Erosion lifetime prediction for a radial turbine operation in a low dust stream

This study is part of the work performed for the pilot plant of an innovative indirect fired gas turbine cycle for biomass incineration using the Pebble-Heater technology and a microturbine. A Pebble-Heater is a regenerative heat exchanger consisting of an annular bed with radial flow, which implements an unsteady counterflow heat exchanger. It captures the flue gas enthalpy during the atmospheric heating phase and heats compressed air for the turbine during the blowing phase. During the heating phase ash particles from the incinerator deposit on the bulk material and are segregated by periodic exchange and cleaning of the bed. Like this a high degree of de-dusting and cooling of the flue gas is achieved upstream of the emission control filters. In the blowing phase, however, a small amount of ash (< 3mg/m_N³) is re-suspended and condensables re-sublimate into the hot air stream, which feeds the turbine. In this paper we develop a simple procedure to estimate how the size distribution and concentration of the dust load decrease the lifetime of the radial turbine due to erosion. It is based on the consideration of single particles following the gas flow. To obtain a lifetime estimate the focus was set on the erosion of the radial turbine blade tips, which are identified as the most critical parts with respect to particle impacts. On the basis of the particle relative velocity and trajectory determined with the flow model the turbine lifetime was calculated with an erosion rate model. The paper discusses the influence of particle size and distribution on turbine lifetime. For the current turbine it was found that equivalent particle diameters above 3-4μm and dust loads greater than 1 mg/m_N³ reduce the nominal turbine lifetime drastically.