Characterization and fermentation of waste paper sludge for bioethanol production

Boshoff, Sonja (2015-12)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The need for renewable energy sources are at an unprecedented high due to the world population and energy demand increasing drastically past the point that the remaining fossil fuels are able to supply. Biomass is a sustainable and renewable source of energy with the potential to mitigate greenhouse gas emissions and to serve as an alternative to fossil fuels when converted into biofuels, such as bioethanol or biodiesel. Paper sludge (PS) is a biomass waste stream from the paper and pulp industry that is often landfilled. By converting PS into bioethanol, landfill can be avoided and an energy stream can be produced to be used at the mill or sold. This study investigated the conversion of PS into ethanol and how the nature of the sludge influenced a high solid loading fermentation process. Paper sludge samples from various paper and pulp mills in South Africa were collected and characterized into categories according to chemical composition and the feed utilized at each mill. Significant variation was observed in the chemical composition between mills, whereas clear correlations were observed within categories utilizing the same feed. Screening for fermentation performance also revealed substantial variation due to the differences in digestibility of the samples Based on characterization and screening data, samples from two categories, namely corrugated recycle mills and virgin pulping mills were chosen for further investigation and optimization. Selecting PS samples with high digestibility to ensure maximum ethanol yield and productivity is a critical requirement for process efficiency. However, the PS samples differed substantially in terms of viscosity. Virgin pulp PS, originating from a chemical pulping process, had a significantly higher water holding capacity and viscosity compared to corrugated recycle PS, originating mainly from recycling and repulping operations. These differences affected the maximum solid loading that could be achieved in fermentations, and inherently, the enzymatic hydrolysis of the material where high viscosity would limit enzyme accessibility to the fibers. Given the viscous nature of virgin pulp PS, solids loadings of between 3 to 9% (w/w) achieved the maximum PS hydrolysis to sugar, whereas for corrugated recycle PS the maximum enzymatic hydrolysis was achieved at substantially greater solids loadings of 15% (w/w) and higher. The optimised process with corrugated recycle PS resulted in an ethanol concentration and yield of 45.5 g/L and 78.2 %, respectively, at a solid loading of 27% (w/w) and an enzyme dosage of 11 FPU/gram dry sludge. The optimised process for the virgin pulp PS required a significantly higher enzyme dosage of 20 FPU/gram dry sludge at a lower solid loading of 18% (w/w), to achieve the optimum ethanol concentration and yield of 34.2 g/L and 66.9% (w/w), respectively. The virgin pulp PS was highly viscous at 18% (w/w) and required high agitation of 1500 rpm that, in turn, had a negative effect on enzyme activity from shear stress of the agitator. This study demonstrated that corrugated recycle PS is more suited for bioethanol production compared to virgin pulp PS, primarily due to water holding capacity, viscosity and shear stress associated with high agitation rates, which had a major influence on high solids loading fermentation processes.

AFRIKAANSE OPSOMMING: Die behoefte aan hernubare energiebronne het ongekende hoogtes bereik weens die aanwas in die wêreldpopulasie asook die vraag na energie wat die punt drasties oorskry het waar fossielbrandstowwe aan hierdie aanvraag kan voldoen. Biomassa is ʼn volhoubare energiebron met die potensiaal om groenhuisgasvrystelling te bekamp en om as alternatief tot fossielbrandstowe te dien wanneer dit na biobrandstof omgeskakel word. Papierslyk (PS) is ʼn biomassa-ryke afvalstroom van die papier en pulp industrie wat gebruiklik vir stortingsterreine bestem is. Hierdie vermorsing van potensiële energie kan verhoed word deur PS na bioetanol om te skakel. ʼn Energiestroom kan dus gegenereer word wat by meule gebruik of verkoop kan word. In hierdie studie is die omskakeling van PS na etanol ondersoek asook hoe die eienskappe van die slyk die fermentasieproses by hoë soliedemateriaalladings beïnvloed het. Papierslykmonsters is van Suid Afrikaanse papier en pulp meule ingesamel en volgens chemiese samestelling, en die aard van die voer by die meule, in kategorieë gegroepeer. Beduidende variasie in die chemiese samestelling van monsters tussen verskillende meule is waargeneem, terwyl daar duidelike korrelasies binne kategorieë was wanneer dieselfde voer vir meule gebruik was. ʼn Siftingsproses op grond van fermentasiewerkverrigting het ook op aansienlike variasie gedui, weens verskille in die monsters se verteerbaarheid. Op grond van karakteriserings- en siftingsdata is monsters van twee kategorieë, nl. geriffelde hersirkulerende meule en reinpulpmeule vir verdere ondersoek en optimering gekies. Die seleksie van PS monsters met hoë verteerbaarheid is ʼn kritiese vereiste ten einde etanol opbrengs en produktiwiteit te optimeer, en die proses se effektiwiteit te maksimeer. Die PS monsters het taamlik in terme van viskositeit verskil. Reinpulp, wat vanuit die chemiese pulpproses afkomstig is, het ʼn aansienlike hoër waterhouvermoë en viskositeit vergeleke met geriffelde hersirkuleerde PS, wat meestal uit hersirkulerings- en herpulprosesse afkomstig is. Hierdie verskille het die maksimum lading van soliede materiaal moontlik in fermentasieprosesse, en wesenlik die ensiematiese hidroliese van die materiaal beïnvloed, waar hoë viskositeit toegang van ensieme tot die vesels beperk. Gegewe die viskeuse aard van die reinpulp was hidroliese van PS na suikers maksimaal by soliedemateriaalladings van tussen 3 en 9% (m/m), terwyl geriffelde hersirkuleerde PS se hidroliese na suiker maksimaal by aansienlik hoër soliedemateriaalladings van 15% (m/m) en meer was. Die geoptimeerde proses met geriffelde hersirkuleerde PS het ʼn etanol konsentrasie en opbrengs van onderskeidelik 45.5 g/L en 78.2 % by ʼn soliedemateriaallading van 27% (m/m) en ensiemdosering van 11 FPE/gdm tot gevolg gehad. Daarenteen was die proses met reinpulp by ʼn beduidende hoër ensiemdosering van 20 FPE/gdm en laer soliedemateriaallading van 18% (m/m) optimaal, waar ʼn etanol konsentrasie en opbrengs van onderskeidelik 34.2 g/L en 66.9% (m/m) aangeteken is. Die reinpulp was by ʼn lading van 18% (m/m) baie viskeus wat ʼn baie hoë roersnelheid van 1500 opm vereis het. Om die beurt het die hoë roersnelheid hoë sleurkragte tot gevolg gehad wat negatiewe effekte op die stabiliteit van die ensieme gehad. In hierdie studie is gedemonstreer dat vergeleke met reinpulp, geriffelde hersirkuleerde PS meer geskik vir bioetanolproduksie is, hoofsaaklik as gevolg van waterhouvermoë, viskositeit en sleurkragte, saam met hoë roersnelhede, wat ʼn groot invloed op hoësoliedlading fermentasieprosesse gehad het.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98028
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