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Selection of preferred sweet sorghum cultivars and their pretreatment optimisation for bio-ethanol production

McIntosh, Paul Andrew (2013-03)

Thesis (MScEng)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: Currently the world is facing a global energy crisis due to rising energy demands, dwindling fossil fuels and increased greenhouse gas emissions. Countries are therefore looking at reducing fossil fuel reliability, mitigating emissions and meeting sustainable development goals. One such initiative has been the replacement of fossil fuel with bio-ethanol, which is both renewable and sustainable. Locally in the South African context, one of the potential bio-ethanol feed stocks under investigation is sweet sorghum. This feedstock is high in both soluble and non-structural sugars, it is drought resistant, has low input requirements, has high yields and can be grown over a wide range of climatic conditions. Thirty-six sweet sorghum cultivars were therefore collected from the University of Kwa-Zulu Natal, where they were grown, before being transported to the University of Stellenbosch where they were they underwent a selection process coupled with a pretreatment optimization process aimed at maximizing potential bio-ethanol yields. The initial thirty-six cultivars were screened at a small scale with dilute sulfuric acid pretreatment at 170⁰C, 15 minutes and 0.7% H2SO4 combined with enzymatic hydrolysis which was carried out at an enzyme loading of 15FPU/g water insoluble solids (WIS), a pH of 4.8, a temperature of 50⁰C and a residence time of 72 hours Results showed statistically that increasing lignin and ash content negatively affected pretreatment response resulting in a range of combined sugar yields for the initial thirty-six cultivars of between 32.64g/100g raw material and 44.04g/100g raw material. From these results, the thirty six cultivars were reduced to ten by inclusion of pretreatment response yields and agronomic factors into total estimated ethanol yields from the whole plant. The top ten ranking ethanol producing sweet sorghum cultivars selected were SS27, AS254, AS246, AS103, AP6, AS106, MSJH13, AS245, AS248 and AS79. Utilising two low severity pretreatments namely, 190⁰C, 5 minutes, 0.25% (w/w) H2SO4 and 200⁰C, 5 minutes, 0.07% (w/w) H2SO4 combined with two enzyme loadings of 3.75FPU/g WIS and 15FPU/g WIS, the previously selected ten cultivars were further evaluated and reduced to five through ranking of the average total potential ethanol yields for the two pretreatment conditions and their corresponding enzyme loadings. The five cultivars which performed well under these conditions were AP6, SS27, AS103, MSJH13 and AS246 which subsequently underwent further optimization. Conditions investigated were 180 - 190⁰C, 5 – 15 minutes and 0.25% H2SO4 which resulted in an increase in combined sugar yields to between 48.83 and 54.5g/100g raw material on an oven dry basis. Selection of three preferred cultivars was based on the best average total potential ethanol yields calculated for the two seasons at each cultivars optimum pretreatment condition. Further one of the selected cultivars had to have poor pretreatment response at a small scale to allow for the effect of chemical composition to be evaluated in the steam explosion pilot plant. The three selected cultivars were AP6, SS27 and AS246. Of the five previously selected sweet sorghum cultivars, AP6, SS27 and AS246 underwent optimization in a steam explosion reactor under air dried, water soaked and SO2 catalysed conditions. Cultivars performed similarly under air dried and water soaked conditions but varied under SO2 catalysed conditions. A 3% SO2 catalysed steam explosion was most promising with yields of 87.2% to 91.48% of theoretical sugar content in the native biomass for the 3 preferred Sweet Sorghum cultivars. Combined with agronomic data this translated into potential bio-ethanol yields from the whole plant of between 7131 and 8678 L/ha grown under South African conditions. While these results are promising, further development of the three preferred sweet sorghum cultivars, AP6, SS27 and AS246, is necessary to implement these as dedicated bio-energy feed stocks.

AFRIKAANSE OPSOMMING: Tans die wêreld in die gesig staar 'n wêreldwye energiekrisis as gevolg van stygende energie vereistes, kwynende fossielbrandstowwe en verhoogde kweekhuisgasvrystellings. Lande is dus op soek op die vermindering van fossielbrandstof betroubaarheid, die vermindering van die uitstoot en die bereiking van volhoubare ontwikkeling doelwitte. Een so 'n inisiatief is die vervanging van fossielbrandstof met bio-etanol, wat beide hernubare en volhoubare. Plaaslik in die Suid-Afrikaanse konteks, een van die potensiële bio-etanol feed lêers wat ondersoek is Sweet Sorghum. Hierdie grondstof is hoog in beide oplosbare en nie-strukturele suikers, dit is droogtebestand, het 'n lae inset, het 'n hoë opbrengste en kan oor 'n wye verskeidenheid van klimaatstoestande gekweek word. Ses-en-dertig soet sorghum-kultivars is dus versamel van die Universiteit van Kwa-Zulu Natal, waar hulle groot geword het, voordat dit vervoer word na die Universiteit van Stellenbosch waar hulle was dat hulle 'n seleksie proses, tesame met 'n behandeling optimalisering proses wat daarop gemik is die maksimalisering van potensiële bio ondergaan -etanol opbrengste. Die aanvanklike 36 kultivars is gekeur op 'n klein skaal met verdunde swawelsuur suur behandeling by 170⁰C, 15 minute en 0.7% H2SO4 gekombineer met ensiematiese hidrolise wat is uitgevoer uit op 'n ensiem laai van 15FPU / g water onoplosbare vastestowwe (WIS), 'n pH van 4.8, 'n temperatuur van 50⁰C en 'n verblyf tyd van 72 uur Resultate het getoon statisties dat die verhoging van lignien en ash inhoud negatief beïnvloed behandeling reaksie wat lei tot 'n reeks van gekombineerde suiker opbrengste vir die aanvanklike 36 kultivars van tussen 32.64g/100g rou materiaal en 44.04g/100g rou materiaal. Uit hierdie resultate is, is die 36 kultivars tot tien verminder deur die insluiting van behandeling reaksie opbrengste en agronomiese faktore in totale geraamde etanol opbrengste van die hele plant. Die top tien posisie etanol vervaardiging van soet sorghum cultivars gekies was SS27, AS254, AS246, AS103, AP6, AS106, MSJH13, AS245, AS248 en AS79. Benutting van 2 lae erns vir wysigings naamlik, 190⁰C, 5 minute, 0.25% (w / w) H2SO4 en 200⁰C, 5 minute, 0.07% (w / w) H2SO4 gekombineer met 2 ensiem beladings van 3.75FPU /g WIS en 15FPU / g WIS , is verder die voorheen gekies tien cultivars geëvalueer en na vyf verminder deur die posisie van die gemiddelde totale potensiële etanol opbrengste vir die twee behandeling voorwaardes en hul ooreenstemmende ensiem beladings. Die vyf kultivars wat goed presteer onder hierdie toestande was AP6, SS27, AS103, MSJH13 en AS246 wat daarna verdere optimalisering ondergaan. Voorwaardes ondersoek was 180 - 190⁰C, 5 - 15 minute en 0,25% H2SO4 wat gelei het tot 'n toename in die gekombineerde suiker opbrengste tot tussen 48.83 en 54.5g/100g rou materiaal op 'n oond droog basis. Keuse van drie voorkeur kultivars is gebaseer op die beste gemiddelde totale potensiële etanol opbrengste bereken vir die twee seisoene by elke kultivars optimale behandeling toestand. Verdere een van die geselekteerde kultivars het swak behandeling reaksie te hê op 'n klein skaal toe te laat vir die effek van chemiese samestelling om geëvalueer te word in die stoom ontploffing pilot plant. Die drie geselekteerde kultivars was AP6, SS27 en AS246. Van die vyf voorheen gekies soet sorghum cultivars, AP6, SS27 en AS246 optimalisering in 'n stoom-ontploffing reaktor onder lug droog, water geweek en SO2 gekataliseerde voorwaardes ondergaan. Kultivars uitgevoer insgelyks onder lug gedroogde en water geweekte toestande, maar varyied onder SO2 gekataliseerde voorwaardes. 'N 3% SO2 gekataliseerde stoom ontploffing was die mees belowende met opbrengste van 87,2% tot 91.48% van die teoretiese suiker inhoud in die inheemse biomassa vir die dire voorkeur sweet sorghum cultivars. Gekombineer met agronomiese data vertaal in potensiële bio-etanol opbrengste van die hele plant van tussen 7131 en 8678 L / ha gekweek onder Suid-Afrikaanse toestande. Terwyl hierdie resultate is belowend, verdere ontwikkeling van die drie voorkeur soet sorghum kultivars, AP6, SS27 en AS246, is nodig om hierdie as toegewyde bio-energie voer lêers uit te voer.

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