PURPOSE: To prevent the temperature during low oxygen combustion from lowering, by recovering more heat than that radiated from a purifying cylinder from exhaust gas by an air preheater by keeping expansions of two cylinders balanced by designing the air preheater longer than the purifying cylinder, because the purifying cylinder is expanded downward by heat starting from the upper part of a combustion cylinder, while the air preheater is expanded upward by heat starting from the lower part of a purifying cylinder.

CONSTITUTION: An exhaust gas generated in a combustion cylinder 1 is purified by an ash- treating filter 16 and a CO purifying catalyst 17 in a purifying cylinder 14, flowing into an air preheater 18, where it gives heat to combustion air, flowing into a cooling unit 19 at the top of a preheater, where it is cooled down to the atmospheric temperature. The exhaust gas flows into a scrubber 20 which is filled with a substance which adsorbs carbon dioxide, and then is fed into a refrigerator 9. The combustion air is circulated through a combustion part and the refrigerator 9 in order to make the atmosphere in the refrigerator 9 of low oxygen and high carbon dioxide concentration that is suitable to store vegetable and fruit. The purifying cylinder 14 is expanded downward by heat starting from an exhaust pipe 15, so that the upward expansion by heat of an air preheater 18 is cancelled, and excessive deforming stress is not produced between the air preheater 18 and the cooling unit 19. Accordingly the air preheater which has necessary and enough capacity can be mounted on an equipment in order to raise the temperature during combustion.