| OCR Text |
Show NUMERICAL SIMULATIONS Two models have been developed in order to better simulates the real delay of time characterizing the gas mixing, Such models will be referred as: • "Sub-Streamed Model" • "Combined Model". The two Models above will be referred together as "Delayed Models", counter-posed to the "Instantaneous Model". Both models provide a more realistic representation of the reality, still allowing the use of simple simulative tools such ideal reactors. The codes S E N K I N (Lutz et al, 1993) and PSR (Glarborg et al., 1993) of the package CHEMKIN-II (Kee et al., 1993) have been adopted for this purpose. A dimensionless "Index of Nfix-reduction" (Figures) have been defined as the unity decreased by the rate between the harmful nitrogen-compounds at the outlet and those at the inlet of the G T C C . Therefore the "Index of N-reduction" provides the degree of success on the conversion to N 2 along the gas turbine combustion chamber. The Sub-Streamed Model In this model the time delay in mixing is simulated in each stage by distributing the air in a number of sub-streams. Each sub-stream is mixed instantaneously with the main stream but a varying delay of time is assumed in the G T C C before adding the next sub-stream (Fig. 1). Therefore a sequence of Plug Flow Reactors (PFRs) has been adopted, each one simulating a sub-section located between two sub-streams. Figure 1. The principle of the Sub-Streamed Model Instantaneous Mixing. 1 Sub-Stream, mixing completed within 0 ms. i i | i | | Fast Mixing. 5 Sub-Streams, mixing • T T T T completed within 2 ms. ^ ^L ^ ^ ^ i V V \ • • • • w • • • • • • • W WW Slow Mixing. 5 Sub-Streams, mixing completed within 4 ms. 1 Sub-Stream, mixing completed within 4 ms. 2 Sub-Stream, mixing completed within 4 ms. 3 Sub-Stream, mixing completed within 4 ms. 5 Sub-Stream, mixing completed within 4 ms. 10 Sub-Stream, mixing completed within 4 ms. Time evolution [ms| The investigation has been done for the following conditions: > pressure: 1 or 10 bar > temperature: 1000 or 1300 degC > number of stages: 3 > air-fuel stoichiometry at the stages: ^=0.8, A,2= 1.0, ki=2.0 > concentration of dopants in the syngas: • [NH3]= 1000 ppmv or • [HCN]= 1000 ppmv or • [NO]= 1000 ppmv or • [NH3] + [NO] = 500 + 500 ppmv > concentration of methane in the syngas: [CH4]=0, l,3or6vol-%. The variable characterizing the model has been the number of sub-streams: 1, 2, 3, 5 or 10 per stage, each one sharing an equal amount of the total air inlet in the stage. Three speeds for the gas mixing has been compared: > "instantaneous", where the mixing is completed within 0 m s (corresponding to the Instantaneous Mixing cases) > "fast", where the mixing is completed within 2 ms > "slow", where the mixing is completed within 4 ms. The Combined Model In this model the time delay in mixing and the effect of backmixing are simulated in each stage of the G T C C by mixing the additional air in a Continuously Stirred Tank Reactors (CSTRs) during an established period. The residence time in the stage is then completed by a plug flow. Therefore a series of combinations of CSTRs and Plug R o w Reactors (PFRs) has been adopted (Fig. 2). The investigation has been done for the following conditions: > pressure: 1 or 10 bar > temperature: 1000 or 1300 degC > number of stages: 3 > stoichiometry at the stages: X^= 0.8, %?=• 1.0, ?i3= 2.0 > concentration of the dopant into the syngas: 1000 ppmv of N H 3 , or H C N , or N O . The variable characterizing the model has been the sharing of residence time adopted between any C S T R and PFR. Such shares were respectively: 5+0, 1+4, 0.1+4.9, 0.01+4.99, and 0+5 ms (corresponding to the Instantaneous Mixing cases). |
