21st century ESP design- synergism of old and new concepts

Demonstrating the effects of the successful integration of three ESP design technologies - discharge electrode configuration, power supply type, and collecting plate spacing - into the design of two utility ESP’s

Enstreet News Bureau
10/3/2009

Until recently, it was believed that dust emission levels at or below 0.015 lb/MBtu from pulverized coal fired boilers could be routinely achieved only using fabric filters.  Since the turn of this century, there have been advances in electrostatic precipitator (ESP) design, along with the equipment that provides power for the process, that have substantially lowered this imaginary floor to emission levels that rival those delivered by fabric filter technology.  Data provided in this paper will demonstrate the effects of the successful integration of three ESP design technologies - discharge electrode configuration, power supply type, and collecting plate spacing - into the design of two utility ESP’s.  The first application is on an 80 MW utility ESP rebuild, and the second is a 460 MW coal fired ESP retrofit.  Results exceeded expectations by a wide margin.

ESP Discharge Electrode Design

The ELEX/RS discharge electrode has an operating history in the U.S. dating back into the 1970’s.  The characteristics and performance of this electrode have been well documented(1). It is widely recognized that this electrode is among the best current distributing discharge electrodes in the industry, and it typically promotes production of very high current densities.  These characteristics alone lead to increased overall ESP performance.

An additional feature of this electrode is its adaptability to customization.  This capability permits the designer to create an optimum set of electrostatic conditions in several different areas of an ESP.  An example would be doubling the quantity of electrode emitter points in the inlet field(s) of an ESP, enabling production of higher current levels to overcome space charge effects, coupled with a voltage enhancing electrode configuration in the latter fields. 

The ELEX electrode can be further optimized by varying the tip spread of its emitter tabs.  Air load tests have demonstrated corona current density increases of as much as 44% over pipe and spike type electrodes(2).  The ability to effectively control the voltage/current relationship in each area of the ESP through variation of the emitter tip spread is a feature available only with the ELEX electrode, and this reflects a major reason for its superior performance. 

ESP Power Supplies

A second recent advancement in ESP technology is the use of switch mode power supplies (SMPS) in lieu of conventional transformer/rectifier (TR) sets.  Various forms of this technology have been studied for the past several decades, and in recent years this product has become more refined and widely accepted.

In summary the major advantages in electrical performance of a SMPS over a conventional TR set are:

• Improved power factor
• Increased secondary voltage and secondary current
• More efficient delivery of power to the ESP

These effects have been observed and applied on numerous occasions over the past decade or so, such that a clear trend of improved performance using SMPS in conjunction with ELEX electrodes has been established.

Several units employing the switched mode power supplies in conjunction with ELEX electrodes have started up in the past few years, all with very favorable results.  Test results from a 2005 startup whereby the internals (collecting plates and discharge electrodes) of the existing ESP were completely removed and replaced, collecting plate spacing increased from 9 inches to 12 inches, and the TR power supplies replaced with SMPS units, resulted in a 70% improvement in opacity.

Wide Plate Spacing

While wide plate spacing designs have been widely utilized in Europe and Japan or many years, they have not yet enjoyed widespread acceptance in U.S. market.  This is likely due to the relatively slow evolutionary process from the narrow plate spaced ESP’s of the 70’s and 80’s to the more common 12” spacing of today.  A myriad of reasons, such as electrode geometry, size of available power supplies, insulator design, and the need for larger electrical clearances have certainly influenced this trend.  The clearance issue comes into play in numerous rebuild situations, as many of these entail upgrading from 9” plate spacing.  A number of these older designs with wire type electrodes and smaller power supplies were established with no anticipation of conversion to wider plate spacing and its attendant design requirements.

A Small Utility Application

The first startup of a utility ESP employing all of these features occurred in July of 2008 on an 80 MW coal fired unit at the Tecumseh Generating Station (Tecumseh, Kansas) of Westar Energy.  Design and installation of the ESP rebuild was conducted by Southern Environmental, Inc (SEI, Pensacola, FL, USA).

The Tecumseh unit represents the first start-up of a utility ESP employing SEI’s new ESP design concept, whereby all three design improvements discussed herein are married into a single utility ESP installation:        

• Customized ELEX rigid discharge electrodes
• NWL Switched mode power supplies
• Wide (~16”) collecting plate spacing

Test results are as follows: 

Performance Test Results- Particulate Emissions

Average = 0.0038 lb/MBtu

A Full Size Utility Application

The most recent ESP configured in this manner was installed and tested at Ameren Energy Generating Company’s Duck Creek Station in Canton, IL.  The ESP was a retrofit installation on a 460 MW coal-fired unit burning a PRB/Bituminous coal blend,  Installed and tested in May of 2009, this installation  represents the first ESP competitively sized and bid using these design synergies.    

The flue gas conditions and target emissions are as follows:

The proposed ESP was sized as follows:

Performance Test Results- Particulate Emissions                                  

Average Particulate Emissions = 0.0047 lb/mmBtu
Opacity = 2.7%

Fractional Efficiency Data

In addition to the data above, efficiencies throughout the particle size ranges shown below were measured by a cascade impactor, and compared against similar test data from a technical paper (3) presented in 2006:                                 

Note that in all three cases the overall PM collection efficiency is approximately the same value.  When comparing the Duck Creek ESP data against fabric filter  and conventional ESP data, it is clear that the performance of the Duck Creek ESP is exceptional in the finer particle size range (PM_2.5 and PM₁), and clearly represents a very favorable comparison to fabric filter performance across all particle size ranges.

Conclusion

As presented herein, the emissions guarantees and accompanying results extend ESP performance projections to levels previously thought to be limited to fabric filters.  These results clearly project modern ESP’s into that market niche, thereby providing a cost effective alternative to current technologies.

Given this now consistent success in achieving extremely low emission levels with a 21st century design ESP, a companion solution for dealing with mercury control becomes the next step.  The design of the Tecumseh rebuilt ESP dealt with that eventuality by leaving a vacant portion of one of the ESP’s fields for future installation of a Toxecon II carbon injection system. 

REFERENCES

Hatsfelt, Michael W., Chaney, James R., Grieco, Gary J.;  “SEI/ELEX Rigid Discharge Electrodes on Narrow Plate Spacing”  Paper Presented at The 2003 DOE/EPRI/EPA/AWMA Combined Power Plant Air Pollution Control Symposium, Washington, DC, May 19-21, 2003

Chambers, Mick, Grieco, Gary J.;  ”Customized Rigid Discharge Electrodes Show Superior Performance on Utility Flyash Applications”  Paper presented at ICESP VIII, Chicago , IL,  August  20-23, 2001

Yi, Honghong, et al: Journal of AWMA, September, 2006

Contributed by-

Mick Chambers
Director of Pre-Contract Operations
Southern Environmental, Inc.
Pensacola, Florida

James R. Chaney
Generation Engineering
Ameren Energy Resources
St. Louis, Missouri

Gary J. Grieco
Consultant
Air Consulting Associates, LLC
Morris Plains, New Jersey