• Energy Research

Biomasses to be used in the phytoremediation process are generally selected to match agronomic parameters and heavy metals uptake ability. A proper selection can be made greatly effective if knowledge of the properties of the residual char from pyrolysis is available to identify possible valorization routes. In this study a comparative analysis of the yields and characteristics of char obtained from slow pyrolysis of five uncontaminated biomasses (Populus nigra, Salix alba, Fraxinus oxyphylla, Eucalyptus occidentalis and Arundo donax) was carried out under steam atmosphere to better develop char porosity. Moreover, the dependence of the properties of solid residue on the process final temperature was studied for E. occidentalis in the temperature range of 688-967K. The results demonstrate that, among the studied biomasses, chars from P. nigra and E. occidentalis have to be preferred for applications regulated by surface phenomena given their highest surface area (270-300m2/g), whereas char from E. occidentalis is the best choice when the goal is to maximize energy recovery.

Biomass for energy production has been extensively studied in the recent years. To overcome some constraints imposed by the chemical-physical properties of the biomass, several pretreatments have been proposed. Torrefaction is one of the most interesting pretreatments because torrefied biomass holds a wide range of advantages over raw biomass. The devolatilization of water and some oxygenated compounds influences the increase in the calorific value on both a mass and volumetric basis. The increase in the density reduces the transportation costs. Moreover, the decreased moisture content increases the resistance of biomass to biological degradation, thus facilitating its storage for long periods. Under torrefaction conditions, approximately 10-40 wt% of the initial biomass is converted into volatile matter including liquid and non-condensable combustible gases.1,2 The energy efficiency of the process could greatly benefit the exploitation of the energy content of these products. Recent studies and technological solutions have demonstrated the possibility to realize polygeneration systems that integrate torrefaction/pyrolysis to a combustion process with the aim of obtaining torrefied material/biochar and/or energy from biomass. Some examples include Pyreg, Pyreg-Aactor GT3, TorPlant, and Top Process.4 The identification of the main volatiles produced under torrefaction regime is useful for the optimization of the operating conditions of the integrated system. The integrated process raises some concerns when biomass from phytoremediation and wood from demolition and construction activities are used as feedstock because they could contain potential toxic elements (PTEs). During the torrefaction treatment, the fate of PTEs should be controlled in order to avoid their release in the gas phase and to evaluate the extent of their concentration in the torrefied biomass. The present work aims at studying torrefaction as an eco-sustainable process for the combined production of a solid biofuel with improved characteristics with respect to the starting material and a combustible vapor phase, embedded in the gas carrier flow, to be directly burned for energy recovery. Herein, torrefaction tests on Populus nigra L. branches from phytoremediation, and demolition wood were conducted at three temperatures, 250, 270 and 300 °C, at a holding time of 15 min. The energetic content of torrefied materials was determined. At the same time, the fate of the heavy metals (Cd, Pb, and Zn) in the raw biomass at different torrefaction temperatures was studied, and their mobility in the torrefied biomass was investigated and compared to the mobility in the raw biomass.

The main objective of the present work is to model the impact of the presence of potassium chloride on the slow pyrolysis of cellulose in terms of mass loss profiles, product yields formation and gas speciation. In the present work, we propose to modify the cellulose sub-mechanism of the Bio-PoliMi kinetic model by introducing a chemical reaction that considers the potassium chloride catalytic effect during pyrolysis. The kinetic parameters and reaction coefficients of the introduced reaction were fitted to the experimental results using a two-step fitting procedure. The experimental results were obtained from thermogravimetric and pyrolysis tests, up to a temperature of 973 K using a heating rate of 5 K/min, for five cellulose samples doped with different potassium chloride concentrations (pure sample, 0.5 wt.%, 1.09 wt.%, 2.0 wt.% and 3.0 wt.%). The experimental results show that the presence of potassium chloride affects both primary and secondary pyrolysis, affecting the product yields and the gas composition. The modified mechanism proposed in the present work is able to predict with good accuracy the thermogravimetric profiles as well as the total product yields of char, gas and tar. Furthermore, the mechanism is able to predict reasonably well the release characteristics of CO and CO2 gaseous species, even though the final yield is under-predicted. As for CH4 and H2 species, the mechanism is not able to follow their experimental trends since the original mechanism cannot capture the secondary pyrolysis of the cellulose.

The post-treatment of biomass from phytoremediation is not yet a well-established practice due to the risk induced by the presence of Potentially Toxic Elements (PTEs). Pyrolysis is a thermochemical treatment that reduces the volume and weight of contaminated matter producing a combustible vapor phase and a solid residue (char). A key factor enhancing the economic and the environmental sustainability of biomass valorization through pyrolysis is the production of a market value char. A proper choice of the pyrolysis operating conditions should take into account the effect of final temperature on PTEs release, on the char physicochemical properties as well as on the mobility of retained PTEs. In particular, in this work the influence of both the temperature and the plant parts is discussed (branches and leaves of Populus Nigra L. and rhizomes and culms of Arundo donax L.) on the release of Cd, Pb, Cu, and Zn in the temperature range 653-873 K under steam assisted slow pyrolysis conditions. The mobility of the heavy metals retained in the chars was also studied as well as the product yields, the gas composition and char porosity. The results suggested that in presence of Cd it is necessary to operate at low-temperature (lower than 703 K) to obtain a heavy metals free vapor phase fuel, whereas in presence of one or more metals among Pb, Cu, and Zn, it is possible to conduct a pyrolytic treatment at higher temperatures, thus obtaining a char with high BET surface area and lower metals mobility.

One of the main drawbacks of using biomass as pyrolysis feedstock consists of the huge variability of the different biomass resources which undermines the viability of downstream processes. Inherent inorganic elements greatly contribute to enhance the compositional variability issues due to their catalytic effect (especially alkali and alkaline earth metals (AAEMs)) and the technical problems arising due to their presence. Due to the different pretreatments adopted in the experimental investigations as well as the different reactor configurations and experimental conditions, some mechanisms involving interactions between these elements and the biomass organic fraction during pyrolysis are still debated. This is the reason why predicting the results of these interactions by adapting the existing kinetic models of pyrolysis is still challenging. In this work, the most prominent experimental works of the last 10 years dealing with the catalytic effects of biomass inherent metals on the pyrolysis process are reviewed. Reaction pathways, products distributions and characteristics, and impacts on the products utilization are discussed with a focus on AAEMs and on potential toxic metallic elements in hyperaccumulator plants. The literature findings are discussed in relation to the applied laboratory procedures controlling the concentration of inherent inorganic elements, their capability of preserving the chemical integrity of the main organic components, and the ability of resembling the inherent inorganic elements in the raw biomass. The goal is to reveal possible experimental inconsistencies and to provide a clear scheme of the reaction pathways altered by the presence of inherent inorganics. This analysis paves the way for the examination of the proposed modifications of the existing models aiming at capturing the effect of inorganics on pyrolysis kinetics. Finally, the most relevant shortcomings and bottlenecks in existing experimental and modeling approaches are analyzed and directions for further studies are suggested.