Method for operating a fluidized bed boiler

A method for operating a fluidized bed boiler (6), comprising the steps of:

• a) providing fresh rock ilmenite particles having a shape factor of 0.75 or lower as bed material to the fluidized bed boiler (6);
• b) carrying out a fluidized bed combustion process;
• c) removing at least one ash stream comprising ilmenite particles from the fluidized bed boiler;
• d) separating ilmenite particles from the at least one ash stream, wherein the separation includes a step of using a magnetic separator (12) comprising a rare earth roll or rare earth drum magnet and a field strength of 2,000 Gauss or more, wherein the separation efficiency of step d) is at least 0.5 by mass for ilmenite, and wherein the separation is at least one of a one-stage magnetic separation including an axial or radial magnetic field, and a two-stage magnetic separation with a first step using an axial magnetic field and a second step using a radial magnetic field;
• e) recirculating separated ilmenite particles into the bed of the fluidized bed boiler;
• the method further comprises a pre-classification step, in which the particles in the at least one ash stream are pre-classified before magnetic separation of the ilmenite particles from the ash stream, wherein the pre-classification comprises mechanical particle classification comprising sieving with a mesh size from 200 to 1,000 μm,
• wherein the average residence time of ilmenite particles in the fluidized bed is 100 h or more.