Prediction of the autoignition delay time of producer gas from biomass gasification

The greenhouse effect and the increasing demand for energy have encouraged the use of alternative fuels, with agricultural and forestry biomass waste being two of the main renewable sources that European and Spanish policies have been promoting during the last several years. In this article, theoretical results for the chemical autoignition delay time of producer gas obtained from the gasification of lignocellulosic biomass are presented. These results, together with those obtained in a previous work related to the laminar flame speed of the gas, are of great interest both for understanding the chemical kinetic mechanisms that control the fuel oxidation process and for the development of combustion models that provide significant information to be used as a tool for the optimization and design of internal combustion engines. The CHEMKIN software, in conjunction with the GRI-Mech chemical reaction mechanism, has been used to compute the autoignition delay time for different producer gas compositions, different values of pressure and temperature, and different producer gas/air equivalence ratios. Correlations of the delay time as a function of those variables are proposed, and a sensitivity analysis of the main reactions affecting the autoignition process has been carried out. The results have been compared with those obtained for conventional fuels (isooctane and methane), showing the potential of producer gas to reduce the knock tendency in a spark ignition engine and to allow homogeneous charge compression ignition (HCCI) combustion conditions at intake temperatures lower than those typically used in a natural gas HCCI engine. The reliability of the Livengood−Wu integral method, as a way to estimate the ignition timing under engine conditions (variable pressure and temperature), has also been checked. Differences lower than 6% between the Livengood−Wu integral and the CHEMKIN method, the latter using a complete chemical kinetic scheme, have been obtained for different engine operating conditions.