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Novel system for the direct fermentation of pretreated lignocellulosic material to ethanol in a single reactor

Applicant Studer Michael Hans-Peter
Number 121934
Funding scheme Ambizione
Research institution Institut für Verfahrenstechnik ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Other disciplines of Engineering Sciences
Start/End 01.08.2009 - 31.07.2012
Approved amount 685'831.00
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All Disciplines (4)

Discipline
Other disciplines of Engineering Sciences
Mechanical Engineering
Experimental Microbiology
Chemical Engineering

Keywords (9)

cellulolytic enzymes; pretreatment; microbial consortium; lignocellulose; biomass; biofuel; enzymatic saccharification; direct fermentation; microbial consortium

Lay Summary (German)

Lead
Lay summary
Der Ersatz von fossilen Energieträgern mit Treibstoff-Ethanol, hergestellt aus erneuerbaren Lignozellulose-Materialien wie zum Beispiel Stroh oder Holz, ist eine wichtige Massnahme zur Eindämmung der klimaschädlichen CO2-Emissionen. Das Projekt verfolgt eine neue Strategie, welche es erlaubt, Ethanol in einem einfacheren und damit ökonomisch attraktiveren Prozess herzustellen.

Hintergrund: Die derzeitige Klimaerwärmung ist auf den Ausstoss von anthropogenen Treibhausgasen wie CO2 zurückzuführen, von denen ein grosser Teil im Transportwesen erzeugt wird. Eine wichtige Massnahme zur Eindämmung dieser Emissionen ist der vermehrte Einsatz von erneuerbaren Treibstoffen, wie das schon heute verwendete Ethanol, welches aber bisher meist auf Zuckerrohr oder Maisstärke als Rohstoff basiert und somit Nahrungs- und Futtermittel konkurrenziert. Alternativ, aber technisch schwieriger, kann Ethanol auch aus Lignozellulose-Materialien, wie zum Beispiel Stroh, Grüngut-Abfällen oder Holz, hergestellt werden. In dem derzeitigen Prozess wird in einem ersten Schritt die Biomasse thermo-chemisch vorbehandelt, um die nachfolgende enzymatische Spaltung von Zellulose in Einfachzucker zu ermöglichen, welche schliesslich mit Hilfe von Mikroorganismen zu Ethanol vergärt werden. Um die Kommerzialisierung von Lignozellulose-Ethanol voranzutreiben, muss dieser Prozess stark vereinfacht und kostengünstiger werden.

Ziel: In dem Projekt soll ein vereinfachter, integrierter Prozess, welcher die direkte Herstellung von Ethanol aus lignozellulosehaltiger Biomasse ermöglicht, entwickelt und untersucht werden. In dem vorgeschlagenen Verfahren werden die Herstellung der notwendigen Zellulose-spaltenden Enzyme, die Verzuckerung der Biomasse sowie die Fermentation der Zucker und gegebenenfalls die Vorabtrennung des Ethanols in einem einzigen kontinuierlichen Bioreaktor zusammengefasst.

Bedeutung: Die Verminderung von CO2-Emissionen mittels des teilweisen Ersatzes von fossilen mit erneuerbaren Treibstoffen ist ein global formuliertes Ziel, welches auch mit politischen Direktiven in der EU und in den USA erreicht werden soll. Diese Massnahmen erzeugen ein enormes Marktpotenzial und erfordern erhebliche Anstrengungen und Investitionen, um umgesetzt werden zu können. Forschungsprogramme, welche die Entwicklung von alternativen, emissionsarmen Energieträgern mit dem Potential zur kurz- bis mittelfristigen Kommerzialisierung zum Ziel haben, sind hierfür unumgänglich.
Direct link to Lay Summary Last update: 21.02.2013

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Title Type of contribution Title of article or contribution Date Place Persons involved


Communication with the public

Communication Title Media Place Year
New media (web, blogs, podcasts, news feeds etc.) Biotreibstoffe – besser als ihr Ruf ETH-Klimablog German-speaking Switzerland 10.04.2012
Media relations: print media, online media Sprit für besseres Klima ETH Globe Italian-speaking Switzerland Rhaeto-Romanic Switzerland International German-speaking Switzerland Western Switzerland 01.06.2011

Abstract

According to the recent report of the Intergovernmental Panel on Climate Change (IPCC) the warming of the world's climate system is unequivocal and is very likely due to the observed increases in anthropogenic greenhouse gas concentrations. The rising concentration of carbon dioxide (CO2), the most important greenhouse gas, is resulting primarily from fossil fuel use, to which the transportation sector contributes the largest part. Besides the negative environmental impact of fossil fuels, the dramatically rising oil prices and dependency on political unstable oil exporting countries resulted in a significant increase in the world's interest in alternative fuels. A common renewable liquid fuel is bioethanol, which is already produced on a large scale in Brazil and the US using sugar cane or corn starch as raw materials. Alternatively, ethanol can also be produced from much more abundant and sustainable lignocellulosic materials, such as agricultural residues or wood. The first step in the current cellulosic ethanol production process is a thermochemical pretreatment that loosens up the lignin-cellulose fiber entanglement to improve enzyme access to the cellulose. After pretreatment, the solid suspension is exposed to cellulolytic enzymes that hydrolyse the cellulosic and hemicellulosic biomass components to release six- (e.g glucose) and five-carbon sugars (e.g. xylose). Finally the released sugars are fermented by a suitable microorganism to ethanol. The large scale commercialization of the technology, which is essential in order to realize its environmental and societal benefits soon, is still in the early phase due to the inherent techno-economical challenges for such a process: i) The fermenting organism needs to be very robust and more tolerant towards toxic inhibitors which are released during pretreatment. ii) The production costs of the cellulolytic enzymes have to be drastically reduced and enyzme cocktails with maximized activity and stability have to be developed. iii) Industrially suitable organism which completely utilize all 5- and 6-carbon sugars to ethanol in high yields are to be defined and iv) it is essential to simplify the process scheme by integrating as many unit operations as possible and to develop a continuous process.In this proposal, a process concept is presented, which allows the integrated production of cellulolytic enzymes, the hydrolysis of the cellulosic biomass and the fermentation of the resulting sugars to ethanol in a single continuous biofilm reactor. A multi-species bioreactor is designed, which enables simultaneous aerobic and anaerobic processes within one vessel by locally defined aeration through a membrane. This membrane serves the different microorganisms as growth basis, where they form a biofilm. All oxygen, necessary for the growth of the aerobic cellulolytic enzyme producing fungi, is only delivered from the gas-phase through the membrane, which causes an oxygen gradient within the biofilm. The oxygen saturated zone is located on the membrane, whereas the upper part of the biofilm, as well as the growth medium are oxygen depleted. The secreted enzymes, produced by the aerobic growing fungi, hydrolyze the hemicellulose and the cellulose in the biomass to mono-sugars, which are then fermented by yeast or bacteria to ethanol in the anaerobic zones of the reactor. Ultimately, the process will employ tube-shaped membranes and a continuous true plug-flow reactor is created. This rector concept features several advantages over conventional lignocellulosic ethanol processes: i) Microorganisms grow in a biofilm which is a natural and highly effective protection mechanism against harmful environments, thus direct fermentation of pretreatment liquors containing toxic inhibitors is enabled without prior detoxification, even with non-industrial ylose-metabolizing strains. ii) The cellulolytic enzymes are directly produced within the same reaction vessel using a small fraction of the pretreated biomass. Costly enzyme purification is omitted, and highly active enzymes are constantly secreted, thereby circumventing the reduced hydrolysis rates at prolonged reaction times due to enzyme deactivation. Furthermore, fungi secrete an optimized enzyme cocktail by growing on the substrate, which is to be hydrolyzed. iii) Several unit operations are integrated, which reduces capital costs. In order to design and finally set-up and characterize the proposed reaction system, a stepwise approach is planned. In the first phase, the in situ production of lignocellulolytic enzymes, a key process of the proposed system, is investigated. Different fungal strains, e.g. Trichoderma reesei or Aspergillus niger, are grown as a biofilm on an oxygen permeable membrane in an otherwise anaerobic reactor. Pure cellulose (Avicel) and pretreated wheat straw or woody biomass are employed as substrates and optimal conditions for maximum enzyme activity are defined. Next, an ethanol producing microorganism is co-immobilized with a fungal strain in a flat sheet biofilm reactor and the direct fermentation of cellulosic substrate to ethanol is investigated. Finally, after optimization of the microbial consortium, the batch bioreactor system is extended towards a continuous tube reactor.
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