This paper discusses automatic gas-metal-arc-welding (GMAW) overlay cladding for fireside corrosion and erosion/corrosion protection of the waterwall in coal-fired boilers equipped with a low NOx burner system. In addition, the use of overlay cladding for protection against steam soot blower erosion is discussed. Also discussed is the use of the cladding method for protection against damages from water lancing used to remove slag deposits from the waterwall. The overall success of this surface cladding technology for waterwall protection in coal-fired boilers is described.
In a coal-fired boiler, the furnace wall (often referred to as waterwall) is constructed of tubes connected by membranes, surrounding the furnace enclosure for extracting heat for generation of steam. The material for waterwall construction is typically carbon or Cr-Mo steel. The waterwall is subject to high heat flux, fireside corrosion and fly ash erosion/corrosion attack. In general, the fireside corrosion is in a form of oxidation. However, recent installations of low NOx burner systems in boilers for reducing NOx emissions have caused the combustion conditions to change from oxidizing to reducing in the lower furnace. This causes the waterwall corrosion to change from oxidation to sulfidation, thus resulting in a significant increase in tube wall metal wastage rates 1-5. Wastage rates in many boilers have been observed to increase from 0.25 mm/y (10 mpy) or less to up to 2.5 mm/y (100 mpy) or more. The waterwall is also subject to fly ash erosion/corrosion because of dynamic combustion conditions. In addition, constant removal of slag deposits from the waterwall tubes may be needed for maintaining the required heat transfer. Some of the slag removal methods include steam sootblowing and water injection using water lances or water canons. The latter approach utilizes thermal shock to achieve the slag removal. This method can potentially cause the tube to develop thermal fatigue cracking.
In trying to deal with these waterwall materials degradation issues, the industry has been trying to find a cost-effective, long-term solution. Among several corrosion and erosion/corrosion protection methods tried, weld overlay technology involving automatic GMAW overlay cladding has become the most common waterwall protection method for solving the severe tube wastage problems in the boiler 5-10. The automatic GMAW overlay cladding has been applied successfully to mitigate the tube wall thinning problems in both subcritical and supercritical units equipped with low NOx burners. The cladding has also been successfully used for restoring the structural integrity of the waterwalls of many subcritical boilers that have operated for 20 plus years. It generally is more cost-effective to overhaul and restore the existing waterwall on-site by applying a corrosion-resistant weld overlay to prevent further tube wall thinning than replacement with new fabricated waterwall panels. When the thickness of the badly corroded waterwall has been reduced to below the ASME Code allowable, a weld metal build-up using a matching filler metal to that of the tube material can be performed to increase the tube wall thickness to meet the ASME Code before applying a corrosion-resistant overlay. Furthermore, an increasing number of boilers have to resort to water injection with either water lances or water cannons in order to remove slag deposits from boiler tube surfaces due to the use of powder river basin (PRB) coal for reducing emissions and lowering fuel cost. Accordingly, the tubes are more prone to thermal fatigue cracking. The overlay cladding may be an effective protection method against damages from the use of water lances or
