Date of Award

Fall 12-15-2017

Level of Access Assigned by Author

Open-Access Thesis

Language

English

Degree Name

Master of Science in Chemical Engineering (MSChE)

Department

Chemical Engineering

Advisor

William J. DeSisto

Second Committee Member

M. Clayton Wheeler

Third Committee Member

Douglas W. Bousfield

Abstract

The world’s increasing population requires an increase in transportation fuel production. The lack of production of transportation fuels due to the shortage of fossil fuel resources combined with concerns about global emissions of carbon dioxide from fossil fuel combustion are the two major issues that have driven researchers to actively pursue alternative sources for oil production. Biomass is being considered as an alternative feedstock to produce fuel and chemicals due to its abundance and renewability. It has many features that make it suitable as a source of transportation fuel production. However, the bio-oil produced by the fast pyrolysis process has many undesirable characteristics that reduce its quality as a transportation fuel. The major problem that causes these negative properties is mainly the oxygenated groups that are present in the bio-oil.

In this research, the goal was to produce a high-quality bio-oil with low or zero oxygen content that could be suitable for use as a transportation fuel. To do that, four different biomass feedstocks were pyrolyzed at the same operating conditions using a process developed by the UMaine group. This process is called formate-assisted fast pyrolysis or FAsP. Oils produced from the fast pyrolysis of these four feedstocks contained oxygen contents of 16, 21, 26 and 27 wt.% and were generated either by formate-assisted pyrolysis or hot-gas filter pyrolysis of pine sawdust. The generated bio-oils were then hydrotreated over an inexpensive commercially available nickel on silica-alumina catalyst and high hydrogen pressure to produce a hydrocarbon fuel. Hydrotreating experiments were conducted in a downflow trickle bed reactor at temperatures between 300-400 ºC and reactor pressures between 750-1400 psi with a hydrogen flow rate of 100 sccm over several days. Liquid yields, carbon yields, final product oxygen content, and H:C ratio were determined as a function of time-on-stream.

For the 16 wt.% oxygen content bio-oil, the longest time onstream, 345 hours, was achieved at an average bed temperature of 300 °C, reactor pressure of 1400 psi, hydrogen flow rate of 100 sccm and a weight hourly space velocity of 0.06 hr-1. The carbon of the raw bio-oil that ended up in the hydrotreated oil fraction of this experiment was 91.8% with a liquid yield of 95.3%. The highest carbon and hydrogen contents measured for the hydrotreated liquid products from all experiment were 87.0 wt.%, 14.2 wt.% and the lowest were 75.0 wt.%, 10.6 wt.% respectively. Partial deactivation of the catalyst over time was evident due to the quality of the oil product collected, which saw the density, oxygen content and viscosity increase and the H:C ratio and carbon content decrease. The partial deactivation was more pronounced for higher oxygen-containing bio-oil feedstocks and for higher temperatures >300 oC.

Calcium formate pretreatment of biomass prior to pyrolysis produces stable bio-oils with reduced oxygen content. These stable bio-oils can be successfully upgraded into hydrocarbon fuels in a single catalytic hydrotreatment step that ran up to 15 days without significant deactivation and reactor plugging. This improvement eliminates the need for an oil stabilization step prior to hydrotreatment that is required for conventional bio-oil upgrading.

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