The ageing boilers of two units at the Suralaya power station in Indonesia have been successfully upgraded by PT. Indonesia Power, a subsidiary of the state electricity company Perusahaan Listrik Negara. The project, which was funded by the Japan Bank for International Co-operation, was carried out in partnership with Japan’s Marubeni Corporation and Babcock & Wilcox Power Generation Group Inc. (B&W) of the US.
The boilers for Units 1 and 2 are 400 MW pulverised coal fired radiant tower types and were originally supplied by B&W and Marubeni back in the early 1980s. The objective of the project was to extend the life of these units, increase their steam capacity, reduce their nitrogen oxides (NOx) emissions, and restore unit efficiency ” thereby reducing their carbon dioxide (CO2) emissions.
The first unit underwent a rehabilitation outage in October 2010 and successfully passed all its performance guarantees in July 2011. The second unit outage started in July 2011 and successfully completed its performance tests the following February.
Suralaya power station is on the northwest tip of the Indonesian island of Java, close to the city of Cilegon and about 150 km west of the capital city of Jakarta.
Suralaya is PT. Indonesia Power’s largest coal fired power station, with a total generating capacity of 4025 MW: Units 1 to 4 are 400 MW each, Units 5 to 7 are rated at 600 MW each and Unit 8 is 625 MW. The power station was constructed in four stages with Units 1 and 2 entering service in 1983 and 1984 respectively.Units 3 and 4 were commissioned in 1989 and Units 5 to 7 were placed into service in 1997. The last unit, which is Unit 8, entered service in 2011.
The radiant tower boilers of Units 1 and 2 are balanced draft, natural circulation units, with single reheat, and are top supported. The maximum continuous rating (MCR) of each unit is 1,156,040 kg/h of steam at 173 kg/cm2 gauge and 541à‚°C at the secondary superheater outlet. The reheat steam flow at MCR is 997,050 kg/h at the reheater outlet. Steam temperature leaving the superheater and reheater is maintained at 541à‚±6à‚°C at loads between 65 per cent and 100 per cent of MCR.
The boiler units are designed to burn pulverised coal as the main fuel, with light oil as the igniter fuel. Heavy oil guns are provided on all burner levels to assist in the warm-up cycle. The heavy oil atomisers and valve rack systems were also designed to achieve full load operation. There are five B&W Roll Wheelà¢â€ž¢ size 89NR pulverisers per unit. The boilers can operate at both MCR output and valves wide open load conditions with one pulveriser and burner group out of service.
Each unit is equipped with two 50% size 28 secondary air heaters and two 50% size 22 primary air heaters. In addition, each unit is provided with two secondary and primary steam coil air heaters. Each unit includes two 50% forced draft fans, two 50% primary air fans, two 50% induced draft fans and two 50% gas recirculation fans.
The project’s main aim was to upgrade the capacity of Units 1 and 2. It was also the plan to rehabilitate and modernise the boiler island to recover the losses in efficiency due to the age deterioration of the equipment, and to improve performance for the wider range of coals that were now being utilised.
The increased output, improved efficiency, lower O&M costs (from equipment modification and rehabilitation), as well as the usage of a wider range of Indonesian coals would provide an increased revenue stream for PT. Indonesia Power. The additional revenue would then provide a payback within a reasonable timeframe, justifying the capital expenditure. The lower O&M costs would also allow PT. Indonesia Power to improve profitability and deliver additional electricity to the grid at the lowest possible cost.
Life cycle management approach
As a plant ages, it is beneficial to undertake a Life Cycle Management Programme to assess the condition of the equipment. This approach was used to assess the overall condition of Units 1 and 2. PT. Indonesia Power wanted to determine if the facility was in a suitable condition to perform a rehabilitation and capacity upgrade.
There are three key steps in a Life Cycle Management Programme: risk assessment, condition assessment, life cycle decisions.
PT. Indonesia Power performed a risk assessment and identified the probability of an unwanted event at the facility. An unwanted event was defined as site-specific downtime due to equipment issues, as well as the resultant impact on the grid. Based on known history, statistical data, judgment by experts and an evaluation of the consequences and repair or replacement costs of the equipment, PT. Indonesia Power assessed the potential risks and decided to proceed with the next step: the condition assessment.
Unit 1 and 2 boiler inspections were based on the B&W test plan submitted prior to the outages, which included: visual inspections and photo summary; non-destructive evaluation and ultrasonic thickness testing surveys; mechanical checks, destructive testing, sample collection and laboratory analyses. After reviewing the results and the recommendations from the condition assessment and taking into consideration the concerns identified during the risk assessment, PT. Indonesia Power decided that the equipment could be refurbished/rehabilitated more economically than a complete replacement of the boilers and associated equipment.
The rehabilitation would involve the redesign of the convection pass sections of the boilers, a redesign of the combustion systems for NOx emissions compliance, upgrades to the pulverisers, and refurbishment of the primary and secondary air heaters
Varying fuel sources
Suralaya was originally designed to use coal from the Bukit Asam mine in South Sumatra. Bukit Asam fuel specification and corresponding characteristics were therefore used to design the bulk of the fuel handling, pulverisation and combustion equipment for the boiler islands.
But in recent years, the power plant has relied on new coal sources. The wide range and variety of these coals, coupled with the original pulveriser design technology for Units 1 and 2 and current equipment conditions raised O&M concerns.
The pulveriser design modifications and technology enhancements in the capacity upgrade project optimised the B&W 89NR pulveriser performance for a wider selection of Indonesian coals. It was also found that the increase in heat input from the capacity upgrade and the latest Indonesian and World standard for NOx emission limits necessitated replacing the combustion equipment. The replacement burner technology chosen was B&W DRB-XCLà‚® low-NOx burners, which have been successfully applied to a broad range of units with varying fuel characteristics and boiler arrangements in both new and retrofit applications.
Scope of work
The key objective of the boiler island portion of the project was to modify equipment so the boilers achieve steam output under the increased load conditions. In addition, the equipment rehabilitation would improve unit performance and efficiency. Finally, the modernisation of the coal pulveriser and burner systems would allow Units 1 and 2 to use a wider selection of Indonesian coals.
The conceptual design improvements for the capacity upgrade and rehabilitation of Units 1 and 2 were based on the evaluation of the original boiler design, original performance data sheets, new turbine heat balances, setting drawings and the current fuels being used. The equipment and component modifications and enhancements highlighted in the following sections were required to achieve the project objectives.
To meet increased demand from the capacity upgrade and to optimise performance for the wide range of proposed coals the following pulveriser modifications and enhancements were carried out:
- Wearesistorà‚® low profile (LP) grinding elements were selected to reduce pulveriser operating power and allow higher coal flows for the increased boiler capacity, while maintaining the original main pulveriser drive motors.
- An Auto-Springà¢â€ž¢ loading system was introduced to provide a means of varying the roll wheel tensioning pressure with pulveriser load or coal flow, which in turn helps regulate the coal bed depth under the roll wheel tyres. This system also reduces pulveriser pressure drop, improves load response time, and broadens the flexibility of the mills to accept a wider range of coals.
- DSVSà‚® rotating classifiers were included as part of the upgrade package to improve product fineness and maintain combustion efficiency with the new low-NOx combustion system.
To meet improved pulveriser availability and to ensure compatibility with these technology improvements, the following components were required: new roll wheel assemblies, new loading rods and seals, new helical tensioning springs, and new hydraulic loading cylinders. Erosion in the turret top plates was also repaired. Figure 1 illustrates the modifications that were made to the pulverisers.
|Figure 1: Schematic summarising the upgrades to the pulverizer
Due to the increase in burner heat input required to meet the new capacity load conditions, and to meet optimum combustion efficiency and environmental goals, the 35 circular register coal fired burners were replaced with advanced DRB-XCL low-NOx burners. The burners were designed as plug-in replacements into the original burner pressure part openings.
Each unit was equipped with 35 DRB-XCL low-NOx burners for firing pulverised coal. Fourteen of these burners also fire No.6 fuel oil, which can be used for warm up and to achieve 30 per cent MCR. These 14 pulverised coal/oil-fired burners were located at the bottom elevation of the front and rear walls.
The arrangement of burners is three elevations on the front wall and two elevations on the rear wall ” the middle elevation is vacant on the rear wall. The five pulverisers serve seven burners each. The rear wall burners have coal pipes feeding from above, while front wall burners are fed from below. Each level of burners was installed in a compartmented windbox with dampers and airfoils at each compartment entrance.
The DRB-XCL burner design and supply also included: ceramic-lined burner inlet elbows; ceramic-lined coal nozzles; erosion-resistant deflectors; erosion-resistant diffusers; and burner support rails. Each DRB-XCL burner was also equipped with a new CFS oil igniter system. Each igniter system included: CFS oil lighter assembly; oil atomiser; fuel and steam flex hoses; high-energy spark ignition system; high-energy ignition probe; insert and retract pneumatic cylinders; and igniter valve rack assemblies (two per burner elevation).
Each boiler was also equipped with new flame detection systems comprising: 70 flame scanners (one main flame and one igniter flame scanner for each burner) complete with fibre optic extensions; 70 special scanner cables; and one scanner cooling air blower skid with redundant blowers and inlet filters.
Air heater modifications
To increase unit capacity and restore boiler efficiency, all four air heaters on each boiler (two primary and two secondary) must operate at optimum performance. To achieve this and reverse age deterioration, the radial, bypass and axial seals were replaced. New heating surfaces were also supplied. The overall scope of work on the heaters included:
- Air heater leakage control repair work, air heater casing and static seal repairs
- Replaced cold end sector plate in Secondary Air Heater 1A
- Replaced cold end T-bar in Primary Air Heater 1A
- New replacement seals (radial, by-pass, and rotor post seals)
- New replacement heating elements (hot, intermediate, and cold-end baskets). To maximise efficiency, the hot and intermediate heating elements were combined into one layer, increasing the heating surface of the air heaters and raising unit efficiency. The cold-end layers were supplied with enameled coating.
Upgrade of pressure parts
A review of the convection pass surface performance at the increased capacity operating conditions found that American Society of Mechanical Engineers (ASME) code requirements, as well as current B&W standards required upgrading the materials in most of these banks. The reheater outlet sections were replaced for life extension considerations.
Replacing convection heating surface consisted of: primary superheater II (PSH II) inlet sections; PSH II outlet & reheater (RH) intermediate sections; secondary superheater (SSH) & RH outlet sections; SSH intermediate sections; bifurcate and vertical support tube assemblies. Because of the increase in overall steam flow, it was determined that most existing safety valves required replacement to meet ASME Section I Code for overall relieving capacity. The following safety relief valves were replaced with new valves rated for the new steam conditions: steam drum safety valves; SSH outlet safety valves; reheat outlet safety valve; and ERV with manual isolation valve.
All objectives achieved
PT. Indonesia Power’s rehabilitation and capacity upgrade project has met all the initial overall objectives, i.e. to extend the life of the units, increase the steam capacity, reduce NOx emissions, and restore unit efficiency. Restoring unit efficiency cut fuel consumption, combustion air requirements, power needed for the pulverisers, and CO2 emissions.
Table 1 summarises the overall benefits of implementing this project. It is important to note that the improvements listed are based on the increased capacity of the boiler. Performance test results were compared with the baseline test conditions so the improvements would be on an equivalent basis.
Of particular note is the refurbishment of the air heater, which involved replacing worn/corroded air heater baskets, seals and some sector plates. This reduced leakage and lowered the air heater exit gas temperature, which had the following benefits: combined fan power savings of 1000 kW at MCR load for both units and a 0.7 per cent improvement in boiler efficiency, which yields a 0.8 per cent reduction in fuel consumption ” a combined annual reduction of 22,000 tonnes.
The pulveriser enhancements also brought benefits including: increased coal fineness, which improved combustion; reduced vibrations at low mill loading; improved overall performance; and lower power consumption.
In conclusion, the combined effect of all of the modifications is an annual reduction in CO2 emissions by 51,000 tonnes and NOx emissions by 8890 tonnes per year for Unit 1 and Unit 2 combined. To achieve this level of NOx reduction using a selective catalytic reduction system, assuming 2 ppm ammonia slip, would require 3380 tonnes per year of ammonia. At an assumed ammonia cost of $800/tonne, this would equate to an ammonia cost savings of more than $2.7 million per year.
Steve Borsani is manager, International Service Projects, B&W Power Generation Group Incorporated.
He would like to thank Paul Nesbitt, Matthew Fedak and Robert Popelmayar from B&W, as well as Yu Numasawa of Marubani Corporation and Herdiyanto Soekono of PT. Indonesia Power for their invaluable contribution to this article.
The article is based on an award-winning paper presented at POWER-GEN Asia in Bangkok, Thailand, in October this year.