Water-tube boiler

A water-tube boiler comprising:

• a first water tube array made up of a plurality of first water tubes arranged into an annular shape;
• a combustion chamber defined inside the first water tube array;
• a first opening defined at part of the first water tube array;
• a cooling water tube array made up of a plurality of cooling water tubes arranged into an annular shape in a zone within the combustion chamber where burning-reaction ongoing gas is present;
• gaps provided between adjacent cooling water tubes so as to permit the burning-reaction ongoing gas to flow through;
• and
• a burning-reaction continuing zone, where burning reaction is continuously effected, provided in a space between the cooling water tube array and the first water tube array, whereby the burning-reaction ongoing gas generally uniformly contacts the individual cooling water tubes wherein said cooling water tubes, are constructed and arranged so that a temperature of the burning-reaction after contacting said cooling water tubes is lowered to 1400° C. or lower, whereby generation of NOx is reduced.