Sethu Menon, Mitsui Babcock, Crawley, UK
Samar Sen, Mitsui Babcock India, Calcutta, India
Two years ago, Mitsui Babcock was awarded a contract by India’s National Thermal Power Corporation (NTPC) to refurbish and modernize a coal-fired boiler unit at Talcher power plant. Now, the project is nearing completion and Mitsui Babcock is to embark on a further three units at the same plant.
Figure 1. Mitsui Babcock was awarded the Talcher contract in September 1998
Talcher, a 470 MW plant in Orissa state, was constructed in two stages between 1967 and 1983. By 1995, trippings, outages, failures and breakdowns in some of its units had caused its plant load factor to drop to as low as 20 per cent. Keen to restore its operating performance in a cost-effective manner, NTPC decided that the best way forward was through renovation and modernization.
Power plant modernization is an economic way of maximizing generation and improving efficiency, safety, reliability and service life in a power system. It offers an alternative to the burden of installing new greenfield capacity, in terms of both capital cost and project lead times. Renovation and modernization is therefore becoming increasingly popular in India.
Since 1950, India has installed over 90 000 MW of new generating capacity, much of it thermal. As of 1998, India’s installed generating capacity stood at 100 000 MW, and it faces a peaking power shortage of around nine per cent.
Rapid industrial development in India is forcing a high electricity demand growth rate, and the government has projected capacity additions of 47 000 MW during the ninth five-year plan period (from 1997/8 to 2001/2), 57 000 MW during the tenth five-year plan, and 58 500 MW during the eleventh five-year plan.
The capital cost required for these capacity additions is huge, and the government has decided that other cost-effective options for increasing capacity must be undertaken, including maximizing generation from existing plants.
Figure 2. The objectives set by NTPC for the plant improvement were ambitious
The government has therefore assigned high priority to its Renovation and Modernization and Life Extension Programme, which was initially launched in 1984. The programme targets mainly older thermal units whose performance has deteriorated due to factors such as poor coal quality, inadequate milling capacity, erosion of boiler components, non-availability of high pressure (HP) heaters and low condenser vacuum.
One of the plants targeted under the Renovation and Modernization and Life Extension Programme was Talcher.
The Talcher story
Talcher power plant is a coal-based thermal plant located in the Angul district of Orissa state. It comprises two stages: Stage I consists of four 62.5 MW units and Stage II comprises two 110 MW units. The two stages were commissioned in 1967-68 and 1982-83 respectively.
Talcher’s six units are fitted with direct pulverized coal-fired steam generators. The Stage I units are radiant, front wall fired, bi-drum type boilers supplied by Babcock & Wilcox of the USA, supplying steam to GE steam turbines. Each boiler is provided with three mills, one forced draft fan, one induced draft fan and a dedicated hot primary air fan for each mill. The equipment for the Stage II units was supplied by BHEL of India.
The coal for Talcher is currently supplied by the adjacent Jaganath coal fields. However, these mines are likely to be exhausted within the next four years, and the future coal supply for the plant will come from the Hingrvala & Bhubneshwari coal field.
Figure 3. The renovation and modernization work was highly challenging
India’s NTPC took over the ownership and operation of Talcher from the Orissa State Electricity Board (OSEB) in 1995. At this time, the performance of Talcher had deteriorated and the capacity of the Stage I units had been derated to 60 MW, bringing the plant’s total installed capacity down to 460 MW. In addition, the average plant load factor was 20-25 per cent.
On taking over Talcher, NTPC undertook an assessment of the plant’s performance and operating history, which revealed that the poor availability of the steam generators had been one of the major contributing factors to the low plant load factor of the Stage I units. NTPC made an assessment of how the plant’s performance could be improved, and having made a commitment to gradually increase the plant load factor to 60 per cent within five years, decided to undertake major renovation and modernization work, starting on the older Stage I.
Specific problems with the Stage I units at Talcher were numerous. Over the years, the flue gas temperature at the airheater exit had increased from the design value of 145-154
In addition, there had been a deterioration in the coal gross calorific value (GCV) from 4420 Kcal/kg to 3300 Kcal/kg, leading to a limitation in mill throughput and the non-availability of any standby mill for maintenance purposes at near full loads. The sootblowers were non-functional, the burners were eroded, and the flame scanners and burner management/furnace safeguard supervisory system was not installed. Feedback to the control room about flame failure was not available and hence the safe operation of the boiler was not ensured, resulting in unsafe and prolonged start-ups.
NTPC therefore developed a number of main objectives to be achieved by the renovation and modernization of Talcher’s Stage I units:
- To bring back the units to their original name-plate rating in terms of steam generation, final steam pressure and final steam temperature.
- To make the boilers compatible for inferior coal with a GCV of 3300 Kcal/kg.
- To attain a minimum gross thermal efficiency of 86 per cent on the inferior fuel.
- To ensure that the boiler has an annual availability of at least 90 per cent.
- To guarantee a plant life of 25 years.
- To ensure maximum operational flexibility and improved plant maintainability.
- To achieve safe, reliable and sustained commercial operation of the boilers.
NTPC undertook a competitive bidding process, and in September 1998 awarded the UK’s Mitsui Babcock Energy (India) Private Ltd. (MBEIPL), a wholly owned subsidiary of Mitsui Babcock, a contract to renovate and modernize Unit 4 of Stage I. After two years of work, Unit 4 is now undergoing commissioning, and NTPC has awarded MBEIPL a further contract for the renovation of Units 1-3 of Stage I.
The objectives set out by NTPC for the plant improvement were naturally very ambitious and the renovation and modification work involved was highly challenging because of the various uncertainties that surrounded the run/repair/replacement decisions.
Figure 4. The scope of Mitsui Babcock’s work included the installation of control and instrumentation systems
A preliminary Remnant Life Assessment (RLA) had been carried out by NTPC prior to the contract award and this had identified certain plant items for replacement. However, a more detailed post-contract RLA had to be undertaken by Mitsui Babcock to be able to meet the required performance guarantees such as boiler efficiency and flue gas temperatures, and also to achieve the guaranteed 25 year life of the plant.
This detailed assessment involved boroscopic examinations of internal surfaces, analysis of deposits on the tube/pipe internal surfaces, visual inspection of pressure and non-pressure parts, ultrasonic testing, thickness measurements and in-situ metallographic examination to determine the extent of creep damage to pressure parts.
The findings of the two RLAs formed the basis of the extensive modifications and replacements needed to the boiler plant. The execution of these modifications and replacements presented a number of challenges, and innovative ideas had to be developed to overcome them. This was largely due to the various constraints such as the size limitation of the existing furnace, space limitations, the need to use existing structures as far as possible, the need to retain all useful components, minimum availability of underground layout information, various erection limitations and a tight project schedule.
Redesigning the superheating surfaces to reduce flue gas velocities and associated erosion rates was made difficult by the existing closely pitched primary/secondary superheater side by side arrangement, and involved lowering of the rear vestibule floor maintaining its slope. The space limitations necessitated new ideas to be developed for the superheater supporting system. Redesigning of the bi-drum boiler bank to reduce the flue gas velocity involved the lowering of the water drum.
The considerable enlargement of the economizer surface to improve boiler efficiency necessitated major redesign of the boiler bank outlet/economizer inlet flue arrangement and also the installation of a new economizer supporting structure connected to the main supporting structure.
The need to have a standby coal pulverizing mill at boiler maximum continuous rated conditions required the entire redesign of the combustion system comprising coal feeders, mills, primary air fans, pulverized fuel piping and all burners.
The basic scope of Mitsui Babcock’s renovation and modernization work included the redesign and replacement of the superheaters, and the incorporation of superior metallurgy into this component, the redesign and replacement of the economizer, lowering the drum from its original position, and the enhancement of the air heater performance. Mitsui Babcock has also enhanced the capacity of the milling plant, redesigned and replaced the coal/oil burners, installed a burner management system, and replaced the sootblower system.
Also included in the work carried out were penthouse pressurization with dedicated fan, replacement of insulation and casing and installation of control and instrumentation for the furnace safeguard supervisory system, sootblower operation, and electronic remote water level monitoring system. In addition, a light diesel oil (LDO) system has been installed for all four Stage I units in place of the high speed diesel oil system.
An additional radiant platen-type primary superheater, convective secondary superheater and convective tertiary superheater surfaces have been added to contain the furnace exit gas temperature to acceptable levels and to achieve the desired superheating performance. A re-adjustment of the superheater surfaces by incorporating increased pitch and reduction of tube diameters has been carried out, primarily to reduce flue gas velocities.
Flue gas velocities have also been reduced by the reduction of the boiler bank tube diameters and an increase in the boiler bank drum centres (by lowering of the water drum). An increase in the economizer heating surface has brought about an improvement in gross thermal efficiency, and an upgrade in superheater tube metallurgy has enabled longer life to be ensured and top suspension load to be contained despite the increased heating surface.
An increased plant life has also been ensured through a redesign of the top suspension slings with superior material.
The calorific value of the coal used at Talcher has deteriorated over the years from that for which the original combustion equipment was designed. In order to meet the increased consumption of inferior coal by the boiler, together with the requirement of having one mill group as standby at 100 per cent BMCR, the combustion equipment had to be totally redesigned and replaced.
As part of this redesign, the original coal feeders were replaced with drag link-type coal feeders with almost twice the capacity of the existing coal feeders. The original small ball, high speed coal mills were replaced by new large ball, slow speed Babcock E type vertical spindle mills with the following features:
- High component wear life
- Capacity of the mill does not fall with wear life
- Moisture in fuel does not affect milling capacity.
To take care of the increased load of the replaced milling plant, extra-high density haematite concrete, with a density of
3.5 t/m3 was used to contain the size of foundation blocks.
The original pulverized coal/high speed diesel (HSD) fired burners were replaced by Babcock self-stable pulverized coal/LDO fired circular burners of higher capacity, capable of controlling the flame geometry by swirl adjustment.
The original pulverized fuel piping has been replaced by new a set of pulverized fuel pipes with features to ensure a higher capacity, longer life and even pulverized fuel distribution to all the operating burners. The primary air fans and mill seal air fans were replaced to suit the replaced milling plant, while the forced draught fan has been upgraded to suit the upgraded plant’s system resistance.
Talcher moves forward
The complete redesign, manufacture and testing of Unit 4 at Talcher were carried out in line with Indian boiler regulations. From contract award, Mitsui Babcock had 13 months for design and supply activities before a planned six month shutdown of the unit for total field activities.
Mitsui Babcock achieved full load operation of Unit 4 without oil support during July 2000, and later, from 19 September to 22 September, a continuous full load run was achieved.
Figure 5. Mitsui Babcock achieved full load operation of Unit 4 without oil support in July 2000
By the end of 1999, NTPC had taken the decision to proceed with the renovation and modernization of the Stage I Units 1-3. Following a competitive tendering period and the successful modification of Unit 4, NTPC awarded the contract for this work to MBEIPL on 24 July 2000.
Mitsui Babcock has already commenced detailed engineering work on these three units. The project completion dates are 16 months for the first unit, 23 months for the second and 30 months for the third. The total value of this contract is around