Oil shale is a difficult fuel to burn yet is used in Estonia’s largest power plants. The country is embarking on a repowering programme, which through the use of advanced CFB technology will bring enhanced efficiency and environmental performance from its power plant fleet.
Pertti Niemi, Foster Wheeler Energia Oy, Finland and
Mati Uus, Narva Power, Estonia
The Narva power plants in the eastern part of Estonia were constructed in the 1960s to supply electricity to Estonia and the northeast of the Soviet Union as well as heat for Narva town. The gross installed capacity of the plants is currently 2700 MWe and 589 MWth across 20 power plant blocks, and they account for more than 90 per cent of Estonia’s net electricity production.
The plants burn oil shale, which comes from mines in the Narva region. It is quite a difficult fuel to use, with an average ash content of 46 per cent, a chlorine content of more than 0.2 per cent and a heating value of just 8.2 MJ/kg. The existing power plant blocks are each equipped with two pulverized firing boilers.
In 2000, Narva Power decided to repower two of the Narva power plant blocks – Eesti block 8 and Balti block 11 – by constructing new boilers and balance of plant systems, rehabilitating existing turbines and using old cooling water connections, common fuel feeding up to the fuel silos and ash slurry pumping and disposal systems.
The main reasons for the investment were to reduce emissions of SO2, NOx, CO2 and particulates, to improve the efficiency and reliability of the plant, and to reduce operation and maintenance costs. Narva Power awarded the contract for the work to Foster Wheeler Energia Oy in May 2001.
The Eesti plant is the world’s largest oil shale fired power station. The plant consists of eight power generating blocks of about 200 MWe each; blocks 1 to 8 are currently in operation with a capacity of 1410 MWe and 84 MWth. The units were commissioned between 1969 and 1973. Each power block consists of one K200-130 condensing steam turbine with reheat manufactured by LMZ of St. Petersburg, and two pulverized-fired boilers (Models TP-101).
The flue gas cleaning system for each boiler consists of hot cyclones and an electrostatic precipitator. The flue gases from the plant are discharged to the atmosphere through two 250 m stacks.
The oil shale is delivered by rail and crushed prior to storage onsite. The oil shale is subsequently conveyed to four silos per boiler. Generated power is transmitted at 330 kV to the Estonian grid. The generators are connected to the 330 kV switchyard via 220 MVA step-up transformers.
The Narva power plants in the eastern part of Estonia were constructed in the 1960s
Eesti block 8 was commissioned in 1973 but has now been taken out of service for the repowering project.
The Balti plant was constructed in two phases. The older part of the plant, built between 1959 and 1963, originally consisted of eight steam turbines and 18 boilers. It now consists of four 100 MWe steam turbines, two of which have controlled extraction for delivery of district heat, while steam is supplied to the turbines from eight pulverised fuel boilers with a capacity of 53 kg/s each, feeding to a common header.
The newer part of the power plant, built between 1963 and 1966, consists of four blocks (Blocks 9, 10, 11, and 12) each with two pulverised fuel fired boilers manufactured by Taganrog with a total capacity of 78 kg/s, and one 200 MWe steam turbine manufactured by LMZ. The flue gas from these four blocks are discharged to the atmosphere in two 180 m stacks.
The current peak district heating demand for municipal needs is 200 MWth. It also produces process steam for corporate customers in Narva. The volume of the process steam sales is 200 GWh/annum, with a peak demand of 50 MWth. The old boilers need a lot of maintenance work to overcome tube leakage, fouling and wear, which reduce boiler availability and efficiency.
Oil shale for the Balti plant is also received via rail and stored onsite. The water treatment system provides the demineralized water supply. A water softening and deaerating plant provides feed water to the district heating network, which also supplies hot water for consumers. This adds special requirements for district heating water treatment.
The total capacity of the plant is currently 1090 MWe, with one block being repowered, and 505 MWth, which includes hot water for heating and steam for industrial use. Generated power is transmitted from the station at 110, 220, and 330 kV.
Two new blocks
On the basis of a tender process, Eesti Energia selected Foster Wheeler Energia Oy as the main engineering, procurement and construction (EPC) contractor for the project to repower Eesti block 8 and Balti block 11. The contract price is about g250 million and covers modernizing 400 MW of capacity to increase the output to 430 MW. Narva Power’s parent company, Eesti Energia, is responsible for financing the project.
As a result of the project, the electricity generation efficiency of the two blocks will be increased from 30 per cent to 36.5 per cent, which is expected to lead to a saving in fuel costs of over 20 per cent. The gross output of both blocks will be increased from 170 MWe and 180 MWe to 215 MWe. Sulphur emissions will be reduced by about 95 per cent, NOx emissions by around 55 per cent, CO2 by some 23 per cent and particulate emissions by more than 97 per cent. These environmental benefits will be achieved by the installation of advanced circulating fluidized bed (CFB) boilers without the need for a separate desulphurization plant and because oil shale ash contains high levels of limestone.
Advanced CFB boilers
In repowering the two power plants, Narva Power wanted to keep the two-boiler concept in order to guarantee high availability. The new Foster Wheeler CFB boilers are of the single reheat type with a steam production capacity of 90 kg/s each at 540°C and 13.1/2.7 MPa.
Drawing on extensive tests carried out at Foster Wheeler’s R&D centre at Karhula in Finland, the new CFB boilers have been designed to take account of the special requirements imposed by oil shale, with its high corrosion-promoting ash content. CFB technology has shown its ability to burn this fuel efficiently and cleanly. The separators are steam cooled to avoid heavy maintenance and intensive refractory. The last super- and reheaters are of Foster Wheeler’s Intrex design, which effectively protects them from chlorine corrosion because they are not at direct contact with flue gases.
The convective part of the boilers is designed on the basis of experience gained with fine dust conditions in the metallurgical industry, this time drawing on Foster Wheeler’s experience as the world’s largest supplier of waste heat boilers to the non-ferrous metallurgical industry. An acoustic cleaning and spring hammer cleaning system will be used in the boilers, which are designed to operate with a daily load swing of between 40 per cent and 100 per cent of the MCR output.
BOP and turbines
In addition to two new CFB boilers with auxiliaries, the repowering of both units includes the refurbishment of balance of plant equipment such as overhauling of the steam turbines, rebuilding numerous systems, the ESPs, instrumentation and control systems, electrical systems and ash handling.
The power output of the LMZ steam turbines will be increased from 200 to 215 MW by replacing the Bauman-type low pressure turbine rotor and stationary blades with modern ones. The high pressure and intermediate pressure parts are being overhauled by LMZ in St. Petersburg, Russia. The turbine control and protection system will also be upgraded. The lubrication and vacuum system components will be renewed as well as the condenser tubing.
An increase in generator capacity to provide a continuous rating of 215 MWe at a power factor 0.85 will be made by changing the rotor winding. The exciters of the generators will be dismantled and replaced by static excitation systems. The existing generator protection system will be refurbished.
Erection of the two boilers at Balti started in February 2003
Practically all of the balance of plant (BOP) equipment will be replaced: low and high-pressure feed water preheaters, condensate pumps, feedwater pumps, pipelines, and so on. The Balti power plant turbine will be modified to supply district heat to Narva town at a maximum 160 MJ/s capacity.
One ESP will be provided for each boiler. The ESPs will be capable of operating undre all conditions from initial start up to complete shutdown and will achieve a particulate emissions limit of 30 mg/Nm3. The electrification and C&I systems of the two blocks will be totally replaced.
The ash handling system will feature a dry pneumatic system up to the ash silos. After the silos the ash will be mixed with water and fed to existing slurry pumps, which will pump the ash to a disposal area.
Work at Eesti started in October 2001, and at the Balti site six months later. Boiler erection at Eesti started in August 2002, and the boilers there are now mechanically complete. Commissioning work has started, with acid cleaning and magnetite coating already completed. Steam blowing began at the end of August. Hand-over to the customer is scheduled for March 2004.
Erection of the two boilers at the Balti plant started in February this year, and the plant is due to be handed over in September 2004.