Biodiesel from microalgae beats bioethanolYusuf Chistia, aSchool of Engineering, Massey University, Private Bag 11 222, Palmerston North, New Zealand Available online 24 January 2008. Le biodiesel algal lemporte sur le biothanolYusuf Chistia, acole dingnieurs, Universit de Massey, Private Bag 11222, Palmerston North, Nouvelle-Zlande En ligne depuis le 24 janvier 2008.An integrated oil-production processA conceptual process for producing microalgal oils for making biodiesel is shown in Figure 1. The process consists of a microalgal biomass production step that requires light, carbon dioxide, water and inorganic nutrients. The latter are mainly nitrates, phosphates, iron and some trace elements. Sea water supplemented with commercial nitrate and phosphate fertilizers, and a few other micronutrients, is commonly used for growing marine microalgae 5. Fresh and brackish water from lakes, rivers and aquifers can be used. Growth media are generally inexpensive 1. In a 100 tons annum1 facility, cost of producing algal biomass has been estimated to be about $3000ton1 1, but cost per ton declines significantly as the scale of the production operation is increased.Un procd intgr de production dhuileLa figure 1 dcrit un concept de production dhuile de microalgues en vue dobtenir du biodiesel. Ce procd comporte une tape de production de biomasse microalgale qui ncessite de la lumire, du dioxyde de carbone, de leau et des nutriments inorganiques. Ces derniers sont principalement des nitrates, des phosphates, du fer ainsi que quelques lments traces. Habituellement, pour cultiver des microalgues marines, on utilise de leau de mer enrichie en engrais commerciaux base de nitrate et de phosphate ainsi quen autres micronutriments 5. On peut aussi envisager dutiliser de leau douce et saumtre provenant de lacs, rivires et aquifres. En gnral, ces lments sont peu coteux 1. Dans une infrastructure permettant de produire 100tonnes an-1, le cot de production de la biomasse algale a t estim environ 3000$ ton1 1, mais le cot la tonne baisse significativement mesure que lchelle de production augmente.Approximately half of the dry weight of the microalgal biomass is carbon 6, which is typically derived from carbon dioxide. Therefore, producing 100 tons of algal biomass fixes roughly 183 tons of carbon dioxide. This carbon dioxide must be fed continually during daylight hours. Microalgal biomass production can potentially make use of some of the carbon dioxide that is released in power plants by burning fossil fuels 7 and 8. This carbon dioxide is often available at little or no cost.Environ la moiti de la matire sche de la biomasse microalgale est constitue de carbone 6, qui est en gnral driv de dioxyde de carbone. Par consquent, en produisant 100tonnes de biomasse algale, on fixe aux alentours de 183tonnes de dioxyde de carbone. Ce dernier doit tre fourni en continu pendant la journe. La production de biomasse microalgale peut ventuellement tre ralise partir dune partie du dioxyde de carbone rejet par des centrales lectriques lors de la combustion de carburants fossiles 7 et 8. Ce dioxyde de carbone est souvent disponible un cot rduit ou mme gratuitement.The algal broth produced in the biomass production stage needs to be further processed to recover the biomass 9. The water and residual nutrients recovered at this stage can be recycled to the biomass-cultivation stage (Figure 1). The concentrated biomass paste is extracted with a water-immiscible solvent to recover algal oil, which can then be converted to biodiesel using already existing methods 1. The feasibility of oil extraction for microalgal biomass has been previously demonstrated 6 and 10. The extraction solvent (e.g. hexane) is expected to be recovered and recycled.Afin de rcolter la biomasse, il est ncessaire de traiter le milieu de culture obtenu lors de la production des microalgues 9. Leau et les nutriments rsiduels rcuprs au cours de cette tape peuvent tre rinjects dans un nouveau cycle (Figure1). La pte contenant les microalgues concentres est extraite laide dun solvant non miscible leau pour rcolter lhuile dalgue. Cette dernire peut par la suite tre convertie en biodiesel en utilisant les mthodes existantes 1. Il a dj t dmontr que lextraction dhuile partir de biomasse microalgale est ralisable 6 et 10. Il est en outre prfrable que le solvant (exemple: hexane) ncessaire lextraction soit ensuite rcupr et recycl.The biomass residue that remains after extraction of oil could be used partly as high-protein animal feed and, possibly, as a source of small amounts of other high-value microalgal products 5, 11 and 12. In both scenarios, the revenue from selling the biomass residues could defray the cost of producing biodiesel. However, the majority of algal bio