The Problem: A History of Screen Breaches

This unit had proven to be a very difficult application for LPA screens, as two different designs had been attempted previously, with little success, which resulted in screen breaches between outages and extensive LPA accumulation in the SCR.

LPA Screen Installation

The IGS NoNOx LPA screen was installed in 2017, replacing the previous OEM design. The superior design and protective High Velocity Thermal Spray (HVTS) cladding proved successful in this aggressive application, with no reported screen breaches between outages. This provided reliable LPA filtration and protected the SCR. This, along with the SCR air cannons and CatFlow® screens, addressed the SCR ash piling and eliminated SCR related outages and derates.

Ensuring Long-term LPA Screen Reliability

However, similar to brake pads on a car, the LPA screen system does require periodic maintenance to provide long-term performance. During an inspection in early 2023, although no breaches in the LPA screens were found, it was determined that the LPA screens were showing signs of wear, and action would be required to extend their service life and ensure long-term reliability.

The Solution

IGS reviewed the various maintenance options with the plant, and it was decided that a partial screen panel replacement and full system refurbishment was the most cost-effective solution to keep the system in a reliable condition.

The refurbishment process involved media-blasting the LPA screen, which allowed for a detailed inspection of each screen panel while simultaneously preparing the screens for the refurbishment coating application. IGS then inspected the panels closely, looking for indications of severe wear and damage that would require replacement. Worn out panels totalled roughly 20% of the screen panels, which were replaced with new NoNOx LPA screen panels.

The Result

The refurbishment of the LPA screen ensured it would continue to offer reliable protection of the SCR for years to come, while also avoiding a full LPA screen panel replacement. This provided significant cost savings.

Additionally, the refurbished top coat made future inspections quick and easy by making the green pigment a wear indicator. An inspector can now assess and monitor the LPA screen condition quickly and regularly, ensuring proper maintenance and eliminating the potential for LPA screen failures.

The Problem: Catalyst Screen Erosion

Fly ash accumulated leading to further flow disruption and extreme erosion of the catalyst. It was found that the existing LPA screens were in disrepair due to poor design and neglected maintenance, resulting in significant LPA bypass to the SCR. These screens comprised mostly of wire mesh sheets constantly eroded away between outages and allowed large volumes of LPA to pass through.

Popcorn Ash Build-up

This LPA was littered across the first and second catalyst layer, resulting in the flue gas flow being forced through less than half of the designed area. Over time, the resulting catalyst pluggage became so severe that large portions of the catalyst completely eroded away. The damage to the catalyst on layer 1 was beyond repair and attempting to clean the modules was not a viable option.

Action Required to Remain Environmentally Compliant

It became evident that to meet environmental compliance, drastic improvement was required. The situation was so severe there was concern that it would not be economically viable to restore the system, which threatened to shut the unit down permanently, well before its planned retirement date.

The Solution

Due to the remaining life expectancy of the plant, a full system overhaul was not economically viable. However, to save the operation of the unit, three things were clear. The first layer of the SCR catalyst had to be replaced, LPA had to be prevented from reaching the SCR, and the remaining layers had to be cleaned and restored.

Restoring DeNOx Performance

To save the unit and keep the project within a reasonable budget, IGS worked with the plant to devise a hybrid retrofit of the LPA screen, focusing on upgrading the highest risk screen regions.

To restore the DeNOx performance and improve the ash handling capacity of the SCR, the first catalyst layer was replaced with a hybrid layer of DustBuster and a standard honeycomb catalyst, all of which was retrofitted with IGS CatFlow® screens. The screens help to reduce the fly ash accumulation while also providing a last line of defense against combustion and non-combustion LPA.

Lastly, the lower layers of the SCR were cleaned using dry ice blasting and vacuuming. Although the first layer provides the bulk of catalyst potential, restoration of the lower layers was critical to allow the unit to reach its designated end of life, near 2030.

This combined approached proved by working with experienced SCR experts, it was possible to bring the unit back to compliance within a reasonable operating budget.

Proactive Maintenance is Key

The facility chose to invest in a high-quality, long-lasting, and reliable NoNOx LPA screen system to protect the SCR. Other than regular inspections, very little maintenance was required for the first six years of operation. However, similar to high-performance brake pads on a heavy-duty truck, even the best LPA screens wear over time and eventually require routine maintenance and replacement. Screen breaches (as shown in the image) aren’t uncommon but can be costly.

The Solution

Understanding the importance of keeping the SCR free from LPA, this plant adopted a proactive maintenance approach. This involved regular detailed inspections of the system and documentation of wear patterns found on the screen. If a screen panel was found to have significant erosion (shown in the image), it was replaced with spare panels held onsite prior to screen failure, eliminating the risk of LPA screen breaches between outages.

LPA Installation

By 2022, the LPA screen system had reached the end of its expected service life. Although the panels were in relatively good shape for their age, the plant decided to fully replace the LPA screen panel array, to ensure reliable LPA filtration for years to come. The plant also decided to upgrade the system to the two-stage LPA screen coating, which offers better erosion protection and a green visual wear indicator. This visual wear indicator dramatically reduces inspection time while allow improving inspection accuracy.