NUMERICAL SIMULATION OF COMBUSTION OF NATURAL GAS WITH HIGH-TEMPERATURE AIR

ABSTRACT This article deals with Ihe technology of burning natural gas with high-temperature air and large quantities of flue gas. The objective of this work is to assess the potential of several combustion models for their abilities to predict characteristics of the new combustion technology. Three numerical models have been used: (a) an Eddy-Break-Up model with a two-step reaction scheme, (b) an Eddy-Dissipation Concept model with chemical equilibrium and (c) pdf/mixture fraction model with equilibrium, non-adiabatic look up tables for chemistry. The nitric oxide post processors have incorporated thermal and prompt mechanisms as well as the NOx reburning mechanism. The computational results have been compared with both in-furnace measurements (temperature, O2, CO2, CO, CH4, NO, velocities, radiative heat flux) and with the measured furnace exit parameters. All three models have correctly reproduced the characteristics of the high temperature air combustion, namely the uniformity of the temperature field, high radiative fluxes and low NOx and CO emission. With the exception of a small region located within the natural gas jets, the chemical equilibrium assumption with respect to both natural gas combustion and equilibrium O, OH and N radicals have resulted in predictions of very good quality. The Eddy Dissipation Concept model and the pdf/mixture fraction model have provided almost identical results. The tested combustion models cannot describe the chemistry and temperature field in the fuel jet region. This is partially caused by imperfections in predicting the entrainment of the weak methane jet interacting with the strong combustion air stream. A more comprehensive understanding of the chemistry of natural gas combustion under fuel-rich conditions with comburent containing 3-4% oxygen is required to develop appropriate non-equilibrium sub-models for this fuel jet region.

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