**4. Supercritical alcohol process for the production of biodiesel from wet algal biomass**

The utilization of vegetable oils to produce biodiesel has resulted in stress on domestic markets and often disrupted the production capacities to lower levels of the operating plants [19]. Other feed stocks like waste cooking oil and animal fats are not sufficient to meet commercial demands and to make biofuels at profitable scales. Researchers identified microalgae as an alternative crop to produce oils in larger volumes, with smaller areas of land and in shorter periods of time. The active research on algae has started in the 1970s due to the oil crisis. Microalgae are single cell plants which grow in most of the marine environments around the globe. They can be cultivated under autographic and heterotrophic conditions depending upon the species. Due to their faster photosynthesis they grow much faster and consume more CO2 when compared to oil producing energy crops. Algae can be harvested in cycles of 6-14 days depending on the strategies and cultivating conditions.

Microalgae are being cultivated in open race way ponds and in closed photo bioreactors. Open raceway ponds are much cheaper to operate, but very hard to control the conditions within the pond. Open ponds are more vulnerable the atmospheric conditions and other invading species which greatly effect both quantity and quality of algal biomass. On the other hand, photo bioreactors provide a very controlled environment which helps to produce biomass with better quality and quantity than open ponds. But operating costs of photo bioreactors are very high, prohibiting it for the use in biofuels production. Many research institutions and private corporations have developed the best suitable systems for their needs. After cultivation biomass can be harvested with techniques such as centrifugation, flocculation and hydro cyclones etc. The biomass content or water content in the biomass varies for different systems. The extraction of oil is the most energy intensive step in algal biofuels production. The drying step that occurs prior to the solvent extraction of oil consumes nearly 90MJ of energy which is nearly 85% of the total energy consumed to produce 1 kg of biodiesel. Techniques like Supercritical CO2 extraction, pyrolysis and gasification also need dry biomass. Due to this these methods are also not suitable for the production of biofuels with algal biomass. Biodiesel could be produced directly from the algal biomass by using supercritical alcohol transesterification process.

The direct conversion of wet algal biomass into biodiesel is demonstrated with *Nannochloropsis sp.* with supercritical methanol and ethanol as conversion media without using catalyst. The FAMEs and FAEEs can be produced with free fatty acids and triglycerides present in the algal biomass. As the temperature rises from normal room temperature to supercritical conditions, the reduced dielectric constant provides enhanced extraction capabilities to the alcohols to open/break the cell walls and to extract lipids. The cells structure disintegrates into small particles at higher temperatures due to enormous pressure, providing complete conversion of lipids into biodiesel. After reaching the critical points of the alcohols, the transesterification reaction takes place. At higher pressures the cell structure will be destroyed and provides more access to the lipids for transesterification reaction. The cell structures of the biomass before and after conversion are presented in Figure 3. The algal cells containing large globules of lipids (indicated by arrows) along with other cellular organelles (Figure 3a) are completely destroyed and disintegrated (figure 3b) into a network of pieces [20].

between 450-475o

170 Biofuels - Status and Perspective

**algal biomass**

process.

C. The resulting biocrude is then upgraded with commercial nickel catalyst

to produce jet fuel and diesel range hydrocarbons [18]. The schematic of the CH process is shown in Figure 3. The hydrogen utilization during the CH process can be reduced compared to direct catalytic cracking of oils. The fatty acid profile of feed stock, water to oil ratio, rate of

**4. Supercritical alcohol process for the production of biodiesel from wet**

The utilization of vegetable oils to produce biodiesel has resulted in stress on domestic markets and often disrupted the production capacities to lower levels of the operating plants [19]. Other feed stocks like waste cooking oil and animal fats are not sufficient to meet commercial demands and to make biofuels at profitable scales. Researchers identified microalgae as an alternative crop to produce oils in larger volumes, with smaller areas of land and in shorter periods of time. The active research on algae has started in the 1970s due to the oil crisis. Microalgae are single cell plants which grow in most of the marine environments around the globe. They can be cultivated under autographic and heterotrophic conditions depending upon the species. Due to their faster photosynthesis they grow much faster and consume more CO2 when compared to oil producing energy crops. Algae can be harvested in cycles of 6-14 days

Microalgae are being cultivated in open race way ponds and in closed photo bioreactors. Open raceway ponds are much cheaper to operate, but very hard to control the conditions within the pond. Open ponds are more vulnerable the atmospheric conditions and other invading species which greatly effect both quantity and quality of algal biomass. On the other hand, photo bioreactors provide a very controlled environment which helps to produce biomass with better quality and quantity than open ponds. But operating costs of photo bioreactors are very high, prohibiting it for the use in biofuels production. Many research institutions and private corporations have developed the best suitable systems for their needs. After cultivation biomass can be harvested with techniques such as centrifugation, flocculation and hydro cyclones etc. The biomass content or water content in the biomass varies for different systems. The extraction of oil is the most energy intensive step in algal biofuels production. The drying step that occurs prior to the solvent extraction of oil consumes nearly 90MJ of energy which is nearly 85% of the total energy consumed to produce 1 kg of biodiesel. Techniques like Supercritical CO2 extraction, pyrolysis and gasification also need dry biomass. Due to this these methods are also not suitable for the production of biofuels with algal biomass. Biodiesel could be produced directly from the algal biomass by using supercritical alcohol transesterification

The direct conversion of wet algal biomass into biodiesel is demonstrated with *Nannochloropsis sp.* with supercritical methanol and ethanol as conversion media without using catalyst. The FAMEs and FAEEs can be produced with free fatty acids and triglycerides present in the algal biomass. As the temperature rises from normal room temperature to supercritical conditions,

heating and reaction pressure determines the final product properties.

depending on the strategies and cultivating conditions.

**Figure 4.** TEM images of algal cells before (a) and after (b) in supercritical ethanol conversion

The major influencing parameters of the direct conversion are reaction temperature, algae to alcohol ratio (wt.:vol.) and reaction time. The optimum reaction conditions for both methanol and ethanol are presented in Table 4. The algal biomass used in the experi‐ ments has 50% (supercritical methanol) and 52% (supercritical ethanol) of total lipids on ash free dry weight basis. As the temperature increases the extraction takes place below critical point, and transesterification starts from critical point. The maximum yields observed are 84% and 67% at 255o C and 265o C respectively. The short chain containing methanol has produced more biodiesel than ethanol similar to the vegetable oil transesterification. Same amount of alcohol may have been another reason for the lower yields with ethanol. When compared to the supercritical alcohol transesterification of vegetable oils and fats, the reaction temperatures of the direct conversion methods of algae are very low. This is due to the difference in fatty acid profile of algal biomass. The algal biomass used in these studies has more unsaturated (~40-45%) and polyunsaturated fatty acids (PUFAs) (~10%); which are thermally unstable and causes the reduction of biodiesel yields at higher temperatures. The decomposition of PUFAs was observed at higher temperatures above the optimum reaction temperature in both studies [21, 22].


**Table 4.** Reaction conditions for maximum yields of biodiesel with algal biomass

When compared to biodiesel production using vegetable oils by supercritical alcohol process, nearly 2-3 times more alcohol is needed for algal biomass conversion. More energy is required for the separation of the extra alcohol, making the process more energy intensive. The production of biodiesel directly from the wet algal biomass is possible; but supercritical processing of expensive feedstock like algae demands complex infrastructure and higher energy, making production of biofuels less profitable. During this process valuable byproducts like polyunsaturated fatty acid ethyl esters are lost in order to maintain the fuel properties.
