Hydrothermal liquefaction of algae for the production of biofuels

Hydrothermal liquefaction is one of the conversion processes for the production of biofuels that is gaining attention form last decade. Several researchers studied this process for biofuel production using different feedstock. HTL directly converts whole algae into fuel. This process also extracts sugars and lipids when operated at lower temperatures and converts algae directly into fuel intermediate when operated at high temperatures. Hydrothermal liquefaction of Chlorella sorokiniana, Coelastrella and Galdieria Sulphuraria algae. These three algal species were hydrothermally liquefied separately to do a comparative study of different algal species with different chemical composition. The carbon content for all the biocrude oils increased to above 75% where the initial carbon content in the raw materials was less than 55% in all the cases. The nitrogen content in the biocrude oil was lower than the fresh and the oxygen content was significantly reduced during the HTL process. The higher heating value of the biocrude oil was above 34 MJ kg-1 and is higher than the HHV of all the micro algae which were less than 24 MJ kg -1. The HHV's of the biochar ranged from 17 to 23 MJ kg-1. For all the algae, the ammoniacal and total nitrogen levels increased with temperature and the phosphate levels decreased with the temperature rise. The total and ammoniacal nitrogen extracted from galdieria at 300°C are 2-4 times higher than the other two algal species. The phosphate levels extracted at 180°C for galdieria were 7 times higher than the other two species.;The introduction of catalysts into HTL system increases both the biocrude oil yield and the water soluble compounds yield and delivers better quality fuel. Four homogeneous catalysts including two base catalysts and two acid catalysts were used in the catalytic HTL process in this study. A maximum biocrude yield of 21.22% was obtained using catalytic HTL. The HHV's of biocrude oil ranged from 26-32 MJ/Kg without catalysts and increased to 33.76 MJ/Kg with catalysts. The introduction of catalysts into the reaction mixture increased the biocrude oil yield as well as high heating values. The HHV's of biochar ranged from 20 to 27.78 MJ/Kg without catalyst and dropped as 11.74 MJ/Kg with catalysts. The algal species and biocrude oil was analyzed with thermo gravimetric analyzer for the stability. The high heating values of biocrude oil and biochar were determined using bomb calorimeter. Qualitative analysis of biocrude oil using TOFMS revealed the compounds present in the biocrude oil and are convinced with the assumed HTL reaction pathways. Aqueous phase analysis was conducted for the presence of nutrients and valuable co products. Around 20000 mg/L of carbohydrates was extracted during the HTL of Cyanidioschyzon merolae algal species. The increase in temperature increased the extraction of ammonical nitrogen, total nitrogen but decreased the phosphate, carbohydrate and soluble protein levels. The catalyst in the reaction mixture decreased the nutrient extraction and increased the coproducts extraction. Hydrothermal liquefaction of Galdieria sulphuraria algal species was also conducted. A maximum biocrude yield of 18.92% was obtained using direct HTL. The increase in temperature increased the extraction of ammonical nitrogen, total nitrogen but decreased the phosphate, carbohydrate and soluble protein levels. Based on the water analysis, a second step sequential HTL was conducted to extract carbohydrates and soluble proteins at lower temperatures and to convert the remaining residue to biocrude oil at higher temperatures.;Sequential hydrothermal liquefaction process helps extracting valuable co products at lower temperatures and biocrude oil at higher temperatures. The biocrude oil obtained using the 2step process is higher than the biocrude oil obtained using direct HTL when galdieria algal biomass was used. The HHV's of the two biocrude oils are similar. The toxicity studies showed that the biocrude oil is more toxic than the biochar, aqueous phase and the microalgae. The nitrogen levels in the aqueous phase increased with temperature and the phosphate levels increased initially but showed less extraction at higher temperatures. The ICP-OES analysis revealed that the elemental phosphorous in the biochar was less than the raw material at lower temperatures. The phosphorous concentration in the biochar increased with the temperature and further increased with the sequential HTL. A high lipid algal biomass was used for hydrothermal liquefaction in batch and continuous processes. The biomass has 21.2% lipid content with high heating value of 23.86 MJ.kg. The biocrude oil yields obtained were 38.25% for batch process and 21.51% for continuous process based on the ash free dry weight. The reaction times were 30 min and 10 min for batch and continuous process respectively. The continuous process seems to be helpful for solvent free extraction by separating biochar at high temperature and pressure conditions. The compounds detected in the biocrude oils were similar but the major compounds with high area% were different in both the batch and continuous processes.;The effect of coliquefaction and process water recycle on biocrude oil yield was studied with different algal species. In the coliquefaction experiments, the biocrude oil obtained when pure Coelastrella was used is 15.63% and biocrude oil obtained when pure chlorella was used is 24.6%. The 80-20 mixture tend to decrease the biocrude oil yield but the increase in the percentage of chlorella algae in the mixture increased the yield. Synergetic effect was observed with Coe-Chl: 40-60 where the biocrude oil increased to 25.89% which is greater than the biocrude oil when pure species was used. In the process water recycle experiments, the biocrude oil obtained at 300°C with Nannochloropsis and chlorella are 30.44 and 30.17% respectively with the initial run without HTL water recycle. For Nannochloropsis, the biocrude oil increased when the HTL was recycled from 30.44% to 38.87% after three recycles. For chlorella, the biocrude oil yield increased from 30.17 to 40.43 with two recycles and the third recycle reduced the biocrude oil yield to 35.21%. This increase was due to the catalytic effect of the water soluble compounds such as acetic acid on the conversion process.