Characteristics Of Hydrothermal Treatment And Phosphorus Migration And Transformation Between Algae And Duckweed

Hydrothermal treatment is a potential biomass heat treatment method,which can significantly improve the quality of biomass related products.In addition,the aquatic plants（algae）used for water restoration are rich in phosphorus.In order to alleviate the shortage of P resources,recycling them is an effective measure to alleviate the shortage of resources.Recovery of P from aquatic biomass（Duckweed,Nannochloropsis sp,Enteromorpha prolifera）residues can maintain water purification efficiency and obtain alternative phosphorus resources,but there is little research on this.Therefore,this paper first studied the product distribution and component changes of different aquatic biomass at different hydrothermal treatment temperatures,focused on the physical and chemical properties and combustion characteristics of solid products,evaluated its feasibility as fuel,and explored the influence of different temperatures on the element changes and the occurrence state of P in different aquatic biomass The effect of bicarbonate on the hydrothermal transformation of duckweed was studied.Firstly,three kinds of aquatic biomass were transformed by high-temperature and high-pressure hydrothermal reactor.The composition of oil phase and gas phase products were tested by GC-MS and GC respectively,and the influence of hydrothermal temperature（180～300℃）on product distribution and composition was explored.The results showed that hydrothermal temperature had a significant effect on the yield of three kinds of hydrothermal conversion products,and the effect on the three kinds of aquatic biomass had the same trend.With the increase of temperature,the yield of three kinds of biomass decreased from18.6～46.5%to 3.6～20.1%（180～300℃）;The yield of oil and gas increased accordingly.The oil components of duckweed and Nannochloropsis sp were mainly nitrogen-containing heterocyclic compounds and organic acids,while those of Enteromorpha were mainly ketones,aldehydes and organic acids.At 180～200℃,the oil components of Enteromorpha prolifera were mainly 5-methylfurfural,accounting for 60.4%～73.5%.The main components of the gas products are CO₂.Secondly,based on element analysis and thermogravimetric analysis,the physical and chemical properties and combustion characteristics of hydrothermal carbon were studied,and the migration and transformation characteristics of phosphorus at different temperatures were investigated by FTIR,XRD,XPS and sequential extraction.It was found that temperature and species of aquatic biomass jointly affect the basic physical and chemical properties and combustion characteristics of hydrothermal carbon.Compared with biomass raw materials,hydrothermal carbon has higher stability（H/C=0.93～3.40）and energy density（up to 21.76MJ/kg,equivalent to the calorific value of bituminous coal）.Moreover,more oxygen-containing functional groups are retained in the hydrothermal carbon,which makes the hydrothermal carbon have great potential in the application of heavy metal adsorption.Hydrothermal treatment is helpful to the transformation of P into more stable phosphate,and the type of raw materials and hydrothermal temperature jointly determine the evolution path of P in the hydrothermal process.The recovery of continuous phosphorus was in the range of11.9～68.2%,which indicated that continuous extraction was reliable.Finally,in order to improve the yield of solid-phase products and further explore the potential value of solid-phase products,duckweed was selected as raw material,and the effects of adding different bicarbonates（Na HCO₃,KHCO₃,NH₄HCO₃）and different proportions on the product distribution,fuel characteristics,crystal phase composition and pore size distribution of solid-phase products were determined.The results showed that the yield of solid phase product was the highest when 30wt.%KHCO₃ was added,and the growth rate was 21.5%compared with that without KHCO₃（24.2%）.And high KHCO₃ addition ratio is conducive to the improvement of oil phase yield.The oil phase yield was the highest when40wt.%KHCO₃ was added,reaching 28.6%,which increased by 6.6%compared with that without additives.The activation energy of the hydrothermal carbon with Na HCO₃ is 178.5k J/mol,and the combustion reaction is the least difficult.Ca CO₃ is the main mineral in the hot carbon added with hydrogen carbonate brine.KHCO₃ has little effect on the pore structure of duckweed hydrothermal carbon.