OCR Text |
Show 100 90 o NO REBURN 80 V CLOSE COUPLED REBUR (0I I 70 l{ C'l 60 a; 0 u ~ ~ cto..:; 4C 0:: tal ~ x 20 '0 0 0.6 O.B 0.9 1.0 1.1 1.2 REBURN ZONE STOICHIOMETRY Fig 14 BSF Tests at Conditions TYPical of ENEL 660 MW Oil-Fired Units- 72 x 1()6 Btu/hr, Stack CO vs Reburn Zone Stoichiometry reduc tions below baseline levels while at the same time generating less than 30 ppm CO, an emissions leve l which is acceptable in most utility situations . For equivalent NOz emissions, overfire air technology on average requires lower furnace sto i chiometries than gas reburning . This result is shown in Figures 11 , 12 and 15 which presents NOz emission test results plotted versus reburn and main burner zone stoichiometry . For a given low NOz level, furnace stoichiometry in the reburn and main burner zones were on average 0 . 02 to 0 . 10 lower (2 to lOX ) in the over-fire air case than the reburn case . 2SO 24() 220 200 S ISO JI ISO ~ i 14() l 120 100 80 0 NO REBURI\ I--- • STANDARD GAS REBURN E EVATION 0/ 0 / coi / o 0 / / 0 / . y ~ ~.t • - tit v -- .. . 80 0.8 0.7 0.8 0.0 1.0 1.1 1.2 IIWH IUllNER ZONE ST'OOO()WCTRY Fig 15 BSF Tests at Conditions Typical of ENEL's Santa Gilla Unit No. 2- 90 x 1()6 Btu/hr, NOx vs. Main Burner Zone Stoichiometry From a practical boiler operating standpoint , thes e data also confirm one of the major potent i al advantages of reburn NOz control technology . Generally speaking, boilers are designed to operate in an oxidizing environment . When specific zones in the furnace are operated sub-stoichiometrically, changes 7 in boiler tube corrosion characteristics , slagging , furnace gas temperature, and heat transfer can occur . These changes are of course highly dependent on the type of boiler fuel employed as well as a specific boiler's design and operating characteristics ; and the changes can range from negligible (as has been the case in most field demonstrations to date) to having a measurable potential for creating boiler operating and maintenance problems. It is commonly believed (5) (but not substantially confirmed by limited available f i eld data to date) that furnace operation at or slightly below the stoichiometric point (1 .0 - 0 . 9) can avo i d problems which could occur under more substantial reducing conditions (less than 0.9) . From the BSF experience to date, NOz reductions up to 64X have been achieved with reburn zone stoichiometries in the 0 .9 - 1.0 stoichiometry range (Figures 11 and 12) . Residence Time Maximum NOz emissions reductions occurred in the BSF trials when maximizing reburn zone residence times at minimum stoichiometry . Figure 16 plots reburn zone residence time versus NOz reduction in order to illustrate this effect . zw :J w ~ ~ ~ ~ ~ cr !z w !f UJ II. 70 6S ""*' 1~OfA 45 ... QASIUl 0.15 0.2 0.25 0.3 0.3.5 0.4 0.'5 0.5 0.55 REB URN ZONE RESIDENCE nME, SEC. Fig. 16 NOx Reduction vs Residence Time at Stoichiometry < 1 0 for Different BSF Thermal Conditions 06 Residence time was increased by moving the overfire air injectors to a higher position in the BSF. In all cases, percent NOz reductions were increased as residence time increased. An increase in furnace gas residence time under reducing conditions increases reaction time (between the reburn fuel and bulk furnace gases) for converting reactive nitrogen species to molecular nitrogen . These data confirm that the most effective reburn NOz reduction strategy would include maximizing bulk furnace gas residence time in the reb urn (reducing) zone. Reburn zone residence time was also maximized by locating reburn fuel injectors within the main tangential firing system windbox. This design called wclose-coupled reburningW produced consistently low NOz emissions (Figure 12), Close-coupled reburn could be a very practical option for field retrofit applications, offering NOz reductions similar to standard reburning, while reducing retrofit costs . |