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Show Ansaldo Energia s.p.a. must be done before a definite answer can be given. To be finally noted that the proper re-arrangement of windbox air compartments, an example of which is given in fig. 2, is another key element of success in air and fuel staging retrofits, being the proper balancing of windbox/OFA free areas a mandatory action for primary combustion control under staged operation. Summing up for T F boilers Ansaldo and E N EL have cumulated experience for oil over oil reburning in boilers rated from 35 to 660 M W e and reburning has proven capable of assuring the respect of the law emission limits without significant operational problems or other constraints. Reburning in gas over oil configuration is a much easier job. For this applicaiton Ansaldo/ENEL experience is limited to the 320 M W e size. Experience from wall fired boilers First Ansaldo application of Reburning on front fired boilers goes back to 1990 (Cassano 1) followed by E N E L Units Monfalcone 3 (1995), Casella 4 (1996) and recently by Casella 3 (1997), Tavazzano 5 and Keratsini 8 (1998). Another 320 M W e unit (Cassano 2), in the gas over oil configuration, is presently under commissioning. Results and problems did not differ significantly from those obtained and encoutered in T F boilers although more difficult to be got because of the intrinsic "non mixing" behavior of a wall fired boiler as compared with the "mixing" behavior of the T F design. Also for these boilers was however possible to demonstrate the reliability of the technology and the capability to satisfy the very stringent Italian Regulations. The most interesting results for the above boilers are presented with the aim of outlining both problems and solutions. Cassano # 1 The boiler is a front fired unit rated 75 MWe operated by A E M (Azienda Elettrica Municipale di Milano) firing oil and natural gas in the oldest A N S A L D O reburning configuration (fig. 9). Fig. 9 - Cassano # 1 - Layout The combustion system consists of two rows of three T E A # 1 burners, one row of three reburn injectors and one row of three circular OFA's; all the components are housed on the front wall of the boiler. The system has been designed when in field experimental data were not yet available, specially those concerning the importance of mixing and combustion system's component location as successful factors. Nevertheless significant results were obtained in emission reduction. Figure 10 shows the experimental data obtained firing natural gas at full load. The N O x reduction, in baseline configuration was about 6 0 % (usual for T E A burner) but it increased up to 8 5 % as a function of the gas mixing flowrate. •6- |