Effect of Alkali and Alkaline Earth Metals on in-Situ Catalytic Fast Pyrolysis of Lignocellulosic Biomass: A Microreactor Study

Abstract

In-situ catalytic fast pyrolysis (CFP) is considered to be a promising pathway to produce aromatic hydrocarbons from lignocellulosic biomass. However, the presence of variable amounts of inorganic ash in biomass in the form of alkali and alkaline earth metals (AAEMs) is a concern while using in situ catalysts because AAEMs could influence product distribution from CFP while also being a major reason for catalyst deactivation. In this study, the effect of four alkali and alkaline earth metals (K, Na, Mg, and Ca) commonly found in biomass was investigated to understand their individual influence on the fate of primary pyrolysis products as well as their effect on the selectivity of products from in situ CFP using ZSM-5 catalyst. Experiments were performed in a microreactor (Py-GC/MS) with ZSM-5 catalyst using AAEM-impregnated biomass. It was found that the type of AAEM as well as the concentration were significant, with Mg appearing to be relatively inert when compared to the stronger catalytic activity of K, Na, and Ca. The influence of AAEMs on the formation of pyrolysis products from cellulose, hemicellulose, lignin, and its subsequent influence on CFP is discussed. From noncatalytic pyrolysis experiments, even the lowest concentration of AAEMs (0.1 wt %) was observed to have a significant influence on the thermal decomposition behavior of biomass, promoting the formation of lower molecular weight cellulose and lignin-derived products. AAEMs were found to be influencing CFP product distribution by reducing the carbon yield of desired aromatic hydrocarbons and olefins, while it accelerated pathways resulting in increased yields of thermally derived char and noncondensable gases. The effect of AAEMs on CFP followed the order: Na > K > Ca > Mg.

Blended Feedstocks for Thermochemical Conversion: Biomass Characterization and Bio-Oil Production From Switchgrass-Pine Residues Blends

“…Increased relative yields of CO 2 and methanol in 2PN are indicated by negative PC1 loadings, which can be attributed to the higher ash and inorganics content in 2PN. Ash and particularly AAEMs are more reactive than other inorganics and effectively catalyze ring-breaking, depolymerization, and fragmentation reaction, which results in increased production of non-condensable gases, organic acids, and other light oxygenated compounds (Carpenter et al, 2014;Mahadevan et al, 2016b). Similar trends are observed in the PCA results obtained from the Py-GC/MS data for SG, 6PN, and their blends ( Figure S1).…”

“…In addition, previous work demonstrates that the presence of ash, especially AAEMs, results in greater formation of organic acids, non-condensable gases, and char during pyrolysis (Fahmi et al, 2008;Patwardhan et al, 2010;Mahadevan et al, 2016b).…”

“…The higher ash content in switchgrass can be largely attributed to its high silicon content, which is commonly reported in switchgrass (Vassilev et al, 2010). Silicon is inert under pyrolysis conditions, and while high content is undesirable due to the reduced amount of carbon available for conversion, it is not detrimental to the conversion reaction like the AAEMs (Fahmi et al, 2008;Patwardhan et al, 2010;Mahadevan et al, 2016b). Carbon content is the highest in 2PN (53.5%), followed by 6PN (52.0%), and SG (49.2%) and the same order is observed for nitrogen, where nitrogen content is higher in 2PN (0.5%) compared to 6PN (0.2%) and SG (0.2%).…”

“…This is probably related to the relatively lower inorganics content in the 6PN compared to 2PN. Mahadevan et al (2016b) demonstrated that increased concentrations of AAEMs result in decreased coniferyl alcohol yields in pyrolysis vapors. When comparing the SG/2PN and SG/6PN blends, the trends and % peak areas for acetic acid, 2,3-dihydrobenzofuran, isoeugenol, and levoglucosan are very similar between the two.…”

Development of Heterogeneous Catalysts for Thermo-Chemical Conversion of Lignocellulosic Biomass

“…On the other hand, the investigations performed by Mahadevan et al [44] indicated that contamination of the catalyst surface by alkali or alkaline earth metals (especially Na, K, or Ca) present in biomass feedstock resulted in a reduction in the amount of aromatic hydrocarbons and olefins, with a simultaneous increase in the yield of permanent gases and char. The presented literature data demonstrate that an introduction of metals on the surface of zeolites or mesoporous silicas (i.e., by impregnation or ion-exchange methods) leads to an increase in the value of the bio-oil produced.…”