A plant for burning waste which has a main combustion chamber to be chargedith the waste to be burnt and with fresh gases for burning the waste. From this main combustion chamber the burnt material and unburnt residual material including gases pass through a grate into a post-combustion chamber, the bottom of which is adapted to be closed, and which has lateral outlets for the waste gas. The post-combustion chamber which has a volume at least equalling the volume of the main combustion chamber has a wall structure extending from above into said post-combustion chamber for guiding gases passing through the grate toward the bottom of the post-combustion chamber. This wall structure is open at its lower end and is of highly heat resistant material as e.g. steel. The post-combustion chamber has a waste gas outlet through which the waste gas from the post-combustion chamber is discharged after it has, prior thereto, been purified by passing through filters between an outer wall and inner wall of the post-combustion chamber.