I. Background Overview
Nitrogen oxides and particulate matter are among the main pollutants from coal-fired power plants, and with sulfur dioxide emissions gradually being brought under control, they are increasingly attracting national attention.
In 2011, the Ministry of Environmental Protection issued a new standard—the Emission Standard for Air Pollutants from Thermal Power Plants (GB13223-2011). The standard required that, starting from January 1, 2012, the nitrogen oxide emissions of newly built thermal power units must reach 100 mg/m³; and starting from July 1, 2014, except for special units that are required to reach 200 mg/m³, all others are also required to reach 100 mg/m³.
In order to implement the "Law of the People's Republic of China on the Prevention and Control of Atmospheric Pollution", improve the quality of the atmospheric environment and build a sustainable economic development, all newly built coal-fired power units are required to simultaneously construct denitrification projects, and existing coal-fired power units are also launching denitrification projects. The technical transformation of dust removal technology also urgently requires the technical transformation of corresponding equipment such as air preheaters and induced draft fans.
Meanwhile, along with denitrification upgrades, major power plants are also actively promoting dust removal upgrades, installing low-temperature economizers, and secondary desulfurization upgrades, resulting in continuously increasing resistance in flue gas systems. Against this backdrop, how to equip induced draft fans is a topic of great concern to us.
Currently, considering only the denitrification resistance and the original induced draft resistance, the total pressure of the induced draft fan is around 5500-6500Pa; considering the denitrification resistance, the original induced draft resistance, and the desulfurization system resistance, the total pressure of the induced draft fan is around 8000-10500Pa; if dust removal retrofitting is added, the total pressure of the induced draft fan in some power plants can even reach 12000Pa.
II. Renovation Plan
According to incomplete statistics in 2012, 96 power plants with 337 units and 674 fans have been retrofitted or have clear plans to retrofit large thermal power units of 300MW or above. Except for a few imported units that still insist on using centrifugal fans due to insufficient space near the original centrifugal induced draft fans, large thermal power units generally use dynamic adjustment and static adjustment for combined fans, with static adjustment having a slight advantage.
According to statistics, large thermal power units that only perform denitrification and induced draft ventilation mostly use statically controlled fans. However, if a combined fan integrating denitrification, induced draft ventilation, and desulfurization (the so-called "three-in-one") is used, there are different configuration options.
300MW-class units, due to their small flow rate, high total pressure, and low specific speed, generally have turbine parameters that exceed the static adjustment range, and therefore often employ dynamic adjustment.
The static and dynamic performance of the combined wind turbines in 600MW-class units is equally impressive.
Most 1000MW-class units use adjustable stationary blades and speed regulation mode for combined wind turbines, while a small number use dynamic adjustment.
In the context of advocating energy conservation and emission reduction, simple centrifugal and static fans, due to their narrow high-efficiency range and low efficiency under low load, obviously cannot represent the most advanced productivity. At present, the configuration of large-scale induced draft fans in the market is generally dynamic or static + speed regulation.
The high-efficiency zone of the centrifugal fan is elliptical, with its short axis almost parallel to the system resistance characteristic curve. The high-efficiency zone is narrow, resulting in significant throttling losses. Efficiency is low at low loads, and only 30-40% at 50% load.
The high-efficiency zone of a statically controlled fan is almost circular, and its range is generally small. Its efficiency is higher than that of a centrifugal fan but lower than that of a dynamically controlled fan under low load. When operating at 50% load, its efficiency is about 50-60%. Static control combined with speed regulation can keep the fan at an efficiency of over 80% under almost all loads.
The high-efficiency zone of the dynamically adjustable fan is elliptical, with its major axis almost parallel to the system resistance characteristic curve. It has a wide high-efficiency zone and relatively high efficiency at low loads. Even when running at 50% load, its efficiency is still around 60-65%.
III. Technology Comparison
Lubrication and cooling
● The static fan uses grease lubrication and air cooling, resulting in a clean and pollution-free environment, but the cooling effect is average.
● The dynamic adjustment fan uses oil sump lubrication and circulating oil cooling, which provides good cooling effect, but the quality of domestically produced seals is unstable, and there is a possibility of oil leakage on site.
abrasion resistance
● Due to its meridional acceleration characteristics, the static-adjustable fan has a narrow flow channel area only at the rear end, making it less prone to wear. Even if wear occurs, it can be repaired by welding, resulting in a long impeller service life. The rear guide vanes are replaceable and can be replaced without shutting down the machine. If speed regulation is used, long-term operation at low to medium speeds greatly improves wear resistance.
● Dynamically adjustable fans have a higher linear velocity, resulting in lower wear resistance at the same speed compared to statically adjustable fans. Furthermore, once worn, their efficiency drops more rapidly, requiring replacement of the entire blade assembly. The rear guide vanes must be replaced along with the entire casing.
Maintenance and repair
● The replacement of the impeller and main bearing of the static adjustment fan is very convenient and simple, and can be completed within 24 hours. Maintenance costs are low, requiring only monthly grease application, with virtually no maintenance costs.
●Replacing and disassembling the blades of a dynamically adjustable fan requires at least 48 hours. The assembly and disassembly of the main bearing can be completed within 48 hours. The hydraulic system components and seals have higher quality requirements, resulting in higher maintenance costs compared to statically adjustable fans.
Operating efficiency
● The static fan has a maximum efficiency of 86-87%, with lower efficiency at low loads. After adding speed regulation, the efficiency at all loads is generally above 80%.
● The maximum efficiency of the dynamic adjustment fan can reach 90%, and the efficiency at low loads is relatively high, reaching over 60%.
reliability
●The static control fan has a simple structure, few parts, and high reliability. Even if a frequency converter is added, it can still operate at the power frequency in the event of a frequency converter failure, and its reliability is not adversely affected.
● The dynamic adjustment fan has many parts and requires a lot of maintenance. If the maintenance is not done well or is done carelessly, the accident rate is high and the reliability is relatively poor.
IV. Economic Comparison
Taking a 600MW unit denitrification retrofit project as an example, a combined fan is proposed to overcome the resistance of denitrification, induced draft, and desulfurization. After hot-state performance testing, based on the power plant's actual operation in 2012, the unit's annual load is divided into three sections: high, medium, and low. The fan parameters for each of the three load sections represent the fan operating parameters for those sections. The power consumption difference between the two types of fans in the three load sections is calculated by using the difference in shaft power between the dynamic and static fans.
Operating cost and input cost analysis
Energy saving: A single wind turbine with static adjustment + frequency conversion saves approximately 1.175 million kWh of electricity per year compared to dynamic adjustment. Calculated at a grid electricity price of 0.4 yuan/kWh, a single wind turbine saves 470,100 yuan in electricity costs per year.
Investment costs: A single HA46036-8Z static fan costs 120W and the frequency converter costs 165W; a HU16050-02 dynamic fan costs 145W. The initial investment for a static fan + frequency converter is 140W higher per unit than that for a dynamic fan.
Maintenance costs: The average annual maintenance cost for a static fan is about 30,000 yuan, and for a frequency converter it is about 70,000 yuan; the average annual maintenance cost for a dynamic fan is 200,000 yuan.
Using the dynamic theory that takes into account the time value of money, the payback period n for the investment difference is calculated as follows:
ΔC=ΔZ(A/P,i,n)
The meanings of each symbol are as follows:
ΔZ: The increase in investment due to static adjustment + frequency conversion compared to dynamic adjustment;
ΔC: Annual differential revenue, consisting of electricity savings costs plus increased operating and maintenance costs;
i: Benchmark rate of return, calculated at 8%;
n: The period for recovering the difference.
Substituting the data into the formula, we find that the payback period n = 3.1, meaning that the increased investment cost can be recovered in about 3 years using the static adjustment + frequency conversion scheme.
Therefore, from the perspective of long-term operational economy, static regulation + frequency conversion is undoubtedly the most economical option.
Due to their simple structure, low maintenance, and low failure rate, statically controlled fans have been the preferred choice for induced draft fans in large thermal power units since the 1990s. In recent years, however, their market share has declined somewhat due to the implementation of national energy conservation and emission reduction policies. But with the continuous decrease in inverter prices, the combination of static control and speed regulation—often jokingly referred to as the "golden pairing" in the industry—is becoming increasingly advantageous.
For large domestic thermal power units that simply use denitrification and induced draft fans, the basic approach is to make partial modifications to the original static adjustment fans, with static adjustment as the main focus.
300MW-class combined wind turbines are generally selected based on dynamic control as they exceed the static control range. However, some power plants are exploring alternative approaches, such as the Weihe Power Plant's 300MW unit which uses a single static control turbine. Currently, in response to market demand for static control turbines from 300MW combined wind turbine users, Chengdu Electric Machinery Plant, along with relevant units and universities, is researching ways to increase the turbine speed from the current 1000 rpm to 1200-1300 rpm by overclocking the frequency converter and expanding the motor capacity to meet parameter requirements.
The main point of contention lies in the combination of 600MW units and wind turbines. Power plants should choose the appropriate turbine type based on their specific circumstances. Data shows that static control + variable frequency drives offer virtually no energy savings above 80% load, and may even increase power consumption. Energy savings only become apparent below 75% load, with the savings becoming more pronounced at lower loads. Given the generally low load rates of domestic units and the limited operational and maintenance capabilities of power plants, the unparalleled reliability of static control turbines is a significant advantage.
1000MW units combined with wind turbines primarily employ static control and speed regulation, with turbine speed regulation being the most common, although variable frequency speed regulation and dynamic control are also used in some cases. As unit capacity increases, parameters become more suitable for static control selection, and turbine speed regulation reduces induced draft fan plant power consumption to almost zero, resulting in highly attractive economic and political benefits. Simultaneously, with increasing unit capacity, the energy savings shorten the payback period for turbine investment; based on the experience of several currently operational 1000MW units, the investment cost can generally be recovered in 2-3 years.
V. Conclusions and Recommendations
To stay at the forefront of technology, we are currently researching and developing technologies related to 1300MW-class combined wind turbines. Static regulation + speed regulation should be the preferred solution for 1300MW-class combined wind turbines.
In conclusion, the static control + speed regulation scheme for induced draft fans of large thermal power units has significant advantages, conforms to the national policy of energy conservation and emission reduction, and is in the best interests of power plants. It should be the preferred solution for all power generation companies.
