Optimisation of thermal treatment of invasive alien plants

199 Pages·2017·4.35 MB·English

Optimisation of thermal
treatment of invasive alien
plants (IAPs) for char
production for use in
combustion applications
by
Jhonnah Mundike
Dissertation presented for the Degree
of
DOCTOR OF PHILOSOPHY
(Chemical Engineering)
in the Faculty of Engineering
at Stellenbosch University
Supervisor
Prof. Johann Görgens
Co-Supervisor
Dr. François-Xavier Collard
March 2018
Stellenbosch University https://scholar.sun.ac.za
Declaration
By submitting this dissertation electronically, I declare that the entirety of the work contained therein
is my own, original work, that I am the sole author thereof (save to the extent explicitly otherwise
stated), that reproduction and publication thereof by Stellenbosch University will not infringe any
third-party rights and that I have not previously in its entirety or in part submitted it for obtaining any
qualification.
Date: March 2018
This dissertation includes [02] original papers published in peer-reviewed journals or books and [01]
unpublished publications. The development and writing of the papers (published and unpublished)
were the principal responsibility of myself and, for each of the cases where this is not the case, a
declaration is included in the dissertation indicating the nature and extent of the contributions of co-
authors.
Copyright © 2018 Stellenbosch University
All rights reserved
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Abstract
Due to the popular worldwide demand for need to use cleaner fuels, lignocellulosic-derived char is
gaining importance as a possible component in co-firing with coal. In order to avoid deforestation of
indigenous forests in Zambia for char production, possibilities of using alternative feedstocks from
invasive alien plants (IAPs) were investigated.
In the present study, torrefaction and slow pyrolysis were used for char production from IAPs for
energy applications. Both processes were optimised individually at milligram-scale in a
thermogravimetric analyser (TGA) for char yield and higher heating value (HHV), through
manipulation of the temperature, heating rate and holding time. Two IAPs, namely Lantana camara
(LC) and Mimosa pigra (MP), from Zambia were used as feedstock materials. The feedstock particle
size distribution (PSD) used was from 425 to 600 µm. The optimisation results for torrefaction and
slow pyrolysis showed that temperature majorly influenced char yield and HHV. In case of
torrefaction, operating at temperatures ≤ 300 ˚C, heating rate and hold time also influenced char HHV,
while neither parameters had a statistically-significant influence on char yield and HHV during slow
pyrolysis.
During torrefaction at 300 ˚C, LC recorded a higher char yield of 43 wt.%, and a corresponding HHV
of 27.0 MJ kg-1, mainly due to increased hemicelluloses content, compared with MP that had a char
yield of 52 wt.% with HHV of 24.4 MJ kg-1. In case of slow pyrolysis, MP recorded the highest char
HHV of 31.0 MJ kg-1 at 580 ˚C, due to increased lignin, in comparison with LC that had a highest
char HHV of 30.0 MJ kg-1 at 525 ˚C.
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Based on optimised conditions from milligram-scale, LC and MP samples of PSD from 850 to 2800
µm were used for char production at gram-scale in a bench-scale reactor. Scaling-up promoted
secondary char formation due to mass and heat transfer limitations in larger particles and increased
sample size, thereby increasing char yields for both biomasses. Char yields were increased by 4 and
2 wt.% for MP and LC, respectively, due to scale-up. The highest HHVs at bench-scale were 30.8
MJ kg-1 (614 ˚C) and 31.6 MJ kg-1 (698 ˚C) for LC and MP, respectively.

For the purposes of coal substitution and co-firing, a combustion study was conducted in a TGA
reactor using LC and MP chars (torrefied and pyrolysed) from gram-scale of PSD from 850 to 2800
µm. LC and MP chars were blended with three South African coals between 5 to 90 wt.% (biomass
char). The combustion characteristic results showed that LC chars were more reactive than MP chars,
with significantly lower combustibility temperatures than the coals. During co-combustion, the
combustion indices for blends < 30% were similar to those of the individual coals, showing that
partial coal substitution could be done without significant modifications to existing equipment. There
was better combustion performance through increased combustion indices for blends > 60%, though
probably with a likelihood of modifications to existing reactors that were initially designed for coal
combustion, as the conversion was much faster.

In summary, this study has shown that LC and MP IAPs could be valorised through torrefaction and
slow pyrolysis to produce char for direct energy applications and co-firing with coal. LC samples
torrefied at 300 ˚C were found to be equivalent to high volatile bituminous C coal, while pyrolysed
chars for LC and MP were equivalent to high volatile bituminous B coal. To confirm the practicality
of co-firing possibilities, it is recommended that scale-up studies to pilot-scale be conducted in
order to assess overall energy efficiency, techno-economics, operating conditions of industrial
reactors and a life cycle assessment.
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Opsomming
Weens die populêre, wêreldwye behoefte om skoner brandstof te gebruik, is daar ʼn toename in die
belangrikheid van houtskool afkomstig vanaf lignosellulose as ʼn moontlike komponent in die
gesamentlike-verbranding met steenkool. Om ontbossing van inheemse bosse in Zambië vir
houtskool produksie te voorkom, die moontlikheid om indringer uitheemse plante as grondstof te
gebruik, is ondersoek.

In die huidige studie is die lae-temperatuur rooster, ook bekend as torrefaksie, en stadige pirolise
benut vir die produksie van houtskool wat in energietoepassings gebruik kan word. Die eerste teiken
was om houtskool produksie deur torrefaksie en stadige pirolise vir energietoepassings te optimeer,
deur die houtskool-opbrengs en hoër-verhittings-waarde (HVW) te maksimeer, deur die optimering
van temperatuur, verhittingstempo, en hou-tyd op milligram-skaal in ʼn termo-gravimetriese
analiseerder (TGA). Twee indringer uitheemse plante in Zambië, naamlik Lantana camara (LC) en
Mimosa pigra (MP), was as grondstof gebruik. Vir die grondstof was ʼn partikel-grootte-verspreiding
(PGV) van 425 tot 600 µm gebruik. Die optimiserings-resultate vir torrefaksie en stadige pirolise het
getoon dat temperatuur ʼn groot invloed op houtskool-opbrengs en HVW gehad het. In die geval van
torrefaksie was dit bevind dat vir temperature ≤ 300 ˚C, die verhittingstempo en hou-tyd ook die
houtskool-HVW beïnvloed, terwyl vir stadige pirolise het beide veranderlikes geen statistiese
merkbare invloed op die houtskool-opbrengs of HVW gehad nie.

Gedurende torrefaksie by 300 ˚C het LC ʼn hoër houtskool-opbrengs van 43 massa% en ʼn
ooreenstemmende HVW van 27.0 MJ kg-1 gehad, grootliks as gevolg van die verhoogde hoeveelheid
hemisellulose. Dit is in vergelyking met MP wat ʼn houtskool-opbrengs van 52 massa% en ʼn HVW
van 24.4 MJ kg-1 gehad het. In die geval van stadige pirolise het MP ʼn hoër maksimum HVW van
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31.0 MJ kg-1 by 580 ˚C getoon as gevolg van verhoogde lignien inhoud. Dit is in vergelyking met LC
wat ʼn optimale houtskool HVW van 30.0 MJ kg-1 by 525 ˚C gehad het.

Gebaseer op die ge-optimiseerde kondisies op milligram-skaal, was LC en MP monsters met PGV
van 850 tot 2800 µm gebruik vir houtskool produksie op gram-skaal in ʼn laboratoriumskaal reaktor.
Opskalering het sekondêre-houtskool-produksie bevorder as gevolg van massa-en-hitte-oordrag
beperkinge in groter partikels en groter monster-groottes en gevolglik het beide monsters verhoogde
houtskool-opbrengs getoon. Verhoogde houtskool-opbrengs verskille tot 4 en 2 massa% was verkry
vir MP en LC onderskeidelik. Die opgeskaleerde optimale HVW resultate was 30.8 MJ kg-1 (614 ˚C)
en 31.6 MJ kg-1 (698 ˚C) vir LC en MP onderskeidelik.

ʼn Verbrandingstudie was gedoen op LC en MP houtskool (vanaf torrefaksie en pirolise) met gram-
skaal-PGV van 850 tot 2800 µm in ʼn TGA reaktor met die doel van steenkool substitusie. LC en MP
houtskool was gemeng met drie Suid-Afrikaanse steenkool in ʼn verhouding van 5 tot 90 massa%
(biomassa houtskool). Die verbrandingseienskappe-resultate het getoon dat LC houtskool meer
reaktief is as MP houtskool, aangesien dit die verbrandingstemperatuur van al die steenkool
aansienlik verlaag het. Gedurende gesamentlike-verbranding was dit bevind dat vir mengsels < 30%
is die verbrandingsindekse soortgelyk was die van die individuele steenkool, wat wys dat gedeeltelike
substitusie moontlik is sonder om merkbare veranderinge aan die bestaande toerusting hoef te maak.
Vir mengsels > 60% was die verbrandings-bedrywe beter met verhoogde verbrandingsindekse, maar
aangesien die verbranding baie vinniger was, sal daar waarskynlik veranderinge aan die bestaande
toerusting gemaak moet word.

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In opsomming, die studie het gewys dat indringer uitheemse plante, veral LC en MP, gebruik kan
word deur torrefaksie en stadige pirolise om houtskool te produseer vir direkte energietoepassings
deur gesamentlike-verbranding met steenkool. Dit was bevind dat LC monsters van torrefaksie by
300 ˚C, gelykwaardig is aan hoogsvlugtige bitumeniese C steenkool, terwyl gepiroliseerde houtskool
van LC en MP gelykwaardig was aan hoogsvlugtige bitumeniese B steenkool. Om die praktiese
moontlikehede van gesamentlike verbranding te bevestig, word dit voorgestel dat studies rakende die
opskalering na loodskaal gedoen word om die algehele energie-doeltreffendheid, tegno-ekonomiese,
en bedryftoestande van industriële reaktore te evalueer, sowel as ʼn lewens-siklus-evaluasie.

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Dedication
This thesis work is dedicated to my family, who have been very supportive during the entire long
journey!

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Acknowledgements
Primarily, I would like to thank my supervisor Prof. Johann Görgens for the professional and technical
guidance as well as the opportunity given for me to enrol for this work. I am especially indebted to
my co-supervisor Dr. François-Xavier Collard for his guidance, encouragement and support
throughout the course of this degree.

I would like to thank the Department of Process Engineering at Stellenbosch University and the
Copperbelt University from Zambia for their financial support which added value to this research
work. For all the various laboratory analyses conducted, I would like to thank the following technical
staff; Hanlie Botha, Levine Simmers and Alvin Petersen of Process Engineering, Cynthia Sanchez-
Garrido of Soil Science and Henry Solomon of Forestry and Wood Science.

Finally, but not the least, special thanks to the thermochemical process development research group
for all the memorable experiences we have had, good and bad, frustrating and up-lifting, they all
added value to this work. A special friendly thank you to David Naron, Frank Nsaful, Logan Brown,
Malusi Mkize and Angelo JJ Ridout (Dr.), who we shared technical and professional experiences in
our research group as we journeyed through this period of study.
Special thanks to Salomie Van der Westhuizen for the translation of the English version of the abstract
into Afrikaans.

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Table of contents
Declaration ........................................................................................................................................ i
Abstract ............................................................................................................................................ ii
Opsomming ..................................................................................................................................... iv
Dedication ...................................................................................................................................... vii
Acknowledgements ....................................................................................................................... viii
Table of contents ............................................................................................................................. ix
List of Figures ............................................................................................................................... xvi
List of Tables ................................................................................................................................ xix
List of acronyms and abbreviations .............................................................................................. xxi
Chapter 1 Introduction ...................................................................................................................... 1
1.1 Contextual background .............................................................................................................. 1
1.2 Thesis outline ............................................................................................................................. 4
1.3 References .................................................................................................................................. 6
Chapter 2 Literature review ........................................................................................................... 11
2.1 General overview ..................................................................................................................... 11
2.2 Use of IAPs as feedstock materials for char production .......................................................... 13
2.2.1 Lignocellulosic biomass for torrefaction and pyrolysis .................................................... 14
2.2.1.1 Cellulose..................................................................................................................... 15
2.2.1.2 Hemicelluloses ........................................................................................................... 16
2.2.1.3 Lignin ......................................................................................................................... 16
2.2.1.4 Extractives and inorganics ......................................................................................... 17
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