Comparison of the Properties of Activated Carbons Produced in One-Stage and Two-Stage Processes

Davide Bergna,Henrik Romar,Toni Varila,Ulla Lassi

doi:10.3390/c4030041

Davide Bergna, Henrik Romar + Show 2 more

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https://doi.org/10.3390/c4030041

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Abstract

Activated carbons (ACs) can be produced from biomass in a thermal process either in a direct carbonization-activation process or by first carbonizing the biomass and later activating the bio-chars into activated carbons. The properties of the ACs are dependent on the type of process used for production. In this study, the properties of activated carbons produced in one-stage and two-stage processes are considered. Activated carbons were produced by physical activation of two types of starting materials: bio chars produced from spruce and birch chips in a commercial carbonization plant and from the corresponding raw chips. The activated carbons produced were characterized regarding specific surfaces, pore volumes, and pore size distributions. The un-activated bio chars had varying surface areas, 190 and 140 m2 g−1 for birch and spruce, respectively, and pore volumes of 0.092 and 0.067 cm3 g−1, respectively. On the other hand, 530–617 and 647–679 m2 g−1 for activated bio chars from birch and spruce, respectively, and pore volumes 0.366–0.509 and 0.545–0.555 cm3 g−1, respectively, were obtained. According to the results obtained, two slightly different types of activated carbons are produced depending on whether a one-stage or a two-stage carbonization and activation process is used. The ACs produced in the one-stage process had higher specific surface areas (SSA), according to the BET-model (Brunauer–Emmett–Teller), compared to the ones produced in a two-stage process (761–940 m2 g−1 vs. 540–650 m2 g−1, respectively). In addition, total pore volumes were higher in ACs from the one-stage process, but development of micro-pores was greater compared to those of the two-stage process. This indicates that the process can have an influence on the ACs’ porosity. There was no significant difference in total carbon content in general between the one-stage and two-stage processes for spruce and birch samples, but some differences were seen between the starting materials. Especially in the one-stage procedure with 2 and 4 h steam activation, there was nearly a 10% difference in carbon content between the spruce and birch samples.

Highlights

Activated carbons (ACs) are produced by thermal treatment or a combination of chemical and thermal treatments of carbon-rich sources like lignite or, in the case of bio-mass based carbon, from lignocellulosic materials
The pre-carbonization leads to rather high yields (62.6% and 43.9% for birch at 2 and 4 h, respectively, and 56.0% and 38.7% for spruce at 2 and 4 h, respectively) for the activation step because most of the volatile fractions have been removed in the carbonization, but the total yields are at the same level for both procedures
Longer steam activations lead to lower yields; this was more obvious for the one-stage process than the two-stage process

Summary

Introduction

Activated carbons (ACs) are produced by thermal treatment or a combination of chemical and thermal treatments of carbon-rich sources like lignite or, in the case of bio-mass based carbon, from lignocellulosic materials. Typical bio-based raw materials used in AC production are coconut shells, peat, and wood chips or saw dust [1,2]. Most of the raw materials used in the production of bio-chars and activated carbons can be considered as waste fractions from industrial or agricultural production [3,4,5,6]. The biomasses used in the present study, spruce and birch, fulfil the criteria presented in Table 1, especially in the Nordic countries if waste fractions of wood such as thinnings and residues from wood harvesting are used

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