Double wall extension

A fluidized bed reactor having a top, said fluidized bed reactor comprising:

• a fluidized bed;
• tubed membrane walls cooled by a coolant fluid, said tubed membrane walls defining side walls of a combustion chamber having a bottom;
• at least one partition wall extending between parallel tubed membrane walls to divide the combustion chamber;
• and
• tubed extension panels extending within the combustion chamber through which flows a coolant fluid by single pass forced circulation, said extension panels are paired to form a double wall extension, the tubed extension panels including tubes and membranes each tube is contacted by a membrane,
• wherein the tubed membrane walls and the tubed extension panels are exposed to heated material of the fluidized bed on only one side,
• wherein said extension panels are arranged on and extend from the at least one partition wall to a point within an inner area of the combustion chamber between the partition wall and one of the parallel tubed membrane walls, and
• wherein the at least one partition wall is a double wall partition formed of tubed panels paired together.