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Show Ansaldo Energia s.p.a. Some interesting considerations apply : • As long as N O x levels between 250 and 300 mg/Nm3 3 % 0 2 (0.16 + 0.195 lb/mmBTU) are pursued, reburn stochiometry can be reasonably high (> 0,85) and therefore C O and particulate are very easily controllable. In this range simple air staging (OFA) and reburning are quite equivalent. If much lower values are pursued staging at main burners must be high (order of 0,85, corresponding to a S R Z ~ 0,75) and therefore problems of C O and particulate m ay be encountered depending upon the plant characteristic, type of fuel oil and installed ESP performance • With the improved O F A system the 200 m g limit becomes marginally reachable with pure air staging but reburning always provides lower N O x and lower particulate • Fuel quality plays a very important role, specifically for particulate control. In this respect it is very important to use advanced atomizers capable of providing fine droplets and the most constant S M D over the whole boiler load range. According Ansaldo experience atomizers of the internal mixing type are the most appropriate [10] (see fig. 7). Fig. 7 - Spray quality versus fuel flow for Y Jet and I mix atomizers -Y-J« LUli run now T w o questions are normally raised when contemplating a reburn installation O Is a transport medium for reburn fuel necessary or not? O H o w overall boiler performance will be impacted by the new combustion system? Response to the first question is quite easy: the use of recirculated flue gas as transport medium is mandatory if oil is the reburn fuel, this because oil droplets alone have not enough momentum to sufficiently penetrate and mix. If gas is the reburn fuel, simplified injection schemes can be designed injecting gas alone although slightly reduced N O x reduction efficiency will be got. All the experimental data presented refer to the E N E L plants where the reference design is oil over oil configuration, so that gas recirculation is present also when gas is the reburn fuel. Fig. 8 provides experimental confirmation of the importance of the use of flue gas recirculation with different reburn fuels and different reburn zone stoichiometry. F* l-ENELTORVALDAUGA I 2 - 320 MU, TAXGENTULLY FIRED BOILER NOx m m/lmedfm <f PC ft REM m/ua iaaaf 1 • Ql Rat SR2 • 0 96 m Oil Rao SR2 • 0 92 • Gat RabSR2« 0.84 2 4 6 • 10 12 14 Racycttd flua gas lo R B fedtctm (\ tow nut gaa now] Second question might result more difficult to respond because it depends upon the boiler characteristics and the original design. Being most of the E N E L T F units designed for oil/gas firing reburn F G R didn't cause any significant operational problem in oil firing. In general reburning causes a reduced heat flux in the burner's area thus allowing to reduce hopper recirculation, if any, so that an average increase of flue gas mass flow of about 5 % in the convective pass can be accommodated into the original boiler design margins. It is obvious that the experience of a boiler designer is a need and that appropriate calculations |
