Biokubo: The search for an alternative feedstock for biodiesel

Part III. The candidates: Used cooking oil and microalgae

In the first and second parts of this series, the need and rationale for an alternative feedstock biodiesel and candidates from plants and animals were discussed. In this third part, I discuss other candidates — used cooking oil and microalgae.

Used cooking oil

It is actually difficult nowadays to avoid reports in the popular media about vehicles being operated with biodiesel from used cooking oil. Indeed, it is an idea that makes sense. Used cooking oil is much less expensive than virgin oil and utilization of the waste oil would mitigate a waste stream. 

Studies have shown that the biodiesel process utilizing waste cooking oils had a better rate of return than a process which used virgin oil, although all of the processes in their papers had negative rates of return. What kept the waste cooking oil process from having a positive rate of return was a higher capital cost. More capital is needed because waste cooking oil usually comes with impurities which have to be removed with additional equipment. Waste oils are expected to be more variable in composition and thus a plant operating with waste cooking oils would require more frequent process adjustments in order to meet quality standards. Nonetheless, the challenges for the full utilization of waste cooking oil seem to be purely logistical, operational and economic. Given the right external conditions, the use of waste cooking oils should be easy to achieve. But while there may be profit achievable on a small scale, the volumes are not large enough to make a dent in the energy problem.

Phototrophic microalgae and other microbial oils

There is no doubt that the challenges of finding an alternative for petroleum are enormous. The challenges become even larger when the volume required of the alternative is considered. The demand for liquid fuel is so large that 40 percent of the arable land on the planet would be needed to grow enough oil palms to completely replace petroleum. Dedicating such a large chunk of the agricultural capacity of the world for its fuel demands is obviously unwise and thus a more radical idea has been proposed. Microalgae (microscopic unicellular aquatic plants) have been cultivated for varied products since the 19th century. They are far more efficient than terrestrial plants in converting sunlight to useful biomass. 

The potential is great and thus in 1978, the United States National Renewable Energy Laboratory embarked on an ambitious program to extract oils from microalgae for the production of biodiesel. The project faced great technical challenges. The oil yield, purity and quality are greatly affected by the growing conditions. The species with the highest oil content grow most slowly. Nutrient deprivation results in a higher oil content but also a slower growth rate.

Processing of the microalgae after harvest is one of the key issues. The oil must ideally be moisture-free and since the algae are aquatic, dewatering is often an expensive and energy-consuming step. Often, the cell has to be disrupted in order to retrieve the metabolite and so this requires yet another energy-consuming processing step. Finally, a solvent extraction step is often used to retrieve the oils from the biomass cake. This usually involves the use of flammable solvents which may themselves be pollutants.

Many microalgal fats contain large amounts of highly unsaturated fats which raise concerns about stability and engine clogging. This has led some to recommend that highly unsaturated fats be hydrogenated or distilled. Hydrogenation or distillation would add another resource-using and pollution-generating process step. In particular, the hydrogenation step necessitates the generation of hydrogen, whose present source is petroleum and other fossil fuels and is very energy-intensive. 

The challenges for commercialization of biodiesel from microalgae are so great that the NREL shut down its program in 1996 after screening more than 3,000 species of microalgae. Economic feasibility had been unproven and the problems of invasion were deemed to be intractable. Our own study, recently published in Applied Energy, for a specific microalgal system shows an energy deficit. That is, more energy is required than is obtained from a system.

In spite of these challenges, it does seem that when one considers the enormous demand for diesel, microalgae may be a fertile area for research and development. The attraction is so great that other new sources of oils have been proposed such as heterotrophic microalgae, macroalgae (seaweed), yeasts, bacteria, fungi and even wastewater treatment plant microorganisms. These systems for producing lipids are even earlier in the development stage. For the Philippines, doing research on microalgae may make sense considering our extensive experience in aquaculture. Indeed, the Southeast Asian Fisheries Development Center (SEAFDEC) in Iloilo has had a long history of microalgal culture albeit with the end of providing fish feed in mind.

The approach of cultivating microalgae and extracting multiple products in a “biorefinery” may ultimately bear fruit. 

In Part IV of this series, some thoughts on the future of this research are discussed.

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Luis F. Razon is a full professor of chemical engineering at De La Salle University. Razon obtained his bachelor’s degree in chemical engineering (magna cum laude) from De La Salle University and his MS and Ph.D. in chemical engineering from the University of Notre Dame, Indiana. His papers on the dynamics and stability of chemically reacting systems are some of the best-cited papers in the chemical engineering literature. He served in the food industry for 14 years, launching several important new products for a major international nutritional products company. He returned to the academe in 2001 and is pursuing research in chemical reactor engineering, alternative fuels and life-cycle assessment. E-mail at [email protected].