Weiterentwicklung und Erprobung eines Online-Verfahrens zur Bestimmung von Teerkonzentrationen in Vergasungsprozessen

Dissertation von Andreas Gredinger
Universität Stuttgart, 2018

Until today, the so-called tars are still one of the major drawbacks in gasification of biomass. One reason why their concentration in gasification processes is not determined on a continuous basis is their extensive, analytical determination with today's established adsorption methods. Furthermore, these methods permit only a time-delayed indication of an average value over a certain period of time. Consequently, the lack of appropriate and available instrumental analysis limits the feasibility of (industrial) gasification projects.

In order to close this gap in gas measurement technology and to provide an alternative to conventional adsorption methods, a prototype of an online measurement device was developed at the Institute of Combustion and Power Plant Technology of the University of Stuttgart in the past. In this work, the device is further developed and extensively tested to reach marketability. Its analytic is based on a differential measurement method, in which the tars are separated from the gaseous hydrocarbons of the producer gas on a suitable condensate separator. A FID is used as hydrocarbon detector.

Firstly, the necessary technical changes of the measurement device and the further developments of its underlying analytic are described. Investigations on the accuracy of the newly developed detector showed a good repeatability of the results of individual measurement cycles with deviations from their mean value of less than 1 %. The detection limit of the tar concentration is dependent on the selected measuring range and determined at 50 mgCstp, 250 mgCstp and 500 mgCstp. However, it is independent of the hydrocarbon concentration of the measured gas. The influences of different operational parameters of the measurement device on the sensitivity of the detector are also shown and ideal operational settings are specified.

As a further step, an optimum tar condensate separator for separation of the hydrocarbons at the molecular boundary of the tar definition (differential measurement) was identified. It is found that inert condensate traps are not sufficient to separate light hydrocarbons, such as BTX, from the remaining producer gas stream. Therefore, adsorptive materials are used to solve this problem. The most promising materials are naturally occurring (layer) silicates. They adsorb, except for toluene, all tar substances completely in a large concentration range, but let gaseous hydrocarbons pass completely. The tar definition in this work is made at the molecular mass bigger than benzene, however, benzene is also partially adsorbed. Experiments showed, that the partial adsorption of toluene and benzene can balance each other at certain concentration ratios; thus permitting the measuring of the tar concentration according to the tar definition.

The conduction of comparative measurements with the standardized, conventional Tar Protocol confirms the findings obtained in the prior laboratory investigations. The results' deviations of the on-line measuring device and the GC-MS analyses of the Tar Protocol in producer gases of an air gasification are in the range of ± 20 %. Same results (with corrections of the influence of hydrogen and vapour on the measuring method) were obtained in steam gasification. The absolute deviations never exceed the mark of 2.5 gCstp.

The measurement cycle time of 60-90 seconds and the direct output of the measured values allow the direct monitoring of the total tar concentrations of a gasification process in the case of tar peaks occurrences or similar events. Therefore, a fast intervention in the process management is then possible. To complete the work, the further developed measurement device was demonstrated on a semi-industrial gasifier. Goal of the demonstration was the fast determination of optimized gasifier operational parameters with the aim of minimizing tar production. The results from the tests at IFK and the demonstration finally confirm the findings and statements gained and made during the course of the present work with regard to an instrumental replacement of conventional tar measurement methods.

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