Combustion of Biogas-like Mixtures in Coflow of Cold Air and of Hot Lean Combustion Products

 

Summary of the MSc Thesis Sustainable Energy Technology by Metin Çelik presented on July 18, 2012

Combustion of biogas is of great interest because it is a CO2 neutral source of energy. Nevertheless, due to its low calorific value, it is fairly difficult to extract usable thermal energy from it. Current research aims at better understanding of its combustion characteristics at different conditions in order to enable development of better combustion systems.

The present study focuses on the flame luminescence and velocity flow field of DNG-CO2mixtures with various dilution levels through a series of luminescence and PIV experiments in the Delft Jet-in-Hot Coflow burner. The examination is made by comparing the biogas-like fuel obtained by the CO2 dilution of Dutch natural gas (DNG) to the higher calorific value fuel of pure DNG and occasionally to two other intermediate diluted fuels. The fuel mass flow rates are kept constant at a level where the Reynolds number is set to 4000 (calculated with transport properties at 298 K). Cold coflow consisted of pure air. Three different hot coflows are used, having different levels of O2 dilution and/or temperature.

The results with cold coflow revealed less stable flames, increasing lift-off heights, decreasing flame lengths and higher flame base mobility with increasing dilution level of the fuel. The axial luminescence distributions of flames with hot coflows were more uniform than those formed with cold coflow. As dilution increased, the intensity of the combustion decreased. With hot coflow, both pure DNG fuel and DNG-CO2mixtures stabilized as a result of auto-ignition kernels. The location of the auto-ignition kernel formation is found to be related to the coflow temperature, whereas the fuel or coflow dilution has no substantial influence on this location. The frequency of auto-ignition kernel formation is estimated to rise with increasing fuel and coflow dilution and higher temperatures. It is disclosed that kernels obtained with diluted fuels have more homogeneous structure and undergo less intense combustion. The same influences were observed for lower temperatures and O2 levels in the coflow. No correlation is found between the fuel dilution and the auto-ignition kernel propagation speed. The average axial auto-ignition kernel velocities are measured to range from 0.125 SL to 0.265 SL. Earlier and slightly stronger entrainment of the hot coflow is observed with the pure DNG case, compared to the 30% CO2 case, due to its higher velocity resulting in higher momentum. The heat release is found to damp the turbulence and vorticity, resulting in slightly higher turbulence and vorticity levels with low calorific value (LCV) fuels.