On the thermodynamics of algal biodiesel production systems

(Based on our forthcoming paper “Net Energy Analysis of the Production of Biodiesel and Biogas from the Microalgae: Haematococcus pluvialis and Nannochloropsis” to appear in the journal Applied Energy.)

Biodiesel is the term used for derivatives of vegetable oils and animal fats that have been chemically modified so that their physical properties resemble those of diesel fuel. This chemical modification ensures that they can be used as fuels for diesel engine without posing significant risk of damage, and without any drastic changes in performance. There are many environmental benefits that have been attributed to biodiesel use, including biodegradability and reduction of emissions of greenhouse gases, sulfur dioxide (an acid rain precursor) and particulates. Critics, on the other hand, point to various environmental impacts resulting from its consumption, including possibly slightly increased levels of smog precursors (i.e., NOx) and the consumption of land and water resources. The latter issue has proven to be particularly contentious, since such resources also perform the arguably more important function of providing food crops for the world’s ever growing population.

Quantitative metrics have thus been developed over the years to help decision makers evaluate the viability of various potential systems for producing biofuels, including biodiesel. One of the simplest, and oldest, approaches is net energy analysis (which was described in detail in the article entitled “Measuring the Sustainability of Energy Systems” that appeared in this column last November). In simple terms, net energy analysis may be based on the computation of the ratio of total useful energy output from a system (i.e., in the form of fuels or electricity) to the total non-renewable energy inputs. The latter includes all direct and indirect energy flows, such as fuel used for operating farm equipment and irrigation pumps, energy inputs for the manufacture of fertilizers or pesticides. Note that only non-renewable flows are counted as inputs, since renewable energy streams, which are derived from sunlight, are assumed to be freely available. Thus, using this definition, any viable bioenergy system will have an energy ratio greater than one (i.e., it produces more useful energy than it consumes). Such a system can be visualized as an “energy amplifier” which serves to leverage a given amount of energy commodity into an even larger quantity of biomass-derived energy. Note that this does not violate the First Law of Thermodynamics, since the apparent energy gain is derived from the conversion of diffuse solar energy into commercially useful forms of energy.

One of the main problems with biofuel systems in general, and biodiesel production chains in particular, is that these are relatively inefficient and thus consume inordinate amounts of valuable land and water resources for a given level of energy output. The photosynthetic process of converting radiation from the sun into chemical energy is the main bottleneck. Consider, for instance, how much of the sunlight that falls onto a coconut tree is converted into the oily flesh from which coconut oil is pressed, relative to the huge mass of the entire tree itself. The same argument can be made for other biodiesel feedstocks. Only a tiny fraction of most plants’ biomass consists of the oils which can then be extracted and converted into biodiesel. It is this weakness which has led many researchers in recent years to look at oil-bearing marine or aquatic algae as an alternative feedstock for the production of biodiesel. The technology for cultivating such algae is still under development. Various schemes, ranging from open ponds to enclosed “photobioreactors” made of transparent plastic, have been proposed. Because oils make up a larger proportion of the total biomass of these algae, as compared to terrestrial plants, such systems have much higher photosynthetic efficiency levels; this advantage then suggests that better energy ratios may be achieved from biodiesel production systems utilizing such algae.

Full-chain energy analysis of such systems has been done in recent years by researchers all over the world. Thus far, the results have been inconclusive, with some work showing favorable energy balances, and still others showing unfavorable profiles. A survey of the literature shows that the net energy available from such systems is highly sensitive to the operational assumptions made by the analysts. Our own contribution to this field involves the net energy analysis of production systems based on two types of oil-bearing algae (Haematococcus pluvialis and Nannochloropsis). This work will appear in a forthcoming issue of the journal Applied Energy dedicated to microalgal biodiesel. Our calculations show that the energy ratios, based on the technology being proposed for commercial operation of such systems, fall far short of thermodynamic break-even. These are our findings even when we make highly optimistic assumptions about operations (e.g., we assume that the residual algal biomass after oil removal is used to produce biogas, which is in turn may be used to produce both electricity and process steam). This time, the main bottleneck lies in the large energy demand needed to separate the valuable oil from water and from the rest of the algal biomass. It is also significant to note that the biomass itself contains high levels of moisture, which further increases the processing energy requirement. Thus, the key implications of our results is that significant innovation is needed in developing less energy-intensive techniques for separating oils from algal biomass, perhaps by converting the oil to biodiesel in situ, or while still in solution. Other possibilities would be to obtain multiple products from the algae such that the energy usage allocations are spread over a wider range of useful products; this is an example of the “biorefinery” concept that has gained a lot of international research interest lately. It thus appears that the aspect of downstream processing will determine whether algal biodiesel becomes thermodynamically viable in the future. 

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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 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 and alternative fuels. E-mail at [email protected].

Raymond R. Tan is a full professor of chemical engineering and university fellow at De La Salle University. His main research interests are process systems engineering (PSE), life cycle assessment (LCA) and pinch analysis. He received his BS and MS in chemical engineering and Ph.D. in mechanical engineering from De La Salle University, and is the author of more than 60 articles in ISI-indexed journals in the fields of chemical, environmental and energy engineering. He is a member of the editorial board of the journal “Clean Technologies and Environmental Policy,” and co-editor of the forthcoming book “Recent Advances in Sustainable Process Design and Optimization.” He is also the recipient of multiple awards from the Philippine National Academy of Science and Technology and the National Research Council of the Philippines. E-mail at [email protected].