Level switches keep electrostatic precipitators online

A Midwest US power plant has eliminated spillover of fly ash from its hoppers, by enlisting Endress+Hauser to assist in the upgrade of its level switch technology.

Fly ash is composed primarily of inorganic, non-combustible minerals such as alumina, silica and iron and is one of the non-combustible materials that remain after burning coal to produce electricity. Along with the other by-products, which include bottom ash, and flue gas desulphurisation gypsum, fly ash requires disposal but has proved to be an economical, high-quality raw material feedstock for several other industries. For example, the materials can be used in structural fill and in land development, buildings, metals recovery and highways, and to manufacture agricultural fertilisers, carpeting and plastics filler.

Although most modern power plants collect their coal combustion products to meet environmental regulations, coal-fired power plants have discovered they can be profitably recycled. Many plants now sell combustion products to add to their bottom line.

The US power plant in question was built around four decades ago and is boiler rated for just less than 500 MW. Over the last decade, it has invested hundreds of millions of dollars into environmental equipment designed to reduce SO₂ emissions by over 95%. It is now a leader in its re-use program for fly ash, bottom ash and FGD scrubber material. For example, 100% of the gypsum produced from the plant scrubbers is used to manufacture wallboard.

Typical electrostatic precipitator at a power station.

One remaining issue, however, was that spillover from fly ash hoppers was creating issues with Environmental Protection Agency/Occupational Safety and Health Administration reporting as well as causing a major hazardous cleanup procedure. The old capacitance probes that were being used to measure the level of fly ash collected in the hoppers of the electrostatic precipitator (ESP) were so unreliable that plant operators did not trust the control information the devices were supposed to be providing.

Failures were frequent, and when they happened, the precipitator would often clog up, resulting in downtime and repair costs. On one occasion the failure of an older-design point level device resulted in damage to a precipitator that cost $100,000 to repair. This did not include the lost electricity production, which was worth several times the cost of repairs.

The low reliability of these older technology devices was most likely a result of high temperatures, insufficient surface area on the probe for the low dielectric constant, inability of the devices to withstand the physical rigours of the application or a combination of these reasons.

The key element of the upgrade project was Endress+Hauser’s site visit to review the point of installation to ensure the instrument was properly configured and set up. Thereafter it was important to ensure probes were delivered on time as the power plant had a window of time for which it had scheduled maintenance.

For applications such as this, a level switch that is designed for point level detection at high temperatures in bulk solids is the best choice. The switch must also be able to withstand the harsh conditions of abrasive fly ash. After considering its control device options, the utility installed the Endress+Hauser Solicap S-FTI77 level switch on the precipitator to replace the failed capacitance sensor.

There were a number of reasons that made the FTI77 a suitable choice. The switch features active build-up compensation for when fly ash accumulates around the probe, facilitating safe switching. It can also withstand a maximum load or lateral stress of up to 800 Nm, making it suitable for fly ash hopper applications.

In addition, the FTI77 has a sword probe instead of a round probe to increase the surface area of capacitance, which improves performance in low dielectric applications such as fly ash level measurement. The design of the switch prohibits fly ash from entering the electronics housing should probe damage occur, eliminating the possibility of any static electricity discharges.

Space was also a consideration in this project. The precipitator hoppers were grouped tightly together with limited clearance. The FTI77 requires probes to be only a minimum of 508 mm apart, with the threaded coupling as short as possible as condensation or product build-up can adversely affect operation in long-threaded couplings.

The installation of the FTI77 switch was so successful in enabling the plant to eliminate the spillover of fly ash from its hoppers, it is now planning to install two FTI77 switches on all of its precipitators.