A Review of Recent Research on Catalytic Biomass Pyrolysis and Low-Pressure Hydropyrolysis

Abstract

Direct catalytic upgrading of biomass-derived fast pyrolysis vapors can occur in different process configurations, under either inert or hydrogen-containing atmospheres. This review summarizes the myriad of different catalysts studied and benchmarks their deoxygenation performance by also taking into account the resulting decrease in bio-oil yield compared to a thermal pyrolysis oil. Generally, catalyst modifications aim at improving the initial selectivity of the catalyst to more desirable oxygen-free hydrocarbons and/or improving the catalysts’ stability against deactivation by coking. Optimizing the pore structure and acid site density/distribution of solid acid catalysts can slow down deactivation and prolong activity. Basic catalysts such as MgO and Na2O/γ-Al2O3 are excellent ketonization catalysts favoring oxygen removal via decarboxylation, whereas solid acid catalysts such as zeolites primarily favor decarbonylation and dehydration. Basic catalysts can therefore produce bio-oils with higher H/C ratios. However, since their coke formation per surface area is higher, compared to a microporous HZSM-5 zeolite, precoking (or imperfect regeneration) of these basic catalysts and operating for longer time-on-stream can be approaches to improve the oil yield. In-line vapor-phase upgrading with a dual bed comprising a solid acid catalyst followed by a basic catalyst active in ketonization and aldol condensation further improves deoxygenation while maintaining high bio-oil carbon recovery. Also low-cost catalysts such as iron-rich red mud have deoxygenation activity. An improved bio-oil carbon recoverycompared at a similar level of oxygen removalcan be obtained when changing from an inert atmosphere to a hydrogen-containing atmosphere and using an effective hydrodeoxygenation (HDO) catalyst. To keep costs low, this can be conducted at near-atmospheric pressure conditions. Pt/TiO2 and MoO3/TiO2 showed high activity and reduced coke formation. Stable performance has been demonstrated using Pt/TiO2 for 100+ reaction/regeneration cycles with woody biomass feedstock. If future works can demonstrate the same durability for lower-cost biomass containing higher contents of ash, N, and S, this would considerably boost the commercial viability of near-atmospheric pressure HDO. Further research should be directed to testing the durability of lower-cost HDO catalysts such as MoO3/TiO2 and further improving the activity and stability of lower-cost catalysts.

An effective strategy to produce highly amenable cellulose and enhance lignin upgrading to aromatic and olefinic hydrocarbons

“…50,51 Monomers 3, 4, and 5 are formed through the cleavage of b-O-4 linkages in lignin mediated by the supported metal catalyst. 52 Only p-coumaric (8) and ferulic acid (9) were observed for lignin pretreated in the presence of HY zeolite catalyst. Moreover, the yield was lower than for samples deconstructed through solvolysis.…”

“…Unfortunately, raw bio-oil presents adverse properties due to its high oxygen content (about 40%), namely poor thermal and chemical stability, corrosivity from high acidity, low heating value, and high viscosity compared to petroleum-derived oils. 6,8,9 Several strategies were proposed to lower the bio-oils' oxygen content, overcome its adverse properties, and improve its compatibility with existing infrastructure in the chemical industry. Most of these strategies focus on the production of aromatic hydrocarbons because of their high energy density and important role in chemical manufacturing.…”

From Biomass to Fuel Blendstocks via Catalytic Fast Pyrolysis and Hydrotreating: An Evaluation of Carbon Efficiency and Fuel Properties for Three Pathways

“…CFP may take place in either an in situ or ex situ configuration. ,− During in situ CFP, the catalyst is placed directly in the pyrolysis reactor, while in ex situ CFP, the pyrolysis products are catalytically upgraded in a separate downstream reactor. The in situ configuration offers a simpler process with fewer reactors, but the catalyst is in contact with the biomass, char, and ash, which may lead to irreversible catalyst deactivation by biomass contaminants. − The ex situ configuration has a higher capital cost due to the added upgrading reactor, but the catalyst is not in direct contact with the biomass.…”

“…The deactivation caused by the carbon deposits can be reversed by thermal oxidation of the catalyst, and commercial-scale upgrading over zeolites is envisioned to take place in riser reactors with continuous regeneration of the spent catalysts . Bench- and pilot-scale experiments of biomass CFP over HZSM-5-based catalysts have shown carbon efficiencies of 21–33% for producing CFP oils with 18–24 wt % oxygen. ,,, While limited differences in the performance of HZSM-5 catalyst in the in situ and ex situ configuration have been found in short-term bench-scale experiments, , longer experiments and studies of metal addition have pointed to irreversible deactivation of HZSM-5 and lower CFP oil yields caused by alkali and alkaline earth metals (e.g., Na, K, Ca) found in biomass. , This suggests that without feed demineralization, ex situ CFP may be better suited than in situ CFP for upgrading over zeolite catalysts.…”

Perspectives of Biomass Catalytic Fast Pyrolysis for Co-refining: Review and Correlation of Literature Data from Continuously Operated Setups

“…In the last two decades, the CFP of biomass has been studied extensively and several literature reviews have been published with specific attention given to various aspects of the technology, i.e., chemistry and catalysts, ,,,− the process, ,− overview and advances in the technology, ,,,− and the techno-economic analysis (TEA) and process economics. − Just a few review articles, − in which the performances of CFP process units with varying scales are compared, contributed to the efforts shown for process improvement/development. This review, on the other hand, follows a novel methodology.…”