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Influence of type of burner on NO emissions for pulverized coal preheating method
Author: Liu Changchun | Print | Close | Text Size: A A A | 2016-10-29

Coal preheating method has demonstrated great potential to reduce NOx emissions. Interestingly, whether this method is suitable for all types of burner remains to be elucidated. Four types of burner with different air staging schemes were studied in the present experiments. With coal preheating method, numerous nitrogen (N) intermedias (HCN), soot and hydrocarbons () were yielded in the preheating chamber. However, in oxygen-rich conditions the N-intermedias were easily converted to NO, and the most soot and hydrocarbons () were oxidized before reducing the NO to N2. The type of burner and the way of air staging had a significant effect on NO emissions. Without air staging NO emissions even increased using a bad burner, and the NO emissions were almost unchanged as the speed of outer secondary air increased from 23 m/s to 118 m/s. Three fitting formulas were summarized based on a large number of experimental data. The close-coupled over fire air played a more important role when the speed of outer secondary air was lower or the volatile content of coal was lower. The residence time in the reducing zone was also a key to reduce NO emissions.

The influence of type of burner on NO emissions for pulverized coal preheating method was studied. In this study, some important conclusions are listed, as follows:

1. Not all types of burner could get satisfactory result of NO emissions using coal preheating method. With coal preheating method, NO emissions may be higher using a bad burner such as burner A.

2. Numerous HCN, soot, and hydrocarbons () are yielded using coal preheating method. However, in oxygen-rich conditions the N-intermedias are easily converted to NO, and most soot and hydrocarbon () are oxidized before reducing the NO to N2. So NO emissions were almost unchanged as the velocity of outer secondary air increased from 23 m/s to 118 m/s for non-staging combustion since there was enough oxygen in the environment in the combustion. However, with air staging combustion, the high velocity can play an important role to reduce the NO emissions as there was less oxygen in the environment in the combustion zone.

3. When SOFA was used, NO emissions decreased with lSR decreased for all three types of burners. NO emissions reached the minimum at lSR0.95. It is encouraging to find that the NO emissions could be substantially reduced at lSR > 1 for highspeed jet of outer secondary air (burner D), which could reduce slagging and corrosion as they are seriously considered in reducing atmosphere.

4. Three fitting formulas were summarized from a large number of experimental data and were analyzed. Regardless of high volatile coal and low volatile coal, CCOFA and SOFA could offer much effective help to reduce NO emissions for coal preheating method. The CCOFA offered more help to reduce NO emissions when the velocity of outer secondary air or the volatile content of coal was lower. The effect of stoichiometric ratio on NO emissions was less for the burner of high speed of outer secondary air at stoichiometric ratio of about 1.0.

5. The residence time on the reducing zone on NO emissions was a key to reduce NO emissions. To obtain higher denitrification efficiency, a suggestion is proposed to prolong NO residence time in the reducing zone.

The results have been published on APPLIED THERMAL ENGINEERING Volume:85 Pages:278-286.

 
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