It is well understand that Bangladesh is facing severe power crisis since long. Most of our power plants become older and substantially de-rated even from its low capacity and efficiency (Maximum 35%). Thus, we are also loosing our existing power capacity as many of existing power stations are frequently out of operations although overhauling and scheduled maintenance were performed. On the other hand setting up of complete new power stations are difficult in terms of price and execution time. More over we are also facing the shortage of gas supply, which creates significant hindrance for setting up new power stations. New Power Stations need separate land, huge land development cost, fuel supply pipelines and other environmental effects. Considering all the effects to set up the new power station, our country must go for the new concept of “REPOWERING” existing older power stations which can not only provide huge extension /increase of capacity and but also reduce the cost of project. The REPOWERING of older existing plants of Bangladesh shall also increase the efficiency ( up to 60%) of the plant which ensure the generations of higher capacity power without additional supply of fuel. The concept also reduces the emission of gases that pollutes the environment and may get the benefits from the developed countries by reducing the global warming.
The advantages of Re-powering which mainly are:
1. Increased Output with minimum investment
a. Reduced investment cost per MW – compared to New Plant saving of over 30%
b. No additional Land required
2. Efficiency Increase from ~35% to ~ 57% for all the units
a. With marginal increase in gas consumption the Re-powered Plant can generate over 400 MW.( If the unit Capacity is 210 MW)
b, C02 emission by -40% for same power generated.
For understandings the concept Repowering, the details are as follows:
Repowering is the process of replacing older power stations with newer ones that have a greater Installed Capacity and more efficiency which results in a net increase of power output.
As power plants age, the efficiency of steam production equipment declines and/or the equipment becomes technologically obsolete, even with regular maintenance. Rather than replace aging steam production equipment in kind, some utilities have chosen to replace it with new technology. This process is called “repowering” and offers the opportunity to increase the efficiency of the process through the installation of new, improved equipment. Repowering with more efficient equipment can reduce net emissions of greenhouse gases and air pollutants compared to operating the original equipment. Repowering with equipment that uses lower-carbon fuels can further reduce net greenhouse gas emissions.
Repowering can take several forms. It can involve the replacement of the boiler with a new steam-producing facility, or it can involve a totally new steam production process involving a combustion turbine and heat steam generator and additional generating equipment.
Repowering with a natural gas-fired combustion turbine and adding a heat recovery steam generator improves overall efficiency, and often increases the electrical output of the facility. Greenhouse gas emissions could be reduced, as natural gas is lower in carbon than the fuel previously used. Repowering with a combustion turbine also has environmental benefits of more effective control of SO2, NOx, and particulate emissions.
Deregulation and competition are further fueling the demand for new power generation equipment worldwide. Due to the availability and cleanliness of gas, and the ease of consent, gas turbine applications have increased over the last few years. This development is driven by the addition of capacity, but also by major replacement programs. In this market in particular, it has been realizing that repowering of existing units is taken into consideration due to its reduced land, infrastructure, existing permits, capital cost and time requirement compared to greenfield projects.
Especially with the expected reform of the Bangladesh power Plants into a competitive, liberal-ized , repowering of existing units is an attractive opportunity not only for extending the lifetime of existing plants, but also for reducing the life-cycle costs .
The fact that Bangladesh has a fleet of gas-fired steam power plants which went into commercial operation more than 20 years ago and major equipment of these plants exceeded or is close to their design lifetime even more supports the idea of repowering. Last but not least the rising electricity demand and Bangladesh’s ratification of the Kyoto Treaty will have implications to power plant economics as well.
Repowering targets existing gas power plants under certain conditions to make such an effort viable for competitive power generation costs. The levers to fulfill the goal of modernization and repowering to increase the economics and dispatchability of exist-ing power assets are:
Increase of efficiency and power output
Extension of lifetime
Increase of availability and reliability and reduction of O&M costs
Increase of operational flexibility
Reduction of specific emissions
2 Repowering Concepts
There are several alternatives to combine and integrate a gas turbine into an existing steam power plant. A detailed assessment of the existing plant equipment as well as technical and economic boundary conditions of a specific project determines the optimal approach and the selection of the repowering concept. The main alternatives of repowering are illustrated in Figure 1.
• Full Repowering:
Full repowering is defined as complete replacement of the original boiler with a com-bination of one or more gas turbines (GT) and heat-recovery steam generators (HRSG), and is widely used with very old plants with boilers at the end of their life-time. It is considered as one of the simplest ways of repowering an existing plant. In most cases repowering projects include the modernization of the steam turbine and I&C.
• Parallel Repowering:
In the parallel repowering concept the boiler stays in operation for peak and interme-diate load. The steam from the boiler is added to the steam from the HRSGs at several pressure levels depending on the condition and capability of the existing steam tur-bine.
The figure shows an existing plant that was repowered as full Repowering.
Before Repowering After Repowering
The figure shows an existing plant that was repowered as Parallel Repowering
Before Repowering After Repowering
3 Steam Turbine Capability for Repowering
For economical reasons, it is recommended that the original capability of the ST and the asso-ciated BOP systems (transformer, cooling systems etc.) are utilized as much as possible. For full repowering, the available HRSG steam defines the achievable ST output within narrow limitations. As a rule of thumb the thermal cycle of the repowered unit targets approx. 70-80% of the rated steam turbine output in order to achieve a reasonable amount of steam flow through the HP-section of the steam turbine. In combined cycle applications as well as for full repowering concepts the steam flow across the steam turbine increases due to the added steam from the IP- and/or LP-HRSG and the fact that the steam extractions for condensate and feed-water heating are put out of service. Repowering therefore requires detailed assessment of the LP-turbine and the condenser capability to cope with the increased flow.
In many applications the end of the design life of the ST has been reached, and modernization of the ST will take the modified steam flow along the steam path into consideration..
For parallel repowering, the ST capacity is arbitrary. The steam flow characteristics enable full utilization of the ST and BoP systems. The combined-cycle (CC) mode of operation cor-responds to full repowering. In the hybrid mode the existing boiler supplies additional steam up to the capacity limit of the ST. This intermediate-load reserve is far higher than what can be achieved by duct firing in the HRSG. The attractiveness of parallel repowering lies in the tremendous load reserve in the hybrid mode at an efficiency that is far higher than simple-cycle GT plants or steam plants.
Repowering is an ex-cellent concept for improving the efficiency and flexibility of older existing fired power plants to survive.
There are several benefits, which can be derived from such a repowering project:
• The existing station can still be operated while the new part of the plant is being con-structed.. e.g. power from the gas turbine in advance.
• Efficiency increases to competitive levels.
• Specific emission levels are reduced due to the efficiency increase and the low-NOx combustion technology utilized in the state-of-the-art gas turbines.
• Existing assets are further utilized leading to reduced capital costs and lower recruit-ment costs (as the personnel of the existing plant can operate the repowered plant) compared to new plant on a Greenfield site.
• It sometimes might be easier for BPDB to obtain permissions to upgrade a plant rather than to construct a new plant on a greenfield site.
• Repowering and modernization projects result in economic benefits, while each project has its own unique facets.