Sugar to Lipid Conversion and Bio-hydrogen Production from Hydrolyzed Lignocellulosic Residues and Organic Fraction of Municipal Solid Waste



The severity of the global climate crisis is not to mistake, and we need to take action to save our planet. To do so, the Paris Agreement aims to limit the global temperature rise to a maximum of 2ºC and preferably 1.5ºC (Rogelj et al., 2016). To meet the objectives of this significant agreement, all sectors of the global economy must take action to reduce the emission of greenhouse gasses. One of the essential contributors to the emissions is the transport sector, which in 2018 was responsible for about 27% of the carbon dioxide emission, the most dangerous and prevalent greenhouse gas (Santos, 2017). The most obvious way of reducing emissions in the transport sector is to substitute fossil fuels with electricity generated by renewable energy. However, electrification might not be the most suitable solution for heavy transport, such as aviation and shipping. With the demand for air travel predicted to double by 2050, the need for alternative solutions to emission reduction is increasingly relevant (Appl-Scorza et al., 2018). One potential solution is the substitution of fossil fuels and petro-based materials by biofuels.
Biofuels are classified as any alternative fuel derived from biologically based materials such as cereal crops, crop residues, and waste biomasses. Its gaseous form, biogas, and liquid forms, bioethanol, and biodiesel, have been used globally as renewable energy resources alternatives to fossil fuels (Hirani et al., 2018)
Production of biofuels for the transport sector entails several concerns: greenhouse gas reduction potential, biomass availability, land-use change, and cost-competitiveness with fossil fuels. The sustainability of the produced biofuels mostly depends on the type of feedstock used for the production. Currently, most biofuels are produced from first-generation feedstocks such as plant oil and starch from grains. This feedstock is the easiest to convert to biofuels, but these are also costly and potential foods (Hirani et al., 2018). Second-generation feedstocks, such as municipal waste, wood residuals, and straw, on the other hand, are abundantly produced, have a low cost, do not compete with food, and are CO2 neutral. Second-generation feedstocks emit fewer greenhouse gasses than first-generation; the feedstock does not need to be cultivated as it is a waste product. Therefore, this biomass has a way shorter harvest lifetime than first-generation biomass, allowing for continuous production of biofuels (Srivastava et al., 2019) (Saini et al., 2014). The disadvantage is that second-generation feedstocks require more extensive pretreatment processes to recover the fermentable sugars, and therefore require extensive research to become a thoroughly efficient and feasible solution (Mat Aron et al., 2020).
In this project, we utilize that the feedstocks contain large amounts of carbohydrates that can be broken down to glucose, which can then be converted to lipids, which are superior bio-crudes for producing hydrocarbon fuels (Scaldaferri & Pasa, 2019) (Meng et al., 2009). The sugars are converted to lipids using microorganisms, namely yeast. A class of yeast classified as oleaginous, which means that they have a very high lipogenesis and can store more than 20% of lipids compared to their cell weight, will be exploited for the sugar to lipid conversion (Patel et al., 2020). Before the lipid generation by yeast, the feedstocks must undergo different fractionation processes to obtain a valuable glucose fraction for the subsequent yeast fermentation (Patel et al., 2019) (Salimi et al., 2019).
The fractionation processes depend on the type of feedstock. Many processes have been suggested for the fractionation of lignocellulosic feedstocks, e.g., organosolv fractionation, fractionation using ionic liquid 1-butylimidazolium hydrogen sulfate, and ultrafast fractionation by microwave-assisted deep eutectic solvent pretreatment (Li et al., 2016) (Verdía et al., 2014) (Chen & Wan, 2018). In this project feedstocks of wood and agricultural origin that contain large amounts of lignocellulose, will be separated into three fractions: Lignin, hemicellulose, and cellulose by organosolv fractionation (done by Luleå University, not AAU).
The cellulose fraction and the municipal solid waste, which to a large extend contains starch and cellulose, can be hydrolyzed both by enzymes and acids (Salimi et al., 2019) (Spets et al., 2010). This project will focus on enzymatic hydrolysis. The enzymatic hydrolysis will break down the starch and cellulose into glucose molecules. Following enzymatic hydrolysis, the oleaginous yeast will use glucose as a building block for lipids (Patel et al., 2020). These lipids then must be extracted in the most sustainable way possible. For lipids to be converted to aviation fuels, hydrogen is needed to remove oxygen and form hydrocarbons (Shi et al., 2018). In this project, we aim at producing sustainable bio-hydrogen using a microbial electrolysis cell (Jafary et al., 2019). The MEC will run on side streams from the yeast fermentation and electricity generated by windmills. In the electrolysis cell, specific bacteria will convert the substrate to bio-hydrogen that can be harvested and used in the hydrogenation process of the lipids to produce bio-fuel ready for testing in aviation systems.
This Ph.D. project will focus on the characterization of the second-generation feedstocks/organic waste products to better understand their composition. This will be followed by optimizing methods for efficient enzymatic hydrolysis of the carbohydrates in the feedstocks to release glucose. After successful hydrolysis, the next area of focus will be the conversion of the released sugars to lipids using oleaginous yeast. The produced lipids are then to be extracted in a way that is as sustainable as possible. This project will also focus on the production of bio-hydrogen by using side products from yeast fermentation in combination with a MEC.

Funding: H2020, EU, GA 10100713  
Effektiv start/slut dato01/10/202130/09/2024


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