Screw Pyrolysis of Biogenic Feedstock with Integrated Hot Gas Filtration

Dissertation von Marco Tomasi Morgano
Universität Stuttgart, 2019

The Institute for Technical Chemistry of the Karlsruhe Institute of Technology developed an innovative screw pyrolysis reactor (STYX) with integrated hot gas filtration, which enables the production of particle-free vapors, and respective condensate and gas. To support the development of the technology, experimental and theoretical work aiming at understanding and controlling the thermal and chemical processes undergoing during pyrolysis of biogenic residues at the STYX is necessary. The absence of solids in the vapors minimizes the typical issues of pyrolysis, i.e. clogging and blocking in the hot pipelines and in the condensation unit, and it is of immense advantage for the following direct applications such as combustion of the vapors for heat and power conversion and for upgrading of the liquids into fuels and chemicals. Moreover, a solid hygienized and carbonized product, the pyrolysis char, is always produced during pyrolysis. It finds applications in the energy industry as fuel and in agricultural industry as soil enhancer and fertilizer, depending on the initial content of nutrients of the feedstock. Robustness and flexibility are the keys of the economic feasibility of the STYX technology.

The effects of the pyrolysis process parameters on the products yields and properties of beech wood as a reference biogenic material were investigated experimentally at STYX. The feedstock was characterized in detail with focus on the chemical composition. The temperature of the reactor was adopted as fundamental parameter since it influences the thermodynamic equilibrium and ultimately defines the yields of the products. Mass, elemental and energy balances were carried out at the reactor for selected conditions globally and regarding local situations. The pyrolysis chars and oils were investigated for further utilization as fuels. Moreover, the pyrolysis oils were characterized for further upgrading applications. The aqueous condensate is often considered the by-product of pyrolysis processes. In this work, it was identified as feedstock for the production of valuable chemicals. The permanent gas, or non-condensable gas, was characterized as fuel as well.

Successively, the pyrolysis of different biogenic feedstock with high inert and inorganic contents was investigated in the STYX reactor at well-defined process conditions. Low-grade biogenic feedstock, such as solid manure and dried sewage sludge, have a distinctive behavior from typical lignocellulosic biomass. The inert content is responsible for higher yields of pyrolysis solids compared to the reference feedstock. The yields of the pyrolysis oils are comparable or higher than those of lignocellulosic feedstock, whereas the yields of the aqueous condensate and of the permanent gas are considerably reduced. Furthermore, the presence of nitrogen, sulfur and chlorine as well as of minerals and metals implies the necessity for removal concepts or feasible recovery of potential pollutants.

Transport mechanisms of granular solids and the hydrodynamics of the gas in screw reactor where evaluated experimentally in the STYX reactor and adopted as basis for the development of a thermochemical model of screw reactors. Consequently, the reactor was modelled as a cascade of perfectly mixed reactors for both the solid and the gas/vapor phases. The processes taking place in the reactor were described on the basis of available models obtained from literature. The mechanical behavior of the solids was modelled in terms of mixing quality. The heat exchange between the reactor and the vapors was described taking into account forced convection and radiative heat transfer mechanisms. The heat exchange between the reactor and the granular solid was described by the penetration model, regarding the granular solid with interstitial gas as a continuous system. Pyrolysis mechanism was described by the chemical kinetics for the decomposition of model compounds present in wood biomass. On the basis of the reactor model, an extensive sensitivity analysis was carried out assessing the most important parameters.

Finally, the thermochemical model was validated experimentally at the STYX reactor. Experiments were carried out to describe the heat transfer of an inert dry granular solid. The temperature of the solids was measured online adopting temperature-sensitive dyes; thus the real temperature of the solid surface was measured. The drying mechanisms of an inert porous granular solid was investigated experimentally and compared with the numerical results. The pyrolysis mechanism was studied extensively validating the decomposition of beech wood and the secondary gas phase reactions along the reactor as well as the overall yields distributions and the compositions as a function of the process parameters. The thermochemical model developed in this work could be shown a validated and flexible tool for the scale-up of pyrolysis reactor based on the STYX technology.

Because filtration plays a decisive role for the quality of the vapors and of the condensate, the fundamental mechanisms involved during filtration need to be investigated thoroughly. For future implementation in the thermochemical model of the reactor filtration should be modelled as well. The pyrolysis mechanisms of non-lignocellulosic model compounds should be integrated into the numerical tool in order to extend its capabilities to more attractive feedstock for pyrolysis applications at decentral scale.

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