Evaluation of advanced weld overlay, thermal spray and laser clad coatings in an operating waste wood fired biomass boiler.

Different surface protection technologies were investigated in a full scale commercial waste wood fired bubbling bed fluidised boiler. This biomass fuel environment is more aggressive than those firing virgin wood, due to the elevated presence of sodium, potassium, lead and zinc, leading to the deposit of alkali and heavy metal chlorides in conjunction with ash on boiler tube surfaces. As laboratory tests are seldom representative of the complex firing, chemistry, temperature and local heat flux in true operating conditions, five different commercial, near commercial and development coatings were applied to a 1m length of PG235GH plain carbon steel tubing used in the furnace walls. The coatings were fully characterised and measured prior to installation and after exposure. Advanced Iron and Nickel based weld overlays, two high velocity thermal spray coatings and a laser clad nanosteel type coating were tested. After exposure, the tube was extracted from the boiler with the structure and circumferential distribution of the corrosion scales and material loss rates evaluated in comparison to the unprotected tube material. The technologies considered here could be employed to reduce current wastage rates in existing units and provide protection against uncertain future fuel mixes.