Characterization of Chemically and Physically Activated Carbons from Lignocellulosic Ethanol Lignin-Rich Stream via Hydrothermal Carbonization and Slow Pyrolysis Pretreatment

Edoardo Miliotti,Andrea Maria Rizzo,Luca Rosi,Lorenzo Bettucci,Giulia Lotti,David Chiaramonti

doi:10.3390/en13164101

Edoardo Miliotti, Andrea Maria Rizzo + Show 4 more

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https://doi.org/10.3390/en13164101

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Abstract

The aim of the present work is to investigate the possibility of producing activated carbons from the residual lignin stream of lignocellulosic ethanol biorefineries, as this represents an optimal opportunity to exploit a residual and renewable material in the perspective of sustainable bioeconomy, increasing biorefinery incomes by producing value-added bioproducts in conjunction with biofuels. Activated carbons (ACs) were produced via chemical (KOH) and physical (CO2) activation. Char samples were obtained by slow pyrolysis (SP) and hydrothermal carbonization (HTC). Several HTC experiments were carried out by varying residence time (0.5–3 h) and reaction temperature (200–270 °C), in order to evaluate their influence on the product yield and on the morphological characteristics of the hydrochar (specific surface area, total pore volume and pore size distribution). ACs from hydrochars were compared with those obtained from pyrochar (via physical activation) and from the raw lignin-rich stream (via chemical activation). In both cases, by increasing the HTC temperature, the specific surface of the resulting activated carbons decreased from 630 to 77 m2 g−1 for physical activation and from 675 to 81 m2 g−1 for chemical activation, indicating that an increase in the severity of the hydrothermal pretreatment is deleterious for the activated carbons quality. In addition, the HTC aqueous samples were analyzed, with GC-MS and GC-FID. The results suggest that at low temperatures the reaction mechanisms are dominated by hydrolysis, instead when the temperature is increased to 270 °C, a more complex network of reactions takes place among which decarboxylation.

Highlights

In order to face the challenges of climate change, our dependence from fossil fuels has to be heavily reduced: one of the most relevant sectors where this need to be urgently achieved is transports, which represent the most difficult one to decarbonize [1]
From most of the reported studies, which deal with technical lignin or lignin from pulp and paper, here the conversion of residual lignin from a demo-scale lignocellulosic ethanol plant was investigated in order to produce activated carbons by slow pyrolysis and hydrothermal carbonization (HTC) pretreatment
Residual lignin from ethanol biorefineries greatly differs from pulp and paper lignin, as more than 30% w/w (d.a.f.) of structural sugars are still retained in the material and lignin only accounts for nearly 50%

Summary

Introduction

In order to face the challenges of climate change, our dependence from fossil fuels has to be heavily reduced: one of the most relevant sectors where this need to be urgently achieved is transports, which represent the most difficult one to decarbonize [1]. Energies 2020, 13, 4101 as reported by Lynd [2], this technology showing slower growth than initially expected. This is partly because of overestimation of technological readiness, partly due to the global financial crisis and collapse of oil prices, and mostly because of a lack of adequate supporting policies. As a result many advanced biofuel startups have failed and those who survived have changed their focus from biofuels to biomass-derived value-added products. In a zero-waste biorefinery concept, all products derived from biomass should be exploited and the production of biofuels (low selling price) should follow that of biochemicals (high selling price)

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