Characterizing combustion instability in non-premixed methane combustion using internal flue gas recirculation

Combustion instability (CI) has been widely reported and studied in lean premixed combustion (LPM), however, there is limited information on the instability aspects in non-premixed methane combustion using internal flue gas recirculation (IFGR), a frequently used low-NOx technique in boilers. The existence and nonlinear dynamic features of combustion instability in non-premixed methane combustion are experimentally examined by varying IFGR nozzle structures and global equivalence ratios (Φ). The experimental observations reveal that the flammability limits and stability maps are transited according to different IFGR nozzle structures. Combustion instabilities are prone to occur in lean conditions due to the intensified axial and tangential heat release rate oscillations. Multiple combustion states and two bifurcations in thermoacoustic instability by decreasing global equivalence ratios are found. In the discussion and analysis, a computational method of convection time is established for non-premixed combustion. To compensate for the lack of experimental data of local species/flow fields, numerical simulations are conducted to achieve quantitative data of convection time. A convection time of 34 ms triggers the limit cycle oscillation modes in the experiments, corresponding to the 2nd thermoacoustic delay time in the combustion system. Varying IFGR structures and global equivalence ratios have impacts on the flow velocity in the oxidant mixture, and thus, the convection time is changed. When the convection time is coupled with the thermoacoustic delay time in the combustion system, combustion instability occurs. This work establishes a link between IFGR nozzles and combustion instability in non-premixed methane combustion, and highlights the influence of varying nozzle structures and global equivalence ratios on convection time and the flow velocity in the oxidant mixture in non-premixed combustion, which can provide a foundation for further studies on combustion instability in various low-NOx combustion types.