By Feng Weizhong
Shanghai Waigaoqiao No. 3 Power Generation Co., Ltd,
Shanghai Shenergy Energy Technology Co., Ltd
In today’s international community the strategic importance of energy efficiency and moving toward a low-carbon economy continues to gain prominence. In China, it is predicted that coal-fired power will remain the principal contributor to the power industry for the long term; therefore, today there is a strong focus on improving the efficiency of China’s coal-fired power plants.
To improve the overall efficiency of the thermal power industry, the Chinese government has, since 2004, shut down about 10,000 MW of low-efficiency plants, often those smaller than 200 MW. Simultaneously, construction of new supercritical and ultra-supercritical units, with capacities ranging from 600 MW to 1000 MW, has increased; these plants utilize imported technologies that are domestically manufactured.
One of China’s first such projects, the Shanghai Waigaoqiao No. 3 power plant, includes two 1000-MW ultra-supercritical power generation units. This landmark project was initiated in July 2005 with the two units being placed into commercial operation in March and June 2008. Throughout the project, emphasis was placed on optimization and technological innovation as related to design, equipment selection, construction, commissioning, startup, and operation. As a result, the units’ overall performance has been greatly improved with a net efficiency of 45.03% (LHV basis throughout this article), as demonstrated during the performance test (circulating water temperature of 19°C) prior to commercial operation. This efficiency far surpassed the design value and that of the other similar units. During the first year of commercial operation, the actual net efficiency was 42.73% at an average operational load of 74%.
Since the units were placed into commercial operation, the pace of technological innovation has continued through a series of new energy-saving and emission reduction R&D projects, such as utilization of waste heat upstream of flue gas desulfurization (FGD) and selective catalytic reduction (SCR) to reduce NOx with any load rate. Any modifications associated with these projects have been implemented during planned annual maintenance. Through such modifications, the net efficiency and environmental performance have improved considerably each year.1 For example, in 2009 and 2010 the plant average net efficiency increased to 43.53% and 43.97%, respectively, again with a 74–75% average load rate. In 2011, the average net efficiency further improved to 44.5% (including FGD and SCR). This means the unit net efficiency including FGD and SCR has reached above 46.5% at rated conditions (i.e., full load).
EFFICIENY CONTRIBUTIONS FROM MAJOR COMPONENTS
Shanghai Electric imported Alstom’s boiler technology for the basis of the plant. The boiler is an ultra-supercritical tower-type design with single reheat, built-in separator, spiral water wall, sliding operation, single furnace, corner tangential firing, open arrangement, balancing ventilation, solid slag disposal, and pulverized coal firing. The main boiler parameters are provided in Table 1.
Shanghai Electric imported Siemens technology for the 1000-MW, single-shaft turbine with four cylinders and four exhausts, and double backpressures. The main parameters for the turbine are provided in Table 2.
LIMITATIONS TO IMPROVING EFFICIENCY
A number of conventional approaches are used to improve the efficiency of modern thermal power plants:
Improving the steam parameters (temperature and pressure): The maximum parameters are limited by materials technology. Currently, the most advanced steam temperature and pressure are 600/620°C (main/reheat) and 28–30 MPa. Research and development are progressing globally toward advanced ultra-supercritical plants (with steam temperatures of 700°C and beyond), but widespread commercial prospects are unlikely within the next 10 years.
Reducing the turbine exhaust parameters: For example, the Nordjylland power plant Unit 3 takes advantage of naturally cold water available for cooling (average temperature of 10°C). Thus, the 411-MW double reheat, ultra-supercritical unit can operate with a backpressure of only 2.3 kPa. However, it is restricted by the turbine exhaust area, exhaust steam humidity, length of the last blading, natural conditions, etc.
Adopting double reheat: Currently the most popular cycle for power plants is single reheat. Double reheat, which can improve the unit efficiency, is seldom adopted due to system complexity, the large investment required, and concerns related to balancing the performance and costs.
Promoting boiler efficiency: This is accomplished primarily by reducing the amount of excess air and increasing the use of the flue gas heat. The designed efficiency for bituminous coal-fired boilers has reached ~95%. This can be limited by the burn-off rate and coal type, and especially by the sulfur content.
ROUTE TO HIGH EFFICIENCY AND LOW EMISSIONS AT WAIGAOQIAO NO. 3
Efficiency improvements are also possible for existing power plants. To find such opportunities, losses in efficiency due to equipment and operation must be identified and the economic feasibility of modifications must be examined. There are several areas on which to focus.
Reducing Flue Gas Heat Losses
Heat loss from flue gas is a critical area because 80% of all boiler losses can be attributed to flue gas. In addition, the enthalpy and temperature (maximum 10°C) of the flue gas can be increased due to the induced fans and FGD booster fans. With flue gas temperatures normally around 130°C or lower, the energy available for recovery is limited. Compounding this is the important issue of erosion of the recovery heat exchanger surface caused by SO2 and even SO3 and NH4HSO4 after installing SCR denitration, which results in reduced heat recovery from the flue gas. In addition, fly ash can adhere to the surface of the exchanger. The combination of the alkaline ash and the sulfur acid dew can form a concrete-like substance that is difficult to remove and hinders heat exchanger operation.
This problem has been addressed through several different approaches at Waigaoqiao No. 3. Sulfur dew deposition on the heat exchanger surface is prevented, assisted by the use of anti-acidic materials in the lower temperature sections. In addition, the heat exchanger is placed in the low-ash zone between the booster fan and FGD tower, which prevents abrasion and reduces ash accumulation. Also in this arrangement, the heat generated by the induced fan and booster fan is recovered. As well, a fin-type heat exchanger has been adopted for improving heat exchange. The recovered heat is transferred to the condensate and, thus, low-pressure extraction steam can be returned to the turbine to reduce the turbine heat rate.
The FGD flue gas heat recovery systems for the two units were placed into operation in June and October 2009. To date, they have performed well with minimal erosion detected. The performance test revealed that the unit efficiency was improved by 0.4 percentage points with the operation of this device, and the water used for the FGD was reduced by 45 tonnes/hr.
Improving the Air Preheater Seal
Air leakage is an important efficiency consideration because it can lead to increased power consumption by all fans due to the increased air and gas flow. In addition, the heat exchanger efficiency can decrease with leakage, leading to a decrease in boiler efficiency.
Similar to most large modern boilers, the two units at the Waigaoqiao No. 3 power plant use rotating air preheaters (diameter 17 m, height 2.5 m). Although such air preheaters have many advantages generally, a nonlinear “mushroom” deformation of the rotor can occur during operation. The clearances between the rotating and stationary parts are not easily controlled, leading to an increase in air leakage. At Waigaoqiao No. 3, the designed air leakage was less than 5%. After the first year of operation the actual air leakage was less than 6%.
To reduce the air leakage, a round, flexible sealing technology was researched and developed without changing the structure of the air preheater. The sealing device is contacting, flexible, and wear controlled. The flexibility of this new seal compensates for the nonuniform changes in the clearance between the rotating and stationary parts of the air preheater. This technology has led to a significant reduction in air leakage. The service power consumption, including FGD and SCR, was below 3.5 percentage points. The boiler efficiency was increased by 0.29% with the increased air temperature for the boiler as a result of reducing air leakage.
Optimization of Turbine Steam Parameters and Operation
Supercritical and ultra-supercritical turbines supplied by Siemens adopt no control stage design; the basic mode of operation is that of a sliding pressure run. To address the need for turbine load variation, Siemens introduced ultra-supercritical turbines in China with an overload valve suitable for grid frequency regulation and overload control. There is an additional port for steam injection in the high-pressure turbine immediately after the fifth stage of blading. The overload valve is placed between the additional port and downstream of the main steam stop valves. During normal operation, the main steam control valve and the overload valve work together to control load. Changes in load can be met by opening the overload valve or closing the control valves. Turbine efficiency deteriorates as the overload valve opens. In cases where the overload valve is frequently opened and closed or is maintained slightly open, erosion and leakage are likely.
To avoid this, the design parameters and the control mode were optimized. First, the main steam pressure was selected and the opening point of the overload valve was set to the rated load point which corresponds to the maximum cooling water temperature (during the summer season). Thus, opening of the overload valve can be avoided throughout the year as the load demand will always be equal to or lower than the rated output.
Second, focusing on the turbine load control mode, an energy-saving turbine load adjustment technology based on adjusting the extraction steam was developed. In this way, the main control valves are fully open while the overload valve is always closed. Therefore, the throttling loss across the valves can be eliminated. This method of operation changes the turbine load basis by adjusting the condensate flow to indirectly change the amount of extraction steam, supported by controlling the extraction steam for the feedwater preheaters. Through this approach, transient turbine output can be obtained and the load requirements set by demand will be satisfied by adjusting the boiler combustion system.
By applying this approach, a fast response to changes in load demand can be achieved and a relatively large range of loads can be handled. This new, successful frequency regulation mode was demonstrated in practice at the two units at Waigaoqiao No. 3 power plant. Currently, the load change rate for frequency control reaches or surpasses 15 MW/min.
These methods have improved the unit operational efficiency by 0.22 percentage points. However, these benefits cannot be detected during performance tests.
Boiler Feedwater Pump Turbine
Based on success at other projects abroad, a 1×100% turbine-driven feedwater pump was adopted at Waigaoqiao No. 3 power plant—a first in China. The plant thus eliminated the motor-driven pump. This boiler feed pump turbine (BFPT), with its own condenser, is able to start up independently using steam from the neighboring boiler.
In addition to successfully developing and implementing the new technology so that the boiler feedwater pump can operate at a wide range of speeds, this technology saves a tremendous amount of energy during startup. It also simplifies the system control strategy and eliminates the risk of minimum flow valve leakage and enhances equipment safety. Compared with competing options, this specific boiler feedwater pump increases the unit efficiency by 0.117 percentage points.
FLEXIBLE HEAT REGENERATIVE TECHNOLOGY
Most power plants today utilize heat regenerative technology wherein steam is extracted from the turbines to heat the boiler feedwater. However, regenerative technology as applied at Waigaoqiao No. 3 has been reformed from standard practice by expanding the regenerative media from only water to water, air, and coal. Based on this new approach, a series of new heat regenerative technologies has been developed. As a result, unit efficiency has improved.
One of these technologies is flexible regenerative technology, which adapts an adjustable high-pressure extraction steam to the additional feedwater heater; the feedwater temperature (into the boiler) can be sustained and the flue gas temperature decreases downstream of the boiler economizer can be minimized during low-load operation. Thus, the SCR can be used at low load without difficulty. Another benefit is the ability to increase the potential rate of load change to respond to changes in demand. Also, by increasing the fraction of bleeding steam at low-load operation, unit efficiency is also improved. Finally, an increase in the combustion air temperature, and the water temperature at the inlet of the water wall during low load, improves combustion stability and efficiency, and the fluid dynamics. At 75% of full load, applying flexible regenerative technology can improve unit efficiency by 0.2 percentage points.
MAINTAINING HIGH EFFICIENCY
Supercritical and ultra-supercritical power plants offer the benefits associated with higher efficiency and lower emissions; to maintain these benefits, however, stable operation and high efficiency must be maintained. Some challenges, such as steam-side oxidization of the boiler tubes and subsequent solid particle erosion (SPE) of the turbine blades, substantially threaten safe and economic operation. These issues often occur during startup. The oxidized scales in the boiler tubes (steam side) can peel off due to heat shock (see Figure 1). This mass can then be deposited in the tubes or form larger particles that move with the steam.7 A major problem can occur when steam tubes burst due to a major deposition. Erosion of the turbine blades can be caused by the particles carried by the steam, and the power plant efficiency will be irreversibly decreased. Furthermore, the particles can also erode the sealing surface of the bypass valve plug during startup, causing leakage and allowing for steam to bypass the turbine, further decreasing efficiency. Recently, several such problems have occurred at many supercritical and ultra-supercritical units in China—in some cases the efficiency has decreased by 4% after only two years of operation.
Therefore, to maintain high-efficiency operation it is important to prevent or slow the oxidization on the steam side of the boiler heat exchanger. This requires comprehensive prevention, incorporating this concern during design, equipment selection, installation and commissioning, and normal operation. After 10 years of research, a comprehensive approach to preserving the high efficiency of the power unit of Waigaoqiao No. 3 has been developed and implemented. Some of the most important components of this effort include:
- Apply dry steam blowouts with highly superheated steam in the outlet area of the water wall, effectively increasing the force and resulting cleaning during blowouts.
- Deploy a large-capacity bypass system designed to bypass the turbine during unit startup and also implement a high-momentum flushing procedure to send oxides directly to the condenser during startup.
- Develop a new configuration design and control strategy to avoid eroding the bypass valve plugs.
- Use steam to heat the feedwater that is used to heat the boiler during startup and also during low-load operation.
After 30 months of operation with these strategies in place, the tubes in the third superheater and the second reheater of a boiler at Waigaoqiao No. 3 were inspected. There were no indications of oxidation or deposits. Samples of the second reheater tubes are shown in Figure 2; the results of the inspection of the third superheater were similar.
At the same time, the first blading of the intermediate-pressure turbine was checked. As illustrated in Figure 3, the blading was also undamaged. Notably, a performance test indicated that the turbine interior efficiency has not deteriorated since the initial power plant startup.
In order to rapidly decrease the carbon emissions from the electricity industry, the potential to save energy and increase efficiency through all means possible must constantly be evaluated. This effort includes researching and developing new technologies, challenging efficiency limitations, and developing the most efficient thermal power based on available technologies.
Operational experience with the Waigaoqiao No. 3 power plant demonstrates that much room remains to save energy through equipment selection, design, commissioning, operation, emission control strategies, etc. Through optimization, improvement, and innovation, tapping the energy-saving potential is the best solution to achieve lower cost, lower risk, and near-term results for efficiency improvements. For new plants, the strategies implemented at Waigaoqiao No. 3 can incrementally improve net power plant efficiency by over three percentage points, which is equivalent to increasing the steam temperature of an ultra-supercritical plant from 600°C to 700°C. Although these technologies are principally used for newly constructed units, they can also be employed for existing units, resulting in an efficiency improvement of more than two percentage points.
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