Disclosed are a process and apparatus for effecting chemical and/or physical reactions by means of solid-gas contact. The apparatus has a shaftlike reaction chamber widening in upward direction and divided into compartments by means of horizontal grids. The process is performed by introducing a particulate solid of about 0.5 to 5 millimeters particle size into the reaction chamber and passing a gas stream in upward direction through that chamber at an output sufficient to keep the particles in floating condition. A floating bed of solid particles will be formed in the reactor then and excellent solid-gas contact suitable for effecting chemical and physical reactions is provided hereby. A special embodiment relates to the removal of nitrogen oxides from flue gases with the aid of a particulate catalyst.