Laminar burning velocity measurements of methane and carbon dioxide mixtures (biogas) over wide ranging temperatures and pressures

Biogas is an important renewable energy resource that is mostly methane but contains up to 40% carbon dioxide by volume. As the carbon dioxide is a diluent, it reduces the laminar burning velocity, the flame stability and the flammability range, all of which can have adverse effects on combustion system performance. When used in gas turbines, spark ignited gas engines and dual fuel engines it is important to have laminar burning velocity data for high temperatures and pressures. These data have been obtained by using the pressure rise data from constant volume combustion, so that a single experiment yields burning velocity data (linked by the isentropic compression) for a range of pressures and temperatures. The pressure/temperature dependency has been decoupled by conducting experiments with different initial temperatures and pressures. The results from experiments with different carbon dioxide fractions and equivalence ratios have been fitted to a correlation with twelve coefficients. Data have been obtained with up to 40% carbon dioxide, pressures of up to 18 bar, temperatures of up to 660 K and equivalence ratios in the range 0.7-1.4. Results showed a decrease in burning velocity with pressure and an increase with temperature. The effect of dilution with carbon dioxide was a reduction in burning velocity, to around 65% of that of pure methane when the carbon dioxide fraction was 40% for stoichiometric mixtures. There is only a very limited amount of prior burning velocity data for high temperatures and pressures, but where possible, comparisons have been made with published data.