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Effect of process variables on producing biocoals by hydrothermal carbonisation of pine Kraft lignin at low temperatures

Musa, Umaru; Castro-Díaz, Miguel; Uguna, Clement N.; Snape, Colin E.


Umaru Musa

Miguel Castro-Díaz

Professor of Chemical Technology & Chemical Eng


Lignin from pulping is recovered in a wet form, making it ideal for hydrothermal carbonisation (HTC). This study investigates the effects of temperature (200–280 °C), residence time (1–6 h), and water to biomass mass ratio (2:1–6:1) on the composition of the resultant biocoal and the extent of alkaline and alkaline metal removal during HTC of pine Kraft lignin (PKL). Consistent with studies on other biomass materials, temperature exhibited the most significant effect on biocoal yield and properties, followed by residence time showing a marginal effect and varying the water to lignin mass only affecting alkaline and alkaline earth metals removal. Biocoal yields were in the range of 76–95 % (80–97 % on a carbon basis) and, compositionally, the biocoal obtained corresponded to sub-bituminous (220–260 °C) and high volatile bituminous coals (280 °C). Gas yields are low with the gas comprising mainly CO2 (98 v/v). The optimum temperature, time and biomass to water ratio for alkaline and alkaline earth metals removal was 260 °C, 3 h and 5:1 under which 93–95 % for sodium (Na) and potassium (K) and 75–80 % for magnesium (Mg) and calcium (Ca) removal were achieved. Solid-state 13C Nuclear Magnetic Resonance (NMR) revealed that during HTC, the oxygen-containing bonds of esters, ethers, and carboxylic groups were dissociated to produce lower-molecular weight including phenolic compounds dissolved in the process water. These findings have shown that the HTC is a promising alternative thermochemical route for coalification of wet PKL to a low ash coal-like fuel similar to sub-bituminous coal in rank that has potential applications for carbonisation.

Journal Article Type Article
Acceptance Date Jun 2, 2022
Online Publication Date Jun 16, 2022
Publication Date Oct 1, 2022
Deposit Date Jun 22, 2022
Journal Fuel
Print ISSN 0016-2361
Publisher Elsevier BV
Peer Reviewed Peer Reviewed
Volume 325
Article Number 124784
Keywords Organic Chemistry; Energy Engineering and Power Technology; Fuel Technology; General Chemical Engineering
Public URL
Publisher URL
Additional Information This article is maintained by: Elsevier; Article Title: Effect of process variables on producing biocoals by hydrothermal carbonisation of pine Kraft lignin at low temperatures; Journal Title: Fuel; CrossRef DOI link to publisher maintained version:; Content Type: article; Copyright: © 2022 The Author(s). Published by Elsevier Ltd.