The Effect Of Pretreatment On Catalytic Hydrothermal Liquefaction Of Low-lipid Microalgae

Ever since the shortages of petroleum resources emerged alongwith the global energy crisis in 1970s,considerable attentions have been focused on the development of alternative fuels.As a promising renewable energy,biomass energy resources are suitable for conversion into alternative fuels such as gas,liquid and solid.Biomass is considered a promising substitute for fossil fuels due to its accessibility.Among the diverse types of biomass,microalgae have aroused extensive interests owing to their multiple advantages,including fast growth rate,high photosynthesis efficiency,high productivity,no competition with food crops and arable lands,and the production of valuable co-products.Compared to the traditional conversion routes（pyrolysis and enzymolysis）,hydrothermal liquefaction（HTL）is considered to be the ideal conversion route as a biomass dewatering process is no longer required.The HTL conversion efficiency of microalgae depends not only on the conversion processes,but also on the origin feedstock composition.High-lipid algal species usually achieves high bio-oil yields,however,these high-lipid microalga species generally have low productivity and need to be cultivated under nutrient-rich conditions.Meanwhile,low-lipid microalgae are able to thrive under extreme environmental conditions such as saline water,brackish water,and wastewater with high biomass productivity.The utilization of low-lipid microalgae may allow the production of bio-oil on a large scale and simultaneously provide an environmental treatment mechanism for wastewater.The presence of protein in algae leads to a high nitrogen content in the bio-oil.To obtain high-quality bio-oil products,in this paper,the effects of hydrothermal catalytic liquefaction of pretreated and low-algal algae on bio-oil upgrading were studied from the perspective of raw materials and conversion pathway of low-lipid algae Spirulina,in order to provide data foundation and theoretical basis for industrial applications for the hydrothermal liquefaction of algae.The main research contents are as follows:1.Hydrothermal pretreatmentHydrothermal synthesis reactors were introduced for the first time in the pretreatment step,and the pretreatment process is set in the range of 150-225°C for 10-50 min.HTL of pretreated algae was performed at 340°C for 2 h with 11 wt%pretreated algae feed concentration.The experimental results show that the use of hydrothermal synthesis reactor for the pretreatment reaction can produce the high-yield,low-nitrogen content bio-oil products.Pretreatment conditions have a great influence on the denitrification effect.When the pretreatment conditions are mild,the denitrification efficiency is not satisfying.With proper prolongation of the pretreatment time,the nitrogen and oxygen contents were reduced by 37%and 36%,respectively.Furthermore,the analysis of bio-oil revealed that the content of desired long-chain hydrocarbons increased to 30%.These findings demonstrate that great improvements in the quality of bio-oil can be achieved by the pretreatment of the algae.Considering the yield and quality of bio-oil,the best pretreatment condition is at 225°C for 10 min.2.Catalytic HTL of pretreated algaeWe investigated the effects of bimetallic catalytic ability of noble metals and non-noble metal catalysts on the HTL of pretreated algae.The experimental results show that when the bimetallic catalyst is Pd-Pt,the corresponding bio-oil yield is 46.73-51.18wt%,which is higher than the bio-oil yield of a single noble metal catalyst.When the Pd content in the bimetallic catalyst system is 20%,the bio-oil yield is highest 51.18wt%.and the nitrogen content in the bio-oil is in the range 4.39-5.62%and reduced by23%,in comparison of HTL of pretreated algae without catalyst.When Pd-Rh is performed,the bio-oil yield is 40.75-56.51 wt%.When the ratio of Pd:Rh is 3:2,the yield of algae bio-oil is the highest,which is 65%higher than that of the HTL without catalyst.At this time,the nitrogen content in bio-oil is only 2.94%,49%lower than that of bio-oil obtained from the HTL without catalyst.These findings indicate that the use of the bimetallic catalyst is more effective than the single metal catalyst in improving the quality and yield of the algae bio-oil.When Ni₁₆Cu₄-ZrO₂ was used,the obtained bio-oil had the lowest nitrogen and oxygen content,which was 32.75%and 9.54%,respectively,lower than that of the bio-oil without catalyst.Compared with noble metal catalysts,Ni₁₆Cu₄-ZrO₂ has better hydrodenitrogenation effect.