In the future, synthetic fuels made from biomass or coal can be an economical and environmentally friendly alternative to petroleum-based fuels. This will require processes that are one the one hand flexible with respect to the feed stock, on the other hand efficient in the increase of the raw material’s energy density.
The design and optimization of these processes in industrial plants is accomplished by means of numerical computational methods (Computational Fluid Dynamics, CFD), which, among others, have to describe the transition of the source material to a gaseous fuel (high-temperature pyrolysis). The major challenge of this task is to handle the complexity of the raw material: It can be liquid or solid or a suspension, often referred to as "slurry", and basically consists of variety of different species. Also, a size distribution of the pyrolysis fuel has to be taken into account.
Previous approaches in the numerical calculation of the transition of this complex mixture of substances in the gas phase are either (a) the use of computationally very time-consuming detailed chemical reaction mechanisms which are coupled with sub-models for the particle-specific processes or (b) simplistic models with inaccurate display of events in reality. A mediating approach of computation between these extremes is currently being developed in the Chemical Kinetics Department - the ‘Virtual Particle Model’ (VPM).
Virtual Particle Model (VPM)
The VPM is a gas-phase reaction model which represents a middle way between the detailed and therefore computationally expensive resolution of the chemical-kinetic processes in a multi-phase reactive flow and the substitution of these processes by a simplistic one-step model in which the source material is transformed into volatile gaseous products in a step.
By contrast, in the VPM species of a complex source material are divided in specific size-dependent classes, so-called. BIN-classes.
The thermochemical properties of these classes are estimated by quantum chemical methods. In this model reactions from three different categories are defined:
The reaction rate coefficients associated with the reaction types have to be adjusted on the basis of experimental data for the respective source material (conditioning). This allows the VPM with relatively little computing effort to model the transition of a complex source material into the gas phase. The data for the conditioning of the VPM for a particular feedstock composition are determined by the ‘High Temperature Flash Pyrolysis Reactor’ (HTFPR) for different operating conditions.
High Temperature Flash Pyrolysis Reactor (HTFPR)
The HTFPR is the main tool for conditioning the VPM. This allows the reactor to make the relevant process parameters - process temperature, heating rate and residence time – variable within a wide range and thus to create an environment in which a flash-pyrolysis of to be investigated biomass takes place.
For this purpose the HTFPR is divided into a burner module, in which the biomass experiences a sudden increase into a high-temperature region ("flash") in the exhaust gas of a hydrogen/oxygen flame and in an electrically heated plug flow reactor downstream. Based on the process temperature profile being monitored and the analyzed product distribution, the virtual particle model is adapted to the specific biomass composition.