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Show £_JV7 NITROGEN OXIDE REDUCTION BY STAGED COMBUSTION OF BIOMASS GAS IN GAS TURBINES - A MODELING STUDY OF THE EFFECT OF MIXING Edgardo G. Coda Zabetta Pia T. Kilpinen Mikko M. Hupa Abo Akademi University, C C R G , Lemminkaisenkatu 14-18 B, Turku, FIN-20520, Finland Phone: +358 2 215 31 Fax: +358 2 215 4780 E-Mail: ezabetta® mail.abo.fi Jukka K. Leppalahti VTT Energy FIN-02044 VTT, Finland C. Krister O. Stahl Sydkraft A B S-205 09 Malmo, Sweden Michael F. Cannon ALSTOM Gas Turbines Ltd P.O.Box 1, Lincoln LN2 5DJ, United Kingdom Jorma Nieminen Foster Wheeler Energia Oy P.O.Box 201, FIN-78201 Varkaus, Finland ABSTRACT Detailed chemical kinetic modeling has been used to study the reduction of nitrogen oxides at gas turbine combustion chamber conditions. A biomass-derived gasification gas from an air-blown IGCC plant has been used as the fuel. Air has been the oxidizer. A staged combustion technique has been adopted. In our previous study a simple plug flow model was used to study the effects of many process parameters like pressure, temperature, number of air addition stages, air-fuel stoichiometry, and fuel composition on the N O x emission. The calculations showed pressure and temperature as the main affecting variables. In this work the effect of the various mixing models of air and fuel are studied. A varying number of air sub-streams are mixed with the fuel gas during different time periods. Alternatively, a combination of a perfectly mixed zone followed by a plug flow zone is illustrated. The results showed that models predict an enhanced nitrogen oxide reduction when a mixing delay between air and fuel is considered as compared to an instantaneous mixing. NOMENCLATURE CSTR Continuously Stirred Tank Reactor G T C C Gas Turbine Combustion Chamber PFR Plug Flow Reactor SCO Selective Catalytic Oxidation X Air-fuel stoichiometry INTRODUCTION Advanced designs for combustion systems are nowadays under development in order to increase the electricity production efficiency of energy conversion plants. At present, combined cycle with gas turbine is one of the most promising technologies. The suited new generation of such system will perform the energy conversion from a wide typology of fuels including wood (SYDKRAJFT, 1998), peat, coals, and organic wastes. Beside the economical aspect, the environmental targets must be achieved. New emission related problems follow the new technology. Proper approaches to solve the specific problems are under development. In this work the legislated limits on nitrogen oxide emissions are under theoretical and experimental investigations. The reduction on nitrogen emissions from a gasification power plant can be intended via the addition of a reactor for the Selective Catalytic Oxidation (SCO) of the fuel gases before entering the gas turbine (Leppalahti et al., 1997). Another possibility for nitrogen reduction consists in an improvement of one or more of the plant devices, e.g., the gasifier and the gas turbine. Encouraging results have been obtained elsewhere, e.g., adopting a staged technique in the Gas Turbine Combustion Chamber (GTCC). The problems related to burning low Btu fuel gases in gas turbines had been analysed by several authors, but only for coal gasification gas: Sato et al. (1989), Kelsall et al. (1991), Bevan et al. (1994). The knowledge especially about systems burning wood gasification gas is still limited. |