Power 101: Improving the Performance of Boiler Auxiliaries, Part III

Part I of this three-part series, "Improving the Performance of Boiler Auxiliaries," explored the air preheater (APH) and important performance calculations. Part II discussed the performance degradation resulting from corrosion and fouling caused by coal-combustion flue gas constituents plus the effects of ammonia and sodium bisulfite injection for SO3 mitigation. This last installment examines ways to reduce auxiliary power usage by using variable-speed drives instead of constant-speed motors.

The definition of net unit heat rate, expressed in terms of its component parts (Figure 1), provides a set of potential heat rate improvement options. Reducing auxiliary power use is an important option for improving net unit heat rate.

1. Options for plant heat rate (efficiency) improvement. Source: Energy Research Center, Lehigh University (ERC)

To illustrate the magnitude of this effect, a sensitivity analysis was performed for an 800-MW (gross) power plant with a turbine cycle heat rate of 8,000 Btu/kWh and boiler efficiency of 85%. The baseline auxiliary power use was 80 MW, and baseline net unit heat rate was 10,458 Btu/kWh. The results, presented in Figure 2, show that a 10% reduction in auxiliary power use will improve net unit heat rate by approximately 1.1%.

2. Effect of change in auxiliary power use on net unit heat rate. Source: ERC

The breakdown of auxiliary power loads for a large coal-fired power plant is presented in the Table for today’s and future technology. The feedwater system loads are determined by operating parameters of the Rankine cycle (where boiler feedwater pump power depends on the main steam pressure), and there is little opportunity for reducing these auxiliary power loads except in the selection of drives. The feedwater system loads include the main feedwater pumps and condensate booster pumps.

Auxiliary power use in coal-fired power plants by technology. The numbers shown are in percent of plant auxiliary load. Source: EPRI

The cooling water system loads include circulating water pumps and, in case of mechanical-draft towers, cooling water fans. The cooling water fans consume about 6.5% of the total auxiliary power. The pollution control system includes the electrostatic precipitator, selective catalytic reduction (SCR), and flue gas desulfurization. Combustion air and flue gas handling includes the forced draft (FD), primary air, and induced draft (ID) fan(s). Fuel handling includes pulverizers and the coal-handling system. Other loads include the ash-handling system, steam turbine auxiliaries, water treatment, transformer and cabling losses, and controls and lighting.

There might be some opportunity for achieving incremental reductions in auxiliary loads by streamlining power plant ductwork and by careful engineering of the feedwater and steam pipes to minimize pressure drops. These efficiency improvement measures can be applied to new or retrofit units.

Additionally, replacing the mechanical draft towers with natural draft towers would eliminate cooling tower fan load. A natural draft cooling tower designed to achieve the same cooling as a mechanical draft tower (at the same approach temperature) would result in a heat rate improvement of approximately 0.7% (75 Btu/kWh in this example).

Most auxiliary loads are powered by electric motor drives, particularly lower power loads. Because electric motor efficiency and cost are directly related, the long service life of motors in power plants commonly justifies the use of high-efficiency motors, particularly for larger loads.

In coal-fired power plants, the motors for loads such as fans, pumps, pulverizers, and compressors are larger than 200 kW. These motors typically have efficiencies higher than 93%. The very large motors, such as those used for boiler feedpumps, run with efficiencies up to 97%. Electrical motors smaller than 200 kW are used extensively in power plants, but their total power consumption is relatively small compared with larger loads.
Constant-speed electrical motors operate at highest efficiency between 60% and 100% of rated load. Efficiency drops off sharply below 50% load. In utility applications, it has commonly been the practice to specify oversized motors for small and intermediate loads. Oversized motors not only operate less efficiently compared with correctly sized motors, but they are also more expensive. For a new power plant, proper sizing of motors is likely to be more critical than specifying high-efficiency, oversized motors. The same argument goes for existing power plants. When an existing motor needs to be replaced, care should be taken to ensure its replacement is a properly sized high-efficiency motor.

Steam turbines may be used to drive auxiliaries in place of electric motor drives. This is common practice for feedwater pumps, the largest auxiliary load, and on occasion, on the FD and ID fans. Steam turbine drives commonly use intermediate-pressure (IP) or low-pressure (LP) steam and may share a condenser with the main LP turbine or might be installed with their own condenser. Another possibility would be to employ a back-pressure turbine as a drive for the main feedwater pump and use its exhaust for feedwater heating. The efficiency of such a design needs to be evaluated.

Typically, there is a little difference in heat rate penalty between the electrically driven and steam turbine-driven main feedwater pumps. This higher efficiency of the main steam turbine is offset by losses such as generator, transformer, cabling, motor, and hydraulic coupling losses (hydraulic coupling is used for pump speed control). Therefore, other factors, such as unit availability, capital cost, part-load performance, and performance during unit start-up (black start especially) and shutdown, and sudden load loss need to be considered both for new units and retrofits.

Variable-Speed (Frequency) Drives

As presented in the table, auxiliary power use for handling of the combustion air and flue gas represents a substantial part of the total auxiliary load use. Depending on how the plant is dispatched, these fans can run anywhere from part- to full-load. Capacity control of the fans (control of combustion air or flue gas flow through the fan) can be achieved in two ways: controlling the flow into or within the fan or controlling the speed of the fan. Please note that fans are volumetric machines that operate on their performance (fan) curve, that is, a given flow rate of fluid through the fan corresponds to the pressure rise across the fan. The fan performance curve is affected by temperature of the fluid and maintenance status of the fan. The control of flow is achieved by throttling of fan discharge pressure (fan pressure rise), which must be higher than the total system resistance. This results in fan power losses.

The system resistance curve is a function of flow rate, and changes due to changes in coal quality (both higher heating value and composition) and fouling of the APH and SCR catalyst. Also, in the SCR retrofit applications, an oversized ID fan is usually specified to allow for pressure drop in additional catalyst layer(s) that might be added in the future, so additional throttling of the fan discharge pressure is needed, leading to increased fan power losses.

Traditional methods for fan flow control include:

  • Inlet damper control (IDC), which controls the flow into the fan.
  • Inlet guide-vane control (IGVC), which controls flow within the fan.
  • Two-speed motors, which control the speed of the fan.

With IDC and IGVC for flow control and a given flow rate of combustion air or flue gas through the fan, fan discharge pressure (fan pressure rise) is matched to the system resistance by adjusting (reducing) pressure drop across the flow control device (Figure 3). IGVC tends to be slightly more efficient than IDC.

3. Typical fan curve. Source: ERC

Two-speed motors have two fan curves, one for each speed. The system resistance curve intersects each fan curve at a different flow, depending on the speed of the motor (Figure 4). The final adjustment of fan pressure is achieved by using the IDC or IGVC flow controls. Many two-speed motors have been applied to fans to gain the overall high efficiencies available from two running speeds. Two-speed motors are also the least expensive and least complex control method. However, operators are often reluctant to change to a lower speed until far below the design load limit, which negates the economy of the system.

4. Typical two-speed motor fan curve. Source: ERC

VSDs Offers Many Advantages

Variable-speed (or frequency) drives (VSDs or VFDs) allow fan pressure rise to be matched exactly to the system resistance without throttling of the fan discharge pressure. This is accomplished by varying the speed of the drive (electric motor) until the fan curve intersects the system resistance curve at the required flow rate of combustion air or flue gas.

Other possible applications of VFDs in utility power plants include gas recirculation  fans, circulating water pumps, fan drives for mechanical draft cooling towers, and other large motors.

Despite their obvious advantages compared with traditionally used methods of fan flow control, use of VSDs in utility power plants has been relatively rare. Some common misconceptions about VSDs are that they are expensive, difficult to maintain, and have a long payback period.

A typical argument against VSDs is that the power plant is baseloaded. Baseloaded power plants are some of the largest generating units in the U.S., and their efficiency is of critical importance. Even when operating at full-load, some throttling of the fan pressure rise takes place in order to control the flow of combustion air and the combustion process in general. Also, many baseloaded units cycle at night and on the weekends. The baseload units may also have once-through cooling systems or mechanical draft cooling towers. The cooling water systems in both of these cases have large motors and flows, which have seasonal variations (temperature and flow) that might justify VSD installations. These units also need to be investigated to determine if there are economic benefits to using VSDs. In addition, more baseload units will be operating as cycling units in the future as more alternative energy suppliers come online.

As previously mentioned, some of the largest auxiliary power loads in a coal-fired power plant are FD and ID fan drives. FD fans are used to provide combustion air to the furnace, and they normally follow unit load. As combustion air and fuel flow vary with load, the ID fan flow varies to maintain a constant furnace pressure. VSDs on the FD and ID fans would provide precise linear speed control over the entire operating range without any power losses.

The use of VSDs for these fans is an excellent use of the technology due to the fans’ large power consumption. This is even more evident when the ID fans are used in conjunction with an SCR system. In this application, the ID fans are generally oversized to account for future operation with additional catalyst layers and for the eventual APH fouling. When a new ID fan is first installed, it will only run at a fraction of its design full flow capability. This is extremely inefficient and unnecessarily adds to the station parasitic load. Some stations add booster fans instead of replacing the existing ID fans when retrofitting an SCR system. This is also an inefficient approach and adds serious fan coordination and control problems between the booster and the ID fans. There can even be significant performance loss from non-uniform flow from the ID fan into the booster fan.

Other power plant applications that should be considered for VSDs include SO2 scrubber booster fans drive and, as previously mentioned, mechanical draft cooling tower fan drives. Hundreds of VSDs have been installed on a large number of power plant fans, ranging from 600 hp to 11,000 hp.

VSD Economic Benefits

Atmospheric and process flow conditions affect fan flow requirements. Of the available controls that can be retrofitted, the least energy efficient is the inlet damper control (IDC) and the most energy efficient is the VSD (Figure 5).

5. Power requirements for IDC and VSD on power plant fans. Source: ERC

As the flow rate of combustion air or flue gas through the fan decreases below 80% of rated flow, the power savings from a VSD increase greatly over the use of IDC. The reason for this is that the performance of a centrifugal fan is controlled by fan laws that state:

  • Flow is directly proportional to the fan speed.
  • Fan pressure rise varies with the square of the fan speed.
  • Fan power varies as the cube of fan speed.

An example of a centrifugal fan curve is shown in Figure 6. Also shown in the figure are system curves, which show the operating points at different fan speeds. As discussed earlier, the traditional way of changing the operating point is using a fan inlet damper or inlet vane control, which reduce fan pressure rise to match it to the system resistance curve at required flow. This reduction of fan pressure represents power loss. An alternative, much more efficient method is to change the fan speed using a VSD so that the system curve will intersect the fan curve at a required flow, without incurring power losses due to throttling.

6. Matching fan pressure rise and system resistance through variation of fan speed. Source: ERC

An important aspect in considering a VSD retrofit is the economic benefits. Vacon PLC, which develops and produces variable-speed AC drives globally, has energy savings calculators http://www.vacon.com/Default.aspx?id=450399 that allow you to estimate online how much a VSD can save in energy and cost compared with traditional control methods in pump and fan applications. The fan drives savings calculator is used to determine (and print) the savings for VSD versus other type of regulation methods. The input data for the calculator is shown in Figure 7.

7. The Fan Drive Calculator. Source: Vacon PLC

For example, the economics of a VSD can be compared with vane control. At one coal-fired power plant, SCR systems were added to the units. In the case study below, the scrubber booster fan motor was upgraded to a 1,000 hp (746 kW) VSD. Typical results using the calculator are shown in Figure 8. Because specific information about the fan profile was not available in the case study, the only variable changed in the calculator for this example was the fan flow rate to illustrate the order of magnitude in savings.

8. The potential savings using the VSD versus vane control was calculated. Source: Vacon PLC

In this example, note that changing from vane control to VSD would save about 1,067,177 kWh for the year. The energy cost savings would be about $64,031 per year. Actual savings for any installation are site-specific.

Case Study 1: VSD Replacement

The Abbott Power Plant at the University of Illinois at Urbana-Champaign  is a coal-fired unit with SO2 scrubbers that supplies energy for campus buildings’ heat and air conditioning. The existing scrubber booster fan was regulated by inlet vane control. In an effort to reduce operating and maintenance costs, the 1,000 hp (746 kW) fixed-speed scrubber booster fan drive was replaced with a medium-voltage VSD. The replacement VSD was installed and started-up over a two-day period.

The VSD supplier estimated that benefits from the retrofit included:

  • Energy savings of $63,000 per year. This was a 25% improvement over the inlet vane control fan.
  • Maintenance and hardware cost savings saving of $10,000 per year.
  • Total process controllability.
  • Quieter operation.
  • 24-month payback on investment.

Case Study 2: Upgrade ID Fans with VFDs

A feasibility study was performed for a 427-MW coal-fired power plant where the benefits of retrofitting constant-speed motor drives and IGV control for ID fans was evaluated by the VFD manufacturer. The ID fan motor is rated at 1,500 hp. The comparison in fan power savings per ID fan is presented in Figure 9. As discussed earlier, with a VFD, fan power is significantly reduced. In order to evaluate actual power savings, the plant load profile needs to be accounted for. In this case study, the plant load profile is illustrated (Figure 10) in terms of operating time at a given load (expressed as a percentage of maximum continuous rating [MCR]). As the data show, this plant runs at 100% MCR approximately 30% of the time, 38% of the time at 70% MCR, approximately 16% of the time at 30% MCR, and for the remainder of the time (16%) is offline for maintenance..

9. The difference in auxiliary power required for an ID fan is shown for the IGVC and VFD options in Case Study 2. Source: ERC

10. The typical utility plant load profile used in Case Study 2. Source: ERC

11. The potential ID fan power savings, per fan, in Case Study 2. Source: ERC

The power savings per ID fan, taking the load profile into the account, are presented in Figure 11. The cumulative annual savings per ID fan for this unit are 2,353 MWh, or 44%. Assuming an electricity price of $40/MWh, this corresponds to annual savings of more than $94,000 per ID fan and an internal rate of return of more than 43%.

Assuming the same plant is operating at 100% MCR 74% of the time and is off-line for outages 16% of the time, cumulative annual savings per ID fan are 1,095 MWh, or 18%. Assuming an electricity price of $40/MWh, this corresponds to annual savings of more than $43,800 per ID fan.

Nenad Sarunac is principal research engineer and associate director at the Energy Research Center, Lehigh University. The Illinois Clean Coal Institute funded a portion of this work.

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