Integrated bioenergy and animal feed production from AFEX TM and steam exploded sugarcane residues

Mokomele, Thapelo (2019-03)

Thesis (PhD)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Current and future trends demonstrate that the increasing world population, dwindling arable land, changing human diets and increased demand for (bio)energy present an opportunity to redesign the way land is used to meet the future food, feed and bioenergy demands. The sustainable integration of bioenergy and highly digestible livestock feed production systems has been touted as a potential avenue to increase the economic returns to agriculture and simultaneously promote energy security, particularly in developing countries. To this end, post-harvest residues from sugarcane processing (i.e. sugarcane bagasse (SCB) and cane leaf matter (CLM)) have emerged as candidate feedstock for integrated bioenergy (e.g. bio-ethanol and biogas) and animal feeds production in South Africa and Brazil. The principal aim of this dissertation was to perform a systematic comparison of the potential use of steam explosion (StEx) and ammonia fiber expansion (AFEXTM) as pretreatment technologies to overcome biomass recalcitrance, thereby generating highly digestible animal feeds, and cellulosic ethanol and biogas production feedstocks from sugarcane residues for future integrated biofuel-animal feed systems. A side-by-side comparison of the effect of StEx and AFEXTM pretreatment of sugarcane residues revealed AFEXTM to be the better pretreatment for maximising ethanol yields per Mg raw dry material (RDM) from both SCB and CLM. Under industrially relevant solids loadings of 16% and dosages of 9.8 mg protein/g RDM, AFEXTM pretreated sugarcane residues generated ethanol yields up to 324 litres/Mg RDM, the highest ethanol yields reported in literature from sugarcane residues. In contrast, ethanol yields from steam exploded sugarcane residues were limited to the range 205 to 257 litres/Mg RDM primarily due to the compounded effect of carbohydrate degradation during pretreatment, enzyme inhibition and microbial inhibition of recomninant Saccharomyces cerevisiae 424A (LNH-ST) during fermentation. To debottleneck microbial inhibition during the fermentation of non-detoxified StEx whole slurry’s, the potential use of industrial xylose-fermenting S. cerevisiae strains as efficient and inhibitor tolerant ethanologens was evaluated. S. cerevisiae strains CelluXTM 4 and TP-1 demonstrated near complete glucose and xylose consumption, with high acetate resistance, furan detoxification and phenolic aldehyde detoxification phenotypes. Ultimately, both strains facilitated the generation of 224 litres/Mg RDM from non-detoxified StEx SCB whole slurry under a pre-hydrolysis simultaneous saccharification and co-fermentation (PSSCF) configuration. In comparison, the same yeast strains generated moderately higher ethanol yields (254 litres/Mg RDM) during the PSSCF of highly fermentable AFEXTM-treated SCB, demonstrating that the difference in the potential ethanol yields that can be recovered from the two pretreatment technologies can be significantly reduced by using inhibitor-tolerant ethanologens. With both AFEXTM and StEx-treated sugarcane residues requiring enzyme dosages of 9.8mg protein/g RDM to achieve high ethanol yields, the potential use a room-temperature Cellulose IIIIactivation (CIIII-activation) process to enhance the digestibility of StEx- or AFEXTM-treated sugarcane residue pellets was investigated as a potential strategy to minimise the enzyme cost contribution per unit volume ethanol produced. Coupling AFEXTM sugarcane lignocelluloses with CIIII-activation reduced of the enzyme dosage requirements by more than 60% (to ~3 mg protein/g RDM), whilst achieving ethanol yields greater than 280 litres/Mg RDM. These results represented the lowest enzyme dosage to achieve ethanol yields of 280 L/Mg RDM reported in literature. In contrast, upgrading StEx-treated sugarcane residue pellets could only facilitate ethanol yields up to 201 litres/Mg RDM at an enzyme dosage of ~3 mg protein/g RDM. Besides ethanol production, both AFEXTM and StEx also demonstrated significant improvements in the animal feed value of SCB and CLM. AFEXTM-treated sugarcane residues were characterized by 230% increase in the non-protein nitrogen content of the biomass, and up to 69% and 26% improvement in the in-vitro true digestibility (IVTD) and metabolizable energy (ME), respectively, relative to untreated controls (P < 0.05). Although StEx did not increase the nitrogen content of the pretreated sugarcane residues, the IVTD and ME of StEx-treated SCB and CLM were improved by 54% and 7%, respectively (P < 0.05). These results demonstrated that both AFEX and StEx pretreatment can simultaneously generate highly digestible animal feeds and enhanced cellulosic ethanol feedstocks from sugarcane residues. The combination of the near optimal C/N ratios and structural modifications of AFEXTM-treated sugarcane residues also facilitated biogas production with methane yields up to 299 L CH4/kg VS, with or without co-digestion with dairy cow manure (DCM). To obtain comparable methane yields, untreated and steam exploded (StEx) sugarcane residues had to be co-digested with DCM, at mass ratios providing initial C/N ratios in the range of 18 to 35. Furthermore, the solid digestates recovered from the co-digestion of the sugarcane lignocelluloses with DCM were enriched in nitrogenphosphate-potassium (NPK), suggesting that they could be used as biofertilizers or partial replacements for the CLM that is typically left on the field during green cane harvesting. The results from this dissertation showed that both AFEXTM and StEx successfully enhanced the ethanol production potential, methane production potential, and animal feed value of sugarcane residues, providing alternative models for the sugarcane industry to create bioenergy and food value from sugarcane residues. Ultimately, these results provide essential information and insights for future techno-economic and life-cycle analyses that are required to establish the preferred pretreatment technology and processing strategies to enable economically viable and environmentally sustainable integrated bioenergy and animal feed production from South African sugarcane residues.

AFRIKAANSE OPSOMMING: Huidige en toekomstige tendense dui daarop dat die toename in die wêreldbevolking, drastiese afname in bewerkbare grond, verandering in menslike diëte en verhoging in die vraag na (bio-)energie ’n geleentheid bied vir die herontwerp van grondgebruik om in toekomstige voedsel-, voer- en bio-energiebehoeftes te voorsien. Die volhoubare integrasie van produksiestelsels vir bioenergie en hoogs verteerbare veevoer word as ’n moontlikheid beskou om die ekonomiese opbrengs van landbou te verhoog en terselfdertyd energiesekerheid te bevorder, veral in ontwikkelende lande. Die naoesreste van suikerrietverwerking (d.w.s. suikerrietbagasse (SRB) en rietblaarmateriaal (RBM)) word as kandidaatvoerstof vir geïntegreerde bio-energie- (bv. bio-etanol- en biogas-) en veevoerproduksie in Suid-Afrika en Brasilië beskou. Die hoofdoel van hierdie verhandeling was om ’n stelselmatige vergelyking te onderneem van die moontlike gebruik van stoomontploffing (“StEx”) en ammoniakveseluitsetting (AFEXTM) as voorbehandelingstegnologieë om enersyds die biomassaweerspannigheid van suikerrietreste te bowe te kom en sodoende hoogs verteerbare veevoer te skep, en andersyds sellulosiese etanol- en biogasproduksievoerstof uit suikerrietreste te vervaardig vir toekomstige geïntegreerde biobrandstof-veevoerstelsels. Wanneer die uitwerking van StEx- en AFEXTM-voorbehandeling van suikerrietreste naas mekaar beskou word, blyk AFEXTM die beter voorbehandeling te wees vir maksimum etanolproduksie per Mg onverwerkte droëmateriaal (ODM) vir sowel SRB as RBM. Met industrieel relevante vastestofladings van 17% en ’n dosis van 9,8 mg proteïen/g ODM, bied AFEXTM-voorbehandelde suikerrietreste ’n etanollewering van tot 324 liter/Mg ODM, synde die hoogste etanollewering uit suikerrietreste wat tot nog toe in die literatuur aangemeld is. Daarteenoor is die etanollewering van stoomontplofte suikerrietreste beperk tot tussen 205 en 257 liter/Mg ODM, hoofsaaklik weens die saamgestelde uitwerking van koolstofafbreking gedurende voorbehandeling, ensieminhibisie en mikrobiese inhibisie van Saccharomyces cerevisiae 424A (LNH-ST) gedurende fermentasie. Om die bottelnek van mikrobiese inhibisie gedurende die fermentasie van niegedetoksifiseerde StEX-ru-flodder uit die weg te probeer ruim, is die potensiële gebruik van industriële xilose-fermenterende S. cerevisiae-stamme ook ondersoek. Die S. cerevisiae-stamme CelluXTM 4 en TP1 het byna volledige glukose- en xiloseverbruik, hoë asetaatweerstandigheid, furaandetoksifikasie- én fenoliese-aldehied-detoksifikasiefenotipes getoon, en het uiteindelik ’n etanollewering van 224 liter/Mg ODM gebied in ’n konfigurasie van pre-hidrolise- gelyktydige versuikering en gesamentlike fermentasie (“PSSCF”). Daarteenoor het die PSSCF van hoogs fermenteerbare AFEXTMbehandelde SRB ’n effens hoër etanollewering getoon (254 liter/Mg ODM), wat daarop dui dat die verskil in die moontlike etanollewering van die twee voorbehandelingstegnologieë beduidend verminder kan word met behulp van inhibitorverdraagsame etanologene. Daarbenewens is daar ondersoek ingestel na die opgradering van verpilde StEx- of AFEXTMbehandelde suikerrietreste deur middel van ’n CIIII-aktiveringsproses (CIIII) by kamertemperatuur om die bottelnek van die hoë ensiemdosisvereistes verbonde aan hoë etanollewering te probeer verwyder. Die kombinasie van AFEXTM-suikerrietlignosellulose en CIIII het die ensiemdosisvereistes met meer as 60% verlaag (tot ~3 mg proteïen/g ODM) en etanollewering tot meer as 280 liter/Mg ODM verhoog. Hierdie resultaat is die laagste ensiemdosis vir ’n etanollewering van 280 L/Mg ODM wat tot dusver in die literatuur aangemeld is. Daarteenoor het die opgradering van verpilde StEx-behandelde suikerrietreste ’n etanollewering van slegs 201 liter/Mg ODM by ’n ensiemdosis van ~3 mg proteïen/g ODM teweeggebring. Benewens die uitwerking op etanolproduksie, blyk sowel AFEXTM as StEx ook aansienlike verbeteringe in die veevoerwaarde van SRB en RBM tot gevolg te hê. Die biomassa van AFEXTMbehandelde suikerrietreste het tipies oor ’n 230% hoër nieproteïenstikstofinhoud beskik, en in vitroware verteerbaarheid (IVWV) en metaboliseerbare energie (ME) was onderskeidelik 69% en 26% hoër as by onbehandelde kontroles (P < 0,05). Hoewel StEx nie die stikstofinhoud van die voorbehandelde suikerrietreste verhoog het nie, het die IVWV en ME van StEx-behandelde SRB en RBM met onderskeidelik 54% en 7% verhoog (P < 0,05). Hierdie resultate toon dat sowel AFEX- as StExvoorbehandeling terselfdertyd hoogs verteerbare veevoer én beter sellulosiese etanolvoerstof uit suikerrietreste kan oplewer. Die kombinasie van die byna optimale C/N-verhoudings en strukturele aanpassings van AFEXTMbehandelde suikerrietreste het ook biogasproduksie teweeggebring, met ’n metaanlewering van tot 299 L CH4/kg VS, met óf sonder gesamentlike vertering met melkbeesmis (MBM). Om vergelykbare metaanlewering te verkry, moes onbehandelde en stoomontplofte (StEx-) suikerrietreste saam met MBM verteer word, wat op massaskaal aanvanklike C/N-verhoudings van tussen 18 en 35 gelewer het. Daarbenewens was die vaste digestate wat uit die gesamentlike vertering van die suikerrietlignosellulose en MBM herwin is, ryk in stikstof-fosfaat-kalium (NPK), wat daarop dui dat dit as biobemesting of gedeeltelike plaasvervanger kan dien vir die RBM wat gewoonlik gedurende groen oesting op landerye agterbly. Die resultate van hierdie studie toon dat sowel AFEXTM as StEx die etanolproduksiepotensiaal, metaanproduksiepotensiaal en veevoerwaarde van suikerrietreste suksesvol verhoog, en sodoende die suikerrietbedryf van alternatiewe modelle voorsien om bio-energie en voedselwaarde te skep. Die bevindinge bied noodsaaklike inligting en insigte vir toekomstige tegno-ekonomiese en lewensiklusontledings om te bepaal watter voorbehandelingstegnologie en verwerkingstrategieë die beste sal werk om geïntegreerde bio-energie- en veevoerproduksie uit Suid-Afrikaanse suikerrietreste ekonomies lewensvatbaar en omgewingsvolhoubaar te maak.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105670
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