Catalytic pyrolysis of lignin to produce fuels and functional carbon materials

Time: Wed 2020-06-10 10.00

Location: publikt via ZOOM, Stockholm (English)

Subject area: Materials Science and Engineering

Doctoral student: Tong Han , Energi- och ugnsteknik, Energy and Furnace Technology

Opponent: Professor Anker Jensen,

Supervisor: Docent Weihong Yang, Energi- och ugnsteknik, Materialvetenskap, Tillämpad termodynamik och kylteknik; Professor Pär Jönsson, Materialens processteknologi


Development of renewable energy carriers and green adsorbents is an essential step in creating a fossil-free and toxin-free future of the world. Lignin is the second highest component of biomass and the only renewable resource of aromatics in nature. Currently, around 70 million tons of lignin are produced annually from the pulp and paper industries word-wide, while only 1-2% of them can be upgraded into value-added products. Pyrolysis is one of the most promising technologies for lignin conversion to produce value-added products. After a lignin pyrolysis process, biooil, biogas, and biochar can be produced. Wherein, after upgrading, biogas and biooil can be used as alternatives to fossil based energy carries to produce fuels or chemicals; biochar can be used as carbon source to produce green adsorbents for pollutants removal. 

This dissertation provides a systematic research focusing on the catalytic pyrolysis of lignin to produce upgraded biofuels and magnetic activated carbons (MACs). First of all, two specific issues i.e. sulfur and melting unique to lignin pyrolysis process are studied to achieve a thorough understanding of the lignin pyrolysis processes. Investigation of sulfur evolution during the lignin pyrolysis process is the study carried out first. Understanding lignin melting characteristics is the study carried out subsequently. Hereafter, in situ catalytic pyrolysis of lignin over low-cost catalysts is studied to produce upgraded biooils. Low-cost catalysts with different textural and acidic properties screening is the study carried out first. Development of a self-sufficient catalytic pyrolysis of lignin process via using activated carbons (ACs) derived from the same lignin pyrolysis process as catalysts is the study carried out subsequently. At last, pyrolysis and subsequent steam gasification of metal dry impregnated lignin is studied to produce MACs and H2-rich syngas. Development of a streamlined process to produce high-quality MACs for phosphorous adsorption is the study carried out first. Pyrolysis and subsequent steam gasification of metal dry impregnated lignin to co-produce MACs and H2-rich syngas is the study carried out subsequently.

The study of sulfur evolution during the lignin pyrolysis process implies that sulfur-containing radicals are more likely to combine with other small radicals during a fast pyrolysis process. As a result, the main detected sulfur-containing compounds are small molecular gases or liquids with low boiling points and the main compounds in liquid phase are sulfur-free. The study of lignin melting characteristics at pre-pyrolysis temperature implies that the degree of cross-linked of the lignin structure determines its melting characteristics. Lignin extracted from pulping process has a less cross-linked structure. Therefore, it melts and softens to a flow state after a glass transition. Lignin extracted from hydrolysis process has a more cross-linked structure. Therefore, it does not melt but rather decompose after a glass transition.

The study of low-cost catalysts with different acidic and textural properties screening for in situ catalytic pyrolysis of lignin implies that the use of only commercial AC as a catalyst induces the enhanced yield of monocyclic aromatic hydrocarbons (MAHs) among all low-cost catalysts. Bentonite and red mud catalysts have strong surface acidity but poor porous properties. This determines that produced reactive intermediates are easy to repolymerize to form char or coke without the blocking effect of pore wall. Commercial AC has an abundant porous structure as well as a surface acidity with a certain strength. The produced reactive intermediates could be isolated by pore walls and therefore induce the of MAHs production. A subsequent study of in situ catalytic pyrolysis of lignin over ACs from the same lignin pyrolysis process implies that the use of only AC that has more mesopores than micropores as catalyst could induce a significant decrease of the tarry oil yield and a significant increase of the phenols concentration in aqueous and liquid phase oils. The diffusion efficiency of the reactive intermediates determined by pore size is supposed to be the most crucial parameter that determines the catalytic performance of ACs. The pore sizes of mesopores are much bigger than the sizes of reactive intermediates. Therefore, these pores could allow most of the reactive intermediates to diffuse quickly and to react within their pores.

The study of the streamlined MACs production process development implies that iron species can be embedded into a carbon matrix via a lignin melting process. After the pyrolysis/carbonization of lignin and FeSO4 mixture under a nitrogen atmosphere, FeSO4 is decomposed and further reduced to form hagg iron carbide, which is buried into carbon matrix of biochars after a lignin melting. During subsequent steam gasification/activation process, iron species are gradually exposed from the carbon via the pore drilling and widening effect of steam. At the same time, the bare part of iron species are oxidized by steam to form magnetite. The maximum phosphorous adsorption capacity of produced MAC sample calculated using the best-fit Langmuir-Freundlich model is estimated to be 21.18 mg P/g. Further study of pyrolysis and subsequent steam gasification of metal dry impregnated lignin to produce MACs and H2-rich syngas implies that during the pyrolysis of FeSO4 impregnated lignin process, H2 is produced via the catalytic cracking of the volatiles. During the subsequent steam gasification of solid residues, H2 was mainly produced via the steam carbon reactions and the steam gas shift reactions. The maximum overall H2 yield of the integrated process is as high as 42.73 mol/kg-lignin. Also, approximately 70% of phosphorous in real domestic wastewater can be adsorbed by MACs produced from the same process after a treatment for 2 hours.