Insights into the scalability of catalytic upgrading of biomass pyrolysis vapors using micro and bench-scale reactors†
Despite a vast body of literature dedicated to catalyst screening using micro-scale reactor systems for the catalytic deoxygenation of biomass-derived fast pyrolysis vapors, the quantitative scalability of the results obtained at the microscale to a continuous industrial process remains questionable. In this work, the product yields from ex situ vapor upgrading in a micro-pyrolyzer utilizing mg quantities of catalysts and a bench-scale system using ∼100 g of the catalyst and continuous biomass feeding are compared for nine different catalysts in an inert atmosphere and three different hydrodeoxygenation catalysts in a hydrogen-containing atmosphere. Besides the comparison of product yields, important quality parameters of the bio-oil collected at the bench-scale such as oxygen content, TAN, molar H/C and O/C ratio, effective hydrogen index (EHI), and higher heating value (HHV) were compared with the properties of the non-condensed vapors detected at the micro-scale. A higher deoxygenation severity and HHV and higher yields of CO and light hydrocarbons (both alkanes and alkenes) were measured at the bench-scale compared to the micro-scale. Catalytic vapor upgrading at the bench-scale produced ∼125% more coke per catalyst surface area compared to the micro-pyrolyzer studies, and generally for acidic catalysts tested under an inert atmosphere the yield of aromatics was higher for bench-Py tests compared to μ-Py tests. These differences are attributed to the approximately two orders of magnitude higher catalyst mass/gas flow rate ratios (W/F) applied at the bench-scale compared to the micro-scale, resulting in higher conversions over the catalytic bed and likely additional secondary cracking reactions. At both scales, with an increasing amount of pyrolysis vapors fed over the catalyst the yield of deoxygenated products (aliphatics and monoaromatics) decreased and the selectivity for BTX products decreased when using HZSM-5 containing catalysts. Trends in product selectivity towards aliphatics or aromatics observed for the HDO catalysts tested with the micropyrolyzer agreed with the properties of bio-oil from bench-scale tests. The estimation of the oxygen-content and EHI of the non-condensed vapors at the micro-scale can serve as an indicator for the charring propensity and thus the chemical reactivity of bio-oils obtained at larger scales. Overall, this contribution addresses the implications to consider when applying results obtained at the microscale for catalytic upgrading of fast pyrolysis vapors under an inert or hydrogen-containing atmosphere for predicting the process behavior on a large continuous scale.