Xin Chen

Date of Award

8-2012

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Biological Engineering

Advisor

G. Peter van Walsum

Second Committee Member

M. Clayton Wheeler

Third Committee Member

Adriaan R. P. van Heiningen

Abstract

There is a rising concern about securing sustainable, green house neutral and economic sources of energy. The supply of liquid fuels is of particular concern due to the high prices and limited amount of petroleum. One of the most commonly proposed biological methods of converting biomass to liquid fuels is simultaneous saccharification and fermentation (SSF). But due in part to the requirements of expensive enzymes and sterile operating conditions, SSF has relatively high predicted costs. In contrast, the carboxylate platform processing offers several advantages, such as non-sterile fermentation, the use of inexpensive (plastic or concrete) tanks, adaptability to a wide variety of feedstocks (polysaccharides, proteins and lipids) and no need for added enzyme.

One configuration of carboxylate platform processing makes use of an acidogenic digestion process that applies a mixed culture inoculum for conversion of biomass into carboxylic acids. The products of mixed organic acid fermentation can then be converted into mixed ketones through neutralization and a thermal deoxygenation process. The ketone products can be further processed through chemical means into alcohols or hydrocarbons.

Brown seaweed, one of the most abundant biomass resources in the world, is selected as the feedstock in this project. In a fermentation experiment testing four conditions: temperatures of 25 or 37 °C, with or without lime pretreatment, it was found that 37 °C with lime pretreatment was the most efficient. Another set of fermentations tested the effect of calcium carbonate as the buffer instead of ammonium bicarbonate and the use of a mixed culture enriched with C. acetobutylicum as an aid to decomposing mannitol, which is an abundant carbohydrate in kelp that proved recalcitrant in previous fermentations. The C. acetobutylicum did not significantly increase the total organic acids concentration, but the calcium carbonate as the buffer showed almost the same effectiveness as the ammonium bicarbonate, which will simplify downstream processing and reduce expenses.

Thermal deoxygenation on kelp fermentation salts successfully generated products with high ketone content and comparatively high energy content, which verifies its potential to be precursor to a drop-in fuel. It was also found that feeding lactic acid into thermal deoxygenation decreases the oxygen content and increases the energy density of the final product compared to comparable organic acids. Furthermore, lactic acid produces a final oil product that phase separates from water. The hydrophobic phase has high energy content, low oxygen content and water immiscibility, which lend it high potential to be a liquid fuel. The total yield of liquid TDO product over total volatile solids in fermentation was 20% and the higher heating value of the liquid TDO product was 33 MJ/kg.

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