3D modeling of ammonia co-firing in a 50 kWth CFB test rig: NO pathways and environmental implications for carbon-free power generation

One of the objectives of the new generation of concentrating solar power plants is to increase the actual temperature limit (565 °C) up to or near 1000 °C. An alternative heat transfer fluid that could help to reach this objective is the use of a fluidized dense gas–particle suspension ascending through a vertical tube. Since the predominant cause of rupture in solar receivers is the creep-fatigue damage process, which is mostly influenced by the maximum temperatures and stress experienced along the tube, the main objective of this work is to numerically study the thermomechanical behavior of a non-uniform externally irradiated tube where particles are fluidized and moves upwards. The heat transfer problem in the two media present (i.e. dense suspension of particles and tube wall) was solved separately: a Computational Particles Fluid Dynamic model solves the heat transfer in the particles, whereas a three dimensional Finite Volume model simulates the heat conduction through the tube wall to obtain the temperature profile, which serves as input to calculate the thermal stress of the tube with an analytical method. Higher thermal stresses were obtained for an absorbed power of 250 kW/m² compared to that for an absorbed power of 500 kW/m² due to the lower temperature difference between the front and rear sides of the tube caused by the more pronounced increase of the radiative losses in the front side of the tube compared to that at the rear side. The thermomechanical behavior of a particle receiver was compared to that of a molten salt receiver. For an incident solar flux of 500 kW/m², the particle receiver reduced the maximum thermal stress by 73% compared to the molten salt receiver. However, the tube’s maximum temperature exceeded the working limit for stainless steel tubes. In order for the receiver to survive heat fluxes characteristic of solar power plants, research into technological solutions that allow to improve the effectiveness of the heat transfer rate from the tube to the particle flow is necessary.