Energy & Fuels 2000, 14, 751-761
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Kinetic Modeling Study on the Potential of Staged Combustion in Gas Turbines for the Reduction of Nitrogen Oxide Emissions from Biomass IGCC Plants Edgardo Coda Zabetta, Pia Kilpinen,* and Mikko Hupa Åbo Akademi University, Lemminka¨ isenkatu 14-18 B, FIN-20520 Turku, Finland
Krister Ståhl Sydkraft Ab, S-20509 Malmo¨ , Sweden
Jukka Leppa¨lahti VTT Energy, FIN-02044 VTT, Finland
Michael Cannon ABB Alstom Power, P.O. Box 1, Lincoln LN2/5DJ, U.K.
Jorma Nieminen Foster Wheeler Energia Oy, P.O. Box 201, FIN-78201 Varkaus, Finland Received July 20, 1999
The potential for reduction of nitrogen oxides in gas turbine combustors was studied by detailed chemical kinetic modeling under ideal flow conditions. The investigation focused on turbines burning biomass-derived gasification gas from an air-blown integrated gasification combined cycle plant. The aim was to give detailed information about the parameters that favor reduction of NOx emissions, providing a solid background for designing an air-staged, low-NOx gas turbine. The potential and limitations of the detailed chemical kinetic modeling as a predictive tool for simulating the process were discussed. Instantaneous, delayed, and back-streamed air/fuel mixing models were tested to study the effect of mixing on the emissions. Predictions showed that the nitrogen chemistry was mainly affected by temperature and pressure: low temperatures of about 900-1000 °C and high pressures of about 10-20 bar favored fuel nitrogen conversion to N2. At atmospheric pressure, an increase in the number of air addition stages increased the conversion to N2, but at higher pressure the reduction was more efficient with three-stage addition than with either one- or six-stage addition. The conversion efficiency of NH3 to N2 increased with the inlet NH3 concentration, but the final NOx emission calculated in ppmv increased as well. NOx emission often was higher when HCN replaced ammonia in the gasification gas. The main paths for fuel-NH3 conversion to NOx and N2 were predicted to occur via intermediate formation of amino radicals (NHi). Another important conversion path to N2 was shown to proceed via a H2NO intermediate. Models accounting for delayed mixing led to more realistic predictions, showing the effect of CH4 in the gasification on increased NOx emission by means of its CHi radicals.
Introduction The integrated gasification combined cycle (IGCC) is a promising technology for heat and electricity generation. The figures regarding energy growth worldwide during the next decades reserve a consistent contribution by such a technique, mostly when biomasses are regarded as the primary fuel.1,2 In IGCC a solid fuel is gasified under pressure to form a combustible gas in * To whom correspondence should be addressed. Phone: +358 2 215 31. Fax: +358 2 215 4780. E-mail:
[email protected]. (1) Price, B. Electricity from Biomass; Financial Times Energy: London, 1998. (2) Biomass & Bioenergy; Hall, D. O., et al., Eds.; PergamonElsevier Science Ltd.: Oxford, U.K., 1998; Vol. 15, No. 3.
which carbon monoxide (CO), hydrogen (H2), and methane (CH4) are the major burning compounds. The exact composition of the gas largely depends on the solid fuel selected and strongly affects the chemistry of nitrogen. After purification of the gas to remove components such as dust, alkali metals, and sulfur compounds, the gas is burned to completion in a gas turbine combustor. The flue gas that forms is expanded in the turbine and fed to a steam boiler where its heat is recovered. The IGCC offers a number of advantages over conventional solid fuel combustion technology, including higher efficiency of electricity generation, reduced plant size, and the potential for efficient emission control with reduced costs.
10.1021/ef9901591 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/02/2000
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Energy & Fuels, Vol. 14, No. 4, 2000
Figure 1. Simplified scheme of the fate of fuel-bound nitrogen in the IGCC process.4
There are two major versions of IGCC, and a number of commercial-scale IGCC demonstration plants are under design or commissioning in Europe, the United States, and Japan. The different versions are as follows: (a) Gasification with oxygen and wet gas cleaning. The cleaning takes place at temperatures below 150 °C so that preliminary gas cooling is required. Although cold cleaning methods are well-established and commercially available, the required intermediate cooling reduces the overall cycle efficiency, and the handling of liquid streams resulting from gas scrubbing usually is complicated and costly. Large units (>200 MWe) are expected to be economically more competitive. Plants of this type are existent, for example, in Buggenum (The Netherlands), Puertollano (Spain), Goldenberg (Germany), and Polk (The United States). (b) Gasification with air and gas cleaning at high temperature. The use of air for gasification makes the application of cold cleaning uneconomical, so that hot gas clean up is required. This results in higher efficiency of the cycle but requires the development of a yet unproven hot cleaning technology. This process will probably be applied for smaller units (
