Hydrothermal and fast pyrolysis processes of biomass for valorization of lignin streams

Marc Borrega, Taina Ohra-aho, Tiina Liitiä, Juha Lehtonen, Tarja Tamminen, Ekaterina Kholkina, Narendra Kumar, Dmitry Yu. Murzin, Kristien De Sitter, Kelly Servaes

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Abstract

The LF4Value project investigated the application of the Lignin First philosophy to create the maximum value to lignin-based streams. The Lignin First philosophy, which considers lignin as the main biomass component to be valorized, was implemented by using chemical additives for reactive protection of lignin in two established thermal treatments of biomass: hydrothermal pre-treatments for saccharification, and fast pyrolysis processes.
The addition of 2-naphthol to the hydrothermal pre-treatment of pine wood improved its saccharification by two-fold, presumably because lignin condensation during the pre-treatment was minimized. On the other hand, chemical additives in the pre-treatment of birch and willow wood were not needed to achieve almost quantitative saccharification. The analyses of hydrolysis lignins from birch revealed differences in their composition and structure by the use of 1- and 2-naphthol, consistent with a lower extent of lignin condensation. Selected hydrolysis lignins from pine, birch and willow were successfully used as filler in PLA-based composites at 20% lignin loading. The addition of lignin slightly decreased the tensile strength and strain of PLA without significant increase in tensile stiffness. However, the different PLA/lignin composites exhibited a similar mechanical performance, which seemed to indicate that differences in lignin structure were not relevant.
The addition of chemical additives to fast pyrolysis of wood did not change the distribution of lignin- and carbohydrate-derived degradation products, probably because the reaction times were too short for the additives to react. The addition of 2-naphthol, however, slightly enhanced the stability of the fast pyrolysis bio-oils in long-term storage. The use of slag-based catalysts in fast pyrolysis demonstrated that the catalysts changed the product distribution and the yield of degradation products. However, the catalysts did not appear to prevent the recondensation of lignin pyrolysis products and consequently the yield losses in bio-oil. Both analytical and bench scale pyrolysis trials produced similar trends in product composition, even if the actual values differed, and thus analytical pyrolysis could be used for screening purposes.
The complex composition of the synthesized slag catalysts and the biomass feedstocks challenged the analysis of catalytic trends and structure-performance relationships. Therefore, the slag catalysts were tested in a model chemical reaction to assess their catalytic activity. Such model reaction was the carboxymethylation of cinnamyl alcohol with dimethyl carbonate to produce cinnamyl methyl carbonate. The use of catalysts resulted not only in high conversion and selectivity to the desired product, but also illustrated the dependence of the conversion on the basicity and surface area of the catalysts.
The biochars obtained in bench scale pyrolysis of wood were activated and acid washed prior to their use as catalyst for the post-treatment of degradation vapors in analytical pyrolysis of pine wood. The activated biochars reduced the oxygen content of the pyrolysis degradation products, particularly in those compounds derived from polysaccharides. The most promising catalyst for vapor upgrading was unwashed activated carbon from willow, having high surface area and pore volume together with high mineral contents (due to the presence of bark).
The lignin fraction in the bio-oils from bench scale pyrolysis of wood was precipitated by addition of water, but the membrane fractionation of a reference bio-oil was also investigated. The results from membrane nanofiltration showed that a compromise had to be found with respect to lignin yield in the permeate, permeate purity, and purity of the high MW fraction in the retentate.
The pyrolysis lignins obtained by addition of water to the bio-oils were tested for substitution of phenol in the synthesis of phenol-formaldehyde resins. The lignins were found to be highly reactive and prone for condensation reactions already during the dissolution phase in alkali. Oxidation of catechols appeared to enhance the attachment of phenol into birch pyrolysis lignin, preventing self-condensation and increasing the number of reactive sites for formaldehyde.
With the more reactive pine pyrolysis lignin, the oxidation further enhanced the selfcondensation, and crosslinking of phenol in general was lower.
Finally, the direct upgrade of fast pyrolysis bio-oil into transportation fuels by catalytic HDO was investigated. The carbonyl content decreased with increasing treatment temperature, indicating improved stability of the bio-oil. However, the water content also increased, indicating that considerable amounts of carbon were either cracked to gases or remained in the reactor as coke. This was further supported by the low yields of liquid product. The bio-oil was a very challenging feedstock for upgrading by HDO due to its high oxygen content, with highly reactive oxygenates.
Original languageEnglish
PublisherVTT Technical Research Centre of Finland
Number of pages43
Publication statusPublished - 24 Nov 2020
MoE publication typeD4 Published development or research report or study

Publication series

SeriesVTT Research Report
NumberVTT-R-01428-20

Keywords

  • Hydrothermal pre-treatment
  • fast pyrolysis
  • Slag catalysts
  • saccharification
  • lignin
  • bio-oil fractionation
  • composites
  • resins

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