OCR Text |
Show 4 The recirculation parameter is the measure of axial momentum to overcome local static pressure to furnace pressure rise. This is a measure of the potential for gas recirculation back into the air register, windbox, or in the flame stabilizer vanes. Hot gas ingestion can damage or destroy burner parts. The selected five degree difference of zones calculated a positive recirculation parameter, which avoids hot gas ingestion in the flame stabilizer vanes. The Fluent CFD and NOx module software was utilized on a Sun Sparc 10 computer. The objectives of this portion of the project were to predict the percentage reduction in NOx with the addition of a staged flame stabilizer and to determine whether the combustion in the internal recirculation ring vortex of the burner is approximately stoichiometric. The baseline burner geometry (burner as modified in 1990) is shown in Figure 2. The model predicted NOx emissions of the baseline burner of .33 Ib/mmBtu. The model inputs were altered to reflect the addition of the staged coal flame stabilizer. Predicted NOx emissions of the modified burner were .1 lbl mmBtu. Because of the various modeling assumptions involved (coal volatilization and burnout characteristics, coal sizing and distribution, coal composition, turbulence modeling, combustion rate constants, etc.) the absolute levels of NOx predicted are only approximate. Nevertheless, the computed results are useful in comparing trends and in quantifying the relative effectiveness of the two burner designs. The staged flame stabilizer was mounted on the end of the original burner coal pipe. New coal spreaders were fabricated which provide adjustable primary air velocity, axial flow of the coal jets, and separation of the individual coal jets to coincide with the staged sections of the coal flame stabilizer. The coal pipe was extended to the zero inch reference point of the burner throat. The coal pipe incorporated a conical reducer to provide adjustable velocity control. Northeast Utilities installed an overfire air system in six (6) existing observation doors one elevation above the burner level. Four (4) front-wall ports and two (2) side-wall ports at the rear of the furnace were utilized. Overfire air was taken from the top of the windbox. |