Development of Pollution Control Technology During Coal Combustion - Simultaneous Multi-Pollutants Removal in Flue Gas by Ozone - page 6

1.2.2.2 Staged-Air Combustion Technology
The staged-air combustion technology obtains the minimum NO x formation by
forming oxygen-lean, fuel-rich regions in the main combustion zone through a
multi-level air supply strategy. The injected over-fire air (OFA) makes the
pulverized coal to be burnt out. The parameters which impact on the NO x removal
efficiency are the excessive air ratio, temperature, and residence time in the main
combustion zone. The excess air ratio in the main combustion zone generally
ranges from 0.7 to 0.95. The temperature and residence time mostly depend on the
excess air ratio in the primary combustion zone.
This method is quite simple and effective for either the existing or newly built
boilers. Compared with the outdated boiler models, the advanced, integrated-air
staging combustion technology can reduce NO x by 15% - 30% [16] . However, it
may possibly cause corrosion and slag in the furnace, along with a reduction in the
combustion efficiency.
1.2.2.3 Flue-Gas Recirculation
Flue-gas recirculation is one of a low-NO x combustion technology, widely
employed for gas-fired and oil-fired boilers. It introduces the re-circulate flue gas
into the combustion zone by using a fan to lower the flame temperature and to
dilute the oxygen concentration so as to reduce the NO x formation.
Gas mixing and fuel combustion can be significantly improved by flue-gas
recirculation. Thus, NO emissions can be reduced by about 20%. However, this
method barely inhibits thermal NO x generation. If the circulating flue gas exceeds
to 15 wt.% of the combustion air, the NO x reduction will eventually decrease a lot.
The maximum flue gas recycled is limited by the flame stability. Recirculation
fans must be added once retrofitting is necessary. Flue-gas recycling and
modification is relatively easy and the retrofit investment is low.
1.2.2.4 Reburning Technology
Reburning [17] is an effective and promising alternative method for reducing NO x
emissions. Reburning technology can reduce NO x emissions by 50% - 70%,
which is inferior to SCR and SNCR. Reburning can keep a stable reductive
atmosphere in the fuel-rich zone, low excess air ratio area, high concentrations of