manner that they can move in a horizontal plane these grates 5 5 are normally projecting into the furnace 3 to form a dividing wall and the refuse fed to the first combustion chamber 31 is intercepted by the first set of grates 5 5 provided on both sides of the furnace 3 where said first set of grates 5 5 are disposed are grate compartments 7 7 which are adapted to accommodate the grates 5 5 of the first set when the grates are retreated from the internal space of the furnace 3 these compartments 7 7 are supplied with air 8 at atmospheric temperature the above mentioned air 8 is blown out into the furnace 3 through the clearances 9 9 formed between the furnace 3 and the respective compartments 7 7 to cool the first set of grates 5 5 and at the same time prevent entry of the ashes produced in the furnace 3 into the compartments 7 7 through the clearances 9 9 moreover this air 8 at atmospheric temperature is fed as a secondary air to the top of the first combustion chamber 31 in the furnace 3 grates 6 6 of the second set are installed in such a manner that is can be swung down into the vertical positions indicated by two dots broken line in fig1 as the grates 6 6 of the second set are swung down the ashes in the second combustion chamber 32.