NOVELTY - The furnace (1) has an inner water tube section (20) within an outer water tube section (10) receiving water from outside and heating the water. The outer water tube section is formed in a shape to gradually increases or to provide uniform diameter from bottom towards a middle portion (M). A combustion space defined between the tube sections is formed in a shape to gradually increases and decreases diameter from the bottom towards the middle portion. The space is formed in a serpentine shape from the middle portion towards top.    USE - Furnace for a boiler i.e. pulverized coal fired boiler, of a thermoelectric power plant, that is used for thermal power generation. Can also be used for fluidized bed coal fired boiler, oil fired boiler and gas fired boiler.    ADVANTAGE - The combustion space defined between the tube sections increases the contact area between the water tube sections and the large curved tubular flame, thus improving the temperature of water heated in the water tube sections of the furnace, and hence improving the heat absorption efficiency at the bottom of the furnace. The furnace improves the heat absorption efficiency at the bottom of the furnace by absorbing the convective heat of the convective gas using the tube sections and improves the operation efficiency of a turbine. The furnace reduces the construction cost and greatly reduces the length of the water tubes so as to significantly decrease a load of a feed water pump, thus increasing the thermal efficiency of the entire system. The furnace prevents formation of a fire ball, thus preventing the thermal nitrous oxide (NOx) formation caused by a high-temperature fire ball, and preventing an ash combustion residue from being molten by the high-temperature fire ball to form slag.    DESCRIPTION OF DRAWING(S) - The drawing shows a partially cut perspective view illustrating a furnace of a boiler for a power plant.    Middle portion (M)    Furnace (1)    Furnace wall (2)    Heat insulating material (3)    Outer water tube section (10)    Lower headers (11, 21)    Outer water tube wall (12)    Upper headers (13, 23)    Inner water tube section (20)    Inner water tube wall (22)