Lignocellulosic Ethanol Production: Studies on Sugarcane Bagasse, Paja Brava, Wheat Straw, Quinoa Stalks and Curupaú

Abstract

Lignocellulosic biomass refers to plant material that is composed mainly of cellulose, hemicellulose and lignin. These materials are today of large interest to researchers in the field of biofuel and bioenergy. Residues for forestry and agriculture are particularly interesting since they result from crops with established cultivation procedures and technology for harvest and transportation are already in place. In the current work, the utilisation of a number of biomass materials for bioethanol production was studied. The materials used were sugarcane bagasse, wheat straw, paja brava (the brave straw), quinoa stalks and hardwood (Anadenanthera colubrina). These materials were initially analysed to determine the contents of carbohydrates, lignin, extractives and ash.



The second part of the work dealt with the production of pentose-rich hydrolysates from steam pretreatment of the materials either without a catalyst or using SO2 or H2SO4 to enhance the hydrolysis. Most emphasis was placed on SO2-catalysed steam pretreatment, which was found to give high pentose sugar recovery from feedstocks such as sugarcane bagasse, paja brava, wheat straw, and quinoa stalks. For the pretreatment of Anadenanthera colubrina, known in Bolivia as the Curupaú tree, the use of H2SO4 was also investigated. By-products originating from the degradation of carbohydrates and lignin, were found in only low amounts in the hydrolysates obtained at suitably chosen conditions. The solid, cellulose rich, fraction of four materials was subjected to enzymatic hydrolysis (using commercial cellulases). In most cases, a relatively high degree of hydrolysis (typically > 80%) was obtained, at least at dilute conditions. Specific analytical efforts were made to analyse aromatic compounds in the liquid phase after pretreatment. Glycosylated aromatics, primarily arabinosylated p-coumaric and ferulic acids, were found in the liquid fraction after pretreatment. The concentrations showed a clear dependence on the severity conditions and the type of catalyst agent.



The third part of the work focused on the fermentabilities of sugarcane and paja brava hydrolysates. A genetically engineered xylose-utilizing strain of Saccharomyces cerevisiae (TMB3400) was found to give a high xylose conversion during fermentation of the xylose rich pretreatment hydrolysates from sugarcane bagasse and paja brava, whereas the yeast Pichia stipitis was found less robust in these hydrolysates. For both yeasts, the process option simultaneous saccharification and fermentation (SSF) was found to be preferable to separate hydrolysis and fermentation (SHF) in terms of overall yields.

Abstract (Swedish)

Popular Abstract in English

The world’s resources of fossil fuels are rapidly being depleted, and we now know that the combustion of these fuels leads to the emission of greenhouse gases resulting in climate change. It is therefore imperative that we develop renewable sources of fuel. The production of bioethanol in Brazil is the best example today of the introduction of a renewable fuel on a large scale. This ethanol is obtained from the fermentation of sugar from sugarcane. In fact, all ethanol today is obtained from the fermentation of sugar (from sugarcane or sugar beets), or starch (mainly from corn or wheat). This is sometimes referred to as “first generation” biofuel.



The term “second generation” or “advanced biofuels” refers to fuels derived from lignocellulosic biomass, which means that the whole plant is used (with the exception of the grain). This biomass is available in much larger quantities and does not compete with cultivation for food. However, this material is a complex mixture of macromolecules such as cellulose, hemicellulose and lignin, which are not directly fermentable. The overall resistance of lignocellulosic biomass to degradation is also high, or at least much higher than that of starch. Efficient methods are therefore needed to degrade, in particular, the carbohydrate polymers to obtain sugars that can then be fermented to ethanol, or other desired chemical building blocks, by various micro¬organisms. These questions are addressed in this thesis, through studies on chemical and biochemical methods of obtaining sugars from various kinds of biomass, and the fermentation of these sugars by various kinds of yeasts.



The materials studied here were “waste” biomass, such as sugarcane bagasse, which is the solid part of the sugarcane remaining after sugar extraction, wheat straw, Anadenanthera colubrina and quinoa stalks. Quinoa is a crop grown on the Bolivian Altiplano, as is paja brava (the “brave” straw). A. colubrina is a South American tropical hardwood. To obtain sugars from these kinds of materials, they first have to be subjected to steam treatment at a high temperature for a few minutes. This breaks down part of the carbohydrates, the hemicellulose part, which is a polymer consisting of many sugars, both six-carbon sugars and five-carbon sugars. Various kinds of catalysts, typically acids, can be used, and in this study SO2 was found to be a good catalyst for many feedstocks. After this pretreatment, enzymes can be used to break down the cellulose in the remaining material. Optimal conditions for pretreatment were determined in this work. The hemicellulosic sugar yields obtained from the lignocellulosic materials studied ranged from 67 to 80% of the theoretical yield, which represents about 15-18 g of xylose per 100 g dry matter for a typical straw material.



Studies were also carried out on fermentation, in particular the design of a process allowing co-fermentation of both hexose (six-carbon) and pentose (five-carbon) sugars. Both natural and genetically engineered yeast strains were evaluated. A genetically modified strain of S. cerevisiae was found to be more tolerant to hydrolysates than the natural xylose fermenting yeast P. stipitis, and showed higher ethanol yields (0.40-0.43 g/g) in the Bolivian straw material.



Considerable efforts were made to analyse the liquid fraction after pretreatment. In addition to the sugars and the degradation products of the sugars, aromatic compounds were found in the pretreatment liquid. In this work, glycosylated aromatics were also found, which show binding structures in the original biomass linking the lignin to the hemicellulose.



In general, lignocellulosic materials studied in this work have a great potential as ethanol feedstocks and other biorefinery applications. In particular, fractionation processes of these materials propose ways to produce valuable building blocks, i.e. glycosylated aromatics. These studies in this thesis are believed to contribution to novel biomass conversion processes.

• author: Carrasco, Cristhian
• supervisor: Gunnar Lidén
• opponent: Professor Jönsson, Leif, Department of Chemistry, Umeå University, Sweden
• organization: Division of Chemical Engineering
• publishing date: 2013
• type: Thesis
• publication status: published
• subject: Chemical Engineering
• keywords: Steam pretreatment, ethanol production, SSCF, arabinosylated aromatics, SO2, H2SO4, sugarcane bagasse, paja brava, wheat straw, quinoa stalks, Anadenanthera colubrina
• pages: 210 pages
• defense location: Hall B, Center for Chemistry and Chemical Engineering, Getingevägen 60, Lund, Sweden
• defense date: 2013-05-24 10:00:00
• ISBN: 978-91-7422-320-0
• language: English
• LU publication?: yes
• id: ac176c12-50a7-46c6-a799-a727c47e8483 (old id 3706234)
• date added to LUP: 2016-04-04 09:59:18
• date last changed: 2018-11-21 20:56:03