Hydrothermal Depolymerisation of Kraft Lignin: The Influence of Capping Agents and Residence Time
Doktorsavhandling, 2023

Lignin, the aromatic macromolecule found in wood and other lignocellulosic biomass, is envisioned as a future renewable source of aromatic compounds that can be used as, for example, chemicals, fuel additives and resins. A way in which this can be realised is by depolymerising the lignin once it has been isolated from the lignocellulosic biomass. Hydrothermal methods that employ water as a reaction medium are suggested as ways of accomplishing this. However, reactive fragments prone to repolymerisation, indeed char formation, are formed during depolymerisation, causing operational difficulties in the process as well as yield losses of the desired components. The suggestion has been made to add chemicals, i.e. capping agents, to scavenge the reactive components in order to mitigate these issues. Careful selection of reaction parameters, such as residence time and temperature, is also crucial for the process.

In this work, the depolymerisation of softwood kraft lignin has been investigated under hydrothermal conditions, with additions isopropanol, glycerol and guaiacol, to investigate their potential of functioning as capping agents. A customised batch reactor (99 ml) enabling swift heating of the reaction mixture was employed at 290-335 °C and 250 bar with low residence times (1-12 min).

The result of the hydrothermal depolymerisation of lignin was an aqueous suspension with a strong smoky odour. No apparent separate liquid organic phase was formed, although a char fraction precipitated. Characterisation of the products indicated a rapid depolymerisation of the lignin, since inter-unit ether linkages were cleaved, which was later followed by a slower repolymerisation. Additions of isopropanol, glycerol and guaiacol, reduced the weight average molecular weights (Mw) of the product fractions. Only isopropanol appeared to be able to reduce the amount of char formed from the lignin. Subsequent aftertreatment of the product would be required to obtain a usable product.



capping agents

kraft lignin



hydrothermal liquefaction

VasaA Lecture Hall, Vera Sandbergs Allé 8
Opponent: Prof. Lasse Rosendahl | The Faculty of Engineering and Science, AAU Energy | Aalborg University, Denmark


Anders Ahlbom

Chalmers, Kemi och kemiteknik, Kemiteknik

Towards understanding kraft lignin depolymerisation under hydrothermal conditions

Holzforschung,; Vol. 76(2021)p. 37-48

Artikel i vetenskaplig tidskrift

On the hydrothermal depolymerisation of kraft lignin using glycerol as a capping agent

Holzforschung,; Vol. 77(2023)p. 159-169

Artikel i vetenskaplig tidskrift

Ahlbom A., Maschietti M., Nielsen R., Hasani M., Theliander H.; Using Guaiacol as a Capping Agent in the Hydrothermal Depolymerisation of Kraft Lignin

In the transition to a society that aims at sourcing more sustainable materials, the efficient use of renewable resources, such as wood, is necessary. Lignin, which is a key component of wood, is currently used primarily as a low value fuel in pulping mills. However, advances in the energy performance of these mills make it possible to isolate lignin for other uses: an excellent idea, since the lignin structure contains valuable aromatic groups.

There has been a long-desired goal to break down, i.e. depolymerise, lignin efficiently in order to access its aromatic units: hydrothermal depolymerisation, which employs water as a reaction medium and reactant, is suggested as being suitable for this end. As the lignin is broken down, the products, which are reactive, react and form unwanted char: this can be mitigated, however, by adding chemicals known as capping agents that scavenge reactive components.

In this work, hydrothermal depolymerisation of a type of lignin available on a large scale, namely LignoBoost kraft lignin, was investigated: focus was placed on the effect of adding capping agents as well as on short residence times in the reactor. Not only was it shown that adding isopropanol can reduce the amount of char formed as well as its molecular weight, and thus function as a capping agent, but also that bonds in the lignin structure break swiftly, i.e. the depolymerisation process appears to be very rapid. As such, this work is part of endeavours to valorise lignin.

Från lignin till biobränslen och specialkemikalier

Energimyndigheten (45395-1), 2018-01-01 -- 2021-12-31.




Chalmers materialanalyslaboratorium



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5274



VasaA Lecture Hall, Vera Sandbergs Allé 8


Opponent: Prof. Lasse Rosendahl | The Faculty of Engineering and Science, AAU Energy | Aalborg University, Denmark

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