Uncatalysed steam pretreatment regimes for bagasse and harvest residues in a sugarcane biorefinery

Hamann, Martin Louis (2020-03)

Thesis

ENGLISH ABSTRACT: Biorefining of sugarcane lignocellulosic wastes, i.e. bagasse and harvest residues, at existing cane sugar mills can diversify product ranges to potentially improve profitability. Bagasse is the fibrous residue after juice extraction from the cane and harvest residues are all agricultural wastes generated during sugarcane harvesting. Pretreatment, the first step in bioprocessing of lignocellulose, must ensure maximum yields of desired sugar intermediates (glucose, xylose and arabinose) according to lignocellulosic feedstock and downstream bioconversion requirements. This study chose steam pretreatment for its proven track record in largescale operations and availability of steam at sugar mills, and was studied uncatalysed to allow operation without chemicals addition. The originality of this study firstly included the direct comparison of bagasse and harvest residues pretreatment operability in the same equipment. Uncatalysed steam pretreatment of these feedstocks was optimised by response surface methodology in the ranges of 185 – 215 °C and 5 – 15 min for maximum digestibility (indication of cellulose accessibility to enzymes) of the solids, maximum combined sugar yield (CSY) and maximum hemicellulose recovery to identify preferred pretreatment operating regimes. Secondly, deacetylation (acetyl groups removed from lignocellulose with a mild alkaline extraction) upstream of uncatalysed steam pretreatment was proposed as a detoxification step to remove acetic acid in downstream fermentation. Uncatalysed steam pretreatment of raw and deacetylated bagasse and harvest residues was compared on digestibility, fermentability (portion of maximum theoretical ethanol yield that can be obtained) and dewaterability of the pretreated material. Thirdly, the contribution of the explosion step during steam pretreatment to improving digestibility of the pretreated solids was investigated by comparing material retained and released during the sudden depressurisation at the end of pretreatment. The most significant findings of this study included that sugarcane harvest residues were better suited than bagasse for biorefining via a sugar platform for the following reasons: (1) harvest residues allowed for robust uncatalysed steam pretreatment at a single condition (between 198 and 200 °C, and between 8 and 12 min) to obtain at least 95% of the maximum digestibility, CSY and hemicellulose recovery values, respectively; (2) maximum digestibility was obtained at lower severity (205.8 °C, 8.7 min as opposed to 215 °C, 15 min for bagasse); (3) pretreated harvest residues did not require detoxification at optimum pretreatment conditions; and (4) pretreated harvest residues displayed better dewaterability, especially when pretreated for maximum hemicellulose recovery, compared to bagasse. Furthermore, it was found that the mechanical impingement experienced by steam pretreated material as it is transported during depressurisation, has a significant contribution to improving digestibility. Also, it was found that deacetylation of feedstocks can be performed prior to uncatalysed steam pretreatment as a detoxification step that could potentially increase fermentability of pretreated pressed slurries (not washed). Consequently, it is proposed that bagasse continues to be used to generate steam and electricity in a sugar mill, while attention should be given to the recovery, collection and allocation of sugarcane harvest residues as an attractive sugarcane biorefinery feedstock.

AFRIKAANSE OPSOMMING: Bioraffinering van suikerriet lignosellulose-afval, i.e. bagasse en oesreste, by bestaande suikermeulens kan produkreekse diversifiseer en winsgewendheid moontlik verbeter. Bagasse is die veselreste na ekstraksie van suikersap uit suikerriet, terwyl oesreste alle plantaardige afval wat gegenereer word gedurende die oes van suikerriet insluit. Voorbehandeling, die eerste stap in bioprossesering van lignosellulose, moet die maksimale opbrengs van die verlangde intermediêre suikers (glukose, xilose en arabinose) verseker na gelang van die lignosellulosevoer en stroomaf bio-omsettingvereistes. Hierdie studie het stoomvoorbehandeling gekies vir sy bewese prestasies in grootskaalse bedrywe, asook vir die beskikbaarheid van stoom by suikermeulens, en is ongekataliseerd ondersoek om voorsiening te maak vir bedryf sonder die toevoeging van chemikalië. Die oorspronklikheid van hierdie studie het eerstens die direkte vergelyking in voorbehandelingsbedryfbaarheid tussen bagasse en oesreste in dieselfde toerusting ingesluit. Ongekataliseerde stoomvoorbehandeling van hierdie voere is geoptimeer deur gebruik te maak van responsie-oppervlak metodologie in die bestekke 185 – 215 °C en 5 – 15 min vir maksimum verteerbaarheid (indikasie van sellulose toeganklikheid tot ensieme) van soliede, maksimum gekombineerde suikeropbrengs en maksimum hemisellulose herwinning om voorkeur voorbehandelingsbedryfsregimes te identifiseer. Tweedens, deasetilering (asetielgroep verwydering uit lignosellulose met ‘n matig alkaliese ekstraksie) stroomop van ongekataliseerde stoomvoorbehandeling is as detoksifiseringstap voorgestel vir die verwydering van asynsuur in stroomaf fermentasie. Ongekataliseerde stoomvoorbehandeling van rou en gedeasetileerde bagasse en oesreste is vergelyk op grond van verteerbaarheid, fermenteerbaarheid (gedeelte van maksimum teoretiese etanolopbrengs wat behaal kan word) en ontwatering van die voorbehandelde materiaal. Derdens, die bydra van die ontploffingstap gedurende stoomvoorbehandeling om verteerbaarheid van voorbehandelde materiaal te verbeter, is ondersoek deur materiaal te vergelyk wat teruggehou is en vrygelaat is gedurende die skielike drukontlasting aan die einde van voorbehandeling. Die mees beduidende bevindings van hierdie studie het ingesluit dat suikerriet oesreste meer geskik was as bagasse vir bioraffinering via ‘n suikerplatform weens die volgende redes: (1) oesreste het toegelaat vir robuuste ongekataliseerde stoomvoorbehandeling by ‘n enkele kondisie (tussen 198 en 200 °C, en tussen 8 en 12 min) om ten minste 95% van die maksimum verteerbaarheid, gekombineerde suikeropbrengs en hemisellulose herwinning waardes respektiewelik te behaal; (2) maksimum verteerbaarheid is by ‘n minder strawwe kondisie verkry (205.8 °C, 8.7 min teenoor 215 °C, 15 min vir bagasse); (3) voorbehandelde oesreste het nie detoksifisering by die optimale voorbehandelingskondisies benodig nie; en (4) voorbehandelde oesreste het beter ontwatering getoon, veral wanneer daar voorbehandel is vir hemisellulose herwinning, vergeleke met bagasse. Verder is daar gevind dat die meganiese botsing, soos ondervind deur die voorbehandelde materiaal wanneer dit vervoer word gedurende drukontlasting, beduidend bydra tot die verteerbaarheid. Ook is gevind dat deasetilering van die voerstowwe voor ongekataliseerde voorbehandeling uitgevoer kan word as ‘n detoksifiseringstap vir potensieel verbeterde fermenteerbaarheid van gepersde flodders (ongewas). Gevolglik word dit voorgestel dat bagasse steeds benut word vir die opwekking van stoom en eletrisiteit in ‘n suikermeul, terwyl aandag geskenk moet word aan die herwinning, versameling en toekenning van suikerrietoesreste as ‘n belowende voerstof tot ‘n suikerrietbioraffinadery.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/109744
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