OCR Text |
Show All the simulations show quite similar trends of the specie concentrations in any stage of the G T C C and at any pressure and temperature. Regardless the adopted model, the best final NOx reduction is obtained at higher pressure and lower temperature (Fig. 6). Figure 6. Index of N-Reduction along the GTCC. Instantaneous vs Sub-Streamed Mixing: Effects of the pressure and Temperature. Instantaneous Sub-Streamed: 5 sub-streams, mixing within 4 m s Dopant: [NH3]= 1000 ppmv. 0 5 10 15 residence time [ms] Due to the delay in the oxygen supply, with a Delayed Model the effects of pressure and temperature are lesser than with the Instantaneous Model. A delayed case always deals with a much higher nitrogen conversion to N 2 than an instantaneous one (Fig. 7). More in detail, in a delayed first stage of the G T C C the formation of H C N is enhanced and a noticeable concentration of it is calculated at the inlet of the second stage. Along the second stage the trends look alike for both mixing approaches: delayed and instantaneous. There, the dependence on pressure and temperature deals with the opposite influence of the physical variables on the oxidation of N O and NH3. Also the third stage shows similar trends: only a reduced tendency forming N 0 2 is visible when delayed mixing. Whatever the dopant in the syngas, N H 3 N O or HCN, the delay in the availability of oxygen introduced by a Delayed Model ends up in the same qualitative result. The reducing path of all the species is enhanced respectively by increasing the number of sub-streams (Fig. 8) and by rising the share of residence time in the CSTRs. When the H C N is dopant, such sensitivity to the model's variables is somewhat reduced. With the Instantaneous Model and at 1300 degC temperature the dopant conversion to N 2 is somewhat higher when H C N dopant. With Delayed Models the results are twisted. Always the presence of two dopants, i.e. N H 3 and NO, in the syngas ends up in a nitrogen behavior which mediates between the ones respectively predicted for each dopant separately. inlet Figure 7. The Nitrogen Chemistry: Delayed Mixing 3rd stage NH3 outlet Ni inlet Instantaneous Mixing 2nd stage 3rt stage N, NO N02 outlet NO NOz Figure 8. Index of N-Reduction along the GTCC. Instantaneous vs Sub-Streamed Mixing: Effect of the Dopant in the Syngas. 1.0 Parameter: Number of Sub-Streams, Mixing within 4 ms. ! 1st STAGE ! 2nd STAGE! 3rd STAGE | ^^_-f .«•"' 1 • 4. r-"1"" i • 4 .;, 1 Pressure-10 bar, Temperature-1300 degC 5 10 residence time [ms] 15 Whatever the methane content in the syngas, the Nfix reduction predicted by a Delayed Model is always higher than the one predicted by the Instantaneous Model (Fig.9): |