Inherent Metal Elements in Biomass Pyrolysis: A Review

Abstract

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.

Catalytic effect of alkali and alkaline earth metals on fast pyrolysis pre‐treatment of agricultural waste

“…With the addition of NaCl in the blend, Na metal also increases in the sample. Thus, it was observed a reduction of the available catalytic pores in biomass that collapse reverting the formation of silica compounds 32 . The results demonstrated that Si compounds vanished after adding salt, showing that NaCl promotes ring‐opening / closing, isomerization, decarboxylation, and dehydration reactions, after which these structures disappeared to form cyclic hydrocarbons, and alkanes, leaving silica molecules as a residue.…”

“…It is noticeable that silicon was abundant metal in PS sample (0.28 wt%). This metalloid was excluded from the mineral biomass matter because it could be derived from soil contamination 32 . According to Vassilev et al ., the presence of inorganic elements belonging from biomass collection, transportation or transformation, defined as technogenic minerals, cannot be considered inherent metals 33 …”

“…Acetic acid was therefore obtained in a high proportion in AS, due to the high hemicellulose content in this biomass. Moreover, it is well known that the presence of K increases selectivity in acetic acid and decreases production of other acids 32,37 …”

Strong anti-kinetoplastid activity of silver nanoparticle-coated biochar

“…The onset of Biochar degradation is 274 °C, higher than that of Biochar@Ag which starts to degrade at 200 °C, well below the bare Biochar. This could be due to catalytic thermal degradation of the carbonized matter by silver NPs [22]. Degradation progresses until 377 °C and the mass becomes stable.…”

Catalytic Fast Pyrolysis of Lignocellulosic Biomass to Benzene, Toluene, and Xylenes

“…Alkaline and alkaline earth metal ions (AAEMs) are part of the plant's nutrient to enable the plant to grow [15,126]. Lignocellulose is comprised of four different types of materials: lignin, cellulose, hemicellulose, and inorganics.…”