Prediction and measurement of the critical compression ratio and methane number for blends of biogas with methane, propane and hydrogen

Abstract

Methane number (MN) and the critical compression ratio (CCR) measurements for twelve blends of biogas with methane or propane and hydrogen additions were taken in a Cooperative Fuel Research (CFR) F2 model engine according to the standard. In addition, CHEMKIN simulations of MN and the CCR were performed on these blends at similar conditions to the CFR F2 engine operation. Eight chemical kinetics mechanisms were used; it was concluded that the best mechanism to simulate the CCR is USCII, and the best mechanism to simulate MN is San Diego. In almost all blends that include propane, the best mechanism to predict the CCR and MN is Butane. It was not possible to find an optimal mechanism for all gaseous blends to simulate the CCR and MN. Experimentally, three blends of biogas were found with methane or propane and hydrogen additions with an MN of 100, with the intention to find blends of alternative and conventional fuels to interchange with methane from the viewpoint of knock resistance. For two blends, tests were carried out while changing the equivalence ratio and inlet pressure to evaluate their effect on knocking, measured with the CCR. A correlation between MN and the CCR is presented utilizing data from current and past tests performed at Colorado State University. The MN and CCR characterize the knocking tendency for each blend and indicate the maximal compression ratio to spark ignition engines; a higher CCR will lead to higher thermal efficiencies. These proposed blends use mostly biogas with a high percentage of inert gas, with favorable combustion properties including relatively large energy densities, low heating values, laminar flame speeds and low adiabatic flame temperatures. Blends of biogas with conventional fuels allow desirable combustion characteristics and high resistance to knocking.