Abstract: A combustion air supply apparatus 9 of alternating heat exchange type supplies combustion air and discharges combustion exhaust gas at a flow velocity of 80 to 200 m/sec. A burner assembly 4 is configured in such a manner that low-caloric fuel gas is pre-heated with heat of pre-combusting high-caloric fuel gas before the low-caloric fuel gas reaches a mixing starting space CA in the combustion chamber where the pre-combusting high-caloric fuel gas and the low-caloric fuel gas come to burn together in a full scale in the mixing starting space CA. When an air amount of the combustion air supplied through the high-temperature air supply ports of the plurality of fuel gas combustion apparatuses is defined as Q1 and an air amount of the pre-combustion air to be mixed with the high-caloric fuel gas, supplied from the fuel gas combustion apparatuses, is defined as Q2, a total air amount (Q1+Q2) is 1.02 to 1.10 times more than a theoretical air amount QS required for combustion, and a ratio of Q2/(Q1+Q2) is 0.

Abstract: In a burning air feeding device (9) of alternating heat-exchanging type, the feed of burning air and the discharge of a burned exhaust gas are performed at the speeds of 80 to 200 m/sec. A burner structure (4) is constituted such that a low-calorie fuel gas is preheated with the heat of a precombustion high-calorie fuel gas till the low-calorie fuel gas reaches a mixing starting zone (CA), and such that the precombustion high-calorie fuel gas and the low-calorie fuel gas are burned together in the mixing starting zone (CA). The sum (Q1+Q2) of an air quantity (Q1) fed from the hot air feeding ports of a plurality of fuel gas burning devices and an air quantity (Q2) of a precombustion air to be mixed with the high-calorie fuel gas in the plural fuel gas burning devices is set to 1.02 to 1.10 times as high as the stoichiometric air quantity (Qs) necessary for the combustion, and the ratio (Q2/(Q1+Q2)) is set within the range of 0.011 to 0.047.