In general, four factors significantly affect the formation of NOx in the burner of water tube boilers.
The ability of water tube boilers to be designed without the use of excessively large and thick-walled pressure vessels makes these boilers particularly attractive in applications that require dry, high-pressure, high-energy steam, including steam turbine power generation.
Owing to their superb working properties, the use of water tube boilers is highly preferred in the following major areas:
Besides, they are frequently employed in power generation plants where large quantities of steam (ranging up to 500 kg/s) having high pressures i.e., approximately 16 megapascals (160 bar) and high temperatures reaching up to 550°C are generally required.
A water tube boiler can be defined as a Steam boiler in which the flow of water in the tubes, as well as hot gases, enclose the tubes. Not like fire tube boilers, this boiler attains high-pressures, as well as high-steam capabilities, can be achieved. This is because of condensed tangential pressure on tubes which is known as hoop stress.
Burners have different flames depending on their utilizations. While fire tube boilers require a long flame with a small diameter, water tube boilers often require short flame but large diameter burners. In general, the flame length of water tube burners is 2 to 2.5 times the flame diameter.
Raadman WT burners have a ball shaped flame with substantial swirl (swirl number is higher than 0.6). The flame has hot reverse flow into the center and cold forward flow at sides. There is intense mixing and the secondary jet velocity is more than the primary jet velocity. The flame is used for combustion chambers which are more or less cubicle in shape.