Pyrolysis

Summary

The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. Temperature can be understood as thermal vibration. At high temperatures, excessive vibration causes long chain molecules to break into smaller molecules. The word is coined from the Greek-derived elements pyro "fire", "heat", "fever" and lysis "separating". Pyrolysis is most commonly used in the treatment of organic materials. It is one of the processes involved in charring wood. In general, pyrolysis of organic substances produces volatile products and leaves char, a carbon-rich solid residue. Extreme pyrolysis, which leaves mostly carbon as the residue, is called carbonization. Pyrolysis is considered the first step in the processes of gasification or combustion. The process is used heavily in the chemical industry, for example, to produce ethylene, many forms of carbon, and other chemicals from petroleum, coal, and even wood, or to produc

Official source

https://en.wikipedia.org/wiki/Pyrolysis

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This research validates the viability of a recycling and reusing process for end-of-life glass fiber reinforced wind turbine blades. Short glass fibers from scrap turbine blades are reclaimed and mixed with polylactic acid (PLA) through a double extrusion process to produce composite feedstock with recycled glass fibers for fused filament fabrication (FFF) 3D printing. Reinforced filaments with different fiber contents, as high as 25% by weight, are extruded and used to 3D print tensile specimens per ASTM D638-14. For 25 wt% reinforcement, the samples showed up to 74% increase in specific stiffness compared to pure PLA samples, while there was a reduction of 42% and 65% in specific tensile strength and failure strain, respectively. To capture the level of impregnation of the non-pyrolyzed recycled fibers and PLA, samples made from reinforced filaments with virgin and recycled fibers are fabricated and assessed in terms of mechanical properties and interface. For the composite specimens out of reinforced PLA with recycled glass fibers, it was found that the specific modulus and tensile strength are respectively 18% and 19% higher than those of samples reinforced with virgin glass fibers. The cause for this observation is mainly attributed to the fact that the surface of recycled fibers is partially covered with epoxy particles, a phenomenon that allows for favorable interactions between the molecules of PLA and epoxy, thus improving the interface bonding between the fibers and PLA.

2019

Polymer-derived ceramics (PDCs) exhibit excellent properties and are compatible with many shaping techniques due to the liquid character of the preceramic polymers (PCPs). Furthermore, their properties are adjustable by modifying the precursor composition and controlling the processing conditions. In this thesis, poly(methylvinylsilazane) (PMVSz) is chosen as the precursor basis and varied amounts of divinylbenzene (DVB) are added to increase the carbon content, which significantly increases the electrical conductivity of the silicon carbonitride (SiCN) obtained after pyrolysis. Based on molding and pressureless processing, samples are manufactured with varied homogeneous composition and functional properties as well as with engineered property contrasts, so-called functionally graded ceramics.Starting with shaping aspects, several mold materials, mold fabrication techniques, and the casting and thermal processing are discussed in the first part. The liquid precursor formulation is directly pipetted into the molds, either CNC-machined PTFE for bulk samples or anisotropically etched Si for microsized parts. Thermally cured bulk green bodies (GBs) are removed from the PTFE molds before pyrolysis whereas the monolithic and resilient character of Si allows for an in-mold pyrolysis which eliminates the need for mold release of fragile GBs. While the Si molds define the parts' general shape, sacrificial 3D microstructures are optionally added onto the mold bottom by two-photon polymerization (2PP). After pyrolysis, the PDC parts are obtained loose in the mold with sub-µm resolution features replicated as imprints.The second part focuses on the processing conditions and the tuning of the PDCs' bulk properties. Thanks to a thermal initiator, the PMVSz-based precursors are cured without pressurization within 30 min at 200°C. The pore-free GBs, filled with 0-60 wt.% DVB, are converted to SiCN with varied carbon (C) concentration by pyrolysis at 800-1400°C under Ar atmosphere. Dense and defect-free PDCs are obtained whose free carbon content is adjusted in the range 15-25 wt.%. The influence of the pyrolysis temperature on the ceramics' microstructure is analyzed and correlated to the electrical conductivity, tunable across 10 orders of magnitude up to 0.4 S/cm. Very high characteristic flexural strength of up to 1.65 GPa is observed and the SiCNs' cytocompatibility demonstrated.Thirdly, PCPs with and without DVB-filling are combined by consecutive casting to fabricate functionally graded SiCN parts. The dissimilar precursors are joined in vertical or lateral arrangement in dedicated PTFE molds, typically with an intermediate thermal curing step. Monolithic and defect-free property-contrast PDC parts are obtained after pyrolysis which exhibit seamless transitions and no signs of delamination at the interface. The casting order and extent of intermediate curing are varied, showing that composition gradients are adjustable from sub-µm to mm-range transition lengths. Significant contrasts in C-concentration and consequently electrical conductivity are observed within monolithic samples entirely consisting of SiCN. Finally, a cm-scale plate with two seamlessly integrated contrast regions is designed and fabricated. The all-SiCN parts have an insulating matrix and two conductive feedthroughs. An LED connected to the areas with higher C-concentration is powered through the ceramic and lit up.

EPFL2022

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

EPFL2022

EPFL2012