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Effect of air/fuel nozzle arrangement on the MILD combustion of syngas
Author: Huang Mingming | Print | Close | Text Size: A A A | 2016-10-28

The effect of air/fuel nozzle arrangement on MILD (Moderate or Intense Low-oxygen Dilution) combustion of coal-derived syngas under lean operating condition was evaluated in a parallel jet combustor. The results were presented on flow field using non-reactive numerical calculation and on OH* chemiluminescence images and exhaust emissions using experiments. The increase in equivalence ratio aids in the reduction of peak flame temperature and in the distribution of reaction volume. The critical equivalence ratios above which MILD combustion occurred were identified for four MILD configurations. The MILD configuration with both air and fuel injected from the opposite side of the combustor exit was observed to achieve MILD combustion at leaner condition than other three configurations. The MILD scheme was established for syngas fuel under highly lean condition with low NOx and CO emissions.

Commercial software FLUENT was utilized to calculate the nonreactive flow field inside the combustor with 10 MJ/Nm3 syngas fueled at heat load of 27.5 kW andΦ=0.45. Both air and fuel were 293 K injected. The operating pressure was 1 atm. Full hexahedral grid was constructed in only one-fourth of the combustor geometry. Appropriate refinement of the grid was conducted in the region with high gradients. In grid independence study, both velocity contour in the combustor and oxygen level along the fuel jet centerline were carefully compared, considering the computation time requirements and calculation accuracy, one-fourth geometry with grid size of about 0.45 million cells was modeled. The mixing field was solved using a steady state, implicit, finite-volume based compressible solver, and realizable k-ε model with standard wall functions was used to model turbulence. Realizable k-ε model has been shown to provide more accurate prediction of the jet mixing flow field. Turbulence intensities of the mass flow inlets and pressure outlet were set to 0.05 while SIMPLE algorithm was used for pressure velocity coupling. A second-order upwind discretization scheme was used to solve all governing equations. Convergence was obtained when the residuals for all the variables were less than 10-4. The axial velocity along the air jet centerline obtained from numerical solution was compared with the results predicted by the correlation.

The development of syngas MILD combustion for gas turbine application requires careful examination of the effect of air/fuel nozzle arrangement on the mixing and combustion performance under lean operating condition. Four MILD configurations with different air and fuel nozzle positioning were utilized to examine the role of nozzle arrangement on flow field and combustion behavior. Data on flow field pattern, flame characteristics and pollutant emissions are presented under the heat loads of 12.2-36.7 kW and equivalence ratio range of 0.2-0.6. Numerical simulations revealed stronger gas recirculation and more delayed air/fuel confluence for the cocurrent flow configuration than the opposed. Forward flow configuration has lower oxygen concentration in the combustor than the reverse flow configuration. Experimental observation showed that, at constant heat load, the change over from the opposed and reverse flow configurations to the cocurrent and forward flow configurations results in the decrease of peak flame temperature and NOx formation. For a given MILD configuration, increased equivalence ratio helps to reduce the maximum flame temperature and to distribute the reaction zone across a larger volume. With equivalence ratio increased to exceed a critical value, the reaction zone covers the entire combustion chamber. Towards the critical equivalence, both NOx and CO emissions showa noticeable reduction. The cocurrent forward flow configuration “ABFB” exhibits a significant advantage in realizing MILD combustion with operational lean limit (Φ=0.49), limited NOx formation (4 ppm at Φ=0.49) and minimized CO emissions (39 ppm at Φ=0.49).

The results have been published on APPLIED THERMAL ENGINEERING  Volume: 87  Pages: 200-208.

 

 
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