Biodiesel (fatty acid methyl ester) derived from oleaginous microbes—microalgae, yeast, and bacteria—can effectively displace both petroleum diesel and biodiesel produced from plant oils, according to the findings of a new study by a team from Utah State University.
The researchers, who reported their results in a paper published in the ACS journal Energy & Fuels, examined the properties, engine performance, and emissions for biodiesel produced from the microalgae Chaetoceros gracilis; the yeast Cryptococcus curvatus; and the bacterium Rhodococcus opacus.
While biodiesel derived from plant seed oils has advantages as a replacement for petroleum diesel, there is strong interest in the potential for biodiesel produced from microbial derived oils because of potential use of contaminated water, the diversity of oils that can be produced, use of marginal lands, and potential for higher oil yields per acre. Three different groups of microbes are known to produce high neutral oils including select microalgae, bacteria, and yeast. Here, we have selected a representative from each of these three groups, produced biodiesel, have characterized the properties of the fuels in comparison to biodiesel produced from plant oils.
Plant-based oils, commonly used to produce biodiesel (e.g., soybean, canola, and sunflower) are similar to one another in terms of fatty acid composition, containing primarily C16 and C18 fatty acids with varying degrees of unsaturation. Microbial oils, however, can differ substantially and may contain uncommon fatty acids that differ in both chain length and structure. Each of the microbial sources of oil chosen for this study differ in one way or another from soybean oil, a common feedstock for biodiesel production.
The team determined the key physical properties of each biodiesel and compared them with commercial soybean biodiesel. Each fuel was then used to operate a two-cylinder indirect injection diesel engine attached to an eddy current dynamometer. The horsepower and torque output from the engine under load was reported for each microbial biofuel and compared to outputs obtained with both diesel fuel and soybean biodiesel. Key emissions data were collected for each biofuel without load at a steady 3500 rpm, allowing comparison to emissions from petroleum diesel and soybean biodiesel.
They found that the selected physical properties of the three microbial fuels were comparable to soybean biodiesel and are within ASTM (ASTM D6751) specification.
In engine testing, diesel #2 delivered the highest power output (8.5 hp) of all fuels. Soybean biodiesel registered a power output of 8.2 hp, 96.5% of the value obtained for diesel #2. Of the microbial fuels tested, bacterial biodiesel had the lowest power output at 7.8 hp, still producing 92% and 95% of the output achieved with diesel #2 and soybean biodiesel, respectively. Power output for the engine operated with both yeast and microalgae biodiesel was similar for each fuel achieving 93% and 96% of outputs for diesel #2 and soybean biodiesel, respectively.>
The team found less of a difference in observed torque output. Exhaust gas temperatures were highest for diesel #2, followed by soybean, microalgae, yeast, and bacteria (in order of decreasing temperatures). The biofuels showed higher fuel consumption (BSFC) across the rpm range of the test compared to diesel #2, which is consistent with their lower energy content. Very small differences in BSFC between the biodiesel fuels were observed.
Hydrocarbon and CO emissions were reduced from diesel #2 levels for all microbial and soybean biodiesel fuels. While NOx emissions are elevated relative to diesel #2 in yeast, bacteria, and soybean biodiesel fuels, microalgae biodiesel fuel generated NOx emissions that were significantly lower, they found. They suggested that the low prevalence of polyunsaturated fatty acids and the predominance of shorter chain length fatty acids present in C. gracilis oil likely contribute to its low NOxemissions.
his study demonstrates that microbial-derived biodiesel shows comparable properties in the parameters tested to soybean biodiesel. Future wide scale use of microbial oils as a source for biodiesel will require advances in large scale cultivation, dewatering, and oil extraction.
—Whalen et al.