Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The world’s depleting fossil fuels and increasing greenhouse gas emissions have given rise to much
research into renewable and cleaner energy. Biomass is unique in providing the only renewable source of
fixed carbon. Agricultural residues such as Sugarcane Bagasse (SB) are feedstocks for ‘second generation
fuels’ which means they do not compete with production of food crops. In South Africa approximately 6
million tons of raw SB is produced annually, most of which is combusted onsite for steam generation. In
light of the current interest in bio-fuels and the poor utilization of SB as energy product in the sugar
industry, alternative energy recovery processes should be investigated. This study looks into the
thermochemical upgrading of SB by means of pyrolysis.
Biomass pyrolysis is defined as the thermo-chemical decomposition of organic materials in the absence of
oxygen or other reactants. Slow Pyrolysis (SP), Vacuum Pyrolysis (VP), and Fast Pyrolysis (FP) are
studied in this thesis. Varying amounts of char and bio-oil are produced by the different processes, which
both provide advantages to the sugar industry. Char can be combusted or gasified as an energy-dense fuel,
used as bio-char fertilizer, or upgraded to activated carbon. High quality bio-oil can be combusted or
gasified as a liquid energy-dense fuel, can be used as a chemical feedstock, and shows potential for
upgrading to transport fuel quality. FP is the most modern of the pyrolysis technologies and is focused on
oil production. In order to investigate this process a 1 kg/h FP unit was designed, constructed and
commissioned. The new unit was tested and compared to two different FP processes at
Forschungszentrum Karlsruhe (FZK) in Germany. As a means of investigating the devolatilization
behaviour of SB a Thermogravimetric Analysis (TGA) study was conducted. To investigate the quality of
products that can be obtained an experimental study was done on SP, VP, and FP.
Three distinct mass loss stages were identified from TGA. The first stage, 25 to 110°C, is due to
evaporation of moisture. Pyrolitic devolatilization was shown to start at 230°C. The final stage occurs at
temperatures above 370°C and is associated with the cracking of heavier bonds and char formation. The
optimal decomposition temperatures for hemicellulose and cellulose were identified as 290°C and 345°C,
respectively. Lignin was found to decompose over the entire temperature range without a distinct peak.
These results were confirmed by a previous study on TGA of bagasse.
SP and VP of bagasse were studied in the same reactor to allow for accurate comparison. Both these
processes were conducted at low heating rates (20°C/min) and were therefore focused on char production. Slow pyrolysis produced the highest char yield, and char calorific value. Vacuum pyrolysis produced the
highest BET surface area chars (>300 m2/g) and bio-oil that contained significantly less water compared
to SP bio-oil. The short vapour residence time in the VP process improved the quality of liquids. The
mechanism for pore formation is improved at low pressure, thereby producing higher surface area chars.
A trade-off exists between the yield of char and the quality thereof.
FP at Stellenbosch University produced liquid yields up to 65 ± 3 wt% at the established optimal
temperature of 500°C. The properties of the bio-oil from the newly designed unit compared well to bio-oil
from the units at FZK. The char properties showed some variation for the different FP processes. At the
optimal FP conditions 20 wt% extra bio-oil is produced compared to SP and VP. The FP bio-oil contained
20 wt% water and the calorific value was estimated at 18 ± 1 MJ/kg. The energy per volume of FP bio-oil
was estimated to be at least 11 times more than dry SB. FP was found to be the most effective process for
producing a single product with over 60% of the original biomass energy. The optimal productions of
either high quality bio-oil or high surface area char were found to be application dependent. / AFRIKAANSE OPSOMMING: As gevolg van die uitputting van fossielbrandstofreserwes, en die toenemende vrystelling van
kweekhuisgasse word daar tans wêreldwyd baie navorsing op hernubare en skoner energie gedoen.
Biomassa is uniek as die enigste bron van hernubare vaste koolstof. Landbouafval soos Suikerriet
Bagasse (SB) is grondstowwe vir ‘tweede generasie bio-brandstowwe’ wat nie die mark van
voedselgewasse direk affekteer nie. In Suid Afrika word jaarliks ongeveer 6 miljoen ton SB geproduseer,
waarvan die meeste by die suikermeulens verbrand word om stoom te genereer. Weens die huidige
belangstelling in bio-brandstowwe en ondoeltreffende benutting van SB as energieproduk in die
suikerindustrie moet alternatiewe energie-onginningsprosesse ondersoek word. Hierdie studie is op die
termo-chemiese verwerking van SB deur middel van pirolise gefokus.
Biomassa pirolise word gedefinieer as die termo-chemiese afbreking van organiese bio-materiaal in die
afwesigheid van suurstof en ander reagense. Stadige Pirolise (SP), Vakuum Pirolise (VP), en Vinnige
Pirolise word in hierdie tesis ondersoek. Die drie prosesse produseer veskillende hoeveelhede houtskool
en bio-olie wat albei voordele bied vir die suikerindustrie. Houtskool kan as ‘n vaste energie-digte
brandstof verbrand of vergas word, as bio-houtskoolkompos gebruik word, of kan verder tot geaktiveerde
koolstof geprosesseer word. Hoë kwaliteit bio-olie kan verbrand of vergas word, kan as bron vir
chemikalië gebruik word, en toon potensiaal om in die toekoms opgegradeer te kan word tot
vervoerbrandstof kwaliteit. Vinnige pirolise is die mees moderne pirolise tegnologie en is op bio-olie
produksie gefokus. Om die laasgenoemde proses te toets is ‘n 1 kg/h vinnige pirolise eenheid ontwerp,
opgerig en in werking gestel. Die nuwe pirolise eenheid is getoets en vegelyk met twee verskillende
vinnige pirolise eenhede by Forschungszentrum Karlsruhe (FZK) in Duitsland. Termo-Gravimetriese
Analise (TGA) is gedoen om die ontvlugtigingskenmerke van SB te bestudeer. Eksperimentele werk is
verrig om die kwaliteit van produkte van SP, VP, vinnige pirolise te vergelyk.
Drie duidelike massaverlies fases van TGA is geïdentifiseer. Die eerste fase (25 – 110°C) is as gevolg van
die verdamping van vog. Pirolitiese ontvlugtiging het begin by 230°C. Die finale fase (> 370°C) is met
die kraking van swaar verbindings en die vorming van houtskool geassosieer. Die optimale
afbrekingstemperatuur vir hemisellulose en sellulose is as 290°C en 345°C, respektiewelik, geïdentifiseer.
Daar is gevind dat lignien stadig oor die twede en derde fases afgebreek word sonder ‘n duidelike
optimale afbrekingstemperatuur. Die resultate is deur vorige navorsing op TGA van SB bevestig. SP en VP van bagasse is in dieselfde reaktor bestudeer, om ‘n akkurate vergelyking moontlik te maak.
Beide prosesse was by lae verhittingstempo’s (20°C/min) ondersoek, wat gevolglik op houtskoolformasie
gefokus is. SP het die hoogste houtskoolopbrengs, met die hoogste verbrandingsenergie, geproduseer. VP
het hootskool met die hoogste BET oppervlakarea geproduseer, en die bio-olie was weens ‘n dramatiese
afname in waterinhoud van beter gehalte. Die meganisme vir die vorming van ‘n poreuse struktuur word
deur lae atmosferiese druk verbeter. Daar bestaan ‘n inverse verband tussen die kwantiteit en kwaliteit
van die houtskool.
Vinnige pirolise by die Universiteit van Stellenbosch het ‘n bio-olie opbrengs van 65 ± 3 massa% by ‘n
vooraf vasgestelde optimale temperatuur van 500°C geproduseer. Die eienskappe van bio-olie wat deur
die nuwe vinnige pirolise eenheid geproduseer is het goed ooreengestem met die bio-olie afkomstig van
FZK se pirolise eenhede. Die houtskool eienskappe van die drie pirolise eenhede het enkele verskille
getoon. By optimale toestande vir vinnige pirolise word daar 20 massa% meer bio-olie as by SP en VP
geproduseer. Vinnige pirolise bio-olie het ‘n waterinhoud van 20 massa% en ‘n verbrandingswarmte van
18 ± 1 MJ/kg. Daar is gevind dat ten opsigte van droë SB die energie per enheidsvolume van bio-olie
ongeveer 11 keer meer is. Vinnige pirolise is die mees doeltreffende proses vir die vervaardiging van ‘n
produk wat meer as 60% van die oorspronklike biomassa energie bevat. Daar is gevind dat die optimale
hoeveelhede van hoë kwaliteit bio-olie en hoë oppervlakarea houtskool doelafhanklik is.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/5238 |
| Date | 12 1900 |
| Creators | Hugo, Thomas Johannes |
| Contributors | Knoetze, J. H., Gorgens, J. F., University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 195 p. : ill. |
| Rights | University of Stellenbosch |
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