Air-cooled Corrosion Probe successfully fabricated, installed and operated at Lough Ree Power

By Michael Rocke, Group Safety, Engineering and Environment Manager for the Coal/Peat/Hydro Group of Stations ESB power Generation's peat stations are Lough Ree Power (LRP) in LanESBoro, Co. Longford and West Offaly Power (WOP) in Shannonbridge, Co. Offaly. Both stations are in service since 2004. They both have a Circulating Fluidised Bed boiler design, which is the state-of-the-art design for providing the lowest possible emissions of nitrogen and sulphur oxides and the design also allows for extremely efficient and complete combustion of any fuel, in this case locally harvested peat.

The flue-gas cleaning plant is next to the boiler and is comprised of an absorber and a highly efficient fabric filter bag-house.

Why is the corrosion probe required?
Low-temperature corrosion has been an observed problem in the flue-gas cleaning plant in both stations and it occurs whenever high chlorine and high humidity levels are present in the flue gas. Salt compounds form, they are corrosive to the steel components. This problem also occurs in Waste to Energy plants. As a measure to proactively manage this, a corrosion probe is used to indicate the operating conditions where these salt compounds form, allowing for corrective measures to be taken.

A corrosion probe has now been successfully fitted to the fluegas cleaning plant at Lough Ree Power. This instrument has given the station invaluable information on the optimum running of the unit to minimise the effects of low-temperature corrosion. By simulating low-temperature operating conditions with the probe, it is possible to conduct 'what-if' type analysis for different fuels and operating conditions without the risk of damage to the flue gas cleaning plant.

Description of the corrosion probe
The probe itself consists of a one-metre length steel pipe fitted with thermocouples and cooled by compressed air. The probe must be kept at a constant temperature using a closed-loop controller. All the mechanical components and instrumentation components for the probe were sourced by the maintenance department at Lough Ree Power. Liam Doyle, Maintenance Team Leader, and apprentice fitter Brendan Finnegan assembled the mechanical components while Pat Lane put together the PI controller, thermocouples, data logger and air control valve to precisely control and measure temperatures along the length of the probe.

What is the closed-loop control system?
Metal temperatures must be carefully controlled if the probe is to provide accurate and useful information. Advice on the best type of closed-loop control system was obtained from John Egan in training services. This control system is required to ensure that the probe is able to automatically respond to varying boiler operating conditions such as load and fuel variations (this is similar to when a car fitted with cruise control negotiates going up a hill and needs to maintain a constant speed by increasing fuel supply).

Once fully assembled in the workshop at LRP, the probe was mounted and exposed to the flue gas passing from the absorber into the bag-house for a series of tests, each lasting for about a week in duration.

What has the probe told us? The probe has been shown that the corrosion in the flue gas cleaning plant at Lough Ree Power is significantly reduced when the flue gas is maintained above a certain threshold temperature for a given level of humidity. This has allowed verification of the theoretical safeworking temperatures calculated by experts without any corrosion risk to the plant inherent in varying the actual process temperatures. The probe also clearly demonstrates that operating at the optimum temperature prevents build-up of sticky ash deposits on metal surfaces and further confirms theoretical predictions.

Conclusion The air-cooled probe has proven to provide invaluable information to the staff at Lough Ree Power in ensuring optimum performance and high availability of this 100MW electricity generating unit.