Boiler furnace that avoids thermal no2

The present invention relates to a boiler furnace which avoid NOx and increase thermal efficiency with small boiler. For this purpose, the boiler furnace according to the present invention has a outer water walls and inner water walls. All of the water walls contain water which absorb heat from flame and cool the water tube which act as a container, passage, evaporator of water and water walls with membrane that connect water tubes each other. The spaces between outer water walls and inner water walls make pipe shape space which contain fire and avoid making fire ball which produce nitrogen oxides due to high temperature induced from concentrating of flame.