Staged combustion

Air staging or two-stage combustion, is generally described as the introduction of overfire air into the boiler or furnace. Staging the air in the burner (internal air staging) is generally one of the design features of low NOx burners.

Furnace overfire air (OFA) technology requires the introduction of combustion air to be separated into primary and secondary flow sections to achieve complete burnout and to encourage the formation of N2 rather than NOx. Primary air (70-90%) is mixed with the fuel producing a relatively low temperature; oxygen deficient, fuel-rich zone and therefore moderate amounts of fuel NOx are formed. The secondary (10-30%) of the combustion air is injected above the combustion zone through a special wind-box with air introducing ports and/or nozzles, mounted above the burners. Combustion is completed at this increased flame volume. Hence, the relatively low-temperature secondary-stage limits the production of thermal NOx. The location of the injection ports and mixing of overfire air are critical to maintain efficient combustion. Retrofitting overfire air on an existing boiler involves waterwall tube modifications to create the ports for the secondary air nozzles and the addition of ducts, dampers and the wind-box. This technique is currently used in 116 pulverised coal-fired units, on a total capacity of 50 GWe as a stand-alone measure. It is used in combination with other primary measures for NOx control, in 175 coal-fired units on a total capacity of 53 GWe.[3]

Some characteristics include:

• NO_X reduction rate from 50-75% is possible
• Spatial separation of primary and secondary air
• Control of combustion performance with primary air (sub-stoichiometric conditions, ${\displaystyle \lambda }$ < 1)
• Secondary air supply for burnout (usually, above stoichiometric conditions, ${\displaystyle \lambda }$>1 )
• Simple configuration
• Application at small scale combustions

One form of fuel staged combustion is the procedure known as burner out of service (boos). The technique involves shutting off the fuel flow from one burner or more to create fuel rich and fuel lean zones achieving some NOx emission control (10%). The technique is not widely used in pulverised coal-fired plants (2 units, 350 MWe).

Another method of fuel staged combustion is fuel biasing. In fuel biasing, combustion is staged by diverting fuel from the upper level burners to the lower ones or from the centre to the side burners. The aim is to create fuel-rich lower or central zone and a fuel-lean upper or side zone in order to achieve complete burnout. The technology lowers flame temperature and improves the balance of the oxygen concentration in the furnace. NOx emissions may be reduced by up to 30% using this technology. This technique is used in 13 pulverised coal-fired units on a total capacity of 2.7 GWe.[4]

Characteristics

• NO_X reduction rate from 50-75% is possible
• Minor temperature instabilities at the reduction zone
• Lower temperature necessary
• Complex constructive configuration
• Application at large scale combustions

Staged combustion dates back to the hot bulb engine of the 1890s. The first stage of combustion occurred inside the hot bulb, and the hot gases were then forced out into the cylinder, where they mixed with additional air, and the second stage of combustion took place. During those times, staged combustion was used because it was a convenient method of ignition, and, likely, there was not much concern about air pollution. A modern application of the principle is the Stratified charge engine, in which a spark ignites a rich mixture, and the resulting flame-front ignites the weaker mixture elsewhere in the cylinder.

• Staged combustion cycle (rocket)

• Regenerative Thermal Oxidizer