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Comment on “Biodiesel Production from Freshwater Algae” †

by Gerhard Knothe, James a. Kenar
Energy & Fuels ()

Abstract

and animal fats,1 has received significant interest in recent years as an alternative to petroleum-derived diesel fuel. The advantages of biodiesel, which include domestic origin, re- newability, biodegradability,miscibilitywith petrodiesel at all blend ratios, compatibility with the existing fuel distribution infrastructure, low or no sulfur, high flash point, excellent lubricity, and reduction of most regulated exhaust emissions have been driving forces toward its increased use. Technical problems with biodiesel include elevation of nitrogen oxide exhaust emissions,moderate topoor cold flowproperties,and oxidative stability. Economics and sufficient supply of feed- stock are non-technical concerns with biodiesel. The supply issue has caused the search for additional feedstocks.Among these additional feedstocks, algae are receiving increasing attention because of the claimed high production potential of algal oils.2,3 Accordingly, the number of research papers on the potential of algae as biodiesel feedstock has grown signi- ficantly in recent years. In this connection, a recentpaper entitled“Biodiesel Produc- Biodiesel, defined as the monoalkyl esters of vegetable oils tionfromFreshwaterAlgae”was publishedbyVijayaraghavan and Hemanathan in Energy & Fuels (10.1021/ef9006033; re- ferredto as the“VandHpaper” inthefurtherdiscussion).4The V and H paper is currently ranked highly under the category “MostRead” of this journal. However, an inspection of the V and H paper reveals that it is not concerned with biodiesel. Rather, it appears that the authors obtained a hydrocarbon feedstock and fuel from the algae. Unfortunately, there are numerous other errors and omissions of essential information in the V andHpaper. Themost salient shortcomings of the V andHpaper are summarized here. The authors do not describe the nature of the algae used in their work. This lack of information is essential and by itself significantly reduces the value of the V and H paper for anyone interested in algae-derived fuels. The problems occur- ringintheVandHpaper tosome extent seemtoarise fromthe † Disclaimer: Product names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. *To whom correspondence should be addressed. Telephone: (309) 681-6112. Fax: (309) 681-6524. E-mail: gerhard.knothe@ars.usda.gov. (1) American Society for Testing and Materials (ASTM). Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels; ASTM:West Conshohocken, PA, 2009. (2) Chisti, Y. Biodiesel from microalgae. Biotechnol. Adv. 2007, 25, 294–306. (3) Mata, T. M.; Martins, A. A.; Caetano, N. S. Microalgae for biodiesel production and other applications: A review. Renewable Sustainable Energy Rev. 2010, 14, 217–232. (4) Vijayaraghavan, K.; Hemanathan, K. Biodiesel production from freshwater algae. Energy Fuels 2009, 23, 5448–5453. This article not subject to U.S. Copyright. Published 2010 by the American Chemical Society 3299 authors probably not being aware of the nature of the algae used. The reported density is too low for biodiesel. Biodiesel should have a density of about 0.88 and not 0.801. The flash point is also untypically low for biodiesel that is properly prepared. Instead, these values point in the direction of a non- ester fuel, such as one consisting of hydrocarbons. In section 2.1 (Collection of Algal Sample), the collected algal biomass was stored in a plastic container that was exposed to “open sunlight” (it is not clear what exactly this means) for up to 5 days before use. This is very important whenexaminingthe gas chromatography-mass spectrometry (GC-MS) peak total ion chromatogram (TIC) data pre- sented in Table 2, as discussed below. In section 2.2 (Analytical Methods), other essential infor- mation is lacking, such as the conditions and equipment of GC-MS used for analyses. The “analytical methods” for the fuel properties are not detailed, rather “ASTM procedures” are generically mentioned (are these the ASTM methods prescribed in biodiesel standards or other ASTMmethods?). In conjunction with Table 1 in the V and H paper, essential fuel properties, if the fuel were indeed biodiesel, such as vis- cosity, oxidative stability, and free and total glycerol, are not given. In section 2.4.1 (Effect of Drying and Expression on Algal Biomass), drying of the algae is insufficiently described. The samples were dried for a maximum of 2 h before being expressed. What was the drying temperature? What was the percent moisture remaining in the samples? What was the quantity of the aliquots removed for expression? This is not clear until the Results and Discussion, where it is stated: “A suitable aliquot of air-dried algal biomass was removed from the mesh, on a volume basis, in the form of squares (which averaged to a weight of 255 ( 7g)”. The aliquots were expressed, and its extractable lipid content was monitored; however, it is never mentionedwhat equipment or experimen- tal conditions were employed to express the lipids. How the lipids were extracted after expression is not described either. In section 2.4.2 (Effect ofSoakingAir-Dried AlgalBiomass in Hexane Prior to Expression), it is stated that “on average, 255 ( 7 g of the dried algal biomass was soaked in 50mL of hexane as a solvent at a constant agitation of 25 rpm”. This can hardly be described as soaking, because the algal biomass would be present in excess. Insections 2.4.4 and3.4 (Transesterification), essential experi- mental conditions (temperature) are not mentioned (section 2.4.4). Instead of using the volume ratio, the ratio of triacylgly- cerol/methanol should be presented as the molar ratio (section 3.4). What would the molar ratio have been, if the material “transesterified” were indeed a triacylglycerol-containing oil?

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