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.
The main objective of this work is to evaluate a number of unresolved issues on the kinetic modeling of the pyrolysis of woody and nonwoody biomass fuels. More specifically, this work aims to provide a better understanding of the role of the interactions between the main biomass componentscellulose, hemicellulose, and ligninand the role of the presence of extractives on the pyrolysis of those biomass fuels. To achieve these goals, five biomass samples and the three main biomass components were pyrolyzed under slow pyrolysis conditions, up to 700 °C, in order to obtain a comprehensive experimental database for model validation. Subsequently, the experimental weight loss curves, product yields, and gas composition were compared with the predictions obtained with the Bio-PoliMi mechanism. Experimental mixtures were obtained by superimposing the experimental curves from the single component pyrolysis tests while satisfying the composition of the biomass samples. The Bio-PoliMi mechanism predicted with good accuracy the weight loss curves and the product yields of the single components. Overall, there was a good agreement between the experimental and the predicted characteristic temperatures of CO2 and CO, but the predictions showed a delay in the release of CH4 and H2 that led to major discrepancies in the gas composition. In the case of lignin, the triangulation method was found to contribute to the observed disagreement between measurements and predictions of the CO2 yield. When analyzing mixtures resembling real biomass, a good agreement between the experimental mixtures and predicted product yields was observed when the extractives were not considered for both the woody and nonwoody mixtures. Discrepancies were also observed in the case of the gas species yield, and compensation errors were identified in the case of the CO. When extractives were considered, the Bio-PoliMi mechanism showed a good agreement with the product yields of the real biomass, especially with the total gas yield. However, large discrepancies were observed for the gas composition, meaning that the model is not capable of predicting the pyrolysis gas heating value.
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. 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 GT, TorPlant, and Top Process. The identification of the main volatiles produced under the 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 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 biorefinery concept is growing rapidly for bio-based production of fuels and products, and steam explosion is by far the most applied pre-treatment technology allowing the delignification of lignocellulosic biomass. Within the bioethanol production process, pyrolysis of lignin-rich residue (LRR), for producing char to be used in a wide variety of applications, presents a viable way to recover materials and energy, helping to improve the sustainability of the whole production chain. In the present study, it is shown that yields, elemental composition and porosity characteristics of LLR-char are significantly different from those of char produced from alkali lignin. Both products yields and char composition were more similar to the typical values of woody and herbaceous biomasses. The chemical characterization of the chars’ organic matrices as well as the content of the main inorganic species suggest the opportunity to perform pyrolysis at low temperatures for producing high yields of chars suitable to be used as carbon sink or soil fertilizers. The BET values of the chars obtained at final temperatures in the range 500–700 °C seem to be promising for char-application processes involving surface phenomena (e.g., adsorption, catalyst support), thus encouraging further analyses of char-surface chemistry.
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