Simplified kinetic schemes for oxy-fuel combustion

This work presents simplified kinetic schemes to be adopted in CFD modeling of oxy-fuel combustion. Oxy-fuel combustion is an important technology to control CO2 emissions. The limited nitrogen content during combustion makes easier the CO2 capture for its successive storage. Flue gas recirculations can be adopted for the eventual temperature control. Moreover, oxygen rich combustion finds several applications where high temperatures are required, like iron and steel industry, or when the plants undergo frequent startups and shutdowns, to minimize the transient times. Computational Fluidynamics is also increasing its importance as a tool for industrial design and optimization. One of the main problem when using CFD in the full scale geometry is the dimension of the computational grid and then the large memory and CPU required. For this reason the combustion chemistry has to be drastically reduced down to a few reactions and simple RANS approaches are assumed. On the other side, very simplified approaches, like ‘mixed is burned’ are completely meaningless in the case of oxy-fuel combustion, because the high temperatures promote dissociation reactions. In the typical air combustion cases, when CO profiles are of interest or when temperature estimations have to be improved, simplified mechanisms are available. Typical and well known examples are the schemes of Westbrook and Dryer [1] or Jones and Lindstedt [2]. Turbulence-kinetic interactions are generally managed adopting the EDC approach, as proposed by Magnussen [3]. Unfortunately, the already mentioned temperature peaks of oxy-fuel combustion limit the validity of such mechanisms in these conditions. At temperatures higher then 2500 K, not only CO-CO2 equilibrium is in favor of CO and H2-H2O equilibrium foresees a significant amount of H2, but also the radical pool has a significant impact in limiting the heat release. The simplified mechanisms do not always account for the dissociation reactions [1], but above all non of them include radicals. This paper presents new estimations of the parameters of multistep oxidation mechanisms, specifically conceived for oxy-fuel applications. These evaluations are based on a regression of data with a very effective numerical algorithm [4]. The main novelty is the definition of the set of comparison measures. In order to take into account the quite broad temperature and stoichiometry ranges of a turbulent diffusive flame, the regression analysis is performed over data obtained by a detailed kinetic scheme [5] in laminar diffusive counter-flow flames. These flames are assumed as a good representation of the real flame, still maintaining a simple solution, compatible with the numerical effort required by the regression algorithm. 1 st International Conference on Sustainable Fossil Fuels for Future Energy – S4FE 2009 The global kinetics typically includes non elementary reactions with real reaction orders. These results might cause numerical problems when orders lower than 1 are proposed, because of possible negative values of the concentration. A solution is a linearization of the rate expression when the concentration of the reactants becomes lower than a certain specified value. Different mechanisms are proposed, including also main radicals, and their performances are tested in several conditions typical of oxy-fuel combustion. The simplified mechanism is then applied to simulate experimental data carried out in a 1 MW furnace fed with natural gas in the frame of OXYFLAM project [6]. Despite several uncertainties in the experimental measurements, main trends in the temperature and velocity profiles are caught. Moreover, the NOx formation, coming from nitrogen feeding, is also investigated and reproduced through the use of a postprocessor, which allows the use of detailed kinetic schemes. As a matter of facts, when interested in the formation of pollutants, simplified mechanisms are not any more sufficient and the adoption of a more complex chemistry is absolutely necessary to correctly predict emissions. The Kinetic Post-Processor (KPP) operates assuming the temperature and flow fields as predicted by the CFD and solves mass balances in the cells with detailed chemistry at fixed temperatures [7]. Two major features make this approach advantageous over the direct coupling of detailed kinetics and CFD. The first is the possibility to lump and group several cells and the latter is to fix the temperature inside the cells. CFD solution provides detailed flow, composition and thermal fields allowing the identification of the zones where the detail of the description can be reduced. The fixed temperature inside the cells reduces the high non linearity of the system, mainly related to the reaction rates and to the coupling between mass and energy balances. The KPP uses a detailed kinetic mechanism [5], containing about 100 species, to model combustion and pollutants formation in the gas phase. References: [1] Westbrook, C. K. ; Dryer, F. L. Combust. Sci. Technol. 1981, 27, 31–44. [2] Jones, W. P.; Lindstedt, R. P. Combust. Flame 1988, 73, 233–249 [3] Magnussen B.F. , “On the structure of turbulence and a generalized eddy dissipation concept for chemical reactions in turbulent flows”, 19 th AIAA Aerospace Science Meeting, 1981 [4] A. Cuoci, A. Frassoldati, T. Faravelli, E. Ranzi, “Accuracy and Flexibility of Simplified Kinetic Models for CFD applications”, 32 nd meeting on Combustion, Napoli, April, 2009 [5] http://www.chem.polimi.it/creckmodeling/kinetic.html [6] Lallemant N. , Dugue J., Weber R. , “Analysis of the experimental data collected during the oxyflam-1 and oxyflam-2 experiments. , IFRF doc # F85/Y/4, 1997 [7] Cuoci, A. , Frassoldati, A. , Buzzi Ferraris, G. , Faravelli, T. , Ranzi, E. , Int. J. of Hydrogen Energy, 32: 3486-3500 (2007)