Marco Wellinger
When using natural or waste wood in thermo-chemical conversion processes, the presence of a number of heteroatoms (e.g. sulfur, chlorine, potassium and sodium or heavy metals) may hinder the processes themselves as well as pose a threat to equipment and/or the environment. Due to the often strongly heterogeneous composition of the feedstocks and high dynamics of thermo-chemical reactions, the elemental concentrations in process gases show a high temporal variability. In order to accurately characterize such processes efficient and representative sampling methods and analytical instruments with a high sensitivity and time resolution are necessary. This thesis presents the development and application of devices to analyze trace element contents of product gases from thermo- chemical conversion processes and to characterize the thermal behavior of feedstocks such as wood pellets. In the first part the development of a mobile sampling and measurement system for the analysis of gaseous and liquid samples in the field is presented. An inductively coupled plasma optical emission spectrometer (ICP-OES) which was built into a van, was used as detector. The analytical system was calibrated with liquid and/or gaseous standards. It was shown that identical mass flows of either gaseous or liquid standards resulted in identical ICP-OES signal intensities. In a field measurement campaign trace and minor elements in the raw flue gas of a waste wood combustor were monitored. Sampling was performed with a highly transport efficient liquid quench system which made possible the observation of temporal variations in the elemental process gas composition. After a change in feedstock an immediate change of the element concentrations in the flue gas was detected. A comparison of the average element concentrations during the combustion of the two feedstocks showed a high reproducibility for matrix elements that are expected to be present in similar concentrations. Elements showing strong differences in their concentration in the feedstock were also represented by a higher concentration in the flue gas. Following the temporal variations of different elements revealed strong correlations between a number of elements, such as chlorine with sodium, potassium and zinc, as well as arsenic with lead, and calcium with strontium. Most notably, an online detector for alkalis (SID II) based on the principle of surface ionization has been designed and constructed within the framework this thesis. The detector includes a number of improvements compared to previous designs. Due to a fixed filament geometry, the sensitivity of the SID II is highly reproducible between different filaments. Improvements to the flow geometry of the device, and the installation of an auxiliary heateing element have minimized the overall tar depositions. In contrast to previous designs, the electrical feedthroughs are kept outside the sample gas stream and are therefore protected against tar contamination which can lead to signal artifacts. In numerous measurements at thermal processes up to industrial scale the SID II has demonstrated a high sensitivity and improved characteristics to conduct measurements in tar and particle laden process gases. By using an ultrasonic nebulizer as a source for alkali aerosols it was possible to perform a calibration of the alkali detector over 4 orders of magnitude. In combination with a dilution setup and a sampling lance, we could take online measurements of gasifier product gas with a high degree of reproducibility and a high time resolution of 1 s. In an experimental series with a miniature thermo-chemical reactor the release of alkalis and formation of product gases during pyrolysis and combustion of single commercial-sized wood pellets could both be recorded with a time resolution of approximately 1 s for with the SID II and the a mass spectrometer, respectively. The thermal processing was conducted by using compressed air at temperatures between 400 °C and 900 °C. During the experiments at temperatures of 700 °C and below the majority of the alkali are released after the initial pyrolysis stage. In contrast to that, in the experiments at 800 °C and 900 °C most of the alkali release happened during the pyrolysis stage (during the first 200 s after the insertion of the pellet). The alkali release rates collected at 800°C during the char combustion phase were reproduced using an extended version of the random pore model, resulting in similar numeric values of the fitted model parameters as reported by Struis et al. (2002) for thermo-chemical conversion experiments with pulverized wood char and carbon dioxide at 800°C well. During the late stage of combustion the alkali release rates exhibit the same phenomena as previously determined for char pore surface area. These phenomena, related to pore coalescence and char structure disintegration, gave rise to a number of varied attempts to modify the original random pore model derived by Bhatia and Perlmutter.
EPFL2012
