Development of Pollution Control Technology During Coal Combustion - Simultaneous Multi-Pollutants Removal in Flue Gas by Ozone - page 9

advanced reburning lean technology, advanced reburning rich technology, and
multiple-injection advanced reburning technology [27-29] .
1.2.2.8 Oxygen-Enhanced Combustion
In the oxygen-enhanced combustion technology, pure O 2 is injected into low-NO x
burners or OFA to replace air. Deep-staging technologies can be applied to create a
strong reductive atmosphere to reduce NO x emissions. The coal combustion
intensity will decline and less heat will be released by using low-NO x burners and
OFA for forming air-staging conditions, which decrease the chemical equivalent of
the main combustion zone. This phenomenon often results in a less flame stability,
lower flame temperature, and higher carbon in fly ash. These factors tend to limit
the air-staging degree. When O 2 is applied, the staging degree can be significantly
improved given the superior performance of pure oxygen combustion.
Consequently, the chemical equivalent ratio of burners can be 0.85 or even lower.
While the flame temperature increases, the combustion stability will improve, and
the decomposition rate of volatile N will increase at an early stage to generate low
NO x emissions. Compared with air staging, carbon in fly ash under pure oxygen
combustion conditions will be significantly reduced. Results of Praxair
Corporation in a 44-MWe industrial furnace show that the oxygen-enhanced
combustion can effectively improve the staging intensity of low-NO x burners and
reduce carbon in fly ash and flue-gas opacity, compared with air staging [30] .
Moreover, in an experiment with replacing 5% of the combustion air by O 2 , a 40%
NO x reduction was achieved. Numerical results of the Praxair's oxygen-enhanced
combustion technology [31] suggested that replacing 2% of air with O 2 could
reduce about 35% of NO x emissions without a significant increase in burnout rate
and no obvious change appeared in the slagging rate on water-cooling walls.
1.2.2.9 Hybrid Selective Reduction Technology
In recent years, new NO x control hybrid systems, using a combination of SNCR
and SCR technology, have been developed. These hybrid systems reduce levels of
nitrogen oxides in fossil fuel combustion flue gas with a selective non-catalytic
reduction treatment stage followed by a selective catalytic reduction treatment
stage. The hybrid SNCR/SCR technology can be a cost effective compromise with
the SNCR and SCR system, reducing NO x , and balancing capital and operation