Amato, Davide (2023) Behavior of lead during pyrolysis of contaminated biomass. [Tesi di dottorato]

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Abstract

Pyrolysis of lignocellulosic biomasses is a consolidate process to obtain biofuels and biomaterials. However, the cultivation of biomasses for energetic purposes must be carefully evaluated to avoid conflicts with the cultivation of food crops. To overcome such issues, new kinds of biomasses are being considered, such as contaminated biomasses; heavy metals play a prominent role among the possible biomass contaminants. However, pyrolysis of heavy metals contaminated biomass is a scarcely investigated topic, and the information about heavy metals fate and influence during the pyrolysis process are few. This Ph.D. thesis investigates the transformations of Pb, one of the most common soil contaminants, during pyrolysis in a wide range of pyrolysis temperatures (400-800 °C) in order to give indications on the transport mechanisms that determine its distribution into the different pyrolysis products and its possible effects on the pyrolysis mechanisms. Also, the effects of different Pb chemical forms typically found in the soil and of different mechanisms responsible of biomass contamination are investigated. Poplar (Populus nigra), grown on Pb contaminated soil, is used for the experimental campaign. Pyrolysis tests are conducted at slow heating rate at three different final pyrolysis temperatures (i.e. 465, 600 and 800 °C). In order to explore the devolatilization and the chemical transformation of Pb during the pyrolysis process a reactor configuration is used where solid elutriation phenomena can be considered negligible. Poplar samples are doped to achieve a concentration of Pb of about 1000ppm with two Pb salts, Pb(C2H3O2)2 and Pb(NO3)2, representing the most common chemical form of Pb contamination. Moreover, in order to reproduce different source of contamination (authigenic or detrital or a mix of them) three different doping procedures are adopted, namely wet impregnation, ion exchange and dry mixing. The possible interactions with inorganics intrinsically present in the biomass have also been investigated by using both plain and demineralized poplar as feedstock for Pb doping. All the pyrolysis products (bio-oil, biochar and gas) are characterized extensively to study the distribution of Pb, its chemical form in the solid residue and its possible effect on the pyrolysis mechanisms and products characteristics. Chromatographic techniques are used for the gas and liquid characterization, whereas biochar is analysed through elemental analysis, ashing, porosity, ICP-MS (Inductively Coupled Plasma Mass spectroscopy), XRD (x-ray diffraction) and BCR sequential extraction. The results related to biochar characterization highlighted that both the initial Pb speciation and the type of contamination affect many biochar physical chemical properties and are relevant in Pb volatilization. From the gas and bio-oil analyses emerged that also the other pyrolysis products are influenced by the presence of Pb in terms of yield and composition. The results of the study provide fundamental indications for the optimization of a very robust pyrolysis reactor, namely the auger reactor, typically adopted for both biochar and bio-oil production by taking into account not only the yields of the desired product but also Pb content and stability.

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