Case study: Fast boiler repair at biomass plant

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

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

When the walls of the boiler in Lisahally Power Station, in Northern Ireland, had sustained erosion and corrosion damage, automated weld overlay technology was utilised to extend the service life of the boiler and ensure that the plant could continue to run at peak performance.

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

As a result, regular maintenance is fundamental to maximize the lifespan and reliability of equipment in these power stations as well as ensure plant 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 walls with 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 to generate 6 MW of heat and 18.2 MW of power, 15.8 MWe of which is then used to power 30,000 local homes and businesses.

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

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

As an economical and sustainable alternative to replacing the existing structures within the boiler, it was decided to cover the damaged membrane walls with layers of Inconel 625 to repair and protect the membrane wall. This is an austenic nickel-chromium-based superalloy that is particularly resistant to corrosion and oxidation, even in harsh environments with elevated temperatures, such as in furnaces.

The additional Inconel layer would enable the boiler to efficiently maintain high steam pressure whilst protecting the surfaces from future corrosion and erosion.

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

Using advanced automated technology, highly accurate welding operations were carried out for structures such as boilers, furnaces, vessels and towers.

CladFuse, the technology in question, uses a carriage that travels along a laser levelled track system fixed to the wall that needs repairing. On the carriage, a robotic index arm moves the welding torch and the oscillator in order to create weld Inconel beads with an overlap of approximately 50 per cent between adjacent weld beads. All the weld overlay process parameters, e.g. carriage speed or bead thickness, are controlled by a programmable logic controller.

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

In addition to offering a cost-effective and high-quality method to repair damaged metal surfaces, the solution is extremely fast. This was particularly important for managers and operators at Lisahally, who needed the welding to be completed by the end of a scheduled outage for regular maintenance activities.

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