Untersuchung stationärer und dynamischer Vorgänge bei der Teerentstehung in Biomassevergasern und Gasreinigungen mit Hilfe der kontinuierlichen Teeranalyse (PDF)

Biomass is an option to use renewable energy for the generation of heat, power and secondary
fuels to substitute fossil fuels. The thermochemical gasification is an alternative technology to
conventional biomass combustion. Gasification technology can generate power with high
efficiency even in small scale power plants. It gives the option to produce gaseous and liquid
fuels or basic chemicals from solid biomass.
The major obstacle for the application of biomass gasification is the high tar content of the
generated gas. A number of different approaches in gasification and gas cleaning systems has
been developed in the past. However, tar production in the gasification process and
incomplete removal of tars by the gas cleaning systems are still problems preventing the
technical and economical breakhrough of the biomass gasification technology. Due to these
problems the determination of the tar content is important in terms of research as well as in
industrial application. The established tar analysis methodes are based on wet chemical
sampling and GC-analysis. They are able to give detailed information about the different tar
species existing in the gas. However, realtime process monitoring by measuring tars
continuously is not possible up to now.
An online tar measurement method has been developed at the Institute of Process Engineering
and Power Plant Technology (IVD) of the University of Stuttgart. The method is based on the
comparison of the total hydrocarbon content in the hot gas and the corresponding value of the
gas with all condensable tars removed. The hydrocarbon levels are measured by a flame
ionization detector. The difference in the hydrocarbon levels corresponds with the tar
concentration in the producer gas. The measurement system allows a sequential sampling
from two sampling points, i.e. the producer gas before and after the gas cleaning system. One
complete sampling and analysis run lasts two minutes, allowing a quasi online analysis of the
tar concentration. This allows a continuous process monitoring.
Different types of gasifiers, such as fixed bed gasifiers or fluidised bed gasifiers in laboratory,
pilot- or industrial scale have been investigated with the online measuring system. Biomass in
the form of pellets and wood chips was used as well as special fuels like sewage sludge or
RDF. Experiments have been done by sampling in the raw gas of gasifiers as well as in the
clean gas. This was in order to evaluate the performance of the tar analyser on the one hand
and to increase the knowledge about the dynamics of tar generation in gasification processes
and of tar reduction in gas cleaning devices on the other hand. The results of the measurings
showed variable tar concentrations in a range of a few milligrams up to about 40 g/m³.
Validation with other tar analysing methods was carried out to detect correspondence and
differences. This allows the interpretation of the obtained results. The thesis gives a succesful
demonstration of the online Tar-Analyser being a suitable instrument to be used in research
and industry and finally to help to improve gasifier technology.