Prediction and Measurement of the Product Gas Composition of the Ultra Rich Premixed Combustion of Natural Gas: Effects of Equivalence Ratio, Residence Time, Pressure, and Oxygen Concentration

The ultra rich combustion (partial oxidation) of natural gas to hydrogen and carbon monoxide is theoretically and experimentally investigated. The effect of the process parameters equivalence ratio, residence time, pressure, and composition of the oxidizer is explored. Computations are performed with the use of the chemical kinetics simulation package CHEMKIN. First, the ultra rich combustion process is modeled as a freely propagating flame in order to establish the rich flame propagation properties. An Arrhenius correlation of the laminar flame speed with the adiabatic flame temperature is found with activation temperature 20,000 K. Subsequently, perfectly stirred reactor (PSR) computations were performed. From these, it is concluded that optimal natural gas conversion to hydrogen and carbon monoxide requires a residence time of at least 50 ms and, depending on residence time, an equivalence ratio between 2 and 4. To reach chemical equilibrium in ultra rich mixtures, the residence time is very long (> 1000 ms). The model predictions are validated by experiments on ultra rich combustion of natural gas by means of air enriched to 40% oxygen concentration at up to 3 bar and 300 kW. The effect of equivalence ratio at residence time 50 ms was investigated. Good comparison was found between measurements and model predictions on carbon monoxide, hydrogen, and the soot precursor acetylene. It can be concluded that the model provides reliable information on product gas concentrations as a result of ultra rich combustion of natural gas.