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Sustainable valorization of Biomass into Syngas/H2 via Biocarbon catalyst

Time: Wed 2023-11-29 10.00

Location: F3 (Flodis), Lindstedtsvägen 26 & 28, Stockholm

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Language: English

Subject area: Materials Science and Engineering

Doctoral student: Hanmin Yang , Processer

Opponent: Dr. Chunfei Wu, Queen's University Belfast

Supervisor: Docent Weihong Yang, Tillämpad termodynamik och kylteknik, Processer; Professor Pär Jönsson, Processer; Dr. Linda Sandström, Research Institutes of Sweden

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Environmental issues stemming from the rapid growth in global energy demand and carbon dioxide emissions require urgent resolutions. Biomass represents a viable alternative for displacing fossil fuels, as its energy can be converted into electricity, heat, fuels, and chemical precursors, thus substituting petrochemicals. It is also the only carbon-containing renewable resource with substantial potential to mitigate environmental degradation, attain carbon-negative emissions, and drive sustainable development. Syngas production from biomass pyrolysis and in-line catalytic upgrading has attracted an increased attention, since it is a promising approach for further generating renewable bio-fuels, bio-chemicals, and bio-materials. Hydrogen isolated from bio-syngas is a clean and promising secondary energy source and carrier, capable of advancing a carbon-free energy system across technological, economic, and societal dimensions. 

This dissertation aims to realize a full valorization of biomass into renewable and affordable hydrogen-rich syngas and carbon-based battery anodes (hard carbons) through pyrolysis and in-line catalytic upgrading using biochar as the core of the catalyst strategy. Biochar, a carbon-enriched solid material with a carbon-neutral nature, emerges as a promising catalyst for promoting volatile upgrading owing to its extensively functionalized surface, porous structure, and resistance to coke deactivation.

Optimization of the catalyst strategy using biochar-based catalysts in the catalytic upgrading process to enhance syngas quality is critical for scaling up the proposed process. This dissertation first investigated the effect of Ni-based, biochar, cascaded biochar+Ni-based, and engineered biochar catalysts on the catalytic performance in terms of the syngas yield, hydrogen yield, and gas energy conversion efficiency (GECE). Among them, the cascaded biochar+Ni-based catalyst and the Ni-doped biochar (NiBC) catalyst showed the most excellent catalytic performance. Using NiBC as a reforming catalyst introduced 78.2 wt. % of syngas consisting of an H2+CO proportion of 94.6 vol. % while applying cascaded biochar + NiAlO catalyst strategy resulted in 71 wt. % of syngas with a total H2+CO proportion of 89.5 vol. %. However, after a three-time test repetition, the Ni-doped biochar catalyst showed a slightly decreasing catalytic performance. In contrast, the cascaded biochar+NiAlO catalyst showed a stable promoting performance in terms of syngas and H2 yields after 15 feeding cycles. In addition, plastic waste, being a carbonaceous resource, was also applied in the pyrolysis and in-line catalytic upgrading process for hydrogen and high-value-added carbon production by using biochar as a cracking catalyst. 

In order to evaluate the techno-economic viability of the proposed process, a novel biorefinery concept was simulated and assessed based on the above results, aiming to produce affordable hydrogen and high-quality hard carbons from biomass and to realize negative carbon emissions. The proposed biorefinery was estimated to produce 75 kg of H2, 169 kg of hard carbon, and 891 kg of captured CO2 (95% purity) per metric ton of biomass while realizing a payback period (PBP) within two years at reference prices of 13.7 €/kg and 5 €/kg for HCs and H2. At the same time, a negative emission of the proposed biorefinery could be achieved with -0.89 kg CO2-eq/kg-biomass based on Sweden’s wind electricity, considering the captured CO2.

A pilot-scale system using a continuous pyrolysis reactor was deployed to scale up the capacity of the proposed process. The catalytic performance of biochar was examined in terms of products’ distribution, gas composition and gas properties. Critical parameters, such as the weight hourly space velocity (WHSV), particle size and the morphology of the catalysts, and pressure drop of the catalyst bed, were evaluated. The results showed that a lower WHSV favours a higher syngas yield, a higher H2+CO proportion, and a higher hydrogen yield due to a longer residence time for volatiles-char contacts. Smaller catalyst particle sizes correspond to higher bed pressure drop, which resulted in a higher syngas and hydrogen yield. In addition, biochar particles with larger bulk density and more spherical and rounded shape introduced higher syngas yield, H2 +CO proportion, and H2 yield compared to the particles with elongated and angular shape. The stability of using biochar as a catalyst in a continuous feeding system was also examined and verified in this dissertation, which indicated its great anti-coking performance.