Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2014
Publication Date: 9/22/2014
Citation: Martin, J.A., Mullen, C.A., Boateng, A.A. 2014. Maximizing the stability of pyrolysis oil/diesel fuel emulsions. Energy and Fuels. 28(9):5918-5929. Interpretive Summary: In order to curb greenhouse gas emissions and promote U.S. energy independence, renewable energy sources must be developed to extend or replace nonrenewable fossil fuels. A major potential source of renewable energy is biomass, i.e. nonliving plant and animal matter such as crop residues, woody materials, and animal wastes. Biomass can be converted into a liquid fuel via pyrolysis, in which the biomass undergoes rapid heating in an oxygen-free environment. The major product of this process is a dense, combustible liquid known as pyrolysis oil, or ‘bio-oil’. Bio-oil has several properties that make it unsuitable as a fuel in most combustion systems, but it has the potential to be upgraded into a drop-in transportation fuel similar to diesel fuel. However this is a longer term prospect and a more immediate application of bio-oil would help to sustain its production. One such application is to mix bio-oil with conventional diesel fuel. Normally bio-oil and diesel are not compatible but can be if a special type of mixture known as an emulsion is prepared. In emulsions, a chemical called a surfactant is used to stabilize the mixture, i.e. prevent the bio-oil from sinking to the bottom. In this study, several different surfactants were used to prepare emulsions of 20% bio-oil and 80% diesel fuel, and the stability of each emulsion was investigated. We found that with 2.5% of a surfactant called PEG-DPHS we could make a very stable emulsion with 20% bio-oil that separated and sank at a rate of only 2.4 mm/day, and that this emulsion would require only slight agitation to stay evenly mixed. This study presents a route for renewable bio-oil to displace fossil-derived diesel fuel and increase the level of biofuels in the U.S. fuel supply.
Technical Abstract: Several emulsions consisting of biomass pyrolysis oil (bio-oil) in diesel fuel were produced and analyzed for stability over time. An ultrasonic probe was used to generate microscopic droplets of bio-oil suspended in diesel fuel, and this emulsion was stabilized using surfactant chemicals. The most stable emulsion was produced using a polyethylene glycol dipolyhydroxystearate (PEG-DPHS) surfactant, with a Hydrophilic-Lipophilic Balance (HLB) number of 4.75 and a 32:8:1: ratio of diesel to bio-oil to surfactant, i.e., 20% utilization of bio-oil. This emulsion consisted of uniformly sized droplets with an average diameter of 0.48 µm, with no observed coalescence of droplets after 1 week. If left undisturbed, these droplets would slowly settle to the bottom of the mixture at a rate of 2.4 mm/day. However, slight agitation is sufficient to keep the droplets suspended indefinitely. This level of stability facilitates utilization of 20 wt% of raw bio-oil in diesel as a renewable liquid fuel for spray combustion without the need for costly and energy-intensive upgrading.