Direct Thermochemical Liquefaction


Key Points

  • Lignin has gained interest as a potential source for biomass derived aromatic compounds
  • Pyrolysis of lignin is challenging, but may provide a method for depolymerizing complex lignins
  • Subsequent processing of depolymerized lignins may be considered for aromatic fuels and chemicals

Lignin is the second most abundant component in typical biomass, but more uniquely may be one of the few routes to aromatic compounds not derived from petroleum. As a common product from processes that use biomass as a feed, such as the paper industry, subsequent research has also been common.  Lignin pyrolysis has been studied for nearly 100 years aiming to unravelling the structure of the aromatic biopolymer and produce a reliable source of monomeric phenols.

Reviews of the lignin research from 1920-1980, were performed by Goldstein [1] as well as Allan and Mattila [2].  Review of the period from 1980 to 2000 was covered by Amen-Chen et al revealing that through many decades lignin little consideration has been paid to the use of lignin as a chemical resource [3].

One method of depolymerization of lignin has been demonstrated in research in pyrolysis of lignin.  Degradation mechanisms were evaluated by  analytical pyrolysis methods (Curie point pyrolysis, heated filament, micro-oven) combined with hyphenated separation and detection systems (GC/MS) [4-7]. Mass spectral information was provided by Faix et al. [8]. Influence of pyrolysis conditions on the kinetics of lignin pyrolysis was recently investigated by Britt et al. [9].

With the continued emphasis on seeking holistic solutions in biorefineries, lignin has been considered with renewed interest as a resource for aromatic chemicals that are not sourced from petroleum processing. This is accentuated by the potential availability of larger sources of kraft lignin through employing advanced precipitation technologies [10-12].

However, pyrolysis of lignin is not as straighforward as pyrolysis of neat biomass.  Dry thermal depolymerization of lignin, such as fast and slow pyrolysis, suffers from various challenges:

  • Continuous feeding is difficult, as lignin tends to melt around 90°C;
  • Low yields of monomeric aromatics are obtained (5-15 wt%);
  • There exist a spectrum of alkylated aromatics, which are less prone to further reactions.

These observations became obvious in a recent round robin testing organized by the IEA Task 34 [13].

To increase yields and to simplify the composition of the monomeric products, advanced processes are needed such as hydrocracking. In the presence of a suitable catalyst and under hydrogen pressure better yields and more attractive compositions can be expected [14-19].

Additionally, consideration of hydrothermal liquefaction may prove to address the challenges of feeding to the depolymerization reaction and provide additional advantages to co-mingling catlysis with the depolymerization.


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