Optimization of Separated Overfire Air System for a Utility Boiler from a 3-MW Pilot-Scale Facility

Abstract

A 3-MW pilot-scale facility was used to study the effect of separated overfire air (SOFA) on NO x emissions from utility boilers. Experiment results showed that NO x emissions decreased as the excess air ratio in the main combustion zone first decreased, but remained unchanged when the excess air ratio was above a certain value; the carbon content in fly ash increased with the decrease of the excess air ratio in the main combustion zone; NO x emissions decreased and the heat loss due to unburned carbon increased as the residence time in the reduction zone (the zone in the furnace from the middle of combustion zone to the middle of the SOFA zone) increased. Based on the experiment results, a residence time of 2.12−2.68 s is recommended for the reduction zone. Besides, numerical simulations were conducted about the pilot-scale facility and a 600-MW boiler firing the blended coal of Shenhua coal (80 wt %) and Baode coal (20 wt %). The simulations, together with the experiments, showed that there was a critical excess air ratio with value of ∼0.8 for the minimum NO x emissions with relatively low unburned carbon. This critical value, together with the residence time of 2.35 s, which is in the range recommended by our experiments, were used in the retrofit of the 600-MW boiler by SOFA technology, leading to a reduction of NO x emissions as high as 60%.

Numerical study on combustion characteristics and heat flux distributions of 660‐MW ultra‐supercritical double‐reheat tower‐type boiler

“…The SIMPLE algorithm is carried out to calculate the three conservation equations with time‐averaged method. The standard k – ε model, verified to provide a good balance between accuracy and computational efficiency in pulverized coal combustion simulations, is chosen to describe the turbulence flow 34 . The radiation heat transfer is computed with the P‐1 model, 35 and the absorption coefficient is simulated with the WSGGM‐domain‐based model.…”

Numerical study on the influence of platen super-heater on combustion characteristics in a 600 MW tangentially fired pulverized-coal boiler

“…The NOx production simulation was carried out as a postprocessing procedure after the flow field, temperature distribution, and species concentration in furnace are predicted. These models used in this study just follow in the wake of earlier studies [2,15]. Default kinetics in Fluent was used, as both trendies of cross-sectional average properties along the furnace and properties at furnace boundaries are not affected by kinetics as these parameters are in reasonable scopes [12].…”

“…However, the uneven of flue-gas in a large capacity tangentially boiler is quite large and can lead to local overheating [1]. Moreover, the NOx emission from the traditional tangentially fired boiler did not meet the demand of increasing strict regulations against air pollutants [2,3]. To get over these two shortcomings, many studies have been carried out.…”

Influence of overfire air rate of the two‐channel overfire air on flow and combustion characteristics in a 600‐MWe boiler

“…For example, Liu et al 7 carried out simulations for a 600-MWe boiler and found that the NO x emission decreased and char burnout worsened with increasing OFA rate. Zhang et al 8 found that the emission of NO x decreased and burnout worsened with decreasing excess-air coefficient in the main combustion zone (i.e., increasing OFA rate so that the pulverized coal can be fully burned). When the excess-air coefficient in the main combustion zone was less than .8, there was no decrease in the emission of NO x .…”