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ISBN 978-952-62-2411-4 (Paperback)ISBN 978-952-62-2412-1 (PDF)ISSN 0355-3213 (Print)ISSN 1796-2226 (Online)

U N I V E R S I TAT I S O U L U E N S I SACTAC

TECHNICA

U N I V E R S I TAT I S O U L U E N S I SACTAC

TECHNICA

OULU 2019

C 725

Davide Bergna

ACTIVATED CARBON FROM RENEWABLE RESOURCESCARBONIZATION, ACTIVATION AND USE

UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF TECHNOLOGY;KOKKOLA UNIVERSITY CONSORTIUM CHYDENIUS

C 725

AC

TAD

avide Bergna

C725etukansi.fm Page 1 Tuesday, October 15, 2019 8:46 AM

ACTA UNIVERS ITAT I S OULUENS I SC Te c h n i c a 7 2 5

DAVIDE BERGNA

ACTIVATED CARBON FROM RENEWABLE RESOURCESCarbonization, activation and use

Academic dissertation to be presented with the assent ofthe Doctoral Training Committee of Technology andNatural Sciences of the University of Oulu for publicdefence in the Ulappa auditorium of Kokkola UniversityConsortium Chydenius (Talonpojankatu 2B) on 29November 2019, at 12 noon

UNIVERSITY OF OULU, OULU 2019

Copyright © 2019Acta Univ. Oul. C 725, 2019

Supervised byProfessor Ulla LassiDoctor Henrik RomarDoctor Pekka Tynjälä

Reviewed byDoctor Rafal LukasikDoctor Miguel A. Montes Morán

ISBN 978-952-62-2411-4 (Paperback)ISBN 978-952-62-2412-1 (PDF)

ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)

Cover DesignRaimo Ahonen

JUVENES PRINTTAMPERE 2019

OpponentDocent Yohannes Kiros

Bergna, Davide, Activated carbon from renewable resources. Carbonization,activation and useUniversity of Oulu Graduate School; University of Oulu, Faculty of Technology; KokkolaUniversity Consortium ChydeniusActa Univ. Oul. C 725, 2019University of Oulu, P.O. Box 8000, FI-90014 University of Oulu, Finland

Abstract

Biomass is the most abundant renewable material present on Earth and has been widely used e.g.in energy production. Recently, new applications for biomass utilization have been developed, e.g.the use of biomass as a raw material for synthesizing new chemicals. This research aimed toproduce activated carbon (AC) from waste wood-based materials and peat through carbonizationfollowed by physical or chemical activation. Physical steam activation and chemical activationgenerate the porosity in AC after the carbonization. The desired properties of AC (porosity, poresize distribution, surface functionality) are dependent on the application in which AC is used.

The first part of the research focused on setting up the carbonization and activation device. Themost important variables affecting carbonization and activation and the AC properties werestudied. The process parameters were optimized through the design of experiments (DOE). Theresults showed that in the physical activation, the most important variables affecting thecharacteristics of AC are the holding time, temperature, and the steam feed. Consequently, a modelfor tailoring the microporosity or mesoporosity of AC and maximizing the yield is proposed.

The second part of the research focused on chemical activation using zinc chloride. The aimwas to study the effect of activation variables on the yield and properties of AC. Finally, the useof AC as an adsorbent was studied. Especially, the applicability of birch sawdust based activatedcarbon on the removal of dyes, zinc metal, nitrate, phosphate, and sulfate ions was evaluated.Based on the results, a difference was shown between one and two step process for carbonizationand activation, and a single-step process was suggested to maximize the quality of AC.

Keywords: activated carbon, biomass, chemical activation, peat, physical activation,porosity, process optimization, sawdust

Bergna, Davide, Aktiivihiilen valmistus uusiutuvista raaka-aineista. Hiilestys,aktivointi ja käyttöOulun yliopiston tutkijakoulu; Oulun yliopisto, Teknillinen tiedekunta; Kokkolanyliopistokeskus ChydeniusActa Univ. Oul. C 725, 2019Oulun yliopisto, PL 8000, 90014 Oulun yliopisto

Tiivistelmä

Biomassa on maapallon eniten saatavilla olevaa uusiutuvaa materiaalia, jota on hyödynnetty jopitkään mm. energiantuotannossa. Viime aikoina uusia biomassan käyttökohteita on kehitettylaajalti, kuten esimerkiksi uusien kemikaalien valmistukseen. Tässä tutkimuksessa puupohjaistajätebiomassaa ja turvetta hyödynnetään fysikaalisesti ja kemiallisesti aktivoidun aktiivihiilenvalmistuksessa. Fysikaalinen höyryaktivointi ja kemiallinen aktivointi lisäävät aktiivihiilen huo-koisuutta hiilestyksen jälkeen. Aktiivihiilen halutut ominaisuudet (huokoskokojakauma, pinnantoiminnalliset ryhmät) määräytyvät käyttökohteen mukaan.

Tutkimuksen ensimmäisessä vaiheessa keskityttiin hidas pyrolyysilaitteiston ja aktivointilait-teiston rakentamiseen sekä hiilestyksen ja aktivoinnin kannalta keskeisimpien muuttujien tutki-miseen. Prosessimuuttujien vaikutusten tarkastelussa ja optimoinnissa hyödynnettiin koesuunnit-teluohjelmaa. Tulosten perusteella todettiin, että fysikaalisessa aktivoinnissa olennaisimmatmuuttujat olivat lämpötila, pitoaika sekä höyrysyöttö. Tämän pohjalta esitettiin malli aktiivihii-len mikro- ja mesohuokoisuuden muokkaamiseksi ja saannon maksimoimiseksi.

Tutkimuksen toisessa vaiheessa tutkittiin kemiallista aktivointia hyödyntämällä sinkkiklori-dia aktivointikemikaalina. Tavoitteena oli selvittää eri aktivointimuuttujien vaikutusta saantoonja aktiivihiilen laatuun. Tutkimuksen viimeisessä vaiheessa tutkittiin valmistettujen aktiivihiili-en käyttöä adsorbenttina. Erityisesti tutkittiin koivupurusta valmistetun aktiivihiilen soveltuvuut-ta väriaineiden, metallien ja anionien sidontaan. Tutkimuksen keskeisenä tuloksena voitiin osoit-taa merkittävä ero yksi- ja kaksivaiheisen hiilestyksen ja aktivoinnin välillä, ja ehdotettiin yksi-vaiheista prosessia hiililaadun optimoimiseksi.

Asiasanat: aktiivihiili, biomassa, fysikaalinen aktivointi, huokoisuus, kemiallinenaktivointi, prosessioptimointi, sahanpuru, turve

Bergna, Davide, Carbone attivo da fonti rinnovabili. Carbonizzazione, attivazioneed usoScuola di ricerca dell'Università di Oulu; Università di Oulu, Facoltà di Tecnologia; Universitàdi Kokkola Consorzio ChydeniusActa Univ. Oul. C 725, 2019Università di Oulu, P.O. Box 8000, FI-90014 Università di Oulu, Finlandia

Sommario

La biomassa è il materiale rinnovabile più abbondante presente sulla Terra ed è stata intensamenteusata e.g. nella produzione di energia. Recentemente sono state sviluppate nuove applicazioni perla biomassa, ad esempio come materiale di base per sintetizzare nuovi prodotti chimici. Lo scopodi questa ricerca è produrre carbone attivo (CA) attraverso attivazione fisica e chimica da materialilegnosi di scarto come segatura, cippato e torba. L'attivazione fisica e l'attivazione chimica, creanola porosità nel CA dopo la carbonizzazione. Il prodotto finale può essere usato in differentiapplicazioni in base a diversi fattori tra cui la distribuzione della porosità e la tipologia dei gruppifunzionali presenti sulla superficie.

La prima fase della ricerca è stata dedicata alla progettazione e installazione dell'hardwarenecessario per l'attivazione e nell'individuazione dei parametri di processo più importanti. Iparametri di processo sono stati ottimizzati attraverso il design of experiments (DOE) e sono stateconsiderate le differenti variabili che interagiscono nella formazione del CA. I risultati hannomostrato che i parametri di processo più importanti che influiscono sulle caratteristiche del CAsono il tempo, la temperatura di attivazione e la quantità di vapore iniettato nel reattore. È statoproposto un modello per progettare CA microporoso o mesoporoso con massa finalemassimizzata. La seconda parte della ricerca è stata incentrata sull'attivazione chimica con clorurodi zinco. Lo scopo é stato studiare l'effetto delle variabili di attivazione su massa finale e proprietàdel carbone attivo. Infine, è stata studiato il CA come adsorbente. In particolare è stata consideratal'applicabilità del CA da segatura di betulla per la rimozione di coloranti, zinco metallico, ioni dinitrato, fosfato e solfato. In base a questi risultati, una differenza é stata evidenziata tra il processodi carbonizzazione e attivazione a uno o due stadi, ed il processo a singolo stadio è stato propostoper massimizzare la qualità del CA.

Parole chiave: carbone attivo, biomassa, attivazione fisica, attivazione chimica, torba,porosità, ottimizzazione di processo, segatura

To Satu and Anna

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Acknowledgments

This research work was carried out at Kokkola at the University Consortium

Chydenius facility together with the Research Unit of Sustainable Chemistry of

Oulu in the period between August 2015 and July 2019.

The list of people I must acknowledge for this work is very long, and I

apologize in advance if I forgot someone.

First, let me thank Prof. Ulla Lassi, my dear supervisor, for giving me the

chance to develop, on a rare hot day in Finland in August five years ago, the idea

behind this research. To listen and give credit to a person you don´t know requires

courage: thanks, Ulla, for having had the courage to trust me that day.

In addition, thanks again to Prof. Ulla Lassi, Prof. Timo Fabritius, Prof. Osmo

Hormi, and Dr. Tero Tuuttila for guiding me during the follow-up group meetings.

Dr. Rafal Lukasik from the National Laboratory of Energy and Geology of

Lisbon, Portugal and Dr. Miguel Angel Montes Morán from the National Institute

of Carbon of Oviedo, Spain are deeply acknowledged for the work in reviewing

and commenting my thesis manuscript.

I would like to thank Prof. Laura Prati, Prof. Claudia Letizia Bianchi, and the

staff of the University of Milan for their fruitful collaboration.

I must make a special acknowledgment to Dr. Henrik Romar. He really gave

me the instruments and the tools to make this research practically possible. He has

been guiding me like Virgil with Dante through the “Inferno” that can be a research

laboratory if not properly handled.

I would also like to thank my research colleagues: M.Sc. Juho Välikangas, for

having acquainted me with the workplace; Mr. Jaakko Pulkkinen, for the

multitasking support; M.Sc. Riikka Lahti, for her kindness and help in many

projects; M.Sc. Toni Varila, for his support and unforgettable time spent during the

conference in York; D.Sc. Tuomas Vielma, for having introduced me to the world

of multivariate analysis; M.Sc. Toni Kauppinen, for his help in the laboratory; Dr.

Pekka Tynjäla, Dr. Katja Lappalainen, M.Sc. Henna Lempiäinen, M.Sc. Jiachen

Dong, M.Sc. Marianna Hietaniemi, M.Sc. Pekka Myllymäki, Tuomo Vähätitto,

M.Sc. Anatoli Vaskinov, M.Sc. Janne Keränen, M.Sc. Petteri Laine, Sini Kivioja,

Nuorala Jessica, Qi Ping, Pauliina Seppälä, and Joonas Kallio, for their daily help

and support.

I want to thank the research unit of sustainable chemistry of Oulu in particular

Dr. Hu Tao for the FESEM images, Dr. Hanna Prokkola for the elemental analysis,

Dr. Sari Tuomikoski, Dr. Hanna Runtti, and Dr. Teija Kangas for the review paper,

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M.Sc. Annu Rusanen, M.Sc. Riikka Juhola and M.Sc. Johanna Kutuniva for their

kindness and Dr. Yue Dong and Dr. Laura Schneider for their support as

international doctoral students.

A special thanks goes to the Director of the University Consortium Chydenius,

Dr. Tanja Risikko, and all the Chydenius University staff.

The Central Ostrobothnia Regional Fund, to which goes my deep gratitude, has

financially supported this research.

In the end, I would like to thank my Italian and Finnish family and friends, and

my wife Satu and daughter Anna for giving me strength, joy, and enthusiasm in

everyday life.

Kokkola, 29 November 2019 Davide Bergna

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Abbreviations

AC/ACs Activated carbon/Activated carbons

ASTM American Society for Testing and Materials

AUX Auxiliary components

BET Brunauer, Emmet, Teller theory

BJH Barrett, Joyner, Halenda theory

CGs Condensable gases

DFT Density functional theory

DOE Design of experiments

DP Dry pyrolysis

FESEM Field emission scanning electron microscope

GV Gentian violet (hexamethyl pararosaniline chloride)

HTC Hydrothermal carbonization

MB Methylene blue (methylthioninium chloride)

MFC Mass flow controller

NCGs Noncondensable gases

OII Orange II (2-naphtol orange)

PAH Polycyclic aromatic hydrocarbons

PCM Pyrogenic carbonaceous materials

PLC Programmable logic controller

PLS Partial least square regression

PSD Pore size distribution

PP Process parameters

QSDFT Quenched solid density functional theory

SSA Specific surface area

TC Total carbon

wt.% Weight percentage

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15

Original publications

This thesis is based on the following publications, which are referred to throughout

the text by their Roman numerals:

I Bergna D., Romar H., Tuomikoski S., Runtti H., Kangas T., Tynjälä P., & Lassi, U. (2017). Activated carbon from renewable sources: thermochemical conversion and activation of biomass and carbon residues from biomass gasification. In L. Singh, V. C. Kalia (Eds.), Waste Biomass Management – A Holistic Approach (pp 187-213) Springer.

II Bergna D., Varila T., Romar H., & Lassi, U. (2018). Comparison of the properties of activated carbons produced in one-stage and two-stage processes. C Journal of Carbon Research 4(3): 41.

III Bergna D., Romar H., & Lassi, U. (2018). Physical activation of wooden chips and the effect of particle size, initial humidity, and acetic acid extraction on the properties of activated carbons. C Journal of Carbon Research 4(4): 66.

IV Bergna D., Hu T., Prokkola H., Romar H., & Lassi, U. (2019). Effect of some process parameters on the main properties of activated carbon produced from peat in a lab-scale process. Waste and Biomass Valorization.

V Varila T., Bergna D., Lahti R., Romar H., Hu T., & Lassi, U. (2017). Activated carbon production from peat using ZnCl2: characterization and applications. BioResources 12(4): 8078-8092.

VI Lahti R., Bergna D., Romar H., Tuuttila T., Hu T., & Lassi, U. (2017). Physico-chemical properties and use of waste biomass-derived activated carbons. Chemical Engineering Transactions, 57: 43-48.

Papers I–IV were written by the author of this thesis in collaboration with the other

authors. For Papers II–IV, the author’s main responsibilities were experimentation,

data analysis, and reporting the results. For Paper V and Paper VI, the author

contributed with the experimental part (carbonization and activation) and data

analysis.

Other contributions not included in this thesis:

Lahti R, Bergna D, Romar H, Hu T, Comazzi A, Pirola C, Bianchi CL, & Lassi U

(2017) Characterization of cobalt catalysts on biomass-derived carbon supports.

Topics in Catalysis, 60: 1415-1428.

Prati L, Bergna D, Villa A, Spontoni P, Bianchi CL, Hu T, Romar H, & Lassi U

(2018) Carbons from second generation biomass as sustainable supports for catalytic

systems. Catalysis Today 301: 239-243.

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17

Contents

Abstract

Tiivistelmä

Sommario

Acknowledgments 11 

Abbreviations 13 

Original publications 15 

Contents 17 

1  Introduction 19 

1.1  Background ............................................................................................. 19 

1.2  Aim of the thesis ..................................................................................... 20 

2  Activated carbon 23 

2.1  Definitions of different carbon types ...................................................... 23 

2.2  Activated carbon properties and structure ............................................... 28 

2.2.1  Porosity ......................................................................................... 28 

2.2.2  Functional groups ......................................................................... 29 

2.3  Production methods ................................................................................. 32 

2.3.1  Carbonization (slow pyrolysis) ..................................................... 32 

2.3.2  Physical activation ........................................................................ 36 

2.3.3  Chemical activation ...................................................................... 37 

2.4  Activated carbon use ............................................................................... 38 

2.4.1  Gas and water cleaning ................................................................. 38 

2.4.2  Food processing ............................................................................ 39 

2.4.3  Support for catalysts ..................................................................... 39 

2.4.4  Electrical applications .................................................................. 40 

2.4.5  Other applications ......................................................................... 41 

3  Materials and Methods 43 

3.1  Materials used for carbonization and activation...................................... 43 

3.1.1  Sawdust, peat, and woodchips ...................................................... 43 

3.2  Pre-treatments and activation .................................................................. 44 

3.2.1  Acetic acid extraction ................................................................... 44 

3.2.2  Physical activation ........................................................................ 45 

3.2.3  Chemical activation ...................................................................... 47 

3.3  Characterization ...................................................................................... 48 

3.3.1  Moisture, total carbon content, yield ............................................ 48 

3.3.2  Surface areas and pore size distribution ....................................... 49 

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3.3.3  Adsorptive properties ................................................................... 50 

3.3.4  Other characterizations ................................................................. 51 

3.3.5  Experimental design ..................................................................... 51 

4  Results and discussion 53 

4.1  Research review of recent AC studies ..................................................... 53 

4.2  Difference between one- and two-stage physical activation ................... 54 

4.3  Particle size initial humidity and acetic acid extraction .......................... 56 

4.4  Parameters affecting biomass carbonization and activation .................... 59 

4.4.1  AC from peat design of experiment and parameter

modeling and optimization ........................................................... 62 

4.5  Chemical activation with ZnCl2 .............................................................. 63 

4.6  Properties of biomass waste-derived AC................................................. 64 

5  Conclusions 67 

6  Perspectives 69 

List of references 71 

Original publications 81 

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1 Introduction

1.1 Background

The Paris Agreement – Status of Ratification (2017) represents a milestone in

international agreements regarding greenhouse gas emission mitigation, adaptation,

and finance. In 2018, 195 countries signed the agreement, and 180 have ratified it.

With the noticeable possible withdraw of the United States, for the first time, the

whole world, including most industrialized countries, agreed to the reduction of the

actual environmental pollution trend. In particular, the three main aims of the

agreement are (United Nations Framework Convention on Climate Change, 2016):

– Holding the increase of the global temperature below 2 °C in reference to the

pre-industrial period and pursuing the effort to reach 1.5 °C, recognizing the

impacts of pollution on climate change.

– Increasing the capability to adapt to the adverse impact of climate change,

lowering greenhouse emissions in order to preserve food production.

– Guaranteeing proper financial support to sustain the effort to reduce

greenhouse gas emissions and create a climate-resilient development.

The concentration of CO2 in the atmosphere is considered the main cause of the

greenhouse effect, even though recent studies have demonstrated that black carbon,

reducing the ice albedo and dispersing as an aerosol into the atmosphere, is making

a great contribution to the global warming process (Ramanathan & Carmichael,

2008) (Jacobson, 2001). Historical data suggest that in the past, higher levels of

CO2 have been present in the atmosphere (Lowenstein & Demicco, 2006), probably

due to volcanism (Schmitz et al., 2004).

The official data of Tilastokeskus (Statistics Finland, 2019) report emissions of

CO2 for the year 2018 of 56.5 Mt, with a 2% increase compared to the previous

year. Forest biomass is a significant CO2 sink. According to the 2018 data, the

LULUCF (land use, land use change, and forestry) sector has reduced the CO2 net

balance by 14.2 Mt, a significantly lower performance (-30%) if compared to 2017.

(Statistics Finland, 2019). In Finland, the surface area occupied by forest is about

23 million hectares, or about three-fourths of the total surface area of the country.

If sparse trees and other forestry lands are considered, then this surface area

increases to 86% of the country (10% is occupied by water). The wooden forest

stocks have shown constant growth in the last 40 years (Parviainen & Västilä, 2011).

On the other hand, Finland is covered by 9.3 million hectares of peatlands. Peat

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combustion supplied 4% of Finnish electrical power and 13% of district heating in

2017. Recent government legislation has implemented guidelines for the 2030

energy and climate strategy, including a substantial reduction in greenhouse gas

emissions and deeper and more efficient use of wood-based biomasses and peat

(Huttunen, 2017).

Finland decided to ban the use of coal as a source of energy on May 1st, 2029,

and to reduce the use of peat for energy production by one-half (7.5 TWh)

compared to present consumption by 2025. Thus, different alternatives to peat use

are needed to preserve the existing industrial cluster and relative economy. The

same consideration is valid for wood and sawdust, where a higher value from the

processing of the material can be obtained instead of direct combustion.

AC production from peat, sawdust, and wood chips represents one feasible

technological alternative to combustion and energy use. During the year 2017,

Finland imported 5.5 M€ (est. 3900 tons) of AC and exported 0.34 M€ (est. 217

tons), causing a deficit balance of about 4.9 M€ (Parker, 2015). Currently, AC is

not produced in Finland, but Vapo Oy plans to start peat-based technical carbon

production in Ilomantsi (Vapo Oy, 2018). The present technology can easily

saturate domestic AC needs and could be a possible commodity for export. The

need for AC worldwide is rising, with many economic reviews forecasting a CAGR

(compound average growth rate) over 8% for the next 10 years (Transparency

Market Research, 2018; Verified Market Research, 2018)

1.2 Aim of the thesis

The scope of this thesis concerns the laboratory-scale development of biomass

carbonization and activation and biomass waste residuals (peat and wood

chips/sawdust) and the analysis of process parameters in order to investigate how

they affect the properties of the final carbon product.

Both chemical and physical activation have been studied, with the main interest

focused on the latter, due to its higher environmentally friendly process. The

activating agent used for physical activation is steam; while for chemical activation,

zinc chloride was selected as the activating agent. In the end, some possible

applications for AC are considered.

The original publications (I–VI) of this thesis are presented in Fig. 1.

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Fig. 1. Thesis scheme and research topic diagram.

The main theses of this dissertation are:

1. High-quality AC with a high surface area and controlled porosity can be

produced from biomass waste.

2. Carbonization and activation as a thermal process done in a single stage differs

from that performed in two stages.

3. AC properties depend strongly on the production process and pre-treatment

parameters, and these properties can be tailored.

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2 Activated carbon

There are two types of raw material available for the production of AC: fossil coal,

and biomass-based carbon. In this thesis, only biomass-based carbon is considered.

When it comes to carbon and especially AC, there are a number of solid, carbon-

containing products available on the market. The naming of these products is still

confusing, hence the terminology is explained in the following chapter. AC can be

produced in a number of processes, yet a common route in all these processes

includes a carbonization step and an activation step. A short description of the

individual steps and procedures is presented later on in this chapter. Further, AC

properties and applications are briefly considered.

2.1 Definitions of different carbon types

Typically, AC from coconut shells is used, but some concerns have been raised

regarding the transport, mixing energy, and ethical labor conditions involved in the

countries where it is produced (Arena, Lee, & Clift, 2016). On the industrial scale,

two main methodologies can be identified: In the first method, biomass is first

carbonized and then physically activated. The physical activation occurs at a high

temperature (600–900 °C) under a stream of typical CO2 or steam. This activating

atmosphere determines the slow oxidation of the carbon, creating porosity and

oxygen complexes on the surface (Marsh & Rodríguez-Reinoso, 2006a). The other

method for activating carbon is through chemical activation, in which the biomass

precursor is first impregnated with an activating chemical agent (ZnCl2, H3PO4,

KOH, NaOH, etc.) and then carbonized and activated in one single step at different

temperatures depending on the type of chemical used (Marsh & Rodríguez-Reinoso,

2006b). In Fig. 2, a simplified cycle of converting biomass to AC is proposed.

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Fig. 2. Schematic process diagram of the activated carbon (AC) from biomass.

The nomenclature for carbon produced in thermal processes is to some extent rather

vague, and the same product is often described by multiple names. To some extent,

the naming is regulated by international agreement, but there is still a lack of

agreement and standardization, especially regarding AC. In the following

paragraphs, some of the relevant terminologies are introduced

According to definitions from the International Biochar Initiative, a different

typology of carbon from biomass can be identified (O’Laughlin & McElligott,

2009).

Biochar, sometimes referred to as Biocarbon, is classified as a solid material

obtained from the thermochemical conversion of biomass in an oxygen-limited

environment. In particular, to be considered as biochar, the material must match

specific properties, e.g., H/Corg, heavy metal content limit, etc. (European Biochar

Foundation (EBC), 2018), and be used as a soil amendment in order to remove CO2

from the atmosphere.

Hydrochar is identified as the solid product of hydrothermal carbonization

(HTC) or liquefaction. The process of production is different compared to that for

biochar (lower temperature but higher pressure), and the final product presents

different chemical and morphological characteristics (Kambo & Dutta, 2015).

Pyrogenic Carbonaceous Materials (PCM) comprise all the materials produced

by thermochemical conversion. In this group, everything containing some organic

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C (without any mass limitation), i.e., charcoal, biochar, char, black carbon, soot, or

AC, can be identified.

Char results from incomplete combustion processes that occur in natural and

man-made fires. It differs from charcoal, which is produced by thermochemical

conversion from biomass solely for energy generation.

Each one of these different materials typically contains ashes, defined as the

fraction of biomass or PCM determined through the standard (ASTM E1755-01,

2012), and includes inorganic oxides and carbonates (Nunes, Matias, & Catalão,

2016).

Carbon soot and carbon black are defined as PCMs dispersed in the

environment from wildfires and fossil fuel combustion as well as from secondary

gas condensation products, respectively (Buseck, Adachi, Gelencsér, Tompa, &

Pósfai, 2012). Carbon black is created under controlled conditions, while soot is

casually formed. Typically, the size of the colloidal carbon that produces carbon

black is <1000 nm. Carbon black generally contains high values of polycyclic

aromatic hydrocarbons (PAH) (Tsai, Shieh, Hsieh, & Lee, 2001).

Finally, activated carbon is identified directly as a PCM that has undergone

activation (i.e., using steam, CO2, or chemicals) (Marsh & Rodríguez-Reinoso,

2006c).

In another definition from IUPAC, AC is described as a porous carbon

material—a char which has been subjected to reaction with gases, sometimes with

the addition of chemicals, e.g., ZnCl2, before, during, or after the carbonization, in

order to increase its adsorptive properties (Fitzer, Kochling, Boehm, & Marsh,

1995). In the same IUPAC recommendation, activated charcoal is defined as a

“traditional” term for AC.

The REACH (Registration, Evaluation, Authorization, and Restriction of

Chemicals) divides the AC typology into two categories: ACs with low-density

skeleton and ACs with high-density skeleton. The skeletal density is determined

through helium pycnometry. The low-density skeleton typically has a lower

impurity content and must be handled as a possible fire hazard, especially in high

quantities. On the contrary, AC with a high-density skeleton has a higher mineral

content (SiO2, CaO, MgO, FeO, K2CO3, Al2O3, and CaSO4), so the hazard is more

related to inhalation than flammability.

It is opportune to define the process of pyrolysis as a form of thermolysis (the

breakdown or decomposition of molecules through the action of heat) in the inert

atmosphere.

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Pyrolysis can be divided into fast pyrolysis, where the heating rates reported

are higher than 1000 °C/min with a residence time of a few seconds (Montoya

Arbeláez, Janna, & Garcia-Pérez, 2015), and slow or mild pyrolysis, where the rates

are generally between 1–100 °C/min.

Torrefaction is a pyrolysis process at relatively low temperatures (225–300 °C)

whereby a large portion of the volatiles is removed from the starting material.

Generally, in fast pyrolysis, the production of a liquid fraction is favored, while in

slow pyrolysis, the solid product yield is improved. Another form of thermolysis is

gasification, in which the material is oxidized at relatively high temperatures, with

the low controlled tenor of O2 introducing air or steam. This process favors the

production of syngas with a less solid fraction. Some studies have demonstrated

that gasification with steam can produce an “activated” carbon residue (Benedetti,

Patuzzi, & Baratieri, 2017; García-García et al., 2003). In Table 1, a summary of

some carbon processing methods is presented.

In this thesis, it has been considered correct to describe the final product

produced simply as activated carbon.

27

Ta

ble

1. S

um

ma

ry o

f d

iffe

ren

t n

om

en

cla

ture

s i

n t

he

fu

nc

tio

n o

f th

e p

roc

es

s p

rod

uc

tio

n t

yp

es

.

Ta

rge

t p

rod

uct

T

em

pe

ratu

re

( ℃)

Ca

rbo

niz

atio

n

Act

iva

tion

S

SA

(m

2/g

) A

tm.

So

lid f

ract

ion

yield

wt.%

Refe

rence

Bio

char

300–700

S

low

Pyr

oly

sis

No

Low

<3

00

In

ert

M

ediu

m≈3

0–

50

%

(We

be

r &

Qu

icke

r, 2

01

8)

HT

C

200–250

S

low

Pyr

oly

sis

No

Low

<1

00

20–30

ba

r/st

ea

m -

vola

tile

s

Hig

h≈5

0–70%

(T

u e

t al.,

2019)

Phys

ically

AC

600–900

S

low

Pyr

oly

sis

H2O

/CO

2

Hig

h>

1000

In

ert

/Slo

wO

x ≈1

0–

20

%

(Da

nis

h &

Ah

ma

d,

20

18

a)

Chem

ica

lly A

C

500–800

S

low

Pyr

oly

sis

ZnC

l 2/H

3P

O4/K

OH

/Na

OH

Hig

h>

10

00

In

ert

≈4

0%

(P

rau

chn

er

& R

od

ríg

ue

z-

Rein

oso

, 2012)

Syn

gas

800–1000

S

low

Therm

oly

sis

No

Low

Low

Ox

Low

(C

outo

, R

ou

boa, S

ilva,

Mo

nte

iro

, &

Bo

uzi

an

e,

20

13

)

Pyr

oly

tic o

il 450–650

F

ast

Pyr

oly

sis

No

N

o

Inert

Low

(I

sahak,

His

ham

, Y

arm

o, &

Yun H

in, 2012)

28

2.2 Activated carbon properties and structure

In this section, the structural characteristics and main properties of AC are

presented.

2.2.1 Porosity

The most known peculiarity that characterizes AC is its porosity. Porosity from a

schematic point of view is a cluster of channels, slits, and tunnels of different and

mainly irregular morphology that run through the porous material. In general,

porosity is defined as the fraction of the total pore volume in the particle volume

(Thommes et al., 2015a). The surface of the pores can be schematized as in Fig. 2.

The level closer to the bulk is the van der Waals surface, which is composed by the

external part of the van der Waals spheres. The layer just above is the Connolly

surface, which is where, in the adsorptive material, the process called physisorption

(low energy interaction due mainly to van der Waals force) takes place. In the upper

level, the probe accessible surface, which is defined as the surface designed by

rolling the probe molecule onto the Connolly surface at a probe radius distance, is

found. This surface relates the size of the probe molecule with the adsorptive

material surface (partially explaining why different size molecules have different

adsorption performances).

The morphology and the pore volume are key factors in the characterization of

a porous material. In particular, the pore size distribution can determine different

behaviors in the adsorptive material. For this reason, the IUPAC classification (Sing

et al., 1985) divides the pores according to their diameter size (x) as follows:

– Micropores x < 2 nm.

– Mesopores 2nm < x < 50 nm.

– Macropores x > 50 nm.

The surface area and the pore size distribution, which are the most used parameters

to confront and evaluate the quality of the porous material, are generally calculated

from the nitrogen adsorption-desorption data and from the geometry of the pores.

Referring to AC, these parameters are not a simple determination because of the

typical heterogeneity of the pore geometry.

29

Fig. 3. Pore distribution and porous material surface scheme.

Considering AC specifically, its structure and surface properties are influenced by

two other characteristics: the type of precursor material used, and the type of

process used for activation. Differences in the performance of AC from different

precursors have been demonstrated (De Lima, Quináia, Melquiades, De Biasi, &

Garcia, 2014), as have differences in structural development related to the

activation method (Viswanathan, Neel, & Varadarajan, 2009).

2.2.2 Functional groups

Different material precursors have been used to produce AC, but the most used

precursors are those derived from biomasses—and among them, the most

interesting from an environmental point of view are those derived from waste

biomasses (Schröder et al., 2011). AC, as said before, has a high surface area and

functional groups that can bind with organic and inorganic (e.g., heavy metals)

compounds, separating them from the main stream (Fig. 4). The functional groups

are mainly based on the oxygen, hydrogen, nitrogen, and sulfur present in the

original raw material or developed during the activation process. The most common

oxygen functional groups are carboxyl, carbonyl, phenols, lactones, and quinones

(Bhatnagar, Hogland, Marques, & Sillanpää, 2013).

30

Fig. 4. Typical functional groups of activated carbon: (a) carboxyl, (b) hydroxyl, (c)

lactone, (d) carbonyl, (e) carboxylic anhydride, (f) possible carboxylic adsorption

mechanism of heavy metals, (g) metal adsorbed by the carboxylic functional group.

Treating AC with basic materials enhances the uptake of organics but reduces the

chelating capability of metal ions.

Impregnation with a foreign material is performed to catalyze some reactions

(such as oxidation); on the other hand, occupying part of the porosity with such

material reduces the surface area.

Concerning the physical activation, a different modulation of the heat and

activating agent can increase the surface area but reduces the oxygen functional

groups on the surface.

One of the first works to determine the type of carbon structure produced by

pyrolysis dates to 1951 (Franklin, 1951). Typically, AC from a biomass source is

considered part of non-graphitizing carbon, a type of carbon that cannot be

transformed into a graphitic crystalline structure, even at high temperatures

(3000 °C) (Cheng, Endo, Okabe, Saito, & Zheng, 1999).

31

Fig. 5. Proposed activated carbon structure: (a) disordered and not continuous carbon

rings lattice with functional groups on the surface, (b) a “buckyball”-like structure, (c)

typical sp2 electron configuration of the carbon atom.

Previous studies indicated a possible fragmented fullerene-type structure

composing the non-graphitizing carbon (Harris & Tsang, 1997; Harris, 2005), thus

explaining the microporosity generated through the irregular hexagonal and non-

hexagonal “buckyball”-like structure (Fig. 5). According to these studies, this

particularity gives to AC its peculiar properties (Harris, Liu, & Suenaga, 2008).

Other research has indicated that it is possible to graphitize non-graphitizable raw

materials through catalytic graphitization (Oya & Otani, 1979) or micro-domain

modification and merging (Kim et al., 2017).

This development in the study of AC structure results is particularly important

because it changes the theoretical approach to modeling surface areas and porosity.

In fact, by now, most of the main models are based on chemical and geometrical

homogeneity of the pore walls on a smooth surface (Gor, Thommes, Cychosz, &

Neimark, 2012). Also, the calculation and results of this research work are based

on this approach, but a further analysis could be performed considering new models

like QSDFT (quenched solid density functional theory) (Neimark, Lin, Ravikovitch,

& Thommes, 2009), which considers non-homogeneous AC surfaces.

32

2.3 Production methods

In the following chapter, the activation methods used for this research are presented.

2.3.1 Carbonization (slow pyrolysis)

Carbonization is a process in which an organic-based material increases its carbon

percent content. Typically, this happens through destructive distillation in an inert

atmosphere in which a complex of reactions, such as dehydrogenation,

isomerization, condensation, and hydrogen transfer, takes place. Carbonization is

defined as solid-phase carbonization when the pseudo-morph carbon material

maintains the same shape of the starting material but with lower bulk density.

During the carbonization process, the carbon supramolecular structure moves to

more stable carbon structures (e.g., six atom rings). The spaces created by the

decomposition and subsequent release of heteroatoms and the repositioning of the

carbon atoms initiate the creation of microporosity (i.e., atomic size dimension)

(Marsh & Rodríguez-Reinoso, 2006c).

The main components of the biomass used are cellulose, hemicellulose, and

lignin. Hemicellulose is a composition of saccharides (e.g., galactose, mannose,

xylose) in a casual amorphic structure and has the function of strengthening the cell

walls in trees. Cellulose is a glucose polymer-ordered shape and serves many

functions in trees, e.g., it is the fiber that composes leaves, stems, and branches.

Lignin is a group of aromatic rings with a low hydrophilic level that comprises the

structure of trees, surrounding (like glue) the other two main components and

allowing the efficient distribution of water. In peat, generally, the content of these

three polymers is lower with the increase of humic and fulvic acids due to plant

decay and biodegradation (Brown, Kauri, Kushner, & Mathur, 2010; Pérez, Muñoz-

Dorado, De La Rubia, & Martínez, 2002).

Referring to wood-based material, carbonization occurs thermally,

decomposing the three main components: cellulose, hemicellulose, and lignin,

thereby reducing the non-carbonaceous content of their molecular chains. Cellulose

and hemicellulose complete their degradation in temperatures ranging between

200–375 °C, while lignin, which has a more complex molecular structure,

decomposes between 200 and 700 °C (Huang et al., 2015; Wongsiriamnuay &

Tippayawong, 2010). During pyrolysis, the first component to undergo

decomposition is hemicellulose. Its content in the biomass varies between 23 and

32 wt.% (Paper VI), and the depolymerization starts at temperatures between 220–

33

315 °C (Yang, Yan, Chen, Lee, & Zheng, 2007). As can be seen in Fig. 6, a proposed

but not sole pathway mechanism identifies a thermally induced depolymerization

of hemicellulose to oligosaccharides. The increase of temperature determines

dehydration, the release of water molecules, and a molecular rearrangement to

xylose. A further increase in temperature causes the breaking of hydrogen and

carbon bonds with the release of molecular hydrogen and carbon monoxide. Other

studies (Giudicianni, Cardone, & Ragucci, 2013) have identified the decomposition

of xylans (the main biopolymers of hemicellulose) to furans, pentanone, and

hydroquinone as a possible further pathway. In an oxidative atmosphere, the

decomposition can lead to the formation of acetone and acetic acid (Shen et al.,

2015)

Fig. 6. Decomposition of hemicellulose during thermal distillation in an inert

atmosphere (Reprinted by permission from Liu, Li, Jiang, & Yu, 2017 © American

Chemical Society).

34

The content of cellulose varies from 37–45% (Paper VI) and changes according to

the wood species. During pyrolysis, cellulose starts to decompose at a temperature

between 315–400 °C. In the first part of the decomposition pathway mechanism

proposed (Fig. 7), the cellulose is depolymerized to oligosaccharides in a process

similar to that of hemicellulose. While the temperatures rise, the oligosaccharides

are decomposed into a simple monosaccharide (glucoside) and, after dehydration,

to levoglucosan.

Fig. 7. Cellulose decomposition to oligosaccharides and further to levoglucosan during

pyrolysis (Redrawn from Zhang, Yang, & Blasiak, 2013).

Lignin contains a variety of aromatic rings with different characteristics

determining a more difficult decomposition and wider temperature range (200–

700 °C). The lignin content in the wood can vary by species with the content of e.g.

23–27 wt.% (Paper VI). The products from the thermal decomposition change

according to the initial lignin composition. This different composition in mainly

associated with monolignols (mainly coniferyl alcohol, sinapyl alcohol, and

paracoumaryl alcohol), biosynthesis enzymes, and enzymes that operate lignin

transport (Campbell & Sederoff, 1996). In Fig. 8, (Kosa, Ben, Theliander, &

Ragauskas, 2011) proposed a potential but not sole thermal decomposition

mechanism for lignin that determines the simplified production of 20 different

compounds as follows:

– starting lignin structure (1)

– relatively stable β-5 product in the primary decomposition step (2)

– α-O cleavage product (3)

– methyl guaiacols (4)

– aldehydes (5)

35

– styrenes and 6a (“reduced vinyl-”) ethyl-phenols (6)

– stilbenes (7)

– phenoxy radicals (8)

– guaiacols (9)

– catechols (10),

– transient radicals (11, 13, 14, 17, 19)

– aliphatics (12)

– phenols (15)

– condensation products (e.g. 4-O-5 dimers) (16, 18)

– cresols (20).

Where the R1 cinnamyl group (propanoid unit), R2, R6, R8 γ carbons in cinnamyl

groups, and R3-5, R7, R9-11 H, and CH3 are phenylpropanoid

units/macromolecules.

The typology of compounds produced is dependent on the type of starting

material, the moisture content, the decomposition temperature, and the process

atmosphere (Brebu & Vasile, 2010). For this heterogeneous production during the

thermal decomposition and for its abundance in nature, lignin is considered a

potential source of raw organic chemicals.

Fig. 8. Lignin decomposition during pyrolysis (Reprinted by permission from Kosa, et

al., 2011 © Royal Society of Chemistry)

36

2.3.2 Physical activation

In the activation process, the removal of the carbon atoms from the nanostructure

of the precursor accessing and interconnecting the inherent structural porosity

occurs. In physical activation, the carbonized raw material is activated by the

physical activating agent. Typically, the activating agents used are water vapor

(steam) or carbon dioxide. Additionally, a mixture of water vapor (steam) and

carbon dioxide can be used. The activation temperature varies between 600 °C and

900 °C. The choice of the activation agent and the activation time depends on the

type of raw material and the pore size distribution needed. Generally, CO2

determines a more microporous structure, while steam favors the production of

mesopores (Rodríguez-Reinoso, Molina-Sabio, & González, 1995).

Physical activation, including the carbonization step, is a two-stage process,

and therefore activation is performed after the carbonization process. A physical

activating agent, such as steam, is fed to the precursor material at temperatures of

between 600 °C and 900 °C. During this phase, the carbon changes physically;

additionally, the carbon changes chemically by opening the carbon matrix and

changing its functional groups. The quantity of activating agent used is a function

of the type and the mass of carbon source material. In Fig. 9, a typical temperature

activation profile is presented.

Fig. 9. Scheme of physical activation of carbon.

Generally, the efficiency of this activation is influenced by contact between the

activating agent and the mass of carbon. Therefore, rotating activation reactors are

preferable during this stage. The entire process must be realized under an inert

atmosphere, e.g., nitrogen, because the prepared carbon autoignition temperature

in atmospheric conditions is around 350–400 °C and, moreover, water molecules

37

include hydrogen and oxygen and can thus generate a potentially explosive

atmosphere. Steam activation time is also a parameter that must be optimized

because it can influence the structure and yield of the carbon mass. Typically, higher

activation times correspond to higher yield loss but also to higher porosity. From a

chemical point of view, during steam activation, the following compounds exist:

O2, CO2, H2, and H2O. Simplified reactions for steam activation are listed in Table

2.

Table 2. Simplified reactions occurring during steam activation and relative enthalpy

(Paper IV)

Reactants Products Enthalpy of reaction

Cx(H2O)y xC(s)+yH2O

C + H2O CO + H2 ΔH = 31.14 kcal mol-1

CO + H2O CO2 + H2 ΔH = -9.65 kcal mol-1

C + CO2 2CO ΔH = 40.79 kcal mol-1

C + 2H2 CH4 ΔH = -17.87 kcal mol-1

2.3.3 Chemical activation

Chemical activation can be considered a single-step process in which the

carbonization of raw material and activation are done simultaneously. Substances

which contain alkali and alkaline earth metals and some general acids are typically

used as a chemical activating agent, such as KOH, NaOH, ZnCl2, and H3PO4

(Ahmadpour & Do, 1996). Chemicals during the activation step can act as a

dehydrating agent (e.g., ZnCl2 and H3PO4), and these types of chemicals are

commonly used for the activation of especially lignocellulosic materials. Alkali

hydroxides are used as chemical activating agents for the activation of coal or chars.

In the chemical activation process, the biomass is first impregnated with a solution

containing the chemical activating agent, after which it is dried in an oven and then

heated in a furnace, with rather lower temperatures (400 to 600 °C) compared to

physical activation, under an inert atmosphere (nitrogen atmosphere). In the case

of chemical activation, important factors include the correct mass to carbon ratio

needed to avoid excessive consumption of reagents and the proper activation

temperature, which can vary according to the type of chemical agent used.

Differently from physical activation, a final washing phase to remove from the

38

precursor material from the chemical activating agent and to uncover the porosity

is necessary.

From the mechanism point of view, ZnCl2 acts as a dehydrating agent in the

carbonization phase, determining the creation of charring and aromatics and

leaving the pore structure after acid washing (Liou, 2010). Moreover, ZnCl2,

working as a tarring inhibitor and “salt templating” (Fechler, Fellinger, & Antonietti,

2013) due to the decomposition of phenolic compounds, creates molecular

hydrogen from the precursor hydro-aromatic structure determining further reaction

sites (and thus higher carbon yield) (Uçar, Erdem, Tay, & Karagöz, 2009)

(Ashourirad, Demir, Smith, Gupta, & El-Kaderi, 2018).

2.4 Activated carbon use

In this chapter, the actual main applications of AC-based products are reported.

2.4.1 Gas and water cleaning

A typical and probably most known application of AC is related to the cleaning of

gaseous and liquid streams.

Different ACs with different specific surfaces and different functional groups

show different performances when interacting with different pollutant materials.

Thus, research in recent years has tried to tailor AC in order to increase the

efficiency of removing a target substance (Yin, Aroua, & Daud, 2007). These types

of treatment are as follows:

– Chemical modification through acid, basic treatment, oxidizers, or

impregnation with a foreign material.

– Physical modification through heat treatment (oxygen decrease).

– Biological modification through the growing of biofilm on the surface

(bioadsorption).

Acid treatment is performed to improve the chelation ability of metals, with the

drawback of reducing the surface area and pore volume. Dispersed and not-paired

π electrons and adsorbed molecular oxygen concur with the functionalization.

Acid treatment using nitric acid and ammonium persulphate have shown

incremental effects in carboxylic, lactonic, and phenolic groups for the removal of

copper, mercury cadmium, nickel, zinc, and lead. Treatment with hydrochloric acid

showed an improvement in the chelation of Cr (VI) (Park & Jang, 2002).

39

Considering oxidizers, treatment with hydrogen peroxide showed an increase

in the oxygen groups on the surface of AC (Gómez-Serrano, Acedo-Ramos, López-

Peinado, & Valenzuela-Calahorro, 1994).

Adsorption favors fast oxidation of biodegradable organic material, but the

biofilm can reduce diffusion inside the porosity of AC.

2.4.2 Food processing

One of the most used applications of AC is for food processing. AC is used, e.g.,

for the purification and decolorization of liquid sugars and starch-based sweeteners

used in soft drinks. Decolorization is also important for producing white sugar

crystals from sugar cane syrup (Aljohani et al., 2018; Çelebi & Kincal, 2007). In

the production of alcohol, AC is used to purify the ingredients and eliminate odors

and turbidity from the (Cai, Rice, Koziel, Jenks, & van Leeuwen, 2016; Onuki et

al., 2015). Melanoidins and polyphenols are removed with AC during the

production of juices, while a high purification of gluconates and lactates in

biochemical food products is also achieved with AC (Liakos & Lazaridis, 2016).

PAH and polychlorinated biphenyls (PCBs) are removed from fish and vegetable

oils with AC as well (Gong, Alef, Wilke, & Li, 2007; Maes et al., 2005).

2.4.3 Support for catalysts

The application of carbon and AC as a catalyst have been well known since the

early part of the previous century (Larsen & Walton, 1940). Typically, this process

can be identified as impregnation with a foreign material when a catalytic metal is

placed on the AC surface. To obtain an AC-supported catalyst, different methods

have been developed (e.g., dry impregnation, incipient wetness impregnation,

ultrasonic impregnation, microwave-assisted impregnation, immobilization, etc.).

The applications of AC-based catalysts are referred normally to heterogeneous

catalytic reactions. Some of these reactions are already applied industrially: Some

examples include the Fischer Tropsch process to produce fuels from CO and H2 (on

Co-Fe/C catalyst), fatty acid hydrogenation (e.g., Bazzi process Pd/C), selective

nitrobenzene hydrogenation (Pt-V/C), and the reductive alkylation process to

produce secondary and tertiary amines from primary amines (Pt-Pd/C). Other

processes involving carbon-based catalysts include the hydrogenation of

dinitrotoluene to toluenediamine (Pd/C), butadiene to butadiene (e.g., BP Lurgi

process on Pd-Ag-Re/C catalyst or Mitsubishi process on Pd-Te/C), and the

40

purification of terephthalic acid (base chemical for PET on Pd/C) (Serp &

Figueiredo, 2008).

2.4.4 Electrical applications

AC is also used in applications related to the construction of capacitors and hybrid

battery-capacitor dispositives. The capability of AC to work as charge storage due

to its high surface areas and small distances between the surfaces which increases

the capacitance is very well known (Kumar, Gaikwad, Mayyas, Sahajwalla, & Joshi,

2018). In the charging process, the negatively charged plate attracts the cations

while in the positively charged electrode increases the number of anions. Using

ionic liquids as electrolytes (e.g., tetrafluoroborates) demonstrated the possibility

of increasing the voltage from 0.8 V, which is the typical value for aqueous medium,

to 2.5 V (Sato, Masuda, & Takagi, 2004). The capacitance Eq. (1) shows the

correlation between the permittivity ε of the separator and the electrical double

layer, which lies on the high surface area S at the nanometric distance d.

𝐶 𝜀 (1)

In Fig. 10, a scheme of the electrical charge in a capacitor is shown. Materials with

non-symmetrical morphology composition, such as AC, polymers, or metal oxides,

increase the operating voltage (Frackowiak, 2007)

Fig. 10. Carbon-based capacitor scheme.

41

Furthermore, AC with nitrogen and oxygen groups on the surface showed enhanced

capacitance (Rennie & Hall, 2013). Recent works have tried to couple the higher

energy density of the chemical batteries with the higher power density of the AC-

based capacitors (Sun et al., 2017).

2.4.5 Other applications

A further application for AC concerns carbon dioxide adsorption. As said before,

CO2 emissions represents the major source for greenhouse gas pollutants. Typically,

zeolites have been investigated for this function, but recent studies have shown that

surface-functionalized AC can be an option for selectively separating and adsorbing

carbon dioxide (Shafeeyan, Daud, Houshmand, & Shamiri, 2010). The acidic

nature of CO2 indicates a basic functionalized surface as a possible choice. In

particular, AC with nitrogen groups (introduced mainly through NH3, nitric acid,

or amines) on the surface generated promising results in capturing carbon dioxide

even though this process influences porosity distribution; thus, reducing the

micropores is more suitable for the adsorption (Plaza, Pevida, Arenillas, Rubiera,

& Pis, 2007)

AC also has applications in the military and defense sector, where equipment

protecting against CBRN (chemical, biological, radioactive, and nuclear) agents

has been developed. The same consideration is valid for civil purposes in nuclear

power plant HVAC filtering, where nuclear grade AC is tested for the adsorption of

methyl iodide and is typically impregnated with triethylenediamine, which protects

against ionizing radiation (Park, Park, Lee, & Moon, 1995). The medical use of AC

is very well known e.g., as an adsorbant against poisoning agents and toxins

(Danish & Ahmad, 2018b). Dextran-coated AC has shown potential for use in

hemoperfusion applications (Howell et al., 2016). In laboratory analysis, AC is

used, e.g., for the determination of AOX (adsorbable organic halides).

42

43

3 Materials and Methods

3.1 Materials used for carbonization and activation

The materials (i.e., peat and sawdust) reported in this chapter were used in this

research for carbonization and activation. All the materials were provided by local

companies.

3.1.1 Sawdust, peat, and woodchips

Peat extracted from Finnish peatland was used in this research. The sample was

stored in a sealed plastic container at room temperature before use. The peat

prepared for physical activation was previously dried for 24 h at 105 °C then

introduced carbonized and activated in the reactor.

Wood sawdust from silver birch (Betula pendula), Norway spruce (Picea

abies), and Scots pine (Pinus sylvestris) was used for carbonization, activation, and

extraction. The selection of these tree typologies was made according to the “zero

miles” approach comprising 99% of all Finnish forests (67% Pine, 22% Spruce, 10%

Birch, 1% other broadleaves) (Parviainen & Västilä, 2011). The samples were

stored in plastic bags at room temperature before use. The sawdust, before being

processed, was dried for 24 h at 105 °C, then introduced into the reactor, where it

was carbonized and activated. In Fig. 11, the sawdust and peat samples used are

shown.

Fig. 11. a) birch sawdust, b) spruce sawdust, c) pine sawdust, and d) peat from Northern

Finland peatland.

The actual technical standard normative concerning the wood chips is the UNI EN

ISO 17225. According to the normative, the characterization considers three main

44

aspects: the particle size P defined according to UNI EN ISO 17827-1 (ISO, 2016),

the moisture content M UNI EN ISO 18134-1/2 (ISO, 2015b, 2017) and the ash

content according to UNI EN ISO 18122 (ISO, 2015a). The particle size identified

from the normative varies from 5 to 100 mm, and the moisture content is divided

into three subcategories: M15 containing less than 10 wt.% water; M20 containing

less than 20 wt.% water; and M25 containing less than 25 wt.% water. Wood chips

from birch and spruce were used for carbonization and activation studies. The chips

were sieved to three different sizes: 2 mm, 6 mm, and 2 cm (in Fig. 12, an example

of 2 cm wood chips from spruce is shown). In the carbonization and activation

studies, the chips were processed as received (Ar) with a standard value of M25

and dried (D) with a standard value of M10 for 24 h at 105 °C.

Fig. 12. Spruce woodchips 2 cm in size.

3.2 Pre-treatments and activation

In this section, the pre-treatment of the raw material and the activation methods

performed to obtain the AC are presented.

3.2.1 Acetic acid extraction

Demineralization was performed on the sawdust samples for four hours using 3.5

M of acetic acid (Sigma Aldrich 100% pure glacial) as an extracting agent. Aliquots

45

of each wood species were transferred into cellulose thimbles of the Soxhlet

extraction unit. In Fig. 13, the sawdust extraction setup is shown.

Fig. 13. Acetic acid Soxhlet extraction.

Sawdust less than 2 mm in size was selected for the extraction. After

demineralization, the samples were suction-filtered on glass filters and washed with

distilled water until the pH of the filtrates was neutral. Finally, the samples were

dried for 24 h at 105 °C in an oven equipped with mechanical convection.

3.2.2 Physical activation

The hardware available for the physical activation was as follows:

– stainless-steel screening reactor placed in a tubular oven (Nabertherm GmbH

RT 50/250/13-P320) with the possibility of direct injection of water-steam

through a volumetric pump,

– rotating quartz reactor inserted in a tubular oven (Nabertherm GmbH RSRB

80-750/11) as shown in Fig. 14.

During the carbonization step, the reactor temperature was incremented from room

temperature levels to distinct temperature levels at different heating rates. The

atmosphere inside the reactor remained inert due to the continuous flushing of

protective gas (nitrogen). When the target temperature was reached, the inert

atmosphere was changed to an activating atmosphere by feeding CO2 or steam into

46

the reactor for a specified time. Following the activation step, the samples were

cooled down to room temperature in a nitrogen atmosphere overnight and then

recovered. A mass flow regulator mounted on the input of the reactor tube

controlled the flow of the protective gas.

Fig. 14. Carbonization and activation setup. (1) Nitrogen line, (2) gas control valves, (3)

mass flow controller, (4) nitrogen-pressurized water tank vessel and water line control

valve, (5) evaporator and steam + nitrogen carrier line, (6) oven PLC, (7)

screening/testing reactor and oven, (8) cascade temperature thermocoupler, (9) oven

and quartz reactor, (10) rotating system, (11) first stadium water bubbling separator, (12)

second stadium cooler and cooling condenser system.

47

In Fig. 15, the setup for physical activation is presented. The yield from the process

for each run was calculated by dividing the mass of carbon produced from the initial

mass of the starting material.

Fig. 15. The design scheme of the carbon activation hardware (Under CC BY Paper IV ©

2019 Authors).

The activating gas (steam) was produced in a Controlled Evaporator mixer

(Bronkhorst) maintained at 140 °C, and water was fed into the evaporator through

a mass flow controller (Bronkhorst). The steam was transferred to the reactor

through a heated transfer line using nitrogen gas as a carrier. The process

parameters used for control were holding time, oven temperature, heating rate,

steam flow, rotation speed, nitrogen flow, and initial mass of biomass used.

3.2.3 Chemical activation

In the chemical activation process, typically, the biomass is first impregnated with

a solution containing the chemical activating agent, after which it is dried in an

oven, then heated in a furnace, with rather lower temperatures (400 to 600 °C)

compared to physical activation, under an inert atmosphere (nitrogen atmosphere).

In the case of chemical activation, important factors include the correct mass to

carbon ratio needed to avoid excessive consumption of reagents and a proper

48

activation temperature, which can vary according to the type of chemical agent

used.

In this research, the samples were prepared by adding a known amount of

biomass and zinc chloride solution to an 800 mL beaker. Under mechanical stirring,

the prepared solution was heated to 85 °C for 3 h in order to complete the chemical

impregnation. Deionized water was added into the solution at various time intervals

to prevent it from completely drying out. The impregnated biomass was then dried

at 105 °C for 24 h in an oven with natural convection.

The impregnated samples were crushed in a mortar and placed in a stainless-

steel fixed-bed reactor. The reactor was settled in a tubular heating oven

(Nabertherm GmbH RT 50/250/13-P320) for carbonization. During the heating

process, the reactor was flushed with an inert gas (nitrogen) in order to avoid

oxidation. The oven temperature was increased from room temperature to the final

temperatures to obtain the final AC. The chemical activating agent remaining in the

sample after carbonization was removed by refluxing the sample with 3 M of HCl

for 1 h. The carbon was filtered and washed with distilled water until a neutral

filtrate was obtained. The AC was finally oven-dried overnight at 105 °C.

3.3 Characterization

In this chapter, the methods used for the quantification of the produced material

properties are presented.

3.3.1 Moisture, total carbon content, yield

The moisture content percent was calculated as the dried mass at 105°C for 24 h

and the initial raw sample mass as shown in the Eq. (2).

𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒 % ∗ 100 (2)

The mass yield from each sample was calculated as the mass of AC divided by the

mass of the initial dry sample as shown in Eq. (3).

𝑦𝑖𝑒𝑙𝑑 % ∗ 100 (3)

The percentage of total carbon present in each sample was measured using a Skalar

Primacs MCS instrument (Fig. 16). Dried samples were weighted in quartz

49

crucibles and then combusted at 1100 °C in a pure oxygen atmosphere, after which

the formed CO2 was analyzed by an IR analyzer built in the Skalar Formacs

analyzer. Carbon content values were obtained by reading the signal of the IR

analyzer from a calibration curve derived from known masses of a standard

substance, citric acid. The carbon content of the individual samples was calculated

as a percent of the initially weighted mass.

Fig. 16. Skalar Primacs MCS for the total carbon analysis.

3.3.2 Surface areas and pore size distribution

The specific surface areas and pore size distributions were determined from the

adsorption-desorption isotherms using nitrogen as the adsorbate. Determinations

were performed with a Micromeritics ASAP 2020 instrument (Fig. 17). Portions of

each sample (100–200 mg) were degassed at low pressure (2 µm Hg) and at a

temperature of 140 °C for 2 h to clean the surfaces and remove any adsorbed gas.

Adsorption isotherms were obtained by immersing sample tubes in liquid nitrogen

(-196 °C) to achieve constant temperature conditions. Gaseous nitrogen was then

added to the samples in small doses, and the resulting isotherms were obtained.

Specific surface areas were calculated from adsorption isotherms according to the

BET method (Brunauer, Emmett, & Teller, 1938), while nitrogen adsorption was

used to calculate the pore size distribution using the DFT (Density Functional

Theory) and BJH model (Barrett, Joyner, & Halenda, 1951). For the analysis of the

micropores, deBoer’s t-plot statistical thickness method was chosen (Lippens & de

Boer, 1965).

With the instrumental setup used, micropores down to 1.5 nm in diameter could

be measured even when smaller pores were included in the first point measured.

50

According to the IUPAC notation, the partial volume contributions from

micropores < 2 nm, mesopores 2–50 nm, and macropores > 50 nm were considered

(Thommes et al., 2015b) and related as a percent to the total pore volume.

Fig. 17. Micromeritics ASAP 2020 used for nitrogen adsorption.

3.3.3 Adsorptive properties

The adsorptive properties of the AC were tested using the adsorption of dyes (such

as methylene blue (MB) and orange II (OII)) into the pores. A solution containing

300 mg of dye per liter of H2O was prepared, and 100 mL of this solution was then

transferred into 250 mL Erlenmeyer flasks. Next, 50 mg of AC was added, and the

solutions were continuously agitated for 24 h to achieve equilibrium between

adsorption and desorption of the test dye. Portions of each solution were filtered

and diluted if needed. The absorbance of the solution was measured at different

wavelengths (e.g. 664 nm for MB and 485 nm for OII) on a Shimadzu UV-1700

double-beam spectrophotometer (Kyoto, Japan). The concentrations of the solution

were calculated from calibration lines obtained with known dye concentrations.

The absorbed mass (qm) was calculated for each compound using Eq. (4), and the

percent MB and OII removed was calculated using Eq. (5):

𝑞 𝐶𝑜 𝐶𝑡 ∗ 𝑉/𝑚 (4)

% 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 ∗ 100 (5)

51

where C0 is the initial concentration of the dye (300 mg/L), Ct is the measured

concentration (mg/L) after 24 h, V is the volume of dye solution used (L), and m is

the mass of the AC used (mg).

3.3.4 Other characterizations

Elemental analysis for measuring the hydrogen, nitrogen, and oxygen content was

performed with the Flash 2000 CHNS-O Organic elemental analyzer produced by

Thermo Scientific. The ground sample was first weighted to 1.5–3.5 mg and dried

for 1 h at 105 °C. Then, the sample was placed in the analyzer and mixed with 10

mg of vanadium pentoxide V2O5 to enhance the burning. The prepared sample was

then combusted at a temperature of 960 °C for 600 s using a standard methionine

for the hydrogen and nitrogen, while the standard used for oxygen was BBOT 2,5-

(Bis (5-tert-butyl-2-benzo-axazol-2-yl) thiophene. Plain tin cups were used as a

bypass (3 pcs) when starting the measurements.

The wood chemical composition was determined according to the method

described by Rowell, Pettersen, and Tshabalala (2012), Yokoyama, Kadla, and

Chang (2002), and Styarini, Risanto, Aristiawan, and Sudiyani (2012), and it is

described in detail in Paper VI.

The metal contents of samples (ashes) were measured by inductively coupled

optical emission spectrometry (ICP-OES) using a Perkin Elmer Optima 5300 DV

ICP–OES instrument: 0.10-0.12 g of samples were added in 63% nitric acid and

hydrogen peroxide, then digested in a microwave oven (MARS, CEM Corporation)

at 200 °C for 10 min. After digestion, the solution was diluted to 50 mL and

measured by ICP-OES.

The microstructures of the raw materials, the intermediates, and the AC were

revealed in FESEM images, which were obtained using a Zeiss Sigma field

emission scanning electron microscope (FESEM) in the Centre of Microscopy and

Nanotechnology at the University of Oulu, operated at 5 kV.

3.3.5 Experimental design

The experimental design for optimizing chemical processes is relatively new, even

though the theory of multivariate analysis has been well known for more than 70

years (Craig & Fisher, 1936).

The design of experiments can be defined as a rigorous and systematic

mathematical and statistical tool to organize and minimize experiments in order to

52

understand how parameters affect one or multiple response variables. The design

of experiments generates a model that can be useful for interpreting and optimizing

the process parameters to achieve a statistically predetermined output value.

The fractional factorial design is a design containing combinations of variables.

It is a two-level design that can have III, IV, V or more levels of resolution. The

level of resolution depends on the number of runs selected. The fitting of the model

was made through the partial least square (PLS) method, which takes into account

the covariance of multiple responses to achieve a multivariate analysis.

After completing the selection of the main parameters affecting the whole

process, a minimum and maximum range for each parameter was identified. From

a factor list of seven process parameters, an experimental matrix was generated

from the software following fractional factorial resolution IV fitted with PLS

regression. The number of experiments resulting in the screening mode was 19 (2(7-

3) + 3 repetitions with intermediate values). The values of the process parameters

were specified in a range with minimum and maximum points. The experiments

were performed following a random sequence in order to maintain stochastic

independence between each experiment and avoid bias (Cox & Reid, 2002). Modde

9.1 by Umetrics was used to process the data.

53

4 Results and discussion

4.1 Research review of recent AC studies

AC has been used as an adsorbant for many years (the first historical recognition

of its adsorptive properties dates back to 1773 from Pomeranian chemist Carl

Wilhelm). The first industrial product under the name of Eponit was

commercialized from 1911 by the Austro-Hungarian company, Fanto Works (Gupta,

2018). Recently, new discoveries in other applications have led to growing interest

in the surface chemistry analysis and specific property manipulation of AC.

In particular, new environmental regulations have boosted economic interest

in the field to produce more selective and efficient products and to improve

production and regeneration processes (e.g., microwave irradiation,

electrochemical regeneration, bio-regeneration, etc.) (Ao et al., 2018; El Gamal,

Mousa, El-Naas, Zacharia, & Judd, 2018; McQuillan, Stevens, & Mumford, 2018).

Furthermore, the need to apply the principle of maximum sustainability and cost

saving to production has pushed research in the direction of using waste materials

from other productions.

In Paper, I, a preliminary evaluation of physically AC and chemically AC is

carried out.

Concerning carbonization, the following was compared:

– Hydrothermal carbonization (HTC), where the process takes place at a

relatively low temperature (180–220 °C) in an autoclave (pressurized) with the

raw material in water suspension.

– Dry Pyrolysis (DP), where the process takes place at a higher temperature (>

500 °C) in an inert but non-pressurized atmosphere.

The main advantages of the HTC process are the lower temperature involved and

the higher achievable yield (reduced production of sidestream products, e.g., tars).

Its disadvantages are the high vapor pressure with relative risk and the need to

separate and dry the final carbon from the water.

On the other hand, DP, which has a higher tar production and involves a higher

temperature, results in a process that is easier to scale up. Furthermore, the gas

produced from DP can be reused to produce energy.

Concerning activation, physical and chemical activations were compared in

reference to previous case studies. Testing with carbon residue from gasification

physically activated with CO2 and chemically activated with ZnCl2 has been

54

studied. For chemical activation, different activating agents have been considered,

such as HCl, H2SO4, ZnCl2, KOH, and HNO3. The aim was not only to compare

and verify the process parameters and the effectivity of activation agents but also

to comprehend the point of zero charges (pHZPC), which is an important parameter

for characterizing the anionic or cationic behavior of the AC.

The carbon residue has been tested in the adsorption of different compounds,

such as phosphates, nitrates, iron (II), copper (II), and nickel (II). A high variation

in chemisorption affinity correlated with the starting material type can be seen.

4.2 Difference between one- and two-stage physical activation

A comparison between one-stage and two-stage steam activation was investigated

to determine whether AC characteristics are influenced by the carbonization-to-

activation delayed time. It must be noted, however, that this was only a preliminary

analysis because not many studies in this area have been carried out.

An important assumption, in this case, was made because confronting plant-

scale with laboratory-scale production can introduce some uncertainty about the

comparability of the process. Regardless, comparing industrial production with the

laboratory setup design was an important evaluation.

The two-step activation samples of already carbonized birch and spruce from

a carbonization plant were activated and compared to the AC carbon produced from

the same raw material but from virgin wood. The choice of activating the carbon

through one-step or two-step activation presents some differences. The total yields

are higher for the two-stage process compared to the one-stage process.

Pre-carbonization leads to rather high yields for the activation step because

most of the volatile fractions are removed during carbonization, but the total yields

are at the same level for both procedures. Longer steam activations lead to lower

yields, affecting the one-stage process more than the two-stage process. All AC

produced in the two-stage process is lower in terms of specific surface areas

compared to AC produced in a one-stage process; this difference occurs

independent of the activation times used, even if longer activation times, in general,

produce higher specific surface areas.

55

Fig. 18. A) BET SSA, B) Langmuir SSA, C) t-plot Micropore SSA, D) t-plot External SSA,

E) DFT Pore Volume, F) DFT PSD, G) Adsorption isotherm comparison for spruce, H)

Adsorption isotherm comparison for birch I) Adsorption isotherms of only carbonized

samples. (Under CC BY Paper II © 2018 Authors).

As can be seen in Fig. 18, the BET SSA indicates a higher surface area value for

one-stage activation, which is also confirmed by the Langmuir and t-plot models.

The PSD analysis seems to indicate that mesoporous AC is obtained in a two-stage

process when compared with a one-stage process. This result is also evident

through the DFT model and the adsorption isotherms, where the one-stage process

presents a Type I profile, while the two-stage process presents a Type II profile.

The carbonized material presents a small surface area mainly composed of

micropores.

The subsequent steam activation further develops micro-mesoporosity. But this

process happens in a distinct way when comparing the one-stage and two-stage

processes. While in a one-stage process, the isotherm profile follows a more

uniform development in micropore and mesopore formation, in a two-stage process,

mesoporosity seems to be favored. Why this occurs is not yet clear, and thus only

speculative explanations can be given at present. The most probable explanation is

56

the different chemical composition of the AC due to aging and a possible synergic

effect of decomposition-activation during the one-stage process.

Further investigations in this direction could also explain whether AC is a

chemically stable material or undergoes, to a certain extent, a chemical change

process.

4.3 Particle size initial humidity and acetic acid extraction

In Paper III, possible biomass pre-treatments before carbonization and activation,

as well as how these pre-treatments can affect the final characteristics of the AC,

are investigated.

In particular, grinding (i.e., particle size), moisture content (pre-drying), and

acid extraction have been considered. The choice of these parameters depends on

the possible operative plant-scale scenario used to examine whether this pre-

treatment can somehow affect the quality of the final product. Particle size can be

of interest from the logistical point of view, because moving woodchip-size

particles can be more practical than moving particles in a powder form.

Pre-drying, on the other hand, can be considered an opportune pre-treatment to

avoid an excess of liquids in the out stream and thereby reduce the mass of possible

wastes to treat. Acid treatment has been considered because it is a process used to

reduce mineral content in the final product (ashes). In this particular case, the

extraction was carried out from the raw material, even though post-treatment

directly on the produced AC is still a practical option.

57

Fig. 19. Comparison between different particle sizes and moisture content samples. A)

BET SSA, B) Langmuir SSA, C) t-plot Micropore SSA, D) t-plot External SSA, E) DFT

Pore Volume, F) DFT PSD. (Paper III).

As evidenced in Fig. 19, there is a negligible difference between the AC SSA and

PSD because of initial moisture and particle size. These results can lead to the

conclusion that pre-treatments like milling or drying do not affect the final

produced AC pore size distribution. The morphology and initial moisture content

do not affect the porous structure, which in turn suggests that in potential industrial-

scale production, powder risk or the quantity of liquid wastes can be reduced

without affecting the final product characteristics.

58

Fig. 20. FESEM images of a) untreated birch sawdust, b) untreated spruce sawdust, c)

acetic acid-extracted birch sawdust and d) acetic acid-extracted spruce sawdust.

Another type of pre-treatment considered is the acid extraction of wood. This

investigation was conducted to demonstrate how a possible interaction between an

acid (e.g., from fermentation) could change the final AC characteristics. Acid

extraction is a common method used to reduce the metal content in carbon and also

to separate extractives from wood. In particular, acetic acid has been used because

of its greener nature and its growing use in the treatment of wood through

acetylation, i.e., to preserve wood from degradation. This process takes place by

substituting the free hydroxyls groups present on the wood surface, which generally

reacts with water and is easily accessible to the wood enzymes with less digestible

acetyl groups (Rowell & Dickerson, 2014). In Fig. 20, the FESEM images of the

treated and untreated sawdust are shown.

In Fig. 21, the difference between AC from untreated sawdust and acetic acid-

extracted sawdust is depicted. As can be seen from the DFT pore size distribution

59

and the adsorption isotherms, acetylation induces a higher formation of micropores

in the AC. This result is also confirmed by the t-plot model. The DFT total pore

volume results in much higher mesoporous AC, predicting a high SSA to volume

ratio. In this case, it is possible to observe that the acetic acid pre-treatment allows

the tailoring of more microporous AC, changing the chemical structure of the

starting biomass.

Fig. 21. Comparison between untreated and acetic acid-extracted sawdust. A) BET SSA,

B) Langmuir SSA, C) t-plot Micropore SSA, D) t-plot External SSA, E) DFT Pore Volume,

F) DFT PSD, G) Adsorption Isotherms. (Under CC BY Paper III © 2018 Authors).

4.4 Parameters affecting biomass carbonization and activation

In Paper IV, optimization of the process parameters of physical activation was

performed through the design of experiments. The biomass source material used

was peat. The choice of the parameters was made according to the hardware setup.

These controllable parameters were holding time, oven temperature, steam flow

rate, heating rate, reactor rotation speed, biomass initial mass, and nitrogen flow.

As can be seen in Fig. 22, an experimental matrix was generated that combined the

maximum and minimum level of each parameter (factor). The execution of the

60

experiment was performed following a randomly generated sequence in order to

reduce bias.

Table 3. Experimental matrix generated according to the factorial design. (Under CC BY

Paper IV © 2019 Authors).

Exp

No

Exp

Name

Run

Order

Incl/Ex Holding

time (min)

Oven

temperature

(°C)

Steam

flow

(g/h)

Heating

rate

(°C/min)

Rotation

(rpm)

Biomass

mass initial

(g)

Nitrogen

flow

(ml/min)

1 N1 8 Incl 60 700 30 2.6 4.36 100 100

2 N2 15 Incl 240 700 30 2.6 17.44 100 300

3 N3 18 Incl 60 800 30 2.6 17.44 300 100

4 N4 6 Incl 240 800 30 2.6 4.36 300 300

5 N5 3 Incl 60 700 120 2.6 17.44 300 300

6 N6 17 Incl 240 700 120 2.6 4.36 300 100

7 N7 19 Incl 60 800 120 2.6 4.36 100 300

8 N8 12 Incl 240 800 120 2.6 17.44 100 100

9 N9 11 Incl 60 700 30 13 4.36 300 300

10 N10 7 Incl 240 700 30 13 17.44 300 100

11 N11 14 Incl 60 800 30 13 17.44 100 300

12 N12 13 Incl 240 800 30 13 4.36 100 100

13 N13 4 Incl 60 700 120 13 17.44 100 100

14 N14 5 Incl 240 700 120 13 4.36 100 300

15 N15 1 Incl 60 800 120 13 4.36 300 100

16 N16 9 Incl 240 800 120 13 17.44 300 300

17 N17 10 Incl 150 750 75 7.8 10.9 200 200

18 N18 2 Incl 150 750 75 7.8 10.9 200 200

19 N19 16 Incl 150 750 75 7.8 10.9 200 200

The prepared AC samples were then characterized by a number of methods that

generated the response matrix. Responses included in the matrix were the yield, the

BET surface area, the total carbon, the pore volume by BET, DFT micropores, DFT

mesopores, the DFT total pore volume, and the nitrogen, hydrogen, and oxygen

content. The results were elaborated through the MODDE 9.1 software by Umetrics.

Regarding the holding time, it is possible to observe a direct correlation with

the BET surface. The same effect occurs with the pore size distribution responses

involved with a higher increase in mesopores. This result indicates that the holding

time interval is one of the most important parameters in porosity production. An

61

oven temperature from 700 to 800 °C slightly increases the porosity, with a more

evident effect on mesoporosity. The oven temperature seems to influence the

morphology more so than the absolute value of the pore volume. The impact of

steam flow is peculiar in the process. This result demonstrates that physical

activation with steam is a widening process in which micropores are generated in

the first phase and, in the second phase, mesopores are formed.

Oxygen content is increased by steam flow, indicating that water oxygenates

the AC, likely creating functional groups on the surface. Meanwhile, the heating

rate has an impact on the adsorption properties. The fastest ramping times seem to

induce microporosity formation. The final AC yield result is not affected by the

heating rate. A possible explanation for this is that energy and a time-consuming

ramp (>1h) are not beneficial for obtaining higher AC yields.

The rotation speed has an influence on micro-mesoporosity formation. The

better efficiency of the interface gas–particle surface interaction along with parallel

mechanical milling stress favors pore production. With the lab-scale production, it

is possible to identify a “scale effect” on the quantity of AC produced. The result

shows that it is possible to detect a scaling gradient effect on the order of a 1.4%

yield increase within the interval 100–300 g biomass mass. Inert gas flow (nitrogen)

is another parameter that was investigated in order to determine whether interaction

occurred with the final nitrogen content in the sample. The results showed that at

the process parameter level considered, nitrogen flow does not affect the nitrogen

content in the material. Furthermore, nitrogen does not affect the porosity

parameters, except for a slight reduction in mesopore formation. A better yield was

achieved with a higher nitrogen flow. This result suggests that a higher protective

atmosphere can “smoothen” the oxidation process.

In Fig. 23, the results of normalized process parameter effects on the responses

are reported.

62

Fig. 22. Normalized plot effect of the different process parameters affecting each

response (N= number of experiments, DF= degree of freedom, Cond.no.= condition

number). (Under CC BY Paper IV © 2019 Authors).

4.4.1 AC from peat design of experiment and parameter modeling

and optimization

The final aim of the multivariate analysis was to build a simulation model valuable

for the parameter range selected. To determine whether the model could be used

for tailoring the AC properties, it was set with the target to produce a maximized

yield and either microporous or mesoporous AC. The model simulated the relative

process parameter set values that were subsequently run in a validation experiment.

In Table 3, the input values necessary for the experimental setup are listed;

while in Table 4, a relative comparison between the predicted values and the actual

values produced by the validation experiments is shown. The results achieved show

a very realistic simulation in the microporous AC scenario (75.64% predicted vs.

71.65% observed) and a slight overestimation (around 17%) in the case of

mesoporous AC (63.52% predicted vs. 46.47% observed), with an evident general

increase of mesopores.

63

Table 4. Process parameters of the model to produce optimized yield AC from peat with

either high mesopore or high micropore distribution.

Targets Sample

Holding

time

(min)

Oven

temperature

(°C)

Steam

flow

(g/h)

Heating

rate

(°C/min)

Rotation

(rpm)

Biomass

mass

initial (g)

Nitrogen

flow

(ml/min)

Max Yield Validation1

Max

Micro

Min

Meso

Max

PV Prediction 120.09 752.0 90.45 13.00 4.36 240.95 288.06

Observed 120.00 753.0 90.50 13.00 4.36 241.00 288.00

Max Yield Validation2

Min

Micro

Max

Meso

Max

PV Prediction 90.47 799.6 83.11 6.84 17.44 100.00 278.98

Observed 90.00 800.0 83.10 6.84 17.44 100.00 279.00

Table 5. Predicted model compared to measured results.

4.5 Chemical activation with ZnCl2

The chemical activation was completed using zinc chloride as an activating agent

and peat as a raw material. As said previously, ZnCl2 interacts with the biomass at

a certain temperature level, creating the porous structure. What is presented in

Paper V is an optimization of the different masses of biomass to the mass of the

activating agent ratio and the activation temperature. In Fig. 24, the results of the

different AC produced from peat are reported. As it is possible to see that the TC

increases with higher temperatures, the yield is also affected by high temperatures.

The BET SSA resulted in a high value for an impregnation ratio, from ½ to 1, while

higher ratios tended to reduce the specific surface. High ZnCl2 ratios and high

Targets Sample Yield

(%)

BET

surface

(m2/g)

TC

(%)

Pore

volume

BET

(cm3/g)

DFT

micro

(cm3/g)

DFT

meso

(cm3/g)

DFT

total V

(cm3/g)

DFT

Micro

(%)

DFT

Meso

(%)

Max Yield Validation1

Max

Micro

Min

Meso

Max

PV

Prediction 25.8 497 85.58 0.17 0.16 0.05 0.21 75.64 23.78

Observed 20.0 518 83.96 0.28 0.16 0.06 0.23 71.65 27.99

Max Yield Validation2

Min

Micro

Max

Meso

Max

PV

Prediction 27.4 359 74.87 0.11 0.09 0.16 0.26 36.09 63.52

Observed 21.0 579 87.70 0.39 0.17 0.15 0.31 53.53 46.47

64

temperatures favor mesoporosity formation. In the dye adsorption test, the anionic

dye was more readily adsorbed.

Fig. 23. Comparison of the chemically activated carbon from peat as a function of

different temperature and biomass to chemical mass ratio. A) TC, B) Yield, C) BET SSA,

D) DFT Pore Volume, E) DFT PSD, F) Dye adsorption comparison of methylene blue MB

and Orange II. (Paper V).

4.6 Properties of biomass waste-derived AC

Another part of the research was the study and determination of how some

characteristics of the starting biomass used could affect the PSD and the adsorptive

performances of the AC produced. In this case, sawdust from birch, pine, and

spruce was analyzed. The results are presented in Fig. 25.

The content of hemicellulose-cellulose-lignin was determined through

different methods (Rowell, 2005; Styarini et al., 2012; Yokoyama et al., 2002),

which resulted in a higher level of cellulose and lignin in pine, whereas a higher

65

content of hemicellulose was found in birch. The ash analysis indicates a high

content of Ca, K, and Mg, with a relevant content of P, in the case of birch wood.

Fig. 24. Wood analysis. A) Wood composition, B) Ash content in w/w % on dry wood, C)

Ash metal composition, D) AC from birch adsorption performance of methylene blue

MB, gentian violet GV, metal zinc, nitrate, sulfate, and phosphate ions. (Paper VI).

The biomasses were then carbonized and activated following a physical one-stage

procedure with steam. For comparison, a sample of lignin from the Kraft extraction

process was activated with the same method.

In the case of AC produced from birch, an adsorption test relative to common

dyes and ions was performed, resulting in high adsorption of Zn2+ and MB (cationic

dye), while lower values were achieved for phosphate and sulfate ions (PO43-,

SO42-). Considering the PSD as evidenced in Table 5, a correlation between lignin

and metal content is suggested. According to the BJH model, birch wood, which

has a lower value of lignin content, has a higher development in mesoporosity;

while pine and spruce, both possessing higher lignin content, have increased

microporosity.

66

Table 6. Pore distribution according to the BJH model. (Under CC BY Paper VI © 2017

Authors).

Sample AC BJH micropores (%) BJH mesopores (%) BJH macropores (%)

Birch 14 84 2

Spruce 18 78 4

Pine 18 76 6

The difference in ash content might indicate a possible effect on increased

mesoporosity, with spruce having a higher ash content in comparison with pine.

This suggests that an endogenous metal content in the biomass can induce some

degree of chemical activation in the final AC.

67

5 Conclusions

This research investigated the potential of producing tailored AC from waste

materials for different possible applications. Different types of raw materials were

considered, such as wood sawdust, wood chips (birch, spruce, and pine), and peat.

Physical and chemical activation were used and evaluated, determining a valuable

method to valorize the product.

The activation process can be performed in a single stage, resulting in a more

efficient method for production without reducing the quality of the AC produced.

Physical activation was demonstrated to be a greener process for activating carbon,

requiring heat and steam or carbon dioxide. Chemical activation resulted in a less

eco-friendly method but produced AC with a higher yield and more homogeneous

pore size distribution.

Controlling the production process and the pre-treatment parameters allows for

the tailoring of the final properties of AC. Important factors in the physical

activation with steam included holding time, which particularly affects the surface

area and the micro-mesoporosity, and steam, which also increases the oxygen

concentration in the final product. A pore-widening mechanism has been identified

as in which the pore size distribution develops from micropores to mesopores,

continuing the process during the activation time.

Pre-treatments, like pre-drying, showed no effect on the final carbon

characteristics, while extraction with acetic acid induced a higher formation in

micropores. At an industrial scale, it is recommended that the input mass be pre-

dried in order to avoid high water content in the condensable gas output.

For the chemical activation with zinc chloride, the key role is played by the

mass ratio with the initial biomass and the activation temperature, which is very

important for developing the porosity. The study demonstrated the importance of

detecting the right operative temperature in which the chemical activating agent

can perform better. At the industrial level, the key factor is to recover all or partially

all of the activating chemical after the activation in order to make the whole process

sustainable and environmentally friendly.

Particular importance in the evaluation of the adsorption test resulted to be not

only the specific surface area and the porosity but also the point of zero charge that

affects the functional groups on the surface and subsequently the selectivity of the

AC.

68

To conclude, high-quality activated carbon can be produced starting from local

biomass waste, lending actual technical and scalable feasibility for industrial

production or reconversion.

Table 7. A summary of the possible pros and cons of the approach used (++highly

positive, +positive, -negative, --highly negative).

Parameters Chemical activation Physical activation

Pre-treatments - + Heat consumption + - Yield + - Activating agent recovery

- +

Post washing -- + Parameter control ++ + Porosity control ++ + Process complexity - + Waste hazard (potential) -- +

69

6 Perspectives

Regarding future research, a specific characterization of the functional groups on

the surface using, e.g., FTIR-ATR with Germanium crystals, is warranted. This

characterization can increase understanding of the chelating properties of AC but

also some of its catalytic properties.

In this direction, another interesting research prospect and potential upgrade to

actual technology is the functionalization of carbon in order “to tailor” the surface

to host particular functional groups, such as nitrogen (using ammonia in the

activating stream). Interest in this field is related to the capability of amine groups

to interact with CO2. Furthermore, nitrogen-doped AC showed interesting

electronic properties (Ahmed, Rafat, & Ahmed, 2018; Li et al., 2016).

A further interest is to optimize the side stream of AC production in order to

convert all the phase products (solid liquid and syngas) into valuable products. This

investigation would be made from the perspective of more economically and

environmentally sustainable biorefinery production.

Another interesting area to develop is the understanding and optimization of

the mechanism of conversion of non-graphitizable carbon into graphitic carbon

through catalytic graphitization (e.g., catalytic graphitization using N2 plasma pre-

treated AC with Iron (III) (Shi et al., 2017) to evaluate the possibility of producing

graphene-like structures from a biomass source.

70

71

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Original publications

I Bergna D., Romar H., Tuomikoski S., Runtti H., Kangas T., Tynjälä P., & Lassi, U. (2017). Activated carbon from renewable sources: thermochemical conversion and activation of biomass and carbon residues from biomass gasification. In L. Singh, V. C. Kalia (Eds.), Waste Biomass Management – A Holistic Approach (pp 187-213) Springer.

II Bergna D., Varila T., Romar H., & Lassi, U. (2018). Comparison of the properties of activated carbons produced in one-stage and two-stage processes. C Journal of Carbon Research 4(3): 41.

III Bergna D., Romar H., & Lassi, U. (2018). Physical activation of wooden chips and the effect of particle size, initial humidity, and acetic acid extraction on the properties of activated carbons. C Journal of Carbon Research 4(4): 66.

IV Bergna D., Hu T., Prokkola H., Romar H., & Lassi, U. (2019). Effect of some process parameters on the main properties of activated carbon produced from peat in a lab-scale process. Waste and Biomass Valorization.

V Varila T., Bergna D., Lahti R., Romar H., Hu T., & Lassi, U. (2017). Activated carbon production from peat using ZnCl2: characterization and applications. BioResources 12(4): 8078-8092.

VI Lahti R., Bergna D., Romar H., Tuuttila T., Hu T., & Lassi, U. (2017). Physico-chemical properties and use of waste biomass-derived activated carbons. Chemical Engineering Transactions, 57: 43-48.

Reprinted with permission from Springer (Paper I), AIDIC/CET (Paper VI) and

under Creative Commons CC BY license (Papers II-V).

Original publications are not included in the electronic version of the dissertation.

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ACTIVATED CARBON FROM RENEWABLE RESOURCESCARBONIZATION, ACTIVATION AND USE

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