Beitrag zur Verwertung von carbonfaserverstärkten Kunststoffen in Hochtemperaturprozessen

Stockschläder, Jan; Quicker, Peter (Thesis advisor); Flamme, Sabine (Thesis advisor)

Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023


Carbon fibres (CF) are used in combination with plastics for various high-performance applications, especially in aviation, mobility or wind turbines. Although these carbon-fibre-reinforced-plastics (CFRP) are a niche application in terms of mass technology compared to glass-fibre-reinforced-plastics, continuous growth rates of around 11 w.-% have been recorded in recent years. With the increase in use, the amount of waste containing carbon fibres (WCCF) is also rising. Currently, production waste dominates here, but end-of-life waste also arises with a time lag. In the best case, this waste is collected separately and sent to one of the few recycling plants. However, fibres that cannot be recycled are produced during recycling. There are currently no known energy recovery or feedstock recycling routes for these WCCF. In addition, the missing of separate collection can already lead to inputs into established treatment routes such as municipal waste incineration. This thesis addresses this problem by carrying out both laboratory and large-scale investigations on the use of CFRP in high-temperature processes. The investigations are intended to find out possibilities for their utilisation in high-temperature processes. First, different types of fibres (High Tensile Strength, High Modulus and Ultra High Modulus) were investigated in the laboratory with regard to their thermal behaviour. The reaction mechanism is described by the two-dimensional phase boundary controlled reaction, especially in the first part of the oxidation. An increase in temperature accelerates the oxidative process. In addition, the degradation was investigated by SEM microscopy in different stages of oxidation. In addition to the typical reduction of the fibre diameter, various forms of decomposition such as pitting or poring could be detected. The various types of fibres differed in this respect. In further laboratory investigations, a fuel characterisation of various WCCF was carried out. In addition, the decomposition behavior and loss of volatiles was investigated by using an thermogravimetric analysis. As expected, all WCCF have a high carbon content. However, the ratio of carbon and oxygen or hydrogen shows clear differences compared to other wastes or secondary fuels. In addition, some WCCF have other elements such as nitrogen or sulphur, which have to be taken into account for utilisation in high-temperature processes. With the results, a combustion reaction calculation was carried out so that further differences to secondary fuels could be shown. The thermogravimetric analysis show significant differences in the loss of volatiles and decomposition behaviour. It follow that a suitable high-temperature process should cover a wide spectrum of WCCF. In the large-scale investigations, test campaigns were carried out in different high-temperature processes on an industrial scale. In addition to the three approaches for energy recovery in a municipal waste incineration plant, a hazardous waste incineration plant and a cement kiln plant, feedstock recycling in a electric arc furnace for the production of calcium carbide was investigated. The objectives of the investigations are, firstly, to test the suitability of the process for a complete conversion of CF, secondly, to identify the requirements for flue gas cleaning and, thirdly, to examine the extent to which fibres are produced with WHO criteria (diameter D < 3 μm, length L > 5 µm, L:D ≥ 3:1) (WHO-CF). The established methods of thermal waste treatment with grate or rotary kiln firing are not suitable for a complete oxidation of CF due to the insufficient process conditions (especially temperature and dwell time or mixing of air/solid fuel). The majority was discharged via bottom ash / slag, which cannot be further mechanically processed due to the high CF residues content. The exhaust gas cleaning system retained the CF discharged via the flue gas path. WHO-CF are produced to a small extent, but the possible exposure must be taken into account here. The cement kiln plant showed that the WCCF can only be used in the kiln burner. This requires a reduction to 2D particles with a maximum edge length of 30 x 30 mm. A low concentration of CF was found in the clinker, which should be investigated again in a further long-term test. The campaign time was too short and the specific amount of WCCF used were too low. In the fourth process, CFRP production waste was previously shredded and pelletised together with mixed plastics and replaced coke and coal tar pitch in the calcium carbide production. The recovery rates of CF are low, so that feedstock recycling seems to be possible. However, CF and, to a lesser extent, WHO-CF are discharged, especially via the carbide oven gas. In the exhaust gas cleaning system, the discharged fibres are separated. Due to the external utilisation of these residues, the effects on such utilisation paths should be examined in the case of a potential application. In summary, it can be seen that a systematic investigation of different types of CF with regard to their thermal behaviour should be carried out. Based on the examinations of three different fibres, the fibres should be investigated systematically (for example on the basis of properties such as material characteristics or virgin diameter). In particular, the proportion of potentially emerging WHO-CF should be determined. However, suitable methods are still required. The large-scale campaigns showed that thermal waste treatment in municipal and hazardous waste incineration plants is not suitable. The potential use in the two processes cement kiln plant electric arc furnace for calcium carbide production requires that the WCCF are prepared for utilisation. For this purpose, corresponding treatment processes must be prepared and evaluated. For the overall process evaluation, further bases should be created and evaluated, especially for the upstream. Only then can a final assessment be made for incineration in acement kiln or for feedstock recycling in an electric arc furnace for the production of calcium carbide.


  • Forschungskolleg VERBUND.NRW [080053]
  • Division of Mineral Resources and Raw Materials Engineering [510000]
  • Unit of Technology of Fuels [512220]