Case study: Fast boiler repair at biomass plant

Biomass is becoming an increasingly popular fuel solution forpower generation. It is one of several technologies being implemented toimprove the sustainability of power supplies as demand continues to increase.

As with all facilities, regular maintenance is required toensure operability and any repairs need to be carried out quickly and to a highstandard.

Whenthe walls of the boiler in Lisahally Power Station, in Northern Ireland, hadsustained erosion and corrosion damage, automated weld overlay technology wasutilised to extend the service life of the boiler and ensure that the plantcould continue to run at peak performance.

The fuels used to operate biomass power plants cause corrosionand erosive wear in boilers. In fact, biomass can release contaminants, such asalkali metals, chlorine, sulphur and other corrosive chemicals, when burned.

As a result, regular maintenance is fundamental to maximize thelifespan and reliability of equipment in these power stations as well as ensureplant availability, performance and efficiency.

In an effort to increase the durability of its essential assets,maintenance planners at the power station wanted to repair the boiler wallswith a long-lasting solution that could be completed with minimal disruption.

Fuelling plant efficiency and preserving equipment health The plant, located in Londonderry, Northern Ireland, was built and is operated by Burmeister & Wain Scandinavian Contractor (BWSC).

The facility uses recycled wood as the main feedstock togenerate 6 MW of heat and 18.2 MW of power, 15.8 MWe of which is then used topower 30,000 local homes and businesses.

The key piece of equipment at the Lisahally plant is thetwo-pass biomass grate boiler, which generates high-temperature, high-pressuresteam by combusting the wood-based material fed to its furnace. The steam isthen transferred to a turbine, coupled with a generator to produce electricity.Waste steam is condensed into water and fed back to the boiler via aclosed-loop system.

As corrosion and erosion were affecting the integrity ofLisahally Power Station’s heat exchange system, the plant managers contactedSulzer to help them conduct the necessary repair work.

As an economical and sustainable alternative to replacing theexisting structures within the boiler, it was decided to cover the damagedmembrane walls with layers of Inconel 625 to repair and protect the membranewall. This is an austenic nickel-chromium-based superalloy that is particularlyresistant to corrosion and oxidation, even in harsh environments with elevatedtemperatures, such as in furnaces.

The additional Inconel layer would enable the boiler toefficiently maintain high steam pressure whilst protecting the surfaces fromfuture corrosion and erosion.

The homogeneous deposition of Inconel onto the corroded anderoded surfaces was carried out onsite by automated weld overlay.

Using advanced automated technology, highly accurate weldingoperations were carried out for structures such as boilers, furnaces, vesselsand towers.

CladFuse, the technology in question, uses a carriage thattravels along a laser levelled track system fixed to the wall that needsrepairing. On the carriage, a robotic index arm moves the welding torch and theoscillator in order to create weld Inconel beads with an overlap ofapproximately 50 per cent between adjacent weld beads. All the weld overlayprocess parameters, e.g. carriage speed or bead thickness, are controlled by aprogrammable logic controller.

Operators can communicate with it and adjust the parametersmanually using a human-machine interface.

In addition to offering a cost-effective and high-quality methodto repair damaged metal surfaces, the solution is extremely fast. This wasparticularly important for managers and operators at Lisahally, who needed thewelding to be completed by the end of a scheduled outage for regularmaintenance activities.