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Show Figure 2. The Principle of the Combined Model from stage n-1 to stage n+1 Air addition the third stage. To be noted is that the same nitrogen reductions are predicted regardless the initial time delay and residence time characterizing each sub-stream. The number of sub-streams is the dominant Model's variable: l="instantaneous" and 5="slow" (Fig.3). With the Combined Model, the longer the residence time in the CSTRs the better the Nfix reduction at the outlet of the combustion chamber (Fig. 4). Figure 4. Index of N-Reduction along the G T C C. Instantaneous vs Combined Mixing: Effect of the Residence Time Sharing. R E S U L TS The predictions obtained by the simplistic Instantaneous Mixing Model adopted in the previous study are here compared with those from the Sub-Streamed and the Combined Models. At higher pressure and lower temperature long ignition delays are always predicted. The risk of uncompleted combustion is beyond the objective of this work; therefore it will be not discussed here. With the Sub-Streamed Model the number of sub-streams heavily affects the predictions on Nfix reduction (Fig. 3). Figure 3. Index of N-Reduction along the GTCC. Instantaneous vs Sub-Streamed Mixing: Effect of the Number of Sub-Streams. Parameter: Number of Sub-Streams, Mixing within 4 ms. 1st STAGE ,2nd STAGE 3rd STAGE -* * 10 i slow p= 10 bar, T_ 1300 degC; Dopant: [NH3]= 1000 ppmv. 5 10 residence time [ms] 15 1.0 -f 0.8 I 0.6 1 0.4 0.2 0.0 Parameter: residence time in each CSTR + res. 1 St STAGE p i i ^_____ time in each PFR . 7 5 + 0 i .^--^ 0.1+4.9 : 0.01 + 4.99 i i / ^ ^ ^ ' 2nd STAGE ' 3rd STAGE ' p=10bar, T= 1300 degC Dopant: [NH3]= 1000ppmv. 5 10 residence time [ms] 15 When short residence times (less than 0.1 ms) in the CSTRs, the N O produced along the first stage is the same predicted when only PFRs simulate the G T C C (Instantaneous Model). A better index is anyway predicted here due to a N O conversion along the second and third stages. When longer residence times (more than 1 ms) in the CSTRs, the enhanced nitrogen reduction is predicted because of a lesser production of N O along the first stage. Summarizing, the nitrogen conversion is predicted higher by the Combined Model than by the Sub-Streamed Model and than by the Instantaneous Model (Fig.5). Figure 5. Index of N-Reduction along the G T C C. Instantaneous vs Sub-Streamed vs Combined Mixing. Qualitatively, the more the sub-streams in a stage the more the syngas dopant is converted to N2. When mixing is done through one sub-stream, the dopant species is shortly oxidized to N O , which afterward is hardly reduced to N2. When many sub-streams are introduced, the dopant produces more H C N as an intermediate. The conversion of H C N to N 2 along the first and second stage is usually more effective than the corresponding conversion of NOx. The main conversion of nitrogen-species to N 2 is predicted in the first stage, both in the delayed and instantaneous cases. Conversions of nitrogen-species to N 2 are not visible along - Sub-Streamed: 5 Sub-Streams, Mixing within 4 ms - Combined: 3 CSTRs and no PFRs p_10bar,T_1300degC; Dopant: [NH3]=1000ppmv. S 10 residence time [ms] 15 |