Ash deposition control technology: A strong enabler for performance upgrades in HFO CCGT plants

摘要

Heavy duty gas turbines are known for their high energy performance and their extended fuel flexibility. However, burning low grade fuels such as heavy fuel oil (HFO) in GTCCs results in significant performance penalties due to the fouling of the expansion turbine by ash deposits. There is actually a prospect for considerable performance and profitability gains, if one could suppress or reduce this adverse ash deposition effect. However, this represents a tricky technological challenge because the ash particles fly at very high temperature across the turbine section of the machine and hit the GT hot parts at high speed. In contrast, in a boiler plant, the prime mover, i.e. the steam turbine, is driven by high purity steam, the quality of which is kept under strict control regardless of the quality of the fuel. A further difficulty faced when trying to reduce ash deposition in gas turbines lies in the additional fouling entailed by the “inhibition process”: the combustion of HFO requires the injection of a magnesium-based additive to inhibit the corrosion caused by the vanadium compounds contained in HFOs. This magnesium-based corrosion inhibitor substantially increases the amount of ash crossing the machine and accentuates the decay of performances, namely the pace of power output loss. A joint initiative by LTL Holdings Ltd, GE Power & Water assisted by three academic organizations has resulted in a feasible ash modification solution that has been successfully tested, providing a reduction by 50% of power output decay when running on HFO. This novel family of vanadium inhibitor brings interesting improvements in terms of GT efficiency and plant availability as it enables reducing the frequency of the machine shutdowns that are periodically required to remove the ash deposits by water wash. In addition, it procures a strong soot suppression effect, a second, significant added value of this technology. The paper outlines the development carried out in this project and sets out the successful field test performed in 2016 at the Kerawalapitiya 300 MW power plant in Sri Lanka. It also offers an analysis of the evolution of the performance of gas turbines fired on HFO, which shows the criticality of the cleanliness of the turbine.