تولید بیودیزل از روغن جلبک در اسیدهای چرب آزاد با تبدیل کاتالیزوری دو مرحله ای Biodiesel production from algae oil high in free fatty acids by two-step catalytic conversion

Biodiesel is composed of fatty acid alkyl esters produced from triacylglycerols (TAG), diacylglycerols (DAG), free fatty acids (FFA) and phospholipids (PL), traditionally derived from vegetable oils or animal fats (Leung et al., 2010; Vyas et al., 2010). Compared to conventional diesel, biodiesel generally contains a higher level of oxygen and lower levels of sulfur and nitrogen and therefore, less SOx, NOx, CO, benzene and toluene are released upon combustion (Tica et al., 2010). A major bottleneck limiting the development of the biodiesel industry is supply and price of feedstocks (Greenwell et al., 2010; Naik et al., 2010). A promising source of biodiesel is microalgae, which can grow in fresh water or marine environments, without using arable land and competing with food production (Singh et al., 2011a). Some microalgae have high biomass and oil productivity (Hu et al., 2008; Williams and Laurens, 2010). Biodiesel production from algae is generally done by one of three methods. The first is a two-step protocol in which algae oil is extracted with organic solvent and then converted to biodiesel using a catalyst, such as an acid (Krohn et al., 2011; Nagle and Lemke, 1990), a base (Umdu et al., 2009; Vijayaraghavan and Hemanathan, 2009), or an enzyme (Li et al., 2007). The second method directly produces biodiesel from algae biomass using an acid catalyst at atmospheric pressure and ambient temperature (Ehimen et al., 2010; Johnson and Wen, 2009; Wahlen et al., 2011). The third method is one-step conversion to biodiesel at high pressure and high temperature in the absence of a catalyst (Huang et al., 2011; Patil et al., 2011). Each method has innate advantages and disadvantages. Method 2 requires high concentrations of sulfuric acid since moisture in the biomass is a limiting factor for conversion efficiency (Ehimen et al., 2010; Johnson and Wen, 2009). In contrast, moisture can be ignored under the subcritical or supercritical conditions of method 3 (Patil et al., 2011); however, side reactions happen at subcritical or supercritical conditions that produce organic acids and heterocyclic nitrogen compounds from the degradation of proteins and carbohydrates (Huang et al., 2011). These contaminants lower the quality of biodiesel or interfere with the purification process. From an economics and energy cost point-of-view, oil extraction directly from wet algal slurry is thought to be preferable (Xu et al., 2011), but issues regarding stability of the oils in harvested wet algae still have to be addressed. Cellular lipids in wet algae biomass may be enzymatically degraded by internal enzymes (Singh et al., 2011b). During long-term storage, cellular lipids can be degraded to volatile organic acids (Foree and Mccarty, 1970) or free fatty acid (Alencar et al., 2010). Krohn et al. (2011) reported that free fatty acid in oil extracted from algae biomass can reach as high as 84% (oil weight). Such high levels of FFAs are unlikely to have been present in the algae during growth since they would have had a cytotoxic effect on the cells (Wu et al., 2006). In the current study, changes in FFA and TAG in wet algae biomass stored under various conditions were investigated. Algae oil from the fresh water species Scenedesmus sp., the marine species Nannochloropsis sp. and a heterotrophic Dinoflagellate, containing different free fatty acid levels were converted to biodiesel using a two-step process under optimum conditions. The biodiesel yield and fuel properties were analyzed.