The application of homogeneous reactor models to examine the conversion of Polycyclic Aromatic Hydrocarbons in biomass derived fuel gases

The removal of tar from biomass derived fuel gases is considered to be a bottleneck in the widespread application of biomass gasification. Tar is made up of Polycyclic Aromatic Hydrocarbons (PAHs) consisting of fused aromatic rings. Past research has proven the possibility to remove the PAH species naphthalene from producer gas at high temperature by means of introducing a limited amount of oxidizer. This process, which also can be regarded as partialcombustion, is not yet fully understood. By conducting a fundamental study of the mechanisms involved it will be possible to optimize the process conditions and the reactor geometry. In the work presented here it is examined if partial combustion can be described by homogeneous reactors. The PAH species representing the tar content of the producer gas is naphthalene (in correspondence with the executed experiments). Chemical equilibrium calculations indicate that, if time scales approach infinity, it is possible to convert naphthalene to lighter hydrocarbons at smallvalues of the equivalence ratio, lambda. For lambda <0.2, soot is produced. The influence of chemical kinetics is examined with Perfectly Stirred Reactor (PSR) calculations. The results reveal that the time scales involved in the conversion of naphthalene at small values of the equivalence ratio (0 <lambda <0.2) are considerate. This leads to the conclusion that the process of tar cracking by partial combustion cannot be described by homogeneous reactors alone. The experimental results are not fully explained by the presented results. This indicates that heterogeneous effects should be taken into account. This requires full 2D simulations in which molecular transport is included.