Maximizing solid carbon yield from biomass pyrolysis via acid-catalyzed dehydration for CO2-gasification biofuel production

Authors: Terrell, Evan; MARTINEZ, LINA - Washington State University; GARCIA-PEREZ, MANUEL - Washington State University

Submitted to: Pacifichem Symposium
Publication Type: Abstract Only
Publication Acceptance Date: 6/8/2021
Publication Date: N/A
Citation: N/A

Interpretive Summary: A challenge for making biofuels from lignocellulosic materials (e.g., wood, grass, agriculture residues) is the relatively high oxygen content of biomass, in comparison to typical fossil fuels (e.g., coal, oil, natural gas). This large amount of oxygen already present in biomass limits the amount of heat that can be generated when burning a biofuel. Fossil fuels typically contain only hydrogen and carbon, and therefore they do not have this oxygen problem. One way to overcome this limitation of biomass is to remove its oxygen prior to making a biofuel product. This can be done through treatment with acid (e.g., phosphoric acid), which helps to remove oxygen in the form of water when biomass is heated up (in a process called slow pyrolysis), leaving behind a more carbon-rich product. This carbon-rich char can then be processed at high temperature with carbon dioxide to make a mixture of carbon monoxide and some hydrogen (in a process called gasification). With the help of methane addition, this mixture can then be converted into biofuels using catalysts (in a process called Fischer-Tropsch synthesis). The focus of this work is to measure how well the acid pretreatment is capable of removing oxygen from biomass while leaving behind its carbon. Economic assessment of this entire process has also been conducted, with promising results that suggest it could be a feasible pathway for biofuel production (Tanzil AH, et al. Biomass and Bioenergy (2021), 145, 105942).

Technical Abstract: Biomass pyrolysis is among the leading technologies for producing sustainable green fuels and chemicals. One challenge when working with biomass pyrolysis for the production of fuels is the considerable abundance of oxygen in comparison to fossil-based resources. The oxygen content is responsible for reducing overall heating value, thereby limiting suitability in combustion applications. One promising route for fuel development from biomass is through CO2 gasification of biochar, followed by F-T synthesis. However, in order to maximize syngas carbon, the solid carbon yield in biochar must also be maximized. Therefore, the goal of this work is to present the effect of phosphoric acid treatment, known to catalyze oxyen-removing dehydration reactions, on char and carbon yields during biomass carbonization. Phosphoric acid-pretreated biomass samples were prepared at varying concentrations of acid (~1-6%). For thermogravimetric analysis (TGA), biomass was heated under CO2 from 35-1000 C at heating rates of 5, 10 and 20 C/min. TGA results show that with increasing acid concentration, there is a greater yield of char (max ~50% char at 400 C). The effect of acid tends to diminish with higher concentrations, showing an asymptotic effect. This suggests that there is perhaps an optimal acid concentration less than/near to ~10%, beyond which no further increase in char yield from carbonization can be meaningfully obtained. The kinetic limitation of CO2 gasification is also apparent from TGA at different heating rates. Continuing work will incorporate GC analysis of gases and elemental analysis of char to determine oxygen removal/carbon retention prior to gasification. A hypothetical process combining acid pretreatment, carbonization, and gasification under CO2 will be developed in greater depth. For this process design, CO2-eq emissions will be estimated and TEA will be used to assess economic feasibility for biofuel production following this pathway.