nuclear , foss i l fue l f i red and

hydroelectr ic power plants. The company is Finland's largest sup-plier of heat and power to industrial and ut i l i ty c o m p a n i e s , meet ing some 45 per cent of the national energy consumption.

TVO is owned by a number of individual industrial groups, includ-ing IVO which has a 25 per cent shareholding in TVO. It supplies power to industry and owns and o p e r a t e s the O l k i l u o t o nuc lear power plant.

The decisin to build the Meri-Pori plant was taken at the end of 1989, pre l iminary p lanning was c o m p l e t e d in 1990 , and IVO ordered the main equipment for the power station at the end of May that year.

Construction work started at the end of 1990 and equipment instal-lat ion c o m m e n c e d a year later.

Commissioning is scheduled for summer 1993, with commerc ia l operation to follow in November. Power will be fed into the local 400 kV grid.

P o w e r n e e d e d u r g e n t l y To meet the anticipated growth

in demand for power in Finland, three types of power generating capacity were considered: natural gas, nuclear and coal. Natural gas. It was felt that a sta-ble supply of natural gas could only be g u a r a n t e e d by bu i l d ing a pipeline either from Norway or from the Baren ts Sea. Both o p t i o n s would be uneconomic under cur-rent conditions. Nuclear. There are four nuclear units in Finland - two at Loviisa and two at Olkiluoto - all of which have been operating successfully

Figure 1. The Meri-Pori power station will be one of the world's c leanest and most efficient coal-fired power plants

March 1993 19

for a number of years. Government approval would be needed before further nuclear capacity could be built and a proposal has been put forward to build a fifth unit. No final dec is in has yet been reached about whether to proceed with it but even if it is approved, it will take around seven years to construct, so o ther f o rms of g e n e r a t i n g capacity are needed in the short to mdium term. Coal is already used extensively for power generation in Finland. Coal-f ired power stations can be built relatively quickly and by using the latest emission control technol-ogy, environmental emissions can be minimized.

P l a n t c o n s t r u c t i o n The new plant is being built near

the city of Pori, next to the existing Tahkoluoto power plant. Factors nfluencing the cho ice of Pori i nc luded the fact that the s i te already has a deep water harbour. Also, many facilities from the exist-ing power plant, such as the cool-ing-water channel and road and railway infrastructure, are being re-used.

The first stage of con-s t r u c t i o n i nvo l ved s i te m o d i f i c a t i o n w o r k . The building office of the City of Pori g ranted bu i ld ing p e r m i s s i o n in O c t o b e r 1990, and w o r k on the foundations began the fol-lowing month.

The largest construc -tion contractor at Meri-Pori is the Haka Group. They have undertaken the con-c re te w o r k and s u p p l e -mentary structures for the main buildings. In al l , over a hundred s u b c o n t r a c t o r s have been employed, and during the installa-t ion phase , a round 700 people have worked at the site.

E m i s s i o n c o n t r o l The emission control system for

Meri-Pori was selected at the end of 1990, on the basis of the sulphur removal requirements then prevail-ing, and the p roposed n i t rogen ox ide lev is , in l ine w i th Government guidelines.

The system has cost around $125 million and includes low-NO x burners, plus catalytic converters for further reduction of NO x levis, electrostatic precipitators to remove particulates, and a wet-type desul-phurization system. This combina-tion of systems is the first to be fit-

ted to a coal-fired power station in Finland.

In May 1 9 9 1 , the F inn ish Supreme Court ruled on the emis-sion levis that would apply, and these were in line with the original guidelines. The levis set by the Supreme Court are 140 mg/MJ for sulphur dioxide, 70 mg/MJ for nitro-gen dioxide and 20 mg/MJ for par-ticulate emissions during normal operation of the plant. Wet desulphurization: The desul-phurization plant will be a wet sys-t e m , s u p p l i e d by O u t o k u m p u Ecoenergy of F in land and L&C Steinmller of Germany, with the bulk of the equipment manufac-tured in Finland. It will comprise an absorption reactor, together with water, washing liquor, end product and ash treatment systems.

The flue gases will be fed via the electrostatic precipitators and the heat e x c h a n g e r into the absorption reactor, where the S 0 2 in the flue gas will react with an alkaline washing liquor made up of powdered limestone and water, to form gypsum.

The washing liquor will be fed into the sump of the reactor and pumped from there into the reac-tor's spraying zone. Flue gases will be fed into the reactor abov the s u m p c o n t a i n i n g the w a s h i n g liquor. As the gases move upward, the washing liquor will be sprayed into the tower from a quadruple battery of overlapping spray banks. This will ensure efficient mixing of the gases and the washing liquor. The liquor will collect in the bottom of the reactor, where the reactions will be supported by aeration and agitation.

The upper part of the tower incorporates droplet separators to prevent droplets from leaving the reacto r w i th the f lue gas . The cooled, water-rich gases will be fed to the exit stack through a gas-gas heat exchanger.

appeenranta

The process will remove over 90 per cent of the sulphur contained in the f lue gas. The site 's sulphur dioxide emissions on an average annual basis w i l l be be low the Finnish Government 's guidel ine

valu of 140 mg/MJ with all grades of coal.

The slurry accumulat-ing in the bottom of the reactor w i l l be fed to hydro cyclones, which will increase the solid matter conten from about 10 per cent to 50 per cent. The lighter, smal ler gypsum crystals will be fed from the cyclones to a recy-cling tank and from there back into the reactor. The concentrated slurry gen-erated by the cyc lones

will then travel to a vacuum belt fi l -ter, after which the solid content will be 90 per cent.

The Meri-Pori plant will produce about 60 000 t/yr of gypsum and it is planned that this will be used in the construction industry. Between 4 and 10 t /h of limestone will be used under norma l c o n d i t i o n s , depending on the purity of the lime-stone and the sulphur content of the coal being burned. Some 100 t/h of seawater will also be used, as well as 8 t/h of potable water for gypsum cleaning. Burners: The boiler at Meri-Pori is equipped with a two-stage low-NO x combustin system, incorporating a total of 30 low-NO x burners. The burners s u p p l i e d by T a m p e l l a Power are of the Babcock-Hitachi HTNR (High T e m p e r a t u r e N O x Reduction) type. This design was

Figure 2. The plant is situated near the town of Pori, 250 km north-west of Helsinki

Figure 3. Meri-Pori during construction

Varch 1993 21

chosen for opt imum fuel efficiency and NO x reduc-tion. These swirl burners can run on pulverised coal or oil, and yleld N 0 2 con-c e n t r a t i o n s of b e t w e e n 100 and 200 m g / M J depending on the type of coal used.

The design was inf lu -enced by a n u m b e r of requirements: To maximize the rate at which the volatile elements are evolved from the fuel. To provide an initial oxygen-defi-cient zone to minimize NO x forma-tion, but provide sufficient oxygen to maintain a stable fame To optimise both the residence time and the temperature under fuel - r ich condit ions, to minimize NO x formation To maximize char residence time under fuel-rich conditions, to reduce the potential for the formation of char nitro-gen oxide To add the remaining air in such a way that c o m -plete fuel b u r n - o u t s ensured.

A further requirement is that the fame should have an oxidising envelope, to minimize possible corro -sin of the furnace wall.

Key e l e m e n t s of the HTNR burner design are the in t roduct ion of axia l swirl generation, which is inherently more stable than radial generation, and the use of a stabilizing ring to promote rapid and stable ignition.

Staged combustin, as used in the HTNR burner, is widely regard-ed as the most effective way to minimize N O x p roduct ion in the boiler, as it promotes localised fuel-rich conditions and reduces the for-mation of both thermal and fuel -derived NO x .

The burners will yield N 0 2 con-centrat ions of between 100 and 200 mg/MJ in the flue gas leaving the boiler. These levis will be reduced by subsequent catalytic con-versin. Catalytic converters: Catalytic converters are fit-ted to reduce the NO x lev-is further. The Babcock-Hitachi system supplied by Tampella Power includes an ammon ia process ing unit, a network of spray jets and a catalyt ic con -verter , wh ich is located

between the feedwater preheater and the air preheater. The temper-ature in this rea is around 350C, which is the optimum temperature for the reaction. Additional catalyt-ic layers should be installed after 3-4 years, and the total expected life of the catalyst is seven years.

Ammonia will be fed into the flue gas flow prior to the converter and the process has been designed to keep ammonia concentrations in the post-converter stage as low as

Figure 5. The desulphuriza-tion process

possible, to no more than 4 ppm, which will have no impact on out-side air quality.

After passing through the cat-alytic converter, the flue gas will have an N 0 2 content of no more than 70 mg/MJ and it is expected that overall, the system will achieve an 80 per cent reduction in nitrogen oxide levis compared to older boil-er designs. Removal of particulates: Following the cata lyt ic converter , the f lue gases will be fed to the electrostat-

ic precipitators for dust Figure 4. remova l . These wi l l be Additional dual -chamber, four- f ield coal delivery systems and after pass- equipment ing t h r o u g h t h e m , the has been dust content of the flue added to the gas will be a mximum of existing 100 mg/nm 3 (wet). facilities that

A p r o p o r t i o n of the dellver coal r e m a i n i n g p a r t i c u l a t e to the matter wi l l be removed Tahkoluoto

dur ing the d e s u l p h u r i z a t i o n power plant process, after which the particulate content will be no more than 50 mg /nm 3 (wet), which is equivalent to 20 mg/MJ.

Overall, 99.5 per cent of the fly ash in the flue gas stream will be removed. This will be around 150 000 t/yr and it will be held in two silos, each with a volume of 3000 m 3 , prior to being recycled either for use in the construction industry, or for disposal in a landfill site.

B o i l e r The boiler at Meri-Pori

is a Benson once-through supercritical design and has been s u p p l i e d by T a m p e l l a Power of Finland. It will burn pulver-ized coal, with oil used for start-up, and will produce

440 kg/s of live steam at 240 bar and 540C, while reheat steam conditions will be 48 bar and 560C.

The boiler house is 77 m high and the boiler's

size means that pulverized fuel is the best option. This also has the advantages of good combust in efficiency and low operating cost.

The 30 burners arranged at five levis will be fed with pulverized coal by five MPS milis manufac-tured by Deutsche Babcock, each with a capacity of 52 t /h . These feature a rotary classifier for better particle separation.

At full boiler load four milis will be used mostly, with the fifth as a s tandby . The coal wi l l be pur -

chased w o r l d w i d e , but Figure 6. The main ly f rom C o l u m b i a boiler is fitted and Poland. The boiler with 30 HTNR has been designed for 16 burners, six at different bituminous coal each burner t y p e s , w h i c h wi l l have level ca lo r i f i c v a l e s in the range 26.0 - 32.1 MJ/kg.

S t e a m t u r b i n e The s t e a m t u r b i n e ,

which is being supplied by ABB Stal, comprises f ive react ion ax ia l - f low turbine sections:

March 1993 23

Figure 7. The burners feature axial swirl-generation and a stabilizing ring to promote rapid and stable ignition

one single-f low high pressure (HP) section one double - f low intermedate pressure (IP) section three double-flow low pressure (LP) sections.

These are connected coaxially, by integral forged coupling flanges, to the generator and the slip-ring unit. The whole train is supported on eight bearing pedestals; one at each end of the rotor train and one oetween each individual rotor and slip-ring shaft.

Each bearing pedestal, except the one between the HP and IP sections, has a single Journal bear-ng and is fixed to the foundation in

all directions. The HP/IP pedestal can move axially, and carries the axial thrust bearing, integrated with the journal bearing

The HP and IP casings are sup-ported vertically and guided trans-versely by the bearing pedestals. The casings are axially connected via the thrust bear ing pedesta l , which is free to move axially on the 'oundation.

At the side towards the first LP rasing, the IP casing is fixed axially to its bearing pedestal , which in turn is fixed to the foundation. Thus when warming up, the IP casing, the thrust bearing pedestal and the HP casing will be able to slide axi-al ly. The rotor t ra in fo l lows the movement of the thrust bearing and thus the axial clearance in the LP turbines can be reduced. Steam path: Live steam will enter the HP turb ine through the two valve casings flanged to the outer casing. Each valve casing contains a stop valve and a control valve.

After expansin through the sin-g le - f low b lad ing , the steam wil l eave the exhaust rea for reheat-ng, with some being extracted for

'eedwater heating. The reheated steam will be admitted to the IP tur-bine through the connections to the two combined stop and intercept .alves located on each side of the turbine.

Af ter expans in th rough the double-flow IP turbine blading, the steam wil l enter the IP exhaust rea. A g a i n , s o m e s team f rom

ging of combustin air low-NO x burners

Overfire air

Fuel and

primary air

1000C

1200C Completion of

combustin

800C

CO+ I2 O2 CO2

Secondary Tertiary air a i r NO

Nh N2 + O2 Fuelj<

nitrogen N 0 \ N 2NO

S

Figure 10. The steam turbine which features HP, IP and LP stages is mounted on spring bases to reduce dynamic loads on the foundation structure

C o n t r o l s y s t e m The automation system at Meri-

Pori was supplied by Siemens and is based on the c o m p a n y ' s "e leperm ME technology, devel -:oed for power plant use.

The system is made up of intelli-gent l/O modules, which enable a modular f o r m a t i o n of f u n c t i o n groups to be assembled, depend-i g on the mechanical structure a~d operation of the power plant. In :rs way, the Teleperm ME system :an be applied to all apsects of rower plant automation, including : r otect ion , interlocking, measure-ment, and closed and open loop controls.

The Meri-Pori system comprises tne following elements: Main automation system, includ-ng automation for the boiler, tur-bine and turbine-related process :a-ts

Automation of the desulphuriza-ron plant Coal t r a n s p o r t s y s t e m =Jtomation

Burner and sootblower automa-tion (acting as a subcontractor to the boiler supplier)

i Auxiliary switchgear automation It includes two different bus sys-"s; the CS275 bus and the sys-

m coupling bus, both of which nave redundant st ructures . The system coupl ing bus is used to

nnect all the process stations t need data exchange, such as

rxotection signis and interlocking signis.

The CS275 bus connects the AS 220 EA automation systems, ttie OS256-6 operating systems, t re Simatic S5 programmable logic :ontrol lers f i tted to the auxil iary ooiler plant and the water treatment plant, the workstations for system -a intenance, and the information system (process computer).

Both main systems (boiler and ine) and the auxiliary systems

sulphurization, coal transport, _c) have the i r own buses and

rating systems. The two buses

ing system, one for the information system and one for the process computer.

The whole plant is operated and controlled via the monitors, using the OS256 -6 operat ing system. The OS system also includes a

. Absolute f x point, bearing I 9 casing/outer casing V anchored on foundation

Relative fix point, inner casing fixed inside outer casing

Relative fix point, O positionof rotor train in

thrust bearing casing

Inner "casing ^Outer

casing _ Rotor

train

are linked via a bus connector for data exchange.

C o n t r o l r o o m The control room is designed to

be manned by one operator and one supervisor. However, the con-trol desk s designed so that during start up and shut down, a second operator can be accommodated. Both parts of the contro l board have four monitors for the operat

Sicomp M56 computer, with the IS information system for displaying and storing such information as alarms and limit switch signis. It is also used for displaying curves and functional operating points and as a gateway to other systems.

Due to this extensive automa-tion, when it enters service Meri-Pori will require only around forty operating personnel, plus mainte-

Figure 11. Thermal expansin of the steam turbine train

nance staff. Figure 12. The power plant control room

1993 27

Schematic diagram

| Schematic diagram

HT-NR BURNER

Coal and prlmary air Tertiary Fame

air stabilizing ring

A. Combustin zone of volatile matter B. Production zone of reducing species C. NOx decomposition D. Char oxidizing zone

MERI PORI FINLANDIA 560 MWe

Key 1 Coal yard 23 Gypsum silo 2 Coal conveyor 24 Ash silo 3 Coal silo 25 Main condnsate pump 4 Coal feeder 26 Low-pressure preheater 5 Coal mili 27 Feed-water tank (FW) 6 Coal burners 28 Turbine driven FW-pump 7 Forced-draught fan 29 High-pressure preheater 8 Mill-airfan 30 Economiser 9 Air preheater 31 Evaporator

10 Ammonia supply 32 Superheater 11 NOx catalyst 33 HP turbine 12 Electrostatic precipitator 34 Reheater 13 Induced-draught fan 35 IP-turbine 14 Desulphurization absorber 36 LP-turbine 15 Absorbent circulation 37 Condenser 16 Demisters 38 Cooling water pump 17 Flue gas reheater 39 Turbogenerator 18 Stack 40 Main transformen 19 Gypsum dewatering 41 Control room 20 Effluent 42 Boiler house 21 Process Water 43 Turbine hall 22 Limestone silo 44 Switch yard