Oil extraction is defined as the process of separating triglyceride (TAG) lipids from the harvested and concentrated algal biomass and it could be done through a variety of mechanical or chemical manipulation techniques. Oil extraction using hexane solvent has been considered in this study using a stripper column model [27].
Biodiesel or fatty acid methyl ester (FAME) is formed by transesterification of acylglycerols (glycerides) namely monoacylglycerols (MAG), diacylglycerols (DAG), and triacylglycerols (TAG), as well as free fatty acids (FFA) and phospholipids (PL). Triglycerides are considered to be the most favorable type of lipids with the highest yield for production of biodiesel [50]. Fatty acids are constituents of lipids and, in general, a fatty acid molecule is comprised of a hydrocarbon chain attached to a carboxyl functional group. If the structure consists of at least one double bond, then it is an unsaturated fatty acid, and it can bond with hydrogen; otherwise it is saturated. Vegetable oils and animal fats mostly contain triglycerides which can be broken down by natural enzymes into other acylglycerols and FFA. Various non-acylglycerol types of lipids may be present in the extracted bio-oil such as polar lipids, FFAs, ketones, pigments, etc. These would have an adverse impact on the transesterification process and often there is a need for purification before the transesterification stage [51].
Current commercial transesterification of lipids is in the presence of alkaline (KOH/NaOH) catalysts. Methanol is generally used for large-scale production due to its availability and lower cost compared to other options. The main factors affecting transesterification are the TAG content, the amount of inhibitors and water content. The transesterification process is usually modeled based on industrial procedures that have already been used for conversion of first-generation bio-oils, such as rapeseed and soybean. The extracted oil from these biomasses is considered to have a low FFA content (often less than 1%) and high TAG lipids, respectively. Oil containing more than 2% FFA should undergo a pretreatment step (esterification) with methanol in the presence of a sulfuric catalyst to reduce the FFA content, followed by transesterification with methanol and sodium hydroxide [52,53]. For oil extracted from algae, the percentage of FFA and other inhibitors is usually much higher (up to 14%) than the maximum allowable content [54,55]. Apparently, the inhibitors should be purified and removed from the oil before transesterification in order to prevent saponification and biodiesel hydrolysis. In order to avoid the undesired reactions of saponification and hydrolysis, a two-step transesterification has been suggested [56]. Thus more chemicals and energy are involved in the process. In this study an average value of 7% FFA content in algae oil was assumed. Materials and energy required for processing oil and biodiesel production with 7% FFA using a two-step process (acid esterification and alkali transesterification) modeled and reported in Zhang et al. [52] were used in this LCA study.