Page 293 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

270 9 In-combustion Air Emission Control

9.4.2 Fuel Staging

Fuel staging is also known as reburning or three-stage combustion; it utilizes fuel to
reduce the formation of nitric oxide. Like the air staging technology, fuel staging
may also be applied either to a low-NO x burner or to the entire furnace [11–13]. The
principles are illustrated in Fig. 9.10. In the low-NO x burner, the same fuel is used
as primary and secondary fuel. The method includes three stages, being primary,
secondary stage, and final combustion.
In the primary combustion zone, the primary fuel such as coal or oil is oxidized
by excess air. Both fuel NO and thermal NO are formed in this zone. Secondary
fuel is usually natural gas and it is injected to the furnace at the secondary stage. It
is typically corresponding to 10–20 % of the energy content of the primary fuel. At
this stage, the NO formed in the primary stage is reduced to molecular nitrogen by
complicated chain reactions initiated by hydrocarbon radicals (CH i ), which origi-
nate from the secondary fuel. High temperature (>1,000 °C) is favorable for these
reactions, which may be written as follows.

NO þ CH i ! HCN þ O þ OH ! H i NCO þ H ! NH i þ NO ! N 2 ð9:11Þ
Air is added to the final combustion stage for the completion of fuel combustion.
The nitrogen compounds (e.g., NO, HCN and NH 3 ) are oxidized back to NO and/or
N 2 . Different from the secondary stage, a low temperature (<1,000 °C) is preferred
during the final combustion stage for the formation of molecular nitrogen. How-
ever, the temperature cannot be too low in order to minimize the formation of
laughing gas (N 2 O) and to maximize the oxidation of carbon monoxide. In dem-
onstration plants the NO reduction by fuel staging could reach 30–70 % [22], with
some practical problems such as increased unburned components in flue gas and
increased corrosion of the furnace.

Fig. 9.10 Principle of fuel staging (Left low-NO x burner, Right furnace)