Water-tube boiler

A water-tube boiler comprising:

• a combustion chamber where a combustion reaction takes place;
• a first water tube array including a plurality of first water tubes arranged in said combustion chamber so as to extend along the same direction in an annular shape, said first water tubes having a plurality of gaps between at least some of said water tubes through which a flame associated with the combustion reaction passes, wherein said first water tubes are constructed and arranged within the combustion chamber so that a temperature of the flame after contacting said first water tubes is lowered to below 1400.degree. C., whereby generation of NOx is reduced compared to generation of NOx at a temperature greater than 1400.degree. C.;
• and
• a zone radially outside of said first water tube array in which at least part of the combustion reaction takes place.

A water-tube boiler comprising:

• first and second spaced apart headers;
• a plurality of first water tubes in fluid communication with said first and second headers and being arranged generally along a first annulus, at least some of said first water tubes being spaced apart from one another so as to provide gaps therebetween;
• a combustion device constructed and arranged within said first annulus to emit a flame therefrom, wherein said gaps allow the flame associated with a combustion reaction to pass therebetween, wherein said first water tubes are constructed and arranged so that a temperature of the flame after contacting said first water tubes is lowered to below 1400.degree. C. whereby generation of NOx is reduced compared to generation of NOx at a temperature greater than 1400.degree. C.;
• and
• a plurality of second water tubes in fluid communication with said first and second headers and being arranged generally on a second annulus lying outside of said first annulus;
• wherein said pluralities of first and second water tubes have at least one spacing therebetween.

In a boiler including a burner constructed and arranged to emit flame and a plurality of water tubes extending in the same general direction in an annular shape about the burner, at least some of the water tubes having gaps between themselves, a method of operating the boiler so as to reduce generation of thermal NOx, comprising:

• locating the water tubes sufficiently close to the burner such that a flame from the burner passes through the gaps between the water tubes;
• firing the burner so as to emit a flame having a first temperature above 1400.degree. C.;
• and
• passing the flame through the gaps between the water tubes so as to reduce the flame temperature to a second temperature below 1400.degree. C., thereby reducing generation of thermal NOx.